Plus Therapeutics (NASDAQ: PSTV) launches $17.35M ATM with Canaccord

Rhea-AI Filing Summary

Insights

Q: What does Plus Therapeutics' $100,000,000 shelf registration mean for PSTV shareholders?

Q: What commission does Canaccord receive under the ATM agreement?

Key Figures

Key Terms

Offering Details

It permits the company to offer up to $100,000,000 of various securities over time. The shelf allows future offerings of common stock, preferred stock, warrants, debt, subscription rights or units via prospectus supplements and does not itself issue shares.

Canaccord receives a commission equal to 3.0% of the gross sales price per share for shares sold under the Equity Distribution Agreement, as stated in the prospectus supplement.

Filing Impact

(Neutral)

Filing Sentiment

(Neutral)

Form Type

S-3

Plus Therapeutics, Inc. registered up to $100,000,000 of securities on a shelf registration, including an $17,350,000 at-the-market equity program under an Equity Distribution Agreement with Canaccord Genuity LLC dated June 1, 2026.

The prospectus describes a broad shelf for common stock, preferred stock, warrants, debt securities, subscription rights and units to be offered from time to time, and a separate prospectus supplement governs the ATM sales through Canaccord. The base prospectus states that proceeds use is for general corporate purposes and that specific terms and underwriting arrangements will appear in any prospectus supplement.

Shelf plus ATM provides near-term equity access while preserving flexible issuance paths.

The registration statement establishes a $100,000,000 shelf and an $17,350,000 equity distribution agreement with Canaccord Genuity LLC dated June 1, 2026. The ATM is structured as sales agent-placed offerings, at commission of 3.0 of gross proceeds, permitting sales on Nasdaq or in privately negotiated transactions with prior written approval.

Cash-flow treatment is that the company receives proceeds from primary sales; specific timing, sizes and pricing will be set in prospectus supplements. The actual capital raised will depend on market receptivity and placement pacing under the Equity Distribution Agreement.

Registration supports funding for radiopharmaceutical and diagnostic programs described in the prospectus summary.

The prospectus reiterates the company’s focus on precision diagnostics (CNSide Test) and radiotherapeutics (including 186Re obisbemeda and 188RNL-BAM). It notes CLIA accreditation for the Houston laboratory and payer agreements expanding access to approximately 81 million covered lives.

Use of proceeds is broad—R&D, clinical trials, manufacturing and commercialization are listed; specific allocations will appear in any accompanying prospectus supplement for a given offering.

Shelf capacity: $100,000,000 ATM program size: $17,350,000 Public float: $52,133,434 +4 more

7 metrics

Shelf capacity $100,000,000 aggregate initial offering price of securities registered under the base prospectus

ATM program size $17,350,000 equity distribution agreement with Canaccord Genuity LLC dated June 1, 2026

Public float $52,133,434 public float as of May 27, 2026 based on 6,883,897 shares and $7.62 price

Shares outstanding 6,883,897 shares shares outstanding as of May 27, 2026 (post reverse split)

Reverse stock split ratio 1-for-25 reverse stock split effected April 2, 2026 reducing shares from ~171,550,698 to ~6,862,027

Options outstanding 631,779 shares outstanding options as of May 27, 2026 under equity incentive plans

January 2026 warrants 1,815,784 warrants outstanding warrants issued January 15, 2026 exercisable at $9.50 per share until January 15, 2031

Equity Distribution Agreement, At-the-Market offering, CLIA, Reverse Stock Split

4 terms

Equity Distribution Agreement financial

"‘‘Equity Distribution Agreement’’ with Canaccord Genuity LLC dated June 1, 2026"

An equity distribution agreement is a formal plan between a company and financial institutions to sell newly issued shares of the company's stock to investors over a period of time. It helps the company raise money gradually, similar to filling a container with water in stages, rather than all at once. For investors, it provides an organized way to buy shares and can influence the stock's supply and price.

At-the-Market offering regulatory

"sales by methods deemed to be an “at the market” offering as defined in Rule 415"

An at-the-market offering is a method companies use to sell new shares of stock directly into the open market over time, rather than all at once. This allows them to raise money gradually, similar to selling small pieces of a product instead of a large batch. For investors, it means the company can access funding more flexibly, but it may also increase the supply of shares and influence the stock’s price.

CLIA regulatory

"laboratory in Houston has received a certificate of accreditation from the Centers for Medicare & Medicaid Services (“CMS”) compliant with CLIA"

CLIA stands for the Clinical Laboratory Improvement Amendments, a U.S. federal program that sets quality and safety standards for laboratories that test human samples. For investors, a CLIA certification is like a safety inspection or license for a lab — it signals that test results are produced under accepted quality controls, affects a lab’s ability to sell or bill certain tests, and therefore influences a company’s regulatory risk, market access, and credibility.

Reverse Stock Split financial

"On April 2, 2026, we effected a 1-for-25 reverse split of our common stock"

A reverse stock split is when a company reduces the number of its shares outstanding, making each share more valuable. For example, if you own 100 shares worth $1 each, a 1-for-10 reverse split would turn your 100 shares into 10 shares worth $10 each. Companies often do this to boost their stock price and appear more stable to investors.

Large accelerated filer

Accelerated filer

Non-accelerated filer

Smaller reporting company

Emerging growth company

shelf with ATM equity distribution

Offering

Offering Type shelf with ATM equity distribution

Use of Proceeds general corporate purposes including R&D, clinical trials, manufacturing, commercialization, working capital

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06/01/2026 - 08:58 PM

As filed with the Securities and Exchange Commission on June 1, 2026

Registration No. 333-

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM S-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

PLUS THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

Delaware

33-0827593

(State or other jurisdiction of

incorporation or organization)

(I.R.S. Employer

Identification Number)

6420 Levit Green Boulevard, Suite 310

Houston, Texas 77021

(737) 255-7194

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Andrew Sims

Chief Financial Officer

Plus Therapeutics, Inc.

6420 Levit Green Boulevard, Suite 310

Houston, Texas 77021

(737) 255-7194

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

David E. Danovitch

Charles E. Chambers, Jr.

Sullivan & Worcester LLP

1251 Avenue of the Americas

New York, New York 10020

(212) 660-3060

Approximate date of commencement of proposed sale to the public: From time to time after the effective date of this Registration Statement.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box. ☐

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box. ☒

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box. ☐

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box. ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act:

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ☐

The Registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until this registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

EXPLANATORY NOTE

This registration statement contains:

•

a base prospectus, which covers the offering, issuance and sale of the registrant’s common stock, preferred stock, warrants, debt securities and/or units from time to time in one or more offerings; and

•

an equity distribution agreement prospectus, which covers the offering, issuance and sale by us of up to a maximum aggregate offering price of $17,350,000 of the registrant’s common stock that may be issued and sold from time to time under an equity distribution agreement with Canaccord Genuity LLC.

The base prospectus immediately follows this explanatory note. The specific terms of any other securities to be offered pursuant to the base prospectus, other than the securities to be issued and sold pursuant to the equity distribution agreement, will be specified in one or more prospectus supplements to the base prospectus. The equity distribution agreement prospectus immediately follows the base prospectus. The $17,350,000 of common stock that may be offered, issued and sold under the equity distribution agreement prospectus is included in the $100,000,000 of securities that may be offered, issued and sold by us under the base prospectus. Upon termination of the equity distribution agreement with Canaccord Genuity LLC (the “Equity Distribution Agreement”), any portion of the $17,350,000 included in the equity distribution agreement prospectus that is not sold pursuant to the Equity Distribution Agreement will be available for sale in other offerings pursuant to the base prospectus and a corresponding prospectus supplement, and if no shares are sold under the Equity Distribution Agreement, the full $100,000,000 of securities may be sold in other offerings pursuant to the base prospectus and a corresponding prospectus supplement.

The information contained in this prospectus is not complete and may be changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any state where such offer or sale is not permitted.

SUBJECT TO COMPLETION, DATED JUNE 1, 2026

PROSPECTUS

LOGO

$100,000,000

Common Stock

Preferred Stock

Warrants

Debt Securities

Subscription Rights

Units

We may offer and sell up to $100,000,000 in aggregate initial offering price of the securities identified above from time to time in one or more offerings.

This prospectus provides you with a general description of the securities we may offer and sell and the general manner in which they may be offered. Each time we offer securities pursuant to this prospectus, we will provide one or more supplements to this prospectus or free writing prospectuses containing specific information about the offering and the terms of the securities being sold. The prospectus supplement or free writing prospectus may also add, update or change information contained in this prospectus with respect to that offering. You should carefully read this prospectus, the applicable prospectus supplement, the information incorporated herein and therein by reference, and any free writing prospectus before you invest in any of our securities.

We may offer and sell the securities described in this prospectus and any prospectus supplement to or through one or more underwriters, dealers, and agents, or directly to purchasers, or through a combination of these methods. The names of any underwriters, dealers or agents involved in the sale of any of the securities and the terms of the arrangements with them will be set forth in the applicable prospectus supplement or free writing prospectus. See the sections of this prospectus entitled “About this Prospectus” and “Plan of Distribution” for more information. No securities may be sold without delivery of this prospectus and the applicable prospectus supplement describing the method and terms of the offering of such securities.

INVESTING IN OUR SECURITIES INVOLVES A HIGH DEGREE OF RISK. SEE THE SECTION OF THIS PROSPECTUS ENTITLED “RISK FACTORS” ON PAGE 6 OF THIS PROSPECTUS AND ANY SIMILAR SECTION CONTAINED IN THE APPLICABLE PROSPECTUS SUPPLEMENT CONCERNING FACTORS YOU SHOULD CONSIDER BEFORE INVESTING IN OUR SECURITIES.

Our shares of common stock are listed on the Nasdaq Capital Market (“Nasdaq”) under the symbol “PSTV.” On May 27, 2026, the last reported sale price of our shares of common stock on Nasdaq was $6.64 per share. The applicable prospectus supplement will contain information, where applicable, as to any other listing, if any, on Nasdaq or any securities market or other exchange of the securities covered by the applicable prospectus supplement.

As of May 27, 2026, the aggregate market value of our outstanding common stock held by non-affiliates, or public float, was approximately $52,133,434 based on 6,883,897 shares of outstanding common stock, of which 6,841,658 shares were held by non-affiliates, and a price of $7.62 per share, which was the closing price at which our common stock was last sold on Nasdaq on April 22, 2026. We have offered and sold $0 of our securities pursuant to General Instruction I.B.6 of Form S-3 during the prior 12-calendar-month period that ends on and includes the date of this prospectus. Pursuant to General Instruction I.B.6 of Form S-3, in no event will we sell securities registered on this registration statement in a public primary offering with a value exceeding more than one-third of our public float in any 12-month period so long as our public float remains below $75 million.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is , 2026

TABLE OF CONTENTS


	Page
About this Prospectus		1
Forward-Looking Statements		2
Market, Industry and Other Data		3
Prospectus Summary		4
Risk Factors		6
Selected Financial Data Reflecting Reverse Stock Split		7
Use of Proceeds		8
Plan of Distribution		9
Description of Our Securities		12
Description of Our Common Stock		13
Description of Our Preferred Stock		17
Description of Our Warrants		21
Description of Our Debt Securities		22
Description of Subscription Rights		27
Description of Our Units		28
Where You Can Find More Information		29
Incorporation by Reference		30
Legal Matters		30
Experts		30

ABOUT THIS PROSPECTUS

This prospectus is a part of a registration statement on Form S-3 that we filed with the Securities and Exchange Commission, or the SEC, using a “shelf” registration process. Under this shelf registration process, we may offer to sell any of the securities, or any combination of the securities, described in this prospectus, in each case in one or more offerings, up to a total dollar amount of $100,000,000.

This prospectus provides you only with a general description of the securities that we may offer. Each time securities are sold under this shelf registration statement, we will provide an accompanying prospectus supplement that will contain specific information about the terms of those securities and the terms of that offering. The accompanying prospectus supplement may also add, update or change information contained in this prospectus. If there is any inconsistency between the information in this prospectus and any accompanying prospectus supplement, you should rely on the information in the accompanying prospectus supplement. You should read both this prospectus and any accompanying prospectus supplement, including all documents incorporated by reference herein and therein, together with the additional information described under “Where You Can Find More Information” below.

The information contained in this prospectus is not complete and may be changed. You should rely only on the information provided in or incorporated by reference in this prospectus or in any accompanying prospectus supplement, or documents to which we otherwise refer you. We have not authorized anyone else to provide you with different information.

We have not authorized any dealer, agent or other person to give any information or to make any representation other than those contained or incorporated by reference in this prospectus and any accompanying prospectus supplement. You must not rely upon any information or representation not contained or incorporated by reference in this prospectus or an accompanying prospectus supplement. This prospectus and the accompanying prospectus supplement, if any, do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the registered securities to which they relate, nor do this prospectus and the accompanying prospectus supplement, if any, constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction. You should not assume that the information contained in this prospectus and the accompanying prospectus supplement, if any, is accurate on any date subsequent to the date set forth on the front of such document or that any information we have incorporated by reference is correct on any date subsequent to the date of the document incorporated by reference, even though this prospectus and any accompanying prospectus supplement is delivered or securities are sold on a later date.

References in this prospectus to the terms “the Company,” “Plus Therapeutics,” “we,” “our” and “us” or other similar terms mean Plus Therapeutics, Inc. and our wholly owned subsidiary, unless we state otherwise or the context indicates otherwise.

Unless otherwise indicated, all share amounts and share prices disclosed herein are presented on a post-split basis, giving effect to the reverse stock split of all of the outstanding shares of our common stock on April 2, 2026. Any financial statements incorporated by reference into this prospectus that were filed prior to such reverse stock split present the outstanding shares on a pre-split basis.

FORWARD-LOOKING STATEMENTS

This prospectus and the documents incorporated by reference herein contain, and any prospectus supplement and the documents incorporated therein, may contain forward-looking statements that involve substantial risks and uncertainties. All statements, other than statements of historical facts, included in this prospectus, any prospectus supplement or the documents incorporated herein and therein by reference, including statements regarding our future financial condition, results of operations, business strategy and plans and objectives of management for future operations, industry trends and other future events, are forward-looking statements. In some cases, you can identify forward-looking statements by terminology such as “believe,” “will,” “may,” “estimate,” “continue,” “anticipate,” “intend,” “should,” “plan,” “expect,” “predict,” “project,” “could,” “potentially,” “continue,” “ongoing,” “scheduled” or the negative of these terms or other similar expressions, although not all forward-looking statements contain these identifying terms.

These statements include, without limitation, statements regarding: our anticipated expenditures, including research and development, and general and administrative expenses; our strategic collaborations and license agreements, intellectual property, FDA and EMA approvals and interactions and government regulation; the potential size of the market for our product candidates; our research and development efforts; results from our pre-clinical and clinical studies and the implications of such results regarding the efficacy or safety of our product candidates; the safety profile, pathways, and efficacy of our product candidates and formulations; anticipated advantages of our product candidates over other products available in the market and being developed; the populations that will most benefit from our product candidates and indications that will be pursued with each product candidate; anticipated progress in our current and future clinical trials; plans and strategies to create novel technologies; our IP strategy; future development and/or expansion of our product candidates and therapies in our markets; sources of competition for any of our product candidates; our ability to generate product or development revenues and the sources of such revenue; our ability to effectively manage our gross profit margins; our ability to obtain and maintain regulatory approvals; expectations as to our future performance; portions of the “Liquidity and Capital Resources” section of our annual and quarterly reports filed with the SEC; our need for additional financing and the availability thereof; our ability to continue as a going concern; our ability to remain listed on the Nasdaq Capital Market; our ability to repay or refinance some or all of our outstanding indebtedness and our ability to raise capital in the future; our ability to transfer the drug product manufacture to a contract drug manufacturing organization; and the potential enhancement of our cash position through development, marketing, and licensing arrangements.

We have based these forward-looking statements largely on our current expectations and projections about future events and financial trends that we believe may affect our business, financial condition and results of operations. We cannot guarantee that the results and other expectations expressed, anticipated or implied in any forward-looking statement will be realized. The risks set forth under Item 1A of our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, as revised or supplemented by our Quarterly Reports on Form 10-Q and other documents we file with the SEC, describe material risks to our business, and you should read and interpret any forward-looking statements together with these risks. A variety of factors, including these risks, could cause our actual results and other expectations to differ materially from the anticipated results or other expectations expressed, anticipated or implied in our forward-looking statements. Should known or unknown risks materialize, or should underlying assumptions prove inaccurate, actual results could differ materially from past results and those anticipated, estimated or projected in the forward-looking statements. You should bear this in mind as you consider any forward-looking statements.

You should read this prospectus, any prospectus supplement and the documents that we incorporate by reference herein and therein completely and with the understanding that our actual future results may be materially different from what we expect. The forward-looking statements contained in this prospectus are made as of the date of this prospectus and we do not assume any obligation to update any forward-looking statements, whether as a result of new information, future events or otherwise, except as required by applicable law.

MARKET, INDUSTRY AND OTHER DATA

This prospectus and any applicable prospectus supplement and the documents incorporated by reference herein and therein contain estimates, projections, market research and other information concerning, among other things, our industry, our business, and markets for our product candidates. Unless otherwise expressly stated, we obtain this information from reports, research surveys, studies and similar data prepared by market research firms and other third parties, industry, medical and general publications, government data and similar sources as well as from our own internal estimates and research and from publications, research, surveys and studies conducted by third parties on our behalf. Information that is based on estimates, projections, market research or similar methodologies is inherently subject to uncertainties and actual events or circumstances may differ materially from events and circumstances that are reflected in this information. As a result, you are cautioned not to give undue weight to such information.

PROSPECTUS SUMMARY

This summary highlights selected information from this prospectus and does not contain all of the information that you need to consider in making your investment decision. You should carefully read the entire prospectus, the applicable prospectus supplement and any related free writing prospectus, including the risks of investing in our securities discussed under the heading “Risk Factors” contained in the applicable prospectus supplement and any related free writing prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus. You should also carefully read the information incorporated by reference into this prospectus, including our financial statements, and the exhibits to the registration statement of which this prospectus is a part.

Company Overview

Plus Therapeutics is a U.S. healthcare company developing and commercializing precision diagnostics and targeted radiopharmaceuticals for central nervous system (“CNS”) cancers. CNSide Diagnostics, LLC (“CNSide Diagnostics”) is our wholly owned subsidiary that develops and commercializes proprietary laboratory-developed tests, such as CNSide^®, designed to identify tumor cells that have metastasized to the central nervous system in patients with carcinomas and melanomas. In radiopharmaceuticals, our lead candidate, rhenium (¹⁸⁶Re) obisbemeda, is designed specifically for CNS cancers including recurrent glioblastoma (“GBM”), leptomeningeal metastases (“LM”), and pediatric brain cancers (“PBC”) by direct localized delivery utilizing approved standard-of-care tissue access such as with convection-enhanced delivery (“CED”) and intraventricular brain (Ommaya reservoir) catheters. Our acquired radiotherapeutic candidate, Rhenium-188 NanoLiposome Biodegradable Alginate Microsphere (“¹⁸⁸RNL-BAM”) is designed to treat many solid organ cancers including primary and secondary liver cancers by intra-arterial injection.

Traditional approaches to radiation therapy for cancer, such as external beam radiation, have many disadvantages including continuous treatment for four to six weeks (which is onerous for patients), that the radiation damages healthy cells and tissue, and that the amount of radiation delivered is very limited and, therefore, is frequently inadequate to fully destroy the cancer.

Our novel radioactive drug formulations, medical devices and therapeutic candidates have the potential to overcome these disadvantages as they are designed to deliver safe and effective doses of radiation at the tumors-potentially in a single treatment. To achieve this, we have developed innovative approaches to drug formulation, including encapsulating radionuclides such as rhenium isotopes with nanoliposomes and microspheres. Our formulations are intended to achieve elevated patient-absorbed radiation doses and extend retention times such that the clearance of the isotope occurs after significant and essentially complete radiation decay, which will contribute and provide less normal tissue/organ exposure and improved safety margins.

By minimizing radiation exposure to healthy tissues while simultaneously maximizing locoregional delivery and, thereby, efficacy, we hope to reduce the radiation toxicity for patients, improving their quality of life and life expectancy. Our radiotherapeutic platform, combined with advances in neurosurgery, nuclear medicine, interventional radiology, neuro-oncology, and radiation oncology, affords us the opportunity to target a broad variety of cancer types.

The CNSide Cerebrospinal Fluid Assay is currently being utilized in the ReSPECT-LM clinical trial funded by the Cancer Prevention and Research Institute of Texas (“CPRIT”). In connection with our business plan for developing the CNSide Platform, we formed CNSide Diagnostics and our board of directors appointed a board of managers for CNSide Diagnostics. We re-introduced the CNSide Cerebrospinal Fluid Tumor Cell Enumeration test (the “CNSide Test”), which is a laboratory developed test (“LDT”) in August 2025. The laboratory for the CNSide Test in Houston, Texas has received a certificate of accreditation from the Centers for Medicare &

Medicaid Services (“CMS”) which deems the lab compliant with Clinical Laboratory Improvement Amendments (“CLIA”) regulations. Furthermore, CNSide Diagnostics has signed national agreements to provide the CNSide Test with four payers, including United Healthcare, Humana, Highmark, and Blue Shield of California, expanding patient access nationwide to approximately 81 million.

Corporate Information

In March 2025, we moved our headquarters to Houston, Texas, in proximity to world-class cancer institutions and researchers. Our principal executive offices are located at 6420 Levit Green Boulevard, Suite 310, Houston, Texas 77021, and our telephone number is (737) 255-7194. We maintain a website at www.plustherapeutics.com. We make available free of charge through our website our Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, and amendments to these reports filed or furnished pursuant to Section 13(a) or 15(d) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), as soon as reasonably practicable after we electronically file such material with, or furnish such material to, the SEC. These reports and other information are also available, free of charge, at www.sec.gov. Information contained on, or that can be accessed through, the websites referenced in this prospectus are not a part of, or incorporated by reference into, this prospectus.

RISK FACTORS

Investing in our securities involves a high degree of risk. The prospectus supplement applicable to each offering of our securities will contain a discussion of the risks applicable to an investment in our securities. Prior to making a decision about investing in our securities, you should carefully consider the specific factors discussed under the heading “Risk Factors” in the applicable prospectus supplement, together with all of the other information contained or incorporated by reference in the prospectus supplement or appearing or incorporated by reference in this prospectus. You should also consider the risks, uncertainties and assumptions discussed under the heading “Risk Factors” in our most recent Annual Report on Form 10-K and our Quarterly Reports on Form 10-Q and other documents that we file with the SEC, which are incorporated herein by reference as described in this prospectus under the heading “Where You Can Find More Information”. The risks and uncertainties we have described in such documents are not the only risks that we face. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our operations.

SELECTED FINANCIAL DATA REFLECTING REVERSE STOCK SPLIT

Reverse Stock Split

On April 2, 2026, we effected a 1-for-25 reverse split of our common stock (the “Reverse Stock Split”). As a result of the Reverse Stock Split, every twenty-five (25) shares of our issued and outstanding common stock, par value $0.001, were converted into one (1) share of common stock, par value $0.001, reducing the number of issued and outstanding shares of our common stock from approximately 171,550,698 shares to approximately 6,862,027 shares as of April 2, 2026. The Reverse Stock Split did not alter the par value of the Company’s common stock or modify any voting rights or other terms of the common stock. No fractional shares were issued in connection with the Reverse Stock Split. Except where the context otherwise requires, share numbers in this prospectus reflect the 1-for-25 reverse stock split of our common stock.

The following selected financial data has been derived from our audited financial statements included in our Annual Report on Form 10-K filed with the SEC on March 12, 2026, as adjusted to reflect the reverse stock split for all periods presented. Our historical results are not indicative of the results that may be expected in the future and results of interim periods are not indicative of the results for the entire year.

AS REPORTED (in thousands, except share and per share amounts):


	Years Ended December 31,
	2025			2024
Net loss	$	(22,386	)	$	(12,978	)
Net loss per share of common stock—basic	$	(0.29	)	$	(1.95	)
Weighted average number of shares of common stock outstanding—basic		77,804,612			6,640,251
Net loss per share of common stock—diluted	$	(0.29	)	$	(2.34	)
Weighted average number of shares of common stock outstanding—diluted		77,804,612			7,700,774
Common shares outstanding at year end		138,675,856			5,896,333

AS ADJUSTED FOR 1-FOR-25 REVERSE STOCK SPLIT (unaudited, in thousands, except share and per share amounts):


	Years Ended December 31,
	2025			2024
Net loss	$	(22,386	)	$	(12,978	)
Net loss per share of common stock—basic	$	(7.25	)	$	(48.75	)
Weighted average number of shares of common stock outstanding—basic		3,112,184			265,610
Net loss per share of common stock—diluted	$	(7.25	)	$	(58.50	)
Weighted average number of shares of common stock outstanding—diluted		3,112,184			308,030
Common shares outstanding at year end		5,547,034			235,853

USE OF PROCEEDS

Except as otherwise provided in the applicable prospectus supplement relating to a specific offering, we intend to use the net proceeds from the sale of securities by us under this prospectus and any applicable prospectus supplement for general corporate purposes, which may include research and development expenditures, preclinical study and clinical trial expenditures, manufacturing expenditures, commercialization expenditures, working capital, capital expenditures, acquisitions of new technologies, products or businesses, and investments. Additional information on the use of net proceeds from the sale of securities by us under this prospectus may be set forth in the accompanying prospectus supplement relating to the specific offering.

PLAN OF DISTRIBUTION

We may sell the securities, from time to time pursuant to public offerings, negotiated transactions, block trades, “At the Market Offerings,” within the meaning of Rule 415(a)(4) of the Securities Act of 1933, as amended, or the Securities Act, into an existing trading market, at prevailing market prices, or a combination of these methods. We may sell the securities to or through underwriters or dealers, through agents or remarketing firms, or directly to one or more purchasers. We may distribute securities from time to time in one or more transactions:

•

at a fixed price or prices, which may be changed;

•

at market prices prevailing at the time of sale;

•

at prices related to such prevailing market prices; or

•

at negotiated prices.

A prospectus supplement or supplements (and any related free writing prospectus that we may authorize to be provided to you) will describe the terms of the offering of the securities, including, to the extent applicable:

•

the name or names of the underwriters, dealers or agents, if any;

•

if the securities are to be offered through the selling efforts of brokers or dealers, the plan of distribution and the terms of any agreement, arrangement, or understanding entered into with broker(s) or dealer(s) prior to the effective date of the registration statement, and, if known, the identity of any broker(s) or dealer(s) who will participate in the offering and the amount to be offered through each;

•

the purchase price of the securities or other consideration therefor, and the proceeds, if any, we will receive from the sale;

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if any of the securities being registered are to be offered otherwise than for cash, the general purposes of the distribution, the basis upon which the securities are to be offered, the amount of compensation and other expenses of distribution, and by whom they are to be borne;

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any delayed delivery arrangements;

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any options under which underwriters may purchase additional securities from us;

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any agency fees or underwriting discounts and other items constituting agents’ or underwriters’ compensation;

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any public offering price;

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any discounts, commissions or concessions allowed or reallowed or paid to dealers;

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the identity and relationships of any finders, if applicable; and

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any securities exchange or market on which the securities may be listed.

Only underwriters named in the prospectus supplement will be underwriters of the securities offered by the prospectus supplement.

If underwriters are used in the sale, they will acquire the securities for their own account and may resell the securities from time to time in one or more transactions at a fixed public offering price or at varying prices determined at the time of sale. The obligations of the underwriters to purchase the securities will be subject to the conditions set forth in the applicable underwriting agreement. We may offer the securities to the public through underwriting syndicates represented by managing underwriters or by underwriters without a syndicate. Unless otherwise indicated in the prospectus supplement, subject to certain conditions, the underwriters will be obligated to purchase all of the securities offered by the prospectus supplement, other than securities covered by any purchase option. Any public offering price and any discounts or concessions allowed or reallowed or paid to

dealers may change from time to time. We may use underwriters, dealers or agents with whom we have a material relationship. We will describe in the prospectus supplement, naming the underwriter, dealer or agent, the nature of any such relationship.

We may use a remarketing firm to offer the securities in connection with a remarketing arrangement upon their purchase. Remarketing firms will act as principals for their own account or as agents for us. These remarketing firms will offer or sell the securities pursuant to the terms of the securities. A prospectus supplement will identify any remarketing firm and the terms of its agreement, if any, with us and will describe the remarketing firm’s compensation. Remarketing firms may be deemed to be underwriters in connection with the securities they remarket.

If we offer and sell securities through a dealer, we or an underwriter will sell the securities to the dealer, as principal. The dealer may then resell the securities to the public at varying prices to be determined by the dealer at the time of resale. The name of the dealer and the terms of the transaction will be set forth in the applicable prospectus supplement.

We may sell securities directly or through agents we designate from time to time. We will name any agent involved in the offering and sale of securities and we will describe any commissions we will pay to the agent in the prospectus supplement. Unless the prospectus supplement states otherwise, our agent will act on a best-efforts basis for the period of its appointment.

Dealers and agents participating in the distribution of the securities may be deemed to be underwriters, and compensation received by them on resale of the securities may be deemed to be underwriting discounts. If such dealers or agents were deemed to be underwriters, they may be subject to statutory liabilities under the Securities Act.

We may sell securities directly to one or more purchasers without using underwriters or agents. Underwriters, dealers and agents that participate in the distribution of the securities may be underwriters as defined in the Securities Act, and any discounts or commissions they receive from us and any profit on their resale of the securities may be treated as underwriting discounts and commissions under the Securities Act.

We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the prospectus supplement.

We may provide agents, underwriters and dealers with indemnification against civil liabilities, including liabilities under the Securities Act, or contribution with respect to payments that the agents, underwriters or dealers may make with respect to these liabilities. Agents, underwriters and dealers, or their respective affiliates, may engage in transactions with, or perform services for, us in the ordinary course of business.

All securities we may offer, other than shares of common stock, will be new issues of securities with no established trading market. Any underwriter may make a market in these securities, but will not be obligated to do so and may discontinue any market making at any time without notice. We cannot guarantee the liquidity of the trading markets for any securities.

Any underwriter may engage in purchase options, stabilizing transactions, short-covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. Purchase options involve sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum price. Syndicate-covering or other short-covering transactions involve purchases of the securities, either through exercise of the purchase option or in the open market after the distribution is completed, to cover short positions. Penalty bids permit the

underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a stabilizing or covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.

Any underwriters that are qualified market makers on the Nasdaq Capital Market may engage in passive market making transactions in the shares of common stock on the Nasdaq Capital Market in accordance with Regulation M under the Exchange Act, during the business day prior to the pricing of the offering, before the commencement of offers or sales of the shares of common stock. Passive market makers must comply with applicable volume and price limitations and must be identified as passive market makers. In general, a passive market maker must display its bid at a price not in excess of the highest independent bid for such security; if all independent bids are lowered below the passive market maker’s bid, however, the passive market maker’s bid must then be lowered when certain purchase limits are exceeded. Passive market making may stabilize the market price of the securities at a level above that which might otherwise prevail in the open market and, if commenced, may be discontinued at any time.

DESCRIPTION OF OUR SECURITIES

We may offer and sell, at any time and from time to time:

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shares of our common stock;

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shares of our preferred stock;

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warrants to purchase shares of our common stock, preferred stock or debt securities;

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debt securities consisting of debentures, notes or other evidences of indebtedness;

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subscription rights to purchase any of the foregoing securities;

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units consisting of a combination of the foregoing securities; or

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any combination of these securities.

The terms of any securities we offer will be determined at the time of sale. We may issue debt securities that are exchangeable for and/or convertible into shares of common stock or any of the other securities that may be sold under this prospectus. When particular securities are offered by us, a supplement to this prospectus will be filed with the SEC, which will describe the terms of the offering and sale of the offered securities.

DESCRIPTION OF COMMON STOCK

The following summary of the terms of our shares of common stock is subject to and qualified in its entirety by reference to our composite certificate of incorporation, as amended and restated (the “Charter”), and bylaws, as amended and restated (the “Bylaws”), copies of which are on file with the SEC as exhibits to previous filings with the SEC. Please refer to “Where You Can Find More Information” below for directions on obtaining these documents.

Our Charter authorizes us to issue up to 2,000,000,000 shares of our common stock and to issue and designate the rights of, without stockholder approval, and up to 5,000,000 shares of preferred stock. As of May 27, 2026, there were 6,883,897 shares of common stock outstanding, held by 28 stockholders of record. This figure does not reflect the number of beneficial owners of shares of our common stock as a single stockholder of record often holds shares in nominee name (also referred to as, in “street name”) on behalf of multiple beneficial owners.

Voting Rights

Each holder of shares of our common stock is entitled to one vote for each share held of record on each matter submitted to a vote of stockholders, including the election of directors. Stockholders do not have any right to cumulate votes in the election of directors.

When a quorum is present at any meeting, the vote of the holders of a majority of the voting power of the common stock and entitled to vote present in person or represented by proxy shall decide any question brought before such meeting, unless the question is one upon which by express provision of our Charter, Bylaws or by law, a different vote is required, in which case such express provision shall govern and control the decision of such question. Directors are elected by a plurality of the voting power of the shares present in person or represented by proxy and entitled to vote on the election of directors at a meeting at which a quorum is present, and stockholders are not entitled to cumulate their votes for the election of directors.

Dividend Rights

Subject to preferences that may be granted to the holders of any then outstanding preferred stock, each holder of shares of our outstanding common stock is entitled to share ratably in distributions to stockholders and to receive ratably such dividends as may be declared by our board of directors out of funds legally available therefor.

Liquidation Rights

In the event of our liquidation, dissolution or winding up, the holders of shares of our common stock will be entitled to receive, after payment of all of our debts and liabilities and of all sums to which holders of any then outstanding preferred stock may be entitled, the distribution of any of our remaining assets.

Other Rights and Preferences

Holders of shares of our common stock have no conversion, exchange, sinking fund, redemption or appraisal rights (other than such as may be determined by our board of directors in its sole discretion) and have no preemptive rights to subscribe for any of our securities. The rights, preferences and privileges of the holders of our shares of common stock are subject to, and may be adversely affected by, the rights of shares of any series of our preferred stock that we may classify and issue in the future.

Outstanding Stock Options

As of May 27, 2026, we had outstanding options to purchase 631,779 shares of our common stock, pursuant to our equity incentive plans.

Outstanding Stock Units

As of May 27, 2026, we had 256,524 outstanding restricted stock units and no shares reserved to be granted as restricted stock units, pursuant to our 2020 Stock Incentive Plan, as amended.

Outstanding Warrants

As of May 27, 2026, we had 1,930 Series A common stock purchase warrants outstanding (the “May 2024 Series A Warrants”), which warrants were issued on May 9, 2024 and have an exercise price of $44.75 per share. The May 2024 Series A Warrants are exercisable immediately. Subject to certain ownership limitations, the May 2024 Series A Warrants will be exercisable until May 8, 2029.

As of May 27, 2026, we had 1,815,784 warrants outstanding (the “January 2026 Warrants”), which warrants were issued on January 15, 2026. The January 2026 Warrants have an exercise price of $9.50 per share. The January 2026 Warrants are exercisable immediately. Subject to certain ownership limitations, the January 2026 Warrants will be exercisable until January 15, 2031.

Incentive Plans

As of May 27, 2026, there were 724,522 shares of our common stock reserved for future issuance under our 2020 Stock Incentive Plan, as amended, and 44,840 shares of our common stock reserved for future issuance under our Amended and Restated 2015 New Employment Incentive Plan (as inducement material to certain individuals entering into employment with us in accordance with Nasdaq Listing Rule 5635(c)(4)).

Description of Certain Provisions of Delaware Law and our Charter and Bylaws

Section 203 of the Delaware General Corporation Law

We are subject to the provisions of Section 203 of the Delaware General Corporation Law (the “DGCL”). In general, Section 203 of the DGCL prohibits a publicly held Delaware corporation from engaging in a “business combination” with an “interested stockholder” for a three-year period following the time that this stockholder becomes an interested stockholder, unless the business combination is approved in a prescribed manner. Under Section 203 of the DGCL, a business combination between a corporation and an interested stockholder is prohibited unless it satisfies one of the following conditions:

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prior to the date of the transaction, the board of directors of the corporation approved either the business combination or the transaction which resulted in the stockholder becoming an interested stockholder;

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the interested stockholder owned at least 85% of the voting stock of the corporation outstanding upon consummation of the transaction, excluding for purposes of determining the number of shares outstanding (1) shares owned by persons who are directors and also officers and (2) shares owned by employee stock plans in which employee participants do not have the right to determine confidentially whether shares held subject to the plan will be tendered in a tender or exchange offer; or

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on or subsequent to the consummation of the transaction, the business combination is approved by the board of directors and authorized at an annual or special meeting of stockholders, and not by written consent, by the affirmative vote of at least 66-2/3% of the outstanding voting stock which is not owned by the interested stockholder.

Section 203 of the DGCL defines a business combination to include:

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any merger or consolidation involving the corporation and the interested stockholder;

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any sale, transfer, lease, pledge or other disposition involving the interested stockholder of 10% or more of the assets of the corporation;

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subject to exceptions, any transaction that results in the issuance or transfer by the corporation of any stock of the corporation to the interested stockholder;

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subject to exceptions, any transaction involving the corporation that has the effect of increasing the proportionate share of the stock of any class or series of the corporation beneficially owned by the interested stockholder; and

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the receipt by the interested stockholder of the benefit of any loans, advances, guarantees, pledges or other financial benefits provided by or through the corporation.

In general, Section 203 of the DGCL defines an interested stockholder as any entity or person beneficially owning 15% or more of the outstanding voting stock of the corporation and any entity or person affiliated with or controlling or controlled by the entity or person.

Anti-takeover Provisions

Certain provisions of our Charter and Bylaws, which are summarized below, may have the effect of delaying, deferring or preventing another person from acquiring control of the Company. These provisions may discourage takeovers, coercive or otherwise, and are also designed, in part, to encourage persons seeking to acquire control of the Company to negotiate first with our board of directors. These provisions include the following:

Board Vacancies. Our Bylaws provide that any vacancy or vacancies in the Board shall be deemed to exist in the case of the death, resignation or removal of any director, or if the authorized number of directors be increased. Vacancies may be filled by a majority of the remaining directors, though less than a quorum, or by a sole remaining director, unless otherwise provided in our Charter. The stockholders may elect a director or directors at any time to fill any vacancy or vacancies not filled by the directors.

Stockholder Action; Special Meeting of Stockholders. Pursuant to Section 228 of the DGCL, any action required to be taken at any annual or special meeting of the stockholders may be taken without a meeting, without prior notice and without a vote if a consent or consents in writing, setting forth the action so taken, is signed by the holders of outstanding stock having not less than the minimum number of votes that would be necessary to authorize or take such action at a meeting at which all shares of stock entitled to vote thereon were present and voted, unless our Charter provides otherwise. Our Charter provides that stockholders may not take action by written consent but may only take action at annual or special meetings of stockholders. As a result, a holder controlling a majority of our capital stock would not be able to amend our Bylaws or remove directors without holding a meeting of stockholders called in accordance with our Bylaws. Our Bylaws provide that special meetings of the stockholders may be called only at the request of our president, chief executive officer or chairman of our board of directors or by a majority of our board of directors. These provisions might delay the ability of our stockholders to force consideration of a proposal or for stockholders controlling a majority of our capital stock to take any action, including the removal of directors.

Advance Notice Requirements for Stockholder Proposals and Director Nominations. Our Bylaws provide advance notice procedures for stockholders seeking to bring business before our annual meeting of stockholders or to nominate candidates for election as directors at our annual meeting of stockholders. Our Bylaws specify certain requirements regarding the form and content of a stockholder’s notice and prohibit the conduct of any business at a special meeting other than as specified in the notice for such meeting. These provisions might preclude stockholders from bringing matters before our annual meeting of stockholders or from making nominations for directors at our annual meeting of stockholders if the proper procedures are not followed. These provisions may also discourage or deter a potential acquirer from conducting a solicitation of proxies to elect the acquirer’s own slate of directors or otherwise attempting to obtain control of the Company.

No Cumulative Voting. The DGCL provides that stockholders are denied the right to cumulate votes in the election of directors unless our Charter provides otherwise. Our Charter and Bylaws do not provide for cumulative voting.

Amendment of Charter Provisions and Bylaws. Our Charter provides that our Bylaws may be adopted, amended, altered or repealed by a vote of a majority of the total number of directors of our board of directors.

Issuance of Undesignated Preferred Stock. The authority possessed by our board of directors to issue preferred stock could potentially be used to discourage attempts by third parties to obtain control of our company through a merger, tender offer, proxy contest or otherwise by making such attempts more difficult or more costly. Our board of directors may issue preferred stock with voting rights or conversion rights that, if exercised, could adversely affect the voting power of the holders of common stock.

Authorized but Unissued Shares. Our authorized but unissued shares of common stock and preferred stock will be available for future issuance without stockholder approval. We may use additional shares for a variety of purposes, including future public offerings to raise additional capital, to fund acquisitions and as employee compensation. The existence of authorized but unissued shares of common stock and preferred stock could render more difficult or discourage an attempt to obtain control of us by means of a proxy contest, tender offer, merger or otherwise.

The Nasdaq Capital Market

Our shares of common stock are listed for trading on the Nasdaq Capital Market under the symbol “PSTV.”

Transfer Agent and Registrar

The transfer agent and registrar for our common stock is Nevada Agency and Transfer Company.

DESCRIPTION OF OUR PREFERRED STOCK

We are authorized to issue up to 5,000,000 shares of preferred stock, par value $0.001 per share, of which, as of May 27, 2026, there were 938 outstanding shares of Series C Preferred Stock that can be converted into an aggregate of 1,111 shares of common stock, and 1,014 shares of Series B Convertible Preferred Stock that can be converted into an aggregate of 15 shares of common stock.

Our Board may, without further action by our stockholders, from time to time, direct the issuance of shares of preferred stock in series and may, at the time of issuance, determine and fix the number of shares of such series and the designation of such series, the voting powers, if any, of the shares of such series, the preferences and relative, participating, optional or other special rights, if any, and the qualifications, limitations or restrictions thereof, including without limitation thereof, dividend rights, conversion rights, redemption privileges and liquidation preferences, of the shares of such series. Satisfaction of any dividend preferences of outstanding shares of our preferred stock would reduce the amount of funds available for the payment of dividends on shares of our common stock. Holders of shares of our preferred stock may be entitled to receive a preference payment in the event of any liquidation, dissolution or winding-up of our company before any payment is made to the holders of shares of our common stock. In some circumstances, the issuance of shares of preferred stock may render more difficult or tend to discourage a merger, tender offer or proxy contest, the assumption of control by a holder of a large block of our securities or the removal of incumbent management. Upon the affirmative vote of our board of directors, without stockholder approval, we may issue shares of preferred stock with voting and conversion rights which could adversely affect the holders of shares of our common stock. It is not possible to state the actual effect of the issuance of any shares of preferred stock on the rights of holders of shares of common stock until our board of directors determines the specific rights attached to that preferred stock.

Series B Preferred Stock

Voting Rights

Except as otherwise provided in the certificate of designation for the Series B Preferred Stock or as otherwise required by law, the Series B Preferred Stock has no voting rights.

Dividends

Holders of Series B Preferred Stock are entitled to receive dividends (on an as-if-converted-to-common-stock basis) in the same form as dividends actually paid on shares of the common stock when, as and if such dividends are paid on shares of common stock.

Liquidation Preference

Upon our liquidation, dissolution or winding-up, whether voluntary or involuntary, holders of Series B Preferred Stock will be entitled to receive out of our assets, whether capital or surplus, an amount equal to the $1,000 stated value per share for each share of Series B Preferred Stock before any distribution or payment is paid to the holders of our shares of common stock.

Conversion

Each share of Series B Preferred Stock is convertible, at our option or at the option of the holder, at any time, into the number of shares of our common stock determined by dividing the $1,000 stated value per share of the Series B Preferred Stock by a conversion price of $67,600 per share. In addition, the conversion price per share is subject to adjustment for stock dividends, distributions, subdivisions, combinations or reclassifications. Subject to limited exceptions, a holder of the Series B Preferred Stock will not have the right to convert any portion of the Series B Preferred Stock to the extent that, after giving effect to the conversion, the holder, together with its affiliates, would beneficially own in excess of 9.99% of the number of shares of our common stock outstanding immediately after giving effect to its conversion.

Fundamental Transactions

In the event we effect certain mergers, consolidations, sales of substantially all of our assets, tender or exchange offers, reclassifications or share exchanges in which our shares of common stock are effectively converted into or exchanged for other securities, cash or property, we consummate a business combination in which another person acquires 50% of the outstanding shares of our common stock, or any person or group becomes the beneficial owner of 50% of the aggregate ordinary voting power represented by our issued and outstanding shares of common stock, then, upon any subsequent conversion of the Series B Preferred Stock, a holder of the Series B Preferred Stock will have the right to receive any shares of the acquiring corporation or other consideration it would have been entitled to receive if it had been a holder of the number of shares of common stock then issuable upon conversion in full of the Series B Preferred Stock.

Redemption Rights

We are not obligated to redeem or repurchase any shares of Series B Preferred Stock. Shares of Series B Preferred Stock are not otherwise entitled to any redemption rights.

Series C Preferred Stock

Voting Rights

Except as otherwise provided in the certificate of designation for the Series C Preferred Stock or as otherwise required by law, the Series C Preferred Stock has no voting rights.

Dividends

Holders of Series C Preferred Stock are entitled to receive dividends (on an as-if-converted-to-common-stock basis) in the same form as dividends actually paid on shares of the common stock when, as and if such dividends are paid on shares of common stock.

Liquidation Preference

Upon our liquidation, dissolution or winding-up, whether voluntary or involuntary, holders of Series C Preferred Stock will be entitled to receive out of our assets, whether capital or surplus, an amount equal to the $1,000 stated value per share for each share of Series C Preferred Stock before any distribution or payment shall be made to the holders of any junior securities.

Conversion

Each share of Series C Preferred Stock is convertible, at our option at any time, subject to certain conditions, or at the option of the holder at any time, into the number of shares of our common stock determined by dividing the $1,000 stated value per share of the Series C Preferred Stock by a conversion price of $844.28. In addition, the conversion price per share is subject to adjustment for stock dividends, distributions, subdivisions, combinations or reclassifications. Subject to limited exceptions, a holder of the Series C Preferred Stock does not have the right to convert any portion of the Series C Preferred Stock to the extent that, after giving effect to the conversion, the holder, together with its affiliates, would beneficially own in excess of 9.99% of the number of shares of our common stock outstanding immediately after giving effect to its conversion.

Fundamental Transactions.

acquires 50% of the outstanding shares of our common stock, or any person or group becomes the beneficial owner of 50% of the aggregate ordinary voting power represented by our issued and outstanding shares of common stock, then, upon any subsequent conversion of the Series C Preferred Stock, a holder of the Series C Preferred Stock will have the right to receive any shares of the acquiring corporation or other consideration it would have been entitled to receive if it had been a holder of the number of shares of common stock then issuable upon conversion in full of the Series C Preferred Stock.

Anti-Dilution

Subject to certain exceptions contained in the certificate of designation for the Series C Preferred Stock, including our ability to issue securities in connection with equity awards to service providers, strategic transactions, debt financings, research and development partnerships, an equity line of credit, our “at the market” equity offering program and other customary exceptions, if we issue or sell, or are deemed to have issued or sold, any shares of common stock or Common Stock Equivalents (as defined in the certificate of designation) for a consideration per share lower than the conversion price of the Series C Preferred Stock in effect immediately prior to such issuance or sale, or deemed issuance or sale, then the conversion price of the Series C Preferred Stock then in effect will be reduced to an amount equal to such lower price pursuant to the terms of the certificate of designation.

Redemption Rights

We are not obligated to redeem or repurchase any shares of Series C Preferred Stock. Shares of Series C Preferred Stock are not otherwise entitled to any redemption rights.

If we offer additional series of preferred stock under this prospectus, we will describe the terms of the preferred stock in the prospectus supplement for such offering and will file a copy of the certificate establishing the terms of the preferred stock with the SEC. To the extent required, this description may include:

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the title and stated value;

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the number of shares offered, the liquidation preference per share, and the purchase price;

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the dividend rate(s), period(s), and/or payment date(s), or method(s) of calculation for such dividends;

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whether dividends will be cumulative or non-cumulative and, if cumulative, the date from which dividends will accumulate;

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the provisions for a sinking fund, if any;

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the provisions for redemption, if applicable;

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any listing of the preferred stock on any securities exchange or market;

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whether the preferred stock will be convertible into our shares of common stock or our other securities and, if applicable, the conversion price (or how it will be calculated), the conversion period and any other terms of conversion (including any anti-dilution provisions, if any);

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whether the preferred stock will be exchangeable into debt securities, and, if applicable, the exchange price (or how it will be calculated), the exchange period and any other terms of exchange (including any anti-dilution provisions, if any);

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voting rights, if any, of the preferred stock;

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a discussion of any material and/or special U.S. federal income tax considerations applicable to the preferred stock;

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the relative ranking and preferences of the preferred stock as to dividend rights and rights upon liquidation, dissolution, or winding up of our affairs;

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any material limitations on issuance of any class or series of preferred stock ranking senior to or on a parity with the series of preferred stock as to dividend rights and rights upon our liquidation, dissolution, or winding up; and

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any other affirmative, negative or other covenants or contractual rights which might be attendant with the specific series of preferred stock.

The preferred stock offered by this prospectus, when issued, will not have, or be subject to, any preemptive or similar rights.

Transfer Agent and Registrar

The transfer agent and registrar for any series of preferred stock to be issued in the future will be set forth in each applicable prospectus supplement.

DESCRIPTION OF OUR WARRANTS

We may issue warrants to purchase shares of our common stock, preferred stock or debt securities in one or more series together with other securities or separately, as described in each applicable prospectus supplement. Below is a description of certain general terms and provisions of the warrants that we may offer. Particular terms of the warrants will be described in the applicable warrant agreements and the applicable prospectus supplement for the warrants. We urge you to read the applicable prospectus supplement and any related free writing prospectus, as well as the complete warrant agreements and warrant certificates that contain the terms of the warrants.

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The applicable prospectus supplement will contain, where applicable, the following terms of and other information relating to the warrants:

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the specific designation and aggregate number of, and the price at which we will issue, the warrants;

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the currency or currency units in which the offering price, if any, and the exercise price are payable;

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the designation, amount and terms of the securities purchasable upon exercise of the warrants;

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if applicable, the exercise price for shares of common stock and the number of shares of common stock to be received upon exercise of the warrants;

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if applicable, the exercise price for shares of our preferred stock, the number of shares of preferred stock to be received upon exercise of the warrants, and a description of that series of our preferred stock;

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if applicable, the exercise price for our debt securities, the amount of our debt securities to be received upon exercise of the warrants, and a description of that series of debt securities;

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the date on which the right to exercise the warrants will begin and the date on which that right will expire or, if the warrants may not be continuously exercised throughout that period, the specific date or dates on which the warrants may be exercised;

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whether the warrants will be issued in fully registered form or bearer form, in definitive or global form or in any combination of these forms, although, in any case, the form of a warrant included in a unit will correspond to the form of the unit and of any security included in that unit;

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any applicable material U.S. federal income tax or foreign tax consequences;

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the identity of the warrant agent for the warrants, if any, and of any other depositaries, execution or paying agents, transfer agents, registrars or other agents;

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the proposed listing, if any, of the warrants or any securities purchasable upon exercise of the warrants on any securities exchange or market;

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if applicable, the date from and after which the warrants and the shares of common stock, preferred stock or debt securities will be separately transferable;

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if applicable, the minimum or maximum amount of the warrants that may be exercised at any one time;

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information with respect to book-entry procedures, if any;

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the anti-dilution provisions of the warrants, if any;

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any redemption, put or call provisions;

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whether the warrants are to be sold separately or with other securities as parts of units; and

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any additional terms of the warrants, including terms, procedures and limitations relating to the exchange and exercise of the warrants.

Transfer Agent and Registrar

The transfer agent and registrar, if any, for any warrants will be set forth in the applicable prospectus supplement.

Description of Outstanding Warrants

See “Description of Our Securities—Description of Our Shares of Common Stock—Outstanding Warrants.”

DESCRIPTION OF OUR DEBT SECURITIES

This section describes the general terms and provisions of the debt securities that we may offer under this prospectus, any of which may be issued as convertible or exchangeable debt securities. We will set forth the particular terms of the debt securities we offer in a prospectus supplement. The terms of any debt securities offered under a prospectus supplement may differ from the terms described below. The extent, if any, to which the following general provisions apply to particular debt securities will be described in the applicable prospectus supplement. The following description of general terms relating to the debt securities and the indenture under which the debt securities will be issued are summaries only and therefore are not complete. You should read the indenture and the applicable prospectus supplement and any related free writing prospectus, which may be amended or supplemented from time to time, that contains the terms of the debt securities.

We will issue the debt securities offered by this prospectus and any accompanying prospectus supplement under an indenture to be entered into between us and the trustee identified in the applicable prospectus supplement. The terms of the debt securities will include those stated in the indenture and those made part of the indenture by reference to the Trust Indenture Act of 1939, as in effect on the date of the indenture. We have filed or will file a copy of the form of indenture as an exhibit to the registration statement in which this prospectus is included. The indenture will be subject to and governed by the terms of the Trust Indenture Act of 1939.

We may offer under this prospectus up to an aggregate principal amount of $100,000,000 in debt securities, or if debt securities are issued at a discount, or in a foreign currency, foreign currency units or composite currency, the principal amount as may be sold for an aggregate initial public offering price of up to $100,000,000. Unless otherwise specified in the applicable prospectus supplement, the debt securities will represent direct, unsecured obligations of ours and will rank equally with all of our other unsecured indebtedness.

The following statements relating to the debt securities and the indenture are summaries, qualified in their entirety by reference to the detailed provisions of the indenture and the final form indenture as may be filed with a future prospectus supplement.

General

We may issue the debt securities in one or more series with the same or various maturities, at par, at a premium, or at a discount. We will describe the particular terms of each series of debt securities in a prospectus supplement relating to that series, which we will file with the SEC.

The prospectus supplement will set forth, to the extent required, the following terms of the debt securities in respect of which the prospectus supplement is delivered:

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the title of the series;

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the aggregate principal amount;

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the issue price or prices, expressed as a percentage of the aggregate principal amount of the debt securities;

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any limit on the aggregate principal amount;

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the date or dates on which the debt securities will be issued and on which principal of, and premium, if any, is payable;

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the interest rate or rates (which may be fixed or variable) or, if applicable, the method used to determine such rate or rates;

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the date or dates from which interest will accrue, the interest payment date or dates on which interest will be payable and any regular record date for the interest payable, and the basis upon which interest will be calculated if other than that of a 360-day year of twelve 30-day months;

•

the place or places where principal and, if applicable, premium and interest, is payable;

•

the terms and conditions upon which we may, or the holders may require us to, redeem or repurchase the debt securities;

•

the denominations in which such debt securities may be issuable, if other than a minimum denomination of $2,000 or an integral multiple of $1,000 in excess thereof;

•

the interest rate or rates (which may be fixed or variable) or, if applicable, the method used to determine such rate or rates;

•

whether the debt securities are to be issuable in the form of certificated debt securities (as described below) or global debt securities (as described below);

•

the portion of principal amount that will be payable upon declaration of acceleration of the maturity date if other than the principal amount of the debt securities;

•

the currency of denomination;

•

the designation of the currency, currencies or currency units in which payment of principal and, if applicable, premium and interest, will be made;

•

if payments of principal and, if applicable, premium or interest, on the debt securities are to be made in one or more currencies or currency units other than the currency of denomination, the manner in which the exchange rate with respect to such payments will be determined;

•

if amounts of principal and, if applicable, premium and interest may be determined by reference to an index, including an index based on a currency or currencies other than in which the debt securities are payable, then the manner in which such amounts will be determined;

•

the provisions, if any, relating to any collateral provided for such debt securities;

•

whether the debt securities will be guaranteed by any person or persons and, if so, the identity of such person or persons, the terms and conditions upon which such debt securities shall be guaranteed and, if applicable, the terms and conditions upon which such guarantees may be subordinated to other indebtedness of the respective guarantors;

•

any addition to or change in the covenants described in this prospectus or in the indenture;

•

any events of default, if not otherwise described below under “Events of Default”;

•

the terms and conditions, if any, for conversion into or exchange for shares of common stock or preferred stock;

•

any depositaries, interest rate calculation agents, exchange rate calculation agents or other agents;

•

the terms and conditions, if any, upon which the debt securities shall be subordinated in right of payment to other indebtedness of our company; and

•

any other terms of the debt securities of such series.

We may issue discount debt securities that provide for an amount less than the stated principal amount to be due and payable upon acceleration of the maturity of such debt securities in accordance with the terms of the indenture. We may also issue debt securities in bearer form, with or without coupons. If we issue discount debt securities or debt securities in bearer form, we will describe material U.S. federal income tax considerations and other material special considerations that apply to these debt securities in the applicable prospectus supplement.

We may issue debt securities denominated in or payable in a foreign currency or currencies or a foreign currency unit or units. If we do, we will describe the restrictions, elections, and general tax considerations relating to the debt securities and the foreign currency or currencies or foreign currency unit or units in the applicable prospectus supplement.

Exchange and/or Conversion Rights

We may issue debt securities which can be exchanged for or converted into shares of our common stock or preferred stock. If we do, we will describe the terms of exchange or conversion in the prospectus supplement relating to these debt securities.

Transfer and Exchange

We may issue debt securities that will be represented by either:

•

“book-entry securities,” which means that there will be one or more global securities registered in the name of a depositary or a nominee of a depositary; or

•

“certificated securities,” which means that they will be represented by a certificate issued in definitive registered form.

We will specify in the prospectus supplement applicable to a particular offering whether the debt securities offered will be book-entry or certificated securities.

Certificated Debt Securities

If you hold certificated debt securities issued under an indenture, you may transfer or exchange such debt securities in accordance with the terms of the indenture. You will not be charged a service charge for any transfer or exchange of certificated debt securities but may be required to pay an amount sufficient to cover any tax or other governmental charge payable in connection with such transfer or exchange.

Global Securities

The debt securities of a series may be issued in the form of one or more global securities that will be deposited with a depositary or its nominees identified in the prospectus supplement relating to the debt securities. In such a case, one or more global securities will be issued in a denomination or aggregate denominations equal to the portion of the aggregate principal amount of outstanding debt securities of the series to be represented by such global security or securities.

Unless and until it is exchanged in whole or in part for debt securities in definitive registered form, a global security may not be registered for transfer or exchange except as a whole by the depositary for such global security to a nominee of the depositary and except in the circumstances described in the prospectus supplement relating to the debt securities. The specific terms of the depositary arrangement with respect to a series of debt securities will be described in the prospectus supplement relating to such series.

Protection in the Event of Change of Control

Any provision in an indenture that governs our debt securities covered by this prospectus that includes any covenant or other provision providing for a put or increased interest or otherwise that would afford holders of our debt securities additional protection in the event of a recapitalization transaction, a change of control of the Company, or a highly leveraged transaction will be described in the applicable prospectus supplement.

Covenants

Unless otherwise indicated in this prospectus or the applicable prospectus supplement, our debt securities may not have the benefit of any covenant that limits or restricts our business or operations, the pledging of our assets or the incurrence by us of indebtedness. We will describe in the applicable prospectus supplement any material covenants in respect of a series of debt securities.

Consolidation, Merger, Conveyance, Transfer or Lease

We may agree in any indenture that governs the debt securities of any series covered by this prospectus that we will not consolidate with or merge into any other person or convey, transfer or lease (as lessor) our properties and assets as, or substantially as, an entirety to any person, unless such person and such proposed transaction meets various criteria, which we will describe in detail in the applicable prospectus supplement.

Defaults and Notice

The debt securities of any series will contain events of default to be specified in the applicable prospectus supplement, which may include, without limitation:

•

default in the payment of any interest upon any debt security of that series when it becomes due and payable, and continuance of such default for a period of 30 days;

•

default in the payment of the principal of or any premium on any debt security of that series at its maturity;

•

default in the deposit of any sinking fund payment, when and as due by the terms of a debt security of that series;

•

default in the performance or breach of any other covenants or agreements in the indenture with respect to the debt securities of such series; and

•

certain events relating to our bankruptcy, insolvency or reorganization.

If an event of default with respect to debt securities of any series shall occur and be continuing, we may agree that the trustee or the holders of at least 25% in aggregate principal amount of the then outstanding debt securities of such series may declare the principal amount (or, if the debt securities of such series are issued at an original issue discount, such portion of the principal amount as may be specified in the terms of the debt securities of such series) of all debt securities of such series or such other amount or amounts as the debt securities or supplemental indenture with respect to such series may provide, to be due and payable immediately. Any provisions pertaining to events of default and any remedies associated therewith will be described in the applicable prospectus supplement.

Any indenture that governs our debt securities covered by this prospectus may require that the trustee under such indenture shall, within 90 days after the occurrence of a default, give to holders of debt securities of any series notice of all uncured and unwaived defaults with respect to such series known to it. In the case of a default, however, that results from the failure to make any payment of the principal of, premium, if any, or interest on the debt securities of any series, or in the payment of any sinking or purchase fund installment with respect to debt securities of such series, if any, the trustee may withhold such notice if it in good faith determines that the withholding of such notice is in the interest of the holders of debt securities of such series. Any terms and provisions relating to the foregoing types of provisions will be described in further detail in the applicable prospectus supplement.

Any indenture that governs our debt securities covered by this prospectus will contain a provision entitling the trustee to be indemnified by holders of debt securities before proceeding to exercise any trust or power under the indenture at the request of such holders. Any such indenture may provide that the holders of at least a majority in

aggregate principal amount of the then outstanding debt securities of any series may direct the time, method and place of conducting any proceedings for any remedy available to the trustee, or of exercising any trust or power conferred upon the trustee with respect to the debt securities of such series. However, the trustee under any such indenture may decline to follow any such direction if, among other reasons, the trustee determines in good faith that the actions or proceedings as directed may not lawfully be taken, would involve the trustee in personal liability or would be unduly prejudicial to the holders of the debt securities of such series not joining in such direction.

Any indenture that governs our debt securities covered by this prospectus may endow the holders of such debt securities to institute a proceeding with respect to such indenture, subject to certain conditions, which will be specified in the applicable prospectus supplement and which may include, that the holders of at least a majority in aggregate principal amount of the debt securities of such series then outstanding make a written request upon the trustee to exercise its power under the indenture, indemnify the trustee and afford the trustee reasonable opportunity to act. Even so, such holders may have an absolute right to receipt of the principal of or premium, if any, and interest when due, to require conversion or exchange of debt securities if such indenture provides for convertibility or exchangeability at the option of the holder and to institute suit for the enforcement of such rights. Any terms and provisions relating to the foregoing types of provisions will be described in further detail in the applicable prospectus supplement.

Modification of the Indenture

We and the trustee may modify any indenture that governs our debt securities of any series covered by this prospectus with or without the consent of the holders of such debt securities, under certain circumstances to be described in a prospectus supplement.

Defeasance; Satisfaction and Discharge

The prospectus supplement will outline the conditions under which we may elect to have certain of our obligations under the indenture discharged and under which the indenture obligations will be deemed to be satisfied.

Regarding the Trustee

We will identify the trustee and any relationship that we may have with such trustee, with respect to any series of debt securities, in the prospectus supplement relating to the applicable debt securities. You should note that if the trustee becomes a creditor of us, the indenture and the Trust Indenture Act of 1939 limit the rights of the trustee to obtain payment of claims in certain cases, or to realize on certain property received in respect of any such claim, as security or otherwise. The trustee and its affiliates may engage in, and will be permitted to continue to engage in, other transactions with us and our affiliates. If, however, the trustee acquires any “conflicting interest” within the meaning of the Trust Indenture Act of 1939, it must eliminate such conflict or resign.

Governing Law

The law governing the indenture and the debt securities will be identified in the prospectus supplement relating to the applicable indenture and debt securities.

DESCRIPTION OF SUBSCRIPTION RIGHTS

We may issue subscription rights to purchase debt securities, preferred stock, shares of common stock or other securities. These rights may be issued independently or together with any other security offered hereby and may or may not be transferable by the stockholder receiving the subscription rights in such offering. The applicable prospectus supplement may add, update or change the terms and conditions of the rights as described in this prospectus. We urge you to read the applicable prospectus supplement and any related free writing prospectus, as well as the complete rights agreements and rights certificates that contain the terms of the rights.

The applicable prospectus supplement may describe the specific terms of any offering of rights for which this prospectus is being delivered, including the following:

•

the price, if any, per right;

•

the exercise price payable for debt securities, preferred stock, shares of common stock, or other securities upon the exercise of the rights;

•

the number of rights issued or to be issued to each stockholder;

•

the number and terms of debt securities, preferred stock, shares of common stock, or other securities which may be purchased per right;

•

the extent to which the rights are transferable;

•

any other terms of the rights, including the terms, procedures and limitations relating to the exchange and exercise of the rights;

•

the date on which the holder’s ability to exercise the rights shall commence, and the date on which the rights shall expire;

•

the extent to which the rights may include an over-subscription privilege with respect to unsubscribed securities; and

•

if applicable, the material terms of any standby underwriting or purchase arrangement entered into by us in connection with the offering of such rights.

Holders may exercise rights as described in the applicable prospectus supplement. Upon receipt of payment and the rights certificate properly completed and duly executed at the corporate trust office of the rights agent or any other office indicated in the prospectus supplement, we will, as soon as practicable, forward the applicable securities purchased upon exercise of the rights. If less than all of the rights issued in any rights offering are exercised, we may offer any unsubscribed securities directly to persons other than stockholders, to or through agents, underwriters or dealers or through a combination of such methods, including pursuant to standby arrangements with one or more underwriters or other purchasers, pursuant to which the underwriters or other purchasers may be required to purchase any securities remaining unsubscribed for after such offering, as described in the applicable prospectus supplement.

The description in the applicable prospectus supplement of any rights that we may offer will not necessarily be complete and will be subject, and qualified in its entirety by reference, to the applicable rights agreement and rights certificate, which will be filed with the SEC.

DESCRIPTION OF OUR UNITS

The following description, together with the additional information we include in any applicable prospectus supplement, summarizes the material terms and provisions of the units that we may offer under this prospectus. We may issue units consisting of any combination of the other types of securities offered under this prospectus in one or more series. We may evidence each series of units by unit certificates that we will issue under a separate agreement. We may enter into unit agreements with a unit agent. Each unit agent will be a bank or trust company that we select. We will indicate the name and address of the unit agent in the applicable prospectus supplement relating to a particular series of units. While the terms we have summarized below will generally apply to any future units that we may offer under this prospectus, we will describe the particular terms of any series of units that we may offer in more detail in the applicable prospectus supplement. The terms of any units offered under a prospectus supplement may differ from the terms described below.

We will incorporate by reference into the registration statement of which this prospectus forms a part the form of unit agreement, including a form of unit certificate, if any, that describes the terms of the series of units we are offering before the issuance of the related series of units. The following summaries of material provisions of the units, and the unit agreements, are subject to, and qualified in their entirety by reference to, all the provisions of the unit agreement applicable to a particular series of units. We urge you to read the applicable prospectus supplements related to the units that we sell under this prospectus, as well as the complete unit agreements that contain the terms of the units.

General

We may issue units comprised of the other types of securities offered under this prospectus in any combination. Each unit will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately, at any time or at any time before a specified date.

We will describe in the applicable prospectus supplement the terms of the series of units, including:

•

the designation and terms of the units and of the securities comprising the units, including whether, and under what circumstances, those securities may be held or transferred separately;

•

the rights and obligations of the unit agent, if any;

•

any provisions of the governing unit agreement that differ from those described below; and

•

any provisions for the issuance, payment, settlement, transfer or exchange of the units or of the securities comprising the units.

The provisions described in this section, as well as those described under “Description of Our Shares of Common Stock,” “Description of our Preferred Stock,” “Description of Our Debt Securities,” “Description of Our Warrants” and “Subscription Rights” will apply to each unit and to any shares of common stock, preferred stock, debt securities, warrants and subscription rights included in each unit, respectively.

Issuance in Series

We may issue units in such amounts and in numerous distinct series as we determine.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. The SEC maintains an Internet website at www.sec.gov that contains reports, proxy and information statements, and other information regarding issuers that file electronically with the SEC. Our reports on Forms 10-K, 10-Q and 8-K, and amendments to those reports, are also available for download, free of charge, as soon as reasonably practicable after these reports are filed with, or furnished to, the SEC, at our website at www.plustherapeutics.com. Information contained on or accessible through our website is not a part of this prospectus or any prospectus supplement, and the inclusion of our website address in this prospectus is an inactive textual reference only.

INCORPORATION BY REFERENCE

The SEC allows us to “incorporate by reference” into this prospectus the information in other documents that we file with it. This means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be a part of this prospectus, and information in documents that we file later with the SEC will automatically update and supersede information contained in documents filed earlier with the SEC or contained in this prospectus. We incorporate by reference in this prospectus (i) the documents listed below, (ii) all documents that we file with the SEC under Section 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of the initial filing of the registration statement of which this prospectus is included and prior to the effectiveness of such registration statement, and (iii) and any future filings that we may make with the SEC under Sections 13(a), 13(c), 14, or 15(d) of the Exchange Act prior to the termination of the offering under this prospectus; provided, however, that we are not incorporating, in each case, any documents or information deemed to have been furnished and not filed, including any information that we disclose under Items 2.02 or 7.01 of any Current Report on Form 8-K, in accordance with SEC rules:

•

our Annual Report on Form 10-K for the year ended December 31, 2025 (filed with the SEC on March 12, 2026);

•

our Quarterly Report on Form 10-Q for the quarter ended March 31, 2026 (filed with the SEC on May 15, 2026);

•

our Current Reports on Form 8-K filed with the SEC on January 16, 2026, January 22, 2026, February 13, 2026, March 12, 2026 (the second Current Report on Form 8-K filed on such date), March 26, 2026, April 2, 2026, April 8, 2026, April 9, 2026, April 21, 2026 and May 15, 2026 (the second Current Report on Form 8-K filed on such date) (except for Item 2.02 and Item 7.01 of any Current Report on Form 8-K which are not deemed “filed” for purposes of Section 18 of the Exchange Act and are not incorporated by reference in this prospectus); and

•

the description of our common stock contained in Exhibit 4.1—Description of Securities to our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, filed with the SEC on March 12, 2026.

We will provide without charge to each person, including any beneficial owner, to whom this prospectus is delivered, upon written or oral request, a copy of any or all documents that are incorporated by reference into this prospectus, but not delivered with the prospectus, other than exhibits to such documents unless such exhibits are specifically incorporated by reference into the documents that this prospectus incorporates. You should direct oral or written requests by one of the following methods. Attention: Investor Relations, Plus Therapeutics, Inc., 6420 Levit Green Boulevard, Suite 310, Houston, Texas 77021, (737) 255-7194. You may also access these documents, free of charge on the SEC’s website at www.sec.gov or on the “Investors” page of our website at www.plustherapeutics.com. The information found on our website, or that may be accessed by links on our website, is not part of this prospectus. We have included our website address solely as an inactive textual reference. Investors should not rely on any such information in deciding whether to purchase our common stock.

LEGAL MATTERS

The validity of any securities offered by this prospectus will be passed upon for us by Sullivan & Worcester LLP, New York, New York.

EXPERTS

been so incorporated in reliance on the report of CBIZ CPAs P.C., an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting. The report on the consolidated financial statements contains an explanatory paragraph regarding the Company’s ability to continue as a going concern.

The consolidated financial statements of Plus Therapeutics, Inc. (the Company) as of December 31, 2024 and for the year then ended incorporated by reference in this Prospectus and in the Registration Statement have been so incorporated in reliance on the report of BDO USA P.C., an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting. The report on the consolidated financial statements contains an explanatory paragraph regarding the Company’s ability to continue as a going concern.

LOGO

$100,000,000

Common Stock

Preferred Stock

Warrants

Debt Securities

Subscription Rights

Units

PROSPECTUS

, 2026

The information in this preliminary prospectus is not complete and may be changed. These securities may not be sold until the registration statement filed with the Securities and Exchange Commission is effective. This preliminary prospectus is not an offer to sell nor does it seek an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

SUBJECT TO COMPLETION DATED JUNE 1, 2026

PROSPECTUS SUPPLEMENT

LOGO

Up to $17,350,000 of Common Stock

We have entered into an equity distribution agreement dated as of June 1, 2026, (the “Equity Distribution Agreement”) with Canaccord Genuity LLC (the “Sales Agent” or “Canaccord”) relating to shares of our common stock, $0.001 par value per share, offered by this prospectus supplement and the accompanying prospectus. In accordance with the terms of the Equity Distribution Agreement, we may offer and sell shares of our common stock from time to time up to an aggregate offering price of $17,350,000 through Canaccord, acting as sales agent.

Upon our delivery of a placement notice and subject to the terms and conditions of the Equity Distribution Agreement, Canaccord may sell the common stock by methods deemed to be an “at the market” offering as defined in Rule 415 promulgated under the Securities Act of 1933, as amended (the “Securities Act”), including sales made directly on The Nasdaq Capital Market (“Nasdaq”), on any other existing trading market for the common stock or to or through a market maker other than on an exchange. In addition, with our prior written approval, Canaccord may also sell the common stock by any other method permitted by law, including in privately negotiated transactions. Canaccord will act as sales agent using its commercially reasonable efforts consistent with its normal trading and sales practices and applicable state and federal laws, rules and regulations and the rules of The Nasdaq Capital Market. There is no arrangement for funds to be received in any escrow, trust or similar arrangement.

We will pay Canaccord a commission, or allow a discount, for its services in acting as agent in the sale of common stock equal to 3.0% of the gross sales price per share of all shares sold through it as agent under the Equity Distribution Agreement. See “Plan of Distribution” for information relating to certain expenses of Canaccord to be reimbursed by us.

In connection with the sale of common stock on our behalf, Canaccord may be deemed to be an “underwriter” within the meaning of the Securities Act and the compensation to Canaccord will be deemed to be underwriting commissions or discounts. We have also agreed to provide indemnification and contribution to Canaccord with respect to certain liabilities, including liabilities under the Securities Act.

Our shares of common stock are listed on Nasdaq under the symbol “PSTV.” On May 27, 2026, the last reported sale price of our shares of common stock on Nasdaq was $6.64 per share.

Canaccord Genuity

The date of this prospectus supplement is , 2026

TABLE OF CONTENTS


	Page
ABOUT THIS PROSPECTUS		S-1
FORWARD-LOOKING STATEMENTS		S-3
PROSPECTUS SUPPLEMENT SUMMARY		S-4
THE OFFERING		S-5
RISK FACTORS		S-7
USE OF PROCEEDS		S-9
DILUTION		S-10
PLAN OF DISTRIBUTION		S-11
DESCRIPTION OF SECURITIES WE ARE OFFERING		S-12
LEGAL MATTERS		S-12
EXPERTS		S-12
WHERE YOU CAN FIND MORE INFORMATION		S-13
INFORMATION WE INCORPORATE BY REFERENCE		S-14

No dealer, salesperson or other person is authorized to give any information or to represent anything not contained in this prospectus supplement or the accompanying base prospectus. You must not rely on any unauthorized information or representations. This prospectus supplement and the accompanying base prospectus are an offer to sell only the securities offered hereby, but only under circumstances and in jurisdictions where it is lawful to do so.

The information contained in this prospectus supplement is accurate only as of the date on the front cover page of this prospectus supplement, or other earlier date stated in this prospectus supplement, regardless of the time of delivery of this prospectus supplement or of any sale of our securities.

No action is being taken in any jurisdiction outside the United States to permit a public offering of our securities or possession or distribution of this prospectus supplement and the accompanying base prospectus in that jurisdiction. Persons who come into possession of this prospectus supplement in jurisdictions outside the United States are required to inform themselves about and to observe any restrictions as to this offering and the distribution of this prospectus supplement applicable to that jurisdiction.

ABOUT THIS PROSPECTUS

This document consists of two parts. The first part is the prospectus supplement, including the documents incorporated by reference herein, which describes the specific terms of this offering. The second part, the accompanying prospectus, including the documents incorporated by reference therein, provides more general information. In general, when we refer only to the prospectus, we are referring to both parts of this document combined. Before you invest, you should carefully read this prospectus supplement, the accompanying prospectus, all information incorporated by reference herein and therein, as well as the additional information described under the heading “Where You Can Find More Information.” These documents contain information you should carefully consider when deciding whether to invest in our securities.

This prospectus supplement may add, update or change information contained in the accompanying prospectus. To the extent there is a conflict between the information contained in this prospectus supplement and the accompanying prospectus, you should rely on information contained in this prospectus supplement, provided that if any statement in, or incorporated by reference into, one of these documents is inconsistent with a statement in another document having a later date, the statement in the document having the later date modifies or supersedes the earlier statement. Any statement so modified will be deemed to constitute a part of this prospectus supplement only as so modified, and any statement so superseded will be deemed not to constitute a part of this prospectus supplement.

You should rely only on the information contained in this prospectus supplement, the accompanying prospectus, any document incorporated by reference herein or therein, or any free writing prospectuses we may provide to you in connection with this offering. Neither we nor the Sales Agent have authorized anyone to provide you with any different information. We take no responsibility for and can provide no assurance as to the reliability of, any other information that others may provide to you. The information contained in this prospectus supplement, the accompanying prospectus, and in the documents incorporated by reference herein or therein is accurate only as of the date such information is presented. Our business, financial condition, results of operations and prospects may have changed since that date.

This prospectus supplement and the accompanying prospectus do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the shares of our common stock to which it relates, nor does this prospectus supplement and the accompanying prospectus constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction.

S-1

We note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that is incorporated by reference herein were made solely for the benefit of the parties to such agreement, including, in some cases, for the purpose of allocating risk among the parties to such agreement, and should not be deemed to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as of the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

Unless otherwise indicated, all share amounts and share prices disclosed herein are presented on a post-split basis, giving effect to the reverse stock split of all of the outstanding shares of the Company’s issued and outstanding Common Stock on April 2, 2026. Any financial statements incorporated by reference into this prospectus that were filed prior to the reverse stock split present the outstanding shares on a pre-split basis.

S-2

FORWARD-LOOKING STATEMENTS

This prospectus and the documents incorporated by reference herein may contain forward-looking statements that involve substantial risks and uncertainties. All statements, other than statements of historical facts, included in this prospectus, any prospectus supplement or the documents incorporated herein and therein by reference, including statements regarding our future financial condition, results of operations, business strategy and plans and objectives of management for future operations, industry trends and other future events, are forward-looking statements. In some cases, you can identify forward-looking statements by terminology such as “believe,” “will,” “may,” “estimate,” “continue,” “anticipate,” “intend,” “should,” “plan,” “expect,” “predict,” “project,” “could,” “potentially,” “continue,” “ongoing,” “scheduled” or the negative of these terms or other similar expressions, although not all forward-looking statements contain these identifying terms.

S-3

PROSPECTUS SUMMARY

Company Overview

S-4

(“CLIA”) regulations. Furthermore, CNSide Diagnostics has signed national agreements to provide the CNSide Test with four payers, including United Healthcare, Humana, Highmark, and Blue Shield of California, expanding patient access nationwide to approximately 81 million.

Corporate Information

THE OFFERING

Common Stock offered by us

Shares of our common stock having an aggregate offering price of up to $17,350,000.

Manner of Offering

“At the Market Offering” that may be made from time to time through or to, the Sales Agent, as sales agent. See “Plan of Distribution” on page S-11.

Common stock outstanding immediately before offering

6,883,897 shares of common stock.

Common stock outstanding immediately after offering⁽¹⁾

9,496,849 shares of common stock assuming 2,612,952 shares of common stock are sold, based on an assumed offering price of $6.64 per share, which was the closing price of our common stock on the Nasdaq Capital Market on May 27, 2026. The actual number of shares of our common stock issued will vary depending on the sales price under this offering.

Use of Proceeds

We currently intend to use the net proceeds from this offering for working capital and general corporate purposes. See “Use of Proceeds” on page S-9 of this prospectus supplement.

Risk Factors

Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page S-7 of this prospectus supplement for a discussion of factors you should consider carefully when making an investment decision.

Exchange Listing

Our common stock is traded on Nasdaq under the symbol “PSTV.”

Transfer Agent

The transfer agent and registrar for our common stock is Nevada Agency and Transfer Company.

S-5

⁽¹⁾ Unless otherwise noted, the number of shares of common stock to be outstanding immediately after this offering is based on 6,883,897 shares outstanding as of May 27, 2026, and excludes, as of such date:

•

631,779 shares of common stock issuable upon exercise of stock options outstanding under our equity incentive plans, with a weighted-average exercise price of $17.36 per share;

•

256,524 shares of common stock issuable upon vesting of restricted stock units under our equity incentive plans, with a weighted-average grant date fair value of $9.72 per share;

•

44,840 shares of common stock reserved for future issuance under our 2015 New Employee Incentive Plan;

•

724,522 shares of common stock reserved for future issuance under our 2020 Stock Incentive Plan;

•

15 and 1,111 shares of common stock issuable upon conversion of 1,014 shares of Series B Convertible Preferred Stock and 938 share of Series C Preferred Stock, respectively; and

•

1,817,714 shares of common stock issuable upon the exercise of warrants to purchase common stock, with a weighted-average exercise price of $9.54 per share.

Unless otherwise indicated, all information in this prospectus supplement assumes no exercise of the outstanding options or warrants described above.

S-6

RISK FACTORS

Investing in our securities involves a high degree of risk. Before making an investment decision with respect to our securities, we urge you to carefully consider the risks described below and in the “Risk Factors” sections of our most recent Annual Report on Form 10-K and Quarterly Reports on Form 10-Q, as well as our Current Reports on Form 8-K, filed with the SEC and incorporated by reference in this prospectus, and the other information contained in this prospectus. The risks and uncertainties incorporated by reference into this prospectus or described below are not the only ones we face. Additional risks and uncertainties not presently known or which we consider immaterial as of the date hereof may also materially harm our business and could result in a complete loss of your investment. If any of the matters discussed in the following risk factors were to occur, our business, financial condition, results of operations, cash flows, or prospects could be materially and adversely affected, the market price of our common stock could decline, and you could lose all or part of your investment in our securities.

Our management will have broad discretion over the use of the net proceeds from this offering, you may not agree with how we use the proceeds, and the proceeds may not be invested successfully.

Our management will have broad discretion as to the use of the net proceeds from this offering and could use them for purposes other than those contemplated at the time of commencement of this offering. Accordingly, you will be relying on the judgment of our management regarding the use of these net proceeds, and you will not have the opportunity, as part of your investment decision, to assess whether the proceeds are being used appropriately. It is possible that, pending their use, we may invest the net proceeds in a way that does not yield a favorable, or any, return for us. The failure of our management to use such funds effectively could have a material adverse effect on our business, financial condition, operating results and cash flows.

You will experience immediate and substantial dilution in the net tangible book value per share of the common stock you purchase.

The offering price per share in this offering may exceed the historical and pro forma as adjusted net tangible book value per share of our common stock outstanding prior to this offering. If the price per share of our common stock being offered is substantially higher than the pro forma as adjusted net tangible book value per share of our common stock, you will suffer immediate and substantial dilution in the pro forma net tangible book value of the common stock you purchase in this offering. Assuming that an aggregate of 2,612,952 shares of our common stock are sold at a price of $6.64 per share, the last reported sale price of our common stock on Nasdaq on May 27, 2026, for aggregate gross proceeds of $17,350,000 and after deducting commissions and estimated offering expenses payable by us, you would experience immediate dilution of $3.71 per share, representing the difference between our pro forma as adjusted net tangible book value per share of common stock as of March 31, 2026. If we were to sell shares of our common stock in this offering at a price per share greater than our pro forma net tangible book value, it would result in dilution of your investment. See the section entitled “Dilution” below for a more detailed illustration of the dilution you may incur if you participate in this offering. Because the sales of the shares offered hereby will be made directly into the market or in negotiated transactions, the prices at which we sell these shares will vary and these variations may be significant. Purchasers of the shares we sell, as well as our existing shareholders, will experience significant dilution if we sell shares at prices significantly below the price at which they invested.

You may experience future dilution as a result of future equity offerings and other issuances of our common stock or other securities. In addition, this offering and future equity offerings and other issuances of our common stock or other securities may adversely affect our common stock price.

In order to raise additional capital, we may in the future offer additional shares of our common stock or other securities convertible into or exchangeable for our common stock at prices that may not be the same as the price per share in this offering. We may not be able to sell shares or other securities in any other offering at a

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price per share that is equal to or greater than the price per share paid by the investors in this offering, and investors purchasing shares or other securities in the future could have rights superior to existing stockholders. The price per share at which we sell additional shares of our common stock or securities convertible into common stock in future transactions may be higher or lower than the price per share in this offering. You will incur dilution upon exercise of any outstanding stock options, warrants or upon the issuance of shares of common stock under our stock incentive programs. In addition, the sale of shares of our common stock in this offering and any future sales of a substantial number of shares of our common stock in the public market, or the perception that such sales may occur, could adversely affect the price of our common stock. We cannot predict the effect, if any, that market sales of those shares of common stock or the availability of those shares for sale will have on the market price of our common stock.

The common stock offered hereby will be sold in “at the market offering” and investors who buy shares at different times will likely pay different prices.

Investors who purchase shares of common stock in this offering at different times will likely pay different prices and so may experience different outcomes in their investment results. We will have discretion, subject to market demand, to vary the timing, prices, and numbers of shares of common stock sold, and there is no minimum or maximum sales price. Investors may experience a decline in the value of their shares of common stock as a result of sales of common stock made at prices lower than the prices they paid.

The actual number of shares of common stock we will issue under the Equity Distribution Agreement, at any one time or in total, is uncertain.

Subject to certain limitations in the Equity Distribution Agreement and compliance with applicable law, we have the discretion to deliver a sales notice to the Sales Agent at any time throughout the term of the Equity Distribution Agreement. The number of shares of common stock that are sold by the Sales Agent after delivering a sales notice will fluctuate based on the market price of the common stock during the sales period and limits we set with the Sales Agent. Because the price per share of each share of common stock sold will fluctuate based on the market price of our common stock during the sales period, it is not possible at this stage to predict the number of shares of common stock that will be ultimately issued.

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USE OF PROCEEDS

We may offer and sell shares of our common stock having aggregate sales proceeds of up to $17,350,000 from time to time. The amount of proceeds we receive, if any, will depend on the actual number of shares of our common stock sold and the market price at which such shares are sold. There can be no assurance that we will be able to sell any shares or fully utilize the Equity Distribution Agreement with the Sales Agent as a source of financing. Because there is no minimum offering amount required as a condition to close this offering, the net proceeds to us, if any, are not determinable at this time.

The precise amount and timing of the application of such net proceeds will depend upon our funding requirements and the availability and cost of other funds. Our board of directors and management will have considerable discretion in the application of the net proceeds from this offering, and it is possible that we may allocate the proceeds differently than investors in the offering may desire or that we may fail to maximize the return on these proceeds. You will be relying on the judgment of our management with regard to the use of proceeds from this offering, and you will not have the opportunity, as part of your investment decision, to assess whether the proceeds are being used appropriately.

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DILUTION

If you purchase our common stock in this offering, your ownership interest will be immediately diluted to the extent of the difference between the public offering price per share and the net tangible book value per share of our common stock immediately after this offering. Net tangible book value per share is determined by dividing the number of shares of common stock outstanding as of March 31, 2026 into our total tangible assets less total liabilities.

Our net tangible book value as of March 31, 2026 was $11.3 million, or $1.65 per share, based on 6,862,027 shares of our common stock outstanding as of that date. After giving effect to the sale of 2,612,952 shares of common stock by us at an assumed public offering price of $6.64 per share, the last reported sale price of our common stock on the Nasdaq Capital Market on May 27, 2026, and after deducting the estimated selling discounts and commissions and estimated offering expenses payable by us, our as adjusted net tangible book value as of March 31, 2026 would have been approximately $27.8 million, or approximately $2.93 per share. This represents an immediate increase in net tangible book value of approximately $1.28 per share to existing stockholders and immediate dilution of approximately $3.71 per share to investors purchasing shares of common stock in this offering, as illustrated by the following table:


Assumed offering price per share			$	6.64
Net tangible book value per share as of March 31, 2026	$	1.65
Increase to net tangible book value per share attributable to this offering	$	1.28

As adjusted net tangible book value per share after this offering			$	2.93

Dilution per share to new investors in this offering			$	3.71

An increase of $1.00 per share in the price at which the shares are sold from the assumed offering price of $6.64 per share shown in the table above, assuming all of our common stock in the aggregate amount of $17,350,000 during the term of the Equity Distribution Agreement is sold at that price, would increase the dilution in as adjusted net tangible book value per share to new investors in this offering to $4.60 per share, after deducting commissions and estimated aggregate offering expenses payable by us. A decrease of $1.00 per share in the price at which the shares are sold from the assumed offering price of $6.64 per share shown in the table above, assuming all of our common stock in the aggregate amount of $17,350,000 during the term of the Equity Distribution Agreement is sold at that price, would decrease the dilution in pro forma as adjusted net tangible book value per share to new investors in this offering to $2.84 per share, after deducting commissions and estimated aggregate offering expenses payable by us. This information is supplied for illustrative purposes only.

The table above is based on 6,862,027 shares outstanding as of March 31, 2026, and excludes, as of such date:

•

606,421 shares of common stock issuable upon exercise of stock options outstanding under our equity incentive plans, with a weighted-average exercise price of $18.28 per share;

•

253,042 shares of common stock issuable upon vesting of restricted stock units under our equity incentive plans, with a weighted-average grant date fair value of $10.02 per share;

•

95,640 shares of common stock reserved for future issuance under our 2015 New Employee Incentive Plan;

•

522 shares of common stock reserved for future issuance under our 2020 Stock Incentive Plan;

•

15 and 1,111 shares of common stock issuable upon conversion of 1,014 shares of Series B Convertible Preferred Stock and 938 share of Series C Preferred Stock, respectively; and

•

1,817,714 shares of common stock issuable upon the exercise of warrants to purchase common stock, with a weighted-average exercise price of $9.54 per share.

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PLAN OF DISTRIBUTION

We have entered into a sales agreement with Canaccord Genuity LLC, or Canaccord, under which we may issue and sell shares of our common stock from time to time through or to Canaccord acting as sales agent or principal, subject to certain limitations, having an aggregate gross sales price of up to $17,350,000. Sales of our common stock, if any, under this prospectus supplement and the accompanying prospectus may be made in sales deemed to be “at the market offerings” as defined in Rule 415 promulgated under the Securities Act. We may instruct Canaccord not to sell our common stock if the sales cannot be effected at or above the price designated by us from time to time. We or Canaccord may suspend the offering of our common stock upon notice and subject to other conditions. As our agent, Canaccord will not engage in any transactions that stabilize the price of our common stock.

Each time we wish to issue and sell common stock under the sales agreement, we will notify Canaccord of the number or dollar value of shares to be sold, the dates on which such sales are anticipated to be made, any minimum price below which sales may not be made and other sales parameters as we deem appropriate. Once we have so instructed Canaccord, unless Canaccord declines to accept the terms of the notice, Canaccord has agreed to use its commercially reasonable efforts consistent with its normal trading and sales practices to sell such shares up to the amount specified on such terms. The obligation of Canaccord under the sales agreement to sell shares of our common stock is subject to a number of conditions that we must meet.

We will pay Canaccord a commission equal to 3.0% of the gross proceeds we receive from the sales of our common stock under the sales agreement. Because there is no minimum offering amount required as a condition to closing this offering, the actual total public offering amount, Canaccord’s commission and proceeds to us, if any, are not determinable at this time. In addition, we have agreed to reimburse Canaccord for its reasonable documented out-of-pocket expenses, including fees and disbursements of its counsel, in the amount of $75,000. We estimate that the total expenses of the offering payable by us, excluding commissions payable to Canaccord under the sales agreement, will be approximately $340,000.

Settlement for sales of our common stock will occur on the first trading day following the date on which any sales are made, in return for payment of the net proceeds to us. There is no arrangement for funds to be received in an escrow, trust or similar arrangement.

In connection with the sale of the common stock on our behalf, Canaccord will be deemed to be an “underwriter” within the meaning of the Securities Act, and the compensation of Canaccord will be deemed to be underwriting commission or discount. We have agreed to provide indemnification and contribution to Canaccord against certain liabilities, including liabilities under the Securities Act.

The offering pursuant to the sales agreement will terminate upon the earlier of (i) the issuance and sale of all shares of our common stock subject to the sales agreement, or (ii) the termination of the sales agreement as permitted therein.

Canaccord and its affiliates may in the future provide various investment banking and other financial services for us and our affiliates, for which services they may in the future receive customary fees. To the extent required by Regulation M, Canaccord will not engage in any market making activities involving our common stock while the offering is ongoing under this prospectus supplement.

S-11

DESCRIPTION OF SECURITIES WE ARE OFFERING

Common Stock

We are offering up to $17,350,000 of shares of our common stock in this offering pursuant to this prospectus supplement and the accompanying prospectus. As of May 27, 2026, there were 6,883,897 shares of common stock issued and outstanding, held by approximately 28 stockholders of record. Not reflected in the number of stockholders of record are persons who beneficially own shares of common stock held in nominee or street name. See “Descriptions of Common Stock” beginning on page 13 in the accompanying prospectus for more information regarding our shares of common stock.

LEGAL MATTERS

Certain legal matters in connection with this offering and the validity of the shares of common stock offered by this prospectus supplement will be passed upon for us by Sullivan & Worcester LLP, New York, New York. DLA Piper LLP (US), New York, New York, is acting as counsel to the Sales Agent in connection with this offering.

EXPERTS

The consolidated financial statements of Plus Therapeutics, Inc. (the Company) as of December 31, 2025 and for the year then ended incorporated by reference in this Prospectus and in the Registration Statement have been so incorporated in reliance on the report of CBIZ CPAs P.C., an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting. The report on the consolidated financial statements contains an explanatory paragraph regarding the Company’s ability to continue as a going concern.

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WHERE YOU CAN FIND MORE INFORMATION

We have filed our registration statement on Form S-3 with the SEC under the Securities Act of 1933, as amended, or the Securities Act. We also file annual, quarterly and current reports, proxy statements and other information with the SEC. You can read and copy any document that we file with the SEC, including the registration statement and exhibits to the registration statement, over the internet at the SEC’s website at www.sec.gov.

We are subject to the information reporting requirements of the Exchange Act and we are required to file reports, proxy statements and other information with the SEC. These reports, proxy statements, and other information are available for inspection and copying at the SEC’s website referred to above. These documents may also be accessed on our web site at https://ir.plustherapeutics.com/. Information contained on our web site is not incorporated by reference into this prospectus supplement or the accompanying prospectus and you should not consider information contained on our web site to be part of this prospectus supplement or the accompanying prospectus. Information contained on or accessible through our website is not a part of this prospectus supplement, and the inclusion of our website address in this prospectus is an inactive textual reference only.

This prospectus supplement and the accompanying prospectus are part of a registration statement filed with the SEC and do not contain all of the information in the registration statement. The full registration statement may be obtained from the SEC or us as indicated above. Other documents establishing the terms of the securities offered are filed as exhibits to the registration statement or will be filed through an amendment to our registration statement on Form S-3 or under cover of a Current Report on Form 8-K and incorporated into this prospectus supplement by reference.

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INCORPORATION BY REFERENCE

The SEC permits us to “incorporate by reference” the information contained in documents we have filed with the SEC, which means that we can disclose important information to you by referring you to those documents rather than by including them in this prospectus. Information that is incorporated by reference is considered to be part of this prospectus and you should read it with the same care that you read this prospectus. We have filed with the SEC, and incorporate by reference in this prospectus:

•

our Annual Report on Form 10-K for the year ended December 31, 2025 (filed with the SEC on March 12, 2026);

•

our Quarterly Report on Form 10-Q for the quarter ended March 31, 2026 (filed with the SEC on May 15, 2026);

•

We are not, however, incorporating, in each case, any documents or information that we are deemed to furnish and not file in accordance with SEC rules.

Any statement contained in any document incorporated by reference herein will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained in this prospectus or any prospectus supplement modifies or supersedes such statement. Any statement so modified or superseded will not be deemed, except as so modified or superseded, to constitute a part of this prospectus.

All reports and other documents we subsequently file pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act prior to the termination of this offering, including all such documents we may file with the SEC after the date of the initial registration statement and prior to the effectiveness of the registration statement, but excluding any information furnished to, rather than filed with, the SEC, will also be incorporated by reference into this prospectus and deemed to be part of this prospectus from the date of the filing of such reports and documents.

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LOGO

Up to $17,350,000 of shares of Common Stock

Canaccord Genuity

The date of this prospectus supplement is , 2026.

PART II

INFORMATION NOT REQUIRED IN THE PROSPECTUS

Item 14. Other Expenses of Issuance and Distribution.

Set forth below is an estimate (except in the case of the SEC registration fee) of the amount of fees and expenses to be incurred in connection with the issuance and distribution of the offered securities registered hereby, other than underwriting discounts and commission, if any, incurred in connection with the sale of the offered securities. All such amounts will be borne by Plus Therapeutics, Inc.


	Amount to be paid
SEC registration fee	$	13,810
FINRA filing fee	$	15,500
Printing expenses	$	(1	)
Accounting fees and expenses	$	(1	)
Legal fees and expenses	$	(1	)
Miscellaneous	$	(1	)

Total	$	(1	)

(1)	These fees are calculated based on the securities offered and the number of issuances and accordingly cannot be estimated at this time.

Item 15. Indemnification of Directors and Officers.

The Company is a Delaware corporation. Section 145(a) of the DGCL provides that a Delaware corporation may indemnify any person who was or is a party or is threatened to be made a party to any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrative or investigative, other than an action by or in the right of the corporation, by reason of the fact that such person is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, against expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by the person in connection with such action, suit or proceeding if the person acted in good faith and in a manner the person reasonably believed to be in or not opposed to the best interests of the corporation, and, with respect to any criminal action or proceeding, had no reasonable cause to believe his or her conduct was unlawful.

Section 145(b) of the DGCL provides that a Delaware corporation may indemnify any person who was or is a party or is threatened to be made a party to any threatened, pending or completed action or suit by or in the right of the corporation to procure a judgment in its favor by reason of the fact that such person acted in any of the capacities set forth above, against expenses (including attorneys’ fees) actually and reasonably incurred by such person in connection with the defense or settlement of such action or suit if the person acted in good faith and in a manner the person reasonably believed to be in, or not opposed to, the best interests of the corporation, except that no indemnification shall be made in respect of any claim, issue or matter as to which such person shall have been adjudged to be liable to the corporation, unless and only to the extent that the Court of Chancery or the court in which such action or suit was brought shall determine, upon application, that, despite the adjudication of liability but in view of all the circumstances of the case, such person is fairly and reasonably entitled to indemnity for such expenses which the court shall deem proper.

Further subsections of DGCL Section 145 provide that:

(1)

to the extent a present or former director or officer of a corporation has been successful on the merits or otherwise in the defense of any action, suit or proceeding referred to in subsections (i) and (ii) of Section 145 or in the defense of any claim, issue or matter therein, such person shall be indemnified against

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expenses, including attorneys’ fees, actually and reasonably incurred by such person in connection therewith;

(2)

the indemnification and advancement of expenses provided for pursuant to Section 145 shall not be deemed exclusive of any other rights to which those seeking indemnification or advancement of expenses may be entitled under any bylaw, agreement, vote of stockholders or disinterested directors or otherwise; and

(3)

the corporation shall have the power to purchase and maintain insurance of behalf of any person who is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, against any liability asserted against such person and incurred by such person in any such capacity, or arising out of such person’s status as such, whether or not the corporation would have the power to indemnify such person against such liability under Section 145.

As used in this Item 14, the term “proceeding” means any threatened, pending or completed action, suit or proceeding, whether or not by or in the right of the Company, and whether civil, criminal, administrative, investigative or otherwise.

Section 145 of the DGCL makes provision for the indemnification of officers and directors in terms sufficiently broad to indemnify officers and directors of the Company under certain circumstances from liabilities (including reimbursement for expenses incurred) arising under the Securities Act. The Company’s organizational documents provide, in effect, that, to the fullest extent and under the circumstances permitted by Section 145 of the DGCL, the Company will indemnify any and all of its officers and directors. The Company has entered into indemnification agreements with its officers and directors. The Company may, in its discretion, similarly indemnify its employees and agents. The Company’s Charter also relieves its directors from monetary damages to the Company or its stockholders for breach of such director’s fiduciary duty as a director to the fullest extent permitted by the DGCL. Under Section 102(b)(7) of the DGCL, a corporation may relieve its directors from personal liability to such corporation or its stockholders for monetary damages for any breach of their fiduciary duty as directors except (i) for a breach of the duty of loyalty, (ii) for failure to act in good faith, (iii) for intentional misconduct or knowing violation of law, (iv) for willful or negligent violations of certain provisions in the DGCL imposing certain requirements with respect to stock repurchases, redemptions and dividends or

(v) for any transactions from which the director derived an improper personal benefit.

The Company has purchased insurance policies that, within the limits and subject to the terms and conditions thereof, cover certain expenses and liabilities that may be incurred by directors and officers in connection with proceedings that may be brought against them as a result of an act or omission committed or suffered while acting as a director or officer of the Company.

We have entered into a registration rights agreement with the stockholders of the shares of common stock registered hereby which obligates the parties to indemnify, under certain circumstances, the other party, its officers, directors, and controlling persons within the meaning of the Securities Act against certain liabilities.

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Item 16. Exhibits and Financial Statement Schedules.

Exhibit
Number

Exhibit Title

Filed
with
this
Form
S-3

Incorporated by Reference

Form

File No.

Date Filed

1.1

Equity Distribution Agreement, by and between the Company and Canaccord Genuity LLC, dated as of June 1, 2026.

3.1

Composite Certificate of Incorporation

10-K

001-34375

Exhibit 3.1

03/11/2016

3.2