Spero Therapeutics sets $100M ATM with Jefferies

Spero Therapeutics, Inc. (SPRO) entered into an Open Market Sale Agreement with Jefferies LLC on August 28, 2026, establishing an at-the-market equity program.

Rhea-AI Filing Summary

Spero Therapeutics, Inc. (SPRO) entered into an Open Market Sale Agreement with Jefferies LLC on August 28, 2026, establishing an at-the-market equity program. Once its new Form S-3 universal shelf registration statement for up to $300,000,000 of securities is declared effective, the company may sell up to $100,000,000 of common stock through Jefferies under this agreement.

Sales, if any, will be made as at-the-market offerings, with Jefferies earning up to 3.0% of aggregate gross proceeds as sales agent. On the same date, Spero terminated its prior Cantor Fitzgerald Controlled Equity Offering Sales Agreement, which had allowed up to $75,000,000 of sales; no shares were sold under that program in 2024 or 2025 and no termination penalties were incurred.

Filing Explained

The program creates potential, not committed, financing; eventual share sales would dilute existing holders’ percentage ownership.

The new Jefferies arrangement is financing capacity, not committed funding: Jefferies is not required to sell any specific amount. If shares are sold, the increased share count would reduce an existing holder’s percentage ownership absent offsetting changes.

As of June 30, 2026, the company reported $50,774,000 of cash and equivalents, equal to 862.8 days of the last reported quarterly operating cash use at that rate.

The next state-changing disclosures are effectiveness of the Form S-3 and any subsequent ATM sales; the 8-K leaves the program unsold and conditional until then.

Sources and calculations

• Spero Therapeutics Form 8-K (2026-08-28)
• Spero Therapeutics second-quarter 2026 fundamentals (2026-06-30)
• Dilution definition (2026-07-17)

• Available liquidity against the last reported quarterly operating outflow, in days at that rate $50,774,000 / ($5,355,000 / 91) = 862.8 days

8-K Event Classification

3 items: 1.01, 8.01, 9.01

3 items

Item 1.01 Entry into a Material Definitive Agreement Business

The company signed a significant contract such as a merger agreement, credit facility, or major partnership.

Item 8.01 Other Events Other

Voluntary disclosure of events the company deems important to shareholders but not covered by other items.

Item 9.01 Financial Statements and Exhibits Exhibits

Financial statements, pro forma financial information, or exhibit attachments filed with this report.

Key Figures

Universal shelf capacity: $300,000,000 Jefferies ATM capacity: $100,000,000 Jefferies sales commission: 3.0% +4 more

7 metrics

Universal shelf capacity $300,000,000 Maximum aggregate offering price of securities under new Form S-3

Jefferies ATM capacity $100,000,000 Maximum aggregate offering price of common stock under Open Market Sale Agreement

Jefferies sales commission 3.0% Up to 3.0% of aggregate gross proceeds of shares sold

Cantor ATM capacity $75,000,000 Maximum aggregate offering price of common stock under prior Cantor Sales Agreement

Shares sold under Cantor ATM in 2024 0 shares Common stock sold during fiscal year ended December 31, 2024

Shares sold under Cantor ATM in 2025 0 shares Common stock sold during fiscal year ended December 31, 2025

Termination date of Cantor Sales Agreement August 28, 2026 Effective date of termination of Cantor Controlled Equity Offering Sales Agreement

Key Terms

at-the-market offerings, universal shelf Registration Statement, Open Market Sale Agreement℠, Controlled Equity Offering℠ Sales Agreement, +1 more

5 terms

at-the-market offerings financial

"Sales of the Shares, if any, under the Sale Agreement Prospectus will be made in sales deemed to be “at-the-market offerings”"

An at-the-market offering is a method for a company to sell new shares of its stock directly into the stock market over time, rather than all at once. This approach allows the company to raise money gradually, similar to selling small portions of a product as demand grows. For investors, it can influence stock availability and price, making it an important factor to consider when assessing a company's financial strategy.

universal shelf Registration Statement regulatory

"The Company has filed a universal shelf Registration Statement on Form S-3"

A universal shelf registration statement is a standing registration filed with regulators that lets a company and authorized sellers offer and sell many kinds of securities (stock, bonds, warrants, etc.) over time without filing a new registration each time. For investors it matters because it gives the issuer the flexibility to raise cash or let insiders sell shares quickly, which can change the supply of securities, affect share price and dilution, and influence liquidity—like a store having a pre-approved plan to add new items to its shelves as needed.

Open Market Sale Agreement℠ financial

"entered into an Open Market Sale Agreement℠ (the “Sale Agreement”) with Jefferies LLC"

Controlled Equity Offering℠ Sales Agreement financial

"entered into a Controlled Equity Offering℠ Sales Agreement (the “Cantor Sales Agreement”)"

Registration Statement on Form S-3 regulatory

"Registration Statement on Form S-3 filed with the SEC on August 28, 2026"

A registration statement on Form S‑3 is a short, standardized filing a qualified public company uses to register new securities with regulators so they can be sold to investors; think of it as a pre-approved, reusable permission slip that speeds up future offerings. It matters to investors because it lets the company raise money more quickly and cheaply — which can fund growth or pay debt — but may also lead to share dilution or change in ownership, so it affects value and liquidity.

Offering Details

shelf/ATM

Offering

Offering Type shelf/ATM

FAQ

What new equity offering program did SPRO announce with Jefferies?

Spero Therapeutics entered into an Open Market Sale Agreement with Jefferies LLC, allowing it to sell up to $100,000,000 of common stock in at-the-market offerings once its new Form S-3 shelf registration statement is declared effective.

What is the total size of SPRO’s new universal shelf registration statement?

Spero Therapeutics filed a universal shelf Registration Statement on Form S-3 for the offer and sale of up to a maximum aggregate offering price of $300,000,000 in various securities, including debt, common stock, preferred stock, depositary shares, subscription rights, units and warrants.

How will Jefferies be compensated under SPRO’s new ATM program?

Under the Open Market Sale Agreement, the compensation to Jefferies for sales of Spero Therapeutics’ common stock will be an amount up to 3.0% of the aggregate gross proceeds of any shares sold through the at-the-market program.

What happened to SPRO’s prior Cantor Fitzgerald ATM facility?

Spero Therapeutics terminated its Controlled Equity Offering℠ Sales Agreement with Cantor Fitzgerald effective August 28, 2026. That agreement had permitted up to $75,000,000 of common stock sales, and the company incurred no termination penalties.

Did SPRO sell any shares under the Cantor ATM in 2024 or 2025?

No. As of the effective date of termination of the Cantor Sales Agreement, Spero Therapeutics had sold no shares of common stock pursuant to that agreement during the fiscal years ended December 31, 2025 and 2024.

Can SPRO start selling shares under the new Jefferies ATM immediately?

No. The Form S-3 Registration Statement is not yet effective, and no sales may be made under the Registration Statement or Sale Agreement prospectus until it is declared effective by the SEC.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Available on EDGAR 08/28/2026 - 05:26 PM Accepted by SEC EDGAR 08/28/2026 - 05:25 PM Learn about SEC filing dates

false 0001701108 0001701108 2026-08-28 2026-08-28

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): August 28, 2026

SPERO THERAPEUTICS, INC.

(Exact Name of Registrant as Specified in its Charter)

Delaware		001-38266		46-4590683
(State or Other Jurisdiction of Incorporation)		(Commission File Number)		(IRS Employer Identification No.)

675 Massachusetts Avenue, 14th Floor
Cambridge, Massachusetts		02139
(Address of Principal Executive Offices)		(Zip Code)

Registrant’s Telephone Number, Including Area Code: (857) 242-1600

Not Applicable

(Former Name or Former Address, if Changed Since Last Report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Title of each class		Trading Symbol(s)		Name of each exchange on which registered
Common Stock, $0.001 par value		SPRO		The Nasdaq Global Select Market

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company ☐

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 13(a) of the Exchange Act. ☐

Item 1.01 Entry into a Material Definitive Agreement.

On August 28, 2026, Spero Therapeutics, Inc., a Delaware corporation (the “Company”), entered into an Open Market Sale Agreement℠ (the “Sale Agreement”) with Jefferies LLC, as agent (“Jefferies”), pursuant to which the Company may offer and sell shares (the “Shares”) of its common stock, $0.001 par value per share (the “Common Stock”), from time to time through Jefferies (the “Offering”). The Company has filed a universal shelf Registration Statement on Form S-3 (“Registration Statement”) with the Securities and Exchange Commission (the “SEC”) for the offer and sale of up to a maximum aggregate offering price of $300,000,000 in debt securities, common stock, preferred stock, depositary shares, subscription rights, units and/or warrants (the “Securities”) and, forming a part of such Registration Statement, a sale agreement prospectus in connection with the Offering (the “Sale Agreement Prospectus”). The Registration Statement is not yet effective, and no sales may be made under the Registration Statement or Sale Agreement Prospectus until such time as the Registration Statement is declared effective. Once the Registration Statement is effective, the Company may offer and sell Shares pursuant to the Sale Agreement Prospectus of up to a maximum aggregate offering price of $100,000,000 from time to time under the Sale Agreement with Jefferies.

Sales of the Shares, if any, under the Sale Agreement Prospectus will be made in sales deemed to be “at-the-market offerings” as defined in Rule 415 promulgated under the Securities Act of 1933, as amended (the “Securities Act”). Jefferies is not required to sell any specific amount of securities, but will act as sales agent using commercially reasonable efforts to sell on the Company’s behalf all of the Shares requested to be sold by the Company, consistent with its normal trading and sales practices, on mutually agreed terms between Jefferies and the Company. There is no arrangement for funds to be received in any escrow, trust or similar arrangement.

The compensation to Jefferies for sales of the Shares sold pursuant to the Sale Agreement will be an amount up to 3.0% of the aggregate gross proceeds of any Shares sold under the Sale Agreement. The Sale Agreement contains customary representations, warranties, covenants and agreements, indemnification obligations of the Company and Jefferies, including for liabilities under the Securities Act or the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and termination provisions. The representations, warranties and covenants contained in the Sale Agreement were made only for purposes of such agreement and as of specific dates, were solely for the benefit of the parties to such agreement and may be subject to limitations agreed upon by the contracting parties.

A copy of the Sale Agreement is filed as Exhibit 1.1 hereto and is incorporated herein by reference. The foregoing description of the material terms of the Sale Agreement does not purport to be complete and is qualified in its entirety by reference to such exhibit.

The Shares will be sold pursuant to the Registration Statement, and offerings of the Shares will be made only by means of the Sale Agreement Prospectus. This Current Report on Form 8-K shall not constitute an offer to sell or solicitation of an offer to buy the Shares, nor shall there be any sale of the Shares in any state in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of such state or jurisdiction.

Item 8.01 Other Events.

As previously disclosed, on March 11, 2021, the Company entered into a Controlled Equity Offering℠ Sales Agreement (the “Cantor Sales Agreement”) with Cantor Fitzgerald & Co., as agent (“Cantor”), pursuant to which the Company could offer and sell shares of Common Stock from time to time through Cantor (the “Cantor ATM Offering”). The Company filed a prospectus with the SEC on March 15, 2024 in connection with the Cantor ATM Offering (the “Cantor Prospectus”). The Cantor Prospectus was filed under a universal shelf Registration Statement on Form S-3 filed with the SEC on March 15, 2024, which became effective on March 22, 2024. Pursuant to the Cantor Prospectus, the Company could offer and sell shares of Common Stock of up to a maximum aggregate offering price of $75,000,000 under the Cantor Sales Agreement.

The Company terminated the Cantor Sales Agreement effective as of August 28, 2026. The Company did not incur any termination penalties as a result of the termination of the Cantor Sales Agreement. As of the effective date of the termination of the Cantor Sales Agreement, the Company had sold no shares of Common Stock pursuant to the

Cantor Sales Agreement during the fiscal years ended December 31, 2025 and 2024. The termination of the Cantor Sales Agreement terminated any future sales of Common Stock through the Cantor ATM Offering pursuant to the Cantor Prospectus.

A copy of the Cantor Sales Agreement was filed as Exhibit 10.28 to the Company’s Annual Report on Form 10-K filed with the SEC on March 11, 2021.

Item 9.01. Financial Statements and Exhibits.

(d) Exhibits:

Exhibit No.		Description
1.1		Open Market Sale AgreementSM, dated August 28, 2026, by and between Spero Therapeutics, Inc. and Jefferies LLC (incorporated by reference as Exhibit 1.2 to the Registrant’s Registration Statement on Form S-3 filed with the SEC on August 28, 2026).
104		Cover Page Interactive Data File (embedded within the Inline XBRL document)

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

						SPERO THERAPEUTICS, INC.
Date: August 28, 2026				By:		/s/ Esther Rajavelu
						Esther Rajavelu
						Chief Executive Officer and Chief Financial Officer

Filing Exhibits & Attachments

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