FibroBiologics ups ATM capacity to $7.5M

FibroBiologics filed a prospectus supplement updating its at‑the‑market sales capacity under the Form S-3 registration: it may sell up to $7,500,000 of Common Stock from time to time through H.C.

Rhea-AI Filing Summary

FibroBiologics filed a prospectus supplement updating its at‑the‑market sales capacity under the Form S-3 registration: it may sell up to $7,500,000 of Common Stock from time to time through H.C. Wainwright & Co. The supplement states the company sold approximately $8.14 million of Common Stock in the prior 12 months and sold about $0.17 million under the earlier supplement. The company reports a public float of approximately $47.0 million based on 5,399,112 shares held by non‑affiliates as of May 5, 2026, and notes Nasdaq closing prices of $1.38 (May 4, 2026) and $8.70 (March 6, 2026) used in float calculation. The offering is subject to the Form S-3 General Instruction I.B.6 limitations and the Sales Agreement, with any additional sales requiring a new prospectus supplement.

Insights

Capital Markets Analyst

Supplement raises short‑term ATM capacity to $7.5M while documenting prior $8.14M sales.

The filing updates the available at‑the‑market (ATM) capacity under the existing Sales Agreement with H.C. Wainwright, allowing offers of up to $7,500,000 of Common Stock subject to Form S-3 limits. The supplement also records prior issuance activity of $8.14M over the last 12 months and $0.17M under the prior supplement.

Practical implications: actual issuance pace depends on market opportunities and the company’s decision to place shares through the broker; timing and cash proceeds per sale are not detailed in this excerpt.

Securities & Compliance Attorney

Text preserves Form S-3 constraints and attaches the General Instruction I.B.6 qualifier.

The supplement explicitly ties the offering cap to General Instruction I.B.6 of Form S-3 and the Sales Agreement terms, including the one‑third public float cap when float is below $75,000,000. It notes that additional sales beyond this limit would require another prospectus supplement.

Compliance note: the filing clarifies pricing anchors used for public float calculation and reiterates investor risk disclosures; purchasers should consult the full prospectus and the Sales Agreement for placement mechanics.

Key Figures

ATM capacity: $7,500,000 Prior 12‑month sales: $8,140,000 Prior supplement sales: $170,000 +4 more

7 metrics

ATM capacity $7,500,000 maximum aggregate offering price under this Supplement

Prior 12‑month sales $8,140,000 aggregate sales in prior 12 calendar months (excluded current Supplement)

Prior supplement sales $170,000 aggregate sold pursuant to the Original Prospectus Supplement

Public float (non‑affiliates) $47.0M aggregate market value of outstanding Common Stock held by non‑affiliates as of May 5, 2026

Non‑affiliate shares 5,399,112 shares shares outstanding held by non‑affiliates as of May 5, 2026

Nasdaq close price (May 4, 2026) $1.38 closing sale price on Nasdaq cited in the supplement

Price used for float calc (Mar 6, 2026) $8.70 closing sale price used to calculate public float

Key Terms

At The Market Offering Agreement, General Instruction I.B.6 of Form S-3, public float, emerging growth company

4 terms

At The Market Offering Agreement regulatory

"the At The Market Offering Agreement (the “Sales Agreement”), dated May 1, 2026"

An at-the-market offering agreement is a contract that lets a company sell newly issued shares directly into the open market through a broker, at whatever price the stock is trading at that moment. For investors this matters because it can increase the number of shares available (which may dilute existing ownership) while providing a flexible, often faster way for the company to raise cash without fixing a price, similar to a vendor selling small batches at current market stalls rather than setting a single fixed price.

General Instruction I.B.6 of Form S-3 regulatory

"in accordance with General Instruction I.B.6 of Form S-3"

public float financial

"aggregate market value of our outstanding Common Stock held by non-affiliates"

Public float is the total number of a company's shares that are available for trading by the general public. It excludes shares held by company insiders or large stakeholders who are unlikely to sell them easily. This figure helps investors understand how much of the company's stock is actively available, which can influence its liquidity and how easily its price might change.

emerging growth company regulatory

"We are an “emerging growth company” and a “smaller reporting company”"

An emerging growth company is a recently public or smaller public firm that qualifies for temporary, lighter regulatory and disclosure rules to reduce the cost and effort of being public. For investors, it means the company may provide less historical financial detail and face fewer reporting requirements than larger firms, so it can grow more quickly but also carries higher uncertainty—like buying a promising early-stage product with fewer user reviews.

Offering Details

ATM · $7,500,000

Offering

Offering Type ATM

Securities Offered Common Stock

Offering Amount $7,500,000

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What amount can FibroBiologics (FBLG) sell under this prospectus supplement?

The company may offer up to $7,500,000 of Common Stock under the Sales Agreement. The supplement updates ATM capacity and complies with Form S-3 General Instruction I.B.6 limits.

How much stock did FibroBiologics sell in the prior 12 months?

FibroBiologics sold approximately $8.14 million of Common Stock in the 12 calendar months prior to this supplement. The figure excludes shares offered under the current supplement.

What public float did the company report for the Form S-3 calculation?

The reported aggregate market value of shares held by non‑affiliates was about $47.0 million. That value used 5,399,112 non‑affiliate shares and a March 6, 2026 closing price of $8.70.

Who will sell the shares and by what method?

Shares may be sold from time to time through H.C. Wainwright & Co. under an At‑The‑Market Offering Agreement. Sale methods depend on the Sales Agreement; specific transaction mechanics are not detailed here.

Does this supplement change investor risk disclosures or reporting status?

No substantive change to risk disclosures is stated; FibroBiologics reiterates existing Risk Factors and confirms it is an emerging growth company and a smaller reporting company under federal securities laws.

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

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Accepted by SEC EDGAR 05/05/2026 - 05:39 PM Learn about SEC filing dates

Filed pursuant to Rule 424(b)(5)

Registration No. 333-284663

PROSPECTUS SUPPLEMENT

(to Prospectus Supplement dated May 1, 2026

and Prospectus dated February 10, 2025)

FibroBiologics, Inc.

Up to $7,500,000

Common Stock

This prospectus supplement (this “Supplement”) amends, modifies, supersedes and supplements certain information contained in, and should be read in conjunction with, our prospectus supplement, dated May 1, 2026 (the “Original Prospectus Supplement”), and the accompanying base prospectus, dated February 10, 2025 (the “Base Prospectus” and, collectively with the Original Prospectus Supplement, the “Prospectus”) filed as part of our registration statement on Form S-3 (File No. 333-284663) (the “Registration Statement”), as supplemented by the Prospectus, relating to the offering, issuance and sale by us of our Common Stock, par value $0.00001 per share (the "Common Stock"), from time to time that may be issued and sold under the At The Market Offering Agreement (the “Sales Agreement”), dated May 1, 2026, by and between us and H.C. Wainwright & Co., LLC (“Wainwright”). This Supplement should be read in conjunction with the Prospectus, and is qualified by reference thereto, except to the extent that the information herein amends or supersedes the information contained in the Prospectus. This Supplement is not complete without, and may only be delivered or utilized in connection with, the Prospectus, and any future amendments or supplements thereto.

We are filing this Supplement to amend the Prospectus to update the maximum amount of our Common Stock we are eligible to sell under the Registration Statement pursuant to General Instruction I.B.6 of Form S-3. As a result of these limitations and the current public float of our Common Stock calculated as set forth below, and in accordance with the terms of the Sales Agreement, we may offer and sell our Common Stock having an aggregate offering price of up to $7,500,000 from time to time through Wainwright, which does not include the Common Stock having an aggregate sales price of approximately $8.14 million that were sold during the 12 calendar months prior to, and including, the date of this prospectus supplement(but excluding the shares of common stock offered pursuant to this Supplement)We have sold an aggregate of approximately $0.17 million pursuant to the Original Prospectus Supplement. In the event that we may sell additional amounts under the Sales Agreement and in accordance with General Instruction I.B.6 of Form S-3, we will file another prospectus supplement prior to making such additional sales.

Our Common Stock is traded on the Nasdaq Capital Market (“Nasdaq”), under the symbol “FBLG.” On May 4, 2026, the closing sale price of our Common Stock on Nasdaq was $1.38 per share.

As of the date of this Supplement, the aggregate market value of our outstanding Common Stock held by non-affiliates was approximately $47.0 million, which was calculated based on 5,399,112 shares of outstanding Common Stock held by non-affiliates as of May 5, 2026, at a price per share of $8.70, the closing sale price of our Common Stock on Nasdaq on March 6, 2026. In no event will we sell shares of our Common Stock pursuant to this prospectus supplement and accompanying prospectus with a value exceeding more than one-third of our public float in any 12 calendar month period if, and for so long as, our public float is below $75 million at the date of measurement. During the 12 calendar months prior to, and including, the date of this Supplement (but excluding the shares of common stock offered pursuant to this Supplement), we have sold approximately $8.14 million of shares of our Common Stock pursuant to General Instruction I.B.6 of Form S‑3.

We are an “emerging growth company” and a “smaller reporting company” as defined under the federal securities laws and, as such, have elected to comply with certain reduced public company reporting requirements for this prospectus and may elect to do so in future filings. See “Prospectus Supplement Summary—Implications of Being an Emerging Growth Company and a Smaller Reporting Company” page S-5 of the Original Prospectus Supplement.

Investing in our Common Stock involves a high degree of risk. You should read carefully and consider the information contained in and incorporated by reference under “Risk Factors” beginning on page S-8 of the Original Prospectus Supplement and beginning on page 3 of the base prospectus and under the caption “Risk Factors” in our most recently filed Annual Report on

Form 10-K and most recently filed Quarterly Report on Form 10-Q, each as filed with the Securities and Exchange Commission, which are incorporated herein by reference in their entirety.

Neither the Securities and Exchange Commission (the “SEC”) nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

H.C. Wainwright & Co.

The date of this prospectus supplement is May 5, 2026.