莫尔库林生物科技公司美股招股说明书(2025-12-23版)

Moleculin Biotech, Inc. Up to $8,200,000 Common Stock We have previously entered into an At The Market Offering Agreement (“ATM Agreement”) with Roth Capital Partners, LLC(“Roth”) relating to shares of our common stock offered by this prospectus supplement and the prospectus dated July 11, 2025 (the“July 2025 prospectus”) and the base prospectus dated July 1, 2024 (the “base prospectus”). In accordance with the terms of theATM Agreement, we may offer and sell shares of our common stock, $0.001 par value per share, from time to time through Roth We are filing this prospectus supplement to supplement and amend, as of December 31, 2025, the July 2025 prospectus to increasethe maximum aggregate gross sales price of our common stock that may be offered, issued and sold under the ATM Agreementfrom and after the date hereof to $8,200,000 (the “ATM Offering Size Increase”), not including the shares of common stockpreviously sold. Under the July 2025 prospectus, we initially registered up to $6,500,000 of our common stock for offer and sale The purpose of this prospectus supplement is to update the July 2025 prospectus to reflect the ATM Offering Size Increase. Exceptas modified by this prospectus supplement, the terms of the ATM Offering remain unchanged, and the ATM Agreement remains in Our common stock is listed on The Nasdaq Capital Market (“Nasdaq”), under the symbol “MBRX.” On December 22, 2025, thelast reported sale price of our common stock on Nasdaq was $4.01 per share. As of the date of this prospectus supplement, the aggregate market value of the voting and non-voting common equity held by non-affiliates, our public float, was approximately $39,435,000, based on 3,079,229 shares of outstanding common stock as of the dateof this prospectus supplement, of which 43,454 shares were held by non-affiliates, and at a price of $12.99 per share, the closingsale price of our common stock on November 3, 2025, which is the highest closing sale price of our common stock on Nasdaqwithin the prior 60 days of this prospectus supplement. Pursuant to General Instruction I.B.6 of Form S-3, in no event will we sellsecurities in a public primary offering with a value exceeding more than one-third of our public float in any 12-month period solong as our public float remains below $75.0 million. During the 12 calendar months prior to and including the date of this beginning on page S-9 of the July 2025 prospectus and the base prospectus, respectively, and in the documents incorporatedby reference into the July 2025 prospectus and base prospectus, as well as the risks and uncertainties described in other Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of thesesecurities or passed upon the adequacy or accuracy of this prospectus supplement or the accompanying prospectus. Any Roth Capital Partners The date of this prospectus supplement is December 31, 2025