GT Biopharma Inc美股招股说明书(2026-01-08版)

GT BIOPHARMA, INC.UP TO 54,423,826 SHARES OF COMMON STOCK This prospectus supplement no. 1 (this “Prospectus Supplement”) amends and supplements the prospectus dated July 3, 2025 (asmay be supplemented or amended from time to time, the “Prospectus”), which forms a part of the Registration Statement on Form S-1(Registration No. 333-287963) of GT Biopharma, Inc. (the “Company,” “we,” “us,” or “our”). The purpose of this Prospectus Supplement is to update and supplement the information included in the Prospectus. ThisProspectus Supplement is not considered complete without, and may not be delivered or utilized except in combination with, theProspectus, including any amendments or supplements thereto. This Prospectus Supplement should be read in conjunction with theProspectus, and if there is any inconsistency between the information in the Prospectus and this Prospectus Supplement, you shouldrely on this Prospectus Supplement. As previously disclosed, on May 12, 2025, the Company entered into a securities purchase agreement with the purchasersidentified therein providing for the issuance and sale to the purchasers of certain securities, including up to 6,056 shares of Series L10% Convertible Preferred Stock, par value $0.001 per share (“Series L Preferred Stock”). Also, as previously disclosed, pursuant tothe Certificate of Designation of Preferences, Rights and Limitations of Series L 10% Convertible Preferred Stock (the “Certificate ofDesignation”), the holders of the Series L Preferred Stock are entitled to receive cumulative dividends at the rate per share (as apercentage of the stated value per share) of 10% per annum until May 11, 2026, increasing to 12% per annum thereafter, payablequarterly on January 1, April 1, July 1 and October 1 in cash, shares of Common Stock, par value $0.001 (“Common Stock”) (subjectto the Company’s satisfaction of the conditions set forth in the Certificate of Designation), or a combination thereof.On December 18,2025, the Company provided notice to the holders of the Series L Preferred Stock that the Company would pay its dividend payable onJanuary 1, 2026 on the Series L Preferred Stock in shares of Common Stock in lieu of cash. You should read this prospectus and any prospectussupplementor amendment carefully before you invest in our Common Stock. Our shares of Common Stock are listed on the Nasdaq Capital Market (“Nasdaq”) under the symbol “GTBP.” On January 7, 2026,the closing price of our Common Stock was $0.732 per share. Investing in our securities involves risks that are described in the “Risk Factors” section beginning on page 7of theProspectus. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of thesecurities to be issued under this prospectus or determined if this prospectus is truthful or complete. Any representation to thecontrary is a criminal offense. The date of this prospectus is January 8, 2026.