GT Biopharma, Inc.

Item 1.01 Entry into a Material Definitive Agreement. Private Placement of Preferred Stock and Warrants As previously disclosed, on September 14, 2026, GT Biopharma, Inc. (the “Company”) entered into a Securities Purchase Agreement (the “Securities Purchase Agreement”) with the purchasers identified therein (collectively, the “Original Purchasers”) providing for the issuance and sale to the Original Purchasers of (i) up to 8,277.778 shares of the Company’s Series M 10% Convertible Preferred Stock (the “Preferred Stock”), (ii) warrants to purchase up to a number of shares of common stock of the Company (the “Common Stock”) equal to 100% of the shares of the Company’s Common Stock issuable upon conversion of the shares of Preferred Stock (the “Common Warrants”), and (iii) warrants to purchase up to a number of shares of Company’s Common Stock equal to the number of Greenshoe Conversion Shares (as defined in the Securities Purchase Agreement) issuable upon exercise of the Greenshoe Right (as defined below) (the “Vesting Warrants” and together with the Common Warrants, the “Warrants”), with an aggregate stated value of $8,277,778, for an aggregate purchase price of $7,450,000 (the “Offering”). On September 18, 2026, the Company and the requisite Original Purchasers entered into the First Amendment to Securities Purchase Agreement (the “First Amendment to Securities Purchase Agreement”) with a new purchaser (the “New Purchaser,” and together with the Original Purchasers, the “Purchasers”) to increase the number of shares of Preferred Stock issuable under the Securities Purchase Agreement, as amended, to 8,611.111 shares of Preferred Stock, and thereby increase the accompanying Warrants, with an aggregate stated value of $8,611,111.11, for an aggregate purchase price of $7,750,000, on the same terms and conditions as previously disclosed. Pursuant to the First Amendment to Securities Purchase Agreement, each Purchaser may elect to purchase shares of Preferred Stock with an aggregate stated value of up to $34,675,615 (the “Greenshoe Rights”) for an aggregate purchase price of $31,208,054, subject to adjustments, as further described in the Securities Purchase Agreement, as amended. Each Purchaser is entitled to exercise its respective Greenshoe Rights for an amount of Preferred Stock equal to the ratio of such Purchaser’s original subscription amount to the original aggregate subscription amount of all Purchasers. The securities in the Offering were offered privately pursuant to Rule 506(b) of Regulation D under the Securities Act of 1933, as amended. Registration Rights Agreement As previously disclosed, on September 14, 2026, the Company and the Original Purchasers entered into a registration rights agreement (the “Registration Rights Agreement”) pursuant to which the Company agreed to file a registration statement with the Securities and Exchange Commission (the “SEC”) covering the public resale of the Common Stock issuable upon conversion of the Preferred Stock and upon exercise of the Warrants. The Company has agreed to file a registration statement within thirty (30) days after the initial closing and after each closing of the exercise of a Greenshoe Right in accordance with the Securities Purchase Agreement, as amended, to become effective no later than sixty (60) days after the Closing Date (as defined in the Securities Purchase Agreement) or each Trigger Date (as defined in the Registration Rights Agreement), or in the event of a “full review” by the SEC, ninety (90) days after the Closing Date or each Trigger Date. On September 18, 2026, the New Purchaser executed a joinder agreement to the Registration Rights Agreement on the same terms and conditions (the “Joinder”). Certificate of Increase to Certificate of Designations of Preferences, Rights and Limitations of Series M Convertible Preferred Stock As previously disclosed, on September 14, 2026, the Company filed a Certificate of Designation of Preferences, Rights and Limitations of Series M 10% Convertible Preferred Stock with the Secretary of State of the State of Delaware (the “Certificate of Designations”). On September 18, 2026, the Company filed with the Secretary of State of the State of Delaware a Certificate of Increase (the “Certificate of Increase”) increasing the shares of the Preferred Stock as designated in the Certificate of Designations from 41,778 shares to 43,287 shares. The foregoing descriptions of the Certificate of Increase, First Amendment to Securities Purchase Agreement and Joinder do not purport to be complete and are qualified in their entirety by reference to the full text of such documents, copies of which are attached as Exhibits 3.1, 10.1 and 10.2 to this Current Report on Form 8-K, which are incorporated herein by reference.

Item 3.02 Unregistered Sales of Equity Securities. The information set forth in Item 1.01 is incorporated by reference into this Item 3.02 in its entirety.

Item 3.03 Material Modification to Rights of Security Holders. The information set forth in Item 1.01 is incorporated by reference into this Item 3.03 in its entirety.

Item 5.03 Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year. The information set forth in Items 1.01 is incorporated by reference into this Item 5.03 in its entirety.