Moleculin Biotech, Inc.

The audited financial statements incorporated by reference in this prospectus and elsewhere in the registration statement have been so incorporated by reference in reliance upon the report of Grant Thornton LLP, independent registered public accountants, upon the authority of said firm as experts in accounting and auditing.

Moleculin Biotech, Inc.

Up to 4,566,210 Shares of Common Stock

Up to 4,566,210** Pre-Funded Warrants to Purchase up to 4,566,210**** Shares of Common Stock**

Up to 13,698,630 Common Warrants to Purchase up to 13,698,630 Shares of Common Stock

Up to 18,264,840** Shares of Common Stock underlying the Pre-Funded Warrants and Common Warrants**

Roth Capital Partners

PRELIMINARY PROSPECTUS

_____________________, 2026

PART II -INFORMATION NOT REQUIRED IN PROSPECTUS

Item 13. Other Expenses of Issuance and Distribution.

The following table sets forth the estimated costs and expenses to be incurred in connection with the issuance and distribution of the securities of Moleculin Biotech, Inc. (the “Registrant”) which are registered under this Registration Statement on Form S-1 (this “Registration Statement”), other than placement agent commissions. All amounts are estimates except the Securities and Exchange Commission registration fee and the Financial Industry Regulatory Authority, Inc. (“FINRA”) filing fee.

The following expenses will be borne solely by the Registrant:

Item 14. Indemnification of Directors and Officers.

The Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws of Moleculin Biotech, Inc. (“MBI”) authorize it to indemnify directors, officers, employees and agents of MBI against expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement, actually and reasonably incurred in connection with any action, suit or proceeding, if the party to be indemnified acted in good faith and in a manner that he or she reasonably believed to be in or not opposed to the best interests of MBI, and, with respect to any criminal action or proceeding, such party had no reasonable cause to believe his conduct was unlawful. The Amended and Restated Certificate of Incorporation and the Amended and Restated Bylaws of MBI also authorize it to indemnify directors, officers, employees and agents of MBI who are or were a party to or threatened to be a party to, any threatened, pending, or completed action or suit by or in the right of MBI to procure a judgment in its favor by reason of the fact the he or she was a director, officer, employee or agent of MBI or of another entity at the request of MBI, against expenses (including reasonable attorneys’ fees) actually and reasonably incurred by him or her in connection with the defense or settlement of such action or suit if he or she acted in good faith and in a manner he or she reasonably believed to be in or not opposed to the best interests of MBI.

The Amended and Restated Bylaws also permit MBI to enter into indemnity agreements with its employees and agents. MBI has entered into such agreements with its directors and officers and may in the future enter into such agreements with other employees or agents. These agreements, together with the Amended and Restated Bylaws and Amended and Restated Certificate of Incorporation, may require MBI, among other things, to indemnify employees or agents against certain liabilities that may arise by reason of their status or service as directors (other than liabilities resulting from willful misconduct of a culpable nature), to advance expenses to them as they are incurred, provided that they undertake to repay the amount advanced if it is ultimately determined by a court that they are not entitled to indemnification, and to obtain and maintain directors’ and officers’ insurance if available on reasonable terms.

MBI’s Amended and Restated Certificate of Incorporation provides that directors shall have no personal liability to MBI or its stockholders for monetary damages for breach of fiduciary duty as a director, except (i) for any breach of a director’s duty of loyalty to MBI or its stockholders, (ii) for acts or omissions not in good faith or which involve intentional misconduct or a knowing violation of law, (iii) under section 174 of the General Corporation Law of Delaware as it may from time to time be amended or any successor provision thereto, or (iv) for any transaction from which a director derived an improper personal benefit.

MBI currently has directors’ and officers’ liability insurance. Delaware General Corporation Law, Section 145, and the Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws of MBI provide for the indemnification of officers, directors and other corporate agents in terms sufficiently broad to indemnify such persons, under certain circumstances, for liabilities (including reimbursement of expenses incurred) arising under the Securities Act.

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Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons pursuant to the foregoing provisions, or otherwise, MBI has been advised that in the opinion of the SEC such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable.

Item 15. Recent Sales of Unregistered Securities.

Except as set forth below, in the three years preceding the filing of this Registration Statement, the Registrant has not issued any securities that were not registered under the Securities Act:

On December 20, 2023, we entered into Securities Purchase Agreements (the “December Purchase Agreement”) with an institutional investor and several of our executive officers, advisors, and a member of our board of directors (collectively, the “Investors”) for the sale of: (i) 9,607 shares (the “December Shares”) of our common stock, par value $0.001 per share, and (ii) pre-funded warrants to purchase 9,181 shares of common stock in lieu thereof (the “December Pre-Funded Warrants”) in a registered direct offering (the “December Offering”). In a concurrent private placement (the “December Private Placement”), we also sold to the Investors unregistered warrants to purchase up to an aggregate of 37,573 shares of common stock (the “December Common Warrants”). The combined purchase price of one share of common stock (or December Pre-Funded Warrant in lieu thereof) and accompanying December Common Warrant was $240.00 for the institutional investor, and $258.75 for the executive officers, employees, advisors and the member of our board of directors who participated in the December Offering.

Subject to certain ownership limitations, each of the December Common Warrants becomes exercisable on the effective date of such stockholder approval as may be required by the applicable rules and regulations of the Nasdaq Stock Market with respect to the issuance of all of the December Common Warrants and the common stock upon the exercise thereof. On February 14, 2024, we held a special meeting of stockholders at which our stockholders approved the issuance of up to 37,573 shares of our common stock upon the exercise of the December Common Warrants. Subject to certain ownership limitations, each December Common Warrant will have an exercise price of $240.00 per share, expire five years from the date of stockholder approval and will become exercisable beginning on the effective date of stockholder approval (February 14, 2024) for the shares issuable upon the exercise of the December Common Warrants.

On February 13, 2025, we entered into a warrant exercise inducement offer letter (the Inducement Letter) with a holder of certain existing warrants (the Holder) pursuant to which the Holder received the Series C Warrants (also referred to as “Inducement Warrants”) to purchase up to a number of shares of common stock equal to 200% of the number of warrant shares issued pursuant to the exercise of certain existing warrants to purchase up to 233,143 shares of common stock, which the Holder agreed to exercise for cash at a reduced exercise price of $25.00 per share of common stock in exchange for the Company’s agreement to issue the Inducement Warrants to purchase up to 466,286 shares of the Company’s common stock. The Inducement Warrants were issued on February 14, 2025.

On February 25, 2025, the Company entered into a Securities Purchase Agreement with an institutional investor (the Investor) for the sale by the Company of 46,000 shares of the Company’s common stock and pre-funded warrants to purchase 84,842 shares of common stock (the Pre-Funded Warrants) in a registered direct offering (the February Offering). In a concurrent private placement (the February Private Placement), the Company also sold to the Investor unregistered Series D common stock purchase warrants (the Series D Warrants) to purchase up to an aggregate of 261,683 shares of common stock. The combined purchase price of one share of common stock (or Pre-Funded Warrant in lieu thereof) and accompanying warrants was $26.75. The closing of the February Offering and February Private Placement occurred on February 26, 2025.

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In August 2025, we entered into a warrant exercise inducement offer letter with holders of certain existing warrants pursuant to which the holders agreed to exercise Series E warrants to purchase up to 648,649 shares of common stock with an exercise price of $9.25 per share in exchange for our issuing new Series F warrants to purchase 2,594,595 shares of our common stock. Each Series F warrant had an exercise price of $13.75 per share (which was subsequently adjusted as described below) and is exercisable as of the date of issuance to October 16, 2030. We received gross proceeds of $6.0 million. In addition, during August and September 2025, Series E warrants to purchase 119,848 shares of common stock were exercised for $1.1 million. Upon issuance the Series F warrants were initially classified as liability warrants. Pursuant to the Warrant Amendment described above, the Series F warrants were equity-classified as of December 21, 2025. In total, we received gross proceeds of $7.1 million.

On December 9, 2025, we entered into warrant exercise inducement agreements with holders of certain outstanding Series C, Series D and Series F warrants, pursuant to which such holders agreed to exercise warrants to purchase an aggregate of 1,044,329 shares of our common stock at reduced or existing exercise prices in exchange for the issuance of Series G warrants to purchase up to 2,610,823 shares of common stock, representing 250% of the shares underlying the exercised warrants, with a five-year exercise term. We received gross proceeds of approximately $6.8 million from the warrant exercises. Pursuant to ASC 815, and in accordance with our accounting policy, we determined that Series G warrants met the criteria for equity classification as of their issuance date. Additionally, we utilized our ATM on this date and sold 86,475 shares for gross proceeds of $0.6 million.

On February 19, 2026, we entered into warrant inducement agreements with certain holders of our existing equity-classified Series E and Series F warrants. Pursuant to these agreements, the holders exercised warrants to purchase an aggregate of 2,122,652 shares of our common stock at an exercise price of $3.90 per share, resulting in gross proceeds of approximately $8.3 million. In consideration for such exercises, we issued new Series H warrants to purchase up to 6,367,956 shares of common stock. The Series H warrants became exercisable upon stockholder approval, have a term of five years from such approval, and an exercise price of $2.3976 per share. The Series H warrants contain customary anti-dilution adjustments and beneficial ownership limitations. Due to the completion of this transaction, the exercise price of our outstanding Series E warrants was adjusted to $3.00 per share, the exercise price of our outstanding Series F warrants was adjusted to $2.75 per share, and the exercise price of our outstanding Series G and Series H warrants was adjusted to $2.3976 per share.

Issuance of Warrants to Consultants

In August 2023, the Company issued warrants to purchase 6,667 shares of common stock with an exercise price of $9.30 to an entity providing consulting services, which warrants will vest based on performance of certain services.

In March 2024, the Company granted equity classified warrants to purchase up to 133 shares of common stock with a ten year term and an exercise price of $228.75 per share. These warrants vest annually over four years, subject to continued service.

In October 2024, the Company issued equity classified warrants to consultants to purchase up to 1,440 shares of common stock. Of these warrants, 1,200 vest annually over four years, 200 vest monthly over three years, and 40 vested immediately upon issuance.

In January 2025, the Company granted equity classified warrants to two consultants to purchase up to 2,000 shares of common stock each, with a ten-year term and an exercise price of $41.00 per share. Of these warrants, 2,000 vest upon the performance of specified services and the remaining 2,000 vest annually over four years.

In July 2025, the Company issued 8,000 equity-classified warrants to a consultant, of which 4,000 vested immediately and 4,000 vest upon the execution of a letter of intent or the closing of a licensing transaction with a value of $10.0 million or more.

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In September 2025, the Company granted equity classified warrants to purchase up to 14,000 shares of the Company’s common stock with a ten year term and an exercise price of $10.75 per share. These warrants vest annually over four years, subject to continued service.

All of the securities above were issued in reliance on the exemption from registration provided by Section 4(a)(2) of the Securities Act or Regulation D promulgated thereunder.

Item 16. Exhibits and Financial Statement Schedules.

(a)Exhibits:

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(b)Consolidated Financial Statement Schedules: All schedules are omitted because the required information is inapplicable or the information is presented in the consolidated financial statements and the related notes.

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Item 17. Undertakings

(a) The undersigned Registrant hereby undertakes that:

(1) To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:

(i) To include any prospectus required by Section 10(a)(3) of the Securities Act;

(ii) To reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than 20 percent change in the maximum aggregate offering price set forth in the “Calculation of Registration Fee” table in the effective registration statement;

(iii) To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement;

provided, however, that paragraphs (1)(i), (1)(ii) and (1)(iii) above do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commission by the registrant pursuant to Section 13 or Section 15(d) of the Exchange Act that are incorporated by reference in this Registration Statement or is contained in a form of prospectus filed pursuant to Rule 424(b) that is part of the Registration Statement.

(2) That, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(3) To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

(4) That, for the purpose of determining liability under the Securities Act to any purchaser: each prospectus filed pursuant to Rule 424(b) as part of a registration statement relating to an offering, other than registration statements relying on Rule 430B or other than prospectuses filed in reliance on Rule 430A, shall be deemed to be part of and included in the registration statement as of the date it is first used after effectiveness. Provided, however, that no statement made in a registration statement or prospectus that is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registration statement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale prior to such first use, supersede or modify any statement that was made in the registration statement or prospectus that was part of the registration statement or made in any such document immediately prior to such date of first use.

(5) That, for the purpose of determining liability of the registrant under the Securities Act to any purchaser in the initial distribution of the securities, the undersigned registrant undertakes that in a primary offering of securities of the undersigned registrant pursuant to this registration statement, regardless of the underwriting method used to sell the securities to the purchaser, if the securities are offered or sold to such purchaser by means of any of the following communications, the undersigned registrant will be a seller to the purchaser and will be considered to offer or sell such securities to such purchaser:

(i) Any preliminary prospectus or prospectus of the undersigned registrant relating to the offering required to be filed pursuant to Rule 424;

(ii) Any free writing prospectus relating to the offering prepared by or on behalf of the undersigned registrant or used or referred to by the undersigned registrant;

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(iii) The portion of any other free writing prospectus relating to the offering containing material information about the undersigned registrant or its securities provided by or on behalf of the undersigned registrant; and

(iv) Any other communication that is an offer in the offering made by the undersigned registrant to the purchaser.

(b) The undersigned Registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the registrant’s annual report pursuant to section 13(a) or section 15(d) of the Exchange Act (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to section 15(d) of the Exchange Act) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(c) Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the Registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer, or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered hereunder, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question of whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

(d) The undersigned registrant hereby undertakes that:

(1) For purposes of determining any liability under the Securities Act of 1933, the information omitted from the form of prospectus filed as part of this registration statement in reliance upon Rule 430A and contained in a form of prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4) or 497(h) under the Securities Act shall be deemed to be part of this registration statement as of the time it was declared effective.

(2) For the purpose of determining any liability under the Securities Act of 1933, each post-effective amendment that contains a form of prospectus shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

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SIGNATURES

Pursuant to the requirements of the Securities Act, the registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the city of Houston, Texas, on July 29, 2026.