Moleculin Biotech, Inc.

As filed with the U.S. Securities and Exchange Commission on July 29,** 2026.**

Registration No. 333-____

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORMS-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

**Moleculin Biotech, Inc. **

(Exact name of registrant as specified in its charter)

5300 Memorial Drive, Suite 950

Houston, Texas 77007

(713) 300-5160

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Walter V. Klemp, Chief Executive Officer and Chairman

5300 Memorial Drive, Suite 950

Houston, Texas 77007

(713) 300-5160

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale to the public: As soon as practicable after the effective date of this Registration Statement.

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box. ☒

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Securities Exchange Act. (Check one):

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ☐

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the Registration Statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

The information in this preliminary prospectus is not complete and may be changed. These securities may not be sold until the registration statement filed with the Securities and Exchange Commission is effective. This preliminary prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any state or jurisdiction where the offer or sale is not permitted.

DATED July 29, 2026

SUBJECT TO COMPLETION

PRELIMINARY PROSPECTUS

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

This is a reasonable best efforts public offering of up to 4,566,210 shares of our common stock, together with up to 13,698,630 Series I common warrants to purchase an aggregate of up to 13,698,630 shares of common stock (“common warrants”) at an assumed combined public offering price of $2.19 per share and common warrants, which was the closing price of our common stock as reported on the Nasdaq Capital Market (“Nasdaq”) on July 27, 2026.

Each share of common stock is being offered together with three common warrants, each to purchase one share of common stock. The common warrants will be exercisable beginning on the effective date of such stockholder approval as may be required by the applicable rules and regulations of the Nasdaq Stock Market to permit the exercise of the common warrants (“Warrant Stockholder Approval”) at an assumed exercise price of $2.19 per share (or 100% of the combined price) and will expire five years from the date of the Warrant Stockholder Approval.

If we are unable to obtain any required Warrant Stockholder Approval, the common warrants will not be exercisable and therefore have no value. The shares of common stock and accompanying common warrants will be separately issued and will be immediately separable upon issuance but will be purchased together in this offering. This prospectus also covers the shares of common stock issuable upon exercise of the common warrants sold in this offering.

We are also offering pre-funded warrants to those purchasers, whose purchase of shares of common stock in this offering would result in the purchaser, together with its affiliates and certain related parties, beneficially owning more than 4.99% (or, at the election of the purchaser, 9.99%) of our outstanding common stock immediately following the consummation of this offering in lieu of the shares of our common stock that would result in ownership in excess of 4.99% (or, at the election of the purchaser, 9.99%). Each pre-funded warrant will be exercisable for one share of common stock at an exercise price of $0.001 per share. Each pre-funded warrant is being offered together with the same common warrants, each to purchase one share of common stock described above being offered with each share of common stock. The purchase price of each pre-funded warrant will equal the combined public offering price per share of common stock and common warrants being sold in this offering, less the $0.001 per share exercise price of each such pre-funded warrant. Each pre-funded warrant will be immediately exercisable upon issuance and will expire when exercised in full. The pre-funded warrants and accompanying common warrants will be separately issued and will be immediately separable upon issuance but will be purchased together in this offering. For each pre-funded warrant that we sell, the number of shares of common stock that we are selling will be decreased on a one-for-one basis. This prospectus also covers the shares of common stock issuable from time to time upon the exercise of the pre-funded warrants offered hereby.

There is no established public trading market for the pre-funded warrants or common warrants, and we do not expect a market to develop. We do not intend to apply for listing of the pre-funded warrants or common warrants on any securities exchange or other nationally recognized trading system. Without an active trading market, the liquidity of the pre-funded warrants and common warrants will be limited.

We have engaged Roth Capital Partners, LLC (the "placement agent"), to act as our exclusive placement agent in connection with this offering. The placement agent has agreed to use its reasonable best efforts to arrange for the sale of the securities offered by this prospectus. The placement agent is not purchasing or selling any of the securities we are offering, and the placement agent is not required to arrange the purchase or sale of any specific number or dollar amount of securities. We have agreed to pay to the placement agent the placement agent fees set forth in the table below, which assumes that we sell all the securities offered by this prospectus. There is no arrangement for funds to be received in escrow, trust or similar arrangement. There is no minimum number of shares of securities or minimum aggregate amount of proceeds that is a condition for this offering to close. We may sell fewer than all the securities offered hereby, which may significantly reduce the amount of proceeds received by us, and investors in this offering will not receive a refund if we do not sell all of the securities offered hereby. Because there is no escrow account and no minimum number of securities or amount of proceeds, investors could be in a position where they have invested in us, but we have not raised sufficient proceeds in this offering to adequately fund the intended uses of the proceeds as described in this prospectus. We will bear all costs associated with the offering. See “Plan of Distribution” on page 20 of this prospectus for more information regarding these arrangements. This offering will terminate no later than August 14, 2026, unless we decide to terminate the offering (which we may do at any time in our discretion) prior to that date.

Our common stock is listed on the Nasdaq Capital Market under the symbol “MBRX.” On July 27, 2026, the last reported sale price of our common stock on the Nasdaq Capital Market was $2.19 per share. All share, common warrant and pre-funded warrant numbers are based on an assumed combined public offering price of $2.19 per share or pre-funded warrant, as applicable, and three common warrants. The actual combined public offering price per share and common warrants and the actual combined public offering price per pre-funded warrant and common warrants will be determined between us and investors based on market conditions at the time of pricing and may be at a discount to the current market price of our common stock. Therefore, the assumed public offering price used throughout this prospectus may not be indicative of the final public offering price. Numbers in this prospectus reflect the reverse stock split of our common stock at the reverse split ratio of 1-for-25 that was effected on December 1, 2025.

You should read this prospectus, together with additional information described under the headings “Incorporation of Certain Information By Reference”* *and “Where You Can Find More Information,” carefully before you invest in any of our securities.

Investing in our securities involves a high degree of risk. See the section entitled“Risk Factors”** beginning on page 6**** of this prospectus and in the documents incorporated by reference into this prospectus for a discussion of risks that should be considered in connection with an investment in our securities.**

(1) We have agreed to pay the placement agent a cash placement commission equal to 7% of the aggregate proceeds from this offering. We have also agreed to reimburse the placement agent for certain expenses incurred in connection with this offering. See “Plan of Distribution” for additional information about the compensation payable to the placement agent.

(2) Because there is no minimum number of securities or amount of proceeds required as a condition to closing in this offering, the actual public offering amount, placement agent fees, and proceeds to us, if any, are not presently determinable and may be substantially less than the total maximum offering amounts set forth above.

The delivery of the shares of common stock and any pre-funded warrants and common warrants to purchasers is expected to be made on or about August___, 2026, subject to satisfaction of certain customary closing conditions.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

Roth Capital Partners

The date of this prospectus is , 2026.