As filed with the Securities and Exchange Commission on October 15, 2018.

Registration No. 333-_______

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Moleculin Biotech, Inc.

(Exact Name of Registrant as Specified inIts Charter)

5300 Memorial Drive, Suite 950

Houston, Texas 77007

(713) 300-5160

(Address, Including Zip Code, and TelephoneNumber, Including Area Code, of Registrant’s Principal Executive Offices)

Mr. Walter Klemp, Chief Executive Officer

5300 Memorial Drive, Suite 950

Houston, Texas 77007

(713) 300-5160

(Name, Address, Including Zip Code, and TelephoneNumber, Including Area Code, of Agent For Service)

Copies to:

Cavas S. Pavri

Schiff Hardin LLP

100 N. 18th, Suite 300

Philadelphia, PA 19103

Telephone: (202) 724-6847

Fax: (202) 778-6460

Approximate date of commencement of proposed saleto the public: From time to time after the effective date of this Registration Statement.

If any of the securities being registered onthis form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the followingbox.  x

If this form is filed to register additionalsecurities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Actregistration statement number of the earlier effective registration statement for the same offering.  ¨

If this form is a post-effective amendmentfiled pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statementnumber of the earlier effective registration statement for the same offering.  ¨

If this form is a post-effective amendmentfiled pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statementnumber of the earlier effective registration statement for the same offering.  ¨

Indicate by check mark whether the registrantis a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitionsof “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerginggrowth company” in Rule 12b-2 of the Securities Exchange Act of 1934. (Check one):

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

(1) Pursuant to Rule 457(c) of the Securities Act of 1933, as amended,calculated on the basis of the average of the high and low prices per share of the registrant’s common stock as reportedby The NASDAQ Capital Market on October 12, 2018.

The Registrant hereby amends this RegistrationStatement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendmentwhich 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, actingpursuant to said Section 8(a), may determine.

The information in thispreliminary prospectus is not complete and may be changed. The selling shareholders may not sell these securities until the registrationstatement filed with the Securities and Exchange Commission is effective. This preliminary prospectus is not an offer to sell,nor does it seek an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject to Completion DatedOctober 15, 2018

Moleculin Biotech, Inc.

6,730,526 Shares of Common Stock

This prospectus relates to the offer and saleof up to 6,730,526 shares of our common stock by Lincoln Park Capital Fund, LLC, or Lincoln Park or the selling stockholder.

The shares of common stock being offered by theselling stockholder have been or may be issued pursuant to the Purchase Agreement dated October 4, 2018 that we entered into withLincoln Park. See “The Lincoln Park Transaction” for a description of that agreement and “Selling Stockholder”for additional information regarding Lincoln Park. The prices at which Lincoln Park may sell the shares will be determined by theprevailing market price for the shares or in negotiated transactions.

We are not selling any securities under thisprospectus and will not receive any of the proceeds from the sale of shares by the selling stockholder.

The selling stockholder may sell the shares ofcommon stock described in this prospectus in a number of different ways and at varying prices. See “Plan of Distribution”for more information about how the selling stockholder may sell the shares of common stock being registered pursuant to this prospectus.The selling stockholder is an “underwriter” within the meaning of Section 2(a)(11) of the Securities Act of 1933, asamended.

We will pay the expenses incurred in registeringthe shares, including legal and accounting fees. See “Plan of Distribution”.

Our common stock is currently listed on The NASDAQCapital Market under the symbol “MBRX”. On October 12, 2018, the last reported sale price of our common stock on TheNASDAQ Capital Market was $1.39.

We are an “emerging growth company”as defined in Section 2(a) of the Securities Act of 1933, as amended, and we have elected to comply with certain reduced publiccompany reporting requirements.

Investing in our securities involves a high degree of risk. Seethe section entitled “Risk Factors” appearing on page 5 of this prospectus for a discussion of information that shouldbe considered in connection with an investment in our securities.

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

The date of this prospectus is              , 2018

TABLE OF CONTENTS

PROSPECTUS SUMMARY

This summary highlights information containedelsewhere in this prospectus. This summary does not contain all of the information that you should consider before deciding toinvest in our securities. You should read this entire prospectus carefully, including the “Risk Factors” section inthis prospectus and under similar captions in the documents incorporated by reference into this prospectus. The terms “MBI”,the “Company”, “our”, or “we” refer to Moleculin Biotech, Inc. and, unless the context otherwiserequires, its predecessors.

Overview

We are a clinical-stage pharmaceutical companyorganized as a Delaware corporation in July 2015 to focus on the development of anti-cancer drug candidates, all of which are basedon license agreements with The University of Texas System on behalf of the M.D. Anderson Cancer Center, which we refer to as MDAnderson. We have three core drug technologies: a uniquely designed anthracycline (Annamycin), a portfolio of STAT3 inhibitors(WP1066 Portfolio) and a collection of inhibitors of glycolysis (WP1122 Portfolio). Our clinical stage drugs are Annamycin, ananthracycline designed to avoid multidrug resistance mechanisms with little to no cardiotoxicity being studied for the treatmentof relapsed or refractory acute myeloid leukemia, more commonly referred to as AML, and WP1066, an immuno-stimulating STAT3 inhibitortargeting brain tumors, pancreatic cancer and AML. We are also engaged in preclinical development of additional drug candidates,including additional STAT3 inhibitors and compounds targeting the metabolism of tumors.

We currently have six drug candidates representingthree substantially different approaches to treating cancer. Liposomal Annamycin, which we refer to as Annamycin, is a chemotherapydesigned to inhibit the replication of DNA of rapidly dividing cells. WP1122 is an inhibitor of glycolysis intended to cut of thefuel supply of tumor cells, which are often overly dependent on glycolysis as compared to healthy cells. And, finally, WP1066 andWP1220, have shown capability, in in vivo testing, of altering the cell signaling associated with tumors.

We submitted in October 2017 a request for ClinicalTrial Authorization ("CTA") in Poland which, will enable a Phase I/II clinical trial to study Annamycin for the treatmentof relapsed or refractory AML in Poland. This will be in addition to the previously announced allowance of our IND in the UnitedStates. In December 2017, the Ethics Committee in Poland approved our Phase I/II clinical trial of Annamycin. On June 20, 2018,we announced approval from the Polish National Office to permit the start of clinical trials in Poland.

Recent Business Developments

Below are recent businessdevelopments.

New Data to be Presentedat Neuro-Oncology Annual Scientific Meeting

On October 10, 2018, we announced that newdata relating to our molecule WP1122 will be presented at the upcoming Society for Neuro-Oncology Annual Scientific Meeting. Webelieve that we have discovered new data during our IND-enabling research with animals that appears to confirm a beneficial metabolismof WP1122 and significant organ accumulation of the inhibitor of glycolysis in the brain and also in the pancreas. We believe thisis significant because both brain and pancreatic tumors are highly dependent upon glucose for survival and WP1122 appears to havethe ability to inhibit glycolysis, the primary process by which these tumors convert glucose into energy. We are planning to prepareWP1122 for Investigational New Drug (IND) status.

Brain Cancer Drug Candidate Begins PatientDosing at Clinical Trial Being Conducted at MD Anderson

On September 13, 2018, we announced that oursmall molecule lead drug candidate, WP1066, which is designed to block a critical target for tumors and cross the blood brain barrier,began its first brain cancer patient dosing in a physician sponsored Phase 1 clinical trial at MD Anderson Cancer Center. The compoundhas been shown in animal models to both inhibit the activated form of an important cell signaling protein, STAT3, that is linkedto cell survival, proliferation and other processes critical to tumor development, while also stimulating a beneficial immune response.The first glioblastoma patient has received the initial doses of WP1066, which were, apparently, well tolerated, in this physician-sponsoredIND (investigational new drug) study at MD Anderson Cancer Center.

Other Updates on Clinical Trials for Annamycin

We continue to recruit sites in the US and Poland. In the US, enrollmentis open and patients have been treated with Annamycin. In Poland, while the clinical trial and the first site was approved in June2018, we continue to work towards final approval by two different authorities in Europe and in Poland necessary to ship Annamycindrug product to Poland. Such approval is not necessary for use of Annamycin drug product in the US and we have Annamycin drug productready and available in the US to treat potential patients. We expect to obtain the necessary approvals, ship Annamycin drug productand treat patients in Poland by the end of the first quarter of 2019. Due to this issue, we currently estimate that our recommendedphase 2 dose will not be obtained until 2019. We can provide no assurance that we will obtain the necessary approvals to ship drugproduct or that additional sites, recruitment or treatments will occur in the near term or on a timely basis, if at all.

Our principal executive offices are locatedat 5300 Memorial, Suite 950, Houston, Texas 77007. Our telephone number is (713) 300-5160. Our website address is www.moleculin.com.The information on or accessible through our website is not part of this prospectus.

Equity Offering

On October 4, 2018, we entered into a purchaseagreement with Lincoln Park, which we refer to in this prospectus as the Purchase Agreement, pursuant to which Lincoln Park hasagreed to purchase from us up to an aggregate of $20,000,000 of our common stock (subject to certain limitations) from time totime over the term of the Purchase Agreement. Also on October 4, 2018, we entered into a registration rights agreement with LincolnPark, which we refer to in this prospectus as the Registration Rights Agreement, pursuant to which we have filed with the SEC theregistration statement that includes this prospectus to register for resale under the Securities Act of 1933, as amended, or theSecurities Act, the shares of common stock that have been or may be issued to Lincoln Park under the Purchase Agreement.

We do not have the right to commence any salesof our common stock to Lincoln Park under the Purchase Agreement until certain conditions set forth in the Purchase Agreement,all of which are outside of Lincoln Park’s control, have been satisfied, including that the SEC has declared effective theregistration statement that includes this prospectus, which we refer to in this prospectus as the Commencement. Thereafter, wemay, from time to time and at our sole discretion, on any single business day, direct Lincoln Park to purchase shares of our commonstock in amounts up to 100,000 shares, which amounts may be increased to up to 250,000 shares depending on the market price ofour common stock at the time of sale and subject to a maximum commitment by Lincoln Park of $1,000,000 per single purchase, whichwe refer to in this prospectus as “regular purchases”. In addition, at our discretion, Lincoln Park has committed topurchase up to $1,000,000 on the first Business Day following the date of the Commencement, which is referred in this prospectusas the “tranche purchase”, plus other “accelerated amounts” and/or “additional accelerated amounts”under certain circumstances. We will control the timing and amount of any sales of our common stock to Lincoln Park. The purchaseprice of the shares that may be sold to Lincoln Park in the tranche purchase and regular purchases under the Purchase Agreementwill be based on the market price of our common stock preceding the time of sale as computed under the Purchase Agreement. Thepurchase price per share will be equitably adjusted for any reorganization, recapitalization, non-cash dividend, stock split, orother similar transaction occurring during the business days used to compute such price. We may at any time in our sole discretionterminate the Purchase Agreement without fee, penalty or cost upon one business day notice. There are no restrictions on futurefinancings, rights of first refusal, participation rights, penalties or liquidated damages in the Purchase Agreement or RegistrationRights Agreement, other than a prohibition on our entering into certain types of transactions that are defined in the PurchaseAgreement as “Variable Rate Transactions”. Lincoln Park may not assign or transfer its rights and obligations underthe Purchase Agreement.

In consideration for entering into the PurchaseAgreement, we previously issued to Lincoln Park 243,013 shares of common stock as a commitment fee and shall issue up to an additional121,507 commitment shares, pro rata for no additional consideration, when and if Lincoln Park purchases (at our discretion) the$20,000,000 aggregate commitment. For example, if we elect, at our sole discretion, to require Lincoln Park to purchase $50,000of our stock then we would issue 304 additional commitment shares, which is the product of $50,000 (the amount we have electedto sell) divided by $20,000,000 (total amount we can sell Lincoln Park pursuant to the Purchase Agreement) multiplied by 121,507(the total number of additional commitment shares). The additional commitment shares will only be issued pursuant to this formulaas and when we elect at our discretion to sell stock to Lincoln Park.

As of October 4, 2018, there were 27,104,510shares of our common stock outstanding, of which 20,191,578 shares were held by non-affiliates. Although the Purchase Agreementprovides that we may sell up to $20,000,000 of our common stock to Lincoln Park, only 6,730,526 shares of our common stock arebeing offered under this prospectus, which represents shares which may be issued to Lincoln Park in the future under the PurchaseAgreement, if and when we sell shares to Lincoln Park under the Purchase Agreement. Depending on the market prices of our commonstock at the time we elect to issue and sell shares to Lincoln Park under the Purchase Agreement, we may need to register for resaleunder the Securities Act additional shares of our common stock in order to receive aggregate gross proceeds equal to the $20,000,000total commitment available to us under the Purchase Agreement. If all of the 6,730,526 shares offered by Lincoln Park under thisprospectus were issued and outstanding as of the date hereof, such shares would represent approximately 20% of the total numberof shares of our common stock outstanding and approximately 25% of the total number of outstanding shares held by non-affiliates,in each case as of the date hereof. If we elect to issue and sell more than the 6,730,526 shares offered under this prospectusto Lincoln Park, which we have the right, but not the obligation, to do, we must first register for resale under the SecuritiesAct any such additional shares, which could cause additional substantial dilution to our stockholders. The number of shares ultimatelyoffered for resale by Lincoln Park is dependent upon the number of shares we sell to Lincoln Park under the Purchase Agreement.

Under applicable rules of The NASDAQ CapitalMarket, in no event may we issue or sell to Lincoln Park under the Purchase Agreement more than 19.99% of the shares of our commonstock outstanding immediately prior to the execution of the Purchase Agreement (which is 5,369,613 shares based on 26,861,497 sharesoutstanding immediately prior to the execution of the Purchase Agreement), which limitation we refer to as the Exchange Cap, unless(i) we obtain stockholder approval to issue shares of common stock in excess of the Exchange Cap or (ii) the average price of allapplicable sales of our common stock to Lincoln Park under the Purchase Agreement equals or exceeds $1.6235, such that issuancesand sales of our common stock to Lincoln Park under the Purchase Agreement would be exempt from the Exchange Cap limitation underapplicable NASDAQ rules. In any event, the Purchase Agreement specifically provides that we may not issue or sell any shares ofour common stock under the Purchase Agreement if such issuance or sale would breach any applicable NASDAQ rules.

The Purchase Agreement also prohibits us fromdirecting Lincoln Park to purchase any shares of common stock if those shares, when aggregated with all other shares of our commonstock then beneficially owned by Lincoln Park and its affiliates, would result in Lincoln Park and its affiliates having beneficialownership, at any single point in time, of more than 9.99% of the then total outstanding shares of our common stock, as calculatedpursuant to Section 13(d) of the Securities Exchange Act of 1934, as amended, or the Exchange Act, and Rule 13d-3 thereunder, whichlimitation we refer to as the Beneficial Ownership Cap.

Issuances of our common stock in this offeringwill not affect the rights or privileges of our existing stockholders, except that the economic and voting interests of each ofour existing stockholders will be diluted as a result of any such issuance. Although the number of shares of common stock thatour existing stockholders own will not decrease, the shares owned by our existing stockholders will represent a smaller percentageof our total outstanding shares after any such issuance to Lincoln Park.

The Offering

RISK FACTORS

Investing in shares of our common stock involvesa high degree of risk. Before making an investment decision, you should carefully consider and evaluate the risks described inthe “Risk Factors” section in our most recent Annual Report on Form 10-K, as well as any updates to those risk factorsin our subsequent Quarterly Reports on Form 10-Q, together with all of the other information appearing in or incorporated by referenceinto this prospectus, before deciding whether to purchase any of the common stock being offered. The risks described in these documentsare not the only ones we face, but those that we consider to be material. There may be other unknown or unpredictable economic,business, competitive, regulatory or other factors that could have material adverse effects on our future results. Our business,financial condition or results of operations could be materially adversely affected by any of these risks. The trading price ofshares of our common stock could decline due to any of these risks, and you may lose all or part of your investment. Please alsoread carefully the section entitled “Cautionary Note Regarding Forward-Looking Statements.”

The sale or issuance of our common stockto Lincoln Park may cause dilution and the sale of the shares of common stock acquired by Lincoln Park, or the perception thatsuch sales may occur, could cause the price of our common stock to fall.

On October 4, 2018, we entered into the PurchaseAgreement with Lincoln Park, pursuant to which Lincoln Park has committed to purchase up to $20,000,000 of our common stock. Uponthe execution of the Purchase Agreement, we issued 243,013 Commitment Shares to Lincoln Park as a fee for its commitment to purchaseshares of our common stock under the Purchase Agreement. The remaining shares of our common stock that may be issued under thePurchase Agreement may be sold by us to Lincoln Park at our discretion from time to time over a 36-month period commencing afterthe satisfaction of certain conditions set forth in the Purchase Agreement, including that the SEC has declared effective the registrationstatement that includes this prospectus. The purchase price for the shares that we may sell to Lincoln Park under the PurchaseAgreement will fluctuate based on the price of our common stock. Depending on market liquidity at the time, sales of such sharesmay cause the trading price of our common stock to fall.

We generally have the right to control the timingand amount of any future sales of our shares to Lincoln Park. Additional sales of our common stock, if any, to Lincoln Park willdepend upon market conditions and other factors to be determined by us. We may ultimately decide to sell to Lincoln Park all, someor none of the additional shares of our common stock that may be available for us to sell pursuant to the Purchase Agreement. Ifand when we do sell shares to Lincoln Park, after Lincoln Park has acquired the shares, Lincoln Park may resell all, some or noneof those shares at any time or from time to time in its discretion. Therefore, sales to Lincoln Park by us could result in substantialdilution to the interests of other holders of our common stock. Additionally, the sale of a substantial number of shares of ourcommon stock to Lincoln Park, or the anticipation of such sales, could make it more difficult for us to sell equity or equity-relatedsecurities in the future at a time and at a price that we might otherwise wish to effect sales.

We may require additional financing tosustain our operations and without it we may not be able to continue operations.

We may direct Lincoln Park to purchase up to$20,000,000 worth of shares of our common stock under our agreement over a 36-month period generally in amounts up to 100,000 sharesof our common stock, which may be increased to up to 250,000 shares of our common stock depending on the market price of our commonstock at the time of sale and subject to a maximum limit of $1,000,000 per purchase, on any such business day. Assuming a purchaseprice of $1.56 per share (the closing sale price of the common stock on October 4, 2018) and the purchase by Lincoln Park of the6,730,526 purchase shares, proceeds to us would only be $10.5 million.

The extent we rely on Lincoln Park as a sourceof funding will depend on a number of factors including, the prevailing market price of our common stock and the extent to whichwe are able to secure working capital from other sources. If obtaining sufficient funding from Lincoln Park were to prove unavailableor prohibitively dilutive, we will need to secure another source of funding in order to satisfy our working capital needs. Evenif we sell all $20,000,000 under the Purchase Agreement to Lincoln Park, we may still need additional capital to fully implementour business, operating and development plans. Should the financing we require to sustain our working capital needs be unavailableor prohibitively expensive when we require it, the consequences could be a material adverse effect on our business, operating results,financial condition and prospects.

INCORPORATION BY REFERENCE

The SEC allows us to “incorporate by reference”into this prospectus the information in other documents that we file with it. This means that we can disclose important informationto you by referring you to those documents. The information incorporated by reference is considered to be a part of this prospectus,and information in documents that we file later with the SEC will automatically update and supersede information contained in documentsfiled earlier with the SEC or contained in this prospectus. We incorporate by reference in this prospectus the documents listedbelow and any future filings that we may make with the SEC under Sections 13(a), 13(c), 14, or 15(d) of the Exchange Act priorto the termination of the offering under this prospectus; provided, however, that we are not incorporating, in each case, any documentsor information deemed to have been furnished and not filed in accordance with SEC rules:

Additionally, all documents filed by us withthe SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act after (i) the date of the initial registration statement andprior to effectiveness of the registration statement, and (ii) the date of this prospectus and before the termination or completionof any offering hereunder, shall be deemed to be incorporated by reference into this prospectus from the respective dates of filingof such documents, except that we do not incorporate any document or portion of a document that is “furnished” to theSEC, but not deemed “filed.”

We will provide, without charge, to each personto whom a copy of this prospectus is delivered, including any beneficial owner, upon the written or oral request of such person,a copy of any or all of the documents incorporated by reference herein, including exhibits. Requests should be directed to: Attention:Corporate Secretary, 5300 Memorial Drive, Suite 950, Houston, Texas 77007, telephone (713) 300-5160. The documents incorporatedby reference may be accessed at our website at www.moleculin.com.

CAUTIONARY NOTE REGARDING FORWARD-LOOKINGSTATEMENTS

Some of the information in this prospectus,and the documents we incorporate by reference, contain forward-looking statements within the meaning of the federal securitieslaws. You should not rely on forward-looking statements in this prospectus, and the documents we incorporate by reference. Forward-lookingstatements typically are identified by use of terms such as “anticipate,” “believe,” “plan,” “expect,” “future,” “intend,” “may,” “will,” “should,” “estimate,” “predict,” “potential,” “continue,” and similar words, although someforward-looking statements are expressed differently. This prospectus, and the documents we incorporate by reference, may alsocontain forward-looking statements attributed to third parties relating to their estimates regarding the markets we may enter inthe future. All forward-looking statements address matters that involve risk and uncertainties, and there are many important risks,uncertainties and other factors that could cause our actual results to differ materially from the forward-looking statements containedin this prospectus, and the documents we incorporate by reference.

You should also consider carefully the statementsunder “Risk Factors” and other sections of this prospectus, and the documents we incorporate by reference, which addressadditional facts that could cause our actual results to differ from those set forth in the forward-looking statements. We cautioninvestors not to place significant reliance on the forward-looking statements contained in this prospectus, and the documents weincorporate by reference. We undertake no obligation to publicly update or review any forward-looking statements, whether as aresult of new information, future developments or otherwise.

USE OF PROCEEDS

This prospectus relates to shares of our commonstock that may be offered and sold from time to time by Lincoln Park. We will receive no proceeds from the sale of shares of commonstock by Lincoln Park in this offering. We may receive up to $20,000,000 aggregate gross proceeds under the Purchase Agreementfrom any sales we make to Lincoln Park pursuant to the Purchase Agreement after the date of this prospectus. However, we may notbe registering for sale or offering for resale under the registration statement of which this prospectus is a part all of the sharesissuable pursuant to the Purchase Agreement. As we are unable to predict the timing or amount of potential issuances of all ofthe shares offered hereby (other than the commitment shares), we have not allocated any proceeds of such issuances to any particularpurpose. Accordingly, all such proceeds are expected to be used for working capital and general corporate purposes. It is possiblethat no additional shares will be issued under the Purchase Agreement.

DIVIDEND POLICY

We have never declared or paid any cash dividendson our capital stock, and we do not currently intend to pay any cash dividends on our common stock for the foreseeable future.We expect to retain future earnings, if any, to fund the development and growth of our business. Any future determination to paydividends on our common stock will be at the discretion of our board of directors and will depend upon, among other factors, ourresults of operations, financial condition, capital requirements and any contractual restrictions.

SELLING SHAREHOLDER

This prospectus relates to the possible resaleby the selling stockholder, Lincoln Park, of shares of common stock that have been or may be issued to Lincoln Park pursuant tothe Purchase Agreement. We are filing the registration statement of which this prospectus forms a part pursuant to the provisionsof the Registration Rights Agreement, which we entered into with Lincoln Park on October 4, 2018 concurrently with our executionof the Purchase Agreement, in which we agreed to provide certain registration rights with respect to sales by Lincoln Park of theshares of our common stock that have been or may be issued to Lincoln Park under the Purchase Agreement.

Lincoln Park, as the selling stockholder, may,from time to time, offer and sell pursuant to this prospectus any or all of the shares that we have issued or may sell to LincolnPark under the Purchase Agreement. The selling stockholder may sell some, all or none of its shares. We do not know how long theselling stockholder will hold the shares before selling them, and we currently have no agreements, arrangements or understandingswith the selling stockholder regarding the sale of any of the shares.

The following table presents information regardingthe selling stockholder and the shares that it may offer and sell from time to time under this prospectus. The table is preparedbased on information supplied to us by the selling stockholder, and reflects its holdings as of October 4, 2018. Neither LincolnPark nor any of its affiliates has held a position or office, or had any other material relationship, with us or any of our predecessorsor affiliates. Beneficial ownership is determined in accordance with Section 13(d) of the Exchange Act and Rule 13d-3 thereunder.

(1)       Josh Scheinfeld andJonathan Cope, the Managing Members of Lincoln Park Capital, LLC, are deemed to be beneficial owners of all of the shares of commonstock owned by Lincoln Park Capital Fund, LLC. Messrs. Cope and Scheinfeld have shared voting and investment power over the sharesbeing offered under the prospectus filed with the SEC in connection with the transactions contemplated under the Purchase Agreement.Lincoln Park Capital, LLC is not a licensed broker dealer or an affiliate of a licensed broker dealer.

(2)       Represents (i) the243,013 shares issued to Lincoln Park as Commitment Shares under the Purchase Agreement and being registered under the registrationstatement of which this prospectus is a part and (ii) an aggregate of 830,203 shares beneficially owned by Lincoln Park that werepreviously acquired by Lincoln Park in one or more transactions not related to the transactions contemplated by the Purchase Agreement,which shares are not being registered under the registration statement of which this prospectus is a part. In accordance with Rule13d-3(d) under the Exchange Act, we have excluded from the number of shares beneficially owned prior to the offering all of theshares of common stock that Lincoln Park may be required to purchase pursuant to the Purchase Agreement because the issuance ofsuch shares is solely at our discretion and is subject to certain conditions, the satisfaction of all of which are outside of LincolnPark’s control, including the registration statement of which this prospectus is a part becoming and remaining effective.Furthermore, under the terms of the Purchase Agreement, issuances and sales of shares of our common stock to Lincoln Park are subjectto certain limitations on the amounts we may sell to Lincoln Park at any time, including the Exchange Cap and the Beneficial OwnershipCap. See the description under the heading “The Lincoln Park Transaction” for more information about the Purchase Agreement.

(3)       Based on 27,104,510outstanding shares of our common stock as of October 8, 2018.

(4)       Although the PurchaseAgreement provides that we may sell up to $20,000,000 of our common stock to Lincoln Park in addition to the 243,013 shares thathave already been issued to Lincoln Park, only 6,730,526 shares of our common stock are being offered under this prospectus thatmay be sold by us to Lincoln Park at our discretion from time to time over a 36-month period commencing after the date of the Commencement.Depending on the price per share at which we sell our common stock to Lincoln Park pursuant to the Purchase Agreement, we may needto sell to Lincoln Park under the Purchase Agreement more shares of our common stock than are offered under this prospectus inorder to receive aggregate gross proceeds equal to the $20,000,000 total commitment available to us under the Purchase Agreement.If we choose to do so, we must first register for resale under the Securities Act such additional shares. The number of sharesultimately offered for resale by Lincoln Park is dependent upon the number of shares we sell to Lincoln Park under the PurchaseAgreement.

LINCOLN PARK TRANSACTION

General

On October 4, 2018, we entered into a PurchaseAgreement and Registration Rights Agreement with Lincoln Park. Pursuant to the terms of the Purchase Agreement, Lincoln Park hasagreed to purchase from us up to $20,000,000 of our common stock (subject to certain limitations) from time to time during theterm of the Purchase Agreement. Pursuant to the terms of the Registration Rights Agreement, we have filed with the SEC the registrationstatement that includes this prospectus to register for resale under the Securities Act the shares that have been or may be issuedto Lincoln Park under the Purchase Agreement.

We do not have the right to commence any salesto Lincoln Park under the Purchase Agreement until the Commencement has occurred. Thereafter, we may, from time to time and atour sole discretion, on any single business day, direct Lincoln Park to purchase shares of our common stock in amounts up to 100,000shares, which amounts may be increased to up to 250,000 shares depending on the market price of our common stock at the time ofsale and subject to a maximum commitment by Lincoln Park of $1,000,000 per single purchase, which we refer to in this prospectusas “regular purchases”. In addition, at our discretion, Lincoln Park has committed to purchase in the tranche purchaseup to $1,000,000 on the first Business Day following the date the date of the Commencement, plus other “accelerated amounts”and/or “additional accelerated amounts” under certain circumstances. The purchase price per share sold in the tranchepurchase and regular purchases will be based on the market price of our common stock immediately preceding the time of sale ascomputed under the Purchase Agreement. Lincoln Park may not assign or transfer its rights and obligations under the Purchase Agreement.

In consideration for entering into the PurchaseAgreement, we previously issued to Lincoln Park 243,013 shares of common stock as a commitment fee and shall issue up to an additional121,507 commitment shares, pro rata for no additional consideration, when and if Lincoln Park purchases (at our discretion) the$20,000,000 aggregate commitment. For example, if we elect, at our sole discretion, to require Lincoln Park to purchase $50,000of our stock then we would issue 304 additional commitment shares, which is the product of $50,000 (the amount we have electedto sell) divided by $20,000,000 (total amount we can sell Lincoln Park pursuant to the Purchase Agreement) multiplied by 121,507(the total number of additional commitment shares). The additional commitment shares will only be issued pursuant to this formulaas and when we elect at our discretion to sell stock to Lincoln Park.

Under applicable rules of The NASDAQ CapitalMarket, in no event may we issue or sell to Lincoln Park under the Purchase Agreement share of our common stock in excess of theExchange Cap (which is 5,369,613 shares, or 19.99% of the shares of our common stock outstanding immediately prior to the executionof the Purchase Agreement), unless (i) we obtain stockholder approval to issue shares of common stock in excess of the ExchangeCap or (ii) the average price of all applicable sales of our common stock to Lincoln Park under the Purchase Agreement equals orexceeds $1.6235, such that issuances and sales of our common stock to Lincoln Park under the Purchase Agreement would be exemptfrom the Exchange Cap limitation under applicable NASDAQ rules. In any event, the Purchase Agreement specifically provides thatwe may not issue or sell any shares of our common stock under the Purchase Agreement if such issuance or sale would breach anyapplicable rules NASDAQ rules.

The Purchase Agreement also prohibits us fromdirecting Lincoln Park to purchase any shares of common stock if those shares, when aggregated with all other shares of our commonstock then beneficially owned by Lincoln Park and its affiliates, would result in Lincoln Park exceeding the Beneficial OwnershipCap.

Purchase of Shares Under the Purchase Agreement

Under the Purchase Agreement, upon the firstbusiness day following the date of the Commencement, which we refer to as the Tranche Purchase Date, in our sole discretion, wemay direct Lincoln Park to purchase in a one-time tranche purchase up to $1,000,000 worth of shares of our common stock at a purchaseprice per share equal to the lesser of (i) the lowest sale price of our common stock on the Tranche Purchase Date or (ii) the arithmeticaverage of the three lowest closing prices of our common stock during the 10 consecutive business days ending on the business dayimmediately preceding the Tranche Purchase Date.

Thereafter, we may direct Lincoln Park to purchaseup to 100,000 shares of our common stock in a regular purchase on any business day, provided, however, that (i) the regular purchasemay be increased to up to 200,000 shares, provided that the closing sale price of our common stock is not below $2.25 on the purchasedate (subject to adjustment for any reorganization, recapitalization, non-cash dividend, stock split, reverse stock split or othersimilar transaction as provided in the Purchase Agreement) and (ii) the regular purchase may be increased to up to 250,000 shares,provided that the closing sale price of our common stock is not below $2.75 on the purchase date (subject to adjustment for anyreorganization, recapitalization, non-cash dividend, stock split, reverse stock split or other similar transaction as providedin the Purchase Agreement). In each case, Lincoln Park’s maximum commitment in any single regular purchase may not exceed$1,000,000. The regular purchase share limit is subject to proportionate adjustment in the event of a reorganization, recapitalization,non-cash dividend, stock split or other similar transaction; provided, that if after giving effect to such full proportionate adjustment,the adjusted regular purchase share limit would preclude us from requiring Lincoln Park to purchase common stock at an aggregatepurchase price equal to or greater than $150,000 in any single regular purchase, then the regular purchase share limit will notbe fully adjusted, but rather the regular purchase share limit for such regular purchase shall be adjusted as specified in thePurchase Agreement, such that, after giving effect to such adjustment, the regular purchase share limit will be equal to (or asclose as can be derived from such adjustment without exceeding) $150,000.

The purchase price per share for each such regularpurchase will be equal to the lower of:

In addition to regular purchases described above,we may also direct Lincoln Park, on any business day on which we have properly submitted a regular purchase notice directing LincolnPark to purchase the maximum number of shares of our common stock that we are then permitted to include in a single regular purchasenotice and the closing sale price of our common stock on such business day is not below $1.00 per share (subject to adjustmentfor any reorganization, recapitalization, non-cash dividend, stock split, reverse stock split or other similar transaction as providedin the Purchase Agreement), to purchase an additional amount of our common stock, which we refer to as an accelerated purchase,not to exceed the lesser of:

The purchase price per share for each such acceleratedpurchase will be equal to the lower of:

We may also direct Lincoln Park, not later than1:00 p.m., Eastern time, on a business day on which an accelerated purchase has been completed and all of the shares to be purchasedthereunder (and under the corresponding regular purchase) have been properly delivered to Lincoln Park in accordance with the PurchaseAgreement prior to such time on such business day, and provided that the closing price of our common stock on the business dayimmediately preceding such business day is not less than $1.00 per share (subject to adjustment for any reorganization, recapitalization,non-cash dividend, stock split, reverse stock split or other similar transaction as provided in the Purchase Agreement), to purchasean additional amount of our common stock, which we refer to as an additional accelerated purchase, of up to the lesser of:

We may, in our sole discretion, submit multipleadditional accelerated purchase notices to Lincoln Park prior to 1:00 p.m., Eastern time, on a single accelerated purchase date,provided that all prior accelerated purchases and additional accelerated purchases (including those that have occurred earlieron the same day) have been completed and all of the shares to be purchased thereunder (and under the corresponding regular purchase)have been properly delivered to Lincoln Park in accordance with the Purchase Agreement.

The purchase price per share for each such additionalaccelerated purchase will be equal to the lower of:

In the case of the tranche purchase, regularpurchases, accelerated purchases and additional accelerated purchases, the purchase price per share will be equitably adjustedfor any reorganization, recapitalization, non-cash dividend, stock split, reverse stock split or other similar transaction occurringduring the business days used to compute the purchase price.

Other than as described above, there are notrading volume requirements or restrictions under the Purchase Agreement, and we will control the timing and amount of any salesof our common stock to Lincoln Park.

Events of Default

Events of default under the Purchase Agreementinclude the following:

Lincoln Park does not have the right to terminatethe Purchase Agreement upon any of the events of default set forth above. During an event of default, all of which are outsideof Lincoln Park’s control, we may not direct Lincoln Park to purchase any shares of our common stock under the Purchase Agreement.

Our Termination Rights

We have the unconditional right, at any time,for any reason and without any payment or liability to us, to give notice to Lincoln Park to terminate the Purchase Agreement.In the event of bankruptcy proceedings by or against us, the Purchase Agreement will automatically terminate without action ofany party.

No Short-Selling or Hedging by Lincoln Park

Lincoln Park has agreed that neither it norany of its affiliates shall engage in any direct or indirect short-selling or hedging of our common stock during any time priorto the termination of the Purchase Agreement.

Prohibitions on Variable Rate Transactions

There are no restrictions on future financings,rights of first refusal, participation rights, penalties or liquidated damages in the Purchase Agreement or Registration RightsAgreement other than a prohibition on entering into a “Variable Rate Transaction,” as defined in the Purchase Agreement.

Effect of Performance of the Purchase Agreement on Our Stockholders

All 6,730,526 shares registered in this offeringwhich have been or may be issued or sold by us to Lincoln Park under the Purchase Agreement are expected to be freely tradable.It is anticipated that shares registered in this offering will be sold over a period of up to 36-months commencing on the datethat the registration statement including this prospectus becomes effective. The sale by Lincoln Park of a significant amount ofshares registered in this offering at any given time could cause the market price of our common stock to decline and to be highlyvolatile. Sales of our common stock to Lincoln Park, if any, will depend upon market conditions and other factors to be determinedby us. We may ultimately decide to sell to Lincoln Park all, some or none of the additional shares of our common stock that maybe available for us to sell pursuant to the Purchase Agreement. If and when we do sell shares to Lincoln Park, after Lincoln Parkhas acquired the shares, Lincoln Park may resell all, some or none of those shares at any time or from time to time in its discretion.Therefore, sales to Lincoln Park by us under the Purchase Agreement may result in substantial dilution to the interests of otherholders of our common stock. In addition, if we sell a substantial number of shares to Lincoln Park under the Purchase Agreement,or if investors expect that we will do so, the actual sales of shares or the mere existence of our arrangement with Lincoln Parkmay make it more difficult for us to sell equity or equity-related securities in the future at a time and at a price that we mightotherwise wish to effect such sales. However, we have the right to control the timing and amount of any additional sales of ourshares to Lincoln Park and the Purchase Agreement may be terminated by us at any time at our discretion without any cost to us.

Pursuant to the terms of the Purchase Agreement,we have the right, but not the obligation, to direct Lincoln Park to purchase up to $20,000,000 of our common stock. Dependingon the price per share at which we sell our common stock to Lincoln Park pursuant to the Purchase Agreement, we may need to sellto Lincoln Park under the Purchase Agreement more shares of our common stock than are offered under this prospectus in order toreceive aggregate gross proceeds equal to the $20,000,000 total commitment available to us under the Purchase Agreement. If wechoose to do so, we must first register for resale under the Securities Act such additional shares of our common stock, which couldcause additional substantial dilution to our stockholders. The number of shares ultimately offered for resale by Lincoln Park underthis prospectus is dependent upon the number of shares we direct Lincoln Park to purchase under the Purchase Agreement.

The Purchase Agreement prohibits us from issuingor selling to Lincoln Park under the Purchase Agreement (i) shares of our common stock in excess of the Exchange Cap, unless weobtain stockholder approval to issue shares in excess of the Exchange Cap or the average price of all applicable sales of our commonstock to Lincoln Park under the Purchase Agreement equal or exceed $1.6235, such that the transactions contemplated by the PurchaseAgreement are exempt from the Exchange Cap limitation under applicable NASDAQ rules, and (ii) any shares of our common stock ifthose shares, when aggregated with all other shares of our common stock then beneficially owned by Lincoln Park and its affiliates,would exceed the Beneficial Ownership Cap.

The following table sets forth the amount ofgross proceeds we would receive from Lincoln Park from our sale of shares to Lincoln Park under the Purchase Agreement at varyingpurchase prices:

(1)       Althoughthe Purchase Agreement provides that we may sell up to $20,000,000 of our common stock to Lincoln Park, we are only registering6,730,526 shares under this prospectus, which may or may not cover all the shares we ultimately sell to Lincoln Park under thePurchase Agreement, depending on the purchase price per share. As a result, we have included in this column only those shares thatwe are registering in this offering. If we seek to issue shares of our common stock, including shares from other transactions thatmay be aggregated with the transactions contemplated by the Purchase Agreement under the applicable rules of The NASDAQ CapitalMarket, in excess of 5,369,913 shares, or 19.99% of the total common stock outstanding immediately prior to the execution of thePurchase Agreement, we may be required to seek stockholder approval in order to be in compliance with the rules of The NASDAQ CapitalMarket.

(2)       Thedenominator is based on 27,104,510 shares outstanding as of October 8, 2018, adjusted to include (i) 243,013 commitment sharesissued to Lincoln Park upon the execution of the Purchase Agreement and (ii) the number of shares set forth in the adjacent columnwhich we would have sold to Lincoln Park, assuming the purchase price in the adjacent column. The numerator is based on the numberof shares issuable under the Purchase Agreement at the corresponding assumed purchase price set forth in the adjacent column.

(3)       Theclosing sale price of our shares on October 4, 2018.

PLAN OF DISTRIBUTION

The common stock offered by this prospectusis being offered by the selling stockholder, Lincoln Park. The common stock may be sold or distributed from time to time by theselling stockholder directly to one or more purchasers or through brokers, dealers, or underwriters who may act solely as agentsat market prices prevailing at the time of sale, at prices related to the prevailing market prices, at negotiated prices, or atfixed prices, which may be changed. The sale of the common stock offered by this prospectus could be effected in one or more ofthe following methods:

In order to comply with the securities lawsof certain states, if applicable, the shares may be sold only through registered or licensed brokers or dealers. In addition, incertain states, the shares may not be sold unless they have been registered or qualified for sale in the state or an exemptionfrom the state’s registration or qualification requirement is available and complied with.

Lincoln Park is an “underwriter”within the meaning of Section 2(a)(11) of the Securities Act.

Lincoln Park has informed us that it intendsto use an unaffiliated broker-dealer to effectuate all sales, if any, of the common stock that it may purchase from us pursuantto the Purchase Agreement. Such sales will be made at prices and at terms then prevailing or at prices related to the then currentmarket price. Each such unaffiliated broker-dealer will be an underwriter within the meaning of Section 2(a)(11) of the SecuritiesAct. Lincoln Park has informed us that each such broker-dealer will receive commissions from Lincoln Park that will not exceedcustomary brokerage commissions.

Brokers, dealers, underwriters or agents participatingin the distribution of the shares as agents may receive compensation in the form of commissions, discounts, or concessions fromthe selling stockholder and/or purchasers of the common stock for whom the broker-dealers may act as agent. The compensation paidto a particular broker-dealer may be less than or in excess of customary commissions. Neither we nor Lincoln Park can presentlyestimate the amount of compensation that any agent will receive.

We know of no existing arrangements betweenLincoln Park or any other stockholder, broker, dealer, underwriter or agent relating to the sale or distribution of the sharesoffered by this prospectus. At the time a particular offer of shares is made, a prospectus supplement, if required, will be distributedthat will set forth the names of any agents, underwriters or dealers and any compensation from the selling stockholder, and anyother required information.

We will pay the expenses incident to the registration,offering, and sale of the shares to Lincoln Park. We have agreed to indemnify Lincoln Park and certain other persons against certainliabilities in connection with the offering of shares of common stock offered hereby, including liabilities arising under the SecuritiesAct or, if such indemnity is unavailable, to contribute amounts required to be paid in respect of such liabilities. Lincoln Parkhas agreed to indemnify us against liabilities under the Securities Act that may arise from certain written information furnishedto us by Lincoln Park specifically for use in this prospectus or, if such indemnity is unavailable, to contribute amounts requiredto be paid in respect of such liabilities.

Lincoln Park has represented to us that at notime prior to the Purchase Agreement has Lincoln Park or its agents, representatives or affiliates engaged in or effected, in anymanner whatsoever, directly or indirectly, any short sale (as such term is defined in Rule 200 of Regulation SHO of the ExchangeAct) of our common stock or any hedging transaction, which establishes a net short position with respect to our common stock. LincolnPark agreed that during the term of the Purchase Agreement, it, its agents, representatives or affiliates will not enter into oreffect, directly or indirectly, any of the foregoing transactions.

We have advised Lincoln Park that it is requiredto comply with Regulation M promulgated under the Exchange Act. With certain exceptions, Regulation M precludes the selling stockholder,any affiliated purchasers, and any broker-dealer or other person who participates in the distribution from bidding for or purchasing,or attempting to induce any person to bid for or purchase any security which is the subject of the distribution until the entiredistribution is complete. Regulation M also prohibits any bids or purchases made in order to stabilize the price of a securityin connection with the distribution of that security. All of the foregoing may affect the marketability of the securities offeredby this prospectus.

This offering will terminate on the earlierof (i) termination of the Purchase Agreement or (ii) the date that all shares offered by this prospectus have been sold by LincolnPark.

Our common stock is quoted on The NASDAQ CapitalMarket under the symbol “MBRX”.

DESCRIPTION OF CAPITAL STOCK

The following summary is a description ofthe material terms of our capital stock and is not complete. You should also refer to the Moleculin Biotech, Inc. certificate ofincorporation and bylaws, which are included as exhibits to the registration statement of which this prospectus forms a part, andthe applicable provisions of the Delaware General Corporation Law.

Our amended and restated certificate of incorporationauthorizes us to issue up to 75,000,000 shares of common stock, par value $0.001 per share, and 5,000,000 shares of preferred stock,$0.001 par value per share.

Common Stock

Shares of our common stock have the followingrights, preferences and privileges:

Voting

Each holder of common stock is entitled to onevote for each share of common stock held on all matters submitted to a vote of stockholders. Any action at a meeting at which aquorum is present will be decided by a majority of the voting power present in person or represented by proxy, except in the caseof any election of directors, which will be decided by a plurality of votes cast. There is no cumulative voting.

Dividends

Holders of our common stock are entitled toreceive dividends when, as and if declared by the our board of directors out of funds legally available for payment, subject tothe rights of holders, if any, of any class of stock having preference over the common stock. Any decision to pay dividends onour common stock will be at the discretion of our board of directors. Our board of directors may or may not determine to declaredividends in the future. See “Dividend Policy.” The board’s determination to issue dividends will depend uponour profitability and financial condition any contractual restrictions, restrictions imposed by applicable law and the SEC, andother factors that our board of directors deems relevant.

Liquidation Rights

In the event of a voluntary or involuntary liquidation,dissolution or winding up of the company, the holders of our common stock will be entitled to share ratably on the basis of thenumber of shares held in any of the assets available for distribution after we have paid in full, or provided for payment of, allof our debts and after the holders of all outstanding series of any class of stock have preference over the common stock, if any,have received their liquidation preferences in full.

Other

Our issued and outstanding shares of commonstock are fully paid and nonassessable. Holders of shares of our common stock are not entitled to preemptive rights. Shares ofour common stock are not convertible into shares of any other class of capital stock, nor are they subject to any redemptionor sinking fund provisions.

Preferred Stock

We are authorized to issue up to 5,000,000 sharesof preferred stock. Our certificate of incorporation authorizes the board to issue these shares in one or more series, to determinethe designations and the powers, preferences and relative, participating, optional or other special rights and the qualifications,limitations and restrictions thereof, including the dividend rights, conversion or exchange rights, voting rights (including thenumber of votes per share), redemption rights and terms, liquidation preferences, sinking fund provisions and the number of sharesconstituting the series. Our board of directors could, without stockholder approval, issue preferred stock with voting and otherrights that could adversely affect the voting power and other rights of the holders of common stock and which could have the effectof making it more difficult for a third party to acquire, or of discouraging a third party from attempting to acquire, a majorityof our outstanding voting stock.

Limitations on Liability and Indemnification of Officers andDirectors

Our certificate of incorporation and bylawslimit the liability of our officers and directors and provide that we will indemnify our officers and directors, in each case,to the fullest extent permitted by the Delaware General Corporation Law.

Certificate of Incorporation and Bylaw Provisions

Our certificate of incorporation and bylawsinclude a number of anti-takeover provisions that may have the effect of encouraging persons considering unsolicited tender offersor other unilateral takeover proposals to negotiate with our board of directors rather than pursue non-negotiated takeover attempts.These provisions include:

Advance Notice Requirements. Ourbylaws establish advance notice procedures with regard to stockholder proposals relating to the nomination of candidates for electionas directors or new business to be brought before meetings of stockholders. These procedures provide that notice of stockholderproposals must be timely and given in writing to our corporate Secretary. Generally, to be timely, notice must be received at ourprincipal executive offices not fewer than 120 calendar days prior to the first anniversary date on which our notice of meetingand related proxy statement were mailed to stockholders in connection with the previous year’s annual meeting of stockholders.The notice must contain the information required by the bylaws, including information regarding the proposal and the proponent.

Special Meetings of Stockholders. Ourbylaws provides that special meetings of stockholders may be called at any time by only the Chairman of the Board, the Chief ExecutiveOfficer, the President or the board of directors, or in their absence or disability, by any vice president.

No Written Consent of Stockholders. Ourcertificate of incorporation and bylaws provide that any action required or permitted to be taken by stockholders must be effectedat a duly called annual or special meeting of stockholders and may not be effected by any consent in writing by such stockholders.

Exclusive Forum Provision.Our certificate of incorporation provides that the Court of Chancery of the State of Delaware shall be the sole and exclusive forumfor (i) any derivative action or proceeding brought our behalf, (ii) any action asserting a claim of breach of a fiduciaryduty owed by any of our directors or officers to us or our stockholders, (iii) any action asserting a claim against us arisingpursuant to any provision of the Delaware General Corporation Law (the “DGCL”), or our certificate of incorporationor the bylaws, and (iv) any action asserting a claim against us governed by the internal affairs doctrine. This choice offorum provision may limit a stockholder’s ability to bring a claim in a judicial forum that it finds favorable for disputeswith us or our directors, officers or other employees, which may discourage such lawsuits against us and our directors, officersand employees. Alternatively, a court could find these provisions of our certificate of incorporation to be inapplicable or unenforceablein respect of one or more of the specified types of actions or proceedings, which may require us to incur additional costs associatedwith resolving such matters in other jurisdictions, which could adversely affect our business and financial condition.

Amendment of Bylaws. Ourstockholders may amend any provisions of our bylaws by obtaining the affirmative vote of the holders of a majority of each classof issued and outstanding shares of our voting securities, at a meeting called for the purpose of amending and/or restating ourbylaws.

Preferred Stock.Our certificate of incorporation authorizes our board of directors to create and issue rights entitling our stockholders to purchaseshares of our stock or other securities. The ability of our board to establish the rights and issue substantial amounts of preferredstock without the need for stockholder approval may delay or deter a change in control of us. See “Preferred Stock”above.

Delaware Takeover Statute

We are subject to Section 203 of the DGCLwhich, subject to certain exceptions, prohibits a Delaware corporation from engaging in any “business combination”(as defined below) with any interested stockholder for a period of three years following the date that such stockholder becamean interested stockholder, unless: (1) prior to such date, the board of directors of the corporation approved either the businesscombination or the transaction that resulted in the stockholder becoming an interested stockholder; (2) on consummation ofthe transaction that resulted in the stockholder becoming an interested stockholder, the interested stockholder owned at least85% of the voting stock of the corporation outstanding at the time the transaction commenced, excluding for purposes of determiningthe voting stock outstanding those shares owned (x) by persons who are directors and also officers and (y) by employeestock plans in which employee participants do not have the right to determine confidentially whether shares held subject to thisplan will be tendered in a tender or exchange offer; or (3) on or subsequent to such date, the business combination is approvedby the board of directors and authorized at an annual or special meeting of stockholders, and not by written consent, by the affirmativevote of at least 66 ²⁄₃% of the outstanding voting stock that is not owned by the interested stockholder.

Section 203 of the DGCL defines generally “businesscombination” to include: (1) any merger or consolidation involving the corporation and the interested stockholder; (2) anysale, transfer, pledge or other disposition of 10% or more of the assets of the corporation involving the interested stockholder;(3) subject to certain exceptions, any transaction that results in the issuance or transfer by the corporation of any stockof the corporation to the interested stockholder; (4) any transaction involving the corporation that has the effect of increasingthe proportionate share of the stock of any class or series of the corporation beneficially owned by the interested stockholder;or (5) the receipt by the interested stockholder of the benefit of any loans, advances, guarantees, pledges or other financialbenefits provided by or through the corporation. In general, Section 203 defines an “interested stockholder” asany entity or person beneficially owning 15% or more of the outstanding voting stock of the corporation and any entity or personaffiliated with or controlling or controlled by such entity or person.

Listing

Our common stock is listed on the Nasdaq CapitalMarket under the symbol “MBRX”.

Transfer Agent

The transfer agent for our common stock is VStockTransfer, LLC.

LEGAL MATTERS

The validity of the securities offered herebywill be passed upon for us by Schiff Hardin LLP, Washington, DC.

EXPERTS

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

WHERE YOU CAN FIND MORE INFORMATION

We are subject to the reporting and informationrequirements of the Exchange Act and, as a result, will file periodic and current reports, proxy statements and other informationwith the SEC. We make our periodic reports and other information filed with or furnished to the SEC, available, free of charge,through our website as soon as reasonably practicable after those reports and other information are filed with or furnished tothe SEC. Additionally, these periodic reports, proxy statements and other information will be available for inspection and copyingat the public reference room and SEC’s website at www.sec.gov. You may read and copy any document that we file atthe SEC’s public reference room located at 100 F Street, NE, Washington, D.C. 20549. Please call theSEC at 1-800-SEC-0330 for further information on the public reference rooms. SEC filings are also available to the public at theSEC’s website referred to above.

Moleculin Biotech, Inc.

Common Stock

PROSPECTUS

__________________, 2018

We have not authorized any dealer, salesperson or other person togive any information or to make any representations not contained in this prospectus. You must not rely on any unauthorized information.This prospectus is not an offer to sell these securities in any jurisdiction where an offer or sale is not permitted.

PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 13. Other Expenses of Issuance and Distribution.

The following table sets forth the estimatedcosts 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”).All amounts are estimates except the Securities and Exchange Commission registration fee and the Financial Industry RegulatoryAuthority, Inc. filing fee.

The following expenses will be borne solelyby the Registrant:

Item 14. Indemnification of Directors and Officers.

Pursuant to Section 145 of the DelawareGeneral Corporation Law (the “DGCL”), a corporation shall have the power to indemnify any person who was or is a partyor is threatened to be made a party to any threatened, pending or completed action, suit or proceeding, whether civil, criminal,administrative or investigative (other than a derivative action by or in the right of such corporation) by reason of the fact thatsuch person is or was a director, officer, employee or agent of such corporation, or serving at the request of such corporationin such capacity for another corporation, partnership, joint venture, trust or other enterprise, against expenses (including attorneys’fees), judgments, fines and amounts paid in settlement actually and reasonably incurred in connection with such action, suit orproceeding, if such person acted in good faith and in a manner he or she reasonably believed to be in or not opposed to the bestinterests of such corporation, and, with respect to any criminal action or proceeding, had no reasonable cause to believe his orher conduct was unlawful.

The DGCL also permits indemnification by a corporationunder similar circumstances for expenses (including attorneys’ fees) actually and reasonably incurred by such persons inconnection with the defense or settlement of a derivative action or suit, except that no indemnification shall be made in respectof any claim, issue or matter as to which such person shall have been adjudged to be liable to such corporation unless the DelawareCourt of Chancery or the court in which such action or suit was brought shall determine upon application that such person is fairlyand reasonably entitled to indemnity for such expenses which such court shall deem proper.

To the extent a present or former director orofficer is successful in the defense of such an action, suit or proceeding referenced above, or in defense of any claim, issueor matter therein, a corporation is required by the DGCL to indemnify such person for actual and reasonable expenses incurred inconnection therewith. Expenses (including attorneys’ fees) incurred by such persons in defending any action, suit or proceedingmay be paid in advance of the final disposition of such action, suit or proceeding upon in the case of a current officer or director,receipt of an undertaking by or on behalf of such person to repay such amount if it is ultimately determined that such person isnot entitled to be so indemnified.

The DGCL provides that the indemnification describedabove shall not be deemed exclusive of other indemnification that may be granted by a corporation pursuant to its bylaws, disinteresteddirectors’ vote, stockholders’ vote and agreement or otherwise.

Section 102(b)(7) of the DGCL enables acorporation, in its certificate of incorporation or an amendment thereto, to eliminate or limit the personal liability of a directorto the corporation or its stockholders for monetary damages for violations of the directors’ fiduciary duty, except (i) forany breach of the director’s duty of loyalty to the corporation or its stockholders, (ii) for acts or omissions notin good faith or which involve intentional misconduct or a knowing violation of law, (iii) pursuant to Section 174 ofthe DGCL (providing for liability of directors for unlawful payment of dividends or unlawful stock purchases or redemptions) or(iv) for any transaction from which a director derived an improper personal benefit. The Registrant’s certificate ofincorporation provides for such limitations on liability for its directors.

The DGCL also provides corporations with thepower to purchase and maintain insurance on behalf of any person who is or was a director, officer, employee or agent of such corporation,or is or was serving at the request of such corporation in a similar capacity for another corporation, partnership, joint venture,trust or other enterprise, against any liability asserted against him or her in any such capacity or arising out of his or herstatus as such, whether or not the corporation would have the power to indemnify him or her against such liability as describedabove. In connection with this offering, the Registrant will obtain liability insurance for its directors and officers. Such insurancewould be available to its directors and officers in accordance with its terms.

The Registrant’s certificate of incorporationrequires the Registrant to indemnify and hold harmless, to the fullest extent permitted by applicable law as it presently existsor may hereafter be amended, any person (a “covered person”) who was or is made or is threatened to be made a partyor is otherwise involved in any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrativeor investigative (a “proceeding”) by reason of the fact that he or she is or was a director, officer or member of acommittee of the Registrant, or, while a director or officer of the Registrant, is or was serving at the request of the Registrantas a director or officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise or non-profitentity, including service with respect to employee benefit plans, against all liability and loss suffered and expenses (includingattorneys’ fees), judgment, fines and amounts paid in settlement actually and reasonably incurred by such person in connectionwith a proceeding.

In addition, under the Registrant’s certificateof incorporation, in certain circumstances, the Registrant shall pay the expenses (including attorneys’ fees) incurred bya covered person in defending a proceeding in advance of the final disposition of such proceeding; provided, however, that theRegistrant shall not be required to advance any expenses to a person against whom the Registrant directly brings an action, suitor proceeding alleging that such person (1) committed an act or omission not in good faith or (2) committed an act ofintentional misconduct or a knowing violation of law. Additionally, an advancement of expenses incurred by a covered person shallbe made only upon delivery to the Registrant of an undertaking, by or on behalf of such covered person, to repay all amounts soadvanced if it shall ultimately be determined by final judicial decision from which there is no further right to appeal or otherwisein accordance with Delaware law that such covered person is not entitled to be indemnified for such expenses.

In addition, the Registrant has entered intoindemnification agreements with its directors and executive officers that provide for additional indemnification protections, whichform of agreement has been filed as an exhibit to this registration statement.

Item 15. Recent Sales of Unregistered Securities.

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

In August 2015, Messrs. Klemp, Picker and Priebepurchased 1,100,000 shares, 500,000 shares and 3,000,000 shares of our common stock, respectively, at a purchase price of $0.001per share.

In August 2015, in exchange for the issuanceof 630,000 shares of common stock, we acquired the rights to the license agreement with MD Anderson covering our WP1122 Portfolioheld by IntertechBio Corporation, a company affiliated with Messrs. Priebe and Picker. In August 2015, in exchange for the issuanceof 1,431,000 shares of common stock, we acquired the rights to the Annamycin IND and all data related to the Annamycin IND or thedevelopment of Annamycin held by AnnaMed, Inc., a company affiliated with Mr. Klemp.

In May 2016, Moleculin, LLC, a Texas limitedliability company, was merged with and into MBI, which survived the merger. As a result of the merger, we issued the equity interestsholders of Moleculin, LLC (including the convertible noteholders of Moleculin, LLC) an aggregate of 999,931 shares of our commonstock. Messrs. Klemp, Picker and Priebe are members of Moleculin, LLC and received shares of our common stock as a result of themerger.

In August and September 2015, the Registrantissued two 8% convertible notes in an aggregate of $250,000 in principal amount of convertible notes to one investor, which principaland accrued interest automatically converted into shares of common stock upon the closing of the Company’s initial publicoffering at a conversion rate of $0.1299 per share. In October 2015, the Registrant issued two 8% convertible notes in an aggregateof $200,000 in principal amount of convertible notes to two investors, which principal and accrued interest automatically convertedinto shares of common stock upon the closing of the Company’s initial public offering at a conversion rate of $0.20 per share.At the time of such purchase in October 2015, the convertible note investors and the Registrant agreed that the investors wouldfund the Registrant up to the filing of its initial non-confidential registration statement an additional $165,000 on the sameterms as in the October financing. Pursuant to such agreement, such amounts were received in January 2016.

Between January and May 2016, the Registrantsold 225,963 shares of common stock for a total purchase price of $677,889 to 19 investors.

On June 20, 2016, the Registrant agreed to issue24,000 shares of common stock to an investor relations firm for services provided.

In January 2017, the Registrant agreed to issue79,167 shares of common stock to a third-party in settlement of $237,500 in past due amounts.

In July 2017, the Registrant agreed to issuetwo warrants to purchase 100,000 and 50,000 shares of common stock at exercise prices of $2.41 and $3.00 per share, respectively,to a consultant, subject to approval by Nasdaq of a listing of additional shares application, which was received in August 2017.

In February, 2018, the Registrant entered intoa Securities Purchase Agreement (the “Purchase Agreement”) with certain institutional investors (the “Investors”)for the sale of shares of common stock and, concurrently with such sale, the Registrant also sold warrants to purchase 2,145,000shares of common stock. Subject to certain beneficial ownership limitations, the warrants will be initially exercisable on thesix-month anniversary of the issuance date at an exercise price equal to $2.80 per share of common stock, subject to adjustmentsas provided under the terms of the warrants. The warrants are exercisable for five years from the initial exercise date. The warrantsand the shares issuable upon exercise of the warrants were sold without registration under the Securities Act in reliance on theexemptions provided by Section 4(a)(2) of the Securities Act as transactions not involving a public offering as sales to accreditedinvestors, and in reliance on similar exemptions under applicable state laws. The Registrant also entered into a placement agentagreement (the “Placement Agency Agreement”) with Roth Capital Partners, LLC (“Roth”), pursuant to whichRoth agreed to serve as exclusive placement agent for the issuance and sale of the shares and warrants. Pursuant to the PlacementAgency Agreement, the Registrant agreed to grant to Roth or its designees warrants to purchase up to 3% of the aggregate numberof shares of common stock sold in the transactions (the “Roth Warrants”). The Roth Warrants have substantially thesame terms as the warrants described above, except that the Roth Warrants will expire on February 15, 2023. The Roth Warrants andthe shares issuable upon exercise of the Roth Warrants will be issued in reliance on the exemption from registration provided bySection 4(a)(2) of the Securities Act as transactions not involving a public offering and in reliance on similar exemptions underapplicable state laws.

In May 2018, the Registrant agreed to issuea warrant to purchase 100,000 shares of common stock at an exercise price of $3.00 per share to a consultant, subject to NASDAQlisting of additional shares approval. The consultant is an accredited investor.

In June 2018, the Registrant entered into aSecurities Purchase Agreement (the “Purchase Agreement”) with certain institutional investors (the “Investors”)for the sale of shares of common stock and, concurrently with such sale, the Registrant also sold warrants to purchase 710,212shares of common stock. Subject to certain beneficial ownership limitations, the warrants will be initially exercisable on thesix-month anniversary of the issuance date at an exercise price equal to $2.02 per share of common stock, subject to adjustmentsas provided under the terms of the warrants. The warrants are exercisable for five years from the initial exercise date. The warrantsand the shares issuable upon exercise of the warrants were sold without registration under the Securities Act in reliance on theexemptions provided by Section 4(a)(2) of the Securities Act as transactions not involving a public offering as sales to accreditedinvestors, and in reliance on similar exemptions under applicable state laws. The Registrant also entered into a placement agentagreement (the “Placement Agency Agreement”) with Roth Capital Partners, LLC (“Roth”), pursuant to whichRoth agreed to serve as exclusive placement agent for the issuance and sale of the shares and warrants. Pursuant to the PlacementAgency Agreement, the Registrant agreed to grant to Roth or its designees warrants to purchase up to 3% of the aggregate numberof shares of common stock sold in the transactions (the “Roth Warrants”). The Roth Warrants have substantially thesame terms as the warrants described above, except that the Roth Warrants will expire on June 22, 2023. The Roth Warrants and theshares issuable upon exercise of the Roth Warrants will be issued in reliance on the exemption from registration provided by Section4(a)(2) of the Securities Act as transactions not involving a public offering and in reliance on similar exemptions under applicablestate laws.

In October 2018, the Registrant entered intoa Purchase Agreement with Lincoln Park Capital Fund, LLC, or Lincoln Park, which provides that, upon the terms and subject to theconditions and limitations set forth in the agreement, Lincoln Park is committed to purchase up to an aggregate of $20.0 millionshares of the Registrant’s common stock over the 36-month term of the agreement. The Registrant issued 243,013 shares ofits common stock to Lincoln Park in consideration for entering into the agreement.

None of these transactions involved a publicoffering. We believe that each of the above issuances was exempt from registration under the Securities Act in reliance on Section4(a)(2) of the Securities Act regarding transactions not involving a public offering.

Item 16. Exhibits and Financial Statement Schedules.

(a) Exhibits: Reference is madeto the Exhibit Index following the signature pages hereto, which Exhibit Index is hereby incorporated into this Item.

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

Item 17. Undertakings

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

(i)         Toinclude any prospectus required by Section 10(a)(3) of the Securities Act of 1933;

(ii)        Toreflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recentpost-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information setforth in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (ifthe total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or highend of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule424(b) if, in the aggregate, the changes in volume and price represent no more than a 20% change in the maximum aggregate offeringprice set forth in the “Calculation of Registration Fee” table in the effective registration statement.

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

Provided, however, that Paragraphs (a)(1)(i),(a)(1)(ii) and (a)(1)(iii) of this section do not apply if the registration statement is on Form S-1 and the information requiredto be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commissionby the Registrant pursuant to Section 13 or Section 15(d) of the Securities Exchange Act of 1934 that are incorporated by referencein the registration statement.

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

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

(4)        That, for the purpose of determiningliability under the Securities Act to any purchaser:

(i)         Each prospectus filed pursuant to Rule424(b) as part of a registration statement relating to an offering, other than registration statements relying on Rule 430B orother than prospectuses filed in reliance on Rule 430A, shall be deemed to be part of and included in the registration statementas of the date it is first used after effectiveness. Provided, however, that no statement made in a registration statement or prospectusthat is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registrationstatement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale priorto such first use, supersede or modify any statement that was made in the registration statement or prospectus that was part ofthe registration statement or made in any such document immediately prior to such date of first use.

(b) The undersigned registrant hereby undertakesthat, for purposes of determining any liability under the Securities Act of 1933, each filing of the registrant’s annualreport pursuant to section 13(a) or section 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing ofan employee benefit plan’s annual report pursuant to section 15(d) of the Securities Exchange Act of 1934) that is incorporatedby reference in the registration statement shall be deemed to be a new registration statement relating to the securities offeredtherein, 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 liabilitiesarising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of the registrant pursuantto the provisions referenced in Item 14 of this Registration Statement, or otherwise, the Registrant has been advised thatin the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Actand is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the paymentby the registrant of expenses incurred or paid by a director, officer, or controlling person of the registrant in the successfuldefense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securitiesbeing registered hereunder, the registrant will, unless in the opinion of its counsel the matter has been settled by controllingprecedent, submit to a court of appropriate jurisdiction the question of whether such indemnification by it is against public policyas expressed in the Act and will be governed by the final adjudication of such issue.

SIGNATURES

Pursuant to the requirements of the SecuritiesAct, 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 October 15, 2018.

POWER OF ATTORNEY

KNOW ALL MEN BY THESE PRESENTS, that the undersignedofficers and directors of Moleculin Biotech, Inc., a Delaware corporation, that is filing a registration statement on Form S-1with the Securities and Exchange Commission under the provisions of the Securities Act of 1933, as amended, hereby constitute andappoint Walter V. Klemp and Jonathan P. Foster their true and lawful attorneys-in-fact and agents, with full power of substitutionand re-substitution, for him and in his name, place and stead, in any and all capacities, to sign any or all amendments (includingpost-effective amendments, exhibits thereto and other documents in connection therewith) to the registration statement , any subsequentregistration statement filed by the registrant pursuant to Rule 462(b) of the Securities Act of 1933, as amended, which relatesto this registration statement, and to file the same, with all exhibits thereto, and other documents in connection therewith, withthe Securities and Exchange Commission, granting unto said attorneys-in-fact and agents full power and authority to do and performeach and every act and thing requisite and necessary to be done in and about the premises, as fully to all interests and purposesas they might or could do in person, hereby ratifying and confirming all that said attorneys-in-fact and agents or either of them,or their substitute or substitutes, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the SecuritiesAct of 1933, as amended, this registration statement has been signed below by the following persons in the capacities and on thedates indicated:

EXHIBIT INDEX