Rein Therapeutics, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934

(Amendment No. )

Filed by the Registrant☒

Filed by a Party other than the Registrant☐

Check the appropriate box:

(Name of Registrant as Specified in Its Charter)

(Name of Person(s) Filing Proxy Statement, if Other Than the Registrant)

Payment of Filing Fee (Check the appropriate box):

(737) 802-1989

June 8, 2026

Dear Fellow Stockholders:

You are cordially invited to attend the 2026 annual meeting of stockholders (the “Annual Meeting”) of Rein Therapeutics, Inc., to be held on Monday, July 20, 2026 at 9:00 a.m., Eastern Time.

The purposes of the Annual Meeting are set forth in the accompanying Notice of 2026 Annual Meeting of Stockholders and Proxy Statement.

The Annual Meeting will be a virtual meeting of stockholders, conducted solely online via live webcast. There is no physical location for the Annual Meeting. You will be able to attend and participate in the Annual Meeting online, vote your shares electronically and submit your questions prior to and during the meeting by visiting: www.meetnow.global/MM4MRP6. Further information about how to attend the Annual Meeting online, vote your shares during the meeting and submit questions during the meeting is included in the accompanying Proxy Statement.

The enclosed Notice of 2026 Annual Meeting of Stockholders and Proxy Statement describe the formal business to be transacted at the Annual Meeting. Also enclosed for your review is our Annual Report to stockholders, which contains detailed information concerning our activities and operating performance.

Stockholders of record at the close of business on May 21, 2026, will be entitled to notice of and to vote at the Annual Meeting or any adjournment or postponement thereof.

Your vote is important to us. Whether or not you plan to attend the Annual Meeting, and regardless of the number of shares of Rein Therapeutics that you own, it is important that your shares be represented and voted. Please vote electronically over the Internet, or if you receive a paper copy of the proxy card by mail, by returning your signed proxy card in the envelope provided. We encourage you to vote by proxy so that your shares will be represented and voted at the meeting, whether or not you can attend. If you decide to attend the Annual Meeting, you will still be able to vote online during the Annual Meeting, even if you previously submitted your proxy.

On behalf of the Board of Directors of Rein Therapeutics, Inc., we thank you for your ongoing support and continued interest.

Sincerely,

/s/ Josef H. von Rickenbach

Josef H. von Rickenbach

Chairman of the Board

(737) 802-1989

NOTICE OF 2026 ANNUAL MEETING OF STOCKHOLDERS

Your vote is very important. Please submit your proxy or voting instructions as soon as possible to ensure that your shares will be represented at the Annual Meeting, whether or not you expect to attend the Annual Meeting.

By Order of the Board of Directors,

/s/ Brian Windsor, Ph.D.

Brian Windsor, Ph.D.

President and Chief Executive Officer

Austin, Texas

June 8, 2026

Important Notice Regarding Internet Availability of Proxy Materials: The attached Proxy Statement and our 2025 Annual Report to stockholders, which includes our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, are available atwww.envisionreports.com/RNTX. These documents are also available to any stockholder who wishes to receive a paper copy by calling 1-866-641-4276, or by emailing investorvote@computershare.com with “Proxy Materials Rein Therapeutics” in the subject line, or by submitting a request over the Internet atwww.envisionreports.com/RNTX.

TABLE OF CONTENTS

Rein Therapeutics, Inc. | 2026 Proxy Statementi

PROXY STATEMENT FOR THE

2026 ANNUAL MEETING OF STOCKHOLDERS

TO BE HELD ON JULY 20, 2026

This proxy statement (this “Proxy Statement”) and the proxy card (the “Proxy Card”) are being furnished in connection with the solicitation of proxies by the board of directors (the “Board”) of Rein Therapeutics, Inc. for use at the annual meeting of stockholders to be held on Monday, July 20, 2026, at 9:00 a.m., Eastern Time (the “Annual Meeting”), and any adjournment thereof. The Annual Meeting will be held in a virtual meeting format only via the Internet. At the Annual Meeting, holders of shares of common stock of Rein Therapeutics, Inc. at the close of business on May 21, 2026 (the “Record Date”) will be able to attend, vote and submit questions by visitingwww.meetnow.global/MM4MRP6. Stockholders do not need to register to attend the Annual Meeting. Stockholders who hold their shares through an intermediary, such as a bank, broker or other nominee, who wish to attend the Annual Meeting must submit proof of their legal proxy reflecting their holdings and their name and email address to our transfer agent no later than 5:00 p.m., Eastern Time on July 15, 2026. Stockholders will receive confirmation of their registration by email after our transfer agent receives their registration materials. Further information about how to attend the Annual Meeting online, vote your shares of common stock during the Annual Meeting and submit questions during the Annual Meeting is included in this Proxy Statement.

Except where the context otherwise requires, references to “Rein Therapeutics,” “the Company,” “we,” “us,” “our,” and similar terms refer to Rein Therapeutics, Inc., a Delaware corporation.

The Board is using this Proxy Statement to solicit proxies for use at the Annual Meeting. This Proxy Statement summarizes information about the proposals to be considered at the Annual Meeting and other information you may find useful in determining how to vote. All properly submitted proxies will be voted in accordance with the instructions contained in those proxies.

Instead of mailing a paper copy of this Proxy Statement and our Annual Report to stockholders for the fiscal year ended December 31, 2025 (the “Annual Report” and, together with this Proxy Statement, the “Proxy Materials”), we are providing access to our Proxy Materials over the Internet under the “notice and access” rules of the Securities and Exchange Commission (the “SEC”). As a result, we are mailing stockholders a Notice of Internet Availability of Proxy Materials (the “Notice”) instead of paper copies of the Proxy Materials. We are mailing the Notice on or about June 8, 2026, and it contains instructions on how to access the Proxy Materials over the Internet. The Notice also contains instructions on how stockholders can receive a paper copy of the Proxy Materials.

Important Notice Regarding the Availability of Proxy Materials

for the Annual Meeting of Stockholders to be Held on July 20, 2026:

This Proxy Statement and the Annual Report are available for viewing, printing, and downloading atwww.envisionreports.com/RNTX.

A copy of the Annual Report, except for exhibits, will be furnished without charge to any stockholder upon written or oral request to Rein Therapeutics, Inc., 12407 N. Mopac Expy. Suite 250 #390, Austin, Texas 78758, or by calling 1-866-641-4276, by emailing investorvote@computershare.com with “Proxy Materials Rein Therapeutics” in the subject line, or by submitting a request over the Internet atwww.envisionreports.com/RNTX. This Proxy Statement and the Annual Report are also available on the SEC’s website at www.sec.gov.

Rein Therapeutics, Inc. | 2026 Proxy Statement 1