Material Event (Form 8-K)

On September 16, 2025, Rein Therapeutics, Inc. (the "Company") received additional communication through preliminary meeting comments from the United States Food and Drug Administration ("FDA") regarding the Clinical Hold Letter received on July 8, 2025 relating to its U.S. Phase 2 RENEW clinical trial of LTI-03.The FDA stated that it had evaluated further communications from the Company, including existing GLP toxicology data, supplemental pathology peer review, a new assessment of the localization of the mucous cell hyperplasia/hypertrophy ("MCH/H") within the bronchioles, and other scientific considerations. The FDA communicated that after reviewing the totality of the nonclinical information, it agreed that the MCH/H findings in the GLP rat studies are not dose-limiting for the proposed 24-weekclinical protocol evaluating LTI-03at doses up to 10 mg/day in a limited number of patients with idiopathic pulmonary fibrosis, or IPF.

The FDA recommended that the Company prepare a Complete Response Letter, or CRL, submission addressing the safety concerns in the Clinical Hold Letter, including, among other things, amended study reports and other summaries of the supplemental nonclinical justification provided by the Company to the FDA subsequent to the Clinical Hold Letter. Importantly, the FDA confirmed that the CRL need not include data from a new GLP toxicity study in rats. It was further recommended that the CRL include an updated summary of available clinical safety, pharmacokinetics, and pharmacodynamics data in IPF patients as well as an updated protocol. The Company was advised that an updated protocol should include augmented safety monitoring that includes spirometry assessments similar to Study LTI-03-1002(the Company's Phase 1b study), treatment discontinuation criteria, and Data and Safety Monitoring Board unblinded reviews that incorporate spirometry data.

The Company was also encouraged to submit publications and study reports employing New Approach Methodologies, including human precision-cutlung slices and organoids derived from IPF patients.

The Company is preparing a CRL in accordance with the FDA's recommendations and hopes to submit the CRL in the next few weeks. The Company is hopeful that its CRL submission will lead to the release of the clinical hold in a timely manner.