OS Therapies Provides OST-HER2 UK MHRA and US FDA Osteosarcoma Regulatory Update

• Company updates sequence of OST-HER2 regulatory submissions, prioritizing UK MHRA conditional Marketing Authorisation Application (MAA) submission following positive August 2025 Scientific Advice Meeting feedback from UK MHRA
• In principle, UK MHRA accepts use of historical control arm to support conditional MAA
• UK MHRA conditional MAA and US FDA Biologics Licensing Application (BLA) submissions expected to be completed within 30 days of each other, between December 2025 and January 2026
• Company to propose use of immune activation biomarker coupled with overall survival data as surrogate efficacy endpoint to support Accelerated Approval in Type C Meeting invited by FDA, following positive End of Phase 2 Meeting and follow-up interactions

New York, New York--(Newsfile Corp. - September 30, 2025) - OS Therapies Inc. (NYSE:OSTX) ("OS Therapies" or "the Company"), a clinical-stage cancer immunotherapy and antibody drug conjugate biotechnology company, today provided a regulatory update on its plans for filing marketing authorizations in the United States (US) and United Kingdom (UK) for OST-HER2 in the prevention or delay of recurrent, fully resected, pulmonary metastatic osteosarcoma based on a data generated from its completed Phase 2b clinical trial currently in the long term follow-up phase. The Company has updated its regulatory filing sequence to prioritize the UK Medicines and Healthcare products Regulatory Agency (MHRA) conditional Marketing Authorisation Application (MAA) final module submission to immediately precede its US FDA Biologics Licensing Application (BLA) final module submission under the Accelerated Approval Program (Accelerated Approval). The Company expects to complete the MHRA conditional MAA submission in December 2025, with the FDA BLA Accelerated Approval submission expected in January 2025.

The Company has submitted a conditional MAA Pre-Submission Request following MHRA feedback arising from its August 2025 Scientific Advice Meeting (SAM). The Company expects to receive formal acceptance of a rolling review request to MHRA in the near future. MHRA has aligned around the use of peer-reviewed historical control data as a suitable comparator arm to support the Company's conditional MAA request. MHRA further agreed to support OS Therapies efforts to obtain case-matched external control arm data by providing access to the UK's Clinical Practice Research Datalink (CPRD).

The information in the MHRA conditional MAA submission is closely aligned with the information in the planned US FDA BLA Accelerated Approval request, apart from alignment around an acceptable comparator arm against which to compare the efficacy data from the Company's Phase 2b trial of OST-HER2. After the End of Phase 2 and follow-up conversations with FDA, general alignment was reached on all matters, except for suitable comparators. At the direction of FDA, the Company has requested an additional Type C meeting to review updated overall survival and new biomarker data. The Company intends to confirm whether immune activation biomarker data as the surrogate marker correlated with 2-year interim overall survival as a secondary endpoint would be a suitable surrogate marker of efficacy to support Accelerated Approval.

"We've gained a deeper understanding of what FDA is now looking for to support an efficacy claim as a result of our interactions over the last 45 days," said Paul Romness, MHP, Chairman & CEO of OS Therapies. "The FDA agreed with the Company that OST-HER2 had a strong safety profile and the canine overall survival data correlating with immune activation biomarker data recently published in Journal of Molecular Therapy, attributing clinical benefit to OST-HER2 treatment. We intend to present a similar data package generated from our Phase 2b human clinical trial of OST-HER2 and believe the FDA will support this approach to Accelerated Approval."

About OS Therapies

OS Therapies is a clinical stage oncology company focused on the identification, development, and commercialization of treatments for Osteosarcoma (OS) and other solid tumors. OST-HER2, the Company's lead asset, is an immunotherapy leveraging the immune-stimulatory effects of Listeria bacteria to initiate a strong immune response targeting the HER2 protein. OST-HER2 has received Rare Pediatric Disease Designation (RPDD) from the U.S. Food & Drug Administration and Fast-Track and Orphan Drug designations from the U.S. FDA and European Medicines Agency. The Company reported positive data in its Phase 2b clinical trial of OST-HER2 in recurrent, fully resected, lung metastatic osteosarcoma, demonstrating statistically significant benefit in the 12-month event free survival (EFS) primary endpoint of the study. The Company anticipates submitting a Biologics Licensing Application (BLA) to the U.S. FDA for OST-HER2 in osteosarcoma in 2025 and, if approved, would become eligible to receive a Priority Review Voucher that it could then sell. OST-HER2 has completed a Phase 1 clinical study primarily in breast cancer patients, in addition to showing preclinical efficacy data in various models of breast cancer. OST-HER2 has been conditionally approved by the U.S. Department of Agriculture for the treatment of canines with osteosarcoma.

In addition, OS Therapies is advancing its next-generation Antibody Drug Conjugate (ADC) and Drug Conjugates (DC), known as tunable ADC (tADC), which features tunable, tailored antibody-linker-payload candidates. This platform leverages the Company's proprietary silicone Si-Linker and Conditionally Active Payload (CAP) technology, enabling the delivery of multiple payloads per linker. For more information, please visit www.ostherapies.com.

Forward-Looking Statements

Statements in this press release about future expectations, plans and prospects, as well as any other statements regarding matters that are not historical facts, may constitute forward-looking statements within the meaning of the federal securities laws. These forward-looking statements and terms such as "anticipate," "expect," "intend," "may," "will," "should" or other comparable terms involve risks and uncertainties because they relate to events and depend on circumstances that will occur in the future. Those statements include statements regarding the intent, belief or current expectations of OS Therapies and members of its management, as well as the assumptions on which such statements are based. OS Therapies cautions readers that forward-looking statements are based on management's expectations and assumptions as of the date of this press release and are subject to certain risks and uncertainties that could cause actual results to differ materially, including, but not limited to the approval of OST-HER2 by the U.S. FDA and other risks and uncertainties described in "Risk Factors" in the Company's most recent Annual Report on Form 10-K, most recent Quarterly Report on Form 10-Q and other subsequent documents the Company files with the Securities and Exchange Commission. Any forward-looking statements contained in this press release speak only as of the date hereof, and, except as required by the federal securities laws, OS Therapies specifically disclaims any obligation to update any forward-looking statement, whether as a result of new information, future events or otherwise.

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