OS Therapies Granted Meetings with U.S. FDA, U.K. MHRA, EMA and Australian Therapeutic Goods Administration to Review Global Confirmatory Phase 3 Trial for OST-HER2 in Metastatic Osteosarcoma

• Meetings with all four regulatory agencies scheduled to occur in the second quarter of 2026, with expected Phase 3 commencement in the third quarter of 2026 initially in Australia
• Commencing the Phase 3 confirmatory study is a pre-requisite to being granted a Biologics License Application (BLA) under the Accelerated Approval Program in the U.S. and Conditional Marketing Authorisations (CMAs) in the U.K. & Europe

New York, New York--(Newsfile Corp. - March 27, 2026) - OS Therapies, Inc. (NYSE:OSTX) ("OS Therapies" or "the Company"), the world leader in listeria-based cancer immunotherapies, today announced that it has been granted meetings with the U.S. Food & Drug Administration (FDA), the U.K. Medicines and Healthcare products Regulatory Agency (MHRA), the European Medicines Agency (EMA) and the Australian Therapeutic Goods Administration (TGA) to review the design of the proposed global confirmatory Phase 3 trial for OST-HER2 in metastatic osteosarcoma. Meetings with each of these four regulatory agencies are expected to occur in the second quarter of 2026, with trial enrollment expected to begin in the third quarter of 2026 initially in Australia.

Commencing the Phase 3 trial is a requirement to be granted a Biologics License Application (BLA) under the Accelerated Approval Program in the U.S. and Conditional Marketing Authorisation (CMA) in the U.K. and in Europe. The Company is seeking to receive a BLA in the U.S. and CMAs in the U.K. & in Europe in the second half of 2026. The Company remains on track to deliver to FDA the clinical and biomarker data it requested in preparation for the Company's upcoming Type B Pre-BLA Meeting by the end of the first quarter of 2026. The company has also commenced preparatory market access activities in Europe, including the Health Technology Assessment (HTA) process.

"We are very pleased to see coordination between these global regulatory agencies as it relates to the proposed design of our planned global confirmatory Phase 3 trial design," said Paul Romness, MPH, Chairman & CEO of OS Therapies. "Patients with metastatic osteosarcoma have significant mortality risk and have seen no improvements in the standard of care in over forty years. The immune system activation mechanism OST-HER2 candidate is very well tolerated. We believe the clinical and biomarker data that we have generated are more than sufficient to justify early market access for patients who have no alternatives with this deadly childhood cancer. We look forward to engaging with the regulators on our path to gaining market access in 2026."

OST-HER2 has received Orphan Drug Designation (ODD), Fast Track Designation (FTD) and Rare Pediatric Disease Designation (RPDD) from the FDA, and ODD, FTD and ATMP from the EMA. Under the RPDD program, if the Company receives a Biologics License Application (BLA) in the United States, it will become eligible to receive a Priority Review Voucher (PRV) that it intends to sell. The most recent publicly disclosed PRV transaction occurred in February 2026 at a reported value of $205 million. The Company is seeking to obtain a BLA under the Accelerated Approval Program for OST-HER2 in osteosarcoma in the second half of 2026.

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. The Company is the world leader in listeria-based cancer immunotherapies. 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 Orphan Drug Designation (ODD), Fast Track Designation (FTD) and Rare Pediatric Disease Designation (RPDD) from the U.S. Food & Drug Administration and has received ODD, FTD and ATMP from the 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 and the overall survival (OS) secondary endpoint. The Company anticipates receiving a Biologics License Application (BLA) from the U.S. FDA for OST-HER2 in osteosarcoma in 2026 and, if approved, would become eligible to receive a Priority Review Voucher that it could then sell. The Company also anticipates receiving Conditional Marketing Authorisations from the U.K.'s Medicines and Healthcare products Regulatory Agency and the EMA for OST-HER2 in 2026. 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. The Company also anticipates reading out data from a Phase 1b study of OST-504 in castration resistant prostate cancer in the first half of 2026.

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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