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 American: 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 2026.

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).