OS Therapies Holds Pre-MAA With UK MHRA Regarding Phase 2b Human Clinical Trial Of OST-HER2 In Prevention Or Delay Of Recurrent, Fully-Resected, Pulmonary Metastatic Osteosarcoma

• Alignment achieved on all key points surrounding non-clinical, CMC and post-market authorization confirmatory study design
• Biomarker data advanced as key pre-specified surrogate clinical efficacy endpoint, with pending analysis awaiting alignment with US FDA on biomarker statistical analysis plan to be discussed at upcoming December 11, 2025 FDA Type C Meeting
• Company reiterates end of January 2026 timeline for MAA submission

New York, New York--(Newsfile Corp. - December 9, 2025) - OS Therapies Inc. (NYSE:OSTX) ("OS Therapies" or "the Company"), the world leader in listeria-based cancer immunotherapies, today announced that it held a successful pre-Marketing Authorisation Application (MAA) with the United Kingdom (UK) Medicines and Healthcare Products Regulatory Agency (MHRA) regarding the Phase 2b human clinical trial of OST-HER2 in the prevention or delay of recurrent, fully-resected, pulmonary metastatic osteosarcoma (the 'Metastatic Osteosarcoma Program'). The Company achieved full alignment with its pre-meeting objectives related to non-clinical, CMC (chemistry, manufacturing, and controls) and post-market authorization confirmatory study design.

Additionally, the Company advanced the correlation of upregulated immune response biomarkers shown to be activated in OST-HER2 treated canine patients who achieved long term survival with 2-year overall survival data from the OST-HER2 Phase 2b human clinical trial as a surrogate clinical efficacy endpoint to further support the approval of a conditional MAA in the United Kingdom. The Company is awaiting feedback from its upcoming December 11, 2025 meeting with United States Food & Drug Administration (FDA) before initiating the proposed biomarker data analysis to ensure that the analysis is pre-specified, thereby making the resulting data suitable to support a Biologics Licensing Application (BLA) under the Accelerated Approval Program ('Accelerated Approval') in the United States.

The Company reiterated that it expects to submit a conditional MAA for the Metastatic Osteosarcoma Program to MHRA by the end of January 2026.