OS Therapies Granted FDA Meeting Request To Gain Alignment On Surrogate Endpoint To Support Breakthrough Therapy Designation & Accelerated Approval Of OST-HER2 In Prevention Of Recurrence Of Fully Resected, Lung Metastatic Osteosarcoma

• Written response-only meeting to align analysis plan of surrogate endpoint to support Breakthrough Therapy Designation & Accelerated Approval of OST-HER2 in the Prevention of Recurrence of Fully Resected, Lung Metastatic Osteosarcoma
• Analysis to be featured at keynote presentation closing out major osteosarcoma conference MIB Factor on June 28, 2025 at 3:30pm MDT

OS Therapies (NYSE-A: OSTX) ("OS Therapies" or "the Company"), a clinical-stage immunotherapy and Antibody Drug Conjugate (ADC) biopharmaceutical company, today announced that the US Food & Drug Administration ("FDA") granted the Company's meeting request to gain alignment on the surrogate endpoint to support Breakthrough Therapy Designation & Accelerated Approval of OST-HER2 in the Prevention of Recurrence of Fully Resected, Lung Metastatic Osteosarcoma. FDA granted a written response-only meeting and confirmed that its response would be received by mid-June 2025, in time for the Company to present the statistical analysis as part of the keynote presentation closing out major osteosarcoma conference MIB Factor on June 28, 2025 at 3:30pm MDT.

OST-HER2 has received Rare Pediatric Disease Designation (RPDD) for osteosarcoma from the US FDA, and if it receives a conditional BLA via Accelerated Review prior to September 30, 2026, it will become eligible to receive a Priority Review Voucher (PRV) that it intends to immediately sell. The most recent PRV sale, valued at $150 million, occurred in February 2025.

The osteosarcoma treatment market was estimated at $1.2 billion in 2022 according to Data Bridge Market Research. Approximately 50% of patients are diagnosed with a lung metastasis at some point following surgical resection and chemotherapy. 3-year survival rates in patients who were not diagnosed with a metastasis are 59%. 3-year survival rates in patients who were diagnosed with pulmonary metastasis were 30%. The Company believes the market opportunity for OST-HER2 in the prevention of lung metastases is over $500 million.

OST-HER2, an immunotherapy for osteosarcoma using a HER2 bioengineered form of the bacteria Listeria monocytogenes to trigger a strong immune response against cancer cells expressing HER2, is being featured in the upcoming movie Shelter Me: The Cancer Pioneers. The movie offers a look into canine comparative oncology, a field that compares treatment of cancers in dogs to those in people and covers developing treatments for rare forms of cancer. A trailer for the movie is available here. The movie will air live nationally on PBS and be available via streaming on PBS' website in early May 2025.