OS Therapies Announces Statistically Significant Positive Final 2-Year Overall Survival Data from Phase 2b Trial of OST-HER2 in the Prevention or Delay of Recurrent, Fully-Resected, Pulmonary Metastatic Osteosarcoma

• 75% of OST-HER2 treated patients achieved 2-year overall survival compared with 40% in the historical control group (p < 0.0001)
• 100% of patients who achieved 12 month event free survival achieved 2 year overall survival

New York, New York--(Newsfile Corp. - October 10, 2025) - OS Therapies Inc. (NYSE American: OSTX) ("OS Therapies" or "the Company"), a clinical-stage cancer immunotherapy and antibody drug conjugate biotechnology company, today announced statistically significant positive final 2-year overall survival data from the Company's Phase 2b trial of off-the-shelf immunotherapy candidate OST-HER2 in the prevention or delay of recurrent, fully-resected, pulmonary metastatic osteosarcoma (the "Fully Resected Metastatic Osteosarcoma Trial"). 75% (27 out of 36 evaluable patients; 5 patients were lost to follow-up) of OST-HER2 treated patients achieved 2-year overall survival as measured from most recent pulmonary resection, compared with 40% of historical control patients¹ (p < 0.0001) Subgroup analyses showed that 100% of patients who achieved 12-month Event Free Survival (EFS) achieved 2-year overall survival, whereas 59% of patients who did not achieve EFS achieved 2-year overall survival.

"The overall survival data in this non-randomized OST-HER2 study is encouraging as the results show it was safe and well-tolerated," said Dr. Peter Anderson, pediatric oncologist at Cleveland Clinic Children's. "New treatments are needed for treating metastatic osteosarcoma, so I am hopeful this could become an option in the near future." Dr. Anderson is a scientific advisor for OS Therapies.

"The continued outperformance in overall survival of OST-HER2 when compared with historical control is exactly what we were hoping for when we started the Company in 2018," said Paul Romness, MPH, Chairman & CEO of OS Therapies. "We have had highly productive regulatory meetings with the United Kingdom's (UK) Medicines and Healthcare products Regulatory Agency (MHRA), the United States (US) Food & Drug Administration (FDA) and the European Medicines Agency's (EMA) Dutch rapporteur. Representatives of all three regulatory agencies, in various ways, indicated that overall survival may be an appropriate clinical endpoint to support a conditional marketing authorization, especially when supported with biomarker data. The rationale for overall survival combined with biomarkers is largely a result of the compelling data recently published in canine osteosarcoma by researchers at the University of Pennsylvania."