OS Therapies Provides OST-HER2 Recurrent, Fully Resected, Pulmonary Metastatic Osteosarcoma Program Update Following FDA End of Phase 2 Meeting - Seite 2

• Current single arm, open-label Phase 2b study design, originally recommended by FDA in 2020 as a proof-of-concept study, compares OST-HER2-treated patients with historical clinical outcomes data published by the Children's Oncology Group ("COG") consortium. The Company continues to seek direction on which FDA guidance to rely upon with respect to clinical efficacy for the pending BLA submission
• New FDA Draft Guidance entitled 'Approaches to Assessment of Overall Survival in Oncology Clinical Trials' made public on August 18, 2025 (the "Draft FDA Overall Survival Guidance"), provides potential pathways to evaluate osteosarcoma clinical trial efficacy data. The Company has achieved statistically significant positive final 12-month Event Free Survival (EFS) data (p = 0.0197) and interim 2-year overall survival data (p = 0.0046) from its 40 patient Phase 2b trial
• Final FDA guidance made public in December 2023 entitled 'Rare Diseases: Considerations for the Development of Drugs and Biological Products,' provides additional potential pathways to evaluate osteosarcoma clinical trial efficacy data. From 2015 to 2022, single arm Phase 1 or Phase 2 trials supported 45 out of 84 molecular targeted marketing authorizations in oncology

The Company is continuing to support the expansion of natural history database OST-400, "Recurrent Osteosarcoma after Resection in Children and Young Adults: A Retrospective Longitudinal Study". Data is being sourced from multiple leading US and international oncology research institutions, in addition to real world data sources. OST-400 database is being assembled in order to be able to develop a synthetic control arm suitable to support a randomization process that could serve as comparator arm in the event FDA relies upon its 2023 Rare Diseases guidance. Various FDA-accepted statistical analysis methods reviewed in this Guidance allow for randomization after treatment in single-arm trials. Final decisions on acceptable efficacy outcome measures are needed to support Regenerative Medicine Advanced Therapy ("RMAT") designation, Breakthrough Therapy designation ("BTD") and Accelerated Approval or full approval, and the Company expects significant progress to be made towards that end at the October 10, 2025 FDA/OSI osteosarcoma workshop.