OST-HER2 (AOST-2121)

September 17, 2026: OS Therapies (NYSE: OSTX) said FDA aligned with MHRA and EMA on the adaptive design for the planned Phase 3 trial of OST-HER2 in prevention or delay of recurrence in fully resected pulmonary metastatic osteosarcoma. The Phase 3 study is expected to start in Q4 2026.

For regulatory milestones, the company said its BLA filing for OST-HER2 was initiated in January 2026 and that it expects an invited FDA Type B pre-BLA meeting in Q4 2026 after submitting the clinical section including 3-year overall survival data. It is seeking U.S. accelerated approval and conditional marketing authorization applications in Europe, the U.K., and Australia in Q4 2026.

The company also said it accepted an MHRA request for MHRA to act as the company’s representative in Project Orbis after a Statistical Methods Scientific Advice Meeting; CBER recently re-opened Project Orbis to foreign-sponsored U.S. companies.

Background data cited in the release: OST-HER2 previously showed clinically significant benefit in the Phase 2b trial on the 12-month event-free survival primary endpoint and overall survival secondary endpoint. OST-HER2 has FDA orphan drug, fast track, and rare pediatric disease designations; if a BLA is granted, the company said it could receive a priority review voucher.