New York, NY – October 10, 2025 – OS Therapies Inc. has announced statistically significant positive final results from its Phase 2b clinical trial evaluating OST-HER2, an innovative off-the-shelf immunotherapy for recurrent, fully resected pulmonary metastatic osteosarcoma. The therapy demonstrated a 75% overall survival (OS) rate compared with 40% in historical controls (p < 0.0001), showing a remarkable improvement in long-term survival outcomes for patients facing this rare and aggressive cancer.
Science Significance
The Phase 2b study results highlight OST-HER2’s novel immune-mediated approach, leveraging live-attenuated Listeria monocytogenes as a delivery system for HER2 antigens to activate a strong, targeted immune response. This immunotherapy represents a scientific breakthrough in HER2-positive osteosarcoma, with evidence that 100% of patients achieving 12-month event-free survival maintained 2-year survival. The findings suggest that OST-HER2 could become a first-of-its-kind durable therapy for pediatric and adolescent patients, redefining HER2-targeted treatment beyond monoclonal antibodies.
Regulatory Significance
Following the positive clinical outcomes, OS Therapies has engaged with the U.S. Food and Drug Administration (FDA), U.K. Medicines and Healthcare products Regulatory Agency (MHRA), and European Medicines Agency (EMA) to advance conditional and accelerated approval pathways. The company intends to file a Marketing Authorization Application (MAA) in the United Kingdom by December 2025, followed by a Biologics License Application (BLA) submission in the U.S. in January 2026, and an EMA submission in early 2026. OST-HER2 has previously been granted Rare Pediatric Disease, Orphan Drug, and Fast Track Designations, underscoring its potential clinical and regulatory impact.
Business Significance
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With these results, OS Therapies (NYSE American: OSTX) is positioned as a key innovator in oncology immunotherapy. The company’s tunable antibody-drug conjugate (tADC) platform, built using its proprietary Si-Linker and CAP technologies, enhances its long-term growth prospects. Upon approval, OST-HER2 may qualify for a Priority Review Voucher, creating significant commercial and partnership opportunities while expanding OS Therapies’ presence in rare and pediatric oncology markets.
Patients’ Significance
Patients with metastatic osteosarcoma face a survival plateau that has not improved in over three decades. The 75% two-year overall survival rate achieved in this study represents a major advancement in patient hope and outcomes. The therapy’s favorable safety and tolerability profile makes it suitable for maintenance or adjuvant use following surgical resection, offering a new lifeline for young patients battling this life-threatening disease.
Policy Significance
The success of OST-HER2 reinforces the importance of global harmonization of regulatory frameworks to accelerate rare cancer therapies. Collaborative initiatives between the FDA, MHRA, and EMA demonstrate growing international commitment to fast-track innovative treatments for rare pediatric indications. If approved, OST-HER2 could help shape future policy models for immunotherapy approval and access under orphan drug and rare disease programs.
The positive Phase 2b data for OST-HER2 positions OS Therapies at the forefront of cancer immunotherapy innovation, delivering meaningful clinical benefit in osteosarcoma, one of the most challenging rare cancers. With regulatory filings planned across major health authorities and biomarker results anticipated in November 2025, the company is advancing toward a pivotal moment in transforming pediatric oncology care worldwide.
Source: OS Therapies press release
