OS Therapies' OST-HER2: Regulatory Catalysts and a $160M PRV Drive Near-Term Value

OS Therapies stands on the brink of a transformative moment for its lead candidate, OST-HER2, an immunotherapy targeting recurrent, fully resected pulmonary metastatic osteosarcoma—a rare and aggressive cancer predominantly affecting children and adolescents. With a robust clinical data package and a strategic regulatory roadmap, the company is positioned to deliver near-term catalysts that could unlock significant value. The upcoming FDA End of Phase 2 Meeting, PRV eligibility, and a novel external control strategy form the pillars of this opportunity. Here's why investors should pay attention.

The Regulatory Pathway: Speed and Incentives

The FDA End of Phase 2 Meeting, scheduled for Q3 2025, is the next critical milestone. This meeting will determine whether the FDA accepts the rolling Biologics License Application (BLA) submission process, which allows OS Therapies to file sections of its application incrementally. A favorable outcome here could accelerate approval timelines, potentially leading to Accelerated Approval by late 2025.

OST-HER2's Rare Pediatric Disease Designation (RPDD) adds urgency. If approved by September 30, 2026, the therapy qualifies for a Priority Review Voucher (PRV), which can be sold for immediate cash. Recent PRV sales, including a June 2025 transaction valued at $160 million, underscore the financial upside. This “PRV lottery ticket” alone represents a 20-30% uplift in OS Therapies' current valuation, even before accounting for commercial sales.

Phase 2b Data: Statistically Significant and Clinically Meaningful

The Phase 2b trial's 12-month event-free survival (EFS) data is the backbone of this opportunity. OST-HER2 achieved a 33% EFS rate versus 20% in historical controls (p=0.0158), a statistically significant improvement. Notably, all patients achieving 12-month EFS remained alive at the data cutoff, with 91% overall survival (OS) at 1 year and 61% at 2 years—dramatically outperforming historical trends.

The use of an external control comparator (via the OST-400 retrospective study) is a strategic move. By aggregating over 200 historical cases from the U.S., U.K., and France, OS Therapies addresses the rarity of the disease, which makes randomized trials impractical. The FDA's acceptance of this approach, validated in prior feedback, strengthens the case for accelerated approval.

Canine Oncology Synergy: A Biomarker Boost

OST-HER2's development leverages comparative oncology insights, a field where 96% genetic homology between human and canine osteosarcoma allows cross-species data sharing. The company's subsidiary, OS Animal Health, has already secured conditional USDA approval for OST-HER2 in canine osteosarcoma. Data from this program may identify predictive biomarkers for human treatment response, potentially expanding the therapy's label to include limb-sparing approaches and unresectable lung metastases. This dual-track development not only diversifies revenue streams but also strengthens the human drug's clinical profile.

The Investment Case: Buy Ahead of Q3 Catalysts

The combination of statistically robust data, regulatory tailwinds, and the PRV's tangible value creates a compelling risk-reward profile. Key catalysts are:
1. Q3 2025 FDA End of Phase 2 Meeting: A green light for rolling BLA submission.
2. PRV monetization by 2026: A $160M windfall if approval timelines are met.
3. Clinical data presentation at MIB Factor (June 2025): Solidifies investor confidence ahead of the FDA meeting.

Risks include FDA hesitation on external controls or manufacturing hurdles, but the data's strength and the company's $160M cash runway (funding through 2026) mitigate these concerns.

Conclusion: A High-Conviction Buy

OS Therapies is at a pivotal inflection point. With a $160M PRV, FDA-friendly regulatory strategy, and clinically validated data, the stock is primed for a catalyst-driven rally. Investors should consider a buy rating, with a price target reflecting both PRV monetization and commercial potential. The next 12 months will test the thesis, but the pieces are in place for a rare win in pediatric oncology—a win for patients, and a lucrative one for shareholders.