OST-HER2 Shows 71.2% 3-Year OS in Lung-Metastatic Osteosarcoma

Treatment with OST-HER2 was associated with a statistically significant improvement in interim 3-year overall survival (OS) compared with a combined historical control population in patients with fully resected, metastatic pulmonary osteosarcoma.¹

An interim analysis of a phase 2b trial (NCT04974008) showed a 3-year OS rate of 71.2% among OST-HER2–treated patients compared with 45.8% in a comparable combined historical control group (90% CI, 11.9%-38.9%; P =.002).¹

“We believe the increasing survival benefit as time goes on for OST-HER2–treated patients when compared with any and all available published literature in [patients with] fully resected metastatic osteosarcoma, including data published as recently as 2026, presents a compelling case for early market access for [patients with] osteosarcoma who have not seen a new drug approved in the last forty years,” Craig Eagle, MD, chief medical advisor and director of OS Therapies, stated in a news release.

Trial Background and Updated Results

OST-HER2 is a gene-edited, Listeria-based immunotherapy designed to stimulate an immune response against 2 mutated extracellular epitopes and 1 mutated intracellular epitope of the HER2 oncogene; only 1 of the 3 targeted epitopes needs to be present in a tumor or micrometastasis to trigger the intended response.

In the phase 2b trial, patients received OST-HER2 at a dose of 1 × 10⁹ colony-forming units every 3 weeks for 48 weeks.² The company previously reported a clinically significant benefit on the trial’s primary end point of 12-month event-free survival (EFS), with a 33% EFS rate compared with 20% in a matched historical control group (P =.0156), and on the OS secondary end point.

The historical weighted control included findings from 5 prior clinical trials, the smallest enrolling 10 patients and the largest enrolling 339, whose 3-year OS rates ranged from 36.0% to 54.7%.¹ Three of these trials were in the same setting of recurrent completely resected pulmonary-metastatic osteosarcoma, whereas 2 others were for recurrent, completely resected metastases without specific focus on pulmonary metastases.

The trial enrolled 41 patients; at the time of interim analysis, 2 patients had not yet reached the 3-year follow-up timepoint, with monitoring visits scheduled for September and early October 2026, and 6 patients were reported as lost to follow-up.

Regulatory Pathway for OST-HER2

OS Therapies is scheduled to participate in a Type C statistical methods meeting with the FDA’s Center for Biologics Evaluation and Research in mid-September 2026.³ The meeting is expected to focus on the appropriateness of using pooled, nonconcurrent published historical control data to evaluate the trial’s efficacy data, and to request rolling review of the ongoing biologics license application. The company stated it is seeking accelerated approval or the equivalent in the United States, Europe, United Kingdom, and Australia in the fourth quarter of 2026 with a goal of commercial availability in 2027.¹

REFERENCES

1. OS Therapies Achieves Statistically Significant Benefit for OST-HER2-Treated Patients in Interim 3-Year Overall Survival Analysis of Phase 2b Pulmonary Metastatic Osteosarcoma Trial. News release. OS Therapies, Inc. September 8, 2026. Accessed September 8, 2026. https://tinyurl.com/44khsb2c

2. OS Therapies announces phase 2b clinical trial of OST-HER2 achieves primary endpoint with statistical significance in the prevention of recurrent, fully resected, lung metastatic osteosarcoma. News release. OS Therapies, Inc. January 15, 2025. Accessed September 8, 2026. https://tinyurl.com/ya4a4cby

3. OS Therapies Granted U.S. FDA Type C Statistical Methods Meeting to Review 2.5-Year Overall Survival Data for OST-HER2 in the Prevention or Delay of Recurrence in Fully Resected, Pulmonary Metastatic Osteosarcoma. News release. OS Therapies, Inc. August 13, 2026. Accessed September 8, 2026. https://tinyurl.com/mwb8x7ta