FDA Grants Type C Meeting for OST-HER2 OS Data in Osteosarcoma

OS Therapies, will take part in a type C statistical methods meeting with the FDA to review 2.5-year overall survival (OS) data from a phase 2b trial (NCT04974008) of OST-HER2 (OST31-164) as maintenance in patients with fully resected, pulmonary metastatic osteosarcoma.¹

The meeting, expected to occur in mid-September 2026, will include members of a division within the FDA’s Center for Biologics Evaluation and Research (CBER), which is responsible for regulatory approval of gene-edited products such as OST-HER2. It will run in parallel with a separate, previously scheduled biomarker data review meeting with the Center for Drug Evaluation and Research’s biomarker qualification program. The company also expects to announce interim 3-year OS data in early September 2026 and reported that the OS benefit observed at the 2-year timepoint remained unchanged at 2.5 years.

“We expect this type C statistics methods meeting with [the] FDA to focus on the appropriateness of using pooled nonconcurrent published historical control data, as compared with concurrent natural history data generated via OST-400, to evaluate the clinical efficacy data generated from the OST-HER2 metastatic osteosarcoma trial,” Craig Eagle, MD, chief medical advisor and a member of the OS Therapies board of directors, stated in the news release.

Regulatory Pathway

The FDA meeting is intended in part to help determine the appropriate comparator arm for interpreting the trial’s clinical efficacy data, a decision the company said will also inform its pending regenerative medicine advanced therapy (RMAT) designation request and eligibility for FDA rolling review. OS Therapies is pursuing a biologics license application (BLA) for OST-HER2 under the accelerated approval program by the end of 2026.

Concurrently, the UK Medicines and Healthcare Products Regulatory Agency (MHRA) has granted the company a scientific advice meeting, expected in September 2026, to review the statistical methods supporting a planned conditional marketing authorization application (CMAA), including whether the submission can be finalized using 2.5-year OS data. The company stated it is also planning CMAA submissions in Europe and Australia.

Trial Background

The phase 2b trial evaluated OST-HER2, a gene-edited, Listeria-based immunotherapy vaccine designed to stimulate an immune response against 2 mutated extracellular epitopes and 1 mutated intracellular epitope of the HER2 oncogene, in patients with fully resected, pulmonary metastatic osteosarcoma. Only 1 of the 3 targeted epitopes needs to be present in a tumor or micrometastasis to trigger the intended immune response, according to the company. Patients were treated with a dose of 1 × 10⁹ colony-forming units every 3 weeks for 48 weeks. OS Therapies has reported what it describes as clinically significant benefit on the trial’s primary end point of 12-month event-free survival (EFS) and its secondary end point of OS. The 12-month EFS rate of 33% was compared with a matched historical control group which had a 20% rate (P =.0156).²

Regulatory Designations and Broader Pipeline

OST-HER2 has received orphan drug designation, fast track designation, and rare pediatric disease designation (RPDD) from the FDA, along with corresponding designations from the European Medicines Agency and MHRA.¹ Under the RPDD program, the company would become eligible to receive a priority review voucher upon BLA approval, which it has stated it intends to sell; the FDA has accepted the company’s letter of intent for a commissioner’s national priority review voucher, with a final decision pending full BLA submission.

OST-HER2 has also completed a phase 1 study primarily in patients with breast cancer, has shown preclinical activity in breast cancer models, and has received conditional approval from the US Department of Agriculture for treatment of osteosarcoma in dogs. The company has separately completed dosing in a phase 1 study of OST-504 in castration-resistant prostate cancer.

REFERENCES

1. 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 August 14, 2026. https://tinyurl.com/mwb8x7ta

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 August 14, 2026. https://tinyurl.com/ya4a4cby