OS Therapies Reports Positive OST-HER2 Survival Data at Osteosarcoma Conference
OS Therapies a clinical-stage biopharmaceutical company specializing in immunotherapy and Antibody Drug Conjugates (ADCs), has announced encouraging clinical data from its ongoing Phase 2b clinical trial of OST-HER2, a promising immunotherapy candidate for osteosarcoma. The company presented new one-year event-free survival (EFS), overall survival (OS), and safety data during the MIB Agents Factor Osteosarcoma Conference held on June 28, 2025, in Salt Lake City, Utah.
Statistically Significant Improvement in Event-Free Survival
The Phase 2b clinical trial includes a 40-patient treatment arm targeting a specific high-risk population: patients with fully resected lung metastatic osteosarcoma. In this challenging subgroup, which traditionally has few therapeutic options and poor prognosis, OST-HER2 showed a statistically significant improvement in 1-year event-free survival.
According to Dr. Damon Reed, Principal Investigator, 35% (14 out of 40) of patients treated with OST-HER2 were event-free after one year. This compares to just 20% in a matched external control population sourced from peer-reviewed publications identified by the Children’s Oncology Group (COG). The statistical significance of this finding (p = 0.0194) provides compelling early evidence of OST-HER2’s potential clinical benefit.
“The updated OST-HER2 data presented at MIB Factor showed EFS data statistically significantly favoring OST-HER2-treated patients when compared with leading peer-reviewed publications on historical outcomes in this osteosarcoma subpopulation,” said Dr. Robert Petit, Chief Medical & Scientific Officer of OS Therapies.
Strong Safety Profile With No Treatment Discontinuations
In addition to efficacy, OST-HER2 also demonstrated a favorable safety profile. Out of the 40 patients treated:
- 13 patients experienced severe adverse events (SAEs).
- Of those, 7 SAEs were categorized as treatment-associated (TSAEs).
- All 7 TSAEs were Grade 3, meaning serious but manageable.
- Importantly, no Grade 4 or Grade 5 TSAEs (life-threatening or fatal) occurred.
- None of the patients discontinued treatment due to TSAEs.
This safety profile is particularly notable in the context of osteosarcoma, where standard treatments are often associated with harsh side effects.
“The favorable safety profile of OST-HER2 compared with standard of care is also an important quality of life factor when assessing potential new treatment options for this difficult-to-treat patient population,” Dr. Petit added.
Regulatory Path Forward: Accelerated Approval Strategy
Alongside the clinical update, OS Therapies shared progress on its regulatory engagement with the U.S. Food and Drug Administration (FDA). The company is actively pursuing Accelerated Approval for OST-HER2, supported by external control data in lieu of a traditional randomized controlled trial (RCT)—an increasingly accepted approach for rare diseases.
FDA Interaction and Guidance
Informed by a recent Type D Meeting with the FDA, OS Therapies submitted updated materials addressing:
- Historical external control data from published studies
- Matched external control data based on patient characteristics
- Real-world evidence (RWE) drawn from comparable populations
These data sources are in line with guidance from FDA documents such as:
- ICH E10: Choice of Control Group in Clinical Trials
- Demonstrating Substantial Evidence of Effectiveness for Human Drug and Biological Products (2019)
- Rare Diseases – Natural History Studies for Drug Development (2019)
The company aims to finalize discussions and reach full alignment with the FDA during an End of Phase 2 (EOP2) meeting, which it has formally requested.
“The feedback received from FDA regarding the use of external control comparators in settings where placebo-controlled randomization trials are not feasible increases the avenues available for OST-HER2 to gain Accelerated Approval,” said Paul Romness, Chairman & CEO of OS Therapies.
Special Designations and Potential for Priority Review Voucher
OST-HER2 has already received several regulatory designations from the FDA, including:
- Orphan Drug Designation (ODD)
- Rare Pediatric Disease Designation (RPDD)
- Fast Track Designation
These designations highlight the unmet need and therapeutic promise of OST-HER2 in pediatric osteosarcoma. If OS Therapies secures Accelerated Approval before September 30, 2026, the company will be eligible to receive a Priority Review Voucher (PRV).
Such vouchers are highly valuable; the most recent PRV sale in June 2025 was valued at $160 million. OS Therapies has indicated plans to sell its PRV if granted.
MHRA and Global Regulatory Support
Beyond the United States, OS Therapies is expanding its regulatory groundwork internationally. The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) has agreed to support OS Therapies in leveraging the Clinical Practice Research Datalink (CPRD).
The CPRD is one of the world’s largest patient health databases, containing longitudinal data on over 60 million individuals, including 18 million active patients. This real-world dataset is expected to help support global marketing authorizations for OST-HER2, including in the UK, EU, and US.
A follow-up to the company’s Scientific Advice Meeting with the MHRA, held on July 31, 2025, is anticipated by mid-August 2025.
Innovative Mechanism: HER2-Targeted Immunotherapy via Bioengineered Listeria
OST-HER2 is not a conventional immunotherapy. It employs a bioengineered form of Listeria monocytogenes that expresses the HER2 antigen, enabling the body’s immune system to recognize and attack HER2-expressing osteosarcoma cells. This unique delivery method triggers a potent immune response that may prevent or delay tumor recurrence.
This platform represents an innovative shift in the treatment of pediatric cancers, especially those resistant to chemotherapy.
Public Awareness Through Media Spotlight
OST-HER2’s development has also gained attention through its inclusion in the film “Shelter Me: The Cancer Pioneers”, which explores the use of canine comparative oncology. This field draws insights from treating naturally occurring cancers in dogs to accelerate development of therapies for rare human cancers, like osteosarcoma.
The film highlights how insights from canine patients are helping researchers better understand tumor biology and immunotherapy response, further validating OST-HER2’s real-world potential.
Looking Ahead: A New Hope for Osteosarcoma Patients
With strong early efficacy signals, a favorable safety profile, and broad regulatory support, OS Therapies appears well-positioned to bring OST-HER2 closer to market approval. The company’s approach—combining innovative science, real-world data, and regulatory flexibility—offers renewed hope for patients and families facing lung-metastatic osteosarcoma, a historically intractable cancer.
“We were very pleased with the reception the presentation received from the osteosarcoma community,” said Mr. Romness. “We remain committed to advancing OST-HER2 as quickly and responsibly as possible.”
As OS Therapies continues to work with the FDA and international health authorities, OST-HER2 could become a first-in-class, life-extending therapy for a rare pediatric cancer in urgent need of better options.
The movie is available via streaming on PBS’ website.
The most recent data updated regarding the OST-HER2 canine osteosarcoma program is available at this link.
About OS Therapies
OS Therapies is a clinical stage oncology company focused on the identification, development, and commercialization of treatments for Osteosarcoma (OS) and other solid tumors. OST-HER2, the Company’s lead asset, is an immunotherapy leveraging the immune-stimulatory effects of Listeria bacteria to initiate a strong immune response targeting the HER2 protein. OST-HER2 has received Rare Pediatric Disease Designation (RPDD) from the US Food & Drug Administration and Fast-Track and Orphan Drug designations from the US FDA and European Medicines Agency. The Company positive data in its Phase 2b clinical trial of OST-HER2 in recurrent, fully resected, lung metastatic osteosarcoma demonstrating statistically significant benefit in the 12-month event free survival (EFS) primary endpoint of the study. The Company anticipates submitting a Biologics Licensing Application (BLA) to the US FDA for OST-HER2 in osteosarcoma in 2025 and, if approved, would become eligible to receive a Priority Review Voucher that it could then sell. OST-HER2 has completed a Phase 1 clinical study primarily in breast cancer patients, in addition to showing preclinical efficacy data in various models of breast cancer. OST-HER2 has been conditionally approved by the U.S. Department of Agriculture for the treatment of canines with osteosarcoma.
In addition, OS Therapies is advancing its next-generation Antibody Drug Conjugate (ADC) and Drug Conjugates (DC), known as tunable ADC (tADC), which features tunable, tailored antibody-linker-payload candidates. This platform leverages the Company’s proprietary silicone Si-Linker and Conditionally Active Payload (CAP) technology, enabling the delivery of multiple payloads per linker. For more information, please visit www.ostherapies.com.
