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The immunotherapy OST-HER2 combined with palliative radiation has shown positive data in preventing or delaying amputation during primary osteosarcoma treatment in dogs with unresected appendicular osteosarcoma, according to a recent announcement by OS Therapies—a clinical-stage immunotherapy and antibody drug conjugate biopharmaceutical company behind the development of OST-HER2. Findings from a study suggest OST-HER2 could be used as a frontline treatment for osteosarcoma in humans, potentially reducing the need for chemotherapy, according to OS Therapies.1
“It has been my dream since founding the company that OST-HER2 could potentially change the standard of care in osteosarcoma, potentially limiting the need for amputation or surgical resection of the primary tumor,” Paul Romness, CEO of OS Therapies, said in a news release.1 “With today’s data, we believe we are taking the first steps towards this given that our comparative oncology approach, as a result of the 96% genetic homology between human and canine osteosarcoma, leads us to believe there is significant potential for this canine data to translate to humans in the treatment of frontline and primary metastatic osteosarcoma, similarly to how it has in recurrent, fully-resected, lung metastatic osteosarcoma.”
OST-HER2 is an immunotherapy that uses a bioengineered form of the Listeria monocytogenes vaccine to trigger an immune response against cancer cells expressing human epidermal growth factor receptor 2 (HER2).1 The therapy has received conditional approval from the US Department of Agriculture for the treatment of canines with osteosarcoma. It has also completed a Phase 1 trial in human patients with breast cancer and shown preclinical efficacy data in various models of breast cancer. According to OS Therapies, the treatment has also “demonstrated positive data” in its Phase 2b clinical trial in recurrent, fully resected, lung metastatic osteosarcoma.1
In a recent study—Immunological responses and clinical outcomes in dogs with osteosarcoma receiving standard therapy and a Listeria vaccine expressing HER2— 118 dogs with appendicular osteosarcoma were enrolled.2 Participants in this clinical trial received standard therapy followed by OST-HER2.
According to the study’s results, the OST-HER2 therapy did not significantly extend overall survival or disease-free intervals in all participants. However, immune responses in dogs after receiving the vaccine were stronger in long-term survivors—those with a disease-free interval of more than 490 days.2 Specifically, results showed that the therapy triggered strong innate and cytotoxic immune responses starting at the first dose of a 3-dose regimen, which was linked to the prevention of metastasis in dogs with resected primary osteosarcoma.1 According to OS Therapies, the study also found that short-term survivors had defective immune responses to OST-HER2 that improved with the second and third doses.1
Currently, the study on OST-HER2 combined with palliative radiation as a therapy for unresected primary osteosarcoma in dogs is being prepared for publication.1 Unpublished data from the study shows the treatment resulted in clinical and radiographic arrest of the primary tumor in 5 of 15 dogs. Treatment also delayed pulmonary metastases in dogs that did not undergo surgery or chemotherapy and prolonged survival of greater than 500 days in these 5 participants.
According to OS Therapies, the data suggest that OST-HER2 could be used as a frontline treatment for human osteosarcoma, potentially reducing the need for chemotherapy and helping prevent or delay limb amputation, tumor resection, and lung metastasis.1 According to OS Therapies, the company plans to submit the new data from its to-be-published study and new information on its manufacturing process to the USDA with the goal of launching sales in 2025. Following this, OS Therapies intends to conduct a pivotal clinical trial with the aim of obtaining full approval in 2026.1
In both studies, treatment with OST-HER was found to be safe and well tolerated. “Our newly published canine data and unpublished radiation combination canine data expands the potential use of OST-HER2 into delay/prevention of metastasis and PFS following resected primary osteosarcoma and raises the intriguing possibility of combination radiation therapy and OST-HER2 in the treatment of unresectable osteosarcoma,” Nicola J. Mason, BVetMed, PhD, DACVIM, the Paul A. James and Charles A. Gilmore endowed professor, at the University of Pennsylvania, School of Veterinary Medicine in Philadelphia, said in a company release.1
“We are laser focused on getting an accelerated approval for OST-HER2 in recurrent, fully resected, lung metastatic human osteosarcoma…by year-end 2025 and then using funds obtained from the sale of our pending Priority Review Voucher to expand the potential clinical uses of OST-HER2 throughout the human osteosarcoma treatment paradigm,” said Romness.1 “In parallel, we believe the data from these 2 studies support the use of OST-HER2 in all phases of canine osteosarcoma treatment and we are hopeful to obtain conditional approval with our newly improved, patented manufacturing for OST-HER2 later this year so that we can launch the product at specialized cancer centers and our 4-legged patients can begin getting treatment very soon.
References
- OS Therapies announces positive data for OST-HER2 in the treatment of unresected osteosarcoma in dogs, opening the potential for use as front-line therapy in humans. News release. OS Therapies. April 10, 2025. Accessed April 14, 2025. https://www.businesswire.com/news/home/20250410123762/en/OS-Therapies-Announces-Positive-Data-for-OST-HER2-in-the-Treatment-of-Unresected-Osteosarcoma-in-Dogs-Opening-the-Potential-for-Use-as-Front-Line-Therapy-in-Humans
- Mason NJ, Selmic L, Ruple A, et al. Immunological responses and clinical outcomes in dogs with osteosarcoma receiving standard therapy and a listeria vaccine expressing HER2. Molecular Therapy. 2025;33(4):1674-1686. doi:10.1016/j.ymthe.2025.02.023
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