By Melane Sampson
OS Therapies (NYSE American: OSTX) is addressing the urgent need for innovative therapies to kill aggressive cancers that express HER2, including osteosarcoma—a rare and often fatal bone cancer—as well as other solid tumors affecting children, adults, and canine patients. The company has pioneered a novel approach designed to activate the immune system, enabling cytotoxic T cells to identify and attack tumor-associated antigens with greater precision.
“There have been no new treatments for osteosarcoma in more than 40 years, and there has never been any approved therapies for recurrent osteosarcoma,” Paul Romness, Chair, CEO, and President of OS Therapies, says in an interview with BioTuesdays.
“We are paving the way for next-generation cancer treatments aimed at improving overall survival outcomes,” he adds.
Mr. Romness explains that the current standard of care for osteosarcoma (OS) and similar aggressive cancers typically involves surgical resection when feasible, followed by intensive chemotherapy, intending to prevent recurrence. “The challenge is that micrometastases—tiny clusters of cancer cells—can spread to other organs, such as the lungs and brain. This is where we see a significant decrease in survival rates and long-term outcomes.”
A recognized leader in Listeria-based cancer immunotherapies, OS Therapies’ lead asset is OST-HER2 LM Vaccine. This immunotherapy leverages the immune-stimulatory properties of Listeria bacteria to initiate a strong and sustained immune response targeting the human epidermal growth factor receptor 2 (HER2), a protein that is present at low levels in normal cells but becomes overexpressed in many tumors.
“Due to the function of our novel approach, when we target HER2-expressing cancers, the immune system aims to activate killer T cells against a broad range of cancer antigens that the tumor expresses,” Mr. Romness points out.
He adds that OST-HER2 works under checkpoint inhibitors, which help repress tumor defenses, allowing the therapy to more effectively eliminate cancer cells. “Within one hour of IV administration of OST-HER2, there is a robust immune response. At approximately hour four, we intentionally shut down the OST-HER2 treatment and administered an antibiotic. We have observed that our treatment performs very well when combined with standard-of-care checkpoint inhibitors.”
From a regulatory standpoint, OST-HER2 has received Rare Pediatric Disease, Fast-Track, and Orphan Drug designations from the FDA, along with Orphan Drug status from the European Medicines Agency (EMA). The company expects to submit a Biologics License Application (BLA) for osteosarcoma later this year. If approved, the therapy would qualify for a Priority Review Voucher, which the company could potentially monetize. It also anticipates conditional marketing authorizations from both the UK’s Medicines and Healthcare products Regulatory Agency and the EMA within a similar timeframe.
“In the U.S. alone, approximately 1,000 children are diagnosed with osteosarcoma each year, underscoring the significant unmet medical need in this rare disease,” Mr. Romness notes.
Clinical data from the company’s open-label Phase 2b trial in pediatric osteosarcoma demonstrated promising outcomes. The study, conducted across 21 leading pediatric oncology centers in the U.S., enrolled 41 patients. Results showed overall survival exceeding 80% at two years, with event-free survival of approximately 35%, indicating that a meaningful portion of patients experienced no disease recurrence during the study period.
The therapy’s mechanism of action involves rapid immune activation following intravenous administration. Within hours, the immune system is stimulated to identify and attack micrometastatic disease—referred to by the company as “cancer seeds”—before they can establish secondary tumors in vital organs.
Mr. Romness highlights the growing visibility of the company’s platform, including national media coverage and its feature in the Daytime Emmy-nominated documentary titled Shelter Me: The Cancer Pioneers, which explores comparative oncology between humans and dogs.
The company has completed clinical development for its canine osteosarcoma program through Phase 3. Notably, the therapy has received conditional approval by the U.S. Department of Agriculture for the treatment of canine osteosarcoma, pending a bacterial clearance study. “There are approximately 30,000 to 50,000 dogs diagnosed with osteosarcoma each year in the U.S.—that’s a significant market opportunity,” Mr. Romness contends.
He clarifies, however, that OS Therapies plans to spin off its canine OST-HER2 program following full regulatory approval, allowing the company to maintain focus on its core mission: advancing treatments for osteosarcoma and other solid tumors in human patients.
OST-HER2 has also completed a Phase 1 clinical study, primarily in breast cancer patients, and has demonstrated preclinical efficacy across multiple breast cancer models.
Additionally, the company is currently advancing its next-generation antibody drug conjugate (ADC) platform, known as tunable ADC (tADC). This platform incorporates a proprietary silicon-based Si-Linker and Conditionally Active Payload (CAP) technology, enabling the attachment and controlled release of multiple therapeutic payloads per linker.
“We have several advantages around our linker technology because we can mix and match different payloads and targeting ligands. We can incorporate multiple payloads in sets of three, six, or nine, and we can adjust the targeting ligands to direct therapies toward specific cancer types. This differentiates us in a crowded market where many existing treatments are relatively similar to one another,” Mr. Romness says.
“The analogy I use to explain the process is moving a piano through a house and wrapping it with bubble wrap so that it doesn’t bump against the walls. In the body, we are effectively wrapping payloads with silicone. Without that protection, contact with healthy tissue could result in toxic side effects.”
Mr. Romness emphasizes that the company has a highly experienced management team with members who have successfully guided therapies through regulatory approval. “We also have the founding scientists for both of our platforms on our team, as well as a very active and engaged patient advocacy group.”
Looking ahead, the company intends to expand its clinical focus beyond osteosarcoma. Upon potential regulatory approval, plans include initiating studies in additional solid tumors, including breast, colorectal, esophageal, lung, ovarian, and pancreatic cancers.
“We’ve reached several major inflection points with regulators, and we are hopeful for approval by the end of 2026. Our goal is to be on the market in 2027,” Mr. Romness concludes.
“We are anticipating accelerated approval in the U.S., as well as in the EU and UK, which would pivot us toward broader solid tumor applications and enable us to continue working to deliver these therapies to the millions of patients worldwide who urgently need new treatment options.”
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