OST-HER2 Exhibits Survival Benefit in Pulmonary Metastatic Osteosarcoma

Of 36 patients evaluable for efficacy, 75% were alive as measured from the most recent pulmonary resection, compared with 40% of historical control patients.

OST-HER2 (OST31-164), an off-the-shelf immunotherapy agent, exhibited statistically significant positive 2-year overall survival (OS) data in patients with recurrent, fully-resected, pulmonary metastatic osteosarcoma in a phase 2b trial (NCT04974008), according to a news release from the developer, OS Therapies Inc.1

Specifically, of 36 patients evaluable for efficacy, 27 (75%) were alive as measured from the most recent pulmonary resection, compared with 40% of historical control patients (P <.0001). Additionally, subgroup analyses revealed that 100% of patients with 12-month event-free survival (EFS) achieved 2-year OS vs 59% of patients who did not achieve EFS.

"The [OS] data in this non-randomized OST-HER2 study [are] encouraging as the results show it was safe and well-tolerated," Peter Anderson, MD, pediatric oncologist at Cleveland Clinic Children's Hospital and member of the scientific advisory board for OS Therapies, stated in the news release on the 2-year OS data.1 "New treatments are needed for treating metastatic osteosarcoma, so I am hopeful this could become an option in the near future."

Developers plan to hold an investor conference call on October 13, 2025, to discuss the data collected from the ongoing trial. Additionally, they plan to outline a regulatory path to conditional marketing authorization for OST-HER2 in the UK, US, and EU.

"We are now prospectively analyzing patient samples from the clinical trial to confirm that clinical outcomes are indeed correlated with immune system biomarker activation. We expect that data to be available to us in November 2025, in time for our meetings we expect to hold with each of these regulatory agencies in December 2025,” Robert Petit, MD, chief medical & scientific officer for OS Therapies, said in the news release.1 “We remain on track to file a conditional Marketing Authorization Application (MAA) to [The Medicines and Healthcare products Regulatory Agency] in December 2025, a biologics licensing application (BLA) under the Accelerated Approval Program to the FDA in January 2026, and a MAA to [the European Medicines Agency] in the first quarter of 2026."

OST-HER2 previously received Rare Pediatric Disease Designation (RPDD) from the FDA and fast track and orphan drug designations from the FDA and European Medicines Agency. Developers plan to submit a BLA to the FDA for OST-HER2 in osteosarcoma later this year.

Previous data presented at the 2025 MIB Factor Osteosarcoma Conference revealed that OST-HER2 produced an EFS rate of 35% (n = 14/40) among patients with fully resected lung metastatic osteosarcoma.2 Additionally, safety data revealed 13 patients experiencing serious adverse effects (SAEs), 7 of whom experienced treatment-associated SAEs (TSAEs). All TSAEs were grade 3, no grade 4 or 5 TSAEs were observed, and no TSAEs leading to study discontinuation occurred.

Patients ages 12 to 39 with histologically confirmed osteosarcoma and 1 or more instance of disease recurrence in the lungs in the open-label, multicenter study received OST-HER2 monotherapy every 3 weeks for 48 weeks.3 Four doses constituted a treatment cycle, with a cycle being 12 weeks in length. A dose of 1x109 colony-forming units (CFU) was given up until week 48 in the absence of disease progression, unacceptable toxicity, or other discontinuation criteria.

The primary end point of the trial was EFS. Secondary end points included OS, with safety observed as an additional prespecified end point.

Eligibility criteria for trial enrollment included a weight of at least 40 kg at diagnosis, an ECOG performance status of 0 to 2, adequate organ function, and full recovery from acute toxic effects of prior treatment. Grounds for exclusion from trial entrance included having clinically evident metastatic or recurrent disease, having concurrent pulmonary recurrence and local recurrence, having primary refractory disease with primary tumor progression, or having central nervous system or extramedullary disease involvement at the most recent instance of disease recurrence.

References

1. OS Therapies announces statistically significant positive final 2-Year overall survival data from phase 2b trial of OST-HER2 in the prevention or delay of recurrent, fully-resected, pulmonary metastatic osteosarcoma. News release. OST Therapeutics. October 10, 2025. Accessed October 10, 2025. https://tinyurl.com/3nzbtpky
2. OS Therapies presents statistically significantly positive 1-Year event free survival, overall survival and safety clinical data updates for OST-HER2 at the MIB Agents Factor Osteosarcoma Conference. News release. OST Therapeutics. June 30, 2025. Accessed October 10, 2025. https://tinyurl.com/ysmsw7pp
3. Osteosarcoma maintenance therapy with OST31-164 (OST-164-01). ClinicalTrials.gov. Updated January 22, 2025. Accessed October 10, 2025. https://tinyurl.com/mptma4ub