Mifamurtide for Osteosarcoma in the United States: A Single-Center Real-World Experience

Purpose: Mifamurtide (liposomal muramyl tripeptide phosphatidylethanolamine [L-MTP-PE] is approved in many countries for osteosarcoma but remains unavailable for routine use in the United States (US). We describe the feasibility and outcomes of patients with osteosarcoma treated with mifamurtide at Memorial Sloan Kettering Cancer Center (MSK) through expanded access (EAP) and compassionate use (SPU) pathways. Methods: Medical records were retrospectively reviewed for patients with completely resected, high-grade osteosarcoma treated with mifamurtide at MSK from 2016-2026 through EAP or SPU protocols. Mifamurtide 2 mg per meter squared was administered intravenously twice weekly for 12 weeks, followed by weekly administration for 24 weeks. Outcomes were evaluated by treatment setting and, for upfront therapy, metastatic status and histologic necrosis. Results: Fifty-seven patients received mifamurtide (median age, 14 years; range, 2-60); 72% had localized and 28% metastatic disease at diagnosis. No serious adverse events were considered possibly or probably related to mifamurtide, and no new grade 3-4 safety signals emerged. Five patients with localized disease and [&ge;]90% necrosis at resection had no events, with 100% progression-free survival (PFS) and overall survival (OS) at median follow-up of 29 months. Among 22 patients with localized disease and <90% necrosis treated upfront, PFS was 82%, 66%, and 59% at 12, 24, and 48 months, respectively; OS was 95% at 12, 24, and 60 months. Among 8 patients with metastatic disease treated upfront, PFS was 73%, 58%, and 39% at 12, 24, and 36 months, respectively; OS was 100%, 86%, and 86%. Among 22 patients treated after relapse, PFS was 34%, 23%, and 17% at 12, 24, and 36 months, respectively; OS was 91%, 70%, and 35% at 12, 24, and 60 months. Conclusions: In this real-world US cohort, mifamurtide was feasible and safe to administer. Despite limitations of the EAP/SPU setting, single-institution administration, and patient heterogeneity, overall survival outcomes were encouraging.