Pre-clinical validation of OSCAR-3 mRNA-based CAR T cells for targeting Osteosarcomas

Osteosarcoma (OS) is a rare cancer affecting children and young adults with a good prognosis when localised. Nevertheless, primary metastases or metastatic relapse are associated with high mortality, underlying the need for novel treatment strategies. Immunotherapy, and more recently Chimeric Antigen Receptor (CAR)-based therapy, is being investigated as a new approach for the management of solid tumours, although clinical results in OS have so far been limited. We developed a second-generation CAR (OSCAR-3), derived from the TP-3 hybridoma and targeting ALPL-1, an isoform of alkaline phosphatase selectively expressed in OS. Stable OSCAR-3 expression in T cells demonstrated potent antitumour activity in preclinical models. To enable safer clinical translation, we further engineered OSCAR-3 as an mRNA-based CAR. OSCAR-3 mRNA CAR T cells maintained cytotoxic activity in vitro and in vivo , and, importantly, delayed tumour progression in OS patient-derived xenograft models. These findings support the further development of OSCAR-3 mRNA CAR T cells as a strategic approach that prioritizes safety over persistence for first-in-human studies, while preserving the ability to reduce tumour growth.