Vir Biotechnology and Astellas agreed to co-develop and co-commercialize VIR-5500, a PRO-XTEN masked T-cell engager targeting prostate-specific membrane antigen (PSMA). The upfront and near-term payments to Vir included $100 million and potential development, regulatory and sales milestones pushing total value past $1.7 billion. VIR-5500 is a Phase I masked CD3 T-cell engager designed to reduce off-target toxicity by remaining inactive until it reaches the tumor microenvironment. Vir will share proceeds with Sanofi for PRO-XTEN-derived economics and plans to leverage the mask technology across additional TCE candidates. The deal accelerates Vir’s pivot from infectious disease toward solid-tumor immuno-oncology and brings Astellas a clinic-stage, prostate-focused TCE with a tumor-activated safety design. Financial backing and partner scale aim to de-risk early development and broaden combination and indication strategies.