Astellas Pharma and Vir Biotechnology announced a co‑development and co‑commercialization agreement worth up to $1.7 billion for VIR‑5500, a PSMA‑targeted masked T‑cell engager (TCE) derived from Vir’s PRO‑XTEN platform. The program uses tumor‑masked TCE technology designed to limit off‑tumor activity and improve the therapeutic index in prostate cancer. VIR‑5500 is in early clinical development; the collaboration follows Vir’s broader pivot into oncology and leverages PRO‑XTEN’s masking approach to reduce systemic toxicity for potent TCEs. Vir retains additional PRO‑XTEN programs and expects the partnership to expand development resources and global reach for the PSMA program. The deal underlines Big Pharma interest in next‑generation masked TCEs and will be watched for clinical tolerability and efficacy readouts that validate the masking strategy.