Astellas Pharma and Vir Biotechnology agreed to co-develop and co-commercialize Vir’s PRO‑XTEN masked T‑cell engager targeting PSMA, in a collaboration that could deliver over $1.3 billion to Vir and up to $1.7 billion in combined payments depending on reports. The deal covers VIR‑5500, a Phase I dual‑masked CD3 T‑cell engager engineered to remain inactive until it reaches the tumor microenvironment, aiming to reduce off‑tumor toxicity while engaging cytotoxic T cells. Vir will advance the PRO‑XTEN platform’s clinical assets in prostate cancer and retain programs in other solid tumors; Astellas gains co‑development rights and commercialization support. Marianne De Backer, Vir’s CEO, framed the PRO‑XTEN mask as a platform-level safety lever that can be applied across multiple targets, signaling a broader industry push to make potent T‑cell engagers safer and more programmable. For drug developers, the pact represents continued pharma interest in modular masking strategies to expand the therapeutic index of T‑cell engagers and accelerate clinical development through large partner funding and commercialization infrastructure.