Astellas Pharma and Vir Biotechnology agreed to co‑develop and co‑commercialize VIR‑5500, a PRO‑XTEN masked CD3 T‑cell engager targeting PSMA, under a deal potentially worth more than $1.7 billion to Vir. VIR‑5500 is in Phase I and uses a masking technology designed to keep T‑cell engaging activity quiescent until exposure to the tumor microenvironment. Vir will receive up‑front and near‑term payments and remain eligible for development, regulatory and sales milestones; the company will share a portion of proceeds with Sanofi based on prior PRO‑XTEN licensing terms. Vir represents this as a continuation of its pivot into oncology from infectious disease, expanding a PRO‑XTEN portfolio that includes VIR‑5525 and VIR‑5818. Markets reacted favorably: Vir’s shares and analyst commentary cited validation of the PRO‑XTEN approach and early clinical signals. Marianne De Backer, Vir’s CEO, described the mask as “plug‑and‑play,” highlighting the platform’s potential to apply a common masking strategy across multiple targets.