UPDATE: VIR-5500 Receives FDA Fast Track Designation Following Promising Phase 1 Results in mCRPC / mAPMR

The US Food and Drug Administration (FDA) has granted Fast Track designation to VIR-5500 (formerly AMX-500), an experimental immunotherapy for patients with heavily pretreated metastatic castration-resistant prostate cancer (mCRPC). The designation is intended to accelerate clinical development and regulatory review, but does not constitute drug approval.

VIR-5500 is a bispecific T-cell engager designed to direct immune cells against PSMA-expressing prostate cancer cells. Its dual-masking technology aims to keep the treatment largely inactive in the bloodstream, allowing it to become activated primarily within the tumour environment and potentially reducing systemic toxicity.

Updated phase 1 results from the ongoing trial showed encouraging activity. Among 17 patients receiving doses of at least 3,000 µg/kg every three weeks, 82% achieved a PSA reduction of at least 50% (PSA50), while 53% achieved a reduction of at least 90% (PSA90). Objective tumour responses were observed in 45% (5 of 11) of patients with measurable disease.

Regarding safety, 12% of the 58 evaluated patients experienced treatment-related adverse events of grade 3 or higher. Cytokine release syndrome occurred in 50%, although most cases were mild. No dose-limiting toxicities were reported.

The clinical programme is also expanding into earlier stages of metastatic prostate cancer, including combinations with enzalutamide, docetaxel and darolutamide. Phase 3 development is planned for 2027.

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