Vir Biotechnology announced that the US Food and Drug Administration (FDA) has granted Fast Track designation to VIR-5500, an investigational prostate-specific membrane antigen (PSMA)-targeted, PRO-XTEN dual-masked T-Cell Engager (TCE), for the treatment of late-line metastatic Castration-Resistant Prostate Cancer (mCRPC).

VIR-5500 is being co-developed by Vir Biotechnology and Astellas Pharma under a global strategic collaboration. The Fast Track designation is intended to facilitate the development and review of therapies targeting serious conditions with unmet medical needs.

The designation comes as the companies prepare to advance VIR-5500 into pivotal Phase 3 development, which is anticipated to begin in 2027. Astellas also highlighted the need for new treatment options that could provide meaningful and durable benefits for people with advanced prostate cancer.

VIR-5500 is currently being evaluated in an open-label, non-randomised Phase 1 clinical trial designed to assess its safety, pharmacokinetics and preliminary efficacy in patients with mCRPC. The study includes six dose-expansion cohorts covering monotherapy and combination treatment approaches.

The monotherapy cohorts include patients with mCRPC who have not received taxane chemotherapy, those who are radioligand therapy-naïve and those previously exposed to radioligand therapy. Combination cohorts are evaluating VIR-5500 with enzalutamide in early-line mCRPC, with docetaxel in early-line mCRPC, and with darolutamide in metastatic hormone-sensitive prostate cancer (mHSPC).

Phase 1 data reported earlier in 2026 showed that VIR-5500 was generally well tolerated and demonstrated preliminary anti-tumour activity in patients with mCRPC.

VIR-5500 combines a bispecific TCE that binds to PSMA on prostate cancer cells and CD3 on T cells with Vir Biotechnology’s PRO-XTEN masking technology. The technology is designed to keep the drug inactive until it reaches the tumour microenvironment, where tumour-associated proteases remove the mask and activate the T-cell engager. This mechanism aims to direct T cells towards cancer cells while limiting activity in healthy tissues.

The PRO-XTEN platform is also designed to extend the time the molecule remains in circulation in its inactive form, potentially improving tumour delivery and enabling less frequent dosing. The approach seeks to address toxicity challenges that have limited the application of T-cell engagers in solid tumours, although its clinical benefits and safety profile require further evaluation.

Prostate cancer remains a major cause of cancer-related mortality among men. Although hormonal treatments and other therapies can control the disease in earlier stages, many patients eventually progress to mCRPC, where treatment options may provide limited durable disease control. The development of targeted therapies with improved efficacy and tolerability remains an important area of research.

The FDA’s Fast Track designation supports closer engagement between the regulator and the developers during the drug development process. VIR-5500 remains investigational, and its safety and efficacy have not yet been established for regulatory approval.