Vir Biotechnology has dosed the first patient in one of three expansion cohorts in its ongoing Phase I clinical trial of VIR-5500 for metastatic prostate cancer.

The study is assessing the safety and efficacy of VIR-5500, a prostate-specific membrane antigen (PSMA)-targeted, PRO-XTEN dual-masked T-cell engager, as a monotherapy in late-line metastatic castration-resistant prostate cancer (mCRPC).

It is also evaluating VIR-5500 in combination with an androgen receptor pathway inhibitor (ARPI) in early-line mCRPC and metastatic hormone-sensitive prostate cancer (mHSPC).

The first monotherapy expansion cohort in late-line mCRPC has started enrolment based on favourable safety and preliminary anti-tumour activity demonstrated in monotherapy dose-escalation data.

Presented at the 2026 American Society of Clinical Oncology (ASCO) Genitourinary Cancers Symposium in February, these results led to the selection of a step-up dosing regimen of Q3W 800/2000/3500 µg/kg for the expansion cohort.

This stage will measure safety and efficacy endpoints, including Prostate-Specific Antigen response rate and objective response rate.

It will enrol patients with mCRPC who are refractory following multiple prior lines of therapy, including at least one second-generation androgen receptor pathway inhibitor and one taxane regimen, as well as standard-of-care radioligand-based therapy.

Dose-escalation of VIR-5500 in combination with enzalutamide continues in early-line mCRPC. Future combination dose-expansion cohorts are planned for early-line mCRPC and mHSPC, with pivotal Phase III trials anticipated in 2027.

VIR-5500, a bispecific PSMA and CD3 binding T-cell engager utilising PRO-XTEN masking technology, remains under investigation in this open-label, non-randomised Phase I trial.

Vir Biotechnology CEO Marianne De Backer said: “The initiation of the VIR-5500 expansion cohorts underscores the significant momentum behind this programme and the enthusiasm we are seeing across the clinical community.

“We are encouraged by the promising anti-tumour activity shown in the Phase I data announced earlier this year and look forward to collaborating with Astellas after closing of the transaction to explore VIR-5500’s potential to make a meaningful difference across the spectrum of metastatic prostate cancer.”

In July 2025, Vir Biotechnology dosed the first subject in the non-randomised Phase I trial to assess VIR-5525, the investigational dual-masked T-cell engager (TCE) that targets epidermal growth factor receptor (EGFR).