On April 13, 2026 Vir Biotechnology, Inc. (Nasdaq: VIR) reported that the first patient has been dosed in one of three expansion cohorts in the Phase 1 trial evaluating VIR-5500, a prostate-specific membrane antigen (PSMA)-targeted, PRO-XTEN dual-masked T-cell engager (TCE) for metastatic prostate cancer (NCT05997615). The Phase 1 trial is measuring the safety and efficacy of VIR-5500 monotherapy in late-line mCRPC, and of VIR-5500 in combination with an androgen receptor pathway inhibitor (ARPI) in early-line mCRPC and mHSPC.

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"The initiation of the VIR-5500 expansion cohorts underscores the significant momentum behind this program and the enthusiasm we are seeing across the clinical community," said Marianne De Backer, M.Sc., Ph.D., MBA, Chief Executive Officer, Vir Biotechnology. "We are encouraged by the promising anti-tumor activity shown in the Phase 1 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."

The monotherapy expansion cohort in late-line mCRPC is the first to begin enrollment based on the monotherapy dose-escalation data that showed VIR‑5500 has a favorable safety profile and promising anti-tumor activity in mCRPC. These safety and efficacy data were presented at the 2026 American Society of Clinical Oncology (ASCO) (Free ASCO Whitepaper) Genitourinary Cancers Symposium in February. Based on these data, the selected dose regimen to be evaluated in this monotherapy expansion cohort is Q3W 800/2000/3500 µg/kg step-up dosing. This expansion cohort will measure safety and efficacy, including Prostate-Specific Antigen (PSA) response rate and Objective Response Rate (ORR) in patients with mCRPC who are refractory following treatment with multiple prior lines of therapy including at least one second-generation ARPI and one taxane regimen in addition to standard-of-care radioligand-based therapy.

"Building on the encouraging Phase 1 dose-escalation monotherapy data, this milestone represents an important step in further evaluating VIR-5500’s best-in-class potential," said Anthony Jarkowski, Primary Focus Lead, Immuno-oncology, Astellas. "We look forward to starting our collaboration with Vir Biotechnology to potentially benefit more people living with prostate cancer, where there remains a significant unmet medical need."

Dose-escalation of VIR-5500 in combination with enzalutamide continues in early-line mCRPC patients. The Company anticipates dosing of first patients in the combination dose-expansion cohorts in both early-line mCRPC and mHSPC over the coming months, followed by pivotal Phase 3 trials in 2027.

About Advanced Prostate Cancer

Prostate cancer remains a significant global health burden, representing the second leading cause of cancer-related mortality in men behind lung cancer.1 While diagnostic and therapeutic advances like androgen-directed therapy can improve outcomes in earlier settings, most patients ultimately relapse and develop metastatic hormone sensitive prostate cancer (mHSPC).2 mHSPC is characterized by its responsiveness to intensified hormonal interventions designed to reduce androgen levels or block their action. The majority of these patients eventually progress to metastatic castration-resistant prostate cancer (mCRPC).3 This stage is associated with poor clinical outcomes, including limited durability of existing therapies, with a 5-year survival rate of approximately 30%.4 There is a critical need for safer, more effective, and precisely targeted therapies capable of improving long term disease control and quality of life across the prostate cancer continuum.

About VIR-5500

T-cell engagers (TCEs) are powerful anti-tumor agents that can direct the immune system, specifically T-cells, to destroy cancer cells. VIR-5500 is an investigational PRO-XTEN dual-masked TCE currently being evaluated in an open-label, non-randomized Phase 1 clinical trial (NCT05997615) designed to assess the safety, pharmacokinetics and preliminary efficacy in participants with metastatic castration-resistant prostate cancer (mCRPC). VIR-5500 is the only dual-masked PSMA-targeting TCE in clinical evaluation.

VIR-5500 combines a bispecific PSMA and CD3 binding TCE with the PRO-XTEN masking technology. The PRO-XTEN masking technology is designed to keep the TCEs inactive (or masked) until they reach the tumor microenvironment, where tumor-specific proteases cleave off the mask and activate the TCEs, leading to killing of cancer cells by T-cells. By confining the activity to the tumor microenvironment, we aim to circumvent the traditionally high toxicity associated with unmasked TCEs and increase their efficacy and tolerability. Additionally, the mask is designed to help drug candidates stay in the bloodstream longer in their inactive form, allowing them to better reach the site of action and potentially allowing for less frequent dosing regimens.

(Press release, Vir Biotechnology, APR 13, 2026, View Source [SID1234664336])