Vir Biotechnology, Inc. (NASDAQ:VIR) today announced new data from the ongoing Phase 1 clinical trial of VIR-5500, a prostate-specific membrane antigen (PSMA)-targeting, PRO-XTEN® dual-masked T-cell engager (TCE) being evaluated in patients with advanced metastatic castration-resistant prostate cancer (mCRPC) who have progressed after multiple lines of therapy (NCT05997615). These data suggest that VIR-5500 monotherapy is well tolerated and exhibits promising anti-tumor activity. Data will be presented in an oral presentation at the 2026 American Society of Clinical Oncology (ASCO) Genitourinary Cancers Symposium on February 26 in San Francisco, CA (Oral Abstract #17).

"We are encouraged by VIR-5500's safety and tolerability profile and the early signals of durable anti-tumor activity in a heavily pre-treated population, which validate our PRO-XTEN® masking strategy aimed at achieving a differentiated therapeutic index," said Marianne De Backer, M.Sc., Ph.D., MBA, Chief Executive Officer, Vir Biotechnology. "Based on these data, we are advancing dose-expansion cohorts and plan to initiate our registrational trial in 2027. We want to thank the patients in our Phase 1 program and their families for participating in the development of VIR-5500."

Data across all patients receiving VIR-5500 monotherapy in the Phase 1 trial (n=58) show that VIR‑5500 was generally well tolerated with no dose‑limiting toxicities (DLTs) observed to date. Grade ≥3 treatment‑related adverse events occurred in 12% (7/58) of patients and were manageable. Limited cytokine release syndrome (CRS) was observed in 50% (29/58) of patients, with events generally restricted to Grade 1 (fever only). Prophylactic steroids were not required and were only explored in a small cohort of three patients. Enrolled patients were heavily pre-treated (median of four prior lines) and a substantial proportion presented with high tumor burden, including nearly one half with visceral metastases.

Dose‑dependent activity was observed across the entire treatment group as measured by both prostate-specific antigen (PSA) declines and radiographic responses. Efficacy data were reported in the highest dose cohorts (≥3,000 µg/kg Q3W; n=22/58) as of the January 9, 2026 data cut-off. In these cohorts, PSA50 declines occurred in 82% (14/17) and PSA90 declines in 53% (9/17) of PSA-evaluable patients. Among RECIST (Response Evaluation Criteria in Solid Tumors)‑evaluable patients, objective responses were seen in 45% (5/11). Of the five responders, four achieved confirmed responses with one patient pending confirmation. Reductions on PSMA‑PET (positron emission tomography) affirm PSA declines and radiographic responses, with tumor shrinkage observed across multiple lesions, including visceral metastases. These findings support proof‑of‑concept and further evaluation in expansion cohorts.

"It is remarkable to see these early signs of profound anti-tumor activity in heavily pre-treated mCRPC patients, and the favorable tolerability with minimal CRS to date means VIR-5500 could play a role in treating earlier disease," said Dr. Johann de Bono, Principal Investigator and Director of the Drug Development Unit and Head of Prostate Cancer Targeted Therapy Group at the Institute of Cancer Research. "For patients with metastatic prostate cancer who have long faced limited treatment choices, VIR-5500 may offer a renewed sense of hope and a potential path to better outcomes."

Vir Biotechnology has concluded QW and Q3W monotherapy dose-escalation in late-line mCRPC and has defined a preliminary go-forward dose and regimen recommendation for expansion. In parallel, dose-escalation of VIR-5500 in combination with enzalutamide continues in early-line mCRPC patients. The Company anticipates initiating monotherapy dose-expansion cohorts in late-line mCRPC and combination dose-expansion cohorts in both early-line mCRPC and metastatic hormone-sensitive prostate cancer (mHSPC) in the second quarter of 2026 followed by pivotal Phase 3 trials in 2027.