Vir Biotechnology Unveils Promising Phase 1 Data for Novel Prostate Cancer Therapy
New clinical trial results for VIR-5500, an investigational T-cell engager targeting prostate-specific membrane antigen (PSMA), demonstrate a favorable safety profile and encouraging signs of anti-tumor activity in patients with advanced metastatic castration-resistant prostate cancer (mCRPC). The data, presented at the 2026 American Society of Clinical Oncology (ASCO) Genitourinary Cancers Symposium, suggest VIR-5500 could offer a new therapeutic avenue for a patient population with limited treatment options.
In the Phase 1 dose-escalation study, which included 58 patients, VIR-5500 monotherapy was generally well-tolerated, with no dose-limiting toxicities observed. Treatment-related adverse events of Grade 3 or higher occurred in 12% of patients and were manageable. Cytokine release syndrome (CRS), a common concern with T-cell engagers, was limited, with most events being Grade 1 and characterized solely by fever. The data also indicated that prophylactic steroids were not required for most patients, further underscoring the therapy’s tolerability.
The study enrolled heavily pre-treated patients, with a median of four prior lines of therapy and many presenting with high tumor burden and visceral metastases. Despite this challenging patient profile, dose-dependent anti-tumor activity was observed. In the higher dose cohorts (≥3,000 µg/kg every three weeks), significant prostate-specific antigen (PSA) declines were noted, with 82% of patients achieving at least a 50% reduction (PSA50) and 53% achieving at least a 90% reduction (PSA90). Radiographic responses, as measured by RECIST criteria, were also notable, with an objective response rate (ORR) of 45% in evaluable patients. Positron emission tomography (PET) imaging with PSMA tracers further supported these findings, showing tumor shrinkage across multiple lesions, including in visceral metastases.
“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,” stated Marianne De Backer, M.Sc., Ph.D., MBA, Chief Executive Officer of Vir Biotechnology. “Based on these data, we are advancing dose-expansion cohorts and plan to initiate our registrational trial in 2027.”
The PRO-XTEN® technology employed in VIR-5500 is designed to mask the T-cell engager until it reaches the tumor microenvironment. This targeted activation aims to minimize systemic exposure and associated toxicities, potentially leading to an improved therapeutic window compared to conventional T-cell engagers. This masking approach, combined with the dual-targeting of PSMA and CD3, represents a novel strategy in the development of prostate cancer therapies.
Dr. Johann de Bono, Principal Investigator and Director of the Drug Development Unit at the Institute of Cancer Research, commented on the findings, saying, “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. 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 is planning to initiate 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. Pivotal Phase 3 trials are anticipated to commence in 2027. The company is also evaluating VIR-5500 in combination with enzalutamide in early-line mCRPC patients.
**Understanding the Landscape of Advanced Prostate Cancer**
Prostate cancer remains a significant global health challenge. While advancements have been made, patients with advanced disease, particularly mCRPC, continue to face substantial unmet medical needs. mCRPC is characterized by cancer that has spread to other parts of the body and no longer responds to hormone therapies designed to lower testosterone levels. This stage of the disease is associated with poor outcomes and limited effective treatment options, highlighting the critical need for novel and more effective therapies.
The development of PSMA-targeted therapies, including imaging agents and therapeutics, has been a major focus in recent years. PSMA, a protein that is often overexpressed on prostate cancer cells, serves as a valuable biomarker for both diagnosis and treatment. PSMA-PET scans, for instance, have revolutionized the staging and management of prostate cancer by providing highly sensitive visualization of disease spread. T-cell engagers represent another promising class of immunotherapy designed to harness the patient’s own immune system to combat cancer. VIR-5500 integrates these approaches, aiming for a more precise and potent anti-cancer effect.
Vir Biotechnology is a clinical-stage biopharmaceutical company dedicated to developing medicines for serious infectious diseases and cancer. Its pipeline includes programs targeting chronic hepatitis delta and multiple T-cell engagers for solid tumors.
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