New Immunotherapy Breakthrough in Advanced Prostate Cancer Research

Advanced prostate cancer has historically been difficult to treat with immune-based approaches, often causing severe treatment-related side effects. However, a promising new immunotherapy drug is generating excitement among prostate cancer researchers, after early trial results showed positive results in men with advanced prostate cancer. The phase I trial results were shared during the American Society of Clinical Oncology (ASCO) Genitourinary Cancers Symposium last month, in February 2026.

The immunotherapy drug funded by Vir Biotechnology, known as VIR-5500, was tested in a phase I trial across 8 sites around the world. Led by the Institute of Cancer Research, London and the Royal Marsden MHS Foundation Trust, the trial tested the drug on 58 patients with advanced prostate cancer.

The treatment is a type of T-cell engager — a therapy that links cancer cells to T-cells, the immune system’s natural “killer” cells, prompting them to attack the tumor. While T-cell engagers have shown success in some blood cancers, they have struggled in solid tumors like prostate cancer, due to severe side effects caused by immune activation throughout the body.

This new drug incorporates a biochemical “invisibility cloak” that remains inactive while circulating in the bloodstream and becomes activated only when it reaches the tumor environment. This targeted design aims to reduce inflammation and damage to healthy tissue, which was a major limitation of earlier immune therapies for advanced prostate cancer.

Professor Johann de Bono, who led the study, described the approach as a potential breakthrough for prostate cancer, which is often resistant to immunotherapy.

Among the patients involved in the study, nearly half experienced measurable tumor shrinkage. At the highest dose tested, 82% of the participants saw their prostate-specific antigen (PSA) levels fall by at least 50%, and 53% experienced reductions of 90% or more.

In some cases, tumor regression was observed in metastatic sites, including the liver. Researchers reported that one patient with multiple liver metastases experienced complete resolution of 14 cancerous liver lesions after six treatment cycles.

Earlier T-cell engager treatments have been associated with serious immune-related side effects. However, in this study, 88% of patients did not experience severe treatment-related side effects typically seen with similar immunotherapies.

The cloaking design appears to allow the drug to circulate longer before activating at the tumor site, potentially reducing toxicity while maintaining anti-cancer potency.

Researchers emphasize that while the current results are positive and encouraging, these are early-phase results and the clinical trial is still ongoing. Larger trials will be needed to assess the drug’s impacts on patients’ long-term outcomes.