A new drug for prostate cancer is producing results so striking that researchers are struggling to find ordinary words for them.
'Remarkable.' 'Stunning.' 'Unprecedented.'
These were the words used by scientists presenting early-stage results for VIR-5500 at the 2026 American Society of Clinical Oncology Genitourinary Cancers Symposium in San Francisco — a Phase 1 clinical trial for men with advanced metastatic castration-resistant prostate cancer (mCRPC), a form of the disease that had stopped responding to every other treatment available.
Prostate cancer has historically been resistant to immunotherapy. The immune system's T cells, which are natural killers of tumour cells, often fail to recognise or attack prostate cancer effectively. Previous attempts to use immunotherapy drugs that were effective in other cancers — such as melanoma or lung cancer — showed little benefit in prostate cancer patients.
VIR-5500 is designed differently. It's what scientists call a dual-masked T-cell engager: a molecule engineered to remain 'cloaked' and inactive in healthy tissue, only unlocking its tumour-killing power when it reaches the right environment inside the cancer. Think of it as a guided missile that stays inert until it reaches its target. This precision approach is the key to avoiding the dangerous inflammatory overreactions — cytokine release syndrome — that have plagued earlier T-cell engaging drugs.
'The cloaking device technology allows the drug to activate specifically within the tumour environment,' researchers from the Institute of Cancer Research explained.
The trial enrolled 58 men with advanced disease who had already progressed through multiple lines of therapy. The results at the highest dose were striking.
Of the 11 patients at the highest dose with evaluable tumour response data, 45% saw tumour shrinkage. In the broader highest-dose cohort of 17 patients, 82% experienced their PSA levels — a key marker of disease activity — halved. Fifty-three percent experienced a 90% drop in PSA. One 77-year-old man achieved completely undetectable PSA levels after 17 treatment cycles.
But the case that has captured the most attention is that of a 63-year-old man with liver metastases — cancer that had spread to his liver and formed 14 separate lesions. After six cycles of VIR-5500, every single one of those lesions had completely resolved. Gone.
'Complete resolution of 14 cancerous liver lesions.' Those are not words that appear often in oncology reports.
Tolerance was equally encouraging. Eighty-eight percent of patients experienced only very mild side effects. No dose-limiting toxicities have been reported. The cloaking mechanism appears to be working exactly as designed.
Vir Biotechnology has since announced a major global collaboration with Japanese pharmaceutical giant Astellas to advance and commercialise VIR-5500, with Astellas leading commercialisation in the U.S. and holding exclusive rights internationally. A registrational trial — the definitive study needed for regulatory approval — is planned for 2027.
Prostate cancer is the most common cancer in men in many countries and the second leading cause of cancer death in men. Historically, immunotherapy simply didn't work for it. These results suggest that barrier may be falling.
For the men who have run out of other options, a drug that stays hidden until it reaches the target — and then clears 14 liver lesions — is not just promising. It's the kind of result that changes what hope looks like. 💙