More than 12,000 men lose their lives to prostate cancer every year in the UK alone. But right now, researchers are making progress, and the latest results from a clinical trial have left leading experts describing them as "stunning."

A new experimental drug called VIR-5500 has shown remarkable results in an early-stage trial involving men with the most advanced form of prostate cancer. For patients who had already exhausted other treatment options, the findings offer some hope.

What Is VIR-5500 and How Does It Work?

VIR-5500 is a new type of immunotherapy - a treatment that uses your own immune system to fight cancer, rather than relying solely on chemotherapy or radiation.

It belongs to a class of drugs called T-cell engagers. T-cells are the immune system's frontline fighters, designed to seek out and destroy harmful cells. A T-cell engager acts as a bridge, connecting those T-cells directly to cancer cells so the immune system can mount a targeted attack.

The problem with previous T-cell engagers has been severe side effects. When they activate the immune system across the entire body, they can trigger dangerous inflammatory responses. This has made them very difficult to use for prostate cancer patients.

VIR-5500 takes a different approach. It comes with a built-in "cloaking device" that keeps it inactive as it travels through the body. It only switches on when it arrives at the tumour, where proteins in the cancer's immediate environment remove the mask and activate the drug. We can think of it as a guided missile that only arms itself when it reaches the target. This not only reduces side effects, it also allows the drug to stay in the bloodstream longer, which may mean patients need fewer doses.

What Did the Trial Find?

The Phase 1 trial, funded by Vir Biotechnology and led by Professor Johann de Bono of the Institute of Cancer Research and the Royal Marsden NHS Foundation Trust, enrolled 58 men with advanced prostate cancer who had stopped responding to other treatments. Results were presented at the American Society of Clinical Oncology (ASCO) Genitourinary Cancers Symposium in San Francisco in February 2026.

The findings were striking. Among the 17 men given the highest doses:

• 82% saw their PSA levels fall by at least half - a key indicator that the cancer is responding
• 53% saw PSA levels drop by 90% or more
• 29% saw a fall of 99% or more

Of the 11 patients in the highest dose group whose tumours could be measured on scans, 5 showed tumour shrinkage. In one particularly notable case, a 63-year-old man whose cancer had spread to his liver saw 14 cancerous liver lesions completely resolve after 6 cycles of treatment.

Crucially, 88% of patients experienced only very mild side effects - a significant achievement given that prostate cancer has traditionally been considered resistant to immunotherapy, described by researchers as "immune-cold."

"We do need more data, but the results are stunning," said Professor de Bono. "We believe that such treatments may in the long term lead to cures."

What Comes Next?

These results have not yet been peer-reviewed, and this was an early-phase trial primarily designed to assess safety and dosing. Larger trials are now being planned.

Researchers are also exploring whether VIR-5500 could be combined with existing treatments such as chemotherapy or hormone therapy for even stronger results. The masked T-cell engager approach is also showing early promise in trials for other cancers, including pancreatic, colorectal and lung cancers.

Simon Grieveson, Assistant Director of Research at Prostate Cancer UK, called the results "extremely promising," adding: "I look forward to seeing this now tested in larger trials, with the hope that this treatment will offer men more valuable time with their loved ones."

Early Detection Remains the Most Powerful Tool We Have

Breakthroughs like this give men with advanced prostate cancer reasons for hope. But the most powerful thing any man can do right now is not wait until the disease reaches this stage.

Prostate cancer caught early, before it spreads, has a near 100% survival rate. A simple PSA blood test can detect warning signs years before symptoms appear. It is quick, straightforward, and could make all the difference.

GFCT runs affordable PSA testing events across the UK. If you are a man over 40, or have a family history of prostate cancer, please don't put it off.

Book your PSA test at mypsatests.org.uk

Sources: Professor Johann de Bono, Institute of Cancer Research / Royal Marsden NHS Foundation Trust; Simon Grieveson, Prostate Cancer UK; ASCO GU Symposium, San Francisco, February 2026; The Guardian; Black Enterprise; Sunday Guardian Live; The Independent