An experimental drug called VIR-5500 has produced encouraging results in men with advanced prostate cancer.
Researchers reported tumour shrinkage and major biomarker reductions in an early clinical trial.
Prostate cancer is the most common cancer in men in many countries.
About 1.5 million people receive a diagnosis worldwide each year.
VIR-5500 is an engineered antibody that redirects killer T-cells toward tumour cells.
The drug activates mainly inside the tumour, which helps limit harmful immune reactions.
This design may also allow longer circulation in the blood and fewer treatments.
Fifty-eight men whose disease no longer responded to standard therapies received the drug.
Most participants experienced only mild side effects.
At the highest dose, prostate-specific antigen levels fell by at least half in most patients.
More than half saw reductions of ninety percent or more.
Nearly one-third recorded falls of at least ninety-nine percent.
Among patients with measurable disease at that dose, almost half showed tumour shrinkage.
One man with cancer spread to the liver had all detectable lesions disappear after treatment.
Researchers called the findings unprecedented for a cancer long considered resistant to immunotherapy.
They are now planning larger trials to confirm effectiveness and safety.
Independent experts described the results as highly promising but stressed the need for broader studies.
They also urged inclusion of patients from diverse ethnic backgrounds.
Scientists hope this new approach could extend survival and eventually lead to cures for advanced disease.
