An experimental drug called VIR-5500 has produced striking early results in men with advanced prostate cancer.
Researchers reported tumour shrinkage and major biomarker reductions in patients who had exhausted other treatments.
Prostate cancer affects about 1.5 million men worldwide each year.
Immunotherapy has transformed care for several cancers but has rarely worked in this disease.
VIR-5500 uses engineered antibodies to link killer T-cells directly to tumour cells.
The drug activates mainly inside the tumour, which reduces harmful immune reactions.
Its longer circulation in the bloodstream may also limit dosing frequency.
In the phase one trial, 58 men received the treatment.
Most experienced only mild side effects.
At the highest dose, 82% saw their PSA levels fall by at least half.
More than half recorded a drop of 90% or greater.
Doctors measured tumours in a smaller group at the top dose.
Nearly half of those patients showed visible tumour shrinkage.
One man with liver metastases saw all detectable lesions disappear after six cycles.
Researchers described the findings as unprecedented for a cancer long considered resistant to immunotherapy.
They are now planning larger trials to confirm safety and effectiveness.
Independent experts called the results highly encouraging but urged broader studies.
They stressed the need to include diverse patient groups and longer follow-up.
Scientists hope this approach could open a new treatment class and extend survival for men with advanced disease.
