The Institute of Cancer Research (ICR), London, carried out the study on Olaparib drug with the Royal Marsden NHS Foundation Trust and other funding bodies.
The drug was identified to benefit about a third of patients with prostate cancer, including many who did not inherit cancer genes but whose tumours had acquired defects in DNA repair.
Under the trial, 49 men with treatment-resistant, advanced prostate cancer received olaparib. 16 of them responded, as defined by a set of clinical criteria.
Researchers said Olaparib halted prostate cancer growth, generating lasting falls in prostate specific antigen (PSA) levels, falls in circulating tumour cell counts in the blood, and radiological responses on CT scans and MRI.
The clinical trial found that about 30% of men with advanced prostate cancer had tumours with defects in their systems for repairing DNA detected by genomic testing.
Trial chief investigator Johann de Bono, head of drug development at the ICR and the Royal Marsden, said: "Our trial marks a significant step forward in the treatment of prostate cancer, showing that olaparib is highly effective at treating men with DNA repair defects in their tumours.
"It also proves the principle that we can detect prostate cancers with specific targetable mutations using genomic sequencing to deliver more precise cancer care by matching treatment to those men most likely to benefit.
The researchers plan to confirm the findings in a second trial, which will test the theory of precision medicine for prostate cancer patients further.