A gene-targeting pill for women may provide a highly effective treatment for up to a third of men with advanced prostate cancer, trial results have shown.
Olaparib, the first marketed drug to tackle inherited cancer mutations, was licensed last year for women with ovarian cancer who have faulty BRCA genes.
The new trial, described as a "milestone" by the Institute of Cancer Research, found it could also halt tumour growth in many men with treatment-resistant prostate cancer.
Of the 49 men taking part, a third responded to the drug. Cancers stopped growing, numbers of circulating tumour cells in the blood fell, and clinically positive results were obtained from scans.
Levels of PSA - the blood marker used to track the progress of prostate cancer - also plunged by up to 96%.
Chief investigator Professor Johann de Bono, from the Institute of Cancer Research in London, 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.
"I hope it won't be long before we are using olaparib in the clinic to treat prostate cancer, or before genomic stratification of cancers becomes a standard in this and other cancers."
Sixteen patients had detectable faults in genes that play a role in repairing damaged DNA, including BRCA 1 and 2. Of these, 14 responded very well to olaparib.
A second stage of the trial is now planned which will only recruit men with prostate cancer who have similar detectable gene defects.
If this study also yields positive results, olaparib could become a new treatment option for men with genetically-driven advanced prostate cancer, say the researchers.
However it remains to be seen whether men will qualify for the drug on the NHS due to its high cost. The drug has a list price of £4,740 per month, according to NHS England.
Currently olaparib is not approved for use on the NHS by the National Institute for Health and Care Excellence (Nice) or the Cancer Drugs Fund. Nice is still in the process of considering whether to recommend the drug for women with ovarian cancer.
Women with breast cancer and defective BRCA genes are also known to respond to olaparib.
The new findings are published in the New England Journal of Medicine.
Each year more than 41,700 men are diagnosed with prostate cancer in the UK and nearly 11,000 die from the disease.
In 84% of cases, patients live 10 or more years, but once the disease has spread and stopped responding to hormone treatments the chances of survival are much slimmer.
Dr Aine McCarthy, science information officer at Cancer Research UK, one of a number of organisations that contributed funding for the trial, said: "This trial is exciting because it could offer a new way to treat prostate cancer by targeting genetic mistakes in cancers that have spread.
"The hope is that this approach could help save many more lives in the future."
Dr Iain Frame, director of research at Prostate Cancer UK, which also provided funding, said: "Sometimes we find answers to research questions from unexpected sources.
"These results demonstrate just how much we can benefit from the success of treatments for other diseases and it's exactly the type of research we want to see more of.
"Although it's still early days, this drug has already been approved for use for other cancers, so it's more likely it will get into the hands of the men who need it more quickly if it's proven to be successful in further trials."
The research also received support from the Movember Foundation, which raises funds and cancer awareness by promoting the growth of moustaches.