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A blood test can determine whether prostate cancer patients are likely to respond to drugs, scientists say.

Tests on 97 men, described in Science Translational Medicine, were able to tell whether tumours were already resistant to the drug abiraterone.

Doctors at the Institute of Cancer Research in London will now trial the test to see if it can extend lives.

Prostate Cancer UK said such a test would also avoid unnecessary side-effects for men.

Abiraterone is a potent drug able to shrink tumours, but only some men benefit.

Mutated tumours

Cancers can evolve resistance to drugs over time, so the team of scientists set about looking for evidence in the fragments of tumour DNA that float in the blood stream.

Abiraterone works by hitting the male hormone receptor on prostate cancer cells.

But the team discovered that mutations affecting the male hormone receptor stopped the drug from working.

The men were more than seven times more likely to respond to treatment if they did not have these mutations.

Dr Gerhardt Attard, from the Institute of Cancer Research, told the BBC News website: "We have identified a group that should not have abiraterone and another set who have great benefit.

"This is the first study in prostate cancer to predict which patients are going to respond, it's very compelling data to suggest we could have a test."

Image copyright SPL Image caption Prostate cancer can spread to bone (in green)

Only 17 men in the trial had tumours shrinking in response to the therapy.

Fifteen of them had normal male hormone receptor, and only two had the abiraterone-resistance mutation.

And those two, who seemingly responded, had only a short-lived effect and their cancer rapidly rebounded.

The researchers are now trailing the test in 600 men to see if those with abiraterone-resistant tumours would be better off being given chemotherapy straight away.

'Exciting time'

Dr Attard said several new drugs were being developed for advanced prostate cancer.

"It's a very exciting time in prostate cancer, but we do not know which treatments to give to which patient," he said.

Blood tests - also known as liquid biopsies - are seen as crucial for determining which patients will respond to therapy.

One of the key advantages they have is the ability to picks up mutations from every tumour throughout the body.

A traditional biopsy can test only the area that has been sampled.

Clock ticking

The drug abiraterone was rejected for use by the NHS in England because of the cost.

A test to determine who will respond could make the drug more cost-effective.

Dr Iain Frame, the director of research at the Prostate Cancer UK charity, said: "We know that a one-size-fits-all approach to treating prostate cancer doesn't work.

"When the clock is ticking for a man with advanced prostate cancer, finding out early that his treatment needs changing can not only save precious time, but can also help avoid unpleasant side-effects from a treatment that no longer works for him."

Dr Emma Smith, from Cancer Research UK, said: "If these important early results bear up in larger clinical trials it could lead to a test which would indicate which patients might benefit more from trying other therapies instead."

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