Scientists have developed a three–in–one blood test that could transform treatment of advanced prostate cancer through use of precision drugs designed to target mutations in the BRCA genes.

By testing cancer DNA in the bloodstream, researchers found they could pick out which men with advanced prostate cancer were likely to benefit from treatment with exciting new drugs called PARP inhibitors.

They also used the test to analyse DNA in the blood after treatment had started, so people who were not responding could be identified and switched to alternative therapy in as little as four to eight weeks.

And finally, they used the test to monitor a patient’s blood throughout treatment, quickly picking up signs that the cancer was evolving genetically and might be becoming resistant to the drugs.

The researchers, at The Institute of Cancer Research, London, and The Royal Marsden NHS Foundation Trust, say their test is the first developed for a precision prostate cancer therapy targeted at specific genetic faults within tumours.

It could in future allow the PARP inhibitor olaparib to become a standard treatment for advanced prostate cancer, by targeting the drug at the men most likely to benefit, picking up early signs that it might not be working, and monitoring for the later development of resistance.

The study was published in the journal Cancer Discovery.

The test could help to extend or save lives, by targeting treatment more effectively, while also reducing the side–effects of treatment and ensuring patients don’t receive drugs that are unlikely to do them any good.

The new study is also the first to identify which genetic mutations prostate cancers use to resist treatment with olaparib. The test could potentially be adapted to monitor treatment with PARP inhibitors for other cancers.

Researchers at the ICR and The Royal Marsden collected blood samples from 49 men at The Royal Marsden with advanced prostate cancer enrolled in the TOPARP–A phase II clinical trial of olaparib.

Olaparib is good at killing cancer cells that have errors in genes that have a role in repairing damaged DNA such as BRCA1 or BRCA2. Some patients respond to the drug for years, but in other patients, the treatment either fails early, or the cancer evolves resistance.

Looking at the levels of cancer DNA circulating in the blood, the researchers found that patients who responded to the drug had a median drop in the levels of circulating DNA of 49.6 per cent after only eight weeks of treatment, whereas cancer DNA levels rose by a median of 2.1 per cent in patients who did not respond.

Men whose blood levels of DNA had decreased at eight weeks after treatment survived an average of 17 months, compared with only 10.1 months for men whose cancer DNA levels remained high.

The researchers also performed a detailed examination of the genetic changes that occurred in cancer DNA from patients who had stopped responding to olaparib. They found that cancer cells had acquired new genetic changes that cancelled out the original errors in DNA repair – particularly in the genes BRCA2 and PALB2 – that had made the cancer susceptible to olaparib in the first place.