A new pill offers the potential of effective personalised treatments for patients with inherited breast and ovarian cancers.
The drug, olaparib, was found in proof-of-concept trials to shrink tumours that had resisted several rounds of earlier chemotherapy.
It works by specifically attacking cancer cells containing defective BRCA1 or BRCA2 genes.
Women who inherit one of the abnormal genes have about a 60pc risk of developing breast cancer during their lifetime.
Defective BRCA1 and BRCA2 genes also account for about 10pc of ovarian cancers, and in men BRCA2 is believed to increase the risk of early-onset prostate cancer.
Until now, knowing that a woman has a BRCA mutation has not affected the choice of her cancer therapy. But olaparib, and other drugs like it, raise the future possibility of treatments tailored to suit such patients.
The drug blocks an enzyme called poly (ADP-ribose) polymerase (PARP) that helps cells with faulty BRCA genes stay alive.
In the trials, an international team of researchers studied two groups of breast and ovarian cancer patients with BRCA mutations for six months.
More than 100 women were given either a low dose 100 milligram pill or a high dose 400 milligram pill twice a day. The higher dose was most effective in both studies.
Among breast cancer patients, taking the 400mg pill led to more than 40pc of tumours reducing significantly in size and not growing for the length of the trial. Ovarian cancer patients on the higher dose had a similar outcome, with a third of their tumours shrinking. In both cases, patients suffered relatively minor side effects such as fatigue and nausea.
Lead researcher Dr Andrew Tutt, director of the Breakthrough Breast Cancer Research Unit at King's College London, said: "This new type of treatment is showing great promise for patients whose cancer is caused by this specific genetic fault.
"It was remarkable to see that olaparib benefited women with advanced breast and ovarian cancer who had already been treated with several different chemotherapy drugs.
"However, it is important to remember this drug is at an early stage of development."