The first of a new class of drugs for cancer is showing promise in patients with inherited forms of the disease, researchers report.

Called olaparib, the new drug comes in pill form and targets cancer cells caused by the faulty BRCA1 or BRCA2 genes, which affects around 1,500 of the nearly 46,000 women diagnosed with breast cancer in the UK each year.

Women who inherit one of the abnormal genes have about a 60 per cent risk of developing breast cancer. They also sharply increase the risk of ovarian cancer, and of other cancers including cervical, uterine and bowel cancer. Although only a small group of patients are affected by the BRCA genes, the drugs have been shown in the laboratory to have potential benefits for a wider range of cancer types that share similar characteristics.

Results from two early stage studies published in The Lancet show that olaparib significantly reduced the size of tumours in over 40 per cent of women with advanced breast cancer given the highest dose, and over 33 per cent of those with advanced ovarian cancer. In both cases tumours were prevented from progressing for an average of six months.

Andrew Tutt, director of the Breakthrough Breast Cancer Research Unit at King's College London, who led the breast cancer study, said: "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, this drug is at an early stage of development, and further clinical trials will be required."

In a separate study, researchers have found that men who carry a faulty copy of the BRCA2 gene have a one in 15 chance of developing breast cancer by the time they are 70. Breast cancer is rare in men, with about 300 cases in Britain each year compared with more than 45,000 among women. But the lifetime risk among men with a faulty BRCA2 gene is 8.4 per cent by the age of 80, the researchers say in the Journal of Medical Genetics.