The drug could 'vaccinate' women against the disease
If given regularly to those with a strong family history of the cancer, researchers say it could effectively "vaccinate" them against a disease they are almost certain to develop.
The drug, which attacks tumours caused by genetic flaws, could spare those who have the rogue genes the trauma of having their breasts removed.
Currently, a high proportion of women told they have inherited the rogue genes choose to have a mastectomy as a preventative measure.
Researchers hope such a "vaccine" will be available within a decade. Flawed BRCA genes, which are passed from mother to daughter, are responsible for around 2,000 of the 44,000 cases of breast cancer each year in the UK.
Women with the rogue genes have an 85 per cent chance of developing the disease - eight times that of the average woman.
Initial tests suggest that the drug, known only as AGO14699, could also be free of the side-effects associated with other cancer treatments, including pain, nausea and hair loss.
The drug, which is being tested on patients in Newcastle upon Tyne, works by exploiting the "Achilles' heel" of hereditary forms of breast cancer - which is its limited ability to repair damage to its DNA.
Normal cells have two ways of fixing themselves, allowing them to grow and replicate, but cells in BRCA tumours have only one.
The drug, which is part of the class of anti-cancer medicines called PARP inhibitors, blocks this mechanism and stops the tumour cells from multiplying.
The researchers say the drug could also be used against other forms of cancer, including prostate and pancreatic, although further tests are needed.
Researcher Dr Ruth Plummer, senior lecturer in medical oncology at Newcastle University, said: "The implications for women and their families are huge because if you have the gene, there is a 50 per cent risk you will pass it on to your children. You are carrying a time bomb."