Campaigners claim the drug offers a 'precious lifeline' for women with the most aggressive form of the disease, who have tried other treatments.
Known as T-DM1, it combines the 'wonder' drug Herceptin with a potent chemotherapy agent, the Daily Mail reported.
T-DM1 is designed to seek out and destroy cancerous cells while sparing healthy tissue from unnecessary damage.
Results from a major trial show the drug prolonged the lives of patients with advanced HER2-positive breast cancer by 30.9 months compared with 25.1 months on standard therapy.
Paul Ellis, professor of cancer medicine at King's College London, said the trial results were remarkable in patients with advanced disease who had relapsed on existing treatment.
"T-DM1 contains an extremely potent form of chemotherapy that's been around 20 years which we haven't been able to use before because it's so toxic. Clever new technology has allowed these two older drugs to be linked so that the chemotherapy is not released until it reaches the target," Ellis was quoted as saying by the paper.
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The international trial recruited 991 patients, including mother-of-two Emma Barnes, 36, who has been battling HER-2 positive breast cancer for nine years.
"HER-2 positive breast cancer is very aggressive and once it progresses to the advanced stage it becomes very difficult to treat," Ellis said.
She joined the trial in May 2010 at the Christie Hospital Manchester and has been having infusions of T-DM1 every three weeks.
Although she became resistant to Herceptin after five years of treatment, the new drug has stopped any new secondary tumours.
T-DM1 seeks out and destroys cancerous cells in a two-stage attack. It attaches to the tumour cell and blocks signals that encourage the cancer to spread.
Then it breaches the outer defences and releases chemotherapy to destroy it from within. This spares healthy tissue from unnecessary damage.
The cancer's return is also delayed and side effects from chemotherapy such as diarrhoea and hair loss are significantly reduced.
The drug's manufacturer, Roche, is applying for a licence in Europe, which could mean it will be available for patients before the end of 2013. The price is not yet known.
The results were released at the European Society for Medical Oncology in Vienna, Austria.