A new breast cancer drug from Roche could significantly lengthen the lives of women battling invasive forms of breast cancer.

Women with a type of cancer called HER2 positive (which makes up a quarter of all breast cancer cases) were found to experience "unprecedented" benefits as a result of the drug Perjeta, Reuters reported. Women who took the drug in combination with the medicine Herceptin and chemotherapy lived an average of 15.7 months longer than those who did not receive the new treatment.

The breakthrough is significant because 15.7 months is the longest life extension ever seen in clinical trials regarding metastatic breast cancer.

"The results, I think, are phenomenal," lead researcher Sandra Swain from the Washington Hospital Center told the European Society for Medical Oncology (ESMO) annual congress in Madrid, Reuters reported. "The survival improvement of nearly 16 months ... is unprecedented among studies of metastatic breast cancer."

For the study, Perjeta was given to 800 women. Researchers observed the survival rate was at an average of 56.5 months for those given Perjeta compared with 40.8 months for patients given only chemotherapy and the older drug.

"I think these data are really compelling," Eric Van Cutsem of the University of Leuven, who was not involved in the research, told Reuters. "When you see in breast cancer such a big change in survival with not a lot of cardio-toxicities then that is really practice-changing."

Both Perjeta and Herceptin come with side effects such as rash and diarrhea and may have a potential effect on heart health, but taking the drugs in combination is not believed to worsen these effects.

In the United States, the monthly price of Perjeta will be about $5,900, and Herceptin costs around $5,300, the prices are expected to be lower in Europe, Reuters reported.