A study has shown that a new drug has extended the lives of some patients with advanced breast cancer by almost 16 months.

A drug developed by Swiss giant Roche to treat an aggressive form of breast cancer has been shown to extend patients' lives by almost 16 months, researchers say.

Patients who took the new Perjeta drug in combination with chemotherapy and Roche's older anti-cancer drug Herceptin lived a median of 56.5 months, compared to 40.8 months for people in the trial who weren't on Perjeta, Roche said on Sunday.

"Adding Perjeta to treatment with Herceptin and chemotherapy resulted in the longest survival observed to date in a clinical study of people with HER2-positive metastatic breast cancer," said Sandra Horning, Roche's chief medical officer and head of global product development.

The 15.7-month longer survival time marked "a magnitude of improvement we rarely see in clinical trials in advanced cancer", she added.

The trial involved more than 800 patients with previously untreated HER2-positive metastatic breast cancer.

HER2 is a protein that makes breast cancer cells grow. HER2-positive cancer makes up about 15 to 20 per cent of breast cancers, and tends to be more difficult to treat.

Metastatic cancer is a cancer that has spread to other parts of the body.

Copyright AAP 2014