Adding Herceptin (trastuzumab) to hormonal therapy increases significantly the length of time certain women with advanced breast cancer live without their disease progressing, researchers said on Monday.

Postmenopausal women whose breast cancer is positive for hormone receptors and HER2 had an average progression-free survival of 4.8 months when given both Roche’s Herceptin and AstraZeneca’s Arimidex anastrozole.

In women on Arimidex alone, the cancer progressed after an average of just 2.4 months, according to results of a phase III clinical trial sponsored by Roche.

Around two thirds of postmenopausal women with breast cancer have hormone-sensitive disease and up to one quarter of them will also be HER2-positive, which means they have a particularly aggressive form of cancer with a higher risk of relapse.

Roche first announced in May that the Herceptin plus Arimidex combination produced good results, but the full findings were only released on Monday at the European Society for Medical Oncology Congress in Istanbul.

“The results are very positive,” Dr. Bella Kaufman of Israel’s Chaim Sheba Medical Center, who led the research, told the meeting. “In breast cancer, there are not many trials that show double progression-free survival.”

In patients receiving both drugs, overall survival was also prolonged to an average 28.5 months compared with 23.9 months for those on Arimidex alone - but this difference was not statistically significant.

Roche said in May it would file the results of the clinical trial with health authorities around the world, with the objective of securing approval to use the two drugs in tandem.