CHICAGO ( TheStreet) -- An experimental antibody-drug conjugate therapy from the Genentech unit of Roche ( RHHBY) delayed by more than three months the re-growth of breast cancer compared to the current standard of care, according to a phase III study being presented today at the American Society of Clinical Oncology (ASCO) annual meeting. The drug, known as T-DM1, will be filed for regulatory approval in the U.S. and Europe later this year, Roche said. T-DM1 is a second-generation version of Genentech's blockbuster breast cancer drug Herceptin. The drug consists of Herceptin (also known as trastuzumab) linked to a tumor-killing chemotherapy payload developed by ImmunoGen ( IMGN). The phase III enrolled 1,000 women with advanced, metastatic Her-2 positive breast cancer who were ready for second-line therapy, having been previously treated with Herceptin and chemotherapy. The women were randomized to treatment with either TDM-1 or a combination of Roche's Xeloda and GlaxoSmithKline's ( GSK) Tykerb, which is approved for second-line breast cancer treatment. Breast cancer patients treated with TDM-1 lived a median of 9.6 months before their cancer progressed compared to 6.4 months for the patients treated with the Xeloda-Tykerb combination. The benefit, which equates to a 35% reduction in the risk of disease progression favoring TDM-1, was statistically significant. After two years of follow-up, 65.4% of TDM-1 patients were alive compared to 47.5% of patients treated with Xeloda-Tykerb. This improvement in survival is not yet statistically significant but Genentech continues to follow these patients and additional survival analyses will be conducted, the company said. TDM-1 caused more patients to suffer from reduced platelet counts but the Xeloda-Tykerb combination caused more diarrhea, hand-foot syndrome and vomiting. Sixteen percent of patients had to reduce the dose of TDM-1 during treatment compared to 53% of patients treated with Xeloda-Tykerb. "The drug TDM-1 worked. It was significantly better than a very effective approved therapy for HER2 overexpressing metastatic breast cancer," said Dr. Kimberly Blackwell, professor of medicine at Duke University's Cancer Institute. "Also, as a clinician who takes care of a lot of breast cancer patients, I’m pleased that this drug has very little dose-limiting toxicity. Patients don't lose their hair from this drug. For patients facing metastatic breast cancer, this is a breakthrough."