SUNDAY, June 3 (HealthDay News) -- An experimental drug designed to treat patients with a specific kind of breast cancer known as HER2-positive appeared to boost survival compared to the standard treatment, a new study shows.

The drug, known as trastuzumab emtansine (T-DM1), is in the final stage of research necessary before the U.S. Food and Drug Administration can approve its sale. For now, it is only available in clinical trials.

"The drug worked. It was significantly better than a very effective approved therapy for HER2-overexpressing metastatic breast cancer," study author Dr. Kimberly Blackwell, a professor of medicine and an assistant professor of radiation oncology at Duke Cancer Institute in Durham, N.C., said in a news release from the American Society of Clinical Oncology.

"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," she added. "Patients don't lose their hair from this drug. For patients facing metastatic breast cancer, this is a breakthrough."

Patients with HER2-positive breast cancer have a protein called human epidermal growth factor receptor 2 that promotes cancer cell growth.

The drug T-DM1 is a dual drug made up of the antibody trastuzumab (Herceptin) and the cytotoxic drug emtansine (DM1).

In the study, nearly 1,000 patients received either T-DM1 or a regimen of capecitabine (Xeloda) and lapatinib (Tykerb), a combination referred to as XL. They took the assigned treatment until the disease got worse or side effects became unmanageable.

After two years, 65.4 percent of those who took T-DM1 were alive, compared to 47.5 percent of those who took the other treatment.

The median progression-free survival time was 9.6 months for those who got T-DM1, compared to 6.4 months for the others.

Several side effects were more common in the T-DM1 patients, including a low platelet count, but the regimen was generally well-tolerated, the researchers said. Those who got the standard treatment were more likely to experience diarrhea, stomach upset and redness, swelling and pain in their palms and the soles of their feet.

Dr. Daniel Hayes, clinical director of the breast oncology program at the University of Michigan Comprehensive Cancer Center in Ann Arbor, said the study "suggests that T-DM1 will provide us with yet another effective and meaningful agent to use in women with HER2-positive breast cancer."

The study was scheduled to bepresented Sunday at the American Society of Clinical Oncology annual meeting in Chicago.

Data and conclusions presented at medical meetings should be considered preliminary until published in a peer-reviewed medical journal.

More information

For more about breast cancer, see the U.S. National Library of Medicine.