Women with very advanced breast cancer may have a new treatment option. A combination of two drugs that more precisely target tumors significantly extended the lives of women who had stopped responding to other treatments, doctors reported Friday.

The study is the first big test of combining Herceptin and Tykerb. In the study of 300 patients, women receiving both drugs lived 20 weeks longer than those given Tykerb alone. Doctors expect the combo to make an even bigger difference for women with less advanced disease.

The medicines aim at a protein that is made in abnormally large quantities in about one-fourth of all breast cancers. One drug blocks the protein inside a cell and the other does the same on the cell's surface.

"It's kind of like having a double brake on your tumor. If the first one fails, the second one does the job," said the Dr. Kimberly Blackwell of Duke University. She led the study and has consulted for its sponsor, British-based GlaxoSmithKline PLC, which makes Tykerb, and for Genentech, which makes Herceptin.

Women in the study had already received Herceptin alone or in combination with various chemotherapy drugs and still were getting worse. They were randomly assigned to receive only Tykerb or Tykerb plus Herceptin.

Median survival was analyzed after about three-fourths of the women had died — roughly two years after the study began. It was 61 weeks in the combo group versus 41 for those taking only Tykerb.

That likely underestimates the combo's true benefit because women on Tykerb alone were allowed to add Herceptin partway through the study if they continued to worsen, and many of them did, Blackwell said.

Results were presented Friday at the San Antonio Breast Cancer Symposium.