(AP) - A combination of two new-generation cancer drugs modestly delayed the time it took for cancer to worsen in a study of 300 women with very advanced disease who had stopped responding to other treatments.

It was the first test of these two highly targeted drugs, Herceptin and Tykerb. Both aim at a protein called HER-2/neu that is made in abnormally large quantities in about one-fourth of all breast cancers. Herceptin blocks the protein on the cell's surface; Tykerb does it inside the cell.

In the study, women getting the combo survived 12 weeks without their cancer worsening compared to eight weeks for those receiving Tykerb alone.

"Really blocking that pathway is an effective strategy. This gives us a new treatment option for patients," said Dr. Joyce O'Shaughnessy of Baylor-Sammons Cancer Center in Dallas.

She led the study and has consulted in the past for its sponsor, British-based GlaxoSmithKline PLC, which makes Tykerb. California-based Genentech and Swiss-based Roche Holding AG market Herceptin.

Side effects were more common with the combo - 5 percent had a modest decrease in pumping power of the main chamber of the heart compared to 2 percent of those taking only Tykerb. That side effect also has been seen with Herceptin.

Liver problems also have been tied to Tykerb, which is currently approved to be used in combination with the chemo drug Xeloda for breast cancer patients who have failed treatment with Herceptin and other drugs.

Results of the study were released Thursday by the American Society of Clinical Oncology and will be presented at the group's annual meeting later this month.