A combination of drugs that acts as a sort of "smart bomb" against breast cancer cells without damaging healthy ones has undergone successful early testing.

In a key test involving nearly 1,000 women with very advanced disease, the experimental treatment extended by several months the time women lived without their cancer getting worse.

More importantly, the treatment seems likely to improve survival. After two years, 65 percent of women who received it were still alive versus 47 percent of those in a comparison group given two standard cancer drugs.

That margin fell just short of the very strict criteria researchers set for stopping the study and declaring the new treatment a winner. In fact, so many women on the new treatment are still alive that researchers cannot yet determine average survival for the group.

Doctors planned to report results at a cancer conference in Chicago.

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