A drug that targets only diseased cells has proved astonishingly effective against an aggressive form of early breast cancer – a long-sought breakthrough that has doctors talking about curing thousands of women each year in this country alone.

The drug, Herceptin, is already used for advanced cancer. But in three studies involving thousands of women with early-stage disease, it cut the risk of a relapse in half.

Several experts used words like “revolutionary,” “stunning” and “jaw-dropping” to describe the findings.

“In 1991, I didn’t know that we would cure breast cancer, and in 2005, I’m convinced we have,” exulted Dr. Jo Anne Zujewski, head of breast cancer therapeutics at the government’s National Cancer Institute.

However, an official at the American Cancer Society warned that it is far too early to suggest this amounts to a cure, since the women studied were followed for only three years at the most.

Moreover, Herceptin is only for the estimated 20 percent of breast cancer cases in which tumors churn out too much of a protein known as HER2. Even then, the drug does not help everyone.

Still, Herceptin could be the biggest thing in cancer drugs since research a decade ago demonstrated the extraordinary effectiveness of tamoxifen, another medicine that transformed the treatment of the disease by homing in on cancer cells but sparing healthy ones.

Herceptin, made by Genentech, appears to have “changed one of the most worrisome kinds of cancers into one that may have a relatively good prognosis,” said Dr. Ed Romond of the University of Kentucky.

He was one of the researchers who reported findings from three Herceptin studies today in The New England Journal of Medicine. One was an international study sponsored by Herceptin’s European marketer, Roche. The others were North American studies sponsored by the National Cancer Institute.