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,” the head of breast cancer therapeutics at the government’s National Cancer Institute, Dr. Jo Anne Zujewski, said.

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% 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,” Dr. Ed Romond of the University of Kentucky said.

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.

The researchers followed a total of more than 6,500 women with early stage breast cancer. Women received Herceptin along with the standard treatments, including surgery and chemotherapy.

In the first study, 220 women undergoing standard therapy for a year either developed breast cancer again, showed other kinds of tumors, or died. Only 127 did when Herceptin was added.

The two other studies, partly funded by Genentech, reached similar findings in their combined results. At three years, patients on Herceptin showed a disease-free survival rate that was 12 percentage points higher than without it.

About 200,000 women are diagnosed with breast cancer each year in this country, and 40,000 die. About 30,000 American women will probably be taking Herceptin for breast cancer within a couple of years, curing perhaps 7,000 who would otherwise relapse, some doctors predicted.