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.
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.

Doctors cautioned that some women get better without Herceptin, especially when there is little evidence that the cancer is spreading within the breast. Also, a small number taking the drug suffer heart failure.