The drug Herceptin (trastuzumab) holds huge promise for women with a particularly aggressive type of breast cancer, experts say.

Herceptin is currently used to treat women with advanced cancer whose tumors have too much of a protein, called HER-2, that causes the cancer to grow and spread faster. About 20%-30% of women (both before and after menopause) have these types of tumors.

Last month, the National Cancer Institute announced that the drug cuts recurrences in half when given with chemotherapy to women with early-stage disease. On Monday, researchers released more details of their findings to thunderous applause from thousands of cancer specialists attending the annual meeting of the American Society of Clinical Oncology, held in Orlando.

"The results were spectacular," said Len Lichtenfeld, MD, deputy chief medical officer of the American Cancer Society. "I don't think anyone left the room with any doubt this is the new standard of care."

The findings came from two large studies of women with early-stage, HER-2-positive breast cancer. After initial treatment of the tumor, some women received standard chemotherapy, while others were given chemotherapy plus Herceptin. After about 2 years, those treated with Herceptin had a 52% lower risk of their breast cancer coming back.

"Up until this point, these women have had a poor prognosis, even with the chemotherapy," said Lichtenfeld, who was not involved in the research. "What these studies have done is show clearly and convincingly that there is a substantial improvement in their outlook when Herceptin is added to the chemotherapy after primary treatment."

Women who take the drug must be carefully monitored for heart damage, however, because Herceptin can sometimes cause congestive heart failure, as can some of the chemotherapy drugs used to treat this aggressive breast cancer.

About 3%-4% of the women in the studies developed this side effect, and it was serious in some of them. In some women, the symptoms went away after stopping treatment.

"The caveat is, these are selected patients who were healthy, and those who had a prior [history of heart disease] were excluded," said Gabriel Hortobagyi, MD, FACP, director of the breast cancer research program at the University of Texas M.D. Anderson Cancer Center. He briefed reporters on the findings over the weekend.

Researchers must do more studies to know for sure whether any heart damage can really be completely reversed, he added.

"Women are going to have to make the decision on whether or not they choose to accept that risk," said Lichtenfeld, "but the reality is that the risk of death from breast cancer in these women is much greater than the risk of dying from heart failure."

The chair of one of the studies, Edith Perez, MD, said she would begin treating appropriate patients with Herceptin right away.

"There's a high probability that with this therapy, we're going to cure a lot of women," said Perez, a medical oncologist at the Mayo Clinic in Jacksonville, Fla. "We will need to do follow-up research to prove that point, but we will continue to do other trials, and we hope we'll receive continued collaboration from medical personnel and patients to see if we can do even better."

Hortobagyi and Lichtenfeld both agreed more studies will be important to learning the best strategy for giving Herceptin to women with early breast cancer, and learning how to minimize the side effects on the heart.

"Because of people's willingness to participate in these [initial] clinical trials, we were able to find answers, and the answers were remarkable," Lichtenfeld said.

"These types of studies don't come along very often," he added. "These are the types of studies that are incredibly positive and have an immediate effect on how women are treated, and as a result, it appears that many of these women may enjoy a life free from a recurrence of their breast cancer."