WASHINGTON - Neva Hart's mother, grandmother and aunt all battled breast cancer. So when Hart heard about an experiment testing a pill that might protect women like her from the malignancy, she jumped at the chance to volunteer.

"I wish science could find something aside from chopping off parts of the body to fight this disease,"said Hart, 57, of Wirtz, Va. "I decided to donate my body so people might not have to go through that kind of torture."

The study that Hart joined and a similar one based in Europe are raising hope that a new class of drugs may offer women at high risk of breast cancer a safe way to protect themselves. But the experiments also raise thorny questions about whether the potential benefits outweigh the risks.

"Studies like this raise serious ethical issues," said Michael Grodin, a bioethicist at the Boston University School of Public Health. "What you're comparing here is the risk of getting cancer, which is unknown, against the potential risk of side effects from the prevention, which also is unknown. You're not treating people who are sick. These are healthy women."

These kinds of questions are becoming increasingly common as researchers focus more and more on trying to prevent, rather than treat, disease - often testing powerful drugs on people who have theoretical risks based on nebulous attributes such as age and family history rather than more concrete risk factors such as high cholesterol or high blood pressure.

"It's not like someone has the disease. In this case you're treating a statistic - the chance of getting the disease," Grodin said. "If you knew this drug had no side effects, then it wouldn't be as concerning. But you don't know that."

Breast cancer is diagnosed in about 211,000 U.S. women each year, and about 40,000 die from the disease, making breast cancer the most common cancer and the second-biggest killer cancer, after lung cancer, among women. Surgery, radiation, chemotherapy and estrogen-blocking drugs have cut the breast cancer death rate. But with the incidence still rising, researchers and women at risk for the disease desperately want ways to prevent the cancer in the first place.

The study that Hart joined, dubbed the ExCel trial, is testing a drug known as exemestane, or Aromasin, one of a class of new drugs called aromatase inhibitors. Aromatase inhibitors block formation of estrogen, which can fuel the growth of breast cancer cells.

Aside from sometimes causing symptoms similar to those experienced during menopause, exemestane so far appears very safe. Nevertheless, some advocates in the field are worried about giving healthy women a powerful hormone-blocking drug for years before long-term studies have been conducted to ensure its safety.

"There is abundant evidence that inhibiting estrogen can prevent new primary breast cancers. We estimate that we may be able to reduce the risk by 60 to 80 percent,"said Paul Goss, director of breast cancer research at Massachusetts General Hospital in Boston, who is leading the ExCel study.

Researchers have started recruiting what they hope will be about 4,500 postmenopausal women in the United States, Canada and Spain who are considered at high risk for breast cancer. For five years, half the women will take the drug while the other half take a placebo. The study is being sponsored by the Canadian Cancer Society, the National Cancer Institute of Canada and the drug's maker, Pfizer Inc.; the company is not directly involved in the study.

"I understand the excitement about these drugs. I hope they prove to be everything everyone hopes them to be. But the lack of long-term use and side-effect data are a big problem," said Barbara Brenner of Breast Cancer Action, a patient advocacy group based in San Francisco. "Without long-term side-effect data on this drug, you can't really get informed consent from people you are trying to enroll in the study."

Some evidence suggests that aromatase inhibitors may, for example, increase the risk of the bone-thinning disease osteoporosis, which makes women prone to potentially serious fractures.

"Any drug that is powerful enough to reduce your risk for breast cancer is almost inevitably going to have side effects," Brenner said. "What we could end up having is disease substitution instead of disease prevention."

The ExCel trial originally planned to test whether the painkiller Celebrex also prevents breast cancer, but that part of the study was discontinued after evidence surfaced that drugs of its class may cause heart problems. Skeptics also point to once-popular hormone replacement therapy as an example of a drug treatment designed to prevent disease that initially appeared to be safe.

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