One of a new class of breast cancer drugs was so successful in a major international trial that scientists stopped the test half way through to allow all the patients to take advantage of the stunning success rate, the lead author of a new medical report said on Thursday.

The five-year study of about 5,200 women with the most common form of breast cancer was halted midway because Femara, made by Swiss drugmaker Novartis, cut by 43 percent the risk of the disease's recurrence, compared with the risk for patients taking a placebo, scientists said.

"This is a sea change in the treatment of the disease," Dr. Paul Goss of Princess Margaret Hospital in Toronto, who led the research, told a news conference.

The New England Journal of Medicine released the findings ahead of its Nov. 6 print issue because of their significance.

Goss said the "dramatic results" prompted doctors to halt the trial so that nearly 2,600 women in the placebo group could start taking the once-daily Femara pill immediately.

Doctors said the study offers a new direction to treat patients who have the hormome-receptor positive form of breast cancer in which estrogen fuels the cancer. But the report also raises questions.

"How long should one take (Femara)? What are the long-term toxicities," said Dr. James Ingles, with the Mayo Clinic in Rochester, Minnesota, another major participant in the study, which was conducted in nine countries, including Canada, the United States, England and Belgium.

Stopping the trial meant researchers were unable to study Femara's effect over the trial's full five-year period.

The most widely used treatment for breast cancer now is a drug called tamoxifen. But tamoxifen loses its effectiveness after five years, a cause of worry for patients with the disease, which strikes again in one out of two women five or more years after diagnosis.

The trial offered Femara, whose generic name is letrozole, to women who had been off tamoxifen for less than three months after taking tamoxifen for an average of five years.

The question of the effectiveness of the new drug for women who have been off tamoxifen for more than three months is still unanswered. Also, the study offered no definitive answers for women who have recently been diagnosed with breast cancer and might want to take letrozole.

"There are studies looking at taking (letrozole) right away (when you are diagnosed)," said Goss. "I would argue 'take letrozole' to any woman who has previously been exposed to tamoxifen."

Letrozole is one of a class of cancer drugs called aromatase inhibitors, which suppress the production of estrogen. AstraZeneca has a drug of the same type called Arimidex.

Half of the women in the blind trial got letrozole and half got a placebo, but neither group knew what they were getting.

After a little more than two years, the cancer returned in 75 of the women who took letrozole, compared with 132 in group taking the placebo. The researchers said 42 women who took the placebo died and 31 women who got letrozole died.

Of these deaths, however, only 17 women in the placebo group died of breast cancer, and nine taking letrozole died from the disease. The remaining deaths were from other causes, such as old age.

After lung cancer, breast cancer is the most common cancer killer in women across the industrialized world. The World Health Organization says more than 1.2 million people will be diagnosed with breast cancer this year.

Tamoxifen works by blocking the estrogen receptor on cells -- the molecular doorway that estrogen uses to get into cells. But it can also, paradoxically, stimulate this receptor.

Scientists believe that over time, this stimulation becomes greater, interfering with tamoxifen's ability to prevent cancer.

Aromatase inhibitors such as letrozole suppress the production of estrogen. Side effects include increased risk of osteoporosis, hot flashes, night sweats and pain in the bones, joints or back.

Sales of Femara, which totaled $108 million in the first half of 2003, are expected to reach $417 million by 2007, according to Merrill Lynch.

In Switzerland, Novartis shares closed up 2.4 percent, or 1.25 Swiss francs, at 52.65 Swiss francs on Thursday. ($1=$1.33 Canadian)

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