A NEWER drug prevented far more breast cancers from recurring in older women than the old standby tamoxifen and with far fewer side effects, doctors in the US said yesterday, citing a new study.

Cancer specialists said Arimidex is likely to become the first-choice treatment for most women who have had the disease, and they predicted a wider role for similar drugs of its type, called aromatase inhibitors.

New research suggests Arimidex might be able to prevent 70 per cent to 80 per cent of the most common type of tumors that occur in women after menopause, compared with the 50 per cent that tamoxifen is credited with warding off.

Women with early-stage breast cancer who took Arimidex for five years were less likely to have cancer recur, develop in the other breast, or spread throughout their bodies than women who took tamoxifen.

"Arimidex is a more effective treatment. This is a better drug," said Dr Aman Buzdar, a specialist at the University of Texas' M.D. Anderson Cancer Center. He headed the US portion of the study, which involved about 6,000 women in 21 countries

Results were reported Wednesday at a meeting in Texas of breast cancer experts and were published online by the British medical journal The Lancet.

Studies involving other aromatase inhibitors showed that women who switched to them after two or three years of tamoxifen also fared better than those who took tamoxifen for the standard five years.

The studies were largely funded by the makers of the newer drugs, but experts not involved in the work were impressed by the results.

"The big message is, women will have a better outcome if they take aromatase inhibitors," said Dr Paul Goss of Massachusetts General Hospital, an expert on the drugs.

None of the studies change tamoxifen's status as the drug of choice for women who get breast cancer before menopause, because the newer drugs aren't thought to be effective then.

Tamoxifen revolutionised breast cancer treatment when it came into use some three decades ago and studies showed it could cut recurrence risk in half. It blunts the effects of estrogen, a hormone that promotes the growth of about three-fourths of the tumors that occur in postmenopausal women.

Aromatase inhibitors prevent estrogen from being made in the first place, and don't raise the risk of blood clots and endometrial cancer as tamoxifen does. Three are available: AstraZeneca PLC's Arimidex, Pfizer Inc.'s Aromasin and Novartis Pharmaceuticals' Femara.

Excitement for Arimidex grew three years ago, when early results from a company-funded study of more than 9000 women suggested it was better than tamoxifen at preventing recurrence.

But many doctors were reluctant to recommend it - based on these results alone - instead of tried-and-true tamoxifen, which has long been available in cheap, generic form.

The new five-year results will persuade many, doctors said. Women on Arimidex were 13 percent more likely to be alive and cancer-free than those on tamoxifen.

Arimidex cut the risk of cancer developing in the other breast by 42 per cent over tamoxifen, and of spreading to other places in the body by 14 per cent.

"It really cements the role of these agents," said Dr Eric Winer of Boston's Dana Farber Cancer Institute, who led a guidelines panel studying the drugs for the American Society of Clinical Oncology.

Dr Kathy Albain, a breast cancer specialist at Loyola University in Chicago, said she now would recommend Arimidex for patients with estrogen-sensitive tumors as front-line therapy in place of tamoxifen.

"The benefit in the first five years justifies offering treatment as early as possible," said Dr Anthony Howell of the Univesity of Manchester in England, who led the study.

Yet they fall short of meeting the toughest standard for proving a drug's value - improving overall survival. Doctors say that women in the study had very early cancers and therefore the best possible prognosis, so seeing a difference in survival likely will take longer than five years.

In fact, fewer cancer deaths occurred among Arimidex users, but the trend wasn't strong enough to say it couldn't have resulted from chance alone, Buzdar said.

"I don't think you have to show a survival advantage to change practice habits," because of Arimidex's many other benefits, said Goss of Massachusetts General.

They include fewer cases of endometrial cancer, blood clots, hot flashes and vaginal bleeding and discharge than among tamoxifen users. Women on Arimidex had more joint pain and bone fractures, though the latter can be treated with other drugs.

Arimidex also proved superior when women were switched to it after two or three years on tamoxifen, tests on 3,224 women found. With about two years of followup information, women on Arimidex had a 40 per cent lower risk of recurrence than those who stuck with tamoxifen, said Dr Raimund Jakesz of Vienna Medical School.

A Pfizer-sponsored study of its aromatase inhibitor, Aromasin, found similar benefits to switching in a study of nearly 5,000 women. Those who got Aromasin after two or three years on tamoxifen had 32 per cent less risk of recurrence or new cancers than those who got tamoxifen for five years, said Dr. Charles Coombes of the Imperial College of Medicine in England