By Julie Steenhuysen

CHICAGO, June 1 (Reuters) - The estrogen-blocking drug Aromasin worked better than the long-standing therapy tamoxifen at keeping cancers from returning in younger women with early stage breast cancer, a finding that may change the way the patients are treated, U.S. researchers said on Sunday.

Aromasin, a drug developed by Pfizer Inc that is sold generically as exemestane, is in a class of treatments called aromatase inhibitors that are typically used in post-menopausal women with low levels of estrogen.

Aromatase is an enzyme that converts the hormone androgen into small amounts of estrogen.

The drugs have largely been off-limits for younger women with working ovaries that produce estrogen.

In premenopausal women with hormone-sensitive cancers, the standard for preventing recurrence is five years of treatment with a drug called tamoxifen. For high-risk women, doctors in some countries recommend exemestane plus some form of therapy to shut down the ovaries, cutting off the supply of estrogen. That practice is not typically followed in the United States because there has not been enough evidence to show a benefit.

International researchers tested whether aromatase inhibitors combined with ovary-blocking treatments would do a better job than tamoxifen in two clinical trials involving 4,690 premenopausal women.

All women in the studies underwent some treatment to block the function of their ovaries, either through a drug called triptorelin, the surgical removal of the ovaries or the use of radiation. In addition, one group tested exemestane and the other tested tamoxifen.

A joint analysis of the two international trials known as SOFT and TEXT, presented at the American Society of Clinical Oncology, or ASCO, meeting in Chicago, showed exemestane did a better job of keeping cancer at bay when women's ovarian function was suppressed, said Dr. Olivia Pagani of the Oncology Institute of Southern Switzerland.

Her study was presented at the meeting and will be published this week in the New England Journal of Medicine.

Dr. Clifford Hudis, a breast cancer expert and president of ASCO, said that with ovarian suppression, aromatase inhibitors offered an option for further cutting the risk of recurrence.

But some questions remain.

Although aromatase inhibitors showed a benefit in preventing recurrence, the researchers have yet to present data showing whether the treatment also extends survival compared with tamoxifen.

It is also still not clear whether shutting down the ovaries adds a benefit to women who are being treated with tamoxifen alone, which is the current standard of care in the United States.