Eli Lilly

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will ask regulators to approve its osteoporosis drug Evista as a breast cancer preventer, thanks primarily to the positive findings of a study sponsored by the federal National Cancer Institute.

The study, whose preliminary results were released Monday, said Evista is equally effective to tamoxifen in reducing the risk of invasive breast cancer in postmenopausal women. Both drugs cut the breast cancer risk by about 50%.

The now-generic tamoxifen is the only drug approved for preventing invasive breast cancer in postmenopausal women. It's also is prescribed for women who haven't gone through menopause.

Women taking Evista had fewer side effects such as uterine cancer, blood clots and cataracts than women taking tamoxifen, according to the NCI study. Tamoxifen is also sold by

AstraZeneca

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under the brand name Nolvadex. AstraZeneca recently said it would stop selling Nolvadex in the U.S. in June because generic competition had drastically eroded its sales.

The study "represents an important milestone in cancer research," said Dr. Steven Paul, Lilly's executive vice president of science and technology, in a prepared statement. Evista was Lilly's fourth-biggest drug last year with sales of $1.04 billion. However, sales grew only 2% from 2004.

Lilly didn't say when it would file its application to the Food and Drug Administration. Analysts had speculated that good news from the NCI-sponsored test would lead to a filing this year.

Shares of Indianapolis-based Lilly lost 26 cents and closed at $53.01.

The news marks "a significant step in breast cancer prevention," said Dr. John Niederhuber, the acting director of NCI, in a prepared statement. The study enrolled 19,747 women and is following them for five years of treatment. Each was given a daily pill of Evista or tamoxifen.

The NCI paid $88 million for the study, and Lilly contributed $30 million. The study was conducted by the National Surgical Adjuvant Breast and Bowel Project, a group of researchers, financed by the NCI, that has spent 40 years designing and conducting clinical trials.

Women in the trial taking Evista had 29% fewer blood clots in major veins or in the lungs, and the number of strokes in the Evista and tamoxifen groups were statistically equivalent. The Evista group had 36% fewer uterine cancers than the tamoxifen group. Uterine cancer is a rare but dangerous side effect.

The tamoxifen label carries a black box warning -- the strongest FDA alert -- pointing out that uterine cancer, stroke and blood clots can occur two to four times more often in patients receiving tamoxifen than in those receiving a placebo. However, the label notes that these side effects occur in fewer than 1% of women, adding that the benefits for most women outweigh the chance of side effects.

Although the Evista label warns about blood clots and other side effects, the drug doesn't carry a black box. The Evista dosage used in the NCI clinical trial is the same as the dose recommended for osteoporosis.

"For many women,

Evista's benefits will outweigh its risks in a way that tamoxifen's benefits do not," said Dr. Leslie Ford, associate director of clinical research for the NCI's division of cancer prevention, in a prepared statement.

In addition, the clinical trial showed that tamoxifen reduces the risk of less dangerous, noninvasive breast cancers by 50% whereas Evista has no effect on such cancers. This result wasn't a surprise. In 2004, a clinical trial found that the rates of these cancers were similar in women taking Evista vs. women receiving a placebo.

NCI researchers pointed out on Monday that tamoxifen has a longer track record than Evista. Tamoxifen has been on the market for nearly 30 years as a breast cancer treatment. In 1998, the FDA approved it for reducing the risk of breast cancer in women at a high risk of contracting the disease.

Evista was first approved by the FDA in late 1997 as an osteoporosis prevention for postmenopausal women.