NEW YORK (Reuters Health) - The results of a small study of women in the UK and China suggest that the abortion pill mifepristone could be developed into an estrogen-free oral contraceptive.

Researchers found that in low daily doses, mifepristone--also known as RU-486--suppressed ovulation in a majority of the women they studied. And no pregnancies occurred during the 4-month study period, according to findings published in the January issue of the Journal of Clinical Endocrinology and Metabolism.

When used with another drug called misoprostol, mifepristone can induce an abortion if taken within 7 weeks of a woman's last menstrual period. Mifepristone works by blocking the effects of the female hormone progesterone.

It is this anti-progesterone activity that makes the drug potentially useful in birth control, according to the new study's authors, led by Dr. David T. Baird of the University of Edinburgh, Scotland.

They studied 50 women in Edinburgh and 40 in Shanghai, all of whom received either 5- or 2-milligram doses of mifepristone each day for about 4 months.

Overall, the drug prevented a majority of ovulations, but was more effective among the Chinese women. Among UK women, 38% of those on the lower dose ovulated once during treatment. On the higher dose, 19% ovulated once. Among the Shanghai women, there were only three instances of ovulation, according to the report.

In addition, treatment halted monthly periods in a majority of women--although this was more common among those in China--and two in the UK experienced extended bleeding. Within a month of going off mifepristone, all women resumed ovulation, Baird's team reports.

The researchers suggest that the greater ovulation suppression in Chinese women may have to do with their diet and lower body mass.

Although there were no pregnancies during the study, Baird's team points out that these preliminary findings do not indicate how effective mifepristone might be as a contraceptive. Moreover, they note, more research is needed see whether daily doses of mifepristone might pose a health hazard to the endometrium--the lining of the uterus.

There is some concern, the authors explain, that progesterone-blocking agents might cause abnormal overgrowth of the endometrium or even promote cancer, as such drugs would allow the tissue to be continuously exposed to estrogen levels that are unbalanced by progesterone.

However, the researchers note, there were no signs of endometrial enlargement in this short-term study.

``We conclude that mifepristone in low daily doses inhibits ovulation...and has the potential to be developed as a novel estrogen-free oral contraceptive pill,'' Baird and colleagues write.

SOURCE: Journal of Clinical Endocrinology and Metabolism 2002;87:63-70.

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