Effect of low daily doses of mifepristone on ovarian function and endometrial development

The effects of low daily doses of the antiprogestin mifepristone (RU 486) on ovarian and endometrial function were studied. The study included one control cycle, three treatment cycles and one follow-up cycle. During the treatment cycles, either 0.1 (n = 5) or 0.5 (n = 5) mg of mifepristone was administered once daily. Urine samples were collected three times weekly during the control and treatment cycles and pregnanediol glucuronide and oestrone glucuronide and luteinizing hormone (LH) were quantified by radioimmunoassay. Blood samples for cortisol measurement were collected once weekly and for serum glycodelin at the onset of menstruation. An endometrial biopsy was obtained in the mid-luteal phase in the control cycle and in the first and third treatment cycles and analysed by morphometric and histochemical methods. Binding of Dolichus biflorus agglutinin (DBA) lectin was measured and expression of progesterone and oestrogen receptors and glycodelin were analysed immunohistochemically. All cycles studied were ovulatory with an LH peak and elevated pregnanediol glucuronide concentrations. Follicular development seemed normal as judged by ultrasound examination. The length of the menstrual cycle and the menstrual bleeding were not significantly altered. Following administration of 0.5 mg mifepristone/day, endometrial development appeared to be slightly retarded and glandular diameter was significantly reduced. Furthermore, significant decreases in DBA lectin binding and endometrial expression of glycodelin were observed. Daily doses of 0.1 mg did not have any significant effect on the endometrium. No differences in oestrogen or progesterone receptor immunoactivity between control and treatment cycles were seen. This study provides further evidence that endometrial function is sensitive even to doses of antiprogestin that are low enough not to disturb ovulation. It remains to be established whether these effects are sufficient to prevent implantation.