Why does RU486 fail to prevent implantation despite success in inducing menstruation?

ABSTRACT

The administration of RU38486 (RU486) in the luteal phase may induce menstruation, but it may not be associated with shedding of the functional layer of the endometrium. This provides an explanation why, in some cases of successful menstrual induction by RU486, pregnancy continues undisturbed. The ability of RU486 to interrupt a very early pregnancy is more likely to be related to its ability to cause shedding of the endometrium than its ability to induce menstruation.

ABSTRACT

PIP To see why RU-486, given in luteal phase, induces menstruation but does not always prevent pregnancy, daily progesterone assays and 3 endometrial biopsies were taken from 2 volunteers who received a single dose. Progesterone was assayed by direct radioimmunoassay from saliva samples. The 1st subject was a 36-year old multipara who took a single oral dose of 150 mg RU-486 on Day 4 after the LH surge. She menstruated apparently normally 2 days later, and again 10 days after RU-486, at the expected time of menstruation. Her endometrial biopsy resembled secretory phase, and her progesterone profile did not indicate luteolysis. The 2nd volunteer was a 38-year old multipara. She took 75 mg RU-486 on Day 6 after her LH surge, and had apparently normal menses 2 days later. The endometrial histology was suggestive of menstruation, and the progesterone profile had fallen to baseline, indicating luteolysis. Menstruation did not recur at the normal time. In this woman who had complete shedding of the endometrium, RU-486 had been taken later in the luteal phase, and in a smaller dosage.