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The effects of the antiprogestin RU 486 on the human endometrium were investigated. Seventeen postmenopausal women were injected with estradiol (E2) benzoate (0.625 mg/day) for 15 days. Progesterone (P) (25 mg/day) and/or RU 486 (100 or 200 mg/day) were given to groups of 2-3 women during the last 6 days of E2 benzoate treatment. Serial blood samples were drawn for the measurement of plasma E2, P, and LH and FSH. An endometrial biopsy was performed on the last day of treatment, and processed for histology or for assays of DNA polymerase alpha, E2-dehydrogenase (E2DH), and P receptor (PR). Treatment with E2 benzoate alone resulted in a marked decrease of plasma gonadotropins; in those patients who received either P, RU 486, or both, in addition to E2 benzoate, the concentrations of plasma LH and FSH were further decreased to premenopausal levels. In absence of glycerol, the affinity of RU 486 for the endometrial PR (Kd = 0.8 nM) was higher than that of P (Kd = 1.2 nM). Glycerol decreased markedly the affinity of RU 486, whereas the affinity of P for the PR was unchanged. RU 486 had negligible affinity for plasma transcortin. Either P or RU 486, but not both together, induced secretory changes in the endometrium as determined from histologic sections of tissue biopsies. Either P or RU 486 decreased DNA polymerase alpha and increased E2-DH activities in the endometrium. Unexpectedly, when P and RU 486 were given together. E2-DH activity remained at the level found in E2-treated women. In vitro cultures of proliferative endometrium treated with the synthetic progestagen R 5020 or with RU 486 also had increased E2-DH activity; RU 486 counteracted R 5020 effects. We conclude that, contrary to previous results with experimental animals, the anti-P RU 486 has some progestomimetic activity in humans under specific conditions. Paradoxically, when given together with P, RU 486 lost most of its progestomimetic activity in the endometrium and behaved as a pure antagonist.
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