Effects of the antiprogesterone RU 486 in normal women. II. Administration in the late follicular phase.

RU 486, a synthetic steroid with antiprogesterone receptor activity, was used to investigate the importance of progesterone on gonadotropin secretory dynamics in the midcycle of the normal menstrual cycle. Six normally cycling women were followed for three consecutive cycles. During each cycle, blood samples were obtained beginning on day 10 and continued until menses. After a control cycle, 100 mg RU 486 was given orally between days 10 and 17. The patients were followed for a posttreatment cycle with no medication. When RU 486 was given before the midcycle, the luteinizing hormone surge was delayed by 15.0 +/- 2.1 days after ingestion of the last pill, resulting in cycles of 40.6 +/- 2.6 compared with 28.0 +/- 2.3 days (p less than 0.01). During RU 486 administration and at the time a normal luteinizing hormone surge was anticipated, an attenuated luteinizing hormone/follicle-stimulating hormone surge was noted that was not followed by a rise in progesterone. After the attenuated surge a normal luteinizing hormone/follicle-stimulating hormone level occurred, with a normal rise in progesterone. Estradiol levels during RU 486 administration decreased during treatment, indicating a possible direct action of RU 486 on the ovary.