Danazol has progestin-like actions on the human endometrium

The administration of danazol, 200 mg three times daily, from the 3rd to the 23rd day of the cycle to normally menstruating women exhibited the following actions on the human endometrium: significantly reduced cytosol oestrogen and progestin receptor concentrations, and declined 17 beta-hydroxysteroid dehydrogenase activity. Very similar results were obtained during medroxyprogesterone acetate (100 mg daily) treatment for the same period of time. Danazol administration did not decrease circulating gonadotrophin levels but clearly suppressed luteal serum oestradiol and progesterone concentrations. Danazol was found to bind in vitro to endometrial progestin receptor with an affinity approximately 3% of that of progesterone. These findings are compatible with the notion that a local progestin-like rather than a systemic action of danazol is the way by which its therapeutic effect is exerted. This may be potentiated by the suppression of circulating oestradiol levels.