Mifepristone (RU-486) is used as a medical abortifacient and in Cushing syndrome. Its mechanism is:

• A Competitive antagonism at progesterone receptors (and glucocorticoid receptors at higher doses), blocking progesterone-dependent decidual support and sensitising the myometrium to prostaglandins ✓
• B Selective inhibition of 11β-hydroxylase, blocking cortisol synthesis in the adrenal cortex
• C Stimulation of uterine oxytocin receptors, directly inducing uterine contractions
• D Inhibition of GnRH pulse frequency, reducing LH and FSH and withdrawing progesterone support

Explanation

Mifepristone is a 19-nor-steroid that competitively antagonises progesterone receptors (and glucocorticoid receptors at high doses). In early pregnancy, it blocks progesterone-mediated decidual maintenance and promotes cervical ripening. The myometrium becomes sensitised to prostaglandins; subsequent misoprostol (a PGE1 analogue) completes expulsion. In Cushing syndrome, its glucocorticoid receptor antagonism (at higher doses) is used to control clinical features in inoperable cases. It does not inhibit 11β-hydroxylase (metyrapone), stimulate oxytocin receptors, or suppress GnRH.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.