Mifepristone (RU-486) is used for medical termination of pregnancy. Beyond progesterone receptor antagonism, mifepristone also has which additional clinically relevant receptor activity?

• A Estrogen receptor partial agonism
• B Androgen receptor agonism causing virilization
• C Oxytocin receptor agonism facilitating myometrial contractions
• D Glucocorticoid receptor antagonism (anti-glucocorticoid effect) ✓

Explanation

Mifepristone is a competitive antagonist at both progesterone receptors (PR) and glucocorticoid receptors (GR). At therapeutic doses, its anti-progesterone effect terminates pregnancy, but its significant anti-glucocorticoid activity can elevate ACTH and cortisol (secondary to GR block). This glucocorticoid receptor antagonism is exploited clinically in the treatment of Cushing's syndrome (approved indication: hyperglycemia secondary to endogenous Cushing's). Mifepristone also has weak anti-androgen activity but is not an estrogen or oxytocin agonist.

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

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