Mifepristone (RU-486) is used for medical termination of pregnancy. Its mechanism at the molecular receptor level is:

• A Competitive agonist at progesterone receptors, mimicking progesterone action at lower concentrations
• B Irreversible covalent binding to progesterone receptors, permanently inactivating them
• C Inhibitor of progesterone synthesis by blocking 3-beta-hydroxysteroid dehydrogenase in the corpus luteum
• D Competitive antagonist at progesterone (and glucocorticoid) receptors — binds with high affinity but induces a receptor conformation that blocks co-activator recruitment and gene transcription ✓

Explanation

Mifepristone binds the progesterone receptor (and glucocorticoid receptor) with high affinity but acts as a competitive antagonist by inducing a receptor conformation distinct from that induced by progesterone. This altered conformation does not permit co-activator binding (e.g., SRC-1), preventing gene transactivation while blocking endogenous progesterone signalling. In early pregnancy, this removes progesterone support for the decidua, resulting in endometrial shedding; misoprostol (a prostaglandin E1 analogue) is added to cause uterine contractions.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.