Mifepristone (RU-486) is used for medical termination of pregnancy and is followed by misoprostol 24-48 hours later. Mifepristone's mechanism of action at the progesterone receptor level is:

• A Competitive antagonist at progesterone receptors; prevents progesterone-dependent decidual maintenance and sensitizes myometrium to prostaglandins ✓
• B Inverse agonist causing receptor internalization preventing progesterone from binding in subsequent cycles
• C Partial agonist providing some progesterone effect to prevent uterine hypercontractility before misoprostol
• D Irreversible progesterone receptor antagonist requiring 72 hours for de novo receptor synthesis

Explanation

Mifepristone is a competitive progesterone receptor antagonist (and partial glucocorticoid receptor antagonist) that occupies the progesterone receptor without activating it, blocking progesterone's essential role in decidual support of the embryo and in maintaining uterine quiescence. This causes decidual breakdown and cervical softening. The resulting progesterone withdrawal also upregulates myometrial prostaglandin receptors, enhancing uterine sensitivity to the subsequent prostaglandin E1 analogue misoprostol, which then induces 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.