Mifepristone (RU486) is used for medical abortion. It is also a glucocorticoid receptor antagonist. Its primary mechanism for termination of early pregnancy is:
- A Inhibition of progesterone synthesis in the corpus luteum
- B Competitive antagonism at progesterone receptors in the endometrium and decidua, causing decidual breakdown and increased uterine sensitivity to prostaglandins ✓
- C Direct oxytocic action on uterine smooth muscle
- D Inhibition of hCG secretion from the trophoblast
Correct answer: B. Competitive antagonism at progesterone receptors in the endometrium and decidua, causing decidual breakdown and increased uterine sensitivity to prostaglandins
Explanation
Mifepristone competitively blocks progesterone receptors in the uterine decidua and myometrium. Progesterone is essential for maintaining pregnancy by promoting decidual support and suppressing uterine contractility. Blocking it causes decidual necrosis, increased prostaglandin release, and increased sensitivity of the myometrium to exogenous prostaglandins (like misoprostol). Mifepristone alone is followed 24–48 hours later by misoprostol (a PGE1 analog) for >95% efficacy in terminating pregnancy up to 10 weeks.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.