Mifepristone (RU-486) terminates early pregnancy. It acts as a competitive antagonist at which receptor, and what is its additional anti-glucocorticoid action?
- A Estrogen receptor antagonist; no glucocorticoid action
- B Progesterone receptor antagonist (type II nuclear receptor) with additional high-affinity antagonism at glucocorticoid receptors ✓
- C Oxytocin receptor antagonist; additionally inhibits prostaglandin synthesis
- D HCG receptor antagonist; prevents luteotropic support of the corpus luteum
Correct answer: B. Progesterone receptor antagonist (type II nuclear receptor) with additional high-affinity antagonism at glucocorticoid receptors
Explanation
Mifepristone binds progesterone receptors (both PR-A and PR-B) as a competitive antagonist, blocking progesterone's action on the endometrium and cervix, leading to decidual breakdown and cervical softening; it also has high affinity for glucocorticoid receptors (GR) and is used for Cushing's syndrome at higher doses. It has low affinity for estrogen and androgen receptors. When combined with misoprostol (PGE1), complete abortion rates reach >95% in pregnancies up to 63 days.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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