Ulipristal acetate (UPA) is a selective progesterone receptor modulator (SPRM) approved for uterine fibroids. Compared to mifepristone, UPA is preferred for fibroid therapy because:

• A UPA has no anti-progestogenic activity whatsoever
• B UPA competitively blocks estrogen receptors in fibroid tissue
• C UPA inhibits aromatase, reducing local estrogen production in fibroids
• D UPA has mixed agonist/antagonist activity with tissue-selective effects — inhibiting fibroid growth while sparing endometrial atrophy and suppressing the hypothalamo-pituitary axis less aggressively ✓

Explanation

Ulipristal acetate is a selective progesterone receptor modulator with tissue-specific mixed agonist-antagonist activity. In fibroids, it inhibits cell proliferation by receptor-mediated apoptosis. At the hypothalamic-pituitary level, it suppresses the LH surge and GnRH stimulation but less profoundly than GnRH agonists, avoiding severe hypoestrogenism and menopausal symptoms. Endometrial effects are initially benign (PAEC — progesterone receptor modulator-associated endometrial changes) though reversible. Mifepristone's stronger anti-glucocorticoid and anti-progestogen properties make it less suitable for long-term fibroid therapy.

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

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Written and medically reviewed by the StethoPrep medical team.