The pharmacologic action of Ulipristal Acetate

Ulipristal acetate is a selective progesterone receptor modulator.5 It is a partial agonist, exerting agonist and antagonist activity at the progesterone receptor. While it has been used in the medical management of uterine fibroids,6 it is more commonly used for emergency contraception. Ulipristal acetate is taken as a single 30 mg dose as soon as possible after unprotected intercourse, but has continuing efficacy up to five days later (120 hours), compared with 72 hours for levonorgestrel ECP.

Pharmacologic action

Depending on when ulipristal acetate is taken during the menstrual cycle, it will either prevent or delay ovulation.8 It does this more effectively than levonorgestrel.8 Ulipristal acetate delays ovulation by at least five days when given before the onset of the luteinising hormone (LH) surge in 100% of cycles; in comparison, levonorgestrel only achieves this in 25% of cycles.8 When administered once the LH surge has commenced, but before it peaks, ulipristal acetate delays ovulation in 79% of cycles, whereas levonorgestrel delays ovulation in only 14% of cycles.8 Once the LH surge has reached its peak, neither ulipristal acetate nor levonorgestral has any effect on ovulation.8

Product features

The evidence suggests that ulipristal acetate is a more effective form of emergency contraception than levonorgestrel (Table 1).9 Ulipristal acetate is twice as likely as levonorgestrel to prevent pregnancy when used within 72 hours or within five days of unprotected intercourse.9 When taken within the first 24 hours after intercourse, it reduces unplanned pregnancies by two-thirds when compared with levonorgestrel.

Side effects

Common side effects include nausea, abdominal pain, emesis, dysmenorrhea, pelvic pain, breast tenderness, headache, dizziness, mood swings, myalgia, and fatigue.

Interactions

Ulipristal acetate is metabolized by CYP3A4 in vitro. Ulipristal acetate is likely to interact with substrates of CYP3A4, like rifampicin, phenytoin, St John's wort, carbamazepine or ritonavir, therefore concomitant use with these agents is not recommended.It might also interact with hormonal contraceptives and progestogens such as levonorgestrel and other substrates of the progesterone receptor, as well as with glucocorticoids.