Pharmacology & pharmacokinetics

This section includes the mechanism of action, drug interactions, and other important pharmacokinetic information for health professionals.

Pharmacology

Mariprist contains one 200 mg tablet of mifepristone and four 200mcg tablets of misoprostol.

Mifepristone is a synthetic steroid with an antiprogestational action as a result of competition with progesterone at the progesterone receptors. It is almost exclusively used for termination of pregnancy.

Misoprostol is a synthetic prostaglandin E1 analogue. In comparison to other prostaglandin analogues, misoprostol is cheap, widely available, stable at room temperature, and has few side effects.

Its clinical applications include medical abortion,treatment of incomplete abortion and miscarriages,cervical priming prior to surgical procedures, induction of labour, and prevention and treatment management of postpartum haemorrhage. As well as its obstetric and gynaecological indications, it is also used in the prevention and treatment of peptic ulcers.

Pharmacokinetics

The plasma elimination half-life for mifepristone is 30-40 hours, and for misoprostol is 20-40 minutes. After oral administration, mifepristone is rapidly and almost completely absorbed. Plasma concentrations peak at about 45 minutes.

Mifepristone is primarily metabolised by CYP3A4 enzyme.

After sublingual administration, misoprostol dissolves and is absorbed rapidly, with peak concentrations occurring at about 30 minutes. Bioavailability is very high.

Drug interactions

Few drug interactions studies have been performed with mifepristone. However, as a substrate and weak inhibitor of CYP3A4 enzyme, mifepristone may theoretically have clinically significant drug interactions with CYP3A4 inducers, inhibitors or substrates. Caution is advised in clients using any such medications.

Misoprostol has not been shown to have an effect on hepatic P450 enzyme systems.

Antacids may decrease the bioavailability of misoprostol, and antacids containing magnesium may aggravate diarrhoea caused by misoprostol. Otherwise, no clinically significant drug interactions have been identified with misoprostol.

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