Early Options: Mifepristone and Misoprostol - Pharmacology and Mechanism of Action

Early Options - A Provider's Guide to Medical Abortion

After reading this page, you should be able to answer the following question:
Misoprostol is a prostaglandin analogue. What are its characteristics?

Mifepristone is classified as an antiprogestin because it binds avidly to the progesterone receptor without activating it.¹ Mifepristone is similar to norethindrone, a progestin used in some oral contraceptives. Mifepristone has a long side-chain at the 17-carbon position. This makes the molecule bind to the progesterone receptor with an affinity equal to or greater than progesterone itself.²

The addition of a bulky side chain at the 11-carbon position renders mifepristone inactive. As a result of these structural modifications, the mifepristone molecule blocks the action of progesterone at the receptor level. Click here to view Figure 1.

About 18 hours after mifepristone administration during early pregnancy, the trophoblast separates from the decidua, levels of ß-hCG in the maternal circulation decline, and prostaglandins are released locally.³ Uterine bleeding results. Mifepristone also induces cervical ripening and rhythmic uterine contractions.

When used alone, mifepristone is not adequately effective; at best, roughly 80% of women up to 49 days' gestation will completely abort.⁴ For this reason, researchers explored the effect of adding a prostaglandin to the regimen.⁵ In the United States, misoprostol is the prostaglandin analogue most widely used to augment the medical abortion process by stimulating uterine contractions. Click here to view Figure 2.

Unlike some earlier prostaglandin analogues studied as abortifacients, misoprostol is orally active, stable at room temperature, and relatively inexpensive. In addition, it is well absorbed following oral, vaginal, buccal or sublingual administration and has a proven safety record. (Note: Only the oral route of administration is FDA-approved.)

Click here to check your understanding.

Proceed to The Standard, FDA-Approved Regimen.

References for this module