Misoprostol

Mechanism

Mechanism of action

The gastric parietal cell has specific receptors for prostaglandins of the E series type 3 (EP₃) in its basolateral membrane. These receptors are functionally coupled to the G_{α i} protein. Misoprostol, after being metabolized to its active acid form, binds and activates the EP₃ receptor, resulting in direct inhibition of adenylate cyclase.

This molecular blockade significantly reduces the intracellular production of cAMP induced by physiological stimuli (such as histamine or food), inhibiting cellular kinase activity and stopping the translocation of H⁺/K⁺-ATPase pumps to the canaliculi membrane.

At the mucosal level (Cytoprotection), the binding of misoprostol to EP receptors on gastric epithelial cells stimulates the active secretion of bicarbonate (HCO₃^-) and gastric mucin mucus, improves mucosal blood flow through local vasodilation and promotes regeneration of the epithelium after chemical attacks.

Pharmacokinetics

Pharmacokinetics

• Absorption: Extremely rapid and extensive oral absorption (88%). It undergoes immediate hepatic first-pass metabolism. The presence of high-fat foods delays T_max but does not decrease the area under the curve (AUC).
• Distribution: Low apparent volume of distribution. Plasma protein binding of misoprostol acid (active metabolite) of ≈ 80% to 90%.
• Metabolism: Rapid and extensive metabolic deesterification in the liver and other body tissues to form misoprostol acid (biologically active metabolite responsible for the entire cytoprotective and uterotonic response).
• Excretion: Predominantly renal (≈ 73% of water-soluble metabolites are eliminated in urine; <1% unchanged). Fecal elimination of 15%.
• Half-life: Short. Of the original misoprostol it is a few minutes; of the active acid metabolite is approximately 20 to 40 minutes. Requires repeated doses to maintain the gastric protective effect.

Indicators and dose

Approved and Off-label indications

• Prevention and prophylaxis of gastric and duodenal ulcers induced chronically by therapy with non-steroidal anti-inflammatory drugs (NSAIDs) in high-risk patients.
• Cervical ripening and labor induction at term (Off-label/Controlled obstetric indication).
• Medical treatment of missed or incomplete abortion and prophylaxis/treatment of postpartum hemorrhage secondary to uterine atony (Off-label/Obstetrics).

Dosage and Clinical Adjustment

Adults (Prevention of ulcers due to NSAIDs):

• 200 micrograms (µg) orally four times a day, with meals and at bedtime, to ensure optimal local protection of the mucosa.
• If the patient tolerates the starting dose poorly, it can be reduced to 100 µg four times a day or 200 µg every 12 hours, although with less proven cytoprotective efficacy.

Adjustment in Renal Failure: No systematic dose adjustment is required in patients with renal failure. However, if renal clearance is severely decreased (GFR < 15 mL/min), the maximum plasma concentrations of the active metabolite may double, so it is recommended to start with a dose of 100 µg and closely monitor tolerance.

Adjustment in Liver Failure: Does not require dose modifications.

Security

Teratogenicity and Abortive Effect Alert (Black Box)

Misoprostol directly stimulates EP receptors present on uterine myometrial smooth muscle cells, inducing intense uterine contractions, cervical softening and severe uterine hemorrhages. Its administration during pregnancy is causally associated with a high risk of spontaneous abortion and severe congenital fetal malformations (such as Moebius syndrome: bilateral paralysis of the VI and VII cranial nerves, and limb reduction defects). It is strictly contraindicated in women of childbearing age who do not use effective contraceptive measures.

Contraindications

• Absolute: Active pregnancy in patients who wish to continue the pregnancy. Women of childbearing potential without a highly effective contraceptive method or with positive urine or serum pregnancy tests at the start of treatment. Hypersensitivity to prostaglandins.
• Relative: Active inflammatory bowel disease (can exacerbate basal inflammation and diarrhea). Severe coronary artery disease or cerebral vascular insufficiency (transient hypotension induced by vasodilation may compromise coronary or cerebral flow).

Adverse effects (ADRs)

Drug interactions

• Magnesium-based Antacids: Concomitant administration with antacids containing magnesium salts exacerbates and increases the incidence and severity of misoprostol-induced diarrhea. Recommendation: Preferably use antacids based on calcium carbonate or aluminum hydroxide if rapid symptomatic relief is required.
• NSAIDs: Beneficial therapeutic synergy; They do not alter the pharmacokinetic parameters of misoprostol.

Breastfeeding Safety

Misoprostol acid is excreted in breast milk in small and inconsistent amounts. Exposed infants may develop transient clinical diarrhea or other gastrointestinal symptoms. If maternal therapeutic use is required, it is advisable to suspend breastfeeding or postpone feedings for at least 4-5 hours after each dose.