Nifedipine

Mechanism

Mechanism of Action

Nifedipine is a first-generation dihydropyridine that acts as a selective blocker of L-type voltage-gated calcium channels (α₁ subunit) located in the membranes of uterine myometrial cells and peripheral arterial vascular smooth muscle cells.

Mechanism of Myometrial Quiescence due to Calcium Blockade

1. Inhibition of Calcium entry: By blocking the L-type calcium channel pore, nifedipine decreases the basal entry of extracellular calcium into the gravid myometrial myocyte induced by depolarization.
2. Inactivation of MLCK: Without this massive influx, intracellular calcium levels decrease, preventing the activation of myosin light chain kinase and the formation ofACTOMYOSIN cross-bridges.
3. Effective tocolysis: The uterus ceases its coordinated contractile activity. Secondarily, relaxation of arterial smooth muscle reduces systemic vascular resistance.

Pharmacokinetics

Pharmacokinetics

• Rout of administration: Exclusively oral (the sublingual route for rapid control is contraindicated due to the risk of sudden hypotension and placental hypoperfusion). For tocolysis, conventional quick-release capsules are preferred; for chronic hypertension, extended-release tablets.
• Bioavailability (F): 40-50% due to a significant gastric and hepatic first-pass metabolic effect.
• Metabolism: Hepatic oxidation catalyzed almost exclusively by the isoenzyme CYP3A4 to inactive metabolites. The induction of CYP3A4 by gestational levels of progesterone physiologically accelerates its hepatic clearance.
• Half-life (t_1/2): Short for conventional capsules (approx. 2 to 3 hours); It extends to 10-12 hours in delayed release formulations (retard/OROS).
• Excretion: Renal 80% as inactive metabolites; 20% via bile-fecal route.

Indicators and dose

Indications

• First-line tocolytic: Inhibition of uterine contractions in imminent preterm delivery (between 24 and 34 weeks of gestation) to allow the transfer of the pregnant woman to a tertiary center and complete the cycle of fetal lung maturation with corticosteroids.
• Treatment of chronic or gestational arterial hypertension in pregnancy (with extended-release formulations).

Dosing and Precision Adjustment

Standard tocolytic protocol (Immediate release):

• Initial loading dose: 10 to 20 mg orally (tablet swallowed whole, never crushed or sublingual). A dose of 10-20 mg can be repeated after 30 minutes if regular contractions persist (maximum 40 mg in the first hour).
• Maintenance dose: 10 to 20 mg orally every 6 to 8 hours, adjusting according to uterine dynamics and maternal hemodynamic tolerance, up to a maximum of 120 mg/day. Do not extend more than 48 hours.

Pregnancy and Breastfeeding

FDA Category C. Extensive clinical experience demonstrating an acceptable safety profile in the second and third trimester for tocolysis and gestational hypertension. Compatible with breastfeeding; It is excreted in milk in minimal concentrations incapable of altering the infant's blood pressure.

Security

Contraindications

Absolute

• Hypersensitivity to nifedipine or dihydropyridine derivatives.
• Severe baseline maternal hypotension (SBP < 90 mmHg).
• Severe maternal heart disease (clinically significant aortic stenosis, decompensated heart failure).
• Maternal sepsis or active chorioamnionitis.

Relative

• Simultaneous coadministration with magnesium sulfate (synergistic risk of neuromuscular blockade and cardiovascular collapse).
• Advanced liver disease (markedly slowed clearance that increases plasma levels).

Adverse Effects (ADR)

• Maternal Cardiovascular: Arterial hypotension (usually moderate), reflex tachycardia (compensatory for the decrease in peripheral resistance), temporary facial flushing, pulsating headache and peripheral edema in the lower extremities (microvascular drip without renal origin).
• Fetal / Uteroplacental: The primary risk is transient placental hypoperfusion if a sudden drop in maternal blood pressure is generated, which may induce fetal bradycardia.

Interactions

CYP3A4 inhibitors (e.g., erythromycin, grapefruit juice, protease inhibitors) exponentially elevate nifedipine plasma concentrations, requiring close monitoring. Magnesium sulfate and beta-blockers enhance its hypotensive and myocardial block effect.