Labetalol

Mechanism

Mechanism of Action

Competitive antagonist drug that exerts a non-selective blockade of β adrenergic receptors and a selective blockade of postsynaptic α₁ receptors. The estimated ratio of α to β blocking potency is approximately 1:3 after oral administration and 1:7 after direct intravenous injection. It does not have significant intrinsic sympathomimetic activity, but it has mild membrane stabilizing activity. α₁ blockade induces peripheral vasodilation with reduction in peripheral vascular resistance, while simultaneous β₁ blockade limits the development of compensatory reflex tachycardia.

Pharmacokinetics

Key Pharmacokinetics

• Routes: Oral (tablets), intravenous (slow intermittent boluses or scheduled continuous infusion).
• Metabolism: Hepatic through extensive conjugation with glucuronide acid at the level of first-pass clearance; with low oral bioavailability (25%) and highly variable between individuals.
• Onset of effect: After direct IV injection, the maximum antihypertensive effect is observed between 5 to 15 minutes.
• Elimination half-life: 6 to 8 hours orally, somewhat shorter by IV route.
• Excretion: Renal, eliminating more than 60% of the conjugated metabolites in the urine.

Indicators and dose

Approved and Off-label Indications

Approved: Treatment of moderate to severe essential arterial hypertension; emergencies and hypertensive urgencies in the critical range (hypertensive crises with target organ damage, ischemic or hemorrhagic stroke with indication of lowering BP); pregnant arterial hypertension, including the clinical management of severe intrapartum preeclampsia.

Off-label: Controlled hypotension induced by intraoperative anesthetic route.

Dosage and Adjustments

Hypertensive Crisis (Intravenous): Direct IV injection of 20 mg administered slowly over a 2-minute interval. Additional boluses of 40 to 80 mg may be injected every 10 minutes up to a maximum cumulative dose of 300 mg as needed. Alternatively, start continuous IV infusion at an initial rate of 2 mg/min (titrating between 1 to 8 mg/min).

Gestational Hypertension (Oral): 100 mg twice daily orally initially, increasing progressively every other day to a usual maintenance dose of 200-400 mg twice daily.

Renal adjustment: Does not require initial dose reduction guidelines.

Hepatic adjustment: Start with the minimum effective dose available orally due to the marked metabolic decrease anticipated.

Security

Contraindications

Absolute: Symptomatic bronchial asthma, second or third degree AV block, clinically severe sinus bradycardia (< 45-50 bpm), profound cardiogenic shock, acute decompensated heart failure.

Relative: Uncontrolled pheochromocytoma with prior block.

Adverse Effects (ADR)

Common: Postural vertigo of orthostatic origin, postbolus nausea, transient headache, sudden nasal congestion, generalized fatigue, scalp paresthesias (specific due to local α₁ blockade), ejaculatory dysfunction.

Serious: High-grade AV block, severe acute bronchospasm in predisposed patients, severe idiosyncratic hepatotoxicity of an acute nature (hepatocellular necrosis), symptomatic hypotension with a refractory profile.

Interactions

General anesthetic agents (e.g., halothane): Profound hypotensive synergy and refractory myocardial depression.

Non-dihydropyridine calcium antagonists: High risk of induction of severe bradycardia with advanced atrioventricular block.

Pregnancy and Breastfeeding

FDA Classification: Category C. It is considered of choice, safe and effective for the management of preeclampsia and severe gestational hypertension during pregnancy. It is excreted in breast milk in minimal proportions and is considered compatible with natural breastfeeding.