Phenoxybenzamine

Mechanism

Mechanism of Action

Non-selective adrenergic blocker of α₁ and α₂ receptors that exerts its action through an irreversible covalent bond. Chemically, it undergoes spontaneous intracellular cyclization to form a highly reactive aziridinium molecule that binds covalently to the amino acid residues of the α-adrenergic receptor. This alkylation blocks the receptor definitively, preventing the competitive binding of norepinephrine and adrenaline. Restoration of the normal tonic adrenergic response requires de novo synthesis of new receptors by the vascular smooth muscle cell, which prolongs the biological effect of the drug for a period of 3 to 4 days after cessation of administration.

Pharmacokinetics

Key Pharmacokinetics

• Routes: Oral (capsules).
• Bioavailability: Low and irregular orally (20-30%).
• Distribution: High accumulation in reserve adipose tissue due to its high structural lipophilicity.
• Metabolism: Hepatic through desakilation and oxidative conjugation.
• Elimination half-life: Approximately 24 hours in plasma, but its biological effect half-life is 3 to 4 days due to the nature of its covalent binding.
• Excretion: Renal and biliary.

Indicators and dose

Approved and Off-label Indications

Approved: Preoperative management of patients diagnosed with pheochromocytoma to control and mitigate paroxysmal hypertensive episodes induced by surgical manipulation of the adrenal gland.

Off-label: Symptomatic treatment of hypertensive crises secondary to massive catecholamine overload (e.g., overdose of cocaine, amphetamines, or abrupt discontinuation of clonidine).

Dosage and Adjustments

Management of Pheochromocytoma (Preoperative): Initial dose of 10 mg orally twice daily. Increase the dose progressively by 10-20 mg every other day until stable optimal blood pressure control is achieved without experiencing disabling orthostatic hypotension. The usual maintenance range is 40-120 mg/day divided into two or three doses, ideally administered during the 10 to 14 days prior to the scheduled intervention.

Renal/hepatic adjustment: Requires close clinical monitoring of blood pressure function and hydration due to the risk of accumulation.

Security

Contraindications

Absolute: Clinical scenarios where it is required to maintain elevated mean arterial pressure and tonic systolic afterload (e.g., deep cardiogenic shock, severe mitral or aortic stenosis).

Relative: Advanced chronic renal failure, severe active coronary artery disease.

Adverse Effects (ADR)

Common: Severe disabling orthostatic hypotension, pronounced reflex sinus tachycardia (caused by concomitant blockade of presynaptic α₂ receptors that increases the release of cardiac noradrenaline), bilateral nasal mucosal congestion, pupillary miosis, transient anejaculation due to contractile failure of the seminal duct.

Serious: Refractory myocardial ischemia induced by extreme reflex tachycardia in susceptible patients, profound orthostatic syncope with loss of consciousness.

Interactions

β-adrenergic agonists (e.g., adrenaline): Irreversible blockade of α receptors allows adrenaline to exclusively activate vasodilatory β₂ receptors, inducing an extreme paradoxical drop in systemic blood pressure ("Dale reversal" or reverse vasoconstriction).

Direct vasodilators: Deep refractory hypotensive potentiation.

Pregnancy and Breastfeeding

FDA Classification: Category C. It crosses the placenta and the induction of arterial hypotension and severe neonatal respiratory distress has been described. It is completely discouraged during active breastfeeding.