Phenoxybenzamine hydrochloride

An alpha-adrenergic antagonist used in the treatment of pheochromocytoma

Phenoxybenzamine hydrochloride is a non-selective, irreversible alpha-adrenergic antagonist. It is primarily used in the management of pheochromocytoma, a rare tumor of the adrenal gland tissue that can lead to excessive production of catecholamines, causing hypertension and other cardiovascular symptoms.

Mechanism of Action

Phenoxybenzamine works by binding covalently to alpha-adrenergic receptors, leading to a long-lasting blockade. This action prevents the binding of endogenous catecholamines such as norepinephrine and epinephrine, resulting in vasodilation and a decrease in blood pressure. The irreversible nature of the binding means that the effects of phenoxybenzamine persist even after the drug is cleared from the bloodstream, until new receptors are synthesized.

Pharmacokinetics

Phenoxybenzamine is administered orally and has a bioavailability of 20-30%. It is highly protein-bound (90%) and undergoes hepatic metabolism. The elimination half-life is approximately 24 hours, and it is excreted primarily through the kidneys.

Clinical Uses

The primary indication for phenoxybenzamine is the treatment of hypertension and sweating associated with pheochromocytoma. It is used preoperatively to control blood pressure and prevent hypertensive crises during surgical removal of the tumor. Occasionally, it may be used in the management of benign prostatic hyperplasia (BPH) due to its ability to relax smooth muscle in the bladder neck and prostate.

Side Effects

Common side effects of phenoxybenzamine include orthostatic hypotension, tachycardia, nasal congestion, and miosis. Due to its non-selective action, it can also cause reflex tachycardia and other cardiovascular effects. Long-term use may lead to tolerance and the need for dose adjustments.

Contraindications

Phenoxybenzamine is contraindicated in patients with known hypersensitivity to the drug. Caution is advised in patients with severe coronary artery disease or cerebrovascular disease due to the risk of exacerbating these conditions.

Also see

• Pheochromocytoma
• Alpha-adrenergic antagonist
• Hypertension
• Benign prostatic hyperplasia

Receptor Antagonists
Receptor Type	Example Antagonists
Adrenergic receptor	Propranolol, Prazosin
Cholinergic receptor	Atropine, Scopolamine
Dopamine receptor	Haloperidol, Clozapine
Histamine receptor	Ranitidine, Diphenhydramine
Serotonin receptor	Ondansetron, Risperidone
Glutamate receptor	Memantine, Ketamine
GABA receptor	Flumazenil, Bicuculline
Opioid receptor	Naloxone, Naltrexone
Angiotensin receptor	Losartan, Valsartan