PHENOXYBENZAMINE HYDROCHLORIDE
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PHENOXYBENZAMINE HYDROCHLORIDE (PHENOXYBENZAMINE HYDROCHLORIDE).
Phenoxybenzamine hydrochloride is a nonselective, irreversible alpha-adrenergic antagonist. It blocks both alpha-1 and alpha-2 receptors, leading to vasodilation, decreased peripheral vascular resistance, and relaxation of smooth muscle in the bladder neck and prostate.
| Metabolism | Phenoxybenzamine undergoes hepatic metabolism via oxidation and conjugation. The major metabolic pathway involves CYP450 enzymes, particularly CYP3A4 and CYP2D6, with formation of active and inactive metabolites. Excretion is primarily renal as metabolites. |
| Excretion | Primarily renal (approximately 50% as unchanged drug and metabolites); biliary/fecal excretion accounts for a minor portion (<10%). |
| Half-life | Terminal elimination half-life is approximately 24 hours (range 12–48 hours), allowing once-daily dosing after titration. Clinical context: Steady-state is reached in 4–5 days. |
| Protein binding | Highly bound (>99%) to plasma proteins, primarily albumin and alpha-1-acid glycoprotein. |
| Volume of Distribution | Approximately 0.6 L/kg (range 0.3–1.0 L/kg), indicating extensive tissue distribution. |
| Bioavailability | Oral: Approximately 20–30% due to incomplete absorption and extensive first-pass metabolism; IV: 100%. |
| Onset of Action | Oral: 1–2 hours. Intravenous: 1–2 minutes (rapid alpha-blockade). Clinical effect (e.g., blood pressure reduction) is seen within 1–2 hours after oral administration. |
| Duration of Action | Oral: 12–48 hours (up to 72 hours with continued use); intravenous: 5–10 minutes for initial effect, but residual blockade may last 24–48 hours due to irreversible binding to alpha-receptors. Clinical notes: The drug forms a covalent bond, so effect persists beyond plasma clearance. |
10 mg orally twice daily, increase by 10 mg every 4 days to a maximum of 240 mg/day in divided doses.
| Dosage form | CAPSULE |
| Renal impairment | No dose adjustment required for renal impairment. |
| Liver impairment | Child-Pugh class A: no adjustment; Child-Pugh class B or C: reduce dose by 50%. |
| Pediatric use | 0.2-0.4 mg/kg orally twice daily, not to exceed 10 mg per dose. |
| Geriatric use | Initiate at 5 mg orally twice daily, titrate slowly due to increased risk of hypotension. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for PHENOXYBENZAMINE HYDROCHLORIDE (PHENOXYBENZAMINE HYDROCHLORIDE).
| Breastfeeding | Unknown if excreted in human milk. Due to potential for serious adverse reactions in nursing infants, including hypotension and drowsiness, caution is advised. The M/P ratio is not reported. Consider discontinuing nursing or the drug, taking into account maternal importance. |
| Teratogenic Risk | Pregnancy Category C. Animal studies have demonstrated embryotoxicity and fetotoxicity at doses 2-3 times the maximum human dose. In humans, limited data suggest an increased risk of fetal respiratory depression and hypotension if administered near term. First trimester exposure may be associated with a slight increase in congenital anomalies, but definitive human studies are lacking. |
■ FDA Black Box Warning
None
| Serious Effects |
["Hypersensitivity to phenoxybenzamine or any component","Concurrent use with drugs that block both alpha and beta receptors (e.g., labetalol) due to risk of severe hypotension","Hypotension or shock","Recent myocardial infarction or stroke (relative contraindication due to risk of severe hypotension)"]
| Precautions | ["May cause orthostatic hypotension, syncope, and reflex tachycardia","May exacerbate heart failure, angina, or cerebrovascular disease due to rapid hypotension","Intravenous administration requires careful monitoring of blood pressure and heart rate","May cause nasal congestion, miosis, and inhibition of ejaculation","Use with caution in patients with renal or hepatic impairment"] |
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| Fetal Monitoring | Monitor maternal blood pressure and heart rate closely, especially during initiation and dose titration. In pregnancy, monitor fetal heart rate and uterine tone if used for hypertensive crisis. Assess for maternal and fetal signs of hypotension and bradycardia. Evaluate renal function and electrolyte status periodically. |
| Fertility Effects | In animal studies, oral administration of phenoxybenzamine to male rats at doses of 30 mg/kg/day for 60 days resulted in decreased fertility due to impaired ejaculation. Effects on human fertility are unknown; however, alpha-blockade can cause ejaculatory dysfunction in males. No significant effects on female fertility have been reported. |