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Phenoxybenzamine

Pronunciation

(fen oks ee BEN za meen)

Index Terms

  • Phenoxybenzamine Hydrochloride

Dosage Forms

Excipient information presented when available (limited, particularly for generics); consult specific product labeling.

Capsule, Oral, as hydrochloride:

Dibenzyline: 10 mg [contains fd&c yellow #6 (sunset yellow)]

Generic: 10 mg

Brand Names: U.S.

  • Dibenzyline

Pharmacologic Category

  • Alpha1 Blocker
  • Antidote

Pharmacology

Produces long-lasting noncompetitive alpha-adrenergic blockade of postganglionic synapses in exocrine glands and smooth muscle; relaxes urethra and increases opening of the bladder

Distribution

Distributes to and may accumulate in adipose tissues

Excretion

Primarily in urine and bile

Onset of Action

Within 2 hours; Maximum effect: Within 4 to 6 hours

Duration of Action

IV: ≥3 to 4 days

Half-Life Elimination

IV: ~24 hours

Use: Labeled Indications

Pheochromocytoma: Treatment of sweating and hypertension associated with pheochromocytoma.

Use: Unlabeled

Micturition problems associated with neurogenic bladder, functional outlet obstruction, and partial prostate obstruction; treatment of hypertensive crisis caused by sympathomimetic amines

Contraindications

Hypersensitivity to drug or any component of the formulation; conditions in which a fall in blood pressure may be undesirable.

Dosing: Adult

Pheochromocytoma: Oral: Initial: 10 mg twice daily, may increase slowly every other day until optimal blood pressure response is achieved; usual dosage range: 20 to 40 mg 2 to 3 times/day. Doses up to 240 mg/day have been reported (Kinney 2000).

Micturition disorders (off-label use): Oral: 10 to 20 mg 1 to 2 times/day (Te 2002)

Dosing: Geriatric

Refer to adult dosing.

Dosing: Pediatric

Pheochromocytoma (off-label uses): Oral: Initial: 0.25 to 1 mg/kg/day; increase slowly to blood pressure control (Kinney 2002)

Dosing: Renal Impairment

There are no dosage adjustments provided in the manufacturer's labeling; use with caution.

Dosing: Hepatic Impairment

There are no dosage adjustments provided in the manufacturer's labeling.

Extemporaneously Prepared

Hazardous agent; use appropriate precautions for handling and disposal (NIOSH 2014 [group 2]).

A 2 mg/mL oral suspension may be made with capsules, propylene glycol 1%, and citric acid 0.15% in distilled water. Prepare the vehicle by dissolving 150 mg citric acid in a minimal amount of distilled water. Add 1 mL propylene glycol and mix well; add quantity of distilled water sufficient to make 100 mL (only a small portion of this vehicle will be used to make the final product). Grind the contents of two phenoxybenzamine 10 mg capsules in a mortar and reduce to a fine powder. Add a small portion of the vehicle and mix to a uniform paste; transfer to a graduated cylinder, rinse mortar with vehicle, and add quantity of prepared vehicle sufficient to make 10 mL. Transfer to an amber glass prescription bottle with tight-fitting cap; label "shake well" and "refrigerate". Stable for 7 days when stored in amber glass prescription bottles and refrigerated.

A stock solution of 10 mg/mL in propylene glycol was stable for 30 days refrigerated. When this stock solution was diluted 1:4 (v/v) with syrup (66.7% sucrose) to 2 mg/mL, the preparation was stable for 1 hour refrigerated. Note: Although the stock solution is stable for 30 days, it must be diluted before administration to decrease the amount of propylene glycol delivered to the patient.

Lim LY, Tan LL, Chan EW, et al, "Stability of Phenoxybenzamine Hydrochloride in Various Vehicles," Am J Health Syst Pharm, 1997, 54(18):2073-8.9377206

Administration

Hazardous agent; use appropriate precautions for handling and disposal (NIOSH 2014 [group 2]).

Storage

Store at 25°C (77°F); excursions permitted to 15°C to 30°C (59°F to 86°F).

Drug Interactions

Alpha-/Beta-Agonists: Alpha1-Blockers may diminish the vasoconstricting effect of Alpha-/Beta-Agonists. Similarly, Alpha-/Beta-Agonists may antagonize Alpha1-Blocker vasodilation. Monitor therapy

Alpha1-Agonists: Alpha1-Blockers may diminish the vasoconstricting effect of Alpha1-Agonists. Similarly, Alpha1-Agonists may antagonize Alpha1-Blocker vasodilation. Monitor therapy

Alpha1-Blockers: May enhance the antihypertensive effect of other Alpha1-Blockers. Avoid combination

Amifostine: Blood Pressure Lowering Agents may enhance the hypotensive effect of Amifostine. Management: When amifostine is used at chemotherapy doses, blood pressure lowering medications should be withheld for 24 hours prior to amifostine administration. If blood pressure lowering therapy cannot be withheld, amifostine should not be administered. Consider therapy modification

Amphetamines: May diminish the antihypertensive effect of Antihypertensive Agents. Monitor therapy

Antipsychotic Agents (Second Generation [Atypical]): Blood Pressure Lowering Agents may enhance the hypotensive effect of Antipsychotic Agents (Second Generation [Atypical]). Monitor therapy

Barbiturates: May enhance the hypotensive effect of Blood Pressure Lowering Agents. Monitor therapy

Beta-Blockers: May enhance the orthostatic hypotensive effect of Alpha1-Blockers. The risk associated with ophthalmic products is probably less than systemic products. Exceptions: Levobunolol; Metipranolol. Monitor therapy

Brimonidine (Topical): May enhance the hypotensive effect of Blood Pressure Lowering Agents. Monitor therapy

Calcium Channel Blockers: Alpha1-Blockers may enhance the hypotensive effect of Calcium Channel Blockers. Monitor therapy

Dapoxetine: May enhance the orthostatic hypotensive effect of Alpha1-Blockers. Monitor therapy

Diazoxide: May enhance the hypotensive effect of Blood Pressure Lowering Agents. Monitor therapy

DULoxetine: Blood Pressure Lowering Agents may enhance the hypotensive effect of DULoxetine. Monitor therapy

Herbs (Hypertensive Properties): May diminish the antihypertensive effect of Antihypertensive Agents. Monitor therapy

Herbs (Hypotensive Properties): May enhance the hypotensive effect of Blood Pressure Lowering Agents. Monitor therapy

Hypotension-Associated Agents: Blood Pressure Lowering Agents may enhance the hypotensive effect of Hypotension-Associated Agents. Monitor therapy

Levodopa: Blood Pressure Lowering Agents may enhance the hypotensive effect of Levodopa. Monitor therapy

Methylphenidate: May diminish the antihypertensive effect of Antihypertensive Agents. Monitor therapy

Molsidomine: May enhance the hypotensive effect of Blood Pressure Lowering Agents. Monitor therapy

Nicorandil: May enhance the hypotensive effect of Blood Pressure Lowering Agents. Monitor therapy

Nitroprusside: Blood Pressure Lowering Agents may enhance the hypotensive effect of Nitroprusside. Monitor therapy

Obinutuzumab: May enhance the hypotensive effect of Blood Pressure Lowering Agents. Management: Consider temporarily withholding blood pressure lowering medications beginning 12 hours prior to obinutuzumab infusion and continuing until 1 hour after the end of the infusion. Consider therapy modification

Pentoxifylline: May enhance the hypotensive effect of Blood Pressure Lowering Agents. Monitor therapy

Phosphodiesterase 5 Inhibitors: May enhance the hypotensive effect of Alpha1-Blockers. Management: Ensure patient is stable on one agent prior to initiating the other, and always initiate combination using the lowest possible dose of the drug being added. When tadalafil is used for treatment of BPH, concurrent alpha 1-blockers are not recommended. Consider therapy modification

Prostacyclin Analogues: May enhance the hypotensive effect of Blood Pressure Lowering Agents. Monitor therapy

Quinagolide: May enhance the hypotensive effect of Blood Pressure Lowering Agents. Monitor therapy

Yohimbine: May diminish the antihypertensive effect of Antihypertensive Agents. Monitor therapy

Adverse Reactions

Frequency not defined.

Cardiovascular: Orthostatic hypotension, tachycardia

Central nervous system: Drowsiness, fatigue

Gastrointestinal: GI irritation

Genitourinary: Inhibition of ejaculation

Ocular: Miosis

Respiratory: Nasal congestion

Warnings/Precautions

Concerns related to adverse effects:

• Cardiovascular effects: An exaggerated hypotensive response and tachycardia may occur when administered concurrently with compounds that stimulate both alpha- and beta-adrenergic receptors.

Disease-related concerns:

• Cardiovascular disease: Use with caution in patients with marked cerebral or coronary atherosclerosis.

• Renal impairment: Use with caution in patients with renal impairment.

• Respiratory infection: May aggravate symptoms of respiratory infections.

Concurrent drug therapy issues:

• Drug-drug interactions: Potentially significant interactions may exist, requiring dose or frequency adjustment, additional monitoring, and/or selection of alternative therapy. Consult drug interactions database for more detailed information.

Dosage form specific issues:

• Benzyl alcohol and derivatives: Some dosage forms may contain benzyl alcohol; large amounts of benzyl alcohol (≥99 mg/kg/day) have been associated with a potentially fatal toxicity ("gasping syndrome") in neonates; the "gasping syndrome" consists of metabolic acidosis, respiratory distress, gasping respirations, CNS dysfunction (including convulsions, intracranial hemorrhage), hypotension, and cardiovascular collapse (AAP ["Inactive" 1997]; CDC, 1982); some data suggest that benzoate displaces bilirubin from protein binding sites (Ahlfors, 2001); avoid or use dosage forms containing benzyl alcohol with caution in neonates. See manufacturer's labeling.

Special handling:

• Hazardous agent: Use appropriate precautions for handling and disposal (NIOSH 2014 [group 2]).

Other warnings/precautions:

• Long-term use: Not recommended for long-term use due to case reports of cancer in humans; carefully weigh the risk and benefits before use.

Monitoring Parameters

Blood pressure, pulse, orthostatics

Pregnancy Risk Factor

C

Pregnancy Considerations

Adequate animal reproduction studies have not been conducted. It is not known whether phenoxybenzamine can cause fetal harm when administered to a pregnant woman or can affect reproduction capacity.

Patient Education

• Discuss specific use of drug and side effects with patient as it relates to treatment. (HCAHPS: During this hospital stay, were you given any medicine that you had not taken before? Before giving you any new medicine, how often did hospital staff tell you what the medicine was for? How often did hospital staff describe possible side effects in a way you could understand?)

• Patient may experience fatigue, rhinitis, nausea, or loss of strength and energy. Have patient report immediately to prescriber severe dizziness, passing out, tachycardia, sexual dysfunction, or small pupils (HCAHPS).

• Educate patient about signs of a significant reaction (eg, wheezing; chest tightness; fever; itching; bad cough; blue skin color; seizures; or swelling of face, lips, tongue, or throat). Note: This is not a comprehensive list of all side effects. Patient should consult prescriber for additional questions.

Intended Use and Disclaimer: Should not be printed and given to patients. This information is intended to serve as a concise initial reference for health care professionals to use when discussing medications with a patient. You must ultimately rely on your own discretion, experience, and judgment in diagnosing, treating, and advising patients.

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