Drug Interactions between amyl nitrite/sodium nitrite/sodium thiosulfate and phenelzine

MONITOR CLOSELY: Sodium nitrite can cause methemoglobin formation, which diminishes oxygen-carrying capacity of the blood. Coadministration with other agents that are also associated with methemoglobinemia including local anesthetics (e.g., benzocaine, lidocaine, prilocaine), antimalarials (e.g., chloroquine, primaquine, quinine, tafenoquine), nitrates and nitrites, sulfonamides, aminosalicylic acid, dapsone, dimethyl sulfoxide, flutamide, metoclopramide (primarily in infants), nitrofurantoin (primarily in infants), phenazopyridine, phenobarbital, phenytoin, and rasburicase may increase the risk. Additional risk factors include very young age, anemia, cardiac/pulmonary disease, peripheral vascular disease, shock, sepsis, acidosis, and genetic predisposition (e.g., NADH cytochrome-b5 reductase deficiency; glucose-6-phosphate dehydrogenase deficiency; hemoglobin M). When sodium nitrite is administered to humans, a wide range of methemoglobin concentrations may occur. Methemoglobin concentrations as high as 58% have been reported after administration of two 300 mg doses to an adult. There have been reports of methemoglobinemia, coma, and death in patients without life-threatening cyanide poisoning but who were treated with injection of sodium nitrite at dosages less than twice those recommended for the treatment of cyanide poisoning.

MANAGEMENT: Sodium nitrite should be used with caution in the presence of other methemoglobin-inducing drugs. Patients should be closely monitored to ensure adequate perfusion and oxygenation during treatment with sodium nitrite. Methemoglobin levels should be monitored and oxygen administered whenever possible. Signs and symptoms of methemoglobinemia may be delayed some hours after drug exposure. Patients or their caregivers should be advised to seek medical attention if they notice signs and symptoms of methemoglobinemia such as slate-grey cyanosis in buccal mucous membranes, lips, and nail beds; nausea; headache; dizziness; lightheadedness; lethargy; fatigue; dyspnea; tachypnea; tachycardia; anxiety; and confusion. In severe cases, patients may progress to central nervous system depression, stupor, seizures, acidosis, cardiac arrhythmias, syncope, and shock. Methemoglobinemia should be considered if central cyanosis is unresponsive to oxygen. Calculated oxygen saturation and pulse oximetry are generally not accurate in the setting of methemoglobinemia. The diagnosis can be confirmed by an elevated methemoglobin level of at least 10%. If patient does not respond to administration of oxygen, clinically significant methemoglobinemia should be treated with methylene blue 1 to 2 mg/kg by slow intravenous injection over 5 minutes.

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MONITOR: Monoamine oxidase inhibitors (MAOIs) may potentiate the hypotensive effect of some medications. MAOIs alone quite commonly produce orthostatic hypotension. This effect may stem from a gradual MAOI-induced accumulation of false neurotransmitters in peripheral adrenergic neurons that have minimal activity at alpha- and beta-adrenergic receptors, resulting in a functional block of sympathetic neurotransmission. The interaction has been reported with the concomitant use of beta-blockers. In one report, a young woman developed marked orthostatic hypotension following the addition of pindolol 2.5 mg three times a day to an existing regimen of tranylcypromine. The pindolol dosage was reduced to 2.5 mg twice a day until her blood pressure stabilized, then slowly increased to 5 mg three times a day.

MANAGEMENT: Caution is advised during coadministration of MAOIs and other medications with hypotensive effects, especially during the first few weeks of treatment. Close monitoring for development of hypotension is recommended. Ambulatory patients should be advised to avoid rising abruptly from a sitting or recumbent position and to notify their physician if they experience dizziness, lightheadedness, syncope, orthostasis, or tachycardia.

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