Drug Interactions between InnoPran XL and mephobarbital

MONITOR: Coadministration with barbiturates may decrease the plasma concentrations and pharmacologic effects of certain beta-blockers when administered orally. The proposed mechanism is barbiturate induction of hepatic microsomal and first-pass metabolism. The interaction has been studied with alprenolol, metoprolol and timolol, but probably can occur with any beta-blocker that is primarily metabolized in the liver such as propranolol. In six healthy volunteers, pretreatment with pentobarbital (100 mg daily for 10 days) reduced the plasma levels of alprenolol (200 mg single oral dose) and its metabolite 4-hydroxyalprenolol by approximately 40% without altering their plasma half-lives. The inhibition of exercise-induced tachycardia during a 7-hour period following alprenolol administration was reduced from 14% to 10.7% by pentobarbital, and the reduction was proportional to the decreased drug levels. In another study, pentobarbital reduced alprenolol levels by 59% and 4-hydroxyalprenolol levels by 24% in 6 hypertensive patients treated with alprenolol 400 mg twice daily. The effect of pentobarbital was significant after 3 doses and declined over 4 to 5 days after discontinuation. The decreases were associated with a 6% increase in pulse rate and 8% increase in systolic and 9% increase in diastolic blood pressure, as well as an 18% reduction in inhibition of exercise tachycardia by alprenolol. In eight healthy subjects, administration of metoprolol (100 mg single oral dose) with pentobarbital (100 mg daily for 10 days) resulted in a mean 32% reduction in metoprolol systemic exposure (AUC) compared to administration alone. The same dose of pentobarbital given for 7 days reduced AUC of timolol (10 mg) by just 24% in 12 healthy volunteers.

MANAGEMENT: Barbiturates may variously reduce the effects of certain beta-blockers when given for more than a few days. Pharmacologic response to beta-blockers should be monitored more closely whenever a barbiturate is added to or withdrawn from therapy, and the beta-blocker dosage adjusted as necessary. Renally excreted beta-blockers such as atenolol, carteolol, nadolol, or sotalol are not expected to interact.

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