Drug Interaction Report

GENERALLY AVOID: Coadministration with potent inducers of CYP450 3A4 may significantly decrease the plasma concentrations of macitentan, which is primarily metabolized by the isoenzyme. In ten healthy male subjects pretreated with macitentan (30 mg initially, followed by 10 mg once daily) for 5 days, coadministration with the potent CYP450 3A4 inducer rifampin (600 mg once daily) on days 6 through 12 reduced macitentan systemic exposure (AUC) by 79% and trough plasma concentration (Cmin) by 93% compared to macitentan administered alone. An initial increase in the Cmin of the active metabolite of macitentan was observed following the addition of rifampin on day 6, but a gradual decrease occurred with continued dosing, resulting in concentrations on day 12 that were similar to those observed with macitentan alone. There was also no significant effect of rifampin on the AUC of the active metabolite, which has been reported to be approximately 5-fold less potent than macitentan in vitro, but whose systemic exposure in human is 2.5-fold higher than that of macitentan.

MANAGEMENT: Concomitant use of macitentan with potent CYP450 3A4 inducers should generally be avoided.

GENERALLY AVOID: Alcohol may potentiate some of the pharmacologic effects of carbamazepine. Use in combination may result in additive central nervous system depression and/or impairment of judgment, thinking, and psychomotor skills.

In a small, randomized, crossover study, the administration of carbamazepine with grapefruit juice (compared to water) increased plasma drug concentrations by approximately 40%. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits.

MANAGEMENT: Patients receiving carbamazepine should be advised to avoid or limit consumption of alcohol. Given the drug's narrow therapeutic index, patients receiving carbamazepine therapy should preferably avoid the regular consumption of grapefruits and grapefruit juice to prevent any undue fluctuations in plasma drug levels. Patients should be advised to report signs of carbamazepine toxicity (nausea, visual disturbances, dizziness, or ataxia) to their physicians.