Drug Interaction Report

MONITOR: Concomitant use of moderate CYP450 3A4 inducers with tretinoin (systemic) may decrease the plasma concentration of tretinoin, which is a CYP450 3A4 substrate. Reduced efficacy of tretinoin may occur. Studies document the interaction between endogenous retinoids and CYP450 3A4 inducers (e.g., carbamazepine, phenytoin, phenobarbital) showing decreased concentration of the retinoids and reduced efficacy. While clinical data specific to tretinoin are not available, similar effects may occur.

MANAGEMENT: The manufacturer recommends monitoring patients for reduced efficacy of tretinoin.

ADJUST DOSING INTERVAL: Coadministration with food increases the oral bioavailability of etravirine. The mechanism is unknown. Compared to administration following a meal, the systemic exposure (AUC) to etravirine was decreased by about 50% when the drug was administered under fasting conditions. The types of meal studied (ranging from 345 kilocalories containing 17 grams fat to 1160 kilocalories containing 70 grams fat) did not appear to make a difference with respect to impact on etravirine bioavailability.

MANAGEMENT: Etravirine should always be administered following a meal.