Drug Interaction Report

GENERALLY AVOID: Coadministration with potent inducers of CYP450 3A4 may decrease the plasma concentrations of vemurafenib, which has been shown in vitro to be a substrate of the isoenzyme. According to the product labeling, administration of a single 960 mg dose of vemurafenib with the potent CYP450 3A4 inducer rifampin at a dosage of 600 mg once daily resulted in a 40% decrease in vemurafenib systemic exposure (AUC) relative to vemurafenib administered alone, with no effect on peak plasma concentration (Cmax). Reduced therapeutic efficacy of vemurafenib may occur. In addition, when two or more medications with similar adverse effect profiles are given concurrently, the likelihood of experiencing these adverse reactions may be increased. For example, coadministration with other agents that can prolong the QT interval (e.g., apalutamide, encorafenib, enzalutamide) may result in additive effects and an increased risk of ventricular arrhythmias like torsade de pointes.

MANAGEMENT: Concomitant use of vemurafenib with potent CYP450 3A4 inducers should generally be avoided. Alternative therapeutic agents with less enzyme induction potential should be considered whenever possible during treatment with vemurafenib. If coadministration is required, the manufacturer recommends increasing the dose of vemurafenib by 240 mg (one tablet) as tolerated. Close monitoring for toxicities (e.g., QT prolongation, hepatotoxicity, nephrotoxicity, photosensitivity, dermatologic reactions, uveitis) is advised following dose increase. Two weeks after discontinuation of the strong CYP450 3A4 inducer, the vemurafenib dose that was taken prior to initiating the inducer may be resumed.

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ADJUST DOSING INTERVAL: Phenytoin bioavailability may decrease to subtherapeutic levels when the suspension is given concomitantly with enteral feedings. The mechanism may be related to phenytoin binding to substances in the enteral formula (e.g., calcium, protein) and/or binding to the tube lumen. Data have been conflicting and some studies have reported no changes in phenytoin levels, while others have reported significant reductions.

MONITOR: Acute consumption of alcohol may increase plasma phenytoin levels. Chronic consumption of alcohol may decrease plasma phenytoin levels. The mechanism of this interaction is related to induction of phenytoin metabolism by ethanol during chronic administration. Other hydantoin derivatives may be similarly affected by ethanol.

MANAGEMENT: Some experts have recommended interrupting the feeding for 2 hours before and after the phenytoin dose, giving the phenytoin suspension diluted in water, and flushing the tube with water after administration; however, this method may not entirely avoid the interaction and is not always clinically feasible. Patients should be closely monitored for clinical and laboratory evidence of altered phenytoin efficacy and levels upon initiation and discontinuation of enteral feedings. Dosage adjustments or intravenous administration may be required until therapeutic serum levels are obtained. In addition, patients receiving phenytoin therapy should be warned about the interaction between phenytoin and ethanol and they should be advised to notify their physician if they experience worsening of seizure control or symptoms of toxicity, including drowsiness, visual disturbances, change in mental status, nausea, or ataxia.

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