Drug Interaction Report

MONITOR: Coadministration with fidaxomicin may increase the plasma concentrations of some orally administered drugs that are substrates of the P-glycoprotein (P-gp) transporter. In vitro, fidaxomicin and its main metabolite are substrates and inhibitors of P-gp, an efflux transporter expressed in the gastrointestinal tract. When digoxin, a P-gp substrate, was coadministered with fidaxomicin (200 mg twice daily) in healthy volunteers, digoxin peak plasma concentration (Cmax) and systemic exposure (AUC) increased by 14% and 12%, respectively. Although the effect of fidaxomicin on digoxin exposure is not considered clinically relevant, a larger effect on P-gp substrates with lower bioavailability that may be more sensitive to intestinal P-gp inhibition, such as dabigatran etexilate, cannot be excluded.

MANAGEMENT: Caution is advised when fidaxomicin is used in combination with P-gp substrates that may be sensitive to intestinal P-gp inhibition.

GENERALLY AVOID: Grapefruit juice may increase the plasma concentrations of pazopanib. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. Although not studied, the interaction may increase the risk of QT interval prolongation and torsade de pointes arrhythmia as well as severe and fatal hepatotoxicity associated with the use of pazopanib.

ADJUST DOSING INTERVAL: Food increases the oral bioavailability of pazopanib. The mechanism of interaction is unknown. Administration of pazopanib with a high-fat or low-fat meal results in an approximately 2-fold increase in peak plasma concentration (Cmax) and systemic exposure (AUC).

MANAGEMENT: Patients treated with pazopanib should avoid consumption of grapefruit, grapefruit juice, and any supplement containing grapefruit extract. Pazopanib should be administered at least one hour before or two hours after a meal.