Drug Interaction Report

MONITOR CLOSELY: Coadministration with lapatinib may significantly increase the plasma concentrations of digoxin. The proposed mechanism is lapatinib inhibition of the P-glycoprotein-mediated intestinal efflux and/or renal tubular secretion of digoxin. When single-dose digoxin was coadministered with lapatinib, digoxin systemic exposure (AUC) increased by approximately 2.8-fold.

MANAGEMENT: Caution is advised if digoxin is used in combination with lapatinib. Serum digoxin levels and pharmacologic effects should be closely monitored during coadministration and following the discontinuation of lapatinib, and the digoxin dosage adjusted as necessary. If digoxin serum concentration is above 1.2 ng/mL, the digoxin dosage should be reduced by one-half. Patients should be advised to notify their physician if they experience signs of digoxin toxicity such as nausea, anorexia, visual disturbances, slow pulse, or irregular heartbeat.

GENERALLY AVOID: Grapefruit juice may increase the plasma concentrations of lapatinib. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits.

ADJUST DOSING INTERVAL: Food can significantly increase the oral bioavailability of lapatinib. According to the manufacturer, lapatinib peak plasma concentration (Cmax) was approximately 2.5- and 3-fold higher and systemic exposure (AUC) 3- and 4-fold higher when administered with a low fat meal (5% fat; 500 calories) or with a high-fat meal (50% fat; 1000 calories), respectively, compared to fasting. Dividing the daily dose also resulted in an approximately 2-fold higher systemic exposure at steady state compared to the same total dose administered once daily.

MANAGEMENT: Patients treated with lapatinib should preferably avoid the consumption of grapefruit or grapefruit juice. The manufacturer recommends that lapatinib be administered at least one hour before or one hour after a meal. The lapatinib dose is administered once daily and should not be divided.

Administration of digoxin with a high-fiber meal has been shown to decrease its bioavailability by almost 20%. Fiber can sequester up to 45% of the drug when given orally. Patients should be advised to maintain a regular diet without significant fluctuation in fiber intake while digoxin is being titrated.

Grapefruit juice may modestly increase the plasma concentrations of digoxin. The mechanism is increased absorption of digoxin due to mild inhibition of intestinal P-glycoprotein by certain compounds present in grapefruits. In 12 healthy volunteers, administration of grapefruit juice with and 30 minutes before, as well as 3.5, 7.5, and 11.5 hours after a single digoxin dose (0.5 mg) increased the mean area under the plasma concentration-time curve (AUC) of digoxin by just 9% compared to administration with water. Moreover, P-glycoprotein genetic polymorphism does not appear to influence the magnitude of the effects of grapefruit juice on digoxin. Thus, the interaction is unlikely to be of clinical significance.