Drug Interaction Report

MONITOR: Coadministration with gilteritinib may increase the plasma concentrations and the risk of adverse effects of orally administered drugs that are substrates of the intestinal P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and/or organic cation transporter 1 (OCT1) transporters, such as dabigatran, digoxin, rosuvastatin, methotrexate, and metformin. The proposed mechanism, based on in vitro data, is decreased clearance due to gilteritinib-mediated inhibition of intestinal P-gp, BCRP, and/or OCT1 efflux transport proteins.

MANAGEMENT: Until more information is available, caution is advised if gilteritinib is used concomitantly with drugs that are substrates of the intestinal P-gp, BCRP, and/or OCT1 transport proteins, particularly those with a narrow therapeutic range. Dosage adjustments as well as clinical and laboratory monitoring should be considered whenever gilteritinib is added to or withdrawn from therapy with these drugs. Patients should be monitored for the development of adverse effects.

GENERALLY AVOID: Grapefruit juice may increase the plasma concentrations of guanfacine. The risk of adverse reactions such as hypotension, bradycardia, and sedation may increase. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruit. Ketoconazole, a potent CYP450 3A4 inhibitor, has been reported to increase guanfacine peak plasma concentration (Cmax) and systemic exposure (AUC) by approximately 2- and 3-fold, respectively. A computer simulation suggests that fluconazole, a moderate CYP450 3A4 inhibitor, would increase guanfacine Cmax and AUC by about 1.5- and 2-fold, respectively. In general, the effect of grapefruit juice is concentration-, dose- and preparation-dependent, and can vary widely among brands. Certain preparations of grapefruit juice (e.g., high dose, double strength) have sometimes demonstrated potent inhibition of CYP450 3A4, while other preparations (e.g., low dose, single strength) have typically demonstrated moderate inhibition.

GENERALLY AVOID: Alcohol may enhance the sedative and hypotensive effects of guanfacine.

GENERALLY AVOID: Administration of extended-release guanfacine with a high-fat meal may increase the bioavailability of guanfacine. When a single 4 mg dose of extended-release guanfacine was administered to adult volunteers with a high-fat breakfast, mean guanfacine peak plasma concentration (Cmax) and systemic exposure (AUC) increased by approximately 75% and 40%, respectively, compared to dosing in a fasted state.

MANAGEMENT: Patients treated with guanfacine should avoid consumption of grapefruit and grapefruit juice. In addition, it is preferable to avoid or limit the use of alcohol during treatment. Patients should be advised against driving or operating hazardous machinery until they know how the medication affects them. The extended-release formulation of guanfacine should not be taken together with a high-fat meal.