Drug Interaction Report

GENERALLY AVOID: Coadministration with potent inhibitors of CYP450 3A4 may significantly increase the plasma concentrations of cobimetinib, which is primarily metabolized by the isoenzyme. In 15 healthy volunteers given a single 10 mg dose of cobimetinib with the potent CYP450 3A4 inhibitor itraconazole (200 mg once daily for 14 days), mean cobimetinib peak plasma concentration (Cmax) and systemic exposure (AUC) increased by 3.2- and 6.7-fold, respectively, compared to cobimetinib administered alone. The risk and/or severity of adverse effects such as diarrhea, nausea, vomiting, stomatitis, hemorrhage, cardiomyopathy, rash, photosensitivity, retinopathy, retinal vein occlusion, liver enzyme abnormalities, and rhabdomyolysis may be increased.

MANAGEMENT: Concomitant use of cobimetinib with potent CYP450 3A4 inhibitors should generally be avoided. Some authorities recommend avoiding concomitant use of cobimetinib during and for 2 weeks after treatment with itraconazole.

ADJUST DOSING INTERVAL: Food may reduce the systemic bioavailability of amprenavir from fosamprenavir oral suspension. The mechanism of interaction has not been described. According to the product labeling, administration of fosamprenavir oral suspension (1400 mg single dose) with a high-fat meal (967 kcal, 67 g fat, 33 g protein, 58 g carbohydrate) reduced amprenavir peak plasma concentration (Cmax) by 46% and systemic exposure (AUC) by 28% compared to administration in a fasted state. The time to reach peak plasma level (Tmax) was delayed by 0.72 hours. In contrast, the same high-fat meal did not affect the pharmacokinetics of amprenavir from fosamprenavir tablets.

MANAGEMENT: Fosamprenavir suspension should be administered on an empty stomach in adults, but with food in pediatric patients to aid palatability and compliance. If emesis occurs within 30 minutes after dosing the suspension, the dose should be repeated. Fosamprenavir tablets may be taken with or without food.

MONITOR: Grapefruit juice may increase the plasma concentrations of orally administered drugs that are substrates of the CYP450 3A4 isoenzyme, such as cobimetinib. However, the interaction seems to affect primarily those drugs that undergo significant presystemic metabolism by CYP450 3A4 (i.e., drugs with low oral bioavailability), presumably due to the fact that grapefruit juice inhibits primarily intestinal rather than hepatic CYP450 3A4. Because pharmacokinetic interactions involving grapefruit juice are often subject to a high degree of interpatient variability, the extent to which a given patient may be affected is difficult to predict.

MANAGEMENT: Patients who regularly consume grapefruit or grapefruit juice should be monitored for adverse effects and altered plasma concentrations of cobimetinib. Grapefruit and grapefruit juice should be avoided if an interaction is suspected. Orange juice is not expected to interact with these drugs.