Drug Interaction Report

MONITOR: Coadministration with inhibitors of CYP450 3A4 and/or P-glycoprotein (P-gp) may increase the plasma concentrations of cobimetinib, which is a substrate of both the isoenzyme and the efflux transporter. In 15 healthy volunteers given a single 10 mg dose of cobimetinib with the potent CYP450 3A4 and P-gp 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. Simulations using physiologically-based pharmacokinetic (PBPK) models suggest that steady-state concentrations of cobimetinib given at a reduced dose of 20 mg with short-term (less than 14 days) use of a moderate CYP450 3A4 inhibitor would be similar to steady-state concentrations observed following a 60 mg dose given alone.

MANAGEMENT: Caution is advised when cobimetinib is prescribed with CYP450 3A4 and/or P-gp inhibitors. Patients should be monitored for adverse effects such as diarrhea, nausea, vomiting, stomatitis, hemorrhage, cardiomyopathy, rash, photosensitivity, retinopathy, retinal vein occlusion, liver enzyme abnormalities and rhabdomyolysis, and the cobimetinib dosage adjusted accordingly or treatment discontinued as necessary.

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.