Drug Interaction Report

GENERALLY AVOID: Coadministration with potent inhibitors of CYP450 3A4 and/or P-glycoprotein (P-gp) may significantly increase the plasma concentrations of sirolimus following oral administration. Sirolimus is a substrate of both CYP450 3A4 isoenzyme and P-gp efflux transporter, thus their inhibition in the intestine can enhance the absorption of sirolimus. In 23 healthy volunteers, administration of a single 5 mg dose of sirolimus with the potent dual CYP450 3A4/P-gp inhibitor ketoconazole (200 mg/day orally for 10 days) increased mean sirolimus peak plasma concentration (Cmax) and systemic exposure (AUC) by approximately 4- and 11-fold, respectively. Likewise, posaconazole (400 mg oral suspension twice a day for 16 days) increased mean Cmax and AUC of a single 2 mg dose of sirolimus by nearly 7- and 9-fold, respectively, while voriconazole (400 mg orally every 12 hours for 1 day, then 200 mg every 12 hours for 8 days) increased the same values by 7- and 11-fold, respectively. Another dual inhibitor, boceprevir (800 mg three times a day for 11 days), increased the Cmax and AUC of a single 2 mg dose of sirolimus by 10- and 17-fold, respectively. When sirolimus 2 mg once a day was coadministered with the moderate dual inhibitor erythromycin (ethylsuccinate salt 800 mg every 8 hours) in 24 study subjects, sirolimus Cmax and AUC increased by more than 4-fold each, while erythromycin Cmax and AUC also increased by more than 1.5-fold each.

MANAGEMENT: Concomitant use of sirolimus with potent CYP450 3A4 and/or P-gp inhibitors should generally be avoided. The manufacturers of posaconazole and voriconazole consider coadministration with sirolimus to be contraindicated. Some authorities recommend avoiding concomitant use of sirolimus during and for 2 weeks after treatment with itraconazole.

ADJUST DOSING INTERVAL: Food significantly increases the absorption of saquinavir.

MONITOR: Coadministration with grapefruit juice may increase the plasma concentrations of saquinavir. The primary mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. In eight healthy volunteers, ingestion of 400 mL of grapefruit juice prior to administration of a 600 mg dose of saquinavir mesylate increased the area under the plasma concentration-time curve and oral bioavailability of saquinavir by 50% and 100%, respectively, compared to water; however, the increase is not considered clinically relevant. A high degree of intersubject variability in the grapefruit juice effect was also observed. The extent to which this interaction may occur with the saquinavir free base soft gelatin capsule is unknown. However, the saquinavir soft gelatin capsule formulation is no longer commercially available.

MANAGEMENT: Saquinavir mesylate should be taken with meals or within 2 hours after eating to enhance bioavailability. Patients should be advised to avoid the consumption of large amounts of grapefruit and grapefruit juice during saquinavir therapy unless otherwise directed by their doctor, as the interaction is unreliable and subject to a high degree of interpatient variation.

ADJUST DOSING INTERVAL: Consumption of food can decrease the rate and extent of gastrointestinal absorption of sirolimus. Also, the consumption of grapefruit juice may result in increased sirolimus trough concentrations.

MANAGEMENT: Experts recommend that this drug be taken either at least one hour prior to eating or consistently with or without food to avoid variations in sirolimus blood levels. The manufacturer recommends against using grapefruit juice for dilution of sirolimus doses. Patients should be monitored for clinical and laboratory evidence of altered immunosuppressant effects.