Drug Interactions between Onxol and saquinavir

MONITOR: Coadministration with inhibitors of CYP450 3A4, such as grapefruit juice, may increase the plasma concentrations of paclitaxel, which is a substrate of the isoenzyme. Current data suggest that consumption of large quantities of grapefruit juice inhibit both intestinal and hepatic CYP450 3A4 due to certain compounds present in grapefruit. Specific data for paclitaxel are lacking; however, in a case report of a 52-year-old woman with esophageal squamous cell carcinoma receiving a twice weekly chemotherapy regimen including intravenous docetaxel (40 mg/m2) reported that docetaxel systemic exposure (AUC) increased by 65% compared with the AUC target of 1.96 mg*h/L and clearance decreased by 63%, with a 71% reduction in the patient's neutrophil count. In the absence of other CYP450 3A4 inhibitors, these effects were attributed to daily consumption of 250 mL of grapefruit juice, which the patient had been consuming for at least 3 months. Two weeks after the patient ceased the grapefruit juice, the docetaxel AUC was closer to the target value and the neutrophil count reduction was less than 35%. In addition, in a pharmacokinetic study consisting of 7 cancer patients, mean dose-normalized docetaxel AUC increased by 2.2-fold and clearance decreased by 49% when intravenous docetaxel was given at a reduced dosage of 10 mg/m2 in combination with the potent CYP450 3A4 inhibitor ketoconazole (200 mg orally once daily for 3 days) compared to docetaxel administered alone at 100 mg/m2.

MANAGEMENT: Caution is recommended if paclitaxel is to be used in combination with grapefruit and grapefruit juice. Patients should be closely monitored for the development of paclitaxel toxicity, including diarrhea, mucositis, myelosuppression, and peripheral neuropathy and dose adjustment considered per local treatment protocols.

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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.

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