Drug Interaction Report

MONITOR CLOSELY: Coadministration of fostamatinib with potent inhibitors of CYP450 3A4 may significantly increase exposure to the active metabolite known as R406, the predominant moiety in the systemic circulation following fostamatinib administration. Fostamatinib is metabolized in the gut by alkaline phosphatase to R406, which then undergoes oxidation via CYP450 3A4 and glucuronidation via UGT1A9. When a single 80 mg dose of fostamatinib (0.53 times the 150 mg dosage) was administered with the potent CYP450 3A4 inhibitor ketoconazole (200 mg twice daily for 3.5 days), R406 peak plasma concentration (Cmax) and systemic exposure (AUC) increased on average by 37% and 102%, respectively, compared to fostamatinib administered alone. Increased exposure to R406 may result in increased risk of adverse effects such as diarrhea, hepatotoxicity, hypertension, and neutropenia.

MANAGEMENT: Patients should be monitored for toxicities of fostamatinib during concomitant use of potent CYP450 3A4 inhibitors, and the fostamatinib dosage adjusted as necessary in accordance with the product labeling.

Grapefruit juice may delay the gastrointestinal absorption of clarithromycin but does not appear to affect the overall extent of absorption or inhibit the metabolism of clarithromycin. The mechanism of interaction is unknown but may be related to competition for intestinal CYP450 3A4 and/or absorptive sites. In an open-label, randomized, crossover study consisting of 12 healthy subjects, coadministration with grapefruit juice increased the time to reach peak plasma concentration (Tmax) of both clarithromycin and 14-hydroxyclarithromycin (the active metabolite) by 80% and 104%, respectively, compared to water. Other pharmacokinetic parameters were not significantly altered. This interaction is unlikely to be of clinical significance.