Drug Interactions between emtricitabine / nelfinavir / tenofovir and fostamatinib

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.

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MONITOR: Coadministration with fostamatinib may increase the plasma concentrations of drugs that are substrates of the CYP450 3A4 isoenzyme and/or P-glycoprotein (P-gp) transporter. The proposed mechanism is decreased clearance in the intestine and/or liver due to inhibition of CYP450 3A4-mediated metabolism and P-gp-mediated efflux by fostamatinib. According to the product labeling, administration of a single 40 mg dose of the CYP450 3A4 substrate simvastatin with fostamatinib 100 mg twice daily increased simvastatin peak plasma concentration (Cmax) and systemic exposure (AUC) by 113% and 64%, respectively. In addition, simvastatin acid Cmax increased by 83% and AUC increased by 64%. When the P-gp substrate digoxin (0.25 mg once daily) was administered with fostamatinib (100 mg twice daily), digoxin Cmax and AUC increased by 70% and 37%, respectively.

MANAGEMENT: Caution is advised when fostamatinib is used concurrently with drugs that are known substrates of CYP450 3A4 and/or P-gp, particularly those with a narrow therapeutic range. Dosage adjustments as well as clinical and laboratory monitoring may be appropriate for some drugs whenever fostamatinib is added to or withdrawn from therapy.

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