Drug Interactions between Phenytoin Sodium, Extended Release and suzetrigine

GENERALLY AVOID: Grapefruit and/or grapefruit juice may increase the plasma concentrations of suzetrigine and M6-SUZ, a major active metabolite whose systemic exposure (AUC) at steady state is approximately 3 times that of the parent drug but exhibits 3.7-fold less potency in blocking the NaV1.8 voltage-gated sodium channels responsible for transmission of pain signals to the spinal cord and brain. The proposed mechanism for the interaction is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruit. Inhibition of hepatic CYP450 3A4 may also contribute. The interaction has not been studied with grapefruit juice but has been reported for other CYP450 3A4 inhibitors. When a single dose of suzetrigine was administered with itraconazole, a potent CYP450 3A4 inhibitor, mean suzetrigine peak plasma concentration (Cmax) and systemic exposure (AUC) increased by 1.5- and 4.8-fold, respectively, while mean Cmax of M6-SUZ decreased by 32% and AUC increased by 4.4-fold. Coadministration of fluconazole, a moderate CYP450 3A4 inhibitor, with suzetrigine dosed according to the manufacturer's recommended dosage modification is predicted to increase the mean Cmax and AUC of suzetrigine by 1.4- and 1.5-fold, respectively, while the mean Cmax and AUC of M6-SUZ are predicted to increase by 1.1- and 1.2-fold, respectively, compared to suzetrigine administered at the regular recommended dosage without fluconazole. In general, the effects of grapefruit products are concentration-, dose- and preparation-dependent, and can vary widely among brands. Certain preparations of grapefruit (e.g., high dose, double strength) have sometimes demonstrated potent inhibition of CYP450 3A4, while other preparations (e.g., low dose, single strength) have typically demonstrated moderate inhibition.

ADJUST DOSE INTERVAL: Food may delay the absorption of suzetrigine without impacting the overall systemic exposures to parent drug and M6-SUZ. Administration of suzetrigine 100 mg (the first dose) with a high-fat meal (800 to 1000 calories; 50% from fat), a moderate-fat meal (600 calories; 30% from fat), and a low-fat meal (<=500 calories; <=25% from fat) resulted in decreased initial plasma concentrations of suzetrigine and M6-SUZ compared to administration in a fasted state. The median time to reach peak plasma concentration (Tmax) for suzetrigine and M6-SUZ when administered with either a high-fat or moderate-fat meal was 5 hours and 24 hours, respectively, versus 3 hours and 8 to 10 hours, respectively, when administered in the fasted state. The Cmax and AUC of suzetrigine and M6-SUZ were not affected by any of the meal conditions, including a high-fat meal consumed one hour after suzetrigine. Administration of the second suzetrigine dose of 50 mg with or without regard to meals is also predicted to have no effect on the systemic exposures of suzetrigine and M6-SUZ.

MANAGEMENT: Patients should avoid consumption of foods or drinks containing grapefruit during treatment with suzetrigine. The starting dose of 100 mg should be taken on an empty stomach at least 1 hour before or 2 hours after food, although clear liquids (e.g., water, apple juice, vegetable broth, tea, black coffee) may be consumed during this time. Subsequent doses may be taken with or without food.

ADJUST DOSING INTERVAL: Phenytoin bioavailability may decrease to subtherapeutic levels when the suspension is given concomitantly with enteral feedings. The mechanism may be related to phenytoin binding to substances in the enteral formula (e.g., calcium, protein) and/or binding to the tube lumen. Data have been conflicting and some studies have reported no changes in phenytoin levels, while others have reported significant reductions.

MONITOR: Acute consumption of alcohol may increase plasma phenytoin levels. Chronic consumption of alcohol may decrease plasma phenytoin levels. The mechanism of this interaction is related to induction of phenytoin metabolism by ethanol during chronic administration. Other hydantoin derivatives may be similarly affected by ethanol.

MANAGEMENT: Some experts have recommended interrupting the feeding for 2 hours before and after the phenytoin dose, giving the phenytoin suspension diluted in water, and flushing the tube with water after administration; however, this method may not entirely avoid the interaction and is not always clinically feasible. Patients should be closely monitored for clinical and laboratory evidence of altered phenytoin efficacy and levels upon initiation and discontinuation of enteral feedings. Dosage adjustments or intravenous administration may be required until therapeutic serum levels are obtained. In addition, patients receiving phenytoin therapy should be warned about the interaction between phenytoin and ethanol and they should be advised to notify their physician if they experience worsening of seizure control or symptoms of toxicity, including drowsiness, visual disturbances, change in mental status, nausea, or ataxia.