Drug Interaction Report

MONITOR: Coadministration with potent inhibitors of CYP450 3A4 may increase the plasma concentrations of ospemifene, which is partially metabolized by the isoenzyme. In 12 postmenopausal women given the potent CYP450 3A4 inhibitor ketoconazole 400 mg once daily for 8 days, administration of ospemifene 60 mg after breakfast on day 5 of ketoconazole treatment resulted in 1.5- and 1.4-fold increases in ospemifene peak plasma concentration (Cmax) and systemic exposure (AUC), respectively, compared to administration of ospemifene alone. The potential for increased risk of adverse events such as hot flush, thromboembolism, and endometrial or breast cancer should be considered.

MANAGEMENT: Caution is advised when ospemifene is used with potent CYP450 3A4 inhibitors. Patients should be advised to seek medical attention if they develop potential signs and symptoms of thromboembolism such as chest pain, shortness of breath, and pain or swelling in the arms or legs.

ADJUST DOSING INTERVAL: Food may reduce the systemic bioavailability of amprenavir from fosamprenavir oral suspension. The mechanism of interaction has not been described. According to the product labeling, administration of fosamprenavir oral suspension (1400 mg single dose) with a high-fat meal (967 kcal, 67 g fat, 33 g protein, 58 g carbohydrate) reduced amprenavir peak plasma concentration (Cmax) by 46% and systemic exposure (AUC) by 28% compared to administration in a fasted state. The time to reach peak plasma level (Tmax) was delayed by 0.72 hours. In contrast, the same high-fat meal did not affect the pharmacokinetics of amprenavir from fosamprenavir tablets.

MANAGEMENT: Fosamprenavir suspension should be administered on an empty stomach in adults, but with food in pediatric patients to aid palatability and compliance. If emesis occurs within 30 minutes after dosing the suspension, the dose should be repeated. Fosamprenavir tablets may be taken with or without food.

ADJUST DOSING INTERVAL: Food significantly enhances the oral bioavailability of ospemifene. In a cross-study comparison, administration of a single 60 mg dose of ospemifene with a high-fat/high-calorie meal (860 kcal) in postmenopausal women increased ospemifene peak plasma concentration (Cmax) and systemic exposure (AUC) by 2.3- and 1.7-fold, respectively, compared to administration under fasted condition. Elimination half-life and time to maximum concentration (Tmax) were not altered. In two separate food effect studies where different ospemifene tablet formulations were given to healthy male volunteers, ospemifene Cmax and AUC increased by 2.3- and 1.8-fold, respectively, with a low-fat/low-calorie meal (300 kcal) and 3.6- and 2.7-fold, respectively, with a high-fat/high-calorie meal (860 kcal) relative to fasting.

MANAGEMENT: Ospemifene should be taken once daily with food.