Drug Interactions between fenfluramine and praziquantel

ADJUST DOSING INTERVAL: Administration with food increases the oral bioavailability of praziquantel. The mechanism has not been described. In nine healthy volunteers, administration of praziquantel (1800 mg single oral dose) following a high-fat meal increased the mean praziquantel peak plasma concentration (Cmax) and area under the concentration-time curve (AUC) by 243% and 180%, respectively, compared to administration under fasting conditions. Administration with a high-carbohydrate meal increased these values by 515% and 271%, respectively, compared to fasting. Overall, the relative bioavailability was increased by a factor of 2.72 and 3.98 with the high-fat and high-carbohydrate meals, respectively. The time to reach peak concentration (Tmax) and elimination half-life (T1/2) were not significantly altered.

Coadministration with grapefruit juice may increase the oral bioavailability of praziquantel. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall induced by certain compounds present in grapefruit. In 18 healthy volunteers, administration of praziquantel (1800 mg single oral dose) with 250 mL of commercially squeezed grapefruit juice resulted in increases in the mean praziquantel Cmax and AUC of 63% and 90%, respectively, compared to administration with water. The Tmax and T1/2 were not significantly altered. The pharmacokinetics of praziquantel were subject to a high degree of interpatient variability with and without grapefruit juice.

MANAGEMENT: To ensure maximal oral absorption, praziquantel should be administered with meals. Administration with grapefruit juice may further increase pharmacologic effects of praziquantel, including adverse effects such dizziness, abdominal discomfort, and nausea.

GENERALLY AVOID: Alcohol may potentiate the central nervous system and cardiovascular effects of centrally-acting appetite suppressants. In one study, concurrent administration of methamphetamine (30 mg intravenously) and ethanol (1 gm/kg orally over 30 minutes) increased heart rate by 24 beats/minute compared to methamphetamine alone. This increases cardiac work and myocardial oxygen consumption, which may lead to more adverse cardiovascular effects than either agent alone. Subjective effects of ethanol were diminished in the eight study subjects, but those of methamphetamine were not affected. The pharmacokinetics of methamphetamine were also unaffected except for a decrease in the apparent volume of distribution at steady state.

MANAGEMENT: Concomitant use of centrally-acting appetite suppressants and alcohol should be avoided if possible, especially in patients with a history of cardiovascular disease. Patients should be counselled to avoid hazardous activities requiring complete mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.