Drug Interaction Report

MONITOR: Coadministration with inhibitors of CYP450 3A4 may increase the plasma concentrations of quetiapine, which is primarily metabolized by the isoenzyme. In 12 healthy volunteers, administration of a single 25 mg dose of quetiapine with the potent CYP450 3A4 inhibitor ketoconazole (200 mg once daily for 4 days) increased mean quetiapine peak plasma concentration (Cmax) and systemic exposure (AUC) by 3.4- and 6.2-fold, respectively, and decreased mean oral clearance by 84% compared to quetiapine administered alone. A case report describes a patient treated with quetiapine 700 mg/day who developed severely impaired consciousness and respiratory depression requiring intensive care surveillance following two 500 mg doses of clarithromycin, another potent CYP450 3A4 inhibitor. Quetiapine plasma level was found to be nearly 5 times the high end of the recommended therapeutic range. The patient recovered a week after quetiapine was withdrawn. The interaction was also suspected in a case report of two patients receiving quetiapine with ritonavir-boosted atazanavir. One patient experienced significant increases in appetite and serum glucose and a weight gain of more than 22 kg over six months. The patient's weight returned to baseline five months after stopping both treatments. The second patient had increased sedation and mental confusion, which resolved several days following self-discontinuation of quetiapine.

MANAGEMENT: Pharmacologic response to quetiapine should be monitored more closely whenever a CYP450 3A4 inhibitor is added to or withdrawn from therapy, and the quetiapine dosage adjusted as necessary. Patients should be monitored for potentially increased adverse effects such as dizziness, drowsiness, dry mouth, constipation, increased appetite, weight gain, extrapyramidal symptoms, tardive dyskinesia, hyperglycemia, dyslipidemia, hyperprolactinemia (galactorrhea, amenorrhea, gynecomastia), orthostatic hypotension, blood pressure increases (in children and adolescents), QT prolongation, cognitive and motor impairment, dysphagia, and heat-related illnesses due to disruption of body temperature regulation.

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GENERALLY AVOID: Alcohol may potentiate some of the pharmacologic effects of CNS-active agents. Use in combination may result in additive central nervous system depression and/or impairment of judgment, thinking, and psychomotor skills.

MANAGEMENT: Patients receiving CNS-active agents should be warned of this interaction and advised to avoid or limit consumption of alcohol. Ambulatory patients should be counseled 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.

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