Steady-State Interaction Between Amiodarone and Phenytoin in Normal Subjects

Document Type

Article

Publication Date

5-15-1990

Description

Amiodarone has been reported to increase phenytoin levels. This study was designed to evaluate the pharmacokinetic basis of this interaction at steady-state. Pharmacokinetic parameters for phenytoin were determined after 14 days of oral phenytoin, 2 to 4 mg/kg/day, before and after oral amiodarone, 200 mg dairy for 6 weeks in 7 healthy male subjects. During amiodarone therapy, area under the serum concentration time curve for phenytoin was increased from 208 ± 82.8 (mean ± standard deviation) to 292 ± 108 mg · hr/liter (p = 0.015). Both the maximum and 24-hour phenytoin concentrations were increased from 10.75 ± 3.75 and 6.67 ± 3.51 μg/ml to 14.26 ± 3.97 (p = 0.016) and 10.27 ± 4.67 μg/ml (p = 0.012), respectively, during concomitant amiodarone treatment. Amiodarone caused a decrease in the oral clearance of phenytoin from 1.29 ± 0.30 to 0.93 ± 0.25 liters/ hr (p = 0.002). These results were due to a reduction in phenytoin metabolism by amiodarone as evidenced by a decrease in the urinary excretion of the principal metabolite of phenytoin, 5-(p-hydroxyphenyl)-5-phenylhydantoin, 149 ± 39.7 to 99.3 ± 40.0 mg (p = 0.041) and no change in the unbound fraction of the total phenytoin concentration expressed as a percentage, 10.3 ± 2.7 versus 10.7 ± 2.1% (p = 0.28) during coadministration of amiodarone. The alterations in phenytoin pharmacokinetics suggest that steady-state doses of phenytoin of 2 to 4 mg/kg/day should be reduced at least 25% when amiodarone is concurrently administered. All dosage reductions should be guided by clinical and therapeutic drug monitoring.

Link to Full Text

Share

COinS