Pharmacokinetics of phenytoin (diphenylhydantoin) in horses

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Abstract
The pharmacokinetics of the anticonvulsant phenytoin were investigated in clinically healthy horses after oral (p.o.) and i.v. administration. A single dose of phenytoin (8.8 mg/kg body wt) was given i.v. as a bolus to 9 horses and 1 horse received 13.2 mg/kg. A 2 compartment open model was used to describe the disposition of phenytoin. Of the horses that received an i.v. dose (3 at 8.8 and 1 at 13.2 mg/kg), 4 were then given the same dose 3 days later by the oral route. Phenytoin achieved a peak concentration in serum within 1-4 h after p.o. administration and was poorly absorbed with a bioavailability of 34.5 .+-. 8.6%. Oral dosage regimens were calculated on the basis of a dosing interval of 8 h to provide average serum steady-state concentrations of 5 and 10 .mu.g/ml for phenytoin.