Application of a simplified method to determine bioavailability of an oral dose of phenytoin.

The bioavailability of capsules of phenytoin was determined by two methods: a method involving the numerical integration of the Michaelis-Menten equation and an alternative method involving fitting the time course of plasma concentrations, following the administration of the reference intravenous dosage, to an empirical quadratic function of time. The latter procedure requires much simpler computations. The two methods yielded very similar estimates of the rate and extent of absorption of phenytoin. Total absorption was 0.90 +/- 0.05 and 0.89 +/- 0.05 (mean +/- SE, n = 6) using the methods of numerical integration and quadratic curve fitting, respectively. Both methods indicated that the rate of absorption of phenytoin was inconsistent and slow. Half the total absorption of phenytoin occurred over 2.5 +/- 0.3 hr but the remainder was absorbed very slowly over a period of about 30 hr. Empirical functions may be more generally useful in the determinations of the bioavailability of drugs, particularly if some aspects of the disposition are saturable.