Sensitivity analysis of the effect of bioavailability or dosage form content on mean steady state phenytoin concentration.

Stochastic simulations were used to examine the sensitivity of mean phenytoin steady state concentrations (Css) to changes in the effective dosing rate produced by differences in average product content or bioavailability. Changes of +/- 4%, +/- 6%, +/- 8% and +/- 10% in the effective dosing rate (based on a starting dose yielding a Css of 15 mg/L) were examined. Monte Carlo simulations were performed for each change in dosing rate assuming a one-compartment open model with parallel Michaelis-Menten and first-order elimination. Parameter sets were comprised of a combination of values for maximal rate of saturable elimination (410 or 510 mg/day), the concentration at which the rate of saturable elimination is half maximum (Km, 4.4 or 5.7 mg/L), and linear clearance (CL, 0.15 or 1.5 L/day). These parameters were assumed to be log-normally distributed with coefficients of variation of 30%, 50%, and 15%. The percentages of "individuals" who would be predicted to have Css of less than 10 mg/L following a reduction in the effective dosing rate increased with decreasing Km and CL values. For a Km of 5.7 mg/L and CL of 1.5 L/day, 5% of the "individuals" had Css values of less than 10 mg/L with an 8% decrease in the dosing rate. If the dosing rate was reduced by 10%, then 14-16% of the "individuals" were predicted to have concentrations of less than 10 mg/L. All other combinations of Km and CL values yielded higher percentages of "individuals" with Css of less than 10 mg/L.(ABSTRACT TRUNCATED AT 250 WORDS)