Theoretical model for both saturable rate and extent of absorption: simulations of cefatrizine data.

A pharmacokinetic model incorporating saturable rate of absorption of the Michaelis-Menten type was recently developed to fit cefatrizine (CFZ) plasma concentrations with time following oral administration of 500-mg capsules to humans. This model (MM) was statistically superior to models incorporating either first-order or zero-order absorption. However, the MM model does not predict the reduction in extent of absorption with dose observed in vivo. In this study, a model is proposed in which a time constraint, delta t, is added to the MM model. This new model (MM-delta t) is tested with data following doses of 250, 500, and 1000 mg of CFZ. When delta t is set to 1.5 hr, the predicted relative changes with dose in bioavailability, F, peak plasma concentration, Cmax, the time at which the peak concentration occurs tmax, and the mean absorption time, MAT, are generally in good agreement with the experimental data. The time interval of 1.5 hr is compatible with passage by a limited region within the small intestine where drug is absorbed by a facilitated transport mechanism. Influence of each absorption model parameter (Vmax, Km, and delta t) on total area under the concentration versus time curve (AUC), F, Cmax, and tmax, is assessed by simulation. The MM-delta t model is able to summarize the nonlinerity observed in both rate and extent of absorption.