Mechanistic Models Describing Active Renal Reabsorption and Secretion: A Simulation-based Study

The objective of the present study was to evaluate mechanistic pharmacokinetic models describing active renal secretion and reabsorption over a range of Michaelis-Menten parameter estimates and doses. Plasma concentration and urinary excretion profiles were simulated and renal clearance (CLr) was calculated for two pharmacokinetic models describing active renal reabsorption (R1/R2), two models describing active secretion (S1/S2), and a model containing both processes. A range of doses (1-1,000 mg/kg) was evaluated, and Vmax and Km parameter estimates were varied over a 100-fold range. Similar CLr values were predicted for reabsorption models (R1/R2) with variations in Vmax and Km. Tubular secretionmodels (S1/S2) yielded similar relationships between Michaelis-Menten parameter perturbations and CLr , but the predicted CLr values were threefold higher for model S1. For both reabsorption and secretion models, the greatest changes in CLr were observed with perturbations in Vmax , suggesting the need for an accurate estimate of this parameter. When intrinsic clearance was substituted for Michaelis-Menten parameters, it failed to predict similar CLr values even within the linear range. For models S1 and S2, renal secretion was predominant at low doses, whereas renalclearance was driven by fraction unbound in plasma at high doses. Simulations demonstrated the importance of Michaelis-Menten parameter estimates (especially Vmax ) for determining CLr. Km estimates can easily be obtained directly from in vitro studies. However, additional scaling of in vitro Vmax estimates using in vitro/in vivo extrapolation methods are required to incorporate these parameters into pharmacokinetic models.