Harma Ellens, Zhou Meng, Sylvain J. Le Marchand, Joe Bentz

Introduction In vitro transporter kinetics are typically analyzed by steady-state Michaelis-Menten (MM) approximations. However, no clear evidence exists that these approximations, applied to multiple transporters in biological membranes, yield system-independent mechanistic parameters needed for reliable in vivo hypothesis generation and testing. Areas covered The classical mass action model has been developed for P-glycoprotein mediated transport across confluent polarized cell monolayers. Numerical integration of the mass action equations for transport using a stable global optimization program, yields fitted elementary rate constants that are system-independent. The efflux active P-gp was defined by the rate at which P-gp delivers drugs to the apical chamber, since as many as 90% of drugs effluxed by P-gp partition into nearby microvilli prior to reaching the apical chamber. The efflux active P-gp concentration was 10-fold smaller than the total expressed P-gp for Caco-2 cells, due to their microvilli membrane morphology. The mechanistic insights from this analysis are readily extrapolated to P-gp mediated transport in vivo. Expert opinion In vitro system-independent elementary rate constants for transporters are essential for the generation and validation of robust mechanistic PBPK models. Our modeling approach and programs have broad application potential. They can be used for any drug transporter with minor adaptations.