Mechanistic Understanding of P-gp Mediated Transport and Inhibition Kinetics across a Confluent monolayer of MDCKII-hMDR1 cells using a Mass Action Kinetic Model

The purpose of this work is to investigate several aspects of transporter-mediated interactions in a physiologically relevant model system and to gain a clear mechanistic understanding of the same using a mass action kinetic model. We have first shown that the parameters determining the potency of an inhibitor, IC50 & KI are not the same for P-gp and that IC50 overestimates KI. The quantitative relationship between them is derived and the physical mechanism of overestimation, explained. This sensitive modeling tool was further used for the kinetic identification of other transporters in addition to P-gp in several confluent cell monolayer systems. It was shown that a simple IC50 plot has more than one inhibition events convolved in them and the model was used to separate the individual contributions. This is of clinical relevance for the accurate assessment of transporter mediated drug-drug interaction risks. We have also derived an exact equation for measuring the passive permeability coefficients of compounds that show significant drug loss. Such measurements would help us carefully delineate the passive component from the total transport and describe the P-gp mediated transport of these compounds unambiguously. In summary, such an understanding of drug -transporter relations in vitro obtained as a part of this thesis work is valuable for making accurate predictions of drug disposition in vivo, aiding the ability to make informed decisions during the several stages of drug discovery and development.