Abstract
Membrane transporters are essential for the transfer of hydrophobic molecules across the plasma membrane of cells; therefore active and facilitated transport processes can determine the absorption, distribution, metabolism and excretion profile of drugs. Preclinical assays are used to determine the effects of transport on the pharmacokinetics and toxicology of a new compound during drug development. However cells in culture often give erroneous results due to the differential expression of important proteins within them. It was therefore the aim of this work to assess the differences in drug transporter expression in a variety of human and rat hepatocellular systems compared to liver. RT-PCR analysis of drug transporter levels in vitro and in vivo showed huge differences, with influx transporters generally being under-expressed and efflux transporters overexpressed in vitro compared to in vivo. Further study established that lack of the influx transporter OATP1B1 in Huh7 cells could limit the cellular access of pravastatin. This work also showed that pravastatin could increase the expression of the drug transporter Abcc2 but not ABCB1 or GYP3A4, all of which are regulated by PXR suggesting differential gene regulation by the same nuclear receptor is ligand dependent. Expression analysis of rat hepatocytes showed that sandwich cultured cells expressed a more ‘liverlike’ level of drug transporters than those grown in monolayer. Further work showed that the Abcc family have similar regulatory pathways involving PXR, while Abcg2 was shown to be regulated by PPARα. Finally, an in silico model of carboxydichlorofluroscein transport through a rat hepatocyte was established. In conclusion, differential expression of drug transporters in cell lines may affect the disposition of drugs, however information regarding expression can be used to make informed predictions on the functional effect such transport processes may have. Once in the cell, compounds may activate nuclear receptors to increase levels of transporter expression suggesting a system of coordinate regulation, as the same nuclear receptors are known to regulate the expression of drug metabolising enzymes.