Membrane transporter proteins may play a role both in the pharmacokinetics of several drugs (when expressed in normal tissues) and/or resistance to several anticancer agents (when expressed in cancers). Several of these proteins belong to the ABC (ATP Binding Cassette) protein superfamily, including, P-glycoprotein (Pgp, ABCB1), the Multidrug Resistance Protein (MRP1, ABCC1), the Mitoxantrone Resistance Protein (MXR, ABCG2), the canalicular Multispecific Organic Anion Transporter (cMOAT, ABCC2), the Bile Salt Export Pump (BSEP, ABCB11). Other transporter proteins, such the Organic Cation transporters (OCT1 and OCT2) and the Organic Anionic Transporter (OAT) do not belong to the ABC superfamily. These proteins seem to share several substrate drugs including, possibly, some anticancer agents. The partial sharing of anticancer and non-anticancer substrates between different transporters may explain some of the side effects of drugs used to inhibit the function of specific transporters. For example, Cyclosporin A has been tested clinically as an inhibitor of Pgp-caused multidrug resistance. One of its side effects is jaundice, which might be caused by its interference of the transport of conjugated bilirubin by cMOAT. So, the clinical efficacy of transporter inhibitors (whether used to reverse anticancer drug resistance, or to modulate pharmacokinetics) may be improved by optimizing their transporter selectivity. Pgp is the transporter that has been most thoroughly studied. Pgp effluxes its substrates from cells by transporting them from the cell membrane inner to the outer leaflet or to the extracellular space. Central to the biology of Pgp is its ability to bind a wide array of diverse substrates and inhibitors, suggesting the possible existence of multiple binding sites. The lack of definition of these sites and the unavailability of a crystal structure for Pgp have so far hindered a rational design of drugs targeting Pgp function. We propose that affinity chromatography can be used as a means to characterize the binding sites and transport cycle of the different transporters and, ultimately, to define effective and protein-selective pharmacophores for the pharmacological inhibition of transporter function. We are presently characterizing and optimizing affinity chromatography models of Pgp. This process will also provide a prototype for the modeling of other ABC and non-ABC transporters. Membranes containing Pgp were obtained from a stabley trnasfected cell line (MDA435/LCC6MDR1 human breast cancer cells)and membranes which do not contain Pgp were obtained from the MDA435/LCC6 cell line). The membranes were immobilized on the surface of a 100 micron i.d. glass tube to form open tubnular columns, Pgp(+)-OT and Pgp(-)-OT. The columns were placed in a chromatopgraphic system containing a mass spectrometer and rapid frontal corhomatographic studies were performed with known Pgp substates and compounds known not to bind to Pgp. The results indicate that the difference in the chromatographic retention on the two columns (the delta-time)can be used to classify compounds as Pgp substrates. The results are correlatable with data obtained using cellular methods such as Caco-2 cell premeability and the method is a rapid, reproducible and accurate appraoch to screen for Pgp substrates. Cell lines which express and do not express the human organic transporter (hOCT1)have been obtained and used to prepare hOCT1(+)-IAM and hOCT1(-)-OCT columns. The columns were prepared by immobilizing membranes obtained from the respective cell lines on the surface of immobilized artificial membrane (IAM) liquid chromatography stationary phases. Frontal affinity chromatography has demonstrated that the hOCT1(+)-IAM column can be used to identify substrates and inhibitors of the hOCT1. The studies have also demonstrated that interactions with the hOCT1 are enantioselective and has been used to createa pharmacophore that describes inhibitor binding to the hOCT1 and can be used to indentify these inhibitors in silico. Columns containing mutated hOCT1 transporters created by single point mutations have been created and used to characterize the effects of these mutations on binding affintiy, selectivity and transport. An initial column containing an immobilized organic anion transporter has been developed and the column is being characterized.
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