Cell-based systems to assess nuclear receptor activation and their use in drug development

The evolution of scientific information relating to the regulation of xenobiotic disposition has extended to the discovery of an intricate group of receptor systems now recognized as master regulators. These ligand-activated transcription factors are commonly designated as "nuclear receptors", and include CAR (NR1I3), PXR (NR1I2), PPAR (NR1C1, NR1C2, and NR1C3) and AhR (HLHE76). As regulators of gene expression, activation of these receptors can elicit a plethora of drug-drug interactions. The aforementioned nuclear receptors bind a wide range of structurally-unrelated ligands, such as steroid hormones, bile acids, and small drug-type molecules. A pivotal nuclear receptor with regards to regulation of drug-drug interactions is the pregnane X receptor (PXR). Gene expression profiling has demonstrated that PXR regulates over 60 human genes that are involved not only in physiological functions but also in the metabolism of xenobiotics. Moreover, chemical library screening suggests that about 10% of the compounds comprising the U. S. Food and Drug Administration 1 and 2, Sigma-Aldrich LOPAC collection, Biomol, and Tocris/TimTec bioactive collection libraries exhibit some form of PXR binding. For these reasons, efficient, rapid and economical systems have been developed to identify nuclear receptor ligands. Cell-based assays encompassing transiently and stably-transfected cells and mammalian two-hybrid systems are currently being employed by the pharmaceutical industry to screen compounds for binding to and/or activation of nuclear receptors. Overall, these systems have the ability to predict in vivo responses to receptor activation that culminate in drug-drug interactions and adverse drug effects.