Cytochrome P450 2C9 (CYP2C9) expression is regulated by multiple nuclear receptors including the constitutive androstane receptor (CAR) and pregnane X receptor (PXR). We compared coregulation of CYP2C9 with CYP2B6 and CYP3A4, prototypical target genes for human CAR and PXR using human hepatocyte cultures treated for three days with the PXR activators clotrimazole, rifampin, and ritonavir; the CAR/PXR activator phenobarbital (PB); and the CAR-selective agonists CITCO, (6-(4-chlorophenyl)imidazo[2,1-beta][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime) and phenytoin. Clotrimazole, rifampin, ritonavir, phenytoin, and phenobarbital induced CYP2C9 consistent with previous findings for CYP3A4. We observed EC(50) values of 519 microM (phenobarbital), 11 microM (phenytoin), and 0.75 microM (rifampin), similar to those for CYP3A4 induction. Avasimibe, a potent PXR activator, produced nearly identical concentration-dependent CYP2C9 and CYP3A4 activity profiles and EC(50) values. In 17 donors, rifampin increased mean basal CYP2C9 activity from 59 +/- 43 to 143 +/- 68 pmol/mg protein/min; fold induction ranged from 1.4- to 6.4-fold. Enzyme activity and mRNA measurements after rifampin, CITCO and PB treatment demonstrated potency and efficacy consistent with CYP2C9 regulation being analogous to CYP3A4 rather than CYP2B6. We demonstrate that hepatic CYP2C9 is differentially regulated by agonists of CAR and PXR, and despite sharing common regulatory mechanisms with CYP3A4 and CYP2B6; this enzyme exhibits an induction profile more closely aligned with that of CYP3A4.
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