Meclizine is not an inverse agonist or antagonist of human constitutive androstane receptor

Certain tissues express both pregnane X receptor (PXR) and constitutive androstane receptor (CAR), and various target genes are cross‐regulated by these receptors. Meclizine was reported to be an inverse agonist and antagonist of human CAR (hCAR) and an activator of human PXR (hPXR) in cell‐based reporter gene assays. Therefore, activation of hPXR by meclizine may attenuate its apparent hCAR inverse agonistic effects. To test this hypothesis, we investigated whether meclizine would still be capable of suppressing hCAR target gene expression ( CYP2B6 ) in cultured human hepatocytes, which are known to express hCAR and hPXR. Meclizine (0.03–60 μM) did not decrease constitutive CYP2B6 mRNA expression or attenuate hCAR agonist‐mediated increase in CYP2B6 mRNA and CYP2B6‐catalyzed bupropion hydroxylation levels. These findings reflect hPXR agonism and the lack of hCAR inverse agonism and antagonism by meclizine, which were assessed by a reporter gene assay and a mammalian two‐hybrid assay in transfected HepG2 cells. Control experiment indicated that PK11195, which is a hCAR inverse agonist and antagonist, decreased constitutive and hCAR agonist‐mediated hCAR activity. In conclusion, meclizine does not act as a hCAR inverse agonist or antagonist in human hepatocytes. Therefore, it is not appropriate to use this drug as a pharmacological tool to study hCAR function in this cell type.[Supported by CIHR and MSFHR]