The effects of cyclosporine on drug metabolism in rats and its mechanism
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Immunosuppressant cyclosporine (CsA) is primarily metabolized by cytochrome P450 3A (CYP3A), and a substrate for P-glycoprotein (P-gp). The large variability in CsA pharmacokinetics makes predicting CsA effects and toxicity difficult, which may be related to CsA-induced alterations in CYP3A or Pgp. Based on recent studies on the pregnane X receptor (PXR) in CYP3A induction and the role of cAMP in CYP regulation, we hypothesized that cAMP regulates PXR and CYP3A induction and that PXR and cAMP/CREB pathways are involved in CsA-induced CYP3A alterations. The objective of this work was to study the effects of chronic CsA administration on drug metabolism in rats and the possible mechanisms of CsA-induced CYP3A alterations in cells. Our in vivo results demonstrated that large doses of CsA significantly suppressed hepatic CYP3A but induced hepatic P-gp after rats were given CsA orally or subcutaneously for 28 days. In contrast, chronic oral administration of CsA increased small intestinal CYP3A. These data suggested that the differential effects of CsA on CYP3A and P-gp in the liver and small intestine may contribute to the variability of CsA pharmacokinetics. Our in vitro data demonstrated that CsA significantly decreased CYP3A expression and cAMP levels in CV-1 cells. To study whether PXR and/or cAMP/CREB pathways are involved in CsA-induced CYP3A modulation, we transfected a mouse PXR expression vector and the (CYP3A1DR3)2-tk-CAT reporter gene into HepG2, Caco-2, and CV-1 cells and measured the levels of phosphorylated cAMP response element-binding protein (phosphoCREB) in CV-1 cells. The results showed that CsA significantly suppressed PXR activation in these three cell lines and elevated phosphoCREB in CV-1 cells. Moreover, cAMP analog 8-Br-cAMP significantly upregulated phosphorylation of CREB and induced PXR activation and CYP3A expression. These results suggest that PXR and the cAMP/CREB pathways are critical in CYP3A alterations. However, CsA decreased cAMP content but increased phosphoCREB levels, thus suggesting that another pathway might be involved in CsA-induced CYP3A alteration. Together, these studies identified factors that are involved in the variability of CsA pharmacokinetics and elucidated mechanisms of CsA-induced CYP3A alterations in drug metabolism, thus providing a mechanistic interpretation of metabolism-mediated variation of CsA pharmacokinetics.