|dc.description.abstract||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.||