| Summary: | Studies on pharmacokinetic drug–drug interactions have highlighted the importance of P-glycoprotein (P-gp) because of its involvement in substrate drug transport. This study aimed to investigate the role of chicken xenobiotic receptor (CXR) in the regulation of P-gp and its influences on pharmacokinetics of P-gp substrate sulfadiazine. ALAS1 and CYP2C45, the prototypical target genes of CXR, were used as a positive indicator for CXR activation in this study. Results show that ABCB1 gene expression was upregulated, and transporter activity was increased when exposed to the CXR activator metyrapone. Using ectopic expression techniques and RNA interference to manipulate the cellular CXR status, we confirmed that ABCB1 gene regulation depends on CXR. In vivo experiments showed that metyrapone induced ABCB1 in the liver, kidney, duodenum, jejunum and ileum of chickens. In addition, metyrapone significantly changed the pharmacokinetic behavior of orally administered sulfadiazine, with a C<sub>max</sub> (8.01 vs. 9.61 μg/mL, <i>p</i> < 0.05) and AUC<sub>0-t</sub> (31.46 vs. 45.59 h·mg/L, <i>p</i> < 0.01), as well as a higher T<sub>1/2λ</sub> (2.42 vs.1.67 h, <i>p</i> < 0.05), Cl/F (0.62 vs. 0.43 L/h/kg, <i>p</i> < 0.01) and Vz/F (2.16 vs.1.03 L/kg, <i>p</i> < 0.01). Together, our data suggest that CXR is involved in the regulation of P-gp, and, consequently, the CXR activator can affect, at least in part, the pharmacokinetic behavior of orally administered sulfadiazine.
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