Repeated administration of oxycodone modifies the gene expression of several drug metabolising enzymes in the hepatic tissue of male Sprague-Dawley rats, including glutathione S-transferase A-5 (rGSTA5) and CYP3A2

Objectives Clinical use and illicit abuse of the potent opioid agonist oxycodone has dramatically increased over the past decade. Yet oxycodone remains one of the least studied opioids, particularly its interactions on the genomic level. The aim of this study was to examine potential alterations in gene expression of drug metabolising enzymes in the liver tissue of male Sprague-Dawley rats chronically treated with oxycodone. Methods Rats were administered saline or oxycodone 15 mg/kg i.p. twice a day for 8 days. Changes in RNA levels were detected using microarray analysis validated by quantitative real-time PCR; consequent changes in protein expression and functionality were further assessed by Western blotting and activity assays. Key findings The expression of several drug metabolising enzymes was modulated by oxycodone treatment: cytochrome P450 (CYP) 2132, CYP2C13, CYP17A1, epoxide hydrolase 2, carboxylesterase 2, flavin-containing monooxygenase 1, glutathione S-transferase alpha 5 (rGSTA5) and CYP3A2. In particular, the mRNA level of rGSTA5 (formerly GSTYc(2)) was up-regulated by approximately 6.5 fold and CYP3A2 was down-regulated by approximately 7.0 fold. Immunoblotting assays demonstrated a corresponding significant elevation of rGSTA5 protein and repression of CYP3A2 protein. The apparent cytosolic GST activity towards 1-chloro-2,4-dinitrobenzene conjugation and reduction of cumene hydroperoxide were significantly higher in liver from oxycodone-treated rats than that of saline-treated rats. In addition, the microsomal activity of CYP3A2, measured via 6 beta-hydroxylation of testosterone, was significantly decreased in oxycodone-treated rats. Conclusions Repeated oxycodone administration is associated with a significant upregulation of rGSTA5 and concomitant down-regulation of CYP3A2 mRNA, protein expression and functionality. These results support further in-vivo studies into the clinical impact of our findings.