Exploring the role of mu opioid receptor (OPRM1) and CYP2B6 gene variations for methadone pharmacogenomics. Can these variations be used to advance toxicological interpretation post-mortem?.

Abstract

Methadone is increasingly involved in drug overdose cases and the molecular actions of the drug in vivo are largely unknown requiring elucidation. This study set out to examine the relationship between methadone toxicity and CYP2B6 and mu (μ) opioid receptor (OPRM1) single nucleotide polymorphisms (SNPs). Using SNP genotyping, the association between OPRM1 A118G and CYP2B6 T750C, G516T, and A785G variations and post-mortem methadone concentrations were investigated. The allele frequencies of OPRM1 and CYP2B6 variants were then studied in a control population of live non-methadone using subjects, to determine the prevalence and distribution of specific variations in post-mortem and living subjects. Further in vitro study was conducted to assist in interpreting the association between OPRM1 and CYP2B6 variations and individual susceptibility to methadone. Cloning strategies were designed for the studies of promoter activities affected by the T750C promoter SNP on CYP2B6 expression, and the role of the OPRM1 A118G variation for receptor internalisation following methadone treatment was investigated. A significant association was identified between high post-mortem methadone concentrations and G561T and A785G (CYP2B6*6) variations reflecting poor methadone metabolism. Furthermore, the OPRM1 A118G SNP significantly correlated with higher post-mortem methadone concentrations and the in vitro analysis of A118G indicated that this could be due to a reduction in receptor internalisation in 118 AG subjects. The findings from the research contribute to pre-determining, in part, individual susceptibility to methadone accumulation and toxicity. Specific screening to identify CYP2B6*6 and OPRM1 A118G carriers prior to addiction treatment could therefore be valuable as part of a cost-effective risk management strategy. Furthermore, CYP2B6*6 and A118G could be used to interpret toxicology results identifying subjects with poor metabolism.