Pharmacogenomics of ABCB1 in maintenance pharmacotherapies for opioid dependence.

Abstract

Opioid dependence is a significant public health problem. Whilst long-term opioid maintenance is the most cost-effective approach for treating opioid dependence, the safe and effective use of substitution opioids like methadone and buprenorphine is complicated by their narrow therapeutic indices and a considerable, as yet unexplained, interindividual variability in their dose-effect relationships. Since there is evidence that the P-glycoprotein efflux transporter may influence the plasma pharmacokinetics and CNS distribution of opioids, it was hypothesised that genetic variability in the ABCB1 gene (encoding P-glycoprotein) could play a major role in the interindividual variability in opioid maintenance treatment response. Therefore, the primary aim of this thesis was to investigate ABCB1 genetic variability as a determinant of opioid requirements during maintenance therapy, as well as treatment outcome. This thesis also set out to identify the relationship between ABCB1 genetic variability and the risk of illicit opioid use and dependence, as well as develop new methods for investigating the dynamic interactions between ABCB1 genetic variability, P-glycoprotein expression/function and opioid exposure. For the first major study of this thesis, opioid-dependent methadone maintenance treatment (MMT, n = 78) and buprenorphine maintenance treatment (BMT, n = 30) subjects, as well as non-opioid-dependent healthy controls (n = 98), were retrospectively genotyped and haplotyped for 5 common single nucleotide polymorphisms (SNPs) of ABCB1 (A61G, G1199A, C1236T, G2677T and C3435T). Whilst no link was observed between ABCB1 genetic variability and the risk of opioid dependence, the wild-type AGCGC (61A-1199G-1236C-2677G-3435C) haplotype was associated with significantly higher maintenance opioid requirements among both MMT and BMT subjects. In addition, MMT subjects carrying one of the variant haplotypes, AGCTT, required significantly less methadone, presumably due to a decreased P-gp activity at the blood-brain-barrier. Interestingly, a second retrospective study of a specific cohort of 21 (very) high-dose (≥180 mg/day) MMT subjects could not replicate these findings, suggesting that dose range and/or clinic policy may be important factors influencing the clinical significance of ABCB1 genetic variability. The third major study of this thesis incorporated the development and validation of new methods for quantifying ex vivo P-glycoprotein expression (mRNA and protein) and function in specific lymphocyte subsets (CD4+, CD56+ and CD8+) of healthy and opioid-dependent subjects, with the aim of determining the combined effects of ABCB1 genetic variability and opioid exposure on P-glycoprotein function. Applying these new methods in a pilot study of 6 MMT subjects, CD4+ lymphocyte ABCB1 mRNA and P-glycoprotein expression were found to be positively associated with methadone requirements, and were lowest in the only subject homozygous for the AGCTT haplotype (providing potential mechanistic support for the link between AGCTT haplotypes and low MMT dose requirements). Therefore, this thesis provides the first evidence that ABCB1 haplotypes contribute to variability in substitution opioid requirements. However, ABCB1 genetic variability should not be considered alone, and a combined interpretation of multiple genetic and environmental factors will be required to provide a more complete picture of the factors governing the successful treatment of opioid dependence.