Andrew Byrne

Hepatitis C virus prevalence and outcomes among injecting drug users on opioid replacement therapy

Objectives:  To determine hepatitis C virus (HCV) prevalence among injecting drug users (IDUs) receiving opioid replacement therapy in a referred office setting, and assess potential needs for hepatitis C treatment and care.

ABCB1 haplotype and OPRM1 118A > G genotype interaction in methadone maintenance treatment pharmacogenetics

Background: Genetic variability in ABCB1, encoding the P-glycoprotein efflux transporter, has been linked to altered methadone maintenance treatment dose requirements. However, subsequent studies have indicated that additional environmental or genetic factors may confound ABCB1 pharmacogenetics in different methadone maintenance treatment settings. There is evidence that genetic variability in OPRM1, encoding the mu opioid receptor, and ABCB1 may interact to affect morphine response in opposite ways. This study aimed to examine whether a similar gene-gene interaction occurs for methadone in methadone maintenance treatment. Methods: Opioid-dependent subjects (n = 119) maintained on methadone (15–300 mg/day) were genotyped for five single nucleotide polymorphisms of ABCB1 (61A > G; 1199G > A; 1236C > T; 2677G > T; 3435C > T), as well as for the OPRM1 118A > G single nucleotide polymorphism. Subjects’ methadone doses and trough plasma (R)-methadone concentrations (Ctrough) were compared between ABCB1 haplotypes (with and without controlling for OPRM1 genotype), and between OPRM1 genotypes (with and without controlling for ABCB1 haplotype). Results: Among wild-type OPRM1 subjects, an ABCB1 variant haplotype group (subjects with a wild-type and 61A:1199G:1236C:2677T:3435T haplotype combination, or homozygous for the 61A:1199G:1236C:2677T:3435T haplotype) had significantly lower doses (median ± standard deviation 35 ± 5 versus 180 ± 65 mg/day, P < 0.01) and Ctrough (78 ± 22 versus 177 ± 97 ng/mL, P < 0.05) than ABCB1 wild-type subjects. Among subjects with the most common ABCB1 haplotype combination (wild-type with 61A:1199G:1236T:2677T:3435T), the OPRM1 118 A/G genotype was associated with a significantly higher Ctrough than 118 A/A (250 ± 126 versus 108 ± 36 ng/mL, P = 0.016). No ABCB1 haplotype group or OPRM1 genotype was associated with dose or Ctrough without taking into account confounding genetic variability at the other locus. Therefore, two interacting pharmacogenetic determinants of methadone maintenance treatment response were identified, ie, ABCB1, where variants are associated with lower methadone requirements, and OPRM1, where the variant is associated with higher methadone requirements. Conclusion: These opposing pharmacogenetic effects therefore need to be considered in combination when assessing methadone maintenance treatment pharmacogenetics.

Hepatitis C virus incidence among injecting drug users on opioid replacement therapy

Objective: To determine hepatitis C virus (HCV) incidence among injecting drug users (IDUs) receiving opioid replacement therapy (ORT).

Cannabis and benzodiazepines as determinants of methadone trough plasma concentration variability in maintenance treatment: a transnational study

PurposeTo assess tobacco, alcohol, cannabis and benzodiazepine use in methadone maintenance treatment (MMT) as potential sources of variability in methadone pharmacokinetics.MethodsTrough plasma (R)- and (S)-methadone concentrations were measured on 77 Australian and 74 Swiss MMT patients with no additional medications other than benzodiazepines. Simple and multiple regression analyses were performed for the primary metric, plasma methadone concentration/dose.ResultsCannabis and methadone dose were significantly associated with lower 24-h plasma (R)- and (S)-methadone concentrations/dose. The models containing these variables explained 14–16% and 17–25% of the variation in (R)- and (S)-methadone concentration/dose, respectively. Analysis of 61 patients using only CYP3A4 metabolised benzodiazepines showed this class to be associated with higher (R)-concentration/dose, which is consistent with a potential competitive inhibition of CYP3A4.ConclusionCannabis use and higher methadone doses in MMT could in part be a response to—or a cause of—more rapid methadone clearance. The effects of cannabis and benzodiazepines should be controlled for in future studies on methadone pharmacokinetics in MMT.

Harm reduction, hepatitis C and opioid pharmacotherapy: an opportunity for integrated hepatitis C virus-specific harm reduction

While harm reduction advocates, policy makers and practitioners have a right to be proud of the impact of interventions such as needle and syringe programmes on HIV risk, we can be less sanguine about the ongoing high levels of HCV transmission among injecting drug users (IDUs) and the expanding burden of hepatitis C virus (HCV)-related liver disease. In this Harm Reduction Digest Drs Byrne and Hallinan from the Redfern Clinic and Dr Dore from the National Centre in HIV Epidemiology and Clinical Research offer a model of integrated HCV prevention and treatment services within the setting of opioid pharmacotherapy. In their experience, this common-sense approach provides an opportunity to reduce the burden of HCV and improve overall patient management. They believe that the key elements of a HCV-specific harm reduction model include: regular HCV testing; clinical assessment and determination of need for HCV treatment referral; use of broader HCV treatment inclusion criteria; and flexibility in opioid pharmacotherapy dosing. In an environment when our macro harm reduction interventions seem to have, at best, modest impact on HCV transmission, good clinical practice may be our most effective strategy against the HCV epidemic. This paper provides some practical suggestions as to how this can be done.