Methadone inhibits CYP2D6 and UGT2B7/2B4 in vivo: a study using codeine in methadone- and buprenorphine-maintained subjects.

AIMS: To compare the O-demethylation (CYP2D6-mediated), N-demethylation (CYP3A4-mediated) and 6-glucuronidation (UGT2B4/7-mediated) metabolism of codeine between methadone- and buprenorphine-maintained CYP2D6 extensive metabolizer subjects. METHODS: Ten methadone- and eight buprenorphine-maintained subjects received a single 60 mg dose of codeine phosphate. Blood was collected at 3 h and urine over 6 h and assayed for codeine, norcodeine, morphine, morphine-3- and -6-glucuronides and codeine-6-glucuronide. RESULTS: The urinary metabolic ratio for O-demethylation was significantly higher (P= 0.0044) in the subjects taking methadone (mean ± SD, 2.8 ± 3.1) compared with those taking buprenorphine (0.60 ± 0.43), likewise for 6-glucuronide formation (0.31 ± 0.24 vs. 0.053 ± 0.027; P < 0.0002), but there was no significant difference (P= 0.36) in N-demethylation. Similar changes in plasma metabolic ratios were also found. In plasma, compared with those maintained on buprenorphine, the methadone-maintained subjects had increased codeine and norcodeine concentrations (P < 0.004), similar morphine (P= 0.72) and lower morphine-3- and -6- and codeine-6-glucuronide concentrations (P < 0.008). CONCLUSION: Methadone is associated with inhibition of CYP2D6 and UGTs 2B4 and 2B7 reactions in vivo, even though it is not a substrate for these enzymes. Plasma morphine was not altered, owing to the opposing effects of inhibition of both formation and elimination; however, morphine-6-glucuronide (analgesically active) concentrations were substantially reduced. Drug interactions with methadone are likely to include drugs metabolized by various UGTs and CYP2D6.

(R)- and (S)-methadone and buprenorphine concentration ratios in maternal and umbilical cord plasma following chronic maintenance dosing in pregnancy.

AIMS: The aim of this study was to compare the transfer of buprenorphine and methadone between maternal and cord blood in women under chronic dosing conditions and to determine if differences exist in the transfer of the two methadone enantiomers. METHODS: Maternal and cord blood samples were collected at delivery from women maintained on methadone (35, 25-140 mg day⁻¹) (median; range) or buprenorphine (6.00, 2-20 mg day⁻¹) during pregnancy. Plasma concentration ratios are presented as an indicator of foetal exposure relative to the mother. RESULTS: Methadone was quantified in all samples, with cord : maternal plasma methadone concentration ratios (n= 15 mother-infant pairs) being significantly higher (P < 0.0001; mean difference (MD) 0.07; 95% confidence interval (CI) 0.048, 0.092) for the active (R)-methadone enantiomer (0.41; 0.19, 0.56) (median; range) compared with (S)-methadone (0.36; 0.15, 0.53). (R)- : (S)-methadone concentration ratios were also significantly higher (P < 0.0001; MD 0.24 95% CI 0.300, 0.180) for cord (1.40; 0.95, 1.67) compared with maternal plasma (1.16; 0.81, 1.38). Half the infant buprenorphine samples were below the assay lower limit of quantification (LLOQ) (0.125 ng ml⁻¹). The latter was four-fold lower than the LLOQ for methadone (0.50 ng ml⁻¹). The cord : maternal plasma buprenorphine concentration ratio (n= 9 mother-infant pairs) was 0.35; 0.14, 0.47 and for norbuprenorphine 0.49; 0.24, 0.91. CONCLUSIONS: The transfer of the individual methadone enantiomers to the foetal circulation is stereoselective. Infants born to buprenorphine maintained women are not exposed to a greater proportion of the maternal dose compared with methadone and may be exposed to relatively less of the maternal dose compared with infants born to women maintained on methadone during pregnancy.

Opioid effects and opioid withdrawal during a 24 h dosing interval in patients maintained on buprenorphine.

In maintenance patients methadone has been shown to produce considerable changes in opioid effects and withdrawal over the dosing interval. As a partial agonist buprenorphine may be expected to produce smaller changes, but the nature and magnitude of these changes have only been described for single doses. In the present study opioid effects and withdrawal were described in patients maintained on buprenorphine. Twenty four opioid dependent subjects were administered 16 mg buprenorphine tablets sublingually for 10 days. On day 10 plasma samples were collected and physiological, subjective and observer-rated measures collected pre-dose and at 14 time points during the dosing interval. No significant respiratory depression was observed. Consistent with the partial agonist properties of buprenorphine, other physiological and subjective changes were also of small magnitude. However, even at a once daily dose of 16 mg some patients experienced significant opioid withdrawal that was maximal at the end of the dosing interval. Buprenorphine maintenance should be associated with a high level of safety and a low level of disruption caused by changing opioid effects over the dosing interval, but some patients may require high doses or other strategies to completely suppress withdrawal.

Patterns of symptom reporting during pregnancy comparing opioid maintained and control women.

OBJECTIVE: To characterize the range of symptoms experienced by pregnant methadone-maintained (MM) and buprenorphine-maintained (BM) women to determine whether these differ from those experienced by a control group of nonopioid exposed pregnant women. Opioid-maintained (OM) patients report high rates of symptoms related to direct opioid effects and withdrawal. Pregnancy is associated with a range of symptoms, some overlapping with opioid effects and withdrawal. METHODS: Prospective, nonrandomized, open-label comparison study undertaken in a large teaching maternity hospital in South Australia. Pregnant BM (n = 25), MM (n = 25) and nonopioid exposed controls (n = 25) were recruited and matched for age, parity, gravidity, alcohol consumption, and smoking status. Symptom report patterns, maternal withdrawal, and additional substance use were assessed. RESULTS: MM women reported 10 and BM women reported 2 symptoms throughout pregnancy at rates greater than controls. Methadone-maintained women reported significantly (P < 0.05) more symptoms than BM women compared to controls throughout pregnancy. Methadone-maintained women reported 8 and BM women reported 3 symptoms in the third trimester at rates greater than controls. Methadone-maintained women reported greater opioid withdrawal than controls; this did not occur in BM women. Additional substance use was comparable between BM and MM women but greater than controls. CONCLUSIONS: Patterns of symptom reports may have clinical implications for maternal and fetal health during pregnancy for OM women including optimization of opioid dosing regimens, education regarding maternal nutritional intake and preventing postnatal depression, thereby ensuring maternal health and fetal development during pregnancy and enhancing mother-infant bonding and healthy child development postnatally.

Opioid maintenance: a comparative review of pharmacological strategies.

The use of opioids outside of medical practice is a significant health problem with important social and political implications. Although treatment of opioid dependence is traditionally focused on heroin users, there is increasing recognition that a large number of people become dependent through the use of prescription opioids. The necessity for long-term treatment has also been increasingly recognised. At present, there are several pharmacotherapies available for maintenance treatment, including drugs that are full agonists at the opioid receptor (e.g., methadone, slow-release oral morphine), a partial agonist (buprenorphine) and an opioid antagonist (naltrexone). This review examines the existing strategies, highlights problems associated with their use and discusses the opportunities for new treatment approaches, particularly the use of long-acting formulations.