Stereoselective Quantification of Methadone and a d6-labeled Isotopomer Using High Performance Liquid Chromatography-Atmospheric Pressure Chemical Ionization Mass-Spectrometry: Application to a Pharmacokinetic Study in a Methadone Maintained Subject

Deuterium- and Tritium-Labelled Compounds: Applications in the Life Sciences

Hydrogen isotopes are unique tools for identifying and understanding biological and chemical processes. Hydrogen isotope labelling allows for the traceless and direct incorporation of an additional mass or radioactive tag into an organic molecule with almost no changes in its chemical structure, physical properties, or biological activity. Using deuterium-labelled isotopologues to study the unique mass-spectrometric patterns generated from mixtures of biologically relevant molecules drastically simplifies analysis. Such methods are now providing unprecedented levels of insight in a wide and continuously growing range of applications in the life sciences and beyond. Tritium (³ H), in particular, has seen an increase in utilization, especially in pharmaceutical drug discovery. The efforts and costs associated with the synthesis of labelled compounds are more than compensated for by the enhanced molecular sensitivity during analysis and the high reliability of the data obtained. In this Review, advances in the application of hydrogen isotopes in the life sciences are described.

Inhibition of CYP2D6-mediated tramadol O-demethylation in methadone but not buprenorphine maintenance patients

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Management of pain in opioid dependent individuals is problematic due to numerous issues including cross-tolerance to opioids. Hence there is a need to find alternative analgesics to classical opioids and tramadol is potentially one such alternative. Methadone inhibits CYP2D6 in vivo and in vitro. We aimed to investigate the effect of methadone on the pathways of tramadol metabolism: O-demethylation (CYP2D6) to the opioid-active metabolite M1 and N-demethylation (CYP3A4) to M2 in subjects maintained on methadone or buprenorphine as a control.

WHAT THIS STUDY ADDS

Compared with subjects on buprenorphine, methadone reduced the clearance of tramadol to active O-desmethyl-tramadol (M1) but had no effect on N-desmethyltramadol (M2) formation. Similar to other analgesics whose active metabolites are formed by CYP2D6 such as codeine, reduced formation of O-desmethyltramadol (M1) is likely to result in reduced analgesia for subjects maintained on methadone. Hence alternative analgesics whose metabolism is independent of CYP2D6 should be utilized in this patient population.

AIMS

To compare the O- (CYP2D6 mediated) and N- (CYP3A4 mediated) demethylation metabolism of tramadol between methadone and buprenorphine maintained CYP2D6 extensive metabolizer subjects. METHODS Nine methadone and seven buprenorphine maintained subjects received a single 100 mg dose of tramadol hydrochloride. Blood was collected at 4 h and assayed for tramadol, methadone, buprenorphine and norbuprenorphine (where appropriate) and all urine over 4 h was assayed for tramadol and its M1 and M2 metabolites.

RESULTS

The urinary metabolic ratio [median (range)] for O-demethylation (M1) was significantly lower (P= 0.0002, probability score 1.0) in the subjects taking methadone [0.071 (0.012-0.103)] compared with those taking buprenorphine [0.192 (0.108-0.392)], but there was no significant difference (P= 0.21, probability score 0.69) in N-demethylation (M2). The percentage of dose [median (range)] recovered as M1 was significantly lower in subjects taking methadone compared with buprenorphine (0.069 (0.044-0.093) and 0.126 (0.069-0.187), respectively, P= 0.04, probability score 0.19), M2 was significantly higher in subjects taking methadone compared with buprenorphine (0.048 (0.033-0.085) and 0.033 (0.014-0.049), respectively, P= 0.04, probability score 0.81). Tramadol was similar (0.901 (0.635-1.30) and 0.685 (0.347-1.04), respectively, P= 0.35, probability score 0.65).

CONCLUSIONS

Methadone inhibited the CYP2D6-mediated metabolism of tramadol to M1. Hence, as the degree of opioid analgesia is largely dependent on M1 formation, methadone maintenance patients may not receive adequate analgesia from oral tramadol.

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.

Applications of stable isotopes in clinical pharmacology

This review aims to present an overview of the application of stable isotope technology in clinical pharmacology. Three main categories of stable isotope technology can be distinguished in clinical pharmacology. Firstly, it is applied in the assessment of drug pharmacology to determine the pharmacokinetic profile or mode of action of a drug substance. Secondly, stable isotopes may be used for the assessment of drug products or drug delivery systems by determination of parameters such as the bioavailability or the release profile. Thirdly, patients may be assessed in relation to patient-specific drug treatment; this concept is often called personalized medicine. In this article, the application of stable isotope technology in the aforementioned three areas is reviewed, with emphasis on developments over the past 25 years. The applications are illustrated with examples from clinical studies in humans.

(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.