Article date: May 2012
By: Eloise A. Gelston, Janet K. Coller, Olga V. Lopatko, Heather M. James, Helmut Schmidt, Jason M. White, Andrew A. Somogyi, in Volume 73, Issue 5, pages 786-794
WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT
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.
DOI: 10.1111/j.1365-2125.2011.04145.x
View this article