Characterization of the human cytochrome P450 enzymes involved in the metabolism of dihydrocodeine.
OBJECTIVE
Using human liver microsomes from donors of the CYP2D6 poor and extensive metabolizer genotypes, the role of individual cytochromes P-450 in the oxidative metabolism of dihydrocodeine was investigated.


METHODS
The kinetics of formation of N- and O-demethylated metabolites, nordihydrocodeine and dihydromorphine, were determined using microsomes from six extensive and one poor metabolizer and the effects of chemical inhibitors selective for individual P-450 enzymes of the 1A, 2A, 2C, 2D, 2E and 3A families and of LKM1 (anti-CYP2D6) antibodies were studied.


RESULTS
Nordihydrocodeine was the major metabolite in both poor and extensive metabolizers. Kinetic constants for N-demethylation derived from the single enzyme Michaelis-Menten model did not differ between the two groups. Troleandomycin and erythromycin selectively inhibited N-demethylation in both extensive and poor metabolizers. The CYP3A inducer, alpha-naphthoflavone, increased N-demethylation rates. The kinetics of formation of dihydromorphine in both groups were best described by a single enzyme Michaelis-Menten model although inhibition studies in extensive metabolizers suggested involvement of two enzymes with similar Km values. The kinetic constants for O-demethylation were significantly different in extensive and poor metabolizers. The extensive metabolizers had a mean intrinsic clearance to dihydromorphine more than ten times greater than the poor metabolizer. The CYP2D6 chemical inhibitors, quinidine and quinine, and LKM1 antibodies inhibited O-demethylation in extensive metabolizers; no effect was observed in microsomes from a poor metabolizer.


CONCLUSIONS
CYP2D6 is the major enzyme mediating O-demethylation of dihydrocodeine to dihydromorphine. In contrast, nordihydrocodeine formation is predominantly catalysed by CYP3A.
Kirkwood
L C
LC
Centre for Pharmaceutical Research, School of Pharmacy and Medical Sciences, University of South Australia, Adelaide.
Nation
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RL
Somogyi
A A
AA
D000701:Analgesics, Opioid / Q000493:pharmacokinetics*; D019138:Antibodies, Blocking / Q000494:pharmacology; D001189:Aryl Hydrocarbon Hydroxylases; D001711:Biotransformation; D003061:Codeine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D019389:Cytochrome P-450 CYP2D6 / Q000096:biosynthesis / Q000378:metabolism; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000378:metabolism*; D003640:Dealkylation; D004790:Enzyme Induction / Q000187:drug effects / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology; D010089:Oxidoreductases, N-Demethylating / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000378:metabolism
D000701:Analgesics, Opioid; D019138:Antibodies, Blocking; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System; D001189:Aryl Hydrocarbon Hydroxylases; D019389:Cytochrome P-450 CYP2D6; D051544:Cytochrome P-450 CYP3A; D010089:Oxidoreductases, N-Demethylating; C014481:dihydrocodeine; D003061:Codeine