EVIDENCE FOR CYP2D1-MEDIATED PRIMARY AND SECONDARY O-DEALKYLATION OF ETHYLMORPHINE AND CODEINE IN RAT-LIVER MICROSOMES

The purpose of the present study was to investigate the role of specif ic CYPs responsible for the O-dealkylation of ethylmorphine (EM) and c odeine (CD) to morphine (M), as well as that of norethylmorphine (NEM) and norcodeine (NCD) to normorphine (NM) in rat liver microsomes. Liv er microsomes metabolize EM and CD to M, and NEM and NCD to NM, in the presence of an NADPH-generating system. The metabolites of EM and CD were determined by HPLC with UV and electrochemical detection. In the present study, the role of CYP2D1 in O-dealkylation of EM/NEM and CD/N CD was investigated by use of specific antiCYP antibodies. When testin g rabbit antirat CYP2D1, 2E1, 2C11, and 3A2 antibodies, only the antiC YP2D1 antibody inhibited the EM/NEM and CD/NCD O-dealkylase activities significantly. The maximum inhibition achieved was similar to 80% at a protein ratio (IgG to microsomes) of 10:1, p = 0.001. The contributi on of CYP2D1 to the O-dealkylation of EM/NEM and CD/NCD was further co nfirmed by use of the specific CYP2D1 inhibitors quinine and propafeno ne. Five mu M of quinine inhibited the EM/NEM and CD/NCD O-dealkylase activities by similar to 80%. The CYP3A inhibitor troleandomycin (TAO) failed to inhibit the CYP2D1 catalyzed reaction, but did inhibit the N-demethylation of EM and CD. The O-dealkylation of NEM and NCD was al so impaired in Dark Agouti rat (DA) liver microsomes. Taken together, the immunoinhibition and chemical-inhibitor studies of rat liver micro somes provided convincing evidence for the involvement of CYP2DL, the rat counterpart of human CYP2D6, in the metabolism of EM/NEM and CD/NC D to the corresponding O-dealkylated metabolites. (C) 1997 Elsevier Sc ience Inc.