Mechanism-based inactivation of CYP2D6 by 5-fluoro-2-[4-[(2-phenyl-1H-imidazol-5-yl)methyl]-1-piperazinyl]pyrimidine

SCH 66712 [5-fluoro-2-[4-[(2-phenyl-1H-imidazol-5yl)methyl]-1-piperazinyl]p yrimidine] caused a time- and NADPH-dependent loss of CYP2D6 activity, The inactivation of human liver (HL) microsomal dextromethorphan O-demethylase activity, a prototype marker for CYP2D6, was characterized by a K-I of 4.8 muM and a maximal rate constant of inactivation (k(inact)) of 0.14 min(-1), The inactivation of the recombinant CYP2D6 in Supersomes (r-CYP2D6) was ch aracterized by a K-I of 0.55 muM and a k(inact) of 0.32 min(-1). Extensive dialysis of the SCH 66712-inhibited enzyme failed to restore the activity t o control levels (dialyzed reaction mixture lacking SCH 66712) for both HL microsomes and r-CYP2D6. Addition of glutathione, superoxide dismutase, or mannitol to the reaction mixture failed to protect CYP2D6 against SCH 66712 -NADPH-catalyzed inactivation. Addition of quinidine, a reversible inhibito r of CYP2D6, to a preincubation mixture consisting of SCH 66712, HL microso mes, or Supersomes and NADPH partially protected CYP2D6 from inactivation. SCH 66712 also inhibited HL microsomal CYP3A4, CYP2C9, and CYP2C19; however , the concentrations required to inhibit those isoforms were 5- to 10-fold higher than those required to inhibit CYP2D6, These results demonstrate tha t SCH 66712 is a potent and fairly selective mechanism-based inhibitor of C YP2D6.