EFFECT OF CIMETIDINE ON HEPATIC CYTOCHROME-P450 - EVIDENCE FOR FORMATION OF A METABOLITE-INTERMEDIATE COMPLEX

Cimetidine is an inhibitor of hepatic cytochrome P450 (CYP) in vivo an d in vitro in both rats and humans, However, the concentrations of cim etidine required to inhibit drug metabolism in hepatic microsomes in v itro are 100-1000-fold higher than those associated with a similar deg ree of inhibition in vivo, Recently, we provided evidence indicating t hat cimetidine selectively inhibits CYP2C11 in the rat and that it act s by forming a metabolite-intermediate (MI) complex, To investigate th is further, optical-difference spectroscopy was performed with the use of microsomes from uninduced and phenobarbital-induced male rats trea ted with cimetidine in vivo or, after a preincubation step, in vitro, In microsomes from uninduced rats treated with cimetidine in vivo or i n vitro, a spectral peak at 428-430 nm was observed that was not disso ciated by the addition of potassium ferricyanide. The spectral peak ob tained with cimetidine in vitro was dependent on the presence of NADPH and on the concentration of cimetidine in the reaction mixture, indic ating the presence of an MI complex, The magnitude of this complex was reduced in microsomes from phenobarbital induced rats that were admin istered cimetidine either in vivo or in vitro, The formation of the co mplex was also inhibited by preincubation of the microsomes with anti- rat CYP2C11 immunoglobulin. These results are consistent with the hypo thesis that cimetidine inhibits CYP in vivo in the rat by forming an M I complex, largely with CYP2C11, and that this complex can be generate d in vitro by incubating microsomes with cimetidine in the presence of NADPH.