Abstract

Cimetidine is an inhibitor of hepatic cytochrome P450 (CYP) in vivo and in vitro in both rats and humans. However, the concentrations of cimetidine required to inhibit drug metabolism in hepatic microsomes in vitro are 100-1000-fold higher than those associated with a similar degree of inhibition in vivo. Recently, we provided evidence indicating that cimetidine selectively inhibits CYP2C11 in the rat and that it acts by forming a metabolite-intermediate (MI) complex. To investigate this further, optical-difference spectroscopy was performed with the use of microsomes from uninduced and phenobarbital-induced male rats treated with cimetidine in vivo or, after a preincubation step, in vitro. In microsomes from uninduced rats treated with cimetidine in vivo or in vitro, a spectral peak at 428-430 nm was observed that was not dissociated by the addition of potassium ferricyanide. The spectral peak obtained with cimetidine in vitro was dependent on the presence of NADPH and on the concentration of cimetidine in the reaction mixture, indicating the presence of an MI complex. The magnitude of this complex was reduced in microsomes from phenobarbital induced rats that were administered cimetidine either in vivo or in vitro. The formation of the complex was also inhibited by preincubation of the microsomes with anti-rat CYP2C11 immunoglobulin. These results are consistent with the hypothesis that cimetidine inhibits CYP in vivo in the rat by forming an MI complex, largely with CYP2C11, and that this complex can be generated in vitro by incubating microsomes with cimetidine in the presence of NADPH.