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Differential inhibition of hepatic cytochromes P-450 by cimetidine in adult male rats Chang, Thomas Kwok Hung


The cytochrome P-450 enzymes are a family of hemoproteins that play an important role in drug metabolism in man and animals. Cimetidine is a histamine H₂-receptor antagonist used in the treatment of peptic ulcers and other gastric acid-related disorders. It is thought that this drug is a general inhibitor of cytochrome P-450 enzymes. However, a detailed analysis of the literature indicates substantial, but indirect, evidence that certain cytochrome P-450 enzymes may not be inhibited by cimetidine. Also, it is apparent that the observed inhibition of hepatic microsomal cytochrome P-450-mediated enzyme activities by in vitro cimetidine administration does not adequately explain the inhibition observed following the in vivo administration of the drug to intact animals or to humans. A major objective of the present study was to determine whether cimetidine, when administered in vivo, differentially inhibits cytochrome P-450 enzymes in hepatic microsomes from adult male rats. Uninduced, phenobarbital-induced and dexamethasone-induced rats were sacrificed 90 min after a single intraperitoneal dose of cimetidine HCl (150 mg/kg) or saline. Based on the results from the in vivo cimetidine experiments using enzyme-specific substrates and immunoinhibition experiments with monospecific anti-cytochrome P450IIC11 antibody, it was concluded that cimetidine administration to adult male rats inhibited hepatic cytochrome P450IIC11. Indirect evidence also indicated that unidentified enzymes other than cytochrome P450IIC11 were inhibited by cimetidine in microsomes from uninduced adult male rats. However, the enzyme activities specific for cytochrome P450IIA1, cytochromes P450IIB1/2 and cytochromes P450IIIA1/2 were not affected by in vivo cimetidine. It is possible that these enzymes are not inhibited by cimetidine or that the lack of effect is related to the particular substrate used. In some cases, the extent of inhibition of enzyme activities by in vivo cimetidine administration depended on prior treatment with an inducer. This can be explained by the increasing contribution to such activities by inducible enzymes which were not subject to inhibition by cimetidine. Another objective was to determine whether the differential inhibition of cytochrome P-450 by in vivo cimetidine is observed when the drug is administered in vitro. Cimetidine, at concentrations of up to 10 mM, did not affect the catalytic function of cytochrome P450IIA1. In contrast, it did inhibit enzyme activities that were specific for cytochrome P450IIC11, cytochromes P450IIB1/2 and cytochromes P450IIIA1/2, with IC₅₀ values in the range of 1.0 - 7.4 mM. The discrepancy in the inhibition of cytochrome P-450 by in vivo and in vitro cimetidine administration was further characterized in enzyme kinetic experiments. Based on Lineweaver-Burk plots of the data, the cytochrome P450IIC11-mediated testosterone 2⍺-hydroxylase activity was inhibited non-competitively by in vivo cimetidine, but competitively by in vitro cimetidine. To further investigate the inhibition of cytochrome P-450 enzymes by in vivo and in vitro cimetidine, preincubation experiments were performed. Hepatic microsomes were preincubated with a low concentration (0.05 mM) of cimetidine and 1 mM NADPH for 15 min prior to the initiation of substrate (testosterone) oxidation. Under these conditions, cimetidine resulted in the inhibition of the enzyme activities specific for cytochrome P450IIC11, but it had no effect on those specific for cytochrome P450IIA1, cytochromes P450IIB1/2 and cytochromes P450IIIA1/2. This differential inhibition by in vitro cimetidine required the presence of NADPH in the preincubation medium, suggesting that a catalysis-dependent process is involved. Thus, preincubation of hepatic microsomes with NADPH and a relatively low concentration (0.05 mM) of cimetidine in vitro results in a pattern of inhibition of cytochrome P-450 enzymes similar to that following the in vivo administration of cimetidine.

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