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The effect of cimetidine on dextromethorphan O-demethylase activity of human liver microsomes and recombinant CYP2D6 Madeira, Maria


Cimetidine is a liistamine H₂-receptor antagonist used in the treatment of peptic ulcer disorders. There have been numerous clinical observations of cimetidine drug-drug interactions, in some cases involving known substrates of cytochrome P450 (P450) 2D6. One explanation for these observations is impaired hepatic metabolism via inhibition o f P450s, the primary mediators of hepatic phase I metabolism of drugs. Cimetidine has been demonstrated to inhibit various P450 enzymes in vitro, including CYP2D6, but the inhibition constants are generally higher than the plasma concentrations measured following in vivo administration. In rats, the in vitro selectivity and potency of cimetidine can be increased by preincubating the inhibitor and NAPDH with hepatic microsomes, and the formation of a metabolite-intermediate complex has been implicated. The effect of preincubation on inhibition of CYP2D6 by cimetidine has not been investigated. The objective of this project was to further characterize the effect of cimetidine on CYP2D6 in vitro using human liver microsomes and recombinant CYP2D6. Dextromethorphan O-demethylase activity was used as a probe of CYP2D6 activity. Metabolite formation was quantified using a high performance liquid chromatography assay with fluorescence detection. The V[sub max] and K[sub m] of this reaction were 0.82+0.06 nmol/min/nmol P450 and 4.1 ±0.1 μM, respectively, in microsomes; and 15.9±0.8 nmol/min/nmol P450 and 1.4±0.6 μM, respectively, with recombinant CYP2D6. With human liver microsomes, cimetidine was a competitive inhibitor of CYP2D6 with a K, of 38.0±5.3 μM; the observed inhibition was not effected by preincubation. With the recombinant enzyme, cimetidine acted as a mixed inhibitor with a K[sub i] of 102.5±16.8 μM. Preincubation with the inhibitor and NADPH led to a further decrease in activity of 11% to 18% in a manner that was consistent with mechanism-based inactivation. The effect of preincubation was attenuated by including quinidine in the preincubation reaction. The k[sub inact] and K[sub I] were estimated to be 0.03 min⁻¹ and 19.2 μM, respectively, while the t [sub ½] of inactivation was 25 minutes. In the present study, cimetidine acted as an inhibitor of dextromethorphan O-demethylation in both in vitro systems. With recombinant CYP2D6, loss of activity was enhanced with preincubation. In both cases, the effect of cimetidine as an inhibitor of CYP2D6 was modest and does not fully explain the clinical findings.

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