Abstract

Clinically, cimetidine therapy impairs the clearance of various drugs metabolized by CYP2D6, such as desipramine and sparteine. Cimetidine is known to reversibly inhibit CYP2D6 in vitro; however, K_i values are greater than plasma concentrations observed in vivo. There is evidence suggesting that this drug may act as an inactivator of cytochrome P450 (P450) enzymes after metabolic activation. Therefore, the purpose of this study was to determine whether cimetidine acts as a mechanism-based inactivator of CYP2D6. Dextromethorphan O-demethylation was used as a probe of CYP2D6 activity. The V_max and K_m of this reaction were 0.82 ± 0.06 nmol/min/nmol of P450 and 4.1 ± 0.1 μM, respectively, in pooled human liver 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 competitively inhibited CYP2D6 (K_i = 38 ± 5 μM) and was a mixed inhibitor of recombinant CYP2D6 (K_i = 103 ± 17 μM). Preincubation of human liver microsomes with cimetidine and NADPH did not increase the inhibitory potency of cimetidine; however, preincubation with recombinant CYP2D6 resulted in enzyme inactivation that could be attenuated by the CYP2D6 inhibitor quinidine. The K_I and k_inact were estimated to be 77 μM and 0.03 min^-1, respectively, and the half-life of inactivation was 25 min. Therefore, cimetidine may represent a class of compounds capable of inactivating specific cytochromes P450 in vivo, but for which conditions may not be achievable in vitro using human liver microsomes.