Pharmacogenetic determinants of human liver microsomal aminopyrine metabolism and the role of cytochrome P450 2D6

Aminopyrine (AM) has been used as a model substrate for investigation of drug metabolism. The major metabolic route is N-demethylation that was confirmed in liver microsomes. The aim of the present study was to identify the human cytochrome P-450 enzyme (CYP) mediating the N-demethylation of 4-di-methylaminoantipyrine (4-DMAA) to 4-methylaminoantipyrine (4-MAA). The contribution of human CYP to the metabolism of (4-DMAA) to (4-MAA) in human was investigated using virus expressed human CYP, human liver microsomes with chemical inhibition studies. The substrate of 4-dimethylaminantipyrine was employed at different concentrations (11.5, 23, 46, 115 and 230 µmol per l) with varying concentrations of selective inhibitors of CYP (CYP1A2), (CYP3A4), (CYP2C8),(CYP2A6), (CYP2D6), (CYP2C19) and (CYP1A1). 4-DMAA and 4-MAA were analysed by HPLC and enzyme kinetic parameters (K_m and V_max) were calculated from the concentration data. The transformation of 4-dimethylaminoantipyrine to 4-methyaminoantipyrine by microsomes prepared from baculovirus-expressed human CYP was pronounced with CYP2D6. The metabolism of 4-dimethylaminoantipyrine was inhibited by 60.0% and 55.17% by a concentration of 100 µmo per l of the known CYP2D6 inhibitors quinidine and moclobemide. The corresponding K_i values were 0.050 and 0.11 mM, respectively. The corresponding IC₅₀ values were 0.06 and 0.13 mM, respectively. The enzyme CYP2D6 apparently has an important role in N-demethyl-ation of 4-dimethylaminoantipyrine.