Abstract

The metabolic rate of (S)-(−)-2,8-dimethyl-3-methylene-1-oxa-8-azaspiro [4,5] decane-l-tartarate monohydrate (YM796), an antidementia agent, was determined by use of 12 different human liver microsomal samples. The metabolism of YM796 was shown to consist of three components; one high-affinity (K _m1 = 1.67 μM), one low-affinity (K _m2 = 654 μM) and a nonsaturable component. Good correlations were observed between the individual CYP3A4 content in 12 different human liver microsomal samples and kinetic parameters such as CL_{int, all}, the high-affinity component clearance (V _max1/K _m1) and the low-affinity component clearance (V _max2/K _m2). Anti-human CYP3A4/5 antibodies inhibited the metabolism of YM796 at 1 μM by up to 75%. In addition, ketoconazole, an inhibitor of CYP3A4, inhibited YM796 metabolism by >90%. The metabolic clearance of YM796 in each of the 12 human liver microsomal samples was successfully predicted from the kinetic parameters obtained with the recombinant microsomes by taking into consideration the CYP3A4 content in each microsomal sample. Based on the CL_{int, all}estimated from the in vitro experiments, the area under the plasma concentration-time curve after oral administration (AUC_oral) of YM796 was also predicted by taking into account the hepatic blood flow rate (Q _h), the unbound fraction of YM796 in human plasma (f_p) and the fraction absorbed from the gut. In addition, AUC_oral was determined in six healthy male volunteers. The predicted AUC_oral was similar to the observed valuein vivo, which suggests that the in vitrometabolism data obtained with human liver microsomes are useful for quantitatively predicting human liver metabolism in vivo and that recombinant microsomes are also available when the particular isozyme is almost completely responsible for the metabolism of the drug, the variation in P-450 content of human liver is known and the experimental conditions such as the amount of CYP reductase and cytochrome b₅ are carefully optimized to mimic the activity found in native microsomes, as for YM796.