PREDICTION OF IN-VIVO HEPATIC METABOLIC-CLEARANCE OF YM796 FROM IN-VITRO DATA BY USE OF HUMAN LIVER-MICROSOMES AND RECOMBINANT P-450 ISOZYMES

The metabolic rate of (S)-(-)-2,8-dimethyl-3-mettylene-1-oxa-8-azaspir o [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 mu M), one low-affinity (K-m2 = 654 mu M) and a nonsaturable component. Good correlations were observed between the individual CYP3A4 content in 12 different human liver microsomal s amples and kinetic parameters such as CLint,all all, the high-affinity component clearance (V-max1/K-m1) and the low-affinity component clea rance (V-max2/K-m2). Anti-human CYP3A4/5 antibodies inhibited the meta bolism of YM796 at 1 mu M 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 w as successfully predicted from the kinetic parameters obtained with th e recombinant microsomes by taking into consideration the CYP3A4 conte nt in each microsomal sample. Based on the CLint,all estimated from th e in vitro experiments, the area under the plasma concentration-time c urve after oral administration (AUG(oxal)) of YM796 was also predicted by taking into account the hepatic blood flow rate (Q(h)), the unboun d fraction of YM796 in human plasma (f(p)) and the fraction absorbed f rom the gut. In addition, AUG(oral) was determined in six healthy male volunteers. The predicted AUG(oral) was similar to the observed value in vivo, which suggests that the in vitro metabolism data obtained wi th human liver microsomes are useful for quantitatively predicting hum an liver metabolism in vivo and that recombinant microsomes are also a vailable when the particular isozyme is almost completely responsible for the metabolism of the drug, the variation in P-450 content of huma n liver is known and the experimental conditions such as the amount of CYP reductase and cytochrome b(5) are carefully optimized to mimic th e activity found in native microsomes, as for YM796.