Characterization of humanized liver from chimeric mice using coumarin as a human CYP2A6 and mouse CYP2A5 probe

Coumarin 7-hydroxylation (COH), which is catalyzed almost solely by human CYP2A6 and mouse CYP2A5, shows large differences in activity (humans >> mice) and inhibitor specificity between mice and humans. To differentiate human and mouse liver functions of chimeric mice (CM1, CM2 and CM3) prepared with hepatocytes from 3 donors, the microsomal COH activities were measured with and without benzaldehyde and undecanoic gamma-lactone as a specific inhibitor of human CYP2A6 and mice CYP2A5, respectively. The replacement % to human hepatocytes designated as replacement index (RI) was calculated from human specific cytokeratin 8/18 expression in the liver section. The COH activities correlated well with RIs in CM2 (R-2= 0.98) and CM3 (R-2= 0.94), except CM1 whose genotype of donor is CYP2A6*4/*4. However, the COH activities expressed as % of donor activities were not always coincident with RIs, and the inhibition pattern of CM2 and CM3 was human-type after RI exceeded approximately 50%. Subsequently, our attempts to use % of COH activities or inhibition patterns as an accurate functional replacement index were unsuccessful. Since the detection of human CYP2A6 protein in the liver and the steep increase of human albumin (hAlb) levels in the blood were begun from almost RI = 50% similarly to the changes of inhibition pattern, RI = 50% is the turning point for chimeric mice to have humanized liver function.