Drug Metab Dispos. 2026 May 12;54(6):100322. doi: 10.1016/j.dmd.2026.100322. Online ahead of print.
ABSTRACT
Tree shrews (Tupaia belangeri) are nonprimate animal species available for use in biomedical studies. Because the molecular and enzymatic properties of their cytochromes P450 (P450s or CYPs) are similar to human P450s, tree shrews are a promising animal model for preclinical studies. However, in tree shrews, the properties of dextromethorphan O-demethylation, a marker reaction for human CYP2D6, have not yet been fully elucidated. In this study, we characterized dextromethorphan O-demethylation in tree shrews. We observed biphasic Michaelis-Menten kinetics for dextromethorphan O-demethylation mediated by tree shrew liver microsomes. However, in the narrow dextromethorphan concentration range 0.69-17 μM, monophasic kinetics were observed. Of the 11 recombinant tree shrew P450s analyzed, CYP2A13, CYP2Cs (CYP2C18, CYP2C76a, and CYP2C76b), and CYP2D8a showed dextromethorphan O-demethylation activity; of these, tree shrew CYP2D8a showed the highest activity (22 nmol/min per nmol P450). The quantification of tree shrew CYP2D8a protein by immunoblotting showed levels ranging from 2.3 to 9.2 pmol/mg protein in liver microsomes from 5 tree shrews. The intrinsic clearance of the high-affinity component of dextromethorphan O-demethylation was significantly correlated (r = 0.95, P < .01) with the hepatic protein content of tree shrew CYP2D8a. The intrinsic clearance of the low-affinity component of dextromethorphan O-demethylation was tentatively correlated with both tree shrew CYP2A13 and CYP2C activities, although the correlations did not reach statistical significance. These results suggest that CYP2D8a is the primary enzyme responsible for dextromethorphan O-demethylation in tree shrews. SIGNIFICANCE STATEMENT: We observed biphasic kinetics of dextromethorphan O-demethylation in tree shrew liver microsomes and the activities of recombinant CYP2A13, CYP2C18, CYP2C76a, CYP2C76b, and CYP2D8a. The intrinsic clearance of the high-affinity component was significantly correlated with CYP2D8a protein content. The intrinsic clearance of the low-affinity component was tentatively correlated with both CYP2A13 and CYP2C activities, but the correlations were not statistically significant. These results suggest that CYP2D8a is the primary enzyme responsible for dextromethorphan O-demethylation in tree shrews.
PMID:42241735 | DOI:10.1016/j.dmd.2026.100322
