Effect of 22 CYP2D6 variants found in the Chinese population on tolterodine metabolism in vitro.

Cytochrome P450 2D6 (CYP2D6) is an important member of the cytochrome P450 enzyme superfamily. We recently identified 22 novel variants in the Chinese population using PCR and bidirectional sequencing methods. The aim of this study is to characterize the enzymatic activity of these variants and their effects on the metabolism of the antimuscarinic drug tolterodine in vitro. A baculovirus-mediated expression system was used to express wild-type CYP2D6 and 24 variants (CYP2D62, CYP2D610, and 22 novel CYP2D6 variants) at high levels. The insect microsomes expressing CYP2D6 proteins were incubated with 0.1-50 μM tolterodine at 37 °C for 30 min and the metabolites were analyzed by high-performance liquid chromatography-tandem mass spectrometry system. Of the 24 CYP2D6 variants tested, 2 variants (CYP2D692 and CYP2D696) were found to be catalytically inactive, 4 variants (CYP2D694, F164L, F219S and D336N) exhibited markedly increased intrinsic clearance values (V/K) compared with the wild-type (from 66.34 to 99.79%), whereas 4 variants (CYP2D610, *93, *95 and E215K) exhibited significantly decreased values (from 49.02 to 98.50%). This is the first report of all these rare alleles for tolterodine metabolism and these findings suggest that more attention should be paid to subjects carrying these infrequent CYP2D6 alleles when administering tolterodine in the clinic.