Polymorphic hydroxylation of perhexiline in vitro

Aims The aims of this study were to examine the in vitro enzyme kinetics and CYP isoform selectivity of perhexiline monohydroxylation using human liver microsomes. Methods Conversion of rac-perhexiline to monohydroxyperhexiline by human liver microsomes was assessed using a high-performance liquid chromatography assay with precolumn derivatization to measure the formation rate of the product. Isoform selective inhibitors were used to define the CYP isoform profile of perhexiline monohydroxylation. Results The rate of perhexiline monohydroxylation with microsomes from 20 livers varied 50-fold. The activity in 18 phenotypic perhexiline extensive metabolizer (PEM) livers varied about five-fold. The apparent K-m was 3.3 +/- 1.5 mum, the V-max was 9.1 +/- 3.1 pmol min(-1) mg(-1) microsomal protein and the in vitro intrinsic clearance (V-max/K-m) was 2.9 +/- 0.5 mul min(-1) mg(-1) microsomal protein in the extensive metabolizer livers. The corresponding values in the poor metabolizer livers were: apparent K-m 124 +/- 141 mum; V-max 1.4 +/- 0.6 pmol min(-1) mg(-1) microsomal protein; and intrinsic clearance 0.026 mul min(-1) mg(-1) microsomal protein. Quinidine almost completely inhibited perhexiline monohydroxylation activity, but inhibitors selective for other CYP isoforms had little effect. Conclusions Perhexiline monohydroxylation is almost exclusively catalysed by CYP2D6 with activities being about 100-fold lower in CYP2D6 poor metabolizers than in extensive metabolizers. The in vitro data predict the in vivo saturable metabolism and pharmacogenetics of perhexiline.