The kinetic deuterium isotope effect as applied to metabolic deactivation of imatinib to the des-methyl metabolite, CGP74588

There has recently been a burgeoning interest in impeding drug metabolism by replacing hydrogen atoms with deuterium to invoke a kinetic isotope effect. Imatinib, a front-line therapy for both chronic myeloid leukemia and of gastrointestinal stromal tumours, is often substantially metabolised via N-demethylation to the significantly less active CGP74588. Since deuterium-carbon bonds are stronger than hydrogen-carbon bonds, we hypothesised that the N-trideuteromethyl analogue of imatinib might be subject to a reduced metabolic turnover as compared to imatinib and lead to different pharmacokinetic properties, and hence improved efficacy, in vivo. Consequently, we investigated whether the N-trideuteromethyl analogue would maintain target inhibition and show a reduced propensity for N-demethylation in in vitro assays with liver microsomes and following oral administration to rats. The N-trideuteromethyl compound exhibited similar activity as a tyrosine kinase inhibitor as imatinib and similar efficacy as an antiproliferative in cellular assays. In comparison to imatinib, the trideuterated analogue also showed reduced N-demethylation upon incubation with both rat and human liver microsomes, consistent with a deuterium isotope effect. However, the reduced in vitro metabolism did not translate into increased exposure of the N-trideuteromethyl analogue following intravenous administration of the compound to rats and no significant difference was observed for the formation of the N-desmethyl metabolite from either parent drug.