In vivo CYP3A activity and pharmacokinetics of imatinib in relation to therapeutic outcome in chronic myeloid leukemia patients

• Introduction: The hepatic enzymes CYP3A4 and CYP3A5 metabolize the tyrosine kinase inhibitor imatinib into a large number of metabolites including the pharmacologically active N-desmethyl imatinib (CGP74588). Because the metabolic activity of CYP3A varies considerably between individuals and a previous pilot study suggested an inverse association between in vivo CYP3A metabolic activity and therapeutic outcome of imatinib, the primary aim of this study was to investigate the influence of CYP3A metabolic activity on the outcome of imatinib therapy in chronic myeloid leukemia patients.Methods: Fifty-five patients were included and CYP3A activity was estimated in vivo using quinine as a probe drug. Imatinib and CGP74588 trough concentrations in the plasma were determined at steady state in 34 patients. Cytogenetic and molecular responses after 12 months of first-line imatinib were retrospectively collected from patients’ medical records.Results: Patients with optimal response to imatinib (complete cytogenetic response (CCgR) or molecular response of BCR-ABL <1%) did not have different levels of CYP3A activity compared to non-optimal responders. Similar results were found when analyzing the molecular response and CCgR separately. Neither the imatinib trough concentration nor the CGP74588/imatinib ratio were significantly associated with CYP3A activity.Conclusion: CYP3A enzyme activity, as measured by quinine metabolic ratio, does not correlate with the plasma concentrations of imatinib or CGP74588 and is not predictive of imatinib therapeutic outcome. These results indicate that even though imatinib is metabolized by CYP3A enzymes, this activity is not the   ratelimiting step in imatinib metabolism and excretion. Future studies should focus on other pharmacokinetic processes such as plasma protein binding or transport protein activity to look for the major contributor to patient variability in imatinib plasma concentration.

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