Imatinib mesylate has demonstrated excellent efficacy as first-line therapy for treatment of chronic phase chronic myelogenous leukemia (CML-CP). The drug has a high inter-patient variability in pharmacokinetics (PK), and several factors are presumed to influence its PK. Imatinib is a substrate of CYP3A4 and CYP3A5 enzymes. However, the clinical significance of its metabolism on pharmacokinetics and therapeutic response is still unclear. Significantly higher in vivo CYP3A activity was reported recently in patients achieving complete molecular response than those who did not in a small cohort study. Several investigators in the past have tried to optimize imatinib dosing based on trough levels after the fourth treatment week. Picard S et al (Blood, 2007) showed that threshold plasma concentration of 1002 ng/ml on day 29 was associated with higher incidence of major molecular response. The primary objective of this study was to evaluate the association between CYP3A4 polymorphisms, trough plasma concentration of imatinib on day 8 and day 29 of treatment, and cytogenetic response. The secondary objective was to evaluate the potential of early (day 8) trough level monitoring to discriminate between responders and non-responders.
All patients received standard 400 mg OD dose of imatinib. A single blood sample was collected on day 8 and day 29 prior to imatinib dosing for the determination of trough plasma concentration. Imatinib level was determined using High Performance Liquid Chromatography (HPLC). Cytogenetic response was assessed at 3 monthly intervals by Fluorescence In Situ Hybridization (FISH). Complete cytogenetic response (CCR) was defined as 0% of Philadelphia chromosome–positive cells in the bone marrow aspirate. Pharmacogenetic sample was collected from each patient at the beginning of the study to look for CYP3A4 polymorphism. Genotyping was carried out for *4 (rs55951658), *5 (rs55901263) and *18 (rs28371759) variants by PCR-RFLP technique. About 10% samples were chosen randomly for direct sequencing to confirm the PCR-RFLP findings. Median imatinib concentration in the CCR and non CCR groups was compared using non-parametric Mann-Whitney test. Receiver Operating Characteristic (ROC) curve analysis was performed to assess the discrimination potential of trough imatinib plasma levels for complete cytogenetic response (CCR).
Ninety eight patients were enrolled on this study, 70 males and 28 females. The median age was 33 years (Range 6 – 71 years). High interpatient variability in the trough concentration of imatinib was observed (Coefficient of variation = 64% and 56% for day 8 and day 29 respectively). However, none of the patients were found to have *4, *5 or *18 variant alleles. Genotype–PK or genotype-response association could not be evaluated as a result. Data on cytogenetic response at 12 months was available for 75 patients (60 CCR, 15 without CCR), 46 of whom also had trough imatinib levels measured on day 8 and day 29 (36 CCR, 10 without CCR). Median day 8 and day 29 trough concentrations were not significantly different in the group with CCR (1.71 and 1.91 μg/mL respectively) and the group without (1.31 and 1.64 μg/mL respectively). Regarding the discrimination potential of trough levels for CCR, day 8 concentrations failed to discriminate between responders and non-responders, whereas for the Day 29 levels, the area under the ROC curve (AUC) was 0.554, with best sensitivity (75%) and specificity (61%) at plasma threshold of 1.69 μg/mL.
In this cohort of patients, common CYP3A4 variants did not contribute to the high inter-patient variability of imatinib levels. Early monitoring on Day 8 failed to discriminate responders from non-responders. Day 29 levels are a better predictor of complete cytogenetic response as shown in previous studies.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.