Determination of Imatinib Plasma Levels by High Performance Liquid Chromatography (HPLC): Evaluation of Pharmacokinetic Variability and Haematological Consequences.

Imatinib exerts a potent and selective inhibition of tyrosine kinase Bcr-Abl. It is currently used for treatment of chronic myeloid leukaemia (CML) and acute leukaemia (AL). Pharmacokinetic data indicate large variability especially in plasma exposure with many factors which could be involved (absorption, metabolism, drug-drug interactions). Compliance must also be taken in count. The aim of this study was to determine the variability of imatinib plasma concentrations in a « natural clinical setting» (outpatients on long-term treatment for CML or AL). Secondary objective was to perform a pilot assessment of the relationship between imatinib plasma concentration and clinical outcomes.

Analysis was performed by HPLC. Sample treatment (500 μL of patient plasma) consisted of a liquid-solid extraction. Imatinib and internal standard (CGP 53716) were eluted on a Lichrosorb^® RP8 column with methanol-THF-sodium acetate buffer 0.1M pH 5. Detection was performed with UV spectrophotometer (262 nm). The limit of quantification was set at 200 ng/mL. Within-day and inter-day precisions were lower than 15%.

Blood samples were collected at steady-state (trough and peak values, ie before and 3h after drug administration) in outpatients treated for CML or AL with one imatinib daily dose, at clinical visits. Clinical evolution was considered as successful when following responses were obtained: complete haematological response within 3 months, or major cytogenetic response within 6 months, or complete cytogenetic response within 12 months, or major molecular response within 18 months in the CML group, and complete cytogenetic response after induction in the AL group.

68 imatinib measurements were obtained from 24 patients (13 CML and 11 AL). Mean age was 49 years (range 21–74) and weight was 73 kg (49–100). In patients receiving imatinib 400mg daily dose, trough and peak concentrations were respectively 1.60 ±1.28 (0.5–5.1) μg/mL (n=19), and 3.51 ±3.04 (1.0–11.1) μg/mL (n=9). In patients treated with a daily dose 600mg, trough and peak concentrations were respectively 2.62 ±2.10 (0.8–6.5) μg/mL (n=22), and 5.85 ±4.15 (1.2–15.1) μg/mL (n=18).

Variability of trough concentrations of imatinib were 80% in both groups (400 and 600mg). 18 patients were evaluable. 15 efficient responses were obtained and all these patients showed imatinib trough levels above 0.5 μg/mL (target plasma concentration required to induce death of leukaemic cells). 3 patients with failure showed nevertheless imatinib trough levels of 1.5, 1.8 and 4.5 μg/mL. 3 patients showed accumulation of imatinib (trough levels higher than 5.0 μg/mL) due to hepatic impairment but without side effects.

The method of analysis is very simple, sensitive and specific. Our results confirm the large variability of imatinib plasma concentrations and are consistent with previous results. These preliminary results showed a « safe » imatinib exposure which leads to therapeutic concentrations (higher than 0.5 μg/mL). Therapeutic drug monitoring seems to be not systematicaly necessary but could be reserved to specific cases of poor compliance, major risks of interaction or cases of failure or resistance. Studies of correlation with a larger cohort of patients is necessary to clarify the role of imatinib therapeutic drug monitoring for improving its use.