Imatinib Plasma Levels Correlate with Molecular Response in CML Patients

Measurement of imatinib plasma levels is recommended for CML patients on Glivec therapy with suboptimal response or failure. Recently, reports suggesting that imatinib plasma levels correlate both with cytogenetic and molecular response of the patient and may be used for routine management of the treatment were published. We have introduced the examination of imatinib plasma levels into our laboratory follow-up of CML patients in 2007. All patients have signed informed consent before sampling. New capillary electrophoretic method was developed for determination of imatinib plasma levels. Plasma samples were deproteinated by methanol, evaporated, resuspended and directly analyzed. Separation was performed in buffer consisting of 50 mmol/L citrate adjusted with γ-amino-n-butyric acid to pH 4.0. Imatinib was quantified by UV detection in the range 250 – 270 nm (limit of detection is 10 nmol/L). BCR-ABL transcript levels were monitored by TaqMan-based real-time quantitative PCR, the baseline value of BCR-ABL/ABL% was 60%. Here we report comparison of plasma drug levels in CML patients with optimal and suboptimal molecular response. Total 238 samples from 100 CML patients were examined. Patients with major and complete molecular response (n = 68) and patients with suboptimal molecular response (n = 35) were chosen for the final comparison. The actual sample was eligible for analysis providing patient had been treated with standard dose 400 mg daily for at least one year, there was no other evident cause of the suboptimal response (presence of additional cytogenetic abnormalities or BCR/ABL mutation) and the sample was taken 24±6 hours after the ingestion of the drug. Other medication, the actual duration of the therapy (providing it was longer than one year), time interval between the dose and the meal ingestion, weight, body-mass index, body surface area and co-morbidity of patients were not taken into the account. All patients in the suboptimal group have achieved the complete cytogenetic response. Three patients in the suboptimal group have progressed, while there were any such patients in the optimal group. The results showed considerable inter-individual variability between the patients. However, the imatinib plasma levels found in the optimal group significantly higher (median of 2.34 umol/l; range of 0.46 – 6.23) than in the suboptimal group (median of 1.75 umol/l; range of 0.84 – 4.63) p < 0.003. Moreover, patients with negativity in real-time quantitative PCR testing showed a trend for even higher drug plasma levels (median of 3.18 umol/l; range of 0.46 – 6.23, n = 36) p < 0.001. We can conclude that higher imatinib plasma levels are associated with better molecular response to therapy and laboratory measurements may be used for modification of the imatinib dosage.