Changes in imatinib plasma trough level during long-term treatment of patients with advanced gastrointestinal stromal tumors

306 Background: Pharmacokinetic study in patients with gastrointestinal stromal tumors (GISTs) suggested that plasma concentrations of imatinib decrease following long-term exposure. We therefore measured changes in imatinib plasma trough levels (Cmin) after long-term exposure. Methods: Between November 2009 and May 2010, follow-up (FU) imatinib Cmin was measured in 65 patients who received the same dose of imatinib for at least 9 months after a previous baseline (BL) measurement. Total 244 blood samples were obtained (127 at BL and 117 at FU) and plasma level was measured by liquid chromatography-tandem mass spectrometry. Results: Median patient age was 54 years (range, 28–76 years) and 42 (64.6%) patients were male. Sixty-one (93.8%) patients were treated with 400 mg/day imatinib and 4 (6.2%) with 300 mg/day. The median interval from initiation of imatinib to BL test was 6.4 months (range, 0.5–66.6 months), and the median interval between BL and FU test was 13.1 months (range, 9.6–18.4 months). The mean ± standard deviation imatinib Cmin was significantly higher at FU than at BL (1442 ± 693 ng/mL vs 1221 ± 624 ng/mL, pmin and that of albumin (r=-0.397, p=0.001). In per-sample analysis, imatinib Cmin was significantly correlated with age, hemoglobin, albumin, creatinine clearance, previous major gastrectomy and time between initiation of imatinib and plasma level tests. Conclusions: Steady-state imatinib Cmin did not decrease but remained stable in most GIST patients during long-term treatment. Changes in imatinib Cmin were associated with changes in albumin concentration. Monitoring of imatinib Cmin only for concerns about time-dependent decreases in imatinib exposure is not necessary. [Table: see text]