Pharmacokinetics of escalated dose of imatinib in patients with advanced gastrointestinal stromal tumors.

Abstract

10085 Background: Although escalated dose (ED, 800 mg daily) of imatinib (IM) is one of important options in resistant gastrointestinal stromal tumors (GIST) on standard dose (SD, 400mg daily), there has been lack of pharmacokinetic data at ED. We therefore explore the potential relationship between IM plasma trough level (Cmin), and clinical outcomes or toxicities in patients treated with ED. Methods: Between 2008 and 2011, steady-state IM Cmin was measured in 66 patients with GIST who received ED of IM using liquid chromatography-tandem mass spectrometry. Percent change from IM Cmin at SD was calculated in 43 patients that both SD and ED IM Cmin were measured. The association with efficacy and toxicity was analyzed with grouping patients into quartiles according to IM Cmin at ED and percent change. Results: Median age was 59 years (range, 36–77) and 40 patients were male. KIT exon 11 and 9 mutation was detected in 32 and 18 patients, respectively. At ED, partial response was achieved in 4 patients (6%) and stable disease was in 32 patients (48%). The median progression-free survival was 4.2 months (95% CI, 1.8-6.6) and overall survival was 38.5 months (95% CI, 7.6-69.3). The mean (± standard deviation) IM Cmin at ED was 3552 (± 1540) ng/mL. IM Cmin was significantly increased after dose escalation (p <0.001), and mean percent change from IM Cmin at SD was 162% (± 100). Body surface area (p=0.01), hemoglobin (p=0.001), and neutrophil count (p=0.006) were significant covariates for IM Cmin at ED in multivariate analysis. The group of quartiles of IM Cmin at ED and percent change was not associated with response and survival outcomes. However, grade 3-4 hematologic or grade 2-4 non-hematologic toxicity was observed in 9% (1/11) of patients in the lowest percent change quartile (Q1, <90%) compared with 56% (18/32) in Q2 to Q4 (p=0.012), although absolute values were not associated with toxicity. Conclusions: In GIST patients treated with ED of IM, IM Cmin was not associated with response and survival. Clinically significant toxicity following dose escalation was correlated with percent change of IM Cmin, rather than absolute values. Monitoring of IM Cmin might help to predict or manage ED of IM induced toxicity.