Trough imatinib plasma levels in patients treated for advanced gastrointestinal stromal tumors evidence of large interpatient variations under treatment with standard doses

10564 Background: Large interpatient variations of trough imatinib level have been reported in patients with CML given imatinib 400- 600 mg/d; moreover, the 24h residual plasma level of imatinib was found correlated with imatinib efficacy. Methods: Trough plasma levels of imatinib were determined by high performance liquid chromatography-tandem mass spectrometry in patients with advanced GIST, seen at IB 24h (imatinib 400 mg/d) or 12h (imatinib 400 mg bid) after the last dose intake. Results: From March 2007 to October 2007, 31 dosages were performed in 24 patients. Imatinib dose was 400 mg/d (median duration 26 months), for 24 determinations, and 800 mg/d for 7 (dose increased for tumor progression in 6). There were 11 female and 13 male patients and the median age was 66 years (range: 45–90); treatment was for liver (6), peritoneal 7) metastasis or both (8); 3 patients had inoperable primaries. Mean and median trough plasma levels were 799 and 450ng/ml (range: 188–3,321) for a prescribed dose of 400 mg/...