Imatinib plasma concentrations during long-term effective treatment of metastatic gastrointestinal stromal tumor.

10543 Background: Imatinib concentrations at one month after the start of imatinib have shown an association with clinical benefit in pts with gastrointestinal stromal tumors (GISTs) and a significant decrease of 30% within twelve months. This retrospective study investigated imatinib plasma concentrations in pts with metastatic GISTs in a long-term (≥2 years) effective treatment. Methods: 31 metastatic GIST pts, who had been treated with imatinib for two or more years in our hospital, were enrolled in this study. 11 pts were excluded because of adjuvant setting (n=5) and rejection to concentration testing (n=6). 20 pts were included in this study. GISTs were immunohistochemically diagnosed, and radiologically assessed as effective GIST by Choi’s criteria without relapse. Results: 10 pts were male, median age was 63 years (range, 31-84), and median imatinib exposure time was 3.5 year (range, 2-12). The median plasma concentration was 1093 ng/ml (range, 417-2091) and minimal level after at least five years...