Imatinib plasma levels during successful long-term treatment of metastatic-gastrointestinal stromal tumors.

Abstract To investigate whether imatinib dosage correlated with effective plasma levels and clinical characteristics for Japanese patients undergoing long-term (≥2 years) imatinib therapy for GISTs. Twenty-five patients who received imatinib for a metastatic pathologically diagnosed GISTs at our hospital were enrolled. Imatinib response was assessed according to Choi’s criteria. Blood samples were collected 22–26 h after the previous imatinib dose before the next scheduled dose. Results: Fourteen patients were male and the median age was 65 years. The median duration of imatinib therapy was 3.8 years (range, 2.0–11.5 years). The median plasma level of imatinib was 1098 ng/ml and the minimal plasma level after ≥5 years of therapy was 789 ng/ml. Imatinib dosage was significantly correlated with history of gastrectomy. The minimum body surface area of patients who received 400-mg/day imatinib dosage was 1.560 m2. The minimum level in all patients showing response for ≥5 years of treatment was 789 ng/ml, suggesting an effective plasma imatinib level of ≥800 ng/ml. Our results suggest that imatinib dosage of 400 mg/day is recommended for a patient with a large BSA (≥1.56 m2) and that of 300 mg/day might be sufficient for patients who have undergone a gastrectomy.