Imatinib mesylate (trade names: Glivec® and Gleevec®, imatinib, formerly STI571) is a signal transduction inhibitor targeting several protein-tyrosine kinases that are believed to play a role in the proliferation of tumor cells. In the Phase II study of imatinib [CSTI571B 2222] in 147 patients with recurrent or metastatic GIST, the partial response rates were 67% and 66% in patients treated at 400 mg/d and 600 mg/d, respectively. Skin rash and elevated transaminases were the most common reason for drug discontinuation. The most frequently reported AEs were mild nausea, vomiting, diarrhea, superficial edema (primarily periorbital or lower limb), myalgia and muscle cramps. Grade 3/4 events included fluid retention (pleural or pericardial effusions, ascites, and pulmonary edema), skin rash, liver toxicity and gastrointestinal (GI) hemorrhage. Myelosuppression (neutropenia and thrombocytopenia) was a consistent finding. Also, a tumor lysis-like syndrome occurred in some patients leading to gastrointestinal (GI) and/or intratumoral hemorrhage.