Imatinib Mesylate in Treating Patients With Unresectable or Metastatic Gastrointestinal Stromal Tumor

DISEASE CHARACTERISTICS:

* Histologically confirmed gastrointestinal stromal tumor (GIST)

  * Metastatic or unresectable disease
* Immunohistochemical confirmation of KIT (CD117) expression by tumor as documented by DAKO antibody staining
* Measurable or non-measurable disease by conventional imaging (CT scan or MRI) or physical examination

  * If a target lesion has been previously embolized or irradiated, there must be objective evidence of progression to be considered for response assessment
* No known brain metastasis

PATIENT CHARACTERISTICS:

* WHO performance status 0-3
* Bilirubin ≤ 1.5 times upper limit of normal (ULN)
* AST and ALT ≤ 2.5 times ULN (≤ 5 times ULN if hepatic metastases are present)
* Creatinine ≤ 1.5 times ULN
* ANC ≥ 1,000/mm³
* Platelet count ≥ 100,000/mm³
* Hemoglobin ≥ 9 g/dL (transfusions allowed)
* Not pregnant or nursing
* Negative pregnancy test
* Fertile patients must use effective barrier contraception during and for up to 3 months after completion of study therapy
* No NYHA class III-IV cardiac disease
* No congestive heart failure or myocardial infarction within the past 2 months
* No severe and/or uncontrolled concurrent medical disease (e.g., uncontrolled diabetes, uncontrolled chronic renal or liver disease, or active uncontrolled infection \[e.g., HIV\])
* No other prior malignancy except adequately treated basal cell or squamous cell skin cancer, carcinoma in situ of the cervix, adequately treated stage I or II cancer from which the patient is currently in complete remission, or any other cancer from which the patient has been disease-free for 5 years
* No medical, psychological, familial, sociological, or geographical condition that, in the opinion of the investigator, may preclude the patient's ability to tolerate or complete study treatment, comply with study protocol and follow-up schedule, or give reliable informed consent

PRIOR CONCURRENT THERAPY:

* Recovered from all prior therapy
* More than 28 days since prior chemotherapy, biologic therapy, or any other investigational drug
* More than 14 days since prior major surgery
* No concurrent therapeutic anticoagulation with coumarin derivatives

  * Concurrent therapeutic anticoagulation with low-molecular weight heparin allowed
  * Concurrent mini-dose coumarin derivatives (i.e., equivalent to 1 mg of oral warfarin daily) as prophylaxis allowed
* No concurrent cytokines (i.e., filgrastim \[G-CSF\] or sargramostim \[GM-CSF\]) to support blood counts
* No other concurrent investigational drugs
* No other concurrent anticancer agents, including chemotherapy, radiotherapy, or anticancer biologic therapy