Exatecan Mesylate in Treating Patients With Ewing's Sarcoma, Primitive Neuroectodermal Tumor, or Desmoplastic Small Round Cell Tumor

DISEASE CHARACTERISTICS: One of the following histologically confirmed diagnoses: Relapsed or refractory Ewing's sarcoma or primitive neuroectodermal tumor Desmoplastic small round cell tumor Measurable disease The following are not considered measurable disease: Ascites Pleural effusion Lytic bone lesions No symptomatic brain metastases PATIENT CHARACTERISTICS: Age Any age Performance status ECOG 0-2 (over 10 years of age) Lansky 60-100% (10 years of age and under) Life expectancy At least 12 weeks Hematopoietic Absolute neutrophil count at least 750/mm^3 Platelet count at least 75,000/mm^3 Hemoglobin at least 8.5 g/dL Hepatic Bilirubin no greater than 2.0 mg/dL AST or ALT no greater than 2.5 times upper limit of normal (ULN) (5 times ULN if liver metastases are present) Albumin at least 2.8 g/dL Renal Creatinine less than 1.5 times ULN Other Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception No active serious infection No other malignancy within the past 5 years except nonmelanoma skin cancer or carcinoma in situ of the cervix No overt psychosis or mental disability that would preclude informed consent No other life-threatening illness within the past 6 months PRIOR CONCURRENT THERAPY: Biologic therapy At least 3 months since prior autologous bone marrow or stem cell transplantation No concurrent biologic therapy Chemotherapy Recovered from prior systemic chemotherapy Prior topoisomerase I inhibitor therapy allowed No other concurrent chemotherapy Endocrine therapy Not specified Radiotherapy More than 4 weeks since prior cranial, spinal, or whole pelvis radiotherapy More than 4 weeks since prior radiotherapy to 25% of bone marrow reserve No concurrent radiotherapy Surgery At least 4 weeks since prior major surgery and recovered No concurrent surgery Other More than 28 days since prior investigational drugs (including analgesics or antiemetics) No more than 2 prior treatment regimens for this disease No other investigational drugs during and for 28 days after study therapy No other concurrent anticancer therapy No concurrent grapefruit or grapefruit juice