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