Irinotecan and Celecoxib in Treating Patients With Unresectable or Metastatic Colorectal Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed colorectal cancer * Metastatic or unresectable disease * Failed first-line or second-line therapy OR recurred after receiving fluorouracil-based adjuvant chemotherapy * No known brain metastases PATIENT CHARACTERISTICS: Age * 18 and over Performance status * ECOG 0-2 OR * Karnofsky 60-100% Life expectancy * More than 12 weeks Hematopoietic * Absolute neutrophil count ≥ 1,500/mm\^3 * Platelet count ≥ 100,000/mm\^3 * WBC ≥ 3,000/mm\^3 Hepatic * AST and ALT ≤ 2.5 times upper limit of normal * Bilirubin normal * No Gilbert's disease Renal * Creatinine normal OR * Creatinine clearance ≥ 60 mL/min Cardiovascular * No symptomatic congestive heart failure * No unstable angina pectoris * No cardiac arrhythmia Gastrointestinal * Able to receive oral medications * No prior inflammatory bowel disease * No active ulcer disease or gastritis * No contraindications for sigmoidoscopy * No active colostomy Other * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception during and for 3 months after study participation * No prior allergic reaction to compounds of similar chemical or biological composition to irinotecan, celecoxib, sulfonamides, or other study agents * No active or ongoing infection * No other concurrent uncontrolled illness * No psychiatric illness or social situation that would preclude study compliance PRIOR CONCURRENT THERAPY: Biologic therapy * No concurrent filgrastim (G-CSF) or sargramostim (GM-CSF) Chemotherapy * See Disease Characteristics * No more than 2 prior different chemotherapy regimens, including adjuvant therapy * More than 4 weeks since prior chemotherapy (6 weeks for nitrosoureas or mitomycin) and recovered * No prior irinotecan Endocrine therapy * Not specified Radiotherapy * More than 4 weeks since prior radiotherapy and recovered * No prior radiotherapy of more than 3,000 cGy * No prior radiotherapy to extended marrow-generating fields * No prior abdomino-pelvic irradiation Surgery * No prior abdominoperineal resection Other * More than 2 weeks since prior cyclo-oxygenase-2 (COX-2) inhibitors * No concurrent combination antiretroviral therapy for HIV-positive patients * No other concurrent investigational agents * No other anticancer therapy * No other concurrent COX-2 inhibitors * Low-dose aspirin allowed