DISEASE CHARACTERISTICS: Histologically proven malignant solid tumor that has failed prior conventional chemotherapy or for which no established therapy exists Recurrent or metastatic gastrointestinal adenocarcinoma with no prior therapy eligible No evidence of CNS metastases (brain or leptomeningeal disease)

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-1 Life expectancy: At least 12 weeks Hematopoietic: WBC at least 3,500/mm3 Neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 1.8 mg/dL SGOT and SGPT less than 3 times upper limit of normal (ULN) Alkaline phosphatase less than 3 times ULN (5 times ULN for metastatic disease) Renal: Creatinine no greater than 1.8 mg/dL OR Creatinine clearance at least 60 mL/min Cardiovascular: No congestive heart failure Other: No uncontrolled diabetes Not pregnant or nursing Negative pregnancy test No other serious medical illness (e.g.

clinically active infectious process) No psychiatric illness precluding study No ascites or pleural effusion

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: At least 4 weeks since prior chemotherapy and recovered No more than 1 prior adjuvant chemotherapy regimen No more than 1 prior chemotherapy regimen for recurrent or metastatic disease No prior irinotecan or raltitrexed At least 8 weeks since prior nitrosourea or mitomycin and recovered Endocrine therapy: Not specified Radiotherapy: At least 4 weeks since prior radiotherapy and recovered Prior radiotherapy limited to no greater than 25% of bone marrow Surgery: Not specified