PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent colony stimulating factors Chemotherapy: At least 4 weeks since prior chemotherapy (at least 6 weeks since melphalan or mitomycin) At least 3 months since suramin No prior nitrosoureas or irinotecan Endocrine therapy: No concurrent steroids (except as antiemetic for chemotherapy) Radiotherapy: At least 4 weeks since prior radiotherapy Surgery: At least 4 weeks since major surgery Other: No concurrent medications known to affect hepatic or renal function (e.g., antiseizure medication or nonsteroidal antiinflammatory drugs)