PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: At least 4 weeks since chemotherapy At least 6 weeks since nitrosoureas or mitomycin At least 3 months since suramin Endocrine therapy: No concurrent steroids Radiotherapy: At least 4 weeks since prior large field radiation therapy (greater than 20% of total bone marrow) Surgery: At least 14 days since major surgery No major upper gastrointestinal surgery Other: No concurrent anticonvulsant medications