PRIOR CONCURRENT THERAPY: Biologic therapy: At least 4 weeks since prior biologic therapy and recovered No prior allogeneic, syngeneic, or autologous bone marrow transplantation No prior peripheral blood stem cell transplantation No concurrent biologic therapy Chemotherapy: No more than 2 prior chemotherapy regimens At least 4 weeks since prior chemotherapy (6 weeks since mitomycin or nitrosoureas) and recovered No other concurrent chemotherapy Endocrine therapy: Not specified Radiotherapy: At least 4 weeks since prior radiotherapy and recovered No prior wide field radiation (radiotherapy to at least 25% of bone marrow, including pelvic irradiation) No concurrent radiotherapy Surgery: Prior major gastrointestinal surgery allowed if recovered Other: At least 4 weeks since other prior investigational therapy and recovered