PRIOR CONCURRENT THERAPY: Biologic therapy: Prior growth factor treatment allowed if blood counts normal Chemotherapy: At least 3 weeks since prior chemotherapy and recovered At least 6 weeks since prior nitrosoureas or mitomycin and recovered Prior hydroxyurea allowed and recovered Endocrine therapy: At least 2 weeks since prior hormonal therapy Radiotherapy: At least 3 weeks since prior radiotherapy and recovered No prior radiotherapy to greater than 30% of the bone marrow Surgery: At least 2 weeks since any prior major surgery Other: At least 4 weeks since other prior investigational agents and recovered No other concurrent investigational agents without consent of sponsor