PRIOR CONCURRENT THERAPY: Biologic therapy: At least 4 weeks since prior immunotherapy At least 2 weeks since prior filgrastim (G-CSF) or sargramostim (GM-CSF) No concurrent immunotherapy No concurrent biological response modifiers Chemotherapy: See Disease Characteristics No more than 2 prior cytotoxic regimens in the advanced or metastatic setting At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas and mitomycin) No other concurrent chemotherapy Endocrine therapy: See Disease Characteristics Radiotherapy: At least 4 weeks since prior radiotherapy At least 4 weeks since prior radiopharmaceuticals At least 2 weeks since prior palliative radiotherapy No concurrent radiotherapy except local radiotherapy for pain or solitary brain metastasis if not progressing systemically Surgery: At least 4 weeks since prior major surgery Other: Recovered from prior therapy Prior adjuvant therapy allowed At least 30 days since prior investigational drugs No other concurrent investigational agents