PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent immunotherapy Chemotherapy: Only one prior chemotherapy regimen in the adjuvant or metastatic setting allowed At least 3 weeks since prior chemotherapy (6 weeks for nitrosoureas) No other concurrent chemotherapy Endocrine therapy: No concurrent hormonal therapy (except contraceptives, appetite stimulants and replacement steroids) Radiotherapy: At least 3 weeks since prior radiotherapy No concurrent radiation therapy No concurrent palliative radiation therapy to only site of measurable disease Surgery: Not specified Other: At least 1 month since prior use of any other investigational agent No concurrent use of experimental medications