PRIOR CONCURRENT THERAPY: Biologic therapy: At least 3 months since prior immunotherapy No concurrent cytokines Chemotherapy: At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas, mitomycin, and high dose chemotherapy) Endocrine therapy: Not specified Radiotherapy: At least 4 weeks since prior radiotherapy (6 weeks for extensive radiotherapy) No concurrent radiotherapy Surgery: Not specified Other: No other concurrent investigational or antitumor drugs