PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: At least 3 weeks since prior chemotherapy (6 weeks for nitrosoureas and mitomycin) and recovered No other concurrent antineoplastic therapy Endocrine therapy: Not specified Radiotherapy: At least 3 weeks since prior radiotherapy (except short courses (no greater than 5 fractions) of nonmyelosuppressive, palliative radiotherapy) and recovered No concurrent radiotherapy Surgery: Not specified Other: At least 3 weeks since prior investigational therapy No other concurrent investigational therapy