PRIOR CONCURRENT THERAPY: Biologic therapy: Recovered from all prior immunotherapy No concurrent prophylactic hematopoietic growth factors Chemotherapy: At least 3 weeks since prior myelosuppressive chemotherapy (6 weeks for nitrosourea) and recovered No other concurrent cancer chemotherapy Endocrine therapy: Stable or decreasing dosage of dexamethasone for intracranial pressure within 2 weeks of study entry No concurrent dexamethasone used as an antiemetic Radiotherapy: At least 8 weeks since prior radiotherapy to evaluable lesions and recovered Surgery: At least 4 months since prior radiosurgery Other: No other concurrent investigational agents