PRIOR CONCURRENT THERAPY: Biologic therapy: At least 1 week since prior interferon At least 1 week since prior thalidomide Chemotherapy: Recovered from prior chemotherapy At least 4 weeks since prior cytotoxic therapy (2 weeks for vincristine, 3 weeks for procarbazine, or 6 weeks for nitrosoureas) Endocrine therapy: At least 1 week since prior tamoxifen Prior steroids allowed if on stable or decreasing dose for at least 5-7 days before baseline MRI If steroid dose is increased between date of baseline MRI and initiation of study drug, a new baseline MRI is required Radiotherapy: Not specified Surgery: See Disease Characteristics No concurrent surgery Other: At least 1 week since any prior noncytotoxic agents (e.g., isotretinoin) No other concurrent anticancer therapy, including other investigational drugs