PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent biologic therapy No prophylactic colony stimulating factors to prevent neutropenia (except when neutropenia fever occurs despite dose reduction) Chemotherapy: See Disease Characteristics No greater than 3 prior chemotherapy regimens for metastatic breast cancer or as either adjuvant or neoadjuvant therapy At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas and mitomycin) and recovered No other concurrent cytotoxic chemotherapy Endocrine therapy: Exogenous hormonal therapy for stage IV disease and/or as adjuvant therapy allowed At least 3 weeks since prior hormonal therapy except for: Patients who are highly unlikely to have a withdrawal response to cessation of hormonal therapy (e.g., patients with disease that is primarily resistant to hormonal therapy, patients without prior partial response, or stabilization of disease lasting less than 6 months) Patients with new or extensive visceral metastases Patients with rapidly progressive or symptomatic metastases during hormonal therapy Radiotherapy: See Disease Characteristics At least 4 weeks since prior radiotherapy and recovered No prior radiotherapy to greater than 50% of bone marrow No concurrent radiotherapy Surgery: See Disease Characteristics At least 4 weeks since prior major surgery and recovered No concurrent surgery Other: No other concurrent anticancer treatment At least 28 days since other prior investigational drugs, including analgesics or antiemetics No other investigational drugs during and for 28 days after the study No drugs that induce or inhibit CYP3A enzyme