PS-341 in Treating Women With Metastatic Breast Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed adenocarcinoma of the breast * Metastatic disease * At least 1 unidimensionally measurable lesion * At least 20 mm by conventional techniques OR * At least 10 mm by spiral CT scan * Previously treated with 1 prior chemotherapy regimen (with or without trastuzumab \[Herceptin\]) for metastatic disease * Relapse during or within 6 months after completion of adjuvant chemotherapy considered treatment failure * No known brain metastases * Hormone receptor status: * Not specified PATIENT CHARACTERISTICS: Age: * 18 and over Sex: * Female Menopausal status: * Not specified Performance status: * ECOG 0-2 Life expectancy: * More than 3 months Hematopoietic: * WBC at least 3,000/mm\^3 * Absolute neutrophil count greater than 1,500/mm\^3 * Platelet count at least 100,000/mm\^3 Hepatic: * Bilirubin normal * SGOT/SGPT no greater than 2.5 times upper limit of normal Renal: * Creatinine normal OR * Creatinine clearance at least 60 mL/min Cardiovascular: * No symptomatic congestive heart failure * No unstable angina pectoris * No cardiac arrhythmia Other: * No uncontrolled concurrent illness * No ongoing or active infection * No psychiatric illness or social situation that would preclude study * No prior allergic reaction attributed to compounds of similar chemical or biological composition to bortezomib * No other prior malignancy within the past 5 years unless curatively treated and disease free * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy: * See Disease Characteristics Chemotherapy: * See Disease Characteristics * At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas or mitomycin) and recovered Endocrine therapy: * Prior hormonal therapy for metastatic disease allowed * At least 4 weeks since prior hormonal therapy and recovered Radiotherapy: * At least 4 weeks since prior radiotherapy and recovered Surgery: * Not specified Other: * Prior adjuvant therapy allowed * At least 4 weeks since prior investigational drugs and recovered * No other concurrent investigational or commercial agents or therapies for breast cancer * No concurrent combination antiretroviral therapy for HIV