Inclusion Criteria

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