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