DISEASE CHARACTERISTICS:

• Histologically or cytologically confirmed invasive breast cancer

  • Clinical and/or radiological evidence of stage IV disease
• Relapsed or resistant disease within 6-12 months after completion of prior chemotherapy with doxorubicin or epirubicin and/or paclitaxel or docetaxel for advanced disease or in the adjuvant setting

• At least 1 unidimensionally measurable lesion

  • At least 20 mm by conventional techniques OR
  • At least 10 mm by spiral CT scan
  • No bone metastases as only measurable site
  • Pleural or peritoneal effusions not acceptable as measurable disease
• No known brain metastases

• Hormone receptor status:

  • Estrogen receptor-negative OR
  • Estrogen receptor-positive

PATIENT CHARACTERISTICS:

Age:

• 18 and over

Sex:

• Female

Menopausal status:

• Not specified

Performance status:

• ECOG 0-2
• Karnofsky 60-100%

Life expectancy:

• More than 12 weeks

Hematopoietic:

• WBC at least 3,000/mm^3
• Absolute neutrophil count at least 1,500/mm^3
• Platelet count at least 100,000/mm^3

Hepatic:

• Bilirubin normal
• AST or ALT no greater than 2.5 times upper limit of normal

Renal:

• Creatinine normal OR
• Creatinine clearance at least 60 mL/min

Cardiovascular:

• No acute ischemia or significant conduction abnormality by EKG
• No symptomatic congestive heart failure
• No unstable angina pectoris
• No cardiac arrhythmia
• LVEF greater than 50%

Other:

• No uncontrolled concurrent illness
• No psychiatric illness or social situation that would preclude study
• No ongoing or active infection
• No prior allergic reaction(s) to compounds of similar chemical or biologic composition to PS-341
• No other malignancy within the past 5 years except carcinoma in situ of the cervix or basal cell skin cancer
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective barrier-method contraception

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• See Chemotherapy

Chemotherapy:

• See Disease Characteristics

• No more than 1 prior chemotherapy regimen for metastatic disease

  • High-dose regimen or bone marrow transplantation considered 1 prior regimen
• At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas or mitomycin) and recovered

Endocrine therapy:

• Prior hormonal therapy for metastatic disease or in adjuvant setting allowed

Radiotherapy:

• Prior localized radiotherapy allowed if it does not influence the signal evaluable lesion
• At least 4 weeks since prior radiotherapy and recovered

Surgery:

• At least 2 weeks since prior minor surgery and recovered
• At least 4 weeks since prior major surgery and recovered

Other:

• No other concurrent investigational agent
• No other concurrent investigational or commercial agents or therapies to treat this malignancy