Inclusion Criteria:

- 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
foradvanced disease or in the adjuvant setting

- At least 1 unidimensionally measurable lesion

- At least 20 mm by conventional techniques

- 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

- Estrogen receptor-positive

- Female

- Performance status - ECOG 0-2

- Performance status - Karnofsky 60-100%

- More than 12 weeks

- WBC at least 3,000/mm^3

- Absolute neutrophil count at least 1,500/mm^3

- Platelet count at least 100,000/mm^3

- Bilirubin normal

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

- Creatinine normal

- Creatinine clearance at least 60 mL/min

- 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%

- 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

- See 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

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

- Prior localized radiotherapy allowed if it does not influence the signal evaluable
lesion

- At least 4 weeks since prior radiotherapy and recovered

- At least 2 weeks since prior minor surgery and recovered

- At least 4 weeks since prior major surgery and recovered

- No other concurrent investigational agent

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