Inclusion Criteria:

- Male and Female patients must be at least 18 years of age

- Pathologically confirmed diagnosis of breast cancer

- Metastatic or advanced stage breast cancer

- Prior treatment with at least one and no more than three lines of biologic and/or
chemotherapy for metastatic breast cancer (excluding hormonal therapy)

- Patients with HER2+ disease must have received prior treatment with Trastuzumab

- Patients with ER and/or PR+ disease must have received prior treatment with hormonal
therapy

- Off cytotoxic chemotherapy or biologic therapy (excluding Hormonal therapy) ≥ 3 weeks

- Measurable disease by RECIST 1.1

- Central nervous system metastases are permitted if treated and radiographically and
clinically stable for at least 4 weeks prior to first dose of STA-9090

- Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1

- Life expectancy of at least 3 months

- Adequate hematologic function as defined by:

- Absolute neutrophil count ≥1,500 cells/μL

- Platelets ≥100,000/μL

- Hemoglobin ≥ 9.0g/dL

- Adequate hepatic function as defined by:

- Serum bilirubin ≤ 1.5 X upper limit of normal (ULN);

- Adequate renal function as defined by a serum creatinine ≤ 1.5 x ULN

- AST, ALT, and alkaline phosphatase ≤ 3 × ULN except for:

- Patients with hepatic metastases: ALT and AST ≤ 5 × ULN

- Patients with hepatic and/or bone metastases: alkaline phosphatase ≤ 5 × ULN

- Patients with Gilbert's disease: serum bilirubin < 5 mg/dL

- Willingness and ability to comply with scheduled visits, treatment plans, laboratory
tests, and other study procedures

- Female subjects of childbearing potential and males must agree to use adequate
contraception (e.g., hormonal or barrier method of birth control; abstinence) for the
duration of study treatment

- Female subjects of childbearing age must have a negative serum pregnancy test at
study entry.

Exclusion Criteria:

- Surgery, radiotherapy, or lesion ablative procedure to the only area of measurable
disease

- Major surgery within 4 weeks prior to first dose of STA-9090

- Poor peripheral venous access for study drug administration. Study drug
administration via indwelling catheters allowed only if the catheter is made of
silicone material.

- History of severe (grade 3 or 4) allergic or hypersensitivity reactions to excipients
(e.g., Polyethylene glycol [PEG] 300 and Polysorbate 80)

- Baseline QTc > 470 msec

- Ventricular ejection fraction (EF) <50% at baseline

- Treatment with chronic immunosuppressants (e.g., cyclosporine following
transplantation)

- Women who are pregnant or lactating

- Uncontrolled intercurrent illness including, but not limited to, human
immunodeficiency virus (HIV)-positive subjects receiving combination antiretroviral
therapy, ongoing or active infection, symptomatic congestive heart failure, unstable
angina pectoris, ventricular arrhythmia, or psychiatric illness/social situations
that would limit compliance with study requirements

- Other medications, or severe acute/chronic medical or psychiatric condition, or
laboratory abnormality that may increase the risk associated with study participation
or study drug administration, or may interfere with the interpretation of study
results in the judgment of the investigator

- Seizure disorder or requirement for seizure medication

- Prior treatment with an HSP90 inhibitor

- persistent adverse events of prior therapies that are > 1 grade 1 in severity

- history of or current coronary artery disease, myocardial infarction, angina
pectoris, angioplasty or coronary bypass surgery

- history of or current uncontrolled dysrhythmias, or requirement for antiarrhythmic
medications, or Grade 2 or greater left bundle branch block

- New York Heart Association class II/III/IV congestive heart failure with a history of
dyspnea, orthopnea, or edema that requires current treatment with angiotensin
converting enzyme inhibitors, angiotensin II receptor blockers, beta blockers or
diuretics