Phase 2a, Open-Label, Randomized, Noncomparative Study of BIIB021 in Combination With Exemestane in Women With Hormone Receptor-Positive, Advanced Metastatic Breast Cancer Who Have Progressed on a Nonsteroidal Aromatase Inhibitor

Inclusion Criteria:

- Age 18 years of age

- Must have histologically or cytologically confirmed estrogen receptor- positive or
progesterone receptor-positive, incurable, locally advanced, or metastatic breast
cancer.

- Must have disease progression during treatment with a nonsteroidal AI for locally
advanced or metastatic disease, or relapse during treatment or within 12 months of
discontinuation of treatment in the adjuvant setting.

- Must be a postmenopausal female.

- Must have measurable or evaluable disease.

- Measurable disease is defined as >=1 lesion with a diameter of >=10 mm

- Evaluable disease is defined as bone lesions evaluable by plain X ray, CT scan,
or MRI. Lesions identified only by radionuclide bone scan are not allowed.

- One prior chemotherapy regimen for advanced mBC is allowed.

- Prior radiotherapy is allowed.

- Must be able to swallow and retain oral medication.

- ECOG performance status of <=2

- Required laboratory values

- Plasma cortisol and adrenocorticotropic hormone (ACTH) levels that are not
suggestive of adrenal insufficiency unless on replacement therapy for known
adrenal insufficiency.

Exclusion Criteria:

- HER2 overexpressing tumor.

- History of central nervous system (CNS) metastasis.

- Previous treatment with exemestane or treatment with an Hsp90 inhibitor.

- Use of proton pump inhibitors.

- Known history of or positive test result for hepatitis B or C or HIV.

- History of gastrectomy or major surgery to small intestine.