Anti-Proliferative Effects and Genomic Alterations of Abiraterone Acetate Compared to an Aromatase Inhibitor in Post-menopausal HR+ Operable Breast Cancer

Inclusion Criteria: 1. Woman greater than or equal to 18 years of age and postmenopausal determined by one of the following: * bilateral surgical oophorectomy * age greater than or equal 60 years * age \<60 years, with amenorrhea greater than or equal 24 months and follicle-stimulating hormone and luteinizing hormone concentrations within postmenopausal range 2. Subjects with ER+ (Allred 3-8), HER2 - primary operable T1-T3 breast cancer with a primary tumor size of ≥ 1.5 cm on physical examination or imaging studies 3. Eastern Cooperative Oncology Group (ECOG) performance status score of less than or equal to 1 4. Criterion modified per amendment 7.1 Clinical laboratory values during Screening: * hemoglobin greater than or equal 10.0 g/dL * neutrophils greater than or equal 1.5 x 109/L * platelets greater than or equal100 x 109/L * total bilirubin less than or equal to 1.5x upper limit of normal (ULN) - except for a known diagnosis of Gilbert's syndrome * alanine (ALT) and aspartate (AST) aminotransferase less than or equal to 1.5xULN * alkaline phosphatase less than or equal to 1.5xULN * serum creatinine \<1.5xULN or creatinine clearance greater than or equal 45 mL/min * serum potassium greater than or equal 3.5 mM * serum albumin greater than or equal 3.0 g/dL * INR (or PT) and partial thromboplastin time (PTT) within normal limits 5. Systolic blood pressure \<180 mm Hg and diastolic blood pressure \<100 mm Hg \[Note: Hypertension controlled by antihypertensive therapy is permitted\]. 6. Willing and able to adhere to prohibitions and restrictions specified in this protocol 7. Signs an informed consent document within 4 weeks before randomization indicating she understands the purpose of and procedures required for the study and is willing to participate in the study Exclusion Criteria: 1. Prior treatment with ketoconazole, aminoglutethimide or a CYP17 inhibitor. \[Note: Prior treatment with ketoconazole for ≤7 days is permitted and topical formulations of ketoconazole are permitted\] 2. Anticancer immunotherapy, investigational agent, anticancer radiotherapy or anticancer endocrine therapy within 12 weeks before randomization 3. Use of hormone replacement therapy within the past 4 weeks 4. Serious or uncontrolled nonmalignant disease, including active or uncontrolled infection 5. Clinical or biochemical evidence of hyper-aldosteronism or hypopituitarism 6. Any condition that, in the opinion of the investigator, would compromise the well-being of the patient or that could prevent, limit, or confound the protocol-specified assessments 7. Major thoracic or abdominal surgery or significant traumatic injury with 4 weeks before randomization \[Note: Patients with planned surgical procedures to be conducted under local anesthesia are not excluded from the study (e.g. intravascular device insertion)\] 8. Gastrointestinal disorder interfering with study drug absorption 9. Positive serology for hepatitis B surface antigen or hepatitis C antibody 10. Active or symptomatic viral hepatitis or chronic liver disease 11. History of clinically significant heart disease, ie, myocardial infarction or arterial thrombotic event within 6 months, severe or unstable angina, or New York Heart Association Class III or IV heart disease 12. Known allergies, hypersensitivity, or intolerance to abiraterone acetate, prednisone, or their excipients 13. Received an investigational drug (including investigational vaccines) or used an invasive investigational medical device within 4 weeks before the planned first dose of study drug or is currently enrolled in an investigational study