A Study of Abiraterone Acetate Plus Prednisone With or Without Exemestane in Postmenopausal Women With Estrogen Receptor-Positive (ER+) Metastatic Breast Cancer Progressing After Letrozole or Anastrozole Therapy

This is a randomized (study drug assigned by chance), open-label (all participants will know the identity of the assigned study drug) study divided into three phases, screening, treatment, and follow-up. During screening, potential patients will be assessed for study eligibility after providing signed informed consent. The treatment phase will comprise a series of 28-day cycles with continuous study treatment until breast cancer progression, when an end-of-treatment visit will be completed before the follow-up phase begins. The duration of participation in the study for an individual patient may be up to approximately 7 years, including follow-up evaluations. Patients will be evaluated for the safety and effectiveness of study treatments. During the treatment phase, patients will take the following study drugs by mouth once daily: abiraterone acetate, 1 g/day, as four 250-mg tablets, on an empty stomach, and patients must not eat for at least 1 hour after abiraterone acetate; prednisone (prednisolone when prednisone is not available), 5 mg/day; and exemestane, 25 mg/day, as a single tablet. The treatment phase will consist of a series of 28-day cycles with continuous study treatment until breast cancer progression. At the planned interim analysis, the Data Review Committee has recommended that further randomization to the abiraterone acetate alone group be stopped and that the study is to be continued otherwise.

Inclusion criteria

• Female patients must be postmenopausal
• ER+, Human epidermal growth factor receptor 2 (Her2) negative metastatic breast cancer
• Disease must have been sensitive to anastrozole or letrozole therapy prior to disease progression
• No more than two prior lines of therapy in the metastatic setting, of which no more than one was chemotherapy
• Eastern Cooperative Oncology Group (ECOG) performance status score of <=1
• Patients with disease confined only to bone may be included, but patients with purely sclerotic lesions may not participate in the study

Exclusion criteria

• Prior treatment with exemestane, ketoconazole, aminoglutethimide, or a CYP17 inhibitor. Prior treatment with ketoconazole for <= 7 days is permitted and topical formulations of ketoconazole are permitted
• Potential patients must not have taken anastrozole, letrozole, fulvestrant, or any chemotherapy for at least 2 weeks (bevacizumab for at least 3 weeks) before randomization
• Anticancer immunotherapy or investigational agent within 4 weeks before randomization, or anticancer radiotherapy (except palliative) or anticancer endocrine therapy within 2 weeks before randomization
• Serious or uncontrolled nonmalignant disease, including active or uncontrolled infection
• Clinical or biochemical evidence of hyperaldosteronism or hypopituitarism
• 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