Evaluation Of Biomarker Association With Efficacy For Abiraterone Acetate (Aa) Plus Prednisone (P) With Or Without Exemestane (E) In Postmenopausal Patients (Pts) With Estrogen Receptor-Positive (Er Plus ) Metastatic Breast Cancer (Mbca) Progressing After A Nonsteroidal Aromatase Inhibitor (Nsai).

520 Background: AA treatment may suppress androgens and estrogens that stimulate BCa growth. We conducted a biomarker analysis of circulating tumor cells (CTCs) and formalin-fixed paraffin-embedded tissues (FFPETs) from a phase 2 study of the efficacy and safety of AA in postmenopausal ER+ BCa pts to analyze serum steroid concentrations and identify subgroups with AA sensitivity. Methods: Treatments: AA (1 g/d) + P (5 mg/d) vs AA + P + E (25 mg/d) vs E alone. FFPETs from diagnosis and blood samples collected at baseline, during treatment, and at the end of treatment were used for molecular characterization and serum endocrine marker analysis. Expression of androgen receptor (AR), estrogen receptor (ER), Ki-67, CYP17, CYP19, and other biomarkers was evaluated in CTCs and/or FFPETs. Cox regression analysis stratified by number of prior therapies and type of therapy was used to evaluate biomarker associations with progression-free survival (PFS). Statistical comparisons were performed for treatment effect in...