Abiraterone acetate, exemestane or the combination in postmenopausal patients with estrogen receptor-positive metastatic breast cancer

Androgen receptor (AR) signaling and incomplete inhibition of estrogen signaling may contribute to metastatic breast cancer (MBC) resistance to a nonsteroidal aromatase inhibitor (NSAI; letrozole or anastrozole). We assessed whether combined inhibition of androgen biosynthesis with abiraterone acetate plus prednisone and estradiol synthesis with exemestane (E) may be of clinical benefit to postmenopausal patients with NSAI-pretreated estrogen receptor-positive (ER+) MBC. Patients (N = 297) were stratified by the number of prior therapies for metastatic disease (0-1 versus 2) and by prior NSAI use (adjuvant versus metastatic), and randomized (1 : 1 : 1) to receive oral once daily 1000 mg abiraterone acetate plus 5 mg prednisone (AA) versus AA with 25 mg E (AAE) versus 25 mg E alone (E). Each treatment arm was well balanced with regard to the proportion of patients with AR-positive breast cancer. The primary end point was progression-free survival (PFS). Secondary end points included overall survival, clinical benefit rate, duration of response, and overall response rate. There was no significant difference in PFS with AA versus E (3.7 versus 3.7 months; hazard ratio [HR] = 1.1; 95% confidence interval [CI] 0.82-1.60; P = 0.437) or AAE versus E (4.5 versus 3.7 months; HR = 0.96; 95% CI 0.70-1.32; P = 0.794). Increased serum progesterone concentrations were observed in both arms receiving AA, but not with E. Grade 3 or 4 treatment-emergent adverse events associated with AA, including hypokalemia and hypertension, were less common in patients in the E (2.0% and 2.9%, respectively) and AA arms (3.4% and 1.1%, respectively) than in the AAE arm (5.8% for both). Adding AA to E in NSAI-pretreated ER+ MBC patients did not improve PFS compared with treatment with E. An AA-induced progesterone increase may have contributed to this lack of clinical activity. NCT01381874.