Multicenter study of pathologic complete response rates with pembrolizumab plus chemotherapy for ER-low/HER2-negative breast cancer

ER-low breast cancer (BC) present clinicopathological features and disease behavior resembling triple-negative (TN) BC, but have been frequently excluded from pivotal TNBC trials. Neoadjuvant pembrolizumab plus chemotherapy (P+CT) is the new standard of care for stage II-III TNBC. The Neo-Real study, at ten centers in Brazil, evaluated six cycles of neoadjuvant P+CT for ER-low/HER2-negative BC. Pathologic complete response (pCR) rate was the primary endpoint. The study included 20 patients (median age 40 years; range 28-64; 90% grade 3 tumors; 70% stage II tumors; median Ki67 index 75%; range 30-95). All patients underwent surgery, with pCR observed in 12 patients (pCR rate 60%). Receiving less than 6 cycles of P+CT was associated with a trend toward worse pCR rate (20% versus 73.3%). The authors conclude that the response to neoadjuvant P+CT in this stage II-III ER-low/HER2-negative BC was similar to what is seen in TNBC.