Impact of neoadjuvant HER2-directed therapy on HER2 status in breast cancer.

Abstract

e12130 Background: Neoadjuvant chemotherapy (NAC) with HER2-directed therapy has become standard-of-care for most women with potentially curable HER2-positive (HER2+) breast cancer and is associated with a high pathologic complete response (pCR) rate. The HER2 status of residual disease after NAC is not well characterized and could potentially inform clinical decisions about additional systemic therapy. We describe tumoral HER2 status before and after NAC with HER2-directed therapy. Methods: An institutional database was screened to identify patients with stage 1-3 HER2+ breast cancer by fluorescence in situ hybridization (FISH) and/or immunohistochemistry (IHC) who received NAC with HER2-directed therapy followed by resection between 2011 and 2015. Clinicopathologic data was collected. Change in HER2 status by FISH and IHC following treatment was described. Results: 99 patients were identified. Median age was 49 years (range 26-85). Pre-treatment median HER2/CEP17 copy number ratio (CNR) for all tumors was 6.3 (range 1.9-20.7) by FISH and 84 (84.8%) tumors were IHC 3+. 44 (44.4%) patients achieved a pCR. Of the 55 patients with residual disease, 35 had sufficient residual tumor to evaluate HER2 status and 14/35 (40%) were HER2- by FISH and IHC (table). Tumors converting from HER2+ to HER2- had lower pre-treatment median HER2 copy numbers (11.9, range 4.6-22) compared to tumors that remained HER2+ (18.3, range 5.1-48.6; p=0.04) after neoadjuvant therapy. Additionally, pre-treatment median HER2/CEP17 CNR was lower among tumors that converted from HER2+ to HER2- (3.0, range 2.2-8.2) compared to those remaining HER2+ (6.8, range 2-15.7; p=0.02). Conclusions: While pCR rates are high with NAC and HER2-directed therapy, many patients still have residual tumor. In this cohort, 40% of patients with evaluable residual disease after NAC had HER2+ tumors that became HER2-. HER2 conversion was associated with lower pre-treatment HER2 copy numbers and HER2/CEP17 CNR. Conversion from HER2+ to HER2- in patients undergoing neoadjuvant therapy may have clinical significance and biological implications. [Table: see text]