Correlation Of Quantitative P95her2, Her2, And Her3 Protein Expression With Pathologic Complete Response (Pcr) In Her2-Positive Breast Cancer Patients (Pts) Treated With Neoadjuvant Chemotherapy And Trastuzumab

137 Background: Elevated p95HER2 [HER2-M611-CTF (carboxy-terminal-fragment) also known as p95] expression has been correlated with poor outcomes in HER2+ pts with metastatic breast cancer treated with trastuzumab (T); however, limited data is available on the correlation between p95 and pCR to T in the neoadjuvant (NEO) setting, where p95 was measured by immunohistochemistry. The current study aims to determine whether quantitative p95, HER3 and HER2 expression correlated with pCR in pts treated with T + chemotherapy in the NEO setting.pCR data and quantitative HER2 (H2T), p95, and HER3 (H3T) results by HERmark/VeraTag assays were available in 45 patient cases with pre-therapy, formalin-fixed, paraffin-embedded breast tumors. pCR was defined as the absence of invasive disease in the breast. Quantitative biomarker data were correlated with pCR according to previously published or presented biomarker cutoffs.The overall pCR rate was 46.7% (ER+: 14.3% vs. ER-: 75%; p<0.0001) and was significantly associated with higher H2T levels (p=0.02) and lower H3T levels (p=0.04). In ER- subjects (N=24), no difference in H2T levels was observed between pCR vs non-pCR groups (median H2T=111.5 vs 150.5, respectively; p=0.721). However, within the ER+ group (N=21), H2T levels were significantly higher in the pCR group vs non-pCR group (median H2T=254 vs 37.3; p=0.024). Using multivariate logistic regression, increasing log(H2T) (p = 0.012), ER-negativity (p = 0.027) and low p95 (p = 0.074) were found to correlate or trend with pCR.pCR was significantly associated with high H2T, particularly in ER+ HER2+ breast cancer pts who received NEO therapy with T + chemotherapy. Lower H3T was also associated with pCR. A trend towards pCR was seen in tumors with low p95. These data suggest that quantitative H2T, H3T and p95 may provide additional information on response to T-based regimens in breast cancer stratified by ER status. Additional investigation into the relationship between quantitative H2T, p95 and H3T expression and T response in the NEO setting in larger cohorts is warranted.