Abstract PO1-27-03: Studies on factors for pathological complete response after neoadjuvant chemotherapy in relation to HER2 status in primary breast cancer

Abstract Background: Neoadjuvant chemotherapy (NAC) has been increasingly used as the frontline therapy for the management of high-risk localized breast cancer (BC). Achieving pathological complete response (pCR) following NAC is associated with significantly better disease-free (DFS) and overall survival (OS), particularly for triple negative (TN) and HER2+ breast cancer. Among HER2 negative BC, HER2-low BC is a newly defined subset that has a HER2 immunohistochemical (IHC)-1+ or 2+/ISH negative phenotype. HER2 status is divided into 3 categories: HER2 positive, HER2-low, and HER2-zero. In this study, we investigated factors for pCR in relation to HER2 status and survival after NAC. Methods: A total of 197 cases with primary BC from January 2013 to July 2023 were enrolled in this study. There were 83 HER2 positive cases (including 7 HER2 2+/ISH+ cases) and 114 HER2 (78 HER2-low and 36 HER2-0) negative cases. HER2 3+ was dichotomized according to the staining cell rate (cut-off point; 90%) into the following two groups; HER2 3+/90% (70 cases) and HER2 3+/ 90% (6 cases). The chemotherapy regimen was anthracycline and taxane with the addition of anti-HER2 therapy if the cases were HER2-positive. The clinicopathological factors examined were age, nodal status, tumor size, ER/PgR (cut-off points; 1% and 10%), and the Ki-67 index value (cut-off points: 20% and 40%). pCR was defined as the absence of residual invasive cancer on H90%) had a significantly higher pCR rate than HER2 3+(≤90%) and HER2 2+(ISH+). Multivariate analysis revealed that HER2 3+( 90%) and tumor size (≤5cm) were significant factors for pCR in HER2 positive BC cases. 3. Patients with pCR after NAC had significantly better DFS in HER2 positive BC cases. 4. There was a significant correlation between the pCR rate and ER≤10%, PgR≤10% and tumor size 5cm in the HER2 negative BC cases. There was no relationship between the pCR rate and the HER2 status; 19.4% in HER2-0 and 12.8% in HER2-low. 5. Multivariate analysis revealed that the PgR status and tumor size were significant factors for pCR in the HER2 negative BC cases. 6. There was no significant difference in DFS between patients with pCR and non-pCR, especially in the luminal type BC cases. Moreover, no significant difference was observed between HER2-low and HER2-0. Conclusion: pCR rate after NAC was 57.8% in the HER2 positive cases and 14.9% in the HER2 negative cases. Moreover, the HER2 3+( 90%) group had a significantly higher pCR rate than the HER2 3+(≤90%) and HER2 2+(ISH+) groups. There was no correlation between the HER2 status (HER2-low and HER2-0) and pCR rate in the HER2 negative BC cases. In addition, the ER/PgR negativity (≤10%) was a significant factor for pCR in the HER2 negative cases. These findings suggest that the IHC data on the HER2 and ER/PgR status may be effective in predicting the pCR rate after NAC in BC cases. Citation Format: Nobuyuki Arima, Reiki Nishimura, Kazuhiro Muramoto, Atsushi Inayoshi, Tomio Tanigawa, Sugiko Watanabe. Studies on factors for pathological complete response after neoadjuvant chemotherapy in relation to HER2 status in primary breast cancer abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-27-03.