Abstract PO4-03-05: Prognostic value of Ki67 expression in post-treatment tumors of patients with residual triple-negative breast cancer after neoadjuvant chemotherapy

Abstract Introduction Patients with triple-negative breast cancer (TNBC) who do not achieve pathological complete response (pCR) against neoadjuvant chemotherapy (NAC) have a poor prognosis. The prognosis of these patients can be differentiated using various parameters, including the Residual Cancer Burden (RCB) index and tumor-infiltrating lymphocytes. In this study, we took a different angle and aimed to evaluate the prognostic value of post-treatment Ki67 expression, which was examined in tumors following NAC in patients with non-pCR TNBC. Methods A retrospective review of clinical records from February 2008 to December 2021 was conducted of patients from Gangnam Severance Hospital and Asan Medical Center, both in Seoul, South Korea. We included patients with non-metastatic TNBC who underwent NAC including anthracycline and taxane regimens, followed by curative surgery, regardless of adjuvant capecitabine receipt. Ki67 expression and the RCB index were evaluated using residual tumors obtained from surgery. High Ki67 expression was defined as ≥20%. The 5-year recurrence-free survival (RFS) was estimated using Kaplan-Meier analysis, and hazard ratios (HR) were calculated using Cox regression analysis. Results This study included 338 patients with available Ki67 expression data in residual tumors. Among them, 110 patients (30.9%) had low Ki67 expression, while 228 patients (64%) had high Ki67 expression. At a median follow-up of 34 months, those with low Ki67 expression had a 5-year RFS rate of 82.6%, while those with high Ki67 expression had a rate of 58.3%, with a HR of 0.345 (95% CI 0.210-0.565, p 0.001). When stratified by the RCB index, the 5-year RFS of RCB-I patients was 93%, RCB-II patients 70.2%, and RCB-III patients 40.5%. When assessing survival outcomes based on both Ki67 expression and the RCB index, patients with RCB-I and low Ki67 (n=26) had a 100% 5-year RFS rate. Patients with RCB-I and high Ki67 (n=24) had a rate of 83.3%. Notably, in patients with RCB-III and high Ki67 (n=62), the 5-year RFS rate was 35.5% and there was no significant difference in RFS based on the receipt of adjuvant capecitabine (median RFS: capecitabine - 18 months, observation - 15 months, p-value=0.994). Conclusions Our findings demonstrate the prognostic value of Ki67 expression in residual TNBC after NAC, indicating that cases with low Ki67 expression have a favorable outcome. Combining Ki67 expression with the RCB index allows identification of patients with good residual disease (i.e. low Ki67 expression and RCB-I) who could be potential candidates for de-escalating adjuvant treatments such as capecitabine or pembrolizumab. However, for patients with high Ki67 expression and RCB-III, novel strategies are required to improve survival outcomes beyond the use of capecitabine. Citation Format: Yoonwon Kook, Soo-Young Lee, Seung Ho Baek, Min Ji Kim, Junghyun Kim, Sohyun Moon, Seung Eun Lee, Soong June Bae, Joon Jeong, Sae Byul Lee, Sung Gwe Ahn. Prognostic value of Ki67 expression in post-treatment tumors of patients with residual triple-negative breast cancer after neoadjuvant chemotherapy 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 PO4-03-05.