Abstract PO4-05-08: Primary endocrine therapy resistance in patients with de novo HR+/HER2- metastatic breast Cancer: a National Cancer Information Database Analysis in China

Abstract Background: Endocrine therapy resistance was classifed as primary and secondary, guiding the first-line (1L) treatment choices for hormone receptor positive, HER2-negative (HR+/Her2-) metastatic breast cancer (mBC). However, it is unknown whether it is primary ET resistance or not to the stage IV de novo patients. Therefore, We performed a retrospective study to predict primary resistance in advance in de novo patients and have a more appropriate treatment strategy. Methods: We identified newly diagnosed with stage IV de novo HR+/HER2- mBC patients received single-agent endocrine therapy (ET) or ET plus CDK4/6 inhibitors (CDK4/6i) in 1L setting from January 1, 2020 to Jun 30, 2022, using the National Cancer Information Database (NCID) in China. Primary ET resistance (PER) was defined as switching to second-line treatment within 6 months after receiving the 1L ET±CDK4/6i, whereas secondary ET resistance (SER) patients were those who received 1L ET±CDK4/6i treatment more than six months. The Chi-Square test was used to compare the baseline characteristics of PER and SER patients. Result: A total of 217 patients who met the inclusion criteria were included in the analysis, among which 161 patients received single-agent ET and 56 patients received ET plus CDK4/6i. 18.4% of the patients were classified as PER patients, whereas 81.6% showed SER patients. PER patients were younger (median age of 52 years vs. 58 years in SER patients, p=0.01). In terms of clinicopathological characteristics, patients with PER have a higher percentage of liver metastases (14/40 (35%) vs. 20/177 (11.3%) in SER, p 0.01), distant lymph node metastases (19/40 (34%) vs. 45/177 (25.4%) in SER, p = 0.01), and bone metastases (30/40(75%) vs. 97/177(54.8%) in SER, p = 0.02). Furthermore, the PER rate was 21.7% (35/161) in patients received single-agent ET, compare with 8.9% (5/56) in ET plus CDK4/6i patients, p=0.04. Regarding the following treatment pattern in the PER patients, a high percentage of patients received chemotherapy, accounting for 50%, 30% still received another endocrine therapy, and the remaining 20% patients received CDK4/6i combined with endocrine therapy. Conclusions: There is still a significant proportion of de novo HR+/HER2- mBC patients with primary endocrine resistance, a relatively low proportion of PER in patients receiving endocrine in combination with CDK4/6i. Table. The proportion of primary endocrine resistance across treatment regimens in de novo HR+/HER2- metastatic breast Cancer Citation Format: Zhanhong Chen, Xiying Shao, Yabing Zheng, Wenming Cao, Junqing Chen, Weizhu Wu, Ting Wang, Xiaojia Wang. Primary endocrine therapy resistance in patients with de novo HR+/HER2- metastatic breast Cancer: a National Cancer Information Database Analysis in China 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 PO4-05-08.