Abstract PO3-05-04: Characteristics and clinical outcome of patients with HR positive HER2 low metastatic breast cancer treated with CDK 4/6 inhibitors

Abstract Introduction Breast cancer (BC) is a disease characterized by significant intra- and intertumoral heterogeneity. Hence, it is not surprising that new subtypes with distinct biological features are being discovered, even among previously well-defined BC groups. Recently, HER2-low BC emerged as a new entity with specific clinical behavior, response to treatment and prognosis. HER2-low is a subset of HER2-negative BC, with HER2 immunohistochemical (IHC) score of 1+ or 2+, without HER2 gene amplification measured by in situ hybridization (ISH). As new therapeutic options become available for HER2-low patients, the best treatment sequence is yet to be determined. Furthermore, it is important to distinguish whether there is a difference in the response to standard treatment lines, such as CDK 4/6 inhibitors in metastatic HR positive BC patients, depending on HER2-low status. Methods A retrospective study of 369 metastatic BC (mBC) cases who started CDK 4/6 inhibitor therapy from January 2018 through December 2022 at University Hospital Centre Zagreb was conducted, with prior Ethics Committee approval. All patients with HR positive HER2 negative mBC, determined by standard IHC and ISH, were included in the research. Patient demographics and clinical presentation, tumor characteristics and treatment information were collected. Progression-free survival (PFS) analysis was done with the final data cut-off date being June 1st, 2023. Type 1 right censoring was performed. The data was analyzed using the Kaplan-Meier method and Cox proportional-hazards regression for clinically relevant covariates (age, line of treatment, de novo metastatic disease, endocrine resistance, liver metastases, and detected PIK3CA mutation). Results Median follow-up was 23 months. Of the 283 patients included, 146 (51.59%) had HER2-low disease. A change in HER2 expression between primary tumor and metastasis was found in 16.96% (N=48) patients. Of them,10.25% (N=29) who were initially HER2-low, were found to be HER2-0 in metastatic disease. Meanwhile, 6.71% (N=19) of patients had a change in HER2 expression from 0 to low upon becoming metastatic. In the HER2-low group, 47.06% (N=45) patients had a PIK3CA mutation as opposed to 33.33% (N=30) in the HER2-0 group. Odds ratio for a PIK3CA mutation in HER2-low patients was 1.86 (95% confidence interval (CI): 1.01-3.43, p-value 0.046). Median PFS in the HER2-low group was 18 months (95% confidence interval (CI): 14-24) versus 23 (95% CI: 18-30) in the HER2-0 group. Using multivariable analysis an adjusted hazard ratio of 1.15 (95% CI:0.84-1.57; p-value 0.389) was calculated. Covariates associated with a statistically significant increased risk of disease progression were a higher line of therapy (HR 1.39, 95% CI 1.36-1.71, p-value 0.002) and the presence of liver metastases (HR 2.17, 95% CI 1.42-3.32, p-value 0.0004). A covariate associated with a statistically significant longer PFS was de novo metastatic disease (HR 0.63, 95% CI 0.41-0.97, p-value 0.034). Conclusion There was a trend toward worse PFS in HER2-low mBC that did not reach statistical significance. HER2-low patients were more likely to harbor PIK3CA mutations than HER2-0 patient group. Longer follow-up and a larger cohort are needed in order to make definitive conclusions. Citation Format: Katarina Čular, Kristina Kanceljak, Ana Magdalena Glas, Dora Gudelj, Marija Križić, Marina Popović, Natalija Dedić Plavetić, Maja Sirotković-Skerlev, Stjepko Pleština, Tajana Silovski. Characteristics and clinical outcome of patients with HR positive HER2 low metastatic breast cancer treated with CDK 4/6 inhibitors 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 PO3-05-04.