The association of hormone receptor expression level and clinical efficacy of CDK4/6 inhibitor in hormone receptor-positive, HER2-negative metastatic breast cancer.

e13069 Background: In patients with hormone receptor-positive, HER2-negative metastatic breast cancer (HR+ HER2- MBC), the correlation between the level of expression of HR and the clinical efficacy of CDK4/6 inhibitors remains undefined. Methods: We identified consecutive patients with HR+ HER2- MBC starting a CDK4/6 inhibitor (palbociclib, ribociclib or abemaciclib) plus either an aromatase inhibitor or fulvestrant between Jan 2018 - Oct 2023 from an institutional cancer registry. Expression level of HR was stratified as high (H) (Allred score 7-8), moderate (M) (Allred score 5-6) or low (L) (Allred score 3-4) for both estrogen (ER) and progesterone receptors (PR). Patients were evaluated from start of treatment to December 2023, last visit or death, whichever came earlier. Median progression-free survival (mPFS) and overall survival (mOS) were endpoints. We evaluated the association of HR expression levels with mPFS and mOS using log-rank test and multivariate Cox regression modelling. Results: We examined data from 436 women with HR+ HER2- MBC. Median age at treatment was 60.5 yr. Majority of patients received the treatment in the first-line setting (72.5%), with palbociclib (54.1%), ribociclib (31.9%) and abemaciclib (14.0%) as the CDK4/6 inhibitor. In the population that received first-line treatment, ER expression levels were H (83.5%), M (11.4%) and L (5.1%) respectively. ER expression levels were strongly associated with mPFS and mOS. For ER H, M and L expressions, mPFS were 29.4m, 9.2m and 2.9m ( p<0.001); mOS were 52.1m, 40.6m and 11.6m ( p<0.001), respectively. In multivariate analysis, ER expression levels remain strongly predictive for both mPFS and mOS after adjustment for clinicopathological risk factors (ER L vs H, mPFS HR 10.6, p<0.001; mOS HR 4.66, p<0.001; ER M vs H, mPFS HR 4.90, p<0.001; mOS HR 2.06, p=0.05). Findings were similar for the second line and beyond population. In the ER-high expression cohort, the PR expression levels also correlate strongly with treatment efficacy and survival, with mPFS of 40.8m, 24.5m, 24.0m, 14.1m ( p<0.001) and mOS of NR, 53.3m, 48.1m, 26.9m ( p<0.001) respectively for PR H, M, L and zero expressions. Conclusions: In this real-world cohort of patients with HR+ HER2- MBC, we observed that the semi-quantitative HR expression level subgrouping is a strong predictor for the efficacy of treatment with CDK4/6 inhibitors. In contrast to patients with high HR expression levels, the small population with ER-low expression derived very limited benefits from the standard endocrine therapy with CDK4/6 inhibitors. Our findings provided a practical tool to guide the selection of the appropriate systemic therapy for individual patients.