Is HER2 expression related to CDKi efficacy in HER2-negative luminal metastatic breast cancer?

Abstract

e13073 Background: Cyclin kinase inhibitors (CDKi) are the standard of care in the first line of luminal (Lum) metastatic breast cancer (MBC). HER2 overexpression is a classic risk factor in BC, however, the use of targeted therapy has changed the prognosis of this disease, even with anti-HER2 efficacy data in cases of low HER2 expression. We wonder if the level of HER2 expression can be a predictor of efficacy in treatment with CDKi. Methods: Descriptive, observational, retrospective and analytical study of patients with MBC treated with CDKi, in the period 2018-2022. Descriptive and analytical statistical analysis (IBM SPSS Statistics 22) using logistic regression and Kaplan-Meier with long-rank test to analyze the relationship between CKi efficacy and HER2 expression (negative = 0+; HER2 low = 1+ and 2 + with SISH negative). Results: We analyzed 152 patients (p) with Lum MBC under treatment with CDKi (median 58 years), in first and successive lines. HER2 expression: 31% 46p (0+), 54% 81p (1+) and 14% 21p (2+ and SISH negative). 41% were treated with palbociclib, 34% with ribociclib, and 26% with abemaciclib. Radiological response (RR) to CDKi was achieved by 34.1% (CRR 2.4%, PRR 31.7), and the rest stable disease (SD) or progressive disease (PD): SD 42% and PD 22%. In the overall OR, there was no evidence of a risk association between the response to CDKi and the level of HER2 expression. We obtained similar results in the analysis stratified by type of CDKi. In the multivariate study, for the variables CDKi type and HER2 expression, the logistic regression model was not statistically significant either. Regarding the Kaplan-Meier analysis, we compared the median PFS and OS using the Long-rank test, based on the expression of HER2. In the complete series we did not find statistically significant differences between the curves. In the palbociclib subgroup the curves split in favor of HER2 0+ vs. HER2 low: 44 months 95%CI (7.6-80) vs 16 months 95%CI (7.9-24); p = 0.01. Regarding OS, we found no significant differences. With ribociclib, the curves separate in favor of HER2 low versus HER2 0+: 33 months 95%CI (7.2-18) vs. 13 months 95%CI (23-42); p = 0.03. In OS we did not find significant differences either. In the subgroup with abemaciclib, we observed a trend towards greater survival in HER2 0+ patients, but with no significant difference between the PFS or OS curves. Conclusions: Based on our study, the expression of HER2 could be considered a predictor of efficacy depending on the CDKi used. We assume greater survival with palbociclib in HER2 negative patients (0+) and with ribociclib in HER2 low patients. We consider it essential to develop studies with a larger sample size and clinical trials that allow us to demonstrate the hypothesis of our work with scientific evidence.