Efficacy of Fulvestrant in Metastatic HR+, HER2-low Breast Cancer : An Observational Retrospective Study

Abstract

                           Recently, there is a new type of breast cancer called HER2-low subtype. The biology and prognosis of this subtype of breast cancer is not well established. The results of previous studies have been inconsistent. This is the first origin article compare the difference in the effectiveness of fulvestrant between HER2-negative and HER2-low breast cancer groups. by retrospective study by collecting data on hormone-positive metastatic breast cancer patients who used fulvestrant at the National Cancer Institute from December 18, 2018 to June 20, 2024. We compared efficacy between HER2-negative versus HER2-low groups. The primary end point was Progression Free Survival (PFS). Survival was analyzed using the Kaplan–Meier (KM) method to compare the differences in survival curves between the 2 population groups. The statistical tests used were log-rank test and multivariable Cox regression modeling. results : A total of 32 patients were included, of which 11 patients (34%) were in the HER2-negative group and 21 patients (66%) were in the HER2-low group. The median PFS in the HER2-negative group was 6.4 months (95% CI 1.16-11.67 months), while the median PFS in the HER2-low group was 9.2 months (95% CI 4.63-12.60 months). P = 0.0863, which was not statistically significant. However, when multivariate analysis was used to exclude other factors that might affect the median PFS, patients in the HER2-low group had a significantly better median PFS than the HER2-negative group (hazard ratio [HR], 0.34; 95%CI, 0.13-0.88; P = .026). The study result showed that metastatic breast cancer patients who used fulvestrant, the HER2-low group tended to have a longer median PFS than the HER2-negative group. This study will help us better understand the biology of HER2-low breast cancer and select the most appropriate medication for treating patients. However, further studies with a larger population are needed to make more accurate conclusions.