74. EFFICACY OF HER2-TARGETED THERAPY IN HER2-POSITIVE BREAST CANCER BRAIN METASTASES: A NATIONAL ANALYSIS

BACKGROUND Breast cancer brain metastases (BCBM) commonly develop in human epidermal growth factor 2-positive (HER2+) breast cancer, but BCBM patients are underrepresented in clinical trials, leading to a lack of knowledge on the efficacy of HER2-targeted therapy in this population. METHODS We analyzed clinical characteristics and outcomes of HER2+ BCBM patients from the National Cancer Database 2010–2016, comprising 70% of newly-diagnosed cancers in the U.S, to assess overall survival (OS) associated with HER2-targeted monoclonal antibody therapy (HER2-mab; i.e. trastuzumab, pertuzumab, and trastuzumab emtansine; encoded as of 2013). Survival was estimated with Kaplan-Meier techniques and compared with landmark analysis and Cox regression. The landmark timepoint was selected at which 75% of HER2-mab patients received HER2-mab, which was within 58 days of diagnosis. RESULTS 1,059 HER2+ BCBM patients were identified, 717 (67.7%) patients were estrogen receptor negative (ER-) and 342 (32.3%) were ER+. Median follow-up was 12.0 months, at the end of which 73.8% of patients were deceased. Median OS was 12.2 and 22.1 months for ER- and ER+ patients, respectively. HER2-mab usage for BCBM patients rose from 53.6% in 2013 to 71.7% in 2016. 420 BCBM patients had complete data for landmark analyses: 70.0% (n=294) received HER2-mab and 30.0% (n=126) did not, in which HER2-mab was associated with significantly improved OS in both ER- (median 22.2 months, 95%CI: 18.2–25.4; vs. 9.5 mos, 95%CI: 6.3–10.7; p=0.0001) and ER+ (median 25.7 months, 95%CI: 21.4-not reached; vs. 19.6 months, 95%CI: 11.1–35.2; p=0.02) patients. In multivariable Cox landmark analysis adjusted for ER status, age at diagnosis, extracranial disease, chemotherapy, radiotherapy, and metastasectomy; HER2-mab demonstrated significantly improved OS (hazard ratio 0.59 vs. no HER2-mab, 95%CI: 0.44–0.77; p<0.001). CONCLUSIONS In this large, national study, HER2-mab was associated with substantially improved overall survival in BCBM patients.