HER2-LOW BREAST CANCER BRAIN METASTASES: INCIDENCE AND TREATMENT IMPLICATIONS

Brain metastases (BrM) are a major cause of morbidity and mortality among women with breast cancer (BC). Central nervous system (CNS)-penetrating systemic therapies for patients with HER2-negative BrM are lacking; given CNS activity of trastuzumab deruxtecan, efficacy for patients with HER2-low BrM is of interest. METHODS: A retrospective study of two cohorts of consecutive patients who underwent surgery for BC BrM at Sunnybrook Health Sciences Centre were identified. BrM subtype was assessed based on 2018 ASCO/CAP guidelines. HER2-zero was defined as immunohistochemistry (IHC) 0; HER2-low was defined as IHC 1+ or 2+ and fluorescence in situ hybridization (FISH) negative status. HER2-positive was defined as IHC 3+ or IHC 2+ with positive FISH. We also assessed the prognostic association between extent of HER2 expression and i) brain-specific progression free survival (bsPFS), as well as ii) overall survival (OS). RESULTS: Out of 137 patients with resected BrM, tissue for HER2 assessment was available in 74.5% (n=102) of cases. In this cohort, the median age at BrM diagnosis was 53.5 (range, 32- 85). 68.6% (n=70) had extracranial disease. 53% (n=54) of the BrM were HER2-positive; 29.4% (n=30) were HER2-low and 17.6% (n=18) had HER2-zero status. Among BrM that were triple negative based on ASCO/CAP guidelines, 14 out of 22 cases (63.6%) were re-classified as being HER2-low. 15/25 (60%) BrM that were hormone receptor positive/HER2 negative (HR+/HER2-) based on ASCO/CAP guidelines were re-classified as being HER2-low. There was no significant association between the extent of HER2 expression (HER2-zero, HER2-low versus HER2-positive status) in BrM and either bsPFS or OS. CONCLUSION: Among patients with surgically resected BrM, a high proportion of those with metastatic TNBC and HR+/HER2-negative disease have “HER2-low” BrM with potential to benefit from HER2-targeted therapy.