HER2-low breast cancer brain metastases: Incidence and treatment implications.

person Rania Chehade University of Toronto, Sunnybrook Hospital, Toronto, ON, Canada info_outline Rania Chehade, Sharon Nofech-Mozes, Anna Plotkin, Veronika Moravan, Katarzyna Joanna Jerzak

Authors person Rania Chehade University of Toronto, Sunnybrook Hospital, Toronto, ON, Canada info_outline Rania Chehade, Sharon Nofech-Mozes, Anna Plotkin, Veronika Moravan, Katarzyna Joanna Jerzak Organizations University of Toronto, Sunnybrook Hospital, Toronto, ON, Canada, Sunnybrook Health Sciences Center, Toronto, ON, Canada, Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, ON, Canada, VM stats, Toronto, ON, Canada, Sunnybrook Odette Cancer Centre, University of Toronto, Toronto, ON, Canada Abstract Disclosures Research Funding No funding received None. Background: 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; this is particularly problematic for patients with triple negative disease (TNBC) who have a high likelihood of developing BrM. Given CNS activity of trastuzumab deruxtecan, efficacy for patients with HER2-low BrM is of interest. Methods: A retrospective study of two cohorts of patients who underwent surgery for BC BrM at Sunnybrook Health Sciences Centre between 1999-2013 and 2008-2018 were identified. Estrogen receptor, progesterone receptor, and human epidermal growth factor receptor-2 (HER2) status were assessed based on 2018 ASCO/CAP guidelines. HER2-zero was defined as immunohistochemistry (IHC) 0; HER2-low was defined as IHC 1+ or IHC 2+ with fluorescence in situ hybridization (FISH) negative status. HER2-positive was defined as IHC 3+ or IHC 2+ with positive FISH. Clinicopathological features were recorded. 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). 18.6% (n=19) had leptomeningeal disease and 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. In total, 51 patients had matched primary breast and BrM tissue available; results of HER2 status when categorized as HER2-zero, HER2-low and HER2-positive were concordant in 82.3% (n= 42/51) of cases (Cohen’s kappa 0.58, p=0.0719). In 7 cases, the primary breast tissue was HER2-zero whereas the BrM was either HER2-low (n=5) or HER2-positive (n=2). In only one case (2%), expression of HER2 was lower in the BrM (HER2-low) compared to the primary BC (HER2-positive). Median time from primary BC to the development of BrM was 35 months (IQR, 14-69) in the overall cohort; patients with HER2-zero BrM had a numerically longer time to development of BrM (45.8 months), compared to those with HER2-low (35 months) and HER2-positive (35 months) BrM (p=0.948). There was no significant association between HER2-zero, HER2-low and HER2-positive status in BrM and either bsPFS or OS. Conclusions: 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.