Abstract PS5-01-19: Mechanisms of resistance to anti-HER2 therapies in brain metastatic derivatives of inflammatory HER2-positive breast cancer models

Abstract Background: HER2-positive (HER2+) breast cancer (BC) is an aggressive subtype with 30-50% incidence of brain metastases in metastatic patients. The HER2-selective tyrosine kinase inhibitor (TKI) tucatinib (Tuca) and the pan-HER TKI neratinib (Nrb), mostly used in the late line setting, are effective, including in treating brain metastases. The new antibody drug conjugate T-DXd is also highly effective in the brain metastatic setting. It is crucial to understand the mechanisms of resistance to these brain permeable anti-HER2 therapies. Materials and Methods: GFP/Luc tagged SUM190-BR3 (SUM190Br), a brain-tropic derivative of the HER2+/ER- inflammatory SUM190 cell model (harboring the PIK3CA H1047R mutation), was used (PMID: 27245829). A long-term highly Tuca resistant (TucaR) derivative of this line was developed through exposure to increasing doses of Tuca up to 1uM. Characterization of these resistant and naïve cells was performed by Western blot (WB), DNA-seq, and RNA-seq. Drug efficacy studies involved methylene blue-based cell growth and IC50 assays, and included the Akt inhibitor (i), capivasertib (Capi, 1uM), T-DXd (5ug/mL), and the EGFR-specific TKI gefitinib (Gef, 1uM) or monoclonal antibody cetuximab (Cetux, 10ug/mL). To characterize the in vivo tumorigenicity and metastatic capacity of our resistant model, immunocompromised mice were injected with cells via subcutaneous or intracardiac routes and monitored for primary tumor growth and metastasis by bioluminescence imaging. Results: Interestingly, the naïve SUM190Br cells are intrinsically resistant to Tuca (IC50>600nM), but not Nrb (IC50