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BackgroundHuman epidermal growth factor 2 (HER2/ERBB2) is frequently amplified/mutated in cancer. The tyrosine kinase inhibitors (TKIs) lapatinib, neratinib, and tucatinib are FDA-approved for the treatment of HER2-positive breast cancer. Direct comparisons of the preclinical efficacy of the TKIs have been limited to small-scale studies. Novel biomarkers are required to define beneficial patient populations.MethodsIn this study, the anti-proliferative effects of the three TKIs were directly compared using a 115 cancer cell line panel. Novel TKI response/resistance markers were identified through cross-analysis of drug response profiles with mutation, gene copy number and expression data.ResultsAll three TKIs were effective against HER2-amplified breast cancer models; neratinib showing the most potent activity, followed by tucatinib then lapatinib. Neratinib displayed the greatest activity inHER2-mutant andEGFR-mutant cells. High expression ofHER2,VTCN1,CDK12, andRAC1correlated with response to all three TKIs. DNA damage repair genes were associated with TKI resistance.BRCA2mutations were correlated with neratinib and tucatinib response, and high expression ofATM,BRCA2, andBRCA1were associated with neratinib resistance.ConclusionsNeratinib was the most effective HER2-targeted TKI againstHER2-amplified, -mutant, andEGFR-mutant cell lines. This analysis revealed novel resistance mechanisms that may be exploited using combinatorial strategies.