HER2 heterogeneity and treatment response-associated profiles in HER2-positive breast cancer in the NCT02326974 clinical trial

HER2-targeted therapies including antibody drug conjugates have shown great efficacy in HER2-positive breast cancer. However, resistance to treatment in part due to pre-existing HER2 heterogeneity (HET) is a significant clinical challenge and requires the use of rationally-designed combination therapies. Here we describe transcriptomic profiling of 287 biopsies from 129 patients in a phase II neoadjuvant clinical trial using a combination of T-DM1 and pertuzumab for early-stage HER2-positive breast cancer, we investigated the mechanisms driving T-DM1 resistance. In pre-treatment tumors from patients without a complete response (pCR), distinct molecular features were identified: HER2 HET tumors retained PI3K pathway activation and a basal-like phenotype, while HER2 Non-HET tumors exhibited lower PI3K pathway enrichment. T-DM1 treatment universally reduced HER2 protein expression and copy number heterogeneity while increasing PI3K pathway enrichment and luminal identity in residual tumors. Importantly, HER2 HET tumors showed minimal transcriptomic response compared to HER2 Non-HET tumors, in line with the lesser clinical response. The response to T-DM1 was not associated with hotspot PIK3CA/ERBB2 mutations but rather correlated with immune infiltration. HER2 Non-HET tumors exhibited less effective immune surveillance at baseline but became immune activated with T-DM1 treatment, while HER2 HET tumors exhibited a more immune suppressed microenvironment upon treatment. Our study highlights the multifaceted nature of T-DM1 resistance driven by HER2 heterogeneity and provides essential evidence for optimizing therapeutic strategies for these patients.