Abstract
T-DXd is clinically beneficial in HER2 positive and HER2 low metastatic breast cancer. However, therapeutic resistance emerges over time in most patients, with poorly defined resistance mechanisms. Through a molecular characterization of paired pre- and post-T-DXd treatment patient specimens, we found that 49% cases had major decreases in HER2 expression at progression, among them, 52% exhibited complete HER2 loss. Using isogenic model systems, we demonstrated that decreases in HER2 expression corresponded to reductions in T-DXd internalization and major increases in drug IC50 for tumor growth inhibition. We further identified and validated HER2 mutations in the trastuzumab binding interface (V597M and P593R) that promoted T-DXd resistance. As a strategy to overcome impaired T-DXd binding and internalization, we tested low dose combinations of T-DXd with TROP2-directed ADCs and found these could more uniformly deliver DXd payloads and thereby overcome resistance mediated by HER2 loss.