A degrader of HER2 and EGFR abolishes p95HER2 and shows robust antitumor efficacy in HER2-positive breast cancer

p95HER2 is commonly expressed in human epidermal growth factor receptor 2 (HER2)-positive breast cancer (BC) and is generated primarily from shedding of the extracellular domain of HER2. p95HER2 is oncogenic, driving aggressive tumor growth, and conferring drug resistance. Targeting p95HER2 is challenging. Clinically approved HER2 inhibitors either cannot bind to p95HER2 or show limited inhibitory effects. We used cell lines and orthotopic tumor models (cell line xenografts and patient derived xenografts) to investigate the effects of HER2 inhibitors on p95HER2 and other key signaling proteins in HER2-positive BC, and to compare the therapeutic activities of different HER2 inhibitors. The HER2 inhibitors represent different mechanisms of actions, including trastuzumab, pertuzumab, tucatinib, and lapatinib, all of which are clinically approved, as well as PEPDG278D, a recombinant human protein which was previously shown to induce the degradation of HER2 and epidermal growth factor receptor (EGFR). EGFR is closely related to HER2 and is implicated in drug resistance in HER2-positive BC. HER2-positive BC cells and tumors, despite p95HER2 expression, are exquisitely sensitive to targeted degradation of HER2 and EGFR by PEPDG278D. PEPDG278D-induced degradation of HER2 also consistently eliminates p95HER2. PEPDG278D is far more effective than the clinically approved HER2 inhibitors in inhibiting the oncogenic signaling and growth of HER2-positive BC cells and tumors expressing p95HER2. Most notably, in an orthotopic PDX of HER2-positive BC expressing p95HER2, which is insensitive to trastuzumab and tucatinib, PEPDG278D achieved complete and lasting tumor elimination. Despite p95HER2 expression and resistance to current HER2 inhibitors, HER2-positive BC cells and tumors are highly vulnerable to PEPDG278D-induced degradation of HER2 and EGFR. By inducing HER2 degradation, PEPDG278D eliminates p95HER2 in HER2-positive BC cells and tumors.