Abstract 336: A recombinant human protein targeting HER2 overcomes drug resistance in HER2-positive breast cancer

Background & Goals: HER2 is an oncogenic receptor tyrosine kinase (RTK) implicated in several types of human cancer. It is strongly expressed in about 20% of breast cancer (BC), known as HER2-positive BC (HER2-BC), due to gene amplification. HER2 amplification or overexpression is a strong predictor of poor disease prognosis. Several HER2-targeting drugs are available for treating HER2-BC, including monoclonal antibodies trastuzumab (Ttzm) and pertuzumab, T-DM1 (Ttzm coupled to a microtubule inhibitor), and tyrosine kinase inhibitors (TKIs) lapatinib and neratinib. While these agents have greatly improved disease outcomes, primary and acquired drug resistance, including cross-resistance, is common. Overcoming drug resistance remains a major unmet medical need in HER2-BC. Notably, HER2 remains overexpressed in drug-resistant HER2-BC cells. We recently found that recombinant human peptidase D (PEPD), also known as prolidase, strongly downregulates HER2 and EGFR in cancer cells in vitro and in vivo via its binding to HER2 and EGFR. The enzymatic activity of PEPD plays no role in its modulation of HER2 and EGFR, and we subsequently focused on recombinant PEPDG278D, an enzymatically inactive mutant (point mutation at codon 278). Our objective in the present study is to determine whether PEPDG278D is active in preclinical models of HER2-BC resistant to clinically available HER2 inhibitors and if so, its mechanism of action. Results: PEPDG278D was exceedingly effective against HER2-BC cells that are resistant to Ttzm and other HER2 inhibitors. The cells carried activating PIK3CA mutations, low expression of PTEN, and/or cyclin E overexpression. All the cell lines also overexpressed MUC4 which shields HER2 from Ttzm and stabilizes HER2. However, PEPDG278D rapidly bound to and freed HER2 from MUC4, and rapidly disrupted the interaction of HER2 with other RTKs, including MET and IGF1R. PEPDG278D subsequently induced persistent downregulation of HER2 via internalization and lysosomal degradation. PEPDG278D also downregulated EGFR. Thus, PEPDG278D suppresses multiple RTKs either directly or indirectly in HER2-BC cells. In contrast, Ttzm cannot perform any of these functions of PEPDG278D. PEPDG278D was highly active in mouse tumor models of HER2-BC resistant to Ttzm and also enhanced the therapeutic efficacy of paclitaxel. However, adverse effects of PEPDG278D were not detected in the animal studies. Conclusions & Therapeutic Relevance: The therapeutic activity of PEPDG278D highlights that HER2 remains a critical target in drug-resistant HER2-BC. The strong therapeutic activity of PEPDG278D in preclinical models of drug-resistant HER2-BC, which carry clinically relevant molecular changes that confer resistance to current HER2 inhibitors provides the scientific premise for potential evaluation of this recombinant human protein in patients with drug-resistant HER2-BC. Citation Format: Lu Yang, yun Li, Arup Bhattacharya, yuesheng Zhang. A recombinant human protein targeting HER2 overcomes drug resistance in HER2-positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 336.