Abstract 553: Melanoma subtypes that emerge during adaptive resistance to therapy are targets for bispecific T cell engager (BiTE®) antibody constructs directed to CDH19 and DLL3

The treatment landscape for metastatic melanoma has changed considerably over the last several years with the approval of targeted therapy, such as BRAF inhibitors, and immunotherapy, such as immune checkpoint inhibitors that block cytotoxic T lymphocyte associated protein 4 (CTLA-4) and programmed cell death 1 (PD-1). However, approximately 40% of patients do not respond to the current standard of care, and up to one-third of patients who initially respond to therapy can relapse. Novel therapies that act by a distinct mechanism of action may be needed to increase durable response rates in metastatic melanoma. Bispecific T cell engager (BiTE®) antibody constructs directly connect T cells to tumor cells and redirect T cell lysis to achieve tumor cell killing. BiTE®-mediated cytotoxicity is not dependent on a specific T cell receptor or peptide-MHC complex and has the potential to circumvent common immune resistance mechanisms. We have generated a BiTE® with half-life extension (HLE) that targets the atypical cadherin CDH19 on melanoma cells and CD3 on T cells (anti-CDH19/CD3 HLE BiTE®). We have also generated an HLE BiTE® that targets delta-like ligand 3 (DLL3) on tumor cells and CD3 on T cells, anti-DLL3/CD3 HLE BiTE®, or AMG 757. CDH19 and DLL3 are expressed on about half of primary and metastatic melanoma tumors prior to treatment. In this study, we investigated the expression of CDH19 and DLL3 and the activity of the corresponding BiTE® antibody constructs in a panel of 14 melanoma cell lines that were derived from patients that were resistant to either targeted therapy or immune checkpoint therapy and expanded the spectrum of recently defined multistage differentiation melanoma subtypes from melanocytic, transitional, neural crest-like and de-differentiated subtypes (Tsoi et al. Cancer Cell 2017). The cell surface expression of CDH19 and DLL3 was determined by flow cytometry. We then used T cell-dependent cytotoxicity (TDCC) assays to assess the activity of the anti-CDH19/CD3 HLE BiTE® and AMG 757 against the cell lines. 13/14 (93%) of the treatment-resistant melanoma cell lines expressed CDH19 on the cell surface, and 9/14 (64%) of the cell lines expressed cell surface DLL3. The anti-CDH19/CD3 HLE BiTE® and AMG 757 demonstrated potent cell killing of CDH19-positive or DLL3-positive melanoma cells, respectively, in TDCC assays, with EC50 values in the low picomolar range. These data show that melanoma cells that become resistant to targeted therapy and/or immune checkpoint blockade can be effectively targeted with BiTE® antibody constructs and suggest that the anti-CDH19/CD3 HLE BiTE® and AMG 757 may be potential novel therapies for treatment of relapsed and/or refractory metastatic melanoma. Citation Format: Julie M. Bailis, Fei Lee, Michael Giffin, Paul Hughes, Jennifer Tsoi, Lidia Robert, Thomas G. Graeber, Antoni Ribas, Angela Coxon. Melanoma subtypes that emerge during adaptive resistance to therapy are targets for bispecific T cell engager (BiTE®) antibody constructs directed to CDH19 and DLL3 [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 553.