Abstract 554: Tumor-antigen 5T4-dependent activation of the CD137 costimulatory pathway by bispecific 5T4 x CD137 x CD137 TRIDENT™ molecules

Introduction: Trophoblast glycoprotein 5T4 is expressed on the cell surface of multiple cancers yet sparingly on normal adult tissues. CD137 (4-1BB) is a co-stimulatory molecule expressed by activated T and NK cells that support cell activation, proliferation and survival. Previously we described bispecific tumor associated antigen x CD137 DART® molecules limiting CD137 mediated immunostimulation to the tumor microenvironment (AACR2017) as an approach to reduce the unwanted systemic CD137 effects associated with agonistic anti-CD137 mAbs. We subsequently identified a bispecific TRIDENT format bearing bivalent CD137 and monovalent tumor antigen engagement that provides maximal CD137 activation in a tumor antigen anchored manner. Here we apply that format to generate a 5T4 x CD137 x CD137 TRIDENT molecule that promotes CD137 activation in proximity to 5T4-expressing tumor cells leading to enhanced T-cell co-stimulation, proliferation and tumor cell killing activity when combined with CD3-engaging tumor-targeting bispecific DART molecules. Methods: TRIDENT molecules were constructed comprising bivalent CD137 binding and monovalent 5T4 binding. Binding properties were evaluated by surface plasmon resonance (SPR) and flow cytometry. Signaling was assessed using a NF-κB luciferase reporter cell line expressing CD137. Co-stimulatory activity was characterized with primary human or cynomolgus monkey T cells. T cells were incubated with or without antigen-expressing cells and submaximal concentrations of either anti-CD3 bead or CD3-engaging tumor-targeting bispecific DART molecules. Results: SPR and flow cytometry analyses demonstrate that 5T4 x CD137 x CD137 TRIDENT molecule binds human and monkey target antigens. The 5T4 x CD137 x CD137 TRIDENT molecule induces CD137 signaling in a reporter cell line and promotes cytokine release in primary human & cynomolgus monkey T-cell assays in the presence of 5T4+ tumor cells. In the absence of 5T4+ tumor cells, the TRIDENT molecule lacks agonistic activity. Furthermore, 5T4 x CD137 x CD137 TRIDENT molecule enhances T-cell proliferation and tumor cell cytolysis induced by CD3-targeted DART molecules. Consistent with preferential induction of CD137 by CD8 T-cells, 5T4 x CD137 x CD137 TRIDENT increases the fraction of CD8+ central memory and effector memory T cells in the presence 5T4+ tumor cells. Conclusions: 5T4 x CD137 x CD137 TRIDENT molecule promotes T-cell co-stimulation in a tumor antigen-dependent manner offering an opportunity to target CD137 immunostimulation, while potentially limiting non-specific systemic T-cell activation and related side effects. Citation Format: Liqin Liu, Ling Huang, Vatana Long, Yinhua Yang, Robert Burns, Jonathan Li, Jennifer DiChiara, Qihong Xu, Arin Whiddon, Chia-Ying K Lam, Jim Tamura, Valentina Ciccarone, Syd Johnson, Ezio Bonvini, Paul Moore. Tumor-antigen 5T4-dependent activation of the CD137 costimulatory pathway by bispecific 5T4 x CD137 x CD137 TRIDENT™ molecules [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 554.