Abstract 6067: Preclinical evaluation of XB628: A novel PD-L1 x NKG2A bispecific antibody

Abstract Background: Programmed death ligand-1 (PD-L1) is highly expressed by tumor cells and upon binding to PD-1 on T cells, inhibits T cells from killing tumor cells. NKG2A is expressed by NK cells and a subset of CD8+ T cells; upon binding to HLA-E on tumor cells, NKG2A inhibits NK cell-mediated cytotoxicity against tumor cells. Thus, engagement of the PD-L1 and NKG2A immune checkpoint pathways allows tumor cells to evade both adaptive and innate immune surveillance. A phase 2 clinical trial showed a significant clinical benefit with an anti-PD-L1 and anti-NKG2A antibody combination versus an anti-PD-L1 antibody alone, 1 suggesting that dual inhibition of the PD-L1/PD-1 and NKG2A/HLA-E axes are complementary strategies. XB628 is a bispecific antibody with two distinct targeted monovalent binding domains, one each for PD-L1 and NKG2A, and is generated using the B-Body® platform. It is designed to kill tumor cells by acting as a dual checkpoint inhibitor targeting PD-L1 on tumor cells and NKG2A on NK cells, and by recruiting NK cells to tumor cells. Preclinical characterization of XB628 is reported here. Methods: Binding of XB628 to PD-L1 and NKG2A was assessed using cell lines expressing PD-L1 and NKG2A. PD-L1/PD-1 reporter and HLA-E tetramer competition assays were used to assess XB628-induced release of PD-L1/PD-1- and NKG2A/HLA-E-mediated signaling blockade, respectively. Co-localization of tumor and NK cells with XB628 was assessed by flow cytometry using cell lines expressing PD-L1 and NKG2A. T-cell activation was evaluated by measuring interferon-gamma and granzyme B secretion. The ability of XB628 to promote NK-cell mediated tumor cell killing was assessed at varying ratios of tumor and NK cells. In vivo efficacy of XB628 was assessed using a genetically engineered mouse model. Results: XB628 bound to PD-L1+ cell lines with high affinity and released PD-L1/PD-1 inhibition in a dose-dependent manner; it also bound to NKG2A+ cell lines and released NKG2A/HLA-E inhibition. XB628 induced a high degree of colocalization of PD-L1+ tumor cells and NK cells; an intrinsic property of XB628 not observed with an anti-PD-L1 and anti-NKG2A monoclonal antibody combination. XB628-induced activation of CD8+ T cells. XB628 promotion of NK cell-mediated tumor cell cytotoxicity was greater than anti-PD-1 or anti-NKG2A antibodies alone or in combination. XB628 promoted tumor cell death even when tumor cells significantly outnumbered NK cells. In a genetically engineered mouse model, XB628 demonstrated a significant dose-dependent reduction in tumor volume. Conclusions: XB628 is a first-in-class, bispecific antibody that simultaneously inhibits both adaptive and innate immune checkpoints by engaging PD-L1 and NKG2A, while also recruiting NK cells to tumor cells. XB628 is highly efficacious in tumor cell killing in vitro and in vivo, supporting further clinical development. Reference:1. Aggarwal C et al. J Clin Oncol. 2024;42(Suppl.):8046. Citation Format: Bee-Cheng Sim, Bonnie Hammer, Dileep Pulukkunat, Jing Chen, Annika Lee, Mark Bercher, Hanying Li, Jeffrey Higaki, Anna Ritter, Seema Kantak. Preclinical evaluation of XB628: A novel PD-L1 x NKG2A bispecific antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6067.