Background: Acute myeloid leukemia (AML) is a malignant clonal disorder linked to a broad spectrum of molecular alterations. It is still an incurable cancer, the prognosis is even worse in elderly patients, where overall survival (OS) at 1 year is approximately 10-15%. Now, several CD3 bispecific immunotherapies that harness T cells against AML are at various stages of preclinical and clinical development, but most of them are hampered by the lack of specific targets to distinguish tumor cells from normal hematopoietic stem cells (HSC). Leukocyte immunoglobulin-like receptor B4 (LILRB4) is an immune checkpoint inhibitory receptor, which is highly expressed on FAB M4 and M5 AML cells with a lack of expression on normal HSC and progenitor cells. LILRB4 expression level inversely correlates with OS of patients with M4 and M5 AML. All of these indicate that LILRB4 represents a promising target for developing a T cell redirecting bispecific antibody to treat patients with AML, especially for more aggressive M4 and M5 subtype. Herein a novel bispecific antibody targeting LILRB4 and CD3, LBL-043, is developed to specifically kill LILRB4 positive tumor cells by engaging T cells.

Methods: LBL-043 was constructed using our proprietary LeadsBodyTM T cell engager platform, which is a 2:1 format bispecific antibody with two VHH arms targeting LILRB4 with high affinity and one scFv arm targeting CD3 with fine-tuned low affinity. The binding affinity of LBL-043 to LILRB4 and CD3 was determined with Fortebio, while the activity was measured using several cell based assays including reporter gene and TDCC assays. The anti-tumor activity of LBL-043 was investigated in B-NDG/hu-PBMC reconstructed mice implanted with MOLM-13 (human AML-M5a) tumor model.

Results: The binding affinity of LBL-043 to LILRB4 and CD3 protein was 0.724 nM and 35.7 nM, respectively. In CD3 reporter gene assays, LBL-043 could activate the NFAT reporter signaling through LILRB4 binding dependent CD3 cross-linking with an EC50 value of 2.459 nM. LBL-043 was shown a potent TDCC activity in different LILRB4 expression level MOLM-13 cells and HL-60 cells with T cell activation and cytokine release. In B-NDG/hu-PBMC reconstructed mice implanted with MOLM-13 tumor cells, LBL-043 was shown significant anti-tumor activity. Simultaneously, LBL-043 can effectively improve the survival rate and maintain relatively stable body weight in the study compared to the vehicle group.

Conclusion: LBL-043, a novel bispecific antibody targeting CD3 and LILRB4 with affinity differentiation, induces T cell killing of AML cells by simultaneously binding LILRB4 expressed on tumor cells and CD3 on T cells, redirecting T cells to efficiently killing of AML cells. It is also shown a great anti-tumor efficacy in CDX mouse models. These data support LBL-043 as a novel therapeutic bispecific antibody for the treatment of LILRB4 positive AML and CMML leukemia patients.

Citation Format: Yujia Dang, Xiao Huang, Yurong Qin, Xiaoya Liu, Min Chen, Duqing Jiang, Guojin Wu, Mi Ye, Jianming Sun, Baohui Wang, Jing Guan, Tingting Li, Jordan Zhu, Shoupeng Lai, Xiaoqiang Kang, Hong Ling. LBL-043, a novel LILRB4xCD3 T cell engager, for the treatment of LILRB4 positive leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6712.