Abstract 5304: ABL407, a LILRB4x4-1BB bispecific antibody with a wild type Fc, exhibits potent antitumor activity by modulating immune system in multiple ways involving T cells, myeloids, and regulatory T cells

Abstract Tumor-associated myeloid cells such as myeloid-derived suppressor cells (MDSC) accumulate in the tumor microenvironment (TME) and suppress anti-tumor immune responses in a broad range of cancers. LILRB4 (Leukocyte Immunoglobulin-Like Receptor Subfamily B Member 4), an emerging myeloid target, is specifically expressed in tumor-associated myeloid cells and suppresses the immune response, thus promoting cancer progression and metastasis. ABL407 is a First-in-Class bispecific antibody designed to simultaneously target LILRB4 and 4-1BB, enabling LILRB4 expression-dependent 4-1BB activation. By antagonizing LILRB4 signaling, ABL407 inhibited LILRB4 mediated immunosuppression, restoring T cell activity. In addition, ABL407 inhibited fibronectin, a potential LILRB4 ligand, -mediated suppression of myeloid cells. To evaluate the efficacy of ABL407, we established EL4 murine tumor model overexpressing LILRB4 (EL4/LILRB4) in h4-1BB transgenic mice and hLILRB1/4-h4-1BB triple transgenic mice. We observed LILRB4 was overexpressed in M-MDSC in the blood and liver-metastasized tumors. A significant decrease of M-MDSC population and tumor growth inhibition was observed in mice treated with anti-LILRB4 antibody and ABL407. ABL407 treatment resulted in a proportion increment of T cells. However, the percentage of Treg cells and M-MDSC in the liver and blood was reduced by ABL407 treatment in hLILRB1/4-h4-1BB triple transgenic mice. Mice with complete remission (CR) were further protected from the rechallenge of previously exposed tumor after 3 months of cessation of ABL407 treatment, implicating that immunological memory might be generated. In conclusion, our findings suggest that ABL407, LILRB4x4-1BB bispecific antibody, may be a promising strategy for the treatment of cancer. It can be applied as a "LILRB4 antagonist" as well as a "LILRB4-dependent 4-1BB agonist" across a broad spectrum of cancers. Citation Format: Minkyu Seon, Yangsoon Lee, Hyunseong Youn, Yelim Park, Sora Kim, Kyungjin Park, Jonghwa Won. ABL407, a LILRB4x4-1BB bispecific antibody with a wild type Fc, exhibits potent antitumor activity by modulating immune system in multiple ways involving T cells, myeloids, and regulatory T cells [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 5304.