AbstractTumor-associated macrophages (TAMs) primarily exist in the M2-like phenotype in the tumor microenvironment (TME). M2-TAMs contribute to tumor progression by establishing an immunosuppressive environment. However, TAM targeting is hindered, mainly owing to a lack of specific biomarkers for M2-TAMs. Herein, we verified that TB511, a fusion peptide comprising a TAM-targeting peptide and the pro-apoptotic peptide dKLA, could preferentially bind to active CD18 in M2-TAMs, inducing apoptosis; it was less toxic to other tissue-resident macrophage lineages. TB511 suppressed tumor progression in various cancer models and humanized mouse models. Removal of M2-TAMs by TB511 significantly increased the infiltration of tumor-killing cells, such as granzyme B-positive CD8+ T cells and natural killer cells, in the TME. Collectively, our findings suggest that targeting CD18 in M2-TAMs using peptide drugs is a potential strategy for immunotherapy in various tumors, including non-small cell lung cancer, colorectal cancer, hepatocellular carcinoma, renal cell carcinoma, and prostate cancer.Citation Format: Ilsoeb Choi, Ik-Hwan Han, Soyoung Kim, Juwon Yang, Hye Yeon Kim, Daehwan Park, Hongseo Choi, Jeongyoon Choi, Heekyung Lee, Jin Sun Shin, Jinho Kim, Jihwan Moon, Hyunsu Bae. The apoptotic peptide TB511 targets activated CD18 and suppresses solid tumor progression [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 2901.