Cyproheptadine and Derivatives Inhibit Tumor Growth via NK Cell-Mediated Responses

Bladder cancer, the second most common malignancy of the urothelial system, exhibits a strong capacity to evade innate immune surveillance through multiple pathways, underscoring the need for innovative therapeutic strategies. We previously observed that cyproheptadine (CPH) induces anti-tumor effects in bladder cancer, although the underlying mechanism remained unclear. In this study, RNA-seq analysis of CPH-treated BFTC905 bladder cancer cells revealed significant enrichment of natural killer (NK) cell-mediated anti-tumor immune responses following CPH exposure. Using in vitro co-culture systems, we demonstrated that pre-treatment with CPH or CPH-SAHA conjugates markedly enhanced NK cell-mediated killing of bladder cancer cell lines by NK-92 cells and by human primary NK cells. This enhancement results from the epigenetic activation of NKG2D ligands, driven by increased H3K27ac and reduced H3K27me3 enrichment at the ULBP2 promoter in bladder cancer cells. In vivo, treatment with CPH in a syngeneic MB49 bladder cancer mouse model significantly suppressed tumor growth and increased NK cell infiltration. This elevated infiltration is likely mediated by upregulated expression of the NK-recruiting chemokine CCL3 in CPH-treated urothelial cancer cells. In conclusion, our findings demonstrate that CPH suppresses tumor growth and enhances immune surveillance by modulating epigenetic pathways that activate NK cell responses. These results highlight CPH and its derivatives as promising candidates for developing new immunomodulatory therapies for bladder cancer. Citation Format: Himani Kumari, Chin Pui Chan, Ciao-Ni Chen, Hsin-An Shih, Tsung-Yu Tseng, Chao-Ling Yao, Steven Lin, Cheng-Huang Shen, Michael W.Y. Chan. Cyproheptadine and its derivatives inhibit tumor growth by eliciting NK cell-mediated anti-tumor responses in bladder cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1594.