FL116, a PD-1/IL-18 bispecific antibody, enables cis-activation of PD-1⁺ T cells and reshapes the suppressive tumor microenvironment (TME)

Background: IL-18 is a potent immune cytokine that promotes CD8⁺ T-cell and NK-cell-mediated antitumor responses but is neutralized by IL-18BP. We previously engineered a panel of IL-18 variants which are completely resistant to IL-18BP inhibition. FL116 is a dual-function immunocytokine, in which an anti-PD-1 IgG was fused with one such IL-18 variant to achieve cis-delivery of IL-18R agonism to PD-1⁺ T cells while bypassing IL-18BP in TME. The molecule aims to re-ignite pre-existing tumor immunity within the TME with minimal systemic toxicity. Methods: FL116 was constructed by fusing the DNA coding sequence of an IL-18 variant that fully escapes IL-18BP neutralization with that of one heavy chain of an anti-PD-1 IgG to produce recombinant protein. Both binding domains of FL116 were confirmed by ELISA. The cis-acting of FL116 was evaluated by an engineered mammalian cell line which expresses both IL-18 receptors and PD-1. In vitro, the IFN-gamma-inducing activity of FL116 was further validated using PBMC assays. In vivo, FL116 was assessed in MC38, CT26 and B16-F10 tumor models to evaluate its antitumor efficacy and safety. Results: Both parts of FL116 preserved their binding and biological functions as validated by ELISA and PBMC assays. In vivo, FL116 produced strong and durable antitumor activity across tumor models. In MC38, FL116 achieved 98% tumor growth inhibition (TGI) and 2/5 complete response(CR). In CT26, FL116 showed 93% TGI and 3/5 CR. CT26 rechallenge revealed cured mice fully rejected tumor re-implantation without further treatment, indicating durable immune memory. In B16-F10 tumors, FL116 induced dose-dependent tumor regression and elevated intratumoral IFN-gamma(8-fold) compared to the vehicle treatment, consistent with type-1 immune activation. TIL profiling showed substantial expansion of CD8⁺ Tem(6-fold) and NK cells, increased cytotoxic markers(granzyme B), and enhanced DC activation. Suppressive myeloid cells were significantly reduced, with PMN-MDSC decreasing ~5-fold, leading to improved effector/suppressor ratios (CD8/MDSC 20-fold, CD8/Treg 6-fold). Peripheral blood composition remained stable, with slightly increases in CD8⁺ Tem and NK maturation, indicating localized pharmacodynamic amplification rather than systemic inflammation. No body-weight loss was observed, and only mild, pharmacologically driven hepatosplenic enlargement occurred. Conclusions: FL116 achieves potent antitumor activities, high CR rates, and rechallenge-confirmed immune memory while maintaining systemic immune homeostasis. By delivering decoy resistant IL-18 variant directly to PD-1⁺ T cells, FL116 re-ignites intratumoral cytotoxic immunity and reshapes the TME toward a highly inflamed, effector-dominant state. These results support FL116 as a promising candidate for next-generation cytokine-fusion immunotherapy.