Abstract 6080: Preferential tumor uptake and retention of EGL-001, an anti-CTLA4 -IL2 mutein fusion antibody achieving selective tumor Treg depletion

Abstract Background: Regulatory T cells (Tregs) play a pivotal role in the immunosuppressive tumor microenvironment (TME), contributing significantly to resistance against immune checkpoint inhibitors. Targeting Tregs could reinvigorate anti-tumor immune responses. Tumor-associated Tregs express CTLA-4 and CD25 at levels markedly higher than their peripheral counterparts. EGL-001 consists of an anti-CTLA-4 antibody fused to an IL-2 mutein with antagonistic properties. It blocks CD80/86 interaction with CTLA-4 and inhibits IL-2-induced STAT-5 signaling in Tregs. Importantly, EGL-001 induces CTLA-4-dependent CD25 internalization and degradation in early endosomes. Deprived of IL-2 signaling, CD25-negative Tregs undergo apoptosis. Further studies were performed to investigate EGL-001 biodistribution. Methods: The ability of EGL-001 to selectively localize in the TME, downregulate CD25 on tumor-associated Tregs in a CTLA-4-dependent manner, deplete Tregs, and restore effector T cell (Teff) activity was investigated through in vitro studies by flow cytometry and radiolabeling. Results: After administering therapeutic doses in the MC38 colorectal carcinoma model, EGL-001 binds to tumor-associated Tregs but not Tregs in the periphery. It was not binding to other T cell types in the TME. Radiolabeling with 125-Iodine enabled the study of distribution in the tumor and other organs. Following the infusion of subtherapeutic doses of 125-I-EGL-001, EGL-001 was largely cleared from all organs by Day 7, whereas tumor retention remained visible up to Day 10. Additionally, Indium labeling of EGL-001 was conducted to longitudinally and quantitatively investigate tumor uptake. 111-In-EGL-001 localized to MC38 tumor sites within 1 hour and accumulated until day 2 when it represented 5% of the infused material. EGL-001 persisted in the TME for several days while being rapidly cleared from peripheral tissues. An assessment of intratumoral distribution revealed that EGL-001 did not diffuse uniformly throughout the tumor tissue but instead concentrated in defined zones, likely corresponding to mononuclear cell infiltrates. Conclusions: Preclinical studies have demonstrated that EGL-001 exhibits potent anti-tumor activity by depleting Tregs and reactivating anti-tumor immunity. The current data indicates that EGL-001 preferentially acts within the TME, a phenomenon attributed to the strong overexpression of CTLA-4 and CD25 by tumor-associated Tregs compared to other Treg populations. This unique tumor selectivity sets EGL-001 apart from other Treg-targeting agents, presenting a promising strategy to overcome resistance to immune checkpoint blockade. EGL-001 is currently assessed in a First-In-Human clinical trial (NCT NCT06622486). Citation Format: Fiorella Kotsias, Alvaro Lladser, Marine Dubois, Caroline Janot-Sardet, Erica Russo, Anupama Karnam, Yann Courbebaisse, Mercedes Tkach, Reno Winter, Monica Gostissa, Pejvack Motlagh, Bernard Vanhove. Preferential tumor uptake and retention of EGL-001, an anti-CTLA4 -IL2 mutein fusion antibody achieving selective tumor Treg depletion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6080.