Abstract 3538: A CTLA-4 targeted ETB for Treg depletion shows favorable preclinical efficacy and safety

Abstract

Abstract CTLA-4-targeted engineered toxin bodies (ETBs) are designed to deplete immune-suppressive regulatory T cells (Tregs) in the tumor microenvironment (TME) directly and in a manner independent of Fc-mediated effector functions, offering a unique approach to CTLA-4 targeted therapy that may show efficacy where blocking antibodies have failed. ETBs are large molecule proteins consisting of an antibody fragment genetically fused to a proprietary de-immunized (DI) form of the Shiga-like toxin-1 subunit A (SLTA). Once engaged to the specific cell surface target of interest, ETBs internalize, route to the cytosol, and permanently inactivate ribosomes through an irreversible enzymatic process. This results in cell death via apoptotic mechanisms. Here we describe the preclinical characterization of a candidate CTLA-4-targeted ETB. The candidate ETB kills gain-of-function model cell lines that express CTLA-4 at levels observed on primary human tumor Tregs. Notably, the potency of the candidate ETB is limited when CTLA-4 is expressed at low levels, thereby allowing a mechanism to spare CD8+ cytotoxic T lymphocytes (CTLs) and effector CD4+ T cells from targeted depletion. The candidate ETB binds CTLA-4 with high affinity utilizing a unique biparatopic binding domain. Using in vitro systems, we show that the candidate ETB can block CTLA-4:B7 interactions and repress Treg-mediated T-cell suppression. In a syngeneic mouse pharmacodynamic model, the candidate ETB depletes tumor-resident Tregs and increases the CD8:Treg ratio in the TME. A toxicological study in naive NHP demonstrate that the candidate ETB, which is cross-reactive to NHP, is well-tolerated at 450 mcg/kg (highest dose tested) administered intravenously once weekly for 4-weeks and does not significantly alter the CTLA-4 low/null peripheral T cell populations. Overall, the candidate ETB is a unique CTLA-4 targeting modality that is designed to deplete Tregs selectively and directly in the tumor, spare CD8+ CTLs, and represents an alternative treatment option to traditional blocking antibodies. Development activities are underway to support the upcoming IND application. Citation Format: Asis Sarkar, Rebecca Martin, Lauren R. Byrne, Caleigh Howard, Swati Khanna, Lilia A. Rabia, Diana Adhikari, Michaela M. Sousares, Alvaro Aldana, Garrett L. Robinson, Jay Zhao, Chris B. Moore, Aimee Iberg. A CTLA-4 targeted ETB for Treg depletion shows favorable preclinical efficacy and safety [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3538.