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Ipilimumab is a fully human immunoglobulin G1 (IgG1) monoclonal antibody against cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), an inhibitory receptor expressed on activated effector T cells and regulatory T cells (Treg), that inhibits the binding of CTLA4 to B7 ligands. BMS-986218 and BMS-986249 are second generation molecules that share the same amino acid sequence and ligand blocking properties as ipilimumab, but are mechanistically distinct. BMS-986218 is non-fucosylated (NF) and has an increased affinity for the activating Fcγ receptor (FcγR, CD16) affording the possibility of increased anti-tumor activity via depletion of Treg in the tumor. In contrast, BMS-986249 is a Probody™ of ipilimumab that has a masking peptide covering the active antigen-binding site of the antibody which is clipped by specific proteases within tumors, exposing the fully active antibody, and potentially offering reduced systemic toxicity liabilities with comparable efficacy to ipilimumab. In 1-month toxicity studies in monkeys (n = 5/sex/group) at weekly doses of 3, 15, or 75 mg/kg IV of BMS-986218 or 10 or 50 mg/kg IV of BMS-986249 or ipilimumab, profound enhancement of peripheral T-cell activation occurred in a dose-dependent manner for all 3 compounds following neoantigen immunization (keyhole limpet hemocyanin [KLH], HIV necessary and enforcing factor [Nef], and HIV group specific antigen [Gag] peptides), consistent with target pharmacology. Consistent with the intended mechanistic differences, peripheral T cell activation was generally increased at corresponding doses of BMS-986218 compared to ipilimumab, and was delayed and reduced in monkeys given BMS-986249. BMS-986218, ipilimumab, and BMS-986249 were clinically tolerated by monkeys at doses up to 3, 10, and 50 mg/kg, respectively, with generally mild, loose feces in some monkeys and/or minimal body weight decrease. At higher doses, early euthanasia occurred for 1 and 6 monkeys at 15 and 75 mg/kg BMS-986218, respectively, from Days 22-53 and 1 monkey at 50 mg/kg ipilimumab on Day 55 due to profound clinical toxicity. The predominant microscopic finding was generally dose-related lymphohistiocytic inflammation within a variety of tissues at all doses for all compounds, with BMS-986218 resulting in the greatest incidence, severity, and distribution of tissues and BMS-986249 having the least effects. The GI tract (stomach, cecum, and colon) and the kidney were the most severely and consistently affected, whereas additional organs were affected at higher doses. Most changes were partially or fully reversible during an 8-week recovery period with the exceptions of one monkey at 75 mg/kg BMS-986218 and one monkey at 50 mg/kg ipilimumab that were euthanatized on Days 53 or 55 due to unresolved GI toxicity, persistent lymphohistocytic inflammation, and/or unchanged or progressive increases in AST and ALT. Based on the tolerability and generally mild severity of lymphohistiocytic tissue inflammation, the highest non-severely toxic doses (HNSTD) for BMS-986218, ipilimumab, and BMS-986249 in monkeys following 1 month of dosing were 3 mg/kg (mean AUC 0-168h = 11,300 μg•h/mL), 10 mg/kg (AUC[0-168h] =44,600 µg·h/mL), and 50 mg/kg (AUC[0-168h] =205,000 µg·h/mL), respectively. Overall, these results support the potential of these 2nd generation anti-CTLA4 antibodies to offer an improved risk /benefit profile with increased activity of the NF variant and improved safety of the Probody™ relative to ipilimumab. Citation Format: Karen D. Price, Frank Simutis, Anthony Fletcher, Lila Ramaiah, Rima Srour, John Kozlosky, Jean Sathish, John Engelhardt, Annette Capozzi, James Crona, Courtni Newsome, Jennifer Wheeler, Daniel Szatkowski, Austin Thekkumthala, Bojing Wang, Wendy Freebern, Helen Haggerty, Todd Bunch, Michael Graziano. Nonclinical safety evaluation of two distinct second generation variants of anti-CTLA4 monoclonal antibody, ipilimumab, in monkeys [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr LB-B33.