Abstract 2665: Fc silencing of commonly used surrogate anti-PD-1 RMP1.14 increases its therapeutic profile in MC38 syngeneic model

Abstract Introduction: Anti-PD1 treatment has become a backbone treatment in several cancers such as melanoma and lung carcinoma. Adding new drugs to PD-1 requires careful evaluation of combination treatments in preclinical models to evaluate efficacy and tolerability. Syngeneic tumor model remains one of the best way of preclinical evaluation of combinations, as such evaluation requires fully immune competent animals to correctly assess both the PD-1 contribution and the added value or synergistic effect of new drugs in the combination. Anti mouse PD-1 such as RMP1.14, a rat IgG2a, is widely used in these models as anti-PD-1 mAb surrogate for such studies. Although RMP1.14 demonstrated efficacy in several models such as MC38, and CT26, it may not fully recapitulate the use of blocking mAb in the human as it may be immunogenic in mice and may retain some agonist activity due to its rat isotype. Here, we demonstrate that engineering this antibody with a mice Fc silent backbone increases its activity in MC38 model. Methods: VH/VL from RMP1.14 rat IgG2a (mPD1) were sequenced and engineered with a mouse Fc silent IgG1 backbone (N297Q) and produced in CHO. Both antibodies (mPD-1: rat IgG2a; and MOS2-mPD-1: Fc silent mouse IgG1) were purified on protein A using standard procedures. MC38 (1million cells) were injected subcutaneously in C57BL/6 mice, and randomized when tumor volume reached 100mm3. Dose effect of mPD-1 and MOS2-mPD-1 and their corresponding isotype controls were tested in this MC38 model at doses of 0.5; 1.25; 2.5 and 12.5 mg/kg at day 0, day 3, and day 7 post randomization (8 mice per group). Treatment route effect was tested in the same model by comparing intraperitoneal and intravenous injections in an additional experiment using both antibodies at 1.25 mg/kg and 12.5 mg/kg doses. Results: Both mPD-1 and MOS2-mPD-1 demonstrated efficacy in survival in MC38 model at all tested doses (p<0,05 Mantel-Cox test) compared to their isotype controls. MOS2-mPD-1 demonstrated increased efficacy in terms of complete responses in the above escalated doses (CR of 12.5, 12.5, 12.5 and 37.5 % and 0, 75, 50 and 62.5 % for mPD-1 and MOS2-mPD-1 respectively). Tumor volumes and survival were also always lower in the MOS2-mPD-1 groups compared to dose corresponding mPD-1 groups and were statistically different in two independent experiments at medium doses (P<0,05 Dunn’s test performed at 1.25mg/kg at day 27, and Mantel-Cox test). IP and IV routes were equivalent in efficacy at the tested doses. Conclusion: Mouse Fc engineered surrogate have significantly different behavior in mice models compared to the commonly used surrogates usually from rat origin. These new formatted antibodies should better mimick the efficacy and tolerability of antibodies used in human, and should be preferred when new combinations of treatments are tested. Citation Format: Ester Morgado, Laurent Pouyet, Olivier Deas, François Romagne. Fc silencing of commonly used surrogate anti-PD-1 RMP1.14 increases its therapeutic profile in MC38 syngeneic model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2665.