Abstract 552: Anti-tumor effects of anti-neoantigen antibodies: Tumor growth retardation and increased survival in a syngeneic model of cancer

This study was designed to decrease tumor growth by treatment with multiple anti-neoantigen antibodies (Abs) combined with PD1 inhibition. Dramatic reduction in tumor growth was observed. Despite tremendous excitement of checkpoint inhibitors only a minority of patients have durable complete responses. Success of checkpoint inhibitors appears related to the immune response to neoantigens. We investigated the potential enhancement of checkpoint inhibitors by directing a cocktail of antibodies to multiple neoantigens simultaneous with PD1 inhibition. The choice of neoantigens was simply based on the potential rabbit humoral immunogenicity of a mutated epitope present at the tumor cell surface or secretary proteins, without assigning any weightage to the protein function. Rabbits were vaccinated with each of the 9 selected mutated epitopes of B16-F10 melanoma tumor proteins for generating antibodies. Rabbit polyclonal Abs were affinity purified for each antigen with EC50s mostly in picomolar range. All 9 polyclonal antibodies were mixed equally as a cocktail for treatment experiments. The tumor model was C57BL/J strain of mice implanted with 3x10 5 B16-F10 melanoma tumor cells in their dorsal flank region. The treatment with anti-neoantigen Abs cocktail were administered in two different schedules; 1) a single treatment by implanting tumor cells suspended in 0.2 mg Abs cocktail/mouse and 2) local injection (0.2 mg Abs cocktail/mouse) into the tumor implantation site on 3, 6, 9 and 13 days post-tumor implantation. Except for a completely untreated control group all mice received 4 intraperitoneal injections (0.2 mg/mouse) of anti-mouse PD1 Ab (clone RMP1-14) or its isotype control Ab (IgG2a, k). Normal rabbit polyclonal IgG was used as control against the 9 anti-neoantigen Abs cocktail. No treatment resulted in rapid growth of large tumors and these untreated mice either died or were euthanized by day 20. The treatment with anti-PD1 Ab or its isotype control Ab alone did not affect tumor growth and survival in comparison to untreated mice. Furthermore, PD1 inhibition combined with normal rabbit IgG had no effect on tumor growth or survival. However, the single treatment of tumor cells with the 9 Abs cocktail during implantation combined with PD1 inhibition significantly retarded tumor growth and increased survival. The results were even more dramatic with the group of mice that were implanted with tumor cells in PBS and received 4 treatments of the 9 Abs cocktail in combination to PD1 inhibition. All the animals in this group had substantial retardation of tumor growth and 50% of the mice have shown no tumor whatsoever out to post-implantation day sixty. This is the first study to show anti-tumor effects of multiple anti-neoantigen Abs. These results are important particularly in a poorly immunogenic tumor model which is non-responsive to PD1 checkpoint inhibitor therapy alone. Citation Format: Girja S. Shukla, Yu-Jing Sun, Stephanie C. Pero, David N. Krag. Anti-tumor effects of anti-neoantigen antibodies: Tumor growth retardation and increased survival in a syngeneic model of cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 552.