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Background Clinical data demonstrates increased antigen presentation diversity is a key factor in determining response rates to checkpoint inhibitors.1 In addition to tumour mutational burden/microsatellite instability, increased HLA heterozygosity and HLA evolutionary diversity are non-overlapping factors recently identified to further diversify the immunopeptidome and improve clinical response to checkpoint therapies.2 3 Endoplasmic reticulum aminopeptidase 1 (ERAP1) is an enzyme that trims peptides loaded into classical and nonclassical class I MHC molecules.4 5 Ablation of mouse ERAAP modifies the immunopeptidome, resulting in improved immunogenicity, generation of CD8 T cell responses and tumor growth inhibition.6 7 Recently identified selective small molecules potently inhibit ERAP1 across key species and haplotypes.8 We report the further profiling of lead candidate ERAP1 inhibitors in human primary T cell in vitro assays and in vivo tumor models in mice. Methods Human cancer cell lines treated with ERAP1 inhibitors in vitro or in vivo in xenograft mouse models were assessed by immunopeptidomics9 to profile peptide repertoire changes. Novel or upregulated peptides were also tested in human immunogenicity assays. FACS analysis of T cells stimulated with Tyrosinase mRNA transfected human dendritic cells ± ERAP1 inhibition was to assess T cell repertoire changes. ERAP1 inhibitor and anti PD-1 mAb combination was assessed in syngeneic mouse tumor models to investigate tumour growth inhibition and PD end-points (e.g. IHC). Results Analysis of human cervical, lung, colorectal and melanoma cell lines carrying distinct HLA haplotypes demonstrates a consistent and profound effect of ERAP1 inhibition on the immunopeptidome. Novel and upregulated cancer associated antigens identified in association with multiple different HLA-A and B alleles stimulate IFNγ production in primary naive human T cell immunogenicity assays. The impact of ERAP1 inhibition on the T cell repertoire to the melanoma antigen tyrosinase is ongoing. The combination of ERAP1 inhibitor and anti PD-1 mAb led to significant tumor growth inhibition in the CT26 syngeneic mouse tumor model that correlated with increased infiltration of T cells to the tumor. Further PD end-points to be analysed include immune gene array and TCR Vbeta repertoire. Conclusions Grey Wolf ERAP1 inhibitors significantly modify the immunopeptidome both in vitro and in vivo across a broad range of HLA and tumor types. Combination of these inhibitors with anti PD-1 leads to significant T cell infiltration and tumor growth inhibition. Thus, ERAP1 mediated modulation of the immunopeptidome has the potential to drive anti tumor T cell responses and be a transformative immunotherapy. References Rizvi N, Hellmann MD, Snyder A, et al. Mutational landscape determines sensitivity to PD-1 blockade in non–small cell lung cancer. Science. 2015;348(6230):124–128. Chowell D, Morris LGT, Grigg CM, et al. Patient HLA class I genotype influences cancer response to checkpoint blockade immunotherapy. Science 2018;359 (6375):582–587. Chowell D, Krishna C, Pierini F, et al. Evolutionary divergence of HLA class I genotype impacts efficacy of cancer immunotherapy. Nature Medicine 2019;25(11):1715–1720. Shastri N, Nagarajan N, Lind KC, et al. Monitoring peptide processing for MHC class I molecules in the endoplasmic reticulum. Curr Opin Immunol 2014; 26:123–127. Mpakali A, Maben Z, Stern LJ, et al. Molecular pathways for antigenic peptide generation by ER aminopeptidase 1. Mol Immunol 2018; 13:50–57. James E, Bailey I, Sugiyarto G, et al. Induction of protective antitumor immunity through attenuation of ERAAP function. J Immunol 2013;190(11):5839–5846. Manguso RT, Pope HW, Zimmer MD, et al. In vivo CRISPR screening identifies Ptpn2 as a cancer immunotherapy target. Nature 2017;547(7664):413–418. Leonard, H Remtulla A, Poynton F, et al. AACR Annual Meeting 2020. Purcell AW, Ramarathinam SH, Ternette N. Mass spectrometry–based identification of MHC-bound peptides for immunopeptidomics. Nat Protoc 2019;14(6):1687–1707.

Immune checkpoint therapy has changed the cancer treatment paradigm, however the majority of patients respond poorly to T cell checkpoint blockade alone. Emerging evidence suggests factors that increase the diversity of the immunopeptidome such as high tumor mutation burden and HLA-I evolutionary divergence (HED), improve clinical response to checkpoint blockade providing an impetus to develop strategies that increase the presentation of cancer associated antigens to the immune system. Endoplasmic reticulum aminopeptidase 1 (ERAP1) is an aminopeptidase that trims peptides loaded into classical and nonclassical MHC molecules. Human genome-wide association studies have identified single nucleotide polymorphisms within ERAP1 that are associated with immune-related diseases, such as ankylosing spondylitis, providing human genetic validation for ERAP1's role in human disease and antigen presentation. Furthermore, in mice, genetic ablation, reduction or pharmacological inhibition of ERAP1 changes the immune repertoire, creating novel antigens with improved immunogenicity that lead to the generation of productive CD8 T cell responses and tumor growth inhibition in various syngeneic models. We have generated and profiled highly potent, species cross reactive and selective ERAP1 inhibitors through a suite of in vitro and in vivo assays to develop these as novel immunotherapy agents. In vitro CETSA measurements show potent cellular target engagement that correlates with in vitro SPR and enzyme potency. Using the model antigen SIINFEKL and Class I MHC expression as key pharmacodynamic measurements, we demonstrate Class I modulation in vitro and in vivo in line with published data from ERAP1 knockout mice. These assays are being used to elucidate the kinetics and temporal dynamics of the antigen change to optimise dosing schedules for in vivo studies. Implementation of our immunopeptidomics pipeline has demonstrated treatment with ERAP1 inhibitors both generate novel antigens in vitro and in vivo and lead to profound and consistent increased overall immunopeptidome length in mouse and human cells. To establish the functional consequence of these changes, we developed a human primary T cell immunogenicity platform. The immunogenic potential of these novel cancer associated antigens has been confirmed by the ability to stimulate IFNγ production in naïve T cells and suggests responses to these antigens could reinvigorate anti-tumor responses. Extensive assessment of mouse and human CD8 T cells responses is ongoing, in order to characterise and select Grey Wolf Therapeutics' first lead ERAP1 inhibitor for use as monotherapy or in combination with other immunotherapies such as checkpoint blockade. Citation Format: Jamie Ware, Patrick McIntyre, Kristopher Clark, Carmen Tong, Jason Shiers, Elisa Lori, Camila de Almeida, Emma Reeves, Henry Leonard, Alihussein Remtulla, Michael Ford, Nicola Ternette, Fergus Poynton, Edd James, Lesley Young, Martin Quibell, Peter I. Joyce. Potent oral ERAP1 inhibitors modify the immunopeptidome in vivo and are novel immunotherapy agents [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5551.