The MEK inhibitor selumetinib complements CTLA-4 blockade by reprogramming the tumor immune microenvironment

Edmund Poon,Marie Cumberbatch,Margaret Veldman-Jones,Amanda Watkins,Ross Stewart,Vasu Sah,Alwin Schuller,Geoffrey S Williams,Corrine Reimer,Paul D Smith,Jens-Oliver Koopmann,Robert W Wilkinson,Stefanie Mullins,Shaun E Grosskurth,Gianfranco Di Genova,Simon J Dovedi

doi:10.1186/s40425-017-0268-8

Abstract

BackgroundT-cell checkpoint blockade and MEK inhibitor combinations are under clinical investigation. Despite progress elucidating the immuno-modulatory effects of MEK inhibitors as standalone therapies, the impact of MEK inhibition on the activity of T-cell checkpoint inhibitors remains incompletely understood. Here we sought to characterize the combined effects of MEK inhibition and anti-CTLA-4 mAb (anti-CTLA-4) therapy, examining effects on both T-cells and tumor microenvironment (TME).MethodsIn mice, the effects of MEK inhibition, via selumetinib, and anti-CTLA-4 on immune responses to keyhole limpet haemocyanin (KLH) immunization were monitored using ex vivo functional assays with splenocytes. In a KRAS-mutant CT26 mouse colorectal cancer model, the impact on the tumor microenvironment (TME) and the spleen were evaluated by flow cytometry. The TME was further examined by gene expression and immunohistochemical analyses. The combination and sequencing of selumetinib and anti-CTLA-4 were also evaluated in efficacy studies using the CT26 mouse syngeneic model.ResultsAnti-CTLA-4 enhanced the generation of KLH specific immunity following KLH immunization in vivo; selumetinib was found to reduce, but did not prevent, this enhancement of immune response by anti-CTLA-4 in vivo. In the CT26 mouse model, anti-CTLA-4 treatment led to higher expression levels of the immunosuppressive mediators, Cox-2 and Arg1 in the TME. Combination of anti-CTLA-4 with selumetinib negated this up-regulation of Cox-2 and Arg1, reduced the frequency of CD11+ Ly6G+ myeloid cells, and led to the accumulation of differentiating monocytes at the Ly6C+ MHC+ intermediate state in the tumor. We also report that MEK inhibition had limited impact on anti-CTLA-4-mediated increases in T-cell infiltration and T-cell activation in CT26 tumors. Finally, we show that pre-treatment, but not concurrent treatment, with selumetinib enhanced the anti-tumor activity of anti-CTLA-4 in the CT26 model.ConclusionThese data provide evidence that MEK inhibition can lead to changes in myeloid cells and immunosuppressive factors in the tumor, thus potentially conditioning the TME to facilitate improved response to anti-CTLA-4 treatment. In summary, the use of MEK inhibitors to alter the TME as an approach to enhance the activities of immune checkpoint inhibitors warrants further investigation in clinical trials.

Highlights

T-cell checkpoint blockade and Mitogen-activated protein kinase kinase and/or 2 (MEK) inhibitor combinations are under clinical investigation
Selumetinib inhibits T-cell activation in response to Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) blockade in vitro Since RAS-Mitogenactivated protein kinases (MAPK) signalling via MEK is downstream of the T-cell receptor [18], we first confirmed the inhibitory effect of selumetinib on T-cell activation
We demonstrated for the first time that MEK inhibition by selumetinib resulted in the accumulation of CD11b+ Ly6C+ MHCII+ cells within the tumor microenvironment (TME) of tumor-bearing mice; this is a subset of myeloid cells associated with an intermediate state in the differentiation of infiltrating monocytes into macrophages at sites of inflammation [21], or into tumor-associated macrophages (TAM) in cancer [22,23,24]

Summary

Introduction

T-cell checkpoint blockade and MEK inhibitor combinations are under clinical investigation. Antibodies targeting T-cell checkpoint molecules, such as anti-PD-1/PD-L1 and anti-CTLA-4, have been shown to deliver long-term benefits for a subset of cancer patients [1, 2]. Small molecule inhibitors of MEK have been used in a range of cancer indications and have shown activity [5]. In these cancer settings, patients may benefit from MEK inhibitors used in combination with checkpoint blockade. Recent in vivo studies in mouse models of KRAS-mutant colorectal cancer [7, 8], BRAFV600E-mutant melanoma [9], and triplenegative breast cancer [10] demonstrated that the combination of MEK inhibitors and antibodies targeting PD1 or PD-L1 resulted in superior anti-tumor efficacy compared to single agents. The MEK inhibitor trametinib was found to synergise with adoptive T-cell transfer [9] and anti-CTLA-4 [8] therapies in mouse tumor models

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