The monoclonal antibodies ipilimumab (anti-CTLA-4) and nivolumab (anti-PD-1) have shown remarkable antitumor activity in an increasing number of cancers. When combined, ipilimumab and nivolumab have demonstrated superior activity in patients with metastatic melanoma (CHECKMATE-067). Here we describe the preclinical development strategy that predicted these clinical results. Synergistic antitumor activity in mouse MC38 and CT26 colorectal tumor models was observed with concurrent, but not sequential CTLA-4 and PD-1 blockade. Significant antitumor activity was maintained using a fixed dose of anti-CTLA-4 antibody with decreasing doses of anti-PD-1 antibody in the MC38 model. Immunohistochemical and flow cytometric analyses confirmed that CD3+ T cells accumulated at the tumor margin and infiltrated the tumor mass in response to the combination therapy, resulting in favorable effector and regulatory T-cell ratios, increased pro-inflammatory cytokine secretion, and activation of tumor-specific T cells. Similarly, in vitro studies with combined ipilimumab and nivolumab showed enhanced cytokine secretion in superantigen stimulation of human peripheral blood lymphocytes and in mixed lymphocyte response assays. In a cynomolgus macaque toxicology study, dose-dependent immune-related gastrointestinal inflammation was observed with the combination therapy; this response had not been observed in previous single agent cynomolgus studies. Together, these in vitro assays and in vivo models comprise a preclinical strategy for the identification and development of highly effective antitumor combination immunotherapies.

Recent advances in cancer treatment with checkpoint blockade of receptors such as CTLA-4 and PD-1 have demonstrated that combinations of agents with complementary immunomodulatory effects have the potential to enhance antitumor activity as compared to single agents. We investigated the efficacy of immune-modulatory interleukin-21 (IL-21) combined with checkpoint blockade in several syngeneic mouse tumor models. After tumor establishment, mice were administered recombinant mouse IL-21 (mIL-21) alone or in combination with blocking monoclonal antibodies against mouse PD-1 or CTLA-4. In contrast to monotherapy, IL-21 enhanced antitumor activity of mCTLA-4 mAb in four models and anti-PD-1 mAb in two models, with evidence of synergy for one or both of the combination treatments in the EMT-6 and MC38 models. The enhanced efficacy was associated with increased intratumoral CD8+ T cell infiltrates, CD8+ T cell proliferation, and increased effector memory T cells, along with decreased frequency of central memory CD8+ T cells. In vivo depletion of CD8+ T cells abolished the antitumor activities observed for both combination and monotherapy treatments, further supporting a beneficial role for CD8+ T cells. In all studies, the combination therapies were well tolerated. These results support the hypothesis that the combination of recombinant human IL-21 with CTLA-4 or PD-1 monoclonal antibodies could lead to improved outcomes in cancer patients.

The activity of ipilimumab as a single agent and in combination with nivolumab (anti-PD-1) in melanoma, as well as the use of ipilimumab/nivolumab combinations in other malignancies, has confirmed the importance of CTLA-4 blockade in immunotherapy. The antitumor effect of this treatment also results in significant immune-related adverse events that limit dosing and result in patient discontinuation. We have taken two approaches to alter the activity of ipilimumab so as to improve its potency and its safety profile. One approach is to enhance the antibody-dependent cellular cytotoxicity (ADCC) activity of ipilimumab in order to increase the potential for Treg depletion at the tumor site; this would be expected to increase the activity of the antibody. The second approach is to produce a prodrug form of ipilimumab (an anti-CTLA-4 Probody therapeutic) that will have reduced activity systemically, but will become proteolytically cleaved at the tumor site to produce the fully functional antibody; the goal of this approach is to reduce the adverse event profile while retaining the antitumor activity of ipilimumab. It has previously been shown that antitumor activity of anti-CTLA-4 antibodies in mouse models of cancer is dependent on the ability of the antibody to bind activating FcγRs and mediate ADCC against Tregs at the tumor site (1, 2). Although human IgG1 Abs have been shown to be effective mediators of ADCC in patients with hematologic malignancies, it is still unclear whether ipilimumab mediates Treg depletion in solid tumors. Using in vitro ADCC assays, we have found that a nonfucosylated (NF) version of ipilimumab (ipilimumab-NF) has increased activity compared to ipilimumab. Ipilimumab-NF also demonstrates increased IL-2 secretion in peripheral mononuclear cells treated with the superantigen SEB as compared to ipilimumab. Transgenic mice that express human FcγRs in the place of mouse FcγRs were used to investigate the activity of anti-mouse surrogate CTLA-4 antibody engineered with either a human IgG1 or human IgG1-NF Fc region in a mouse tumor model. In these mice, the IgG1-NF version of anti-mouse CTLA-4 was found to significantly increase antitumor activity and Treg depletion at the tumor site compared to the IgG1. These data suggest that the clinical activity of ipilimumab could be enhanced by use of the nonfucosylated version of the Ab. In addition, ipilimumab-NF was tested for its ability to enhance a vaccine response in Mauritian cynomolgus macaques. Ipilimumab-NF was shown to result in increased vaccine-induced T-cell responses compared to ipilimumab using two replication-incompetent adenovirus serotype 5 viral vectors encoding SIV antigens as assessed by MHC-I tetramers and IFN-gamma ELISPOT in Mauritian cynomolgus macaques expressing the common allele, Mafa-A1*063. In a second approach, using Probody platform technology developed by CytomX, we have developed an anti-CTLA-4 Probody therapeutic (Probody Tx) based on ipilimumab. Probody Txs utilize a masking peptide that binds to the antigen-binding site of the Ab to reduce target binding. The mask extends from the light chain of the Ab via a linker sequence that contains cleavage sites for proteases preferentially active at the tumor site relative to healthy tissue. The ipilimumab Probody Tx binds to CTLA-4 with significantly lower affinity than the parental antibody and has reduced activity in in vitro assays. When tested in a mouse tumor model using human CTLA-4 KI mice, the ipilimumab-Probody Tx has comparable antitumor activity and Treg depletion at the tumor compared to ipilimumab. In contrast, ipilimumab-Probody Tx-treated mice show reduced levels of activated peripheral Tregs compared to ipilimumab-treated mice, even at doses 8-fold higher than are required for antitumor efficacy, consistent with reduced activity of the Probody Tx outside the tumor microenvironment. The development of next-generation anti-CTLA-4 antibodies holds promise for improving the utility of ipilimumab for single-agent or combination therapy. The two improvements to ipilimumab outlined above could each lead to a superior therapeutic outcome and merit further investigation. References 1. Selby MJ, Engelhardt JJ, Quigley M, Henning KA, Chen T, Srinivasan M, et al. Anti-CTLA-4 antibodies of IgG2a isotype enhance antitumor activity through reduction of intratumoral regulatory T cells. Cancer Immunol Res 2013;1:32-42. 2. Simpson TR, Li F, Montalvo-Ortiz W, Sepulveda MA, Bergerhoff K, Arce F, et al. Fc-dependent depletion of tumor-infiltrating regulatory T cells co-defines the efficacy of anti-CTLA-4 therapy against melanoma. J Exp Med 2013;210:1695-710. 1. Citation Format: Alan J. Korman, John Engelhardt, John Loffredo, Jose Valle, Rahima Akter, Raja Vuyyuru, Natalie Bezman, Paula So, Robert Graziano, Kimberly Tipton, James West, Bryan Irving, Mark Selby. Next-generation anti-CTLA-4 antibodies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr SY09-01. doi:10.1158/1538-7445.AM2017-SY09-01

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3019 Background: Interleukin 21 (IL-21), a γc chain family cytokine, is produced primarily by CD4+ T cells and has many effects on immune cells, including enhancing CD8+ T cell and NK cell proliferation and cytotoxicity. Recombinant IL-21 (rIL-21) therapy resulted in objective responses in ~20% of melanoma and renal cell carcinoma patients. In mouse models, monoclonal antibody (mAb) blockade of CTLA-4 prolongs antigen-specific T cell responses, while blockade of programmed death 1 (PD-1) reverses tumor induced T cell suppression. Ipilimumab, a CTLA-4 blocking mAb, significantly improved overall survival in patients with metastatic melanoma in 2 phase III trials, and in phase I studies a PD-1 blocking mAb (nivolumab) has antitumor activity in various cancers. Side effect profiles for each mAb have been related to their mechanism and are generally manageable. It was hypothesized that combination of IL-21 plus CTLA-4 or PD-1 blockade may enhance antitumor responses, potentially leading to improved clinical activity. Methods: Preclinical studies were conducted to test the antitumor activity of mouse IL-21 (mIL-21) in combination with an anti-mouse PD-1 (mPD-1) mAb (4H2-IgG1) or with an anti-mCTLA-4 blocking mAb (9D9-IgG2b) in syngeneic mouse tumor models, including MC38, CT-26, EMT-6, and B16F10. mIL-21 was tested at doses ranging from 50-200 μg/dose, administered up to 3d/wk. mCTLA-4 mAb or mPD-1 mAb were administered 3-4x total at 200-300 μg/dose. Results: Combination treatments produced enhanced antitumor activity vs. monotherapy. In the MC38 model, mIL-21 treatment led to 30% median tumor growth inhibition (TGI) by d29, while mPD-1 mAb produced 60% median TGI and 1/10 tumor-free mice. Combination of both agents led to synergistic antitumor activity, with complete regressions (CR) in 7/10 mice and 99.9% median TGI (p=0.046). CTLA-4 mAb + mIL-21 also produced synergistic activity in the MC38 model. By d21, mIL-21 monotherapy induced 34% TGI while CTLA-4 mAb resulted in 28% TGI, with no CR in either group. Combination resulted in 6/8 mice with CR and 86% TGI (p<0.05). Conclusions: These results support the use of rIL-21+nivolumab and rIL-21+ipilimumab in recently initiated clinical trials.