Marin‑Acevedo et al. J Hematol Oncol (2021) 14:45 https://doi.org/10.1186/s13045‑021‑01056‑8 REVIEW Open Access Next generation of immune checkpoint inhibitors and beyond Julian A. Marin‑Acevedo1, ErinMarie O. Kimbrough2 and Yanyan Lou2* Abstract The immune system is the core defense against cancer development and progression. Failure of the immune system to recognize and eliminate malignant cells plays an important role in the pathogenesis of cancer. Tumor cells evade immune recognition, in part, due to the immunosuppressive features of the tumor microenvironment. Immuno‑ therapy augments the host immune system to generate an antitumor efect. Immune checkpoints are pathways with inhibitory or stimulatory features that maintain self‑tolerance and assist with immune response. The most well‑ described checkpoints are inhibitory in nature and include the cytotoxic T lymphocyte‑associated molecule‑4 (CTLA‑ 4), programmed cell death receptor‑1 (PD‑1), and programmed cell death ligand‑1 (PD‑L1). Molecules that block these pathways to enhance the host immunologic activity against tumors have been developed and become stand‑ ard of care in the treatment of many malignancies. Only a small percentage of patients have meaningful responses to these treatments, however. New pathways and molecules are being explored in an attempt to improve responses and application of immune checkpoint inhibition therapy. In this review, we aim to elucidate these novel immune inhibi‑ tory pathways, potential therapeutic molecules that are under development, and outline particular advantages and challenges with the use of each one of them. Keywords: Cancer, Immunotherapy, Tumor microenvironment, Immune evasion, Cytotoxic T lymphocytes, Immunotherapy, Immune checkpoint therapy, Inhibitory pathways Background signaling maintained by immune checkpoints. Tese Until recently, chemotherapy, radiation, and surgery immune checkpoints are a set of inhibitory and stimula- were considered the cornerstones of cancer treatment. tory pathways that directly afect the function of immune In 2011, with the approval of ipilimumab [1], immune cells [2]. Malignant cells disrupt this balance by promot- checkpoint inhibitors were added to the therapeutic arse- ing an immunosuppressive state that favors immune nal and revolutionized cancer management. Tese drugs evasion and tumor growth [2, 3]. Cancer cells recruit not only introduced a new mechanism to treat cancer but regulatory T cells (Tregs), downregulate tumor antigen also, in select cases, allowed for durable responses with a expression, induce T cell tolerance and/or apoptosis, less toxic profle. and produce immune suppressive cytokines that stimu- In contrast to old cytotoxic therapies, immune check- late inhibitory immune checkpoints [3]. Tis leads to a point inhibitors augment the host immune system to unique and highly immunosuppressive tumor microen- fght cancer. Under homeostatic conditions, there is a bal- vironment (TME) [4]. In an attempt to overcome these ance between pro-infammatory and anti-infammatory immunosuppressive conditions, immune checkpoint inhibitors act by blocking the efects of selected inhibi- *Correspondence: [email protected] tory pathways [2, 3, 5]. 2 Division of Hematology and Oncology, Mayo Clinic, 4500 San Pablo Te best described inhibitory immune checkpoints Road, Jacksonville, FL 32224, USA are cytotoxic T lymphocyte-associated molecule-4 Full list of author information is available at the end of the article © The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/publi cdoma in/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Marin‑Acevedo et al. J Hematol Oncol (2021) 14:45 Page 2 of 29 (CTLA-4), programmed cell death receptor-1 (PD-1), from ClinicalTrials.gov. We included information from and programmed cell death receptor-1 ligand (PD-L1). T February 1, 2018, through June 1, 2020. We focused on cell receptors (TCR) activate T cells. CTLA-4 is a mole- phase I and phase II clinical trials using novel agents cule that is upregulated on the surface of active T cells to that block inhibitory immune checkpoints (e.g., LAG-3, prevent excessive stimulation by the TCR. CTLA-4 com- TIM3) or pathways that act on other inhibitory immune petes with CD28, a TCR co-stimulatory receptor, to bind mechanisms (e.g., CCL2/CCR2, IL-1, Ang2). Our data ligands like B7-1 and B7-2. Tis prevents CD28-mediated summarizes both preliminary results of ongoing trials, T cell activation [6]. PD-1 is also upregulated on activated as well as completed clinical trials. We excluded phase T cells. PD-1 binds to its ligand, PD-L1, and transmits a III or later stage clinical trials, trials that explored well- negative costimulatory signal that limits T cell activation described targets such as CTLA-4, PD-1, and/or PD-L1, [6]. Te oncogenic and immunosuppressive phenotype of immune stimulatory agents, vaccines, viruses, immune the TME is characterized by overexpression of PD-L1 by cellular therapy, and clinical trials involving the pediatric cancer cells and overexpression of PD-1 and CTLA-4 by population. A total of 36 phase I, 9 phase I/II, and 7 phase T cells [7]. Blockade of these molecules leads to immune- II clinical trials were included in this review. A summary mediated anti-tumor response. of the results can be found in Table 2. Ipilimumab, an anti-CTLA-4 monoclonal antibody, was the frst FDA-approved immune checkpoint inhibi- Inhibitory pathways tor and was used in patients with advanced melanoma As mentioned previously, cell growth and immune eva- [1]. Anti-PD-1 agents (nivolumab, pembrolizumab, cemi- sion by malignant cells result from Treg recruitment, plimab) and anti-PD-L1 agents (atezolizumab, avelumab, promotion of chronic infammation and exhaustion of T durvalumab) were developed later [6]. Tese agents have cells, and expression of molecules like PD-L1 or CTLA- been approved for use in multiple solid and hematologic 4, which induce a state of anergy among immune cells malignancies [6]. Tey have improved treatment out- located in the TME [3, 7, 14, 15]. Other inhibitory mole- comes, and durable response has been seen even after cules have been described. We classifed these molecules discontinuation of therapy [8]. Teir efcacy, however, is as inhibitory immune checkpoints or inhibitory targets limited to a small number of patients [9]. beyond immune checkpoints. Tis depends on whether In an attempt to improve response to therapy, combi- the manipulation of the pathway has direct or indirect nation strategies have been utilized. Anti-CTLA-4 agents repercussions on T cell efects [13, 15, 16]. Figure 1 out- have been used in conjunction with anti-PD-1/PD-L1 lines the inhibitory pathways described below and their therapies. Although improved responses have been seen, efects on immune-cell function and tumorigenesis. the incidence and severity of toxicities is a concern [6, 7]. In particular, overactivation of the immune system leads Inhibitory immune checkpoint targets to autoimmune-like side efects that can afect any organ LAG‑3 (CD223) and may require discontinuation of therapy, hospital Lymphocyte activation gene-3 (LAG-3, CD223) is a admission, or management with systemic immunosup- molecule that interacts with major histocompatibility pressive drugs [10, 11]. complex (MHC) class II and is expressed by activated New inhibitory checkpoints and their target molecules T cells, natural killers (NK) cells, B cells, and dendritic are being investigated to expand the use and efcacy of cells (DCs) [13, 17]. Although the mechanism of action existing immune checkpoint inhibition therapy [12, 13]. of LAG-3 is incompletely understood, its interaction with In this review, we focus on these new investigational mol- MHC class II causes downregulation of T cell cytokine ecules (phase I and II clinical trials) and immune check- production, CD4 and CD8 T cell expansion, and favors point inhibitory pathways that have emerged within the Treg phenotype adoption to prevent tissue damage and last 3 years. Table 1 compares these new investigational autoimmunity [17] T cells located in the TME, known therapies to existing anti-CTLA-4, anti-PD-1, and anti- as tumor-infltrating lymphocytes (TILs), overexpress PD-L1 drugs. Tis is an update from a prior review of LAG-3 which results in cell dysfunction, immune exhaus- novel investigational molecules in immune checkpoint tion, and favorable conditions for tumor growth [18]. therapy published in 2018 [13]. Tus, LAG-3 blockade favors immune activation against We conducted a PubMed search using the keywords malignant cells, while