CA-170 - A Potent Small-Molecule PD-L1 Inhibitor or Not?

SAR study of small molecule inhibitors of the programmed cell death-1/programmed cell death-ligand 1 interaction

The development of small molecule inhibitors of programmed cell death-1/programmed cell death-ligand 1 (PD-1/PD-L1) has drawn research interest for the treatment of cancer. Recently, we reported the discovery of a novel dimeric core small molecule PD-1/PD-L1 inhibitor. In an effort to discover more potent inhibitors, we further explored the dimeric core scaffold. Our investigations of the structure-activity-relationship revealed that introduction of lipophilic substituents onto one of the di-alkoxylated phenyl rings improved binding affinities to PD-L1, and inhibitory activities of PD-1/PD-L1 in cellular assays. Furthermore, conversion of the ether linker part to an olefin linker not only improved binding affinity but also led to slow dissociation binding kinetics. We also explored more potent, as well as downsized, scaffolds. Compounds bearing a linear chain in place of one of the di-alkoxylated phenyl rings exhibited good binding affinity with improved ligand efficiency (LE). Representative compounds demonstrated potent inhibitory activities of PD-1/PD-L1 in the submicromolar range in cellular assays as well as cellular function in the mixed lymphocyte reaction (MLR) assay with efficacy comparable to anti-PD-1 antibody. Our results provide applicable information for the design of more potent inhibitors targeting PD-1/PD-L1 pathway.

Immune functions as a ligand or a receptor, cancer prognosis potential, clinical implication of VISTA in cancer immunotherapy

Since cancer immunotherapy with immune checkpoint inhibitors of PD/PDL-1 and CTLA-4 limited efficacy to the patients due to resistance during the current decade, novel target is required for customized treatment due to tumor heterogeneity. V-domain Ig-containing suppressor of T cell activation (VISTA), a programmed death protein-1(PD-1) homolog expressed on T cells and on antigen presenting cells(APC), has emerged as a new target in several cancers. Though VISTA inhibitors including CA-170 are considered attractive in cancer immunotherapy to date, the information on VISTA as a potent biomarker of cancer prognosis and its combination therapy is still lacking to date. Thus, in this review, we discussed extracellular domain, ligands, expression, immune functions and clinical implications of VISTA and finally suggested conclusion and perspectives.

Terphenyl-Based Small-Molecule Inhibitors of Programmed Cell Death-1/Programmed Death-Ligand 1 Protein-Protein Interaction

We describe a new class of potent PD-L1/PD-1 inhibitors based on a terphenyl scaffold that is derived from the rigidified biphenyl-inspired structure. Using in silico docking, we designed and then experimentally demonstrated the effectiveness of the terphenyl-based scaffolds in inhibiting PD-1/PD-L1 complex formation using various biophysical and biochemical techniques. We also present a high-resolution structure of the complex of PD-L1 with one of our most potent inhibitors to identify key PD-L1/inhibitor interactions at the molecular level. In addition, we show the efficacy of our most potent inhibitors in activating the antitumor response using primary human immune cells from healthy donors.

Inhibition of lung tumorigenesis by a small molecule CA170 targeting the immune checkpoint protein VISTA

Expressed on cells of the myeloid and lymphoid lineages, V-domain Ig Suppressor of T cell Activation (VISTA) is an emerging target for cancer immunotherapy. Blocking VISTA activates both innate and adaptive immunity to eradicate tumors in mice. Using a tripeptide small molecule antagonist of VISTA CA170, we found that it exhibited potent anticancer efficacy on carcinogen-induced mouse lung tumorigenesis. Remarkably, lung tumor development was almost completely suppressed when CA170 was combined with an MHCII-directed KRAS peptide vaccine. Flow cytometry and single-cell RNA sequencing (scRNA-seq) revealed that CA170 increased CD8+ T cell infiltration and enhanced their effector functions by decreasing the tumor infiltration of myeloid-derived suppressor cells (MDSCs) and Regulatory T (Treg) cells, while the Kras vaccine primarily induced expansion of CD4+ effector T cells. VISTA antagonism by CA170 revealed strong efficacy against lung tumorigenesis with broad immunoregulatory functions that influence effector, memory and regulatory T cells, and drives an adaptive T cell tumor-specific immune response that enhances the efficacy of the KRAS vaccine.

New emerging targets in cancer immunotherapy: the role of VISTA

The immune surveillance system is complex and regulated by different actors. Programmed death protein-ligand 1 (PD-L1), the only approved biomarker in clinical practice, has proven to be imperfect in selecting patients to immune checkpoint inhibitors treatment. Therefore, new biomarkers, and new therapeutic targets, are needed to maximise the efficacy of immunotherapy. V-domain Ig Suppressor of T-cell Activation (VISTA) is a programmed death protein-1 (PD-1) homolog expressed on T cells and on antigen-presenting cells, which regulates processes of activation and repression of the immune system with not yet completely clarified mechanisms. Its blockage has demonstrated in vitro and in vivo antitumour activity. The clinical research of VISTA antagonists is ongoing. Particularly, CA-170, an orally delivered dual inhibitor of VISTA and PD-L1, has shown to have clinical efficacy in phase I and II clinical trials in different advanced solid tumour types. Further data are needed to define whether this drug class can become a new therapeutic option for patients with VISTA expressing cancers.

Advance investigation on synthetic small-molecule inhibitors targeting PD-1/PD-L1 signaling pathway

Immune checkpoint blockade has displayed substantial anti-tumor resistance in a variety of forms of cancer, but the fundamental regulation role remains unclear, and several questions continue to be addressed. PD-1/PD-L1 has been recognized as an anti-cancer drug target for several years, and through targeting the PD-1/PD-L1 signaling pathway, many monoclonal antibodies have thus far produced promising results in cancer therapy. The discovery of small-molecule inhibitors focused on the PD-1/PD-L1 signaling pathway is steadily reviving over decades, owing to the intrinsic shortcomings of the antibodies. PD-1 function and its PD-L1 or PD-L2 ligands are essential for the activation, proliferation, and cytotoxic secretion of T-cells in cancer to degenerate anti-tumor immune response. The axis PD-1/PD-L1 is important for the immune escape of cancer which has an immense impact on cancer treatment. In this review, we summarize the function of PD-1 and PD-L1 in cancer and aiming to enhance cancer therapy.

Immunotherapy in Breast Cancer Patients: A Focus on the Use of the Currently Available Biomarkers in Oncology

Immune checkpoint inhibitors (ICIs) have remarkably modified the way solid tumors are managed, including breast cancer. Unfortunately, only a relatively small number of breast cancer patients significantly respond to these treatments. To maximize the immunotherapy benefit in breast cancer, several efforts are currently being put forward for the identification of i) the best therapeutic strategy (i.e. ICI monotherapy or in association with chemotherapy, radiotherapy, or other drugs); ii) the optimal timing for administration (e.g. early/advanced stage of disease; adjuvant/neoadjuvant setting); iii) the most effective and reliable predictive biomarkers of response (e.g. tumor-infiltrating lymphocytes, programmed death-ligand 1, microsatellite instability associated with mismatch repair deficiency, and tumor mutational burden). This article reviews the impacts and gaps in the characterization of immune-related biomarkers raised by clinical and translational research studies with immunotherapy treatments. Particular emphasis has been put on the documented evidence of significant clinical benefits of ICI in different randomized clinical trials, along with preanalytical and analytical issues in predictive biomarkers pathological assessment.

VISTA: A Promising Target for Cancer Immunotherapy?

Agents targeting the B7 family co-inhibitory receptors cytotoxic T-lymphocyte antigen-4 (CTLA-4) and programmed cell death protein-1 (PD-1), or its ligand (PD-L1), have a pivotal role in clinical practice. V-domain Ig suppressor of T-cell activation (VISTA) is a protein highly conserved between species, with a similar amino acid sequence to the B7 family members, characterized by a particularly structural homology to PD-1. It has been counted as an emerging target within the list of novel targetable immune checkpoints in oncology. Physiologically, VISTA exerts a regulatory function on the immune system at several levels, particularly by modulating T cells activation. Its altered activity plays a role in many autoimmune diseases, and its expression has been found to be prognostically implicated in different cancer types in preclinical models. We hereby present the main evidence on the value of VISTA as an immune checkpoint in solid and hematological malignancies. We also review its value as a potential target for cancer immunotherapy, by reporting the results of Phase I and II clinical trials assessing the use of drugs targeting VISTA. The complexity of its pathway, along with some unclear biological aspects concerning its molecular interactions, currently represent a limit to the applicability of VISTA as an effective biomarker for immunotherapy in oncology. A deeper characterization of this immune checkpoint may help defining its value within immune signatures of solid and hematological malignancies, and to design future therapeutic strategies.

Identification of the FDA-Approved Drug Pyrvinium as a Small-Molecule Inhibitor of the PD-1/PD-L1 Interaction

Immune checkpoint blockade involving inhibition of the PD-1/PD-L1 interaction has provided unprecedented clinical benefits in treating a variety of tumors. To date, a total of six antibodies that bind to either PD-1 or PD-L1 protein and in turn inhibit the PD-1/PD-L1 interaction have received clinical approvals. Despite being highly effective, these expensive large biotherapeutics possess several inherent pharmacokinetic limitations that can be successfully overcome through the use of low-molecular-weight inhibitors. One such promising approach involves small-molecule induced dimerization and sequestration of PD-L1, leading to effective PD-1/PD-L1 inhibition. Herein, we present the discovery of such potential bioactive PD-L1 dimerizers through a structure- and ligand-based screening of a focused library of approved and investigational drugs worldwide. Pyrvinium, an FDA-approved anthelmintic drug, showed the highest activity in our study with IC₅₀ value of ∼29.66 μM. It is noteworthy that Pyrvinium, being an approved drug, may prove especially suitable as a good starting point for further medicinal chemistry efforts, leading to design and development of even more potent structural analogs as selective PD-1/PD-L1 inhibitors. Furthermore, the adopted integrated virtual screening protocol may prove useful in screening other larger databases of lead- and drug-like molecules for hit identification in the domain of small-molecule PD-1/PD-L1 inhibitors.

PD-1 derived CA-170 is an oral immune checkpoint inhibitor that exhibits preclinical anti-tumor efficacy

Small molecule immune checkpoint inhibitors targeting PD-1 and other pathways may offer advantages including ease of dosing, ability to manage immune-related adverse events (irAEs) due to their shorter pharmacokinetic exposure and opportunity to target more than one pathway for improving efficacy. Here we describe the identification and characterization of CA-170, an amino acid inspired small molecule inhibitor of PD-L1 and VISTA derived from the interface of PD-1 and PD-L1. CA-170 exhibited potent rescue of proliferation and effector functions of T cells inhibited by PD-L1/L2 and VISTA with selectivity over other immune checkpoint proteins as well as a broad panel of receptors and enzymes. Observed blocking of PD-L1 signaling and binding to PD-L1 in the cellular context without preventing the assembly of PD-1:PD-L1 complex support the formation of a defective ternary complex as the mechanism of action of CA-170. Oral administration of CA-170 resulted in increased proliferation and activation of T cells in the tumor, and significant anti-tumor efficacy in a number of immunocompetent mouse tumor models either as a single agent or in combination with approved therapeutics. These results prompted the advancement of CA-170 to human clinical trials.

Design, synthesis, and biological evaluation of 1-methyl-1H-pyrazolo[4,3-b]pyridine derivatives as novel small-molecule inhibitors targeting the PD-1/PD-L1 interaction

Blockade of the programmed cell death-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) signalling pathway is a promising tumour immunotherapeutic approach, and small molecule drugs have more advantages than monoclonal antibody macromolecules in clinical applications. Therefore, a series of 1-methyl-1H-pyrazolo[4,3-b]pyridine derivatives as PD-1/PD-L1 interaction novel small-molecule inhibitors were designed employing a ring fusion strategy. The inhibitory activity of compounds was evaluated by the HTRF assay, among which D38 was identified as the most potent PD-1/PD-L1 interaction inhibitor with an IC₅₀ value of 9.6 nM. Furthermore, D38 exhibited prominent inhibitory activity against the PD-1/PD-L1 interaction with an EC₅₀ value of 1.61 μM in a coculture model of PD-L1/TCR activator-expressing CHO cells and PD-1-expressing Jurkat cells. In addition, the preliminary structure-activity relationships (SARs) of compounds were elucidated, and the binding mode of D38 with the PD-L1 dimer was analysed by molecular docking. Overall, D38 could be employed as a prospective lead compound of PD-1/PD-L1 interaction inhibitors for further development.

Design, Synthesis, and Biological Evaluation of Imidazopyridines as PD-1/PD-L1 Antagonists

The PD-1/PD-L1 axis has proven to be a highly efficacious target for cancer immune checkpoint therapy with several approved antibodies. Also, small molecules based on a biphenyl core can antagonize PD-1/PD-L1, leading to the in vitro formation of PD-L1 dimers. However, their development remains challenging, as we do not yet fully understand their mode of action. In this work, we designed a new scaffold based on our previously solved high-resolution structures of low-molecular-weight inhibitors bound to PD-L1. A small compound library was synthesized using the Groebke–Blackburn–Bienaymé multicomponent reaction (GBB-3CR), resulting in the structure–activity relationship of imidazo[1,2-a]pyridine-based inhibitors. These inhibitors were tested for their biological activity using various biophysical assays giving potent candidates with low-micromolar PD-L1 affinities. An obtained PD-L1 cocrystal structure reveals the binding to PD-L1. Our results open the door to an interesting bioactive scaffold that could lead to a new class of PD-L1 antagonists.

Clinical Insights Into Novel Immune Checkpoint Inhibitors

The success of immune checkpoint inhibitors (ICIs), notably anti-cytotoxic T lymphocyte associated antigen-4 (CTLA-4) as well as inhibitors of CTLA-4, programmed death 1 (PD-1), and programmed death ligand-1 (PD-L1), has revolutionized treatment options for solid tumors. However, the lack of response to treatment, in terms of de novo or acquired resistance, and immune related adverse events (IRAE) remain as hurdles. One mechanisms to overcome the limitations of ICIs is to target other immune checkpoints associated with tumor microenvironment. Immune checkpoints such as lymphocyte activation gene-3 (LAG-3), T cell immunoglobulin and ITIM domain (TIGIT), T cell immunoglobulin and mucin-domain containing-3 (TIM-3), V-domain immunoglobulin suppressor of T cell activation (VISTA), B7 homolog 3 protein (B7-H3), inducible T cell costimulatory (ICOS), and B and T lymphocyte attenuator (BTLA) are feasible and promising options for treating solid tumors, and clinical trials are currently under active investigation. This review aims to summarize the clinical aspects of the immune checkpoints and introduce novel agents targeting these checkpoints.

Discovery of 4-Arylindolines Containing a Thiazole Moiety as Potential Antitumor Agents Inhibiting the Programmed Cell Death-1/Programmed Cell Death-Ligand 1 Interaction

Through specific structural modification of a 4-phenylindoline precursor, new 4-arylindolines containing a thiazole moiety were developed and found to be promising modulators of the programmed cell death-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) axis. Compound A30 exhibited outstanding biochemical activity, with an IC₅₀ of 11.2 nM in a homogeneous time-resolved fluorescence assay. In the cell-based assay, A30 significantly promoted IFN-γ secretion and rescued T-cell proliferation, which were inhibited by PD-1 activation. Furthermore, A30 showed favorable in vivo antitumor activity in a mouse 4T1 breast carcinoma model. Moreover, in mouse CT26 colon carcinoma models, A30 potently suppressed the growth of CT26/PD-L1 tumor but did not obviously affect the growth of CT26/vector tumor. The results of flow cytometry analysis indicated that A30 inhibited tumor growth by activating the immune microenvironment. We concluded that A30 is a new starting point for further development of PD-1/PD-L1 interaction inhibitors as antitumor agents.

Peptide-based and small molecule PD-1 and PD-L1 pharmacological modulators in the treatment of cancer

Cancer immunotherapy is an option to enhance physiological defence mechanism to fight cancer, where natural substances (e.g., antigen/antibody) or small synthetic molecule can be utilized to improve and restore the immune system to stop or slacken the development of malignant cells, stop metastasis and/or help the immune response with synthetic monoclonal antibodies (mAbs) and tumour-agnostic therapy to eliminate cancer cells. Interaction between the programmed cell death ligand 1 (PD-L1) and its receptor (programmed cell death protein 1, PD-1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA4) linked signalling pathways have been identified as perilous towards the body's immune mechanism in regulating the progression of cancer. It is known that certain cancers use these pathways to evade the body's defence mechanism. The immune system is capable of responding to cancer by stalling these trails with specific synthetic antibodies or immune checkpoint inhibitors, which can ultimately either stop or slow cancer cell development. Recent findings and data suggested that using such inhibitors invigorated a new approach to cancer treatment. These inhibitors usually activate the immune system to identify and eliminate cancer cells rather than attacking tumour cells directly. PD-1/PD-L1 inhibitors have already been substantiated for their efficacy in over twenty variations of cancer through different clinical trials. Studies on molecular interaction with existing PD-1/PD-L1 inhibitors that are mainly dominated by antibodies are constantly generating new ideas to develop novel inhibitors. This review has summarised information on reported and/or patented small molecules and peptides for their ability to interact with the PD-1/PD-L1 as a potential anticancer strategy.

Design, synthesis and biological evaluation of isoxazole-containing biphenyl derivatives as small-molecule inhibitors targeting the programmed cell death-1/ programmed cell death-ligand 1 immune checkpoint

Monoclonal antibodies targeting the programmed cell death-1/ programmed cell death-ligand 1 (PD-1/PD-L1) immune checkpoint have achieved enormous success in cancer immunotherapy. But the antibody-based immunotherapies carry a number of unavoidable deficiencies such as poor pharmacokinetic properties and immunogenicity. Small-molecule PD-1/PD-L1 inhibitors offer the superiority of complementarity with monoclonal antibodies and represent an appealing alternative. A novel series of isoxazole-containing biphenyl compounds were designed, synthesized and evaluated as PD-1/PD-L1 inhibitors in this paper. The structure-activity relationship of the novel synthesized compounds indicated that the ring-closure strategy of introducing isoxazole could be employed and the 3-cyanobenzyl group was significant for the inhibitory activity against the PD-1/PD-L1 protein-protein interactions. Molecular docking studies were performed to help understand the binding mode of the small-molecule inhibitor with the PD-L1 dimer. In particular, compound II-12 was a promising anti-PD-1/PD-L1 inhibitor with the IC₅₀ value of 23.0 nM, providing valuable information for future drug development.

Synthesis and pharmacological evaluation of novel resorcinol biphenyl ether analogs as small molecule inhibitors of PD-1/PD-L1 with benign toxicity profiles for cancer treatment

Programmed death protein 1 (PD-1)/programmed death protein ligand 1 (PD-L1) pathway is one of the most actively pursued targets in cancer immunotherapy. In a continuation of our research interest in this pathway, we synthesized and evaluated the pharmacological activities of a series of resorcinol biphenyl ether analogs as small molecule PD-1/PD-L1 inhibitors for cancer treatment. Among the 27 newly synthesized compounds, CH1 was found to have the highest inhibitory effect against PD-1/PDL-1 with an IC₅₀ value of 56.58 nM in the HTRF (homogenous time-resolved fluorescence) assay. In addition, CH1 dose-dependently promoted HepG2 cell death in a co-culture model of HepG2/hPD-L1 and Jurkat T cells. Furthermore, molecular modeling study indicated that CH1 binds with high affinity to the binding interface of PD-L1. Moreover, CH1 effectively inhibited tumor growth (TGI of 76.4% at 90 mg/kg) in an immune checkpoint humanized mouse model with no obvious toxicity. Finally, CH1 did not cause in vivo cardiotoxicity and bone marrow suppression (myelosuppression) to BALB/c mice. Taken together, these results suggest that CH1 deserves further investigation as a potent and safe PD-1/PDL-1 inhibitor for cancer treatment.

Design, synthesis, and structure-activity relationship of programmed cell death-1/programmed cell death-ligand 1 interaction inhibitors bearing a benzo[d]isothiazole scaffold

Blockade of the programmed cell death-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) pathway is an attractive strategy for immunotherapy. A novel series of compounds bearing a benzo[d]isothiazole scaffold were developed, among which CH20 exhibited promising activity, with an IC₅₀ value of 8.5 nM. Further cell-based PD-1/PD-L1 blockade bioassays indicated that CH20 can inhibit the PD-1/PD-L1 interaction at the cellular level, with an EC₅₀ value of 5.6 μM CH20 could have better potency in restoring the activity of effector cells, as the maximal luminescence values (RLU_max) of CH20 were equivalent to those of PD-L1 mAbs. The docking analysis of CH20 with the PD-L1 dimer complex (PDB ID: 6R3K) confirmed that CH20 is a promising lead compound for the development of inhibitors of the PD-1/PD-L1 interaction. The preliminary structure-activity relationship was investigated in this paper, with the aim of future drug development.

Insights into non-peptide small-molecule inhibitors of the PD-1/PD-L1 interaction: Development and perspective

The development of immune checkpoint inhibitors has become a research hotspot in cancer immunotherapy in recent years. Anti-PD-1/PD-L1 monoclonal antibodies (mAbs), such as pembrolizumab and nivolumab have been approved for treating different types of cancer. Many peptides, peptidomimetics and non-peptide small-molecule inhibitors targeting the PD-1/PD-L1 axis have been published so far. In comparison with mAbs, small-molecule inhibitors have the potential to overcome inherent shortcomings of mAbs, such as poor oral bioavailability, low tumor penetration, and high manufacturing costs. In this article, we mainly review non-peptide small-molecule inhibitors of the PD-1/PD-L1 interaction, their cocrystal structures, docking studies, and biological activities are also included to guide future study. In addition, we propose several strategies for designing more effective small-molecule modulators of the PD-1/PD-L1 pathway.

Development of a Non-IgG PD-1/PD-L1 Inhibitor by in Silico Mutagenesis and an In-Cell Protein-Protein Interaction Assay

Inhibiting the programmed death-1 (PD-1)/programmed death ligand 1 (PD-L1) axis by monoclonal antibodies (mAbs) is a successful cancer immunotherapy. However, mAb-based drugs have various disadvantages including high production costs and large molecular sizes, which motivated us to develop a smaller alternative drug. Since PD-L1 binds PD-1 with moderate affinity, a higher affinity PD-1 variant should serve as a competitive inhibitor of the wild-type PD-1/PD-L1 interaction. In this report, we conducted in silico point mutagenesis of PD-1 to identify potent PD-1 variants with a higher affinity toward PD-L1 and refined the in silico results using a luciferase-based in-cell protein-protein interaction (PPI) assay. As a result, a PD-1 variant was developed that had two mutated amino acids (T76Y, A132V), termed 2-PD-1. 2-PD-1 could bind with PD-L1 at a dissociation constant of 12.74 nM. Moreover, 2-PD-1 successfully inhibited the PD-1/PD-L1 interaction with a half maximal inhibitory concentration of 19.15 nM and reactivated the T cell with a half maximal effective concentration of 136.1 nM. These results show that in silico mutagenesis combined with an in-cell PPI assay verification strategy successfully prepared a non-IgG inhibitor of the PD-1/PD-L1 interaction.

Discovery of a new inhibitor targeting PD-L1 for cancer immunotherapy

Blockade of the PD-1/PD-L1 immunologic checkpoint using monoclonal antibodies has provided breakthrough therapies against cancer in the recent years. Nevertheless, intrinsic disadvantages of therapeutic antibodies may limit their applications. Thus, blocking of the PD-1/PD-L1 interaction by small molecules may be a promising alternative for cancer immunotherapy. We used a docking-based virtual screening strategy to rapidly identify new small molecular inhibitors targeting PD-L1. We demonstrated that a small molecule compound (N-[2-(aminocarbonyl)phenyl][1,1′-biphenyl]-4-carboxamide [APBC]) could effectively interrupt the PD-1/PD-L1 interaction by directly binding to PD-L1, presenting the K_D and IC₅₀ values at low-micromolar level. Molecular docking study revealed that APBC may have function through a PD-L1 dimer-locking mechanism, occluding the PD-1 interaction surface of PD-L1. We further confirmed the ligand blocking activity and T cell-reinvigoration potency of APBC using cell-based assays. APBC could dose-dependently elevate cytokine secretions of the primary T-lymphocytes that are cocultured with cancer cells. Importantly, APBC displayed superior antitumor efficacy in hPD-L1 knock-in B16F10-bearing mouse model without the induction of observable liver toxicity. Analyses on the APBC-treated mice further revealed drastically elevated levels of infiltrating CD4⁺ and CD8⁺ T cells, and inflammatory cytokines production in tumor microenvironment. The APBC compound could serve as a privileged scaffold in the design of improved PD pathway modulators, thus providing us promising drug candidates for tumor immunotherapy.

Recent advance of peptide-based molecules and nonpeptidic small-molecules modulating PD-1/PD-L1 protein-protein interaction or targeting PD-L1 protein degradation

Tumor immunotherapy has made great progress in recent years. In the tumor microenvironment, the binding of PD-1 and its ligand PD-L1 can promote tumor immune escape and tumor survival. Clinical studies have indicated that antibodies blocking PD-1 and PD-L1 have reliable effects on many advanced malignant tumors. However, no small-molecule inhibitors have been approved so far, indicating that the development of marketable small-molecules PD-1/PD-L1 targeted therapy drugs is a challenging process. Small-molecule inhibitors can overcome the limitations of monoclonal antibodies, including poor oral bioavailability, high cost, poor tissue and tumor penetration and long half-life, which prompt researchers to turn their attention to the development of peptide molecules and small-molecule inhibitors modulating PD-1/PD-L1 to overcome some disadvantages of monoclonal antibodies or targeting PD-L1 protein degradation as potential alternatives or supplements. In this review, we will focus on the peptide-based and nonpeptidic molecules against PD-1/PD-L1 base on the structural classification. More importantly, we also focus on the latest research progress of small-molecules mediated PD-L1 degradation mechanism.

Discovery of Novel and Highly Potent Resorcinol Dibenzyl Ether-Based PD-1/PD-L1 Inhibitors with Improved Drug-like and Pharmacokinetic Properties for Cancer Treatment

A series of programmed cell death-1 (PD-1)/programmed cell death ligand 1 (PD-L1) inhibitors based on the resorcinol diphenyl ether scaffold were discovered by incorporating hydrophilic moieties into the side chain and converting into the corresponding hydrochloride salt. Among these compounds, P18 showed the highest inhibitory activity against PD-1/PD-L1 with an IC50 value of 9.1 nM in a homogeneous time-resolved fluorescence binding assay. Besides, P18 promoted HepG2 cell death dose dependently in a HepG2/PD-L1 and Jurkat/PD-1 coculture cell model. Further, P18 demonstrated significantly higher water solubility (17.61 mg/mL) and improved pharmacokinetics (e.g., t1/2 of ∼20 h and oral bioavailability of 12%) than the previous analogues. Moreover, P18 was highly effective in suppressing tumor growth in an immune checkpoint humanized mouse model without apparent toxicity. Collectively, these results suggest that compound P18 represents a promising PD-1/PD-L1 inhibitor worthy of further investigation as a potential anticancer agent.

Conjugation of biphenyl groups with poly(ethylene glycol) to enhance inhibitory effects on the PD-1/PD-L1 immune checkpoint interaction

Monoclonal antibodies have been developed as anticancer agents to block immune checkpoint pathways associated with programmed cell death 1 (PD-1) and its ligand PD-L1. However, the high cost of antibodies has encouraged researchers to develop other inhibitor types. Here, biphenyl compounds were conjugated with poly(ethylene glycol) (PEG) to enhance the activity of small molecular inhibitors. Immunoassay results revealed the decrease in the inhibition activity following conjugation with linear PEG, suggesting that the PEG moiety reduced the interaction between the biphenyl structure and PD-L1. However, the inhibitory effect on PD-1/PD-L1 interaction was further enhanced by using branched PEG conjugates. The increase in the number of conjugated biphenyl compounds resulted in increased inhibitory activity. The highest IC50 value was 0.33 μM, which was about 5 times higher than that observed for a non-conjugated monovalent compound. The inhibitory activity was more than 20 times the activity reported for the starting compound. Considering the increase in the inhibition activity, this multivalent strategy can be useful in the design of new immune checkpoint inhibitors.

Virtual Screening and In Vitro Evaluation of PD-1 Dimer Stabilizers for Uncoupling PD-1/PD-L1 Interaction from Natural Products

Genetic mutations accumulated overtime could generate many growth and survival advantages for cancer cells, but these mutations also mark cancer cells as targets to be eliminated by the immune system. To evade immune surveillance, cancer cells adopted different intrinsic molecules to suppress immune response. PD-L1 is frequently overexpressed in many cancer cells, and its engagement with PD-1 on T cells diminishes the extent of cytotoxicity from the immune system. To resume immunity for fighting cancer, several therapeutic antibodies disrupting the PD-1/PD-L1 interaction have been introduced in clinical practice. However, their immunogenicity, low tissue penetrance, and high production costs rendered these antibodies beneficial to only a limited number of patients. PD-L1 dimer formation shields the interaction interface for PD-1 binding; hence, screening for small molecule compounds stabilizing the PD-L1 dimer may make immune therapy more effective and widely affordable. In the current study, 111 candidates were selected from over 180,000 natural compound structures through virtual screening, contact fingerprint analysis, and pharmacological property prediction. Twenty-two representative candidates were further evaluated in vitro. Two compounds were found capable of inhibiting the PD-1/PD-L1 interaction and promoting PD-L1 dimer formation. Further structure optimization and clinical development of these lead inhibitors will eventually lead to more effective and affordable immunotherapeutic drugs for cancer patients.

B Cells versus T Cells in the Tumor Microenvironment of Malignant Lymphomas. Are the Lymphocytes Playing the Roles of Muhammad Ali versus George Foreman in Zaire 1974?

Malignant lymphomas are a heterogeneous group of malignancies that develop both in nodal and extranodal sites. The different tissues involved and the highly variable clinicopathological characteristics are linked to the association between the lymphoid neoplastic cells and the tissues they infiltrate. The immune system has developed mechanisms to protect the normal tissue from malignant growth. In this review, we aim to explain how T lymphocyte-driven control is linked to tumor development and describe the tumor-suppressive components of the resistant framework. This manuscript brings forward a new insight with regard to intercellular and intracellular signaling, the immune microenvironment, the impact of therapy, and its predictive implications. A better understanding of the key components of the lymphoma environment is important to properly assess the role of both B and T lymphocytes, as well as their interplay, just as two legendary boxers face each other in a heavyweight title final, as was the case of Ali versus Foreman.

Small-molecule PD-L1 inhibitor BMS1166 abrogates the function of PD-L1 by blocking its ER export

Therapeutic monoclonal antibodies against the PD-L1/PD-1 (programmed death ligand-1/programmed cell death protein-1) axis have achieved great successes in cancer treatments, but the development of small-molecule immunomodulators of the pathway has lagged far behind. We established a cellular coculture assay with two stable transfectant cell lines, a PD-L1-expressing tumor cell line PC9/PD-L1 and a PD-1-expressing T cell line Jurkat/PD-1. Western blotting analyses were used to monitor the PD-L1/PD-1 interaction and signaling. We analyzed PD-L1 glycosylation by lectin binding assay and glycosidase digestion, and examined subcellular localization of PD-L1 by immunocytochemical staining. Luciferase assay and real-time PCR were used to evaluate T cell activation in the coculture experiments. We found that coculturing of the PC9/PD-L1 cells with the Jurkat/PD-1 cells induced a lysosomal degradation of PD-1. A small-molecule PD-L1 inhibitor BMS1166 developed by Bristol-Myers Squibb inhibited the coculture-induced PD-1 degradation through a unique mechanism. BMS1166 specifically affected PD-L1 glycosylation and prevented transporting of the under-glycosylated form of PD-L1 from endoplasmic reticulum (ER) to Golgi, leading to accumulation of PD-L1 in ER. In doing so, BMS1166 blocked PD-L1/PD-1 signaling. Coculturing PD-L1-expressing cells with PD-1-expressing cells induced degradation of PD-1, which could be used as a readout to identify inhibitors of PD-L1/PD-1 signaling. The small-molecule PD-L1 inhibitor BMS1166 abolished the glycosylation and maturation of PD-L1 by blocking its exporting from ER to Golgi. Our study discovered a new strategy to identify inhibitors of the PD-L1/PD-1 signaling pathway and to develop new drugs for the treatment of cancer.

Di-bromo-Based Small-Molecule Inhibitors of the PD-1/PD-L1 Immune Checkpoint

Immune checkpoint blockade is one of the most promising strategies of cancer immunotherapy. However, unlike classical targeted therapies, it is currently solely based on expensive monoclonal antibodies, which often inflict immune-related adverse events. Herein, we propose a novel small-molecule inhibitor targeted at the most clinically relevant immune checkpoint, PD-1/PD-L1. The compound is capable of disrupting the PD-1/PD-L1 complex by antagonizing PD-L1 and, therefore, restores activation of T cells similarly to the antibodies, while being cheap in production and possibly nonimmunogenic. The final compound is significantly smaller than others reported in the literature while being nontoxic to cells even at high concentrations. The scaffold was designed using a structure–activity relationship screening cascade based on a new antagonist-induced dissociation NMR assay, called the weak-AIDA-NMR. Weak-AIDA-NMR finds true inhibitors, as opposed to only binders to the target protein, in early steps of lead compound development, and this process makes it less time and cost consuming.

Recent advances in the development of protein-protein interactions modulators: mechanisms and clinical trials

Protein-protein interactions (PPIs) have pivotal roles in life processes. The studies showed that aberrant PPIs are associated with various diseases, including cancer, infectious diseases, and neurodegenerative diseases. Therefore, targeting PPIs is a direction in treating diseases and an essential strategy for the development of new drugs. In the past few decades, the modulation of PPIs has been recognized as one of the most challenging drug discovery tasks. In recent years, some PPIs modulators have entered clinical studies, some of which been approved for marketing, indicating that the modulators targeting PPIs have broad prospects. Here, we summarize the recent advances in PPIs modulators, including small molecules, peptides, and antibodies, hoping to provide some guidance to the design of novel drugs targeting PPIs in the future.

Discovery and Optimization of Small-Molecule Ligands for V-Domain Ig Suppressor of T-Cell Activation (VISTA)

V-domain Ig suppressor of T-cell activation (VISTA) is an immune checkpoint that affects the ability of T-cells to attack tumors. A FRET-based high throughput screening identified NSC622608 as the first small-molecule ligand for VISTA. Investigation of the interaction of NSC622608 with VISTA using STD NMR and molecular modeling enabled the identification of a potential binding site in VISTA for NSC622608. Screening NSC622608 against a library of single-point VISTA mutants revealed the key residues in VISTA interacting with NSC622608. Further structural optimization resulted in a lead with submicromolar VISTA binding affinity. The lead compound blocked VISTA signaling in vitro, enhanced T-cell proliferation, and restored T-cell activation in the presence of VISTA-expressing cancer cell lines. This work would enable future development of small molecules targeting VISTA as immunomodulators and imaging probes.

Research Status and Outlook of PD-1/PD-L1 Inhibitors for Cancer Therapy

PD-1/PD-L1 inhibitors are a group of immune checkpoint inhibitors as front-line treatment of multiple types of cancer. However, the serious immune-related adverse reactions limited the clinical application of PD-1/PD-L1 monoclonal antibodies, despite the promising curative effects. Therefore, it is urgent to develop novel inhibitors, such as small molecules, peptides or macrocycles, targeting the PD-1/PD-L1 axis to meet the increasing clinical demands. Our review discussed the mechanism of action of PD-1/PD-L1 inhibitors and presented clinical trials of currently approved PD-1/PD-L1 targeted drugs and the incidence of related adverse reactions, helping clinicians pay more attention to them, better formulate their intervention and resolution strategies. At last, some new inhibitors whose patent have been published are listed, which provide development ideas and judgment basis for the efficacy and safety of novel PD-1/PD-L1 inhibitors.

Small molecules as antagonists of co-inhibitory pathways for cancer immunotherapy: a patent review (2018-2019)

INTRODUCTION

Therapeutic antibodies blocking co-inhibitory pathways do not attack tumor cells directly, but instead bind to their targeted proteins and mobilize the immune system to eradicate tumors. However, only a small fraction of patients with certain cancer types can benefit from the antibodies. Additionally, antibodies have shown serious immune-related adverse events in certain patients. Small-molecule antagonists may be a complementary and potentially synergistic approach to antibodies for patients with various cancers.

AREAS COVERED

The authors review the small molecules as antagonists of co-inhibitory pathway proteins, summarize their preliminary SARs, discuss biochemistry assays used in patents for the development of small molecules as novel antagonists.

EXPERT OPINION

The disclosed pharmacophores of small molecules as co-inhibitory pathway antagonists are represented by biphenyl derivatives, biaryl derivatives, teraryl derivatives, quateraryl derivatives, and oxadiazole/thiadiazole derivatives. However, these antagonists are still inferior to therapeutic antibodies in their inhibitory activities due to relatively flat of human co-inhibitory pathways proteins. Allosteric modulators may be an alternative approach. The more safety and efficacy evaluation trials of small-molecule antagonists targeting co-inhibitory pathways should be performed to demonstrate the proof-of-principle that small-molecule antagonists can result in sustained safety and antitumor response in the near future.

A PD-1 peptide antagonist exhibits potent anti-tumor and immune regulatory activity

Antibodies blocking the PD-1/PD-L1 pathway have achieved great success. However, some disadvantages of antibodies have been found, which limit their clinical applications. Peptide antagonists are alternatives to antibodies in PD-1/PD-L1 blockage, but successful studies in this area are limited. A PD-1 targeting peptide, P-F4, was identified using phage display. P-F4 bound PD-1 with an affinity of 0.119 μM, inhibited PD-1/PD-L1 interaction at the cellular level and modulated T cell activity in vitro. We have overcome the poor solubility and rapid degradation problems of this peptide by packaging P-F4 in nanoparticles. In vivo experiments demonstrated that P-F4 nanoparticles could strongly inhibit tumor growth in a CT26 mouse model. Further research revealed that treatment of P-F4 nanoparticles increased CD8+T cells and reduced Tregs in the tumor microenvironment and tumor-draining lymph nodes. It was shown that treatment of P-F4 nanoparticles also increased lymphocytic activities, including proliferation, cytokine secretion and cytolytic activity. Moreover, computer modeling suggested that the P-F4 binding site to PD-1 overlaps with the PD-L1 binding surface. In this study, a peptide candidate for cancer immunotherapy was provided, and its working mechanisms were studied.

Competition NMR for Detection of Hit/Lead Inhibitors of Protein-Protein Interactions

Screening for small-molecule fragments that can lead to potent inhibitors of protein–protein interactions (PPIs) is often a laborious step as the fragments cannot dissociate the targeted PPI due to their low μM–mM affinities. Here, we describe an NMR competition assay called w-AIDA-NMR (weak-antagonist induced dissociation assay-NMR), which is sensitive to weak μM–mM ligand–protein interactions and which can be used in initial fragment screening campaigns. By introducing point mutations in the complex’s protein that is not targeted by the inhibitor, we lower the effective affinity of the complex, allowing for short fragments to dissociate the complex. We illustrate the method with the compounds that block the Mdm2/X-p53 and PD-1/PD-L1 oncogenic interactions. Targeting the PD-/PD-L1 PPI has profoundly advanced the treatment of different types of cancers.

Preparation of Biphenyl-Conjugated Bromotyrosine for Inhibition of PD-1/PD-L1 Immune Checkpoint Interactions

Cancer immunotherapy has been revolutionized by the development of monoclonal antibodies (mAbs) that inhibit interactions between immune checkpoint molecules, such as programmed cell-death 1 (PD-1), and its ligand PD-L1. However, mAb-based drugs have some drawbacks, including poor tumor penetration and high production costs, which could potentially be overcome by small molecule drugs. BMS-8, one of the potent small molecule drugs, induces homodimerization of PD-L1, thereby inhibiting its binding to PD-1. Our assay system revealed that BMS-8 inhibited the PD-1/PD-L1 interaction with IC₅₀ of 7.2 μM. To improve the IC₅₀ value, we designed and synthesized a small molecule based on the molecular structure of BMS-8 by in silico simulation. As a result, we successfully prepared a biphenyl-conjugated bromotyrosine (X) with IC₅₀ of 1.5 μM, which was about five times improved from BMS-8. We further prepared amino acid conjugates of X (amino-X), to elucidate a correlation between the docking modes of the amino-Xs and IC₅₀ values. The results suggested that the displacement of amino-Xs from the BMS-8 in the pocket of PD-L1 homodimer correlated with IC₅₀ values. This observation provides us a further insight how to derivatize X for better inhibitory effect.

Kaempferol and Its Glycoside, Kaempferol 7-O-Rhamnoside, Inhibit PD-1/PD-L1 Interaction In Vitro

Kaempferol (KO) and kaempferol 7-O-rhamnoside (KR) are natural products from various oriental herbs such as Geranii Herba. Previous studies have reported some biological activities of KO and KR; however, their effects on PD-1/PD-L1 interaction have not been reported yet. To elucidate their inhibitory activities on PD-1/PD-L1 protein–protein interaction (PPI), biochemical assays including competitive ELISA and biolayer interferometry (BLI) systems were performed. Cellular PD-1/PD-L1 blocking activity was measured in a co-culture system with PD-1 Jurkat and PD-L1/aAPC CHO-K1 cells by T-cell receptor (TCR) activation-induced nuclear factor of activated T cells (NFAT)-luciferase reporter assay. The detailed binding mode of action was simulated by an in silico docking study and pharmacophore analysis. Competitive ELISA revealed that KO and its glycoside KR significantly inhibited PD-1/PD-L1 interaction. Cellular PD-1/PD-L1 blocking activity was monitored by KO and KR at non-cytotoxic concentration. Surface plasmon resonance (SPR) and biolayer interferometry (BLI) analysis suggested the binding affinity and direct inhibition of KR against PD-1/PD-L1. An in silico docking simulation determined the detailed mode of binding of KR to PD-1/PD-L1. Collectively, these results suggest that KR could be developed as a potent small molecule inhibitor for PD-1/PD-L1 blockade.

Small Molecular Immune Modulators as Anticancer Agents

After decades of intense effort, immune checkpoint inhibitors have been conclusively demonstrated to be effective in cancer treatments and thus are revolutionizing the concepts in the treatment of cancers. Immuno-oncology has arrived and will play a key role in cancer treatment in the foreseeable future. However, efforts to find novel methods to improve the immune response to cancer have not ceased. Small-molecule approaches offer inherent advantages over biologic immunotherapies since they can cross cell membranes, penetrate into tumor tissue and tumor microenvironment more easily, and are amenable to be finely controlled than biological agents, which may help reduce immune-related adverse events seen with biologic therapies and provide more flexibility for the combination use with other therapies and superior clinical benefit. On the one hand, small-molecule therapies can modulate the immune response to cancer by restoring the antitumor immunity, promoting more effective cytotoxic lymphocyte responses, and regulating tumor microenvironment, either directly or epigenetically. On the other hand, the combination of different mechanisms of small molecules with antibodies and other biologics demonstrated admirable synergistic effect in clinical settings for cancer treatment and may expand antibodies' usefulness for broader clinical applications. This chapter provides an overview of small-molecule immunotherapeutic approaches either as monotherapy or in combination for the treatment of cancer.

Development of bispecific antibodies in China: overview and prospects

A bispecific antibody (bsAb) can simultaneously bind two different epitopes or antigens, allowing for multiple mechanistic functions with synergistic effects. BsAbs have attracted significant scientific attentions and efforts towards their development as drugs for cancers. There are 21 bsAbs currently undergoing clinical trials in China. Here, we review their platform technologies, expression and production, and biological activities and bioassay of these bsAbs, and summarize their structural formats and mechanisms of actions. T-cell redirection and checkpoint inhibition are two main mechanisms of the bsAbs that we discuss in detail. Furthermore, we provide our perspective on the future of bsAb development in China, including CD3-bsAbs for solid tumors and related cytokine release syndromes, expression and chemistry, manufacturing and controls, clinical development, and immunogenicity.

VISTA: Coming of age as a multi-lineage immune checkpoint

The immune response is governed by a highly complex set of interactions among cells and mediators. T cells may be rendered dysfunctional by the presence of high levels of antigen in the absence of co-stimulation while myeloid cells may be programmed towards an immunosuppressive state that promotes cancer growth and metastasis while deterring tumor immunity. In addition, inhibitory programs driven by immune checkpoint regulators dampen anti-tumor immunity. The ideal cancer immunotherapy treatment will improve both cross-priming in the tumor microenvironment and relieve suppression by the inhibitory checkpoints. Recently, blockade of programmed cell death 1 (PD-1) and cytotoxic T lymphocyte antigen 4 (CTLA-4) has elicited impressive results, but not in all patients, so additional targets are under investigation. V-set immunoglobulin domain suppressor of T cell activation (VISTA) is a novel immunoregulatory receptor that is broadly expressed on cells of the myeloid and lymphoid lineages, and is frequently implicated as a poor prognostic indicator in multiple cancers. Importantly, antibody targeting of VISTA uniquely engages both innate and adaptive immunity. This, combined with the expression of VISTA and its non-redundant activities compared to other immune checkpoint regulators, qualifies VISTA to be a promising target for improving cancer immunotherapy.

Immunotherapy for mesothelioma: a critical review of current clinical trials and future perspectives

At the clinical level the role of immunotherapy in cancer is currently at a pivotal point. Therapies such as checkpoint inhibitors are being approved at many levels in cancers such as non-small cell lung cancer (NSCLC). Mesothelioma is a rare orphan disease associated with prior exposure to asbestos, with a dismal prognosis. Various clinical trials for checkpoint inhibitors have been conducted in this rare disease, and suggest that such therapies may play a role as a treatment option for a proportion of patients with this cancer. Most recently approved as a salvage therapy in mesothelioma was granted in Japan, regulatory approval for their use in the clinic elsewhere lags. In this article we review the current pertinent clinical trials of immunotherapies in malignant mesothelioma, discuss the current issues that may affect the clinical outcomes of such therapies and further evaluate potential candidate new avenues that may become future targets for immunotherapy in this cancer.

Small molecules-Giant leaps for immuno-oncology

Immuno-oncology therapies are revolutionizing the oncology landscape with checkpoint blockade becoming the treatment backbone for many indications. While inspiring, much work remains to increase the number of cancer patients that can benefit from these treatments. Thus, a new era of immuno-oncology research has begun which is focused on identifying novel combination regimes that lead to improved response rates. This review highlights the significance of small molecules in this approach and illustrates the huge progress that has been made to date.

Proteasomal and lysosomal degradation for specific and durable suppression of immunotherapeutic targets

Cancer immunotherapy harness the body’s immune system to eliminate cancer, by using a broad panel of soluble and membrane proteins as therapeutic targets. Immunosuppression signaling mediated by ligand-receptor interaction may be blocked by monoclonal antibodies, but because of repopulation of the membrane via intracellular organelles, targets must be eliminated in whole cells. Targeted protein degradation, as exemplified in proteolysis targeting chimera (PROTAC) studies, is a promising strategy for selective inhibition of target proteins. The recently reported use of lysosomal targeting molecules to eliminate immune checkpoint proteins has paved the way for targeted degradation of membrane proteins as crucial anti-cancer targets. Further studies on these molecules’ modes of action, target-binding “warheads”, lysosomal sorting signals, and linker design should facilitate their rational design. Modifications and derivatives may improve their cell-penetrating ability and the in vivo stability of these pro-drugs. These studies suggest the promise of alternative strategies for cancer immunotherapy, with the aim of achieving more potent and durable suppression of tumor growth. Here, the successes and limitations of antibody inhibitors in cancer immunotherapy, as well as research progress on PROTAC- and lysosomal-dependent degradation of target proteins, are reviewed.

Structure and Optimization of Checkpoint Inhibitors

With the advent of checkpoint inhibitor treatment for various cancer types, the optimization of drug selection, pharmacokinetics and biomarker assays is an urgent and as yet unresolved dilemma for clinicians, pharmaceutical companies and researchers. Drugs which inhibit cytotoxic T-lymphocyte associated protein-4 (CTLA-4), such as ipilimumab and tremelimumab, programmed cell death protein-1 (PD-1), such as nivolumab and pembrolizumab, and programmed cell death ligand-1 (PD-L1), such as atezolizumab, durvalumab and avelumab, each appear to have varying pharmacokinetics and clinical activity in different cancer types. Each drug differs in terms of dosing, which becomes an issue when drug comparisons are attempted. Here, we examine the various checkpoint inhibitors currently used and in development. We discuss the antibodies and their protein targets, their pharmacokinetics as measured in various tumor types, and their binding affinities to their respective antigens. We also examine the various dosing regimens for these drugs and how they differ. Finally, we examine new developments and methods to optimize delivery and efficacy in the field of checkpoint inhibitors, including non-fucosylation, prodrug formations, bispecific antibodies, and newer small molecule and peptide checkpoint inhibitors.

Progress in PD-1/PD-L1 pathway inhibitors: From biomacromolecules to small molecules

Programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1) is a negative immune checkpoint pathway that inhibit immune responses, and upregulation of this pathway has implications in many malignancies. The search for effective PD-1/PD-L1 inhibitors has been at the forefront of academic and industrial medicinal chemistry, leading to 16 clinical candidates and the launch of six monoclonal antibodies (mAbs) drugs. Despite the unprecedented success achieved, the limitations of mAbs, including poor tissue and tumor penetration, long half-life time, poor oral bioavailability, and expensive production costs, impelled researchers to turn their attention to the development of peptide-based and non-peptide small-molecule inhibitors as potential alternatives or supplements to mAbs. However, no small-molecule inhibitors have been approved so far, indicating a challenging process of developing marketable small-molecule PD-1/PD-L1 targeted therapeutics. This review will summarize and provide insight into recent advances in the PD-1/PD-L1 pathway, including its structural basis and biology, along with the crystal structures with mAbs, peptides and small molecules. We place great emphasis on design strategies underlying reported small-molecule inhibitors and attempt to provide an outlook at the future of small-molecule PD-1/PD-L1inhibitors.

Immune Checkpoint PD-1/PD-L1 CTLA-4/CD80 are Blocked by Rhus verniciflua Stokes and its Active Compounds

The bark of Rhus verniciflua Stokes (RVS) has been used to treat cancer in Korean herbal medicine. When we screened for PD-1 and CTLA-4 immune checkpoint inhibitors (PD-1/PD-L1 CTLA-4/CD80) from around 800 herbal extracts using competitive Enzyme-Linked Immunosorbent Assay (ELISA), we found that RVS blocked both the PD-1/PD-L1 and the CTLA-4/CD80 interactions. To identify the active compounds from RVS, we performed bioactivity-guided fractionation, and the ethyl acetate (EtOAc) fraction of RVS proved to be the most effective at blocking the PD-1/PD-L1 and CTLA-4/CD80 interactions. In addition, we isolated and identified 20 major compounds in the EtOAc fraction of RVS and then examined the blocking effects of these 20 compounds on PD-1/PD-L1 and CTLA-4/CD80. Among them, four compounds [eriodictyol (7) > fisetin (9) > quercetin (18) > liquiritigenin (13)] blocked the interaction of PD-1/PD-L1 on competitive ELISA. In addition, four different compounds [protocatechuic acid (2) > caffeic acid (19) > taxifolin (5) > butin (6)] blocked the interaction of CTLA-4/CD80. Our findings suggest that RVS and its components could be used as a potential immune checkpoint inhibitor blockade and could be developed for immuno-oncological therapeutics.