Design, Synthesis, Evaluation, and Structural Studies of C2-Symmetric Small Molecule Inhibitors of Programmed Cell Death-1/Programmed Death-Ligand 1 Protein-Protein Interaction

Progress in small-molecule inhibitors targeting PD-L1

PD-L1 is a transmembrane protein overexpressed by tumor cells. It binds to PD-1 on the surface of T-cells, suppresses T-cell activity and hinders the immune response against cancer. Clinically, several monoclonal antibodies targeting PD-1/PD-L1 have achieved significant success in cancer immunotherapy. Nevertheless, their disadvantages, such as unchecked immune responses, high cost and long half-life, stimulated pharmacologists to develop small-molecule inhibitors targeting PD-1/PD-L1. After a batch of excellent inhibitors with a biphenyl core structure were firstly reported by BMS, more and more researchers focused on small-molecule inhibitors targeting PD-L1 rather than PD-1. Numerous small-molecule inhibitors were extensively designed and synthesized in the past few years. In this paper, the structural characteristics of PD-L1 and complexes of PD-L1 with its inhibitors are elaborated and small molecule inhibitors developed in the last decade are summarized as well. This paper aims to provide insights into further designing and synthesis of small molecule inhibitors targeting PD-L1.

Design, Synthesis, and Evaluation of 8-(o-Tolyl)quinazoline Derivatives as Small-Molecule PD-1/PD-L1 Antagonists

Small-molecule inhibitors targeting programmed cell death-1/programmed cell death-ligand 1 (PD-1/PD-L1) interactions can compensate for the shortcomings of antibody-based inhibitors and have attracted considerable attention, some of which have already entered clinical trials. Herein, based on our previous study on small-molecule PD-L1 inhibitors, we reported a series of 8-(o-tolyl)quinazoline derivatives by the skeleton merging strategy. Homogenous time-resolved fluorescence (HTRF) assay against PD-1/PD-L1 interaction identified compound A5, which showed the most potent inhibition with an IC50 value of 23.78 nM. Meanwhile, based on the results of HTRF assay, the structure-activity relationships (SARs) of the tail were focused on. Cell-based PD-1/PD-L1 blockade assay further revealed that A5 significantly blocked the PD-1/PD-L1 interaction at 1.1 μM in the co-culture system of Jurkat-NFAT-PD-1 cells and Hep3B-OS8-hPD-L1 cells with no significant cytotoxicity on Jurkat cells. Moreover, the proposed binding mode of A5 was investigated by a docking analysis. These results indicate that compound A5 is a promising lead compound that deserves further investigation.

Small-molecule modulators of tumor immune microenvironment

In recent years, tumor immunotherapy, aimed at increasing the activity of immune cells and reducing immunosuppressive effects, has attracted wide attention. Among them, immune checkpoint blocking (ICB) is the most commonly explored therapeutic approach. All approved immune checkpoint inhibitors (ICIs) are clinically effective monoclonal antibodies (mAbs). Compared with biological agents, small-molecule drugs have many unique advantages in tumor immunotherapy. Therefore, they also play an important role. Immunosuppressive signals such as PD-L1, IDO1, and TGF-β, etc. overexpressed in tumor cells form the tumor immunosuppressive microenvironment. In addition, the efficacy of multi-pathway combined immunotherapy has also been reported and verified. Here, we mainly reviewed the mechanism of tumor immunotherapy, analyzed the research status of small-molecule modulators, and discussed drug candidates' structure-activity relationship (SAR). It provides more opportunities for further research to design more immune small-molecule modulators with novel structures.

Computational analysis of PD-L1 dimerization mechanism induced by small molecules and potential dynamical properties

The design of small molecule inhibitors that target the programmed death ligand-1 (PD-L1) is a forefront issue in immune checkpoint blocking therapy. Small-molecule inhibitors have been shown to exert therapeutic effects by inducing dimerization of the PD-L1 protein, however, the specific mechanisms underlying this dimerization process remain largely unexplored. Furthermore, there is a notable lack of comparative studies examining the binding modes of structurally diverse inhibitors. In view of the research gaps, this work employed molecular dynamics simulations to meticulously examine the interactions between two distinct types of inhibitors and PD-L1 in both monomeric and dimeric forms, and predicted the dimerization mechanism. The results revealed that inhibitors initially bind to a PD-L1 monomer, subsequently attracting another monomer to form a dimer. Notably, symmetric inhibitors observed superior binding efficiency compared to other inhibitors. Key residues, including Ile54, Tyr56, Met115 and Tyr123 played a leading role in binding. Structurally, symmetric inhibitors were capable of thoroughly engaging the binding pocket, promoting a more symmetrical formation of PD-L1 dimers. Furthermore, symmetric inhibitors formed more extensive hydrophobic interactions with protein residues. The insights garnered from this research are expected to significantly contribute to the rational design and optimization of small molecule inhibitors targeting PD-L1.

Small molecule and PROTAC molecule experiments in vitro and in vivo, focusing on mouse PD-L1 and human PD-L1 differences as targets

In recent years, several monoclonal antibodies (mAbs) targeting PD-L1 have been licensed by the FDA for use in the treatment of cancer, demonstrating the effectiveness of blocking immune checkpoints, particularly the PD-1/PD-L1 pathway. Although mAb-based therapies have made great strides, they still have their limitations, and new small-molecule or PROTAC-molecule inhibitors that can block the PD-1/PD-L1 axis are desperately needed. Therefore, it is crucial to translate initial in vitro discoveries into appropriate in vivo animal models when creating PD-L1-blocking therapies. Due to their widespread availability and low experimental expenses, classical immunocompetent mice are appealing for research purposes. However, it is yet unclear whether the mouse (m) PD-L1 interaction with human (h) PD-1 in vivo would produce a functional immunological checkpoint. In this review, we summarize the in vitro and in vivo experimental studies of small molecules and PROTAC molecules, particularly the distinctions between mPD-L1 as a target and hPD-L1 as a target.

Exploring Hydrophilic PD-L1 Radiotracers Utilizing Phosphonic Acids: Insights into Unforeseen Pharmacokinetics

Immune checkpoint inhibitor therapy targeting the PD-1/PD-L1 axis in cancer patients, is a promising oncological treatment. However, the number of non-responders remains high, causing a burden for the patient and the healthcare system. Consequently, a diagnostic tool to predict treatment outcomes would help with patient stratification. Molecular imaging provides said diagnostic tool by offering a whole-body quantitative assessment of PD-L1 expression, hence supporting therapy decisions. Four PD-L1 radioligand candidates containing a linker-chelator system for radiometalation, along with three hydrophilizing units-one sulfonic and two phosphonic acids-were synthesized. After labeling with 64Cu, log D7.4 values of less than -3.03 were determined and proteolytic stability confirmed over 94% intact compound after 48 h. Binding affinity was determined using two different assays, revealing high affinities up to 13 nM. µPET/CT imaging was performed in tumor-bearing mice to investigate PD-L1-specific tumor uptake and the pharmacokinetic profile of radioligands. These results yielded an unexpected in vivo distribution, such as low tumor uptake in PD-L1 positive tumors, high liver uptake, and accumulation in bone/bone marrow and potentially synovial spaces. These effects are likely caused by Ca2+-affinity and/or binding to macrophages. Despite phosphonic acids providing high water solubility, their incorporation must be carefully considered to avoid compromising the pharmacokinetic behavior of radioligands.

Heterogeneity in M. tuberculosis β-lactamase inhibition by Sulbactam

For decades, researchers have elucidated essential enzymatic functions on the atomic length scale by tracing atomic positions in real-time. Our work builds on possibilities unleashed by mix-and-inject serial crystallography (MISC) at X-ray free electron laser facilities. In this approach, enzymatic reactions are triggered by mixing substrate or ligand solutions with enzyme microcrystals. Here, we report in atomic detail (between 2.2 and 2.7 Å resolution) by room-temperature, time-resolved crystallography with millisecond time-resolution (with timepoints between 3 ms and 700 ms) how the Mycobacterium tuberculosis enzyme BlaC is inhibited by sulbactam (SUB). Our results reveal ligand binding heterogeneity, ligand gating, cooperativity, induced fit, and conformational selection all from the same set of MISC data, detailing how SUB approaches the catalytic clefts and binds to the enzyme noncovalently before reacting to a trans-enamine. This was made possible in part by the application of singular value decomposition to the MISC data using a program that remains functional even if unit cell parameters change up to 3 Å during the reaction.

Discovery of Novel d-(+)-Biotin-Conjugated Resorcinol Dibenzyl Ether-Based PD-L1 Inhibitors for Targeted Cancer Immunotherapy

In this work, we rationally designed, synthesized, and evaluated a series of novel d-(+)-biotin-conjugated PD-L1 inhibitors for targeted cancer therapy. Among them, SWS1 exhibited the highest anti-PD-1/PD-L1 activity with an IC50 of 1.8 nM. In addition, SWS1 dose-dependently promoted tumor cell death in a HepG2/Jurkat cell co-culture model. Importantly, SWS1 displayed high antitumor efficacy in a B16-F10 mouse model with tumor growth inhibition of 66.1%, which was better than that of P18 (44.3%). Furthermore, SWS1 exerted antitumor effects by increasing the number of tumor-infiltrating lymphocytes and reducing the expression of PD-L1 in tumor tissues. Moreover, tissue distribution studies revealed a substantial accumulation of SWS1 in tumors (404.1 ng/mL). Lastly, the safety profiles of SWS1 were better (e.g., less immune-mediated colitis) than those of P18, indicating the advantages of biotin-enabled tumor targeting capability. Taken together, our results suggest that these novel tumor-targeted PD-L1 inhibitors are worthy of further investigation as potential anticancer agents for targeted cancer immunotherapy.

Theoretical and experimental studies on the interaction of biphenyl ligands with human and murine PD-L1: Up-to-date clues for drug design

Today it is widely recognized that the PD-1/PD-L1 axis plays a fundamental role in escaping the immune system in cancers, so that anti-PD-1/PD-L1 antibodies have been evaluated for their antitumor properties in more than 1000 clinical trials. As a result, some of them have entered the market revolutionizing the treatment landscape of specific cancer types. Nonetheless, a new era based on the development of small molecules as anti PD-L1 drugs has begun. There are, however, some limitations to advancing these compounds into clinical stages including the possible difficulty in counteracting the PD-1/PD-L1 interaction in vivo, the discrepancy between the in vitro IC₅₀ (HTFR assay) and cellular EC₅₀ (immune checkpoint blockade co-culture assay), and the differences in ligands' affinity between human and murine PD-L1, which can affect their preclinical evaluation. Here, an extensive theoretical study, assisted by MicroScale Thermophoresis binding assays and NMR experiments, was performed to provide an atomistic picture of the binding event of three representative biphenyl-based compounds in both human and murine PD-L1. Structural determinants of the species' specificity were unraveled, providing unprecedented details useful for the design of next generation anti-PD-L1 molecules.

Design and Biological Evaluation of Small-Molecule PET-Tracers for Imaging of Programmed Death Ligand 1

Noninvasive molecular imaging of the PD-1/PD-L1 immune checkpoint is of high clinical relevance for patient stratification and therapy monitoring in cancer patients. Here we report nine small-molecule PD-L1 radiotracers with solubilizing sulfonic acids and a linker-chelator system, designed by molecular docking experiments and synthesized according to a new, convergent synthetic strategy. Binding affinities were determined both in cellular saturation and real-time binding assay (LigandTracer), revealing dissociation constants in the single digit nanomolar range. Incubation in human serum and liver microsomes proved in vitro stability of these compounds. Small animal PET/CT imaging, in mice bearing PD-L1 overexpressing and PD-L1 negative tumors, showed moderate to low uptake. All compounds were cleared primarily through the hepatobiliary excretion route and showed a long circulation time. The latter was attributed to strong blood albumin binding effects, discovered during our binding experiments. Taken together, these compounds are a promising starting point for further development of a new class of PD-L1 targeting radiotracers.

Inside PD-1/PD-L1,2 with their inhibitors

This review summarizes current knowledge in the development of immune checkpoint inhibitors, including antibodies and small molecules.

Discovery of Small-Molecule Autophagy Inhibitors by Disrupting the Protein-Protein Interactions Involving Autophagy-Related 5

One possible strategy for modulating autophagy is to disrupt the critical protein-protein interactions (PPIs) formed during this process. Our attention is on the autophagy-related 12 (ATG12)-autophagy-related 5 (ATG5)-autophagy-related 16-like 1 (ATG16L1) heterotrimer complex, which is responsible for ATG8 translocation from ATG3 to phosphatidylethanolamine. In this work, we discovered a compound with an (E)-3-(2-furanylmethylene)-2-pyrrolidinone core moiety (T1742) that blocked the ATG5-ATG16L1 and ATG5-TECAIR interactions in the in vitro binding assay (IC₅₀ = 1-2 μM) and also exhibited autophagy inhibition in cellular assays. The possible binding mode of T1742 to ATG5 was predicted through molecular modeling, and a batch of derivatives sharing essentially the same core moiety were synthesized and tested. The outcomes of the in vitro binding assay and the flow cytometry assay of those newly synthesized compounds were generally consistent. This work has validated our central hypothesis that small-molecule inhibitors of the PPIs involving ATG5 can tune down autophagy effectively, and their pharmaceutical potential may be further explored.

Heterogeneity in the M. tuberculosis β-Lactamase Inhibition by Sulbactam

For decades, researchers have been determined to elucidate essential enzymatic functions on the atomic lengths scale by tracing atomic positions in real time. Our work builds on new possibilities unleashed by mix-and-inject serial crystallography (MISC) 1-5 at X-ray free electron laser facilities. In this approach, enzymatic reactions are triggered by mixing substrate or ligand solutions with enzyme microcrystals 6 . Here, we report in atomic detail and with millisecond time-resolution how the Mycobacterium tuberculosis enzyme BlaC is inhibited by sulbactam (SUB). Our results reveal ligand binding heterogeneity, ligand gating 7-9 , cooperativity, induced fit 10,11 and conformational selection 11-13 all from the same set of MISC data, detailing how SUB approaches the catalytic clefts and binds to the enzyme non-covalently before reacting to a trans- enamine. This was made possible in part by the application of the singular value decomposition 14 to the MISC data using a newly developed program that remains functional even if unit cell parameters change during the reaction.

Approaching the Dimerization Mechanism of Small Molecule Inhibitors Targeting PD-L1 with Molecular Simulation

Inhibitors blocking the PD-1/PD-L1 immune checkpoint demonstrate impressive anti-tumor immunity, and small molecule inhibitors disclosed by the Bristol-Myers Squibb (BMS) company have become a hot topic. In this work, by modifying the carbonyl group of BMS-202 into a hydroxyl group to achieve two enantiomers (MS and MR) with a chiral center, we found that this is an effective way to regulate its hydrophobicity and thus to reduce the negative effect of polar solvation free energy, which enhances the stability of PD-L1 dimer/inhibitor complexes. Moreover, we studied the binding modes of BMS-200 and BMS-202-related small molecule inhibitors by molecular dynamics simulation to explore their inhibitory mechanism targeting PD-L1 dimerization. The results showed that the size exclusion effect of the inhibitors triggered the rearrangement of the residue _ATyr56, leading to the formation of an axisymmetric tunnel-shaped pocket, which is an important structural basis for improving the binding affinity of symmetric inhibitors with PD-L1. Furthermore, after inhibitor dissociation, the conformation of _ATyr123 and _BMet115 rearranged, which blocked the entrance of the binding pocket, while the reverse rearrangements of the same residues occurred when the PD-L1 monomer was complexed with the inhibitors, preparing PD-L1 for dimerization. Overall, this study casts a new light on the inhibitory mechanism of BMS inhibitors targeting PD-L1 dimerization and provides an idea for designing novel small molecule inhibitors for future cancer immunotherapy.

In vitro characterization of immune modulating drug-eluting immunobeads towards transarterial embolization in cancer

Hepatocellular carcinoma (HCC) is an aggressive liver cancer with limited effective treatment options. In this study, we selected TLR agonists imiquimod (IMQ), gardiquimod (GARD), GS-9620 and DSR 6434, and a small molecule checkpoint inhibitor, BMS-202, for characterization of drug loading and release from radiopaque embolic beads (DC Bead LUMI) for potential use in image-guided transarterial embolization (TACE) of HCC. The maximum drug loading capacity and amount of drug released over time were determined by high performance liquid chromatography and compared with the commonly used anthracycline, doxorubicin hydrochloride (Dox). Maximum drug loading was 204.54 ± 3.87, 65.28 ± 3.09, 65.95 ± 6.96, 65.97 ± 1.54, and 148.05 ± 2.24 mg of drug per milliliter of DC Bead LUMI for Dox, GARD, DSR 6434, IMQ, and BMS-202, respectively. Fast loading and subsequent rapid release in saline were observed for IMQ, GARD, and DSR 6434. These drugs could also be partially removed from the beads by repeated washing with de-ionized water suggesting weak interaction with the beads. Aggregation of IMQ was observed in water and saline. GS-9620 partially decomposed in the solubilizing solution, so loading and release were not characterized. Compared to TLR agonists, slower loading and release were observed for Dox and BMS-202. Potential factors influencing drug loading into and release from DC Bead LUMI including steric hinderance, hydrophobicity, drug pKa, and the electrostatic nature of the beads are discussed. The maximum loading capacity of BMS-202 and Dox in DC Bead LUMI exceeded the maximum theoretical loading capacity of the beads expected from ionic interaction alone suggesting additional drug-bead or drug-drug interactions may play a role. Slightly more release was observed for BMS-202 at early time points followed by a slower release compared to Dox. Further study of these drug-bead combinations is warranted in search of new tools for locoregional delivery of immune-modulating agents for treatment of HCC via drug-eluting bead chemoembolization.

Novel phthalimides regulating PD-1/PD-L1 interaction as potential immunotherapy agents

Programmed cell death 1(PD-1)/programmed cell death ligand 1(PD-L1) have emerged as one of the most promising immune checkpoint targets for cancer immunotherapy. Despite the inherent advantages of small-molecule inhibitors over antibodies, the discovery of small-molecule inhibitors has fallen behind that of antibody drugs. Based on docking studies between small molecule inhibitor and PD-L1 protein, changing the chemical linker of inhibitor from a flexible chain to an aromatic ring may improve its binding capacity to PD-L1 protein, which was not reported before. A series of novel phthalimide derivatives from structure-based rational design was synthesized. P39 was identified as the best inhibitor with promising activity, which not only inhibited PD-1/PD-L1 interaction (IC₅₀ = 8.9 nmol/L), but also enhanced killing efficacy of immune cells on cancer cells. Co-crystal data demonstrated that P39 induced the dimerization of PD-L1 proteins, thereby blocking the binding of PD-1/PD-L1. Moreover, P39 exhibited a favorable safety profile with a LD₅₀ > 5000 mg/kg and showed significant in vivo antitumor activity through promoting CD8⁺ T cell activation. All these data suggest that P39 acts as a promising small chemical inhibitor against the PD-1/PD-L1 axis and has the potential to improve the immunotherapy efficacy of T-cells.

Exploiting the Innate Plasticity of the Programmed Cell Death-1 (PD1) Receptor to Design Pembrolizumab H3 Loop Mimics

Checkpoint blockade of the immunoreceptor programmed cell death-1 (PD1) with its ligand-1 (PDL1) by monoclonal antibodies such as pembrolizumab provided compelling clinical results in various cancer types, yet the molecular mechanism by which this drug blocks the PD1/PDL1 interface remains unclear. To address this question, we examined the conformational motion of PD1 associated with the binding of pembrolizumab. Our results revealed that the innate plasticity of both C'D and FG loops is crucial to form a deep binding groove (371 Å3 ) across several distant epitopes of PD1. This analysis ultimately provided a rational-design to create pembrolizumab H3 loop mimics [RDYRFDMGFD] into β-hairpin scaffolds. As a result, a 20-residue long β-hairpin peptide 1 e was identified as a first-in-class potent PD1-inhibitor (EC50 of 0.29 μM; Ki of 41 nM).

Discovery of small molecule inhibitors that effectively disrupt IQGAP1-Cdc42 interaction in breast cancer cells

The small GTPase Cdc42 is an integral component of the cytoskeleton, and its dysregulation leads to pathophysiological conditions, such as cancer. Binding of Cdc42 to the scaffold protein IQGAP1 stabilizes Cdc42 in its active form. The interaction between Cdc42 and IQGAP1 enhances migration and invasion of cancer cells. Disrupting this association could impair neoplastic progression and metastasis; however, no effective means to achieve this has been described. Here, we screened 78,500 compounds using a homogeneous time resolved fluorescence-based assay to identify small molecules that disrupt the binding of Cdc42 to IQGAP1. From the combined results of the validation assay and counter-screens, we selected 44 potent compounds for cell-based experiments. Immunoprecipitation and cell viability analysis rendered four lead compounds, namely NCGC00131308, NCGC00098561, MLS000332963 and NCGC00138812, three of which inhibited proliferation and migration of breast carcinoma cells. Microscale thermophoresis revealed that two compounds bind directly to Cdc42. One compound reduced the amount of active Cdc42 in cells and effectively impaired filopodia formation. Docking analysis provided plausible models of the compounds binding to the hydrophobic pocket adjacent to the GTP binding site of Cdc42. In conclusion, we identified small molecules that inhibit binding between Cdc42 and IQGAP1, which could potentially yield chemotherapeutic agents.

Biphenyl-based small molecule inhibitors: Novel cancer immunotherapeutic agents targeting PD-1/PD-L1 interaction

The immune checkpoint proteins are those key to the body's immunity which can either boost the immune system to protect the body from pathogens; or suppress the body's immunity system for the goal of self-tolerance. Cancer cells have evolved some mechanisms to boost the immuno-inhibitory checkpoints to bypass the immune system of the body. The binding of Programmed Cell Death-1 (PD-1) protein with its ligand Programmed Cell Death Ligand-1 (PD-L1) promotes this kind of immune-inhibitory signal. The discovery of immune checkpoint inhibitors was started in the early 21st century; with some success through monoclonal antibodies, peptides, and small molecules. Being the most reliable and safest way to target immune checkpoints, the scientific community is exploring possibilities to develop small molecule inhibitors. Among the different scaffolds of the small molecule, the most exposed and researched core molecule is Biphenyl-based scaffolds. We have described all of the possible biphenyl-based small molecules in this article, as well as their interactions with various amino acids in the binding cavity. The link between the in silico, in vitro, and in vivo activities of the PD-1/PD-L1 inhibitors are well connected. The Tyr56, Met115, Ala121, and Asp122 were detected as the crucial amino acids of the PD-1/PD-L1 inhibition. Additionally, a detailed binding pocket analysis of the PD-L1 receptor was carried out, where it was observed and confirmed that the binding pocket is tunnel-shaped and hydrophobic in nature. Finally, the structure-activity relationship of the biphenyl-based small molecule inhibitors was developed based on their activity and the binding interactions.

Generation, secretion and degradation of cancer immunotherapy target PD-L1

Cancer immunotherapy is a rapidly developing and effective method for the treatment of a variety of malignancies in recent years. As a significant immune checkpoint, programmed cell death 1 ligand 1 (PD-L1) and its receptor programmed cell death protein 1 (PD-1) play the most significant role in cancer immune escape and cancer immunotherapy. Though PD-L1 have become an important target for drug development and there have been various approved drugs and clinic trials targeting it, and various clinical response rate and adverse reactions prevent many patients from benefiting from it. In recent years, combination trials have become the main direction of PD-1/PD-L1 antibodies development. Here, we summarized PD-L1 biofunctions and key roles in various cancers along with the development of PD-L1 inhibitors. The regulators that are involved in controlling PD-L1 expression including post-translational modification, mRNA level regulation as well as degradation and exosome secretory pathway of PD-L1 were focused. This systematic summary may provide comprehensive understanding of different regulations on PD-L1 as well as a broad prospect for the search of the important regulator of PD-L1. The regulatory factors of PD-L1 can be potential targets for immunotherapy and increase strategies of immunotherapy in combination.

Small Molecule Agents Targeting PD-1 Checkpoint Pathway for Cancer Immunotherapy: Mechanisms of Action and Other Considerations for Their Advanced Development

Pioneering success of antibodies targeting immune checkpoints such as programmed cell death protein 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) has changed the outlook of cancer therapy. Although these antibodies show impressive durable clinical activity, low response rates and immune-related adverse events are becoming increasingly evident in antibody-based approaches. For further strides in cancer immunotherapy, novel treatment strategies including combination therapies and alternate therapeutic modalities are highly warranted. Towards this discovery and development of small molecule, checkpoint inhibitors are actively being pursued, and the efforts have culminated in the ongoing clinical testing of orally bioavailable checkpoint inhibitors. This review focuses on the small molecule agents targeting PD-1 checkpoint pathway for cancer immunotherapy and highlights various chemotypes/scaffolds and their characterization including binding and functionality along with reported mechanism of action. The learnings from the ongoing small molecule clinical trials and crucial points to be considered for their clinical development are also discussed.

Discovery of 4-phenylindolines containing a (5-cyanopyridin-3-yl)methoxy moiety as potent inhibitors of the PD-1/PD-L1 interaction

With the great success of anti-programmed cell death-1 (PD-1)/programmed cell death ligand-1 (PD-L1) monoclonal antibodies in clinical applications, blocking the PD-1/PD-L1 pathway has become the most compelling strategy in the field of tumor immunotherapy. In this study, a novel series of 4-phenylindolines containing a (5-cyanopyridin-3-yl)methoxy moiety were developed, and their structure-activity relationships were preliminarily discussed. Among them, compounds M17 and M23 exhibited the most potent ability to disrupt the PD-1/PD-L1 interaction, demonstrating IC₅₀ values of 60.1 nM and 53.2 nM, respectively. The binding mode of M23 was further explored by molecular docking analysis with dimeric PD-L1. Therefore, M17 and M23 are promising lead compounds for developing potent inhibitors of the PD-1/PD-L1 axis.

Small Molecule Inhibitors of Programmed Cell Death Ligand 1 (PD-L1): A Patent Review (2019-2021)

INTRODUCTION

The blockade of immune checkpoints, especially the PD-1/PD-L1 pathway with therapeutic antibodies, has shown success in treating cancers in recent years. Seven monoclonal antibodies (mAbs) targeting PD-1 or PD-L1 have been approved by FDA. However, mAbs exhibit several disadvantages as compared to small molecules such as poor permeation, high manufacturing costs, immunogenicity as well as lacking oral bioavailability. Recently, small-molecule inhibitors targeting PD-L1 have been disclosed with the ability to modulate the PD-1/PD-L1 pathway.

AREAS COVERED

The authors reviewed small molecules targeting PD-L1 that block the PD-1/PD-L1 protein-protein interaction for the treatment of various diseases.

EXPERT OPINION

Compared with mAbs, PD-1/PD-L1 small-molecule inhibitors show several advantages such as improved tissue penetration, low immunogenicity, well-understood formulation and lower manufacturing costs. They can serve as complementary or synergistically with mAbs for immune therapy. However, at this time most of the reported inhibitors are still inferior to therapeutic antibodies in their inhibitory activities due to smaller molecular weight. Therefore, better small molecules need to be developed to improve their potencies. Moreover, although several PD-L1 small-molecule inhibitors have shown excellent preclinical results, their safety and efficacy in the clinic still awaits further validation.

YPD-30, a prodrug of YPD-29B, is an oral small-molecule inhibitor targeting PD-L1 for the treatment of human cancer

PD-1 and PD-L1 antibodies have brought about extraordinary clinical benefits for cancer patients, and their indications are expanding incessantly. Currently, most PD-1/PD-L1 agents are administered intravenously, which may be uncomfortable for some cancer patients. Herein, we develop a novel oral-delivered small molecular, YPD-29B, which specifically targets human PD-L1. Our data suggested that YPD-29B could potently and selectively block the interaction between PD-L1 and PD-1, but did not inhibit any other immune checkpoints. Mechanistically, YPD-29B induced human PD-L1 dimerization and internalization, which subsequently activated T lymphocytes and therefore overcomes immunity tolerance in vitro. YDP-29B was modified as the YPD-30 prodrug to improve druggability. Using humanized mice with human PD-1 xenografts of human PD-L1 knock-in mouse MC38 cancer cells, we demonstrated that YPD-30 exhibited significant antitumor activity and was well tolerated in vivo. Taken together, our results indicate that YPD-30 serves as a promising therapeutic candidate for anti-human PD-L1 cancer immunotherapy.

Cancer Immunotherapy: An Overview on Small Molecules as Inhibitors of the Immune Checkpoint PD-1/PD-L1 (2015-2021)

In 2018, James Allison and Tasuku Honjo received the Nobel Prize in Physiology or Medicine for their discovery of tumor therapy inhibition of negative immune regulation. Immunotherapy stimulates T-cells to fight cancer cells by blocking different immune checkpoint pathways. The interaction between programmed cell death 1 (PD-1) and its ligand PD-L1 (Programmed cell death ligand 1), is one of the main immune checkpoints. Of note, interfering with this pathway is already exploited in clinical cancer therapy, demonstrating that it is one of the key factors involved in cancer immune escape. The development of monoclonal antibodies (mAbs) that possess the ability to inhibit the interactions between PD-1/PD-L1 has radically made the difference in cancer immunotherapy. Yet, because of the many drawbacks that this therapy possesses, the research moved its efforts towards the development of novel small molecules. This may constitute a hope, but also an arduous challenge in fighting cancer. This paper reviews the recent primary literature concerning the development of novel small molecules able to blockade the interaction between PD-1 and its ligand PD-L1.

Advances of biphenyl small-molecule inhibitors targeting PD-1/PD-L1 interaction in cancer immunotherapy

Immunotherapy inhibiting the programmed death-1/programmed death ligand-1 (PD-1/PD-L1) interaction has emerged as one of the most attractive cancer treatment strategies. So far, the clinically used PD-1/PD-L1 inhibitors are monoclonal antibodies, but monoclonal antibodies have several limitations, such as poor pharmacokinetic properties, unchecked immune responses and high production cost. The development of small-molecule inhibitors targeting PD-1/PD-L1 interaction is showing great promise as a potential alternative or complementary therapeutic approach of monoclonal antibodies. In this article, the authors classify the reported biphenyl small-molecule inhibitors into symmetrical and asymmetrical types based on their structural features and further review their representative inhibitors and biological activities, as well as the binding models for providing insight into further exploration of more potent biphenyl small-molecule inhibitors targeting PD-1/PD-L1 interaction.

Predicting a Kind of Unusual Multiple-States Dimerization-Modes Transformation in Protein PD-L1 System by Computational Investigation and a Generalized Rate Theory

The new cancer immunotherapy has been carried out with an almost messianic zeal, but its molecular basis remains unclear due to the complexity of programmed death ligand 1 (PD-L1) dimerization. In this study, a new and integral multiple dimerization-modes transformation process of PD-L1s (with a new PD-L1 dimerization mode and a new transformation path discovered) and the corresponding mechanism are predicted using theoretical and computational methods. The results of the state analysis show that 5 stable binding states exist in system. A generalized inter-state transformation rate (GITR) theory is also proposed in such multiple-states self-assembly system to explore the kinetic characteristics of inter-state transformation. A “drug insertion” path was identified as the dominant path of the PD-L1 dimerization-modes transformation. Above results can provide supports for both the relative drug design and other multiple-states self-assembly system from the theoretical chemistry perspective.

Design, Synthesis, and Pharmacological Evaluation of Biaryl-Containing PD-1/PD-L1 Interaction Inhibitors Bearing a Unique Difluoromethyleneoxy Linkage

Blockade of immune checkpoint PD-1/PD-L1 has been a promising anticancer strategy; however, clinically available PD-1/PD-L1 small-molecule inhibitors are lacking. In view of the high potency of compound 2 (BMS-1002), structural fine tuning of the methoxy linkage together with diverse modification in the solvent interaction region was conducted. A series of novel derivatives featuring a difluoromethyleneoxy linkage were designed. Compound 43 was identified as the most promising PD-1/PD-L1 inhibitor with an IC₅₀ value of 10.2 nM in the HTRF assay. This compound is capable of promoting CD8⁺ T cell activation through inhibiting PD-1/PD-L1 cellular signaling. Moreover, in the Hepa1-6 syngeneic mouse model, administration of compound 43 at 1 mg/kg dosage promoted CD8⁺ T cell activation and delayed the tumor growth with good tolerance. Notably, the tumor in one mouse of the compound 43-treated group was completely regressed. These results indicate that compound 43 is a promising candidate worthy of further investigation.

Discovery of quinazoline derivatives as novel small-molecule inhibitors targeting the programmed cell death-1/programmed cell death-ligand 1 (PD-1/PD-L1) interaction

Development of small molecule PD-1/PD-L1 inhibitors as a novel immunotherapy strategy exhibits great promise. Herein, a novel series of quinazoline derivatives were designed, synthesized and their inhibitory activity against the PD-1/PD-L1 interaction was evaluated through a homogenous time-resolved fluorescence (HTRF) assay. Among them, the compound 39 exhibited the most potent inhibitory activity with an IC₅₀ value of 1.57 nM. Furthermore, the cellular level assays revealed that 39 could inhibit the PD-1/PD-L1 interaction and restore T-cell function, and showed low toxicity on the PBMCs. In addition, the structure-activity relationships (SARs) of the novel quinazoline derivatives were explored and the binding mode of 39 with dimeric PD-L1 was analyzed by molecular docking. This work demonstrates that incorporation of pyrimidine group between the 2 and 3-positions of the biphenyl structure is an effective strategy for designing novel and more potent small molecule PD-1/PD-L1 inhibitors, and 39 can be regarded as a promising lead compound for further investigation.

Interfering with the Tumor-Immune Interface: Making Way for Triazine-Based Small Molecules as Novel PD-L1 Inhibitors

The inhibition of the PD-1/PD-L1 axis by monoclonal antibodies has achieved remarkable success in treating a growing number of cancers. However, a novel class of small organic molecules, with BMS-202 (1) as the lead, is emerging as direct PD-L1 inhibitors. Herein, we report a series of 2,4,6-tri- and 2,4-disubstituted 1,3,5-triazines, which were synthesized and assayed for their PD-L1 binding by NMR and homogeneous time-resolved fluorescence. Among them, compound 10 demonstrated to strongly bind with the PD-L1 protein and challenged it in a co-culture of PD-L1 expressing cancer cells (PC9 and HCC827 cells) and peripheral blood mononuclear cells enhanced antitumor immune activity of the latter. Compound 10 significantly increased interferon γ release and apoptotic induction of cancer cells, with low cytotoxicity in healthy cells when compared to 1, thus paving the way for subsequent preclinical optimization and medical applications.

PD-L1 Inhibitors: Different Classes, Activities, and Mechanisms of Action

Targeting the programmed cell death protein 1/programmed cell death 1 ligand 1 (PD-1/PD-L1) interaction has become an established strategy for cancer immunotherapy. Although hundreds of small-molecule, peptide, and peptidomimetic inhibitors have been proposed in recent years, only a limited number of drug candidates show good PD-1/PD-L1 blocking activity in cell-based assays. In this article, we compare representative molecules from different classes in terms of their PD-1/PD-L1 dissociation capacity measured by HTRF and in vitro bioactivity determined by the immune checkpoint blockade (ICB) co-culture assay. We point to recent discoveries that underscore important differences in the mechanisms of action of these molecules and also indicate one principal feature that needs to be considered, which is the eventual human PD-L1 specificity.

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.

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.

Discovery of phenyl-linked symmetric small molecules as inhibitors of the programmed cell death-1/programmed cell death-ligand 1 interaction

Programmed cell death-1/programmed cell death ligand 1 (PD-1/PD-L1) is one of the most promising targets in the field of immune checkpoint blockade therapy. Beginning with our exploration of linkers and structure-activity relationship research, we found that the aromatic ring could replace the linker and aryl group to maintain the satisfactory activity of classic triaryl scaffold inhibitor. Based on previous studies, we designed and synthesized a series of C₂-symmetric phenyl-linked compounds, and further tail optimization afforded the inhibitors, which displayed promising inhibitory activity against the PD-1/PD-L1 interaction with IC₅₀ value at the single nanomolar range (C13-C15). Further cell-based PD-1/PD-L1 blockade bioassays indicated that these C₂-symmetric molecules could significantly inhibit the PD-1/PD-L1 interaction at the cellular level and restore T cells' immune function at the safety concentrations. The discovery of these phenyl-linked symmetric small molecules showed the potential of simplified-linker and C₂-symmetric strategy and provided a basis for developing symmetric small molecule inhibitors of PD-1/PD-L1 interaction. Moreover, C13 and C15 performed stable binding modes to PD-L1 dimeric after computational docking and dynamic simulation, which may serve as a good starting point for further development.

Discovery of Novel Small-Molecule Inhibitors of PD-1/PD-L1 Interaction via Structural Simplification Strategy

Blockade of the programmed cell death 1 (PD-1)/programmed cell death-ligand 1 (PD-L1) interaction is currently the focus in the field of cancer immunotherapy, and so far, several monoclonal antibodies (mAbs) have achieved encouraging outcomes in cancer treatment. Despite this achievement, mAbs-based therapies are struggling with limitations including poor tissue and tumor penetration, long half-life time, poor oral bioavailability, and expensive production costs, which prompted a shift towards the development of the small-molecule inhibitors of PD-1/PD-L1 pathways. Even though many small-molecule inhibitors targeting PD-1/PD-L1 interaction have been reported, their development lags behind the corresponding mAb, partly due to the challenges of developing drug-like small molecules. Herein, we report the discovery of a series of novel inhibitors targeting PD-1/PD-L1 interaction via structural simplification strategy by using BMS-1058 as a starting point. Among them, compound A9 stands out as the most promising candidate with excellent PD-L1 inhibitory activity (IC₅₀ = 0.93 nM, LE = 0.43) and high binding affinity to hPD-L1 (K_D = 3.64 nM, LE = 0.40). Furthermore, A9 can significantly promote the production of IFN-γ in a dose-dependent manner by rescuing PD-L1 mediated T-cell inhibition in Hep3B/OS-8/hPD-L1 and CD3-positive T cells co-culture assay. Taken together, these results suggest that A9 is a promising inhibitor of PD-1/PD-L1 interaction and is worthy for further study.

Novel Biphenyl Pyridines as Potent Small-Molecule Inhibitors Targeting the Programmed Cell Death-1/Programmed Cell Death-Ligand 1 Interaction

With the successful clinical application of anti-programmed cell death-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) monoclonal antibodies (mAb), targeting the PD-1/PD-L1 interaction has become a promising method for the discovery of cancer therapy. Due to the inherent limitations of antibodies, it is necessary to search for small-molecule inhibitors against the PD-1/PD-L1 axis. We report the design, synthesis, and evaluation in vitro and in vivo of a series of novel biphenyl pyridines as the inhibitors of PD-1/PD-L1. 2-(((2-Methoxy-6-(2-methyl-[1,1'-biphenyl]-3-yl)pyridin-3-yl)methyl)amino)ethan-1-ol (24) was found to inhibit the PD-1/PD-L1 interaction with an IC₅₀ value of 3.8 ± 0.3 nM and enhance the killing activity of tumor cells by immune cells. Compound 24 displays great pharmacokinetics (oral bioavailability of 22%) and significant in vivo antitumor activity in a CT26 mouse model. Flow cytometry and immunohistochemistry data indicated that compound 24 activates the immune activity in tumors. These results suggest that compound 24 is a promising small-molecule inhibitor against the PD-1/PD-L1 axis and merits further development.

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 Evaluation of o-(Biphenyl-3-ylmethoxy)nitrophenyl Derivatives as PD-1/PD-L1 Inhibitors with Potent Anticancer Efficacy In Vivo

Two series of novel o-(biphenyl-3-ylmethoxy)nitrophenyl compounds (A1-31 and B1-17) were designed as programmed cell death protein 1 (PD-1)/PD-ligand 1 (PD-L1) inhibitors. All compounds showed significant inhibitory activity with IC₅₀ values ranging from 2.7 to 87.4 nM except compound A17, and compound B2 displayed the best activity. Further experiments showed that B2 bound to the PD-L1 protein without obvious toxicity in Lewis lung carcinoma (LLC) cells. Furthermore, B2 significantly promoted interferon-gamma secretion in a dose-dependent manner in vitro and in vivo. Especially, B2 exhibited potent in vivo anticancer efficacy in an LLC-bearing allograft mouse model at a low dose of 5 mg/kg, which was more active than BMS-1018 (tumor growth inhibition rate: 48.5% vs 17.8%). A panel of immunohistochemistry and flow cytometry assays demonstrated that B2 effectively counteracted PD-1-induced immunosuppression in the tumor microenvironment, thereby triggering antitumor immunity. These results indicate that B2 is a promising PD-1/PD-L1 inhibitor worthy of 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.

Discovery of a novel, potent and selective small-molecule inhibitor of PD-1/PD-L1 interaction with robust in vivo anti-tumour efficacy

BACKGROUND AND PURPOSE

PD-1/PD-L1 antibodies have achieved great success in clinical treatment. However, monoclonal antibody drugs also have challenges, such as high manufacturing costs, poor diffusion, low oral bioavailability and limited penetration into tumour tissue. The development of small-molecule inhibitors of PD-1/PD-L1 interaction represents a promising perspective to overcome the above challenges in cancer immunotherapy.

EXPERIMENTAL APPROACH

We explored structural activity relationships and used biochemical assays to generate a lead compound (ZE132). CD8+ T-cells killing assay and Ifng expression assay were used to verify the in vitro cellular activity of ZE132. Off-target study was performed to verify the selectivity. Syngeneic mouse models were used to verify the in vivo activity of ZE132 in tumour immune microenvironment (TIME). We also performed pharmacokinetics profiling in mice and The Cancer Genome Atlas database analysis.

KEY RESULTS

ZE132 can effectively inhibit the PD-1/PD-L1 interactions in vitro, and it has a potent affinity to PD-L1. ZE132 shows robust anti-tumour effects in vivo, better than anti-PD-1 antibody. In the analysis of TIME, we found that ZE132 treatment promotes cytotoxic T-cell tumour infiltration and induces IL-2 expression. In addition, ZE132 elicits strong inhibitory effects on the mRNA expression of TGF-β, which may serve as a potential biomarker to predict responsiveness to PD-1/PD-L1 immunotherapies.

CONCLUSION AND IMPLICATIONS

We identified a new lead compound ZE132 targeting PD-1/PD-L1 interactions, not only showing favourable drug-like properties in vitro and in vivo but also showing the advantage of overcoming the barrier of TIME compared to anti-PD-1 antibody.

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.

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.

If small molecules immunotherapy comes, can the prime be far behind?

Anti-cancer immunotherapy, which includes cellular immunotherapy, immune checkpoint inhibitors and cancer vaccines, has transformed the treatment strategies of several malignancies in the past decades. Immune checkpoints blockade (ICB) is the most commonly tested therapy and has the potential to induce a durable immune response in different types of cancers. However, all approved immune checkpoint inhibitors (ICIs) are monoclonal antibodies (mAbs), which are fraught with disadvantages including lack of oral bioavailability, prolonged tissue retention and poor membrane permeability. Therefore, the research focus has shifted to developing small molecule inhibitors to obviate the limitations of mAbs. Given the complexity of the tumor micro-environment (TME), the combination of ICIs with various small molecule agonists/inhibitors are currently being tested in clinical trials to improve treatment outcomes and prevent tumor recurrence. In this review, we have summarized the mechanisms and therapeutic potential of several molecular targets, along with the current status of small molecule inhibitors.

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.

Chemical characterization of a PD-1/PD-L1 inhibitory activity fraction of the ethanol extract from Gymnadenia conopsea

Searching for PD-1/PD-L1 inhibitor from medicinal plants has become a potential method to discover small molecular cancer immunotherapy drugs. Using PD-1/PD-L1 inhibitory activity assay in vitro, a bioactive fraction was obtained from the ethanol extract of Gymnadenia conopsea. A sensitive UPLC-HRMS/MS method was established for the rapid screening and identification of compositions from bioactive fraction. Based on the characteristic fragmentation patterns of standards analysis and extracted ion chromatogram (EIC) method, 46 compounds were rapidly screened and identified (including 35 succinic acid ester glycosides and 11 other compounds), among which 17 compounds were tentatively identified as new compounds.

Checkpoint inhibition through small molecule-induced internalization of programmed death-ligand 1

Programmed death-ligand 1 is a glycoprotein expressed on antigen presenting cells, hepatocytes, and tumors which upon interaction with programmed death-1, results in inhibition of antigen-specific T cell responses. Here, we report a mechanism of inhibiting programmed death-ligand 1 through small molecule-induced dimerization and internalization. This represents a mechanism of checkpoint inhibition, which differentiates from anti-programmed death-ligand 1 antibodies which function through molecular disruption of the programmed death 1 interaction. Testing of programmed death ligand 1 small molecule inhibition in a humanized mouse model of colorectal cancer results in a significant reduction in tumor size and promotes T cell proliferation. In addition, antigen-specific T and B cell responses from patients with chronic hepatitis B infection are significantly elevated upon programmed death ligand 1 small molecule inhibitor treatment. Taken together, these data identify a mechanism of small molecule-induced programmed death ligand 1 internalization with potential therapeutic implications in oncology and chronic viral infections.

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.