Small-molecule inhibitors of PD-1/PD-L1 immune checkpoint alleviate the PD-L1-induced exhaustion of T-cells

Bioactive compounds derived from marine source: a potential immunotherapy treatment

Immunotherapy using checkpoint inhibitors blocks the checkpoint proteins (programmed cell death receptor-1; PD-1) from binding with their corresponding ligands (programmed cell death receptor ligand-1; PD-L1) to regulate cell signaling pathways. The marine environment holds a huge source of small molecules that are understudied which can be developed as an inhibitor. Hence, this study investigated the inhibitory effect of 19 algae-derived small molecules against PD-L1 by using molecular docking, absorption, distribution, metabolism, and elimination (ADME) properties and molecular dynamics simulations (MDS). The molecular docking revealed that the binding energy of the six best compounds ranges from -11.1 to -9.1 kcal/mol. Fucoxanthinol, in particular, has the strongest binding energy at -11.1 kcal/mol with three hydrogen bonds (ASN:63A, GLN:66A, and ASP:122A). Meanwhile, the MDS demonstrated that the ligands were strongly bound to the protein, indicating the stability of the complexes. In summary, the identified compounds are potential PD-L1 inhibitors in immunotherapy.Communicated by Ramaswamy H. Sarma.

Enhancing cancer immunotherapy: Exploring strategies to target the PD-1/PD-L1 axis and analyzing the associated patent, regulatory, and clinical trial landscape

Cancer treatment modalities and their progression is guided by the specifics of cancer, including its type and site of localization. Surgery, radiation, and chemotherapy are the most often used conventional treatments. Conversely, emerging treatment techniques include immunotherapy, hormone therapy, anti-angiogenic therapy, dendritic cell-based immunotherapy, and stem cell therapy. Immune checkpoint inhibitors' anticancer properties have drawn considerable attention in recent studies in the cancer research domain. Programmed Cell Death Protein-1 (PD-1) and its ligand (PD-L1) checkpoint pathway are key regulators of the interactions between activated T-cells and cancer cells, protecting the latter from immune destruction. When the ligand PD-L1 attaches to the receptor PD-1, T-cells are prevented from destroying cells that contain PD-L1, including cancer cells. The PD-1/PD-L1 checkpoint inhibitors block them, boosting the immune response and strengthening the body's defenses against tumors. Recent years have seen incredible progress and tremendous advancement in developing anticancer therapies using PD-1/PD-L1 targeting antibodies. While immune-related adverse effects and low response rates significantly limit these therapies, there is a need for research on methods that raise their efficacy and lower their toxicity. This review discusses various recent innovative nanomedicine strategies such as PLGA nanoparticles, carbon nanotubes and drug loaded liposomes to treat cancer targeting PD-1/PD-L1 axis. The biological implications of PD-1/PD-L1 in cancer treatment and the fundamentals of nanotechnology, focusing on the novel strategies used in nanomedicine, are widely discussed along with the corresponding guidelines, clinical trial status, and the patent landscape of such formulations.

Construction and Activity Evaluation of Novel Bifunctional Inhibitors and a COF Carrier Based on a Fungal Infection Microenvironment

Faced with increasingly serious fungal infections and drug resistance issues, three different series of novel dual-target (programmed death ligand 1/14 α-demethylase) compounds were constructed through the fragment combination pathway in the study. Their chemical structures were synthesized, characterized, and evaluated. Among them, preferred compounds 10c-1, 17b-1, and 18b-2 could efficiently exert their antifungal and antidrug-resistant fungal ability through blocking ergosterol biosynthesis, inducing the upregulation of reactive oxygen species level, and triggering apoptosis. Especially, compound 18b-2 exhibited the synergistic function of fungal inhibition and immune activation. Moreover, the covalent organic framework carrier was also generated based on the acidic microenvironment of fungal infection to improve the bioavailability and targeting of preferred compounds; this finally accelerated the body's recovery rate.

Regulatory mechanisms of PD-1/PD-L1 in cancers

Immune evasion contributes to cancer growth and progression. Cancer cells have the ability to activate different immune checkpoint pathways that harbor immunosuppressive functions. The programmed death protein 1 (PD-1) and programmed cell death ligands (PD-Ls) are considered to be the major immune checkpoint molecules. The interaction of PD-1 and PD-L1 negatively regulates adaptive immune response mainly by inhibiting the activity of effector T cells while enhancing the function of immunosuppressive regulatory T cells (Tregs), largely contributing to the maintenance of immune homeostasis that prevents dysregulated immunity and harmful immune responses. However, cancer cells exploit the PD-1/PD-L1 axis to cause immune escape in cancer development and progression. Blockade of PD-1/PD-L1 by neutralizing antibodies restores T cells activity and enhances anti-tumor immunity, achieving remarkable success in cancer therapy. Therefore, the regulatory mechanisms of PD-1/PD-L1 in cancers have attracted an increasing attention. This article aims to provide a comprehensive review of the roles of the PD-1/PD-L1 signaling in human autoimmune diseases and cancers. We summarize all aspects of regulatory mechanisms underlying the expression and activity of PD-1 and PD-L1 in cancers, including genetic, epigenetic, post-transcriptional and post-translational regulatory mechanisms. In addition, we further summarize the progress in clinical research on the antitumor effects of targeting PD-1/PD-L1 antibodies alone and in combination with other therapeutic approaches, providing new strategies for finding new tumor markers and developing combined therapeutic approaches.

Natural Killer Cells Reprogram Myeloid-Derived Suppressor Cells to Induce TNF-α Release via NKG2D-Ligand Interaction after Cryo-Thermal Therapy

In our previous studies, a novel cryothermal therapy (CTT) was developed to induce systemic long-term anti-tumor immunity. Natural killer (NK) cells were found to play an important role in CTT-induced long-term immune-mediated tumor control at the late stage after CTT, but the underlying mechanism is unclear. Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells that have potent immunosuppressive effects on T cells and weaken the long-term benefits of immunotherapy. Consequently, overcoming MDSC immunosuppression is essential for maintaining the long-term efficacy of immunotherapy. In this study, we revealed that NK cells considerably diminish MDSC accumulation at the late stage after CTT, boost T cell production, increase T cell activation, and promote MDSC maturation, culminating in Th1-dominant CD4+ T cell differentiation and enhancing NK and CD8+ T cell cytotoxicity. Additionally, NK cells activate ERK signaling in MDSCs through NKG2D-ligand interaction to increase the activity of tumor necrosis factor (TNF)-α converting enzyme (TACE)-cleaved membrane TNF-α. Furthermore, Increased TACE activity releases more soluble TNF-α from MDSCs to promote MDSC maturation. In our studies, we propose a novel mechanism by which NK cells can overcome MDSC-induced immunosuppression and maintain CTT-induced persistent anti-tumor immunity, providing a prospective therapeutic option to improve the performance of cancer immunotherapy.

Improving tumor sensitivity by the introduction of an ester chain to triaryl derivatives targeting PD-1/PD-L1

PD-1/PD-L1 pathway blockade is a promising immunotherapy for the treatment of cancer. In this manuscript, a series of triaryl compounds containing ester chains were designed and synthesized based on the pharmacophore studies of the lead BMS-1. After several SAR iterations, 22 showed the best biochemical activity binding to hPD-L1 with an IC50 of 1.21 nM in HTRF assay, and a KD value of 5.068 nM in SPR analysis. Cell-based experiments showed that 22 effectively promoted A549 cell death by restoring T-cell immune function. 22 showed significant in vivo antitumor activity in a 4T1 mouse model without obvious toxicity, with a TGI rate of 67.8 % (20 mg/kg, ip). Immunohistochemistry data indicated that 22 activates the immune activity in tumors. These results suggest that 22 is a promising compound for further development of PD-1/PD-L1 inhibitor for cancer therapy.

Polymer-based antibody mimetics (iBodies) target human PD-L1 and function as a potent immune checkpoint blocker

Immune checkpoint blockade (ICB) using monoclonal antibodies against programmed cell death protein 1 (PD-1) or programmed death-ligand 1 (PD-L1) is the treatment of choice for cancer immunotherapy. However, low tissue permeability, immunogenicity, immune-related adverse effects, and high cost could be possibly improved using alternative approaches. On the other hand, synthetic low-molecular-weight (LMW) PD-1/PD-L1 blockers have failed to progress beyond in vitro studies, mostly due to low binding affinity or poor pharmacological characteristics resulting from their limited solubility and/or stability. Here, we report the development of polymer-based anti-human PD-L1 antibody mimetics (α-hPD-L1 iBodies) by attaching the macrocyclic peptide WL12 to a N-(2-hydroxypropyl)methacrylamide copolymer. We characterized the binding properties of iBodies using surface plasmon resonance, enzyme-linked immunosorbent assay, flow cytometry, confocal microscopy, and a cellular ICB model. We found that the α-hPD-L1 iBodies specifically target human PD-L1 (hPD-L1) and block the PD-1/PD-L1 interaction in vitro, comparable to the atezolizumab, durvalumab, and avelumab licensed monoclonal antibodies targeting PD-L1. Our findings suggest that iBodies can be used as experimental tools to target hPD-L1 and could serve as a platform to potentiate the therapeutic effect of hPD-L1-targeting small molecules by improving their affinity and pharmacokinetic properties.

Molecular glues for protein-protein interactions: Progressing toward a new dream

The modulation of protein-protein interactions with small molecules is one of the most rapidly developing areas in drug discovery. In this review, we discuss advances over the past decade (2014-2023) focusing on molecular glues (MGs)-monovalent small molecules that induce proximity, either by stabilizing native interactions or by inducing neomorphic interactions. We include both serendipitous and rational discoveries and describe the different approaches that were used to identify them. We classify the compounds in three main categories: degradative MGs, non-degradative MGs or PPI stabilizers, and MGs that induce self-association. Diverse, illustrative examples with structural data are described in detail, emphasizing the elements of molecular recognition and cooperative binding at the interface that are fundamental for a MG mechanism of action.

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.

Targeted therapy, immunotherapy, and small molecules and peptidomimetics as emerging immunoregulatory agents for melanoma

Primary cutaneous melanoma is the most lethal of all skin neoplasms and its incidence is increasing. Clinical management of advanced melanoma in the last decade has been revolutionised by the availability of immunotherapies and targeted therapies, used alone and in combination. This article summarizes advances in the treatment of late-stage melanoma including use of protein kinase inhibitors, antibody-based immune checkpoint inhibitors, adoptive immunotherapy, vaccines and more recently, small molecules and peptidomimetics as emerging immunoregulatory agents.

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.

Recent advances and mechanisms of action of PD-L1 degraders as potential therapeutic agents

PD-L1 is an important immune checkpoint protein that can bind to T cells' PD-1 receptor, thereby promoting immune escape from tumors. In recent years, many researchers have developed strategies to degrade PD-L1 to improve the effect of immunotherapy. The study of degrading PD-L1 provides new opportunities for immunotherapy. Here, we mainly summarize and review the current active molecules and mechanisms that mediate the degradation of immature and mature PD-L1 during the post-translational modification stages, involving PD-L1 phosphorylation, glycosylation, palmitoylation, ubiquitination, and the autophagy-lysosomal process. This review expects that by degrading PD-L1 protein, we will not only gain a better understanding of oncogenic mechanisms involving tumor PD-L1 protein but also provide a new way to improve immunotherapy.

Development and In Vivo Evaluation of Small-Molecule Ligands for Positron Emission Tomography of Immune Checkpoint Modulation Targeting Programmed Cell Death 1 Ligand 1

A substantial portion of patients do not benefit from programmed cell death protein 1/programmed cell death 1 ligand 1 (PD-1/PD-L1) checkpoint inhibition therapies, necessitating a deeper understanding of predictive biomarkers. Immunohistochemistry (IHC) has played a pivotal role in assessing PD-L1 expression, but small-molecule positron emission tomography (PET) tracers could offer a promising avenue to address IHC-associated limitations, i.e., invasiveness and PD-L1 expression heterogeneity. PET tracers would allow for improved quantification of PD-L1 through noninvasive whole-body imaging, thereby enhancing patient stratification. Here, a large series of PD-L1 targeting small molecules were synthesized, leveraging advantageous substructures to achieve exceptionally low nanomolar affinities. Compound 5c emerged as a promising candidate (IC50 = 10.2 nM) and underwent successful carbon-11 radiolabeling. However, a lack of in vivo tracer uptake in xenografts and notable accumulation in excretory organs was observed, underscoring the challenges encountered in small-molecule PD-L1 PET tracer development. The findings, including structure-activity relationships and in vivo biodistribution data, stand to illuminate the path forward for refining small-molecule PD-L1 PET tracers.

Discovery of Novel PD-L1 Small-Molecular Inhibitors with Potent In Vivo Anti-tumor Immune Activity

Programmed death-ligand 1 (PD-L1) has surfaced as a promising therapeutic target for various cancers due to its pivotal role in facilitating tumor immune evasion. Herein, we report a series of novel small-molecule PD-L1 inhibitors exhibiting remarkable inhibitory activity against the PD-1/PD-L1 interaction (X18: IC50 = 1.3 nM) and reinstating the suppressive effect of PD-L1 on T cells (X18: EC50 = 152.8 nM). Crystallographic studies revealed the binding mode of X18 and PD-L1. Through a rational prodrug design approach, we have successfully optimized the oral pharmacokinetic properties of X22, effectively addressing the poor oral pharmacokinetic profile of PD-L1 small-molecule inhibitors. Notably, X22 demonstrated significant antitumor efficacy in murine models of MC38 and CT26 colon cancer through the upregulation of tumor infiltration and cytotoxicity of CD8+ T cells partially. These findings offer promising prospects for the advancement of PD-L1 inhibitors as innovative agents in cancer immunotherapy.

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.

Uncovering the colorectal cancer immunotherapeutic potential: Evening primrose (Oenothera biennis) root extract and its active compound oenothein B targeting the PD-1/PD-L1 blockade

BACKGROUND

The emergence of immune checkpoint inhibitors, a novel class of immunotherapy drugs, represents a major breakthrough in cancer immunotherapy, substantially improving patient survival post-treatment. Blocking programmed death-ligand 1 (PD-L1) and programmed death protein-1 (PD-1) has demonstrated promising clinical results in various human cancer types. The US FDA has recently permitted only monoclonal antibody (mAb)-based PD-L1 or PD-1 blockers. Although these antibodies exhibit high antitumor efficacy, their size- and affinity-induced side effects limit their applicability.

PURPOSE

As small-molecule-based PD-1/PD-L1 blockers capable of reducing the side effects of antibody therapies are needed, this study focuses on exploring natural ingredient-based small molecules that can target hPD-L1/PD-1 using herbal medicines and their components.

METHODS

The antitumor potential of evening primrose (Oenothera biennis) root extract (EPRE), a globally utilized traditional herbal medicine, folk remedy, and functional food, was explored. A coculture system was established using human PD-L1-expressed murine MC38 cells (hPD-L1-MC38s) and CD8+ tumor-infiltrating T lymphocytes (CD8+ TILs) expressing humanized PD-1. The in vivo experiments utilized a colorectal cancer (CRC) C57BL/6 J mouse model bearing MC38 cells expressing humanized PD-L1 and PD-1 proteins.

RESULTS

EPRE and its active compound oenothein B effectively hindered the molecular interaction between hPD-L1 and hPD-1. EPRE stimulated tumor-specific T lymphocytes of a hPD-L1/PD-1 CRC mice. This action resulted in the elevated infiltration of cytotoxic CD8+T lymphocytes and subsequent tumor growth reduction. Moreover, the combined therapy of oenothein B, a PD-1/PD-L1 blocker, and FOLFOX (5-fluorouracil plus oxaliplatin) cooperatively suppressed hPD-L1-MC38s growth in the ex vivo model through activated CD8+ TIL antitumor immune response. Oenothein B exhibited a high binding affinity for hPD-L1 and hPD-1. We believe that this study is the first to uncover the inhibitory effects of EPRE and its component, oenothein B, on PD-1/PD-L1 interactions.

CONCLUSION

This study identified a promising small-molecule candidate from natural products that blocks the hPD-L1/PD-1 signaling pathway. These findings emphasize the potential of EPRE and oenothein B as effective anticancer drugs.

Tetrazole derivatives as potent immunomodulatory agents in tumor microenvironment

Twenty-seven compounds bearing a tetrazole ring as a central unit have been designed, synthetized and biologically evaluated. Studies have been performed in order to compare the effect of tetrazole derivatives bearing amine electron-donor or nitro electron-acceptor groups. The antiproliferative activity has been determined in monoculture studies on tumor cell lines HT-29, A-549, MCF-7 and on non-tumor cell line HEK-293 as well as in co-culture studies (HT-29/THP-1). All the compounds have been studied as PD-L1 (Programmed Death Ligand 1), VEGFR-2 (Vascular Endothelial Growth Factor 2), CD-47 (Cluster of Differentiation 47) and c-Myc inhibitors. The effect on TNF-α secretion has also been determined. Bromoderivatives 23, 24 and chloroderivatives 26, 27 have demonstrated an apoptotic effect on HT-29 cancer cells. Compounds bearing an amine group have shown very promising effects as TME immunomodulatory agents.

Effect of Fragment 1 on the Binding of Epigallocatechin Gallate to the PD-L1 Dimer Explored by Molecular Dynamics

Blocking the interaction between programmed cell death-1 (PD-1) and programmed cell death-ligand 1 (PD-L1) by directly targeting the PD-L1 dimer has emerged as a hot topic in the field of cancer immunotherapy. Epigallocatechin gallate (EGCG), a natural product, has been demonstrated binding to the PD-L1 dimer in our previous study, but has a weaker binding capacity, moreover, EGCG is located at the end of the binding pocket of the PD-L1 dimer. The inhibitor fragment 1 (FRA) lies at the other end. So, we proposed that the introduction of FRA might be able to improve the binding ability. To illuminate this issue, molecular dynamics (MD) simulation was performed in the present study. Binding free energy calculations show that the binding affinity is significantly increased by 17 kcal/mol upon the introduction of FRA. It may be due to the energy contributions of emerging key residues ATyr56, AMet115, BTyr123, AIle54 and the enhanced contributions of initial key residues ATyr123 and BVal68. Binding mode and non-bonded interaction results indicate that FRA_EGCG (EGCG in combination with FRA) binds to the C-, F- and G-sheet of the PD-L1 dimer. Importantly, the introduction of FRA mainly strengthened the nonpolar interactions. The free energy landscape and secondary structure results further show that FRA_EGCG can interact with the PD-L1 dimer more stably. These data demonstrated here provide the theoretical basis for screening two or more natural products with additive inhibitory effect on this pathway and therefore exerting more effective anticancer immunity.

Advances in Immunotherapy for Hepatocellular Carcinoma (HCC)

Hepatocellular carcinoma (HCC) is the second most common cause of cancer-related deaths in the world. More than half of patients with HCC present with advanced stage, and highly active systemic therapies are crucial for improving outcomes. Immune checkpoint inhibitor (ICI)-based therapies have emerged as novel therapy options for advanced HCC. Only one third of patients achieve an objective response with ICI-based therapies due to primary resistance or acquired resistance. The liver tumor microenvironment is naturally immunosuppressive, and specific mutations in cell signaling pathways allow the tumor to evade the immune response. Next, gene sequencing of the tumor tissue or circulating tumor DNA may delineate resistance mechanisms to ICI-based therapy and provide a rationale for novel combination therapies. In this review, we discuss the results of key clinical trials that have led to approval of ICI-based therapy options in advanced HCC and summarize the ongoing clinical trials. We review resistance mechanisms to ICIs and discuss how immunotherapies may be optimized based on the emerging research of tumor biomarkers and genomic alterations.

Discovery of novel resorcinol biphenyl ether-based macrocyclic small molecules as PD-1/PD-L1 inhibitors with favorable pharmacokinetics for cancer immunotherapy

Programmed death protein 1 (PD-1)/programmed death protein ligand 1 (PD-L1) is one of the most promising immune checkpoints (ICs) in tumor immunology and has been actively pursued as a target for anticancer drug discovery. Based on our previous research in small molecule PD-1/PD-L1 modulators, we designed and synthesized a series of resorcinol biphenyl ether-bearing macrocyclic compounds and evaluated their anti-PD-1/PD-L1 activities. Among them, compound 8d exhibited the highest inhibitory activity against PD-1/PD-L1 interaction with IC50 of 259.7 nM in the homogenous time-resolved fluorescence (HTRF) assay. In addition, 8d displayed in vitro immunomodulatory effects by promoting HepG2 cell death in a HepG2/Jurkat cell co-culture model. Furthermore, 8d effectively inhibited tumor growth (TGI = 74.6% at 40 mg/kg) in a melanoma tumor model in mice without causing obvious toxicity. Moreover, 8d exhibited favorable pharmacokinetics [e.g. high stability, reasonable half-life, and good oral bioavailability (F = 21.5%)]. Finally, molecular modeling studies showed that 8d bound to PD-L1 with high affinity. These results suggest that 8d may serve as a starting point for further development of macrocyclic small molecule-based PD-1/PD-L1 inhibitors for cancer treatment.

A multitier virtual screening of antagonists targeting PD-1/PD-L1 interface for the management of triple-negative breast cancer

Immunotherapies are promising therapeutic options for the management of triple-negative breast cancer because of its high mutation rate and genomic instability. Of note, the blockade of the immune checkpoint protein PD-1 and its ligand PD-L1 has been proven to be an efficient and potent strategy to combat triple-negative breast cancer. To date, various anti-PD-1/anti-PD-L1 antibodies have been approved. However, the intrinsic constraints of these therapeutic antibodies significantly limit their application, making small molecules a potentially significant option for PD-1/PD-L1 inhibition. In light of this, the current study aims to use a high-throughput virtual screening technique to identify potential repurposed candidates as PD-L1 inhibitors. Thus, the present study explored binding efficiency of 2509 FDA-approved compounds retrieved from the drug bank database against PD-L1 protein. The binding affinity of the compounds was determined using the glide XP docking programme. Furthermore, prime-MM/GBSA, DFT calculations, and RF score were used to precisely re-score the binding free energy of the docked complexes. In addition, the ADME and toxicity profiles for the lead compounds were also examined to address PK/PD characteristics. Altogether, the screening process identified three molecules, namely DB01238, DB06016 and DB01167 as potential therapeutics for the PD-L1 protein. To conclude, a molecular dynamic simulation of 100 ns was run to characterise the stability and inhibitory action of the three lead compounds. The results from the simulation study confirm the robust structural and thermodynamic stability of DB01238 than other investigated molecules. Thus, our findings hypothesize that DB01238 could serve as potential PD-L1 inhibitor in the near future for triple-negative breast cancer patients.

Virtual Screening and Biological Evaluation of Potential PD-1/PD-L1 Immune Checkpoint Inhibitors as Anti-Hepatocellular Carcinoma Agents

Blockade of the programmed cell death-1/programmed cell death ligand-1 (PD-1/PD-L1) immune checkpoint pathway is an efficient immunotherapeutic modality that provided significant advances in cancer treatment especially in solid tumors highly resistant to traditional therapy. Monoclonal antibodies (mAbs) and small-molecule inhibitors are the two main strategies used to block this axis with mAbs suffering from many limitations. Accordingly, the current alternative is the development of small-molecule PD-1/PD-L1 inhibitors. Here, we present a sequential virtual screening (VS) protocol involving pharmacophore screening followed by molecular docking for the discovery of novel PD-L1 inhibitors. The VS protocol resulted in the discovery of eight novel compounds. A 100 ns MD simulation showed two compounds, H4 and H6, exhibiting a stable binding mode at the PD-L1 dimer interface. Upon evaluation of their immunological activities, the two compounds induced higher cytokines levels (IL-2, IL-6, and INF-γ) relative to BMS-202, 72 h post treatment of PBMCs of HCC patients. Thus, the discovered hits represent potential leads for the development of novel classes targeting the PD-L1 receptor as anti-hepatocellular carcinoma agents.

Structural and biological characterization of pAC65, a macrocyclic peptide that blocks PD-L1 with equivalent potency to the FDA-approved antibodies

Recent advances in immuno-oncology have opened up new and impressive treatment options for cancer. Notwithstanding, overcoming the limitations of the current FDA-approved therapies with monoclonal antibodies (mAbs) that block the PD-1/PD-L1 pathway continues to lead to the testing of multiple approaches and optimizations. Recently, a series of macrocyclic peptides have been developed that exhibit binding strengths to PD-L1 ranging from sub-micromolar to micromolar. In this study, we present the most potent non-antibody-based PD-1/PD-L1 interaction inhibitor reported to date. The structural and biological characterization of this macrocyclic PD-L1 targeting peptide provides the rationale for inhibition of both PD-1/PD-L1 and CD80/PD-L1 complexes. The IC50 and EC50 values obtained in PD-L1 binding assays indicate that the pAC65 peptide has potency equivalent to the current FDA-approved mAbs and may have similar activity to the BMS986189 peptide, which entered the clinical trial and has favorable safety and pharmacokinetic data. The data presented here delineate the generation of similar peptides with improved biological activities and applications not only in the field of cancer immunotherapy but also in other disorders related to the immune system.

Raloxifene, a SERM targets PD-L1: an in-silico study

OBJECTIVES

Immunotherapeutic interventions in cancer have been considerably successful and widely accepted for cancer treatment, but are costly and cannot be afforded by all patients. Because of the high cost, the pharmaceutical research groups across the world are sufficiently motivated to discover or design small molecule inhibitors to treat cancer through inhibition of the immune checkpoint proteins previously targeted with monoclonal antibodies. The presented study was designed with an aim to establish raloxifene, a selective estrogen receptor modulator (SERM) as a potential ligand of the immune checkpoint protein Programmed death ligand-1 (PD-L1).

METHODS

In the presented study, the in-silico approach was used for identifying a lead molecule against PD-L1. The hits were screened using the similarity-search method, and drug-likeliness analysis, and the leads were identified through ligand-docking using Autodock. In-vitro cytotoxicity analysis was carried out using the standard sulphorhodamine B (SRB) assay and the wound healing analysis to show the inhibition of cellular migration was performed using the standard scratch assay.

RESULTS

The in-silico study revealed that raloxifene showed a high drug likelihood and higher binding affinity with PD-L1 as compared to the positive control (BMS-1166; BMS is Bristol Myers Squibb). The binding of raloxifene was shown to occur in the same region as the FDA-approved monoclonal antibodies atezolizumab and durvalumab, indicating the potential of raloxifene for PD1/PD-L1 blockade. In the in-vitro studies, raloxifene showed a time-dependent reduction in IC50 values for the cell line HCT116 (colon cancer). The scratch assay also revealed that raloxifene significantly reduced the migratory potential of HCT-116 cells in-vitro.

CONCLUSIONS

PD-L1 is a potential target of the SERM raloxifene in-silico. Overall, this study is one step further towards immune checkpoint blockade using small-molecule inhibitors.

Design, Synthesis, and Antitumor Activity Evaluation of 2-Arylmethoxy-4-(2,2'-dihalogen-substituted biphenyl-3-ylmethoxy) Benzylamine Derivatives as Potent PD-1/PD-L1 Inhibitors

Novel 2-arylmethoxy-4-(2,2'-dihalogen-substituted biphenyl-3-ylmethoxy) benzylamine derivatives were designed, synthesized, and evaluated in vitro and in vivo against cancers as PD-1/PD-L1 inhibitors. Through the computer-aided structural optimization and the homogeneous time-resolved fluorescence (HTRF) assay, compound A56 was found to most strongly block the PD-1/PD-L1 interaction with an IC50 value of 2.4 ± 0.8 nM and showed the most potent activity. 1H NMR titration results indicated that A56 can tightly bind to the PD-L1 protein with KD < 1 μM. The X-ray diffraction data for the cocrystal structure of the A56/PD-L1 complex (3.5 Å) deciphered a novel binding mode in detail, which can account for its most potent inhibitory activity. Cell-based assays further demonstrated the strong ability of A56 as an hPD-1/hPD-L1 blocker. Especially in an hPD-L1 MC38 humanized mouse model, A56 significantly inhibited tumor growth without obvious toxicity, with a TGI rate of 55.20% (50 mg/kg, i.g.). In conclusion, A56 is a promising clinical candidate worthy of further development.

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.

Progress in programmed cell death-1/programmed cell death-ligand 1 pathway inhibitors and binding mode analysis

Programmed cell death protein 1 (PD-1)/programmed cell death protein ligand 1 (PD-L1) plays an important role in negative regulating immunity. The search for effective PD-1/PD-L1 inhibitors has been at the cutting-edge of academic and industrial medicinal chemistry, leading to the emergence of 16 clinical candidate drugs and the launch of six monoclonal antibodies (mAbs) drugs. However, due to the unclear mechanism of the interaction between drugs and substances in vivo, the screening of preclinical drugs often takes a long time. In order to shorten the time of drug development as much as possible, the binding mode analysis that can simulate the interaction between drugs and substances in vivo at the molecular level can significantly shorten the drug development process. This paper reviews the mechanism of PD-1/PD-L1 signaling pathway at the molecular level, as well as the research progress and obstacles of inhibitors. Besides, we analyzed the binding mode of recently reported PD-1/PD-L1 inhibitors with PD-1 or PD-L1 protein in detail in order to provide ideas for the development of PD-1/PD-L1 inhibitors.

Design, Synthesis, and Biological Evaluation of 2-Hydroxy-4-phenylthiophene-3-carbonitrile as PD-L1 Antagonist and Its Comparison to Available Small Molecular PD-L1 Inhibitors

In search of a potent small molecular PD-L1 inhibitor, we designed and synthesized a compound based on a 2-hydroxy-4-phenylthiophene-3-carbonitrile moiety. Ligand's performance was tested in vitro and compared side-by-side with a known PD-L1 antagonist with a proven bioactivity BMS1166. Subsequently, we modified both compounds to allow ¹⁸F labeling that could be used for PET imaging. Radiolabeling, which is used in drug development and diagnosis, was applied to investigate the properties of those ligands and test them against tissue sections with diverse expression levels of PD-L1. We confirmed biological activity toward hPD-L1 for this inhibitor, comparable with BMS1166, while holding enhanced pharmacological properties.

Vitamin D Receptor Antagonist MeTC7 Inhibits PD-L1

Small-molecule inhibitors of PD-L1 are postulated to control immune evasion in tumors similar to antibodies that target the PD-L1/PD-1 immune checkpoint axis. However, the identity of targetable PD-L1 inducers is required to develop small-molecule PD-L1 inhibitors. In this study, using chromatin immunoprecipitation (ChIP) assay and siRNA, we demonstrate that vitamin D/VDR regulates PD-L1 expression in acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) cells. We have examined whether a VDR antagonist, MeTC7, can inhibit PD-L1. To ensure that MeTC7 inhibits VDR/PD-L1 without off-target effects, we examined competitive inhibition of VDR by MeTC7, utilizing ligand-dependent dimerization of VDR-RXR, RXR-RXR, and VDR-coactivators in a mammalian 2-hybrid (M2H) assay. MeTC7 inhibits VDR selectively, suppresses PD-L1 expression sparing PD-L2, and inhibits the cell viability, clonogenicity, and xenograft growth of AML cells. MeTC7 blocks AML/mesenchymal stem cells (MSCs) adhesion and increases the efferocytotic efficiency of THP-1 AML cells. Additionally, utilizing a syngeneic colorectal cancer model in which VDR/PD-L1 co-upregulation occurs in vivo under radiation therapy (RT), MeTC7 inhibits PD-L1 and enhances intra-tumoral CD8+T cells expressing lymphoid activation antigen-CD69. Taken together, MeTC7 is a promising small-molecule inhibitor of PD-L1 with clinical potential.

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.

Stapled peptide PROTAC induced significantly greater anti-PD-L1 effects than inhibitor in human cervical cancer cells

Introduction

Immune checkpoint inhibitors (ICIs) are monoclonal antibodies that target immune checkpoints that suppress immune cell activity. Low efficiency and high resistance are currently the main barriers to their clinical application. As a representative technology of targeted protein degradation, proteolysis-targeting chimeras (PROTACs) are considered to have potential for addressing these limitations.

Methods

We synthesized a stapled peptide-based PROTAC (SP-PROTAC) that specifically targeted palmitoyltransferase ZDHHC3 and resulted in the decrease of PD-L1 in human cervical cancer cell lines. Flow cytometry, confocal microscopy, protein immunoblotting, Cellular Thermal Shift Assay (CETSA), and MTT assay analyses were conducted to evaluate the effects of the designed peptide and verify its safety in human cells.

Results

In cervical cancer celllines C33A and HeLa, the stapled peptide strongly downregulated PD-L1 to < 50% of baseline level at 0.1 μM. DHHC3 expression decreased in both dosedependentand time-dependent manners. MG132, the proteasome inhibitor, can alleviate the SP-PROTAC mediated degradation of PD-L1 in human cancer cells. In a co-culture model of C33A and T cells, treatment with the peptide induced IFN-γ and TNF-α release in a dose-dependent manner by degrading PD-L1. These effects were more significant than that of the PD-L1 inhibitor, BMS-8.

Conclusions

Cells treated with 0.1 μM of SP-PROTAC or BMS-8 for 4 h revealed that the stapled peptide decreased PD-L1 more effectively than BMS-8. DHHC3-targeting SP-PROTAC decreased PD-L1 in human cervical cancer more effectively than the inhibitor BMS-8.

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.

Computational design of a cyclic peptide that inhibits the CTLA4 immune checkpoint

Proteins involved in immune checkpoint pathways, such as CTLA4, PD1, and PD-L1, have become important targets for cancer immunotherapy; however, development of small molecule drugs targeting these pathways has proven difficult due to the nature of their protein-protein interfaces. Here, using a hierarchy of computational techniques, we design a cyclic peptide that binds CTLA4 and follow this with experimental verification of binding and biological activity, using bio-layer interferometry, cell culture, and a mouse tumor model. Beginning from a template excised from the X-ray structure of the CTLA4:B7-2 complex, we generate several peptide sequences using flexible docking and modeling steps. These peptides are cyclized head-to-tail to improve structural and proteolytic stability and screened using molecular dynamics simulation and MM-GBSA calculation. The standard binding free energies for shortlisted peptides are then calculated in explicit-solvent simulation using a rigorous multistep technique. The most promising peptide, cyc(EIDTVLTPTGWVAKRYS), yields the standard free energy -6.6 ± 3.5 kcal mol-1, which corresponds to a dissociation constant of ∼15 μmol L-1. The binding affinity of this peptide for CTLA4 is measured experimentally (31 ± 4 μmol L-1) using bio-layer interferometry. Treatment with this peptide inhibited tumor growth in a co-culture of Lewis lung carcinoma (LLC) cells and antigen primed T cells, as well as in mice with an orthotropic Lewis lung carcinoma allograft model.

Novel PD-L1-Targeted Phenyl-Pyrazolone Derivatives with Antioxidant Properties

Orally-active anticancer small molecules targeting the PD-1/PD-L1 immune checkpoint are actively searched. Phenyl-pyrazolone derivatives with a high affinity for PD-L1 have been designed and characterized. In addition, the phenyl-pyrazolone unit acts as a scavenger of oxygen free radicals, providing antioxidant effects. The mechanism is known for the drug edaravone (1) which is also an aldehyde-reactive molecule. The present study reports the synthesis and functional characterization of new molecules (2-5) with an improved anti-PD-L1 activity. The leading fluorinated molecule 5 emerges as a potent checkpoint inhibitor, avidly binding to PD-L1, inducing its dimerization, blocking PD-1/PD-L1 signaling mediated by phosphatase SHP-2 and reactivating the proliferation of CTLL-2 cells in the presence of PD-L1. In parallel, the compound maintains a significant antioxidant activity, characterized using electron paramagnetic resonance (EPR)-based free radical scavenging assays with the probes DPPH and DMPO. The aldehyde reactivity of the molecules was investigated using 4-hydroxynonenal (4-HNE), which is a major lipid peroxidation product. The formation of drug-HNE adducts, monitored by high resolution mass spectrometry (HRMS), was clearly identified and compared for each compound. The study leads to the selection of compound 5 and the dichlorophenyl-pyrazolone unit as a scaffold for the design of small molecule PD-L1 inhibitors endowed with antioxidant properties.

Novel Combination Therapy for Triple-Negative Breast Cancer based on an Intelligent Hollow Carbon Sphere

Triple-negative breast cancer (TNBC) is a subtype of breast cancer with high mortality, and the efficacy of monotherapy for TNBC is still disappointing. Here, we developed a novel combination therapy for TNBC based on a multifunctional nanohollow carbon sphere. This intelligent material contains a superadsorbed silicon dioxide sphere, sufficient loading space, a nanoscale hole on its surface, a robust shell, and an outer bilayer, and it could load both programmed cell death protein 1/programmed cell death ligand 1 (PD-1/PD-L1) small-molecule immune checkpoints and small-molecule photosensitizers with excellent loading contents, protect these small molecules during the systemic circulation, and achieve accumulation of them in tumor sites after systemic administration followed by the application of laser irradiation, thereby realizing dual attack of photodynamic therapy and immunotherapy on tumors. Importantly, we integrated the fasting-mimicking diet condition that can further enhance the cellular uptake efficiency of nanoparticles in tumor cells and amplify the immune responses, further enhancing the therapeutic effect. Thus, a novel combination therapy "PD-1/PD-L1 immune checkpoint blockade + photodynamic therapy + fasting-mimicking diet"was developed with the aid of our materials, which eventually achieved a marked therapeutic effect in 4T1-tumor-bearing mice. The concept can also be applied to the clinical treatment of human TNBC with guiding significance in the future.

Novel technologies for applying immune checkpoint blockers

Cancer cells develop several ways to subdue the immune system among others via upregulation of inhibitory immune checkpoint (ICP) proteins. These ICPs paralyze immune effector cells and thereby enable unfettered tumor growth. Monoclonal antibodies (mAbs) that block ICPs can prevent immune exhaustion. Due to their outstanding effects, mAbs revolutionized the field of cancer immunotherapy. However, current ICP therapy regimens suffer from issues related to systemic administration of mAbs, including the onset of immune related adverse events, poor pharmacokinetics, limited tumor accessibility and immunogenicity. These drawbacks and new insights on spatiality prompted the exploration of novel administration routes for mAbs for instance peritumoral delivery. Moreover, novel ICP drug classes that are adept to novel delivery technologies were developed to circumvent the drawbacks of mAbs. We therefore review the state-of-the-art and novel delivery strategies of ICP drugs.

Chlorin e6-associated photodynamic therapy enhances abscopal antitumor effects via inhibition of PD-1/PD-L1 immune checkpoint

We hypothesized that photodynamic therapy (PDT) with Chlorin e6 (Ce6) enhances antitumor abscopal effects via inhibition of the programmed cell death-1/programmed death-ligand 1 (PD-1/PD-L1) immune checkpoint. By using syngeneic melanoma and pancreatic tumor mouse models, we studied the Ce6-PDT-induced immune responses in local and distant tumor microenvironments. In addition, the Ce6-PDT's target in the PD-1/PD-L1 interaction was analyzed in MC38-hPD-L1 colon cancer and PD-1 expressing Jurkat T cell coculture. The tumors in the irradiated and non-irradiated sites in the abscopal effective (Abs_eff) group of both mouse models were regressed, proving the abscopal effect. The immunogenic effect in the Abs_eff group was associated with an expansion of T cell and other immune cells infiltration without changes in the CD39⁺ population in either the right or left tumors compared to control group. Furthermore, the abscopal ineffective (Abs_ineff) group demonstrated lesser increase of T cells, decreased immune cell infiltration, and increased CD39-expressing Treg cells without suppression of tumor growth. In the coculture with PD-1-expressing Jurkat T cell, Ce6-PDT efficiently suppressed the PD-1/PD-L1 interactions by increasing the proliferation and cytotoxic activity of CD8⁺ T cells while decreasing CD39-expressing Treg cells in a dose-dependent manner. Likewise, the inhibition of PD-1/PD-L1 interactions was also correlated with the increased production of IL-2 and Granzyme B. Our findings imply that Ce6-PDT is a promising immunotherapy with the potential to improve the abscopal effect.

Turning a Tumor Microenvironment Pitfall into Opportunity: Discovery of Benzamidoxime as PD-L1 Ligand with pH-Dependent Potency

PD-1/PD-L1 protein complex is attracting a great deal of interest as a drug target for the design of immune therapies able to block its assembly. Although some biologic drugs have entered clinical use, their poor response rate in patients are demanding further efforts to design small molecule inhibitors of PD-1/PD-L1 complex with higher efficacy and optimal physicochemical properties. Dysregulation of pH in the tumor microenvironment is indeed one of the key mechanisms promoting drug resistance and lack of response in cancer therapy. Integrating computational and biophysical approaches, herein we report a screening campaign that has led to identifying VIS310 as a novel ligand of PD-L1, with physicochemical properties enabling a pH-dependent binding potency. Additional optimization efforts by analogue-based screening have been instrumental to disclosing VIS1201, which exhibits improved binding potency against PD-L1 and is able to inhibit PD-1/PD-L1 complex formation in a ligand binding displacement assay. While providing preliminary structure–activity relationships (SARs) of a novel class of PD-L1 ligands, our results lay the foundation for the discovery of immunoregulatory small molecules resilient to tumor microenvironmental conditions for escaping drug-resistance mechanisms.

Identification of CBPA as a New Inhibitor of PD-1/PD-L1 Interaction

Targeting of the PD-1/PD-L1 immunologic checkpoint is believed to have provided a real breakthrough in the field of cancer therapy in recent years. Due to the intrinsic limitations of antibodies, the discovery of small-molecule inhibitors blocking PD-1/PD-L1 interaction has gradually opened valuable new avenues in the past decades. In an effort to discover new PD-L1 small molecular inhibitors, we carried out a structure-based virtual screening strategy to rapidly identify the candidate compounds. Ultimately, CBPA was identified as a PD-L1 inhibitor with a K_D value at the micromolar level. It exhibited effective PD-1/PD-L1 blocking activity and T-cell-reinvigoration potency in cell-based assays. CBPA could dose-dependently elevate secretion levels of IFN-γ and TNF-α in primary CD4⁺ T cells in vitro. Notably, CBPA exhibited significant in vivo antitumor efficacy in two different mouse tumor models (a MC38 colon adenocarcinoma model and a melanoma B16F10 tumor model) without the induction of observable liver or renal toxicity. Moreover, analyses of the CBPA-treated mice further showed remarkably increased levels of tumor-infiltrating CD4⁺ and CD8⁺ T cells and cytokine secretion in the tumor microenvironment. A molecular docking study suggested that CBPA embedded relatively well into the hydrophobic cleft formed by dimeric PD-L1, occluding the PD-1 interaction surface of PD-L1. This study suggests that CBPA could work as a hit compound for the further design of potent inhibitors targeting the PD-1/PD-L1 pathway in cancer immunotherapy.

PD-1/PD-L1 and DNA Damage Response in Cancer

The application of immunotherapy for cancer treatment is rapidly becoming more widespread. Immunotherapeutic agents are frequently combined with various types of treatments to obtain a more durable antitumor clinical response in patients who have developed resistance to monotherapy. Chemotherapeutic drugs that induce DNA damage and trigger DNA damage response (DDR) frequently induce an increase in the expression of the programmed death ligand-1 (PD-L1) that can be employed by cancer cells to avoid immune surveillance. PD-L1 exposed on cancer cells can in turn be targeted to re-establish the immune-reactive tumor microenvironment, which ultimately increases the tumor's susceptibility to combined therapies. Here we review the recent advances in how the DDR regulates PD-L1 expression and point out the effect of etoposide, irinotecan, and platinum compounds on the anti-tumor immune response.

Establishment of Cell-Based Assay System for Evaluating Cytotoxic Activity Modulated by the Blockade of PD-1 and PD-L1 Interactions with a Therapeutic Antibody

BACKGROUND

Therapeutic antibodies targeting the PD-1/PD-L1 immune checkpoint are widely used in cancer therapy and are under active further development. Historically, the antitumor activity of PD-1/PD-L1 immune checkpoint inhibitors has been evaluated using in vivo and ex vivo test methods; however, a simple in vitro assay method to evaluate antitumor activity accurately is needed for the efficient development of new therapeutic agents. In the present study, we attempted to establish a simple cell-based assay system to evaluate the modulating effect of PD-1/PD-L1 immune checkpoint inhibitors on cytotoxic activity.

METHODS

We established a new natural killer (NK) cell line stably transfected with the PD-1 and IL-2 genes and a new NK-sensitive target cell line stably transfected with the PD-L1 gene. Then, the assay system was established by co-cultivation of the established cell lines and measurement of the cytotoxic activities using the europium release assay. To confirm the performance of the established assay system, model therapeutic antibodies to block the PD-1/PD-L1 signal, nivolumab and atezolizumab were added to the co-culture system and the modulating effect on the cytotoxic activities were evaluated.

RESULTS

Nivolumab and atezolizumab clearly showed a modulating effect on cytotoxic activity in a dose-dependent manner in our assay system, whereas a human IgG isotype control antibody did not show any modulating effect on the assay system.

CONCLUSION

The newly established cell-based assay system can quantitatively evaluate the modulating effect of PD-1/PD-L1 immune checkpoint inhibitors by measuring cytotoxic activity, playing an important role in antitumor effects as innate immunity.

PD-L1 small-molecule modulators: A new hope in epigenetic-based multidrug cancer therapy?

Programmed death-ligand 1 (PD-L1) is an immune checkpoint protein the overexpression of which results in an inhibitory signal that induces T cell exhaustion responsible for immune escape in tumors. Immunotherapy strategies targeting the PD-L1 pathway have achieved remarkable success in treating various types of cancer. More recently, numerous advances in understanding the complex PD-L1 biology have been made, and the first small-molecule inhibitors have been described in the literature. In this review, we highlight the most promising recent advances in understanding the complex regulation mechanisms focusing on small-molecule modulators, which could be used in rational therapy combinations with other epigenetic chemotherapeutic agents.

It's ok to be outnumbered - sub-stoichiometric modulation of homomeric protein complexes

An arsenal of molecular tools with increasingly diversified mechanisms of action is being developed by the scientific community to enable biological interrogation and pharmaceutical modulation of targets and pathways of ever increasing complexity. While most small molecules interact with the target of interest in a 1 : 1 relationship, a noteworthy number of recent examples were reported to bind in a sub-stoichiometric manner to a homomeric protein complex. This approach requires molecular understanding of the physiologically relevant protein assemblies and in-depth characterization of the compound's mechanism of action. The recent literature examples summarized here were selected to illustrate methods used to identify and characterize molecules with such mechanisms. The concept of one small molecule targeting a homomeric protein assembly is not new but the subject deserves renewed inspection in light of emerging technologies and increasingly diverse target biology, to ensure relevant in vitro systems are used and valuable compounds with potentially novel sub-stoichiometric mechanisms of action aren't overlooked.

Near-Infrared Blinking Carbon Dots Designed for Quantitative Nanoscopy

Blinking carbon dots (CDs) have attracted attention as a probe for single molecule localization microscopy (SMLM), yet quantitative analysis is limited because of inept blinking and low signal-to-noise ratio (SNR). Here we report the design and synthesis of near-infrared (NIR) blinking CDs with a maximum emission of around 750 nm by weaving a nitrogen-doped aromatic backbone with surplus carboxyl groups on the surface. The NIR-CDs allow conjugation to monovalent antibody fragments for labeling and imaging of cellular receptors as well as afford increases of 52% in SNR and 33% in localization precision over visible CDs. Analysis of fluorescent bursts allows for accurate counting of cellular receptors at the nanoscale resolution. Using NIR-CDs-based SMLM, we demonstrate oligomerization and internalization of programmed cell death-ligand 1 by a small molecule inhibitor for checkpoint blockade. Our NIR-CDs can become a generally applicable probe for quantitative nanoscopy in chemistry and biology.

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.

Review: PD-L1 as an emerging target in the treatment and prevention of keratinocytic skin cancer

Recent advances in the understanding and targeting of immune checkpoints have led to great progress in immune therapies against many forms of cancer. While many types of immune checkpoints are currently targeted in the clinic, this review will focus on recent research implicating the programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) axis as an emerging focus for the treatment of keratinocytic tumors. PD-L1 is of particular interest in nonmelanoma skin cancer (NMSC), as it is not only upregulated in these tumors but is stimulated by environmental ultraviolet exposure. This response may also make PD-L1 an excellent target for photochemoprevention using topically applied small molecule inhibitors. Here, we summarize recent investigations on PD-L1 expression and clinically relevant immune checkpoint inhibitor treatment in cutaneous squamous cell carcinoma, basal cell carcinoma, and head and neck squamous cell carcinoma, as well as small molecule agents targeting PD-L1 that may be useful for clinical development aiming at treatment or prevention of NMSC.

Insights into small molecule inhibitor bindings to PD-L1 with residue-specific binding free energy calculation

Targeting the immunological checkpoint PD-1/PD-L1 with antibodies has shown opportunities to improve cancer treatment in recent years. However, antibody therapy is a double-edged sword with high cost, low patient tolerance, lack of oral bioavailability, and a reaction to most solid tumors that prevents the adoption of antibodies. Advancement of small-molecule PD-1/PD-L1 inhibitors that could overwhelm these drawbacks is sluggish because of the poor pharmacodynamic properties and shallow pocket of the PD-1/PD-L1 binding interface. Recently, a number of compounds have been discovered to bind the PD-L1/PD-L1 dimer interface, providing an excellent alternative to inhibit the interaction between PD-1/PD-L1 and small molecules. Quantitative characterization of PD-L1 interactions with these inhibitors will advance the design of novel and efficient inhibitors in the future. Here, the binding free energies of 35 PD-L1 dimer inhibitors have been calculated using the alanine-scanning-interaction-entropy (AS-IE) method. Hotspot residues on PD-L1 and potential modification groups on the inhibitors were identified. The experimental results for the AS-IE method were better correlated than the classical MM/GBSA method. These results may set the stage for the design the more powerful PD-L1 inhibitors.Communicated by Ramaswamy H. Sarma.

Clinical potential of PD-1/PD-L1 blockade therapy for renal cell carcinoma (RCC): a rapidly evolving strategy

Programmed death-1 (PD-1)/programmed death-ligand 1 (PD-L1) blockade therapy has become a game-changing therapeutic approach revolutionizing the treatment setting of human malignancies, such as renal cell carcinoma (RCC). Despite the remarkable clinical activity of anti-PD-1 or anti-PD-L1 monoclonal antibodies, only a small portion of patients exhibit a positive response to PD-1/PD-L1 blockade therapy, and the primary or acquired resistance might ultimately favor cancer development in patients with clinical responses. In light of this, recent reports have signified that the addition of other therapeutic modalities to PD-1/PD-L1 blockade therapy might improve clinical responses in advanced RCC patients. Until, combination therapy with PD-1/PD-L1 blockade therapy plus cytotoxic T lymphocyte antigen 4 (CTLA-4) inhibitor (ipilimumab) or various vascular endothelial growth factor receptors (VEGFRs) inhibitors axitinib, such as axitinib and cabozantinib, has been approved by the United States Food and Drug Administration (FDA) as first-line treatment for metastatic RCC. In the present review, we have focused on the therapeutic benefits of the PD-1/PD-L1 blockade therapy as a single agent or in combination with other conventional or innovative targeted therapies in RCC patients. We also offer a glimpse into the well-determined prognostic factor associated with the clinical response of RCC patients to PD-1/PD-L1 blockade therapy.

Effects of small molecule-induced dimerization on the programmed death ligand 1 protein life cycle

The programmed death 1 (PD-1)/programmed death ligand 1 (PD-L1) checkpoint blockade is central to Immuno-Oncology based therapies, and alternatives to antibody blockers of this interaction are an active area of research due to antibody related toxicities. Recently, small molecule compounds that induce PD-L1 dimerization and occlusion of PD-1 binding site have been identified and developed for clinical trials. This mechanism invokes an oligomeric state of PD-L1 not observed in cells previously, as PD-L1 is generally believed to function as a monomer. Therefore, understanding the cellular lifecycle of the induced PD-L1 dimer is of keen interest. Our report describes a moderate but consistent increase in the PD-L1 rate of degradation observed upon protein dimerization as compared to the monomer counterpart. This subtle change, while not resolved by measuring total PD-L1 cellular levels by western blotting, triggered investigations of the overall protein distribution across various cellular compartments. We show that PD-L1 dimerization does not lead to rapid internalization of neither transfected nor endogenously expressed protein forms. Instead, evidence is presented that dimerization results in retention of PD-L1 intracellularly, which concomitantly correlates with its reduction on the cell surface. Therefore, the obtained data for the first time points to the ability of small molecules to induce dimerization of the newly synthesized PD-L1 in addition to the protein already present on the plasma membrane. Overall, this work serves to improve our understanding of this important target on a molecular level in order to guide advances in drug development.

RCSB Protein Data bank: Tools for visualizing and understanding biological macromolecules in 3D

Now in its 52nd year of continuous operations, the Protein Data Bank (PDB) is the premiere open-access global archive housing three-dimensional (3D) biomolecular structure data. It is jointly managed by the Worldwide Protein Data Bank (wwPDB) partnership. The Research Collaboratory for Structural Bioinformatics Protein Data Bank (RCSB PDB) is funded by the National Science Foundation, National Institutes of Health, and US Department of Energy and serves as the US data center for the wwPDB. RCSB PDB is also responsible for the security of PDB data in its role as wwPDB-designated Archive Keeper. Every year, RCSB PDB serves tens of thousands of depositors of 3D macromolecular structure data (coming from macromolecular crystallography, nuclear magnetic resonance spectroscopy, electron microscopy, and micro-electron diffraction). The RCSB PDB research-focused web portal (RCSB.org) makes PDB data available at no charge and without usage restrictions to many millions of PDB data consumers around the world. The RCSB PDB training, outreach, and education web portal (PDB101.RCSB.org) serves nearly 700 K educators, students, and members of the public worldwide. This invited Tools Issue contribution describes how RCSB PDB (i) is organized; (ii) works with wwPDB partners to process new depositions; (iii) serves as the wwPDB-designated Archive Keeper; (iv) enables exploration and 3D visualization of PDB data via RCSB.org; and (v) supports training, outreach, and education via PDB101.RCSB.org. New tools and features at RCSB.org are presented using examples drawn from high-resolution structural studies of proteins relevant to treatment of human cancers by targeting immune checkpoints.