Development of Inhibitors of the Programmed Cell Death-1/Programmed Cell Death-Ligand 1 Signaling Pathway

Harnessing the capacity of phytochemicals to enhance immune checkpoint inhibitor therapy of cancers: A focus on brain malignancies

Brain cancers, particularly glioblastoma multiforme (GBM), are challenging health issues with frequent unmet aspects. Today, discovering safe and effective therapeutic modalities for brain tumors is among the top research interests. Immunotherapy is an emerging area of investigation in cancer treatment. Since immune checkpoints play fundamental roles in repressing anti-cancer immunity, diverse immune checkpoint inhibitors (ICIs) have been developed, and some monoclonal antibodies have been approved clinically for particular cancers; nevertheless, there are significant concerns regarding their efficacy and safety in brain tumors. Among the various tools to modify the immune checkpoints, phytochemicals show good effectiveness and excellent safety, making them suitable candidates for developing better ICIs. Phytochemicals regulate multiple immunological checkpoint-related signaling pathways in cancer biology; however, their efficacy for clinical cancer immunotherapy remains to be established. Here, we discussed the involvement of immune checkpoints in cancer pathology and summarized recent advancements in applying phytochemicals in modulating immune checkpoints in brain tumors to highlight the state-of-the-art and give constructive prospects for future research.

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.

Enhanced Electrochemical Characterization of the Immune Checkpoint Protein PD-L1 using Aptamer-Functionalized Magnetic Metal-Organic Frameworks

Programmed death ligand 1 (PD-L1) is highly expressed in cancer cells and participates in the immune escape process of tumor cells. However, as one of the most promising biomarkers for cancer immunotherapy monitoring, the key problem ahead of practical usage is how to effectively improve the detection sensitivity of PD-L1. Herein, an electrochemical aptasensor for the evaluation of tumor immunotherapy is developed based on the immune checkpoint protein PD-L1. The fundamental principle of this method involves the utilization of DNA nanotetrahedron (NTH)-based capture probes and aptamer-modified magnetic metal-organic framework nanocomposites as signaling probes. A synergistic enhancement is observed in the electrocatalytic effect between Fe3O4 and UiO-66 porous shells in Fe3O4@UiO-66 nanocomposites. Therefore, the integration of aptamer-modified Fe3O4@UiO-66@Au with NTH-assisted target immobilization as an electrochemical sensing platform can significantly enhance sensitivity and specificity for target detection. This method enables the detection of targets at concentrations as low as 7.76 pg mL-1 over a wide linear range (0.01 to 1000 ng mL-1). The authors have successfully employed this sensor for in situ characterization of PD-L1 on the cell surface and for monitoring changes in PD-L1 expression during drug therapy, providing a cost-effective yet robust alternative to highly expensive and expertise-dependent flow cytometry.

Discovery of Highly Potent Small-Molecule PD-1/PD-L1 Inhibitors with a Novel Scaffold for Cancer Immunotherapy

Inhibition of the PD-1/PD-L1 interaction through small-molecule inhibitors is a promising therapeutic approach in cancer immunotherapy. Herein, we utilized BMS-202 as the lead compound to develop a series of novel PD-1/PD-L1 small-molecule inhibitors with a naphthyridin scaffold. Among these compounds, X14 displayed the most potent inhibitory activity for the PD-1/PD-L1 interaction (IC50 = 15.73 nM). Furthermore, X14 exhibited good binding affinity to both human PD-L1 (KD = 14.62 nM) and mouse PD-L1 (KD = 392 nM). In particular, X14 showed favorable pharmacokinetic properties (oral bioavailability, F = 58.0%). In the 4T1 (mouse breast cancer cells) syngeneic mouse model, intragastric administration of X14 at 10 mg/kg displayed significant antitumor efficacy (TGI = 66%). Mechanistic investigations revealed that X14 effectively enhanced T-cell infiltration within the tumor microenvironment. Our study demonstrates that compound X14 exhibits potential as a candidate compound for the development of orally effective small-molecule inhibitors targeting PD-1/PD-L1.

Delivery to, and Reactivation of, the p53 Pathway in Cancer Cells Using a Grafted Cyclotide Conjugated with a Cell-Penetrating Peptide

Peptides are promising drug modalities that can modulate protein-protein interactions, but their application is hampered by their limited ability to reach intracellular targets. Here, we improved the cytosolic delivery of a peptide blocking p53:MDM2/X interactions using a cyclotide as a stabilizing scaffold. We applied several design strategies to improve intracellular delivery and found that the conjugation of the lead cyclotide to the cyclic cell-penetrating peptide cR10 was the most effective. Conjugation allowed cell internalization at micromolar concentration and led to elevated intracellular p53 levels in A549, MCF7, and MCF10A cells, as well as inducing apoptosis in A549 cells without causing membrane disruption. The lead peptide had >35-fold improvement in inhibitory activity and increased cellular uptake compared to a previously reported cyclotide p53 activator. In summary, we demonstrated the delivery of a large polar cyclic peptide in the cytosol and confirmed its ability to modulate intracellular protein-protein interactions involved in cancer.

Challenges Coexist with Opportunities: Spatial Heterogeneity Expression of PD-L1 in Cancer Therapy

Cancer immunotherapy using anti-programmed death-ligand 1 (PD-L1) antibodies has been used in various clinical applications and achieved certain results. However, such limitations as autoimmunity, tumor hyperprogression, and overall low patient response rate impede its further clinical application. Mounting evidence has revealed that PD-L1 is not only present in tumor cell membrane but also in cytoplasm, exosome, or even nucleus. Among these, the dynamic and spatial heterogeneous expression of PD-L1 in tumors is mainly responsible for the unsatisfactory efficacy of PD-L1 antibodies. Hence, numerous studies focus on inhibiting or degrading PD-L1 to improve immune response, while a comprehensive understanding of the molecular mechanisms underlying spatial heterogeneity of PD-L1 can fundamentally transform the current status of PD-L1 antibodies in clinical development. Herein, the concept of spatial heterogeneous expression of PD-L1 is creatively introduced, encompassing the structure and biological functions of various kinds of PD-L1 (including mPD-L1, cPD-L1, nPD-L1, and exoPD-L1). Then an in-depth analysis of the regulatory mechanisms and potential therapeutic targets of PD-L1 is provided, seeking to offer a solid basis for future investigation. Moreover, the current status of agents is summarized, especially small molecular modulators development directed at these new targets, offering a novel perspective on potential PD-L1 therapeutics strategies.

Discovery of Novel PD-L1 Inhibitors That Induce the Dimerization, Internalization, and Degradation of PD-L1 Based on the Fragment Coupling Strategy

Tumor cells can evade immune surveillance through overexpressing programmed cell death-ligand 1 (PD-L1) to interact with programmed cell death-1 (PD-1). Besides, tumor-intrinsic PD-L1 is involved in tumor progression without interaction with PD-1, which provides more challenges for the discovery of PD-L1 inhibitors. Herein, we report the discovery of novel PD-L1 inhibitors using the fragment coupling strategy. Among them, B9 was found to inhibit the PD-1/PD-L1 interaction with the best IC50 value of 1.8 ± 0.7 nM. Beyond the blockade of the PD-1/PD-L1 axis, B9 promotes the dimerization, internalization, and degradation of PD-L1. Furthermore, B9 displayed high in vivo antitumor efficacy in the CT26 mouse model and activated the immune microenvironment and induced PD-L1 degradation of PD-L1 in the tumor. These results show that B9 is a promising lead PD-L1 inhibitor through the blockade of PD-1/PD-L1 interaction and functional inhibition of the PD-L1 signal pathway.

Mechanisms of the PD-1/PD-L1 pathway in itch: From acute itch model establishment to the role in chronic itch in mouse

Programmed cell death receptor/ligand 1 (PD-1/PD-L1) blockade therapy for various cancers induces itch. However, few studies have evaluated the mechanism underlying PD-1/PD-L1 inhibitor-induced itch. This study aimed to establish and evaluate a mouse model of acute itch induced by PD-1/PD-L1 inhibitors and to explore the role of the PD-1/PD-L1 pathway in chronic itch. The intradermal injection of the PD-1/PD-L1 small molecule inhibitors, or anti-PD-1/PD-L1 antibodies in the nape of the neck in the mice elicited intense spontaneous scratches. The model was evaluated using pharmacological methods. The number of scratches was reduced by naloxone but not by antihistamines or the transient receptor potential (TRP) channel inhibitor. Moreover, the PD-1 receptor was detected in the spinal cord of the mouse models of chronic itch that exhibited acetone, diethyl ether, and water (AEW)-induced dry skin, imiquimod-induced psoriasis, and 1-fluoro-2,4-dinitrobenzene (DNFB)-induced allergic contact dermatitis. Intrathecal PD-L1 (1 μg, 4 times a week for 1 week) suppressed the activation of the microglia in the spinal dorsal horn to relieve the chronic itch that was elicited by imiquimod-induced psoriasis and DNFB-induced allergic contact dermatitis. Although the activation of the microglia in the spinal dorsal horn was not detected in the AEW-treated mice, intrathecal PD-L1 still reduced the number of scratches that were elicited by AEW. Our findings suggest that histamine receptor inhibitors or TRP channel inhibitors have limited effects on PD-1/PD-L1 inhibitor-induced itch and that spinal PD-1 is important for the spinal activation of the microglia, which may underlie chronic itch.

99mTc-Labeled Cyclic Peptide Targeting PD-L1 as a Novel Nuclear Imaging Probe

Recent cancer therapies have focused on reducing immune suppression in the tumor microenvironment to prevent cancer progression and metastasis. PD-1 is a checkpoint protein that stops the immune response and is expressed on immune T cells. Cancer cells express a PD-1 ligand (PD-L1) to bind to the T-cell surface and activate immunosuppressive pathways. This study aimed to design, synthesize, and evaluate a 99mTc-labeled PD-L1-targeting cyclic peptide inhibitor (99mTc-iPD-L1) as a novel SPECT radiopharmaceutical for PD-L1 expression imaging. AutoDock software (version 1.5) was used to perform molecular docking for affinity calculations. The chemical synthesis was based on the coupling reaction of 6-hydrazinylpyridine-3-carboxylic acid with a 14-amino-acid cyclic peptide. iPD-L1 was prepared for 99mTc labeling. Radio-HPLC was used to verify radiochemical purity. The stability of the radiopeptide in human serum was evaluated by HPLC. iPD-L1 specificity was assessed by SDS-PAGE. [99mTc]Tc-iPD-L1 cellular uptake in PD-L1-positive cancer cells (HCC827 and HCT116) and biodistribution in mice with induced tumors were also performed. One patient with advanced plantar malignant melanoma received [99mTc]Tc-iPD-L1. The iPD-L1 ligand (AutoDock affinity: -6.7 kcal/mol), characterized by UPLC mass, FT-IR, and UV-Vis spectroscopy, was obtained with a chemical purity of 97%. The [99mTc]Tc-iPD-L1 was prepared with a radiochemical purity of >90%. In vitro and in vivo analyses demonstrated [99mTc]Tc-iPD-L1 stability (>90% at 24 h) in human serum, specific recognition for PD-L1, high uptake by the tumor (6.98 ± 0.89% ID/g at 1 h), and rapid hepatobiliary and kidney elimination. [99mTc]Tc-iPD-L1 successfully detected PD-L1-positive lesions in a patient with plantar malignant melanoma. The results obtained in this study warrant further dosimetric and clinical studies to determine the sensitivity and specificity of [99mTc]Tc-iPD-L1/SPECT for PD-L1 expression imaging.

Synthesis and preliminary biological evaluation of a novel 99mTc-labeled small molecule for PD-L1 imaging

In recent years, PD-1/PD-L1 checkpoint blockade immunotherapy with remarkable efficacy has set off a heat wave. The expression level of PD-L1, which plays a predictive role in anti-PD-1/PD-L1 therapy, could be quantified by noninvasive imaging with radiotracers. Herein, we introduced the synthesis and preliminary biological evaluation of a novel 99mTc-labeled small molecule radiotracer [99mTc]G3C-CBM for PD-L1 imaging. [99mTc]G3C-CBM was achieved with high radiochemical purity (>96 %) and remained good stability in PBS and FBS. In competitive combination experiment, [99mTc]G3C-CBM was displaced by increasing concentrations of unlabeled G3C-CBM, resulting in an IC50 value of 41.25±2.23 nM for G3C-CBM. The uptake of [99mTc]G3C-CBM in A375-hPD-L1 cells (17.51±2.08 %) was approximately 6.47 folds of that in A375 cells (2.71±0.36 %) after co-incubation for 2 h. The biodistribution results showed that the radioactivity uptake in A375-hPD-L1 tumor reached the maximum (0.35±0.01 %ID/g) at 2 h post injection, and the optimum tumor/muscle ratio of 2.94±0.29 occurred at the same time. In addition, [99mTc]G3C-CBM was quickly cleared from the blood with a clearance half-life of just 119.25 min. These results indicate that [99mTc]G3C-CBM is a potential SPECT PD-L1 imaging agent and is worthy of further study.

Garcinia mangostana L. Pericarp Extract and Its Active Compound α-Mangostin as Potential Inhibitors of Immune Checkpoint Programmed Death Ligand-1

α-Mangostin, a major xanthone found in mangosteen (Garcinia mangostana L., Family Clusiaceae) pericarp, has been shown to exhibit anticancer effects through multiple mechanisms of action. However, its effects on immune checkpoint programmed death ligand-1 (PD-L1) have not been studied. This study investigated the effects of mangosteen pericarp extract and its active compound α-mangostin on PD-L1 by in vitro and in silico analyses. HPLC analysis showed that α-mangostin contained about 30% w/w of crude ethanol extract of mangosteen pericarp. In vitro experiments in MDA-MB-231 triple-negative breast cancer cells showed that α-mangostin and the ethanol extract significantly inhibit PD-L1 expression when treated for 72 h with 10 µM or 10 µg/mL, respectively, and partially inhibit glycosylation of PD-L1 when compared to untreated controls. In silico analysis revealed that α-mangostin effectively binds inside PD-L1 dimer pockets and that the complex was stable throughout the 100 ns simulation, suggesting that α-mangostin stabilized the dimer form that could potentially lead to degradation of PD-L1. The ADMET prediction showed that α-mangostin is lipophilic and has high plasma protein binding, suggesting its greater distribution to tissues and its ability to penetrate adipose tissue such as breast cancer. These findings suggest that α-mangostin-rich mangosteen pericarp extract could potentially be applied as a functional ingredient for cancer chemoprevention.

Discovery and Crystallography Study of Novel Biphenyl Ether and Oxadiazole Thioether (Non-Arylmethylamine)-Based Small-Molecule PD-1/PD-L1 Inhibitors as Immunotherapeutic Agents

Current small-molecule PD-1/PD-L1 inhibitors are mainly based on the arylmethylamine/biphenyl core scaffold. Herein, we designed for the first time a series of non-arylmethylamine analogues (oxadiazole thioether derivatives) as small-molecule PD-1/PD-L1 inhibitors. Among them, compound LP23 exhibited the most potent PD-L1 inhibitory activity with an IC50 of 16.7 nM, 3.2-fold better than the lead BMS-202 (IC50 = 53.6 nM). The X-ray crystal structure of LP23 in complex with PD-L1 was solved at a resolution of 2.6 Å, which further confirmed the high binding affinity of LP23 to PD-L1. In the HepG2/Jurkat T cell co-culture model, LP23 effectively promoted HepG2 cell death by restoring the immune function of T cells. In addition, LP23 showed excellent in vivo antitumor efficacy (TGI = 88.6% at 30 mg/kg) and benign toxicity profiles in a B16-F10 tumor model by modulating PD-L1. In summary, LP23 represents the first non-arylmethylamine-based small-molecule PD-1/PD-L1 inhibitor worthy of further investigation.

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.

Linear Peptide-Based PET Tracers for Imaging PD-L1 in Tumors

Programmed cell death receptor 1 (PD-1) and its ligand PD-L1 are particularly interesting immune checkpoint proteins for human cancer treatment. Positron emission tomography (PET) imaging allows for the dynamic monitoring of PD-L1 status during tumor progression, thus informing patients' response index. Herein, we report the synthesis of two linear peptide-based radiotracers, [64Cu]/[68Ga]HKP2201 and [64Cu]/[68Ga]HKP2202, and validate their utility for PD-L1 visualization in preclinical models. The precursor peptide HKP2201 was derived from a linear peptide ligand, CLP002, which was previously identified by phage display and showed nanomolar affinity toward PD-L1. Appropriate modification of CLP002 via PEGylation and DOTA conjugation yielded HKP2201. The dimerization of HKP2201 generated HKP2202. The 64Cu and 68Ga radiolabeling of both precursors was studied and optimized. PD-L1 expression in mouse melanoma cell line B16F10, mouse colon cancer cell line MC38, and their allografts were assayed by immunofluorescence and immunohistochemistry staining. Cellular uptake and binding assays were conducted in both cell lines. PET imaging and ex vivo biodistribution studies were employed in tumor mouse models bearing B16F10 and MC38 allografts. [64Cu]/[68Ga]HKP2201 and [64Cu]/[68Ga]HKP2202 were obtained with satisfactory radiocharacteristics. They all showed lower liver accumulation compared to [64Cu]/[68Ga]WL12. B16F10 and MC38 cells and their tumor allografts sections were verified to express PD-L1. These tracers demonstrated a concentration-dependent cell affinity and a comparable half-maximal effect concentration (EC50) with radiolabeled WL12. Competitive binding and blocking studies demonstrated the specific target of these tracers to PD-L1. PET imaging and ex vivo biodistribution studies revealed notable tumor uptake in tumor-bearing mice and rapid clearance from blood and major organs. Importantly, [64Cu]/[68Ga]HKP2202 showed higher tumor uptake compared to [64Cu]/[68Ga]HKP2201. Of note, [64Cu] labeled tracers showed longer retention in tumors than [68Ga] labeled traces, indicating advantages in the long-term tracking of PD-L1 dynamics. In comparison, [68Ga]HKP2201 and [68Ga]HKP2202 showed lower liver accumulation, enabling its great potential in the fast detection of both primary and metastatic tumors, including hepatic carcinoma. [64Cu]/[68Ga]HKP2201 and [64Cu]/[68Ga]HKP2202 are promising PET tracers for visualizing PD-L1 status. Notably, their combination would cooperate in rapid diagnosis and subsequent treatment guidance. Future assessment of the radiotracers in patients is needed to fully evaluate their clinical value.

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.

Probing the origins of programmed death ligand-1 inhibition by implementing machine learning-assisted sequential virtual screening techniques

PD-L1 is a key immunotarget involved in binding to its receptor PD-1. PD-L1/PD-1 interface blocking using antibodies (or small molecules) is the central area of interest for tumor suppression in various cancers. Blocking the PD-L1/PD-1 pathway in the tumor cells results in its immune activation and destruction, and thereby restoring the T-cell proliferation and cytokine production. The active binding site interface residues of PD-L1/PD-1 were experimentally known and proven by structural biology and site-directed mutagenesis studies. Structure-based molecular design technique was employed to identify the inhibitors for blocking the PD-L1/PD-1 interface. Nine hits to leads were identified from the SPECS small molecule database by machine learning, molecular docking, and molecular dynamics simulation techniques. Following this, a machine learning-assisted QSAR modeling approach was implemented using ChEMBL database to gain insights into the inhibitory potential of PD-L1 inhibitors and predict the activity of our previously screened nine hit molecules. The best leads identified in the present study bind strongly with the active sites of PD-L1/PD-1 interface residues, which include A121, M115, I116, S117, I54, Y56, D122, and Y123. These computational leads are considered promising molecules for further in vitro and in vivo analysis to be developed as potential PD-L1 checkpoint inhibitors to cure different types of cancers.

Design, synthesis, and evaluation of PD-1/PD-L1 small-molecule inhibitors bearing a rigid indane scaffold

Discovery of small-molecule inhibitors against programmed cell death-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) axis provides a promising alternative to overcome the inevitable defects of PD-1/PD-L1 monoclonal antibodies (mAbs). Here, we report a series of indanes as novel small-molecule inhibitors of PD-1/PD-L1 interaction. Thirty-one indanes were synthesized and the structure-activity relationships (SARs) demonstrated that conformational restriction with (S)-indane is superior in potency to inhibit the interaction of PD-1 and PD-L1. Compound D3 was found to be the most potent inhibitor with an IC₅₀ value of 2.2 nM against PD-1/PD-L1 interaction. Cell-based assay showed that D3 significantly induced immune activity of peripheral blood mononuclear cells (PBMCs) against MDA-MB-231 cells and could restore the immune function of T cells by promoting secretion of the IFN-γ. The above results indicate that compound D3 is a promising PD-1/PD-L1 inhibitor that deserves further development.

Bioactive Peptide Nanodrugs Based on Supramolecular Assembly for Boosting Immunogenic Cell Death-Induced Cancer Immunotherapy

Immunogenic cell death (ICD)-induced immunotherapy holds promise for complete elimination and long-term protective immune responses against cancer by combining direct tumor cell killing and antitumor immune response. Some therapeutic approaches (such as hyperthermia, photodynamic therapy, or radiotherapy) and inducers (certain chemotherapy drugs, oncolytic viruses) have been devoted to initiating and/or boosting ICD, leading to the activation of tumor-specific immune responses. Recently, supramolecular assembled bioactive peptide nanodrugs have been employed to improve the efficacy of ICD-induced cancer immunotherapy by increasing tumor targeted accumulation as well as responsive release of ICD inducers, directly inducing high levels of ICD and realizing the simultaneous enhancement of immune response through the immune function of the active peptide itself. Here, the authors review bioactive peptide nanodrugs based on supramolecular assembly, mainly as an intelligent delivery system, a direct ICD inducer and an immune response enhancer, for boosting ICD induced cancer immunotherapy. The functions of diverse bioactive peptides used in the construction of nanodrugs are described. The design of a supramolecular assembly, the mechanism of boosting ICD, and synergetic effects of bioactive peptides combined immunotherapy are critically emphasized.

A Three-in-One Nanoscale Coordination Polymer for Potent Chemo-Immunotherapy

The addition of immune checkpoint blockade to standard chemotherapy has changed the standards of care for some cancer patients. However, current chemo-immunotherapy strategies do not benefit most colorectal cancer patients and many triple-negative breast cancer patients. Here, the design of a three-in-one nanoscale coordination polymer (NCP), OX/GC/CQ, comprising prodrugs of oxaliplatin (OX), gemcitabine (GC), and 5-carboxy-8-hydroxyquinoline (CQ) for triple-modality chemo-immunotherapy is reported. OX/GC/CQ exhibits optimal pharmacokinetics and enhanced particle accumulation and drug release in acidic tumor tissues, wherein CQ greatly enhances immunogenic cell death induced by OX/GC and downregulates programmed cell death-ligand 1  expression in cancer cells. Consequently, OX/GC/CQ efficiently promotes infiltration and activity of cytotoxic T lymphocytes, while decreasing the proportion of immunosuppressive regulatory T cells. Intravenous injection of OX/GC/CQ reduces the growth of colorectal carcinoma and triple-negative breast cancer, prevents metastasis to lungs, and extends mouse survival by 30-40 days compared to free drugs. This work highlights the potential of NCPs in co-delivering synergistic chemo-immunotherapeutics for the treatment of advanced and aggressive cancers.

Phenothiazine-Based LSD1 Inhibitor Promotes T-Cell Killing Response of Gastric Cancer Cells

Histone lysine specific demethylase 1 (LSD1) has been recognized as an important epigenetic target for cancer treatment. Although several LSD1 inhibitors have entered clinical trials, the discovery of novel potent LSD1 inhibitors remains a challenge. In this study, the antipsychotic drug chlorpromazine was characterized as an LSD1 inhibitor (IC₅₀ = 5.135 μM), and a series of chlorpromazine derivatives were synthesized. Among them, compound 3s (IC₅₀ = 0.247 μM) was the most potent one. More importantly, compound 3s inhibited LSD1 in the cellular level and downregulated the expression of programmed cell death-ligand 1 (PD-L1) in BGC-823 and MFC cells to enhance T-cell killing response. An in vivo study confirmed that compound 3s can inhibit MFC cell proliferation without significant toxicity in immunocompetent mice. Taken together, our findings indicated that the novel LSD1 inhibitor 3s tethering a phenothiazine scaffold may serve as a lead compound for further development to activate T-cell immunity in gastric cancer.

Discovery of small-molecule PD-1/PD-L1 antagonists through combined virtual screening and experimental validation

Inhibition of the interaction between the PD-1 protein on activated lymphocytes and the PD-L1 protein on tumors represents a novel therapeutic approach for selective activation of the innate immune response against a variety of cancers. Therefore, the present study utilized a combined virtual and experimental screening approach to screen databases of both lead-like and larger molecules for identification of novel inhibitors of PD-1/PD-L1 interaction. First, high-throughput virtual screening of ∼3.7 million lead-like molecules using a rigid-receptor docking approach against both human PD-1 and PD-L1 proteins revealed possible small-molecule tractability of PD-1, but not PD-L1, binding interface. The subsequent work, therefore, involved screening of the National Cancer Institute (NCI) compound database against the PD-1 pocket. Several NCI compounds were identified with potential to bind to the PD-1 pocket and in turn inhibit the PD-1/PD-L1 interaction. The dynamic binding behavior of these molecules was further investigated using long 100 ns molecular dynamics (MD) stimulation revealing NSC631535 to be a potentially stable binder at PD-1 interface pocket. In support of these MD data, the experimental testing of NSC631535 exhibited 50% inhibition at ∼15 μM test concentration. The observed activity of this compound is promising as despite its relatively low molecular weight (415.5 g/mol) it is still capable of inhibiting the PD-1/PD-L1 interaction having a large interface area (∼1970 Å²). In summary, our integrated computational and experimental screening led to identification of a novel PD-1 antagonist that may serve as a starting point for further optimization into more potent small-molecule PD-1/PD-L1 inhibitors for cancer immunotherapy.

Engaging stemness improves cancer immunotherapy

Intra-tumoral immune cells promote the stemness of cancer stem cells (CSCs) in the tumor microenvironment (TME). CSCs promote tumor progression, relapse, and resistance to immunotherapy. Cancer stemness induces the expression of neoantigens and neo-properties in CSCs, creating an opportunity for targeted immunotherapies. Isolation of stem-like T cells or retaining stemness in T clonotypes strategies produces exhaustion-resistance T cells with superior re-expansion capacity and long-lasting responses after adoptive cell therapies. Stem cells-derived NK cells may be the next generation of NK cell products for immunotherapy. Here, we have reviewed mechanisms by which stemness factors modulated the immunoediting of the TME and summarized the potentials of CSCs in the development of immunotherapy regimens, including CAR-T cells, CAR-NK cells, cancer vaccines, and monoclonal antibodies. We have discussed the natural or genetically engineered stem-like T cells and stem cell-derived NK cells with increased cytotoxicity to tumor cells. Finally, we have provided a perspective on approaches that may improve the therapeutic efficacy of these novel adoptive cell-based products in targeting immunosuppressive TME.

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.

Cancer Metastasis and Treatment Resistance: Mechanistic Insights and Therapeutic Targeting of Cancer Stem Cells and the Tumor Microenvironment

Cancer metastasis and treatment resistance are the main causes of treatment failure and cancer-related deaths. Their underlying mechanisms remain to be fully elucidated and have been attributed to the presence of cancer stem cells (CSCs)-a small population of highly tumorigenic cancer cells with pluripotency and self-renewal properties, at the apex of a cellular hierarchy. CSCs drive metastasis and treatment resistance and are sustained by a dynamic tumor microenvironment (TME). Numerous pathways mediate communication between CSCs and/or the surrounding TME. These include a paracrine renin-angiotensin system and its convergent signaling pathways, the immune system, and other signaling pathways including the Notch, Wnt/β-catenin, and Sonic Hedgehog pathways. Appreciation of the mechanisms underlying metastasis and treatment resistance, and the pathways that regulate CSCs and the TME, is essential for developing a durable treatment for cancer. Pre-clinical and clinical studies exploring single-point modulation of the pathways regulating CSCs and the surrounding TME, have yielded partial and sometimes negative results. This may be explained by the presence of uninhibited alternative signaling pathways. An effective treatment of cancer may require a multi-target strategy with multi-step inhibition of signaling pathways that regulate CSCs and the TME, in lieu of the long-standing pursuit of a 'silver-bullet' single-target approach.

Exploring the Surface of the Ectodomain of the PD-L1 Immune Checkpoint with Small-Molecule Fragments

Development of small molecules targeting the PD-L1/PD-1 interface is advancing both in industry and academia, but only a few have reached early-stage clinical trials. Here, we take a closer look at the general druggability of PD-L1 using in silico hot spot mapping and nuclear magnetic resonance (NMR)-based characterization. We found that the conformational elasticity of the PD-L1 surface strongly influences the formation of hot spots. We deconstructed several generations of known inhibitors into fragments and examined their binding properties using differential scanning fluorimetry (DSF) and protein-based nuclear magnetic resonance (NMR). These biophysical analyses showed that not all fragments bind to the PD-L1 ectodomain despite having the biphenyl scaffold. Although most of the binding fragments induced PD-L1 oligomerization, two compounds, TAH35 and TAH36, retain the monomeric state of proteins upon binding. Additionally, the presence of the entire ectodomain did not affect the binding of the hit compounds and dimerization of PD-L1. The data demonstrated here provide important information on the PD-L1 druggability and the structure-activity relationship of the biphenyl core moiety and therefore may aid in the design of novel inhibitors and focused fragment libraries for PD-L1.

Development of Radiotracers for Imaging of the PD-1/PD-L1 Axis

Immune checkpoint inhibitor (ICI) therapy has emerged as a major treatment option for a variety of cancers. Among the immune checkpoints addressed, the programmed death receptor 1 (PD-1) and its ligand PD-L1 are the key targets for an ICI. PD-L1 has especially been proven to be a reproducible biomarker allowing for therapy decisions and monitoring therapy success. However, the expression of PD-L1 is not only heterogeneous among and within tumor lesions, but the expression is very dynamic and changes over time. Immunohistochemistry, which is the standard diagnostic tool, can only inadequately address these challenges. On the other hand, molecular imaging techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) provide the advantage of a whole-body scan and therefore fully address the issue of the heterogeneous expression of checkpoints over time. Here, we provide an overview of existing PET, SPECT, and optical imaging (OI) (radio)tracers for the imaging of the upregulation levels of PD-1 and PD-L1. We summarize the preclinical and clinical data of the different molecule classes of radiotracers and discuss their respective advantages and disadvantages. At the end, we show possible future directions for developing new radiotracers for the imaging of PD-1/PD-L1 status in cancer patients.

Computational discovery of small drug-like compounds as potential inhibitors of PD-1/PD-L1 interactions

The programmed cell death ligand protein 1 (PD-L1) is a strong immunosuppressive molecule that inactivates tumor-specific T cells by binding to the programmed cell death- 1 protein (PD-1). Cancer immunotherapy based on the monoclonal antibodies targeting the PD-1/PD-L1 pathway has demonstrated therapeutic responses without precedent over a wide range of cancers. However, the antibody-based immunotherapies have several limitations such as high production cost or the induction of severe immune-related adverse effects. Small-molecule inhibitors of the PD-1/PD-L1 pathway are a promising alternative or complementary therapeutic to antibodies. Currently, the field of developing anti-PD-1/PD-L1 small-molecule inhibitors is intensively explored. In the present study a pharmacophore model was generated based on previously developed compounds and their atomistic structures with the PD-L1 dimer. Structure-based affinity-based virtual screening of small-molecule inhibitors of the PD-1/PD-L1 pathway according to the pharmacophore model followed by a screening in terms of drug-likeness resulted in ten hit compounds of high affinity towards the PD-L1 dimer and the satisfaction to all of the drug-likeness rules. Molecular dynamics (MD) simulations showed that nine of ten compounds formed stable complexes with the PD-L1 dimer as evidenced by the analysis of MD trajectories. Molecular mechanics Poisson- Boltzmann surface area (MM-PBSA) calculation revealed very low binding energies (<-46 kcal/mol) for the interactions of these ligands with the PD-L1 dimer, suggesting that identified compounds may serve as good scaffolds for the design of novel agents of antitumor immunotherapy able to target the PD-1/PD-L1 interactionCommunicated by Ramaswamy H. Sarma.

Stimuli-responsive nano vehicle enhances cancer immunotherapy by coordinating mitochondria-targeted immunogenic cell death and PD-L1 blockade

Induction of immunogenic cell death promotes antitumor immunity against cancer. However, majority of clinically-approved drugs are unable to elicit sufficient ICD. Here, our study revealed that mitochondria-targeted delivery of doxorubicin (DOX) massively amplified ICD via substantial generation of reactive oxygen species (ROS) after mitochondrial damage. The underlying mechanism behind increased ICD was further demonstrated to be ascribed to two pathways: (1) ROS elevated endoplasmic reticulum (ER) stress, leading to surface exposure of calreticulin; (2) ROS promoted release of various mitochondria-associated damage molecules including mitochondrial transcription factor A. Nevertheless, adaptive upregulation of PD-L1 was found after such ICD-inducing treatment. To overcome such immunosuppressive feedback, we developed a tumor stimuli-responsive nano vehicle to simultaneously exert mitochondrial targeted ICD induction and PD-L1 blockade. The nano vehicle was self-assembled from ICD-inducing copolymer and PD-L1 blocking copolymer, and possessed long-circulating property which contributed to better tumor accumulation and mitochondrial targeting. As a result, the nano vehicle remarkably activated antitumor immune responses and exhibited robust antitumor efficacy in both immunogenic and non-immunogenic tumor mouse models.

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.

Multifunctional Lipid Bilayer Nanocarriers for Cancer Immunotherapy in Heterogeneous Tumor Microenvironments, Combining Immunogenic Cell Death Stimuli with Immune Modulatory Drugs

In addition to the contribution of cancer cells, the solid tumor microenvironment (TME) has a critical role in determining tumor expansion, antitumor immunity, and the response to immunotherapy. Understanding the details of the complex interplay between cancer cells and components of the TME provides an unprecedented opportunity to explore combination therapy for intervening in the immune landscape to improve immunotherapy outcome. One approach is the introduction of multifunctional nanocarriers, capable of delivering drug combinations that provide immunogenic stimuli for improvement of tumor antigen presentation, contemporaneous with the delivery of coformulated drug or synthetic molecules that provide immune danger signals or interfere in immune-escape, immune-suppressive, and T-cell exclusion pathways. This forward-looking review will discuss the use of lipid-bilayer-encapsulated liposomes and mesoporous silica nanoparticles for combination immunotherapy of the heterogeneous immune landscapes in pancreatic ductal adenocarcinoma and triple-negative breast cancer. We describe how the combination of remote drug loading and lipid bilayer encapsulation is used for the synthesis of synergistic drug combinations that induce immunogenic cell death, interfere in the PD-1/PD-L1 axis, inhibit the indoleamine-pyrrole 2,3-dioxygenase (IDO-1) immune metabolic pathway, restore spatial access to activated T-cells to the cancer site, or reduce the impact of immunosuppressive stromal components. We show how an integration of current knowledge and future discovery can be used for a rational approach to nanoenabled cancer immunotherapy.

Design, Synthesis, and Evaluation of PD-1/PD-L1 Antagonists Bearing a Benzamide Scaffold

Several antibodies targeting programmed cell death-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) have been approved by the U.S. Food and Drug Administration (FDA) for cancer therapy. Although many small-molecule inhibitors of the PD-1/PD-L1 pathway have been reported, no small-molecule inhibitors have been approved for cancer treatment. In this work, a series of novel benzamide derivatives were designed, synthesized, and evaluated to find effective inhibitors of the PD-1/PD-L1 interaction. The most potent compound D2 exhibited better activity than that of BMS202, with an IC₅₀ of 16.17 nM. D2 could activate the antitumor immunity of T cells efficiently in PBMCs. The proposed binding mode of compound D2 was investigated by docking analysis. These results indicate that compound D2 is a promising lead compound that can be used for further development.

Discovery of Small-Molecule Inhibitors of the PD-1/PD-L1 Axis That Promote PD-L1 Internalization and Degradation

Several monoclonal antibodies targeting the programmed cell death-1/programmed cell death-ligand 1 (PD-1/PD-L1) pathway have been used successfully in anticancer immunotherapy. Inherent limitations of antibody-based therapies remain, however, and alternative small-molecule inhibitors that can block the PD-1/PD-L1 axis are urgent needed. Herein, we report the discovery of compound 17 as a bifunctional inhibitor of PD-1/PD-L1 interactions. 17 inhibits PD-1/PD-L1 interactions and promotes dimerization, internalization, and degradation of PD-L1. 17 promotes cell-surface PD-L1 internalized into the cytosol and induces the degradation of PD-L1 in tumor cells through a lysosome-dependent pathway. Furthermore, 17 suppresses tumor growth in vivo by activating antitumor immunity. These results demonstrate that 17 targets the PD-1/PD-L1 axis and induces PD-L1 degradation.

Immunomodulatory potential of natural products from herbal medicines as immune checkpoints inhibitors: Helping to fight against cancer via multiple targets

Immunotherapy sheds new light to cancer treatment and is satisfied by cancer patients. However, immunotoxicity, single‐source antibodies, and single‐targeting stratege are potential challenges to the success of cancer immunotherapy. A huge number of promising lead compounds for cancer treatment are of natural origin from herbal medicines. The application of natural products from herbal medicines that have immunomodulatory properties could alter the landscape of immunotherapy drastically. The present study summarizes current medication for cancer immunotherapy and discusses the potential chemicals from herbal medicines as immune checkpoint inhibitors that have a broad range of immunomodulatory effects. Therefore, this review provides valuable insights into the efficacy and mechanism of actions of cancer immunotherapies, including natural products and combined treatment with immune checkpoint inhibitors, which could confer an improved clinical outcome for cancer treatment.

A Novel Small Cyclic Peptide-Based 68Ga-Radiotracer for Positron Emission Tomography Imaging of PD-L1 Expression in Tumors

In the tumor microenvironment, programmed death protein 1 and programmed death protein ligand 1 (PD-L1) signaling pathways help tumors escape the immune system. We designed a gallium-68 (⁶⁸Ga)-labeled small-molecule peptide-targeting PD-L1 and used positron emission tomography/computed tomography (PET/CT) to detect and dynamically monitor the expression level of PD-L1 in tumors. S-Cyclo(ETSK)-SF-NH₂ (SETSKSF) is a cyclic peptide inhibitor comprising seven amino-acid residues. We connected it with the chelating agent DOTA, labeled DOTA-SETSKSF, with the short half-life nuclide Ga-68, and measured the stability of ⁶⁸Ga-2,2',2″-(10-(2-((S)-1-((3S,6S,9S,18S)-18-((S)-1-((S)-1-amino-1-oxo-3-henylpropan-2-ylamino)-3-hydroxy-1-oxopropan-2-ylcarbamoyl)-6-((R)-1-hydroxyethyl)-3-(hydroxymethyl)-2,5,8,12-tetraoxo-1,4,7,13-tetraazacyclooctadecan-9-ylamino)-3-ydroxy-1-oxopropan-2-ylamino)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (⁶⁸Ga-DOTA-SETSKSF) in normal saline (NS), phosphate-buffered saline (PBS), and fetal bovine serum (FBS) in vitro. We conducted the ⁶⁸Ga-DOTA-SETSKSF affinity test, cell-specific uptake experiments, time-combined experiments, western blotting, and laser confocal experiments to confirm the expression and localization of PD-L1 at the cell level and determine the uptake. Biodistribution and imaging experiments were performed using the H1975, B16F10, and A549 tumor models. ⁶⁸Ga-DOTA-SETSKSF was successfully synthesized, and the radiochemical purity was >99% after purification. The in vitro stability of ⁶⁸Ga-DOTA-SETSKSF was >95% in NS, PBS, and FBS at 37 °C after 4 h of incubation. Cell-binding experiments confirmed that ⁶⁸Ga-DOTA-SETSKSF exhibited high uptake in H1975 tumors with high PD-L1 expression and low uptake in A549 tumors with low PD-L1 expression. The clear half-life (T_1/2) of ⁶⁸Ga-DOTA-SETSKSF from the blood was 14.48 ± 3.26 min. The percentages of the injected dose per gram of tissue (%ID/g) for H1975 and A549 tumors were 5.29 ± 0.21 and 0.89 ± 0.10 at 1 h after injection, respectively. The H1975 tumor-to-muscle and tumor-to-blood ratios were 41.79 ± 5.81 and 4.75 ± 0.19 at 4 h, respectively. Apart from the H1975 tumor, the kidney and the bladder showed high accumulation because ⁶⁸Ga-DOTA-SETSKSF was excreted through the urinary system. PET/CT images showed high accumulation of ⁶⁸Ga-DOTA-SETSKSF in H1975 tumors and low uptake in A549 tumors, which was consistent with the results of biodistribution experiments. ⁶⁸Ga-DOTA-SETSKSF is convenient to prepare, has high stability, can be used to monitor the expression of PD-L1, and has an extremely high clinical application value.

Small-molecule inhibitor PD-1/PD-L1 interaction for colorectal cancer treatment

Inhibition of the PD-1/PD-L1 pathway is a target for the development of new therapies. US10710986 patent describes a small molecule that targets PDL-1/PD-1 interactions and triggers antitumor activity against colorectal cancer. However, it does not describe biological assays that allow us to suppose that this small molecule may be active in other types of cancer. So far, there are no reports of clinical trials to evaluate the safety, toxicity and efficacy, but it will be of great interest to analyze in the future if this compound surpasses the action of therapy in cancer.

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.

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.

Design, synthesis, and structure-activity relationship of PD-1/PD-L1 inhibitors with a benzo[d]isoxazole scaffold

Blocking the programmed cell death protein 1 (PD-1) and programmed death-ligand (PD-L1) interaction has emerged as one of the most promising treatments for cancer immunotherapy. A novel series of compounds bearing a benzo[d]isoxazole scaffold was developed as PD-1/PD-L1 inhibitors, among them, compound P20 exhibited the most potent inhibitory activity, with an IC₅₀ value of 26.8 nM. The preliminary structure-activity relationship was also investigated. The docking analysis of compound P20 with the PD-L1 dimer complex (PDB ID: 5j89) indicated that compound P20 was bound to the PD-L1 dimer with high affinity. These results suggest that compound P20 is a promising lead compound for the development of inhibitors of the PD-1/PD-L1 interaction.

Discovery of Novel Inhibitors From Medicinal Plants for V-Domain Ig Suppressor of T-Cell Activation

V-domain Ig suppressor of T cell activation (VISTA) is an immune checkpoint and is a type I transmembrane protein. VISTA is linked to immunotherapy resistance, and it is a potential immune therapeutic target, especially for triple-negative breast cancer. It expresses at a high concentration in regulatory T cells and myeloid-derived suppressor cells, and its functional blockade is found to delay tumor growth. A useful medicinal plant database for drug designing (MPD3), which is a collection of phytochemicals from diverse plant families, was employed in virtual screening against VISTA to prioritize natural inhibitors against VISTA. Three compounds, Paratocarpin K (PubChem ID: 14187087), 3-(1H-Indol-3-yl)-2-(trimethylazaniumyl)propanoate (PubChem ID: 3861164), and 2-[(5-Benzyl-4-ethyl-1,2,4-triazol-3-yl)sulfanylmethyl]-5-methyl-1,3,4-oxadiazole (PubChem ID: 6494266), having binding energies stronger than -6 kcal/mol were found to have two common hydrogen bond interactions with VISTA active site residues: Arg54 and Arg127. The dynamics of the compound-VISTA complexes were further explored to infer binding stability of the systems. Results revealed that the compound 14187087 and 6494266 systems are highly stable with an average RMSD of 1.31 Å. Further affirmation on the results was achieved by running MM-GBSA on the MD simulation trajectories, which re-ranked 14187087 as the top-binder with a net binding energy value of -33.33 kcal/mol. In conclusion, the present study successfully predicted natural compounds that have the potential to block the function of VISTA and therefore can be utilized further in experimental studies to validate their real anti-VISTA activity.

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.

Rational Design of Potent Peptide Inhibitors of the PD-1:PD-L1 Interaction for Cancer Immunotherapy

Peptides have potential to be developed into immune checkpoint inhibitors, but the target interfaces are difficult to inhibit. Here, we explored an approach to mimic the binding surface of PD-1 to design inhibitors. Mimicking native PD-1 resulted in a mimetic with no activity. However, mimicking an affinity-optimized PD-1 resulted in the peptide mimetic MOPD-1 that displayed nanomolar affinity to PD-L1 and could inhibit PD-1:PD-L1 interactions in both protein- and cell-based assays. Mutagenesis and structural characterization using NMR spectroscopy and X-ray crystallography revealed that binding residues from the high affinity PD-1 are crucial for the bioactivity of MOPD-1. Furthermore, MOPD-1 was extremely stable in human serum and inhibited tumor growth in vivo, suggesting it has potential for use in cancer immunotherapy. The successful design of an inhibitor of PD-1:PD-L1 using the mimicry approach described herein illustrates the value of placing greater emphasis on optimizing the target interface before inhibitor design and is an approach that could have broader utility for the design of peptide inhibitors for other complex protein-protein interactions.

Structure-activity relationship and biological evaluation of 12 N-substituted aloperine derivatives as PD-L1 down-regulatory agents through proteasome pathway

Twenty-nine 12 N-substituted aloperine derivatives were synthesized and screened for suppression on PD-L1 expression in H460 cells, as a continuation of our work. Systematic structural modifications led to the identification of compound 6b as the most active PD-L1 modulator. Compound 6b could significantly down-regulate both constitutive and inductive PD-L1 expression in NSCLC cells, and successively enhance the cytotoxicity of co-cultured T cells against tumor cells at the concentration of 20 μM. Also, it exhibited a moderate in vivo anticancer efficacy against Lewis tumor xenograft with a stable PK and safety profile. The mechanism study indicated that 6b mediated the degradation of PD-L1 through a proteasome pathway, rather than a lysosome route. These results provided the powerful information for cancer immunotherapy of aloperine derivatives with unique endocyclic skeleton by targeting PD-L1 to activate immune cells to kill cancer cells.

DTX@VTX NPs synergy PD-L1 immune checkpoint nanoinhibitor to reshape immunosuppressive tumor microenvironment for enhancing chemo-immunotherapy

Immunosuppressed tumor microenvironment (TME) is a major cause of the low response rate in solid tumor patients during PD-1/PD-L1 checkpoint blockade therapy. In this study, a series of small molecule nanomedicines with a 100% drug loading rate were prepared via the nanoprecipitation method. They were used in synergistic chemo-immunotherapy for 4T1 tumors. Among four PD-L1 small-molecule nanoinhibitors, BMS-1 NP with the best anti-tumor performance was selected to replace the therapeutic PD-L1 antibody. The core-shell small-molecule nanomedicine DTX@VTX NP (DTX: Docetaxel and VTX: VTX-2337 or Motolimod) was used to reverse immunosuppressed TME through the depletion of myeloid-derived suppressor cells (MDSCs) and the polarization of macrophages from an M2-like phenotype to M1-like phenotype. Thus, the frequency of cytotoxic CD8⁺ T cells was significantly increased, resulting in an effective attack on cancer cells. Combining BMS-1 NPs with DTX@VTX NPs, synergistic chemo-immunotherapy of 4T1 tumors was performed, and the results indicate that the inhibition rates of primary and rechallenge tumors achieved 90.5% and 94.3%, respectively. These results indicate that DTX@VTX NPs can synergize PD-L1 nanoinhibitor BMS-1 NPs to reshape the immunosuppressive tumor microenvironment for enhancing the anti-tumor effect of chemo-immunotherapy for breast.

Identification of active small-molecule modulators targeting the novel immune checkpoint VISTA

Background

Cancer immunotherapy has gained increasing popularity as a novel approach to treat cancer. A member of the B7 family, V-domain immunoglobulin suppressor of T-cell activation (VISTA) is a novel immune checkpoint that regulates a broad spectrum of immune responses. VISTA is an acidic pH-selective ligand for P-selectin glycoprotein ligand-1(PSGL-1). CA-170, a first-in-class small-molecule dual antagonist of VISTA/PD-L1, was collaboratively developed by Aurigene Discovery Technologies Limited and Curis, Inc. It is currently in Phase I clinical trial.

Results

In this study, we develop homology modeling for the VISTA 3D structure and subsequent virtual screening for VISTA small-molecule hit ligands. Visualization of the binding postures of docked ligands with the VISTA protein indicates that some small molecular compounds target VISTA. The ability of antagonist to disrupt immune checkpoint VISTA pathways was investigated though functional studies in vitro.

Conclusions

Affinity active molecule for VISTA was obtained through virtual screening, and the antagonist compound activity to VISTA was assayed in cellular level. We reported a small molecule with high VISTA affinity as antagonist, providing ideas for development VISTA-targeted small molecule compound in cancer immunotherapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12865-021-00446-4.

Structural Characterization of a Macrocyclic Peptide Modulator of the PD-1/PD-L1 Immune Checkpoint Axis

The clinical success of PD-1/PD-L1 immune checkpoint targeting antibodies in cancer is followed by efforts to develop small molecule inhibitors with better penetration into solid tumors and more favorable pharmacokinetics. Here we report the crystal structure of a macrocyclic peptide inhibitor (peptide 104) in complex with PD-L1. Our structure shows no indication of an unusual bifurcated binding mode demonstrated earlier for another peptide of the same family (peptide 101). The binding mode relies on extensive hydrophobic interactions at the center of the binding surface and an electrostatic patch at the side. An interesting sulfur/π interaction supports the macrocycle-receptor binding. Overall, our results allow a better understanding of forces guiding macrocycle affinity for PD-L1, providing a rationale for future structure-based inhibitor design and rational optimization.

Rational design of a potent macrocyclic peptide inhibitor targeting the PD-1/PD-L1 protein-protein interaction

We report optimization by rational design of JMPDP-027, a potent cyclic peptide that interferes with the PD-1/PD-L1 protein-protein interaction. JMPDP-027 shows a potent restoring ability towards T-cells with an EC50 of 5.9 nM that is comparable to that of the anti-PD-1 monoclonal antibody pembrolizumab. In addition, JMPDP-027 shows not only high resistance to enzymatic hydrolysis in human serum but also no observable toxicity and potent in vivo anticancer activity comparable to that of the mouse PD-L1 antibody in a colon carcinoma (CT26) model. Cyclic peptide antagonists of this sort may provide novel drug candidates for cancer immunotherapy.