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

PD-1: A critical player and target for immune normalization

Immune system imbalances contribute to the pathogenesis of several different diseases, and immunotherapy shows great therapeutic efficacy against tumours and infectious diseases with immune-mediated derivations. In recent years, molecules targeting the programmed cell death protein 1 (PD-1) immune checkpoint have attracted much attention, and related signalling pathways have been studied clearly. At present, several inhibitors and antibodies targeting PD-1 have been utilized as anti-tumour therapies. However, increasing evidence indicates that PD-1 blockade also has different degrees of adverse side effects, and these new explorations into the therapeutic safety of PD-1 inhibitors contribute to the emerging concept that immune normalization, rather than immune enhancement, is the ultimate goal of disease treatment. In this review, we summarize recent advancements in PD-1 research with regard to immune normalization and targeted 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.

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.

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.

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

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

The future of cancer immunotherapy: DNA vaccines leading the way

Immuno-oncology has revolutionized cancer treatment and has opened up new opportunities for developing vaccination methods. DNA-based cancer vaccines have emerged as a promising approach to activating the bodily immune system against cancer. Plasmid DNA immunizations have shown a favorable safety profile and there occurs induction of generalized as well as tailored immune responses in preclinical and early-phase clinical experiments. However, these vaccines have notable limitations in immunogenicity and heterogeneity and these require refinements. DNA vaccine technology has been focusing on improving vaccine efficacy and delivery, with parallel developments in nanoparticle-based delivery systems and gene-editing technologies such as CRISPR/Cas9. This approach has showcased great promise in enhancing and tailoring the immune response to vaccination. Strategies to enhance the efficacy of DNA vaccines include the selection of appropriate antigens, optimizing insertion in a plasmid, and studying combinations of vaccines with conventional strategies and targeted therapies. Combination therapies have attenuated immunosuppressive activities in the tumor microenvironment and enhanced the capability of immune cells. This review provides an overview of the current framework of DNA vaccines in oncology and focuses on novel strategies, including established combination therapies and those still under development.The challenges that oncologists, scientists, and researchers need to overcome to establish DNA vaccines as an avant-garde approach to defeating cancer, are also emphasized. The clinical implications of the immunotherapeutic approaches and the need for predictive biomarkers have also been reviewed upon. We have also tried to extend the role of Neutrophil extracellular traps (NETs) to the DNA vaccines. The clinical implications of the immunotherapeutic approaches have also been reviewed upon. Ultimately, refining and optimizing DNA vaccines will enable harnessing the immune system's natural ability to recognize and eliminate cancer cells, leading the world towards a revolution in cancer cure.

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

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

Clinical cancer immunotherapy: Current progress and prospects

Immune checkpoint therapy via PD-1 antibodies has shown exciting clinical value and robust therapeutic potential in clinical practice. It can significantly improve progression-free survival and overall survival. Following surgery, radiotherapy, chemotherapy, and targeted therapy, cancer treatment has now entered the age of immunotherapy. Although cancer immunotherapy has shown remarkable efficacy, it also suffers from limitations such as irAEs, cytokine storm, low response rate, etc. In this review, we discuss the basic classification, research progress, and limitations of cancer immunotherapy. Besides, by combining cancer immunotherapy resistance mechanism with analysis of combination therapy, we give our insights into the development of new anticancer immunotherapy strategies.

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

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

Structure-Based Discovery of MDM2/4 Dual Inhibitors that Exert Antitumor Activities against MDM4-Overexpressing Cancer Cells

Despite recent clinical progress in peptide-based dual inhibitors of MDM2/4, small-molecule ones with robust antitumor activities remain challenging. To tackle this issue, 31 (YL93) was structure-based designed and synthesized, which had MDM2/4 binding K_i values of 1.1 and 642 nM, respectively. In three MDM4-overexpressing cancer cell lines harboring wild-type p53, 31 shows improved cell growth inhibition activities compared to RG7388, an MDM2-selective inhibitor in late-stage clinical trials. Mechanistic studies show that 31 increased cellular protein levels of p53 and p21 and upregulated the expression of p53-targeted genes in RKO cells with MDM4 amplification. In addition, 31 induced cell-cycle arrest and apoptosis in western blot and flow cytometry assays. Taken together, dual inhibition of MDM2/4 by 31 elicited stronger antitumor activities in vitro compared to selective MDM2 inhibitors in wild-type p53 and MDM4-overexpressing cancer cells.

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.