Development of small-molecule immune checkpoint inhibitors of PD-1/PD-L1 as a new therapeutic strategy for tumour immunotherapy

Abstract

Cancer immunotherapy has been increasingly utilised to treat advanced malignancies. The signalling network of immune checkpoints has attracted considerable attention. Immune checkpoint inhibitors are revolutionising the treatment options and expectations for patients with cancer. The reported clinical success of targeting the T-cell immune checkpoint receptors PD-1/PD-L1 has demonstrated the importance of immune modulation. Indeed, antibodies binding to PD-1 or PD-L1 have shown remarkable efficacy. However, antibody drugs have many disadvantages, such as their production cost, stability, and immunogenicity and, therefore, small-molecule inhibitors of PD-1 and its ligand PD-L1 are being introduced. Small-molecule inhibitors could offer inherent advantages in terms of pharmacokinetics and druggability, thereby providing additional methods for cancer treatment and achieving better therapeutic effects. In this review, we first discuss how PD-1/PD-L1-targeting inhibitors modulate the relationship between immune cells and tumour cells in tumour immunotherapy. Second, we discuss how the immunomodulatory potential of these inhibitors can be exploited via rational combinations with immunotherapy and targeted therapy. Third, this review is the first to summarise the current clinical and preclinical evidence regarding small-molecule inhibitors of the PD-1/PD-L1 immune checkpoint, considering features and responses related to the tumours and to the host immune system.

Trial registration: ClinicalTrials.gov identifier: NCT02812875.     

Current status and development of anti-PD-1/PD-L1 immunotherapy for lung cancer

Lung cancer is the leading cause of cancer deaths worldwide, mainly because it is usually in the advanced stage at the time of diagnosis. Although great progress has been made in the diagnosis and treatment of lung cancer in the past 25 years, the prognosis of lung cancer patients remains unsatisfactory. Agents targeting immune checkpoints have shown potential to improve therapeutic outcomes in patients with lung cancer. Inhibitors of PD-1/PD-L1 have been approved for the treatment of different types of lung cancer by FDA. Nevertheless, with the increasing number of clinical trails, the adverse events have emerged. Therefore, attention has been paid to finding out the factors influencing the therapeutic effect of anti-PD-1/PD-L1 therapy and reducing the occurrence of adverse events. Combination therapy may be an effective strategy to reduce the adverse events and improve the therapeutic effect. In this review, we summarized the current status and progress of anti-PD1/PD-L1 agents in lung cancer treatment.     

Self-assembled FeS-based cascade bioreactor with enhanced tumor penetration and synergistic treatments to trigger robust cancer immunotherapy

Major challenges for cancer treatment are how to effectively eliminate primary tumor and sufficiently induce immunogenic cell death (ICD) to provoke a robust immune response for metastasis control. Here, a self-assembled cascade bioreactor was developed to improve cancer treatment with enhanced tumor penetration and synergistic therapy of starvation, chemodynamic (CDT) and photothermal therapy. Ultrasmall FeS-GOx nanodots were synthesized with glucose oxidase (GOx) as template and induced by paclitaxel (PTX) to form self-assembling FeS-GOx@PTX (FGP) via hydrophobic interaction. After accumulated at tumor sites, FGP disassembles to smaller FeS-GOx for enhanced deep tumor penetration. GOx maintains high enzymatic activity to catalyze glucose with assistant of oxygen to generate hydrogen peroxide (H₂O₂) as starvation therapy. Fenton reaction involving the regenerated H₂O₂ in turn produced more hydroxyl radicals for enhanced CDT. Following near-infrared laser at 808 nm, FGPs displayed pronounced tumor inhibition in vitro and in vivo by the combination therapy. The consequent increased exposure to calreticulin amplified ICD and promoted dendritic cells maturation. In combination with anti-CTLA4 checkpoint blockade, FGP can absolutely eliminate primary tumor and avidly inhibit distant tumors due to the enhanced intratumoral infiltration of cytotoxic T lymphocytes. Our work presents a promising strategy for primary tumor and metastasis inhibition.     

Chemo-photothermal immunotherapy for eradication of orthotopic tumors and inhibition of metastasis by intratumoral injection of polydopamine versatile hydrogels

Cancer remains one of the leading causes of death globally and metastasis always leads to treatment failure. Here, we develop a versatile hydrogel loading photothermal agents, chemotherapeutics, and immune-adjuvants to eradicate orthotopic tumors and inhibit metastasis by combinational therapy. Hydrogel networks were synthesized via the thiol-Michael addition of polydopamine (PDA) with thiolated hyaluronic acid. PDA acted as a cross-linking agent and endowed the hydrogel with excellent photothermal property. Meanwhile, a chemotherapeutic agent, doxorubicin (DOX), was loaded in the hydrogel via π?π stacking with PDA and an immune-adjuvant, CpG-ODN, was loaded via electrostatic interaction. The release of DOX from the hydrogel was initially slow but accelerated due to near infrared light irradiation. The hydrogels showed remarkably synergistic effect against 4T1 cancer cells and stimulated plenty of cytokines secreting from RAW264.7 cells. Moreover, the hydrogels eradicated orthotopic murine breast cancer xenografts and strongly inhibited metastasis after intratumoral injection and light irradiation. The high anticancer efficiency of this chemo-photothermal immunotherapy resulted from the strong synergistic effect of the versatile hydrogels, including the evoked host immune response. The combinational strategy of chemo-photothermal immunotherapy is promising for highly effective treatment of breast cancer.     

Alterations of DNA damage response pathway: Biomarker and therapeutic strategy for cancer immunotherapy

Genomic instability remains an enabling feature of cancer and promotes malignant transformation. Alterations of DNA damage response (DDR) pathways allow genomic instability, generate neoantigens, upregulate the expression of programmed death ligand 1 (PD-L1) and interact with signaling such as cyclic GMP–AMP synthase-stimulator of interferon genes (cGAS–STING) signaling. Here, we review the basic knowledge of DDR pathways, mechanisms of genomic instability induced by DDR alterations, impacts of DDR alterations on immune system, and the potential applications of DDR alterations as biomarkers and therapeutic targets in cancer immunotherapy.