Structure-Activity Relationship of PAD4 Inhibitors and Their Role in Tumor Immunotherapy.

Protein arginine deiminase 4 (PAD4) plays an important role in cancer progression by participating in gene regulation, protein modification, and neutrophil extracellular trap (NET) formation. Many reversible and irreversible PAD4 inhibitors have been reported recently. In this review, we summarize the structure-activity relationships of newly investigated PAD4 inhibitors to bring researchers up to speed by guiding and describing new scaffolds as optimization and development leads for new effective, safe, and selective cancer treatments. In addition, some recent reports have shown evidence that PAD4 inhibitors are expected to trigger antitumor immune responses, regulate immune cells and related immune factors, enhance the effects of immune checkpoint inhibitors, and enhance their antitumor efficacy. Therefore, PAD4 inhibitors may potentially change tumor immunotherapy and provide an excellent direction for the development and clinical application of immunotherapy strategies for related diseases.

Chitosan nanomedicine containing RGD peptide and PAD4 inhibitor based on phenyl boronate coupling inhibition of primary tumor growth and lung metastasis.

Stimulus-responsive nanodrugs have been extensively studied and their structural changes in the cells are important for controlled intracellular drug release. Histone citrullination of peptidylarginine deiminase 4 (PAD4) regulates the expression of tumor suppressor genes. In our previous study, compounds such as YW3-56 (356) were developed as potent PAD4 inhibitors with excellent anti-tumor activity in vitro and in vivo. To enhance the antitumor activity and improve the bioavailability, we further optimized the structure by modifying the phenylboronic acid moiety to the PAD4 inhibitor (4B). Taking advantage of the oxidative stress responsiveness of the phenylboronic acid moiety, in this study, we covalently attached 4B to RGD sequence peptide modified chitosan (K-CRGDV) to construct this new oxidative stress responsive nanodrug (K-CRGDV-4B). The modification of RGD sequence peptide conferred the nanodrug the ability to actively target tumors. The release mechanism was verified by UV-Vis spectroscopy, NMR. The anti-tumor and anti-metastatic properties of K-CRGDV-4B were demonstrated by in vitro cytotoxicity assay and in vivo mouse Lewis lung cancer metastasis model. In addition, K-CRGDV-4B modulates the ratio of immune cells in LLC tumor-bearing mice. Immunosuppressive proteins such as PD1 were inhibited, while IFN-γ and IFN-ß, which are stimulators of tumor immune responses, were upregulated. Overall, K-CRGDV-4B is a stimulus-responsive nanodrug that responds to the tumor microenvironment by inhibiting PAD4 activity, blocking the formation of neutrophil extracellular traps (NETs), and improving the tumor immune microenvironment.