Beyond PAD Inhibition: Emerging Avenues and Natural Products for Targeting Citrullination in Immune Diseases

Immune-mediated inflammatory diseases, such as rheumatoid arthritis, multiple sclerosis, and systemic lupus erythematosus, impose a severe and growing global health burden, where current therapies are limited by poor specificity and significant side effects. The peptidylarginine deiminase (PAD)/citrullination axis, in which protein citrullination catalyzed by PADs drives autoantigen generation and sustains inflammation, has emerged as a critical therapeutic target. This review outlines a comprehensive strategy for targeting this axis using natural products. We first detail the established role of natural compounds as direct PAD inhibitors, covering their chemical diversity, inhibitory mechanisms, and therapeutic applications in disease models. Subsequently, the discussion extends to their broader, indirect modulatory functions, highlighting how these compounds can suppress pathogenic citrullination by regulating upstream processes like NETosis and inflammatory signaling. Furthermore, the review introduces the innovative substrate-centric intervention strategy, which represents a paradigm shift toward shielding key arginine residues on autoantigens, thereby preventing the formation of immunogenic neoepitopes. The translational challenges and future directions for each of these avenues are outlined, addressing persistent obstacles including achieving isoform selectivity and biomarker validation. By integrating these multifaceted strategies, from direct inhibition and indirect modulation to substrate protection, this work provides a strategic roadmap for advancing the next generation of more precise, effective, and safe anti-citrullination therapies, ultimately moving beyond conventional enzyme inhibition toward targeted immunomodulation in immune-mediated inflammatory diseases.