Pharmacology of pruritus (pathophysiology, therapeutic targets)

Pruritus (itch) is defined as an unpleasant sensation provoking the urge to scratch and can be understood both as a symptom and, when lasting more than six weeks, as a disease entity termed chronic pruritus (CP). CP is characterized by complex neurobiological changes and can persist independently of the initial trigger, significantly impairing quality of life. Advances since the 1990s in understanding itch pathophysiology have led to the development of numerous targeted therapies, although many treatments remain off-label. Pruritus is classified into dermatological, systemic, neuropathic, and psychogenic categories, often with overlapping etiologies. Effective management prioritizes identifying and treating the underlying cause. Pathophysiologically, itch originates from specialized peripheral sensory neurons (pruriceptors), which transmit signals through histaminergic and non-histaminergic pathways. While histamine plays a role mainly in urticaria, chronic itch is largely mediated by non-histaminergic mechanisms involving cytokines, neuropeptides, proteases, transient receptor potential (TRP) channels, and Mas-related G-protein-coupled receptors (MRGPRs). Keratinocytes, mast cells, and immune mediators such as IL-31, IL-4, and IL-13 actively participate in itch signaling, creating neuroimmune feedback loops. Peripheral and central sensitization, spinal disinhibition, and altered pain-itch interactions sustain chronic itch and the itch-scratch cycle, particularly in chronic prurigo (CPG), a distinct disease model for CP. Therapeutic strategies target multiple levels of itch processing. Traditional treatments include antihistamines, topical anesthetics, calcineurin inhibitors, antidepressants, gabapentinoids, and immunosuppressants, though efficacy varies. Recent breakthroughs include biologics targeting TH2 pathways (dupilumab, nemolizumab), JAK inhibitors, κ-opioid receptor agonists (difelikefalin), and IBAT inhibitors (for cholestatic pruritus). Novel targets under investigation include MRGPRs, PARs, KIT receptors, OX-40, and sodium channels.