Current and emerging systemic treatments targeting the neural system for chronic pruritus

Chronic pruritus: from pathophysiology to drug design

Pruritus, the most common symptom in dermatology, is an innate response capable of protecting skin against irritants. Nonetheless, when it lasts more than six weeks it is assumed to be a chronic pathology having a negative impact on people's lives. Chronic pruritus (CP) can occur in common and rare skin diseases, having a high prevalence in global population. The existing therapies are unable to counteract CP or are associated with adverse effects, so the development of effective treatments is a pressing issue. The pathophysiological mechanisms underlying CP are not yet completely dissected but, based on current knowledge, involve a wide range of receptors, namely neurokinin 1 receptor (NK1R), Janus kinase (JAK), and transient receptor potential (TRP) ion channels, especially transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential ankyrin 1 (TRPA1). This review will address the relevance of these molecular targets for the treatment of CP and molecules capable of modulating these receptors that have already been studied clinically or have the potential to possibly alleviate this pathology. According to scientific and clinical literature, there is an increase in the expression of these molecular targets in the lesioned skin of patients experiencing CP when compared with non-lesioned skin, highlighting their importance for the development of potential efficacious drugs through the design of antagonists/inhibitors.

Pruritus: A Sensory Symptom Generated in Cutaneous Immuno-Neuronal Crosstalk

Pruritus or itch generated in the skin is one of the most widespread symptoms associated with various dermatological and systemic (immunological) conditions. Although many details about the molecular mechanisms of the development of both acute and chronic itch were uncovered in the last 2 decades, our understanding is still incomplete and the clinical management of pruritic conditions is one of the biggest challenges in daily dermatological practice. Recent research revealed molecular interactions between pruriceptive sensory neurons and surrounding cutaneous cell types including keratinocytes, as well as resident and transient cells of innate and adaptive immunity. Especially in inflammatory conditions, these cutaneous cells can produce various mediators, which can contribute to the excitation of pruriceptive sensory fibers resulting in itch sensation. There also exists significant communication in the opposite direction: sensory neurons can release mediators that maintain an inflamed, pruritic tissue-environment. In this review, we summarize the current knowledge about the sensory transduction of pruritus detailing the local intercellular interactions that generate itch. We especially emphasize the role of various pruritic mediators in the bidirectional crosstalk between cutaneous non-neuronal cells and sensory fibers. We also list various dermatoses and immunological conditions associated with itch, and discuss the potential immune-neuronal interactions promoting the development of pruritus in the particular diseases. These data may unveil putative new targets for antipruritic pharmacological interventions.

Mechanistic Insights into the Anti-Pruritic Effects of Lebrikizumab, an Anti-IL-13 Monoclonal Antibody

BACKGROUND

Atopic dermatitis (AD) is a chronic, inflammatory skin disease with persistent and severe itch among its hallmark features. Significant increases in type 2 cytokines (i.e., IL-4, IL-13, IL-31) have been documented in acute AD lesions and lead to multi-faceted downstream effects, including inflammation, epidermal barrier dysfunction, and itch.

OBJECTIVE

The primary objective of preclinical studies reported here was to test direct effects of IL-13 and the anti-IL-13 monoclonal antibody, lebrikizumab, in a human dorsal root ganglion (hDRG) model in itch amplification, neuronal excitability, and transcriptional downstream targets.

METHODS

Neuroactive effects were assessed via live cell calcium imaging, electric field stimulation, and RNA sequencing of hDRGs stimulated with IL-13 alone or in combination with lebrikizumab.

RESULTS

These preclinical findings suggest that IL-13 has a direct enhancer role in multiple itch and neuroactive pathways, as well as transcriptional downstream effects, and provide key insights into the mechanistic basis for lebrikizumab's anti-itch effects.

CONCLUSION

IL-13 is a potent enhancer of neuronal responses to different itch stimuli, consistent with the neuro-immune axis contributing to chronic itch-associated inflammatory skin disease, and that blockade of this cytokine pathway may provide a therapeutic approach.

Nocturnal pruritus and sleep disturbance associated with dermatologic disorders in adult patients

Nocturnal pruritus (NP) is a relatively common reason for dermatologic consultation. Its pathophysiology is partially understood. Skin conditions such as atopic dermatitis, psoriasis, urticaria, and prurigo nodularis are well-described causes of NP. The most distressing sequela of NP is sleep deficit, which can lead to physical and mental disturbances (e.g., daytime somnolence and fatigue) and negative emotional states that profoundly affect quality of life. However, this aspect of NP is often overlooked by dermatologists. It is essential to assess sleep quality in such patients and adopt appropriate measures to arrest the problem at an early stage. We conducted an evidence-based literature review to highlight the pathogenetic mechanisms of NP, identify dermatologic etiologies, and explore methods that have been used to assess the quality of sleep. Furthermore, we performed a systematic review of studies on sleep disturbance relevant to NP in patients with dermatologic conditions. Finally, we discuss the evidence on treatment options for NP and indicate therapies that may target both NP and sleep disturbance.

Approach to the Patient with Chronic Pruritus

Chronic pruritus (itch lasting ≥6 weeks) is a bothersome chief complaint that may present in a broad variety of diseases. Most itch-causing diagnoses fit into 1 of 5 categories (inflammatory, secondary to systemic disease, neuropathic, chronic pruritus of undetermined origin, and psychogenic itch) and this broad differential can be narrowed using key findings in the history and physical. In this article, we discuss which key findings are most pertinent for narrowing this differential and guiding further workup and treatment, as well as how to treat many itchy conditions.

Involvement of Neuro-Immune Interactions in Pruritus With Special Focus on Receptor Expressions

Pruritus is a common, but very challenging symptom with a wide diversity of underlying causes like dermatological, systemic, neurological and psychiatric diseases. In dermatology, pruritus is the most frequent symptom both in its acute and chronic form (over 6 weeks in duration). Treatment of chronic pruritus often remains challenging. Affected patients who suffer from moderate to severe pruritus have a significantly reduced quality of life. The underlying physiology of pruritus is very complex, involving a diverse network of components in the skin including resident cells such as keratinocytes and sensory neurons as well as transiently infiltrating cells such as certain immune cells. Previous research has established that there is a significant crosstalk among the stratum corneum, nerve fibers and various immune cells, such as keratinocytes, T cells, basophils, eosinophils and mast cells. In this regard, interactions between receptors on cutaneous and spinal neurons or on different immune cells play an important role in the processing of signals which are important for the transmission of pruritus. In this review, we discuss the role of various receptors involved in pruritus and inflammation, such as TRPV1 and TRPA1, IL-31RA and OSMR, TSLPR, PAR-2, NK1R, H1R and H4R, MRGPRs as well as TrkA, with a focus on interaction between nerve fibers and different immune cells. Emerging evidence shows that neuro-immune interactions play a pivotal role in mediating pruritus-associated inflammatory skin diseases such as atopic dermatitis, psoriasis or chronic spontaneous urticaria. Targeting these bidirectional neuro-immune interactions and the involved pruritus-specific receptors is likely to contribute to novel insights into the underlying pathogenesis and targeted treatment options of pruritus.

An evaluation of difelikefalin as a treatment option for moderate-to-severe pruritus in end stage renal disease

Introduction: Chronic kidney disease-associated pruritus (CKD-aP), or uremic pruritus, is a severely distressing condition that occurs in greater than 60% of patients undergoing dialysis. However, there are currently no FDA approved treatments for CKD-aP in the United States or Europe. Difelikefalin (DFK) is a kappa opioid receptor agonist with limited central nervous system (CNS) penetration that aims to fill this void by effectively and safely reducing itch in these patients.Areas covered: Through a review of the current literature (using PubMed and Google Scholar keyword searches of difelikefalin, CR845, pruritus, itch, opioids, hemodialysis, chronic kidney disease, uremic pruritus), the authors review DFK's mechanism of action and use published clinical trial data to evaluate its effectiveness in treating CKD-aP both individually and comparatively to other treatment alternatives.Expert opinion: DFK's IV formulation seems to provide safe, rapid-acting and effective itch reduction in hemodialysis patients without many of the negative mu opioid receptor (MOR)- or CNS- related side effects or drug-drug interactions of other currently available opioids. Its administration through IV bolus immediately after dialysis sessions at dialysis centers also increases availability to and ease of drug scheduling for this target population.