Chronic Pruritus: Current and Emerging Treatment Options

ME3183, a novel phosphodiesterase-4 inhibitor, exhibits potent anti-inflammatory effects and is well tolerated in a non-clinical study

Phosphodiesterase 4 (PDE4) inhibitors are expected to exhibit efficacy against inflammatory diseases due to their broad pharmacological activity. The launched PDE4 inhibitors apremilast, crisaborole, and roflumilast have not exhibited sufficient inhibitory potential due to poor margins of effectiveness and tolerability. In this report, we describe the non-clinical efficacy, brain translocation, and vomit-inducing effects of ME3183 compared with apremilast. ME3183 showed extensive cytokine suppression in vitro studies using human peripheral blood mononuclear cells and T cells. ME3183 also significantly suppressed skin inflammation in a chronic oxazolone-induced dermatitis model and showed antipruritic effects in a substance P-induced mouse pruritus model. In these in vitro and in vivo studies, ME3183 also significantly suppressed cytokines, and focusing on tumor necrosis factor-α as a psoriasis-related cytokine and interleukin-4 as an atopic dermatitis-related cytokine, ME3183 potently inhibited both cytokines. ME3183 showed in vivo efficacy at lower doses than apremilast. The brain distribution of ME3183 was sufficiently low in mice and rats. The effective dose of ME3183 for emesis was similar to that of apremilast in ferrets. Given its high-potency inhibitory effects, ME3183 would have a wide margin of efficacy and tolerability. These wide margins demonstrate the effectiveness of ME3183 in treating many inflammatory diseases, such as psoriasis and atopic dermatitis. An on-going phase 2 trial is expected to further demonstrate the efficacy and safety of ME3183.

Gabapentin and oral antidepressants for chronic pruritus: a prospective cohort study evaluating efficacy and side effects in daily dermatological practice

BACKGROUND

Chronic pruritus is frequently seen in daily dermatological practice and is associated with marked impact on quality of life. Research on the use of gabapentin and oral antidepressants in daily dermatological practice is scarce.

OBJECTIVE

To evaluate the efficacy and safety of gabapentin and oral antidepressants in patients with chronic pruritus in daily clinical practice.

METHODS

A prospective observational single-center cohort study was conducted including adult patients with chronic pruritus and an indication for systemic treatment between June 2016 and May 2019.

RESULTS

Systemic treatment with gabapentin and/or antidepressants was initiated in 31 patients with severe chronic pruritus (median average pruritus NRS score 7.0), in which most cases no underlying origin was identified (83.9%). In patients treated with gabapentin 900-1800 mg/day (N = 25), median average pruritus NRS decreased to 5.5 (IQR 3.0) after 4 weeks and remained stable up to 24 weeks of treatment. Efficacy of antidepressants was variable, with the highest response after initiation of amitriptyline, nortriptyline, and mirtazapine. Side effects were frequently observed in both gabapentin and antidepressant treatments; however, were mostly mild and temporary.

LIMITATIONS

This was a single-site observational study, with limited sample size.

CONCLUSION

Treatment with gabapentin and antidepressants should be considered in patients with chronic pruritus unresponsive to conventional treatment.

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.

Adriforant is a functional antagonist of histamine receptor 4 and attenuates itch and skin inflammation in mice

BACKGROUND

Histamine has been postulated to play a role in atopic dermatitis via histamine receptor 4, mediating pruritic and inflammatory effects. The H4R antagonist adriforant (PF-3893787 or ZPL389) indicated clinical efficacy in a Ph2a study in atopic dermatitis. Preclinical investigations of adriforant had been scarce as experiments in transfectants with H4R from several species suggested partial agonism, not seen in human cells.

OBJECTIVE

During the Ph2b trial in AD, we performed experiments to understand the pharmacology of adriforant in primary murine cells and in vivo models. We assessed its effects on ERK phosphorylation and transcriptional changes in bone marrow-derived mast cells, histamine-dependent Ca²⁺ flux in neurons and histamine-induced itch response. In addition, its impact on MC903-induced skin inflammation was evaluated.

RESULTS

We show that, contrary to transfectants, adriforant is a competitive antagonist of the murine histamine receptor 4, antagonizes histamine-induced ERK phosphorylation, normalizes histamine-induced transcriptional changes in mast cells and reduces histamine-dependent Ca²⁺ flux in neurons. Administration to mice reduces acute histamine-induced itch response. In addition, adriforant ameliorates inflammation in the mouse MC903 model.

CONCLUSIONS

Our results suggest that functional inhibition of histamine receptor 4 by adriforant reduces itch and inflammation in vivo. The effects observed in mice, however, did not translate to clinical efficacy in patients as the Ph2b clinical trial with adriforant did not meet pre-specified efficacy endpoints. Given the complex pathogenesis of AD, antagonism of histamine receptor 4 alone appears insufficient to reduce disease severity in AD patients, despite the effects seen in mouse models.

Uremic pruritus: pathophysiology, clinical presentation, and treatments

Uremic pruritus is one of the most common and bothersome symptoms in patients with end-stage renal disease. Most patients with uremic pruritus experience a prolonged and relapsing course and significant impairments of quality of life. The pathophysiology of uremic pruritus is not completely understood. A complex interplay among cutaneous biology and the nervous and immune systems has been implicated, with the involvement of various inflammatory mediators, neurotransmitters, and opioids. Uremic pruritus treatment outcomes are often unsatisfactory. Clinical trials have mostly been small in scale and have reported inconsistent results. Recent evidence shows that gabapentinoids, nalfurafine, and difelikefalin are effective for relieving uremic pruritus in hemodialysis patients. This review provides an overview of the epidemiology and proposed mechanisms of uremic pruritus, then highlights the manifestations of and clinical approach to uremic pruritus. Current evidence regarding treatment options, including topical treatments, treatment of underlying disease, phototherapy, and systemic treatments, is also outlined. With a better understanding of uremic pruritus, more therapeutic options can be expected in the near future.

Complementary and integrative remedies in the treatment of chronic pruritus: A review of clinical trials

BACKGROUND

Chronic pruritus is one of the most common conditions in dermatology and a common manifestation in many systemic diseases. Since the etiology of chronic pruritus remains somewhat unknown, hence, conventional medications may not always show a good therapeutic response. This finding has led both investigators and patients to use herbal and complementary remedies for its treatment. The aim of this study was to review clinical trials in which herbal and complementary medicine was used in the control and treatment of chronic pruritus.

MATERIALS AND METHODS

In this study, we reviewed related articles in this domain, from 2000 to 2020. The search involved electronic databases including PubMed, Scopus, Web of Science, Cochrane, Google Scholar, and SID databases using the keywords "pruritus," "itch," "herb," "complementary medicine," "traditional medicine," "integrative medicine," and their related MeSH terms. Finally, we extracted the pertinent information from these articles and summarized the results.

RESULTS

The findings of this study showed that 17 clinical trials have been conducted till date in order to evaluate the efficacy of herbal remedies and complementary medicines in the treatment of chronic pruritus. Herbal remedies including turmeric, Fumaria parviflora, Avena sativa, capsaicin, sweet almond oil, peppermint oil, violet oil, vinegar, as well as manual therapies including aromatherapy, auricular acupressure, and acupuncture, were significantly effective in the treatment of chronic pruritus.

CONCLUSION

There are only a few studies published on the therapeutic efficacy of herbal remedies and complementary medicine in the treatment of chronic pruritus. Some have shown promising results. Therefore, more evidence-based studies are needed in order to determine if herbal remedies and complementary medicine could be an effective alternative or adjuvant treatment modality in chronic pruritus.

Mild Skin Heating Evokes Warmth Hyperknesis Selectively for Histaminergic and Serotoninergic Itch in Humans

Chronic itch can severely affect quality of life. Patients report that their chronic itch can be exacerbated by exposure to warm conditions (“warmth hyperknesis”). The aim of this mechanistic study was to investigate the effect of mild heating of the skin in humans on various experimental models of itch. A total of 18 healthy subjects were recruited to the study. Itch was provoked by histamine, serotonin, or cowhage in 3 different sessions. The provoked area was heated with an infrared lamp, and the skin temperature was either not altered, or was increased by 4°C or 7°C. Subsequent to induction of itch, the itch intensity was recorded for 10 min while the skin was heated continuously throughout the entire period of itch induction. Heating the skin resulted in a significant increase in itch intensity when provoked by histamine or serotonin. It is possible that thermoception and pruriception interact and selectively produce a higher itch intensity in histaminergic and serotoninergic itch.

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.

CKD-Associated Pruritus: New Insights Into Diagnosis, Pathogenesis, and Management

Chronic kidney disease–associated pruritus (CKD-aP) is a common, troubling and in some cases debilitating problem for patients with CKD and end-stage renal disease. Despite a prevalence rate of approximately 20% in CKD and 40% in end-stage renal disease, and a clear association with poorer psychosocial and medical outcomes, this condition is often underreported by patients and overlooked by health care providers. This is likely due, in part, to uncertainty regarding its pathogenesis and treatment. Most commonly, CKD-aP is attributed to toxin build-up, peripheral neuropathy, immune system dysregulation, or opioid dysregulation. Prior treatment studies of CKD-aP have targeted these potential etiologies but have been limited by noncontrolled design, small sample size, and non-uniform definitions of CKD-aP. Recently, several large, randomized controlled trials targeting opioid dysregulation have yielded promising results. These trials have spurred new hope for understanding and treating this condition.

Spinal cord NLRP1 inflammasome contributes to dry skin induced chronic itch in mice

BACKGROUND

Dry skin itch is one of the most common skin diseases and elderly people are believed to be particularly prone to it. The inflammasome has been suggested to play an important role in chronic inflammatory disorders including inflammatory skin diseases such as psoriasis. However, little is known about the role of NLRP1 inflammasome in dry skin-induced chronic itch.

METHODS

Dry skin-induced chronic itch model was established by acetone-ether-water (AEW) treatment. Spontaneous scratching behavior was recorded by video monitoring. The expression of nucleotide oligomerization domain (NOD)-like receptor protein 1 (NLRP1) inflammasome complexes, transient receptor potential vanilloid type 1 (TRPV1), and the level of inflammatory cytokines were determined by western blot, quantitative real-time PCR, and enzyme-linked immunosorbent assay (ELISA) kits. Nlrp1a knockdown was performed by an adeno-associated virus (AAV) vector containing Nlrp1a-shRNA-eGFP infusion. H.E. staining was used to evaluate skin lesion.

RESULTS

AEW treatment triggers spontaneous scratching and significantly increases the expression of NLRP1, ASC, and caspase-1 and the levels of IL-1β, IL-18, IL-6, and TNF-α in the spinal cord and the skin of mice. Spinal cord Nlrp1a knockdown prevents AEW-induced NLRP1 inflammasome assembly, TRPV1 channel activation, and spontaneous scratching behavior. Capsazepine, a specific antagonist of TRPV1, can also inhibit AEW-induced inflammatory response and scratching behavior. Furthermore, elderly mice and female mice exhibited more significant AEW-induced scratching behavior than young mice and male mice, respectively. Interestingly, AEW-induced increases in the expression of NLRP1 inflammasome complex and the levels of inflammatory cytokines were more remarkable in elderly mice and female mice than in young mice and male mice, respectively.

CONCLUSIONS

Spinal cord NLRP1 inflammasome-mediated inflammatory response contributes to dry skin-induced chronic itch by TRPV1 channel, and it is also involved in age and sex differences of chronic itch. Inhibition of NLRP1 inflammasome may offer a new therapy for dry skin itch.

Nalbuphine, a kappa opioid receptor agonist and mu opioid receptor antagonist attenuates pruritus, decreases IL-31, and increases IL-10 in mice with contact dermatitis

Chronic itch is one of the disturbing symptoms of inflammatory skin diseases. Kappa opioid receptor agonists are effective in suppressing scratching in mice against different pruritogens. Nalbuphine, a nonscheduled kappa opioid receptor agonist and mu opioid receptor antagonist, has been in clinical use for post-operative pain management since the 1980s and recently has been in clinical trials for chronic itch of prurigo nodularis (https://www.trevitherapeutics.com/nalbuphine). We studied whether nalbuphine is effective against chronic scratching induced by rostral neck application of 1-fluoro-2,4-dinitrobenzene (DNFB), an accepted mouse model of contact dermatitis to study pruritoceptive itch. Mice were treated once a week with either saline or nalbuphine 20 min before the third, fifth, seventh, and ninth sensitizations with DNFB and the number of scratching bouts was counted for 30 min. Skin samples from the neck of mice at week 4 were used to measure protein levels and mRNA expressions of chemokines and cytokines. Different sets of mice were used to study sedation and anhedonic-like behavior of nalbuphine. We found that: nalbuphine (a) antagonized scratching in a dose- and time-dependent manner without affecting locomotion, b) decreased IL-31, and increased anti-inflammatory IL-10, and c) induced more elevations in the levels of CCL2, CCL3, CCL12, CXCL1, CXCL2, CXCL9, CXCL10, IL-1β, IL-16, TIMP-1, M-CSF, TREM-1 and M1-type macrophages compared to saline. Increases in chemokines and cytokines and M1 macrophages by nalbuphine suggest an inflammatory phase of healing in damaged skin due to scratching. Our data indicate that nalbuphine is an effective antipruritic in murine model of pruritoceptive itch.

[New findings regarding the neurobiology of pruritus]

Chronic pruritus may arise from different conditions, including dermatological, systemic, neurologic, psychiatric, and psychosomatic diseases, leading to a substantial decrease in the quality of life of affected patients. The neurobiological mechanisms involved in chronic pruritus are not yet fully understood. However, in recent years important achievements have been made in this regard. This article aims to provide an overview of the current understanding of these mechanisms. The complex network of neurons, keratinocytes, inflammatory cells, cytokines, and neurotrophic factors which play a role in the development and maintenance of chronic pruritus are highlighted, as well as the pruritogens involved in pruritic diseases in humans. Additionally, the importance of neuropathy and scratch-induced changes for the pathophysiology of chronic pruritus are discussed. The new findings on the neurobiological mechanisms underlying chronic pruritus have already led to the development of novel therapies, e. g., monoclonal antibodies against specific interleukins, which are important for pruritus transmission. A deeper understanding of the neurobiological mechanisms is necessary in order to develop specifically targeted therapeutic options and thus provide better care for affected patients.

Optimization of Rheological Behaviour and Skin Penetration of Thermogelling Emulsions with Enhanced Substantivity for Potential Application in Treatment of Chronic Skin Diseases

Topical formulations are an important pillar in the therapy of skin diseases. Nevertheless, after application the formulation will be exposed to environmental effects. Contact with other surfaces will reduce the available amount of formulation and drug substance. The resulting consequences for therapy range from reduced effects to therapeutic failure. The removed active ingredient also contaminates patients' environment. The aim of this work was to develop preparations that remain at the application site. These will enhance safety and efficiency and thus improve of skin disease therapies. Therefore, we developed polymer-stabilised emulsions that show thermogelling properties. Emulsions with different methyl cellulose concentrations and macrogols of different molecular weights were investigated. The dispersed phase consisted of nonivamide as the active pharmaceutical ingredient, dissolved in medium-chain triglycerides. Rheological properties, droplet size, substantivity and ex vivo penetration experiments were performed to characterise the developed formulations. Droplet size and rheological parameters were affected by the composition of the preparations. The tested formulations showed benefits in their substantivity compared to a conventional semi-solid cream. We found a residual amount of up to 100% at the application site. The drug levels in viable epidermis were in a therapeutic range. The developed emulsions are a promising vehicle to improve therapy for chronic skin diseases.

Qingpeng Ointment Ameliorates Inflammatory Responses and Dysregulation of Itch-Related Molecules for Its Antipruritic Effects in Experimental Allergic Contact Dermatitis

The pathogenesis of itchy skin diseases including allergic contact dermatitis (ACD) is complicated and the treatment of chronic itch is a worldwide problem. One traditional Tibetan medicine, Qingpeng ointment (QP), has been used in treatment of ACD in China for years. In this study we used HPLC and LC/MS analysis, combined with a BATMAN-TCM platform, for detailed HPLC fingerprint analysis and network pharmacology of QP, and investigated the anti-inflammatory and antipruritic activities of QP on ACD induced by squaric acid dibutylester (SADBE) in mice. The BATMAN-TCM analysis provided information of effector molecules of the main ingredients of QP, and possible chronic dermatitis-associated molecules and cell signaling pathways by QP. In ACD mice, QP treatment suppressed the scratching behavior induced by SADBE in a dose-dependent manner and inhibited the production of Th1/2 cytokines in serum and spleen. Also, QP treatment reversed the upregulation of mRNAs levels of itch-related genes in the skin (TRPV4, TSLP, GRP, and MrgprA3) and DRGs (TRPV1, TRPA1, GRP, and MrgprA3). Furthermore, QP suppressed the phosphorylation of Erk and p38 in the skin. In all, our work indicated that QP can significantly attenuate the pathological alterations of Th1/2 cytokines and itch-related mediators, and inhibit the phosphorylation of MAPKs to treat the chronic itch.

Itching at night: A review on reducing nocturnal pruritus in children

The most common causes of chronic nocturnal itching in children are atopic dermatitis and psoriasis, with lichen simplex chronicus and prurigo nodularis contributing to lesser degrees. Despite the prevalence of nocturnal itching, its pathophysiology remains poorly understood. The most troubling consequence of itching at night is poor quality of sleep. Poor sleep quality in children with nocturnal itching has been linked to adverse neurocognitive, behavioral, and physiologic outcomes, including poor performance in school, attention deficit hyperactivity disorder, short stature, hypertension, obesity, and impaired immune function. There is no consensus on the best management of nocturnal itching in children. We conducted a review of the literature evaluating the efficacy of various treatment options for children with chronic nocturnal pruritus. Our review found three recently conducted randomized controlled trials and one case report demonstrating the efficacy of topical corticosteroids, oral melatonin, and clonidine in reducing nocturnal itching or improving sleep quality in children with nocturnal pruritus. Future research is needed to elucidate the pathophysiology of nocturnal itching to best develop targeted, effective treatment strategies.

Method to determine the impact of substantivity on ex vivo skin-permeation

Topical formulations are the most common therapeutic agents in the treatment of skin diseases. They contain one or more active pharmaceutical ingredients (API) which need to penetrate or permeate the skin in order to exert their effect. However, after application a part of the formulation is removed from the skin due to contact with the environment. Therefore, a part of the active is then not available for penetration and thus, a loss in therapeutic effect will result. To achieve the desired therapeutic outcome a sufficient fraction of the formulation must remain on the skin. The extent to which the loss of preparation affects penetration and permeation is less investigated. This work presents a method to examine the influence of mechanical stress and formulation loss on skin permeation. A movable punch with a defined weight simulated contact between clothing or skin and the applied formulation. Weight of the tool, number of contacts and speed settings were variable and were investigated. Ex vivo permeation experiments were performed in Franz diffusion cells using porcine skin. Three preparations with nonivamide as active ingredient were chosen as model formulations: A semisolid cream, an oil-in-oil emulsion and a film-forming formulation. The last two show sustained permeation profiles. The method uses skin-to-formulation and clothing-to-formulation contact to simulate the removal of the formulations from the skin.

Neuropathic symptoms of the ocular surface: dryness, pain, and itch

PURPOSE OF REVIEW

This review aims to describe the recent findings on epidemiology, pathophysiology, and management of neuropathic symptoms of the ocular surface, with a focus on potential similarities between sensations of dry eye, pain and itch.

RECENT FINDINGS

A narrative review of the literature was undertaken. Key references from research in dry eye, neuropathic symptoms of the ocular surface, ocular pain and itch, as well as general references on itch and pain neurobiology were included. Recent findings suggest aspects of dry eye, chronic ocular pain and itch symptomatology are driven by neuropathic pain mechanisms involving peripheral and central sensitization processes.

SUMMARY

Ocular dryness, pain, and itch are prevalent complaints with several of shared features. Multiple lines of evidence suggest that peripheral and central neuronal sensitization processes are involved in generating and maintaining ocular sensory symptoms. Research is warranted on the epidemiology of ocular sensations, molecular mechanisms involved in nociception and pruriception in the eye, electrophysiological alterations in animal models of eye conditions, and therapeutic modalities that can alleviate unpleasant ocular sensations.

Neuro-immune interactions in allergic diseases: novel targets for therapeutics

Recent studies have highlighted an emerging role for neuro-immune interactions in mediating allergic diseases. Allergies are caused by an overactive immune response to a foreign antigen. The peripheral sensory and autonomic nervous system densely innervates mucosal barrier tissues including the skin, respiratory tract and gastrointestinal (GI) tract that are exposed to allergens. It is increasingly clear that neurons actively communicate with and regulate the function of mast cells, dendritic cells, eosinophils, Th2 cells and type 2 innate lymphoid cells in allergic inflammation. Several mechanisms of cross-talk between the two systems have been uncovered, with potential anatomical specificity. Immune cells release inflammatory mediators including histamine, cytokines or neurotrophins that directly activate sensory neurons to mediate itch in the skin, cough/sneezing and bronchoconstriction in the respiratory tract and motility in the GI tract. Upon activation, these peripheral neurons release neurotransmitters and neuropeptides that directly act on immune cells to modulate their function. Somatosensory and visceral afferent neurons release neuropeptides including calcitonin gene-related peptide, substance P and vasoactive intestinal peptide, which can act on type 2 immune cells to drive allergic inflammation. Autonomic neurons release neurotransmitters including acetylcholine and noradrenaline that signal to both innate and adaptive immune cells. Neuro-immune signaling may play a central role in the physiopathology of allergic diseases including atopic dermatitis, asthma and food allergies. Therefore, getting a better understanding of these cellular and molecular neuro-immune interactions could lead to novel therapeutic approaches to treat allergic diseases.

Drugs on the Horizon for Chronic Pruritus

Chronic pruritus is a common condition that has a detrimental impact on quality of life. As the molecular pathogenesis of itch is elucidated, novel therapies that disrupt itch pathways are being investigated. Emerging treatments include drugs targeting the neural system, drugs targeting the immune system, antihistamines, bile acid transport inhibitors, and topical drugs that work through a variety of mechanisms such as phosphodiesterase-4 inhibition or targeting of nerve ion channels. Many of these therapies show promising results in the treatment of chronic itch of various etiologies, such as atopic dermatitis, psoriasis, uremic pruritus, and cholestatic pruritus.

Chronic Pruritus in the Geriatric Population

Chronic pruritus (>6 week's duration) in the geriatric population (≥65 years old), is an increasing health care problem. The pathophysiologic predisposing factors are abnormalities of the epidermal barrier, immune system, and nervous system. Causes can be dichotomized into histaminergic and nonhistaminergic pruritus. Topical treatments are generally safe. Systemic treatments are chosen depending on the condition, comorbid diseases, and drug interactions. Treatment options are limited. Progress has been made in identifying itch-selective mediators over the last decade. Numerous new medications are currently undergoing clinical trials and they are anticipated to enter the clinics in the near future.

The Genetics of Chronic Itch: Gene Expression in the Skin of Patients with Atopic Dermatitis and Psoriasis with Severe Itch

To identify itch-related mediators and receptors that are differentially expressed in pruritic skin, we used RNA sequencing to analyze the complete transcriptome in skin from paired itchy, lesional and nonitchy, nonlesional skin biopsies from 25 patients with atopic dermatitis and 25 patients with psoriasis and site-matched biopsies from 30 healthy controls. This analysis identified 18,000 differentially expressed genes common between itchy atopic and psoriatic skin compared with healthy skin. Of those, almost 2,000 genes were differentially expressed between itchy and nonitchy skin in atopic and psoriatic subjects. Overexpression of several genes, such as phospholipase A2 IVD, substance P, voltage-gated sodium channel 1.7, and transient receptor potential (TRP) vanilloid 1, in itchy skin was positively correlated with itch intensity ratings in both atopic dermatitis and psoriasis. Cytokines such as IL-17A, IL-23A, and IL-31 had elevated gene transcript levels in both itchy atopic and psoriatic skin. However, expression of genes for TRP vanilloid 2, TRP ankyrin 1, protease-activated receptor 2, protease-activated receptor 4, and IL-10 was found to be increased only in pruritic atopic skin, whereas expression of genes for TRP melastatin 8, TRP vanilloid 3, phospholipase C, and IL-36α/γ was elevated only in pruritic psoriatic skin. This "itchscriptome" analysis will lead to an increased understanding of the molecular mechanisms of chronic pruritus and provide targets for itch treatment irrespective of disease state.