P2X2 knockout mice and P2X2/P2X3 double knockout mice reveal a role for the P2X2 receptor subunit in mediating multiple sensory effects of ATP

Efficacy and safety of eliapixant in endometriosis-associated pelvic pain: the randomized, placebo-controlled phase 2b SCHUMANN study

BACKGROUND

The SCHUMANN study evaluated the efficacy and safety of the selective P2 × 3 antagonist eliapixant in patients with endometriosis-associated pelvic pain (EAPP).

METHODS

SCHUMANN was a randomized, placebo- and active comparator-controlled, double-blind to placebo and open-label to comparator, parallel-group, multicenter, dose-finding phase 2b study. The participants were women with surgically diagnosed endometriosis who fulfilled defined EAPP criteria. Participants were randomized 1:1:1:1 to twice daily (BID) 25 mg, 75 mg, or 150 mg oral eliapixant or a placebo for 12 weeks. An exploratory once-daily elagolix 150 mg treatment group was also included. The primary endpoint was the absolute change in mean worst EAPP from baseline to the end of intervention (EOI).

RESULTS

Overall, 215 participants were randomized for treatment (44 to eliapixant 25 mg, 44 to eliapixant 75 mg, 43 to eliapixant 150 mg, 43 to a placebo, and 41 to elagolix 150 mg). For safety reasons, the study was terminated early; both treatment and enrollment stopped immediately, producing less than 50% of the planned number of completers. The study found no significant differences in EAPP reduction from baseline between groups and no significant dose-response model. The elagolix 150 mg group showed better pain reduction than any of the other groups. No new safety signals were observed, relative to the previously known safety profile of eliapixant, which was generally well tolerated. However, one case of moderate and probably drug-induced liver injury in a participant receiving eliapixant 150 mg BID supported the association between eliapixant and a potential increase in liver function values, defined before the start of the phase 2 program.

CONCLUSIONS

This study did not meet its primary objective as no statistically significant or clinically relevant differences in changes of mean worst EAPP from baseline were observed between treatment groups. The single observed case of moderate, probably drug-induced liver injury was the second case in the eliapixant phase 2 program conducted in the following indications: refractory or unexplained chronic cough, diabetic neuropathic pain, overactive bladder, and EAPP. Due to this, the benefit-risk ratio for the study was no longer considered to be positive.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov identifier NCT04614246; registered November 3, 2020.

P2X3 Receptor Antagonist Eliapixant in Phase I Clinical Trials: Safety and Inter-ethnic Comparison of Pharmacokinetics in Healthy Chinese and Japanese Participants

BACKGROUND

Afferent neuronal hypersensitization via P2X3 receptor signaling has been implicated as a driver of several disorders, including refractory chronic cough, endometriosis, diabetic neuropathic pain, and overactive bladder. Eliapixant, a selective P2X3 receptor antagonist, has been in clinical development for all four disorders.

OBJECTIVE

This paper describes pharmacokinetic (PK) and safety data from two phase I studies of eliapixant in healthy Japanese and Chinese participants and compares those data within the two populations and with previous multiple dose data from Caucasian participants.

METHODS

Two separate phase I, single-center, randomized, placebo-controlled studies were conducted with healthy male participants. The Japanese study was single-blind and the Chinese study was double-blind. Eliapixant was administered as an oral amorphous solid dispersion immediate-release tablet in strengths of 25 mg, 75 mg, and 150 mg. PK characteristics after a single dose (SD) and at steady state (multiple dose [MD], twice daily), adverse events (AEs), and tolerability were evaluated. A post hoc comparison of PK characteristics after SD of eliapixant in Japanese and Chinese participants, and after MD of eliapixant in Japanese, Chinese, and Caucasian participants, was performed.

RESULTS

Overall, 36/39 participants enrolled in the Japanese/Chinese studies, respectively (mean [standard deviation] age 25.4 [6.5] and 26.7 [5.0] years, respectively). After SD administration, maximum plasma concentration (Cmax) was higher among Japanese than Chinese participants in the 25 mg and 75 mg dose groups, but comparable in the 150 mg dose group. The area under the concentration-time curve (AUC) was comparable between Japanese and Chinese participants in the 25 mg and 75 mg dose groups, but lower among Japanese participants in the 150 mg group. Half-lives after SD and MD administration were also comparable in Japanese and Chinese participants. The post hoc analysis included 26 Japanese, 30 Chinese, and 50 Caucasian participants. Comparable exposure (Cmax,md and AUC[0-12]md) was observed after MD administration of eliapixant in Chinese and/or Japanese compared with Caucasian participants (geometric mean inter-ethnic ratios close to 1). The trough plasma concentration after eliapixant 150 mg MD, which was assumed to be relevant to eliapixant efficacy, was comparable across all ethnicity groups. Most AEs reported in the Japanese (eliapixant 75 mg SD, n = 2; eliapixant 150 mg MD, n = 2) and Chinese participants (eliapixant 25 mg SD, n = 7; eliapixant 75 mg SD, n = 6; eliapixant 150 mg SD, n = 7; eliapixant 150 mg MD, n = 9; placebo SD, n = 5; placebo MD, n = 1) were of mild intensity. Higher incidences of AEs in the Chinese population were likely due to differing standards of AE reporting between investigators.

CONCLUSION

Eliapixant was well tolerated by Japanese and Chinese participants. The inter-ethnic evaluation demonstrated similar PK characteristics across Japanese, Chinese, and Caucasian participants.

REGISTRATION

ClinicalTrials.gov identifier numbers: NCT04265781 and NCT04802343.

Increased Purinergic Signaling in Human Dental Pulps With Inflammatory Pain is Sex-Dependent

An enhanced understanding of neurotransmitter systems contributing to pain transmission aids in drug development, while the identification of biological variables like age and sex helps in the development of personalized pain management and effective clinical trial design. This study identified enhanced expression of purinergic signaling components specifically in painful inflammation, with levels increased more in women as compared to men. Inflammatory dental pain is common and potentially debilitating; as inflammation of the dental pulp can occur with or without pain, it provides a powerful model to examine distinct pain pathways in humans. In control tissues, P2X3 and P2X2 receptors colocalized with PGP9.5-positive nerves. Expression of the ecto-nucleotidase NTPDase1 (CD39) increased with exposure to extracellular adenosine triphosphate (ATP), implying CD39 acted as a marker for sustained elevation of extracellular ATP. Both immunohistochemistry and immunoblots showed P2X2, P2X3, and CD39 increased in symptomatic pulpitis, suggesting receptors and the ATP agonist were elevated in patients with increased pain. The increased expression of P2X3 and CD39 was more frequently observed in women than men. In summary, this study identifies CD39 as a marker for chronic elevation of extracellular ATP in fixed human tissue. It supports a role for increased purinergic signaling in humans with inflammatory dental pain and suggests the contribution of purines shows sexual dimorphism. This highlights the potential for P2X antagonists to treat pain in humans and stresses the need to consider sex in clinical trials that target pain and purinergic pathways. PERSPECTIVE: This article demonstrates an elevation of ATP-marker CD39 and of ATP receptors P2X2 and P2X3 with inflammatory pain and suggests the rise is greater in women. This highlights the potential for P2X antagonists to treat pain and stresses the consideration of sexual dimorphism in studies of purines and pain.

Deficiency of purinergic P2X4 receptor alleviates experimental autoimmune hepatitis in mice

Purinergic P2X4 receptor (P2X4R) has been shown to have immunomodulatory properties in infection, inflammation, and organ damage including liver regeneration and fibrosis. However, the mechanisms and pathophysiology associated with P2X4R during acute liver injury remain unknown. We used P2X4R-/- mice to explore the role of P2X4R in three different models of acute liver injury caused by concanavalin A (ConA), carbon tetrachloride, and acetaminophen. ConA treatment results in an increased expression of P2X4R in the liver of mice, which was positively correlated with higher levels of aspartate aminotransferase and alanine aminotransferase in the serum. However, P2X4R gene ablation significantly reduced the severity of acute hepatitis in mice caused by ConA, but not by carbon tetrachloride or acetaminophen. The protective benefits against immune-mediated acute hepatitis were achieved via modulating inflammation (Interleukin (IL)-1β, IL-6, IL-17A, interferon-γ, tumor necrosis factor-α), oxidative stress (malondialdehyde, superoxide dismutase, glutathione peroxidase, and catalase), apoptosis markers (Bax, Bcl-2, and Caspase-3), autophagy biomarkers (LC3, Beclin-1, and p62), and nucleotide oligomerization domain-likereceptorprotein 3(NLRP3) inflammasome-activated pyroptosis markers (NLRP3, Gasdermin D, Caspase-1, ASC, IL-1β). Additionally, administration of P2X4R antagonist (5-BDBD) or agonist (cytidine 5'-triphosphate) either improved or worsened ConA-induced autoimmune hepatitis, respectively. This study is the first to reveal that the absence of the P2X4 receptor may mitigate immune-mediated liver damage, potentially by restraining inflammation, oxidation, and programmed cell death mechanisms. And highlight P2X4 receptor is essential for ConA-induced acute hepatitis.

Role of ecto-5'-nucleotidase in bladder function

Purinergic signaling plays an important role in regulating bladder contractility and voiding. Abnormal purinergic signaling is associated with lower urinary tract symptoms (LUTS). Ecto-5'-nucleotidase (NT5E) catalyzes dephosphorylation of extracellular AMP to adenosine, which in turn promotes adenosine-A2b receptor signaling to relax bladder smooth muscle (BSM). The functional importance of this mechanism was investigated using Nt5e knockout (Nt5eKO) mice. Increased voiding frequency of small voids revealed by voiding spot assay was corroborated by urodynamic studies showing shortened voiding intervals and decreased bladder compliance. Myography indicated reduced contractility of Nt5eKO BSM. These data support a role for NT5E in regulating bladder function through modulation of BSM contraction and relaxation. However, the abnormal bladder phenotype of Nt5eKO mice is much milder than we previously reported in A2b receptor knockout (A2bKO) mice, suggesting compensatory response(s) in Nt5eKO mouse bladder. To better understand this compensatory mechanism, we analyzed changes in purinergic and other receptors controlling BSM contraction and relaxation in the Nt5eKO bladder. We found that the relative abundance of muscarinic CHRM3 (cholinergic receptor muscarinic 3), purinergic P2X1, and A2b receptors was unchanged, whereas P2Y12 receptor was significantly downregulated, suggesting a negative feedback response to elevated ADP signaling. Further studies of additional ecto-nucleotidases indicated significant upregulation of the nonspecific urothelial alkaline phosphatase ALPL, which might mitigate the degree of voiding dysfunction by compensating for Nt5e deletion. These data suggest a mechanistic complexity of the purinergic signaling network in bladder and imply a paracrine mechanism in which urothelium-released ATP and its rapidly produced metabolites coordinately regulate BSM contraction and relaxation.

Efficacy and Safety of Eliapixant in Overactive Bladder: The 12-Week, Randomised, Placebo-controlled Phase 2a OVADER Study

BACKGROUND

Effective, well-tolerated novel treatments for overactive bladder (OAB) are lacking. The P2X3 receptor antagonist eliapixant demonstrated potential to reduce OAB symptoms in preclinical studies.

OBJECTIVE

To evaluate the safety, tolerability, and efficacy of eliapixant in patients with OAB with urgency urinary incontinence (UUI).

DESIGN, SETTING AND PARTICIPANTS

OVADER was a 12-wk, randomised, placebo-controlled, double-blind, parallel-group, multicentre, phase 2a study (NCT04545580) conducted between 2020 and 2022 in private and institutional clinical practices. Eligible patients were aged ≥18 yr with wet OAB symptoms (urgency, urinary frequency, and urinary incontinence) for ≥3 mo before screening.

INTERVENTION

Randomisation (1:1 ratio) to oral eliapixant 125 mg or placebo twice daily.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

The primary endpoint was the mean change from baseline in the mean number of UUI episodes/24 h over weeks 4, 8, and 12 according to an electronic bladder diary, evaluated using a repeated-measurement model in a Bayesian framework.

RESULTS AND LIMITATIONS

Of 202 patients enrolled, 85 were valid for per-protocol analysis. The primary efficacy endpoint was not met. The posterior probability for eliapixant superiority over placebo was 40% (point estimate 0.05, 95% credible interval -∞ to 0.38), which did not meet the predefined criterion of ≥90% probability. Secondary and exploratory endpoints were not met. The incidence of adverse events was similar in the eliapixant (n = 32, 63%) and placebo (n = 27, 56%) groups; most were mild and five led to discontinuation of eliapixant.

CONCLUSIONS

OVADER did not meet its clinical efficacy endpoints. Potential reasons include the nonspecific OAB symptom complex, the poorly understood pathophysiology, and the coinciding COVID-19 pandemic.

PATIENT SUMMARY

We tested whether a new drug called eliapixant would reduce symptoms of overactive bladder in comparison to placebo. We found that the drug did not work. More knowledge on how overactive bladder occurs is needed to find new drugs to treat this condition.

Mass Balance and Metabolic Pathways of Eliapixant, a P2X3 Receptor Antagonist, in Healthy Male Volunteers

BACKGROUND

Overactive adenosine triphosphate signaling via P2X3 homotrimeric receptors is implicated in multiple conditions. To fully understand the metabolism and elimination pathways of eliapixant, a study was conducted to assess the pharmacokinetics, mass balance, and routes of excretion of a single oral dose of the selective P2X3 receptor antagonist eliapixant, in addition to an in vitro characterization.

METHODS

In this single-center open-label non-randomized non-placebo-controlled phase I study, healthy male subjects (n = 6) received a single dose of 50 mg eliapixant blended with 3.7 MBq [14C]eliapixant as a PEG 400-based oral solution. Total radioactivity and metabolites excreted in urine and feces, and pharmacokinetics of total radioactivity, eliapixant, and metabolites in plasma were assessed via liquid scintillation counting and high-performance liquid chromatography-based methods coupled to radiometric and mass spectrometric detection. Metabolite profiles of eliapixant in human in vitro systems and metabolizing enzymes were also investigated.

RESULTS

After administration as an oral solution, eliapixant was rapidly absorbed, reaching maximum plasma concentrations within 2 h. Eliapixant was eliminated from plasma with a mean terminal half-life of 48.3 h. Unchanged eliapixant was the predominant component in plasma (72.6% of total radioactivity area under the curve). The remaining percentage of drug-related components in plasma probably represented the sum of many metabolites, detected in trace amounts. Mean recovery of total radioactivity was 97.9% of the administered dose (94.3-99.4%) within 14 days, with 86.3% (84.8-88.1%) excreted via feces and 11.6% (9.5-13.1%) via urine. Excretion of parent drug was minimal in feces (0.7% of dose) and urine (≈ 0.5%). In feces, metabolites formed by oxidation represented > 90% of excreted total radioactivity. The metabolites detected in the in vitro experiments were similar to those identified in vivo.

CONCLUSION

Complete recovery of administered eliapixant-related radioactivity was observed in healthy male subjects with predominant excretion via feces. Eliapixant was almost exclusively cleared by oxidative biotransformation (> 90% of dose), with major involvement of cytochrome P450 3A4. Excretion of parent drug was of minor importance (~ 1% of dose).

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov: NCT04487431 (registered 27 July 2020)/EudraCT number: 2020-000519-54 (registered 3 February 2020), NCT02817100 (registered 26 June 2016), NCT03310645 (registered 16 October 2017).

Migraine signaling pathways: purine metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a debilitating disorder that afflicts over 1 billion people worldwide, involving attacks that result in a throbbing and pulsating headache. Migraine is thought to be a neurovascular event associated with vasoconstriction, vasodilation, and neuronal activation. Understanding signaling in migraine pathology is central to the development of therapeutics for migraine prophylaxis and for mitigation of migraine in the prodrome phase before pain sets in. The fact that both vasoactivity and neural sensitization are involved in migraine indicates that agonists which promote these phenomena may very well be involved in migraine pathology. One such group of agonists is the purines, in particular, adenosine phosphates and their metabolites. This manuscript explores what is known about the relationship between these metabolites and migraine pathology and explores the potential for such relationships through their known signaling pathways. Reported receptor involvement in vasoaction and nociception.

Pharmacological interaction and immune response of purinergic receptors in therapeutic modulation

Nucleosides and purine nucleotides serve as transmitter and modulator agents that extend their functions beyond the cell. In this context, purinergic signaling plays a crucial role in regulating energy homeostasis and modulating metabolic alterations in tumor cells. Therefore, it is essential to consider the pharmacological targeting of purinergic receptors (PUR), which encompass the expression and inhibition of P1 receptors (metabotropic adenosine receptors) as well as P2 receptors (extracellular ATP/ADP) comprising P2X and P2Y receptors. Thus, the pharmacological interaction between inhibitors (such as RNA, monoclonal antibodies, and small molecules) and PUR represents a key aspect in facilitating the development of therapeutic interventions. Moreover, this review explores recent advancements in pharmacological inhibitors and the regulation of innate and adaptive immunity of PUR, specifically in relation to immunological and inflammatory responses. These responses encompass the release of pro-inflammatory cytokines (PIC), the production of reactive oxygen and nitrogen species (ROS and RNS), the regulation of T cells, and the activation of inflammasomes in all human leukocytes.

Role and therapeutic target of P2X2/3 receptors in visceral pain

Visceral pain (VP) is caused by internal organ disease. VP is involved in nerve conduction and related signaling molecules, but its specific pathogenesis has not yet been fully elucidated. Currently, there are no effective methods for treating VP. The role of P2X2/3 in VP has progressed. After visceral organs are subjected to noxious stimulation, cells release ATP, activate P2X2/3, enhance the sensitivity of peripheral receptors and the plasticity of neurons, enhance sensory information transmission, sensitize the central nervous system, and play an important role in the development of VP. However, antagonists possess the pharmacological effect of relieving pain. Therefore, in this review, we summarize the biological functions of P2X2/3 and discuss the intrinsic link between P2X2/3 and VP. Moreover, we focus on the pharmacological effects of P2X2/3 antagonists on VP therapy and provide a theoretical basis for its targeted therapy.

Mechanotransduction in the urothelium: ATP signalling and mechanoreceptors

The urothelium, which covers the inner surface of the bladder, is continuously exposed to a complex physical environment where it is stimulated by, and responds to, a wide range of mechanical cues. Mechanically activated ion channels endow the urothelium with functioning in the conversion of mechanical stimuli into biochemical events that influence the surface of the urothelium itself as well as suburothelial tissues, including afferent nerve fibres, interstitial cells of Cajal and detrusor smooth muscle cells, to ensure normal urinary function during the cycle of filling and voiding. However, under prolonged and abnormal loading conditions, the urothelial sensory system can become maladaptive, leading to the development of bladder dysfunction. In this review, we summarize developments in the understanding of urothelial mechanotransduction from two perspectives: first, with regard to the functions of urothelial mechanotransduction, particularly stretch-mediated ATP signalling and the regulation of urothelial surface area; and secondly, with regard to the mechanoreceptors present in the urothelium, primarily transient receptor potential channels and mechanosensitive Piezo channels, and the potential pathophysiological role of these channels in the bladder. A more thorough understanding of urothelial mechanotransduction function may inspire the development of new therapeutic strategies for lower urinary tract diseases.

From bench to bedside: The role of cough hypersensitivity in chronic cough

BACKGROUND

Chronic cough is a burdensome condition characterized by persistent cough lasting longer than 8 weeks. Chronic cough can significantly affect quality of life, physical function and productivity, with many people troubled with a cough that lasts for months or even years. People with chronic cough commonly report a persistent urge to cough with frequent bouts of coughing triggered by innocuous stimuli, which has led to the concept of cough hypersensitivity.

MAIN BODY

Both central and peripheral neural pathways regulate cough, and although mechanisms driving development of cough hypersensitivity are not fully known, sensitization of these neural pathways contributes to excessive cough triggering in cough hypersensitivity. Effective therapies that control chronic cough are currently lacking. Recent therapeutic development has focused on several ion channels and receptors involved in peripheral activation of cough (e.g., transient receptor potential channels, P2 × 3 receptors and voltage-gated sodium channels) or central cough processing (e.g., neurokinin-1 [NK-1] receptors and nicotinic acetylcholine receptors).

CONCLUSION

These targeted therapies provide novel insights into mechanisms underlying cough hypersensitivity and may offer new treatment options for people with chronic cough. In this review, we explore preclinical and clinical studies that have improved our understanding of the mechanisms responsible for chronic cough and discuss the most promising targeted approaches to date, including trials of P2 × 3-receptor antagonists and NK-1-receptor antagonists.

Folic acid relieves bone cancer pain by downregulating P2X2/3 receptors in rats

BACKGROUND

Bone cancer pain (BCP) remains a clinical challenge due to the limited and side effects of therapeutic methods. Folic acid has been known as an FDA approved dietary supplement and proved to have an analgesic effect in neuropathic pain. Here we investigate the role and mechanism of folic acid in bone cancer pain of a rat model.

METHODS

Walker 256 tumor cells were inoculated into the left tibia of rats to induce bone cancer pain model. Pain reflex were assessed by paw withdrawal threshold (PWT) response to Von Frey filaments and paw withdrawal latency (PWL) response to thermal stimulation. Folic acid was injected intraperitoneally to evaluate its analgesic effect in rats with bone cancer pain. Western blotting and qPCR were used to determine P2X2/3 receptor protein and mRNA levels in ipsilateral L4-6 dorsal root ganglion (DRG) and spinal dorsal horn (SDH).

RESULTS

The PWT and PWL of rats with bone cancer pain were obviously decreased compared to the naïve and sham rats. Interestingly, continuous folic acid treatment significantly increased the PWT and PWL of rats with bone cancer pain. P2X2 and P2X3 receptors were clearly upregulated at both mRNA and protein expression in L4-6 DRG and SDH of rats with bone cancer pain. P2X2 and P2X3 receptors were mainly localized with CGRP (calcitonin gene-related peptide) or IB4 (isolectin B4) positive neurons in L4-6 DRG of rats with bone cancer pain. Notably, continuous folic acid treatment significantly reduced the expression of P2X2 and P2X3 receptors in L4-6 DRG and SDH of rats with bone cancer pain. Finally, intrathecal injection of A317491 (a selective antagonist of P2X2/3 receptors) markedly elevated the PWT and PWL of rats with bone cancer pain.

CONCLUSION

These results suggest that folic acid has an effective antinociceptive effect on bone cancer pain, which is mediated by downregulating P2X2/3 receptors in L4-6 DRG and SDH of rats with bone cancer pain. Folic acid may be a novel therapeutic strategy in cancer patients for pain relief.

Gabapentin for chronic refractory cough: A system review and meta-analysis

Objective

To evaluate the efficacy and safety of gabapentin in the treatment of chronic refractory cough by Meta-Analysis.

Methods

Literatures were retrieved from PubMed, Embase (OvidIP), Cochrane Library, CNKI, VIP, Wanfang Database and China Biomedical Management System and eligible prospective studies were screened. Data were extracted and analyzed by using RevMan 5.4.1 software.

Results

Six articles (2 RCTs and 4 prospective studies) with 536 participants were finally included. Meta-analysis showed that gabapentin was better than placebo in cough-specific quality of life (LCQ score, MD = 4.02, 95%CI [3.26,4,78], Z = 10.34, P < 0.00001), cough severity (VAS score, MD = -29.36, 95% CI (-39.46, -19.26), Z = 5.7, P < 0.00001), cough frequency (MD = -29.87, 95% CI [- 43.84, -15.91], Z = 4.19, P < 0.0001) and therapeutic efficacy (RR = 1.37,95%CI [1.13,1.65], Z = 3.27, P = 0.001), and equal in safety (RR = 1.32,95%CI [0.47,3.7], Z = 0.53, P = 0.59). Gabapentin was similar to other neuromodulators in therapeutic efficacy (RR = 1.07,95%CI [0.87,1.32], Z = 0.64, P = 0.52), but its safety was better.

Conclusion

Gabapentin is effective in the treatment of chronic refractory cough in both subjective and objective evaluations, and its safety is better than other neuromodulators.

P2 receptor-mediated signaling in the physiological and pathological brain: From development to aging and disease

The purinergic pathway mediates both pro-inflammatory and anti-inflammatory responses, whereas the breakdown of adenosine triphosphate (ATP) is in a critical equilibrium. Under physiological conditions, extracellular ATP is maintained at a nanomolar concentration. Whether released into the medium following tissue damage, inflammation, or hypoxia, ATP is considered a clear indicator of cell damage and a marker of pathological conditions. In this overview, we provide an update on the participation of P2 receptor-mediated purinergic signaling in normal and pathological brain development, with special emphasis on neurodevelopmental psychiatric disorders. Since purinergic signaling is ubiquitous, it is not surprising that it plays a prominent role in developmental processes and pathological alterations. The main aim of this review is to conceptualize the time-dependent dynamic changes in the participation of different players in the purinome in shaping the normal and aberrant developmental patterns and diseases of the central nervous system over one's lifespan.

Diclofenac and other non-steroidal anti-inflammatory drugs (NSAIDs) are competitive antagonists of the human P2X3 receptor

Introduction: The P2X3 receptor (P2X3R), an ATP-gated non-selective cation channel of the P2X receptor family, is expressed in sensory neurons and involved in nociception. P2X3R inhibition was shown to reduce chronic and neuropathic pain. In a previous screening of 2000 approved drugs, natural products, and bioactive substances, various non-steroidal anti-inflammatory drugs (NSAIDs) were found to inhibit P2X3R-mediated currents.

Methods: To investigate whether the inhibition of P2X receptors contributes to the analgesic effect of NSAIDs, we characterized the potency and selectivity of various NSAIDs at P2X3R and other P2XR subtypes using two-electrode voltage clamp electrophysiology.

Results: We identified diclofenac as a hP2X3R and hP2X2/3R antagonist with micromolar potency (with IC50 values of 138.2 and 76.7 µM, respectively). A weaker inhibition of hP2X1R, hP2X4R, and hP2X7R by diclofenac was determined. Flufenamic acid (FFA) inhibited hP2X3R, rP2X3R, and hP2X7R (IC50 values of 221 µM, 264.1 µM, and ∼900 µM, respectively), calling into question its use as a non-selective ion channel blocker, when P2XR-mediated currents are under study. Inhibition of hP2X3R or hP2X2/3R by diclofenac could be overcome by prolonged ATP application or increasing concentrations of the agonist α,β-meATP, respectively, indicating competition of diclofenac and the agonists. Molecular dynamics simulation showed that diclofenac largely overlaps with ATP bound to the open state of the hP2X3R. Our results suggest a competitive antagonism through which diclofenac, by interacting with residues of the ATP-binding site, left flipper, and dorsal fin domains, inhibits the gating of P2X3R by conformational fixation of the left flipper and dorsal fin domains. In summary, we demonstrate the inhibition of the human P2X3 receptor by various NSAIDs. Diclofenac proved to be the most effective antagonist with a strong inhibition of hP2X3R and hP2X2/3R and a weaker inhibition of hP2X1R, hP2X4R, and hP2X7R.

Discussion: Considering their involvement in nociception, inhibition of hP2X3R and hP2X2/3R by micromolar concentrations of diclofenac, which are rarely reached in the therapeutic range, may play a minor role in analgesia compared to the high-potency cyclooxygenase inhibition but may explain the known side effect of taste disturbances caused by diclofenac.

Activation of Intra-nodose Ganglion P2X7 Receptors Elicit Increases in Neuronal Activity

Vagus nerve innervates several organs including the heart, stomach, and pancreas among others. Somas of sensory neurons that project through the vagal nerve are located in the nodose ganglion. The presence of purinergic receptors has been reported in neurons and satellite glial cells in several sensory ganglia. In the nodose ganglion, calcium depletion-induced increases in neuron activity can be partly reversed by P2X7 blockers applied directly into the ganglion. The later suggest a possible role of P2X7 receptors in the modulation of neuronal activity within this sensory ganglion. We aimed to characterize the response to P2X7 activation in nodose ganglion neurons under physiological conditions. Using an ex vivo preparation for electrophysiological recordings of the neural discharges of nodose ganglion neurons, we found that treatments with ATP induce transient neuronal activity increases. Also, we found a concentration-dependent increase in neural activity in response to Bz-ATP (ED₅₀ = 0.62 mM, a selective P2X7 receptor agonist), with a clear desensitization pattern when applied every ~ 30 s. Electrophysiological recordings from isolated nodose ganglion neurons reveal no differences in the responses to Bz-ATP and ATP. Finally, we showed that the P2X7 receptor was expressed in the rat nodose ganglion, both in neurons and satellite glial cells. Additionally, a P2X7 receptor negative allosteric modulator decreased the duration of Bz-ATP-induced maximal responses without affecting their amplitude. Our results show the presence of functional P2X7 receptors under physiological conditions within the nodose ganglion of the rat, and suggest that ATP modulation of nodose ganglion activity may be in part mediated by the activation of P2X7 receptors.

Preclinical and Clinical Pharmacokinetics and Bioavailability in Healthy Volunteers of a Novel Formulation of the Selective P2X3 Receptor Antagonist Eliapixant

BACKGROUND AND OBJECTIVES

The potent, selective P2X3 receptor antagonist eliapixant (BAY 1817080) is under development for conditions characterized by neuronal hypersensitization. As prominent food effects and limited bioavailability in the fasted state were observed with immediate-release eliapixant tablets, a novel formulation was needed. Accordingly, several novel eliapixant formulations were assessed by in vitro and animal studies in a structured way. The most promising of the formulations was then investigated in a phase I study designed to assess its pharmacokinetics, food effect, and bioavailability in healthy volunteers.

METHODS

In vitro non-sink dissolution tests were performed with two amorphous solid dispersion (ASD) granule prototypes compared with pure crystalline eliapixant as a surrogate for the immediate-release formulation. Subsequently, the drug exposure of novel eliapixant formulations under fed and fasted conditions in rats and dogs was assessed to confirm improvements in bioavailability versus the suspension-based formulation. A novel Kollidon VA64®-based eliapixant formulation was identified from the preclinical studies and compared with the original tablet formulation in an open-label, partially randomized, threefold, crossover phase I study, in which healthy males received single oral doses (25-400 mg, fasted/fed). Pharmacokinetic parameters, absolute bioavailability (using an intravenous [¹³C₇¹⁵N]-eliapixant microdose), relative bioavailability (novel versus original formulation), effect of food, and adverse events (AEs) were evaluated.

RESULTS

The non-sink dissolution test demonstrated that the two ASD formulations had an improved dissolution rate compared with pure crystalline eliapixant, with a Kollidon VA64-based prototype having the highest dissolution rate. Further testing of this prototype in animal studies confirmed an approximately twofold higher bioavailability compared with the suspension-based formulation. In the phase I study, 30 subjects were randomized. With the novel Kollidon VA64® formulation (400 mg; fasted), area under the concentration-time curve (AUC) and maximum plasma concentration (C_max) were up to 3.1-fold and 1.7-fold higher, respectively, than with the original formulation (fed). AUC increased dose proportionally between 25 and 100 mg, and less than dose proportionally from 100 to 400 mg. Food had no clinically relevant effect on the novel formulation, with AUC increasing 1.3-fold and C_max 2.1-2.4-fold (time to maximum concentration was delayed by 1.5-2.25 h). Absolute bioavailability with the novel formulation (100 mg) was 50%. AEs occurred in 57% of patients; most were mild in severity.

CONCLUSIONS

The novel eliapixant formulation substantially improved bioavailability compared with immediate-release eliapixant and may be administered with/without food.

CLINICAL TRIAL REGISTRATION

Clinicaltrials.gov: NCT03773068 (initial registration: 12 December 2018).

Effects of Intravesical Electrical Stimulation on Urinary Adenosine Triphosphate and Nitric Oxide in Rats With Detrusor Underactivity Induced By Bilateral Pelvic Nerve Crush Injury: The Possible Underlying Mechanism

PURPOSE

To explore the effect of intravesical electrical stimulation (IVES) on urinary adenosine triphosphate (ATP) and nitric oxide (NO) in rats with detrusor underactivity (DU) induced by bilateral pelvic nerve crush (bPNC), and to determine the underlying peripheral mechanism.

METHODS

Twenty-four female Sprague-Dawley rats were equally divided into 3 groups: sham; bPNC; and IVES. Rats in the IVES group began to receive IVES treatment 10 days after bPNC (20 minutes per day for 14 consecutive days). After the 14th IVES, rat urine was collected and cystometry was performed. The serum creatinine, blood urea nitrogen, and urinary ATP and NO levels were measured, and a routine urinalysis was performed.

RESULTS

The maximum cystometric capacity (MCC), maximum changes in bladder pressure during filling (∆FP), and postvoid residual urine (PVR) in the IVES group were significantly lower than the bPNC group, and the maximum changes in bladder pressure during voiding (∆VP) was significantly higher than the bPNC group. Compared with the sham group, the MCC, ∆FP and PVR were significantly increased, and the maximum voiding pressure (MVP) and ∆VP were significantly decreased in the bPNC group. After bPNC, urinary ATP was significantly decreased, and urinary NO was significantly increased. In IVES-treated rats, urinary ATP was significantly higher than the bPNC group, and NO was significantly lower than the bPNC group. In addition, the ATP-to-NO ratio of the rats in the bPNC group was significantly lower than the sham and IVES groups. Correlation analysis showed that the ATP and NO were not correlated with the MCC, ∆FP, MVP, ∆VP, and PVR.

CONCLUSION

Promoting the release of urothelial ATP and inhibiting the release of urothelial NO may be one of the peripheral mechanisms underlying IVES in the treatment of DU. Specifically, IVES may shift the balance between excitation and inhibition toward excitation.

Carbon Monoxide Signaling: Examining Its Engagement with Various Molecular Targets in the Context of Binding Affinity, Concentration, and Biologic Response

Carbon monoxide (CO) has been firmly established as an endogenous signaling molecule with a variety of pathophysiological and pharmacological functions, including immunomodulation, organ protection, and circadian clock regulation, among many others. In terms of its molecular mechanism(s) of action, CO is known to bind to a large number of hemoproteins with at least 25 identified targets, including hemoglobin, myoglobin, neuroglobin, cytochrome c oxidase, cytochrome P450, soluble guanylyl cyclase, myeloperoxidase, and some ion channels with dissociation constant values spanning the range of sub-nM to high μM. Although CO's binding affinity with a large number of targets has been extensively studied and firmly established, there is a pressing need to incorporate such binding information into the analysis of CO's biologic response in the context of affinity and dosage. Especially important is to understand the reservoir role of hemoglobin in CO storage, transport, distribution, and transfer. We critically review the literature and inject a sense of quantitative assessment into our analyses of the various relationships among binding affinity, CO concentration, target occupancy level, and anticipated pharmacological actions. We hope that this review presents a picture of the overall landscape of CO's engagement with various targets, stimulates additional research, and helps to move the CO field in the direction of examining individual targets in the context of all of the targets and the concentration of available CO. We believe that such work will help the further understanding of the relationship of CO concentration and its pathophysiological functions and the eventual development of CO-based therapeutics. SIGNIFICANCE STATEMENT: The further development of carbon monoxide (CO) as a therapeutic agent will significantly rely on the understanding of CO's engagement with therapeutically relevant targets of varying affinity. This review critically examines the literature by quantitatively analyzing the intricate relationships among targets, target affinity for CO, CO level, and the affinity state of carboxyhemoglobin and provide a holistic approach to examining the molecular mechanism(s) of action for CO.

ATP, an attractive target for the treatment of refractory chronic cough

Chronic cough is the most common complaint in respiratory clinics. Most of them have identifiable causes and some may respond to common disease-modifying therapies. However, there are many patients whose cough lacks effective aetiologically targeted treatments or remains unexplained after thorough assessments, which have been described as refractory chronic cough. Current treatments for refractory chronic cough are limited and often accompanied by intolerable side effects such as sedation. In recent years, various in-depth researches into the pathogenesis of chronic cough have led to an explosion in the development of drugs for the treatment of refractory chronic cough. There has been considerable progress in the underlying mechanisms of chronic cough targeting ATP, and ongoing or completed clinical studies have confirmed the promising antitussive efficacy of P2X3 antagonists for refractory cough. Herein, we review the foundation on which ATP target was developed as potential antitussive medications and provide an update on current clinical progresses.

Purinergic receptors modulators: An emerging pharmacological tool for disease management

Purinergic signaling is mediated through extracellular nucleotides (adenosine 5'-triphosphate, uridine-5'-triphosphate, adenosine diphosphate, uridine-5'-diphosphate, and adenosine) that serve as signaling molecules. In the early 1990s, purines and pyrimidine receptors were cloned and characterized drawing the attention of scientists toward this aspect of cellular signaling. This signaling pathway is comprised of four subtypes of adenosine receptors (P1), eight subtypes of G-coupled protein receptors (P2YRs), and seven subtypes of ligand-gated ionotropic receptors (P2XRs). In current studies, the pathophysiology and therapeutic potentials of these receptors have been focused on. Various ligands, modulating the functions of purinergic receptors, are in current clinical practices for the treatment of various neurodegenerative disorders and cardiovascular diseases. Moreover, several purinergic receptors ligands are in advanced phases of clinical trials as a remedy for depression, epilepsy, autism, osteoporosis, atherosclerosis, myocardial infarction, diabetes, irritable bowel syndrome, and cancers. In the present study, agonists and antagonists of purinergic receptors have been summarized that may serve as pharmacological tools for drug design and development.

First-in-human study of eliapixant (BAY 1817080), a highly selective P2X3 receptor antagonist: tolerability, safety and pharmacokinetics

Aims

Neuronal hypersensitisation due to adenosine triphosphate‐dependent P2X3 receptor signalling plays a significant role in several disorders including chronic cough and endometriosis. This first‐in‐human study of eliapixant (BAY 1817080) investigated the tolerability, safety and pharmacokinetics (PK) of single doses of eliapixant, including the effect of food and coadministration with a CYP3A inhibitor on eliapixant relative bioavailability.

Methods

In this randomised, double‐blind phase I study (NCT02817100), 88 healthy male subjects received single ascending doses of immediate‐release eliapixant (10–800 mg) tablets or placebo under fasted conditions, with food (low‐fat continental or high‐fat American breakfast) or with itraconazole (fasted state). PK parameters, dose proportionality, adverse events and taste assessments (taste strips; dysgeusia questionnaire) were evaluated.

Results

Eliapixant had a long half‐life (23.5–58.9 h [fasted state]; 32.8–43.8 h [high‐fat breakfast]; 38.9–46.0 h [low‐fat breakfast]). Less than dose‐proportional increases in maximum plasma concentrations (C_max) and area under the concentration–time curve from time 0 to infinity (AUC_[0–inf]) were observed with ascending eliapixant doses. We observed a pronounced food effect with the high‐fat breakfast (4.1‐fold increased C_max; 2.7‐fold increased AUC_[0–inf]), a smaller food effect with the low‐fat breakfast and a mild‐to‐moderate effect of itraconazole coadministration on eliapixant (1.1–1.2‐fold increased C_max; 1.7‐fold increased AUC from 0 to 72 h). Eliapixant was well tolerated with minimal impact on taste perception.

Conclusion

The PK profile, particularly the long half‐life, and favourable tolerability with no taste‐related adverse events, supports the further development of eliapixant in disorders with underlying P2X3 receptor‐mediated neuronal hypersensitisation.

Blockage of HCN Channels Inhibits the Function of P2X Receptors in Rat Dorsal Root Ganglion Neurons

Hyperpolarization-activated cyclic nucleotide-gated channels and purinergic P2X receptors play critical roles in the nerve injury-induced pain hypersensitivity. Both HCN channels and P2XR are expressed in dorsal root ganglia sensory neurons. However, it is not clear whether the expression and function of P2X2 and P2X3 receptors can be modulated by HCN channel activity. For this reason, in rats with chronic constriction injury of sciatic nerve, we evaluated the effect of intrathecal administration of HCN channel blocker ZD7288 on nociceptive behavior and the expression of P2X2 and P2X3 in rat DRG. The mechanical withdrawal threshold was measured to evaluate pain behavior in rats. The protein expression of P2X2 and P2X3 receptor in rat DRG was observed by using Western Blot. The level of cAMP in rat DRG was measured by ELISA. As a result, decreased MWT was observed in CCI rats on 1 d after surgery, and the allodynia was sustained throughout the experimental period. In addition, CCI rats presented increased expression of P2X2 and P2X3 receptor in the ipsilateral DRG at 7 d and 14 d after CCI operation. Intrathecal injection of ZD7288 significantly reversed CCI-induced mechanical hyperalgesia, and attenuated the increased expression of P2X2 and P2X3 receptor in rat DRG, which open up the possibility that the expression of P2X2 and P2X3 receptor in DRG is down-regulated by HCN channel blocker ZD7288 in CCI rats. Furthermore, the level of cAMP in rat DRG significantly increased after nerve injury. Intrathecal administration of ZD7288 attenuated the increase of cAMP in DRG caused by nerve injury. Subsequently, effects of HCN channel activity on ATP-induced current (I_ATP) in rat DRG neurons were explored by using whole-cell patch-clamp techniques. ATP (100 μM) elicited three types of currents (fast, slow and mixed I_ATP) in cultured DRG neurons. Pretreatment with ZD7288 concentration-dependently inhibited three types of ATP-activated currents. On the other hand, pretreatment with 8-Br-cAMP (a cell-permeable cAMP analog, also known as an activator of PKA) significantly increased the amplitude of fast, slow and mixed I_ATP in DRG neurons. The enhanced effect of 8-Br-cAMP on ATP-activated currents could be reversed by ZD7288. In a summary, our observations suggest that the opening of HCN channels could enhance the expression and function of P2X2 and P2X3 receptor via the cAMP-PKA signaling pathway. This may be important for pathophysiological events occurring within the DRG, for where it is implicated in nerve injury-induced pain hypersensitivity.

Eliapixant is a selective P2X3 receptor antagonist for the treatment of disorders associated with hypersensitive nerve fibers

ATP-dependent P2X3 receptors play a crucial role in the sensitization of nerve fibers and pathological pain pathways. They are also involved in pathways triggering cough and may contribute to the pathophysiology of endometriosis and overactive bladder. However, despite the strong therapeutic rationale for targeting P2X3 receptors, preliminary antagonists have been hampered by off-target effects, including severe taste disturbances associated with blocking the P2X2/3 receptor heterotrimer. Here we present a P2X3 receptor antagonist, eliapixant (BAY 1817080), which is both highly potent and selective for P2X3 over other P2X subtypes in vitro, including P2X2/3. We show that eliapixant reduces inflammatory pain in relevant animal models. We also provide the first in vivo experimental evidence that P2X3 antagonism reduces neurogenic inflammation, a phenomenon hypothesised to contribute to several diseases, including endometriosis. To test whether eliapixant could help treat endometriosis, we confirmed P2X3 expression on nerve fibers innervating human endometriotic lesions. We then demonstrate that eliapixant reduces vaginal hyperalgesia in an animal model of endometriosis-associated dyspareunia, even beyond treatment cessation. Our findings indicate that P2X3 antagonism could alleviate pain, including non-menstrual pelvic pain, and modify the underlying disease pathophysiology in women with endometriosis. Eliapixant is currently under clinical development for the treatment of disorders associated with hypersensitive nerve fibers.

Synthesis of Sulfonamide Tethered (Hetero)aryl ethylidenes as Potential Inhibitors of P2X Receptors: A Promising Way for the Treatment of Pain and Inflammation

P2X receptors have the ability to regulate various physiological functions like neurotransmission, inflammatory responses, and pain sensation. Such physiological properties make these receptors a new target for the treatment of pain and inflammation. Several antagonists of P2X receptors have been studied for the treatment of neuropathic pain and neurodegenerative disorders but potency and selectivity are the major issues with these known inhibitors. Sulfonamide derivatives were reported to be potent inhibitors of P2X receptors. In this study, sulfonamide carrying precursor hydrazide was synthesized by a facile method that was subsequently condensed with methyl (hetero)arylketones to obtain a series of new (hetero)aryl ethylidenes. These compounds were screened for inhibitory potential against h-P2X2, h-P2X4, h-P2X5, and h-P2X7 receptors to find their potency and selectivity. Computational studies were performed to confirm the mode of inhibition as well as type of interaction between ligand and target site. In calcium signaling experiments, compound 6h was found to be the most potent and selective inhibitor of h-P2X2 and h-P2X7 receptors with IC₅₀ ± standard error of the mean (SEM) values of 0.32 ± 0.01 and 1.10 ± 0.21 μM, respectively. Compounds 6a and 6c exhibited selective inhibition for h-P2X7 receptor, whereas 6e, 7a, and 7b expressed selective inhibitions toward h-P2X2 receptor that were comparable to the positive control suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS).

The discovery and development of gefapixant

Gefapixant is the approved generic name for a compound also known as MK-7264, and prior to that AF-219 and RO-4926219. It is the first-in-class clinically developed antagonist for the P2X3 subtype of trimeric ionotropic purinergic receptors, a family of ATP-gated excitatory ion channels, showing nanomolar potency for the human P2X3 homotrimeric channel and essentially no activity at related channels devoid of P2X3 subunits. As the first P2X3 antagonist to have progressed into clinical studies it has now progressed to the point of successful completion of Phase 3 investigations for the treatment of cough, and the NDA application is under review with US FDA for treatment of refractory chronic cough or unexplained chronic cough. The molecule was discovered in the laboratories of Roche Pharmaceuticals in Palo Alto, California, but clinical development then continued with the formation of Afferent Pharmaceuticals for the purpose of identifying the optimal therapeutic indication for this novel mechanism and establishing a clinical plan for development in the optimal patient populations selected. Geoff Burnstock was a close collaborator and advisor to the P2X3 program for close to two decades of discovery and development. Progression of gefapixant through later stage clinical studies has been conducted by the research laboratories of Merck & Co., Inc., Kenilworth, NJ, USA (MRL; following acquisition of Afferent in 2016), who may commercialize the product once authorization has been granted by regulatory authorities.

Purinergic signalling in the urinary bladder - When function becomes dysfunction

Knowledge of the participation of ATP and related purines in urinary tract physiology has been established over the last five decades through the work of many independent groups, inspired by, and building on the pioneering studies of Professor Geoffrey Burnstock and his coworkers. As part of a series of reviews in this tribute edition, the present article summarises our current understanding of purines and purinergic signalling in modulating and regulating urinary tract function. Purinergic mechanisms underlying the origin of bladder pain; sensations of bladder filling and urinary tract motility; and regulation of detrusor smooth muscle contraction are described, encompassing the relevant history of discovery and consolidation of knowledge as methodologies and pharmacological tools have developed. We consider normal physiology, including development and ageing and then move to pathophysiology, discussing the causal and consequential contribution of purinergic signalling mechanism and their constituent components (receptors, signal transduction, effector molecules) to bladder dysfunction.

The nitric oxide-cyclic guanosine monophosphate pathway inhibits the bladder ATP release in response to a physiological or pathological stimulus

The release of ATP from the epithelium of the urinary bladder (urothelium) in response to mechanical/chemical stimuli contributes to the visceral sensation in the micturition reflex. The nitric oxide (NO)-mediated induction of cyclic guanosine monophosphate (cGMP) has been detected in urothelial cells and may inhibit the micturition reflex. However, the function of the NO-cGMP pathway in the regulation of urothelial ATP release remains poorly understood in contrast to its effects on smooth muscles or primary afferent nerves. Therefore, we investigated the relevance of the NO-cGMP pathway to ATP release on the mucosal side in the present study. The administration of l-arginine (NO precursor) or NOC 12 (NO donor) significantly reduced ATP release to the mucosal side at a physiologically normal urine storage pressure (5 cmH2 O). L-NAME (NO synthase inhibitor) significantly increased the distention-induced release of ATP. The phosphodiesterase-5 inhibitor, sildenafil, which increases cGMP levels, inhibited distention-induced ATP release. Furthermore, sildenafil significantly reduced ATP release in response to the administration of lipopolysaccharide. These results suggest that the NO-cGMP pathway inhibited urothelial ATP release during the storage phase under both physiological and pathological conditions.

BAC transgenic mice to study the expression of P2X2 and P2Y1 receptors

Extracellular purines are important signaling molecules involved in numerous physiological and pathological processes via the activation of P2 receptors. Information about the spatial and temporal P2 receptor (P2R) expression and its regulation remains crucial for the understanding of the role of P2Rs in health and disease. To identify cells carrying P2X2Rs in situ, we have generated BAC transgenic mice that express the P2X2R subunits as fluorescent fusion protein (P2X2-TagRFP). In addition, we generated a BAC P2Y₁R TagRFP reporter mouse expressing a TagRFP reporter for the P2RY1 gene expression. We demonstrate expression of the P2X2R in a subset of DRG neurons, the brain stem, the hippocampus, as well as on Purkinje neurons of the cerebellum. However, the weak fluorescence intensity in our P2X2R-TagRFP mouse precluded tracking of living cells. Our P2Y₁R reporter mice confirmed the widespread expression of the P2RY1 gene in the CNS and indicate for the first time P2RY1 gene expression in mouse Purkinje cells, which so far has only been described in rats and humans. Our P2R transgenic models have advanced the understanding of purinergic transmission, but BAC transgenic models appeared not always to be straightforward and permanent reliable. We noticed a loss of fluorescence intensity, which depended on the number of progeny generations. These problems are discussed and may help to provide more successful animal models, even if in future more versatile and adaptable nuclease-mediated genome-editing techniques will be the methods of choice.

TMEM163 Regulates ATP-Gated P2X Receptor and Behavior

Fast purinergic signaling is mediated by ATP and ATP-gated ionotropic P2X receptors (P2XRs), and it is implicated in pain-related behaviors. The properties exhibited by P2XRs vary between those expressed in heterologous cells and in vivo. Several modulators of ligand-gated ion channels have recently been identified, suggesting that there are P2XR functional modulators in vivo. Here, we establish a genome-wide open reading frame (ORF) collection and perform functional screening to identify modulators of P2XR activity. We identify TMEM163, which specifically modulates the channel properties and pharmacology of P2XRs. We also find that TMEM163 is required for full function of the neuronal P2XR and a pain-related ATP-evoked behavior. These results establish TMEM163 as a critical modulator of P2XRs in vivo and a potential target for the discovery of drugs for treating pain.

Generation and characterization of a P2rx2 V60L mouse model for DFNA41

P2RX2 encodes the P2X2 receptor, which is an adenosine triphosphate (ATP) gated (purinoreceptor) ion channel. P2RX2 c. 178G > T (p.V60L) mutation was previously identified in two unrelated Chinese families, as the cause of human DFNA41, a form of dominant, early-onset and progressive sensorineural hearing loss. We generated and characterized a knock-in mouse model based on human p.V60L mutation that recapitulates the human phenotype. Heterozygous KI mice started to exhibit hearing loss at 21-day-old and progressed to deafness by 6-month-old. Vestibular dysfunction was also observed in mutant mice. Abnormal morphology of the inner hair cells and ribbon synapses was progressively observed in KI animals suggesting that P2rx2 plays a role in the membrane spatial location of the ribbon synapses. These results suggest that P2rx2 is essential for acoustic information transfer, which can be the molecular mechanism related to hearing loss.

Transient receptor potential channels in sensory mechanisms of the lower urinary tract

Disruptions to sensory pathways in the lower urinary tract commonly occur and can give rise to lower urinary tract symptoms (LUTS). The unmet clinical need for treatment of LUTS has stimulated research into the molecular mechanisms that underlie neuronal control of the bladder and transient receptor potential (TRP) channels have emerged as key regulators of the sensory processes that regulate bladder function. TRP channels function as molecular sensors in urothelial cells and afferent nerve fibres and can be considered the origin of bladder sensations. TRP channels in the lower urinary tract contribute to the generation of normal and abnormal bladder sensations through a variety of mechanisms, and have demonstrated potential as targets for the treatment of LUTS in functional disorders of the lower urinary tract.

Manipulation of ion channel gating with photoswitchable tweezers

The modulation of ion channel activity is of central importance within the nervous system, and an in-depth understanding of how such activity occurs on the molecular level is of prime importance for enhancing our understanding of neuronal systems in physiological and pathological states. The use of light as a stimulus has presented the unique opportunity to study these dynamic processes with exquisite spatiotemporal control. We have developed the photoswitchable tweezers method, an optogenetic pharmacology-based technique which relies on the use of a photoswitchable crosslinker as "tweezers" to manipulate the molecular movements involved in ion channel functionalities. Not only does this allow optical control of ion channel activity, but also investigation into the molecular motions and inter-residue distances implicated in such activity. In this chapter we discuss the principles behind the photoswitchable tweezers method, its strategic design and the key experimental steps involved in this technique, using purinergic P2X2 receptor as a case study system.

In utero and lactational PCB exposure drives anatomic changes in the juvenile mouse bladder

Bladder dysfunction, including incontinence, difficulty emptying the bladder, or urgency to urinate is a pervasive health and quality of life concern. However, risk factors for developing these symptoms are not completely understood, and the influence of exposure to environmental chemicals, especially during development, on the formation and function of the bladder is understudied. Environmental contaminants such as polychlorinated biphenyls (PCBs) are known to pose a risk to the developing brain; however, their influence on the development of peripheral target organs, such as bladder, are unknown. To address this data gap, C57Bl/6J mouse dams were exposed to an environmentally-relevant PCB mixture at 0, 0.1, 1 or 6 mg/kg daily beginning two weeks prior to mating and continuing through gestation and lactation. Bladders were collected from offspring at postnatal days (P) 28-31. PCB concentrations were detected in bladders in a dose-dependent manner. PCB effects on the bladder were sex- and dose-dependent. Overall, PCB effects were observed in male, but not female, bladders. PCBs increased bladder volume and suburothelial βIII-tubulin-positive nerve density compared to vehicle control. A subset of these nerves were sensory peptidergic axons indicated by increased calcitonin gene-related protein (CGRP) positive nerve fibers in mice exposed to the highest PCB dose compared to the lowest PCB dose. PCB-induced increased nerve density was also positively correlated with the number of mast cells in the bladder, suggesting inflammation may be involved. There were no detectable changes in epithelial composition or apoptosis as indicated by expression of cleaved caspase 3, suggesting PCBs do not cause overt toxicity. Bladder volume changes were not accompanied by changes in bladder mass or epithelial thickness, indicating that obstruction was not likely involved. Together, these results are the first to suggest that following developmental exposure, PCBs can distribute to the bladder and alter neuroanatomic development and bladder volume in male mice.

Pathogenesis evidence from human and animal models of detrusor underactivity

Detrusor underactivity (DU) is a common urodynamic diagnosis in patients with lower urinary tract symptoms and large post-voiding residual volume. Animal and human studies showed the possible etiologies of DU include central or peripheral nerve injury, bladder outlet obstruction, chronic ischemia, aging, diabetes mellitus, and sympathetic inhibition of micturition reflex. Evidence from animal and human DU studies with various etiologies revealed highly similar gross and histological characteristics in the bladders, including increased bladder weight, bladder wall thickening, inflammation, collagen deposition, and fibrosis. In electron microscopy, smooth muscle destruction, swollen mitochondria, decreased nerve innervation, caveolae, and umbrella cell fusiform vesicles were noted in the DU bladders. Most animal DU models demonstrate detrusor contractility changes from compensatory to the decompensatory stage, and the change was compatible with human DU observation. The cystometry in the DU animal studies is characterized by impaired contractility, prolong intercontraction interval, and hyposensation, while in vitro bladder muscle strips experiment may exhibit normal detrusor contractility. Decreased bladder blood flow and increased oxidative stress in bladders had been proved in different animal DU models, suggesting they should be important in the DU pathogenesis pathway. Sensory receptors mRNA and protein expression changes in DU bladders had been observed in both animal and human studies, including muscarinic receptors M2, M3, adrenergic receptor β3, purinergic receptor P2X1, P2X3, and transient receptor potential vanilloid (TRPV) 1 and TRPV4. Although some of the sensory receptors changes remain controversial, it might be the target for further pharmacologic treatments.

Update of P2X receptor properties and their pharmacology: IUPHAR Review 30

The known seven mammalian receptor subunits (P2X1–7) form cationic channels gated by ATP. Three subunits compose a receptor channel. Each subunit is a polypeptide consisting of two transmembrane regions (TM1 and TM2), intracellular N- and C-termini, and a bulky extracellular loop. Crystallization allowed the identification of the 3D structure and gating cycle of P2X receptors. The agonist-binding pocket is located at the intersection of two neighbouring subunits. In addition to the mammalian P2X receptors, their primitive ligand-gated counterparts with little structural similarity have also been cloned. Selective agonists for P2X receptor subtypes are not available, but medicinal chemistry supplied a range of subtype-selective antagonists, as well as positive and negative allosteric modulators. Knockout mice and selective antagonists helped to identify pathological functions due to defective P2X receptors, such as male infertility (P2X1), hearing loss (P2X2), pain/cough (P2X3), neuropathic pain (P2X4), inflammatory bone loss (P2X5), and faulty immune reactions (P2X7).

Bladder urothelium converts bacterial lipopolysaccharide information into neural signaling via an ATP-mediated pathway to enhance the micturition reflex for rapid defense

When bacteria enter the bladder lumen, a first-stage active defensive mechanism flushes them out. Although urinary frequency induced by bacterial cystitis is a well-known defensive response against bacteria, the underlying mechanism remains unclear. In this study, using a mouse model of acute bacterial cystitis, we demonstrate that the bladder urothelium senses luminal extracellular bacterial lipopolysaccharide (LPS) through Toll-like receptor 4 and releases the transmitter ATP. Moreover, analysis of purinergic P2X₂ and P2X₃ receptor-deficient mice indicated that ATP signaling plays a pivotal role in the LPS-induced activation of L6–S1 spinal neurons through the bladder afferent pathway, resulting in rapid onset of the enhanced micturition reflex. Thus, we revealed a novel defensive mechanism against bacterial infection via an epithelial-neural interaction that induces urinary frequency prior to bacterial clearance by neutrophils of the innate immune system. Our results indicate an important defense role for the bladder urothelium as a chemical-neural transducer, converting bacterial LPS information into neural signaling via an ATP-mediated pathway, with bladder urothelial cells acting as sensory receptor cells.

Nociceptive signaling mediated by P2X3, P2X4 and P2X7 receptors

Chronic pain is a debilitating condition that often occurs following peripheral tissue inflammation and nerve injury. This pain, especially neuropathic pain, is a significant clinical problem because of the ineffectiveness of clinically available drugs. Since Burnstock proposed new roles of nucleotides as neurotransmitters, the roles of extracellular ATP and P2 receptors (P2Rs) in pain signaling have been extensively studied, and ATP-P2R signaling has subsequently received much attention as it can provide clues toward elucidating the mechanisms underlying chronic pain and serve as a potential therapeutic target. This review summarizes the literature regarding the role of ATP signaling via P2X3Rs (as well as P2X2/3Rs) in primary afferent neurons and via P2X4Rs and P2X7Rs in spinal cord microglia in chronic pain, and discusses their respective therapeutic potentials.

The Urothelium: Life in a Liquid Environment

The urothelium, which lines the renal pelvis, ureters, urinary bladder, and proximal urethra, forms a high-resistance but adaptable barrier that surveils its mechanochemical environment and communicates changes to underlying tissues including afferent nerve fibers and the smooth muscle. The goal of this review is to summarize new insights into urothelial biology and function that have occurred in the past decade. After familiarizing the reader with key aspects of urothelial histology, we describe new insights into urothelial development and regeneration. This is followed by an extended discussion of urothelial barrier function, including information about the roles of the glycocalyx, ion and water transport, tight junctions, and the cellular and tissue shape changes and other adaptations that accompany expansion and contraction of the lower urinary tract. We also explore evidence that the urothelium can alter the water and solute composition of urine during normal physiology and in response to overdistension. We complete the review by providing an overview of our current knowledge about the urothelial environment, discussing the sensor and transducer functions of the urothelium, exploring the role of circadian rhythms in urothelial gene expression, and describing novel research tools that are likely to further advance our understanding of urothelial biology.

Molecular Modeling Applied to the Discovery of New Lead Compounds for P2 Receptors Based on Natural Sources

P2 receptors are a family of transmembrane receptors activated by nucleotides and nucleosides. Two classes have been described in mammals, P2X and P2Y, which are implicated in various diseases. Currently, only P2Y12 has medicines approved for clinical use as antiplatelet agents and natural products have emerged as a source of new drugs with action on P2 receptors due to the diversity of chemical structures. In drug discovery, in silico virtual screening (VS) techniques have become popular because they have numerous advantages, which include the evaluation of thousands of molecules against a target, usually proteins, faster and cheaper than classical high throughput screening (HTS). The number of studies using VS techniques has been growing in recent years and has led to the discovery of new molecules of natural origin with action on different P2X and P2Y receptors. Using different algorithms it is possible to obtain information on absorption, distribution, metabolism, toxicity, as well as predictions on biological activity and the lead-likeness of the selected hits. Selected biomolecules may then be tested by molecular dynamics and, if necessary, rationally designed or modified to improve their interaction for the target. The algorithms of these in silico tools are being improved to permit the precision development of new drugs and, in the future, this process will take the front of drug development against some central nervous system (CNS) disorders. Therefore, this review discusses the methodologies of in silico tools concerning P2 receptors, as well as future perspectives and discoveries, such as the employment of artificial intelligence in drug discovery.

Development of a Mouse Reporter Strain for the Purinergic P2X2 Receptor

The ATP-sensitive P2X₂ ionotropic receptor plays a critical role in a number of signal processes including taste and hearing, carotid body detection of hypoxia, the exercise pressor reflex and sensory transduction of mechanical stimuli in the airways and bladder. Elucidation of the role of P2X₂ has been hindered by the lack of selective tools. In particular, detection of P2X₂ using established pharmacological and biochemical techniques yields dramatically different expression patterns, particularly in the peripheral and central nervous systems. Here, we have developed a knock-in P2X₂-cre mouse, which we crossed with a cre-sensitive tdTomato reporter mouse to determine P2X₂ expression. P2X₂ was found in more than 80% of nodose vagal afferent neurons, but not in jugular vagal afferent neurons. Reporter expression correlated in vagal neurons with sensitivity to α,β methylene ATP (αβmATP). P2X₂ was expressed in 75% of petrosal afferents, but only 12% and 4% of dorsal root ganglia (DRG) and trigeminal afferents, respectively. P2X₂ expression was limited to very few cell types systemically. Together with the central terminals of P2X₂-expressing afferents, reporter expression in the CNS was mainly found in brainstem neurons projecting mossy fibers to the cerebellum, with little expression in the hippocampus or cortex. The structure of peripheral terminals of P2X₂-expressing afferents was demonstrated in the tongue (taste buds), carotid body, trachea and esophagus. P2X₂ was observed in hair cells and support cells in the cochlear, but not in spiral afferent neurons. This mouse strain provides a novel approach to the identification and manipulation of P2X₂-expressing cell types.

P2X3-Receptor Antagonists as Potential Antitussives: Summary of Current Clinical Trials in Chronic Cough

Cough is among the most common complaints for which patients worldwide seek medical attention. In a majority of patients with chronic cough (defined as cough of greater than 8 weeks' duration), successful management results from a thorough evaluation and treatment of underlying causes. In a subgroup of patients, however, cough proves refractory to therapeutic trials aimed at known reversible causes of chronic cough. Such patients are appropriately termed as having refractory chronic cough. At present, safe and effective medications are lacking for this challenging patient population. Currently available therapeutic options are usually ineffective or achieve antitussive effect at the expense of intolerable side effects, typically sedation. Fortunately, the past decade has witnessed great progress in elucidating underlying mechanisms of cough. From that knowledge, aided by the development of validated instruments to measure objective and subjective cough-related end points, numerous antitussive drug development programs have emerged. The most active area of inquiry at present involves antagonists of the purinergic P2X receptors. Indeed, four clinical programs (one in Phase 3 and three in Phase 2) are currently underway investigating antagonists of receptors comprised entirely or partially of the P2X3 subunit as potential antitussive medications. Herein we review the foundation on which P2X receptor antagonists were developed as potential antitussive medications and provide an update on current clinical trials.

P2X3-Containing Receptors as Targets for the Treatment of Chronic Pain

Current therapies for the treatment of chronic pain provide inadequate relief for millions of suffering patients, demonstrating the need for better therapies that will treat pain effectively and improve the quality of patient's lives. Better understanding of the mechanisms that mediate chronic pain is critical for developing drugs with improved clinical outcomes. Adenosine triphosphate (ATP) is a key modulator in nociceptive pathways. Release of ATP from injured tissue or sympathetic efferents has sensitizing effects on sensory neurons in the periphery, and presynaptic vesicular release of ATP from the central terminals can increase glutamate release thereby potentiating downstream central sensitization mechanisms, a condition thought to underlie many chronic pain conditions. The purinergic receptors on sensory nerves primarily responsible for ATP signaling are P2X3 and P2X2/3. Selective knockdown experiments, or inhibition with small molecules, demonstrate P2X3-containing receptors are key targets to modulate nociceptive signals. Preclinical studies have identified that P2X3-containing receptors are critical for sensory transduction for bladder function, and clinical studies have shown promise in treatment for bladder pain and pain associated with osteoarthritis. Further clinical characterization of antagonists to P2X3-containing receptors may lead to improved therapies in the treatment of chronic pain.

Functional atropine sensitive purinergic responses in the healthy rat bladder

While acetylcholine is regarded to be the main directly contractile transmitter substance in the urinary bladder, interactions with other transmitters likely occur. Presently, the interplay between purinergic and cholinergic signalling was investigated to unravel the involvement of the urothelium and efferent neurons in the functionally important purinergically evoked release of acetylcholine in vitro. Functional characterization of receptor subtypes involved in this interplay was also performed. In vitro organ bath experiments with electrical field stimulation (EFS) or administration of agonist were performed in the absence and presence of the neurotoxin tetrodotoxin (TTX; 5 × 10^-7 M) and/or receptor antagonists, in intact and urothelium-denuded full thickness rat bladder strip preparations. Interestingly, functional contractions to ATP (10^-6-10^-3 M) remained unaffected by TTX, but were significantly lowered in the presence of the muscarinic antagonist atropine (10^-6 M). However, in urothelium-denuded strip preparations, this latter phenomenon was not present and the ATP response remained unaltered. To rule out purinergic interference caused by break-down of ATP, experiments were performed in which the stable ATP-analogue αβMeATP (10^-7-10^-5 M) gave rise to functional atropine-sensitive contractions. Furthermore, contractions to ATP were not affected by P2Y6 purinoceptor blockade (by MRS2578; 10^-7, 10^-5 M), nor were relaxatory responses to ATP sensitive to atropine, PPADS (3 × 10^-5 M) or αβMeATP. Lastly, relaxations to ADP (10^-6-10^-3 M) or NECA (10^-8-10^-5 M) were unaltered by the presence of atropine. To conclude, purinergic functional contractile, but not relaxatory, responses are supported by the cholinergic transmitter system in vitro, through non-neuronal mechanisms in the urothelium. Involved purinoceptors are of the P2X-subtype, most likely P2X1 and/or P2X3.

In vivo paracrine effects of ATP-induced urothelial acetylcholine in the rat urinary bladder

Mechanical stretch of the urothelium induces the release of ATP that activates bladder afferent nerves. In the rat urinary bladder, ATP is also a contractile co-transmitter in the parasympathetic innervation. In isolated preparations, ATP evokes a urothelial release of acetylcholine that substantially contributes to ATP-evoked contractile responses. Currently we aimed to further examine the interactions of ATP and acetylcholine in the rat urinary bladder in two in vivo models. In the whole bladder preparation, atropine reduced ATP-evoked responses by about 50% in intact but denervated bladders, while atropine had no effect after denudation of the urothelium. In a split bladder preparation, reflex-evoked responses of the contralateral half were studied by applying stimuli (agonists or stretch) to the ipsilateral half. Topical administration of ATP and methacholine as well as of stretch induced contralateral reflex-evoked contractions. While topical administration of atropine ipsilaterally reduced the ATP- and stretch-induced contralateral contractions by 27 and 39%, respectively, the P2X purinoceptor antagonist PPADS reduced them by 74 and 84%. In contrary, the muscarinic M2-(M4)-selective receptor antagonist methoctramine increased the responses by 38% (ATP) and 75% (stretch). Pirenzepine (M1-selective antagonist) had no effect on the reflex. In vitro, in the absence of the reflex, methoctramine did not affect the ATP-induced responses. It is concluded that urothelial ATP potently induces the micturition reflex and stimulates urothelial release of acetylcholine. Acetylcholine subsequently acts on afferents and on the detrusor muscle. While muscarinic M2 and/or M4 receptors in the sensory innervation exert inhibitory modulation, muscarinic M3 receptors cause excitation.