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

The discovery and development of gefapixant as a novel antitussive therapy

INTRODUCTION

Gefapixant, a P2X 3 receptor antagonist, shows considerable potential in managing refractory or unexplained chronic cough. Clinical trials have consistently demonstrated its efficacy in significantly reducing cough frequency and alleviating associated symptoms. However, its adverse effect profile, particularly taste disturbances such as dysgeusia and hypogeusia, the incidence of which is dose-dependent, poses a significant challenge to patient compliance and overall treatment satisfaction.

AREAS COVERED

The authors review the mechanism of action of gefapixant, the dose-dependent nature of its adverse effects and the findings from various clinical trials, including Phase 1, Phase 2, and Phase 3 studies. The authors also cover its regulatory status, post-marketing data, and its main competitors.

EXPERT OPINION

Gefapixant represents a significant advancement in treating chronic cough. However, balancing efficacy and tolerability is crucial. Lower effective doses and potential combination therapies may mitigate taste disturbances. Patient education and close monitoring during treatment are also important for optimal outcomes. Further research is needed to refine dosing strategies to minimize side effects while maintaining therapeutic efficacy. This research and personalized treatment approaches are key to optimizing gefapixant therapy, ensuring improved management of chronic cough while reducing adverse effects. However, pharmaceutical trials and proposals must be adapted to align with each regulatory body's specific requirements and concerns.

A Second Drug Binding Site in P2X3

Purinergic P2X3 receptors form trimeric cation-gated channels, which are activated by extracellular ATP. P2X3 plays a crucial role in chronic cough and affects over 10% of the population. Despite considerable efforts to develop drugs targeting P2X3, the highly conserved structure within the P2X receptor family presents obstacles for achieving selectivity. Camlipixant, a potent and selective P2X3 antagonist, is currently in phase III clinical trials. However, the mechanisms underlying receptor desensitization, ion permeation, principles governing antagonism, and the structure of P2X3 when bound to camlipixant remain elusive. In this study, we established a stable cell line expressing homotrimeric P2X3 and utilized a peptide scaffold to purify the complex and determine its structure using cryo-electron microscopy (cryo-EM). P2X3 binds to camlipixant at a previously unidentified drug-binding site and functions as an allosteric inhibitor. Structure-activity studies combined with modeling and simulations have shed light on the mechanisms underlying the selective targeting and inhibition of P2X3 by camlipixant, distinguishing it from other members of the P2X receptor family.

Benefit-Risk Profile of P2X3 Receptor Antagonists for Treatment of Chronic Cough: Dose-Response Model-Based Network Meta-Analysis

BACKGROUND

Refractory or unexplained chronic cough disrupts quality of life and burdens health care systems around the world. The P2X3 receptor antagonist gefapixant is approved in many countries for its antitussive effects, but taste disturbances are a common adverse effect. Four newer, more selective P2X3 receptor antagonists have been developed to address this problem.

RESEARCH QUESTION

How does the benefit-risk profile vary across the five available P2X3 receptor antagonists?

STUDY DESIGN AND METHODS

A systematic review and network meta-analysis was conducted to evaluate the efficacy of P2X3 receptor antagonists, including gefapixant, sivopixant, eliapixant, camlipixant, and filapixant. Primary outcomes were a reduction rate in 24-hour cough frequency and incidence of taste disturbance. Dose-response curves and median effective dose (ED50) were calculated. Effect size at ED50 was ranked according to the surface under the cumulative ranking curve. The confidence was evaluated by Confidence In Network Meta-Analysis.

RESULTS

Sixteen randomized controlled trials involving 4,904 participants were analyzed. The gefapixant regimen demonstrated an ED50 of 90.7 mg/d for cough frequency reduction. Gefapixant showed the highest antitussive effectiveness at ED50 (reduction rate in 24-hour cough frequency: median, 28.1%; 95% credible interval [CrI], 21.0%-35.6%; ranked 1 of 5; moderate certainty) but the highest prevalence of taste disturbance (absolute risk difference per 100 patients: median, 38; 95% CrI, 27-51; ranked 5 of 5; high certainty) and the highest prevalence of discontinuation. Camlipixant had a well-balanced profile (reduction rate in 24-hour cough frequency: median, 14.7%; 95% CrI, 5.4%-26.0%; ranked 3 of 5; low certainty; and taste disturbance; absolute risk difference per 100 patients; median, 2; 95% CrI, 1-6; ranked 2 of 5; low certainty). Placebo had a mean of 33.1% reduction in 24-hour cough frequency.

INTERPRETATION

When used at safe doses, gefapixant had a favorable risk-benefit profile compared with the other four agents. Camlipixant showed initial promise, which may be further investigated by phase III trials currently underway.

CLINICAL TRIAL REGISTRATION

University Hospital Medical Information Network Center (UMIN-CTR); No. UMIN000050622; URL: https://center6.umin.ac.jp.

Druggable site near the upper vestibule determines the high affinity and P2X3 homotrimer selectivity of sivopixant/S-600918 and its analogue DDTPA

BACKGROUND AND PURPOSE

The P2X3 receptor, a trimeric ionotropic purinergic receptor, has emerged as a potential therapeutic target for refractory chronic cough (RCC). Nevertheless, gefapixant/AF-219, the only marketed P2X3 receptor antagonist, might lead taste disorders by modulating the human P2X2/3 (hP2X2/3) heterotrimer. Hence, in RCC drug development, compounds exhibiting strong affinity for the hP2X3 homotrimer and a weak affinity for the hP2X2/3 heterotrimer hold promise. An example of such a molecule is sivopixant/S-600918, a clinical Phase II RCC candidate with a reduced incidence of taste disturbance compared to gefapixant. Sivopixant and its analogue, (3-(4-([3-chloro-4-isopropoxyphenyl]amino)-3-(4-methylbenzyl)-2,6-dioxo-3,6-dihydro-1,3,5-triazin-1(2H)-yl)propanoic acid (DDTPA), exhibit both high affinity and high selectivity for hP2X3 homotrimers, compared with hP2X2/3 heterotrimers. The mechanism underlying the druggable site and its high selectivity remains unclear.

EXPERIMENTAL APPROACH

To analyse mechanisms that distinguish this drug candidate from other inhibitors of the P2X3 receptors we used a combination of chimera construction, site covalent occupation, metadynamics, mutagenesis and whole-cell recording.

KEY RESULTS

The high affinity and selectivity of sivopixant/DDTPA for hP2X3 receptors was determined by the tri-symmetric site located close to the upper vestibule. Substitution of only four amino acids inside the upper body domain of hP2X2 with those of hP2X3, enabled the hP2X2/3 heterotrimer to exhibit a similar level of apparent affinity for sivopixant/DDTPA as the hP2X3 homotrimer.

CONCLUSION AND IMPLICATIONS

From the receptor-ligand recognition perspective, we have elucidated the molecular basis of novel RCC clinical candidates' cough-suppressing properties and reduced side effects, offering a promising approach to the discovery of novel drugs that specifically target P2X3 receptors.

Mucus Transpiration as the Basis for Chronic Cough and Cough Hypersensitivity

Chronic cough is characterized by a state of cough hypersensitivity. We analyze the process of transpiration, by which water appears to evaporate from laryngeal and tracheal mucus as from the surface of a leaf, as a potential cause of cough hypersensitivity. In this process, osmotic pressure differences form across mucus, pulling water toward the air, and preventing mucus dehydration. Recent research suggests that these osmotic differences grow on encounter with dry and dirty air, amplifying pressure on upper airway epithelia and initiating a cascade of biophysical events that potentially elevate levels of ATP, promote inflammation and acidity, threaten water condensation, and diminish mucus water permeability. Among consequences of this inflammatory cascade is tendency to cough. Studies of isotonic, hypotonic, and hypertonic aerosols targeted to the upper airways give insights to the nature of mucus transpiration and its relationship to a water layer that forms by condensation in the upper airways on exhalation. They also suggest that, while hypertonic NaCl and mannitol may provoke cough and bronchoconstriction, hypertonic salts with permeating anions and non-permeating cations may relieve these same upper respiratory dysfunctions. Understanding of mucus transpiration and its role in cough hypersensitivity can lead to new treatment modalities for chronic cough and other airway dysfunctions promoted by the breathing of dry and dirty air.

Degranulation of human mast cells: modulation by P2 receptors' agonists

Since the late 1970s, there has been an alarming increase in the incidence of asthma and its morbidity and mortality. Acute obstruction and inflammation of allergic asthmatic airways are frequently caused by inhalation of exogenous substances such as allergens cross-linking IgE receptors expressed on the surface of the human lung mast cells (HLMC). The degree of constriction of human airways produced by identical amounts of inhaled allergens may vary from day to day and even hour to hour. Endogenous factors in the human mast cell (HMC)'s microenvironment during allergen exposure may markedly modulate the degranulation response. An increase in allergic responsiveness may significantly enhance bronchoconstriction and breathlessness. This review focuses on the role that the ubiquitous endogenous purine nucleotide, extracellular adenosine 5'-triphosphate (ATP), which is a component of the damage-associated molecular patterns, plays in mast cells' physiology. ATP activates P2 purinergic cell-surface receptors (P2R) to trigger signaling cascades resulting in heightened inflammatory responses. ATP is the most potent enhancer of IgE-mediated HLMC degranulation described to date. Current knowledge of ATP as it relates to targeted receptor(s) on HMC along with most recent studies exploring HMC post-receptor activation pathways are discussed. In addition, the reviewed studies may explain why brief, minimal exposures to allergens (e.g., dust, cat, mouse, and grass) can unpredictably lead to intense clinical reactions. Furthermore, potential therapeutic approaches targeting ATP-related enhancement of allergic reactions are presented.

Characteristics and Management of Patients with Refractory or Unexplained Chronic Cough in Outpatient Hospital Clinics in Spain: A Retrospective Multicenter Study

PURPOSE

Chronic cough (cough that persists for ≥ 8 weeks) can cause a range of physical symptoms and psychosocial effects that significantly impair patients' quality of life. Refractory chronic cough (RCC) and unexplained chronic cough (UCC) are challenging to diagnose and manage, with substantial economic implications for healthcare systems.

METHODS

This retrospective multicenter non-interventional study aimed to characterize the profile and health resource consumption of patients with RCC or UCC who attended outpatient clinics at Spanish hospitals. Data were collected from medical records of patients with RCC or UCC for up to 3 years before study inclusion.

RESULTS

The patient cohort (n = 196) was representative of the chronic cough population (77.6% female, mean age 58.5 years). Two-thirds of patients (n = 126) had RCC. The most frequently visited doctors were pulmonologists (93.4% of patients) and primary care physicians (78.6%), with a mean of 5 visits per patient over three years' observation. The most common diagnostic tests were chest x-ray (83.7%) and spirometry with bronchodilation (77.0%). The most commonly prescribed treatments were proton pump inhibitors (79.6%) and respiratory medications (87.8%). Antibiotics were prescribed empirically to 56 (28.6%) patients. Differences between RCC or UCC groups related mainly to approaches used to manage cough-associated conditions (gastroesophageal reflux disease, asthma) in patients with RCC.

CONCLUSION

RCC and UCC are responsible for high health resource utilization in Spanish hospitals. Specific treatments targeting the pathological processes driving chronic cough may provide opportunities to reduce the associated burden for patients and healthcare systems.

Mouth breathing, dry air, and low water permeation promote inflammation, and activate neural pathways, by osmotic stresses acting on airway lining mucus

Respiratory disease and breathing abnormalities worsen with dehydration of the upper airways. We find that humidification of inhaled air occurs by evaporation of water over mucus lining the upper airways in such a way as to deliver an osmotic force on mucus, displacing it towards the epithelium. This displacement thins the periciliary layer of water beneath mucus while thickening topical water that is partially condensed from humid air on exhalation. With the rapid mouth breathing of dry air, this condensation layer, not previously reported while common to transpiring hydrogels in nature, can deliver an osmotic compressive force of up to around 100 cm H2O on underlying cilia, promoting adenosine triphosphate secretion and activating neural pathways. We derive expressions for the evolution of the thickness of the condensation layer, and its impact on cough frequency, inflammatory marker secretion, cilia beat frequency and respiratory droplet generation. We compare our predictions with human clinical data from multiple published sources and highlight the damaging impact of mouth breathing, dry, dirty air and high minute volume on upper airway function. We predict the hypertonic (or hypotonic) saline mass required to reduce (or amplify) dysfunction by restoration (or deterioration) of the structure of ciliated and condensation water layers in the upper airways and compare these predictions with published human clinical data. Preserving water balance in the upper airways appears critical in light of contemporary respiratory health challenges posed by the breathing of dirty and dry air.