Antitussive effects of the peripherally restricted GABAB receptor agonist lesogaberan in guinea pigs: comparison to baclofen and other GABAB receptor-selective agonists

Gabapentin alleviated the cough hypersensitivity and neurogenic inflammation in a guinea pig model with repeated intra-esophageal acid perfusion

OBJECTIVE

The anti-tussive effect of gabapentin and its underlying neuromodulatory mechanism were investigated via a modified guinea pig model of gastroesophageal reflux-related cough (GERC).

METHODS

Intra-esophageal perfusion with hydrochloric acid (HCl) was performed every other day 12 times to establish the GERC model. High-dose gabapentin (48 mg/kg), low-dose gabapentin (8 mg/kg), or saline was orally administered for 2 weeks after modeling. Cough sensitivity, airway inflammation, lung and esophagus histology, levels of substance P (SP), and neurokinin-1 (NK1)-receptors were monitored.

RESULTS

Repeated intra-esophageal acid perfusion aggravated the cough sensitivity in guinea pigs in a time-dependent manner. The number of cough events was significantly increased after 12 times HCl perfusion, and the hypersensitivity period was maintained for 2 weeks. The SP levels in BALF, trachea, lung, distal esophagus, and vagal ganglia were increased in guinea pigs receiving HCl perfusion. The intensity of cough hypersensitivity in the GERC model was significantly correlated with increased SP expression in the airways. Both high and low doses of gabapentin administration could reduce cough hypersensitivity exposed to HCl perfusion, attenuate airway inflammatory damage, and inhibit neurogenic inflammation by reducing SP expression from the airway and vagal ganglia.

CONCLUSIONS

Gabapentin can desensitize the cough sensitivity in the GERC model of guinea pig. The anti-tussive effect is associated with the alleviated peripheral neurogenic inflammation as reflected in the decreased level of SP.

WAO-ARIA consensus on chronic cough - Part III: Management strategies in primary and cough-specialty care. Updates in COVID-19

Background

Chronic cough management necessitates a clear integrated care pathway approach. Primary care physicians initially encounter the majority of chronic cough patients, yet their role in proper management can prove challenging due to limited access to advanced diagnostic testing. A multidisciplinary approach involving otolaryngologists and chest physicians, allergists, and gastroenterologists, among others, is central to the optimal diagnosis and treatment of conditions which underly or worsen cough. These include infectious and inflammatory, upper and lower airway pathologies, or gastro-esophageal reflux. Despite the wide armamentarium of ancillary testing conducted in cough multidisciplinary care, such management can improve cough but seldom resolves it completely. This can be due partly to the limited data on the role of tests (eg, spirometry, exhaled nitric oxide), as well as classical pharmacotherapy conducted in multidisciplinary specialties for chronic cough. Other important factors include presence of multiple concomitant cough trigger mechanisms and the central neuronal complexity of chronic cough. Subsequent management conducted by cough specialists aims at control of cough refractory to prior interventions and includes cough-specific behavioral counseling and pharmacotherapy with neuromodulators, among others. Preliminary data on the role of neuromodulators in a proof-of-concept manner are encouraging but lack strong evidence on efficacy and safety.

Objectives

The World Allergy Organization (WAO)/Allergic Rhinitis and its Impact on Asthma (ARIA) Joint Committee on Chronic Cough reviewed the recent literature on management of chronic cough in primary, multidisciplinary, and cough-specialty care. Knowledge gaps in diagnostic testing, classical and neuromodulator pharmacotherapy, in addition to behavioral therapy of chronic cough were also analyzed.

Outcomes

This third part of the WAO/ARIA consensus on chronic cough suggests a management algorithm of chronic cough in an integrated care pathway approach. Insights into the inherent limitations of multidisciplinary cough diagnostic testing, efficacy and safety of currently available antitussive pharmacotherapy, or the recently recognized behavioral therapy, can significantly improve the standards of care in patients with chronic cough.

Evidence for Alpha7 Nicotinic Receptor Activation During the Cough Suppressing Effects Induced by Nicotine and Identification of ATA-101 as a Potential Novel Therapy for the Treatment of Chronic Cough

Studies performed in healthy smokers have documented a diminished responsiveness to tussive challenges, and several lines of experimental evidence implicate nicotine as an antitussive component in both cigarette smoke and the vapors generated by electronic cigarettes (eCigs). We set out to identify the nicotinic receptor subtype involved in the antitussive actions of nicotine and to further evaluate the potential of nicotinic receptor-selective agonists as cough-suppressing therapeutics. We confirmed an antitussive effect of nicotine in guinea pigs. We additionally observed that the alpha-4 beta-2 (α ₄ β ₂)-selective agonist Tc-6683 was without effect on evoked cough responses in guinea pigs, while the α ₇-selective agonist PHA 543613 dose-dependently inhibited evoked coughing. We subsequently describe the preclinical evidence in support of ATA-101, a potent and highly selective (α ₇) selective nicotinic receptor agonist, as a potential candidate for antitussive therapy in humans. ATA-101, formerly known as Tc-5619, was orally bioavailable and moderately central nervous system (CNS) penetrant and dose-dependently inhibited coughing in guinea pigs evoked by citric acid and bradykinin. Comparing the effects of airway targeted administration versus systemic dosing and the effects of repeated dosing at various times prior to tussive challenge, our data suggest that the antitussive actions of ATA-101 require continued engagement of α ₇ nicotinic receptors, likely in the CNS. Collectively, the data provide the preclinical rationale for α ₇ nicotinic receptor engagement as a novel therapeutic strategy for cough suppression. The data also suggest that α ₇ nicotinic acetylcholine receptor (nAChR) activation by nicotine may be permissive to nicotine delivery in a way that may promote addiction. SIGNIFICANCE STATEMENT: This study documents the antitussive actions of nicotine and identifies the α₇ nicotinic receptor subtype as the target for nicotine during cough suppression described in humans. We additionally present evidence suggesting that ATA-101 and other α₇ nicotinic receptor-selective agonists may be promising candidates for the treatment of chronic refractory cough.

A Brief History and the Significance of the GABAB Receptor

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. GABA type B (GABA_B) receptors (GABA_BRs) are the only metabotropic G protein-coupled receptors for GABA and can be found distributed not only in the central nervous system, but also in the periphery. This chapter introduces important, fundamental knowledge related to GABA_BR function and the various potential therapeutic applications of the development of novel GABA_BR-active compounds, as documented through extensive studies presented in subsequent chapters of this Current Topic in Behavioral Neurosciences volume on the role of the neurobiology of GABA_BR function. The compounds that have received increased attention in the last few years compared to GABA_BR agonists and antagonists - the positive allosteric modulators - exhibit better pharmacological profiles and fewer side effects. As we continue to unveil the mystery of GABA_BRs at the molecular and cellular levels, we further understand the significance of these receptors. Future directions should aim for developing highly selective GABA_BR compounds for treating neuropsychiatric disorders and their symptomatology.

Effect of centrally and peripherally acting GABAB agonism on the healthy human cough reflex

BACKGROUND

Currently there are no effective licensed anti-tussive therapies. Understanding how the neuronal mechanisms mediating the cough reflex in animal models translate to humans is important for the development of effective therapies. Pre-clinical studies suggest that the activation of GABA_B receptors in both the peripheral and central nervous systems inhibit cough.

OBJECTIVE

To compare the effect of central and peripherally acting GABA_B agonists (lesogaberan and baclofen) on the cough reflex in healthy volunteers.

METHODS

We performed a single center, double-blind, double-dummy, three-way crossover trial in healthy controls comparing single doses of lesogaberan (120 mg MR), with baclofen (40 mg) and placebos. Cough responses to inhaled capsaicin were assessed at screening and 2h post-dose on each study day. The primary endpoint was the maximum number of coughs evoked at any concentration of capsaicin (Emax) and the secondary endpoint was the concentration evoking 50% of the maximal response (ED50).

RESULTS

Fifteen participants enrolled onto the study (median age 29 (IQR 25-44) years; 7 females, mean BMI 24.6(±3.0). Lesogaberan treatment produced a small, statistically significant increase in Emax compared with placebo [mean 13.4coughs (95%CI 10.1-17.9) vs. 11.8coughs (8.8-15.9), p = 0.04], but had no effect on ED50 [geometric mean 47.4 μM (95%CI 24.4-91.7) vs 37.6 μM (95%CI 19.2-73.5), p = 0.37]. In contrast, baclofen had no significant effect on Emax (11.1, 95%CI 8.1-15.4) (p = 0.23), but significantly increased ED50 compared with placebo (geometric mean 75.2 μM (95%CI 37.2-151.8), p = 0.002).

CONCLUSION

This data suggests the anti-tussive actions of GABA_B agonists, in healthy volunteers, occur in the central rather than the peripheral nervous system.

Intra-Arterial, but Not Intrathecal, Baclofen and Codeine Attenuates Cough in the Cat

Centrally-acting antitussive drugs are thought to act solely in the brainstem. However, the role of the spinal cord in the mechanism of action of these drugs is unknown. The purpose of this study was to determine if antitussive drugs act in the spinal cord to reduce the magnitude of tracheobronchial (TB) cough-related expiratory activity. Experiments were conducted in anesthetized, spontaneously breathing cats (n = 22). Electromyograms (EMG) were recorded from the parasternal (PS) and transversus abdominis (TA) or rectus abdominis muscles. Mechanical stimulation of the trachea or larynx was used to elicit TB cough. Baclofen (10 and 100 μg/kg, GABA-B receptor agonist) or codeine (30 μg/kg, opioid receptor agonist) was administered into the intrathecal (i.t.) space and also into brainstem circulation via the vertebral artery. Cumulative doses of i.t. baclofen or codeine had no effect on PS, abdominal muscle EMGs or cough number during the TB cough. Subsequent intra-arterial (i.a.) administration of baclofen or codeine significantly reduced magnitude of abdominal and PS muscles during TB cough. Furthermore, TB cough number was significantly suppressed by i.a. baclofen. The influence of these drugs on other behaviors that activate abdominal motor pathways was also assessed. The abdominal EMG response to noxious pinch of the tail was suppressed by i.t. baclofen, suggesting that the doses of baclofen that were employed were sufficient to affect spinal pathways. However, the abdominal EMG response to expiratory threshold loading was unaffected by i.t. administration of either baclofen or codeine. These results indicate that neither baclofen nor codeine suppress cough via a spinal action and support the concept that the antitussive effect of these drugs is restricted to the brainstem.

Class C GPCRs in the airway

Understanding and targeting of GPCRs remain a critical aspect of airway pharmacology and therapeutics for diseases such as asthma or COPD. Most attention has been on the large Class A GPCRs towards improved bronchodilation and blunting of remodeling. Better known in the central or peripheral nervous system, there is increasing evidence that Class C GPCRs which include metabotropic glutamate and GABA receptors, the calcium sensing receptor, sweet/umami taste receptors and a number of orphan receptors, can contribute to airway structure and function. In this review, we will summarize current state of knowledge regarding the pharmacology of Class C GPCRs, their expression and potential functions in the airways, and the application of pharmacological agents targeting this group in the context of airway diseases.

Transient receptor potential (TRP) channels as molecular targets in lung toxicology and associated diseases

The lungs as the gateways of our body to the external environment are essential for gas exchange. They are also exposed to toxicants from two sides, the airways and the vasculature. Apart from naturally produced toxic agents, millions of human made chemicals were produced since the beginning of the industrial revolution whose toxicity still needs to be determined. While the knowledge about toxic substances is increasing only slowly, a paradigm shift regarding the proposed mechanisms of toxicity at the plasma membrane emerged. According to their broad-range chemical reactivity, the mechanism of lung injury evoked by these agents has long been described as rather unspecific. Consequently, therapeutic options are still restricted to symptomatic treatment. The identification of molecular down-stream effectors in cells was a major step forward in the mechanistic understanding of the action of toxic chemicals and will pave the way for more causal and specific toxicity testing as well as therapeutic options. In this context, the involvement of Transient Receptor Potential (TRP) channels as chemosensors involved in the detection and effectors of toxicant action is an attractive concept intensively discussed in the scientific community. In this review we will summarize recent evidence for an involvement of TRP channels (TRPA1, TRPC4, TRPC6, TRPV1, TRPV4, TRPM2 and TRPM8) expressed in the lung in pathways of toxin sensing and as mediators of lung inflammation and associated diseases like asthma, COPD, lung fibrosis and edema formation. Specific modulators of these channels may offer new therapeutic options in the future and will endorse strategies for a causal, specifically tailored treatment based on the mechanistic understanding of molecular events induced by lung-toxic agents.

Viral laryngitis: a mimic and a monster - range, presentation, management

PURPOSE OF REVIEW

The purpose of this review is to highlight recent literature relating to the diagnosis and treatment of some less common forms of viral laryngitis. The main conditions addressed in this review are chronic cough or postviral vagal neuropathy, varicella zoster infection of the larynx, and a condition increasingly suspected as being virally induced, idiopathic ulcerative laryngitis.

RECENT FINDINGS

Diagnosis of these conditions requires a thorough history and physical exam, and in certain cases referral to other subspecialties such as gastroenterology and pulmonology. Chronic cough due to postviral vagal neuropathy is a diagnosis of exclusion; however, recent literature does suggest that certain studies such as laryngeal electromyography can be of use in reaching a diagnosis. Treatment of this neuropathy has focused on use of neuromodulators. Treatment of laryngeal shingles and idiopathic ulcerative laryngitis has not been well defined because of the rarity of these conditions.

SUMMARY

Recent studies regarding these conditions and potential future treatment options will be discussed.

Chronic cough: a gastroenterology perspective

PURPOSE OF REVIEW

The purpose of this review is to highlight recent work and provide recommendations on the approach for diagnosis and management of chronic cough in a gastroenterology clinic.

RECENT FINDINGS

Chronic cough is a burdensome symptom affecting a large number of patients and contributes significant cost to the healthcare system. Recent work has shown that select patients may benefit from acid-suppressive therapy and even surgery when there is true pathologic evidence of reflux disease with cough. However, judicious use and proper interpretation of diagnostic testing for gastroesophageal reflux in the setting of cough is important to avoid unnecessary or inappropriate therapy.

SUMMARY

Chronic cough remains a vexing problem for many physicians, including gastroenterologists. It is important that physicians approach refractory cough in a multidisciplinary manner. Future research is needed to better understand the likely central hypersensitivity response mediating reflux-related cough and potential alternative approaches to therapy.

Antitussive drugs--past, present, and future

Cough remains a serious unmet clinical problem, both as a symptom of a range of other conditions such as asthma, chronic obstructive pulmonary disease, gastroesophageal reflux, and as a problem in its own right in patients with chronic cough of unknown origin. This article reviews our current understanding of the pathogenesis of cough and the hypertussive state characterizing a number of diseases as well as reviewing the evidence for the different classes of antitussive drug currently in clinical use. For completeness, the review also discusses a number of major drug classes often clinically used to treat cough but that are not generally classified as antitussive drugs. We also reviewed a number of drug classes in various stages of development as antitussive drugs. Perhaps surprising for drugs used to treat such a common symptom, there is a paucity of well-controlled clinical studies documenting evidence for the use of many of the drug classes in use today, particularly those available over the counter. Nonetheless, there has been a considerable increase in our understanding of the cough reflex over the last decade that has led to a number of promising new targets for antitussive drugs being identified and thus giving some hope of new drugs being available in the not too distant future for the treatment of this often debilitating symptom.

The oesophagus and cough: laryngo-pharyngeal reflux, microaspiration and vagal reflexes

Gastro-oesophageal reflux disease is generally considered to be one of the commonest causes of chronic cough, however randomised controlled trials of proton pump inhibitors have often failed to support this notion. This article reviews the most recent studies investigating the mechanisms thought to link reflux and cough, namely laryngo-pharyngeal reflux, micro-aspiration and neuronal cross-organ sensitisation. How recent evidence might shed light on the failure of acid suppressing therapies and suggest new approaches to treating reflux related cough are also discussed.

Investigation of extraesophageal gastroesophageal reflux disease

The most common extraesophageal manifestations of gastroesophageal reflux disease (GERD) include chronic cough, asthma and laryngitis. There are two mechanisms proposed to explain extraesophageal syndromes caused by GERD. The first one is a direct way via irritation and/or microaspiration and the second one is an indirect, vagally mediated way. The investigation of extraesophageal manifestations of GERD is difficult and the empirical therapy with proton pump inhibitors usually double dose for at least three months is still the most common approach.