This study identifies the TRPA1 receptor as a promiscuous receptor, activated by a wide range of stimuli, making it a perfect target for triggering cough and as such one of the most promising targets currently identified for the development of antitussive drugs.

BackgroundCough is the most frequent reason for consultation with a family doctor, or with a general or respiratory physician. Treatment options are limited and a recent meta-analysis concluded that over-the-counter remedies are ineffective and there is increasing concern about their use in children. Endogenous inflammatory mediators such as prostaglandin E2 (PGE2) and bradykinin (BK), which are often elevated in respiratory disease states, are also known to cause cough by stimulating airway sensory nerves. However, how this occurs is not understood.MethodsWe hypothesised that the transient receptor potential (TRP) channels, TRPA1 and TRPV1, may have a role as ‘common effectors’ of tussive responses to these agents. We have employed a range of in vitro imaging and isolated tissue assays in human, murine and guinea pig tissue and an in vivo cough model to support this hypothesis.ResultsUsing calcium imaging we demonstrated that PGE2 and BK activated isolated guinea pig sensory ganglia and evoked depolarisation (activation) of vagal sensory nerves, which was inhibited by TRPA1 and TRPV1 blockers (JNJ17203212 and HC-030031). These data were confirmed in vagal sensory nerves from TRPA1 and TRPV1 gene deleted mice. TRPV1 and TRPA1 blockers partially inhibited the tussive response to PGE2 and BK with a complete inhibition obtained in the presence of both antagonists together in a guinea pig conscious cough model.ConclusionThis study identifies TRPA1 and TRPV1 channels as key regulators of tussive responses elicited by endogenous and exogenous agents, making them the most promising targets currently identified in the development of anti-tussive drugs.

BackgroundAsthma and chronic obstructive pulmonary disease are airway inflammatory diseases characterised by airflow obstruction. Currently approved bronchodilators such as long-acting β2 adrenoceptor agonists are the mainstay treatments but often fail to relieve symptoms of chronic obstructive pulmonary disease and severe asthma and safety concerns have been raised over long-term use. The aim of the study was to identify the receptor involved in prostaglandin E2 (PGE2)-induced relaxation in guinea pig, murine, monkey, rat and human airways in vitro.MethodsUsing an extensive range of pharmacological tools, the relaxant potential of PGE2 and selective agonists for the EP1–4 receptors in the presence and absence of selective antagonists in guinea pig, murine, monkey, rat and human isolated airways was investigated.ResultsIn agreement with previous studies, it was found that the EP2 receptor mediates PGE2-induced relaxation of guinea pig, murine and monkey trachea and that the EP4 receptor mediates PGE2-induced relaxation of the rat trachea. These data have been confirmed in murine airways from EP2 receptor-deficient mice (Ptger2). In contrast to previous publications, a role for the EP4 receptor in relaxant responses in human airways in vitro was found. Relaxant activity of AH13205 (EP2 agonist) was also demonstrated in guinea pig but not human airway tissue, which may explain its failure in clinical studies.ConclusionIdentification of the receptor mediating PGE2-induced relaxation represents a key step in developing a novel bronchodilator therapy. These data explain the lack of bronchodilator activity observed with selective EP2 receptor agonists in clinical studies.

BackgroundSensory nerves innervating the airways play an important role in regulating various cardiopulmonary functions, maintaining homeostasis under healthy conditions and contributing to pathophysiology in disease states. Hypo-osmotic solutions elicit sensory reflexes, including cough, and are a potent stimulus for airway narrowing in asthmatic patients, but the mechanisms involved are not known. Transient receptor potential cation channel, subfamily V, member 4 (TRPV4) is widely expressed in the respiratory tract, but its role as a peripheral nociceptor has not been explored.ObjectiveWe hypothesized that TRPV4 is expressed on airway afferents and is a key osmosensor initiating reflex events in the lung.MethodsWe used guinea pig primary cells, tissue bioassay, in vivo electrophysiology, and a guinea pig conscious cough model to investigate a role for TRPV4 in mediating sensory nerve activation in vagal afferents and the possible downstream signaling mechanisms. Human vagus nerve was used to confirm key observations in animal tissues.ResultsHere we show TRPV4-induced activation of guinea pig airway–specific primary nodose ganglion cells. TRPV4 ligands and hypo-osmotic solutions caused depolarization of murine, guinea pig, and human vagus and firing of Aδ-fibers (not C-fibers), which was inhibited by TRPV4 and P2X3 receptor antagonists. Both antagonists blocked TRPV4-induced cough.ConclusionThis study identifies the TRPV4-ATP-P2X3 interaction as a key osmosensing pathway involved in airway sensory nerve reflexes. The absence of TRPV4-ATP–mediated effects on C-fibers indicates a distinct neurobiology for this ion channel and implicates TRPV4 as a novel therapeutic target for neuronal hyperresponsiveness in the airways and symptoms, such as cough.

What is the current scientific knowledge on this subject? Conclusive data regarding the importance of TRPV1 as a treatment target in chronic cough has been lacking due the absence of a safe, potent and efficacious tool compound for use in clinical studies. In a previous study, a TRPV1 antagonist (SB-705498) failed to improve spontaneous cough frequency in treatment resistant chronic cough patients but the reduction in capsaicin-evoked cough was only small. What does this study add to the field?This study rules out TRPV1 as an effective therapeutic target in refractory chronic cough patients. It also highlights the importance of pharmacodynamic pre-clinical and clinical models in the interpretation of negative clinical trial data, but questions the use of cough challenge models in target identification. Word Count 3917 words METHODS:XEN-D0501 and SB-705498 were profiled against capsaicin in a sensory nerve activation assay and in vivo potency established against capsaicin-induced cough in the guinea pig.Twenty patients with refractory chronic cough participated in a double-blind, randomised, placebocontrolled, crossover study evaluating the effect of 14 days XEN-D0501 (oral, 4mg bd) versus placebo on awake cough frequency (primary outcome), capsaicin-evoked cough and patient reported outcomes. MEASUREMENTS AND MAIN RESULTS:XEN-D0501 was more efficacious and 1000-fold more potent than SB-705498 at inhibiting capsaicin-induced depolarization of guinea pig and human isolated vagus. In vivo, XEN-D0501 completely inhibited capsaicin-induced cough whereas 100-times more SB-705498 was required to achieve the same effect. In patients, XEN-D0501 substantially reduced maximal cough responses to capsaicin (mean change from baseline XEN-D0501 -19.3(±16.4) coughs vs. placebo -1.8(±5.8), p<0.0001), but not spontaneous awake cough frequency (mean change from baseline XEN-D0501 6.7c/h(±16.9) vs. placebo 0.4c/h(±13.7), p =0.41).CONCLUSIONS: XEN-D0501 demonstrated superior efficacy and potency in pre-clinical and clinical capsaicin challenge studies; despite this improved pharmacodynamic profile, spontaneous cough