Effective antitussives for the cough patient: an unmet need

Acute cough in children and adolescents: A systematic review and a practical algorithm by the Italian Society of Pediatric Allergy and Immunology

The current systematic review presented and discussed the most recent studies on acute cough in pediatric age. After that, the Italian Society of Pediatric Allergy and Immunology elaborated a comprehensive algorithm to guide the primary care approach to pediatric patients, such as infants, children, and adolescents, with acute cough. An acute cough is usually consequent to upper respiratory tract infections and is self-resolving within a few weeks. However, an acute cough may be bothersome, and therefore remedies are requested, mainly by the parents. An acute cough may significantly affect the quality of life of patients and their family.Several algorithms for the management of acute cough have been adopted and validated in clinical practice; however, unlike the latter, we developed an algorithm focused on pediatric age, and, also, in accordance to the Italian National Health System, which regularly follows the child from birth to all lifelong. Based on our findings, infants from 6 months, children, and adolescents with acute cough without cough pointers can be safely managed using well-known medications, preferably non-sedative agents, such as levodropropizine and/or natural compounds, including honey, glycerol, and herb-derived components.

The efficacy of specific neuromodulators on human refractory chronic cough: a systematic review and meta-analysis

BACKGROUND

There have been several published reports on the use of orally administered, specific centrally acting medicines for the treatment of idiopathic cough; however, there is no extant systematic review of randomized controlled trials (RCTs) that evaluated their efficacy and safety for the treatment of idiopathic cough in human beings.

METHODS

We conducted a series of definitive systematic reviews and meta-analyses of RCTs. Claims data from the MEDLINE, EMBASE, LILACS, CBM, CNKI, VIP, Wan Fang, and Cochrane Library databases were used. We also reviewed articles and reference lists of relevant articles pertaining to human subjects published prior to March 26, 2016. No language restrictions were imposed. Two authors independently reviewed the titles and abstracts of the retrieved studies, which were matched using Review Manager 5.3 software. Disagreements were resolved by consensus. The outcome data were the number of subjects whose symptoms declined, measured by cough or Leicester Cough Questionnaire (LCQ) score. Random effect meta-analyses were used to pool the findings. Publication bias was assessed using funnel plots.

RESULTS

Three RCTs, regarding the medicines baclofen, amitriptyline, and gabapentin, were conducted involving 92 persons in total. Our reviews confirmed that baclofen, amitriptyline, and gabapentin show promise in the treatment of cough for select cases of refractory chronic cough. After-treatment relief of cough symptoms was significant (risk ratio =2.41; 95% CI: 1.15-5.04, n=84). Each of the medicines was well tolerated with minimal side effects. Methodological biases in the design and execution of cluster randomized trials might contribute to any selection bias in this review.

CONCLUSIONS

Baclofen, amitriptyline, and gabapentin may be effective 'non-specific' antitussives in clinical settings, although none of them are used in medical assessments or routinely included in the anatomic diagnostic protocol.

Interventions for cough in cancer

BACKGROUND

This is an updated version of the original Cochrane review first published in Issue 9, 2010 on "Interventions for cough in cancer". Cough is a common symptom in patients with malignancies, especially in patients with lung cancer. Cough is not well controlled in clinical practice and clinicians have few management options to treat it.

OBJECTIVES

The primary objective was to determine the effectiveness of interventions, both pharmacological and non-pharmacological, (other than chemotherapy and external beam radiotherapy) in the management of cough in malignant disease (especially in lung cancer).

SEARCH METHODS

For this update, we searched for relevant studies in CENTRAL and DARE (The Cochrane Library); MEDLINE; EMBASE; PsycINFO; AMED and CINAHL to 9 June 2014. In addition, we searched for ongoing trials via the metaRegister of controlled trials (mRCT), ClinicalTrials.gov, the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP) and the UK Clinical Research Network Study Portfolio.

SELECTION CRITERIA

We selected randomised controlled trials (RCTs) and clinical trials (quasi-experimental trials and trials where there is a comparison group but no mention of randomisation) in participants with primary or metastatic lung cancer or other cancers.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed the titles and abstracts of all studies for inclusion, and extracted data from all included studies independently before reaching consensus. A third review author arbitrated on any disagreement. Meta-analysis was not attempted due to the heterogeneity of the studies.

MAIN RESULTS

For the original version of the review, 17 studies met the inclusion criteria and examined either brachytherapy, laser or photodynamic therapy (eight studies) or a variety of pharmacological therapies (nine studies). Overall, there was an absence of credible evidence and the majority of studies were of low methodological quality and at high risk of bias. Brachytherapy in a variety of doses seemed to improve cough in selected participants, suggesting that possibly the lowest effective dose should be used to minimise side effects. Photodynamic therapy was examined in one study and, while improvements in cough were observed, its role in relationship to other therapies for cough was unclear. Some indication of positive effect was observed with morphine, codeine, dihydrocodeine, levodropropizine, sodium cromoglycate and butamirate citrate linctus (cough syrup), although all studies had significant risk of bias. For this update, we did not identify any additional trials for inclusion. Two ongoing trials were identified but no study results were available.

AUTHORS' CONCLUSIONS

No new trials were included since the publication of the original version of this review, while 11 new studies that were identified were eventually excluded from this review. Therefore, our conclusions remain unchanged. No practice recommendations could be drawn from this review. There is an urgent need to increase the number and quality of studies evaluating the effects of interventions for the management of cough in cancer.

A cohort description and analysis of the effect of gabapentin on idiopathic cough

BACKGROUND

Chronic idiopathic cough (known as cough hypersensitivity syndrome) is defined by cough in the absence of an identifiable cause. Gabapentin has been suggested as a treatment but evidence is scarce. The aim of our study was to describe the clinical features of patients with unexplained chronic cough and to investigate the effect of gabapentin (600 mg twice a day for a minimal duration of 4 weeks) in reducing cough symptoms.

METHODS

A patient cohort analysis was performed. Patients were retrieved using a query in our medical database for the words 'cough' and 'gabapentin' in 2011. Patients without a clear etiology of cough despite having performed a stepwise diagnostic approach, were included. Medical records of these patients were analyzed. A telephonic survey was performed and patients were asked to retrospectivally rate their cough when they attended the outpatient clinic. They were then asked to rate their cough after treatment with gabapentin. A scale from one to ten was used to score cough severity. They were also questioned about the triggers inducing cough. To evaluate the cough severity score, the results were correlated with questions of the Leicester Cough Questionnaire.

RESULTS

We recruited 51 patients (87% female) with a mean age of onset of 47 years (± 14 y) and an average cough duration of 48 months. The most frequently reported cough triggers included change of temperature (57%), talking (49%) and odours (45%). In 67% of patients, the urge to cough was located in the throat area. Thirty-five patients effectively took the prescribed gabapentin. The average improvement in cough score was 2.8/10 (p<0.0001). Of the 35 patients, 20 achieved improvement of their cough symptoms. Responders had a higher pre-treatment cough severity score (p=0.02) and were more likely to have a history of pre-cough airway infection (p=0.04). Current cough severity score negatively correlated with the Leicester Cough Questionnaire scores (p=0.05).

CONCLUSION

Chronic idiopathic cough were predominantly middle-aged women, frequently reporting various cough triggers. We also demonstrated that gabapentin can significantly improve cough in these patients. Responders tend to have higher pre-treatment severity scores and have a history of an airway infection.

Transient receptor potential genes, smoking, occupational exposures and cough in adults

Background

Transient receptor potential (TRP) vanilloid and ankyrin cation channels are activated by various noxious chemicals and may play an important role in the pathogenesis of cough. The aim was to study the influence of single nucleotide polymorphisms (SNPs) in TRP genes and irritant exposures on cough.

Methods

Nocturnal, usual, and chronic cough, smoking, and job history were obtained by questionnaire in 844 asthmatic and 2046 non-asthmatic adults from the Epidemiological study on the Genetics and Environment of Asthma (EGEA) and the European Community Respiratory Health Survey (ECRHS). Occupational exposures to vapors, gases, dusts, and/or fumes were assessed by a job-exposure matrix. Fifty-eight tagging SNPs in TRPV1, TRPV4, and TRPA1 were tested under an additive model.

Results

Statistically significant associations of 6 TRPV1 SNPs with cough symptoms were found in non-asthmatics after correction for multiple comparisons. Results were consistent across the eight countries examined. Haplotype-based association analysis confirmed the single SNP analyses for nocturnal cough (7-SNP haplotype: p-global = 4.8 × 10^-6) and usual cough (9-SNP haplotype: p-global = 4.5 × 10^-6). Cough symptoms were associated with exposure to irritants such as cigarette smoke and occupational exposures (p < 0.05). Four polymorphisms in TRPV1 further increased the risk of cough symptoms from irritant exposures in asthmatics and non-asthmatics (interaction p < 0.05).

Conclusions

TRPV1 SNPs were associated with cough among subjects without asthma from two independent studies in eight European countries. TRPV1 SNPs may enhance susceptibility to cough in current smokers and in subjects with a history of workplace exposures.

An observational study on cough in children: epidemiology, impact on quality of sleep and treatment outcome

Background

Cough is one of the most frequent symptoms in children and is the most common symptom for which children visit a health care provider.

Methods

This is an observational study on acute cough associated with upper respiratory tract infection (URTI) in children. The study evaluates the epidemiology and impact of cough on quality of sleep and children's activities, and the outcome of cough with antitussive treatments in pediatric routine clinical practice. Study assessments were performed through a pediatric cough questionnaire (PCQ), developed by the Italian Society of Cough Study. A total of 433 children visited by family care pediatricians for acute cough due to a URTI were enrolled in this study, with mean age of 6.1 years (SD 3.6). Cough type, duration, severity and frequency, cough impact on sleep disturbances of children and parents and on school and sport activities were assessed at baseline. In a subset of 241 children who were either treated with antitussive drugs (levodropropizine n = 101, central antitussives n = 60) or received no treatment (n = 80), the outcome of cough after 6 days was analyzed in terms of resolution, improvement, no change, or worsening. Descriptive analysis, χ² test, and multivariate analysis with stepwise logistic regression were performed.

Results

Cough disturbed sleep in 88% of children and 72% of parents. In children treated with cough suppressants, the duration, type, intensity, and frequency cough were similar at baseline in the two groups respectively treated with levodropropizine and central antitussives (cloperastine and codeine). Both levodropropizine and central drugs reduced cough intensity and frequency. However, percentage of cough resolution was higher with levodropropizine than with central antitussives (47% vs. 28% respectively, p = 0.0012).

Conclusions

Acute cough disturbs sleep in most children and their parents. Both levodropropizine and central antitussives reduced cough intensity, with levodropropizine producing a higher cough resolution rate.

Cough management: a practical approach

Cough is one of the most common symptoms for which patients seek medical attention from primary care physicians and pulmonologists. Cough is an important defensive reflex that enhances the clearance of secretions and particles from the airways and protects the lower airways from the aspiration of foreign materials. Therapeutic suppression of cough may be either disease-specific or symptom related. The potential benefits of an early treatment of cough could include the prevention of the vicious cycle of cough. There has been a long tradition in acute cough, which is frequently due to upper respiratory tract infections, to use symptom-related anti-tussives. Suppression of cough (during chronic cough) may be achieved by disease-specific therapies, but in many patients it is often necessary to use symptomatic anti-tussives, too. According to the current guidelines of the American College of Chest Physician on "Cough Suppressants and Pharmacologic Protussive Therapy" and additional clinical trials on the most frequent anti-tussive drugs, it should be possible to diagnose and treat cough successfully in a majority of cases. Among drugs used for the symptomatic treatment of cough, peripherally acting anti-tussives such as levodropropizine and moguisteine show the highest level of benefit and should be recommended especially in children. By improving our understanding of the specific effects of these anti-tussive agents, the therapeutic use of these drugs may be refined. The present review provides a summary of the most clinically relevant anti-tussive drugs in addition to their potential mechanism of action.

A novel approach to studying the relationship between subjective and objective measures of cough

BACKGROUND

Currently, no data are available on the relationship between changes in objective and subjective measures of cough, the magnitude of change in cough frequency perceived by patients as clinically meaningful, or the sample sizes required to show significant changes in cough therapeutic trials. Because patients anecdotally report reductions in cough severity while undergoing esophageal testing, we aimed to address these issues by assessing objective and subjective measures of cough with and without an esophageal catheter.

METHODS

Twenty-four-hour cough monitoring was performed on two occasions, with and without esophageal impedance/pH monitoring in 62 patients with chronic cough (mean age 56.8 years [SD±10.8]; 43 women; median cough duration 3.3 years [interquartile range (IQR), 2.0-10.0]). Cough was assessed objectively measuring coughs per hour and subjectively using a numerical cough score and a visual analog scale (VAS), scored separately for day and night, and then averaged to represent each 24-h period.

RESULTS

Objective cough frequency was reduced by a median of 33.3% (IQR, -68.8% to -13.0%; P<.001) with the catheter. The averaged day and night cough scores and VAS scores also significantly decreased, but changes in these did not correlate with decreases in cough frequency. Sample-size calculations suggested that crossover designs using objective cough frequency may be preferable in therapeutic trials.

CONCLUSIONS

These observations provide useful information on the reduction in objective cough frequency scored as an improvement by patients with chronic cough and offer guidance for the design and powering of future therapeutic trials.

TRIAL REGISTRY

ISRCTN; No.: ISRCTN62337037; URL: http://www.controlled-trials.com.

Interventions for cough in cancer

BACKGROUND

Cough is a common symptom in patients with malignancies, especially in patients with lung cancer. Cough is not well controlled in clinical practice and clinicians have few management options to treat it.

OBJECTIVES

The primary objective of this review was to determine the effectiveness of interventions, both pharmacological and non-pharmacological, (other than chemotherapy and external beam radiotherapy) in the management of cough in malignant disease (especially in lung cancer).

SEARCH STRATEGY

Databases searched included: The Cochrane Central Register of Controlled Trials (CENTRAL) and the Database of Abstracts of Reviews of Effectiveness (DARE) (The Cochrane Library issue 4, 2009); MEDLINE (1966 to May 2010); EMBASE (1980 to May 2010); CINAHL (1980 to May 2010); PSYCHINFO (1980 to May 2010); AMED (1985 to May 2010); SIGLE (1980 to May 2010); British Nursing Index (1985 to May 2010); CancerLit (1975 to May 2010). We searched for cough suppressants, antitussives and other drugs with antitussive activity as well as non-pharmacological interventions (see Appendices 1-4 for search terms).

SELECTION CRITERIA

We selected randomised controlled trials (RCTs) and clinical trials (quasi-experimental trials, and trials where there is a comparison group but no mention of randomisation) in participants with primary or metastatic lung cancer or other cancers.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed titles and abstracts of all studies, and extracted data from all selected studies before reaching consensus. A third review author arbitrated with any disagreement. Meta-analysis was not attempted due to the heterogeneity of studies.

MAIN RESULTS

Seventeen studies met inclusion criteria and examined either brachytherapy, laser or photodynamic therapy (eight studies) or a variety of pharmacological therapies (nine studies). Overall, there was absence of credible evidence and the majority of studies were of low methodological quality and high risk of bias. Brachytherapy seemed to improve cough in a variety of doses in selected participants, suggesting that possibly the lowest effective dose should be used to minimise side effects. Photodynamic therapy was examined in one study, and while improvements in cough were observed, its role over other therapies for cough is unclear. Some indication of effect was observed with morphine, codeine, dihydrocodeine, levodropropizine, sodium cromoglycate and butamirate citrate linctus (cough syrup), although all studies had significant risk of bias.

AUTHORS' CONCLUSIONS

No practice recommendations could be drawn from this review. There is an urgent need to increase the number and quality of studies evaluating the effects of interventions in the management of cough in cancer.

Analysis of the evidence of efficacy and safety of over-the-counter cough medications registered in Brazil

The objective of this study was to analyze the level of evidence regarding the efficacy, effectiveness and safety of over-the-counter (OTC) cough medications registered in Brazil. The National Health Surveillance Agency database was used to identify the drugs. Clinical trials, systematic reviews, meta-analyses, and studies on safety were searched on the Medline baseline, the Cochrane Library and SIETES (System of Essential Information in Therapeutics and Health; database in Spanish). Most drugs (62.5%) were sold as a fixed-dose combination of two or more drugs. Randomized clinical trials were found for only three drugs: bromhexine, dextromethorphan and guaifenesin. No clinical trials were found for fixed-dose combinations. Systematic reviews on Cochrane did not report any evidence in favor of or against the effectiveness of cough drugs. Efficacy is also unclear, especially regarding fixed-dose combinations. The evidence for the efficacy of OTC cough medications available in Brazil is poor due to the lack of quality studies. Pharmacovigilance of OTC cough medications should be encouraged.

Pharmacologic management of cough

This review is an update of recent advances in our understanding of cough suppressants and impairment of cough. Low-dose oral morphine has recently been shown to significantly suppress chronic cough, but the side effect profile of this opioid may limit its widespread utility. Several studies have demonstrated a dissociation between the efficacy of antitussives in some metrics of pathologic cough and their effects on cough sensitivity to inhaled irritants. The relevance of widely used inhaled irritants in understanding pathologic cough and its response to antitussives is questionable. A recent advance in the field is the identification and measurement of an index of sensation related to cough: the urge to cough. This measure highlights the potential involvement of suprapontine regions of the brain in the genesis and potential suppression of cough in the awake human. There are no new studies showing that mucolytic agents are of value as monotherapies for chronic cough. However, some of these drugs, presumably because of their antioxidant activity, may be of use as adjunct therapies or in selected patient populations. The term dystussia (impairment of cough) has been coined recently and represents a common and life-threatening problem in patients with neurologic disease. Dystussia is strongly associated with severe dysphagia and the occurrence of both indicates that the patient has a high risk for aspiration. No pharmacologic treatments ae available for dystussia, but scientists and clinicians with experience in studying chronic cough are well qualified to develop methodologies to address the problem of impaired cough.

Addictive evaluation of cholic acid-verticinone ester, a potential cough therapeutic agent with agonist action of opioid receptor

AIM

The purpose of this work was to search for potential drugs with potent antitussive and expectorant activities as well as a low toxicity, but without addictive properties. Cholic acid-verticinone ester (CA-Ver) was synthesized based on the clearly elucidated antitussive and expectorant activities of verticinone in bulbs of Fritillaria and different bile acids in Snake Bile. In our previous study, CA-Ver showed a much more potent activity than codeine phosphate. This study was carried out to investigate the central antitussive mechanism and the addictive evaluation of CA-Ver.

METHODS

Testing on a capsaicin-induced cough model of mice pretreated with naloxone, a non-selective opioid receptor antagonist, was performed for the observation of CA-Ver's central antitussive mechanism. We then took naloxone-induced withdrawal tests of mice for the judgment of CA-Ver's addiction. Lastly, we determined the opioid dependence of CA-Ver in the guinea pig ileum.

RESULTS

The test on the capsaicin-induced cough model showed that naloxone could block the antitussive effect of CA-Ver, suggesting the antitussive mechanism of CA-Ver was related to the central opioid receptors. The naloxone-urged withdrawal tests of the mice showed that CA-Ver was not addictive, and the test of the opioid dependence in the guinea pig ileum showed that CA-Ver had no withdrawal response.

CONCLUSION

These findings suggested that CA-Ver deserved attention for its potent antitussive effects related to the central opioid receptors, but without addiction, and had a good development perspective.

Cough sensors. III. Opioid and cannabinoid receptors on vagal sensory nerves

Cough is a persistent symptom of many inflammatory airways' diseases. Cough is mediated by receptors sited on sensory nerves and then through vagal afferent pathways, which terminate in the brainstem respiratory centre. Cough is often described as an unmet clinical need. Opioids are the only prescription-based antitussives currently available in the UK. They possess limited efficacy and exhibit serious unwanted side effects, such as physical dependence, sedation, respiratory depression and gastrointestinal symptoms. There are three classical opioid receptors: the mu, kappa and delta receptors. Peripheral opioid receptors are sited on sensory nerves innervating the airways. A greater understanding of the role of the peripheral and centrally sited opioid receptors is necessary to allow the development of targeted treatments for cough. Because of the limited efficacy and the side-effect profile of the opioids, potential new treatments are sought to alleviate cough. One class of compounds that is currently under examination is the cannabinoids. Like the opioids, cannabinoids have peripheral and centrally sited receptors and also suffer from the blight of unwanted centrally mediated side effects such as sedation, cognitive dysfunction, tachycardia and psychotropic effects. Two cannabinoid receptors have been identified, the CB(1) and CB(2) receptors, and their distribution varies throughout the peripheral and central nervous system. Encouragingly, early studies with these compounds suggest that it may be possible to separate their antitussive activity from their centrally mediated side effects, with CB(2) agonists showing potential as putative new treatments for cough. In this chapter, we describe the opioid and cannabinoid receptors, their distribution and the effects they mediate. Moreover, we highlight their potential advantages and disadvantages in the treatment of cough.

Clinical cough VI: the need for new therapies for cough: disease-specific and symptom-related antitussives

Cough is a common symptom that can be self-limiting or persistent. Ideally, treatment of the underlying cause(s) of cough with specific treatments should eliminate cough. This approach may not be successful if no cause can be established or if the treatment of the cause fails. Suppression of cough may be disease-specific or symptom-related. There has been a long tradition in acute cough usually due to upper respiratory tract infections to use symptom-related antitussives. In chronic cough, suppression of cough may be achieved by disease-specific therapies, but in many patients it may be necessary to use symptomatic antitussives. The efficacy of some over-the-counter symptomatic antitussives is often no better than that of a placebo. Currently available cough suppressants include the centrally acting opioids such as morphine, codeine, pholcodeine, and dextromethorphan. Early studies reported success in reducing cough in patients with chronic bronchitis or chronic obstructive pulmonary disease (COPD); however, a carefully conducted blinded controlled study showed no effect of codeine on cough of COPD. Success with these cough suppressants may be achieved at high doses that are associated with side effects. A slow-release preparation of morphine has been shown to have some degree of efficacy, but this should be reserved for the most severe chronic cough patient, and for patients with terminal cancer who may also benefit from its analgesic effects. There are case reports of the success of centrally acting drugs such as amitriptyline, paroxetine, gabapentin, and carbamezepine in chronic cough. New agents derived from basic research such as new opioids such as nociceptin or antagonists of transient receptor potential vanniloid-1 may turn out to have antitussive effects. Efficacy of symptomatic cough suppressants must be tested in double-blind randomized trials using validated measures of cough in patients with chronic cough not responding to specific treatments. Patients with chronic cough need effective antitussives that could be used either on demand or on a long-term basis.

Clinical cough V: complementary and alternative medicine: therapy of cough

We review the actions of complementary and alternative medicines (CAMs) in the treatment of cough and of the conditions associated with it; in particular asthma and upper respiratory tract infections. These therapies may work (1) peripherally, at the sites in the airways and lungs at which cough is being activated, (2) in the brainstem, where the neural "cough center" is situated, or (3) at the cerebral cortex, where cough can be initiated, suppressed or modified by conscious or unconscious controls. Of the large number of trials of CAMs against cough, most are inadequate in design. It may be difficult to randomize selection. Blinding is often impossible both for the patient and the therapist, and adequate placebo controls may be difficult to devise. The patient can usually identify the "active" treatment by the taste or smell of a medicine, or from the approach and apparatus being used. Pure chemicals can be extracted from many of the herbs used as antitussives, and can be shown to be effective in randomized, blind, and controlled trials, but it does not follow that the herb itself, used in the recommended formula and shown to be antitussive, acts by this agency unless a placebo effect is ruled out. A few herbs are identified where the evidence points to a true antitussive action. Of nonherbal treatments, the few positive results are usually outweighed by the larger number of negative ones. Thus, in general, CAMs for cough are welcomed enthusiastically by the patient but lack sound evidence for their efficacy. Antitussive chemicals can be extracted from many herbs, but it is no more than a reasonable hypothesis that the herb itself acts through this pathway.

Chronic cough: future directions in chronic cough: mechanisms and antitussives

Cough is an important defensive reflex of the respiratory tract needed to clear and protect the upper airways; however, it may become exaggerated and interfere with quality of life. Although chronic cough may be successfully treated when associated with the common causes such as asthma and eosinophilic bronchitis, gastrooesophageal reflux disease and post-nasal drip syndrome or rhinosinusitis, increasingly, idiopathic cough or cough with no associated cause is recognised. Chronic cough is often associated with an increased response to tussive agents such as capsaicin, used as an index of the cough reflex. Some airway receptors mediate or influence cough through activation of vagal afferent pathways which converge on brain stem respiratory networks and of supramedullary centres. Plastic changes in intrinsic and synaptic excitability at the brain stem, spinal or ganglionic level may be the mechanism by which the cough reflex is enhanced in chronic cough. Subjective and objective measurements of cough in the clinic are now available but a major unmet need in chronic cough is the availability of effective antitussives. Future directions in chronic cough include the pathophysiological mechanisms of the enhanced cough reflex, and the discovery of effective antitussives that can successfully alleviate chronic cough.

TRPV1 antagonists as potential antitussive agents

Cough is an important defensive pulmonary reflex that removes irritants, fluids, or foreign materials from the airways. However, when cough is exceptionally intense or when it is chronic and/or nonproductive it may require pharmacologic suppression. For many patients, antitussive therapies consist of OTC products with inconsequential efficacies. On the other hand, the prescription antitussive market is dominated by older opioid drugs such as codeine. Unfortunately, "codeine-like" drugs suppress cough at equivalent doses that also often produce significant ancillary liabilities such as GI constipation, sedation, and respiratory depression. Thus, the discovery of a novel and effective antitussive drug with an improved side effect profile relative to codeine would fulfill an unmet clinical need in the treatment of cough. Afferent pulmonary nerves are endowed with a multitude of potential receptor targets, including TRPV1, that could act to attenuate cough. The evidence linking TRPV1 to cough is convincing. TRPV1 receptors are found on sensory respiratory nerves that are important in the generation of the cough reflex. Isolated pulmonary vagal afferent nerves are responsive to TRPV1 stimulation. In vivo, TRPV1 agonists such as capsaicin elicit cough when aerosolized and delivered to the lungs. Pertinent to the debate on the potential use of TRPV1 antagonist as antitussive agents are the observations that airway afferent nerves become hypersensitive in diseased and inflamed lungs. For example, the sensitivity of capsaicin-induced cough responses following upper respiratory tract infection and in airway inflammatory diseases such as asthma and COPD is increased relative to that of control responses. Indeed, we have demonstrated that TRPV1 antagonism can attenuate antigen-induced cough in the allergic guinea pig. However, it remains to be determined if the emerging pharmacologic profile of TRPV1 antagonists will translate into a novel human antitussive drug. Current efforts in clinical validation of TRPV1 antagonists revolve around various pain indications; therefore, clinical evaluation of TRPV1 antagonists as antitussive agents will have to await those outcomes.

The Current State of Cough Research: The Clinician’s Perspective

Cough is one of the most common reasons for medical consultation and it is responsible for a large human and socioeconomic burden. Current guidelines provide a useful framework for cough management and summarize current knowledge of causes and optimal testing sequences and treatments of cough. However, research is needed on the role of noninvasive airway inflammation measurement in assessing etiology; optimal treatment of postinfectious cough or cough due to gastroesophageal reflux disease; protussive treatment; causes of chronic cough in immunocompromised hosts; and characteristics and management of psychogenic cough. The effects of the use or nonuse of evidence-based guidelines should be documented. An empiric, integrative approach to management of chronic cough also needs further validation.