438 REVIEW SERIES Thorax: first published as 10.1136/thx.2003.013490 on 28 April 2004. Downloaded from Cough ? 7: Current and future drugs for the treatment of chronic cough M G Belvisi, P Geppetti ............................................................................................................................... Thorax 2004;59:438–440. doi: 10.1136/thx.2003.013490 There are currently no effective treatments for controlling Orally administered dextromethorphan is as effective as codeine in suppressing cough and the cough response with an acceptable therapeutic ratio. has been used as a constituent of many OTC However, several new mechanisms have been identified preparations. Furthermore, recent reports sug- which may lead to the development of new drugs. gest that patients with an opioid resistant cough achieved symptomatic relief with the peripher- ........................................................................... ally acting non-opioid drug benzonatate.5 Levodropropizine, oxalamine, and prenoxdiazine are available as cough treatments in Europe. Clinical studies have shown favourable data with hronic cough is associated with many levodropropizine in cancer related cough.6 inflammatory airways diseases such as asthma, chronic obstructive pulmonary C Local anaesthetics disease (COPD), post viral infections, pulmonary fibrosis, and bronchiectasis. Indeed, it is the first Local anaesthetics such as lignocaine are deliv- and most persistent symptom of diseases such as ered locally to the airways and have been shown asthma and COPD.12Furthermore, it is the most to attenuate capsaicin induced cough in 7 common respiratory complaint for which medi- humans. However, the effect is transient and cal attention is sought and, although UK annual the antitussive effect is accompanied by orophar- sales of over the counter (OTC) cough remedies yngeal anaesthesia leading to an increased risk of are over £0.5 billion, effective treatments for aspiration of airway secretions and food. This cough are very limited. In fact, a recent study has treatment should be avoided in patients with suggested that OTC medicines for acute cough asthma because it may induce severe broncho- cannot be recommended because there is no constriction. Interestingly, lozenges containing good evidence for their effectiveness given that, local anaesthetics are often used as OTC treat- even in studies where OTC compounds showed ment for acute cough such as that following an http://thorax.bmj.com/ minimal benefits, these were of doubtful clinical upper respiratory tract infection. relevance.3 The identification of new therapeutic targets for the treatment of chronic cough will Menthol therefore be of immense therapeutic benefit and Menthol has been proposed as an antitussive will greatly enhance the quality of life of treatment and has been shown to inhibit citric patients. acid induced cough in normal volunteers, but to a lesser extent than lignocaine.8 However, it has advantages over lignocaine as menthol does not on September 30, 2021 by guest. Protected copyright. CURRENT TREATMENTS result in oropharyngeal anaesthesia and may Opiates therefore be a more selective antitussive treat- The most effective antitussive agents are opioids ment.9 such as morphine, diamorphine, and codeine which, in all probability, act both centrally on brainstem opioid receptors and on receptors NEW TREATMENTS located peripherally on sensory nerve endings New drugs for the treatment of cough may be in the airways.4 However, at their effective doses directed at an extremely heterogeneous group of they also cause physical dependence, respiratory targets. A major distinction in this regard is the depression, and gastrointestinal symptoms. ability of certain drugs to inhibit the underlying Morphine and diamorphine are very addictive inflammatory process that under certain condi- See end of article for but are useful in treating severe distressing tions cause cough—for example, anti-inflamma- authors’ affiliations cough in patients with terminal illness such as tory drugs for the treatment of asthma or COPD ....................... bronchial carcinoma. Several opiate-containing or novel proton pump inhibitors as treatment for Correspondence to: proprietary cough mixtures contain low doses of gastro-oesophageal reflux—or compounds that Professor M G Belvisi, weaker opioids such as codeine, but there is no are targeted to inhibit sensory nerve activity Respiratory Pharmacology strong evidence that these are more effective directly which should, in theory, inhibit cough of Group, Department of than the demulsant vehicle. any aetiology. However, here we would like to Cardiothoracic Surgery, Faculty of Medicine, focus on compounds that can be classified as Imperial College at the Non-narcotic antitussive agents symptomatic antitussive agents which recognise National Heart & Lung Dextromethorphan is the dextro isomer of the as their main targets ion channels, receptors, or Institute, Dovehouse Street, opiate levomethorphan and it has no analgesic or other molecules expressed peripherally in pri- London SW3 6LY, UK; [email protected] sedative properties. Dextromethorphan is the mary sensory neurons or by inhibition of central ....................... most commonly used antitussive in the USA. mechanisms. www.thoraxjnl.com Treatment of chronic cough 439 Ligands acting at G protein coupled receptors Inhibition of prostanoid synthesis/action New opioids The high tussive potency of the prostaglandins in humans Thorax: first published as 10.1136/thx.2003.013490 on 28 April 2004. Downloaded from In addition to the classical opioids such as enkephalins, b- suggests that their local release in various respiratory endorphins, and dynorphins, new opioid peptides have been pathophysiological conditions may be responsible for the described which have functional effects in the airways—for accompanying cough/irritancy.17 20 In patients with asthma example, the endogenous opioid peptide nociceptin/orphanin cough thresholds with indomethacin and OKY-046 (throm- FQ and endomorphins 1 and 2 have recently been isolated.10 boxane synthase inhibitor) treatment were significantly Nociceptin and the endomorphins are structurally different greater than with placebo, which supports the hypothesis from one another and from the classical opioids. that thromboxane A2 may be one of the cyclooxygenase Three classical opioid receptors have been identified products augmenting airway cough sensitivity in asthma.21 pharmacologically: OP1 (formerly the d-opioid receptor), Furthermore, it has been suggested that prostaglandins may OP2 (formerly the k-opioid receptor), and OP3 (formerly the have a role in the genesis of cough induced by ACE inhibitors, m-opioid receptor). The opioids currently used as antitussive and inhibition of prostaglandin synthesis with indometha- 22 23 treatment predominantly bind the OP3 receptor and are cin or a thromboxane antagonist can reduce or abolish the therefore associated with characteristic side effects. New incidence of this side effect. However, although prostaglan- opioid peptides such as the endomorphins also activate the dins may be involved in cough associated with ACE inhibitor OP3 receptor. However, the endogenous opioid peptide therapy, evidenced by the ability of the non-steroidal anti- nociceptin/orphanin FQ has some homology to the dynorphin inflammatory sulindac to inhibit cough in patients on this family but lacks the N-terminal tyrosine residue which is treatment, the same compound was ineffective in patients essential for binding classical opioid receptors. Interestingly, not on ACE inhibitor therapy but with idiopathic dry 24 nociceptin binds to the opioid receptor-like 1 receptor (ORL1). unproductive cough. Furthermore, ORL1 has already been shown to inhibit sensory nerve function in guinea pig airways in vitro and Ion channel modulators may be an excellent target for new antitussive treatments.11 12 Transient receptor potential (TRP) channels Recently, receptors have been cloned on sensory nerves that are activated by thermal stimuli. The cold and menthol Neurokinin receptor antagonists sensitive receptor (CMR1) has recently been characterised Tachykinins such as substance P and neurokinin A have been and cloned and is a member of the TRP family of excitatory shown to elicit cough, and neurokinin receptor antagonists ion channels.25 Interestingly, menthol has been proposed as have been shown to be effective antitussive agents in animal an antitussive treatment and has been shown to inhibit citric models. Data have implicated a role for the NK receptor and 2 acid induced cough in normal volunteers.8 Activators of this possibly the NK receptor in this antitussive action. In fact, 1 particular channel may therefore prove to be useful ther- the NK receptor antagonist SR 48968 has been shown to 2 apeutic agents. The heat sensitive channels VR1 and VRL-1 inhibit citric acid induced cough in conscious guinea pigs,13 14 are TRP channels that detect temperatures over a wide range. but an antitussive effect of NK receptor antagonists is still 1 The VR1 channel is activated by capsaicin, the main pungent debated. Recent data have suggested a role for NK receptor 3 ingredient in hot chilli peppers; however, the related channel activation in evoking a tussive response.15 16 SB 235375, a VRL-1 does not respond to capsaicin but is activated by high affinity selective, reversible, and competitive antagonist, http://thorax.bmj.com/ temperatures exceeding 50 C.26 27 Since the actions