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.