Pharmacology and antitussive efficacy of 4-(3-trifluoromethyl-pyridin-2-yl)-piperazine-1-carboxylic acid (5-trifluoromethyl-pyridin-2-yl)-amide (JNJ17203212), a transient receptor potential vanilloid 1 antagonist in guinea pigs

Acid-sensitive ion channels and receptors

Acidosis is a noxious condition associated with inflammation, ischaemia or defective acid containment. As a consequence, acid sensing has evolved as an important property of afferent neurons with unmyelinated and thinly myelinated nerve fibres. Protons evoke multiple currents in primary afferent neurons, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH, whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Two-pore-domain K(+) (K(2P)) channels are differentially regulated by small deviations of extra- or intracellular pH from physiological levels. Other acid-sensitive channels include TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca(2+) channels, hyperpolarization-activated cyclic nucleotide gated channels, gap junction channels, and Cl(-) channels. In addition, acid-sensitive G protein coupled receptors have also been identified. Most of these molecular acid sensors are expressed by primary sensory neurons, although to different degrees and in various combinations. Emerging evidence indicates that many of the acid-sensitive ion channels and receptors play a role in acid sensing, acid-induced pain and acid-evoked feedback regulation of homeostatic reactions. The existence and apparent redundancy of multiple pH surveillance systems attests to the concept that acid-base regulation is a vital issue for cell and tissue homeostasis. Since upregulation and overactivity of acid sensors appear to contribute to various forms of chronic pain, acid-sensitive ion channels and receptors are considered as targets for novel analgesic drugs. This approach will only be successful if the pathological implications of acid sensors can be differentiated pharmacologically from their physiological function.

Cough sensors. II. Transient receptor potential membrane receptors on cough sensors

The transient receptor potential (TRP) family of channels is represented by at least six members in primary sensory neurons. These include the TRP vanilloid subtypes 1 (TRPV1), 2, 3, and 4, the cold and menthol receptor TRPM8, and TRPA1. Much interest has been directed to the study of the TRPV1, because capsaicin has been instrumental in discovering the unique role of a subset of primary sensory neurons in causing nociceptive responses, in activating reflex pathways including cough, and in producing neurogenic inflammation. TRPV1 is now regarded as an integrator of diverse sensory modalities because it undergoes marked plasticity and sensitization through a variety of mechanisms, including activation of G-protein-coupled or tyrosine kinase receptors. Evidence in experimental animals and in patients with airway diseases indicates a marked hypersensitivity to cough induced by TRPV1 agonists. Recent studies with newly developed high-affinity and selective TRPV1 antagonists have revealed that TRPV1 inhibition reduces cough induced by citric acid or antigen challenge.

TRPV1 receptors in sensitisation of cough and pain reflexes

Preclinical studies suggest that the vanilloid receptor (TRPV1) is an important component of several disease areas such as pain (inflammatory, visceral, cancer and neuropathic), airway disease (including chronic cough), inflammatory bowel disease (IBD), interstitial cystitis, urinary incontinence, pancreatitis and migraine. TRPV1 is a member of a distinct subgroup of the transient receptor potential (TRP) family of ion channels. The neuronally expressed TRPV1 is a non-selective, Ca(2+)-preferring, cation channel. In addition to capsaicin, this channel is activated by a number of different stimuli including heat, acid, certain arachidonic acid derivatives and direct phosphorylation via protein kinase C (PKC). Moreover, there is also evidence that various inflammatory mediators such as adenosine triphosphate (ATP), bradykinin, nerve growth factor (NGF) or prostaglandin E(2) (PGE(2)) may indirectly lead to activation of the TRPV1 channel via activation of their respective receptors. There is strong experimental evidence that the combination of direct and indirect mechanisms finely tune the TRPV1 activity. Each of the different known modes of direct TRPV1 activation (protons, heat and vanilloids) is capable of sensitising the channel to other agonists. Similarly, inflammatory mediators from the external milieu found in disease conditions can indirectly sensitise the receptor. It is this sensitisation of the TRPV1 receptor in inflammatory disease that could hold the key and contribute to the transduction of noxious signalling for normally innocuous stimuli, i.e. either hyperalgesia in the case of chronic pain or airway hyperresponsivness/hypertussive responses in patients with chronic cough. It seems reasonable to suggest that the various mechanisms for sensitisation provide a scenario for TRPV1 to be tonically active and this activity may contribute to the underlying pathology -- providing an important convergence point of multiple pain producing stimuli in the somatosensory system and multiple cough-evoking irritants in the airways. The complex mechanisms and pathways that contribute to the pathophysiology of chronic pain and chronic cough have made it difficult for clinicians to treat patients with current therapies. There is an increasing amount of evidence supporting the hypothesis that the expression, activation and modulation of TRPV1 in sensory neurones appears to be an integral component of pain and cough pathways, although the precise contribution of TRPV1 to human disease has yet to be determined. So the question remains open as to whether TRPV1 therapeutics will be efficacious and safe in man and represent a much needed novel pain and cough therapeutic.

Breathtaking TRP channels: TRPA1 and TRPV1 in airway chemosensation and reflex control

New studies have revealed an essential role for TRPA1, a sensory neuronal TRP ion channel, in airway chemosensation and inflammation. TRPA1 is activated by chlorine, reactive oxygen species, and noxious constituents of smoke and smog, initiating irritation and airway reflex responses. Together with TRPV1, the capsaicin receptor, TRPA1 may contribute to chemical hypersensitivity, chronic cough, and airway inflammation in asthma, COPD, and reactive airway dysfunction syndrome.

Identification and synthesis of 2,7-diamino-thiazolo[5,4-d]pyrimidine derivatives as TRPV1 antagonists

We have identified and synthesized a series of 2,7-diamino-thiazolo[5,4-d]pyrimidines as TRPV1 antagonists. An exploration of the structure-activity relationships at the 2-, 5-, and 7-positions of the thiazolo[5,4-d]pyrimidine led to the identification of several potent TRPV1 antagonists, including 3, 29, 51, and 57. Compound 3 was orally bioavailable and afforded a significant reversal of carrageenan-induced thermal hyperalgesia with an ED(50)=0.5mg/kg in rats.