The information provided herein and references cited are provided solely to assist the understanding of the reader, and does not constitute an admission that any of the references or information is prior art to the present invention.
Purines, acting via cell surface purinoceptors, have been implicated as having a variety of physiological and pathological roles. ATP, and to a lesser extent, adenosine, can stimulate sensory nerve endings resulting in intense pain and irritation and a pronounced increase in sensory nerve discharge. ATP receptors have been classified into two major families, the P2Y- and P2X-purinoreceptors, on the basis of molecular structure, transduction mechanisms, and pharmacological characterization. The P2Y-purinoceptors are G-protein coupled receptors, while the P2X-purinoceptors are a family of ATP-gated cation channels. Purinergic receptors, in particular, P2X receptors, are known to form homomultimers or heteromultimers. To date, cDNAs for seven P2X subunits have been cloned, (P2X1, P2X2, P2X3, P2X4, P2X5, P2X6 and P2X7), each able to produce homotrimeric channels and some able to form heterotrimeric receptors (e.g. P2X2/3, P2X4/6 and P2X1/5). The structure and chromosomal mapping of mouse and human genomic P2X3 receptor subunits have also been described. In vitro, co-expression of P2X2 and P2X3 receptor subunits is necessary to produce ATP-gated currents with the properties seen in some sensory neurons.
P2X3 receptor subunits are found on primary sensory afferents innervating rodent and human organs and tissues. Data exist suggesting that ATP may be released from epithelial/endothelial cells of the hollow organs or from muscle beds as a result of distention, movement, injury infection and inflammation. ATP released in this manner may serve a role in conveying information to nearby sensory neurons located. P2X receptors have been studied in a number of neurons, including sensory, sympathetic, parasympathetic, mesenteric, and central neurons. Some studies indicate that P2X purinergic receptors play a role in afferent neurotransmission from the many organ systems and tissues, and that modulators of P2X receptors are potentially useful in the treatment of functional organ or tissue disorders and attenuate common chronic symptoms and signs of important diseases or conditions.
Evidence also suggests a role of endogenous ATP and purinergic receptors in nociceptive responses in mice. ATP-induced activation of P2X3 receptors on dorsal root ganglion nerve terminals in the dorsal horn of the spinal cord has been shown to stimulate release of glutamate, a key neurotransmitter involved in nociceptive signalling. P2X3 receptors have been identified on nociceptive neurons in the tooth pulp. ATP released from distressed or damaged cells in many tissue systems may thus lead to pain by activating P2X3 containing receptors on nociceptive sensory nerve endings. This is consistent with observations of the induction of pain and discomfort by intradermally applied ATP in the human blister-base model or following its infusion into a muscle bed. P2X antagonists have been shown to be analgesic in many animal models. This evidence suggests that P2X3 containing channels are involved in the sensitization of nerves that drives and maintains heightened nociception signalling, and that modulators of P2X receptors are potentially useful as inhibitors of sensitization and may have applicability as analgesics, anti-pruritics, antitussives and treatments for autonomic hyperresponsiveness.
The use of antagonists of P2X2 and P2X2/3 for the treatment of pain was discussed by Carter, et al., (Bioorganic and Medical Chemistry Letters, 2009, 19(6), 1628-1635; doi:10.1016/j.bmcl.2009.02.003). The structure-activity relationship of a series of diaminopyrimidines was studied. The selectivity of these compounds for P2X3 and P2X2/3 vs. other P2X purinoceptors was also discussed.
Vandenbeuch et al. (J. Physiol, 2015, 593(5), 1113-1125; doi: 10/1113/jphysiol.2014.281014) discuss the role of both P2X3 and P2X2/3 channels in taste transduction.