Selectivity of the P2X3 receptor antagonist Eliapixant, and its potential use in the treatment of endometriosis

Article summary P2X3 receptors, expressed on peripheral afferent neurons, play a significant role in the activation and sensitisation of nerve fibres, and thus in the generation of both chronic neuropathic and inflammatory pain pathways. Eliapixant (BAY1817080), developed by Bayer, is a potent P2X3 antagonist that has recently garnered attention for its potential use as a therapeutic agent for the treatment of hypersensitive nerve fibre disorders. Davenport et al. [1] have recently depicted Eliapixant as a highly selective and potent blockade of the homotrimeric P2X3 receptor, and a potential treatment option for those with endometriosis, a chronic estrogen-dependent inflammatory disease. In this paper published Scientific Reports, they provide evidence for the expression and possible functionality of the P2X3 receptor in rodent models of endometriosis and neurogenic inflammatory pathways, highlighting the receptor as a key mediator within the development of neurogenic inflammation. Eliapixant is currently undergoing phase II clinical trials for its use in several hypersensitive nerve fibre disorders, including refractory and/or unexplained chronic cough (RUCC) (NCT0456215), overactive bladder (OAB) (NCT04545580), endometriosis (NCT04614246), and diabetic neuropathic pain (NCT04641273). Commentary P2X receptors are a family of trimeric adenosine triphosphate (ATP)-gated non-selective ion channels, widely expressed in both excitable and non-excitable cells, and contribute to many important physiological and pathological processes. P2X receptors are capable of assembling as homotrimeric (comprising of identical subunits) or heterotrimeric (comprising of multiple types of P2X subunits) structures in a subunit-dependent fashion in nature. The P2X2/3 heteromeric receptor is the most widely studied, and accepted, heterotrimeric P2X receptor. This heterotrimer, along with the homotrimeric P2X3 receptor, has been extensively implicated in pain and hypersensitivity pathways, and has previously been identified as drug target candidates for the treatment of pain. There is a mounting therapeutic interest in the development of selective and potent P2X3 receptor antagonists for the treatment of disorders associated with hypersensitive nerve fibres. Previous studies have shown reduced nociceptive responses in P2X3 and P2X2/3 receptor double knockout mice [2, 3] particularly in response to ATP. Upregulation of P2X3 receptors in peripheral sensory neurons can be seen following chronic nerve constriction [4], and upregulation of P2X3 receptors or excessive activation of P2X3 has been shown to contribute to neuronal hypersensitivity [5]. These studies, as well as previous papers documenting the nociceptive response in the presence of small molecule antagonists and monoclonal blocking antibodies, provide evidence for the efficacy of P2X3 and P2X2/3 antagonism to reduce unwanted nocifensive responses, and the validation of these purinergic receptors as a therapeutic target. Gefapixant (AF-219; MK7264) is a prominent P2X3 receptor antagonist currently undergoing FDA approval for its use in the treatment of chronic cough. BLU-5937, another P2X3 receptor antagonist, has undergone early stage clinical testing, but failed to achieve sufficient reduction in the reduction of awake cough frequency in an initial RUCC trial (NCT03979638). Dysgeusia (altered taste perception) has been documented in the use of Gefapixant throughout its clinical trials, with a dose-dependent increase in the number of participants affected (10–56%) by this adverse effect [6]. This undesirable side effect has been suggested to be partially due to a lack of selectivity of Gefapixant targeting P2X3 over the P2X2/3 receptor [7]. It is thought that the P2X2/3 receptor is expressed at high levels in tongue sensory afferent nerves, and thus contributes to dysgeusia when blocked. Accepting the theory that the alleviation of symptoms is largely due to the targeting of the P2X3 homomeric receptor and the adverse effects due to P2X2/3 blockade, a more selective P2X3 antagonist may propose a more tolerated alternative in the treatment of hypersensitive nerve disorders. Eliapixant shows high selectivity for the P2X3 homotrimeric receptor in both rat and human orthologues over other P2X receptors, including the P2X2/3 receptor, which has not previously been seen with other available P2X3 antagonists in vitro. Eliapixant displayed a 20-fold lower potency at human P2X3 compared to P2X2/3 receptors (10 nM and 129 nM, respectively). Interestingly, this was not replicated in rat receptors, but this potentially could be due to the model itself rather than any pharmacological difference in rat P2X receptors. The ganglia tissue used to determine potency of the P2X3 antagonist (dorsal root ganglia vs. nodose ganglia) were not shown to represent distinct P2X3-only and P2X2/3-only tissue cultures respectively. DRG neurons have been previously shown to express P2X2/3 receptors in a rat model [3], and so, there may have been a mixed population of P2X receptors present in both tissue samples. During the early phase of clinical trials of Eliapixant, a small percentage (10–20%) of participants developed taste-related side effects following a therapeutic dose [8]. This was reported to be at a slightly reduced level than seen in the comparable Gefapixant trials, but still seen to be present in participants. This suggests that P2X3 homomeric-specific antagonism may help to reduce the incidence of taste-related effects, but cannot completely eliminate the adverse effect previously documented. Endometriosis is a disorder characterised by the growth of endometrial tissue outside the uterine space, affecting approximately 10% of adolescent and adult women of reproductive age. This disorder can severely impact quality of life causing problems with fertility, chronic pelvic pain, and the generation of pain in typically non-painful activities, such as dysuria, dyschezia, and dyspareunia, which can be accentuated during menstruation [9]. There is currently no definitive etiology of the development of endometriosis and the characteristic endometriotic lesions, and diagnostic delays are very common. Current treatment options typically follow a multidisciplinary approach, utilising both surgical and medical approaches to remove the endometriotic lesions and to alleviate the symptoms associated with the disorder respectively. This approach can be inefficient, and lead to reoccurrence in many patients with endometriosis. The potential usefulness of Eliapixant is in alleviating the chronic pain associated with this disorder. Excessive activation of the P2X3 receptor has been suggested to contribute to neurogenic inflammation, a key pathway in several chronic inflammatory disorders including endometriosis. Davenport et al. found that Eliapixant treatment resulted in a significant reduction of neurogenic plasma extravasation following uterine inflammation, showing evidence that P2X3 contributes, or potentially drives, this pathogenic process. This data helps to highlight potential uses for P2X3 antagonism in the therapeutic treatment of chronic inflammatory disorders in the future. Davenport et al. also reported novel evidence of discrete P2X3 expression in the nerve fibres that innervate the endometriotic lesions, which had previously been undetermined. Peptidergic Ad and C-fibre sensory neurons have previously been shown to innervate the characteristic endometriotic lesions seen in endometriosis, and it has been previously suggested that P2X3 receptors would be present on these nerve endings. A previous study, conducted by Ding et al. in 2017 [10], documented an increase in P2X3 receptor expression in endometriotic lesions, but in epithelial and endothelial stromal cells rather than in nerves. This epithelial P2X3 receptor expression was not seen in the present study, and positivity was discretely confined to the nerves, which is more in line with previous reports on the characterisation of the P2X3 receptor. This data set was small, however, so further research into the location of the P2X3 receptor within endometriotic lesions may be required in the future. Nonetheless, the demonstration of P2X3 receptor expression in these lesions does begin to provide evidence to support P2X3 therapeutic targeting in the treatment of endometriosis. Utilising a rat model of dyspareunia (i.e. painful intercourse), the authors of this paper reported that the long term (5 weeks) use of Eliapixant significantly reduced vaginal hyperalgesia. Interestingly, this positive effect in the reduction of hyperalgesia was sustained one week following cessation of the P2X3 antagonist treatment, which suggests that P2X3 antagonism lead to a longer-term adaptive change. This is a very interesting proposition for endometriosis treatment, with P2X3 antagonism potentially contributing to an unanticipated therapeutic effect in addition to pain management. This hypothesis requires further study, as it is currently unclear as to why treatment benefits were observed following withdrawal, but it is hypothesised that it may induce anti-inflammatory effects via an unknown mechanism. In summary, Eliapixant is proposed as a suitable therapeutic agent for the treatment of several hypersensitive nerve fibre disorders including endometriosis, a chronic pain disorder poorly-served by currently available management strategies. Eliapixant is demonstrated to be a selective and potent P2X3 receptor antagonist. Daveport et al. have provided evidence of expression and functionality of the P2X3 receptor in various endometriotic models, highlighting it as a therapeutic target. This helps to provide validation to the clinical development of Eliapixant as a treatment option for endometriosis. Funding MCF was supported by the UKRI Biotechnology and Biological Sciences Research Council (BBSRC) Norwich Research Park Biosciences Doctoral Training Partnership as a CASE award in collaboration with AstraZeneca. Data availability Data sharing is not applicable in this case no datasets were generated or analysed in the generation of this article. Declarations Ethical approval This article does not contain any studies with human participants or animals performed by the author. Conflict of interest The author declares no competing interests. Footnotes Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. References - 1.Davenport AJ, Neagoe I, Bräuer N, Koch M, Rotergi A, Nagel J, Laux-Biehlmann A, Machet F, Coelho AM, Boyce S, Carty N, Gemkow MJ, Hess SD, Zollner TM, Fischer OM. Eliapixant is a selective P2X3 receptor antagonist for the treatment of disorders associated with hypersensitive nerve fibers. 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