Is There a Benefit to Positive Allosteric Modulation of P2X Receptors?

REVIEW published: 12 May 2020 doi: 10.3389/fphar.2020.00627 To Inhibit or Enhance? Is There a Benefit to Positive Allosteric Modulation of P2X Receptors? Leanne Stokes*, Stefan Bidula, Lucˇ ka Bibicˇ and Elizabeth Allum School of Pharmacy, University of East Anglia, Norwich, United Kingdom The family of ligand-gated ion channels known as P2X receptors were discovered several decades ago. Since the cloning of the seven P2X receptors (P2X1-P2X7), a huge research effort has elucidated their roles in regulating a range of physiological and pathophysiological processes. Transgenic animals have been influential in understanding which P2X receptors could be new therapeutic targets for disease. Furthermore, understanding how inherited mutations can increase susceptibility to disorders and diseases has advanced this knowledge base. There has been an emphasis on the discovery and development of pharmacological tools to help dissect Edited by: the individual roles of P2X receptors and the pharmaceutical industry has been involved in Elena Adinolfi, pushing forward clinical development of several lead compounds. During the discovery University of Ferrara, Italy phase, a number of positive allosteric modulators have been described for P2X receptors Reviewed by: and these have been useful in assigning physiological roles to receptors. This review will Günther Schmalzing, RWTH Aachen University, Germany consider the major physiological roles of P2X1-P2X7 and discuss whether enhancement Ralf Schmid, of P2X receptor activity would offer any therapeutic benefit. We will review what is known University of Leicester, about identified compounds acting as positive allosteric modulators and the recent United Kingdom fi *Correspondence: identi cation of drug binding pockets for such modulators. Leanne Stokes Keywords: P2X receptor, allosteric modulator, pharmacology, drug discovery, P2X4, P2X7 [email protected] Specialty section: This article was submitted to INTRODUCTION Experimental Pharmacology and Drug Discovery, Over the last decade we have seen new developments in pharmacological agents targeting P2X4, a section of the journal P2X7, and P2X3 receptors with some candidates entering clinical trials (Keystone et al., 2012; Stock Frontiers in Pharmacology et al., 2012; Eser et al., 2015; Matsumura et al., 2016; Timmers et al., 2018; Muccino and Green, Received: 20 December 2019 2019). Drug discovery for other P2X receptors such as P2X1 and P2X2 is somewhat slower with very Accepted: 21 April 2020 few selective and potent drugs being identified (Burnstock, 2018). Advances in structural biology Published: 12 May 2020 have helped move drug design for P2X receptors forward. Accompanying this is the advance in Citation: knowledge of the types of physiological responses controlled by this family of ion channels and ˇ Stokes L, Bidula S, Bibic L and Allum E clinical areas where such drugs may be therapeutically useful. Much emphasis has been placed on (2020) To Inhibit or Enhance? Is There the development of antagonist agents and relatively little attention has been on the discovery or a Benefit to Positive Allosteric Modulation of P2X Receptors? development of positive modulators. In this review, we take stock of all the evidence regarding the Front. Pharmacol. 11:627. known physiological roles of the major P2X receptors and present what we currently know about doi: 10.3389/fphar.2020.00627 pharmacological agents that can enhance ATP-mediated responses. Frontiers in Pharmacology | www.frontiersin.org 1 May 2020 | Volume 11 | Article 627 Stokes et al. PAMs at P2X Receptors “ ” In receptor function and pharmacology, the word allosteric benzodiazepines act as PAMs at the GABAA receptor and are is commonplace. The historical use of this word is discussed by clinically used as sedatives, anti-convulsants, and anaesthetic Colquhoun and Lape (2012) and the use of the term allosteric in agents (Solomon et al., 2019). Newer developments have seen current pharmacological terminology is used in the context of multiple PAMs for a7 nAchR advance into clinical trials for allosteric modulators, allosteric interactions, and allosteric Alzheimer’s disease, schizophrenia, and ADHD (Yang T. et al., transitions (Neubig et al., 2003). Allosteric transition describes 2017). PAMs for NMDA ionotropic glutamate receptors could the mechanism underlying receptor activation; following ligand be useful for disorders where their hypofunction is implicated binding to an orthosteric site, there is conformational (e.g., schizophrenia) (Yao and Zhou, 2017) and PAMs for AMPA communication through the protein to activate the biological ionotropic glutamate receptors could be useful in several response, for example, ion channel pore opening (Changeux and cognitive disorders (Lee et al., 2016). For the P2X receptors, Christopoulos, 2016). Pharmacologically, multiple sites exist on only ivermectin, a PAM with activity on P2X4, has been assessed receptor proteins where ligands can bind. While agonists bind at in a pilot Phase 1 clinical trial for alcohol-use disorders (Roche orthosteric sites, modulators act at distinct allosteric (other) sites et al., 2016). and typically affect agonist action (Neubig et al., 2003). Allosteric We have recently discovered and characterized novel positive drug interactions can have different outcomes, either positive, allosteric modulators that act on P2X7 and P2X4 receptors negative, or neutral/silent modulatory effects (Neubig et al., 2003; (Helliwell et al., 2015; Dhuna et al., 2019)andthishas Changeux and Christopoulos, 2016). For positive allosteric prompted us to ask questions about whether PAMs hold any modulators (PAMs), these can alter sensitivity to the agonist therapeutic benefit for P2X receptors. We present here a review by shifting the concentration-response curve, alter agonist of the latest literature on P2X receptors focusing on the major efficacy by increasing the maximum response, or alter gating identified subtypes P2X1, P2X2, P2X2/3, P2X4, and P2X7 with kinetics of the ion channel by affecting activation or deactivation well-known physiological roles (P2X5 and P2X6 have no (Chang et al., 2010). At the molecular level, it is thought that assigned physiological roles as homomeric receptors). PAMs reduce the energy barrier for gating thus making it easier for an ion channel to transition into an active, open state (Chang et al., 2010). These two different effects on the agonist actions P2X1 RECEPTOR have been classified by some as Type I (increasing the maximum response or efficacy) and Type II (shifting the concentration- The P2X1 receptor is a fast desensitizing ion channel activated by response curve and thus altering agonist EC50 values) (Hackos ATP, ab-Me-ATP, bg-Me-ATP, 2-MeSATP, BzATP, and Ap4A and Hanson, 2017). These two effects may not always be (North and Surprenant, 2000). Human P2X1 was cloned from separated with some PAMs exhibiting both effects (mixed Type urinary bladder RNA, and later from human platelets (Longhurst I/II) (Figure 1). The therapeutic beauty of PAMs is their reliance et al., 1996; Sun et al., 1998). Mackenzie et al. first recorded P2X1 on the presence of the endogenous agonist for activity. Therefore, currents from human platelets (Mackenzie et al., 1996) and in disease states where agonist-induced receptor signaling is P2X1-dependent Ca2+signaling in platelet activation was perhaps defective, PAMs could help to bring those responses characterized (Sage et al., 1997). In 1999, P2X1 was stated to back into the “normal” range. have no significant role in platelet aggregation (Takano et al., Other ligand-gated ion channels such as the Cys-loop family 1999) however, a later study by Oury et al. demonstrated that (GABAA receptor, glycine receptor, nicotinic acetylcholine P2X1 did act as a positive regulator of platelet responses (Oury receptor, and 5-HT3 receptor) have many drug binding et al., 2001). Further to this, a transgenic mouse over-expressing pockets which are well characterized for both PAMs and P2X1 in megakaryocytes demonstrated increased ab-Me-ATP- negative allosteric modulators (NAMs). The barbiturates and sensitive Ca2+ responses ex vivo and more profound platelet FIGURE 1 | Schematic diagram of types of positive allosteric modulator (PAM) effects on concentration-response curves. Three different types of PAM effect are displayed on concentration-response curves (for illustration purposes only, curves are hand-drawn). A Type I effect is defined as increasing the maximum response without a change in EC50 value. A Type II effect is defined as a left-ward shift in the concentration-response curve (and a reduction in the EC50 value) without affecting efficacy (maximum response). A mixed Type I/II effect is defined as both a left-ward shift in concentration-response curve and an increase in efficacy (maximum response). Frontiers in Pharmacology | www.frontiersin.org 2 May 2020 | Volume 11 | Article 627 Stokes et al. PAMs at P2X Receptors shape changes to this agonist (Oury et al., 2003). Tests on these thought to stabilize the glomerular filtration rate (Guan et al., P2X1 over-expressing platelets revealed an increase in collagen- 2007). P2X1-/- mice have an impairment in this protective induced aggregation and in the transgenic mouse, an increase in autoregulatory behavior (Inscho et al., 2003). In hypertensive fatal pulmonary thromboembolism was observed compared to disorders, this renal autoregulation can be defective and wild-type mice (Oury et al., 2003). This work demonstrates that purinergic receptors may contribute

Load more