Current Topics in Medicinal Chemistry, Volume 4, No. 8, 2004
Contents
Receptors
for Extracellular Nucleosides and Nucleotides
Guest
Editor: Kenneth A. Jacobson
Introduction: P2 Receptors
Pp. 793-803
Geoffrey Burnstock
Molecular Recognition at Purine and Pyrimidine Nucleotide
(P2) Receptors Pp. 805-819
Kenneth A. Jacobson, Stefano Costanzi, Michihiro Ohno, Bhalchandra
V. Joshi, Pedro Besada, Bin Xu and Susanna Tchilibon
Molecular Structure of P2X Receptors Pp. 821-829
Terrance M. Egan, Jane A. Cox and Mark M. Voigt
Neuronal P2 Receptors of the Central Nervous System Pp. 831-838
Peter Illes and Joaquim A. Ribeiro
Partial A1 Adenosine Receptor Agonists From a
Molecular Perspective and their Potential Use as Chronic Ventricular Rate Control
Agents During Atrial Fibrillation (AF) Pp.
839-854
Jeff A. Zablocki, Lin Wu, John Shryock, and Luiz
Belardinelli
Zhan-Guo Gao and Kenneth A. Jacobson
New Highly Potent and Selective Adenosine A3 Receptor Antagonists Pp. 863-870
Neil J. Press, Thomas H. Keller, Pamela Tranter, David Beer, Ken Jones, Alexander Faessler, Richard Heng, Christine Lewis, Trevor Howe, Peter Gedeck, Lazzaro Mazzoni and John R. Fozard
Abstracts
[Back to top] Introduction: P2 Receptors
Geoffrey Burnstock
The current status of ligand gated ion channel P2X and G protein-coupled P2Y receptor subtypes is described. This is followed by a summary of what is known of the distribution and roles of these receptor subtypes. Potential therapeutic targets of purinoceptors are considered, including those involved in cardiovascular, nervous, respiratory, urinogenital, gastrointestinal, musculo-skeletal and special sensory diseases, as well as inflammation, cancer and diabetes. Lastly, there are some speculations about future developments in the purinergic signalling field.
[Back to top]
Molecular Recognition at Purine and Pyrimidine Nucleotide (P2) Receptors
Kenneth A. Jacobson, Stefano Costanzi, Michihiro Ohno, Bhalchandra
V. Joshi, Pedro Besada, Bin Xu and Susanna Tchilibon
In comparison to other classes of cell surface receptors, the medicinal chemistry at P2X (ligand-gated ion channels) and P2Y (G protein-coupled) nucleotide receptors has been relatively slow to develop. Recent effort to design selective agonists and antagonists based on a combination of library screening, empirical modification of known ligands, and rational design have led to the introduction of potent antagonists of the P2X1 (derivatives of pyridoxal phosphates and suramin), P2X3 (A-317491), P2X7 (derivatives of the isoquinoline KN-62), P2Y1 (nucleotide analogues MRS 2179 and MRS 2279), P2Y2 (thiouracil derivatives such as AR-C126313), and P2Y12 (nucleotide/nucleoside analogues AR-C69931X and AZD6140) receptors. A variety of native agonist ligands (ATP, ADP, UTP, UDP, and UDP-glucose) are currently the subject of structural modification efforts to improve selectivity. MRS2365 is a selective agonist for P2Y1 receptors. The dinucleotide INS 37217 potently activates the P2Y2 receptor. UTP-g-S and UDP-b-S are selective agonists for P2Y2/P2Y4 and P2Y6 receptors, respectively. The current knowledge of the structures of P2X and P2Y receptors, is derived mainly from mutagenesis studies. Site-directed mutagenesis has shown that ligand recognition in the human P2Y1 receptor involves individual residues of both the TMs (3, 5, 6, and 7), as well as EL 2 and 3. The binding of the negatively-charged phosphate moiety is dependent on positively charged lysine and arginine residues near the exofacial side of TMs 3 and 7.
[Back to top] Molecular Structure of P2X Receptors
Terrance M. Egan, Jane A. Cox, and Mark M. Voigt
P2X receptors are ligand-gated ion channels that transduce many of the physiological effects of extracellular ATP. There has been a dramatic increase in awareness of these receptors over the past 5 or so years, in great part due to their molecular cloning and characterization. The availability of cDNA clones for the various subunits has led to rapid progress in identifying their tissue-specific expression, resulting in new ideas concerning the functional roles these receptors might play in physiological and pathophysiological processes. In addition, molecular approaches have yielded much information regarding the structure and function of the receptor proteins themeslves. In this review we seek to review recent findings concerning the molecular determinants of receptor-channel function, with particular focus on ligand binding and gating, ion selectivity, and subunit assembly.
[Back to top] Neuronal P2 Receptors of the Central Nervous
System
Peter Illes and Joaquim A. Ribeiro
Neurons of the central nervous system (CNS) are endowed with ATP-sensitive receptors belonging to the P2X (multimeric ligand-gated cationic channels) and P2Y (G protein-coupled 7TM receptors) types. To date seven P2X and eight P2Y receptors of human origin have been molecularly identified and functionally characterized. P2X subunits may occur as homooligomers or as heterooligomeric assemblies of more than one subunit. P2X7 subunits do not form heterooligomeric assemblies and are uniqe in mediating apoptosis and necrosis of glial cells and possibly also of neurons. The P2X2, P2X4, P2X4/P2X6 and P2Y1 receptors appear to be the predominant neuronal types. Whereas a number of P2X receptors mediate fast synaptic responses to the transmitter ATP, P2Y receptors mediate either slow changes of the membrane potential in response to non-synaptically released ATP or the interaction with receptors for other transmitters. The localisation of these receptors may be at the terminal axons (presynaptic) or at the somato-dendritic region (postsynaptic). Whereas presynaptic P2 receptors may be either excitatory (P2X) or inhibitory (P2Y), postsynaptic P2 receptors appear to be without exception excitatory. Finally, the enzymatic degradation of ATP may lead to the local generation of adenosine which can modulate ATP-related neurotransmission via activation of A1 or A2A receptors.
[Back to top] Partial
A1 Adenosine Receptor Agonists From a Molecular Perspective and
their Potential Use as Chronic Ventricular Rate Control Agents During Atrial
Fibrillation (AF)
Jeff A. Zablocki, Lin Wu, John Shryock, and Luiz
Belardinelli
This review provides a molecular
perspective of partial agonism at the A1 adenosine receptor. The
structure-activity relationships (SAR) for affinity and intrinsic efficacy of
analogues of the full agonist N6-cyclopentyladenosine (CPA) are emphasized.
Both general models of activation of G protein-coupled receptors and specific
molecular models of the A1-adenosine receptor are used to interpret
the results of efforts to synthesize and assay effects of partial agonists. The
SAR of affinity and intrinsic efficacy of the 2’, 3’, and especially the
5’-deoxy derivatives of CPA is presented. From this analysis, the nature of the
interactions of specific atoms and substituents of the CPA molecule with the A1-adenosine
receptor are deduced and presented pictorially. As an example of the
therapeutic potential of partial agonists, the design and testing of analogues
of CPA to provide chronic ventricular rate control during atrial fibrillation
is described. The challenges associated with designing a partial A1-adenosine
receptor agonist for providing chronic ventricular rate control during atrial
fibrillation are many. To meet these challenges, further medicinal chemistry
efforts in the area of partial A1- adenosine receptor agonism are
still needed.
Zhan-Guo Gao and Kenneth A. Jacobson
Selective agonists for A3 adenosine receptors (ARs) could potentially be therapeutic agents for a variety of disorders, including brain and heart ischemic conditions, while partial agonists may have advantages over full agonists as a result of an increased selectivity of action. A number of structural determinants for A3AR activation have recently been identified, including the N6-benzyl group, methanocarba substitution of ribose, 2-chloro and 2-fluoro substituents, various 2’- and 3’-substitutions and 4’-thio substitution of oxygen. The 2-chloro substitution of CPA and R-PIA led to A3 antagonism (CCPA) and partial agonism (Cl-R-PIA). 2-Chloroadenosine was a full agonist, while 2-fluoroadenosine was a partial agonist. Both 2’- and 3’- substitutions have a pronounced effect on its efficacy, although the effect of 2’-substitution was more dramatic. The 4-thio substitution of oxygen may also diminish efficacy, depending on other substitutions. Both N6-methyl and N6-benzyl groups may contribute to the A3 affinity and selectivity; however, an N6-benzyl group but not an N6-methyl group diminishes A3AR efficacy. N6-benzyl substituted adenosine derivatives have similar potency for human and rat A3ARs while N6-methyl substitution was preferable for the human A3AR. The combination of 2-chloro and N6-benzyl substitutions appeared to reduce efficacy further than either modification alone. The A2AAR agonist DPMA was shown to be an antagonist for the human A3AR. Thus, the efficacy of adenosine derivatives at the A3AR appears to be more sensitive to small structural changes than at other subtypes. Potent and selective partial agonists for the A3AR could be identified by screening known adenosine derivatives and by modifying adenosine and the adenosine derivatives.
[Back to top] New Highly Potent and Selective Adenosine A3 Receptor Antagonists
Neil J. Press, Thomas H. Keller, Pamela Tranter, David Beer, Ken Jones, Alexander Faessler, Richard Heng, Christine Lewis, Trevor Howe, Peter Gedeck, Lazzaro Mazzoni and John R. Fozard
A class of potent, selective adenosine A3 receptor antagonists was obtained via optimisation of the screening hit N-[4-(4-methoxyphenyl)-thiazol-2-yl]-acetamide. Structural modifications of this hit revealed very quickly that a 5-(pyridin-4-yl) substituent on the 2- aminothiazole ring was optimal for high potency at the adenosine A3 receptor. Structure activity relationship studies led to both potent and selective A3 receptor antagonists, including N-[5-pyridin-4-yl-4-(3,4,5-trimethoxyphenyl)-thiazol-2-yl]-acetamide, a highly potent aden-osine A3 receptor antagonist with greater than 100- fold selectivity against the related adenosine receptors. As well as demonstrating selective in vitro binding on the human A3 adenosine receptor, this compound was also shown to selectively block the rat A3 receptor in vivo. This important new compound can be readily synthesised in four steps from commercially available starting materials.