Current Topics in Medicinal Chemistry - Volume 3, Issue 4, 2003

In this review the latest developments in ligand design for the adenosine A1 receptor are summarized. Novel series of agonists and antagonists are discussed, leading to the conclusion that ligands truly selective for the human adenosine A1 receptor are still scarce.

Adenosine's diverse physiological functions are mediated by four subtypes of receptors (A1, A2A, A2B and A3). The A1 adenosine receptor pharmacology and therapeutic application of ligands for this receptor are the subjects of this review. A1 receptors are present on the surface of cells in organs throughout the body. Actions mediated by A1 receptors include slowing of heart rate and AV nodal conduction, reduction of atrial contractility, attenuation of the stimulatory actions of catecholamines on beta-adrenergic receptors, reduction of lipolysis in adipose tissue, reduction of urine formation, and inhibition of neuronal activity. Although adenosine analogs with high efficacy, affinity, and selectivity for the A1 receptor are available, the ubiquitous distribution and wide range of physiological actions mediated by A1 receptors are obstacles to development of therapeutic agents that activate these receptors. However, it may be possible to exploit the high A1 “receptor reserve” for some actions of adenosine by use of weak (partial) agonists to target these actions while avoiding others for which receptor reserve is low. The presence of high receptor reserves for the anti-arrhythmic and anti-lipolytic actions of adenosine suggests that partial A1 agonists could be used as anti-arrhythmic and anti-lipolytic agents. In addition, allosteric enhancers of the binding of adenosine to A1 receptors could be used therapeutically to potentiate desirable effects of endogenous adenosine. Antagonists of the A1 receptor can increase urine formation, and because they do not decrease renal blood flow, are particularly useful to maintain glomerular filtration in patients having edema secondary to reduced cardiac function.

The search for potent and selective A2A adenosine receptor agonists has been particularly fruitful in the early nineties. A series of 2-amino, 2-alkoxy, 2-alkythio-, 2-alkynyl-, and 2-alkenyl-derivatives of adenosine (Ado, 1) and N-ethylcarboxamidoadenosine (NECA, 30) have been synthesized and tested mainly on different model of rat A1 and A2A receptor subtypes. From these studies some ligands, such as CGS 21680 (33), HENECA (42), and (S)-PHPNECA (46b), showed to possess high A2A affinity combined with good A2A vs A1 selectivity. More detailed characterization of these ligands at the four cloned human adenosine receptor subtypes revealed that none of the prototypical adenosine receptor agonists exhibits at the same time high affinity and selectivity for the human A2AAR subtype. Both NECA and CGS 21680, which are avalaible as radioligands for this subtype, have lower affinity at human than at rat receptor. The 2-alkynylNECA derivatives HENECA an PHPNECA showed high affinity also at human A3 receptors. In particular, (S)-PHPNECA displayed Kis in the low nanomolar range at A1, A2A, and A3 subtypes and an EC50 of 220 nM at human A2B receptor.On the other hand, it is now well known that the coronary vasodilation induced by Ado in different species is mediated by activation of A2AAR and a compound capable of producing coronary vasodilation through activation of A2AAR, but that is devoid of A1- and A3-agonist activity would have advantage over Ado for use in myocardial perfusion imaging studies. Other potential therapeutic applications of selective A2AAR agonists are as anti-aggregatory, anti-inflammatory, anti-psychotic, and anti-Huntington's disease agents.This review is aimed at presenting a complete overview of the medicinal chemistry development of A2A adenosine receptor agonists and at stressing the strong need for more selective ligands at A2A human subtype.

Due to the clearly demonstrated receptor-receptor interaction between adenosine A2A and dopamine D2 receptors in the basal ganglia, the discovery and development of potent and selective A2A adenosine receptor antagonists became, in the last ten years, an attractive field of research to discovery new drugs for the treatment of neurodegenerative disorders, such as Parkinsons disease.Different compounds have been deeply investigated as A2A adenosine receptor antagonists, which could be classified in two great families: xanthine derivatives and nitrogen poliheterocyclic systems. These studies led to the discovery of some highly potent and selective A2A adenosine receptor antagonists such as ZM241385, SCH58261 and some xanthine derivatives (KW6002), which have been used as pharmacological tools for studying this receptor subtype.However, those compounds showed some problems that do not permit their use in clinical studies, such as poor water solubility (SCH58261, and xanthine derivatives) or good affinity for A2B adenosine receptor subtype (ZM241385).In the last few years great efforts have been made to overcome these problems, trying to optimize not only the pharmacological profile but also the pharmacokinetic character of this class of compounds. The aim of this report is to briefly summarize the recent progress made in this attractive field of research.

Adenosine A2A receptors were cloned about ten years ago and are known to be well conserved among mammals. Rather selective agonists and antagonists are available. In addition, two different knock-out mice have been prepared and extensively characterized. A2A receptors are highly enriched in the basal ganglia and on cells involved in inflammatory reactions. At these sites they are likely to play physiologically important roles. Efforts to develop new therapies based on A2A receptors have focused on these topics. However, A2A receptors are found on many other cell types and on them as well agonists can exert effect.

The low affinity A2B adenosine receptor, like any other adenosine receptor subtype, belongs to the super-family of seven transmembrane domain G protein-coupled receptors (7TMs GPCR) and is classified by the GPCR database in the family of rhodopsin like receptors (Class A of GPCR). It has been cloned from various species, including rat and human, and its sequences are highly similar across species, ranging from 85% identity between human and mouse and 95% identity between rat and mouse. The A2B receptors show a ubiquitous distribution, the highest levels are present in cecum, colon and bladder, followed by blood vessels, lung, eye and mast cells. Through A2B receptors adenosine seems to cause mast cells degranulation, vasodilation, cardiac fibroblast proliferation, inhibition of Tumor Necrosis Factor (TNF-α), increased synthesis of interleukin-6 (IL-6), stimulation of Cl- secretion in intestinal epithelia and hepatic glucose production. Hence, A2B adenosine receptor agonists could be useful in the treatment of cardiac diseases like hypertension or myocardial infarction and in the management of septic shock, while antagonists may serve as novel drugs for asthma, Alzheimer's disease, cystic fibrosis and type-II diabetes.No potent and selective A2B agonists have been reported so far, 5'-N-ethylcarboxamidoadenosine (NECA) is one of the most active. The monosubstitution on N6-position of adenosine is well tolerated and that position appears to be a useful site for increasing A2B potency. Among substituents in 2-position of adenosine only 1- alkynyl chains are effective for A2B potency. In particular, the (S)-2-hydroxypropynyl substituents brought about the highest activity demonstrating that the A2B receptors discriminate between (R) and (S) diastereomers.Hence, (S)-2-phenylhydroxypropynylNECA (PHPNECA), with an EC50 = 0.22 μM, proved to be the most potent A2B agonist reported so far.Classical antagonists for adenosine receptors are alkylxanthines which show considerable potency at A2B receptors. Para substituted 1,3-dialkyl-8-phenylxanthines possess high affinity in binding assays, the 3- unsubstituted 1-alkyl analogues resulted more A2 B selective with the 8-[4-[(N-(2- hydroxyethyl)carboxamidomethyl)oxy]phenyl]-1-propylxanthine (60) showing the highest affinity (Ki = 1.2 nM) and selectivity (A1 / A2B = 60, A2A / A2B = 1,790, A3 / A2B = 360) . Among non-xanthine derivatives very promising are substituted purines, in which combination of appropriate substituents in 2-, 8-, and 9-position could lead to very potent and selective A2B antagonists.

A3 Adenosine receptors (ARs) exhibit large species differences. Potent, selective agonists for rat (e.g. Cl-IB-MECA, 5) and human A3 ARs (e.g. PENECA, 17, and analogs) have been developed during the past years. Potent, selective antagonists for human A3 ARs include the imidazopurinones PSB-10 (28) and PSB-11 (29), the pyrazolotriazolopyrimidines MRE-3005F20 (38) and analogs, and the dihydropyridines (e.g. MRS-1334, 50). For rat A3 ARs only moderately potent antagonists have been identified, such as the pyridine derivative MRS-1523 (51) and the flavonoid MRS-1067 (52), both of which exhibit only a low degree of selectivity versus the other AR subtypes. Selective antagonist radioligands for the human A3 receptor, [3H]MRE- 3008F20 and [3H]PSB-11, have been prepared, while A3-selective agonist radioligands are still lacking. Recent developments also include allosteric modulators, irreversibly binding antagonists, fluorescence-labelled agonists, partial agonists and inverse agonists for A3 ARs. Site-directed mutagenesis and molecular modeling studies have been performed in order to obtain information about the ligand binding site and the process of receptor activation. A3 Adenosine receptors have recently attracted considerable interest as novel drug targets. A3 Agonists may have potential as cardioprotective and cerebroprotective agents, for the treatment of asthma, as antiinflammatory and immunosuppressive agents, and in cancer therapy as cytostatics and chemoprotective compounds. A3 AR antagonists might be therapeutically useful for the acute treatment of stroke, for glaucoma, and also as antiasthmatic and antiallergic drugs, since A3 receptors cannot only mediate antiinflammatory, but also proinflammatory responses. The future development of further pharmacological tools, including potent, selective antagonists for rat A3 receptors and selective agonist radioligands for rat and human receptors will facilitate the evaluation of the (patho)physiological roles of A3 receptors and the pharmacological potential of their ligands.

The present study summarizes the biological effects elicit upon A3 adenosine receptor (A3AR) activation in normal and tumor cells. Anti-inflamatory response is mediated upon A3AR activation in neutrophils, eosinophils and macrophages via direct effect on cell degranulation or the production of anti-inflamatory cytokines. In basophils, which highly express A3AR, degranulation and mediator release upon receptor activation lead to pro-inflammatory effects resulting in bronchospasm and asthma. In other normal cells such as cardiomyocytes, neuronal cells and bone marrow cells A3AR activation induces cytoprotective effects in vitro. In vivo, A3AR agonists act as cardio- and neuroprotective agents and attenuate ischemic damage. Furthermore, agonists to A3AR induce granulocyte colony stimulating factor (G-CSF) production and myeloprotective effect in chemotherapy treated mice. Interestingly, A3AR agonists inhibit tumor cell growth both in vitro and in vivo through a cytostatic effect mediated via the de-regulation of the Wnt signaling pathway.The variety of activities elicit by A3AR agonists suggest their potential use as therapeutic agents in inflammation, brain / cardiac ischemia and cancer. Antagonists to A3AR may be implemented to the therapy of asthma and additional allergic conditions.