Adenosine receptors: new opportunities for future drugs

Adenosine: does it have a neuroprotective role after all?

A neuroprotective role for adenosine is commonly assumed. Recent studies revealed that adenosine may unexpectedly, under certain circumstances, have the opposite effects contributing to neuronal damage and death. The basis for this duality may be the activation of distinct subtypes of adenosine receptors, interactions between these receptors, differential actions on neuronal and glial cells, and various time frames of adenosinergic compounds administration. If these aspects are understood, adenosine should remain an interesting target for therapeutical neuroprotective approaches after all.

Tumor-promoting functions of adenosine

Tumor growth is a multifactorial process that, in addition to mutations leading to dysregulated expression of oncogenes and tumor suppressive genes, requires specific conditions that provide a supportive physiological environment at the primary and metastatic sites of the disease. Adenosine is one of the factors potentially contributing to tumor growth that thus far has not received adequate attention, despite evidence for a broad range of cytoprotective, growth-promoting, and immunosuppressive activities. Adenosine accumulates in solid tumors at high concentrations, and has been shown to stimulate tumor growth and angiogenesis and to inhibit cytokine synthesis, adhesion of immune cells to the endothelial wall, and the function of T-cells, macrophages, and natural killer cells. However, the mechanisms whereby adenosine accumulates in cancer and the specific effects that result from this accumulation are not well understood. This article surveys the available evidence that supports an important role of adenosine in cancer.

Radioprotective action of extracellular adenosine on bone marrow cells in mice exposed to gamma rays as assayed by the micronucleus test

The frequency of micronucleated polychromatic erythrocytes (PCEs) in mouse bone marrow was assessed after administration of dipyridamole and/or adenosine monophosphate (AMP) to nonirradiated mice or to mice irradiated 15 min later with a sublethal dose of 6.5 Gy gamma rays. In nonirradiated mice, the administration of the drugs increased the frequency of micronucleated PCEs significantly (by 108%). In contrast, in irradiated mice, the number of radiation-induced micronucleated PCEs was significantly decreased if the mice had been pretreated with dipyridamole or AMP alone (by 24% after administration of each of the compounds) and in particular after administration of the drugs in combination (by 36%).

Adenosine acts as an inhibitor of lymphoma cell growth: a major role for the A3 adenosine receptor

In this study, we demonstrated several mechanisms exploring the inhibitory effect of low-dose adenosine on lymphoma cell growth. Adenosine, a purine nucleoside present in plasma and other extracellular fluids, acts as a regulatory molecule, by binding to G-protein associated cell-surface receptors, A1, A2 and A3. Recently we showed that low-dose adenosine released by muscle cells, inhibits tumour cell growth and thus attributes to the rarity of muscle metastases. In the present work, a cytostatic effect of adenosine on the proliferation of the Nb2-11C rat lymphoma cell line was demonstrated. This effect was mediated through the induction of cell cycle arrest in the G0/G1 phase and by decreasing the telomeric signal in these cells. Adenosine was found to exert its antiproliferative effect mainly through binding to its A3 receptor. The cytostatic anticancer activity, mediated through the A3 adenosine receptor, turns it into a potential target for the development of anticancer therapies.

A PET study of adenosine A1 receptor in anesthetized monkey brain

We demonstrated the distribution of adenosine A1 receptors in the anesthetized monkey brain with positron emission tomography (PET) using [(11)C]KF15372 ([1-propyl-(11)C]8-dicyclopropylmethyl-1, 3-dipropylxanthine). [(11)C]KF15372 was injected intravenously. The regional standardized uptake values and the distribution volume were calculated. We also investigated the effect of carrier on the uptake and regional brain distribution of [(11)C]KF15372. The use of [(11)C]KF15372 with dynamic PET scanning could be an appropriate method to analyze the regional binding potential of adenosine A1 receptors in living brain.

Solid phase synthesis of purines from pyrimidines

In this paper the solid phase synthesis of various substituted purines is described starting from 4,6-dichloro-5-nitropyrimidine. The 4,6-dichloro-5-nitropyrimidine was coupled to Rink amide resin followed by displacement of the second chloride by an amino compound. Reduction of the nitro compound proved to be problematic but was achieved using lithium aluminum hydride/aluminum trichloride. The diamines (13) were then elaborated to purines by three different routes.

Further characterization of a CNS adenosine A2a receptor ligand [11C]KF18446 with in vitro autoradiography and in vivo tissue uptake

PET assessment of the adenosine A2a receptors localized in the striatum offers us a potential new diagnostic tool for neurological disorders. In the present study, we carried out in vitro receptor autoradiography of a newly developed PET ligand [11C]KF18446 ([7-methyl-11C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthin e) with rat brain sections. [11C]KF18446 showed a high striatum/cortex binding ratio (5.0) and low nonspecific binding (<10%), suggesting that [11C]KF18446 has characteristics comparable or slightly superior to [3H]CGS 21680 or [3H]SCH 58261, which are currently available representative A2a receptor ligands. Scatchard analysis indicated a Kd of 9.8 nM and a Bmax of 170 fmol/mm3 tissue in the striatum and a Kd of 16.4 nM and a Bmax of 33 fmol/mm3 tissue in the cortex. Seven xanthine-type and four nonxanthine-type adenosine receptor ligands with an affinity for the adenosine A2a receptors significantly reduced the in vitro binding of [11C]KF18446 to the brain section. The blocking effects were much stronger in the striatum than in the cortex, but did not necessarily parallel their affinity. On the other hand, four xanthine-type ligands and one nonxanthine-type ligand (SCH 58261) of the 11 ligands studied reduced the in vivo uptake of [11C]KF18446 in mice, but other ligands, including A1-selective and nonselective ligands and three nonxanthine-type A2a-selective antagonists did not. We conclude that [11C]KF18446 is a promising adenosine A2a receptor ligand for PET study.

A cautionary note: the actions of adenosine agonists and antagonists may be reversed under certain conditions in primary cultures

It is now generally accepted that adenosine has a neuroprotective role in the central nervous system. Agonists of adenosine such as 2-chloroadenosine (2-ClA) have been shown to be neuroprotective, while antagonists such as 8-phenyltheophylline (8-PT) increase neurotoxicity. However, paradoxical results have been reported with adenosine analogues, especially with respect to length of time of administration. We observe similarly contradictory findings with respect to 2-ClA and 8-PT actions in primary hippocampal cultures exposed to glutamate or kainic acid. We found 8-PT and 2-ClA had antagonist and agonist actions, respectively, only with acute (1 h) treatment; with chronic treatment (24 h), 2-ClA had no effects, while 8-PT had significant agonist actions. We also show that with variations in the type of culturing system, concentration, and pH that 8-PT's neurotoxic antagonist actions could be dramatically changed. We, therefore, present this paper as a cautionary note in experimenting with adenosine analogues.

The discovery and synthesis of highly potent, A2a receptor agonists

A series of N6,2-disubstituted adenosine analogues have been synthesized and their functional activity measured against A2a and A1 receptors. Examples of compounds with both a lipophilic N6-substituent and amino-functionalized 2-position were highly active at the A2a receptor on the human neutrophil.

Water-soluble phosphate prodrugs of 1-propargyl-8-styrylxanthine derivatives, A(2A)-selective adenosine receptor antagonists

Water-soluble prodrugs of potent, A(2A)-selective adenosine receptor (AR) antagonists were prepared. 8-(m-Bromostyryl)-3, 7-dimethyl-1-propargylxanthine (BS-DMPX, 11) and the analogous 8-(m-methoxystyryl)xanthine derivative (MS-DMPX, 5b) were used as starting points. It was found that polar functional groups suitable for the attachment of a prodrug moiety were tolerated on the styryl ring and even better on the 3-substituent. 8-(m-Hydroxystyryl)-DMPX (7) and 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-1-propargylxanthine (5e, MSX-2) were the most potent and A(2A)-selective compounds and were selected for prodrug formation. For the preparation of 5e a new ring-closure method was applied. Treatment of 6-amino-1-(3-hydroxypropyl)-5-(m-methoxycinnamoylamino)-3-propa rgylur acil with hexamethyldisilazane at high temperature resulted in higher yields of the target xanthine than the standard ring-closure procedure using sodium hydroxide. Phosphate prodrugs were prepared by classical phosphorylation using phosphorus oxychloride and alternatively by using a phosphoramidite method. Phosphates of the aliphatic alcohol 5e could be obtained by both methods in similar yields. The phenolic compound 7, however, could be phosphorylated only by using the phosphoramidite method. The disodium salts of the phosphate prodrugs exhibited high water solubility (8-(m-methoxystyryl)-7-methyl-3-[3-O-phosphatylpropyl]-1- propargylxan thine disodium salt, 9b: 17 mM, 9 mg/mL). Prodrug 9b was found to be stable in aqueous solution (pH 7) but readily cleaved by phosphatases to liberate 5e (MSX-2). Compound 5e showed high affinity for rat A(2A) AR (K(i) = 8 nM), human recombinant A(2A) AR (K(i) = 5 nM), and human native A(2A) AR (K(i) = 15 nM) and was highly selective versus rat A(1) AR (110-fold), human recombinant A(2A) AR (500-fold), human A(2B) AR (>2000-fold), and human A(3) AR (>2000-fold).

Novel adenosine A1 receptor antagonists. Synthesis and structure-activity relationships of a novel series of 3-(2-cyclohexenyl-3-oxo-2,3-dihydropyridazin-6-yl)-2-phenylpyrazolo[1,5 -a]pyridines

A novel series of 3-(2-cyclohexenyl-3-oxo-2,3-dihydropyridazin-6-yl)-2-phenylpyrazol o[1,5-a]pyridines was synthesized and evaluated for in vitro adenosine A1 and A2A receptor binding activities. Most of the cyclohexenyl derivatives (7a-e, 8a-s) were found to be potent adenosine A1 receptor antagonists. In a series of analogues of FR166124 (3a), alcohol 7c, nitrile 7e and amide derivatives (7d, 8c, 8r) were found to be more potent A1 antagonists with higher A2A/A1 selectivity than FR166124. Amongst them, 8r showed considerable water solubility (33.3 mg/mL), but lower than that of the sodium salt of FR166124 (> 200 mg/mL). Additionally, FR166124 had strong diuretic activity by both p.o. and iv administration in rats (minimum effective dose=0.1 and 0.032 mg/kg, respectively).

Adenosine receptors as potential therapeutic targets

Recent studies indicate a widening role for adenosine receptors in many therapeutic areas. Adenosine receptors are involved in immunological and inflammatory responses, respiratory regulation, the cardiovascular system, the kidney, various CNS-mediated events including sleep and neuroprotection, as well as central and peripheral pain processes. In this review, the physiological role of adenosine receptors in these key areas is described with reference to the therapeutic potential of adenosine receptor agonists and antagonists.

Central adenosine A(2A) receptors: an overview

Recent advances in molecular biology, biochemistry, cell biology and behavioral pharmacology together with the development of more selective ligands to the various adenosine receptors have increased our understanding of the functioning of central adenosine A(2A) receptors. The A(2A) receptor is one of four adenosine receptors found in the brain. Its expression is highest in striatum, nucleus accumbens and olfactory tubercles, although it also occurs in neurons and microglia in most other brain regions. The receptor has seven transmembrane domains and couples via Gs to adenyl cyclase stimulation. Antagonistic interactions between A(2A) receptors and dopamine D(2) receptors have been described, as stimulation of the A(2A) receptor leads to a reduction in the affinity of D(2) receptors for D(2) receptor agonists. The A(2A) receptor is thought to play a role in a number of physiological responses and pathological conditions. Indeed, A(2A) receptor antagonists may be useful for the treatment of acute and chronic neurodegenerative disorders such as cerebral ischemia or Parkinson's disease. A(2A) receptor agonists may treat certain types of seizures or sleep disorders. This review discusses the characteristics, distribution, pharmacochemical properties and regulation of central A(2A) receptors, as well as A(2A) receptor-mediated behavioural responses and their potential role in various neuropsychiatric disorders.

Selective A1-adenosine receptor antagonists identified using yeast Saccharomyces cerevisiae functional assays

Evaluation of a biased "library" of pyrrolo[2,3-d]pyrimidines using yeast-based functional assays expressing human A1- and A2a-adenosine receptors, led to the A1 selective antagonist 4b. A direct correlation between yeast functional activity and binding data was established. Practical compounds with polar residues at C-4 of the pyrrolopyrimidine system required H-bond donor functionality for high potency.

Discovery of FR166124, a novel water-soluble pyrazolo-[1,5-a]pyridine adenosine A1 receptor antagonist

Novel 3-(2-cycloalkyl and cycloalkenyl-3-oxo-2,3-dihydropyridazin-6-yl)-2-phenylpyrazo lo [1,5-a]-pyridines were synthesized and evaluated for their adenosine A1 receptor binding activities. In this series, FR166124 (3) was found to be the most potent and selective adenosine A1 receptor antagonist, and the double bond of the cyclohexenyl acetic acid group was essential for selectivity of A1 receptor binding. Furthermore, the solubility in water of the sodium salt of FR 166124 was high.