Synthesis and preliminary biological evaluation of [3H]-MRE 3008-F20: the first high affinity radioligand antagonist for the human A3 adenosine receptors

Molecular probes for the human adenosine receptors

Adenosine receptors, G protein-coupled receptors (GPCRs) that are activated by the endogenous ligand adenosine, have been considered potential therapeutic targets in several disorders. To date however, only very few adenosine receptor modulators have made it to the market. Increased understanding of these receptors is required to improve the success rate of adenosine receptor drug discovery. To improve our understanding of receptor structure and function, over the past decades, a diverse array of molecular probes has been developed and applied. These probes, including radioactive or fluorescent moieties, have proven invaluable in GPCR research in general. Specifically for adenosine receptors, the development and application of covalent or reversible probes, whether radiolabeled or fluorescent, have been instrumental in the discovery of new chemical entities, the characterization and interrogation of adenosine receptor subtypes, and the study of adenosine receptor behavior in physiological and pathophysiological conditions. This review summarizes these applications, and also serves as an invitation to walk another mile to further improve probe characteristics and develop additional tags that allow the investigation of adenosine receptors and other GPCRs in even finer detail.

Conjugable A3 adenosine receptor antagonists for the development of functionalized ligands and their use in fluorescent probes

Compounds able to simultaneously bind a biological target and be conjugated to a second specific moiety are attractive tools for the development of multi-purpose ligands useful as multi-target ligands, receptor probes or drug delivery systems, with both therapeutic and diagnostic applications. The human A₃ adenosine receptor is a G protein-coupled receptor involved in many physio-pathological conditions, e.g. cancer and inflammation, thus representing a promising research target. In this work, two series of conjugable hA₃AR antagonists, based on the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine nucleus, were developed. The introduction of an aromatic ring at the 5 position of the scaffold, before (phenylacetamido moiety) or after (1,2,3-triazole obtained by click chemistry) the conjugation is aimed to increase affinity and selectivity towards the hA₃AR receptor. As expected, conjugable compounds showed good affinity towards the hA₃AR. In order to prove their potential in the development of hA₃AR ligands for different purposes, compounds were also functionalized with fluorescent probes. Unfortunately, conjugation decreased affinity and selectivity for the target as compared to the hA_2AAR. Computational studies identified specific non-conserved residues of the extracellular loops which constitute a structural barrier able to discriminate between ligands, giving insights into the rational development of new highly selective ligands.

A3 Adenosine Receptors as Modulators of Inflammation: From Medicinal Chemistry to Therapy

The A3 adenosine receptor (A3 AR) subtype is a novel, promising therapeutic target for inflammatory diseases, such as rheumatoid arthritis (RA) and psoriasis, as well as liver cancer. A3 AR is coupled to inhibition of adenylyl cyclase and regulation of mitogen-activated protein kinase (MAPK) pathways, leading to modulation of transcription. Furthermore, A3 AR affects functions of almost all immune cells and the proliferation of cancer cells. Numerous A3 AR agonists, partial agonists, antagonists, and allosteric modulators have been reported, and their structure-activity relationships (SARs) have been studied culminating in the development of potent and selective molecules with drug-like characteristics. The efficacy of nucleoside agonists may be suppressed to produce antagonists, by structural modification of the ribose moiety. Diverse classes of heterocycles have been discovered as selective A3 AR blockers, although with large species differences. Thus, as a result of intense basic research efforts, the outlook for development of A3 AR modulators for human therapeutics is encouraging. Two prototypical selective agonists, N6-(3-Iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA; CF101) and 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA; CF102), have progressed to advanced clinical trials. They were found safe and well tolerated in all preclinical and human clinical studies and showed promising results, particularly in psoriasis and RA, where the A3 AR is both a promising therapeutic target and a biologically predictive marker, suggesting a personalized medicine approach. Targeting the A3 AR may pave the way for safe and efficacious treatments for patient populations affected by inflammatory diseases, cancer, and other conditions.

Pyrazolo-triazolo-pyrimidines as adenosine receptor antagonists: A complete structure-activity profile

In the last 5 years, many efforts have been conducted searching potent and selective human A(3) adenosine antagonists. In this field several different classes of compounds, possessing very good affinity (nM range) and with a broad range of selectivity, have been proposed. Recently, our group synthesized a new series of pyrazolo-triazolo-pyrimidines bearing different substitutions at the N(5) and N(8) positions, which have been described as highly potent and selective human A(3) adenosine receptor antagonists. The present review summarizes available data and provides an overview of the structure-activity relationships found for this class of human A(3) adenosine receptor antagonists.

New heterocyclic ligands for the adenosine receptors P1 and for the ATP receptors P2

Extracellular adenosine and adenine nucleotides induce various cellular responses through activation of P1 and P2 receptors. P1 receptors preferentially recognize adenosine and four different G protein-coupled receptors (A(1), A(2A), A(2B), and A(3) subtypes) have been identified. On the other hand, P2 receptors are activated by adenine and/or uridine nucleotides and classified into two families: ionotropic P2X and G protein-coupled P2Y receptors. In this article, we summarize our studies which led to development of new potent and selective heterocyclic ligands for the adenosine receptors P1 and for the ATP receptors P2X(7).

Recent developments in the field of A2A and A3 adenosine receptor antagonists

In the last years adenosine receptors have been extensively studied, and mainly at present we understand the importance of A(2A) and A(3) adenosine receptors. A(2A) selective adenosine receptors antagonists are promising new drugs for the treatment of Parkinson's disease, while A(3) selective adenosine receptors antagonists have been postulated as novel anti-inflammatory and antiallergic agents; recent studies also indicated a possible employment of these derivatives as antitumour agents. Lately different classes of compounds have been identified as potent A(2A) and A(3) antagonists. In this article we report the past and present efforts which led to development of more potent and selective A(2A) and A(3) antagonists. Our group has mainly worked on the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine nucleus both as A(2A) and A(3) antagonists, aiming to improve the affinity, selectivity and the hydrophilic profile. In fact, we have synthesised several compounds endowed with high affinity and selectivity versus A(2A) adenosine receptors, as 2, 2a-c (K(i)A(2A)=0.12-0.19 nM), or A(3) adenosine receptors, as 4p (K(i)A(3)=0.01 nM) and 4q (K(i)A(3)=0.04 nM).

[(3)H]8-Ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]-purin-5-one ([(3)H]PSB-11), a novel high-affinity antagonist radioligand for human A(3) adenosine receptors

This study describes the preparation and binding properties of [(3)H]PSB-11, a novel, potent, and selective antagonist radioligand for human A(3) adenosine receptors (ARs). [(3)H]PSB-11 binding to membranes of Chinese hamster ovary (CHO) cells expressing the human A(3) AR was saturable and reversible. Saturation experiments showed that [(3)H]PSB-11 labeled a single class of binding sites with high affinity (K(D)=4.9 nM) and limited capacity (B(max)=3500 fmol/mg of protein). PSB-11 is highly selective versus the other adenosine receptor subtypes. The new radioligand shows an extraordinarily low degree of non-specific binding rendering it a very useful tool for studying the (patho)physiological roles of A(3 )ARs.

Pharmacological and biochemical characterization of adenosine receptors in the human malignant melanoma A375 cell line

1. The present work characterizes, from a pharmacological and biochemical point of view, adenosine receptors in the human malignant melanoma A375 cell line. 2. Adenosine receptors were detected by RT - PCR experiments. A1 receptors were characterized using [3H]-DPCPX binding with a KD of 1.9+/-0.2 nM and Bmax of 23+/-7 fmol x mg(-1) of protein. A2A receptors were studied with [3H]-SCH 58261 binding and revealed a KD of 5.1+/-0.2 nM and a Bmax of 220+/-7 fmol x mg(-1) of protein. A3 receptors were studied with the new A3 adenosine receptor antagonist [3H]-MRE 3008F20, the only A3 selective radioligand currently available. Saturation experiments revealed a single high affinity binding site with KD of 3.3+/-0.7 nM and Bmax of 291+/-50 fmol x mg(-1) of protein. 3. The pharmacological profile of radioligand binding on A375 cells was established using typical adenosine ligands which displayed a rank order of potency typical of the different adenosine receptor subtype. 4. Thermodynamic data indicated that radioligand binding to adenosine receptor subtypes in A375 cells was entropy- and enthalpy-driven. 5. In functional assays the high affinity A2A agonists HE-NECA, CGS 21680 and A2A - A2B agonist NECA were able to increase cyclic AMP accumulation in A375 cells whereas A3 agonists Cl-IB-MECA, IB-MECA and NECA were able to stimulate Ca2+ mobilization. In conclusion, all these data indicate, for the first time, that adenosine receptors with a pharmacological and biochemical profile typical of the A1, A2A, A2B and A3 receptor subtype are present on A375 melanoma cell line.

Molecular approach to adenosine receptors: receptor-mediated mechanisms of tissue protection

Adenosine accumulation during ischemia and inflammation protects tissues from injury. In ischemic tissues adenosine accumulates due to inhibition of adenosine kinase, and in inflamed tissues adenosine is formed from adenine nucleotides that are released from many cells including platelets, mast cells, nerves, and endothelium. Nucleotides are rapidly converted to adenosine by a family of ecto-nucleotidases including CD39 and CD73. Activation of A(1) and possibly A(3) adenosine receptors (ARs) protects heart and other tissues by preconditioning through a pathway including protein kinase C and mitochondrial K(ATP) channels. Activation of A(2A) receptors limits reperfusion injury by inhibiting inflammatory processes in neutrophils, platelets, macrophages and T cells. Adenosine produces proinflammatory responses mediated by receptors that vary among species; A(3) and A(2B) receptors mediate degranulation of rodent and human or canine mast cells, respectively. Novel adenosine receptor subtype-selective ligands have recently been developed. These include MRS1754 (A(2B) blocker), MRS1220 (A(3) blocker), MRE 3008F20 (human A(3) blocker), MRS1523 (rat A(3) blocker), and ATL146e (A(2A) agonist). These new pharmacologic tools will help investigators to sort out how adenosine protects tissues from injury and to identify new therapeutic agents that hold promise for the treatment of inflammatory and ischemic diseases.