(N)-methanocarba 2,N6-disubstituted adenine nucleosides as highly potent and selective A3 adenosine receptor agonists

Synthesis of enantiomerically pure (S)-methanocarbaribo uracil nucleoside derivatives for use as antiviral agents and P2Y receptor ligands

We have developed an approach toward enantiomerically pure (S)-methanocarba ribonucleosides based on several functional group transformations on a sensitive bicyclo[3.1.0]hexane system. D-ribose was transformed into methanocarba alcohol 3 followed by conversion of the OH group to a nitrile with inversion of configuration at C4. The nitrile group was subsequently reduced in two stages to the 5'-hydroxymethyl group. An ester group appended to a tertiary carbon (C1) was transformed to an amino group as a nucleobase precursor.

Selective A(3) adenosine receptor antagonists derived from nucleosides containing a bicyclo[3.1.0]hexane ring system

We have prepared 50-modified derivatives of adenosine and a corresponding (N)-methanocarba nucleoside series containing a bicyclo[3.1.0]hexane ring system in place of the ribose moiety. The compounds were examined in binding assays at three subtypes of adenosine receptors (ARs) and in functional assays at the A3 AR. The H-bonding ability of a group of 9-riboside derivatives containing a 50-uronamide moiety was reduced by modification of the NH; however these derivatives did not display the desired activity as selective A3 AR antagonists, as occurs with 50-N,N-dimethyluronamides. However, truncated (N)-methanocarba analogues lacking a 40-hydroxymethyl group were highly potent and selective antagonists of the human A3 AR. The compounds were synthesized from D-ribose using a reductive free radical decarboxylation of a 50-carboxy intermediate. A less efficient synthetic approach began with L-ribose, which was similar to the published synthesis of (N)-methanocarba A3AR agonists. Compounds 33b-39b (N6-3-halobenzyl and related arylalkyl derivatives) were potent A3AR antagonists with binding Ki values of 0.7-1.4 nM. In a functional assay of [35S]GTPcS binding, 33b (3-iodobenzyl) completely inhibited stimulation by NECA with a KB of 8.9 nM. Thus, a highly potent and selective series of A3AR antagonists has been described.

The A3 adenosine receptor agonist CF502 inhibits the PI3K, PKB/Akt and NF-kappaB signaling pathway in synoviocytes from rheumatoid arthritis patients and in adjuvant-induced arthritis rats

The A(3) adenosine receptor (A(3)AR) is over-expressed in inflammatory cells and was defined as a target to combat inflammation. Synthetic agonists to this receptor, such as IB-MECA and Cl-IB-MECA, exert an anti-inflammatory effect in experimental animal models of adjuvant- and collagen-induced arthritis. In this study we present a novel A(3)AR agonist, CF502, with high affinity and selectivity at the human A(3)AR. CF502 induced a dose dependent inhibitory effect on the proliferation of fibroblast-like synoviocytes (FLS) via de-regulation of the nuclear factor-kappa B (NF-kappaB) signaling pathway. Furthermore, CF502 markedly suppressed the clinical and pathological manifestations of adjuvant-induced arthritis (AIA) in a rat experimental model when given orally at a low dose (100 microg/kg). As is typical of other G-protein coupled receptors, the A(3)AR expression level was down-regulated shortly after treatment with agonist CF502 in paw and in peripheral blood mononuclear cells (PBMCs) derived from treated AIA animals. Subsequently, a decrease in the expression levels of protein kinase B/Akt (PKB/Akt), IkappaB kinase (IKK), I kappa B (IkappaB), NF-kappaB and tumor necrosis factor-alpha (TNF-alpha) took place. In addition, the expression levels of glycogen synthase kinase-3 beta (GSK-3beta), beta-catenin, and poly(ADP-ribose)polymerase (PARP), known to control the level and activity of NF-kappaB, were down-regulated upon treatment with CF502. Taken together, CF502 inhibits FLS growth and the inflammatory manifestations of arthritis, supporting the development of A(3)AR agonists for the treatment of rheumatoid arthritis.

The imaging probe development center and the production of molecular imaging probes

The Imaging Probe Development Center (IPDC), part of the NIH Roadmap for Medical Research Initiative (http://nihroadmap.nih.gov/) recently became fully operational at its newly refurbished laboratories in Rockville, MD. The IPDC (http://nihroadmap.nih.gov/molecularlibraries/ipdc/) is dedicated to the production of known and novel molecular imaging probes, with its services currently being used by the NIH intramural community, although in the future it is intended that the extramural community will also benefit from the IPDC's resources. The Center has been set up with the belief that molecular imaging, and the probe chemistry that underpins it, will constitute key technologies going forward. As part of the larger molecular libraries and imaging initiative, it is planned that the IPDC will work closely with scientists from the molecular libraries effort. Probes produced at the IPDC include optical, radionuclide and magnetic resonance agents and may encompass any type of contrast agent. As IPDC is a trans-NIH resource it can serve each of the 27 Institutes and Centers that comprise NIH so its influence can be expected to impact widely different subjects and disease conditions spanning biological research. IPDC is expected to play a key part in interdisciplinary collaborative imaging projects and to support translational R&D from basic research through clinical development, for all of the imaging modalities. Examples of probes already prepared or under preparation are outlined to illustrate the breadth of the chemistries undertaken together with a reference outline of the diverse biological applications for which the various probes are intended.

Synthesis and pharmacological characterization of [(125)I]MRS1898, a high-affinity, selective radioligand for the rat A(3) adenosine receptor

A known selective agonist of the A₃ adenosine receptors (AR), MRS1898 [(1′R,2′R,3′S,4′R,5′S)-4-{2-chloro-6-[(3-iodophenylmethyl)amino]purin-9-yl}-1-(methylaminocarbonyl)bicyclo[3.1.0]hexane-2,3-diol], was synthesized in radioactive form and characterized pharmacologically. This agonist ligand series, based on nucleoside analogues containing a rigid, bicyclic ring system in place of the ribose moiety, was selected for radiolabeling due to its high A₃AR affinity across species, with nanomolar binding at both rat and human A₃ARs. The radioiodination of MRS1898 on its N⁶–3-iodobenzyl substituent was accomplished in 76% radiochemical yield by iododestannylation of a 3-(trimethylstannyl)benzyl precursor. [¹²⁵I]MRS1898 bound to the rat A₃AR with a K_d value of 0.17 ± 0.04 nM and a B_max value of 0.66 ± 0.15 pmol/mg protein. The competition binding profiles for other agonists and antagonists obtained with this radioligand are similar to those previously obtained with other radioligands. The advantages of [¹²⁵I]MRS1898 compared with previously used radioligands are primarily its high selectivity and affinity for the rat A₃AR and also its facile synthesis and radiochemical stability; however, a relatively high level of nonspecific binding presents a limitation. Thus, we have introduced the first selective radioligand for the rat A₃AR.

Synthesis of ethyl (1S,2R,3S,4S,5S)-2,3-O-(isopropylidene)-4-hydroxy-bicyclo[3.1.0]hexane-carboxylate from L-ribose: a versatile chiral synthon for preparation of adenosine and P2 receptor ligands

Substitution of the ribose moiety of various nucleosides and nucleotides with the (N)-methanocarba ring system increases the potency and selectivity as ligands at certain subtypes of adenosine and P2 receptors. We have prepared a key intermediate in the synthesis of these derivatives, ethyl (1S,2R,3S,4S,5S)-2,3-O-(isopropylidene)-4-hydroxybicyclo[3.1.0]hexane-carboxylate (15), starting from L-ribose (8) as a readily available, enantiopure building block. L-ribose was converted to the corresponding 5'-iodo derivative (9), which was cleaved reductively with Zn. Improvements were made in subsequent steps corresponding to a published route to biologically important (N)-methanocarba 5'-uronamido nucleosides, and new steps were added to prepare related 5'-nucleotides.

Design of (N)-methanocarba adenosine 5'-uronamides as species-independent A3 receptor-selective agonists

2-Chloro-5'-N-methylcarboxamidoadenosine analogues containing the (N)-methanocarba (bicyclo[3.1.0]hexane) ring system as a ribose substitute display increased selectivity as agonists of the human A(3) adenosine receptor (AR). However, the selectivity in mouse was greatly reduced due to an increased tolerance of this ring system at the mouse A(1)AR. Therefore, we varied substituents at the N(6) and C2 positions in search of compounds that have improved A(3)AR selectivity and are species independent. An N(6)-methyl analogue was balanced in affinity at mouse A(1)/A(3)ARs, with high selectivity in comparison to the A(2A)AR. Substitution of the 2-chloro atom with larger and more hydrophobic substituents, such as iodo and alkynyl groups, tended to increase the A(3)AR selectivity (up to 430-fold) in mouse and preserve it in human. Extended and chemically functionalized alkynyl chains attached at the C2 position of the purine moiety preserved A(3)AR selectivity more effectively than similar chains attached at the 3-position of the N(6)-benzyl group.

Translocation of arrestin induced by human A(3) adenosine receptor ligands in an engineered cell line: comparison with G protein-dependent pathways

Structurally diverse ligands were studied in A(3) adenosine receptor (AR)-mediated beta-arrestin translocation in engineered CHO cells. The agonist potency and efficacy were similar, although not identical, to their G protein signaling. However, differences have also been found. MRS542, MRS1760, and other adenosine derivatives, A(3)AR antagonists in cyclic AMP assays, were partial agonists in beta-arrestin translocation, indicating possible biased agonism. The xanthine 7-riboside DBXRM, a full agonist, was only partially efficacious in beta-arrestin translocation. DBXRM was shown to induce a lesser extent of desensitization compared with IB-MECA. In kinetic studies, MRS3558, a potent and selective A(3)AR agonist, induced beta-arrestin translocation significantly faster than IB-MECA and Cl-IB-MECA. Non-nucleoside antagonists showed similar inhibitory potencies as previously reported. PTX pretreatment completely abolished ERK1/2 activation, but not arrestin translocation. Thus, lead candidates for biased agonists at the A(3)AR have been identified with this arrestin-translocation assay, which promises to be an effective tool for ligand screening.

Chemoenzymatic synthesis and antiviral evaluation of conformationally constrained and 3′-methyl-branched carbanucleosides using both enantiomers of the same building block

Starting from both enantiomers of a readily available building block, a straightforward enantioselective approach to constrained 3'-methyl-2',3'-alpha-oxirane-fused and 3'-methyl-3',4'-alpha-oxirane-fused carbanucleosides bearing different purine base analogues is described. The title compounds were evaluated as potential antiviral agents against important viruses. None of the new compounds had significant antiviral activity at a concentration of 100 microg/mL, which was the highest concentration tested.

Emerging adenosine receptor agonists

Adenosine receptors (ARs) are a four-member subfamily of G protein-coupled receptors and are major targets of caffeine and theophylline. There are four subtypes of ARs, designated as A1, A2A, A2B and A3. Selective agonists are now available for all four subtypes. Over a dozen of these selective agonists are now in clinical trials for various conditions, although none has received regulatory approval except for the endogenous AR agonist adenosine itself. A1AR agonists are in clinical trials for cardiac arrhythmias and neuropathic pain. A2AAR agonists are now in trials for myocardial perfusion imaging and as anti-inflammatory agents. A2BAR agonists are under preclinical scrutiny for potential treatment of cardiac ischemia. A3AR agonists are in clinical trials for the treatment of rheumatoid arthritis and colorectal cancer. The present review will mainly cover the agonists that are presently in clinical trials for various conditions and only a brief introduction will be given to major chemical classes of AR agonists presently under investigation.

P2Y1 antagonists: combining receptor-based modeling and QSAR for a quantitative prediction of the biological activity based on consensus scoring

P2Y1 is an ADP-activated G protein-coupled receptor (GPCR). Its antagonists impede platelet aggregation in vivo and are potential antithrombotic agents. Combining ligand and structure-based modeling we generated a consensus model (LIST-CM) correlating antagonist structures with their potencies. We docked 45 antagonists into our rhodopsin-based human P2Y1 homology model and calculated docking scores and free binding energies with the Linear Interaction Energy (LIE) method in continuum-solvent. The resulting alignment was also used to build QSAR based on CoMFA, CoMSIA, and molecular descriptors. To benefit from the strength of each technique and compensate for their limitations, we generated our LIST-CM with a PLS regression based on the predictions of each methodology. A test set featuring untested substituents was synthesized and assayed in inhibition of 2-MeSADP-stimulated PLC activity and in radioligand binding. LIST-CM outperformed internal and external predictivity of any individual model to predict accurately the potency of 75% of the test set.

Design, synthesis, and biological evaluation of LNA nucleosides as adenosine A3 receptor ligands

We have prepared a series of adenosine analogs based on the bicyclo[2.2.1]heptane scaffold of locked nucleic acid (LNA) and tested them for both agonist and antagonist activity at the adenosine A(3) receptor. The design of these derivatives was based on the known A(3) agonist IB-MECA and related compounds. Modifications thus include the 5'-uronamides and N(6)-(3-iodobenzyl) derivatives. In this way we have prepared analogs of known A(3) agonists with the sugar ring restricted in an N-conformation. For comparison we have also prepared 2'-O-methyl derivatives of IB-MECA. The LNA nucleosides showed no agonist activity but some of them are potent antagonists. The 2'-O-methyl derivative of IB-MECA is an agonist with similar potency as the parent compound.

Efficient and selective reduction protocols of the 2,2-dimethyl-1,3-benzodioxan-4-one functional group to readily provide both substituted salicylaldehydes and 2-hydroxybenzyl alcohols

Two complementary procedures have been developed that selectively allow for the synthesis of either substituted salicylaldehydes or the corresponding 2-hydroxylbenzyl alcohols upon treatment of the 2,2-dimethyl-1,3-benzodioxan-4-one functional group with DIBAL-H or LAH, respectively.

Barrier qualities of the mouse eye to topically applied drugs

The mouse eye displays unusually rapid intraocular pressure (IOP) responses to topically applied drugs as measured by the invasive servo-null micropipette system (SNMS). To learn if the time course reflected rapid drug transfer across the thin mouse cornea and sclera, we monitored a different parameter, pupillary size, following topical application of droplets containing 40 microM (0.073 microg) carbachol. No miosis developed from this low carbachol concentration unless the cornea was impaled with an exploring micropipette as used in the SNMS. We also compared the mouse IOP response to several purinergic drugs, measured by the invasive SNMS and non-invasive pneumotonometry. Responses to the previously studied non-selective adenosine-receptor (AR) agonist adenosine, the A(3)-selective agonist Cl-IB-MECA and the A(3)-selective antagonist MRS 1191 were all enhanced to varying degrees, in time and magnitude, by corneal impalement. We conclude that the thin ocular coats of the mouse eye actually present a substantial barrier to drug penetration. Corneal impalement with even fine-tipped micropipettes can significantly enhance entry of topically-applied drugs into the mouse aqueous humor, reflecting either direct diffusion around the tip or a more complex impalement-triggered change in ocular barrier properties. Comparison of invasive and non-invasive measurement methods can document drug efficacy at intraocular target sites even if topical drug penetration is too slow to manifest convincing physiologic effects in intact eyes.

Action of nucleosides and nucleotides at 7 transmembrane-spanning receptors

Ribose ring-constrained nucleosides and nucleotides to act at cell-surface purine recesptors have been designed and synthesized. At the P2Y1 nucleotide receptor and the A3 adenosine receptor (AR) the North envelope conformation of ribose is highly preferred. We have applied mutagenesis and rhodopsin-based homology modeling to the study of purine receptors and used the structural insights gained to assist in the design of novel ligands. Two subgroups of P2Y receptors have been defined, containing different sets of cationic residues for coordinating the phosphate groups. Modeling/mutagenesis of adenosine receptors has focused on determinants of intrinsic efficacy in adenosine derivatives and on a conserved Trp residue (6.48) which is involved in the activation process. The clinical use of adenosine agonists as cytoprotective agents has been limited by the widespread occurrence of ARs, thus, leading to undesirable side effects of exogenously administered adenosine derivatives. In order to overcome the inherent nonselectivity of activating the native receptors, we have introduced the concept of neoceptors. By this strategy, intended for eventual use in gene therapy, the putative ligand binding site of a G protein-coupled receptor is reengineered for activation by synthetic agonists (neoligands) built to have a structural complementarity. Using a rational design process we have identified neoceptor-neoligand pairs which are pharmacologically orthogonal with respect to the native species.

Docking studies of agonists and antagonists suggest an activation pathway of the A3 adenosine receptor

Structural determinants of ligand efficacy in the human A(3) adenosine receptor (AR) were studied using pharmacophore and docking analyses of various categories of A(3) selective ligands: inverse agonist, neutral antagonist (nonnucleoside and nucleoside), and agonist (partial and full). The homology modeling of GPCRs was adapted to provide two templates: the rhodopsin-based resting state for antagonist binding and a putative Meta I state, conformationally altered at a key residue (W6.48), for agonist binding. The preferential binding domains and/or local conformational changes associated with docking of three high affinity A(3)AR ligands were compared: inverse agonist PSB-11 1 ((R)-8-ethyl-4-methyl-2-phenyl-imidazo[2,1-i]purin-5-one); neutral antagonist MRE-3008F20 7 (5-[[(4-methoxyphenyl)amino]carbonyl]amino-8-methyl-2-(2-furyl)pyra-zolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine), and full agonist Cl-IB-MECA 21 (2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarboxamidoadenosine) to define a distinct recognition mode for each. Ribose-containing agonists were more hydrophilic than nonnucleoside antagonists, and H-bonding ability at the ribose 3'- and 5'-positions was required for agonism. From the receptor perspective, common requirements for activation included the destabilization of H-bond networks at W6.48 and H7.43, the specific interactions of the ribose moiety in its putative hydrophilic pocket at T3.36, S7.42, and H7.43, the stabilization of the complex by inward movement of F5.43, and the characteristic rotation of W6.48. By analogy, outward rotation of the W6.48 side-chain upon activation of an internally-crosslinking mutant M(3) muscarinic receptor was indicated by constrained molecular dynamics (MD). Our results are consistent with an anti-clockwise rotation (from the extracellular view) of transmembrane domains 3, 5, 6, and 7, as proposed for other Family A GPCRs. Thus, the putative conformational changes associated with A(3)AR activation indicate a shared mechanism of GPCR activation similar to rhodopsin.

A3 adenosine receptors and mitogen-activated protein kinases in lung injury following in vivo reperfusion

INTRODUCTION

Although activation of A3 adenosine receptors attenuates reperfusion lung injury and associated apoptosis, the signaling pathway that mediates this protection remains unclear. Adenosine agonists activate mitogen-activated protein kinases, and these kinases have been implicated in ischemia/reperfusion injury; the purpose of this study was therefore to determine whether A3 adenosine receptor stimulation with reperfusion modulates expression of the different mitogen-activated protein kinases. In addition, we compared the effect of the A3 adenosine agonist IB-MECA with the newly synthesized, highly selective A3 adenosine receptor agonist MRS3558 on injury in reperfused lung.

METHOD

Studies were performed in an in vivo spontaneously breathing cat model, in which the left lower lobe of the lung was isolated and subjected to 2 hours of ischemia and 3 hours of reperfusion. The selective A3 adenosine receptor agonists IB-MECA (0.05 mg/kg, 0.1 mg/kg, or 0.3 mg/kg) and MRS3558 (0.05 mg/kg or 0.1 mg/kg) were administered before reperfusion.

RESULTS

Both A3 adenosine receptor agonists administered before reperfusion markedly (P < 0.01) attenuated indices of injury and apoptosis, including the percentage of injured alveoli, wet/dry weight ratio, myeloperoxidase activity, TUNEL (in situ TdT-mediated dUTP nick end labeling)-positive cells, and caspase 3 activity and expression. The more pronounced effects at low doses were observed with MRS3558. Increases in phosphorylated c-Jun amino-terminal protein kinase (JNK), p38, and extracellular signal-regulated kinase (ERK)1/2 levels were observed by the end of reperfusion compared with controls. Pretreatment with the A3 agonists upregulated phosphorylated ERK1/2 levels but did not modify phosphorylated JNK and p38 levels.

CONCLUSION

The protective effects of A3 adenosine receptor activation are mediated in part through upregulation of phosphorylated ERK. Also, MRS3558 was found to be more potent than IB-MECA in attenuating reperfusion lung injury. The results suggest not only that enhancement of the ERK pathway may shift the balance between cell death and survival toward cell survival, but also that A3 agonists have potential as an effective therapy for ischemia/reperfusion-induced lung injury.

Adenosine receptors as therapeutic targets

Adenosine receptors are major targets of caffeine, the most commonly consumed drug in the world. There is growing evidence that they could also be promising therapeutic targets in a wide range of conditions, including cerebral and cardiac ischaemic diseases, sleep disorders, immune and inflammatory disorders and cancer. After more than three decades of medicinal chemistry research, a considerable number of selective agonists and antagonists of adenosine receptors have been discovered, and some have been clinically evaluated, although none has yet received regulatory approval. However, recent advances in the understanding of the roles of the various adenosine receptor subtypes, and in the development of selective and potent ligands, as discussed in this review, have brought the goal of therapeutic application of adenosine receptor modulators considerably closer.

The synthesis of highly potent, selective, and water-soluble agonists at the human adenosine A3 receptor

Using a combination of parallel and directed synthesis, the discovery of a highly potent and selective series of adenosine A3 agonists was achieved. High aqueous solubility, required for the intended parenteral route of administration, was achieved by the presence of one or two basic amine functional groups.

Intramolecular Cyclopropanation of Unsaturated Terminal Aziridines

Regio- and stereoselective deprotonation of bishomoallylic terminal N-Bus (Bus=tert-butylsulfonyl)-protected aziridines generate aziridinyl anions that undergo diastereoselective intramolecular cyclopropanation giving trans-2-aminobicyclo[3.1.0]hexanes in good to excellent yields.

Conversion of A3 adenosine receptor agonists into selective antagonists by modification of the 5'-ribofuran-uronamide moiety

The highly selective agonists of the A(3) adenosine receptor (AR), Cl-IB-MECA (2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarboxamidoadenosine), and its 4'-thio analogue, were successfully converted into selective antagonists simply by appending a second N-methyl group on the 5'-uronamide position. The 2-chloro-5'-(N,N-dimethyl)uronamido analogues bound to, but did not activate, the human A(3)AR, with K(i) values of 29 nM (4'-O) and 15 nM (4'-S), showing >100-fold selectivity over A(1), A(2A), and A(2B)ARs. Competitive antagonism was demonstrated by Schild analysis. The 2-(dimethylamino)-5'-(N,N-dimethyl)uronamido substitution also retained A(3)AR selectivity but lowered affinity.

Semi-rational design of (north)-methanocarba nucleosides as dual acting A(1) and A(3) adenosine receptor agonists: novel prototypes for cardioprotection

Ring-constrained adenosine analogues have been designed to act as dual agonists at tissue-protective A(1) and A(3) adenosine receptors (ARs). 9-Ribosides transformed into the ring-constrained (N)-methanocarba-2-chloro-5'-uronamides consistently lost affinity at A(1)/A(2A)ARs and gained at A(3)AR. Among 9-riboside derivatives, only N(6)-cyclopentyl and 7-norbornyl moieties were extrapolated for mixed A(1)/A(3) selectivity and rat/human A(3)AR equipotency. Consequently, 2 was balanced in affinity and potency at A(1)/A(3)ARs as envisioned and dramatically protected in an intact heart model of global ischemia and reperfusion.

A topological function based on spectral moments for predicting affinity toward A3 adenosine receptors

The spectral moment descriptors have been applied to the study of affinity for A(3) adenosine receptors of 32 adenosine analogues. A model, able to describe more than 95% of the variance in the experimental activity, was developed with the use of the above-mentioned approach. The fragment contributions to the activity carried out show that the sulfonamido moiety at the N(6) position and hydrogen bonding play an important role in the interaction with the receptor.

Structure−Activity Relationships of 2-Chloro-N6-substituted-4‘-thioadenosine-5‘-uronamides as Highly Potent and Selective Agonists at the Human A3 Adenosine Receptor

We have established structure-activity relationships of novel 4'-thionucleoside analogues as the A(3) adenosine receptor (AR) agonists. Binding affinity, selectivity toward other AR subtypes, and efficacy in inhibition of adenylate cyclase were studied. From this study, 2-chloro-N(6)-methyl-4'-thioadenosine-5'-methyluronamide (36a) emerged as the most potent and selective agonist at the human A(3) AR. We have also revealed that, similar to 4'-oxoadenosine analogues, at least one hydrogen on the 5'-uronamide moiety was necessary for high-affinity binding at the human A(3) AR, presumably to allow this group to donate a H bond within the binding site. Furthermore, bulky substituents on the 5'-uronamide reduced binding affinity, but in some cases large 5'-uronamide substituents, such as substituted benzyl and 2-phenylethyl groups, maintained moderate affinity with reduced efficacy, leading to A(3) AR partial agonists or antagonists. In several cases for which the corresponding 4'-oxonucleosides have been studied, the 4'-thionucleosides showed higher binding affinity to the A(3) AR.

"Reversine" and its 2-substituted adenine derivatives as potent and selective A3 adenosine receptor antagonists

The dedifferentiation agent "reversine" [2-(4-morpholinoanilino)-N(6)-cyclohexyladenine 2] was found to be a moderately potent antagonist for the human A(3) adenosine receptor (AR) with a K(i) value of 0.66 microM. This result prompted an exploration of the structure-activity relationship of related derivatives, synthesized via sequential substitution of 6-chloro-2-fluoropurine with selected nucleophiles. Optimization of substituents at these two positions identified 2-(phenylamino)-N(6)-cyclohexyladenine (12), 2-(phenylamino)-N(6)-cycloheptyladenine (19), and 2-phenylamino-N(6)-endo-norbornyladenine (21) as potent A(3) AR ligands with K(i) values of 51, 42, and 37 nM, respectively, with 30-200-fold selectivity in comparison to A(1) and A(2A) ARs. The most selective A(3) AR antagonist (>200-fold) was 2-(phenyloxy)-N(6)-cyclohexyladenine (22). 9-Methylation of 12, but not 19, was well-tolerated in A(3) AR binding. Extension of the 2-phenylamino group to 2-benzyl- and 2-(2-phenylethylamino) reduced affinity. In the series of 2-(phenylamino), 2-(phenyloxy), and 2-(phenylthio) substitutions, the order of affinity at the A(3) AR was oxy > or = amino > thio. Selected derivatives, including reversine (K(B) value of 466 nM via Schild analysis), competitively antagonized the functional effects of a selective A(3) AR agonist, i.e., inhibition of forskolin-stimulated cAMP production in stably transfected Chinese hamster ovary (CHO) cells. These results are in agreement with other studies suggesting the presence of a lipophilic pocket in the AR binding site that is filled by moderately sized cycloalkyl rings at the N(6) position of both adenine and adenosine derivatives. Thus, the compound series reported herein comprise an important new series of selective A(3) AR antagonists. We were unable to reproduce the dedifferentiation effect of reversine, previously reported, or to demonstrate any connection between A(3) AR antagonist effects and dedifferentiation.

Purine receptors: GPCR structure and agonist design

An integrated approach to the study of drug-receptor interactions has been applied to adenosine receptors (ARs) and P2Y nucleotide receptors. This approach includes probing the receptor structure through site-directed mutagenesis and molecular modeling, in concert with altering the structure of the agonist ligands. Goals of this structural approach are to generate a testable hypothesis for location of the binding site and subsequently to enable the rational design of new agonists and antagonists. In this manner, receptor subtype selectivity has been increased, and agonists have been converted into partial agonists and antagonists. An approach to receptor engineering (neoceptors) has been explored, in which synthetic small molecule agonists (neoligands) are specifically tailored to activate only receptors in which the putative binding sites have been modified. This orthogonal approach to receptor activation, intended for eventual gene therapy, has been demonstrated for A3 and A2A ARs.