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

Synthesis and biological evaluation of sugar-modified truncated carbanucleosides as A2A and A3 adenosine receptor ligands to explore conformational effect to the receptors

This study investigated the impact of conformation on the binding affinity of carbanucleosides to A2A and A3 adenosine receptors (ARs). A series of nucleosides, including saturated, unsaturated, North (N)-methano, and South (S)-methanocarbanucleosides was prepared, and their binding affinities to A2AAR and A3AR were assessed. Biological evaluations revealed that all synthesized (S)-methanocarbanucleosides had negligible binding to both receptors, and most (N)-methanocarbanucleosides exhibited high binding affinities. Molecular docking analysis showed that the (N)-methanocarbanucleoside 6a exhibited favorable interactions and minimal steric clashes in both A2AAR and A3AR. Conversely, the (S)-methanocarbanucleoside 7a appears to encounter significant steric clashes, which impeded its binding to A2AAR. Furthermore, when adopting the South conformation 7a was unable to bind to A3AR. Expanding upon the (N)-methanocarba moiety, various C8-aromatic groups were introduced to convert A2AAR agonists into antagonists and these modified compounds also exhibited strong binding affinity. These results suggest that the North conformation is favored by both A2AAR and A3AR, and that (N)-methanocarbanucleosides can serve as versatile structural moieties for dual targeting of A2AAR and A3AR. These findings offer promising avenues for the development of dual ligands for therapeutic applications in obesity and immunotherapy.

Structure-Activity Relationship of Truncated 4'-Selenonucleosides: A3 Adenosine Receptor Activity and Binding Selectivity

This study explored the impact of structural modifications on truncated 4'-selenonucleosides as ligands for the A3 adenosine receptor (AR). We synthesized and evaluated a series of these compounds for their binding affinities, functional activities, and structural interactions by using computational modeling. The SAR study revealed that all compounds exhibited selective and notable hA3AR binding, among which 6l (K i = 5.2 nM) and 6m (K i = 5.7 nM) were found as the best binding compounds. The representative N 6-cyclopropyl compound 6m was found to be a partial agonist, contrasting with the antagonist profiles of truncated 4'-oxo and 4'-thionucleosides. Computational docking highlighted 6m's unique interaction with Thr94 at the A3AR binding site. This research not only advances our understanding of A3AR ligand interactions but also highlights the potential of truncated 4'-selenonucleosides as effective A3AR modulators.

Truncated (N)-Methanocarba Nucleosides as Partial Agonists at Mouse and Human A3 Adenosine Receptors: Affinity Enhancement by N6-(2-Phenylethyl) Substitution

Dopamine-derived N6-substituents, compared to N6-(2-phenylethyl), in truncated (N)-methanocarba (bicyclo[3.1.0]hexyl) adenosines favored high A3 adenosine receptor (AR) affinity/selectivity, e.g., C2-phenylethynyl analogue 15 (MRS7591, Ki = 10.9/17.8 nM, at human/mouse A3AR). 15 was a partial agonist in vitro (hA3AR, cAMP inhibition, 31% Emax; mA3AR, [35S]GTP-γ-S binding, 16% Emax) and in vivo and also antagonized hA3AR in vitro. Distal H-bonding substitutions of the N6-(2-phenylethyl) moiety particularly enhanced mA3AR affinity by polar interactions with the extracellular loops, predicted using docking and molecular dynamics simulation with newly constructed mA3AR and hA3AR homology models. These hybrid models were based on an inactive antagonist-bound hA1AR structure for the upper part of TM2 and an agonist-bound hA2AAR structure for the remaining TM portions. These species-independent A3AR-selective nucleosides are low efficacy partial agonists and novel, nuanced modulators of the A3AR, a drug target of growing interest.

Structural Characterization of Agonist Binding to an A3 Adenosine Receptor through Biomolecular Simulations and Mutagenesis Experiments

The adenosine A₃ receptor (A₃R) binds adenosine and is a drug target against cancer cell proliferation. Currently, there is no experimental structure of A₃R. Here, we have generated a molecular model of A₃R in complex with two agonists, the nonselective 1-(6-amino-9H-purin-9-yl)-1-deoxy-N-ethyl-β-d-ribofuranuronamide (NECA) and the selective 1-deoxy-1-[6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-N-methyl-β-d-ribofuranuronamide (IB-MECA). Molecular dynamics simulations of the wild-type A₃R in complex with both agonists, combined with in vitro mutagenic studies revealed important residues for binding. Further, molecular mechanics-generalized Born surface area calculations were able to distinguish mutations that reduce or negate agonistic activity from those that maintained or increased the activity. Our studies reveal that selectivity of IB-MECA toward A₃R requires not only direct interactions with residues within the orthosteric binding area but also with remote residues. Although V169^5.30 is considered to be a selectivity filter for A₃R binders, when it was mutated to glutamic acid or alanine, the activity of IB-MECA increased by making new van der Waals contacts with TM5. This result may have implications in the design of new A₃R agonists.

Probing structure-activity relationship in β-arrestin2 recruitment of diversely substituted adenosine derivatives

In the adenosine receptor (AR) subfamily of G protein-coupled receptors (GPCRs), biased agonism has been described for the human A₁AR, A_2BAR and A₃AR. While diverse A₃AR agonists have been evaluated for receptor binding and G_i-mediated cAMP signalling, the β-arrestin2 (βarr2) pathway has been left largely unexplored. We screened nineteen diverse adenosine derivatives for βarr2 recruitment using a stable hA₃AR-NanoBit^®-βarr2 HEK293T cell line. Their activity profiles were compared with a cAMP accumulation assay in stable hA₃AR CHO cells. Structural features linked to βarr2 activation were further investigated by the evaluation of an additional ten A₃AR ligands. The A₃AR-selective reference agonist 2-Cl-IB-MECA, which is a full agonist in terms of cAMP inhibition, only showed partial agonist behaviour in βarr2 recruitment. Highly A₃AR-selective (N)-methanocarba 5'-uronamide adenosine derivatives displayed higher potency in both cAMP signalling and βarr2 recruitment than reference agonists NECA and 2-Cl-IB-MECA. Their A₃AR-preferred conformation tolerates C2-position substitutions, for increased βarr2 efficacy, better than the flexible scaffolds of ribose derivatives. The different amino functionalities in the adenosine scaffold of these derivatives each seem to be important for signalling as well. In conclusion, we have provided insights into ligand features that can help to guide the future therapeutic development of biased A₃AR ligands with respect to G-protein and βarr2 signalling.

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.

Scaffold Repurposing of Nucleosides (Adenosine Receptor Agonists): Enhanced Activity at the Human Dopamine and Norepinephrine Sodium Symporters

We have repurposed (N)-methanocarba adenosine derivatives (A3 adenosine receptor (AR) agonists) to enhance radioligand binding allosterically at the human dopamine (DA) transporter (DAT) and inhibit DA uptake. We extended the structure-activity relationship of this series with small N6-alkyl substitution, 5'-esters, deaza modifications of adenine, and ribose restored in place of methanocarba. C2-(5-Halothien-2-yl)-ethynyl 5'-methyl 9 (MRS7292) and 5'-ethyl 10 (MRS7232) esters enhanced binding at DAT (EC50 ∼ 35 nM) and at the norepinephrine transporter (NET). 9 and 10 were selective for DAT compared to A3AR in the mouse but not in humans. At DAT, the binding of two structurally dissimilar radioligands was enhanced; NET binding of only one radioligand was enhanced; SERT radioligand binding was minimally affected. 10 was more potent than cocaine at inhibiting DA uptake (IC50 = 107 nM). Ribose analogues were weaker in DAT interaction than the corresponding bicyclics. Thus, we enhanced the neurotransmitter transporter activity of rigid nucleosides while reducing A3AR affinity.

New selective A2A agonists and A3 antagonists for human adenosine receptors: synthesis, biological activity and molecular docking studies

Synthesis and pharmacological characterization of a new series of adenosine derivatives on the four human adenosine receptors are reported.

Extended N(6) substitution of rigid C2-arylethynyl nucleosides for exploring the role of extracellular loops in ligand recognition at the A3 adenosine receptor

2-Arylethynyl-(N)-methanocarba adenosine 5'-methyluronamides containing rigid N(6)-(trans-2-phenylcyclopropyl) and 2-phenylethynyl groups were synthesized as agonists for probing structural features of the A3 adenosine receptor (AR). Radioligand binding confirmed A3AR selectivity and N(6)-1S,2R stereoselectivity for one diastereomeric pair. The environment of receptor-bound, conformationally constrained N(6) groups was explored by docking to an A3AR homology model, indicating specific hydrophobic interactions with the second extracellular loop able to modulate the affinity profile. 2-Pyridylethynyl derivative 18 was administered orally in mice to reduce chronic neuropathic pain in the chronic constriction injury model.

Synthesis and evaluation of N⁶-substituted apioadenosines as potential adenosine A₃ receptor modulators

Adenosine receptors (ARs) trigger signal transduction pathways inside the cell when activated by extracellular adenosine. Selective modulation of the A₃AR subtype may be beneficial in controlling diseases such as colorectal cancer and rheumatoid arthritis. Here, we report the synthesis and evaluation of β-D-apio-D-furano- and α-D-apio-L-furanoadenosines and derivatives thereof. Introduction of a 2-methoxy-5-chlorobenzyl group at N(6) of β-D-apio-D-furanoadenosine afforded an A₃AR antagonist (10c, Ki=0.98 μM), while a similar modification of an α-D-apio-L-furanoadenosine gave rise to a partial agonist (11c, Ki=3.07 μM). The structural basis for this difference was examined by docking to an A₃AR model; the antagonist lacked a crucial interaction with Thr94.

Structural probing of off-target G protein-coupled receptor activities within a series of adenosine/adenine congeners

We studied patterns of off-target receptor interactions, mostly at G protein-coupled receptors (GPCRs) in the µM range, of nucleoside derivatives that are highly engineered for nM interaction with adenosine receptors (ARs). Because of the considerable interest of using AR ligands for treating diseases of the CNS, we used the Psychoactive Drug Screening Program (PDSP) for probing promiscuity of these adenosine/adenine congeners at 41 diverse receptors, channels and a transporter. The step-wise truncation of rigidified, trisubstituted (at N⁶, C2, and 5′ positions) nucleosides revealed unanticipated interactions mainly with biogenic amine receptors, such as adrenergic receptors and serotonergic receptors, with affinities as high as 61 nM. The unmasking of consistent sets of structure activity relationship (SAR) at novel sites suggested similarities between receptor families in molecular recognition. Extensive molecular modeling of the GPCRs affected suggested binding modes of the ligands that supported the patterns of SAR at individual receptors. In some cases, the ligand docking mode closely resembled AR binding and in other cases the ligand assumed different orientations. The recognition patterns for different GPCRs were clustered according to which substituent groups were tolerated and explained in light of the complementarity with the receptor binding site. Thus, some likely off-target interactions, a concern for secondary drug effects, can be predicted for analogues of this set of substructures, aiding the design of additional structural analogues that either eliminate or accentuate certain off-target activities. Moreover, similar analyses could be performed for unrelated structural families for other GPCRs.

Synthesis and anti-renal fibrosis activity of conformationally locked truncated 2-hexynyl-N(6)-substituted-(N)-methanocarba-nucleosides as A3 adenosine receptor antagonists and partial agonists

Truncated N⁶-substituted-(N)-methanocarba-adenosine derivatives with 2-hexynyl substitution were synthesized to examine parallels with corresponding 4′-thioadenosines. Hydrophobic N⁶ and/or C2 substituents were tolerated in A₃AR binding, but only an unsubstituted 6-amino group with a C2-hexynyl group promoted high hA_2AAR affinity. A small hydrophobic alkyl (4b and 4c) or N⁶-cycloalkyl group (4d) showed excellent binding affinity at the hA₃AR and was better than an unsubstituted free amino group (4a). A₃AR affinities of 3-halobenzylamine derivatives 4f–4i did not differ significantly, with K_i values of 7.8–16.0 nM. N⁶-Methyl derivative 4b (K_i = 4.9 nM) was a highly selective, low efficacy partial A₃AR agonist. All compounds were screened for renoprotective effects in human TGF-β1-stimulated mProx tubular cells, a kidney fibrosis model. Most compounds strongly inhibited TGF-β1-induced collagen I upregulation, and their A₃AR binding affinities were proportional to antifibrotic effects; 4b was most potent (IC₅₀ = 0.83 μM), indicating its potential as a good therapeutic candidate for treating renal fibrosis.

Methanocarba ring as a ribose modification in ligands of G protein-coupled purine and pyrimidine receptors: synthetic approaches

Adenosine receptors (ARs) and P2Y receptors for purine and pyrimidine nucleotides have widespread distribution and regulate countless physiological processes. Various synthetic ligands are in clinical trials for treatment of inflammatory diseases, pain, cancer, thrombosis, ischemia, and other conditions. The methanocarba (bicyclo[3.1.0]hexane) ring system as a rigid substitution for ribose, which maintains either a North (N) or South (S) conformation, tends to preserve or enhance the potency and/or selectivity for certain receptor subtypes. This review summarizes recent developments in the synthetic approaches to these biologically important nucleoside and nucleotide analogues.

Rational design of sulfonated A3 adenosine receptor-selective nucleosides as pharmacological tools to study chronic neuropathic pain

(N)-Methanocarba(bicyclo[3.1.0]hexane)adenosine derivatives were probed for sites of charged sulfonate substitution, which precludes diffusion across biological membranes, e.g., blood-brain barrier. Molecular modeling predicted that sulfonate groups on C2-phenylethynyl substituents would provide high affinity at both mouse (m) and human (h) A3 adenosine receptors (ARs), while a N(6)-p-sulfophenylethyl substituent would determine higher hA3AR vs mA3AR affinity. These modeling predictions, based on steric fitting of the binding cavity and crucial interactions with key residues, were confirmed by binding/efficacy studies of synthesized sulfonates. N(6)-3-Chlorobenzyl-2-(3-sulfophenylethynyl) derivative 7 (MRS5841) bound selectively to h/m A3ARs (Ki(hA3AR) = 1.9 nM) as agonist, while corresponding p-sulfo isomer 6 (MRS5701) displayed mixed A1/A3AR agonism. Both nucleosides administered ip reduced mouse chronic neuropathic pain that was ascribed to either A3AR or A1/A3AR using A3AR genetic deletion. Thus, rational design methods based on A3AR homology models successfully predicted sites for sulfonate incorporation, for delineating adenosine's CNS vs peripheral actions.

Structural sweet spot for A1 adenosine receptor activation by truncated (N)-methanocarba nucleosides: receptor docking and potent anticonvulsant activity

A(1) adenosine receptor (AR) agonists display antiischemic and antiepileptic neuroprotective activity, but peripheral cardiovascular side effects impeded their development. SAR study of N(6)-cycloalkylmethyl 4'-truncated (N)-methanocarba-adenosines identified 10 (MRS5474, N(6)-dicyclopropylmethyl, K(i) = 47.9 nM) as a moderately A(1)AR-selective full agonist. Two stereochemically defined N(6)-methynyl group substituents displayed narrow SAR; groups larger than cyclobutyl greatly reduced AR affinity, and those larger or smaller than cyclopropyl reduced A(1)AR selectivity. Nucleoside docking to A(1)AR homology model characterized distinct hydrophobic cyclopropyl subpockets, the larger "A" forming contacts with Thr270 (7.35), Tyr271 (7.36), Ile274 (7.39), and carbon chains of glutamates (EL2) and the smaller subpocket "B" forming contacts between TM6 and TM7. 10 suppressed minimal clonic seizures (6 Hz mouse model) without typical rotarod impairment of A(1)AR agonists. Truncated nucleosides, an appealing preclinical approach, have more druglike physicochemical properties than other A(1)AR agonists. Thus, we identified highly restricted regions for substitution around N(6) suitable for an A(1)AR agonist with anticonvulsant activity.

Truncated Nucleosides as A(3) Adenosine Receptor Ligands: Combined 2-Arylethynyl and Bicyclohexane Substitutions

C2-Arylethynyladenosine-5'-N-methyluronamides containing a bicyclo[3.1.0]hexane ((N)-methanocarba) ring are selective A(3) adenosine receptor (AR) agonists. Similar 4'-truncated C2-arylethynyl-(N)-methanocarba nucleosides containing alkyl or alkylaryl groups at the N(6) position were low-efficacy agonists or antagonists of the human A(3)AR with high selectivity. Higher hA(3)AR affinity was associated with N(6)-methyl and ethyl (K(i) 3-6 nM), than with N(6)-arylalkyl groups. However, combined C2-phenylethynyl and N(6)-2-phenylethyl substitutions in selective antagonist 15 provided a K(i) of 20 nM. Differences between 4'-truncated and nontruncated analogues of extended C2-p-biphenylethynyl substitution suggested a ligand reorientation in AR binding, dominated by bulky N(6) groups in analogues lacking a stabilizing 5'-uronamide moiety. Thus, 4'-truncation of C2-arylethynyl-(N)-methanocarba adenosine derivatives is compatible with general preservation of A(3)AR selectivity, especially with small N(6) groups, but reduced efficacy in A(3)AR-induced inhibition of adenylate cyclase.

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry, and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A(1), A(2A), A(2B), and A(3). Among these G protein-coupled receptors, the A(3) subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A(3) adenosine receptor (AR), together with an overview on the progress of hA(3) AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure-activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA(3) AR and drug design. 

Regio- and stereoselective synthesis of truncated 3'-aminocarbanucleosides and their binding affinity at the A3 adenosine receptor

The stereoselective synthesis of truncated 3'-aminocarbanucleosides 4a-d via a stereo- and regioselective conversion of a diol 9 to bromoacetate 11a and their binding affinity towards the human A(3) adenosine receptor are described.

Truncated (N)-Methanocarba Nucleosides as A(1) Adenosine Receptor Agonists and Partial Agonists: Overcoming Lack of a Recognition Element

A(1) adenosine receptor (AR) agonists are neuroprotective, cardioprotective, and anxiolytic. (N)-Methanocarba adenine nucleosides designed to bind to human A(1)AR were truncated to eliminate 5'-CH(2)OH. This modification previously converted A(3)AR agonists into antagonists, but the comparable effect at A(1)AR is unknown. In comparison to ribosides, affinity at the A(1)AR was less well preserved than at the A(3)AR, although a few derivatives were moderately A(1)AR selective, notably full agonist 21 (N(6)-dicyclopropylmethyl, K(i) 47.9 nM). Thus, at the A(1)AR recognition elements for nucleoside binding depend more on 5'region interactions, and in their absence A(3)AR selectivity predominates. Based on the recently reported agonist-bound AR structure, this difference between subtypes likely correlates with an essential His residue in transmembrane domain 6 of A(1) but not A(3)AR. The derivatives ranged from partial to full agonists in A(1)AR-mediated adenylate cyclase inhibition. Truncated derivatives have more drug-like physical properties than other A(1)AR agonists; this approach is appealing for preclinical development.

Recent developments in adenosine receptor ligands and their potential as novel drugs

Medicinal chemical approaches have been applied to all four of the adenosine receptor (AR) subtypes (A(1), A(2A), A(2B), and A(3)) to create selective agonists and antagonists for each. The most recent class of selective AR ligands to be reported is the class of A(2B)AR agonists. The availability of these selective ligands has facilitated research on therapeutic applications of modulating the ARs and in some cases has provided clinical candidates. Prodrug approaches have been developed which improve the bioavailability of the drugs, reduce side-effects, and/or may lead to site-selective effects. The A(2A) agonist regadenoson (Lexiscan®), a diagnostic drug for myocardial perfusion imaging, is the first selective AR agonist to be approved. Other selective agonists and antagonists are or were undergoing clinical trials for a broad range of indications, including capadenoson and tecadenoson (A(1) agonists) for atrial fibrillation, or paroxysmal supraventricular tachycardia, respectively, apadenoson and binodenoson (A(2A) agonists) for myocardial perfusion imaging, preladenant (A(2A) antagonist) for the treatment of Parkinson's disease, and CF101 and CF102 (A(3) agonists) for inflammatory diseases and cancer, respectively.

Predicted structures of agonist and antagonist bound complexes of adenosine A3 receptor

We used the GEnSeMBLE Monte Carlo method to predict ensemble of the 20 best packings (helix rotations and tilts) based on the neutral total energy (E) from a vast number (10 trillion) of potential packings for each of the four subtypes of the adenosine G protein-coupled receptors (GPCRs), which are involved in many cytoprotective functions. We then used the DarwinDock Monte Carlo methods to predict the binding pose for the human A(3) adenosine receptor (hAA(3)R) for subtype selective agonists and antagonists. We found that all four A(3) agonists stabilize the 15th lowest conformation of apo-hAA(3)R while also binding strongly to the 1st and 3rd. In contrast the four A(3) antagonists stabilize the 2nd or 3rd lowest conformation. These results show that different ligands can stabilize different GPCR conformations, which will likely affect function, complicating the design of functionally unique ligands. Interestingly all agonists lead to a trans χ1 angle for W6.48 that experiments on other GPCRs associate with G-protein activation while all 20 apo-AA(3)R conformations have a W6.48 gauche+ χ1 angle associated experimentally with inactive GPCRs for other systems. Thus docking calculations have identified critical ligand-GPCR structures involved with activation. We found that the predicted binding site for selective agonist Cl-IB-MECA to the predicted structure of hAA(3)R shows favorable interactions to three subtype variable residues, I253(6.58), V169(EL2), and Q167(EL2), while the predicted structure for hAA(2A)R shows weakened to the corresponding amino acids: T256(6.58), E169(EL2), and L167(EL2), explaining the observed subtype selectivity.

2-Dialkynyl derivatives of (N)-methanocarba nucleosides: 'Clickable' A(3) adenosine receptor-selective agonists

We modified a series of (N)-methanocarba nucleoside 5'-uronamides to contain dialkyne groups on an extended adenine C2 substituent, as synthetic intermediates leading to potent and selective A(3) adenosine receptor (AR) agonists. The proximal alkyne was intended to promote receptor recognition, and the distal alkyne reacted with azides to form triazole derivatives (click cycloaddition). Click chemistry was utilized to couple an octadiynyl A(3)AR agonist to azido-containing fluorescent, chemically reactive, biotinylated, and other moieties with retention of selective binding to the A(3)AR. A bifunctional thiol-reactive crosslinking reagent was introduced. The most potent and selective novel compound was a 1-adamantyl derivative (K(i) 6.5nM), although some of the click products had K(i) values in the range of 200-400nM. Other potent, selective derivatives (K(i) at A(3)AR innM) were intended as possible receptor affinity labels: 3-nitro-4-fluorophenyl (10.6), alpha-bromophenacyl (9.6), thiol-reactive isothiazolone (102), and arylisothiocyanate (37.5) derivatives. The maximal functional effects in inhibition of forskolin-stimulated cAMP were measured, indicating that this class of click adducts varied from partial to full A(3)AR agonist compared to other widely used agonists. Thus, this strategy provides a general chemical approach to linking potent and selective A(3)AR agonists to reporter groups of diverse structure and to carrier moieties.

Functionalized congeners of A3 adenosine receptor-selective nucleosides containing a bicyclo[3.1.0]hexane ring system

(N)-Methanocarba nucleosides containing bicyclo[3.1.0]hexane replacement of the ribose ring previously demonstrated selectivity as A(3) adenosine receptor (AR) agonists (5'-uronamides) or antagonists (5'-truncated). Here, these two series were modified in parallel at the adenine C2 position. N(6)-3-Chlorobenzyl-5'-N-methyluronamides derivatives with functionalized 2-alkynyl chains of varying length terminating in a reactive carboxylate, ester, or amine group were full, potent human A(3)AR agonists. Flexibility of chain substitution allowed the conjugation with a fluorescent cyanine dye (Cy5) and biotin, resulting in binding K(i) values of 17 and 36 nM, respectively. The distal end of the chain was predicted by homology modeling to bind at the A(3)AR extracellular regions. Corresponding l-nucleosides were nearly inactive in AR binding. In the 5'-truncated nucleoside series, 2-Cl analogues were more potent at A(3)AR than 2-H and 2-F, functional efficacy in adenylate cyclase inhibition varied, and introduction of a 2-alkynyl chain greatly reduced affinity. SAR parallels between the two series lost stringency at distal positions. The most potent and selective novel compounds were amine congener 15 (K(i) = 2.1 nM) and truncated partial agonist 22 (K(i) = 4.9 nM).

Synthesis and pharmacological characterization of [(125)I]MRS5127, a high affinity, selective agonist radioligand for the A3 adenosine receptor

A recently reported selective agonist of the human A(3) adenosine receptor (hA(3)AR), MRS5127 (1'R,2'R,3'S,4'R,5'S)-4'-[2-chloro-6-(3-iodobenzylamino)-purine]-2',3'-O-dihydroxy-bicyclo-[3.1.0]hexane, was radioiodinated and characterized pharmacologically. It contains a rigid bicyclic ring system in place of a 5'-truncated ribose moiety, and was selected for radiolabeling due to its nanomolar binding affinity at both human and rat A(3)ARs. The radioiodination of the N(6)-3-iodobenzyl substituent by iododestannylation of a 3-(trimethylstannyl)benzyl precursor was achieved in 73% yield, measured after purification by HPLC. [(125)I]MRS5127 bound to the human A(3)AR expressed in membranes of stably transfected HEK 293 cells. Specific binding was saturable, competitive, and followed a one-site binding model, with a K(d) value of 5.74+/-0.97nM. At a concentration equivalent to its K(d), non-specific binding comprised 27+/-2% of total binding. In kinetic studies, [(125)I]MRS5127 rapidly associated with the hA(3)AR (t(1/2)=0.514+/-0.014min), and the affinity calculated from association and dissociation rate constants was 3.50+/-1.46nM. The pharmacological profile of ligands in competition experiments with [(125)I]MRS5127 was consistent with the known structure-activity-relationship profile of the hA(3)AR. [(125)I]MRS5127 bound with similar high affinity (K(d), nM) to recombinant A(3)ARs from mouse (4.90+/-0.77), rabbit (2.53+/-0.11), and dog (3.35+/-0.54). For all of the species tested, MRS5127 exhibited A(3)AR agonist activity based on negative coupling to cAMP production. Thus, [(125)I]MRS5127 represents a new species-independent agonist radioligand for the A(3)AR. The major advantage of [(125)I]MRS5127 compared with previously used A(3)AR radioligands is its high affinity, low degree of non-specific binding, and improved A(3)AR selectivity.

Evaluation of homology modeling of G-protein-coupled receptors in light of the A(2A) adenosine receptor crystallographic structure

Homology modeling of the human A(2A) adenosine receptor (AR) based on bovine rhodopsin predicted a protein structure that was very similar to the recently determined crystallographic structure. The discrepancy between the experimentally observed orientation of the antagonist and those obtained by previous antagonist docking is related to the loop structure of rhodopsin being carried over to the model of the A(2A) AR and was rectified when the beta(2)-adrenergic receptor was used as a template for homology modeling. Docking of the triazolotriazine antagonist ligand ZM241385 1 was greatly improved by including water molecules of the X-ray structure or by using a constraint from mutagenesis. Automatic agonists docking to both a new homology modeled receptor and the A(2A) AR crystallographic structure produced similar results. Heterocyclic nitrogen atoms closely corresponded when the docked adenine moiety of agonists and 1 were overlaid. The cumulative mutagenesis data, which support the proposed mode of agonist docking, can be reexamined in light of the crystallographic structure. Thus, homology modeling of GPCRs remains a useful technique in probing the structure of the protein and predicting modes of ligand docking.

Synthesis and characterization of [76Br]-labeled high-affinity A3 adenosine receptor ligands for positron emission tomography

INTRODUCTION

Bromine-76-radiolabeled analogues of previously reported high-affinity A(3) adenosine receptor (A(3)AR) nucleoside ligands have been prepared as potential radiotracers for positron emission tomography.

METHODS

The radiosyntheses were accomplished by oxidative radiobromination on the N(6)-benzyl moiety of trimethyltin precursors. Biodistribution studies of the kinetics of uptake were conducted in awake rats.

RESULTS

We prepared an agonist ligand {[(76)Br](1'S,2'R,3'S,4'R,5'S)-4'-{2-chloro-6-[(3-bromophenylmethyl)amino]purin-9-yl}-1'-(methylaminocarbonyl)bicyclo[3.1.0]hexane-2',3'-diol (MRS3581)} in 59% radiochemical yield with a specific activity of 19.5 GBq/micromol and an antagonist ligand {[(76)Br](1'R,2'R,3'S,4'R,5'S)-4'-(6-(3-bromobenzylamino)-2-chloro-9H-purin-9-yl)bicyclo[3.1.0]hexane-2',3'-diol (MRS5147)} in 65% radiochemical yield with a specific activity of 22 GBq/micromol. The resultant products exhibited the expected high affinity (K(i) approximately 0.6 nM) and specific binding at the human A(3)AR in vitro. Biodistribution studies in the rat showed uptake in the organs of excretion and metabolism. The antagonist MRS5147 exhibited increasing uptake in testes, an organ that contains significant quantities of A(3)AR, over a 2-h time course, which suggests the presence of a specific A(3)AR retention mechanism.

CONCLUSION

We were able to compare uptake of the [(76)Br]-labeled antagonist MRS5147 to [(76)Br]agonist MRS3581. The antagonist MRS5147 shows increasing uptake in the testes, an A(3)AR-rich tissue, suggesting that this ligand may have promise as a molecular imaging agent.

Medicinal chemistry of the A3 adenosine receptor: agonists, antagonists, and receptor engineering

A(3) adenosine receptor (A(3)AR) ligands have been modified to optimize their interaction with the A(3)AR. Most of these modifications have been made to the N(6) and C2 positions of adenine as well as the ribose moiety, and using a combination of these substitutions leads to the most efficacious, selective, and potent ligands. A(3)AR agonists such as IB-MECA and Cl-IB-MECA are now advancing into Phase II clinical trials for treatments targeting diseases such as cancer, arthritis, and psoriasis. Also, a wide number of compounds exerting high potency and selectivity in antagonizing the human (h)A(3)AR have been discovered. These molecules are generally characterized by a notable structural diversity, taking into account that aromatic nitrogen-containing monocyclic (thiazoles and thiadiazoles), bicyclic (isoquinoline, quinozalines, (aza)adenines), tricyclic systems (pyrazoloquinolines, triazoloquinoxalines, pyrazolotriazolopyrimidines, triazolopurines, tricyclic xanthines) and nucleoside derivatives have been identified as potent and selective A(3)AR antagonists. Probably due to the "enigmatic" physiological role of A(3)AR, whose activation may produce opposite effects (for example, concerning tissue protection in inflammatory and cancer cells) and may produce effects that are species dependent, only a few molecules have reached preclinical investigation. Indeed, the most advanced A(3)AR antagonists remain in preclinical testing. Among the antagonists described above, compound OT-7999 is expected to enter clinical trials for the treatment of glaucoma, while several thiazole derivatives are in development as antiallergic, antiasthmatic and/or antiinflammatory drugs.

Introduction to adenosine receptors as therapeutic targets

Adenosine acts as a cytoprotective modulator in response to stress to an organ or tissue. Although short-lived in the circulation, it can activate four subtypes of G protein-coupled adenosine receptors (ARs): A(1), A(2A), A(2B), and A(3). The alkylxanthines caffeine and theophylline are the prototypical antagonists of ARs, and their stimulant actions occur primarily through this mechanism. For each of the four AR subtypes, selective agonists and antagonists have been introduced and used to develop new therapeutic drug concepts. ARs are notable among the GPCR family in the number and variety of agonist therapeutic candidates that have been proposed. The selective and potent synthetic AR agonists, which are typically much longer lasting in the body than adenosine, have potential therapeutic applications based on their anti-inflammatory (A(2A) and A(3)), cardioprotective (preconditioning by A(1) and A(3) and postconditioning by A(2B)), cerebroprotective (A(1) and A(3)), and antinociceptive (A(1)) properties. Potent and selective AR antagonists display therapeutic potential as kidney protective (A(1)), antifibrotic (A(2A)), neuroprotective (A(2A)), and antiglaucoma (A(3)) agents. AR agonists for cardiac imaging and positron-emitting AR antagonists are in development for diagnostic applications. Allosteric modulators of A(1) and A(3) ARs have been described. In addition to the use of selective agonists/antagonists as pharmacological tools, mouse strains in which an AR has been genetically deleted have aided in developing novel drug concepts based on the modulation of ARs.