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1.  Structure-Activity Relationships of Truncated D- and L-4′-Thioadenosine Derivatives as Species-Independent A3 Adenosine Receptor Antagonists1 
Journal of medicinal chemistry  2008;51(20):6609-6613.
Novel D- and L-4′-thioadenosine derivatives lacking the 4′-hydroxymethyl moiety were synthesized, starting from D-mannose and D-gulonic γ-lactone, respectively, as potent and selective species-independent A3 adenosine receptor (AR) antagonists. Among the novel 4′-truncated 2-H nucleosides tested, a N6-(3-chlorobenzyl) derivative 7c was the most potent at the human A3 AR (Ki = 1.5 nM), but a N6-(3-bromobenzyl) derivative 7d showed the optimal species-independent binding affinity.
PMCID: PMC3616494  PMID: 18811138
2.  Medicinal Chemistry of the A3 Adenosine Receptor: Agonists, Antagonists, and Receptor Engineering 
A3 adenosine receptor (A3AR) ligands have been modified to optimize their interaction with the A3AR. Most of these modifications have been made to the N6 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. A3AR 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)A3AR 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 A3AR antagonists. Probably due to the “enigmatic” physiological role of A3AR, 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 A3AR 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.
PMCID: PMC3413728  PMID: 19639281
A3 adenosine receptor; A3 adenosine receptor agonist; A3 adenosine receptor antagonist; Purines; Structure activity relationship; Nucleoside; G protein-coupled receptor; Neoceptor
3.  Functionalized Congeners of A3 Adenosine Receptor-Selective Nucleosides Containing a Bicyclo[3.1.0]hexane Ring System† 
Journal of medicinal chemistry  2009;52(23):7580-7592.
(N)-Methanocarba nucleosides containing bicyclo[3.1.0]hexane replacement of the ribose ring previously demonstrated selectivity as A3 adenosine receptor (AR) agonists (5′-uronamides) or antagonists (5′-truncated). Here, these two series were modified in parallel at the adenine C2 position. N6-3-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 A3AR agonists. Flexibility of chain substitution allowed the conjugation with a fluorescent cyanine dye (Cy5) and biotin, resulting in binding Ki values of 17 and 36 nM, respectively. The distal end of the chain was predicted by homology modeling to bind at the A3AR extracellular regions. Corresponding l-nucleosides were nearly inactive in AR binding. In the 5′-truncated nucleoside series, 2-Cl analogues were more potent at A3AR 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 (Ki = 2.1 nM) and truncated partial agonist 22 (Ki = 4.9 nM).
PMCID: PMC3109436  PMID: 19499950
4.  Systematic Investigation of Polyamidoamine Dendrimers Surface-Modified with Poly(ethylene glycol) for Drug Delivery Applications: Synthesis, Characterization, and Evaluation of Cytotoxicity 
Bioconjugate chemistry  2008;19(8):1660-1672.
Surface-modification of amine-terminated polyamidoamine (PAMAM) dendrimers by poly(ethylene glycol) (PEG) groups generally enhances water-solubility and biocompatibility for drug delivery applications. In order to provide guidelines for designing appropriate dendritic scaffolds, a series of G3 PAMAM-PEG dendrimer conjugates was synthesized by varying the number of PEG attachments and chain length (shorter PEG550 and PEG750 and longer PEG2000). Each conjugate was purified by size exclusion chromatography (SEC) and the molecular weight (MW) was determined by 1H NMR integration and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). NOESY experiments performed in D2O on selected structures suggested no penetration of PEG chains to the central PAMAM domain, regardless of chain length and degree of substitution. CHO cell cultures exposed to PAMAM-PEG derivatives (≤ 1 µM) showed a relatively high cell viability. Generally, increasing the degree of PEG substitution reduced cytotoxicity. Moreover, compared to G3 PAMAM dendrimers that were N-acetylated to varying degrees, a lower degree of surface substitution with PEG was needed for a similar cell viability. Interestingly, when longer PEG2000 was fully incorporated on the surface, cell viability was reduced at higher concentrations (32 µM), suggesting increased toxicity potentially by forming intermolecular aggregates. A similar observation was made for anionic carboxylate G5.5 PAMAM dendrimer at the same dendrimer concentration. Our findings suggest that a lower degree of peripheral substitution with shorter PEG chains may suffice for these PAMAM-PEG conjugates to serve as efficient universal scaffolds for drug delivery, particularly valuable in relation to targeting or other ligand-receptor interactions.
PMCID: PMC2583177  PMID: 18610944
5.  Enhanced A3 adenosine receptor selectivity of multivalent nucleoside-dendrimer conjugates 
An approach to use multivalent dendrimer carriers for delivery of nucleoside signaling molecules to their cell surface G protein-coupled receptors (GPCRs) was recently introduced.
A known adenosine receptor (AR) agonist was conjugated to polyamidoamine (PAMAM) dendrimer carriers for delivery of the intact covalent conjugate to on the cell surface. Depending on the linking moiety, multivalent conjugates of the N6-chain elongated functionalized congener ADAC (N6-[4-[[[4-[[[(2-aminoethyl)amino]carbonyl]methyl]anilino]carbonyl]methyl]phenyl]-adenosine) achieved unanticipated high selectivity in binding to the cytoprotective human A3 AR, a class A GPCR. The key to this selectivity of > 100-fold in both radioreceptor binding (Ki app = 2.4 nM) and functional assays (EC50 = 1.6 nM in inhibition of adenylate cyclase) was maintaining a free amino group (secondary) in an amide-linked chain. Attachment of neutral amide-linked chains or thiourea-containing chains preserved the moderate affinity and efficacy at the A1 AR subtype, but there was no selectivity for the A3 AR. Since residual amino groups on dendrimers are associated with cytotoxicity, the unreacted terminal positions of this A3 AR-selective G2.5 dendrimer were present as carboxylate groups, which had the further benefit of increasing water-solubility. The A3 AR selective G2.5 dendrimer was also visualized binding the membrane of cells expressing the A3 receptor but did not bind cells that did not express the receptor.
This is the first example showing that it is feasible to modulate and even enhance the pharmacological profile of a ligand of a GPCR based on conjugation to a nanocarrier and the precise structure of the linking group, which was designed to interact with distal extracellular regions of the 7 transmembrane-spanning receptor. This ligand tool can now be used in pharmacological models of tissue rescue from ischemia and to probe the existence of A3 AR dimers.
PMCID: PMC2582240  PMID: 18947419
6.  Application of the functionalized congener approach to dendrimer-based signaling agents acting through A2A adenosine receptors 
Purinergic Signalling  2008;5(1):39-50.
As a continued effort to develop multivalent ligands to enhance the pharmacological effects of monomeric drugs, DITC-APEC, a chemically reactive nucleoside A2A adenosine receptor (AR) agonist, was employed to derivatize the surface of third-generation (G3) polyamidoamine (PAMAM) dendrimers. The resulting conjugates carried multiple copies of the agonist attached through a thiourea linkage and differed in the number of attachments and in the presence of a fluorophore or additional surface modification. Computer modeling studies suggested that these DITC-APEC-loaded dendrimers extended the overall diameter of the previously reported PAMAM-CGS21680 dendrimer derivatives (Kim et al., Bioconjugate Chem 2008 19:406–411) by ca. 20 Å, potentially increasing the conformational flexibility of the appended ligands to achieve optimal geometry for efficient binding at A2A ARs. Increased affinity and selectivity in binding in comparison to the CGS21680 conjugate were envisioned, due to the presence of an extended linker, i.e., a dithioureylenephenyl functionality. In vitro radioligand competition experiments showed effective binding of these PAMAM-DITC-APEC dendrimer conjugates at the human A2A and A3 ARs with submicromolar Ki values and selectivity in comparison to the human A1 AR. Furthermore, these nucleoside-loaded dendrimers exhibited an A2A AR-mediated inhibitory effect on ADP-induced aggregation of human platelets. The present study demonstrates the potential of applying the functionalized congener concept to engineer dendrimer-based multivalent ligands for G protein-coupled receptors.
Electronic supplementary material
The online version of this article (doi:10.1007/s11302-008-9113-3) contains supplementary material, which is available to authorized users.
PMCID: PMC2721767  PMID: 18600474
A2A adenosine receptors; Dendrimers; Functionalized congeners; G protein-coupled receptors; Antithrombotic; Nanotechnology

Results 1-6 (6)