Lipophilic modifications to dinucleoside polyphosphates and nucleotides that confer antagonist properties at the platelet P2Y12 receptor

Beyond Triphosphates: Reagents and Methods for Chemical Oligophosphorylation

Oligophosphates play essential roles in biochemistry, and considerable research has been directed toward the synthesis of both naturally occurring oligophosphates and their synthetic analogues. Greater attention has been given to mono-, di-, and triphosphates, as these are present in higher concentrations biologically and easier to synthesize. However, extended oligophosphates have potent biochemical roles, ranging from blood coagulation to HIV drug resistance. Sporadic reports have slowly built a niche body of literature related to the synthesis and study of extended oligophosphates, but newfound interests and developments have the potential to rapidly expand this field. Here we report on current methods to synthesize oligophosphates longer than triphosphates and comment on the most important future directions for this area of research. The state of the art has provided fairly robust methods for synthesizing nucleoside 5'-tetra- and pentaphosphates as well as dinucleoside 5',5'-oligophosphates. Future research should endeavor to push such syntheses to longer oligophosphates while developing synthetic methodologies for rarer morphologies such as 3'-nucleoside oligophosphates, polyphosphates, and phosphonate/thiophosphate analogues of these species.

Synthetic Strategies for Dinucleotides Synthesis

Dinucleoside 5′,5′-polyphosphates (DNPs) are endogenous substances that play important intra- and extracellular roles in various biological processes, such as cell proliferation, regulation of enzymes, neurotransmission, platelet disaggregation and modulation of vascular tone. Various methodologies have been developed over the past fifty years to access these compounds, involving enzymatic processes or chemical procedures based either on P(III) or P(V) chemistry. Both solution-phase and solid-support strategies have been developed and are reported here. Recently, green chemistry approaches have emerged, offering attracting alternatives. This review outlines the main synthetic pathways for the preparation of dinucleoside 5′,5′-polyphosphates, focusing on pharmacologically relevant compounds, and highlighting recent advances.

Straightforward Ball-Milling Access to Dinucleoside 5',5'-Polyphosphates via Phosphorimidazolide Intermediates

A solvent-assisted mechanochemical approach to access symmetrical and mixed dinucleoside 5,5'-polyphosphates is reported. Under ball-milling conditions, nucleoside 5'-monophosphates were quantitatively activated using 1,1'-carbonyldiimidazole, forming their phosphorimidazolide derivatives. The addition of a nucleoside 5'-mono-, di- or triphosphate directly led to the formation of the corresponding dinucleotides. Benefits of the reported one-pot method include the use of unprotected nucleotides in their sodium or acid form, activation by the eco-friendly 1,1'-carbonyldiimidazole, non-dry conditions, short reaction time, high conversion rates, and easy setup and purification. This work offers new perspectives for the synthesis of nucleotide conjugates and analogues, combining the phosphorimidazolide approach and milling conditions.

2',3'-O-Substituted ATP derivatives as potent antagonists of purinergic P2X3 receptors and potential analgesic agents

Blocking membrane currents evoked by the activation of purinergic P2X3 receptors localized on nociceptive neurons represents a promising strategy for the development of agents useful for the treatment of chronic pain conditions. Among compounds endowed with such antagonistic action, 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP) is an ATP analogue, whose inhibitory activity on P2X receptors has been previously reported. Based on the results of molecular modelling studies performed with homology models of the P2X3 receptor, novel adenosine nucleotide analogues bearing cycloalkyl or arylalkyl substituents replacing the trinitrophenyl moiety of TNP-ATP were designed and synthesized. These new compounds were functionally evaluated on native P2X3 receptors from mouse trigeminal ganglion (TG) sensory neurons using patch clamp recordings under voltage clamp configuration. Our data show that some of these molecules are potent (nanomolar range) and reversible inhibitors of P2X3 receptors, without any apparent effect on trigeminal GABAA and 5-HT3 receptors, whose membrane currents were unaffected by the tested compounds.

Recent Trends in Nucleotide Synthesis

Focusing on the recent literature (since 2000), this review outlines the main synthetic approaches for the preparation of 5'-mono-, 5'-di-, and 5'-triphosphorylated nucleosides, also known as nucleotides, as well as several derivatives, namely, cyclic nucleotides and dinucleotides, dinucleoside 5',5'-polyphosphates, sugar nucleotides, and nucleolipids. Endogenous nucleotides and their analogues can be obtained enzymatically, which is often restricted to natural substrates, or chemically. In chemical synthesis, protected or unprotected nucleosides can be used as the starting material, depending on the nature of the reagents selected from P(III) or P(V) species. Both solution-phase and solid-support syntheses have been developed and are reported here. Although a considerable amount of research has been conducted in this field, further work is required because chemists are still faced with the challenge of developing a universal methodology that is compatible with a large variety of nucleoside analogues.

New highly active antiplatelet agents with dual specificity for platelet P2Y1 and P2Y12 adenosine diphosphate receptors

Currently approved platelet adenosine diphosphate (ADP) receptor antagonists target only the platelet P2Y12 receptor. Moreover, especially in patients with acute coronary syndromes, there is a strong need for rapidly acting and reversible antiplatelet agents in order to minimize the risk of thrombotic events and bleeding complications. In this study, a series of new P(1),P(4)-di(adenosine-5') tetraphosphate (Ap4A) derivatives with modifications in the base and in the tetraphosphate chain were synthesized and evaluated with respect to their effects on platelet aggregation and function of the platelet P2Y1, P2Y12, and P2X1 receptors. The resulting structure-activity relationships were used to design Ap4A analogs which inhibit human platelet aggregation by simultaneously antagonizing both P2Y1 and P2Y12 platelet receptors. Unlike Ap4A, the analogs do not activate platelet P2X1 receptors. Furthermore, the new compounds exhibit fast onset and offset of action and are significantly more stable than Ap4A to degradation in plasma, thus presenting a new promising class of antiplatelet agents.

How To Form a Phosphate Anhydride Linkage in Nucleotide Derivatives

The fundamental roles of nucleoside triphosphates and nucleotide cofactors such as NAD(+) in biochemistry are well known. In recent decades, continuing research has revealed the key role of 5'-capped RNA and 5',5'-dinucleoside polyphosphates in the regulation of vitally important physiological processes. Last but not least, the commercial potential of nucleoside triphosphate synthesis can hardly be overestimated. Nevertheless, despite decades of investigation and the obvious topicality of the research on the chemical synthesis of the nucleotide compounds containing phosphate anhydride linkages, none of the existing procedures can be considered an up-to-date "gold standard". However, there are a number of fruitful synthetic approaches to forming phosphate anhydride linkages in satisfactory yield. These are summarized in this concise review, organized by the type of active phosphorous intermediate and reagents used.

Modeling ligand recognition at the P2Y12 receptor in light of X-ray structural information

The G protein-coupled P2Y12 receptor (P2Y12R) is an important antithrombotic target and of great interest for pharmaceutical discovery. Its recently solved, highly divergent crystallographic structures in complex either with nucleotides (full or partial agonist) or with a nonnucleotide antagonist raise the question of which structure is more useful to understand ligand recognition. Therefore, we performed extensive molecular modeling studies based on these structures and mutagenesis, to predict the binding modes of major classes of P2Y12R ligands previously reported. Various nucleotide derivatives docked readily to the agonist-bound P2Y12R, but uncharged nucleotide-like antagonist ticagrelor required a hybrid receptor resembling the agonist-bound P2Y12R except for the top portion of TM6. Supervised molecular dynamics (SuMD) of ticagrelor binding indicated interactions with the extracellular regions of P2Y12R, defining possible meta-binding sites. Ureas, sulfonylureas, sulfonamides, anthraquinones and glutamic acid piperazines docked readily to the antagonist-bound P2Y12R. Docking dinucleotides at both agonist- and antagonist-bound structures suggested interactions with two P2Y12R pockets. Thus, our structure-based approach consistently rationalized the main structure-activity relationships within each ligand class, giving useful information for designing improved ligands.

Nucleotides Acting at P2Y Receptors: Connecting Structure and Function

Eight G protein-coupled P2Y receptor (P2YR) subtypes are important physiologic mediators. The human P2YRs are fully activated by ATP (P2Y2 and P2Y11), ADP (P2Y1, P2Y12, and P2Y13), UTP (P2Y2 and P2Y4), UDP (P2Y6 and P2Y14), and UDP glucose (P2Y14). Their structural elucidation is progressing rapidly. The X-ray structures of three ligand complexes of the Gi-coupled P2Y12R and two of the Gq-coupled P2Y1Rs were recently determined and will be especially useful in structure-based ligand design at two P2YR subfamilies. These high-resolution structures, which display unusual binding site features, complement mutagenesis studies for probing ligand recognition and activation. The structural requirements for nucleotide agonist recognition at P2YRs are relatively permissive with respect to the length of the phosphate moiety, but less so with respect to base recognition. Nucleotide-like antagonists and partial agonists are also known for P2Y1, P2Y2, P2Y4, and P2Y12Rs. Each P2YR subtype has the ability to be activated by structurally bifunctional agonists, such as dinucleotides, typically, dinucleoside triphosphates or tetraphosphates, and nucleoside polyphosphate sugars (e.g., UDP glucose) as well as the more conventional mononucleotide agonists. A range of dinucleoside polyphosphates, from triphosphates to higher homologs, occurs naturally. Earlier modeling predictions of the P2YRs were not very accurate, but recent findings have provided much detailed structural insight into this receptor family to aid in the rational design of new drugs.

5-alkyl-1,3-oxazole derivatives of 6-amino-nicotinic acids as alkyl ester bioisosteres are antagonists of the P2Y12 receptor

BACKGROUND

Recently, we reported ethyl nicotinates as antagonists of the P2Y12 receptor, which is an important target in antiplatelet therapies. A potential liability of these compounds was their generally high in vivo clearance due to ethyl ester hydrolysis.

RESULTS

Shape and electrostatic similarity matching was used to select five-membered heterocycles to replace the ethyl ester functionality. The 5-methyl and 5-ethyl-oxazole bioisosteres retained the sub-micromolar potency levels of the parent ethyl esters. Many oxazoles showed a higher CYP450 dependent microsomal metabolism than the corresponding ethyl esters. Structure activity relationship investigations supported by ab initio calculations suggested that a correctly positioned alkyl substituent and a strong hydrogen bond acceptor were necessary structural motifs for binding. In rat pharmacokinetics, the low clearance was retained upon replacement of an ethyl ester with a 5-ethyl-oxazole.

CONCLUSION

The use of shape and electrostatic similarity led to the successful replacement of a metabolically labile ethyl ester functionality with 5-alkyl-oxazole bioisosteres.

Efficient microwave-assisted solid phase coupling of nucleosides, small library generation and mild conditions for release of nucleoside derivatives

Nucleosides are essential bio-molecules that participate in a wide array of biological processes involved in maintaining physiologic homeostasis. Recent efforts geared towards the synthesis of nucleoside analogues and development of nucleoside combinatorial libraries using solid phase synthesis has contributed invaluable information towards drug design and development. These studies have provided information concerning the structural requirements of substrate binding pockets of enzymes and evaluation of enzyme kinetics. However, the synthesis of nucleosides and its corresponding analogues remains a challenging and time consuming process. Herein, we report an efficient, microwave assisted solid phase coupling of nucleosides, combinatorial chemistry on the coupled nucleosides to generate small library and mild cleavage conditions to release nucleoside derivatives from its solid support. We anticipate these findings will accelerate the development of synthetic methods or combinatorial library design of nucleoside analogues in similar settings.

Modified diadenosine tetraphosphates with dual specificity for P2Y1 and P2Y12 are potent antagonists of ADP-induced platelet activation

BACKGROUND

Diadenosine 5',5'''-P(1),P(4)-tetraphosphate (Ap(4)A), a natural compound stored in platelet dense granules, inhibits ADP-induced platelet aggregation. Ap(4)A inhibits the platelet ADP receptors P2Y(1) and P2Y(12), is a partial agonist of P2Y(12), and is a full agonist of the platelet ATP-gated ion channel P2X1. Modification of the Ap(4)A tetraphosphate backbone enhances inhibition of ADP-induced platelet aggregation. However, the effects of these Ap(4)A analogs on human platelet P2Y(1), P2Y(12) and P2X1 are unclear.

OBJECTIVE

To determine the agonist and antagonist activities of diadenosine tetraphosphate analogs towards P2Y(1), P2Y(12), and P2X1.

METHODS

We synthesized the following Ap(4)A analogs: P(1),P(4)-dithiotetraphosphate; P(2),P(3)-chloromethylenetetraphosphate; P(1)-thio-P(2),P(3)-chloromethylenetetraphosphate; and P(1),P(4)-dithio-P(2),P(3)-chloromethylenetetraphosphate. We then measured the effects of these analogs on: (i) ADP-induced platelet aggregation; (ii) P2Y(1)-mediated changes in cytosolic Ca(2+); (iii) P2Y(12)-mediated changes in vasodilator-stimulated phosphoprotein phosphorylation; and (iv) P2X1-mediated entry of extracellular Ca(2+).

RESULTS

Ap(4)A analogs with modifications in the phosphate backbone inhibited both P2Y(1) and P2Y(12), and showed no agonist activity towards these receptors. The dithio modification increased inhibition of P2Y(1), P2Y(12), and platelet aggregation, whereas the chloromethylene modification increased inhibition of P2Y(12) and platelet aggregation, but decreased P2Y(1) inhibition. Combining the dithio and chloromethylene modifications increased P2Y(1) and P2Y(12) inhibition. As compared with Ap(4)A, each modification decreased agonist activity towards P2X1, and the dual modification completely eliminated P2X1 agonist activity.

CONCLUSIONS

As compared with Ap(4)A, tetraphosphate backbone analogs of Ap(4)A have diminished activity towards P2X1 but inhibit both P2Y(1) and P2Y(12) and, with greater potency, inhibit ADP-induced platelet aggregation. Thus, diadenosine tetraphosphate analogs with dual receptor selectivity may have potential as antiplatelet drugs.

Studies of Mg2+/Ca2+ complexes of naturally occurring dinucleotides: potentiometric titrations, NMR, and molecular dynamics

Dinucleotides (Np(n)N'; N and N' are A, U, G, or C, n = 2-7) are naturally occurring physiologically active compounds. Despite the interest in dinucleotides, the composition of their complexes with metal ions as well as their conformations and species distribution in living systems are understudied. Therefore, we investigated a series of Mg(2+) and Ca(2+) complexes of Np(n)N's. Potentiometric titrations indicated that a longer dinucleotide polyphosphate (N is A or G, n = 3-5) linker yields more stable complexes (e.g., log K of 2.70, 3.27, and 3.73 for Ap(n)A-Mg(2+), n = 3, 4, 5, respectively). The base (A or G) or ion (Mg(2+) or Ca(2+)) has a minor effect on K(M)(ML) values. In a physiological medium, the longer Ap(n)As (n = 4, 5) are predicted to occur mostly as the Mg(2+)/Ca(2+) complexes. (31)P NMR monitored titrations of Np(n)N's with Mg(2+)/Ca(2+) ions showed that the middle phosphates of the dinucleotides coordinate with Mg(2+)/Ca(2+). Multidimensional potential of mean force (PMF) molecular dynamics (MD) simulations suggest that Ap(2)A and Ap(4)A coordinate Mg(2+) and Ca(2+) ions in both inner-sphere and outer-sphere modes. The PMF MD simulations additionally provide a detailed picture of the possible coordination sites, as well as the cation binding process. Moreover, both NMR and MD simulations showed that the conformation of the nucleoside moieties in Np(n)N'-Mg(2+)/Ca(2+) complexes remains the same as that of free mononucleotides.

A novel series of piperazinyl-pyridine ureas as antagonists of the purinergic P2Y12 receptor

A novel series of P2Y(12) antagonists for development of drugs within the antiplatelet area is presented. The synthesis of the piperazinyl-pyridine urea derivatives and their structure-activity relationships (SAR) are described. Several compounds showed P2Y(12) antagonistic activities in the sub-micromolar range.

Pharmacochemistry of the platelet purinergic receptors

Platelets contain at least five purinergic G protein-coupled receptors, e.g., the pro-aggregatory P2Y(1) and P2Y(12) receptors, a P2Y(14) receptor (GPR105) of unknown function, and anti-aggregatory A(2A) and A(2B) adenosine receptor (ARs), in addition to the ligand-gated P2X1 ion channel. Probing the structure-activity relationships (SARs) of the P2X and P2Y receptors for extracellular nucleotides has resulted in numerous new agonist and antagonist ligands. Selective agents derived from known ligands and novel chemotypes can be used to help define the subtypes pharmacologically. Some of these agents have entered into clinical trials in spite of the challenges of drug development for these classes of receptors. The functional architecture of P2 receptors was extensively explored using mutagenesis and molecular modeling, which are useful tools in drug discovery. In general, novel drug delivery methods, prodrug approaches, allosteric modulation, and biased agonism would be desirable to overcome side effects that tend to occur even with receptor subtype-selective ligands. Detailed SAR analyses have been constructed for nucleotide and non-nucleotide ligands at the P2Y(1), P2Y(12), and P2Y(14) receptors. The thienopyridine antithrombotic drugs Clopidogrel and Prasugrel require enzymatic pre-activation in vivo and react irreversibly with the P2Y(12) receptor. There is much pharmaceutical development activity aimed at identifying reversible P2Y(12) receptor antagonists. The screening of chemically diverse compound libraries has identified novel chemotypes that act as competitive, non-nucleotide antagonists of the P2Y(1) receptor or the P2Y(12) receptor, and antithrombotic properties of the structurally optimized analogues were demonstrated. In silico screening at the A(2A) AR has identified antagonist molecules having novel chemotypes. Fluorescent and other reporter groups incorporated into ligands can enable new technology for receptor assays and imaging. The A(2A) agonist CGS21680 and the P2Y(1) receptor antagonist MRS2500 were derivatized for covalent attachment to polyamidoamine dendrimeric carriers of MW 20,000, and the resulting multivalent conjugates inhibited ADP-promoted platelet aggregation. In conclusion, a wide range of new pharmacological tools is available to control platelet function by interacting with cell surface purine receptors.

P1,P2-diimidazolyl derivatives of pyrophosphate and bis-phosphonates--synthesis, properties, and use in preparation of dinucleoside tetraphosphates and analogs

P(1),P(2)-Diimidazolyl derivatives of pyrophosphate and halomethylene-bis-phosphonates have been synthesized and characterized, and the mechanism of their formation was studied. These reagents enable synthesis of dinucleoside tetraphosphates and tetraphosphonates conveniently and in high yields.

Molecular pharmacology, physiology, and structure of the P2Y receptors

The P2Y receptors are a widely expressed group of eight nucleotide-activated G protein-coupled receptors (GPCRs). The P2Y(1)(ADP), P2Y(2)(ATP/UTP), P2Y(4)(UTP), P2Y(6)(UDP), and P2Y(11)(ATP) receptors activate G(q) and therefore robustly promote inositol lipid signaling responses. The P2Y(12)(ADP), P2Y(13)(ADP), and P2Y(14)(UDP/UDP-glucose) receptors activate G(i) leading to inhibition of adenylyl cyclase and to Gβγ-mediated activation of a range of effector proteins including phosphoinositide 3-kinase-γ, inward rectifying K(+) (GIRK) channels, phospholipase C-β2 and -β3, and G protein-receptor kinases 2 and 3. A broad range of physiological responses occur downstream of activation of these receptors ranging from Cl(-) secretion by epithelia to aggregation of platelets to neurotransmission. Useful structural models of the P2Y receptors have evolved from extensive genetic analyses coupled with molecular modeling based on three-dimensional structures obtained for rhodopsin and several other GPCRs. Selective ligands have been synthesized for most of the P2Y receptors with the most prominent successes attained with highly selective agonist and antagonist molecules for the ADP-activated P2Y(1) and P2Y(12) receptors. The widely prescribed drug, clopidogrel, which results in irreversible blockade of the platelet P2Y(12) receptor, is the most important therapeutic agent that targets a P2Y receptor.

What is the conformation of physiologically-active dinucleoside polyphosphates in solution? Conformational analysis of free dinucleoside polyphosphates by NMR and molecular dynamics simulations

Dinucleoside polyphosphates, or dinucleotides (Np(n)N'; N, N' = A, U, G, C; n = 2-7), are naturally occurring ubiquitous physiologically active compounds. Despite the interest in dinucleotides, and the relevance of their conformation to their biological function, the conformation of dinucleotides has been insufficiently studied. Therefore, here we performed conformational analysis of a series of Np(n)N' Na(+) salts (N = A, G, U, C; N' = A, G, U, C; n = 2-5) by various NMR techniques. All studied dinucleotides, except for Up(4/5)U, formed intramolecular base stacking interactions in aqueous solutions as indicated by NMR. The conformation around the glycosidic angle in Np(n)N's was found to be anti/high anti and the preferred conformation around the C4'-C5', C5'-O5' bonds was found to be gauche-gauche (gg). The ribose moiety in Np(n)N's showed a small preference for the S conformation, but when attached to cytosine the ribose ring preferred the N conformation. However, no predominant conformation was observed for the ribose moiety in any of the dinucleotides. Molecular dynamics simulations of Ap(2)A and Ap(4)A Na(+) salts supported the experimental results. In addition, three modes of base-stacking were found for Ap(2/4)A: α-α, β-β and α-β, which exist in equilibrium, while none is dominant. We conclude that natural, free Np(n)N's (n = 2-5) at physiological pH exist mostly in a folded (stacked), rather than extended conformation, in several interconverting stacking modes. Intramolecular base stacking of Np(n)N's does not alter the conformation of each of the nucleotide moieties, which remains the same as that of the mononucleotides in solution.

Synthesis of uracil nucleotide analogs with a modified, acyclic ribose moiety as P2Y(2) receptor antagonists

A series of new uracil nucleotide analogs (monophosphates, triphosphates, and phosphonates) was synthesized, in which the ribose moiety was replaced by acyclic chains, including branched or linear alkyl or dialkylether linkers. 1-omega-Bromoalkyluracil derivatives (2) were converted to the corresponding alcohols by treatment with sodium hydroxide and subsequently phosphorylated using phosphorus oxychloride followed by hydrolysis to yield the monophosphates, or by coupling with diphosphate to form the triphosphates. Reaction of 2 with triethyl phosphite followed by deprotection with trimethylsilyl bromide led to the omega-phosphonylalkyluracil derivatives. These products could be further phosphorylated by converting them into their imidazolides and subsequent treatment with diphosphate yielding the corresponding UTP analogs. Nucleoside analogs with an oxygen atom in the 2'-position, which are more similar to the natural ribosides, were synthesized from silylated uracil and trimethylsilyl iodide-treated 1,3-dioxolane, or 1,3-dioxane, respectively, and subsequently phosphorylated by standard procedures. The nucleotide analogs were investigated in a functional assay at NG108-15 cells, a neuroblastomaxglioma hybrid cell line which expresses the UTP- and ATP-activated nucleotide receptor subtype P2Y(2). The acyclic nucleotide analogs were generally weaker ligands than UTP, and-in contrast to UTP-they were antagonistic. The most potent compound was diphosphoric 5-(2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)pentylphosphonic anhydride (5c) with an IC(50) value of 92microM showing that the replacement of the alpha-phosphate by phosphonate, which leads to enhanced stability, was well tolerated.

Development of selective agonists and antagonists of P2Y receptors

Although elucidation of the medicinal chemistry of agonists and antagonists of the P2Y receptors has lagged behind that of many other members of group A G protein-coupled receptors, detailed qualitative and quantitative structure–activity relationships (SARs) were recently constructed for several of the subtypes. Agonists selective for P2Y₁, P2Y₂, and P2Y₆ receptors and nucleotide antagonists selective for P2Y₁ and P2Y₁₂ receptors are now known. Selective nonnucleotide antagonists were reported for P2Y₁, P2Y₂, P2Y₆, P2Y₁₁, P2Y₁₂, and P2Y₁₃ receptors. At the P2Y₁ and P2Y₁₂ receptors, nucleotide agonists (5′-diphosphate derivatives) were converted into antagonists of nanomolar affinity by altering the phosphate moieties, with a focus particularly on the ribose conformation and substitution pattern. Nucleotide analogues with conformationally constrained ribose-like rings were introduced as selective receptor probes for P2Y₁ and P2Y₆ receptors. Screening chemically diverse compound libraries has begun to yield new lead compounds for the development of P2Y receptor antagonists, such as competitive P2Y₁₂ receptor antagonists with antithrombotic activity. Selective agonists for the P2Y₄, P2Y₁₁, and P2Y₁₃ receptors and selective antagonists for P2Y₄ and P2Y₁₄ receptors have not yet been identified. The P2Y₁₄ receptor appears to be the most restrictive of the class with respect to modification of the nucleobase, ribose, and phosphate moieties. The continuing process of ligand design for the P2Y receptors will aid in the identification of new clinical targets.

Adenosine analogues as inhibitors of P2Y12 receptor mediated platelet aggregation

Modified adenosine derivatives may lead to the development of P2Y(12) antagonists that are potent, selective, and bind reversibly to the receptor. Analogues of 2',3'-trans-styryl acetal-N6-ureido-adenosine monophosphate were prepared by modification of the 5'-position. The resulting analogues were tested for P2Y(12) antagonism in a platelet aggregation assay.