Identification of hydrolytically stable and selective P2Y(1) receptor agonists

Overview of Biologically Active Nucleoside Phosphonates

The use of the phosphonate motif featuring a carbon-phosphorous bond as bioisosteric replacement of the labile P–O bond is widely recognized as an attractive structural concept in different areas of medicinal chemistry, since it addresses the very fundamental principles of enzymatic stability and minimized metabolic activation. This review discusses the most influential successes in drug design with special emphasis on nucleoside phosphonates and their prodrugs as antiviral and cancer treatment agents. A description of structurally related analogs able to interfere with the transmission of other infectious diseases caused by pathogens like bacteria and parasites will then follow. Finally, molecules acting as agonists/antagonists of P2X and P2Y receptors along with nucleotidase inhibitors will also be covered. This review aims to guide readers through the fundamentals of nucleoside phosphonate therapeutics in order to inspire the future design of molecules to target infections that are refractory to currently available therapeutic options.

Nucleotide P2Y1 receptor agonists are in vitro and in vivo prodrugs of A1/A3 adenosine receptor agonists: implications for roles of P2Y1 and A1/A3 receptors in physiology and pathology

Rapid phosphoester hydrolysis of endogenous purine and pyrimidine nucleotides has challenged the characterization of the role of P2 receptors in physiology and pathology. Nucleotide phosphoester stabilization has been pursued on a number of medicinal chemistry fronts. We investigated the in vitro and in vivo stability and pharmacokinetics of prototypical nucleotide P2Y₁ receptor (P2Y₁R) agonists and antagonists. These included the riboside nucleotide agonist 2-methylthio-ADP and antagonist MRS2179, as well as agonist MRS2365 and antagonist MRS2500 containing constrained (N)-methanocarba rings, which were previously reported to form nucleotides that are more slowly hydrolyzed at the α-phosphoester compared with the ribosides. In vitro incubations in mouse and human plasma and blood demonstrated the rapid hydrolysis of these compounds to nucleoside metabolites. This metabolism was inhibited by EDTA to chelate divalent cations required by ectonucleotidases for nucleotide hydrolysis. This rapid hydrolysis was confirmed in vivo in mouse pharmacokinetic studies that demonstrate that MRS2365 is a prodrug of the nucleoside metabolite AST-004 (MRS4322). Furthermore, we demonstrate that the nucleoside metabolites of MRS2365 and 2-methylthio-ADP are adenosine receptor (AR) agonists, notably at A₃ and A₁ARs. In vivo efficacy of MRS2365 in murine models of traumatic brain injury and stroke can be attributed to AR activation by its nucleoside metabolite AST-004, rather than P2Y₁R activation. This research suggests the importance of reevaluation of previous in vitro and in vivo research of P2YRs and P2XRs as there is a potential that the pharmacology attributed to nucleotide agonists is due to AR activation by active nucleoside metabolites.

Inhibition of nucleotide pyrophosphatase/phosphodiesterase 1: implications for developing a calcium pyrophosphate deposition disease modifying drug

Objectives

Calcium pyrophosphate deposition (CPPD) is associated with osteoarthritis and is the cause of a common inflammatory articular disease. Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (eNPP1) is the major ecto-pyrophosphatase in chondrocytes and cartilage-derived matrix vesicles (MVs). Thus, eNPP1 is a principle contributor to extracellular pyrophosphate levels and a potential target for interventions aimed at preventing CPPD. Recently, we synthesized and described a novel eNPP1-specific inhibitor, SK4A, and we set out to evaluate whether this inhibitor attenuates nucleotide pyrophosphatase activity in human OA cartilage.

Methods

Cartilage tissue, chondrocytes and cartilage-derived MVs were obtained from donors with OA undergoing arthroplasty. The effect of SK4A on cell viability was assayed by the XTT method. eNPP1 expression was evaluated by western blot. Nucleotide pyrophosphatase activity was measured by a colorimetric assay and by HPLC analysis of adenosine triphosphate (ATP) levels. ATP-induced calcium deposition in cultured chondrocytes was visualized and quantified with Alizarin red S staining.

Results

OA chondrocytes expressed eNPP1 in early passages, but this expression was subsequently lost upon further passaging. Similarly, significant nucleotide pyrophosphatase activity was only detected in early-passage chondrocytes. The eNPP1 inhibitor, SK4A, was not toxic to chondrocytes and stable in culture medium and human plasma. SK4A effectively inhibited nucleotide pyrophosphatase activity in whole cartilage tissue, in chondrocytes and in cartilage-derived MVs and reduced ATP-induced CPPD.

Conclusion

Nucleotide analogues such as SK4A may be developed as potent and specific inhibitors of eNPP1 for the purpose of lowering extracellular pyrophosphate levels in human cartilage with the aim of preventing and treating CPPD disease.

Highly Selective and Potent Ectonucleotide Pyrophosphatase-1 (NPP1) Inhibitors Based on Uridine 5'-Pα,α-Dithiophosphate Analogues

Ectonucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) hydrolyzes phosphodiester bonds of nucleotides such as ATP, resulting mainly in the formation of AMP and pyrophosphate. NPP1 activity plays a deleterious function in calcified aortic valve disease and calcium pyrophosphate deposition disease. Thus, inhibitors of NPP1 represent a medical need. We developed novel NPP1 inhibitors based on uridine 5'-P_α,α-dithiophosphate analogues, 9-12. All these analogues potently inhibited hNPP1 (80-100% inhibition) at 100 μM, with no, or minimal, inhibition of NPP3 and other ectonucleotidases (NTPDase1,2,3,8). These compounds showed nearly no activity at uracil-nucleotide sensitive P2Y_2,4,6-receptors and thus represent highly selective NPP1 inhibitors. The most promising inhibitor was diuridine 5'-P_α,α,5″-P_α,α-tetrathiotetraphosphate, 12, exhibiting K_i of 27 nM. Analogues 9-12 proved to be highly stable to air oxidation and to acidic and basic pH. Docking simulations suggested that the enhanced NPP1 inhibitory activity and selectivity of analogue 12 could be attributed to the simultaneous occupancy of two sites (the AMP site and an alternative site) of NPP1 by this compound.

Tools and drugs for uracil nucleotide-activated P2Y receptors

P2Y receptors (P2YRs) are a family of G protein-coupled receptors activated by extracellular nucleotides. Physiological P2YR agonists include purine and pyrimidine nucleoside di- and triphosphates, such as ATP, ADP, UTP, UDP, nucleotide sugars, and dinucleotides. Eight subtypes exist, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄, which represent current or potential future drug targets. Here we provide a comprehensive overview of ligands for the subgroup of the P2YR family that is activated by uracil nucleotides: P2Y₂ (UTP, also ATP and dinucleotides), P2Y₄ (UTP), P2Y₆ (UDP), and P2Y₁₄ (UDP, UDP-glucose, UDP-galactose). The physiological agonists are metabolically unstable due to their fast hydrolysis by ectonucleotidases. A number of agonists with increased potency, subtype-selectivity and/or enzymatic stability have been developed in recent years. Useful P2Y₂R agonists include MRS2698 (6-01, highly selective) and PSB-1114 (6-05, increased metabolic stability). A potent and selective P2Y₂R antagonist is AR-C118925 (10-01). For studies of the P2Y₄R, MRS4062 (3-15) may be used as a selective agonist, while PSB-16133 (10-06) is a selective antagonist. Several potent P2Y₆R agonists have been developed including 5-methoxyuridine 5'-O-((R_p)α-boranodiphosphate) (6-12), PSB-0474 (3-11), and MRS2693 (3-26). The isocyanate MRS2578 (10-08) is used as a selective P2Y₆R antagonist, although its reactivity and low water-solubility are limiting. With MRS2905 (6-08), a potent and metabolically stable P2Y₁₄R agonist is available, while PPTN (10-14) represents a potent and selective P2Y₁₄R antagonist. The radioligand [³H]UDP can be used to label P2Y₁₄Rs. In addition, several fluorescent probes have been developed. Uracil nucleotide-activated P2YRs show great potential as drug targets, especially in inflammation, cancer, cardiovascular and neurodegenerative diseases.

Synthesis and structure-activity relationship of uracil nucleotide derivatives towards the identification of human P2Y6 receptor antagonists

P2Y6 receptor (P2Y6-R) is involved in various physiological and pathophysiological events. With a view to set rules for the design of UDP-based reversible P2Y6-R antagonists as potential drugs, we established structure-activity relationship of UDP analogues, bearing modifications at the uracil ring, ribose moiety, and the phosphate chain. For instance, C5-phenyl- or 3-NMe-uridine-5'-α,β-methylene-diphosphonate, 16 and 23, or lack of 2'-OH, in 12-15, resulted in loss of both agonist and antagonist activity toward hP2Y6-R. However, uridylyl phosphosulfate, 19, selectively inhibited hP2Y6-R (IC50 112 μM) versus P2Y2/4-Rs. In summary, we have established a comprehensive SAR for hP2Y6-R ligands towards the development of hP2Y6-R antagonists.

ATP-γ-S-(α,β-CH2) protects against oxidative stress and amyloid beta toxicity in neuronal culture

Amyloid beta (Aβ) oligomers and oxidative stress, typical of Alzheimer's disease, are highly neurotoxic. Previously we identified ATP-γ-S as a most promising antioxidant and neuroprotectant. To further improve both potency and metabolic stability of ATP-γ-S, we designed a related analogue, ATP-γ-S-(α,β-CH2). We found that ATP-γ-S-(α,β-CH2) effectively inhibited ROS formation in PC12 cells subjected to Fe(II)-oxidation, slightly better than ATP-γ-S (IC50 0.18 and 0.20 μM, respectively). Moreover, ATP-γ-S-(α,β-CH2) rescued primary neurons from Aβ42 toxicity, 4-fold more potently than ATP-γ-S, (IC50 0.2 and 0.8 μM, respectively). In addition, the metabolic stability of ATP-γ-S-(α,β-CH2) in PC12 cells during 4 h of incubation, was up to 20% greater than that of ATP-γ-S and ATP. Previously, we found that ATP-γ-S-(α,β-CH2) resisted hydrolysis by ecto-nucleotidases such as, NPPs and TNAP, and was found to be ∼7-fold more potent agonist than ATP at P2Y11 receptor. Therefore, we propose ATP-γ-S-(α,β-CH2) as a promising agent for rescue of neurons from insults typical of Alzheimer's disease.

Highly potent and selective ectonucleotide pyrophosphatase/phosphodiesterase I inhibitors based on an adenosine 5'-(α or γ)-thio-(α,β- or β,γ)-methylenetriphosphate scaffold

Aberrant nucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) activity is associated with chondrocalcinosis, osteoarthritis, and type 2 diabetes. The potential of NPP1 inhibitors as therapeutic agents, and the scarceness of their structure-activity relationship, encouraged us to develop new NPP1 inhibitors. Specifically, we synthesized ATP-α-thio-β,γ-CH2 (1), ATP-α-thio-β,γ-CCl2 (2), ATP-α-CH2-γ-thio (3), and 8-SH-ATP (4) and established their resistance to hydrolysis by NPP1,3 and NTPDase1,2,3,8 (<5% hydrolysis) (NTPDase = ectonucleoside triphosphate diphosphohydrolase). Analogues 1-3 at 100 μM inhibited thymidine 5'-monophosphate p-nitrophenyl ester hydrolysis by NPP1 and NPP3 by >90% and 23-43%, respectively, and only slightly affected (0-40%) hydrolysis of ATP by NTPDase1,2,3,8. Analogue 3 is the most potent NPP1 inhibitor currently known, Ki = 20 nM and IC50 = 0.39 μM. Analogue 2a is a selective NPP1 inhibitor with Ki = 685 nM and IC50 = 0.57 μM. Analogues 1-3 were found mostly to be nonagonists of P2Y1/P2Y2/P2Y11 receptors. Docking analogues 1-3 into the NPP1 model suggested that activity correlates with the number of H-bonds with binding site residues. In conclusion, we propose analogues 2a and 3 as highly promising NPP1 inhibitors.

Nonhydrolyzable ATP analogues as selective inhibitors of human NPP1: a combined computational/experimental study

Elevated nucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) activity is implicated in health disorders including pathological calcification. Specific NPP1 inhibitors would therefore be valuable for studying this enzyme and as potential therapeutic agents. Here we present a combined computational/experimental study characterizing 13 nonhydrolyzable ATP analogues as selective human NPP1 inhibitors. All analogues at 100 μM inhibited (66-99%) the hydrolysis of pnp-TMP by both recombinant NPP1 and cell surface NPP1 activity of osteocarcinoma (HTB-85) cells. These analogues only slightly altered the activity of other ectonucleotidases, NPP3 and NTPDases. The Ki,app values of the seven most potent and selective inhibitors were in the range of 0.5-56 μM, all with mixed type inhibition, predominantly competitive. Those molecules were docked into a newly developed homology model of human NPP1. All adopted ATP-like binding modes, suggesting competitive inhibition with the endogenous ligand. NPP1 selectivity versus NPP3 could be explained in terms of the electrostatic potential of the two proteins that of NPP1 favoring negatively charged ligands. Inhibitor 2 that had the lowest Ki,app (0.5 μM) was also inactive toward P2Y receptors. Overall, analogue 2 is the most potent and selective NPP1 inhibitor described so far.

Highly efficient biocompatible neuroprotectants with dual activity as antioxidants and P2Y receptor agonists

Currently, there is a need for novel, biocompatible, and effective neuroprotectants for the treatment of neurodegenerative diseases and brain injury associated with oxidative damage. Here, we developed nucleotide-based neuroprotectants acting dually as antioxidants and P2Y-R agonists. To improve the potency, selectivity, and metabolic stability of ATP/ADP, we substituted adenine C2-position by Cl and Pα/Pβ position by borano group, 6-9. Nucleotides 6-9 inhibited oxidation in cell-free systems (Fe(II)-H2O2), as detected by ESR (IC50 up to 175 μM), and ABTS assay (IC50 up to 40 μM). They also inhibited FeSO4-induced oxidative stress in PC12 cells (IC50 of 80-200 nM). 2-Cl-ADP(α-BH3), 7a, was found to be the most potent P2Y1-R agonist currently known (EC50 7 nM) and protected primary cortical neurons from FeSO4 insult (EC50 170 nM). In addition, it proved to be metabolically stable in human blood serum (t(1/2) 7 vs 1.5 h for ADP). Hence, we propose 7a as a highly promising neuroprotectant.

P2Y receptors in the mammalian nervous system: pharmacology, ligands and therapeutic potential

P2Y receptors for extracellular nucleotides are coupled to activation of a variety of G proteins and stimulate diverse intracellular signaling pathways that regulate functions of cell types that comprise the central nervous system (CNS). There are 8 different subtypes of P2Y receptor expressed in cells of the CNS that are activated by a select group of nucleotide agonists. Here, the agonist selectivity of these 8 P2Y receptor subtypes is reviewed with an emphasis on synthetic agonists with high potency and resistance to degradation by extracellular nucleotidases that have potential applications as therapeutic agents. In addition, the recent identification of a wide variety of subtype-selective antagonists is discussed, since these compounds are critical for discerning cellular responses mediated by activation of individual P2Y receptor subtypes. The functional expression of P2Y receptor subtypes in cells that comprise the CNS is also reviewed and the role of each subtype in the regulation of physiological and pathophysiological responses is considered. Other topics include the role of P2Y receptors in the regulation of blood-brain barrier integrity and potential interactions between different P2Y receptor subtypes that likely impact tissue responses to extracellular nucleotides in the CNS. Overall, current research suggests that P2Y receptors in the CNS regulate repair mechanisms that are triggered by tissue damage, inflammation and disease and thus P2Y receptors represent promising targets for the treatment of neurodegenerative diseases.

Identification of a promising drug candidate for the treatment of type 2 diabetes based on a P2Y(1) receptor agonist

The activation by extracellular nucleotides of pancreatic P2Y receptors, particularly, the P2Y(1)R subtype, increases insulin secretion. Therefore, we developed analogues of the P2Y(1)R receptor agonist 2-MeS-ADP, as potential antidiabetic drugs. Analogue 3A was found to be a potent P2Y(1)R agonist (EC(50) = 0.038 μM vs 0.0025 μM for 2-MeS-ADP) showing no activity at P2Y(2/4/6)Rs. Analogue 3A was stable at pH 1.4 (t(1/2) = 7.3 h) and resistant to hydrolysis vs 2-MeS-ADP by alkaline phosphatase (t(1/2) = 6 vs 4.5 h), human e-NPP1 (4% vs 16% hydrolysis after 20 min), and human blood serum (30% vs 50% hydrolysis after 24 h). Intravenous administration of 3A in naive rats decreased blood glucose from 155 mg/dL to normal values, ca. 87 mg/dL, unlike glibenclamide, leading to subnormal values (i.e., 63 mg/dL). Similar observations were made for streptozotocin (STZ)-treated and db(+)/db(-) mouse models. Furthermore, 3A inhibits platelet aggregation in vitro and elongates bleeding time in mice (iv administration of 30 mg of 3A/kg), increasing bleeding time to 16 vs 9 min for Prasugrel. Oral administration of 30 mg/kg 3A to rats increased tail bleeding volume, similar to aspirin. These findings suggest that 3A may be an effective treatment for type 2 diabetes by reducing both blood glucose levels and platelet aggregation.

UDP made a highly promising stable, potent, and selective P2Y6-receptor agonist upon introduction of a boranophosphate moiety

P2Y(6) nucleotide receptor (P2Y(6)-R) plays important physiological roles, such as insulin secretion and reduction of intraocular pressure. However, this receptor is still lacking potent and selective agonists to be used as potential drugs. Here, we synthesized uracil nucleotides and dinucleotides, substituted at the C5 and/or P(α) position with methoxy and/or borano groups, 18-22. Compound 18A, R(p) isomer of 5-OMe-UDP(α-B), is the most potent and P2Y(6)-R selective agonist currently known (EC(50) 0.008 μM) being 19-fold more potent than UDP and showing no activity at uridine nucleotide receptors, P2Y(2)- and P2Y(4)-R. Analogue 18A was highly chemically stable under conditions mimicking gastric juice acidity (t(1/2) = 16.9 h). It was more stable to hydrolysis by nucleotide pyrophosphatases (NPP1,3) than UDP (15% and 28% hydrolysis by NPP1 and NPP3, respectively, vs 50% and 51% hydrolysis of UDP) and metabolically stable in blood serum (t(1/2) = 17 vs 2.4, 11.9, and 21 h for UDP, 5-OMe-UDP, and UDP(α-B), respectively). This newly discovered highly potent and physiologically stable P2Y(6)-R agonist may be of future therapeutic potential.

Structural modifications of UMP, UDP, and UTP leading to subtype-selective agonists for P2Y2, P2Y4, and P2Y6 receptors

A large series of derivatives and analogues of the uracil nucleotides UMP, UDP, and UTP with modifications in various positions of the uracil moiety and/or the phosphate groups were synthesized and evaluated at human P2Y(2), P2Y(4), and P2Y(6) receptors. 2-(Ar)alkylthio substitution of UMP and UDP was best tolerated by the P2Y(2) receptor. 2-Phenethylthio-UMP (13e) showed an EC(50) value of 1.3 μM at P2Y(2) and >70-fold selectivity versus P2Y(4) and P2Y(6) receptors. Substitution of the 2-keto group in UMP by NH (13g, iso-CMP) resulted in the first potent and selective P2Y(4) agonist (EC(50) 4.98 μM, >20-fold selective vs P2Y(2) and P2Y(6)). In contrast, replacement of the 2-keto function in UDP by NH yielded a potent P2Y(2) agonist (12g, iso-CDP, EC(50) = 0.604 μM, >100-fold selective). In an attempt to obtain metabolically stable UTP analogues, β,γ-dichloro- and β,γ-difluoro-methylene-UTP derivatives were synthesized. The triphosphate modifications were much better tolerated by P2Y(2), and in some cases also by P2Y(6), than by P2Y(4) receptors. 4-Thio-β,γ-difluoromethylene-UTP (14g) was a potent P2Y(2) agonist with an EC(50) value of 0.134 μM and >50-fold selectivity. N3-Phenacyl-β,γ-dichloromethylene-UTP (14b) proved to be a potent P2Y(6) receptor agonist (EC(50) 0.142 μM) with high selectivity versus P2Y(4) (50-fold) and moderate selectivity versus P2Y(2) receptors (6-fold).

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.

Diadenosine 5',5''-(boranated)polyphosphonate analogues as selective nucleotide pyrophosphatase/phosphodiesterase inhibitors

Nucleotide pyrophosphatase/phosphodiesterases (NPPs) hydrolyze extracellular nucleotides and dinucleotides and thus control purinergic signaling. Enhanced NPP activity is implicated in health disorders such as osteoarthritis and cancer. We designed novel diadenosine polyphosphonate derivatives as potential NPP inhibitors. Analogues 1-4 bear a phosphonate and/or boranophosphate group and/or a 2'-H atom instead of a 2'-OH group. In comparison to ATP, analogues 1-4 were barely hydrolyzed by human NTPDase1, -2, -3, and -8 (<5% hydrolysis) and NPP1 and -3 (≤ 13%) and were not hydrolyzed by ecto-5'-nucleotidase, unlike AMP. These derivatives did not affect NTPDase activity, and analogues 1 and 2 did not inhibit ecto-5'-nucleotidase. All analogues blocked ∼80% of the NPP2-dependent hydrolysis of pnp-TMP, a specific NPP substrate, and inhibited the catabolism of pnp-TMP (K(i) and IC₅₀ both found to be between 10 and 60 μM), Ap₅A, and ATP by NPP1. The activity of NPP3 was inhibited to a lesser extent by the new analogues, with compounds 1 and 4 being the most effective in that respect. The analogues dramatically reduced the level of hydrolysis of pnp-TMP at the cell surface of both osteocarcinoma and colon cancer cells. Importantly, analogues 1-4 exhibited significantly reduced agonistic activity toward human P2Y₁,₁₁) receptors (except for analogue 1) and no activity with human P2Y₂ receptor. Our data provide strong evidence that analogue 2 is the first specific NPP inhibitor to be described.

Pyrimidine ribonucleotides with enhanced selectivity as P2Y(6) receptor agonists: novel 4-alkyloxyimino, (S)-methanocarba, and 5'-triphosphate gamma-ester modifications

The P2Y(6) receptor is a cytoprotective G-protein-coupled receptor (GPCR) activated by UDP (EC(50) = 0.30 microM). We compared and combined modifications to enhance P2Y(6) receptor agonist selectivity, including ribose ring constraint, 5-iodo and 4-alkyloxyimino modifications, and phosphate modifications such as alpha,beta-methylene and extension of the terminal phosphate group into gamma-esters of UTP analogues. The conformationally constrained (S)-methanocarba-UDP is a full agonist (EC(50) = 0.042 microM). 4-Methoxyimino modification of pyrimidine enhanced P2Y(6), preserved P2Y(2) and P2Y(4), and abolished P2Y(14) receptor potency, in the appropriate nucleotide. N(4)-Benzyloxy-CDP (15, MRS2964) and N(4)-methoxy-Cp(3)U (23, MRS2957) were potent, selective P2Y(6) receptor agonists (EC(50) of 0.026 and 0.012 microM, respectively). A hydrophobic binding region near the nucleobase was explored with receptor modeling and docking. UTP-gamma-aryl and cycloalkyl phosphoesters displayed only intermediate P2Y(6) receptor potency but had enhanced stability in acid and cell membranes. UTP-glucose was inactive, but its (S)-methanocarba analogue and N(4)-methoxycytidine 5'-triphospho-gamma-[1]glucose were active (EC(50) of 2.47 and 0.18 microM, respectively). Thus, the potency, selectivity, and stability of pyrimidine nucleotides as P2Y(6) receptor agonists may be enhanced by modest structural changes.

A novel insulin secretagogue based on a dinucleoside polyphosphate scaffold

Dinucleoside polyphosphates exert their physiological effects via P2 receptors (P2Rs). They are attractive drug candidates, as they offer better stability and specificity compared to nucleotides, the most common P2 receptor ligands. The activation of pancreatic P2Y receptors by nucleotides increases insulin secretion. Therefore, in the current study, dinucleoside polyphosphate analogues (di-(2-MeS)-adenosine-5',5''-P(1),P(4),alpha,beta-methylene-tetraphosphate), 8, (di-(2-MeS)-adenosine-5',5''-P(1),P(4),beta,gamma-methylene-tetraphosphate), 9, and di-(2-MeS)-adenosine-5',5''-P(1),P(3),alpha,beta-methylene triphosphate, 10, were developed as potential insulin secretagogues. Analogues 8 and 9 were found to be agonists of the P2Y(1)R with EC(50) values of 0.42 and 0.46 microM, respectively, whereas analogue 10 had no activity. Analogues 8-10 were found to be completely resistant to hydrolysis by alkaline phosphatase over 3 h at 37 degrees C. Analogue 8 also was found to be 2.5-fold more stable in human blood serum than ATP, with a half-life of 12.1 h. Analogue 8 administration in rats caused a decrease in a blood glucose load from 155 mg/dL to ca. 100 mg/dL and increased blood insulin levels 4-fold as compared to basal levels. In addition, analogue 8 reduced a blood glucose load to normal values (80-110 mg/dL), unlike the commonly prescribed glibenclamide, which reduced glucose levels below normal values (60 mg/dL). These findings suggest that analogue 8 may prove to be an effective and safe treatment for type 2 diabetes.

2-MeS-beta,gamma-CCl2-ATP is a potent agent for reducing intraocular pressure

Extracellular nucleotides can modify the production or drainage of the aqueous humor via activation of P2 receptors and therefore affect the intraocular pressure (IOP). We have synthesized slowly hydrolyzable nucleoside di- and triphosphate analogues, 1, and 8-14. Analogues 8-14 were completely resistant to hydrolysis by alkaline phosphatase over 30 min at 37 degrees C. In human blood serum, analogues 8-14 exhibited high stability, e.g., analogues 9 and 10-14 were only 15% and 0% degraded after 24 h, respectively. Moreover, analogues 8-14 were highly stable at pH 1.4 (t(1/2) 1 h-30 days). Analogues 8-14 were agonists of the P2Y(1) receptor (EC(50) 0.57-9.54 muM). Ocular administration of most analogues into rabbits reduced IOP, e.g., analogue 9 reduced IOP by 32% (EC(50) 95.5 nM). Analogue 9 was more effective at reducing IOP than several common glaucoma drugs and represents a promising alternative to timolol maleate, which cannot be used for the treatment of patients suffering from asthma or cardiac problems.

Human P2Y(14) receptor agonists: truncation of the hexose moiety of uridine-5'-diphosphoglucose and its replacement with alkyl and aryl groups

Uridine-5'-diphosphoglucose (UDPG) activates the P2Y(14) receptor, a neuroimmune system GPCR. P2Y(14) receptor tolerates glucose substitution with small alkyl or aryl groups or its truncation to uridine 5'-diphosphate (UDP), a full agonist at the human P2Y(14) receptor expressed in HEK-293 cells. 2-Thiouracil derivatives displayed selectivity for activation of the human P2Y(14) vs the P2Y(6) receptor, such as 2-thio-UDP 4 (EC(50) = 1.92 nM at P2Y(14), 224-fold selectivity vs P2Y(6)) and its beta-propyloxy ester 18. EC(50) values of the beta-methyl ester of UDP and its 2-thio analogue were 2730 and 56 nM, respectively. beta-tert-Butyl ester of 4 was 11-fold more potent than UDPG, but beta-aryloxy or larger, branched beta-alkyl esters, such as cyclohexyl, were less potent. Ribose replacement of UDP with a rigid North or South methanocarba (bicyclo[3.1.0]hexane) group abolished P2Y(14) receptor agonist activity. alpha,beta-Methylene and difluoromethylene groups were well tolerated at the P2Y(14) receptor and are expected to provide enhanced stability in biological systems. alpha,beta-Methylene-2-thio-UDP 11 (EC(50) = 0.92 nM) was 2160-fold selective versus P2Y(6). Thus, these nucleotides and their congeners may serve as important pharmacological probes for the detection and characterization of the P2Y(14) receptor.