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

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.

Purinergic dysregulation causes hypertensive glaucoma-like optic neuropathy

Glaucoma is an optic neuropathy characterized by progressive degeneration of retinal ganglion cells (RGCs) and visual loss. Although one of the highest risk factors for glaucoma is elevated intraocular pressure (IOP) and reduction in IOP is the only proven treatment, the mechanism of IOP regulation is poorly understood. We report that the P2Y6 receptor is critical for lowering IOP and that ablation of the P2Y6 gene in mice (P2Y6KO) results in hypertensive glaucoma-like optic neuropathy. Topically applied uridine diphosphate, an endogenous selective agonist for the P2Y6 receptor, decreases IOP. The P2Y6 receptor was expressed in nonpigmented epithelial cells of the ciliary body and controlled aqueous humor dynamics. P2Y6KO mice exhibited sustained elevation of IOP, age-dependent damage to the optic nerve, thinning of ganglion cell plus inner plexiform layers, and a reduction of RGC numbers. These changes in P2Y6KO mice were attenuated by an IOP lowering agent. Consistent with RGC damage, visual functions were impaired in middle-aged P2Y6KO mice. We also found that expression and function of P2Y6 receptors in WT mice were significantly reduced by aging, another important risk factor for glaucoma. In summary, our data show that dysfunctional purinergic signaling causes IOP dysregulation, resulting in glaucomatous optic neuropathy.

Identification of Highly Promising Antioxidants/Neuroprotectants Based on Nucleoside 5'-Phosphorothioate Scaffold. Synthesis, Activity, and Mechanisms of Action

With a view to identify novel and biocompatible neuroprotectants, we designed nucleoside 5'-thiophosphate analogues, 6-11. We identified 2-SMe-ADP(α-S), 7A, as a most promising neuroprotectant. 7A reduced ROS production in PC12 cells under oxidizing conditions, IC50 of 0.08 vs 21 μM for ADP. Furthermore, 7A rescued primary neurons subjected to oxidation, EC50 of 0.04 vs 19 μM for ADP. 7A is a most potent P2Y1-R agonist, EC50 of 0.0026 μM. Activity of 7A in cells involved P2Y1/12-R as indicated by blocking P2Y12-R or P2Y1-R. Compound 7A inhibited Fenton reaction better than EDTA, IC50 of 37 vs 54 μM, due to radical scavenging, IC50 of 12.5 vs 30 μM for ADP, and Fe(II)-chelation, IC50 of 80 vs >200 μM for ADP (ferrozine assay). In addition, 7A was stable in human blood serum, t1/2 of 15 vs 1.5 h for ADP, and resisted hydrolysis by NPP1/3, 2-fold vs ADP. Hence, we propose 7A as a highly promising neuroprotectant.

Adenine: an important drug scaffold for the design of antiviral agents

Adenine derivatives, in particular the scaffold bearing the acyclic nucleoside phosphonates (ANPS), possess significant antiviral and cytostatic activity. Till now, several effective adenine derivatives have been marketed for the treatment of HIV, HBV, CMV and other virus-infected diseases. These compounds are represented by tenofovir (PMPA), a medicine for both HIV and HBV, and adefovir as an anti-HBV agent. More than this, other analogs, such as GS9148, GS9131, and GS7340, are also well-known anti-viral agents that have been progressed to the clinical studies for their excellent activity. In general, the structures of these compounds include an adenine nucleobase linked to a phosphonate side chain. Considerable structural modifications on the scaffold itself and the peripheral sections were made. The structure-activity relationships (SARs) of this skeleton will provide valuable clues to identify more effective adenine derivatives as antiviral drugs. Here, we systematically summarized the SARs of the adenine derivatives, and gave important information for further optimizing this template.

Nucleoside 5'-phosphorothioate derivatives as oxidative stress protectants in PC12 cells

Iron-induced oxidative damage of mitochondria contributes to cellular death seen in neurodegenerative diseases, therefore, there is a demand for nontoxic, biocompatible, and effective Fe-ion chelators. We evaluated the chelation of Fe(II) by phosphate derivatives using ferrozine as an indicator. We studied the effect of phosphate derivatives on inhibiting Fe(II)-induced oxidative stress in PC12 cells, and metabolic stability in PC12 cells was evaluated. Nucleotides containing phosphorothioate moieties inhibited ROS formation better than natural nucleotides and were more metabolically stable in PC12 cells. Finally, we elucidated that these nucleotides activate the MAP-kinase pathway that contributes to protection of PC12 cells under oxidative stress.

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.

Sp-2-propylthio-ATP-α-B and Sp-2-propylthio-ATP-α-B,β-γ-dichloromethylene are novel potent and specific agonists of the human P2Y₁₁ receptor

The human P2Y₁₁ nucleotide receptor mRNA was found in virtually all human tissues, and the receptor serves many physiological roles, such as immune response regulation. The Ala-87-Thr-P2Y₁₁ receptor single nucleotide polymorphism was linked to increased risk for acute myocardial infarction. To facilitate the development of new therapeutic applications involving cells expressing several P2 receptor subtypes, the availability of specific and potent agonists is mandatory. Here, we synthesized a series of novel adenine nucleotide derivatives, based upon the potent P2Y₁₁ receptor agonists AR-C67085. Features of the novel nucleotide derivatives are a propylthio substitution at C2-adenine and a Pα-borano or Pα-thio substitution of non-bridging oxygen atom. The latter substitutions introduce a chiral center at the α-phosphate. Sp-isomers of Pα-borano- and Rp-isomers of Pα-thio-substituted nucleotides are preferred by the P2Y₁₁ receptor. As recently reported by us, diastereoselectivity of the P2Y₁₁ receptor is opposite to that of the P2Y₁ receptor. Therefore, we exploit this characteristic to increase nucleotide selectivity. At the P2Y₁₁ receptor, the Sp-isomers of 2-propylthio-ATP-α-B (2B) and 2-propylthio-ATP-α-B,β-γ-dichloromethylene (4B) were the most potent of the novel nucleotide series, with EC₅₀ values of 0.03 μM for both, being ca. 80-fold more potent than 2-propylthio-ATP and ATP (EC₅₀ = 2.6 μM). We conclude that the borano-substitution at the α-phosphate of 2-propylthio-ATP enhances nucleotide potency at the P2Y₁₁ receptor. The combination with a Pβ-Pγ-dichloromethylene group in 4B results in a nucleotide, which shows higher selectivity for the P2Y₁₁ receptor over the P2Y₁₁ receptor than 2B making it the most promising of the novel P2Y₁₁ receptor agonists.

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.

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.

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.