Medicinal chemistry of P2 and adenosine receptors: Common scaffolds adapted for multiple targets

Prof. Geoffrey Burnstock originated the concept of purinergic signaling. He demonstrated the interactions and biological roles of ionotropic P2X and metabotropic P2Y receptors. This review paper traces the historical origins of many currently used antagonists and agonists for P2 receptors, as well as adenosine receptors, in early attempts to identify ligands for these receptors - prior to the use of chemical libraries for screening. Rather than presenting a general review of current purinergic ligands, we focus on common chemical scaffolds (privileged scaffolds) that can be adapted for multiple receptor targets. By carefully analyzing the structure activity relationships, one can direct the selectivity of these scaffolds toward different receptor subtypes. For example, the weak and non-selective P2 antagonist reactive blue 2 (RB-2) was derivatized using combinatorial synthetic approaches, leading to the identification of selective P2Y2, P2Y4, P2Y12 or P2X2 receptor antagonists. A P2X4 antagonist NC-2600 is in a clinical trial, and A3 adenosine agonists show promise, for chronic pain. P2X7 antagonists have been in clinical trials for depression (JNJ-54175446), inflammatory bowel disease (IBD), Crohn's disease, rheumatoid arthritis, inflammatory pain and chronic obstructive pulmonary disease (COPD). P2X3 antagonists are in clinical trials for chronic cough, and an antagonist named after Burnstock, gefapixant, is expected to be the first P2X3 antagonist filed for approval. We are seeing that the vision of Prof. Burnstock to use purinergic signaling modulators, most recently at P2XRs, for treating disease is coming to fruition.

Infection of Rat Osteoblasts with Junin Virus Promotes the Expression of Bone Morphogenetic Protein 6, an Osteogenic Differentiation Inducer

BACKGROUND

The arenavirus Junin virus (JUNV), causative agent of the argentine hemorrhagic fever, is able to modulate several signaling pathways involved in cell survival and multiplication.

OBJECTIVES

We aimed to characterize the infection of rat osteoblasts (OBCs) with JUNV and its consequence on the modulation of osteogenic genes expression, thus studying the ability of this virus to induce cell differentiation. In addition, we evaluated the effect of purinergic agonists on viral replication.

METHOD

Quantification of infectivity by plaque forming unit (PFU) assay, synthesis of viral proteins by western blot and immunofluorescence, and expression of osteogenic differentiation markers (ODM) by quantitative real-time polymerase chain reaction were employed.

RESULTS

Infection of OBCs with JUNV (MOI 0.01 PFU/cell) showed a peak of infectivity, reaching 1.5 × 105 PFU/mL at the second day post-infection (p.i.). A marked restriction in multiplication was detected at day 7 p.i. that did not impair the establishment of a persistent stage of infection in OBCs. Analysis of mRNAs corresponding to ODM such as alkaline phosphatase, bone sialo-protein, and bone morphogenetic proteins (BMPs) 4 and 6 revealed that only the levels of BMP-6 were significantly higher in infected cells. Treatment with the purinergic agonists ATPγS, UTP, ADP, or UDP diminished viral titer and reduced the expression of the viral nucleoprotein. Also, treatment with 10 μM ATPγS reduced the stimulation of BMP-6 expression induced by the infection.

CONCLUSIONS

These data demonstrate that JUNV is capable of infecting OBCs and point out BMP-6 as a key factor during this process.

5-OMe-uridine-5'-O-(α-boranodiphosphate), a novel nucleotide derivative highly active at the human P2Y(6) receptor protects against death-receptor mediated glial apoptosis

P2Y receptors are activated by nucleotides and involved in numerous physiological/pathophysiological processes. However, investigations of specific P2Y receptor functions have been hampered by lack of suitable receptor agonists-antagonists. Recently, we identified the nucleotide 5-OMe-UDP as potent and selective agonist for human P2Y6 receptors. We studied a series of derivatives of this analog with a Pα-borano group substituting a non-bridging oxygen and found increased potency and receptor specificity. Rp-5-OMe-UDPαB (Rp-5-OMe-uridine 5'-O-α-boranodiphosphate) was most potent and selective in inducing intracellular calcium signaling in 1321N1 astrocytoma cells expressing the human P2Y6 receptor. Here, we investigated whether Rp-5-OMe-UDPαB evokes cell protection through human P2Y6 receptors. We tested a well-established model, tumor necrosis factor α (TNFα)-induced cell death in 1321N1 astrocytoma cells. Rp-5-OMe-UDPαB inhibited TNFα-induced cell death even stronger than UDP. These first data of a neuro-protective activity of the human P2Y6 receptor emphasize the potential of the stable, selective, and potent Rp-5-OMe-UDPαB analog for exploiting P2Y6 receptor-mediated cellular functions, like cytoprotection in human tissues, with suitability for future neuro-protective drug development.

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.