Fluorination of Naturally Occurring N⁶-Benzyladenosine Remarkably Increased Its Antiviral Activity and Selectivity

Cytotoxic effects of kinetin riboside and its selected analogues on cancer cell lines

N6-[(Furan-2-yl)methyl]adenosine (kinetin riboside) and its seven synthesized analogues were examined for the ability to inhibit the growth of five human carcinoma cell lines and for comparison of normal human lung fibroblast cell line (MRC-5). Out of the compounds evaluated, 8-azakinetin riboside was shown to exhibit significant cytotoxic activity for 72 h treatment against ovarian OVCAR-3 and pancreatic MIA PaCa-2 cancer cells (IC50 = 1.1 μM) with an observed weaker effect against MRC-5 cells (IC50 = 4.6 μM). Kinetin riboside, as well as its N6-[(furan-3-yl)methyl]- and N6-[(thien-2-yl)methyl]- counterparts, also exhibited cytotoxic activities at low micromolar levels but were non-selective over MRC-5 cells.

Elongation of N6-benzyladenosine scaffold via Pd-catalyzed C-C bond formation leads to derivatives with antiflaviviral activity

Decoration of nucleoside analogues with lipophilic groups often leads to compounds with improved antiviral activity. For example, N6-benzyladenosine derivatives containing elongated lipophilic substituents in the benzyl core efficiently inhibit reproduction of tick-borne encephalitis virus (TBEV), while N6-benzyladenosine itself potently inhibits reproduction of human enterovirus A71 (EV-A71). We have extended a series of N6-benzyladenosine analogues using effective synthetic methods of CC bond formation based on Pd-catalyzed cross-coupling reactions (Sonogashira and Suzuki) in order to study the influence of bulky lipophilic substituents in the N6 position of adenosine on the antiviral activity against flaviviruses, such as TBEV, yellow fever virus (YFV) and West Nile virus (WNV), as well as a panel of enteroviruses including EV-A71, Echovirus 30 (E30), and poliovirus type 2 (PV2). Reproduction of tested flaviviruses appeared to be inhibited by the micromolar concentrations of the compounds, while cytotoxicity in most cases was beyond the detection limit. Time-of-addition studies demonstrated that the hit compounds inhibited the stage of viral RNA synthesis, but not the stages of the viral entry or protein translation. As a result, several new promising antiflaviviral leads have been identified. On the other hand, none of the synthesized compounds inhibited enterovirus reproduction, indicating a possibility of involvement of flavivirus-specific pathways in their mechanism of action.

Nucleoside Analog 2',3'-Isopropylidene-5-Iodouridine as Novel Efficient Inhibitor of HIV-1

Nucleoside reverse transcriptase inhibitors are the first class of drugs to be approved by the FDA for the suppression of HIV-1 and are widely used for this purpose in combination with drugs of other classes. Despite the progress in HIV-1 treatment, there is still the need to develop novel efficient antivirals. Here the efficiency of HIV-1 inhibition by a set of original 5-substituted uridine nucleosides was studied. We used the replication deficient human immunodeficiency virus (HIV-1)-based lentiviral particles and identified that among the studied compounds, 2',3'-isopropylidene-5-iodouridine was shown to cause anti-HIV-1 activity. Importantly, no toxic action of this compound against the cells of T-cell origin was found. We determined that this compound is significantly more efficient at suppressing HIV-1 compared to Azidothymidine (AZT) when taken at the high non-toxic concentrations. We did not find any profit when using AZT in combination with 2',3'-isopropylidene-5-iodouridine. 2',3'-Isopropylidene-5-iodouridine acts synergistically to repress HIV-1 when combined with the CDK4/6 inhibitor Palbociclib in low non-toxic concentration. No synergistic antiviral action was detected when AZT was combined with Palbociclib. We suggest 2',3'-isopropylidene-5-iodouridine as a novel perspective non-toxic compound that may be used for HIV-l suppression.

Rational design of novel nucleoside analogues reveals potent antiviral agents for EV71

Different viruses belonging to distinct viral families, such as enterovirus 71, rely on the host methyltransferase METTL3 for the completion of fundamental cytoplasmic stages of their life cycle. Modulation of the activity of this enzyme could therefore provide a broad-spectrum approach to interfere with viral infections caused by viruses that depend on its activity for the completion of their viral cycle. With the aim to identify antiviral therapeutics with this effect, a series of new nucleoside analogues was rationally designed to act as inhibitors of human METTL3, as a novel approach to interfere with a range of viral infections. Guided by molecular docking studies on the SAM binding pocket of the enzyme, 24 compounds were prepared following multiple-step synthetic protocols, and evaluated for their ability to interfere with the replication of different viruses in cell-based systems, and to directly inhibit the activity of METTL3. While different molecules displayed moderate inhibition of the human methyltransferase in vitro, multiple novel, potent and selective inhibitors of enterovirus 71 were identified.

In Planta, In Vitro and In Silico Studies of Chiral N6-Benzyladenine Derivatives: Discovery of Receptor-Specific S-Enantiomers with Cytokinin or Anticytokinin Activities

Cytokinins, classical phytohormones, affect all stages of plant ontogenesis, but their application in agriculture is limited because of the lack of appropriate ligands, including those specific for individual cytokinin receptors. In this work, a series of chiral N6-benzyladenine derivatives were studied as potential cytokinins or anticytokinins. All compounds contained a methyl group at the α-carbon atom of the benzyl moiety, making them R- or S-enantiomers. Four pairs of chiral nucleobases and corresponding ribonucleosides containing various substituents at the C2 position of adenine heterocycle were synthesized. A nucleophilic substitution reaction by secondary optically active amines was used. A strong influence of the chirality of studied compounds on their interaction with individual cytokinin receptors of Arabidopsis thaliana was uncovered in in vivo and in vitro assays. The AHK2 and CRE1/AHK4 receptors were shown to have low affinity for the studied S-nucleobases while the AHK3 receptor exhibited significant affinity for most of them. Thereby, three synthetic AHK3-specific cytokinins were discovered: N6-((S)-α-methylbenzyl)adenine (S-MBA), 2-fluoro,N6-((S)-α-methylbenzyl)adenine (S-FMBA) and 2-chloro,N6-((S)-α-methylbenzyl)adenine (S-CMBA). Interaction patterns between individual receptors and specific enantiomers were rationalized by structure analysis and molecular docking. Two other S-enantiomers (N6-((S)-α-methylbenzyl)adenosine, 2-amino,N6-((S)-α-methylbenzyl)adenosine) were found to exhibit receptor-specific and chirality-dependent anticytokinin properties.

Synthesis of Fluorine-Containing Analogues of Purine Deoxynucleosides: Optimization of Enzymatic Transglycosylation Conditions

In this work, a comparative analysis of the conditions of transglycosylation reactions catalyzed by E. coli nucleoside phosphorylases was carried out, and the optimal conditions for the formation of various nucleosides were determined. Under the optimized conditions of transglycosylation reaction, fluorine-containing derivatives of N⁶-benzyl-2'-deoxyadenosine, potential inhibitors of replication of enteroviruses in a cell, were obtained starting from the corresponding ribonucleosides.

Cytotoxicity reduction by O-nicotinoylation of antiviral 6-benzylaminopurine ribonucleosides

One of the promising approaches in the development of nucleoside prodrugs is to use the nucleoside analogs containing lipophilic biodegradable residues, which are cleaved to biologically active forms after metabolic transformations in the cell. The introduction of such fragments makes it possible to reduce the general toxicity of the drug candidate and increase its stability in the cell. In order to study the influence of biodegradable lipophilic groups on antiviral activity and cytotoxicity, in this work we synthesized N⁶-benzyl-2',3',5'-tri-O-nicotinoyl adenosine and N⁶-(3-fluorobenzyl)-2',3',5'-tri-O-nicotinoyl adenosine, derivatives of N⁶-benzyladenosine (BAR) and N⁶-(3-fluorobenzyl)adenosine (FBAR), which had previously shown prominent antiviral activity against human enterovirus EV-A71 but appeared to be cytotoxic. As a result, the obtained fully-O-nicotinoylated BAR and FBAR inhibited reproduction of EV-A71 strains BrCr and 46,973 and manifested significantly lower cytotoxicity compared to non-protected compounds. In addition, we performed enzymatic hydrolysis of the fully-O-nicotinoylated FBAR in the presence of esterases (Cal B and PLE) to investigate metabolic degradation of O-nicotinoylated compounds in cells. As a result, both enzymes hydrolyzed the tested substrate to form the corresponding O-deprotected nucleoside that may suggest the role of hydrolase-type enzymes as general participants of metabolic activation of O-nicotinoylated prodrugs in different cells.

Comparative Analysis of Enzymatic Transglycosylation Using E. coli Nucleoside Phosphorylases: A Synthetic Concept for the Preparation of Purine Modified 2′-Deoxyribonucleosides from Ribonucleosides

A comparative analysis of the transglycosylation conditions catalyzed by E. coli nucleoside phosphorylases, leading to the formation of 2'-deoxynucleosides, was performed. We demonstrated that maximal yields of 2'-deoxynucleosides, especially modified, can be achieved under small excess of glycosyl-donor (7-methyl-2'-deoxyguanosine, thymidine) and a 4-fold lack of phosphate. A phosphate concentration less than equimolar one allows using only a slight excess of the carbohydrate residue donor nucleoside to increase the reaction's output. A three-step methodology was elaborated for the preparative synthesis of purine-modified 2'-deoxyribonucleosides, starting from the corresponding ribonucleosides.

Antiviral and Antimicrobial Nucleoside Derivatives: Structural Features and Mechanisms of Action

The emergence of new viruses and resistant strains of pathogenic microorganisms has become a powerful stimulus in the search for new drugs. Nucleosides are a promising class of natural compounds, and more than a hundred drugs have already been created based on them, including antiviral, antibacterial and antitumor agents. The review considers the structural and functional features and mechanisms of action of known nucleoside analogs with antiviral, antibacterial or antiprotozoal activity. Particular attention is paid to the mechanisms that determine the antiviral effect of nucleoside analogs containing hydrophobic fragments. Depending on the structure and position of the hydrophobic substituent, such nucleosides can either block the process of penetration of viruses into cells or inhibit the stage of genome replication. The mechanisms of inhibition of viral enzymes by compounds of nucleoside and non-nucleoside nature have been compared. The stages of creation of antiparasitic drugs, which are based on the peculiarities of metabolic transformations of nucleosides in humans body and parasites, have been considered. A new approach to the creation of drugs is described, based on the use of prodrugs of modified nucleosides, which, as a result of metabolic processes, are converted into an effective drug directly in the target organ or tissue. This strategy makes it possible to reduce the general toxicity of the drug to humans and to increase the effectiveness of its action on cells infected by the virus.

Structure activity relationship of novel antiviral nucleosides against Enterovirus A71

Various (North)-methanocarba adenosine derivatives, containing rigid bicyclo[3.1.0]hexane ribose substitution, were screened for activity against representative viruses, and inhibition was observed after treatment of Enterovirus A71 with a 2-chloro-N6-1-cyclopropyl-2-methylpropan-1-yl derivative (17). µM activity was also seen when testing 17 against other enteroviruses in the Picornaviridae family. Based on this hit, structural congeners of 17, containing other N6-alkyl groups and 5' modifications, were synthesized and tested. The structure activity relationship is relatively narrow, with most modifications of the adenine or the methanocarba ring reducing or abolishing the inhibitory potency. 4'-Truncated 31 (MRS5474), 4'-fluoromethyl 48 (MRS7704) and 4'-chloromethyl 49 nucleosides displayed EC50 ~3-4 µM, and 31 and 48 achieved SI ≥10. However, methanocarba analogues of ribavirin and N6-benzyladenosine, shown previously to have anti-EV-A71 activity, were inactive. Thus, we identified methanocarba nucleosides as a new scaffold for enterovirus inhibition with a narrow structure activity relationship and no similarity to previously published anti-enteroviral nucleosides.

Synthesis of N6 -Substituted Adenosines as Cytokinin Nucleosides

This unit describes preparation of N⁶ -substituted adenosines (cytokinin nucleosides), a unique class of compounds with a wide spectrum of biological activities. Regioselective alkylation of N⁶ -acetyl-2',3',5'-tri-O-acetyladenosine with alkyl halides under basic conditions or alcohols under Mitsunobu conditions followed by deprotection are the methods of choice for the preparation of the cytokinin nucleosides. The attractive feature of this strategy is the possibility of using a broad library of commercially available alkyl halides and alcohols under mild reaction conditions. © 2018 by John Wiley & Sons, Inc.

Semisynthesis, Characterization and Evaluation of New Adenosine Derivatives as Antiproliferative Agents

We describe the semisynthesis and biological effects of adenosine derivatives, which were anticipated to function as agonists for the A₃ receptor. Molecular docking was used to select candidate compounds. Fifteen nucleoside derivatives were obtained through nucleophilic substitutions of the N⁶-position of the nucleoside precursor 6-chloropurine riboside by amines of different origin. All compounds were purified by column chromatography and further characterized by spectroscopic and spectrometric techniques, showing moderate yield. These molecules were then evaluated for their antiproliferative activity in human gastric cancer cells expressing the A₃ receptor. We found that the compounds obtained have antiproliferative activity and that new structural modifications can enhance their biological activity. The ADME (Absorption, Distribution, Metabolism and Excretion) properties of the most active compounds were also evaluated theoretically.

Chemoenzymatic synthesis of cytokinins from nucleosides: ribose as a blocking group

Nucleoside phosphorylases are involved in the salvage pathways of nucleoside biosynthesis and catalyze the reversible reaction of a nucleobase with α-d-ribose-1-phosphate to yield a corresponding nucleoside and an inorganic phosphate. The equilibrium of these reactions is shifted towards nucleosides, especially in the case of purines. Purine nucleoside phosphorylase (PNP, EC 2.4.2.1) is widely used in labs and industry for the synthesis of nucleosides of practical importance. Bacterial PNPs have relatively broad substrate specificity utilizing a wide range of purines with different substituents to form the corresponding nucleosides. To shift the reaction in the opposite direction we have used arsenolysis instead of phosphorolysis. This reaction is irreversible due to the hydrolysis of the resulting α-d-ribose-1-arsenate. As a result, heterocyclic bases are formed in quantitative yields and can be easily isolated. We have developed a novel method for the preparation of cytokinins based on the enzymatic cleavage of the N-glycosidic bond of N⁶-substituted adenosines in the presence of PNP and Na₂HAsO₄. According to the HPLC analysis the conversion proceeds in quantitative yields. In the proposed strategy the ribose residue acts as a protective group. No contamination of the final products with AsO₄^3- has been detected via HPLC-HRMS; simple analytical arsenate detection via ESI-MS has been proposed.