Synthesis and anti-HCV activity of β-d-2'-deoxy-2'-α-chloro-2'-β-fluoro and β-d-2'-deoxy-2'-α-bromo-2'-β-fluoro nucleosides and their phosphoramidate prodrugs

Construction of quaternary stereocenters at carbon 2' of nucleosides

The non-natural nucleosides with a quaternary stereogenic center at C2' are crucial to drug discovery. They have become a cornerstone for the treatment of cancer and various viral infections as exemplified by gemcitabine and sofosbuvir. Major research effort has been expended to gain synthetic access to these nucleoside analogues with a significant steric bulk at C2' in the furanoside ring. The 2'-ketonucleosides and 2'-deoxy-2'-methylenenucleosides emerged as key intermediates in these synthetic strategies. For example, α-face addition of methyl lithium to the 2'-ketonucleosides followed by fluorination of resulting tertiary arabino alcohol with DAST provided 2'-fluoro-2'-C-methyluridine - a core nucleoside component of sofosbuvir. The α-face addition of HCN or HN₃ to the 2'-deoxy-2'-methylene nucleosides gave access to the synthetically versatile 2'-cyano-2'-C-methyl and 2'-azido-2'-C-methyl nucleosides. Likewise, the addition of diazomethane to the 2'-exomethylene group gave access to the 2'-spirocyclopropyl analogue. This review primarily discusses synthetic strategies which employs natural nucleosides as substrates but selected approaches involving coupling of the preelaborated sugar precursors with nucleobases are also examined.

Selective profiling of liver-related specific proteins based on sofosbuvir-modified magnetic separation material

It has great significance in profiling specific proteins throughout for better understanding of complex pathological processes and in-depth pharmacological studies. In this work, an efficient protein profiling strategy was developed based on the specific protein-drug interaction. Sofosbuvir (SOF), as a first-line drug for the treatment of hepatitis C, was modified onto the surface of nanoparticles through stable chemical bonds to fabricate a novel magnetic separation material denoted as Fe₃O₄@SiO₂@PAA@SOF. With sequence coverage as the screening parameter, nine proteins were profiled from fetal bovine serum (FBS) of which eight were liver related. Similarly, the strategy was applied to hepatocellular carcinoma (HCC) patient serum. Eight proteins were profiled and all of them were liver related, demonstrating the superb specificity and selectivity of this strategy for profiling liver-related proteins by virtue of protein-SOF interaction. When serum proteins from HCC patients were compared to those from healthy people, one unique differential protein (D3DQX7) was profiled, which was liver related and was a potential target for ameliorating liver diseases. For further research, this material design concept and protein profiling strategy can be extended to employ other drugs for corresponding studies. Sofosbuvir, as a therapeutic drug for liver diseases, was modified onto the surface of magnetic nanoparticles to fabricate the specific selective separation material (Fe₃O₄@SiO₂@PAA@SOF). Based on protein-SOF interaction, the material was applied to adsorb specific proteins from different serum samples. After MS analysis, specific proteins, most of which were liver related, were successfully profiled from FBS and HCC patient serum, fully demonstrating the superb specificity and selectivity of this protein profiling strategy.

Discovery of a 2'-Fluoro,2'-Bromouridine Phosphoramidate Prodrug Exhibiting Anti-Yellow Fever Virus Activity in Culture and in Mice

Yellow fever virus (YFV) is a potentially lethal, zoonotic, blood-borne flavivirus transmitted to humans and non-human primates by mosquitoes. Owing to multiple deadly epidemics, the WHO classifies YFV as a "high impact, high threat disease" with resurgent epidemic potential. At present, there are no approved antiviral therapies to combat YFV infection. Herein we report on 2'-halogen-modified nucleoside analogs as potential anti-YFV agents. Of 11 compounds evaluated, three showed great promise with low toxicity, high intracellular metabolism into the active nucleoside triphosphate form, and sub-micromolar anti-YFV activity. Notably, we investigated a 2'-fluoro,2'-bromouridine phosphate prodrug (C9), a known anti-HCV agent with good stability in human blood and favorable metabolism. Predictive modeling revealed that C9 could readily bind the active site of the YFV RdRp, conferring its anti-YFV activity. C9 displayed potent anti-YFV activity in primary human macrophages, 3D hepatocyte spheroids, and in mice. In an A129 murine model, shortly after infection, C9 significantly reduced YFV replication and protected against YFV-induced liver inflammation and pathology with no adverse effects. Collectively, this work identifies a potent new anti-YFV agent with strong therapeutic promise.

The Inhibitory Potential of 2'-dihalo Ribonucleotides against HCV: Molecular Docking, Molecular Simulations, MM-BPSA, and DFT Studies

Sofosbuvir is the first approved direct-acting antiviral (DAA) agent that inhibits the HCV NS5B polymerase, resulting in chain termination. The molecular models of the 2′-dihalo ribonucleotides used were based on experimental biological studies of HCV polymerase inhibitors. They were modeled within HCV GT1a and GT1b to understand the structure–activity relationship (SAR) and the binding interaction of the halogen atoms at the active site of NS5B polymerase using different computational approaches. The outputs of the molecular docking studies indicated the correct binding mode of the tested compounds against the active sites in target receptors, exhibiting good binding free energies. Interestingly, the change in the substitution at the ribose sugar was found to produce a mild effect on the binding mode. In detail, increasing the hydrophobicity of the substituted moieties resulted in a better binding affinity. Furthermore, in silico ADMET investigation implied the general drug likeness of the examined derivatives. Specifically, good oral absorptions, no BBB penetration, and no CYP4502D6 inhibitions were expected. Likely, the in silico toxicity studies against several animal models showed no carcinogenicity and high predicted TD50 values. The DFT studies exhibited a bioisosteric effect between the substituents at the 2′-position and the possible steric clash between 2′-substituted nucleoside analogs and the active site in the target enzyme. Finally, compound 6 was subjected to several molecular dynamics (MD) simulations and MM-PBSA studies to examine the protein-ligand dynamic and energetic stability.

In silico design of a novel nucleotide antiviral agent by free energy perturbation

Nucleoside analogs are the backbone of antiviral therapies. Drugs from this class undergo processing by host or viral kinases to form the active nucleoside triphosphate species that selectively inhibits the viral polymerase. It is the central hypothesis that the nucleoside triphosphate analog must be a favorable substrate for the viral polymerase and the nucleoside precursor must be a satisfactory substrate for the host kinases to inhibit viral replication. Herein, free energy perturbation (FEP) was used to predict substrate affinity for both host and viral enzymes. Several uridine 5'-monophosphate prodrug analogs known to inhibit hepatitis C virus (HCV) were utilized in this study to validate the use of FEP. Binding free energies to the host monophosphate kinase and viral RNA-dependent RNA polymerase (RdRp) were calculated for methyl-substituted uridine analogs. The 2'-C-methyl-uridine and 4'-C-methyl-uridine scaffolds delivered favorable substrate binding to the host kinase and HCV RdRp that were consistent with results from cellular antiviral activity in support of our new approach. In a prospective evaluation, FEP results suggest that 2'-C-dimethyl-uridine scaffold delivered favorable monophosphate and triphosphate substrates for both host kinase and HCV RdRp, respectively. Novel 2'-C-dimethyl-uridine monophosphate prodrug was synthesized and exhibited sub-micromolar inhibition of HCV replication. Using this novel approach, we demonstrated for the first time that nucleoside analogs can be rationally designed that meet the multi-target requirements for antiviral activity.

Activity of Selected Nucleoside Analogue ProTides against Zika Virus in Human Neural Stem Cells

Zika virus (ZIKV), an emerging flavivirus that causes neurodevelopmental impairment to fetuses and has been linked to Guillain-Barré syndrome continues to threaten global health due to the absence of targeted prophylaxis or treatment. Nucleoside analogues are good examples of efficient anti-viral inhibitors, and prodrug strategies using phosphate masking groups (ProTides) have been employed to improve the bioavailability of ribonucleoside analogues. Here, we synthesized and tested a small library of 13 ProTides against ZIKV in human neural stem cells. Strong activity was observed for 2′-C-methyluridine and 2′-C-ethynyluridine ProTides with an aryloxyl phosphoramidate masking group. Substitution of a 2-(methylthio) ethyl phosphoramidate for the aryloxyl phosphoramidate ProTide group of 2′-C-methyluridine completely abolished antiviral activity of the compound. The aryloxyl phosphoramidate ProTide of 2′-C-methyluridine outperformed the hepatitis C virus (HCV) drug sofosbuvir in suppression of viral titers and protection from cytopathic effect, while the former compound’s triphosphate active metabolite was better incorporated by purified ZIKV NS5 polymerase over time. These findings suggest both a nucleobase and ProTide group bias for the anti-ZIKV activity of nucleoside analogue ProTides in a disease-relevant cell model.