The war against influenza: discovery and development of sialidase inhibitors

Influenza A Virus Neuraminidase Inhibitors

Depending on the strain, influenza A virus causes animal, zoonotic, pandemic, or seasonal influenza with varying degrees of severity. Two surface glycoprotein spikes, hemagglutinin (HA) and neuraminidase (NA), are the most important influenza A virus antigens. NA plays an important role in the propagation of influenza virus by removing terminal sialic acid from sialyl decoy receptors and thereby facilitating the release of viruses from traps such as in mucus and on infected cells. Some NA inhibitors have become widely used drugs for treatment of influenza. However, attempts to develop effective and safe NA inhibitors that can be used for treatment of anti-NA drugs-resistant influenza viruses have continued. In this chapter, we describe the following updates on influenza A NA inhibitor development: (i) N-acetylneuraminic acid (Neu5Ac)-based derivatives, (ii) covalent NA inhibitors, (iii) sulfo-sialic acid analogs, (iv) N-acetyl-6-sulfo-β-D-glucosaminide-based inhibitors, (v) inhibitors targeting the 150-loop of group 1 NAs, (vi) conjugation inhibitors, (vii) acylhydrazone derivatives, (viii) monoclonal antibodies, (ix) PVP-I, and (x) natural products. Finally, we provide future perspectives on the next-generation anti-NA drugs.

Development and validation of a quantitative method for 15 antiviral drugs in poultry muscle using liquid chromatography coupled to tandem mass spectrometry

The objective of this work was to develop a quantitative multi-residue method for analysing antiviral drug residues and their metabolites in poultry meat samples. Antiviral drugs are not licensed for the treatment of influenza in food producing animals. However, there have been some reports indicating their illegal use in poultry. In this study, a method was developed for the analysis of 15 antiviral drug residues in poultry muscle (chicken, duck, quail and turkey) using liquid chromatography coupled to tandem mass spectrometry. This included 13 drugs against influenza and associated metabolites, but also two drugs employed for the treatment of herpes (acyclovir and ganciclovir). The method required the development of a novel chromatographic separation using a hydrophilic interaction chromatographic (HILIC) BEH amide column, which was necessary to retain the highly polar compounds. The analytes were detected using a triple quadrupole mass spectrometer operating in positive electrospray ionization mode. A range of different sample preparation protocols suitable for polar compounds were evaluated. The most effective procedure was based on a simple acetonitrile-based protein precipitation step followed by a further dilution in a methanol/water solution. The confirmatory method was validated according to the EU 2021/808 guidelines on different species including chicken, duck, turkey and quail. The validation was performed using various calibration curves ranging from 0.1 µg kg^-1to 200 µg kg^-1, according to the analyte. Depending on the analyte sensitivity, decision limits achieved ranged from 0.12 µg kg^-1 for arbidol to 34.7 µg kg^-1 for ribavirin. Overall, the reproducibility precision values ranged from 2.8% to 22.7% and the recoveries from 84% to 127%. The method was applied to 120 commercial poultry samples from the Irish market, which were all found to be residue-free.

Zanamivir-Cholesterol Conjugate: A Long-Acting Neuraminidase Inhibitor with Potent Efficacy against Drug-Resistant Influenza Viruses

Antiviral therapy of influenza virus infections depends heavily on two viral neuraminidase (NA) inhibitors, oseltamivir (OSV) and zanamivir (ZNV). The efficacy of OSV is challenged by the development of viral resistance, while the clinical use of ZNV is limited by its poor pharmacokinetic profile and requirement for twice-daily intranasal administration. We have developed a novel NA inhibitor by conjugating ZNV to cholesterol. The ZNV-cholesterol conjugate showed markedly improved antiviral efficacy and plasma half-life compared with ZNV. Single-dose administration of the conjugate protected the mice from lethal challenges with wild-type or mutant H1N1 influenza viruses bearing an OSV-resistant H275Y-substitution. Mechanistic studies showed that the conjugate targeted the cell membrane and entered the host cells, thereby inhibiting the NA function and the assembly of progeny virions. The ZNV-cholesterol conjugate represents a potential new treatment for influenza infections with sustained effect. Cholesterol conjugation may be an effective strategy for improving the pharmacokinetics and efficacy of other small-molecule therapeutics.

Discovery of novel thiophene derivatives as potent neuraminidase inhibitors

Neuraminidase (NA) is an important target for the treatment of influenza. In this study, a new lead NA inhibitor, 4 (ZINC01121127), was discovered by pharmacophore-based virtual screening and molecular dynamic (MD) simulation. Some novel NA inhibitors containing thiophene ring were synthesized by optimizing the skeleton of the lead compound 4. Compound 4b had the most potent inhibitory activity against NA (IC₅₀ = 0.03 μM), which was better than the positive control oseltamivir carboxylate (IC₅₀ = 0.06 μM). 4b (EC₅₀ = 1.59 μM) also exhibits excellent antiviral activity against A/chicken/Hubei/327/2004 (H5N1-DW), which is superior to the reference drug OSC (EC₅₀ = 5.97 μM). Molecular docking study shows that the thiophene moiety plays an essential role in compound 4b, which can bind well to the active site of NA. The good activity of 4b may be also ascribed to the extending of quinoline ring into the 150-cavity. The results of this study may provide an insightful help for the development of new NA inhibitors.

Bifunctional Inhibitors of Influenza Virus Neuraminidase: Molecular Design of a Sulfonamide Linker

The growing resistance of the influenza virus to widely used competitive neuraminidase inhibitors occupying the active site of the enzyme requires the development of bifunctional compounds that can simultaneously interact with other regulatory sites on the protein surface. When developing such an inhibitor and combining structural fragments that could be located in the sialic acid cavity of the active site and the adjacent 430-cavity, it is necessary to select a suitable linker not only for connecting the fragments, but also to ensure effective interactions with the unique arginine triad Arg118-Arg292-Arg371 of neuraminidase. Using molecular modeling, we have demonstrated the usefulness of the sulfonamide group in the linker design and the potential advantage of this functional group over other isosteric analogues.

The reorganization energy of compounds upon binding to proteins, from dynamic and solvated bound and unbound states

The intramolecular reorganization energy (ΔE_Reorg) of compounds upon binding to proteins is a component of the binding free energy, which has long received particular attention, for fundamental and practical reasons. Understanding ΔE_Reorg would benefit the science of molecular recognition and drug design. For instance, the tolerable strain energy of compounds upon binding has been elusive. Prior studies found some large ΔE_Reorg values (e.g. > 10 kcal/mol), received with skepticism since they imply excessive opposition to binding. Indeed, estimating ΔE_Reorg is technically difficult. Typically, ΔE_Reorg has been approached by taking two energy-minimized conformers representing the bound and unbound states, and subtracting their conformational energy. This is a drastic oversimplification, liable to conformational collapse of the unbound conformer. Instead, the present work applies extensive molecular dynamics (MD) and the modern OPLS3 force-field to simulate compounds bound and unbound states, in explicit solvent under physically relevant conditions. The thermalized unbound compounds populate multiple conformations, not reducible to one or a few energy-minimized conformers. The intramolecular energies in the bound and unbound states were averaged over pertinent conformational ensembles, and the reorganization enthalpy upon binding (ΔH_Reorg) deduced by subtraction. This was applied to 76 systems, including 43 approved drugs, carefully selected for i) the quality of the bioactive X-ray structures and ii) the diversity of the chemotypes, their properties and protein targets. It yielded comparatively low ΔH_Reorg values (median = 1.4 kcal/mol, mean = 3.0 kcal/mol). A new finding is the observation of negative ΔH_Reorg values. Indeed, reorganization energies do not have to oppose binding, e.g. when intramolecular interactions stabilize preferentially the bound state. Conversely, even with competing water molecules, intramolecular interactions can occur predominantly in the unbound compound, and be replaced by intermolecular counterparts upon protein binding. Such disruption of intramolecular interactions upon binding gives rise to occasional larger ΔH_Reorg values. Such counterintuitive larger ΔH_Reorg values may be rationalized as a redistribution of interactions upon binding, qualitatively compatible with binding.

Prognostic Value of Elevated Levels of Plasma N-Acetylneuraminic Acid in Patients With Heart Failure

BACKGROUND

Cardiac sialylation is involved in a variety of physiological processes in the heart. Altered sialylation has been implicated in heart failure (HF) mice. However, its role in patients with HF is unclear, and the potential effect of modulation of cardiac sialylation is worth exploring.

METHODS

We first assessed the association between plasma N-acetylneuraminic acid levels and the incidence of adverse cardiovascular events in patients with HF over a median follow-up period of 2 years. Next, immunoblot analysis and lectin histochemistry were performed in cardiac tissue to determine the expression levels of neuraminidases and the extent of cardiac desialylation. Finally, the therapeutic impact of a neuraminidase inhibitor was evaluated in animal models of HF.

RESULTS

Among 1699 patients with HF, 464 (27%) died of cardiovascular-related deaths or underwent heart transplantation. We found that the elevated plasma N-acetylneuraminic acid level was independently associated with a higher risk of incident cardiovascular death and heart transplantation (third tertile adjusted hazard ratio, 2.11 [95% CI, 1.67-2.66], P<0.001). In addition, in cardiac tissues from patients with HF, neuraminidase expression was upregulated, accompanied by desialylation. Treatment with oseltamivir, a neuraminidase inhibitor, in HF mice infused with isoproterenol and angiotensin II significantly inhibited desialylation and ameliorated cardiac dysfunction.

CONCLUSIONS

This study uncovered a significant association between elevated plasma N-acetylneuraminic acid level and an increased risk of a poor clinical outcome in patients with HF. Our data support the notion that desialylation represents an important contributor to the progression of HF, and neuraminidase inhibition may be a potential therapeutic strategy for HF.

Natural Products & Bioactivity Inspired Synthetic Pursuits Interfacing with Carbohydrates: Ongoing Journey with C-Glycosides

Natural products, remains the most important source for the discovery of new drugs for the treatment of human diseases. This has inspired the synthetic community to design and develop mimics of natural products either to answer important questions in biology or to explore their therapeutic potentials. Glycosides present themselves abundantly in nature, right from the cell surface receptors to natural products of any origin. The O-Glycosides are hydrolytically less stable compared to C-glycosides and this feature has presented a great opportunity for drug discovery. The discovery of Dapagliflozin, an SGLT inhibitor and C-glucoside, for the treatment of diabetes is one such example. Aryl acyl-anion chemistry has been explored for the synthesis of 2-deoxy-C-aryl furanoside/pyranoside/septanosides. Besides success, the studies have provided valuable insight into the natural propensities of the architectural framework for the cascade to furan derivatives. The aryl acyl-anion chemistry has also enabled the synthesis of biologically active diaryl heptanoids. Inspired from sucesss of Dapagliflozin, new analogues have been synthesized with pyridine and isocoumarin heterocycle as the proximal ring. C-glucosides of isoliquiritigenin have been synthesized for the first time and evaluated as an efficient aldose reductase inhibitor. The synthesis and evaluation of acyl-C-β-D-glucosides and benzyl-C-β-D-glucoside as glucose-uptake promoters has revealed promise in small molecules. The concept of building blocks has been used to obtain natural oxylipins, D-xylo and L-xylo-configured alkane tetrols and novel lipophilic ketones with erythro/threo configured trihydroxy polar head-group as possible anti-mycobacterial agents.

Synthesis and Anti-Influenza Virus Effects of Novel Substituted Polycyclic Pyridone Derivatives Modified from Baloxavir

In this work, a series of novel substituted polycyclic pyridone derivatives were designed and synthesized as potent anti-influenza agents. The cytopathic effect (CPE) assay and cytotoxicity assay indicated that all of the compounds possessed potent anti-influenza virus activity and relatively low cytotoxicity; some of them inhibited the replication of influenza A virus (IAV) at picomolar concentrations. Further studies revealed that, at a concentration of 3 nM, three compounds (10a, 10d, and 10g) could significantly reduce the M2 RNA amounts and M2 protein expression of IAV and inhibit the activity of RNA-dependent RNA polymerase (RdRp). Among them, (R)-12-(5H-dibenzo[a,d][7]annulen-5-yl)-7-hydroxy-3,4,12,12a-tetrahydro-1H-[1,4]oxazino[3,4-c]pyrido[2,1-f][1,2,4]triazine-6,8-dione (10a) was found to be a promising anti-influenza drug candidate with good human liver microsomal stability, as well as with better selectivity index and oral bioavailability than Baloxavir.

A study on catalytic and non-catalytic sites of H5N1 and H1N1 neuraminidase as the target for chalcone inhibitors

The H1N1 pandemic in 2009 and the H5N1 outbreak in 2005 have shocked the world as millions of people were infected and hundreds of thousands died due to the infections by the influenza virus. Oseltamivir, the most common drug to block the viral life cycle by inhibiting neuraminidase (NA) enzyme, has been less effective in some resistant cases due to the virus mutation. Presently, the binding of 10 chalcone derivatives towards H5N1 and H1N1 NAs in the non-catalytic and catalytic sites was studied using molecular docking. The in silico study was also conducted for its drug-like likeness such as Lipinski Rule, mutagenicity, toxicity and pharmacokinetic profiles. The result demonstrates that two chalcones (1c and 2b) have the potential for future NA inhibitor development. Compound 1c inhibits H5N1 NA and H1N1 NA with IC₅₀ of 27.63 µM and 28.11 µM, respectively, whereas compound 2b inhibits NAs with IC₅₀ of 87.54 µM and 73.17 µM for H5N1 and H1N1, respectively. The in silico drug-like likeness prediction reveals that 1c is 62% better than 2b (58%) in meeting the criteria. The results suggested that 1c and 2b have potencies to be developed as non-competitive inhibitors of neuraminidase for the future development of anti-influenza drugs.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s13765-021-00639-w.

Substitution of I222L-E119V in neuraminidase from highly pathogenic avian influenza H7N9 virus exhibited synergistic resistance effect to oseltamivir in mice

That the high frequency and good replication capacity of strains with reduced susceptibility to neuraminidase inhibitors (NAIs) in highly pathogenic avian influenza H7N9 (HPAI H7N9) virus made it a significance to further study its drug resistance. HPAI H7N9 viruses bearing NA I222L or E119V substitution and two mutations of I222L-E119V as well as their NAIs-sensitive counterpart were generated by reverse genetics for NA inhibition test and replication capability evaluation in vitro. The attenuated H7N9/PR8 recombinant viruses were developed to study the pathogenicity and drug resistance brought by the above substitutions to mice. The IC₅₀ fold change of oseltamivir to HPAI H7N9 with NA222L-119V is 306.34 times than that of its susceptible strain, and 3.5 times than the E119V mutant virus. HPAI H7N9 bearing NA222L-119V had good replication ability with peak value of more than 6log₁₀ TCID₅₀/ml in MDCK cells. H7N9/PR8 virus bearing NA222L-119V substitutions leaded to diffuse pneumonia, significant weight loss and fatality in mice. NA E119V made H7N9/PR8 virus resistant to oseltamivir, and I222L-E119V had synergistic resistance to oseltamivir in mice. Due to the good fitness of drug resistant strains of HPAI H7N9 virus, it is necessary to strengthen drug resistance surveillance and new drug research.

Sialidase and Sialyltransferase Inhibitors: Targeting Pathogenicity and Disease

Sialidases (SAs) and sialyltransferases (STs), the enzymes responsible for removing and adding sialic acid to other glycans, play essential roles in viruses, bacteria, parasites, and humans. Sialic acid is often the terminal sugar on glycans protruding from the cell surface in humans and is an important component for recognition and cell function. Pathogens have evolved to exploit this and use sialic acid to either “cloak” themselves, ensuring they remain undetected, or as a mechanism to enable release of virus progeny. The development of inhibitors against SAs and STs therefore provides the opportunity to target a range of diseases. Inhibitors targeting viral, bacterial, or parasitic enzymes can directly target their pathogenicity in humans. Excellent examples of this can be found with the anti-influenza drugs Zanamivir (Relenza™, GlaxoSmithKline) and Oseltamivir (Tamiflu™, Roche and Gilead), which have been used in the clinic for over two decades. However, the development of resistance against these drugs means there is an ongoing need for novel potent and specific inhibitors. Humans possess 20 STs and four SAs that play essential roles in cellular function, but have also been implicated in cancer progression, as glycans on many cancer cells are found to be hyper-sialylated. Whilst much remains unknown about how STs function in relation to disease, it is clear that specific inhibitors of them can serve both as tools to gain a better understanding of their activity and form the basis for development of anti-cancer drugs. Here we review the recent developments in the design of SA and ST inhibitors against pathogens and humans.

Amino Acid and Peptide-Based Antiviral Agents

A significant number of antiviral agents used in clinical practice are amino acids, short peptides, or peptidomimetics. Among them, several HIV protease inhibitors (e. g. lopinavir, atazanavir), HCV protease inhibitors (e. g. grazoprevir, glecaprevir), and HCV NS5A protein inhibitors have contributed to a significant decrease in mortality from AIDS and hepatitis. However, there is an ongoing need for the discovery of new antiviral agents and the development of existing drugs; amino acids, both proteinogenic and non-proteinogenic in nature, serve as convenient building blocks for this purpose. The synthesis of non-proteinogenic amino acid components of antiviral agents could be challenging due to the need for enantiomerically or diastereomerically pure products. Herein, we present a concise review of antiviral agents whose structures are based on amino acids of both natural and unnatural origin. Special attention is paid to the synthetic aspects of non-proteinogenic amino acid components of those agents.

Display of the human mucinome with defined O-glycans by gene engineered cells

Mucins are a large family of heavily O-glycosylated proteins that cover all mucosal surfaces and constitute the major macromolecules in most body fluids. Mucins are primarily defined by their variable tandem repeat (TR) domains that are densely decorated with different O-glycan structures in distinct patterns, and these arguably convey much of the informational content of mucins. Here, we develop a cell-based platform for the display and production of human TR O-glycodomains (~200 amino acids) with tunable structures and patterns of O-glycans using membrane-bound and secreted reporters expressed in glycoengineered HEK293 cells. Availability of defined mucin TR O-glycodomains advances experimental studies into the versatile role of mucins at the interface with pathogenic microorganisms and the microbiome, and sparks new strategies for molecular dissection of specific roles of adhesins, glycoside hydrolases, glycopeptidases, viruses and other interactions with mucin TRs as highlighted by examples.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Dual oxidase 1 promotes antiviral innate immunity

Dual oxidase 1 (DUOX1) is an NADPH oxidase that is highly expre-ssed in respiratory epithelial cells and produces H₂O₂ in the airway lumen. While a line of prior in vitro observations suggested that DUOX1 works in partnership with an airway peroxidase, lactoperoxidase (LPO), to produce antimicrobial hypothiocyanite (OSCN^-) in the airways, the in vivo role of DUOX1 in mammalian organisms has remained unproven to date. Here, we show that Duox1 promotes antiviral innate immunity in vivo. Upon influenza airway challenge, Duox1 ^-/- mice have enhanced mortality, morbidity, and impaired lung viral clearance. Duox1 increases the airway levels of several cytokines (IL-1β, IL-2, CCL1, CCL3, CCL11, CCL19, CCL20, CCL27, CXCL5, and CXCL11), contributes to innate immune cell recruitment, and affects epithelial apoptosis in the airways. In primary human tracheobronchial epithelial cells, OSCN^- is generated by LPO using DUOX1-derived H₂O₂ and inactivates several influenza strains in vitro. We also show that OSCN^- diminishes influenza replication and viral RNA synthesis in infected host cells that is inhibited by the H₂O₂ scavenger catalase. Binding of the influenza virus to host cells and viral entry are both reduced by OSCN^- in an H₂O₂-dependent manner in vitro. OSCN^- does not affect the neuraminidase activity or morphology of the influenza virus. Overall, this antiviral function of Duox1 identifies an in vivo role of this gene, defines the steps in the infection cycle targeted by OSCN^-, and proposes that boosting this mechanism in vivo can have therapeutic potential in treating viral infections.

Design of Neuraminidase-Targeted Imaging and Therapeutic Agents for the Diagnosis and Treatment of Influenza Virus Infections

The last step in influenza virus replication involves the assembly of viral components on the infected cell's plasma membrane followed by budding of intact virus from the host cell surface. Because viral neuraminidase and hemagglutinin are both inserted into the host cell's membrane during this process, influenza virus-infected cells are distinguished from uninfected cells by the presence of viral neuraminidase and hemagglutinin on their cell surfaces. In an effort to exploit this difference in cell surface markers for development of diagnostic and therapeutic agents, we have modified an influenza neuraminidase inhibitor, zanamivir, for targeting of attached imaging and therapeutic agents selectively to influenza viruses and virus-infected cells. We have designed here a zanamivir-conjugated rhodamine dye that allows visual monitoring of binding, internalization, and intracellular trafficking of the fluorescence-labeled neuraminidase in virus-infected cells. We also synthesize a zanamivir-^99mTc radioimaging conjugate that permits whole body imaging of the virus's biodistribution and abundance in infected mice. Finally, we create both a zanamivir-targeted cytotoxic drug (i.e., zanamivir-tubulysin B) and a viral neuraminidase-targeted CAR T cell and demonstrate that they are both able to kill viral neuraminidase-expressing cells without damaging healthy cells. Taken together, these data suggest that the influenza virus neuraminidase inhibitor, zanamivir, can be exploited to improve the diagnosis, imaging, and treatment of influenza virus infections.

Gold nanoparticles show potential in vitro antiviral and anticancer activity

AIMS

Gold nanoparticles (AuNPs) have been attracted interests in the various areas of clinical therapeutics. In this study, we investigated the anticancer and antiviral potential activity of AuNPs against influenza A virus and human glioblastoma (GMB) U-87 and U-251 cell lines.

MAIN METHODS

Gold nanoparticles (AuNPs) were synthesized by citrate reduction method. Then, ultraviolet-visible spectrophotometry (UV-vis spectra) and electron microscopy analysis confirmed the type, size (mean diameter of 17 nm) and distribution of the particles. The AuNPs in vitro antiviral and anticancer effects was evaluated by hemagglutination inhibition (HAI), tissue culture infectious dose 50 (TCID₅₀), real-time PCR, MTT, flow cytometry, and scratch assays.

KEY FINDINGS

The AuNPs were synthesized in spherical with a mean diameter of 17 ± 2 nm and an absorbance peak at 520 nm. The AuNPs were well tolerable by MDCK cells at concentrations up to 0.5μg/ml and they significantly inhibited the hemagglutination and virus infectivity, particularly when added pre- or during virus infection. Furthermore, anticancer results indicated that AuNPs treatment caused the marked induction of apoptosis and reduced growth and migration capability of U-87 and U-251 cell lines in a time-dependent manner.

SIGNIFICANCE

The present results suggest that AuNPs provide promising antiviral and anticancer approaches. Further research is needed to fully elucidate the mode of antiviral and anticancer action of AuNPs against influenza virus infection and human glioblastoma cell lines.

Discovery of hydrazide-containing oseltamivir analogues as potent inhibitors of influenza A neuraminidase

Neuraminidase (NA) inhibitors play a prime role in treating influenza. However, a variety of viruses containing mutant NAs have developed severe drug resistance towards NA inhibitors, so it is of crucial significance to solve this problem. Encouraged by urea-containing compound 12 disclosed by our lab, we designed a series of oseltamivir derivatives bearing hydrazide fragment for targeting the 150 cavity. Among the synthesized compounds, compound 17a showed 8.77-fold, 4.12-fold, 203-fold and 6.23-fold more potent activity than oseltamivir carboxylate against NAs from H5N1, H1N1, H5N1-H274Y, H1N1-H274Y, respectively. Meanwhile, the best compound 17a exhibited satisfactory metabolic stability in vitro. This study offers an important reference for the structural optimization of oseltamivir aiming at potent inhibition against H274Y mutant of NAs.

Advancements in macromolecular crystallography: from past to present

Protein Crystallography or Macromolecular Crystallography (MX) started as a new discipline of science with the pioneering work on the determination of the protein crystal structures by John Kendrew in 1958 and Max Perutz in 1960. The incredible achievements in MX are attributed to the development of advanced tools, methodologies, and automation in every aspect of the structure determination process, which have reduced the time required for solving protein structures from years to a few days, as evident from the tens of thousands of crystal structures of macromolecules available in PDB. The advent of brilliant synchrotron sources, fast detectors, and novel sample delivery methods has shifted the paradigm from static structures to understanding the dynamic picture of macromolecules; further propelled by X-ray Free Electron Lasers (XFELs) that explore the femtosecond regime. The revival of the Laue diffraction has also enabled the understanding of macromolecules through time-resolved crystallography. In this review, we present some of the astonishing method-related and technological advancements that have contributed to the progress of MX. Even with the rapid evolution of several methods for structure determination, the developments in MX will keep this technique relevant and it will continue to play a pivotal role in gaining unprecedented atomic-level details as well as revealing the dynamics of biological macromolecules. With many exciting developments awaiting in the upcoming years, MX has the potential to contribute significantly to the growth of modern biology by unraveling the mechanisms of complex biological processes as well as impacting the area of drug designing.

Adverse Effects of Oseltamivir Phosphate Therapy on the Liver of LDLR-/- Mice Without Any Benefit on Atherosclerosis and Thrombosis

ABSTRACT

Desialylation, governed by sialidases or neuraminidases, is strongly implicated in a wide range of human disorders, and accumulative data show that inhibition of neuraminidases, such as neuraminidases 1 sialidase, may be useful for managing atherosclerosis. Several studies have reported promising effects of oseltamivir phosphate, a widely used anti-influenza sialidase inhibitor, on human cancer cells, inflammation, and insulin resistance. In this study, we evaluated the effects of oseltamivir phosphate on atherosclerosis and thrombosis and potential liver toxicity in LDLR-/- mice fed with high-fat diet. Our results showed that oseltamivir phosphate significantly decreased plasma levels of LDL cholesterol and elastin fragmentation in aorta. However, no effect was observed on both atherosclerotic plaque size in aortic roots and chemically induced thrombosis in carotid arteries. Importantly, oseltamivir phosphate administration had adverse effects on the liver of mice and significantly increased messenger RNA expression levels of F4/80, interleukin-1β, transforming growth factor-β1, matrix metalloproteinase-12, and collagen. Taken together, our findings suggest that oseltamivir phosphate has limited benefits on atherosclerosis and carotid thrombosis and may lead to adverse side effects on the liver with increased inflammation and fibrosis.

The clinical impact of glycobiology: targeting selectins, Siglecs and mammalian glycans

Carbohydrates - namely glycans - decorate every cell in the human body and most secreted proteins. Advances in genomics, glycoproteomics and tools from chemical biology have made glycobiology more tractable and understandable. Dysregulated glycosylation plays a major role in disease processes from immune evasion to cognition, sparking research that aims to target glycans for therapeutic benefit. The field is now poised for a boom in drug development. As a harbinger of this activity, glycobiology has already produced several drugs that have improved human health or are currently being translated to the clinic. Focusing on three areas - selectins, Siglecs and glycan-targeted antibodies - this Review aims to tell the stories behind therapies inspired by glycans and to outline how the lessons learned from these approaches are paving the way for future glycobiology-focused therapeutics.

Rapid identification of isoprenylated flavonoids constituents with inhibitory activity on bacterial neuraminidase from root barks of paper mulberry (Broussonetia papyrifera)

This study was to assess the possibility of using competitive and slow binding experiments with affinity-based ultrafiltration UPLC-QTof-MS analysis to identify potent bacterial neuraminidase (bNA) inhibitors from the Broussonetia papyrifera roots extract. To isolate unbound compounds from the enzyme-binding complex, the root bark extracts were either incubated in the absence of bNA, in the presence of bNA, or with the time-dependent bNA before the ultrafiltration was performed. Thirteen flavonoids were separated from the target extract, and their inhibitory activities were tested against bNA. The isolated flavonoids exhibited potent inhibition against NA (IC₅₀ = 0.7-54.0 μM). Our kinetic analysis of representative active flavonoids (1, 2, and 6) showed slow and time-dependent reversible inhibition. Additionally, chalcones exhibited noncompetitive inhibition characteristics, whereas flavonols and flavans showed mixed-type behavior. The computational results supported the experimental behaviors of flavonoids 2, 6, 10, and 12, indicating that bounded to the active site, but flavonoids 6 and 10 binds near but not accurately at the active site. Although this is mixed-type inhibition, their binding can be considered competitive.

A novel aqueous extract from rice fermented with Aspergillus oryzae and Saccharomyces cerevisiae possesses an anti-influenza A virus activity

Human influenza virus infections occur annually worldwide and are associated with high morbidity and mortality. Hence, development of novel anti-influenza drugs is urgently required. Rice Power^® extract developed by the Yushin Brewer Co. Ltd. is a novel aqueous extract of rice obtained via saccharization and fermentation with various microorganisms, such as Aspergillus oryzae, yeast [such as Saccharomyces cerevisiae], and lactic acid bacteria, possessing various biological and pharmacological properties. In our previous experimental screening with thirty types of Rice Power^® extracts, we observed that the 30^th Rice Power^® (Y30) extract promoted the survival of influenza A virus-infected Madin-Darby canine kidney (MDCK) cells. Therefore, to identify compounds for the development of novel anti-influenza drugs, we aimed to investigate whether the Y30 extract exhibits anti-influenza A virus activity. In the present study, we demonstrated that the Y30 extract strongly promoted the survival of influenza A H1N1 Puerto Rico 8/34 (A/PR/8/34), California 7/09, or H3N2 Aichi 2/68 (A/Aichi/2/68) viruses-infected MDCK cells and inhibited A/PR/8/34 or A/Aichi/2/68 viruses infection and growth in the co-treatment and pre-infection experiments. The pre-treatment of Y30 extract on MDCK cells did not induce anti-influenza activity in the cell. The Y30 extract did not significantly affect influenza A virus hemagglutination, and neuraminidase and RNA-dependent RNA polymerase activities. Interestingly, the electron microscopy experiment revealed that the Y30 extract disrupts the integrity of influenza A virus particles by permeabilizing the viral membrane envelope, suggesting that Y30 extract has a direct virucidal effect against influenza A virus. Furthermore, we observed that compared to the ethyl acetate (EtOAc) extract, the water extract of Y30 extract considerably promoted the survival of cells infected with A/PR/8/34 virus. These results indicated that more anti-influenza components were present in the water extract of Y30 extract than in the EtOAc extract. Our results highlight the potential of a rice extract fermented with A. oryzae and S. cerevisiae as an anti-influenza medicine and a drug source for the development of anti-influenza compounds.

Biocatalytic routes to anti-viral agents and their synthetic intermediates

An assessment of biocatalytic strategies for the synthesis of anti-viral agents, offering guidelines for the development of sustainable production methods for a future COVID-19 remedy.

One pot synthesis of thio-glycosides via aziridine opening reactions

thio-Glycosides with a pseudo-disaccharide structure are synthesized via aziridine opening reactions starting from glycosyl thioacetates with a one-pot protocol, which affords glycomimetics equipped for easy and stable conjugation to aglycones.

An overview and future prospects of sialic acids

Sialic acids (Sias) are negatively charged functional monosaccharides present in a wide variety of natural sources (plants, animals and microorganisms). Sias play an important role in many life processes, which are widely applied in the medical and food industries as intestinal antibacterials, antivirals, anti-oxidative agents, food ingredients, and detoxification agents. Most Sias are composed of N-acetylneuraminic acid (Neu5Ac, >99%), and Sia is its most commonly used name. In this article, we review Sias in terms of their structures, applications, determination methods, metabolism, and production strategies. In particular, we summarise and compare different production strategies, including extraction from natural sources, chemical synthesis, polymer decomposition, enzymatic synthesis, whole-cell catalysis, and de novo biosynthesis via microorganism fermentation. We also discuss research on their physiological functions and applications, barriers to efficient production, and strategies for overcoming these challenges. We focus on efficient de novo biosynthesis strategies for Neu5Ac via microbial fermentation using novel synthetic biology tools and methods that may be applied in future. This work provides a comprehensive overview of recent advances on Sias, and addresses future challenges regarding their functions, applications, and production.

SARS-CoV-2 Infection Depends on Cellular Heparan Sulfate and ACE2

We show that SARS-CoV-2 spike protein interacts with both cellular heparan sulfate and angiotensin-converting enzyme 2 (ACE2) through its receptor-binding domain (RBD). Docking studies suggest a heparin/heparan sulfate-binding site adjacent to the ACE2-binding site. Both ACE2 and heparin can bind independently to spike protein in vitro, and a ternary complex can be generated using heparin as a scaffold. Electron micrographs of spike protein suggests that heparin enhances the open conformation of the RBD that binds ACE2. On cells, spike protein binding depends on both heparan sulfate and ACE2. Unfractionated heparin, non-anticoagulant heparin, heparin lyases, and lung heparan sulfate potently block spike protein binding and/or infection by pseudotyped virus and authentic SARS-CoV-2 virus. We suggest a model in which viral attachment and infection involves heparan sulfate-dependent enhancement of binding to ACE2. Manipulation of heparan sulfate or inhibition of viral adhesion by exogenous heparin presents new therapeutic opportunities.

A universal dual mechanism immunotherapy for the treatment of influenza virus infections

Seasonal influenza epidemics lead to 3–5 million severe infections and 290,000–650,000 annual global deaths. With deaths from the 1918 influenza pandemic estimated at >50,000,000 and future pandemics anticipated, the need for a potent influenza treatment is critical. In this study, we design and synthesize a bifunctional small molecule by conjugating the neuraminidase inhibitor, zanamivir, with the highly immunogenic hapten, dinitrophenyl (DNP), which specifically targets the surface of free virus and viral-infected cells. We show that this leads to simultaneous inhibition of virus release, and immune-mediated elimination of both free virus and virus-infected cells. Intranasal or intraperitoneal administration of a single dose of drug to mice infected with 100x MLD₅₀ virus is shown to eradicate advanced infections from representative strains of both influenza A and B viruses. Since treatments of severe infections remain effective up to three days post lethal inoculation, our approach may successfully treat infections refractory to current therapies.

In this study, the authors combine an anti-viral drug and immune system inducer to treat influenza A and B viral infections in vitro and in vivo. They show that the compound outperforms zanamivir alone as it is still able to clear infection three days post infection, and it can be administered via different routes without reduced efficacy.

Inhibiting Sialidase-Induced TGF-β1 Activation Attenuates Pulmonary Fibrosis in Mice

The active form of transforming growth factor-β1 (TGF-β1) plays a key role in potentiating fibrosis. TGF-β1 is sequestered in an inactive state by a latency-associated glycopeptide (LAP). Sialidases (also called neuraminidases (NEU)) cleave terminal sialic acids from glycoconjugates. The sialidase NEU3 is upregulated in fibrosis, and mice lacking Neu3 show attenuated bleomycin-induced increases in active TGF-β1 in the lungs and attenuated pulmonary fibrosis. Here we observe that recombinant human NEU3 upregulates active human TGF-β1 by releasing active TGF-β1 from its latent inactive form by desialylating LAP. Based on the proposed mechanism of action of NEU3, we hypothesized that compounds with a ring structure resembling picolinic acid might be transition state analogs and thus possible NEU3 inhibitors. Some compounds in this class showed nanomolar IC₅₀ for recombinant human NEU3 releasing active human TGF-β1 from the latent inactive form. The compounds given as daily 0.1-1-mg/kg injections starting at day 10 strongly attenuated lung inflammation, lung TGF-β1 upregulation, and pulmonary fibrosis at day 21 in a mouse bleomycin model of pulmonary fibrosis. These results suggest that NEU3 participates in fibrosis by desialylating LAP and releasing TGF-β1 and that the new class of NEU3 inhibitors are potential therapeutics for fibrosis. SIGNIFICANCE STATEMENT: The extracellular sialidase NEU3 appears to be a key driver of pulmonary fibrosis. The significance of this report is that 1) we show the mechanism (NEU3 desialylates the latency-associated glycopeptide protein that keeps the profibrotic cytokine transforming growth factor-β1 (TGF-β1) in an inactive state, causing active TGF-β1 release), 2) we then use the predicted NEU3 mechanism to identify nM IC₅₀ NEU3 inhibitors, and 3) these new NEU3 inhibitors are potent therapeutics in a mouse model of pulmonary fibrosis.

Exploring Amino Sugar and Phosphoenolpyruvate Metabolism to Improve Escherichia coli N-Acetylneuraminic Acid Production

N-acetyl-d-neuraminic acid (NeuAc) has attracted considerable attention because of its wide-ranging applications. The use of cheap carbon sources such as glucose without the addition of any precursor in microbial NeuAc production has many advantages. In this study, improved NeuAc production was attained through the optimization of amino sugar metabolism pathway kinetics and reservation of a phosphoenolpyruvate (PEP) pool in Escherichia coli. N-acylglucosamine 2-epimerase and N-acetylneuraminate synthase from different sources and their best combinations were used to obtain optimized enzyme kinetics and expression intensity, which resulted in a significant increase in NeuAc production. Next, after a design was engineered for enabling the PEP metabolic pathway to retain the PEP pool, the production of NeuAc reached 16.7 g/L, which is the highest NeuAc production rate that has been reported from using glucose as the sole carbon source.

Searching for effective antiviral small molecules against influenza A virus: A patent review

Introduction: Despite the current interest caused by SARS-Cov-2, influenza continues to be one of the most serious health concerns, with an estimated 1 billion cases across the globe, including 3-5 million severe cases and 290,000-650,000 deaths worldwide. Areas covered: This manuscript reviews the efforts made in the development of small molecules for the treatment of influenza virus, primarily focused on patent applications in the last 5 years. Attention is paid to compounds targeting key functional viral proteins, such as the M2 channel, neuraminidase, and hemagglutinin, highlighting the evolution toward new ligands and scaffolds motivated by the emergence of resistant strains. Finally, the discovery of compounds against novel viral targets, such as the RNA-dependent RNA polymerase, is discussed. Expert opinion: The therapeutic potential of antiviral agents is limited by the increasing presence of resistant strains. This should encourage research on novel strategies for therapeutic intervention. In this context, the discovery of arbidol and JNJ7918 against hemagglutinin, and current efforts on RNA-dependent RNA polymerase have disclosed novel opportunities for therapeutic treatment. Studies should attempt to expand the therapeutic arsenal of anti-flu agents, often in combined therapies, to prevent future health challenges caused by influenza virus. Abbreviations: AlphaLISA: amplified luminescent proximity homogeneous assay; HA: hemagglutinin; NA: neuraminidase; RBD: receptor binding domain; RdRp: RNA-dependent RNA polymerase; SA: sialic Acid; TBHQ: tert-butyl hydroquinone; TEVC: two-electrode voltage clamp.

Unraveling the anti-influenza effect of flavonoids: Experimental validation of luteolin and its congeners as potent influenza endonuclease inhibitors

The biological effects of flavonoids on mammal cells are diverse, ranging from scavenging free radicals and anti-cancer activity to anti-influenza activity. Despite appreciable effort to understand the anti-influenza activity of flavonoids, there is no clear consensus about their precise mode-of-action at a cellular level. Here, we report the development and validation of a screening assay based on AlphaScreen technology and illustrate its application for determination of the inhibitory potency of a large set of polyols against PA N-terminal domain (PA-Nter) of influenza RNA-dependent RNA polymerase featuring endonuclease activity. The most potent inhibitors we identified were luteolin with an IC₅₀ of 72 ± 2 nM and its 8-C-glucoside orientin with an IC₅₀ of 43 ± 2 nM. Submicromolar inhibitors were also evaluated by an in vitro endonuclease activity assay using single-stranded DNA, and the results were in full agreement with data from the competitive AlphaScreen assay. Using X-ray crystallography, we analyzed structures of the PA-Nter in complex with luteolin at 2.0 Å resolution and quambalarine B at 2.5 Å resolution, which clearly revealed the binding pose of these polyols coordinated to two manganese ions in the endonuclease active site. Using two distinct assays along with the structural work, we have presumably identified and characterized the molecular mode-of-action of flavonoids in influenza-infected cells.

Profile and generation of reduced neuraminidase inhibitor susceptibility in highly pathogenic avian influenza H7N9 virus from human cases in Mainland of China, 2016-2019

Human infections with highly pathogenic avian influenza (HPAI) H7N9 virus were detected in late 2016. We examined the drug resistance profile of 30 HPAI H7N9 isolates from Mainland of China (2016-2019). Altogether, 23% (7/30) carried neuraminidase inhibitors (NAIs) - resistance mutations, and 13% (4/30) displayed reduced susceptibility to NAIs in neuraminidase (NA) inhibition test. An HPAI H7N9 reassortment virus we prepared was passaged with NAIs for 10 passages. Passage with zanamivir induced an E119G substitution in NA, whereas passage with oseltamivir induced R292K and E119V substitutions that simulated that seen in oseltamivir -treated HPAI H7N9 cases, indicating that the high frequency of resistant strains in the HPAI H7N9 isolates is related to NAIs use. In presence of NAIs, R238I, A146E, G151E and G234T substitutions were found in HA1 region of HA. No amino acid mutations were found in the internal genes of the recombinant virus.

Treating a Global Health Crisis with a Dose of Synthetic Chemistry

The SARS-CoV-2 pandemic has prompted scientists from many disciplines to work collaboratively toward an effective response. As academic synthetic chemists, we examine how best to contribute to this ongoing effort.

SARS-CoV-2 Infection Depends on Cellular Heparan Sulfate and ACE2

We show that SARS-CoV-2 spike protein interacts with cell surface heparan sulfate and angiotensin converting enzyme 2 (ACE2) through its Receptor Binding Domain. Docking studies suggest a putative heparin/heparan sulfate-binding site adjacent to the domain that binds to ACE2. In vitro, binding of ACE2 and heparin to spike protein ectodomains occurs independently and a ternary complex can be generated using heparin as a template. Contrary to studies with purified components, spike protein binding to heparan sulfate and ACE2 on cells occurs codependently. Unfractionated heparin, non-anticoagulant heparin, treatment with heparin lyases, and purified lung heparan sulfate potently block spike protein binding and infection by spike protein-pseudotyped virus and SARS-CoV-2 virus. These findings support a model for SARS-CoV-2 infection in which viral attachment and infection involves formation of a complex between heparan sulfate and ACE2. Manipulation of heparan sulfate or inhibition of viral adhesion by exogenous heparin may represent new therapeutic opportunities.

Reprogramming of the Antibacterial Drug Vancomycin Results in Potent Antiviral Agents Devoid of Antibacterial Activity

Influenza A and B viruses are a global threat to human health and increasing resistance to the existing antiviral drugs necessitates new concepts to expand the therapeutic options. Glycopeptide derivatives have emerged as a promising new class of antiviral agents. To avoid potential antibiotic resistance, these antiviral glycopeptides are preferably devoid of antibiotic activity. We prepared six vancomycin aglycone hexapeptide derivatives with the aim of obtaining compounds having anti-influenza virus but no antibacterial activity. Two of them exerted strong and selective inhibition of influenza A and B virus replication, while antibacterial activity was successfully eliminated by removing the critical N-terminal moiety. In addition, these two molecules offered protection against several other viruses, such as herpes simplex virus, yellow fever virus, Zika virus, and human coronavirus, classifying these glycopeptides as broad antiviral molecules with a favorable therapeutic index.

Modeling mitigation of influenza epidemics by baloxavir

Influenza viruses annually kill 290,000-650,000 people worldwide. Antivirals can reduce death tolls. Baloxavir, the recently approved influenza antiviral, inhibits initiation of viral mRNA synthesis, whereas oseltamivir, an older drug, inhibits release of virus progeny. Baloxavir blocks virus replication more rapidly and completely than oseltamivir, reducing the duration of infectiousness. Hence, early baloxavir treatment may indirectly prevent transmission. Here, we estimate impacts of ramping up and accelerating baloxavir treatment on population-level incidence using a new model that links viral load dynamics from clinical trial data to between-host transmission. We estimate that ~22 million infections and >6,000 deaths would have been averted in the 2017-2018 epidemic season by administering baloxavir to 30% of infected cases within 48 h after symptom onset. Treatment within 24 h would almost double the impact. Consequently, scaling up early baloxavir treatment would substantially reduce influenza morbidity and mortality every year. The development of antivirals against the SARS-CoV2 virus that function like baloxavir might similarly curtail transmission and save lives.

Utilising animal models to evaluate oseltamivir efficacy against influenza A and B viruses with reduced in vitro susceptibility

The neuraminidase (NA) inhibitor (NAI) oseltamivir (OST) is the most widely used influenza antiviral drug. Several NA amino acid substitutions are reported to reduce viral susceptibility to OST in in vitro assays. However, whether there is a correlation between the level of reduction in susceptibility in vitro and the efficacy of OST against these viruses in vivo is not well understood. In this study, a ferret model was utilised to evaluate OST efficacy against circulating influenza A and B viruses with a range of in vitro generated 50% inhibitory concentrations (IC50) values for OST. OST efficacy against an A(H1N1)pdm09 and an A(H1N1)pdm09 virus with the H275Y substitution in neuraminidase was also tested in the macaque model. The results from this study showed that OST had a significant impact on virological parameters compared to placebo treatment of ferrets infected with wild-type influenza A viruses with normal IC50 values (~1 nM). However, this efficacy was lower against wild-type influenza B and other viruses with higher IC50 values. Differing pathogenicity of the viruses made evaluation of clinical parameters difficult, although some effect of OST in reducing clinical signs was observed with influenza A(H1N1) and A(H1N1)pdm09 (H275Y) viruses. Viral titres in macaques were too low to draw conclusive results. Analysis of the ferret data revealed a correlation between IC50 and OST efficacy in reducing viral shedding but highlighted that the current WHO guidelines/criteria for defining normal, reduced or highly reduced inhibition in influenza B viruses based on in vitro data are not well aligned with the low in vivo OST efficacy observed for both wild-type influenza B viruses and those with reduced OST susceptibility.

Avian Influenza Virus Detection by Optimized Peptide Termination on a Boron-Doped Diamond Electrode

The development of a simple detection method with high sensitivity is essential for the diagnosis and surveillance of infectious diseases. Previously, we constructed a sensitive biosensor for the detection of pathological human influenza viruses using a boron-doped diamond electrode terminated with a sialyloligosaccharide receptor-mimic peptide that could bind to hemagglutinins involved in viral infection. Circulation of influenza induced by the avian virus in humans has become a major public health concern, and methods for the detection of avian viruses are urgently needed. Here, peptide density and dendrimer generation terminated on the electrode altered the efficiency of viral binding to the electrode surface, thus significantly enhancing charge-transfer resistance measured by electrochemical impedance spectroscopy. The peptide-terminated electrodes exhibited an excellent detection limit of less than one plaque-forming unit of seasonal H1N1 and H3N2 viruses. Furthermore, the improved electrode was detectable for avian viruses isolated from H5N3, H7N1, and H9N2, showing the potential for the detection of all subtypes of influenza A virus, including new subtypes. The peptide-based electrochemical architecture provided a promising approach to biosensors for ultrasensitive detection of pathogenic microorganisms.

Mesoscale All-Atom Influenza Virus Simulations Suggest New Substrate Binding Mechanism

Influenza virus circulates in human, avian, and swine hosts, causing seasonal epidemic and occasional pandemic outbreaks. Influenza neuraminidase, a viral surface glycoprotein, has two sialic acid binding sites. The catalytic (primary) site, which also binds inhibitors such as oseltamivir carboxylate, is responsible for cleaving the sialic acid linkages that bind viral progeny to the host cell. In contrast, the functional annotation of the secondary site remains unclear. Here, we better characterize these two sites through the development of an all-atom, explicitly solvated, and experimentally based integrative model of the pandemic influenza A H1N1 2009 viral envelope, containing ∼160 million atoms and spanning ∼115 nm in diameter. Molecular dynamics simulations of this crowded subcellular environment, coupled with Markov state model theory, provide a novel framework for studying realistic molecular systems at the mesoscale and allow us to quantify the kinetics of the neuraminidase 150-loop transition between the open and closed states. An analysis of chloride ion occupancy along the neuraminidase surface implies a potential new role for the neuraminidase secondary site, wherein the terminal sialic acid residues of the linkages may bind before transfer to the primary site where enzymatic cleavage occurs. Altogether, our work breaks new ground for molecular simulation in terms of size, complexity, and methodological analyses of the components. It also provides fundamental insights into the understanding of substrate recognition processes for this vital influenza drug target, suggesting a new strategy for the development of anti-influenza therapeutics.