Synthesis of Sulfo-Sialic Acid Analogues: Potent Neuraminidase Inhibitors in Regards to Anomeric Functionality

Antiviral Compounds to Address Influenza Pandemics: An Update from 2016-2022

In recent decades, the world has gained experience of the dangerous effects of pandemic events caused by emerging respiratory viruses. In particular, annual epidemics of influenza are responsible for severe illness and deaths. Even if conventional influenza vaccines represent the most effective tool for preventing virus infections, they are not completely effective in patients with severe chronic disease and immunocompromised and new small molecules have emerged to prevent and control the influenza viruses. Thus, the attention of chemists is continuously focused on the synthesis of new antiviral drugs able to interact with the different molecular targets involved in the virus replication cycle. To date, different classes of influenza viruses inhibitors able to target neuraminidase enzyme, hemagglutinin protein, Matrix-2 (M2) protein ion channel, nucleoprotein or RNAdependent RNA polymerase have been synthesized using several synthetic strategies comprising the chemical modification of currently used drugs. The best results, in terms of inhibitory activity, are in the nanomolar range and have been obtained from the chemical modification of clinically used drugs such as Peramivir, Zanamivir, Oseltamir, Rimantadine, as well as sialylated molecules, and hydroxypyridinone derivatives. The aim of this review is to report, covering the period 2016-2022, the most recent routes related to the synthesis of effective influenza virus inhibitors.

Conformationally locked sugar derivatives and analogues as potential neuraminidase inhibitors

The influenza virus remains a major health concern for mankind because it tends to mutate frequently and cause high morbidity. Influenza prevention and treatment are greatly aided by the use of antivirals. One such class of antivirals is neuraminidase inhibitors (NAIs), effective against influenza viruses. A neuraminidase on the virus's surface serves a vital function in viral propogation by assisting in the release of viruses from infected host cells. Neuraminidase inhibitors are the backbone in stoping such virus propagation thus helps in the treatment of influenza viruses infections. Two NAI medicines are licensed globally: Oseltamivir (Tamiflu™) and Zanamivir (Relanza™). There are two molecules that have acquired Japanese approval recently: Peramivir and Laninamivir, whereas Laninamivir octanoate is in Phase III clinical trials. The need for novel NAIs is due to frequent mutations in viruses and the rise in resistance against existing medication. The NA inhibitors (NAIs) are designed to have (oxa)cyclohexene scaffolds (a sugar scaffold) to mimic the oxonium transition state in the enzymatic cleavage of sialic acid. This review discusses in details and comprises all such conformationally locked (oxa)cyclohexene scaffolds and their analogues which have been recently designed and synthesized as potential neuraminidase inhibitors, thus as antiviral molecules. The structure-activity relationship of such diverese molecules has also been discussed in this review.

Growing impact of sialic acid-containing glycans in future drug discovery

In nature, almost all cells are covered with a complex array of glycan chain namely sialic acids or nuraminic acids, a negatively charged nine carbon sugars which is considered for their great therapeutic importance since long back. Owing to its presence at the terminal end of lipid bilayer (commonly known as terminal sugars), the well-defined sialosides or sialoconjugates have served pivotal role on the cell surfaces and thus, the sialic acid-containing glycans can modulate and mediate a number of imperative cellular interactions. Understanding of the sialo-protein interaction and their roles in vertebrates in regard of normal physiology, pathological variance, and evolution has indeed a noteworthy journey in medicine. In this tutorial review, we present a concise overview about the structure, linkages in chemical diversity, biological significance followed by chemical and enzymatic modification/synthesis of sialic acid containing glycans. A more focus is attempted about the recent advances, opportunity, and more over growing impact of sialosides and sialoconjugates in future drug discovery and development.

Sialidase Inhibitors with Different Mechanisms

Sialidases, or neuraminidases, are enzymes that catalyze the hydrolysis of sialic acid (Sia)-containing molecules, mostly removal of the terminal Sia (desialylation). By desialylation, sialidase can modulate the functionality of the target compound and is thus often involved in biological pathways. Inhibition of sialidases with inhibitors is an important approach for understanding sialidase function and the underlying mechanisms and could serve as a therapeutic approach as well. Transition-state analogues, such as anti-influenza drugs oseltamivir and zanamivir, are major sialidase inhibitors. In addition, difluoro-sialic acids were developed as mechanism-based sialidase inhibitors. Further, fluorinated quinone methide-based suicide substrates were reported. Sialidase product analogue inhibitors were also explored. Finally, natural products have shown competitive inhibiton against viral, bacterial, and human sialidases. This Perspective describes sialidase inhibitors with different mechanisms and their activities and future potential, which include transition-state analogue inhibitors, mechanism-based inhibitors, suicide substrate inhibitors, product analogue inhibitors, and natural product inhibitors.

Synthesis and Neuraminidase Inhibitory Activity of Sialic Acid Analogues with Fluoro, Phosphono, and Sulfo Functionalities

Methods to synthesize influenza virus inhibitors with fluoro, phosphono, and/or sulfo functional groups are described. The resulting sialic acid analogues are produced from the natural substrate N-acetylneuraminic acid as starting material. Fluorescent assay methods for inhibition of influenza neuraminidase and virus proliferation are also provided.

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.

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.

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.

Development of effective anti-influenza drugs: congeners and conjugates - a review

Influenza is a long-standing health problem. For treatment of seasonal flu and possible pandemic infections, there is a need to develop new anti-influenza drugs that have good bioavailability against a broad spectrum of influenza viruses, including the resistant strains. Relenza™ (zanamivir), Tamiflu™ (the phosphate salt of oseltamivir), Inavir™ (laninamivir octanoate) and Rapivab™ (peramivir) are four anti-influenza drugs targeting the viral neuraminidases (NAs). However, some problems of these drugs should be resolved, such as oral availability, drug resistance and the induced cytokine storm. Two possible strategies have been applied to tackle these problems by devising congeners and conjugates. In this review, congeners are the related compounds having comparable chemical structures and biological functions, whereas conjugate refers to a compound having two bioactive entities joined by a covalent bond. The rational design of NA inhibitors is based on the mechanism of the enzymatic hydrolysis of the sialic acid (Neu5Ac)-terminated glycoprotein. To improve binding affinity and lipophilicity of the existing NA inhibitors, several methods are utilized, including conversion of carboxylic acid to ester prodrug, conversion of guanidine to acylguanidine, substitution of carboxylic acid with bioisostere, and modification of glycerol side chain. Alternatively, conjugating NA inhibitors with other therapeutic entity provides a synergistic anti-influenza activity; for example, to kill the existing viruses and suppress the cytokines caused by cross-species infection.

9-Azido-9-deoxy-2,3-difluorosialic Acid as a Subnanomolar Inhibitor against Bacterial Sialidases

A library of 2(a),3(a/e)-difluorosialic acids and their C-5 and/or C-9 derivatives were chemoenzymatically synthesized. Pasteurella multocida sialic acid aldolase (PmAldolase), but not its Escherichia coli homologue (EcAldolase), was found to catalyze the formation of C5-azido analogue of 3-fluoro(a)-sialic acid. In comparison, both PmAldolase and EcAldolase could catalyze the synthesis of 3-fluoro(a/e)-sialic acids and their C-9 analogues although PmAldolase was generally more efficient. The chemoenzymatically synthesized 3-fluoro(a/e)-sialic acid analogues were purified and chemically derivatized to form the desired difluorosialic acids and derivatives. Inhibition studies against several bacterial sialidases and a recombinant human cytosolic sialidase hNEU2 indicated that sialidase inhibition was affected by the C-3 fluorine stereochemistry and derivatization at C-5 and/or C-9 of the inhibitor. Opposite to that observed for influenza A virus sialidases and hNEU2, compounds with axial fluorine at C-3 were better inhibitors (up to 100-fold) against bacterial sialidases compared to their 3F-equatorial counterparts. While C-5-modified compounds were less-efficient antibacterial sialidase inhibitors, 9-N₃-modified 2,3-difluoro-Neu5Ac showed increased inhibitory activity against bacterial sialidases. 9-Azido-9-deoxy-2-(e)-3-(a)-difluoro- N-acetylneuraminic acid [2(e)3(a)DFNeu5Ac9N₃] was identified as an effective inhibitor with a long effective duration selectively against pathogenic bacterial sialidases from Clostridium perfringens (CpNanI) and Vibrio cholerae.

Boronate, trifluoroborate, sulfone, sulfinate and sulfonate congeners of oseltamivir carboxylic acid: Synthesis and anti-influenza activity

Tamiflu readily undergoes endogenous hydrolysis to give oseltamivir carboxylic acid (OC) as the active anti-influenza agent to inhibit the viral neuraminidase (NA). GOC is derived from OC by replacing the 5-amino group with a guanidino group. In this study, OC and GOC congeners with the carboxylic acid bioisosteres of boronic acid, trifluoroborate, sulfone, sulfinic acid, sulfonic acid and sulfonate ester were first synthesized, starting with conversion of OC to a Barton ester, followed by halodecarboxylation to give the iodocyclohexene, which served as a pivotal intermediate for palladium-catalyzed coupling reactions with appropriate diboron and thiol reagents. The enzymatic and cell-based assays indicated that the GOC congeners consistently displayed better NA inhibition and anti-influenza activity than the corresponding OC congeners. The GOC sulfonic acid congener (7a) was the most potent anti-influenza agent, showing EC₅₀ = 2.2 nM against the wild-type H1N1 virus, presumably because the sulfonic acid 7a was more lipophilic than GOC and exerted stronger interactions on the three arginine residues (R118, R292 and R371) in the NA active site. Although the trifluoroborates, sulfones and sulfonate esters did not have acidic proton, they still exhibited appreciable NA inhibitory activity, indicating that the polarized B-F and S→O bonds still made sufficient interactions with the tri-arginine motif.

Inhibition of sialidase activity as a therapeutic approach

The demand for novel anti-influenza drugs persists, which is highlighted by the recent pandemics of influenza affecting thousands of people across the globe. One of the approaches to block the virus spreading is inhibiting viral sialidase (neuraminidase). This enzyme cleaves the sialic acid link between the newly formed virions and the host cell surface liberating the virions from the cell and maintaining the cycle of infection. Viral neuraminidases appear therefore as attractive therapeutic targets for preventing further spread of influenza infection. Compared to ion channel blockers that were the first approved anti-influenza drugs, neuraminidase inhibitors are well tolerated and target both influenza A and B viruses. Moreover, neuraminidase/sialidase inhibitors may be useful for managing some other human pathologies, such as cancer. In this review, we discuss the available knowledge on neuraminidase or sialidase inhibitors, their design, clinical application, and the current challenges.