Influenza virus sialidase: a target for drug discovery

Binding mechanism of oseltamivir and influenza neuraminidase suggests perspectives for the design of new anti-influenza drugs

Oseltamivir is a widely used influenza virus neuraminidase (NA) inhibitor that prevents the release of new virus particles from host cells. However, oseltamivir-resistant strains have emerged, but effective drugs against them have not yet been developed. Elucidating the binding mechanisms between NA and oseltamivir may provide valuable information for the design of new drugs against NA mutants resistant to oseltamivir. Here, we conducted large-scale (353.4 μs) free-binding molecular dynamics simulations, together with a Markov State Model and an importance-sampling algorithm, to reveal the binding process of oseltamivir and NA. Ten metastable states and five major binding pathways were identified that validated and complemented previously discovered binding pathways, including the hypothesis that oseltamivir can be transferred from the secondary sialic acid binding site to the catalytic site. The discovery of multiple new metastable states, especially the stable bound state containing a water-mediated hydrogen bond between Arg118 and oseltamivir, may provide new insights into the improvement of NA inhibitors. We anticipated the findings presented here will facilitate the development of drugs capable of combating NA mutations.

Influenza virus neuraminidase (NA), a viral membrane glycoprotein, plays an important role in the interactions with host cell surface receptors. The emergence and spread of influenza mutants resistant to neuraminidase inhibitors (NAIs), such as oseltamivir, has been of great concern. Despite many improvements to NAIs, no new first-line NAIs are currently in clinical use. Although there have been previous molecular dynamics simulation studies on the binding and dissociation process of oseltamivir-NA, we discovered new binding pathways and states of oseltamivir through larger-scale simulations and more systematic analysis, which may provide new ideas for the improvement of oseltamivir and even a series of NAIs. In our study, we strongly demonstrate that a detailed understanding of the drug−receptor association process is of fundamental importance for drug design and provide methodological references for the improvement of other drugs.

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.

Improvements to the Synthesis of Isofagomine, Noeuromycin, Azafagomine, and Isofagomine Lactam, and a Synthesis of Azanoeuromycin and 'Guanidine' Isofagomine

Improvements in the preparation of a key imidazylate and the reduction of the derived nitrile have led to more efficient syntheses of isofagomine, noeuromycin, azafagomine, and isofagomine lactam. As well, a precursor of azafagomine has been converted into azanoeuromycin, and the nitrogen atom of isofagomine has been incorporated into a guanidine residue.

Zanamivir: an update of its use in influenza

Zanamivir is a potent competitive inhibitor of the neuraminidase glycoprotein, which is essential in the infective cycle of influenza A and B viruses. Zanamivir (10mg by inhalation via the Diskhaler twice daily, or 10mg inhaled plus 6.4mg intranasally two or four times daily, for 5 days) reduced the median time to alleviation of major influenza symptoms by up to 2.5 days compared with placebo. Significant reductions of 1 to 2.5 days versus placebo were observed with inhaled zanamivir in phase III trials involving otherwise healthy adults, high-risk patients or children aged 5 to 12 years. Accelerated return to normal activities, and reduced interference with sleep, consumption of relief medication and incidence of complications leading to antibacterial use were also observed with zanamivir. When used for prophylaxis, inhaled zanamivir 10 to 20 mg/day for 10 days to 4 weeks (plus 6.4 mg/day intranasally in one trial) prevented influenza A in 67% of recipients in a university community, significantly reduced the number of families with new cases of influenza compared with placebo or prevented new cases of influenza in long-term care facilities. The tolerability of inhaled or intranasal zanamivir was similar to that of placebo in otherwise healthy adults, high-risk and elderly patients, and children. Recommended dosages of zanamivir did not adversely affect pulmonary function in patients with respiratory disorders in a well-controlled trial, although there have been rare reports of bronchospasm and/or a decline in respiratory function.

CONCLUSION

Zanamivir (used within 48 hours of symptom development) reduces the duration of symptomatic illness, causes accelerated return to normal activities or reduces complications requiring antibacterial use in adults, high-risk individuals and children with influenza. Vaccination remains the intervention of choice for prophylaxis in selected populations. However, the efficacy, good tolerability profile and lack of resistance with zanamivir make it a useful option, particularly in those not covered or inadequately protected by vaccination, who are able to use the inhalation device. The use of zanamivir in patients with respiratory disorders remains unclear because of concerns regarding its potential for bronchospasm. Prospective cost-effectiveness analyses and investigations of efficacy in preventing serious complications of influenza, particularly in high-risk patients, are required. Zanamivir shows potential for prophylaxis in persons for whom vaccination is contraindicated or ineffective, in elderly or high-risk patients in long-term care facilities and in households.

Syncytium formation and HIV-1 replication are both accentuated by purified influenza and virus-associated neuraminidase

The degree of sialylation has been shown previously to modulate the process of human immunodeficiency virus type-1 (HIV-1) infection by affecting the interaction between the virus and CD4-expressing target cells. In the present study, we investigated whether HIV-1 replication cycle was affected by neuraminidase (NA) derived from the human influenza (flu) virus. We first demonstrate that the level of HIV-1-mediated syncytium formation was greatly enhanced in the presence of purified flu NA. Pretreatment of established monocytic and lymphocytic cell lines as well as primary mononuclear cells with purified flu NA augmented also the process of virus infection. A comparable up-regulating effect was observed when using several strains of UV-inactivated whole flu virus, thereby suggesting that virus-anchored NA enzymes positively modulate the HIV-1 life cycle. Furthermore, flu NA-mediated positive effect on HIV-1 biology was abrogated with zanamivir, a specific flu NA inhibitor. Our results provide a new model allowing the investigation of the potential benefit of using NA inhibitors in the treatment of HIV-1-infected patients suffering from coinfection with NA-bearing pathogens.

Chemical and biological strategies for engineering cell surface glycosylation

Oligosaccharides play a crucial role in many of the recognition, signaling, and adhesion events that take place at the surface of cells. Abnormalities in the synthesis or presentation of these carbohydrates can lead to misfolded and inactive proteins, as well as to several debilitating disease states. However, their diverse structures, which are the key to their function, have hampered studies by biologists and chemists alike. This review presents an overview of techniques for examining and manipulating cell surface oligosaccharides through genetic, enzymatic, and chemical strategies.

Synthesis of delta4-beta-D-glucopyranosiduronic acids as mimetics of 2,3-unsaturated sialic acids for sialidase inhibition

Mimetics of Neu5Ac2en and KDN2en, based on delta4-beta-delta-glucopyranosiduronic acids, have been synthesised. The Neu5Ac2en mimetic 5 showed inhibition of both bacterial and viral sialidases, with inhibition of the viral sialidase being comparable to that of Neu5Ac2en itself.