Approaches and strategies for the treatment of influenza virus infections

Influenza Viruses: Harnessing the Crucial Role of the M2 Ion-Channel and Neuraminidase toward Inhibitor Design

As a member of the Orthomyxoviridae family of viruses, influenza viruses (IVs) are known causative agents of respiratory infection in vertebrates. They remain a major global threat responsible for the most virulent diseases and global pandemics in humans. The virulence of IVs and the consequential high morbidity and mortality of IV infections are primarily attributed to the high mutation rates in the IVs' genome coupled with the numerous genomic segments, which give rise to antiviral resistant and vaccine evading strains. Current therapeutic options include vaccines and small molecule inhibitors, which therapeutically target various catalytic processes in IVs. However, the periodic emergence of new IV strains necessitates the continuous development of novel anti-influenza therapeutic options. The crux of this review highlights the recent studies on the biology of influenza viruses, focusing on the structure, function, and mechanism of action of the M2 channel and neuraminidase as therapeutic targets. We further provide an update on the development of new M2 channel and neuraminidase inhibitors as an alternative to existing anti-influenza therapy. We conclude by highlighting therapeutic strategies that could be explored further towards the design of novel anti-influenza inhibitors with the ability to inhibit resistant strains.

Quantification of Influenza Neuraminidase Activity by Ultra-High Performance Liquid Chromatography and Isotope Dilution Mass Spectrometry

Mounting evidence suggests that neuraminidase's functionality extends beyond its classical role in influenza virus infection and that antineuraminidase antibodies offer protective immunity. Therefore, a renewed interest in the development of neuraminidase (NA)-specific methods to characterize the glycoprotein and evaluate potential advantages for NA standardization in influenza vaccines has emerged. NA displays sialidase activity by cleaving off the terminal N-acetylneuraminic acid on α-2,3 or α-2,6 sialic acid containing receptors of host cells. The type and distribution of these sialic acid containing receptors is considered to be an important factor in transmission efficiency of influenza viruses between and among host species. Changes in hemagglutinin (HA) binding and NA specificity in reassortant viruses may be related to the emergence of new and potentially dangerous strains of influenza. Current methods to investigate neuraminidase activity use small derivatized sugars that are poor models for natural glycoprotein receptors and do not provide information on the linkage specificity. Here, a novel approach for rapid and accurate quantification of influenza neuraminidase activity is achieved utilizing ultra-high performance liquid chromatography (UPLC) and isotope dilution mass spectrometry (IDMS). Direct LC-MS/MS quantification of NA-released sialic acid provides precise measurement of influenza neuraminidase activity over a range of substrates. The method provides exceptional sensitivity and specificity with a limit of detection of 0.38 μM for sialic acid and the capacity to obtain accurate measurements of specific enzyme activity preference toward α-2,3-sialyllactose linkages, α-2,6-sialyllactose linkages, or whole glycosylated proteins such as fetuin.

Anti-influenza virus agents: synthesis and mode of action

Annual epidemics of influenza virus infection are responsible for considerable morbidity and mortality, and pandemics are much more devastating. Considerable knowledge of viral infectivity and replication has been acquired, but many details still have to be elucidated and the virus remains a challenging target for drug design and development. This review provides an overview of the antiviral drugs targeting the influenza viral replicative cycle. Included are a brief description of their chemical syntheses and biological activities. For other reviews, see References1-9.

[Bilateral influenza-triggered panuveitis and subsequent therapy with amantadine and hyperimmunoglobulins]

BACKGROUND

Influenza A is one type of influenza virus that commonly causes acute respiratory illness. Outbreaks of influenza occur every year. Major antigenic variations preclude permanent immunity in the population. Often signs of conjunctivitis or photophobia are common during acute infection. Posterior uveitis is very rare.

PATIENT

A young lady with a diagnosed anterior uveitis was sent for further evaluation to the eye department with a known history of flu.

RESULTS

This patient had a severe ocular manifestation of influenza A infection. There was bilateral panuveitis with keratic precipitates, cells and flare, and an impressive retinopathy in both eyes. Serology was positive for influenza A.

CONCLUSION

The course of an influenza A infection is usually uncomplicated. Severe affection of the choriocapillaris results in a complicated post-influenza retinal pigmentary degeneration. Treatment with amantadine and therapy with hyperimmunoglobulins seem to be useful.

Applications of NMR to structure-based drug design in structural genomics

Structural genomics is poised to have a tremendous impact on traditional structure-based drug design programs. As a result, there is a growing need to obtain rapid structural information in a reliable form that is amenable to rational drug design. In this manner, NMR has been expanding and evolving its role in aiding the design process. A variety of NMR methodologies that cover a range of inherent resolution are described in the context of structure-based drug design in the era of structural genomics.

Use of a pharmacophore model to discover a new class of influenza endonuclease inhibitors

Data from both our own and literature studies of the biochemistry and inhibition of influenza virus endonuclease was combined with data on the mechanism of action and the likely active site mechanism to propose a pharmacophore. The pharmacophore was used to design a novel structural class of inhibitors, some of which were found to have activities similar to that of known influenza endonuclease inhibitors and were also antiviral in cell culture.

In vitro anti-influenza virus activity of the pavine alkaloid (-)-thalimonine isolated from Thalictrum simplex L

The pavine alkaloid (-)-thalimonine (Thl), isolated from the Mongolian plant Thalictrum simplex inhibited markedly the reproduction of influenza virus A/Germany/27, str. Weybridge (H7N7) and A/Germany/34, str. Rostock (H7N1) in cell cultures of chicken embryo fibroblasts. In a number of assays at a non-toxic concentration range of 0.1-6.4 microM the alkaloid inhibited viral reproduction in a selective and specific way (selectivity index = 640, 106.6, respectively). Expression of viral glycoproteins haemagglutinin (HA), neuraminidase (NA) and nucleoprotein (NP) on the surface of infected cells, virus-induced cytopathic effect, infectious virus yields, HA production and virus-specific protein synthesis were all reduced. The inhibition was dose-related and depended on virus inoculum. The time of addition experiments indicated that viral reproduction was markedly inhibited when Thl was added at 4-5 h of infection. No inactivating effect on extracellular virus was found.

An approach to the identification of potent inhibitors of influenza virus fusion using parallel synthesis methodology

Structure-activity studies associated with the salicylic acid-derived inhibitor of influenza fusion, BMY-27709, were examined using a parallel synthesis approach. This SAR survey led to the discovery of potent influenza inhibitory activity in a series of aromatic amides and thioamides derived from 1,3,3-trimethyl-5-hydroxycyclohexylmethylamine. Select compounds were characterized as inhibitors of the H1 subtype of influenza A viruses that act by preventing the pH-induced fusion process, thereby blocking viral entry into host cells. In a plaque-reduction assay, the most potent inhibitors displayed EC(50) values of 0.02-0.14 microg/mL.

Oseltamivir: a review of its use in influenza

Oseltamivir is a prodrug of oseltamivir carboxylate (Ro 64-0802, GS4071), a potent and selective inhibitor of the neuraminidase glycoprotein essential for replication of influenza A and B viruses. Studies in volunteers with experimental human influenza A or B showed that administration of oral oseltamivir 20 to 200 mg twice daily for 5 days reduced both the quantity and duration of viral shedding compared with placebo. Subsequent assessment of the drug at a dosage of 75 mg twice daily for 5 days in otherwise healthy adults with naturally acquired febrile influenza showed that oseltamivir reduced the duration of the disease by up to 1.5 days and the severity of illness by up to 38% compared with placebo when initiated within 36 hours of symptom onset (earlier initiation of therapy was associated with faster resolution). The incidence of secondary complications and the use of antibacterials were also reduced significantly in oseltamivir recipients. A liquid formulation of oseltamivir (2 mg/kg twice daily for 5 days) has been shown to be effective in the treatment of children with influenza, and data presented in abstracts suggest that the drug may also be of use in high-risk populations such as the elderly or those with chronic cardiac or respiratory disease. In addition to treatment efficacy, the drug has demonstrated efficacy when used for seasonal or household prophylaxis. Oral oseltamivir (75 mg once or twice daily for 6 weeks) during a period of local influenza activity significantly prevented the development of naturally acquired influenza by >70% compared with placebo in unvaccinated otherwise healthy adults. The drug also demonstrated efficacy when used adjunctively in previously vaccinated high-risk elderly patients (92% protective efficacy). Short term administration of oseltamivir (75 mg once daily for 7 days) may significantly reduce the risk of illness in household contacts of infected persons when administered within 48 hours of symptom onset in the infected person. Oseltamivir 75 mg twice daily for 5 days was well tolerated in clinical trials in healthy adults and high-risk patients, with nausea and vomiting being the most commonly reported events. Gastrointestinal events were mild and transient and both nausea and vomiting were less likely when oseltamivir was taken with food.

CONCLUSIONS

Oseltamivir is a well tolerated orally active neuraminidase inhibitor which significantly reduces the duration of symptomatic illness and hastens the return to normal levels of activity when initiated promptly in patients with naturally acquired influenza. It therefore represents a useful therapeutic alternative to zanamivir (especially in patients who prefer oral administration or who have an underlying respiratory disorder) and the M2 inhibitors amantadine and rimantadine (because of its broader spectrum of anti-influenza activity and lower likelihood of resistance) in patients with influenza. In addition, although annual vaccination remains the best means of influenza prevention, there may be a place for oseltamivir in providing household prophylaxis or adjunctive prophylaxis in high-risk vaccinated patients during an outbreak of the disease or for use in patients in whom vaccination is unsuitable or ineffective.