Inhibition of the growth of influenza viruses in vitro by 4-guanidino-2,4-dideoxy-N-acetylneuraminic acid

Application of computational methods for anticancer drug discovery, design, and optimization

Developing a novel drug is a complex, risky, expensive and time-consuming venture. It is estimated that the conventional drug discovery process ending with a new medicine ready for the market can take up to 15 years and more than a billion USD. Fortunately, this scenario has recently changed with the arrival of new approaches. Many novel technologies and methodologies have been developed to increase the efficiency of the drug discovery process, and computational methodologies have become a crucial component of many drug discovery programs. From hit identification to lead optimization, techniques such as ligand- or structure-based virtual screening are widely used in many discovery efforts. It is the case for designing potential anticancer drugs and drug candidates, where these computational approaches have had a major impact over the years and have provided fruitful insights into the field of cancer. In this paper, we review the concept of rational design presenting some of the most representative examples of molecules identified by means of it. Key principles are illustrated through case studies including specifically successful achievements in the field of anticancer drug design to demonstrate that research advances, with the aid of in silico drug design, have the potential to create novel anticancer drugs.

Application of computational methods for anticancer drug discovery, design, and optimization

Developing a novel drug is a complex, risky, expensive and time-consuming venture. It is estimated that the conventional drug discovery process ending with a new medicine ready for the market can take up to 15 years and more than a billion USD. Fortunately, this scenario has recently changed with the arrival of new approaches. Many novel technologies and methodologies have been developed to increase the efficiency of the drug discovery process, and computational methodologies have become a crucial component of many drug discovery programs. From hit identification to lead optimization, techniques such as ligand- or structure-based virtual screening are widely used in many discovery efforts. It is the case for designing potential anticancer drugs and drug candidates, where these computational approaches have had a major impact over the years and have provided fruitful insights into the field of cancer. In this paper, we review the concept of rational design presenting some of the most representative examples of molecules identified by means of it. Key principles are illustrated through case studies including specifically successful achievements in the field of anticancer drug design to demonstrate that research advances, with the aid of in silico drug design, have the potential to create novel anticancer drugs.

Neuraminidase Inhibitors and their Role in Avian and Pandemic Influenza

Continuing occurrences of human infections with avian influenza A (H5N1) viruses have ignited increasing fears that the next influenza pandemic is imminent. Fortunately, options for antiviral prophylaxis and treatment have been improved dramatically since the previous pandemics by the availability of neuraminidase inhibitors such as zanamivir and oseltamivir. However, although the prophylactic and therapeutic efficacy of these drugs is well established for uncomplicated seasonal human influenza, clinical effectiveness seems limited for human H5N1 infections despite in vitro susceptibility and efficacy in animal studies. Factors which might contribute to this apparently limited efficacy include suboptimal dosing or routes of administration, suboptimal timing of treatment and the inability of antiviral drugs to interfere with immunopathology, and the development of drug resistance. Efforts to optimize the use of neuraminidase inhibitor treatment in H5N1 disease are urgently needed and might eventually aid in the judicious use of stockpiled neuraminidase inhibitors in the event of a pandemic.

Development of antivirals against influenza

Morbidity and mortality due to influenza virus infections remain a major problem throughout the world. Yearly, medical costs and loss of productivity resulting from influenza infection are estimated to be in the range of 12 dollars bn in the USA. The predicted increases in the elderly and immune-deficient populations will make influenza an even greater threat in the future. Despite the availability of vaccines, they have been least effective in these high-risk populations. Coupled with the requirement for routine revaccination, the need for effective antiviral agents is illustrated. The currently approved drugs, amantadine, rimantadine and ribavirin (in some countries), have limitations. They are only inhibitory against influenza A viruses, are prone to adverse reactions and quickly give rise to resistant virus. This review examines current drug therapies, antivirals in development and possible future opportunities for anti-influenza drugs.

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.

Glycosylation of haemagglutinin and stalk-length of neuraminidase combine to regulate the growth of avian influenza viruses in tissue culture

The influence on virus replication in culture of the presence and location of glycosylation sites on the haemagglutinin (HA) glycoprotein of avian influenza viruses and differences in length of the stalk region of their neuraminidase (NA) glycoprotein was examined using reassortant viruses. Plaque size was measured in the presence or absence of bacterial neuraminidase (CPNA) and/or an influenza virus NA inhibitor, zanamivir, to assess the relative contribution of the NA to replication efficiency in tissue culture. The following conclusions were drawn, (1) HA lacking glycosylation at 158 gives inefficient growth when combined with short-stalked NAs, and efficient growth when combined with long-stalked NAs. (2) Glycosylation at 158 of HA makes the virus less dependent on NA for release from its receptors. (3) HA with glycosylation at 158 gives efficient growth when combined with short-stalked NAs but, when combined with long-stalked NAs, growth is very efficient and excess NA activity is disadvantageous. (4) HA having glycosylation at 158 combined with short-stalked NAs, or HA lacking glycosylation at 158 combined with long-stalked NAs may represent optimal combinations. The results reinforce the importance of a balance of HA and NA activity for efficient virus exit from, and entry into cells.

Zanamivir: a rational approach to influenza B

Influenza B viruses have co-circulated with the HIN1 and H3N2 subtypes of influenza A since 1977. Influenza A viruses are found in various animals, whereas influenza B viruses are probably restricted to humans. The lack of an animal reservoir means that the virus has no potential for genetic reassortment across species. In addition, influenza B viruses are more serologically homogeneous than influenza A viruses. Thus, the chance of influenza B causing a pandemic is much lower than that of influenza A. However, influenza B viruses are still a frequent cause of local disease outbreaks and epidemics as a result of antigenic drift. Any prophylactic or therapeutic measure must, therefore, be effective against both influenza A and B viruses. Zanamivir is the first widely approved neuraminidase inhibitor for the treatment of influenza. It is delivered directly to the primary site of viral replication, the respiratory tract, and is well tolerated and effective in the treatment of both influenza A and B. Data in prophylaxis are also encouraging. Zanamivir is the only drug proven to be clinically effective against both influenza A and B virus infections.

Influenza Aviária: Uma Revisão dos Últimos Dez Anos

A influenza aviária é doença exótica no Brasil. O sistema de vigilância implementado pelo Programa Nacional de Sanidade Avícola (PNSA) mantém monitoração permanente das aves das principais espécies domésticas, tanto do material genético importado para a indústria avícola, por exemplo, da espécie das galinhas (Gallus gallus formadomestica), perus (Meleagris gallopavo formadomestica), codornas (Coturnix coturnix japonica), patos (Anas), primários (elite), bisavós e avós para postura ou corte, como aves de espécies de exploração mais recente, exóticas, por exemplo avestruzes (Struthio camelus) ou nativas, por exemplo emas (Rhea americana). Os plantéis de reprodutores em produção são também acompanhados por amostragens periódicas, conforme previsto no PNSA, além da monitoração das respostas aos programas de vacinação, por exemplo, contra bronquite infecciosa e doença infecciosa bursal. O PNSA estabelece as normas de atuação para o controle e erradicação da doença de Newcastle (ND) e Influenza Aviária (AI) (Projeto de Vigilância, 2001), a saber: I - Notificação de focos da doença (e confirmação laboratorial no LARA-Campinas); II - Assistência a focos; III - Medidas de desinfecção; IV - Sacrifício sanitário; V - Vazio sanitário; VI - Vacinação dos plantéis ou esquemas emergenciais; VII - Controle e fiscalização dos animais susceptíveis; VIII - Outras medidas sanitárias; A vigilância e atenção ao foco exige o diagnóstico laboratorial e diferencial de AI e ND, que segue as normas do PNSA, conforme o sumário abaixo: 1- Interdição e coleta de materiais para exame laboratorial oficial; 2- Registro das aves: espécie(s), categoria(s), número(s), manutenção de aves; utensílios e produtos no local; proibição de trânsito de e para a(s) propriedade(s) em um raio de 10 km; controle de todos os animais e materiais possíveis fontes de propagação; desinfecção de vias de entradas e saídas à(s) propriedade(s); inquérito epidemiológico. 3- Confirmação laboratorial: isolamento de agente letal hemaglutinante em ovos embrionados de galinhas SPF, não inibido (inibição da hemaglutinação) ou não neutralizado (soroneutralização) por soro específico para o vírus da doença de Newcastle; caracterização do agente como vírus da influenza aviária (AIV) por detecção de antígenos da nucleoproteína e/ou matriciais de AIV e de seu subtipo por ensaios específicos para a caracterização da hemaglutinina e neuraminidase (imunodifusão, imunoenzimáticos ou moleculares). 4- Abate e destruição imediata (cremação) de todas as aves, resíduos, carnes e ovos da(s) propriedade(s) atingida(s) e vizinhas (raio de 3 km); limpeza e desinfecção das instalações; vazio sanitário (mínimo 21 dias); 5- Permitir o transporte para o abate ou incubação dentro da zona de vigilância (raio de 10 km). 6- Proibir feiras, exposições, mercados na zona de vigilância (10 km). 7- Aplicar estas medidas por mínimo de 21 dias após a destruição das fontes de propagação e desinfecção das instalações, proibir a retirada de aves e produtos na zona de proteção (3 km) por 21 dias e 15 dias na zona de vigilância (10 km). A certificação de área livre segue as normas da OIE e PNSA, considerando AI exótica no Brasil (país livre), e exige: 1- AI de alta patogenicidade não diagnosticada pelo sistema de vigilância pelos últimos 3 anos; 2- Um período de 6 meses após o abate, destruição das aves e resíduos e desinfecção após surto; O sistema de criação da avicultura predominante no Brasil (galinhas e perus) emprega a mais atual tecnologia e conhecimento científico na produção, no qual os plantéis são gerenciados com biossegurança, avaliação permanente dos pontos críticos, sistema de qualidade total e programas de vacinações que garantem a prevenção de inúmeros problemas sanitários. A prevenção de influenza aviária é especialmente favorecida por essas características. O sistema e tipo de construção (galpões) para o alojamento dos plantéis dessas espécies dificultam também o desafio eventualmente imposto pelas aves de vida livre. A localização geográfica da avicultura nacional, localizada fora das rotas migratórias das aves-reservatório, pode também exercer papel importante na ausência de focos de influenza no Brasil. Além disso, o baixo índice de replicação dos AIV nas aves migratórias durante a estada na região subtropical também influi para a menor ocorrência. As espécies de aves domésticas de importância comercial mais sensíveis à infecção e potencialmente envolvidas no papel de fonte de infecção, conforme citadas na literatura internacional, perus e patos, são mantidas em galpões industriais com sistema de biossegurança e de distribuição geográfica bastante restrita, em contraste com as criações dos países com relatos permanentes de surtos, em que se associam as condições de desafio naturais geográficas ditadas pelas rotas migratórias, mais alta replicação na ave na estação (países temperados) e a criação em campo aberto.

Efficacy of zanamivir against avian influenza A viruses that possess genes encoding H5N1 internal proteins and are pathogenic in mammals

In 1997, an avian H5N1 influenza virus, A/Hong Kong/156/97 (A/HK/156/97), caused six deaths in Hong Kong, and in 1999, an avian H9N2 influenza virus infected two children in Hong Kong. These viruses and a third avian virus [A/Teal/HK/W312/97 (H6N1)] have six highly related genes encoding internal proteins. Additionally, A/Chicken/HK/G9/97 (H9N2) virus has PB1 and PB2 genes that are highly related to those of A/HK/156/97 (H5N1), A/Teal/HK/W312/97 (H6N1), and A/Quail/HK/G1/97 (H9N2) viruses. Because of their similarities with the H5N1 virus, these H6N1 and H9N2 viruses may have the potential for interspecies transmission. We demonstrate that these H6N1 and H9N2 viruses are pathogenic in mice but that their pathogenicities are less than that of A/HK/156/97 (H5N1). Unadapted virus replicated in lungs, but only A/HK/156/97 (H5N1) was found in the brain. After three passages (P3) in mouse lungs, the pathogenicity of the viruses increased, with both A/Teal/HK/W312/97 (H6N1) (P3) and A/Quail/HK/G1/97 (H9N2) (P3) viruses being found in the brain. The neuraminidase inhibitor zanamivir inhibited viral replication in Madin-Darby canine kidney cells in virus yield assays (50% effective concentration, 8.5 to 14.0 microM) and inhibited viral neuraminidase activity (50% inhibitory concentration, 5 to 10 nM). Twice daily intranasal administration of zanamivir (50 and 100 mg/kg of body weight) completely protected infected mice from death. At a dose of 10 mg/kg, zanamivir completely protected mice from infection with H9N2 viruses and increased the mean survival day and the number of survivors infected with H6N1 and H5N1 viruses. Zanamivir, at all doses tested, significantly reduced the virus titers in the lungs and completely blocked the spread of virus to the brain. Thus, zanamivir is efficacious in treating avian influenza viruses that can be transmitted to mammals.

The anti-influenza virus agent 4-GU-DANA (zanamivir) inhibits cell fusion mediated by human parainfluenza virus and influenza virus HA

4-GU-DANA (zanamivir) (as well as DANA and 4-AM-DANA) was found to inhibit the neuraminidase activity of human parainfluenza virus type 3 (HPF3). The viral neuraminidase activity is attributable to hemagglutinin-neuraminidase (HN), an envelope protein essential for viral attachment and for fusion mediated by the other envelope protein, F. While there is no evidence that HN's neuraminidase activity is essential for receptor binding and syncytium formation, we found that 4-GU-DANA prevented hemadsorption and fusion of persistently infected cells with uninfected cells. In plaque assays, 4-GU-DANA reduced the number (but not the area) of plaques if present only during the adsorption period and reduced plaque area (but not number) if added only after the 90-min adsorption period. 4-GU-DANA also reduced the area of plaques formed by a neuraminidase-deficient variant, confirming that its interference with cell-cell fusion is unrelated to inhibition of neuraminidase activity. The order-of-magnitude lower 50% inhibitory concentrations of 4-GU-DANA (and also DANA and 4-AM-DANA) for plaque area reduction and for inhibition in the fusion assay than for reducing plaque number or blocking hemadsorption indicate the particular efficacy of these sialic acid analogs in interfering with cell-cell fusion. In cell lines expressing influenza virus hemagglutinin (HA) as the only viral protein, we found that 4-GU-DANA had no effect on hemadsorption but did inhibit HA2b-red blood cell fusion, as judged by both lipid mixing and content mixing. Thus, 4-GU-DANA can interfere with both influenza virus- and HPF3-mediated fusion. The results indicate that (i) in HPF3, 4-GU-DANA and its analogs have an affinity not only for the neuraminidase active site of HN but also for sites important for receptor binding and cell fusion and (ii) sialic acid-based inhibitors of influenza virus neuraminidase can also exert a direct, negative effect on the fusogenic function of the other envelope protein, HA.

The neuraminidase inhibitor GS4104 (oseltamivir phosphate) is efficacious against A/Hong Kong/156/97 (H5N1) and A/Hong Kong/1074/99 (H9N2) influenza viruses

In 1997, an H5N1 avian influenza A/Hong Kong/156/97 virus transmitted directly to humans and killed six of the 18 people infected. In 1999, another avian A/Hong/1074/99 (H9N2) virus caused influenza in two children. In such cases in which vaccines are unavailable, antiviral drugs are crucial for prophylaxis and therapy. Here we demonstrate the efficacy of the neuraminidase inhibitor GS4104 (oseltamivir phosphate) against these H5N1 and H9N2 viruses. GS4071 (the active metabolite of oseltamivir) inhibited viral replication in MDCK cells (EC(50) values, 7.5-12 microM) and neuraminidase activity (IC(50) values, 7.0-15 nM). When orally administered at doses of 1 and 10 mg/kg per day, GS4104 prevented death of mice infected with A/Hong Kong/156/97 (H5N1), mouse-adapted A/Quail/Hong Kong/G1/97 (H9N2), or human A/Hong Kong/1074/99 (H9N2) viruses and reduced virus titers in the lungs and prevented the spread of virus to the brain of mice infected with A/Hong Kong/156/97 (H5N1) and mouse-adapted A/Quail/Hong Kong/G1/97 (H9N2) viruses. When therapy was delayed until 36 h after exposure to the H5N1 virus, GS4104 was still effective and significantly increased the number of survivors as compared with control. Oral administration of GS4104 (0.1 mg/kg per day) in combination with rimantadine (1 mg/kg per day) reduced the number of deaths of mice infected with 100 MLD(50) of H9N2 virus and prevented the deaths of mice infected with 5 MLD(50) of virus. Thus, GS4104 is efficacious in treating infections caused by H5N1 and H9N2 influenza viruses in mice.

Zanamivir susceptibility monitoring and characterization of influenza virus clinical isolates obtained during phase II clinical efficacy studies

Zanamivir is a highly selective neuraminidase (NA) inhibitor with demonstrated clinical efficacy against influenza A and B virus infections. In phase II clinical efficacy trials (NAIB2005 and NAIB2008), virological substudies showed mean reductions in virus shedding after 24 h of treatment of 1.5 to 2.0 log(10) 50% tissue culture infective doses compared to a placebo, with no reemergence of virus after the completion of therapy. Paired isolates (n = 41) obtained before and during therapy with zanamivir demonstrated no shifts in susceptibility to zanamivir when measured by NA assays, although for a few isolates NA activity was too low to evaluate. In plaque reduction assays in MDCK cells, the susceptibility of isolates to zanamivir was extremely variable even at baseline and did not correlate with the speed of resolution of virus shedding. Isolates with apparent limited susceptibility to zanamivir by plaque reduction proved highly susceptible in vivo in the ferret model. Further sequence analysis of paired isolates revealed no changes in the hemagglutinin and NA genes in the majority of isolates. The few changes observed were all natural variants. No amino acid changes that had previously been identified in vitro as being involved with reduced susceptibility to zanamivir were observed. These studies highlighted problems associated with monitoring susceptibility to NA inhibitors in the clinic, in that no reliable cell-based assay is available. At present the NA assay is the best available predictor of susceptibility to NA inhibitors in vivo, as measured in the validated ferret model of infection.

Genetic analysis reveals that both haemagglutinin and neuraminidase determine the sensitivity of naturally occurring avian influenza viruses to zanamivir in vitro

The basis of differential sensitivity of replication of influenza viruses to the neuraminidase-specific inhibitor zanamivir was examined using four avian influenza viruses and reassortants produced between them. IC(50) values for inhibition of neuraminidase activity by zanamivir were similar for each of the four viruses, whereas the haemagglutinating activity of each of the viruses was relatively insensitive to zanamivir. However, the four viruses showed distinct zanamivir-sensitivity profiles in tissue culture. Analysis of the reassortant viruses showed that sensitivity was determined by the haemagglutinin gene (segment 4) and the neuraminidase gene (segment 6) and was independent of the remaining six RNA segments. Decreased sensitivity to zanamivir was associated with possession of a haemagglutinin that is released from cells with decreased dependence on neuraminidase and with possession of a neuraminidase that has a short stalk region.

Chemotherapy of respiratory viruses: prospects and challenges

Billions of people are infected with respiratory viruses annually. Infants and young children, the elderly, immunocompromised individuals and those debilitated by other diseases or nutritional deficiencies are most at risk for serious disease. There are few vaccines available for use against these viruses, and even where there are (influenza, measles and adenovirus), infections remain common. The continued prevalence of respiratory virus infections has lead to renewed efforts to find safe agents effective against the most medically important respiratory viruses: influenza, respiratory syncytial, parainfluenza, measles, rhino- and adenovirus. Copyright 1999 Harcourt Publishers Ltd.

Safety and efficacy of intravenous zanamivir in preventing experimental human influenza A virus infection

Zanamivir is a potent inhibitor of influenza A and B virus neuraminidases and is active topically in experimental and natural human influenza. We conducted this double-blinded, placebo-controlled study to evaluate the safety and efficacy of intravenously administered zanamivir. Susceptible volunteers were randomized to receive either saline or zanamivir (600 mg) intravenously twice daily for 5 days beginning 4 h prior to intranasal inoculation with approximately 10(5) 50% tissue culture infectious doses (TCID50) of influenza A/Texas/36/91 (H1N1) virus. Reductions in the frequency of viral shedding (0% versus 100% in placebo, P < 0.005) and seroconversion (14% versus 100% in placebo, P < 0.005) and decreases in viral titer areas under the curve (0 versus 11.6 [median] log10 TCID50. day/ml in placebo, P < 0.005) were observed in the zanamivir group, as were reductions in fever (14% versus 88% in placebo, P < 0.05), upper respiratory tract illness (0% versus 100% in placebo, P < 0.005), total symptom scores (1 versus 44 [median] in placebo, P < 0.005), and nasal-discharge weight (3.9 g versus 17.5 g [median] in placebo, P < 0.005). Zanamivir was detectable in nasal lavage samples collected on days 2 and 4 (unadjusted median concentrations, 10.5 and 12.0 ng/ml of nasal wash, respectively). This study demonstrates that intravenously administered zanamivir is distributed to the respiratory mucosa and is protective against infection and illness following experimental human influenza A virus inoculation.

Approaches and strategies for the treatment of influenza virus infections

Influenza A and B viruses belong to the Orthomyxoviridae family of viruses. These viruses are responsible for severe morbidity and significant excess mortality each year. Infection with influenza viruses usually leads to respiratory involvement and can result in pneumonia and secondary bacterial infections. Vaccine approaches to the prophylaxis of influenza virus infections have been problematic owing to the ability of these viruses to undergo antigenic shift by exchanging genomic segments or by undergoing antigenic drift, consisting of point mutations in the haemagglutinin (HA) and neuraminidase (NA) genes as a result of an error-prone viral polymerase. Historically, antiviral approaches for the therapy of both influenza A and B viruses have been largely unsuccessful until the elucidation of the X-ray crystallographic structure of the viral NA, which has permitted structure-based drug design of inhibitors of this enzyme. In addition, recent advances in the elucidation of the structure and complex function of influenza HA have resulted in the discovery of a number of diverse compounds that target this viral protein. This review article will focus largely on newer antiviral agents including those that inhibit the influenza virus NA and HA. Other novel approaches that have entered clinical trials or been considered for their clinical utility will be mentioned.

Synthesis and anti-influenza virus activity of novel pyrimidine derivatives

Efficient synthetic routes of 2-amino-4-(omega-hydroxyalkylamino)pyrimidine derivatives were investigated in relation to the anti-influenza virus activity of these compounds. The derivatives in which cyclobutyl and cyclopentyl groups were introduced to the beta-position of the aminoalkyl group (especially the cyclobutyl group substituted by a phenylalkyl group at the 3'-position) resulted in improved antiviral potency: i.e. an average 50% effective concentration for inhibition of plaque formation (EC50, microM) of 0.1-0.01 microM for both types A and B influenza virus. The antiviral efficacies were in the order of amino group > hydroxyiminomethyl group > halogen substitution at the 5-position, and chlorine or methoxy group > hydrogen at the 6-position of the pyrimidine ring. The antiviral indices of these compounds were 2-6 with respect to the 50% inhibitory concentration for cell proliferation (IC50, microM) for growing cells, but > 500 to > 10(4) with respect to the IC50 for stationary cells, indicating that these compounds may be efficacious for the topical treatment of influenza virus infection.

Characterization of human influenza virus variants selected in vitro in the presence of the neuraminidase inhibitor GS 4071

An oral prodrug of GS 4071, a potent and selective inhibitor of influenza neuraminidases, is currently under clinical development for the treatment and prophylaxis of influenza virus infections in humans. To investigate the potential development of resistance during the clinical use of this compound, variants of the human influenza A/Victoria/3/75 (H3N2) virus with reduced susceptibility to the neuraminidase inhibitor GS 4071 were selected in vitro by passaging the virus in MDCK cells in the presence of inhibitor. After eight passages, variants containing two amino acid substitutions in the hemagglutinin (A28T in HA1 and R124M in HA2) but no changes in the neuraminidase were isolated. These variants exhibited a 10-fold reduction in susceptibility to GS 4071 and zanamivir (GG167) in an in vitro plaque reduction assay. After 12 passages, a second variant containing these hemagglutinin mutations and a Lys substitution for the conserved Arg292 of the neuraminidase was isolated. The mutant neuraminidase enzyme exhibited high-level (30,000-fold) resistance to GS 4071, but only moderate (30-fold) resistance to zanamivir and 4-amino-Neu5Ac2en, the amino analog of zanamivir. The mutant enzyme had weaker affinity for the fluorogenic substrate 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid and lower enzymatic activity compared to the wild-type enzyme. The viral variant containing the mutant neuraminidase did not replicate as well as the wild-type virus in culture and was 10,000-fold less infectious than the wild-type virus in a mouse model. These results suggest that although the R292K neuraminidase mutation confers high-level resistance to GS 4071 in vitro, its effect on viral virulence is likely to render this mutation of limited clinical significance.

New approaches to influenza chemotherapy. Neuraminidase inhibitors

Epidemic influenza continues to be associated with significant morbidity in the general population, and mortality in the elderly and other high risk patients. Although the case fatality rate averages less than 0.01%, tens of thousands of deaths occur each year. Control through immunisation programmes has not been possible due to incomplete protective efficacy and antigenic variations that occur frequently. Currently available anti-influenza medications (amantadine and rimantadine) have had limited success due to underutilisation, lack of activity against influenza B, the rapid development of viral resistance to the drugs, and adverse effects. A new class of antiviral agents designed to inhibit influenza neuraminidase, an important surface glycoprotein, is currently under active development for use in the prophylaxis and treatment of influenza A and B infections. Two of these compounds, zanamivir (GG167) and GS4104 (the ethyl ester prodrug of GS4071) have reached clinical trials. Most studies of zanamivir have involved topical administration by inhalation of dry powder aerosols and/or intranasal doses of aqueous solutions. These routes rapidly provide high local concentrations at the sites of delivery. GS4104 is administered orally, which allows for greater ease of administration, and probably more uniform distribution of the parent compound GS4071 in the respiratory tract. Both have shown potent inhibitory activity against influenza in animal models and experimental human influenza with excellent tolerability profiles. Zanamivir treatment has been shown to reduce the severity and duration of naturally occurring, uncomplicated influenza illness in adults. Clinical resistance to these drugs has not been recognised as a significant problem to date, although strains resistant to each agent have been produced in the laboratory. This class of agents shows considerable promise as a novel approach to prophylaxis and treatment of influenza infections. Ongoing studies should provide the data needed to allow the addition of 1 or more of the neuraminidase inhibitors to the clinician's anti-influenza armamentarium.

The interaction of neuraminidase and hemagglutinin mutations in influenza virus in resistance to 4-guanidino-Neu5Ac2en

We have previously described a 4-guanidino-Neu5Ac2en (zanamivir)-resistant neuraminidase (NA) variant G70C4-G, with an active site mutation Glu 119 to Gly. This variant has been found to also harbor a hemagglutinin (HA) mutation in the receptor binding site, Ser 186 to Phe. Examination of early passages of the G70C4-G virus revealed that this HA mutation had arisen by the first passage. From a subsequent passage two transient variants were isolated which had each acquired a different second HA mutation, Ser 165 to Asn and Lys 222 to Thr. Both were highly drug resistant and drug dependent and their ability to adsorb to and penetrate cells was decreased. Comparison of drug sensitivities between the variant, with the additional HA mutation at Ser 165, and viruses with either mutation alone revealed that these two HA mutations acted synergistically to increase resistance. To determine the contribution to resistance of each of the NA and HA mutations in G70C4-G, the NA mutation was separated from the HA mutation by reassorting. The NA mutation and the HA mutation each conferred low-level resistance to zanamivir, while the two mutations interacted synergistically in the double mutant to give higher resistance in vitro. Infectivity was not adversely affected in the double mutant and while there was a small decrease in sensitivity to zanamivir in the mouse model, there was no detectable resistance to zanamivir in the ferret model.

Drug design against a shifting target: a structural basis for resistance to inhibitors in a variant of influenza virus neuraminidase

BACKGROUND

Inhibitors of the influenza virus neuraminidase have been shown to be effective antiviral agents in humans. Several studies have reported the selection of novel influenza strains when the virus is cultured with neuraminidase inhibitors in vitro. These resistant viruses have mutations either in the neuraminidase or in the viral haemagglutinin. Inhibitors in which the glycerol sidechain at position 6 of 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (Neu5Ac2en) has been replaced by carboxamide-linked hydrophobic substituents have recently been reported and shown to select neuraminidase variants. This study seeks to clarify the structural and functional consequences of replacing the glycerol sidechain of the inhibitor with other chemical constituents.

RESULTS

The neuraminidase variant Arg292-->Lys is modified in one of three arginine residues that encircle the carboxylate group of the substrate. The structure of this variant in complex with the carboxamide inhibitor used for its selection, and with other Neu5Ac2en analogues, is reported here at high resolution. The structural consequences of the mutation correlate with altered inhibitory activity of the compounds compared with wild-type neuraminidase.

CONCLUSIONS

The Arg292-->Lys variant of influenza neuraminidase affects the binding of substrate by modification of the interaction with the substrate carboxylate. This may be one of the structural correlates of the reduced enzyme activity of the variant. Inhibitors that have replacements for the glycerol at position 6 are further affected in the Arg292-->Lys variant because of structural changes in the binding site that apparently raise the energy barrier for the conformational change in the enzyme required to accommodate such inhibitors. These results provide evidence that a general strategy for drug design when the target has a high mutation frequency is to design the inhibitor to be as closely related as possible to the natural ligands of the target.

Generation and characterization of a mutant of influenza A virus selected with the neuraminidase inhibitor BCX-140

Influenza neuraminidase (NA) plays an important role in viral replication, and characterization of viruses resistant to NA inhibitors will help elucidate the role of active-site residues. This information will assist in designing better inhibitors targeted to essential active-site residues that cannot generate drug-resistant mutations. In the present study we used the benzoic acid-based inhibitor BCX-140 to select and characterize resistant viruses. BCX-140 binds to the NA active site in an orientation that is opposite that of a sialic acid-based compound, 4-guanidino-2,4-dideoxy-2,3-dehydro-N-acetylneuraminic acid (GANA). Thus, the guanidino group of BCX-140 binds to Glu-276, whereas in GANA the guanidino group binds to Glu-119. We passaged influenza A/Singapore/1/57 (H2N2) in Madin-Darby canine kidney cells in the presence of BCX-140, and virus resistant to this inhibitor was selected after six passages. The NA of this mutant was still sensitive to inhibition by BCX-140. However, the mutant virus was resistant to BCX-140 in plaque and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. Sequence analysis of hemagglutinin (HA) and NA genes revealed changes in both, although none were in the active site of the NA. Depending on the method of selection of the resistant virus, two types of changes associated with the sialic acid binding site were seen in the HA. One is a change in HA1 of Ala-133 to Thr, a residue close to the binding site, while the other change was Arg-132 of HA1 to Gln, which in HA1 of serotype H3 is a sialic acid contact (Asn-137). Binding studies revealed that both types of resistant viruses had reduced receptor binding affinity compared to that of the wild type. Thus, resistance to BCX-140 was generated by modifying the HA. NA active-site residue 276 may be essential for activity, and thus, it cannot be changed to generate resistance. However, drug-induced changes in the HA can result in a virus that is less dependent on NA activity for growth in cells and, hence, resistant to NA inhibitors.

Safety and Efficacy of Once Daily Intranasal Zanamivir in Preventing Experimental Human Influenza a Infection

Zanamivir, a potent inhibitor of influenza A and B virus neuraminidases, is protective against experimental human influenza when given intranasally twice daily. We conducted two studies to assess the pharmacokinetics and protective efficacy of a reduced frequency dosing regimen of topical zanamivir. In the first study, 36 uninfected volunteers received a single dose of zanamivir by intranasal spray (6.4 mg), intranasal drops (16 mg) or dry powder oral inhalation (10 mg). At 4 h, median nasal wash concentrations were 50-fold higher after intranasal dosing than after inhalation. Substantial levels (spray group, median 4596 ng/ml; drop group, 1239 ng/ml) were detected in nasal wash 48 h after intranasal dosing. In the double-blinded efficacy study, 47 sero-susceptible volunteers were randomized to receive either placebo or zanamivir intranasal spray (6.4 mg). Among the 43 subjects evaluated, decreases in viral shedding occurred in the group receiving one dose of zanamivir 4 h prior to inoculation, whereas no significant benefit was observed in those receiving a single dose 48 h prior to challenge. In the group given three daily doses, reductions were seen in viral shedding and infection. In the two regimens providing zanamivir 4 h prior to inoculation, significant reductions in nasal mucus weight were observed. Decreases in total symptom scores and the incidence of upper respiratory illness also occurred, but they did not reach statistical significance. The efficacy of a single dose of zanamivir given 4 h prior to inoculation supports the hypothesis that influenza virus neuraminidase is essential for initial virus spread through respiratory secretions. These findings indicate that once daily dosing of zanamivir is protective against experimental influenza A infection.

Dihydropyrancarboxamides related to zanamivir: a new series of inhibitors of influenza virus sialidases. 1. Discovery, synthesis, biological activity, and structure-activity relationships of 4-guanidino- and 4-amino-4H-pyran-6-carboxamides

4-Amino- and 4-guanidino-4H-pyran-6-carboxamides 4 and 5 related to zanamivir (GG167) are a new class of inhibitors of influenza virus sialidases. Structure--activity studies reveal that, in general, secondary amides are weak inhibitors of both influenza A and B viral sialidases. However, tertiary amides, which contain one or more small alkyl groups, show much greater inhibitory activity, particularly against the influenza A virus enzyme. The sialidase inhibitory activities of these compounds correlate well with their in vitro antiviral efficacy, and several of the most potent analogues displayed useful antiviral activity in vivo when evaluated in a mouse model of influenza A virus infection. Carboxamides which were highly active sialidase inhibitors in vitro also showed good antiviral activity in the mouse efficacy model of influenza A infection when administered intranasally but displayed modest activity when delivered by the intraperitoneal route.

Oral administration of a prodrug of the influenza virus neuraminidase inhibitor GS 4071 protects mice and ferrets against influenza infection

We have recently described GS 4071, a carbocyclic transition-state analog inhibitor of the influenza virus neuraminidase, which has potent inhibitory activity comparable to that of 4-guanidino-Neu5Ac2en (GG167; zanamivir) when tested against influenza A virus replication and neuraminidase activity in vitro. We now report that GS 4071 is active against several strains of influenza A and B viruses in vitro and that oral GS 4104, an ethyl ester prodrug which is converted to GS 4071 in vivo, is active in the mouse and ferret models of influenza virus infection. Oral administration of 10 mg of GS 4104 per kg of body weight per day caused a 100-fold reduction in lung homogenate viral titers and enhanced survival in mice infected with influenza A or B viruses. In ferrets, a 25-mg/kg dose of GS 4104 given twice daily reduced peak viral titers in nasal washings and eliminated constitutional responses to influenza virus infection including fever, increased nasal signs (sneezing, nasal discharge, mouth breathing), and decreased activity. Consistent with our demonstration that the parent compound is highly specific for influenza virus neuraminidases, no significant drug-related toxicity was observed after the administration of oral dosages of GS 4104 of up to 800 mg/kg/day for 14 days in nonclinical toxicology studies with rats. These results indicate that GS 4104 is a novel, orally active antiviral agent with the potential to be used for the prophylaxis and treatment of influenza A and B virus infections.

Inhibition of influenza virus infections in mice by GS4104, an orally effective influenza virus neuraminidase inhibitor

The carbocyclic transition state sialic acid analog GS4071 ([3R,4R,5S]-4-acetamido-5-amino-3-[1-ethylpropoxy]-1-cyclohexane-1 -carboxylic acid), a potent influenza virus neuraminidase inhibitor, was highly inhibitory to influenza A/NWS/33 (H1N1), A/Victoria/3/75 (H3N2), A/Shangdong/09/93 (H3N2) and B/Hong Kong/5/72 viruses in Madin Darby canine kidney (MDCK) cells. The 50% effective concentrations in these experiments ranged from 1.8 to 59.5 microM, with no cytotoxicity evident at 1000 microM, using inhibition of viral cytopathic effect determined visually and by neutral red dye uptake. The ethyl ester prodrug of GS4071, GS4104, administered by oral gavage (p.o.), had significant inhibitory effects on infections in mice induced by these viruses. Antiviral effects were seen as prevention of death, increase in mean day to death, inhibition of decline of arterial oxygen saturation, lessened lung consolidation and inhibition of infectious virus recovered from the lungs. No toxicity was seen in dosages up to 100 mg/kg/day (highest evaluated). Comparison experiments run versus the influenza A (H1N1) virus-induced infection using GS4104, GS4071 and the neuraminidase inhibitor zanamivir (GG167, 4-guanidino-Neu5Ac2en), all administered p.o., indicated a 10-fold or greater potency for inhibiting the infection by GS4104. The minimum effective dosage for GS4104 was 0.1 mg/kg/day, with the compound administered twice daily for 5 days beginning 4 h pre-virus exposure. Oral therapy with GS4104 could be delayed from 48 to at least 60 h after exposure of mice to influenza A (H1N1) virus and still render a significant antiviral effect, the time of delay being dependent on the viral challenge dose. Intranasal instillation of GS4071 and GG167 to mice infected with influenza virus was highly inhibitory to the infection, the minimum effective dosages to significantly prevent death being 0.01 mg/kg/day for GS4071 and 0.1 mg/kg/day for GG167. Caging of infected mice treated with 10 mg/kg/day of GS4104 with infected saline-treated animals did not transfer any influenza-inhibitory effect to the latter animals. These data provide strong evidence of the potential of orally administered GS4104 for treatment of influenza A and B virus infections in humans.

Influenza virus resistance to neuraminidase inhibitors

The recent reports from Hong Kong of an avian influenza virus which caused the death of six people has served to remind us of the constant threat of pandemics posed by influenza. Recently, structure-based drug design has resulted in the discovery of a new class of antiviral agents which specifically target the influenza virus neuraminidase. Two neuraminidase inhibitors are currently undergoing clinical evaluation for the prevention and treatment of influenza virus infection in man. Here we review the results of in vitro and in vivo studies relating to the potential development of resistance to this new class of compounds.

Characterization of mutants of influenza A virus selected with the neuraminidase inhibitor 4-guanidino-Neu5Ac2en

The development of viral resistance to the neuraminidase (NA) inhibitor, 4-guanidino-Neu5Ac2en, of influenza viruses was studied by serial passage of A/Turkey/Minnesota/833/80 (H4N2) in Madin-Darby canine kidney cells in the presence of increasing concentrations of inhibitor. Resistant mutants selected after eight passages, had a 10,000-fold reduction in sensitivity to the inhibitor in plaque assays, but their affinity (1/Kd) to the inhibitor was similar to that of the parental virus. Electron microscopic analysis revealed aggregation of the mutant virus at the cell surface in the presence of the inhibitor. Sequence analysis established that a substitution had occurred in the NA (Arg-249 to Lys) and in the HA2 subunit of the hemagglutinin (Gly-75 to Glu), in the vicinity of the proposed second sialic acid binding site. The change of residue 249 appears to be a chance mutation, for we were unable to reisolate this mutant, whereas subsequent experiments indicate changes in the hemagglutinin. After 13 passages of the parental virus, mutants that were resistant to the high concentrations of inhibitor tested were obtained. These viruses retained their drug-resistant phenotype even after five passages without the inhibitor. Electron microscopic analysis revealed no aggregation of virus on the surface of infected cells in the presence of the inhibitor. Sequence analysis of the NA gene from these drug-resistant mutants revealed an additional substitution of Glu to Ala at the conserved amino acid residue 119. This substitution is responsible for reducing the affinity of the inhibitor to the NA. Our findings suggest that the emergence of mutants resistant to 4-guanidine-Neu5Ac2en is a multistep process requiring prolonged exposure to the inhibitor.

Determination of the novel sialic acid analog GG167 (GR121167X) in human urine by liquid chromatography: direct injection with column switching

GG167 is a novel compound which selectively inhibits viral neuraminidase and has demonstrated activity against influenza A and B. A liquid chromatography (LC) method for the determination of GG167 in human urine has been developed and validated. The method allows direct injection of urine (7 microliters) using LC column switching followed by UV detection. Initial chromatography is performed using a Nucleosil-Diol column (7 microns, 250 mm x 4.6 mm), eluted with 20 mM phosphate buffer (pH 2.5):acetonitrile (18:82, v/v) at 2.0 ml min-1. GG167 is "heart-cut" to a Spherisorb-SCX column (5 microns, 100 mm x 4.6 mm) and eluted with 35 mM phosphate buffer (pH 2.5):acetonitrile (50:50, v/v) at 1.5 ml min-1 for final separation. GG167 is detected by UV absorbance at lambda = 238 nm. UV detection and peak shape are enhanced at pH < 2.5. The quantitation range of the assay is 0.3-100 micrograms ml-1. The method has demonstrated sufficient ruggedness to be used in support of GG167 clinical trials.

4-Guanidino-Neu5Ac2en fails to protect chickens from infection with highly pathogenic avian influenza virus

The effectiveness of the novel sialidase inhibitor 4-guanidino-Neu5Ac2en, which is highly effective in mouse and ferret models of influenza virus infection (von Itzstein et al. (1993) Nature 363, 418-423), has been assessed as a prophylactic agent in the prevention of infection of chickens with highly pathogenic avian influenza viruses. At best a small delay in the onset of pyrexia and death was observed with one strain of fowl plague virus, but not with two other strains. These results demonstrate that a locally acting drug may be ineffective if virus can escape from the site of inoculation and replicate elsewhere.