treatment THINKSTOCK

44 | Chemistry World | February 2011 www.chemistryworld.org Fighting the flu The threat of pandemic influenza is constantly on the horizon. Clare Sansom explores the latest attempts to tackle an ever-changing foe

least, drug treatments have been design universally effective drugs ‘I had a little bird, limited by resistance. In short and vaccines. The classification of The Ancient Greek physician  The large number of these viruses is complicated. The Its name was Enza, Hippocrates was the first to describe variants and rapid rate broadest classification is into three I opened up the window, the symptoms of influenza, in the of mutation in influenza types: influenza A, B and C. Almost And In Flu Enza’ fourth century BC. The first attempts viruses makes them hard all the threat to human health comes to discern its cause, in the nineteenth to treat or prevent from influenza A, which is also the century, fixed on a bacterium.  Advances in vaccine only type to infect birds as well as This was a very common playground The microbe is known – to the production and design mammals. Influenza A is further rhyme 90 or so years ago. It is not confusion of generations of students could lead to more classified according to the variants of often that an outbreak of infectious – as Haemophilus influenza but is effective protection from haemagglutinin and neuraminidase disease becomes so deeply embedded unrelated to the disease. Influenza a wider range of strains expressed on their surfaces, known in popular culture, but this was no viruses were first discovered in the  Antiviral drugs like as serotypes. So far, sixteen serotypes ordinary outbreak. Between early 1930s. They are roughly spherical Tamiflu and Relenza can of haemagglutinin (named H1 to H16) 1918 and late 1919, pandemic influenza viruses with RNA genomes bearing help stop the spread of and nine of neuraminidase (named or ‘’ killed forty million the characteristic surface proteins disease, but resistance is N1 to N9) have been identified. These people worldwide: almost twice as haemagglutinin and neuraminidase. now emerging differ in the sugars that they bind to, many victims as the first world war. These proteins recognise and bind  Better understanding the range of their bird and mammal Influenza has a smaller place in the to mono- and polysaccharide sugars of the surface proteins hosts, and the clinical course of any public imagination in our generation. on the surface of their target cells. on virus particles could infection. All combinations of H and Any public complacency, however, is They act at opposite ends of the viral inspire new generations N are found in birds, but only some not shared by the health community. life cycle. Haemagglutinin binding of drugs infect humans and other mammals. Physicians have long memories; to cell surface sialic acid leads to the Variations in influenza A viruses influenza pandemics killing hundreds virus adhering to the cell membrane arise through two mechanisms. of thousands worldwide occurred and then being engulfed by the cell, Antigenic drift is the simple in 1957 and 1968, and continue to whereas neuraminidase catalyses accumulation of mutations in the viral threaten today. In 2008, the UK cleavage of sialic acid from newly DNA that arise either at random or in National Risk Register described an formed virus particles, releasing response to drugs or the host immune as ‘the gravest them from the cell. The drugs that response. However, if more than one threat to UK security [greater than are currently widely prescribed type of virus is present in the same terrorist threats or weather-related for influenza – (GSK’s host population (most often birds or emergencies], as it could claim up Relenza) and (marketed pigs), genes from the different viruses to 750 000 lives’. This is despite the by Roche as Tamiflu) – are both can cross over or ‘reassort’ to form wide uptake of both vaccines and neuraminidase inhibitors. new types: this is known as antigenic drugs. Both types of interventions shift. This may create a viral type have proved fairly successful, yet Mix and match that few humans have immunity to, all known drugs and vaccines are Another characteristic of influenza and it is the main cause of human effective only against a limited range viruses is variability, and it is this pandemics. So far, the only viruses of influenza viruses, and so far, at feature that makes it so difficult to known to cause major epidemics www.chemistryworld.org Chemistry World | February 2011 | 45 Influenza treatment

in humans contain haemagglutinin variants H1, H2, H3 and H5. The ‘Spanish flu’ pandemic of 1918–19 coincided with the emergence of a new H1N1 strain; the 1957 pandemic was caused by a new H2N2 strain and the 1968 one by H3N2, which is still the most common viral subtype in circulation worldwide.

The next pandemic The first decade of this century has seen not one but two feared influenza variants emerge, and this has spurred research into all aspects of the disease. The first is the H5N1 (colloquially dubbed ‘bird flu’) and the second a new H1N1 variant that arose in herds of pigs in Mexico, immediately becoming known as ‘swine flu’. One remarkable feature of these two strains is

how different they are in origin, MEDICINE AND HEALTH OF MUSEUM NATIONAL pathogenesis and clinical course. H5N1 avian flu has infected only a Organisation for Animal Health and The 1918-19 ‘Spanish flu’ so strain predictions need to be made few hundred humans since it was first United Nations Food and Agriculture pandemic killed millions. over six months before the vaccine characterised in 1997 in a small Hong Organization Reference Laboratory Predicting or preventing is needed. Until – or unless – the Kong boy, and no cases of human to for Avian Influenza in Padova, the next one is vital much heralded ‘universal influenza human transmission have yet been Italy, recently claimed in a keynote vaccine’ ever becomes available, observed. It is, however, extremely conference lecture that ‘Despite its researchers are studying to improve pathogenic, with a fatality rate close very wide host range, the H5N1 virus the technology to shorten this lead to 50 per cent. In contrast, the H1N1 doesn’t readily reassort with other time, and using bioinformatics variant that arose in Mexico in 2009 is subtypes’: nevertheless, this cannot to improve predictions. A exceptionally infectious but generally be completely ruled out. vaccine strain produced by US causes mild symptoms. pharmaceutical firm Baxter in ‘The greatly feared word Prevention is better than cure African green monkey (Vero) cells “pandemic” is used to mean any Vaccination has been the mainstay has recently been licensed. infection that has spread worldwide’, of influenza treatment for many Vaccines that generate says John McCauley, director of the decades. The technology used for against more conserved regions World Health Organization (WHO) vaccine production, however, has of influenza viruses should Influenza Centre at the National scarcely changed since the 1950s: the also be more widely effective. Institute for Medical Research in vaccine produced for each ‘’ ‘Generating antibodies that target London, UK. ‘It was correct to call consists of the two strains of the haemagglutinin ecto-domain, the 2009 epidemic a pandemic, even influenza A (usually, one H1N1 and which is conserved in many different though few were badly affected.’ one H3N2) and one of influenza Proteins on the virus strains, may be seen as a “conceptual The few people who did contract B predicted to be most highly surface are good targets advance”,’ says Vladimir Brusic of severe illness, however, were young circulating, grown in fertilised for drugs, but resistance the Dana-Farber Cancer Institute, in or middle-aged. This pattern, which chicken eggs. This is a slow process, can develop quickly Boston, US. is opposite to that observed in As well as vaccines, companies seasonal flu epidemics, derives from have looked to small molecule drugs the virus’s H1N1 serotype. ‘H1N1 to suppress the viruses in infected was the main virus in circulation patients. The first anti-influenza before 1957’, says McCaulay. ‘Almost drug, , was licensed in everyone who is now over 60 will the 1960s. This targets the influenza have been exposed to this serotype protein M2, a channel in the virus before and will therefore have membrane that allows protons to acquired some immunity.’ enter the cell; interestingly, it is also Of these emerging influenza an effective drug for Parkinson’s variants, therefore, one is disease. A very similar molecule, highly pathogenic but not very , was licensed to treat transmissible; the other is highly influenza in 1994. Influenza viruses infectious, but causes mild disease. rapidly become resistant to these Any between genes drugs, however, and they are now very from these viruses to produce a rarely prescribed. subtype with the infectivity of H1N1 During the 1970s and early 1980s, and the pathogenicity of H5N1 the UK drug companies Glaxo and would clearly cause a public health Wellcome, now both part of the nightmare. Perhaps reassuringly, giant GSK, were at the forefront

CDC Ilaria Capua from the World of research. There 46 | Chemistry World | February 2011 www.chemistryworld.org seemed, however, to be few reasons mutated to asparagine, and that

ROCHE to focus on influenza. Dick Challand, this mutation confers resistance to a medicinal chemist working at amantadine’, she says. ‘We were very Wellcome at that time, remembers pleased to see amantadine bound to that the disease was thought of this serine in our wild type structure.’ as ‘a ‘white elephant – a largely A lower affinity binding mode with self-limiting infection that could the drug outside the channel has also be well controlled by vaccination’. been observed. Hong’s NMR methods Nevertheless, Wellcome took an can be used to visualise drug-protein inhibitor of viral RNA polymerase binding at high resolution in an into pre-clinical development. environment that mimics the viral Interest switched to the viral membrane. ‘We can observe subtle surface proteins when their three- structural differences between drug dimensional structures became binding modes that may help predict available later in the 1980s. The the potential success of new antiviral zanamivir compounds’, she says. However, was discovered by a group led none of the compounds that have so by Mark von Itzstein at Monash far been tested have been found to University, Australia, using structure- be potent and selective inhibitors of based drug design and developed by amantadine-resistant M2. the Australian biotech company Biota Superficially, at least, and Glaxo (later GlaxoWellcome) haemagglutinin would seem to be in the UK; it was first marketed in a good drug target, in some ways 1999, a year before GSK was formed better than neuraminidase, as it acts by the merger of GlaxoWellcome frequently, influenza researchers Tamiflu (oseltamivir) at the beginning rather than the with SmithKline Beecham. ‘Glaxo have begun to explore new targets. binds to neuraminidase end of the viral life cycle. The main had only a few months’ market lead They are also re-visiting the M2 and prevents the virus difficulty with designing drugs to with zanamivir over Roche’s rival proton channel, helped again by breaking out of host cells inhibit haemagglutinin binding is compound, oseltamivir’, explains new insights into this protein’s that the haemagglutinin receptor Challand. ‘Oseltamivir is much more structure. Several independent binding domain (RBD), which is the prone to developing resistance, but groups have determined atomic obvious target of such molecules, it is currently the market leader resolution structures of M2, but the differs widely between serotypes. because it is orally available [whereas results of these have often proved ‘We cannot expect a drug designed zanamivir must be inhaled].’ inconclusive, particularly in relation to target the haemagglutinin RBD Certainly when, in 2009, many to drug binding. Writing recently in a of one influenza strain to be active governments stockpiled large commentary in Science, Bill DeGrado against other strains’, says Dong quantities of drugs in advance of the of the University of Pennsylvania, US, Xu, assistant professor of chemistry predicted pandemic, oseltamivir claimed that the debate was ‘settling at Boise State University, Idaho, was by far the more popular choice. in favour of a pharmacologically US. Using molecular dynamics Several papers published since relevant [amantadine binding] site simulations, Xu has discovered that the pandemic have documented in the M2 channel pore’, but that all RBDs bind to a common ‘fish- increasing resistance to it. A third there was still controversy about, hook like’ shape on the receptor neuraminidase inhibitor, , among other things, the protein’s surface. ‘We are now searching for is now being developed by BioCryst mechanism of action. compounds that mimic this glycan Pharmaceuticals in Birmingham, Mei Hong and her team at Iowa receptor shape’, he says. His group Alabama, US. State University, US, obtained the is also investigating the viability of M2 structures that resolved this part Antiviral drugs (left) designing molecules to target other Widening the net of the controversy. ‘We know that in and vaccines (right) are sites on haemagglutinin. With resistance to neuraminidase most currently circulating influenza equally important in the So far, it would be safe to say of inhibitors being reported more viruses, one serine residue has been fight against the flu both recent influenza outbreaks, H5N1 ‘avian’ and H1N1 ‘swine’ , that their bark has been ROCHE worse than their bite – at least at population level. Yet pandemics of highly pathogenic influenza remain a serious threat. If the increased media prominence of influenza has inspired any of the innovative work in drug and vaccine discovery currently underway, that is all to the good. Another positive legacy of the avian flu scare has recently been highlighted by Capua: increased cooperation between researchers into human and veterinary infectious disease.

Clare Sansom is a freelance science

writer based in Cambridge, UK LIBRARY PHOTO SCIENCE / IMAGE TEK

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