Since the antiviral agent oseltamivir (Tamiflu) has been touted as the global savior should a bird flu pandemic materialize the idea has been haunted by the specter of Tamiflu resistance. What if H5N1 becomes resistant to the drug? Is all lost? Now it is being reported in the media that the European Centre for Disease Control and Prevention has found that the predominant circulating seasonal flu virus in Europe this year, H1N1, is showing an unexpectedly high rate of Tamiflu resistance (19/148 isolates tested). This is much more than what has been seen in the past and was from patients not treated with the drug, leading to the fear that a mutation conferring resistance also has some survival advantage leading to this sudden emergence and spread. US CDC data reported in the same news article suggests a much lower (2.2%), but still appreciable level of the same resistance associated mutation in the US during this flu season. This sounds like pretty bad news, and it may very well be, but as with everything connected with H5N1 and influenza in general, things are more complicated than they appear at first.
Consider what is meant by “Tamiflu resistance” in these reports. As best I can make out from the Reuters report the resistance was signaled by finding a resistance-associated mutation, H274Y, in the viral isolates (more on the mutations associated with drug resistance here). This particular mutation is thought to interfere with a necessary change in the place where the drug binds to the viral neuraminidase (NA) enzyme. NA is one of two major proteins on the surface of the virus, the other being hemagglutinin (HA). HA is the key player in viral entrance to host cell. Once inside the cell the virus hijacks the host cell’s protein making machinery to make copies of itself. These copies than bud off the surface of the host cell to infect other cells. But in order to cut loose from the surface of the host cell it needs its other protein, NA, which frees it by slicing a connection to the cell’s sialic acid tipped glycan (see our posts here on the subject). Tamiflu gets into a key region of the NA and prevents it from letting the virus shed from the host cell. Tamiflu is a pill. Another neuraminidase inhibitor, zanamivir (trade name Relenza) only is active in inhaled form.
Now things start to get a little more complicated. The NA protein comes in 9 different forms. Humans are routinely infected with the N1 and N2 forms, and these aren’t the same with respect to Tamiflu. The drugs were designed to work against the N2 form, which was the one where we had x-ray crystallography structures. Recently the N1 form was crystalized, permitting an investigation of the differences in how they react to these drugs. For Tamiflu to interfere with NA action it needs to nestle into a pocket. NA can change shape and for Tamiflu, but not Relenza, to work it needs to nestle into a pocket that forms during that shape change. It is thought the H274Y mutation causes Tamiflu resistance by preventing the formation of the pocket (the binding of Tamiflu is produced by rotating an amino acid near position 274 and its binding to another amino acid at position 224; it’s complicated. See Moscona’s Perspective in the New England Journal for more explanation).
All of this was predicted by analyses of the molecular structure of oseltamivir, zanaivir and neuraminidase. But those predictions were also based on snapshots of structures. New work by Landon et al. and Russell reveals the N1 version is more flexible than thought and that once bound the shape can change from a form similar to N2 behavior to a more open form in N1. Thus the tidy story may not be so tidy after all, or at least our explanations need some modification. The claim of increased resistance to oseltamivir in H1N1 is based on the appearance of H274Y, not yet on the demonstration that H274Y is of clinical importance in the treatment of influenza. Is there evidence that would mitigate its importance? Not much, but there is some. There is a body of work to show that the viruses that carry H274Y are not as fit as the ones that don’t. Unfortunately some of this published work comes from the maker, Roche, so there will be well founded suspicion (this is a burden Roche scientists must bear as a result of the behavior of their employer and the whole industry of which it is a part). What that work seems to show (subject to the qualifications just stated) is that H1N1 carrying H274Y replicates a thousand times less well than the unmutated version and that its ability to cause disease in the ferret model (thought to mimic human infection) is also significantly compromised (see Ives et al.). There are little clinical data shedding light on the efficacy of Tamiflu in H274Y carrying H5N1 viruses, although the mutation has been found there in some patients. A paper by de Jong et al. in 2005 described two fatal cases of H5N1 in Vietnamese patients where H274Y viruses replicated to produce moderate to high viral loads even during timely Tamiflu treatment.
An additional argument against the claim that H274Y mutants are less fit is the fact that they are showing up in Europe where use of the drug for treatment of flu is much less common than in Japan where the mutant has been previously seen. This has led to the speculation the mutation might be conferring some survival advantage. However there has also been major and sudden increases in resistance to the older influenza antivirals, Amantadine and Ramantadine (they work by a different mechanism) and this also has happened without much selective pressure. The thought has been that they are hitchhiking (are linked with) other changes that are important to survival and that the resistance is not itself causing incrased replicability or transmissibility of the virus.
So is this new finding of an increased prevalence of H274Y mutations in H1N1 of epidemiological and clinical importance? It certainly could be but we can argue this both ways. Roche obviously doesn’t want this to be the case, nor, for that matter, does anyone else. Now is the time to do some clinical and epidemiological studies on patients infected with H274Y H1N1 in Europe. You would think that is happening.
But often what you would think and what is actually happening are two different things. I hope that is not the case here.