News About Influenza B Drug Resistance That Cannot Be Ignored

During the last 2 years, the medical community has been tensely following the emergence of resistance to oseltamivir in strains of influenza A.¹ The clinical relevance and transmissibility of these resistant variants has been unknown, and planning for epidemic and pandemic influenza thus far has ignored the issue due to lack of evidence that it will be of medical consequence. Neuraminidase inhibitors are presently the only reliable antiviral option for treatment of influenza infection: the usefulness of the adamantanes (amantadine and rimantidine) has been virtually eliminated by the development of resistance.² Until now, little spontaneous resistance to neuraminidase inhibitors has been documented, and no spontaneously resistant influenza viruses were identified in the world prior to the introduction of the drugs. This may now be changing.

In this issue of JAMA, the report from Japan by Hatakeyama and colleagues³ documenting influenza B viruses with decreased sensitivity to oseltamivir and zanamivir increases concerns about the possibility and clinical significance of resistance. The article describes influenza B viruses partially resistant to neuraminidase inhibitors in individuals who had not been treated with antiviral medications and strongly suggests person-to-person transmission of these resistant viruses, either within families or in the community. Japan has the highest use of neuraminidase inhibitors of any country in the world, with more than 90% of the prescriptions being for oseltamivir.^{4 - 5} While the mutations reported in the resistant isolates in the report are mainly those previously associated with selective pressure from oseltamivir, it is not certain whether the mutated viruses came from other treated individuals in the community or if they emerged spontaneously. The emergence of the variants having low-level resistance discussed in the study is also of concern because influenza B has lower baseline sensitivity to neuraminidase inhibitors compared with influenza A, and the presence of low-level resistance sets the stage for selective pressure for development of high-level resistance.

In Australia, 2 influenza viruses with mutations conferring high-level resistance to neuraminidase inhibitors have been isolated from untreated patients. One of these viruses had a mutation in the neuraminidase^{6 - 7} that is known to confer high-level oseltamivir-specific resistance. Since there is little use of neuraminidase inhibitors in Australia,⁴ these viruses likely both arose due to spontaneous mutations.

Part of the complacency about neuraminidase inhibitor–resistant influenza has been fueled by experiments in vitro and in animal models that have generally found neuraminidase inhibitor–resistant influenza viruses to be compromised in infectivity and transmissibility. This has led to the belief that significant transmission is unlikely to occur among humans. A 2003 model of the spread of resistant viruses in a pandemic concluded that community spread of such variants would be negligible because of decreases in biological fitness and transmissibility of drug-resistant viruses.⁸ However, a recent report predicted that, while use of antiviral drugs could significantly slow or stop transmission in the absence of drug resistance, the emergence of resistance could seriously diminish this benefit,⁹ depending on the cost of biological fitness: if there were a modest biological fitness cost and transmissibility were high, the effectiveness of antiviral use would plummet. Even if resistant strains emerge de novo at extremely low frequencies in individuals receiving antiviral drugs for treatment or prophylaxis, these strains may well make a significant contribution in an epidemic or pandemic setting. These concepts, together with the findings reported by Hatakeyama et al³ suggesting that partially-resistant influenza B viruses are circulating, mean it is no longer possible to be confident that resistant strains will have little effect on epidemic or pandemic influenza.

Resistance to oseltamivir has been predicted by in vitro and structural studies to be more likely to arise than resistance to zanamivir under the selective pressure of drug treatment.^{1 ,10} Zanamivir is more structurally similar to the natural substrate of neuraminidase and fits directly into the active site. For oseltamivir, a rearrangement of amino acids in the active site is necessary to accommodate the drug's hydrophobic side chain; mutations that prevent this rearrangement might lead to resistance to oseltamivir but not to zanamivir. It has also been proposed that oseltamivir-resistance mutations that do not affect the catalytic residues are less likely than zanamivir-resistance mutations to have a severe impact on fitness.¹¹ The third, newer, as-yet unlicensed, neuraminidase inhibitor peramivir, like oseltamivir, requires reorientation of amino acids to bind the active site.¹² It also shares a key structural feature of zanamivir, ie, substitution of a guanidinium group at the 4′ position. Therefore, many mutations that confer resistance to oseltamivir or zanamivir also confer resistance to peramivir, and this compound is unlikely to be of significant benefit in the presence of resistance to oseltamivir and zanamivir.

Zanamivir binds more strongly than oseltamivir in the active site of the influenza B virus neuraminidase,^{13 - 14} and differences in the affinity of drug binding may impact clinical efficacy. For oseltamivir to fit into the active site of influenza B neuraminidase, larger conformational changes are needed than those required for influenza A.¹⁴ In young children, oseltamivir is much less effective as a treatment for influenza B than for influenza A virus infection. Sugaya et al¹⁵ recently suggested that use of oseltamivir against influenza B infection in young children should be reconsidered and proposed the options of either increasing the dosage of oseltamivir for influenza B or using zanamivir.

Are these resistant influenza B variants transmissible? The evidence suggests that viruses with reduced sensitivity to drug were transmitted from person to person. If the resistant influenza B viruses in the study by Hatakeyama et al³ were indeed transmitted among family members, it may be significant that transmission took 6 to 9 days for the viruses with a D198N mutation in the neuraminidase (compared to 1-4 days for wild-type viruses) and 7 days for the viruses with an I222T mutation in the neuraminidase. Perhaps these viruses are somewhat less infectious and take longer to reach titers high enough to cause symptoms. If this is the case, then the observed transmission among family members would mirror the experimental transmission of the virus with a H274Y neuraminidase mutation in ferrets.¹⁶ In these particular animal transmission experiments, it took more variant than wild-type virus to infect the animals, and spread to the contact animals was slower, but the virus ultimately reached the same titers in the contact animals.¹⁶ Even if replication takes longer and symptoms in individuals are delayed, if the disease is eventually just as severe then the mutant viruses can be considered pathogenic and transmissible.

The report by Hatakeyama et al³ raises more questions than it answers, including questions about viral evolution, biological fitness, and transmissibility. But some facts are strikingly clear. Influenza B mutants with reduced sensitivity to neuraminidase inhibitors are circulating, and these viruses can cause infections with no difference in duration of symptoms, level of viral shedding, or clinical outcome. Contrary to what had been hoped until now, some resistant variants are vigorous pathogens. Whether these viruses arise by spontaneous mutation or through drug selection, or whether they are transmitted within families or acquired from the community, the resistant variants may be here to stay. In light of the recent observation that oseltamivir may be less effective against influenza B than against influenza A,^{15 ,17} an important concern is whether suboptimal dosing for these viruses will lead to increased selection of viruses with high-level resistance.

Influenza viruses evolve rapidly and nimbly, which compels ongoing investigation of antiviral therapies that use alternative mechanisms of action and target different points in the viral life cycle. The emergence of drug-resistant influenza B should draw attention to the importance of continual monitoring of strains over time and to the need for frequent rethinking of policies for use of antiviral drugs. While the news about resistance is not good and certainly calls into question some of the current assumptions about drug-resistant viruses, an effective response to this news can help contend with the new challenges of influenza.