Will mutant strains hurt prospects for Roche's flu-fighting newcomer Xofluza?

Xofluza, Roche’s bid to replace sales from the now-generic Tamiflu, has yet to catch on in the U.S., and new reports of viral resistance in Japan won’t help the drug’s case.

Some doctors in the country have slowed Xofluza prescribing in light of new evidence that mutant viral flu strains can stop the drug from working as intended in some patients, The Wall Street Journal reported. Tokyo’s National Institute of Infectious Diseases had ID’d six resistant strains as of Feb. 6.

As the institute noted, resistance issues cropped up in clinical trials for developer Shionogi; nearly 25% of children in late-stage trials had resistant strains, meaning the problem isn’t a new one.

But it doesn’t bode well for Roche, which is looking to drive Xofluza growth in the face of copycats to predecessor Tamiflu. The new drug, which runs $150 for patients without insurance, has so far seen limited uptake thanks in part to its price; it generated just $13 million in the fourth quarter after snagging its FDA OK in October.

“Xofluza is very expensive and was just approved, so not many people are taking it,” Susan Wilder, M.D., an Arizona-based family doctor, told the WSJ.

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Of course, Tamiflu is not without its own resistance issues. Xofluza is “extremely promising, especially in treating strains of flu that are resistant to Tamiflu,” Wilder said, adding that “Xofluza has a wider range of efficacy than other available flu drugs.”

But some doctors worry Xofluza will lose its edge if it gets too much play this year.

“There is the possibility that Xofluza will not be effective in the next flu season if it is widely used now, as this will cause drug-resistant viruses to spread quickly,” Naoto Hosokawa, head of the infectious-disease department at Kameda Medical Center near Tokyo, told the WSJ.

Meanwhile, Roche, for its part, pointed out that Xofluza still has some utility in mutant strains. While they showed “reduced susceptibility” to Xofluza, the drug still had some effect on them—meaning they’re not resistant by U.S. authorities’ standards. Further study is needed to determine the scope of the problem and the transmissibility of the strains in question, and a global study of how Xofluza affects flu spread is beginning this year, the company told the Journal.