If the rest this year's flu season follows the pattern of the first round of cases, Roche's best-selling antiviral, Tamiflu (oseltamivir), may be next-to-useless against the dominant flu virus, researchers say. The good news for patients—and GlaxoSmithKline—is that the Tamiflu-resistant strain of H1N1 can be treated with GSK's Relenza (zanamivir).
Tamiflu's firm position in the influenza market weakened when news broke last week that 86 of 88 early-season flu samples tested by the Centers for Disease Control and Prevention (CDC) were resistant to Tamiflu—compared to about 11 percent of samples last year.
"Resistance to Tamiflu has historically been low and is continually monitored," Roche said in an official statement. "The reduced susceptibility observed had no apparent relationship to oseltamivir use or exposure. A spokesperson for Roche told Pharm Exec on Wednesday that - "at this time - only one currently circulating strain of influenza, A (H1N1), is showing resistance to Tamiflu. No resistance to Tamiflu has been identified among other circulating viruses [A(H3N2) and B viruses]."
A CDC advisory released in mid-December recommended GSK's Relenza as treatment for the resistant strain. Because there is no timely way to test for individual strains of flu in a clinic, Relenza may become the drug of choice this year. However, doctors and nursing homes could be less than satisfied with the alternative, as Relenza is an inhaled drug, making it difficult to administer to older and institutionalized patients—a key segment of the flu market.
"There are [several] flus in circulation and one of the strains is resistant to Tamiflu. Luckily we have other treatments," said Tom Skinner, a spokesman for the CDC. "Antiviral-resistant flu is not unheard of. Thankfully, we have a vaccine that seems a good match for this year."
Does this mean Tamiflu is on the ropes? Hardly, say experts. In the long run, CDC and physicians expect to be using Tamiflu again.
Tina Tan, associate professor of Pediatrics at the Feinberg School of Medicine at Northwestern University, said the Tamiflu news is nothing to panic about. "We know that this particular strain was a spontaneous mutation, so it may not happen again," she said. "I still think that Tamiflu is going to remain the drug of choice in most cases."
Tamiflu will still be an important part of government stockpile programs designed to guard against potential outbreaks. "No one can predict what strain will cause a pandemic, said Skinner. "[Tamiflu] still is an effective antiviral against other strains of the flu. It's still a part of our stockpile, and will be until we feel we need to reevaluate."
Even if this is a one-time incident, GlaxoSmithKline is using the gap in the market to their advantage. In a press release issued in response to the CDC's announcement, GSK assured customers that they have a sufficent supply of Relenza to meet the needs of the 2008–09 influenza season.
"Our major strategy is to make sure the pharmacies have the stock they need as people increase prescriptions of Relenza to address concerns," said Judith Ng-Cashin, vice president of Global Clinical Development for Infectious Diseases at GlaxoSmithKline. "We're committed to making sure that enough Relenza is available to the public."