Influenza viruses can become resistant to antiviral drugs by undergoing mutations, and a drug-resistant virus combined with a pandemic could kill millions of people. Masafumi Inoue at the A*STAR Experimental Therapeutics Centre, Timothy Barkham from Tan Tock Seng Hospital in Singapore and co-workers have identified a strain of the influenza A virus subtype H1N1 that developed resistance to the drug oseltamivir more quickly than any case reported previously.
Oseltamivir is an antiviral drug commonly used to fight severe influenza infections. It prevents the spread of viruses by blocking the activity of neuraminidase, a protein found on the surface of influenza viruses that allows them to break away from infected cells. The researchers investigated an H1N1 influenza virus strain that had undergone mutation and substituted the amino acid histidine at position 275 with another amino acid. They found that the mutant was resistant to the effects of oseltamivir and continued to replicate despite treatment.
“The actual mutation occurs not because of the drug but by chance,” explains Inoue. “However, the drug kills off all viruses except those that are resistant, thereby enriching the population of resistant viruses.”
The researchers sequenced respiratory samples from a female patient in Singapore who came down with flu symptoms soon after a trip to Hawaii. They treated her with oseltamivir on the fourth day of her illness, at the height of her fever. All of the pandemic H1N1 influenza virus particles found in the patient’s samples before and 14 hours after antiviral treatment contained the wild-type neuraminidase protein sequence, suggesting that the virus would respond to the antiviral drug. However, 38 hours after antiviral treatment had begun, 24% of the H1N1 influenza virus particles in her respiratory sample were the mutant virus. At 45 hours after treatment, this proportion had risen to 52%. As the patient’s wild-type and mutant viruses were otherwise quite similar, the results suggest that the mutant viruses developed within the patient from the wild-type virus that she had initially been infected with.
Oseltamivir treatment in this case was initiated at the height of the patient’s fever when H1N1 influenza viral replication—and therefore the possibility for replication mistakes—was likely at its peak. The researchers suggest that this may account for the rapid emergence of the mutant virus in this patient. “If a patient fails to respond to oseltamivir, it would be reasonable to suspect resistance, prompting the use of alternative antiviral drugs,” says Inoue. “The findings also remind us that a drug-resistant virus could emerge literally overnight.”
The A*STAR-affiliated researchers contributing to this research are from the Experimental Therapeutics Centre.