A drug already approved to treat covid-19 in some countries is looking even more impressive. Research out this week shows that the medication, called ensitrelvir, can effectively prevent people from catching the viral disease, too.
Scientists in the U.S., Japan, and U.K. conducted a phase III trial of the drug. People who took ensitrelvir (pronounced en-SIH-trel-veer) were substantially less likely to contract covid-19 from someone in their home compared to a placebo, the trial found. The drug’s makers have submitted it to the U.S. Food and Drug Administration for approval as a prophylactic for covid-19, with a decision expected in the next few months.
“In this trial, the prompt initiation of oral ensitrelvir postexposure prophylaxis was effective in preventing Covid-19 in household contacts, including those with risk factors for severe disease, without apparent safety concerns,” the researchers wrote in their paper, published Wednesday in the New England Journal of Medicine.
A newer generation of antiviral
Ensitrelvir was developed by the Japan-based pharmaceutical Shionogi & Co. It’s designed to inhibit the main protease—an enzyme that breaks down proteins—found in coronaviruses, including SARS-CoV-2, the cause of covid-19. By doing so, it hinders the virus’ ability to make more of itself.
There is an existing medication that targets this same protease: nirmatrelvir. It’s part of the current standard antiviral treatment for covid-19 called Paxlovid, which combines nirmatrelvir with the HIV drug ritonavir (ritonavir helps nirmatrelvir stay in a person’s system longer). Though Paxlovid was effective at preventing severe illness and death from covid-19 early in the pandemic, especially for unvaccinated people, its effectiveness has sharply dropped among those vaccinated today. Studies have also failed to show that Paxlovid can prevent covid-19 in people at high risk of exposure.
Ensitrelvir appears to be a step up from nirmatrelvir. It’s already been approved as an treatment for covid-19 in both Japan and Singapore, under the brand name Xocova. Animal studies had suggested that it could be an effective prophylactic as well, leading to the current study.
The double-blinded, randomized, and controlled trial took place between June 2023 and September 2024. It involved roughly 2,000 volunteers in five countries, including the U.S. Volunteers were people who lived in households with a sick person who recently tested positive for covid-19 but had tested negative themselves. They had to be enrolled within 72 hours of their housemate experiencing symptoms. People were randomly assigned to take a once-daily pill of ensitrelvir for five days or a placebo, and their infection status was tracked for ten days.
By the study’s end, 2.9% of people on ensitrelvir had developed covid-19, compared to 9% of the placebo group. The level of adverse events between the two groups was similar as well, supporting the drug’s relative safety and tolerance.
What happens next
Covid-19 obviously isn’t as huge a threat today as it once was, but it’s still sickening and killing a sizeable number of people every year. So simply having another reliable antiviral that can treat and prevent these infections, especially among high-risk groups, is more than worthwhile. A major epidemic or pandemic will also inevitably happen again, and coronaviruses remain high on the list of potential candidates for causing one. So unlike last time, drugs like ensitrelvir will ideally give us a better advantage against such a threat right from the get-go.
Based on these results, Japan already extended its approval of ensitrelvir as a prophylactic earlier this March, and Shionogi has since submitted the drug in the U.S. The FDA is scheduled to issue its decision by this June.
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