Pfizer Inc said on Monday 27 september it has started a large study testing its investigational oral antiviral drug for the prevention of COVID-19 infection among those who have been exposed to the virus.

The drugmaker and its rivals, including U.S.-based Merck & Co Inc and Swiss pharmaceutical Roche Holding AG, have been racing to develop an easy-to-administer antiviral pill for COVID-19.

The mid-to-late-stage study will test Pfizer's drug, PF-07321332, in up to 2,660 healthy adult participants aged 18 and older who live in the same household as an individual with a confirmed symptomatic COVID-19 infection.

In the trial, PF-07321332, designed to block the activity of a key enzyme needed for the coronavirus to multiply, will be administered along with a low dose of ritonavir, an older medication widely used in combination treatments for HIV infection.

"With the continued impact of COVID-19 around the world, we believe that tackling the virus will require effective treatments for people who contract, or have been exposed to, the virus, complementing the impact that vaccines have had in helping quell infections. If successful, we believe this therapy could help stop the virus early - before it has had a chance to replicate extensively - potentially preventing symptomatic disease in those who have been exposed and inhibiting the onset of infection in others," said Mikael Dolsten, MD, PhD., Chief Scientific Officer and President, Worldwide Research, Development and Medical of Pfizer. "Given the continued emergence and evolution of SARS-CoV-2 variants and their immense impact, we continue to work diligently to develop and study new ways that our investigational oral antiviral candidate could potentially lower the impact of COVID-19, not only on patients' lives, but also the lives of their families and household members."

Pfizer has also started another study of PF-07321332 in non-hospitalized, symptomatic adult patients.

Gilead Sciences Inc's intravenous drug Veklury, known generically as remdesivir, is the only approved antiviral treatment for COVID-19 in the United States.

Merck said its new trial will study experimental drug molnupiravir for the prevention of COVID-19 among adults in the same household as someone diagnosed with symptomatic coronavirus infection. Merck and partner Ridgeback Biotherapeutics are already conducting a late-stage trial of the treatment in non-hospitalized patients to see if it reduces the risk of hospitalization or death.

Molnupiravir is a type of antiviral designed to introduce errors into the RNA of the virus that eventually prevent it from replicating.

Merck said in June that the U.S. government agreed to pay about $1.2 billion for 1.7 million courses of molnupiravir, if it is proven to work and authorized by regulators. The company said it expected to file for U.S. emergency use authorization of molnupiravir in the second half of 2021 at the earliest.

Roche and partner Atea Pharmaceuticals in June said early data from a trial of their experimental oral antiviral AT-527 showed that it lowered viral load in hospitalized patients.

RELATED Could a simple pill beat COVID-19? Pfizer is giving it a go

---