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The Pfizer facility in Pleasant Prairie, Wisconsin, on December 1, 2020. - Awaiting approval from the Federal Drug Administration, Pfizer is reported to be utilizing cold storage to hold the Covid-19 vaccine at their distribution site in Pleasant Prairie. Image Credit: AFP

Pfizer Inc. said it has begun human safety testing of a new pill to treat the coronavirus that could be used at the first sign of illness.

If it succeeds in trials, the pill could be prescribed early on in an infection to block viral replication before patients get very sick. The drug binds to an enzyme called a protease to keep the virus from replicating. Protease-inhibiting medicines have been successful in treating other types of viruses, include HIV and Hepatitis C.

"Given the way that SARS-CoV-2 is mutating and the continued global impact of COVID-19, it appears likely that it will be critical to have access to therapeutic options both now and beyond the pandemic," said Mikael Dolsten, Pfizer's Chief Scientific Officer, in a statement.

The new protease inhibitor is the second such medicine Pfizer has brought into human trials to treat COVID-19. Pfizer is testing another given intravenously to hospitalized virus patients.

Easy-to-use treatments are lacking for early-stage COVID-19 patients. While antibody therapies from Eli Lilly & Co. and Regeneron Pharmaceuticals Inc. are authorized in the US for COVID patients who haven't yet been hospitalized but are at high risk of developing severe symptoms, they must be infused in the hospital or at a doctor's office.

That has created logistical challenges that have limited their use. Other therapies are intended for sicker people: Gilead Sciences Inc.'s antiviral drug remdesivir must be infused over several days and is approved only for hospitalized patients.

Among major drugmakers, Merck & Co. has one of the few coronavirus pills that is far along in human testing. Its experimental antiviral drug molnupiravir works by a different mechanism than the Pfizer drug and is in late-stage human trials.

In the test tube, Pfizer's oral protease inhibitor, code-named PF-07321332, had potent antiviral activity both against both SARS-CoV-2 and other coronaviruses, Pfizer said. The company said it would share more data on the compound at the American Chemical Society meeting on April 6.