Pfizer Inc. has begun clinical trials in the U.S. on an oral anti-viral drug to treat COVID-19 patients, the company said on Tuesday.

The drugmaker, which developed the first authorized COVID-19 vaccine in the U.S. with Germany's BioNTech, said the antiviral candidate showed potent activity against SARS-CoV-2, the virus that causes COVID-19, in lab studies, and could be prescribed to patients at the first sign of infection.

Pfizer's candidate, named PF-07321332, is a protease inhibitor that prevents the virus from replicating in cells. Protease inhibitors have been effective at treating other viral pathogens such as HIV and hepatitis C virus, both on their own and in combination with other antivirals, the company said.

Pfizer believes this class of molecules may provide well-tolerated treatments against COVID-19, as currently marketed therapeutics that work on the same lines have not reported safety concerns. The company is also studying an intravenously administered antiviral candidate in an early-stage trial in hospitalized COVID-19 patients.

"Together, the two (oral and intravenous candidates) have the potential to create an end-to-end treatment paradigm that complements vaccination in cases where the disease still occurs," Pfizer's Chief Medical Officer Mikael Dolsten said in a statement.

Pfizer's candidate is behind two other oral antiviral therapies, which are in mid-stage trials – the first being developed by rival Merck & Co with Ridgeback Bio, and a second from Roche Holding and Atea Pharmaceuticals.

Gilead Sciences' remdesivir is currently the only U.S. Food and Drug Administration-approved drug for the treatment of COVID-19.

The FDA has granted an emergency authorization to intravenous therapies from Eli Lilly, bamlanivimab alone and in combination with etesevimab, and a combination therapy from Regeneron.