Pfizer has said that it has begun early-stage clinical trials of an investigational, oral antiviral drug for Covid-19.

The company on Tuesday (local time) said that the candidate "has demonstrated potent in vitro antiviral activity" against the virus that causes COVID-19, as well as activity against other coronaviruses, suggesting the potential for use to address future threats. This Phase 1 trial is being conducted in the United States. As per The Hill, the candidate is a class of drug known as a protease inhibitor, which has been long used to treat HIV and hepatitis C. The drugs work by blocking a critical enzyme, a protease, that the virus needs to replicate.

"Tackling the COVID-19 pandemic requires both preventions via vaccine and targeted treatment for those who contract the virus. 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, Chief Scientific Officer and President, Worldwide Research, Development and Medical of Pfizer in a statement.

Dolsten said the drug candidate could be prescribed at the first sign of infection, without requiring that patients are hospitalized or in critical care.

"We have designed PF-07321332 as a potential oral therapy that could be prescribed at the first sign of infection, without requiring that patients are hospitalized or in critical care. At the same time, Pfizer's intravenous antiviral candidate is a potential novel treatment option for hospitalized patients. Together, the two have the potential to create an end-to-end treatment paradigm that complements vaccination in cases where the disease still occurs," Dolsten added.

The company said that protease inhibitors bind to a viral enzyme (called a protease), preventing the virus from replicating in the cell. Protease inhibitors have been effective at treating other viral pathogens such as HIV and hepatitis C virus, both alone and in combination with other antivirals. Currently marketed therapeutics that target viral proteases are not generally associated with toxicity and as such, this class of molecules may potentially provide well-tolerated treatments against COVID-19.

The Phase 1 trial is a randomized, double-blind, sponsor-open, placebo-controlled, single- and multiple-dose escalation study in healthy adults evaluating the safety, tolerability and pharmacokinetics of PF-07321332.

The initiation of this study is supported by preclinical studies that demonstrated the antiviral activity of this potential first-in-class SARS-CoV-2 therapeutic designed specifically to inhibit replication of the SARS-CoV2 virus.

The structure of PF-07321332, together with the pre-clinical data, will be shared in a COVID-19 session of the Spring American Chemical Society meeting on April 6.

Pfizer is also testing an intravenously administered investigational protease inhibitor for patients who are hospitalized with COVID-19. Gilead Sciences' remdesivir is currently the only Food and Drug Administration-approved drug for the treatment of COVID-19.

According to The Hill, the FDA has granted emergency authorization to two antibody therapies, from Regeneron and Eli Lilly, respectively. Health experts state that there needs to be a range of treatments, not just vaccines, in order to "completely end the pandemic."