Pfizer has kicked off work on an early-stage clinical trial of an oral COVID-19 antiviral therapeutic.

The company said on Tuesday that it was progressing to multiple ascending doses after completing the dosing of single ascending doses in a phase 1 study of adults to look into the safety and tolerability of what they describe as an “investigational, novel oral antiviral therapeutic for SARS-CoV-2,” i.e. the virus that causes COVID-19. The trial is being conducted in the U.S.

Already, the oral antiviral clinical candidate has shown potent in vitro antiviral activity against SARS-CoV-2.

“Tackling the COVID-19 pandemic requires both prevention 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,” Mikael Dolsten, MD, PhD.—the Chief Scientific Officer and President of Worldwide Research, Development, and Medical at Pfizer—said in a press release. “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 disease still occurs.”

The oral antiviral clinical candidate is a protease inhibitor, which means—in short—it could prevent the virus from replicating in cells. The phase 1 trial will incorporate a randomized, double-blind, and placebo-controlled methodology.

Earlier this month, the Pfizer team announced real-world evidence showing “dramatically lower” incidence rates of COVID-19 in those fully vaccinated with the Pfizer x BioNTech vaccine.