Pfizer has begun dosing in a phase 2/3 study evaluating its oral COVID-19 antiviral PF-07321332 in nonhospitalized symptomatic patients who are not at increased risk of progressing to severe disease.

Pfizer said the mid- and late-stage trial will enroll an estimated 1,140 volunteers who will be randomized to receive either PF-07321332 plus ritonavir, an antiretroviral, or placebo every 12 hours for five days.

A protease inhibitor, PF-07321332, works by blocking an enzyme that the SARS-CoV-2 virus needs to replicate, while ritonavir helps slow the metabolic breakdown of PF-07321332 so it can remain in the body longer.