Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of the COVID-19 pandemic. The coronavirus 3-chymotrypsin-like protease (3CLpro) controls virus replication and is therefore considered a major target and promising opportunity for rational-based antiviral discovery with direct acting agents. Here we review first-generation SARS-CoV-2 3CLpro inhibitors PF-07304814, GC-376, and CDI-45205 that are being delivered either by injection or inhalation due to their low intrinsic oral bioavailability. In addition, PF-07321332 is now emerging as a promising second-generation clinical candidate for oral delivery. A key challenge to the development of novel 3CLpro inhibitors is the poor understanding of the predictive value of in vitro potency toward clinical efficacy, an issue complicated by the involvement of host proteases in virus entry. Further preclinical and clinical validation will be key to establishing 3CLpro inhibitors as a bona fide class for future SARS-CoV-2 therapeutics for both hospitalized and outpatient populations.
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