AVI 4773 for COVID-19

The main protease (MPro) of SARS-CoV-2 is crucial for viral replication and is the target of nirmatrelvir (the active ingredient of Paxlovid) and ensitrelvir. The identification of new agents with differentiated pharmacokinetic and drug resistance profiles will increase therapeutic options for COVID-19 patients and bolster pandemic preparedness generally. Starting with a lead-like dihydrouracil chemotype from a large-library docking campaign, we improved MPro inhibition >1,000 fold by engaging additional subsites in the MPro active site, most notably by employing a latent propargyl electrophile to engage the catalytic Cys145. Advanced leads from this series, including AVI 4516 and AVI 4773 show pan-coronavirus antiviral activity in cells, very low clearance in mice, and for AVI 4773 a rapid reduction in viral titers more than a million fold after just three doses, more rapidly and effectively than the approved drugs, nirmatrelvir and ensitrelvir. Both AVI 4516 and AVI 4773 are well distributed in mouse tissues, including brain, where concentrations ten or fifteen-thousand times the EC90, respectively, are observed eight hours after an oral dose. As exemplar of the series, AVI 4516 shows minimal inhibition of major CYP isoforms and human cysteine and serine proteases, likely due to its latent electrophilic warhead. AVI 4516 also exhibits synergy in cellular infection models in combination with the RdRp inhibitor molnupiravir, while related analogs strongly inhibit nirmatrelvir-resistant MPro mutant virus in cells. The in vivo and antiviral properties of this new chemotype are differentiated from existing clinical and pre-clinical MPro inhibitors, and will advance new therapeutic development against emerging SARS-CoV-2 variants and other coronaviruses.