The SARS-CoV-2 main protease (Mpro) is a cysteine protease and a validated antiviral drug target. Paxlovid is an FDA-approved oral COVID-19 antiviral that contains the Mpro inhibitor nirmatrelvir and the metabolic booster ritonavir. The emergence of SARS-CoV-2 variants mutations in the Mpro raised the alarm of potential drug resistance. In this study, we aim to discover Mpro drug resistant mutants from naturally observed polymorphisms. Through analyzing the SARS-CoV-2 sequences deposited in Global initiative on Sharing Avian influenza Data (GISAID) database, we identified 66 prevalent Mpro mutations located at the nirmatrelvir binding site. The Mpro mutant proteins were expressed and characterized for enzymatic activity and nirmatrelvir inhibition. While the majority of the Mpro mutants had reduced enzymatic activity (kcat/Km >10-fold decrease), 11 mutants including S144M/F/A/G/Y, M165T, E166Q, H172Q/F, and Q192T/S/V showed comparable enzymatic activity as the wild-type (kcat/Km <10-fold change) and resistance to nirmatrelvir (Ki > 10-fold increase). We further demonstrate that the enzymatic activity and inhibitor resistance of these single mutations can be enhanced by additional substitutions in a double mutant. X-ray crystal structures were determined for six of the single mutants with and/or without GC-376/nirmatrelvir. The structures illustrate how mutations can reduce ligand binding by impacting the conformational stability of the active site. Overall, our study identified several drug resistant hot spots that warrant close monitoring for possible clinical evidence of Paxlovid resistance.

The main protease (M pro ) is a validated antiviral drug target of SARS-CoV-2. A number of M pro inhibitors have now advanced to animal model study and human clinical trials. However, one issue yet to be addressed is the target selectivity over host proteases such as cathepsin L. In this study we describe the rational design of covalent SARS-CoV-2 M pro inhibitors with novel cysteine reactive warheads including dichloroacetamide, dibromoacetamide, tribromoacetamide, 2-bromo-2,2-dichloroacetamide, and 2-chloro-2,2-dibromoacetamide. The promising lead candidates Jun9-62-2R (dichloroacetamide) and Jun9-88-6R (tribromoacetamide) had not only potent enzymatic inhibition and antiviral activity but also significantly improved target specificity over caplain and cathepsins. Compared to GC-376 , these new compounds did not inhibit the host cysteine proteases including calpain I, cathepsin B, cathepsin K, cathepsin L, and caspase-3. To the best of our knowledge, they are among the most selective covalent M pro inhibitors reported thus far. The cocrystal structures of SARS-CoV-2 M pro with Jun9-62-2R and Jun9-57-3R reaffirmed our design hypothesis, showing that both compounds form a covalent adduct with the catalytic C145. Overall, these novel compounds represent valuable chemical probes for target validation and drug candidates for further development as SARS-CoV-2 antivirals.

No abstract

SARS-CoV-2 is the causative agent of the COVID-19 pandemic. The approval of vaccines and small-molecule antivirals is vital in combating the pandemic. The viral polymerase inhibitors remdesivir and molnupiravir and the viral main protease inhibitor nirmatrelvir/ritonavir have been approved by the U.S. FDA. However, the emergence of variants of concern/interest calls for additional antivirals with novel mechanisms of action. The SARS-CoV-2 papain-like protease (PL pro ) mediates the cleavage of viral polyprotein and modulates the host’s innate immune response upon viral infection, rendering it a promising antiviral drug target. This Perspective highlights major achievements in structure-based design and high-throughput screening of SARS-CoV-2 PL pro inhibitors since the beginning of the pandemic. Encouraging progress includes the design of non-covalent PL pro inhibitors with favorable pharmacokinetic properties and the first-in-class covalent PL pro inhibitors. In addition, we offer our opinion on the knowledge gaps that need to be filled to advance PL pro inhibitors to the clinic.

The SARS-CoV-2 main protease (Mpro) is the drug target of Pfizer’s oral drug nirmatrelvir. The emergence of SARS-CoV-2 variants with mutations in Mpro raised the alarm of potential drug resistance. To identify potential clinically relevant drug-resistant mutants, we systematically characterized 102 naturally occurring Mpro mutants located at 12 residues at the nirmatrelvir-binding site, among which 22 mutations in 5 residues, including S144M/F/A/G/Y, M165T, E166 V/G/A, H172Q/F, and Q192T/S/L/A/I/P/H/V/W/C/F, showed comparable enzymatic activity to the wild-type (k cat/K m < 10-fold change) while being resistant to nirmatrelvir (K i > 10-fold increase). X-ray crystal structures were determined for six representative mutants with and/or without GC-376/nirmatrelvir. Using recombinant SARS-CoV-2 viruses generated from reverse genetics, we confirmed the drug resistance in the antiviral assay and showed that Mpro mutants with reduced enzymatic activity had attenuated viral replication. Overall, our study identified several drug-resistant hotspots in Mpro that warrant close monitoring for possible clinical evidence of nirmatrelvir resistance, some of which have already emerged in independent viral passage assays conducted by others.

Conspectus SARS-CoV-2 is the etiological pathogen of the COVID-19 pandemic, which led to more than 6.5 million deaths since the beginning of the outbreak in December 2019. The unprecedented disruption of social life and public health caused by COVID-19 calls for fast-track development of diagnostic kits, vaccines, and antiviral drugs. Small molecule antivirals are essential complements of vaccines and can be used for the treatment of SARS-CoV-2 infections. Currently, there are three FDA-approved antiviral drugs, remdesivir, molnupiravir, and paxlovid. Given the moderate clinical efficacy of remdesivir and molnupiravir, the drug–drug interaction of paxlovid, and the emergence of SARS-CoV-2 variants with potential drug-resistant mutations, there is a pressing need for additional antivirals to combat current and future coronavirus outbreaks. In this Account, we describe our efforts in developing covalent and noncovalent main protease (M pro ) inhibitors and the identification of nirmatrelvir-resistant mutants. We initially discovered GC376, calpain inhibitors II and XII, and boceprevir as dual inhibitors of M pro and host cathepsin L from a screening of a protease inhibitor library. Given the controversy of targeting cathepsin L, we subsequently shifted the focus to designing M pro -specific inhibitors. Specifically, guided by the X-ray crystal structures of these initial hits, we designed noncovalent M pro inhibitors such as Jun8-76-3R that are highly selective toward M pro over host cathepsin L. Using the same scaffold, we also designed covalent M pro inhibitors with novel cysteine reactive warheads containing di- and trihaloacetamides, which similarly had high target specificity. In parallel to our drug discovery efforts, we developed the cell-based FlipGFP M pro assay to characterize the cellular target engagement of our rationally designed M pro inhibitors. The FlipGFP assay was also applied to validate the structurally disparate M pro inhibitors reported in the literature. Lastly, we introduce recent progress in identifying naturally occurring M pro mutants that are resistant to nirmatrelvir from genome mining of the nsp5 sequences deposited in the GISAID database. Collectively, the covalent and noncovalent M pro inhibitors and the nirmatrelvir-resistant hot spot residues from our studies provide insightful guidance for future work aimed at developing orally bioavailable M pro inhibitors that do not have overlapping resistance profile with nirmatrelvir.

The COVID-19 pandemic spurred a broad interest in antiviral drug discovery. The SARS-CoV-2 main protease (M pro ) and papain-like protease (PL pro ) are attractive antiviral drug targets given their vital roles in viral replication and modulation of host immune response. Structurally disparate compounds were reported as M pro and PL pro inhibitors from either drug repurposing or rational design. Two polyphenols dieckol and 1,2,3,4,6-pentagalloylglucose (PGG) were recently reported as SARS-CoV-2 M pro inhibitors. With our continuous interest in studying the mechanism of inhibition and resistance of M pro inhibitors, we report herein our independent validation/invalidation of these two natural products. Our FRET-based enzymatic assay showed that neither dieckol nor PGG inhibited SARS-CoV-2 M pro (IC 50 > 20 µM), which is in contrary to previous reports. Serendipitously, PGG was found to inhibit the SARS-CoV-2 PL pro with an IC 50 of 3.90 µM. The binding of PGG to PL pro was further confirmed in the thermal shift assay. However, PGG was cytotoxic in 293T-ACE2 cells (CC 50 = 7.7 µM), so its intracellular PL pro inhibitory activity could not be quantified by the cell-based Flip-GFP PL pro assay. In addition, we also invalidated ebselen, disulfiram, carmofur, PX12, and tideglusib as SARS-CoV-2 PL pro inhibitors using the Flip-GFP assay. Overall, our results call for stringent hit validation, and the serendipitous discovery of PGG as a putative PL pro inhibitor might worth further pursuing. Graphical abstract

No abstract