Development of the Safe and Broad-Spectrum Aldehyde and Ketoamide Mpro inhibitors Derived from the Constrained α, γ-AA Peptide Scaffold.

SARS-CoV-2 is still wreaking havoc all over the world with surging morbidity and high mortality. The main protease (Mpro ) is essential in the replication of SARS-CoV-2, enabling itself an active target for antiviral development. Herein, we reported the design and synthesis of a new class of peptidomimetics-constrained α, γ-AA peptides, based on which a series of aldehyde and ketoamide inhibitors of the Mpro of SARS-CoV-2 were prepared. The lead compounds showed excellent inhibitory activity in the FRET-based Mpro enzymatic assay not only for the Mpro of SARS-CoV-2 but also for SARS-CoV and MERS-CoV, along with HCoVs like HCoV-OC43, HCoV-229E, HCoV-NL63 and HKU1. The X-ray crystallographic results demonstrated that our compounds form a covalent bond with the catalytic Cys145. They also demonstrated effective antiviral activity against live SARS-CoV-2. Overall, the results suggest that α, γ-AA peptide could be a promising molecular scaffold in designing novel Mpro inhibitors of SARS-CoV-2 and other coronaviruses.

Validation and Invalidation of SARS-CoV-2 Papain-like Protease Inhibitors.

SARS-CoV-2 encodes two viral cysteine proteases, the main protease (Mpro) and the papain-like protease (PLpro), both of which are validated antiviral drug targets. PLpro is involved in the cleavage of viral polyproteins as well as immune modulation by removing ubiquitin and interferon-stimulated gene product 15 (ISG15) from host proteins. Therefore, targeting PLpro might be a two-pronged approach. Several compounds including YM155, cryptotanshinone, tanshinone I, dihydrotanshinone I, tanshinone IIA, SJB2-043, 6-thioguanine, and 6-mercaptopurine were recently identified as SARS-CoV-2 PLpro inhibitors through high-throughput screenings. In this study, we aim to validate/invalidate the reported PLpro inhibitors using a combination of PLpro target-specific assays including enzymatic FRET assay, thermal shift binding assay (TSA), and cell-based FlipGFP assay. Collectively, our results showed that all compounds tested either did not show binding or led to denaturation of PLpro in the TSA binding assay, which might explain their weak enzymatic inhibition in the FRET assay. In addition, none of the compounds showed cellular PLpro inhibition as revealed by the FlipGFP assay. Therefore, more efforts are needed to search for potent and specific SARS-CoV-2 PLpro inhibitors.

Invalidation of dieckol and 1,2,3,4,6-pentagalloylglucose (PGG) as SARS-CoV-2 main protease inhibitors and the discovery of PGG as a papain-like protease inhibitor.

The COVID-19 pandemic spurred a broad interest in antiviral drug discovery. The SARS-CoV-2 main protease (Mpro) and papain-like protease (PLpro) are attractive antiviral drug targets given their vital roles in viral replication and modulation of host immune response. Structurally disparate compounds were reported as Mpro and PLpro 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 Mpro inhibitors. With our continuous interest in studying the mechanism of inhibition and resistance of Mpro 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 Mpro (IC50 > 20 µM), which is in contrary to previous reports. Serendipitously, PGG was found to inhibit the SARS-CoV-2 PLpro with an IC50 of 3.90 µM. The binding of PGG to PLpro was further confirmed in the thermal shift assay. However, PGG was cytotoxic in 293T-ACE2 cells (CC50 = 7.7 µM), so its intracellular PLpro inhibitory activity could not be quantified by the cell-based Flip-GFP PLpro assay. In addition, we also invalidated ebselen, disulfiram, carmofur, PX12, and tideglusib as SARS-CoV-2 PLpro inhibitors using the Flip-GFP assay. Overall, our results call for stringent hit validation, and the serendipitous discovery of PGG as a putative PLpro inhibitor might worth further pursuing. Graphical abstract.

Invalidation of dieckol and 1,2,3,4,6-pentagalloylglucose (PGG) as SARS-CoV-2 main protease inhibitors and the discovery of PGG as a papain-like protease inhibitor

The COVID-19 pandemic spurred a broad interest in antiviral drug discovery. The SARS-CoV-2 main protease (Mpro) and papain-like protease (PLpro) are attractive antiviral drug targets given their vital roles in viral replication and modulation of host immune response. Structurally disparate compounds were reported as Mpro and PLpro 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 Mpro inhibitors. With our continuous interest in studying the mechanism of inhibition and resistance of Mpro 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 Mpro (IC50 > 20 µM), which is in contrary to previous reports. Serendipitously, PGG was found to inhibit the SARS-CoV-2 PLpro with an IC50 of 3.90 µM. The binding of PGG to PLpro was further confirmed in the thermal shift assay. However, PGG was cytotoxic in 293T-ACE2 cells (CC50 = 7.7 µM), so its intracellular PLpro inhibitory activity could not be quantified by the cell-based Flip-GFP PLpro assay. In addition, we also invalidated ebselen, disulfiram, carmofur, PX12, and tideglusib as SARS-CoV-2 PLpro inhibitors using the Flip-GFP assay. Overall, our results call for stringent hit validation, and the serendipitous discovery of PGG as a putative PLpro inhibitor might worth further pursuing. Graphical

Drug-Repurposing Screening Identified Tropifexor as a SARS-CoV-2 Papain-like Protease Inhibitor.

The global COVID-19 pandemic underscores the dire need for effective antivirals. Encouraging progress has been made in developing small-molecule inhibitors targeting the SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) and main protease (Mpro). However, the development of papain-like protease (PLpro) inhibitors faces several obstacles. Nevertheless, PLpro represents a high-profile drug target given its multifaceted roles in viral replication. PLpro is involved in not only the cleavage of viral polyprotein but also the modulation of host immune response. In this study, we conducted a drug-repurposing screening of PLpro against the MedChemExpress bioactive compound library and identified three hits, EACC, KY-226, and tropifexor, as potent PLpro inhibitors with IC50 values ranging from 3.39 to 8.28 µM. The three hits showed dose-dependent binding to PLpro in the thermal shift assay. In addition, tropifexor inhibited the cellular PLpro activity in the FlipGFP assay with an IC50 of 10.6 µM. Gratifyingly, tropifexor showed antiviral activity against SARS-CoV-2 in Calu-3 cells at noncytotoxic concentrations. Overall, tropifexor represents a novel PLpro inhibitor that can be further developed as SARS-CoV-2 antivirals.

Invalidation of dieckol and 1,2,3,4,6-pentagalloylglucose (PGG) as SARS-CoV-2 main protease inhibitors and the discovery of PGG as a papain-like protease inhibitor.

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 main protease (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 papain-like protease (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.

Cytopathic Effect Assay and Plaque Assay to Evaluate in vitro Activity of Antiviral Compounds Against Human Coronaviruses 229E, OC43, and NL63.

Coronaviruses are important human pathogens, among which the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent for the COVID-19 pandemic. To combat the SARS-CoV-2 pandemic, there is a pressing need for antivirals, especially broad-spectrum antivirals that are active against all seven human coronaviruses (HCoVs). For this reason, we are interested in developing antiviral assays to expedite the drug discovery process. Here, we provide the detailed protocol for the cytopathic effect (CPE) assay and the plaque assay for human coronaviruses 229E (HCoV-229E), HCoV-OC43, and HCoV-NL63, to identify novel antivirals against HCoVs. Neutral red was used in the CPE assay, as it is relatively inexpensive and more sensitive than other reagents. Multiple parameters including multiplicity of infection, incubation time and temperature, and staining conditions have been optimized for CPE and plaque assays for HCoV-229E in MRC-5, Huh-7, and RD cell lines; HCoV-OC43 in RD, MRC-5, and BSC-1 cell lines, and HCoV-NL63 in Vero E6, Huh-7, MRC-5, and RD cell lines. Both CPE and plaque assays have been calibrated with the positive control compounds remdesivir and GC-376. Both CPE and plaque assays have high sensitivity, excellent reproducibility, and are cost-effective. The protocols described herein can be used as surrogate assays in the biosafety level 2 facility to identify entry inhibitors and protease inhibitors for SARS-CoV-2, as HCoV-NL63 also uses ACE2 as the receptor for cell entry, and the main proteases of HCoV-OC43 and SARS-CoV-2 are highly conserved. In addition, these assays can also be used as secondary assays to profile the broad-spectrum antiviral activity of existing SARS-CoV-2 drug candidates.

Discovery of Di- and Trihaloacetamides as Covalent SARS-CoV-2 Main Protease Inhibitors with High Target Specificity.

The main protease (Mpro) is a validated antiviral drug target of SARS-CoV-2. A number of Mpro 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 Mpro 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 Mpro inhibitors reported thus far. The cocrystal structures of SARS-CoV-2 Mpro 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.

Validation and invalidation of SARS-CoV-2 main protease inhibitors using the Flip-GFP and Protease-Glo luciferase assays.

SARS-CoV-2 main protease (Mpro) is one of the most extensively exploited drug targets for COVID-19. Structurally disparate compounds have been reported as Mpro inhibitors, raising the question of their target specificity. To elucidate the target specificity and the cellular target engagement of the claimed Mpro inhibitors, we systematically characterize their mechanism of action using the cell-free FRET assay, the thermal shift-binding assay, the cell lysate Protease-Glo luciferase assay, and the cell-based FlipGFP assay. Collectively, our results have shown that majority of the Mpro inhibitors identified from drug repurposing including ebselen, carmofur, disulfiram, and shikonin are promiscuous cysteine inhibitors that are not specific to Mpro, while chloroquine, oxytetracycline, montelukast, candesartan, and dipyridamole do not inhibit Mpro in any of the assays tested. Overall, our study highlights the need of stringent hit validation at the early stage of drug discovery.

Preclinical characterization of an intravenous coronavirus 3CL protease inhibitor for the potential treatment of COVID19.

COVID-19 caused by the SARS-CoV-2 virus has become a global pandemic. 3CL protease is a virally encoded protein that is essential across a broad spectrum of coronaviruses with no close human analogs. PF-00835231, a 3CL protease inhibitor, has exhibited potent in vitro antiviral activity against SARS-CoV-2 as a single agent. Here we report, the design and characterization of a phosphate prodrug PF-07304814 to enable the delivery and projected sustained systemic exposure in human of PF-00835231 to inhibit coronavirus family 3CL protease activity with selectivity over human host protease targets. Furthermore, we show that PF-00835231 has additive/synergistic activity in combination with remdesivir. We present the ADME, safety, in vitro, and in vivo antiviral activity data that supports the clinical evaluation of PF-07304814 as a potential COVID-19 treatment.

Discovery of SARS-CoV-2 Papain-like Protease Inhibitors through a Combination of High-Throughput Screening and a FlipGFP-Based Reporter Assay.

The papain-like protease (PLpro) of SARS-CoV-2 is a validated antiviral drug target. Through a fluorescence resonance energy transfer-based high-throughput screening and subsequent lead optimization, we identified several PLpro inhibitors including Jun9-72-2 and Jun9-75-4 with improved enzymatic inhibition and antiviral activity compared to GRL0617, which was reported as a SARS-CoV PLpro inhibitor. Significantly, we developed a cell-based FlipGFP assay that can be applied to predict the cellular antiviral activity of PLpro inhibitors in the BSL-2 setting. X-ray crystal structure of PLpro in complex with GRL0617 showed that binding of GRL0617 to SARS-CoV-2 induced a conformational change in the BL2 loop to a more closed conformation. Molecular dynamics simulations showed that Jun9-72-2 and Jun9-75-4 engaged in more extensive interactions than GRL0617. Overall, the PLpro inhibitors identified in this study represent promising candidates for further development as SARS-CoV-2 antivirals, and the FlipGFP-PLpro assay is a suitable surrogate for screening PLpro inhibitors in the BSL-2 setting.

Rational Design of Hybrid SARS-CoV-2 Main Protease Inhibitors Guided by the Superimposed Cocrystal Structures with the Peptidomimetic Inhibitors GC-376, Telaprevir, and Boceprevir.

SARS-CoV-2 main protease (Mpro) is a cysteine protease that mediates the cleavage of viral polyproteins and is a validated antiviral drug target. Mpro is highly conserved among all seven human coronaviruses, with certain Mpro inhibitors having broad-spectrum antiviral activity. In this study, we designed two hybrid inhibitors UAWJ9-36-1 and UAWJ9-36-3 based on the superimposed X-ray crystal structures of SARS-CoV-2 Mpro with GC-376, telaprevir, and boceprevir. Both UAWJ9-36-1 and UAWJ9-36-3 showed potent binding and enzymatic inhibition against the Mpro's from SARS-CoV-2, SARS-CoV, MERS-CoV, HCoV-OC43, HCoV-NL63, HCoV-229E, and HCoV-HKU1. Cell-based Flip-GFP Mpro assay results show that UAWJ9-36-1 and UAWJ9-36-3 inhibited the intracellular protease activity of SARS-CoV-2 Mpro. In addition, UAWJ9-36-1 and UAWJ9-36-3 had potent antiviral activity against SARS-CoV-2, HCoV-OC43, HCoV-NL63, and HCoV-229E, with UAWJ9-36-3 being more potent than GC-376 in inhibiting SARS-CoV-2. Selectivity profiling revealed that UAWJ9-36-1 and UAWJ9-36-3 had an improved selectivity index over that of GC-376 against host cysteine proteases calpain I and cathepsin L, but not cathepsin K. The X-ray crystal structures of SARS-CoV-2 Mpro with UAWJ9-36-1 and UAWJ9-36-3 were both solved at 1.9 Å, which validated our design hypothesis. Overall, hybrid inhibitors UAWJ9-36-1 and UAWJ9-36-3 are promising candidates to be further developed as broad-spectrum coronavirus antivirals.

Expedited Approach toward the Rational Design of Noncovalent SARS-CoV-2 Main Protease Inhibitors.

The main protease (Mpro) of SARS-CoV-2 is a validated antiviral drug target. Several Mpro inhibitors have been reported with potent enzymatic inhibition and cellular antiviral activity, including GC376, boceprevir, calpain inhibitors II, and XII, with each containing a reactive warhead that covalently modifies the catalytic Cys145. Coupling structure-based drug design with the one-pot Ugi four-component reaction, we discovered one of the most potent noncovalent inhibitors, 23R (Jun8-76-3A) that is structurally distinct from the canonical Mpro inhibitor GC376. Significantly, 23R is highly selective compared with covalent inhibitors such as GC376, especially toward host proteases. The cocrystal structure of SARS-CoV-2 Mpro with 23R revealed a previously unexplored binding site located in between the S2 and S4 pockets. Overall, this study discovered 23R, one of the most potent and selective noncovalent SARS-CoV-2 Mpro inhibitors reported to date, and a novel binding pocket in Mpro that can be explored for inhibitor design.

Boceprevir, Calpain Inhibitors II and XII, and GC-376 Have Broad-Spectrum Antiviral Activity against Coronaviruses.

As the COVID-19 pandemic continues to unfold, the morbidity and mortality are increasing daily. Effective treatment for SARS-CoV-2 is urgently needed. We recently discovered four SARS-CoV-2 main protease (Mpro) inhibitors including boceprevir, calpain inhibitors II and XII, and GC-376 with potent antiviral activity against infectious SARS-CoV-2 in cell culture. In this study, we further characterized the mechanism of action of these four compounds using the SARS-CoV-2 pseudovirus neutralization assay. It was found that GC-376 and calpain inhibitors II and XII have a dual mechanism of action by inhibiting both viral Mpro and host cathepsin L in Vero cells. To rule out the cell-type dependent effect, the antiviral activity of these four compounds against SARS-CoV-2 was also confirmed in type 2 transmembrane serine protease-expressing Caco-2 cells using the viral yield reduction assay. In addition, we found that these four compounds have broad-spectrum antiviral activity in inhibiting not only SARS-CoV-2 but also SARS-CoV, and MERS-CoV, as well as human coronaviruses (CoVs) 229E, OC43, and NL63. The mechanism of action is through targeting the viral Mpro, which was supported by the thermal shift-binding assay and enzymatic fluorescence resonance energy transfer assay. We further showed that these four compounds have additive antiviral effect when combined with remdesivir. Altogether, these results suggest that boceprevir, calpain inhibitors II and XII, and GC-376 might be promising starting points for further development against existing human coronaviruses as well as future emerging CoVs.

Structure and inhibition of the SARS-CoV-2 main protease reveal strategy for developing dual inhibitors against Mpro and cathepsin L.

The main protease (Mpro) of SARS-CoV-2 is a key antiviral drug target. While most Mpro inhibitors have a γ-lactam glutamine surrogate at the P1 position, we recently found that several Mpro inhibitors have hydrophobic moieties at the P1 site, including calpain inhibitors II and XII, which are also active against human cathepsin L, a host protease that is important for viral entry. In this study, we solved x-ray crystal structures of Mpro in complex with calpain inhibitors II and XII and three analogs of GC-376 The structure of Mpro with calpain inhibitor II confirmed that the S1 pocket can accommodate a hydrophobic methionine side chain, challenging the idea that a hydrophilic residue is necessary at this position. The structure of calpain inhibitor XII revealed an unexpected, inverted binding pose. Together, the biochemical, computational, structural, and cellular data presented herein provide new directions for the development of dual inhibitors as SARS-CoV-2 antivirals.

Ebselen, Disulfiram, Carmofur, PX-12, Tideglusib, and Shikonin Are Nonspecific Promiscuous SARS-CoV-2 Main Protease Inhibitors.

Among the drug targets being investigated for SARS-CoV-2, the viral main protease (Mpro) is one of the most extensively studied. Mpro is a cysteine protease that hydrolyzes the viral polyprotein at more than 11 sites. It is highly conserved and has a unique substrate preference for glutamine in the P1 position. Therefore, Mpro inhibitors are expected to have broad-spectrum antiviral activity and a high selectivity index. Structurally diverse compounds have been reported as Mpro inhibitors. In this study, we investigated the mechanism of action of six previously reported Mpro inhibitors, ebselen, disulfiram, tideglusib, carmofur, shikonin, and PX-12, using a consortium of techniques including FRET-based enzymatic assay, thermal shift assay, native mass spectrometry, cellular antiviral assays, and molecular dynamics simulations. Collectively, the results showed that the inhibition of Mpro by these six compounds is nonspecific and that the inhibition is abolished or greatly reduced with the addition of reducing reagent 1,4-dithiothreitol (DTT). Without DTT, these six compounds inhibit not only Mpro but also a panel of viral cysteine proteases including SARS-CoV-2 papain-like protease and 2Apro and 3Cpro from enterovirus A71 (EV-A71) and EV-D68. However, none of the compounds inhibits the viral replication of EV-A71 or EV-D68, suggesting that the enzymatic inhibition potency IC50 values obtained in the absence of DTT cannot be used to faithfully predict their cellular antiviral activity. Overall, we provide compelling evidence suggesting that these six compounds are nonspecific SARS-CoV-2 Mpro inhibitors and urge the scientific community to be stringent with hit validation.

Ebselen, disulfiram, carmofur, PX-12, tideglusib, and shikonin are non-specific promiscuous SARS-CoV-2 main protease inhibitors.

There is an urgent need for vaccines and antiviral drugs to combat the COVID-19 pandemic. Encouraging progress has been made in developing antivirals targeting SARS-CoV-2, the etiological agent of COVID-19. Among the drug targets being investigated, the viral main protease (M pro ) is one of the most extensively studied drug targets. M pro is a cysteine protease that hydrolyzes the viral polyprotein at more than 11 sites and it is highly conserved among coronaviruses. In addition, M pro has a unique substrate preference for glutamine in the P1 position. Taken together, it appears that M pro inhibitors can achieve both broad-spectrum antiviral activity and a high selectivity index. Structurally diverse compounds have been reported as M pro inhibitors, with several of which also showed antiviral activity in cell culture. In this study, we investigated the mechanism of action of six previously reported M pro inhibitors, ebselen, disulfiram, tideglusib, carmofur, shikonin, and PX-12 using a consortium of techniques including FRET-based enzymatic assay, thermal shift assay, native mass spectrometry, cellular antiviral assays, and molecular dynamics simulations. Collectively, the results showed that the inhibition of M pro by these six compounds is non-specific and the inhibition is abolished or greatly reduced with the addition of reducing reagent DTT. In the absence of DTT, these six compounds not only inhibit M pro , but also a panel of viral cysteine proteases including SARS-CoV-2 papain-like protease, the 2A pro and 3C pro from enterovirus A71 (EV-A71) and EV-D68. However, none of the compounds inhibits the viral replication of EV-A71 or EV-D68, suggesting that the enzymatic inhibition potency IC 50 values obtained in the absence of DTT cannot be used to faithfully predict their cellular antiviral activity. Overall, we provide compelling evidence suggesting that ebselen, disulfiram, tideglusib, carmofur, shikonin, and PX-12 are non-specific SARS-CoV-2 M pro inhibitors, and urge the scientific community to be stringent with hit validation.

Boceprevir, GC-376, and calpain inhibitors II, XII inhibit SARS-CoV-2 viral replication by targeting the viral main protease.

A new coronavirus SARS-CoV-2, also called novel coronavirus 2019 (2019-nCoV), started to circulate among humans around December 2019, and it is now widespread as a global pandemic. The disease caused by SARS-CoV-2 virus is called COVID-19, which is highly contagious and has an overall mortality rate of 6.35% as of May 26, 2020. There is no vaccine or antiviral available for SARS-CoV-2. In this study, we report our discovery of inhibitors targeting the SARS-CoV-2 main protease (Mpro). Using the FRET-based enzymatic assay, several inhibitors including boceprevir, GC-376, and calpain inhibitors II, and XII were identified to have potent activity with single-digit to submicromolar IC50 values in the enzymatic assay. The mechanism of action of the hits was further characterized using enzyme kinetic studies, thermal shift binding assays, and native mass spectrometry. Significantly, four compounds (boceprevir, GC-376, calpain inhibitors II and XII) inhibit SARS-CoV-2 viral replication in cell culture with EC50 values ranging from 0.49 to 3.37 µM. Notably, boceprevir, calpain inhibitors II and XII represent novel chemotypes that are distinct from known substrate-based peptidomimetic Mpro inhibitors. A complex crystal structure of SARS-CoV-2 Mpro with GC-376, determined at 2.15 Å resolution with three protomers per asymmetric unit, revealed two unique binding configurations, shedding light on the molecular interactions and protein conformational flexibility underlying substrate and inhibitor binding by Mpro. Overall, the compounds identified herein provide promising starting points for the further development of SARS-CoV-2 therapeutics.