Both Boceprevir and GC376 efficaciously inhibit SARS-CoV-2 by targeting its main protease

Identification and evaluation of antiviral activity of novel compounds targeting SARS-CoV-2 virus by enzymatic and antiviral assays, and computational analysis

The viral genome of the SARS-CoV-2 coronavirus, the aetiologic agent of COVID-19, encodes structural, non-structural, and accessory proteins. Most of these components undergo rapid genetic variations, though to a lesser extent the essential viral proteases. Consequently, the protease and/or deubiquitinase activities of the cysteine proteases Mpro and PLpro became attractive targets for the design of antiviral agents. Here, we develop and evaluate new bis(benzylidene)cyclohexanones (BBC) and identify potential antiviral compounds. Three compounds were found to be effective in reducing the SARS-CoV-2 load, with EC50 values in the low micromolar concentration range. However, these compounds also exhibited inhibitory activity IC50 against PLpro at approximately 10-fold higher micromolar concentrations. Although originally developed as PLpro inhibitors, the comparison between IC50 and EC50 of BBC indicates that the mechanism of their in vitro antiviral activity is probably not directly related to inhibition of viral cysteine proteases. In conclusion, our study has identified new potential noncytotoxic antiviral compounds suitable for in vivo testing and further improvement.

A novel method for synthesizing authentic SARS-CoV-2 main protease

The SARS-CoV-2 main protease (Mpro) plays a crucial role in virus amplification and is an ideal target for antiviral drugs. Currently, authentic Mpro is prepared through two rounds of proteolytic cleavage. In this method, Mpro carries a self-cleavage site at the N-terminus and a protease cleavage site followed by an affinity tag at the C-terminus. This article proposes a novel method for producing authentic Mpro through single digestion. Mpro was constructed by fusing a His tag containing TEV protease cleavage sites at the N-terminus. The expressed recombinant protein was digested by TEV protease, and the generated protein had a decreased molecular weight and significantly increased activity, which was consistent with that of authentic Mpro generated by the previous method. These findings indicated that authentic Mpro was successfully obtained. Moreover, the substrate specificity of Mpro was investigated. Mpro had a strong preference for Phe at position the P2, which suggested that the S2 subsite was an outstanding target for designing inhibitors. This article also provides a reference for the preparation of Mpro for sudden coronavirus infection in the future.

Structural basis for the inhibition of the HCoV-NL63 main protease Mpro by X77

Human coronaviruses are a group of pathogens that primarily cause respiratory and intestinal diseases. Infection can easily cause respiratory symptoms, as well as a variety of serious complications. There are several types of human coronaviruses, such as SARS-CoV, MERS-CoV, HCoV-229E, HCoV-OC43, HCoV-NL63, HCoV-HKU1, and SARS-CoV-2. The prevalence of COVID-19 has led to a growing focus on drug research against human coronaviruses. The main protease (Mpro) from human coronaviruses is a relatively conserved that controls viral replication. X77 was discovered to have extremely high inhibitory activity against SARS-CoV-2 Mpro through the use of computer-simulated docking. In this paper, we have resolved the crystal structure of the HCoV-NL63 Mpro complexed with X77 and analyzed their interaction in detail. This data provides essential information for solving their binding modes and their structural determinants. Then, we compared the binding modes of X77 with SARS-CoV-2 Mpro and HCoV-NL63 Mpro in detail. This study illustrates the structural basis of HCoV-NL63 Mpro binding to the inhibitor X77. The structural insights derived from this study will inform the development of new drugs with broad-spectrum resistance to human coronaviruses.

Development of novel antivrial agents that induce the degradation of the main protease of human-infecting coronaviruses

The family of human-infecting coronaviruses (HCoVs) poses a serious threat to global health and includes several highly pathogenic strains that cause severe respiratory illnesses. It is essential that we develop effective broad-spectrum anti-HCoV agents to prepare for future outbreaks. In this study, we used PROteolysis TArgeting Chimera (PROTAC) technology focused on degradation of the HCoV main protease (Mpro), a conserved enzyme essential for viral replication and pathogenicity. By adapting the Mpro inhibitor GC376, we produced two novel PROTACs, P2 and P3, which showed relatively broad-spectrum activity against the human-infecting CoVs HCoV-229E, HCoV-OC43, and SARS-CoV-2. The concentrations of these PROTACs that reduced virus replication by 50 % ranged from 0.71 to 4.6 μM, and neither showed cytotoxicity at 100 μM. Furthermore, mechanistic binding studies demonstrated that P2 and P3 effectively targeted HCoV-229E, HCoV-OC43, and SARS-CoV-2 by degrading Mpro within cells in vitro. This study highlights the potential of PROTAC technology in the development of broad-spectrum anti-HCoVs agents, presenting a novel approach for dealing with future viral outbreaks, particularly those stemming from CoVs.

Progress in SARS-CoV-2, diagnostic and clinical treatment of COVID-19

Background

Corona Virus Disease 2019(COVID-19)is a global pandemic novel coronavirus infection disease caused by Severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2). Although rapid, large-scale testing plays an important role in patient management and slowing the spread of the disease. However, there has been no good and widely used drug treatment for infection and transmission of SARS-CoV-2.

Key findings

Therefore, this review updates the body of knowledge on viral structure, infection routes, detection methods, and clinical treatment, with the aim of responding to the large-section caused by SARS-CoV-2. This paper focuses on the structure of SARS-CoV-2 viral protease, RNA polymerase, serine protease and main proteinase-like protease as well as targeted antiviral drugs.

Conclusion

In vitro or clinical trials have been carried out to provide deeper thinking for the pathogenesis, clinical diagnosis, vaccine development and treatment of SARS-CoV-2.

Study of key residues in MERS-CoV and SARS-CoV-2 main proteases for resistance against clinically applied inhibitors nirmatrelvir and ensitrelvir

The Middle East Respiratory Syndrome Coronavirus (MERS-CoV) is an epidemic, zoonotically emerging pathogen initially reported in Saudi Arabia in 2012. MERS-CoV has the potential to mutate or recombine with other coronaviruses, thus acquiring the ability to efficiently spread among humans and become pandemic. Its high mortality rate of up to 35% and the absence of effective targeted therapies call for the development of antiviral drugs for this pathogen. Since the beginning of the SARS-CoV-2 pandemic, extensive research has focused on identifying protease inhibitors for the treatment of SARS-CoV-2. Our intention was therefore to assess whether these protease inhibitors are viable options for combating MERS-CoV. To that end, we used previously established protease assays to quantify inhibition of SARS-CoV-2, MERS-CoV and other main proteases. Nirmatrelvir inhibited several of these proteases, whereas ensitrelvir was less broadly active. To simulate nirmatrelvir's clinical use against MERS-CoV and subsequent resistance development, we applied a safe, surrogate virus-based system. Using the surrogate virus, we previously selected hallmark mutations of SARS-CoV-2-Mpro, such as T21I, M49L, S144A, E166A/K/V and L167F. In the current study, we selected a pool of MERS-CoV-Mpro mutants, characterized the resistance and modelled the steric effect of catalytic site mutants S142G, S142R, S147Y and A171S.

De novo design of SARS-CoV-2 main protease inhibitors with characteristic binding modes

The coronavirus disease 2019 (COVID-19) is caused by a novel coronavirus called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which spreads rapidly all over the world. The main protease (Mpro) is significant to the replication and transcription of viruses, making it an attractive drug target against coronaviruses. Here, we introduce a series of novel inhibitors which are designed de novo through structure-based drug design approach that have great potential to inhibit SARS-CoV-2 Mproin vitro. High-resolution structures show that these inhibitors form covalent bonds with the catalytic cysteine through the novel dibromomethyl ketone (DBMK) as a reactive warhead. At the same time, the designed phenyl group beside the DBMK warhead inserts into the cleft between H41 and C145 through π-π stacking interaction, splitting the catalytic dyad and disrupting proton transfer. This unique binding model provides novel clues for the cysteine protease inhibitor development of SARS-CoV-2 as well as other pathogens.

SARS-CoV-2 Main Protease Inhibitors That Leverage Unique Interactions with the Solvent Exposed S3 Site of the Enzyme

The main protease (MPro) of SARS-CoV-2 is crucial for the virus's replication and pathogenicity. Its active site is characterized by four distinct pockets (S1, S2, S4, and S1-3') and a solvent-exposed S3 site for accommodating a protein substrate. During X-ray crystallographic analyses of MPro bound with dipeptide inhibitors containing a flexible N-terminal group, we often observed an unexpected binding mode. Contrary to the anticipated engagement with the deeper S4 pocket, the N-terminal group frequently assumed a twisted conformation, positioning it for interactions with the S3 site and the inhibitor component bound at the S1 pocket. Capitalizing on this observation, we engineered novel inhibitors to engage both S3 and S4 sites or to adopt a rigid conformation for selective S3 site binding. Several new inhibitors demonstrated high efficacy in MPro inhibition. Our findings underscore the importance of the S3 site's unique interactions in the design of future MPro inhibitors as potential COVID-19 therapeutics.

In silico screening of potential inhibitors from Cordyceps species against SARS-CoV-2 main protease

Coronavirus disease 2019 (COVID-19) is a result of a retroviral infection of SARS-CoV-2. Due to its virulence and high infection rate, it is a matter of serious concern and a global health emergency. Currently available COVID-19 vaccines approved by regulatory bodies around the world have been shown to provide significant protection against COVID-19. But no vaccine is 100% effective at preventing infection, also they have varying efficacy rates and different side effects. However, the main protease (Mpro) of SARS-CoV-2 has been identified as a key drug target due to its essential role in viral infection and its minimal similarity with human proteases. Cordyceps mushrooms have been found to have various therapeutic properties that could effectively combat SARS-CoV-2, including improve lung functioning, anti-viral, immunomodulators, anti-infectious, and anti-inflammatory. The present study aims to screen and evaluate the inhibitory potential of the bioactive molecules from the Cordyceps species against the Mpro of SARS-CoV-2. The bioactive molecules were screened based on their docking score, molecular interactions in the binding pocket, ADME properties, toxicity, carcinogenicity, and mutagenicity. Among all the molecules that were tested, cordycepic acid was the most effective and promising candidate, with a binding affinity of -8.10 kcal/mol against Mpro. The molecular dynamics (MD) simulation and free binding energy calculations revealed that the cordycepic acid-Mpro complex was highly stable and showed fewer conformational fluctuations. These findings need to be investigated further through in-vitro and in-vivo studies for additional validation.Communicated by Ramaswamy H. Sarma.

Kinetic comparison of all eleven viral polyprotein cleavage site processing events by SARS-CoV-2 main protease using a linked protein FRET platform

The main protease (Mpro) remains an essential therapeutic target for COVID-19 post infection intervention given its critical role in processing the majority of viral proteins encoded by the genome of severe acute respiratory syndrome related coronavirus 2 (SARS-CoV-2). Upon viral entry, the +ssRNA genome is translated into two long polyproteins (pp1a or the frameshift-dependent pp1ab) containing all the nonstructural proteins (nsps) required by the virus for immune modulation, replication, and ultimately, virion assembly. Included among these nsps is the cysteine protease Mpro (nsp5) which self-excises from the polyprotein, dimerizes, then sequentially cleaves 11 of the 15 cut-site junctions found between each nsp within the polyprotein. Many structures of Mpro (often bound to various small molecule inhibitors or peptides) have been detailed recently, including structures of Mpro bound to each of the polyprotein cleavage sequences, showing that Mpro can accommodate a wide range of targets within its active site. However, to date, kinetic characterization of the interaction of Mpro with each of its native cleavage sequences remains incomplete. Here, we present a robust and cost-effective FRET based system that benefits from a more consistent presentation of the substrate that is also closer in organization to the native polyprotein environment compared to previously reported FRET systems that use chemically modified peptides. Using this system, we were able to show that while each site maintains a similar Michaelis constant, the catalytic efficiency of Mpro varies greatly between cut-site sequences, suggesting a clear preference for the order of nsp processing.

Computational Screening Using a Combination of Ligand-Based Machine Learning and Molecular Docking Methods for the Repurposing of Antivirals Targeting the SARS-CoV-2 Main Protease

BACKGROUND

COVID-19 is an infectious disease caused by SARS-CoV-2, a close relative of SARS-CoV. Several studies have searched for COVID-19 therapies. The topics of these works ranged from vaccine discovery to natural products targeting the SARS-CoV-2 main protease (Mpro), a potential therapeutic target due to its essential role in replication and conserved sequences. However, published research on this target is limited, presenting an opportunity for drug discovery and development.

METHOD

This study aims to repurpose 10692 drugs in DrugBank by using ligand-based virtual screening (LBVS) machine learning (ML) with Konstanz Information Miner (KNIME) to seek potential therapeutics based on Mpro inhibitors. The top candidate compounds, the native ligand (GC-376) of the Mpro inhibitor, and the positive control boceprevir were then subjected to absorption, distribution, metabolism, excretion, and toxicity (ADMET) characterization, drug-likeness prediction, and molecular docking (MD). Protein-protein interaction (PPI) network analysis was added to provide accurate information about the Mpro regulatory network.

RESULTS

This study identified 3,166 compound candidates inhibiting Mpro. The random forest (RF) molecular access system ML model provided the highest confidence score of 0.95 (bromo-7-nitroindazole) and identified the top 22 candidate compounds. Subjecting the 22 candidate compounds, the native ligand GC-376, and boceprevir to further ADMET property characterization and drug-likeness predictions revealed that one compound had two violations of Lipinski's rule. Additional MD results showed that only five compounds had more negative binding energies than the native ligand (- 12.25 kcal/mol). Among these compounds, CCX-140 exhibited the lowest score of - 13.64 kcal/mol. Through literature analysis, six compound classes with potential activity for Mpro were discovered. They included benzopyrazole, azole, pyrazolopyrimidine, carboxylic acids and derivatives, benzene and substituted derivatives, and diazine. Four pathologies were also discovered on the basis of the Mpro PPI network.

CONCLUSION

Results demonstrated the efficiency of LBVS combined with MD. This combined strategy provided positive evidence showing that the top screened drugs, including CCX-140, which had the lowest MD score, can be reasonably advanced to the in vitro phase. This combined method may accelerate the discovery of therapies for novel or orphan diseases from existing drugs.

New Thiazolidine-4-One Derivatives as SARS-CoV-2 Main Protease Inhibitors

It has been more than four years since the first report of SARS-CoV-2, and humankind has experienced a pandemic with an unprecedented impact. Moreover, the new variants have made the situation even worse. Among viral enzymes, the SARS-CoV-2 main protease (Mpro) has been deemed a promising drug target vs. COVID-19. Indeed, Mpro is a pivotal enzyme for viral replication, and it is highly conserved within coronaviruses. It showed a high extent of conservation of the protease residues essential to the enzymatic activity, emphasizing its potential as a drug target to develop wide-spectrum antiviral agents effective not only vs. SARS-CoV-2 variants but also against other coronaviruses. Even though the FDA-approved drug nirmatrelvir, a Mpro inhibitor, has boosted the antiviral therapy for the treatment of COVID-19, the drug shows several drawbacks that hinder its clinical application. Herein, we report the synthesis of new thiazolidine-4-one derivatives endowed with inhibitory potencies in the micromolar range against SARS-CoV-2 Mpro. In silico studies shed light on the key structural requirements responsible for binding to highly conserved enzymatic residues, showing that the thiazolidinone core acts as a mimetic of the Gln amino acid of the natural substrate and the central role of the nitro-substituted aromatic portion in establishing π-π stacking interactions with the catalytic His-41 residue.

Anti-Coronavirus Potential of Polyether Ionophores: The New Application of Veterinary Antibiotics in Livestock

Coronaviruses have consistently posed a major global concern in the field of livestock industry and public health. However, there is currently a lack of efficient drugs with broad-spectrum antiviral activity to address the challenges presented by emerging mutated strains or drug resistance. Additionally, the method for identifying multitarget drugs is also insufficient. Aminopeptidase N (APN) and 3C-like proteinase (3CLpro) represent promising targets for host-directed and virus-directed strategies, respectively, in the development of effective drugs against various coronaviruses. In this study, maduramycin ammonium demonstrated a broad-spectrum antiviral effect by targeting both of the proteins. The binding domains 4 Å from the ligand of both target proteins shared a structural similarity, suggesting that screening and designing drugs based on these domains might exhibit broad-spectrum and highly effective antiviral activity. Furthermore, it was identified that the polyether ionophores' ability to carry zinc ion might be one of the reasons why they were able to target APN and exhibit antiviral effect. The findings of this experiment provide novel perspectives for future drug screening and design, while also offering valuable references for the utilization of polyether ionophores in the management of livestock health.

Azapeptides with unique covalent warheads as SARS-CoV-2 main protease inhibitors

The main protease (MPro) of SARS-CoV-2, the causative agent of COVID-19, is a pivotal nonstructural protein critical for viral replication and pathogenesis. Its protease function relies on three active site pockets for substrate recognition and a catalytic cysteine for enzymatic activity. To develop potential SARS-CoV-2 antivirals, we successfully synthesized a diverse range of azapeptide inhibitors with various covalent warheads to target MPro's catalytic cysteine. Our characterization identified potent MPro inhibitors, including MPI89 that features an aza-2,2-dichloroacetyl warhead with a remarkable EC50 value of 10 nM against SARS-CoV-2 infection in ACE2+ A549 cells and a selective index of 875. MPI89 is also remarkably selective and shows no potency against SARS-CoV-2 papain-like protease and several human proteases. Crystallography analyses demonstrated that these inhibitors covalently engaged the catalytic cysteine and used the aza-amide carbonyl oxygen to bind to the oxyanion hole. MPI89 stands as one of the most potent MPro inhibitors, suggesting the potential for further exploration of azapeptides and the aza-2,2-dichloroacetyl warhead for developing effective therapeutics against COVID-19.

An ascidian Polycarpa aurata-derived pan-inhibitor against coronaviruses targeting Mpro

Coronaviruses (CoVs) are responsible for a wide range of illnesses in both animals and human. The main protease (Mpro) of CoVs is an attractive drug target, owing its critical and highly conserved role in viral replication. Here, we developed and refined an enzymatic technique to identify putative Mpro inhibitors from 189 marine chemicals and 46 terrestrial natural products. The IC50 values of Polycarpine (1a), a marine natural substance we studied and synthesized, are 30.0 ± 2.5 nM for SARS-CoV-2 Mpro and 0.12 ± 0.05 μM for PEDV Mpro. Our research further demonstrated that pretreatment with Polycarpine (1a) inhibited the betacoronavirus SARS-CoV-2 and alphacoronavirus PEDV multiplication in Vero-E6 cells. As a result, Polycarpine (1a), a pan-inhibitor of Mpro, will function as an effective and promising antiviral option to combat CoVs infection and as a foundation for further therapeutic research.

Discovery of First-in-Class PROTAC Degraders of SARS-CoV-2 Main Protease

We have witnessed three coronavirus (CoV) outbreaks in the past two decades, including the COVID-19 pandemic caused by SARS-CoV-2. Main protease (MPro), a highly conserved protease among various CoVs, is essential for viral replication and pathogenesis, making it a prime target for antiviral drug development. Here, we leverage proteolysis targeting chimera (PROTAC) technology to develop a new class of small-molecule antivirals that induce the degradation of SARS-CoV-2 MPro. Among them, MPD2 was demonstrated to effectively reduce MPro protein levels in 293T cells, relying on a time-dependent, CRBN-mediated, and proteasome-driven mechanism. Furthermore, MPD2 exhibited remarkable efficacy in diminishing MPro protein levels in SARS-CoV-2-infected A549-ACE2 cells. MPD2 also displayed potent antiviral activity against various SARS-CoV-2 strains and exhibited enhanced potency against nirmatrelvir-resistant viruses. Overall, this proof-of-concept study highlights the potential of targeted protein degradation of MPro as an innovative approach for developing antivirals that could fight against drug-resistant viral variants.

Statine-based peptidomimetic compounds as inhibitors for SARS-CoV-2 main protease (SARS-CoV‑2 Mpro)

COVID-19 is a multisystemic disease caused by the SARS-CoV-2 airborne virus, a member of the Coronaviridae family. It has a positive sense single-stranded RNA genome and encodes two non-structural proteins through viral cysteine-proteases processing. Blocking this step is crucial to control virus replication. In this work, we reported the synthesis of 23 statine-based peptidomimetics to determine their ability to inhibit the main protease (Mpro) activity of SARS-CoV-2. Among the 23 peptidomimetics, 15 compounds effectively inhibited Mpro activity by 50% or more, while three compounds (7d, 8e, and 9g) exhibited maximum inhibition above 70% and IC50 < 1 µM. Compounds 7d, 8e, and 9g inhibited roughly 80% of SARS-CoV-2 replication and proved no cytotoxicity. Molecular docking simulations show putative hydrogen bond and hydrophobic interactions between specific amino acids and these inhibitors. Molecular dynamics simulations further confirmed the stability and persisting interactions in Mpro's subsites, exhibiting favorable free energy binding (ΔGbind) values. These findings suggest the statine-based peptidomimetics as potential therapeutic agents against SARS-CoV-2 by targeting Mpro.

Synthesis, SARS-CoV-2 main protease inhibition, molecular docking and in silico ADME studies of furanochromene-quinoline hydrazone derivatives

Seven furanochromene-quinoline derivatives containing a hydrazone linker were synthesized by condensing a furanochromene hydrazide with quinoline 2-, 3-, 4-, 5-, 6-, and 8-carbaldehydes, including 8-hydroxyquinoline-2-carbaldehye. Structure-activity correlations were investigated to determine the influence of the location of the hydrazone linker on the quinoline unit on SARS-CoV-2 Mpro enzyme inhibition. The 3-, 5-, 6- and 8-substituted derivatives showed moderate inhibition of SARS-CoV-2 Mpro with IC50 values ranging from 16 to 44 μM. Additionally, all of the derivatives showed strong interaction with the SARS-CoV-2 Mpro substrate binding pocket, with docking energy scores ranging from -8.0 to -8.5 kcal/mol. These values are comparable to that of N3 peptide (-8.1 kcal/mol) and more favorable than GC-373 (-7.6 kcal/mol) and ML-188 (-7.5 kcal/mol), all of which are known SARS-CoV-2 Mpro inhibitors. Furthermore, in silico absorption, distribution, metabolism, and excretion (ADME) profiles indicate that the derivatives have good drug-likeness properties. Overall, this study highlights the potential of the furanochromene-quinoline hydrazone scaffold as a SARS-CoV-2 Mpro inhibitor.

Isolation, characterization and SARS-CoV-2 3CL protease inhibitory activity of a new methylsulfinyl-butanyl derivative from Raphani Semen

A new compound named raphanised A (1), along with two known methylsulfinyl -butanyl derivatives (2-3) and seven known indole derivatives (4-10), were isolated from the Raphani Semen. Among the indole derivatives, 5 was identified as a new natural product, and 4, 6, 7, 8, 9, 10 were isolated from the genus of Raphanus for the first time. Their structures were elucidated based on the NMR and HR-EI-MS analysis. Additionally, the inhibitory activity of methylsulfinyl-butanyl derivatives 1-3 on SARS CoV-2 3CL protease was evaluated. The results showed that 1-3 exhibited inhibition of SARS-CoV-2 3CL protease activity at concentrations ranging from 3.3 to 30 µM.

Exploiting high-energy hydration sites for the discovery of potent peptide aldehyde inhibitors of the SARS-CoV-2 main protease with cellular antiviral activity

Small-molecule antivirals that prevent the replication of the SARS-CoV-2 virus by blocking the enzymatic activity of its main protease (Mpro) are and will be a tenet of pandemic preparedness. However, the peptidic nature of such compounds often precludes the design of compounds within favorable physical property ranges, limiting cellular activity. Here we describe the discovery of peptide aldehyde Mpro inhibitors with potent enzymatic and cellular antiviral activity. This structure-activity relationship (SAR) exploration was guided by the use of calculated hydration site thermodynamic maps (WaterMap) to drive potency via displacement of waters from high-energy sites. Thousands of diverse compounds were designed to target these high-energy hydration sites and then prioritized for synthesis by physics- and structure-based Free-Energy Perturbation (FEP+) simulations, which accurately predicted biochemical potencies. This approach ultimately led to the rapid discovery of lead compounds with unique SAR that exhibited potent enzymatic and cellular activity with excellent pan-coronavirus coverage.

Quantitative Assessment of Energetic Contributions of Residues in a SARS-CoV-2 Viral Enzyme/Nanobody Interface

The highly conserved protease enzyme from SARS-CoV-2 (MPro) is crucial for viral replication and is an attractive target for the design of novel inhibitory compounds. MPro is known to be conformationally flexible and has been stabilized in an extended conformation in a complex with a novel nanobody (NB2B4), which inhibits the dimerization of the enzyme via binding to an allosteric site. However, the energetic contributions of the nanobody residues stabilizing the MPro/nanobody interface remain unresolved. We probed these residues using all-atom MD simulations in combination with alchemical free energy calculations by studying the physical residue-residue interactions and discovered the role of hydrophobic and electrostatic interactions in stabilizing the complex. Specifically, we found via mutational analysis that three interfacial nanobody residues (Y59, R106, and L109) contributed significantly, two residues (L107 and P110) contributed moderately, and two residues (H112 and T113) contributed minimally to the overall binding affinity of the nanobody. We also discovered that the nanobody affinity could be enhanced via a charge-reversal mutation (D62R) that alters the local interfacial electrostatic environment of this residue in the complex. These findings are potentially useful in designing novel synthetic nanobodies as allosteric inhibitors of MPro.

Disulfiram Inhibits Opsonin-Independent Phagocytosis and Migration of Human Long-Lived In Vitro Cultured Phagocytes from Multiple Inflammatory Diseases

Disulfiram (DSF), an anti-alcoholism medicine, exerts treatment effects in patients suffering from persistent Borreliosis and also exhibits anti-cancer effects through its copper chelating derivatives and induction of oxidative stress in mitochondria. Since chronic/persistent borreliosis is characterized by increased amounts of pro-inflammatory macrophages, this study investigated opsonin-independent phagocytosis, migration, and surface marker expression of in vivo activated and in vitro cultured human monocyte-derived phagocytes (macrophages and dendritic cells) with and without DSF treatment. Phagocytosis of non-opsonized Dynabeads® M-450 and migration of macrophages and dendritic cells were monitored using live cell analyzer Juli™ Br for 24 h, imaging every 3.5 min. To simultaneously monitor phagocyte function, results were analyzed by a newly developed software based on the differential phase contrast images of cells before and after ingestion of Dynabeads. DSF decreased the phagocytic capacities exhibited by in vitro enriched and long-lived phagocytes. Although no chemotactic gradient was applied to the test system, vigorous spontaneous migration was observed. We therefore set up an algorithm to monitor and quantify both phagocytosis and migration simultaneously. DSF not only reduced phagocytosis in a majority of these long-lived phagocytes but also impaired their migration. Despite these selective effects by DSF, we found that DSF reduced the expression densities of surface antigens CD45 and CD14 in all of our long-lived phagocytes. In cells with a high metabolic activity and high mitochondrial contents, DSF led to cell death corresponding to mitochondrial oxidative stress, whereas metabolically inactive phagocytes survived our DSF treatment protocol. In conclusion, DSF affects the viability of metabolically active phagocytes by inducing mitochondrial stress and secondly attenuates phagocytosis and migration in some long-lived phagocytes.

Structural Basis for Coronaviral Main Proteases Inhibition by the 3CLpro Inhibitor GC376

The main protease (Mpro) of coronaviruses participates in viral replication, serving as a hot target for drug design. GC376 is able to effectively inhibit the activity of Mpro, which is due to nucleophilic addition of GC376 by binding covalently with Cys145 in Mpro active site. Here, we used fluorescence resonance energy transfer (FRET) assay to analyze the IC50 values of GC376 against Mpros from six different coronaviruses (SARS-CoV-2, HCoV-229E, HCoV-HUK1, MERS-CoV, SARS-CoV, HCoV-NL63) and five Mpro mutants (G15S, M49I, K90R, P132H, S46F) from SARS-CoV-2 variants. The results showed that GC376 displays effective inhibition to various coronaviral Mpros and SARS-CoV-2 Mpro mutants. In addition, the crystal structures of SARS-CoV-2 Mpro (wide type)-GC376, SARS-CoV Mpro-GC376, MERS-CoV Mpro-GC376, and SARS-CoV-2 Mpro mutants (G15S, M49I, S46F, K90R, and P132H)-GC376 complexes were solved. We found that GC376 is able to fit into the active site of Mpros from different coronaviruses and different SARS-CoV-2 variants properly. Detailed structural analysis revealed key molecular determinants necessary for inhibition and illustrated the binding patterns of GC376 to these different Mpros. In conclusion, we not only proved the inhibitory activity of GC376 against different Mpros including SARS-CoV-2 Mpro mutants, but also revealed the molecular mechanism of inhibition by GC376, which will provide scientific guidance for the development of broad-spectrum drugs against SARS-CoV-2 as well as other coronaviruses.

Calpain-2 mediates SARS-CoV-2 entry via regulating ACE2 levels

Since the beginning of the coronavirus disease 2019 (COVID-19) pandemic, much effort has been dedicated to identifying effective antivirals against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A number of calpain inhibitors show excellent antiviral activities against SARS-CoV-2 by targeting the viral main protease (Mpro), which plays an essential role in processing viral polyproteins. In this study, we found that calpain inhibitors potently inhibited the infection of a chimeric vesicular stomatitis virus (VSV) encoding the SARS-CoV-2 spike protein but not Mpro. In contrast, calpain inhibitors did not exhibit antiviral activities toward the wild-type VSV with its native glycoprotein. Genetic knockout of calpain-2 by CRISPR/Cas9 conferred resistance of the host cells to the chimeric VSV-SARS-CoV-2 virus and a clinical isolate of wild-type SARS-CoV-2. Mechanistically, calpain-2 facilitates SARS-CoV-2 spike protein-mediated cell attachment by positively regulating the cell surface levels of ACE2. These results highlight an Mpro-independent pathway targeted by calpain inhibitors for efficient viral inhibition. We also identify calpain-2 as a novel host factor and a potential therapeutic target responsible for SARS-CoV-2 infection at the entry step.

IMPORTANCE

Many efforts in small-molecule screens have been made to counter SARS-CoV-2 infection by targeting the viral main protease, the major element that processes viral proteins after translation. Here, we discovered that calpain inhibitors further block SARS-CoV-2 infection in a main protease-independent manner. We identified the host cysteine protease calpain-2 as an important positive regulator of the cell surface levels of SARS-CoV-2 cellular receptor ACE2 and, thus, a facilitator of viral infection. By either pharmacological inhibition or genetic knockout of calpain-2, the SARS-CoV-2 binding to host cells is blocked and viral infection is decreased. Our findings highlight a novel mechanism of ACE2 regulation, which presents a potential new therapeutic target. Since calpain inhibitors also potently interfere with the viral main protease, our data also provide a mechanistic understanding of the potential use of calpain inhibitors as dual inhibitors (entry and replication) in the clinical setting of COVID-19 diseases. Our findings bring mechanistic insights into the cellular process of SARS-CoV-2 entry and offer a novel explanation to the mechanism of activities of calpain inhibitors.

An orally bioavailable SARS-CoV-2 main protease inhibitor exhibits improved affinity and reduced sensitivity to mutations

Inhibitors of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) main protease (Mpro) such as nirmatrelvir (NTV) and ensitrelvir (ETV) have proven effective in reducing the severity of COVID-19, but the presence of resistance-conferring mutations in sequenced viral genomes raises concerns about future drug resistance. Second-generation oral drugs that retain function against these mutants are thus urgently needed. We hypothesized that the covalent hepatitis C virus protease inhibitor boceprevir (BPV) could serve as the basis for orally bioavailable drugs that inhibit SARS-CoV-2 Mpro more efficiently than existing drugs. Performing structure-guided modifications of BPV, we developed a picomolar-affinity inhibitor, ML2006a4, with antiviral activity, oral pharmacokinetics, and therapeutic efficacy similar or superior to those of NTV. A crucial feature of ML2006a4 is a derivatization of the ketoamide reactive group that improves cell permeability and oral bioavailability. Last, ML2006a4 was found to be less sensitive to several mutations that cause resistance to NTV or ETV and occur in the natural SARS-CoV-2 population. Thus, anticipatory design can preemptively address potential resistance mechanisms to expand future treatment options against coronavirus variants.

Licorice extract inhibits porcine epidemic diarrhea virus in vitro and in vivo

Porcine epidemic diarrhea virus (PEDV) causes severe diarrhea and even death in piglets, resulting in significant economic losses to the pig industry. Because of the ongoing mutation of PEDV, there might be variations between the vaccine strain and the prevailing strain, causing the vaccine to not offer full protection against different PEDV variant strains. Therefore, it is necessary to develop anti-PEDV drugs to compensate for vaccines. This study confirmed the anti-PEDV effect of licorice extract (Le) in vitro and in vivo. Le inhibited PEDV replication in a dose-dependent manner in vitro. By exploring the effect of Le on the life cycle of PEDV, we found that Le inhibited the attachment, internalization, and replication stages of the virus. In vivo, all five piglets in the PEDV-infected group died within 72 h. In comparison, the Le-treated group had a survival rate of 80 % at the same time, with significant relief of clinical symptoms, pathological damage, and viral loads in the jejunum and ileum. Our results suggested that Le can exert anti-PEDV effects in vitro and in vivo. Le is effective and inexpensive; therefore it has the potential to be developed as a new anti-PEDV drug.

Fourteen immunomodulatory alkaloids and two prenylated phenylpropanoids with dual therapeutic approach for COVID-19: molecular docking and dynamics studies

The pandemic outbreak of COVID-19 caused by the new severe acute respiratory syndrome coronavirus (SARS-CoV-2) is a global health burden. To date, there is no highly effective antiviral therapy to eradicate the virus; as a result, researchers are racing to introduce new potential therapeutic agents. Alternatively, traditional immunity boosters and symptomatic treatment based on natural bioactive compounds are also an option. The 3-chymotrypsin-like protease (3CLpro) crystal structure, the main proteolytic enzyme of SARS-CoV-2, has been unraveled, allowing the development of effective protease inhibitors via in silico and biological studies. In COVID-19 infected patients, the loss of lung function, and mortality are reported to be linked to several inflammatory mediators and cytokines. In this context, the approach of introducing immunomodulatory agents may be considered a dual lifesaving strategy in combination with antiviral drugs. This study aims to provide immunomodulatory natural products exhibiting potential protease inhibitory activities. Selected groups of alkaloids of different classes and two prenylated phenylpropanoids from the Brazilian green propolis were in silico screened for their ability to inhibit COVID-19 3CLpro protease. Results showed that compounds exhibited binding energy scores with values ranging from -6.96 to -3.70 compared to the reference synthetic protease inhibitor O6K with a binding energy score of -7.57. O6K binding energy was found comparable with lead phytochemicals in our study, while their toxicity and drug-likeness criteria are better than that of O6K. The activities of these molecules are mainly ascribed to their ability to form hydrogen bonding with 3CLpro crucial amino acid residues of the catalytic site. In addition, the molecular dynamics simulations further showed that some of these compounds formed stable complexes as evidenced by the occupancy fraction measurements. The study suggested that the major immunomodulators 3β, 20α-diacetamido-5α-pregnane, (20S)-(benzamido)-3β-(N,N-dimethyamino)-pregnane, and baccharin are 3CLpro inhibitors. Biological screenings of these phytochemicals will be valuable to experimentally validate and consolidate the results of this study before a rigid conclusion is reached, which may pave the way for the development of efficient modulatory bioactive compounds with dual bioactions in COVID-19 intervention. Communicated by Ramaswamy H. Sarma.

Syringa reticulata potently inhibits the activity of SARS-CoV-2 3CL protease

The ongoing coronavirus infectious disease (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) still urgently requires effective treatments. The 3C-like (3CL) protease of SARS-CoV-2 is a highly conserved cysteine protease that plays an important role in the viral life cycle and host inflammation, providing an ideal target for developing broad-spectrum antiviral drugs. Herein, we describe the discovery of a large number of herbs mainly produced in Heilongjiang Province, China, that exhibited different inhibitory activities against SARS-CoV-2 3CL protease. We confirmed that Syringa reticulata, which is used for clinical treatment of chronic bronchitis and asthma, is a specific and potent inhibitor of 3CL protease. A 70 % ethanol extract of S. reticulata dose-dependently inhibited the cleavage activity of 3CL protease in a fluorescence resonance energy transfer assay with an IC50 value of 0.0018 mg/mL, but had minimal effect in pseudovirus-based cell entry and luciferase-based RNA-dependent RNA polymerase assays. These results suggest that S. reticulata will be a potential leading candidate for COVID-19 treatment.

Identification of antiviral phytochemicals from cranberry as potential inhibitors of SARS-CoV-2 main protease (Mpro)

Cranberry phytochemicals are known to possess antiviral activities. In the current study, we explored the therapeutic potential of cranberry against SARS-CoV-2 by targeting its main protease (Mpro) enzyme. Firstly, phytochemicals of cranberry origin were identified from three independent databases. Subsequently, virtual screening, using molecular docking and molecular dynamics simulation approaches, led to the identification of three lead phytochemicals namely, cyanidin 3-O-galactoside, β-carotene and epicatechin. Furthermore, in vitro enzymatic assays revealed that cyanidin 3-O-galactoside had the highest inhibitory potential with IC50 of 9.98 μM compared to the other two phytochemicals. Cyanidin 3-O-galactoside belongs to the class of anthocyanins. Anthocyanins extracted from frozen cranberry also exhibited the highest inhibitory potential with IC50 of 23.58 μg/ml compared to the extracts of carotenoids and flavanols, the class for β-carotene and epicatechin, respectively. Finally, we confirm the presence of the phytochemicals in the cranberry extracts using targeted LC-MS/MS analysis. Our results, therefore, indicate that the identified cranberry-derived bioactive compounds as well as cranberry could be used for therapeutic interventions against SARS-CoV-2.

EvoAI enables extreme compression and reconstruction of the protein sequence space

Designing proteins with improved functions requires a deep understanding of how sequence and function are related, a vast space that is hard to explore. The ability to efficiently compress this space by identifying functionally important features is extremely valuable. Here, we first establish a method called EvoScan to comprehensively segment and scan the high-fitness sequence space to obtain anchor points that capture its essential features, especially in high dimensions. Our approach is compatible with any biomolecular function that can be coupled to a transcriptional output. We then develop deep learning and large language models to accurately reconstruct the space from these anchors, allowing computational prediction of novel, highly fit sequences without prior homology-derived or structural information. We apply this hybrid experimental-computational method, which we call EvoAI, to a repressor protein and find that only 82 anchors are sufficient to compress the high-fitness sequence space with a compression ratio of 1048. The extreme compressibility of the space informs both applied biomolecular design and understanding of natural evolution.

Potent 3CLpro inhibitors effective against SARS-CoV-2 and MERS-CoV in animal models by therapeutic treatment

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Middle East respiratory syndrome coronavirus (MERS-CoV) are zoonotic betacoronaviruses that continue to have a significant impact on public health. Timely development and introduction of vaccines and antivirals against SARS-CoV-2 into the clinic have substantially mitigated the burden of COVID-19. However, a limited or lacking therapeutic arsenal for SARS-CoV-2 and MERS-CoV infections, respectively, calls for an expanded and diversified portfolio of antivirals against these coronavirus infections. In this report, we examined the efficacy of two potent 3CLpro inhibitors, 5d and 11d, in fatal animal models of SARS-CoV-2 and MERS-CoV to demonstrate their broad-spectrum activity against both viral infections. These compounds significantly increased the survival of mice in both models when treatment started 1 day post infection compared to no treatment which led to 100% fatality. Especially, the treatment with compound 11d resulted in 80% and 90% survival in SARS-CoV-2 and MERS-CoV-infected mice, respectively. Amelioration of lung viral load and histopathological changes in treated mice correlated well with improved survival in both infection models. Furthermore, compound 11d exhibited significant antiviral activities in K18-hACE2 mice infected with SARS-CoV-2 Omicron subvariant XBB.1.16. The results suggest that these are promising candidates for further development as broad-spectrum direct-acting antivirals against highly virulent human coronaviruses.IMPORTANCEHuman coronaviruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Middle East respiratory syndrome coronavirus (MERS-CoV) continue to have a significant impact on public health. A limited or lacking therapeutic arsenal for SARS-CoV-2 and MERS-CoV infections calls for an expanded and diversified portfolio of antivirals against these coronavirus infections. We have previously reported a series of small-molecule 3C-like protease (3CLpro) inhibitors against human coronaviruses. In this report, we demonstrated the in vivo efficacy of 3CLpro inhibitors for their broad-spectrum activity against both SARS-CoV-2 and MERS-CoV infections using the fatal animal models. The results suggest that these are promising candidates for further development as broad-spectrum direct-acting antivirals against highly virulent human coronaviruses.

Development of a GC-376 Based Peptidomimetic PROTAC as a Degrader of 3-Chymotrypsin-like Protease of SARS-CoV-2

We have applied a proteolysis targeting chimera (PROTAC) technology to obtain a peptidomimetic molecule able to trigger the degradation of SARS-CoV-2 3-chymotrypsin-like protease (3CLPro). The PROTAC molecule was designed by conjugating a GC-376 based dipeptidyl 3CLPro ligand to a pomalidomide moiety through a piperazine-piperidine linker. NMR and crystallographic data complemented with enzymatic and cellular studies showed that (i) the dipeptidyl moiety of PROTAC binds to the active site of the dimeric state of SARS-CoV-2 3CLPro forming a reversible covalent bond with the sulfur atom of catalytic Cys145, (ii) the linker and the pomalidomide cereblon-ligand of PROTAC protrude from the protein, displaying a high degree of flexibility and no interactions with other regions of the protein, and (iii) PROTAC reduces the protein levels of SARS-CoV-2 3CLPro in cultured cells. This study paves the way for the future applicability of peptidomimetic PROTACs to tackle 3CLPro-dependent viral infections.

BRD4354 Is a Potent Covalent Inhibitor against the SARS-CoV-2 Main Protease

Numerous organic molecules are known to inhibit the main protease (MPro) of SARS-CoV-2, the pathogen of Coronavirus Disease 2019 (COVID-19). Guided by previous research on zinc-ligand inhibitors of MPro and zinc-dependent histone deacetylases (HDACs), we identified BRD4354 as a potent inhibitor of MPro. The in vitro protease activity assays show that BRD4354 displays time-dependent inhibition against MPro with an IC50 (concentration that inhibits activity by 50%) of 0.72 ± 0.04 μM after 60 min of incubation. Inactivation follows a two-step process with an initial rapid binding step with a KI of 1.9 ± 0.5 μM followed by a second slow inactivation step, kinact,max of 0.040 ± 0.002 min-1. Native mass spectrometry studies indicate that a covalent intermediate is formed where the ortho-quinone methide fragment of BRD4354 forms a covalent bond with the catalytic cysteine C145 of MPro. Based on these data, a Michael-addition reaction mechanism between MPro C145 and BRD4354 was proposed. These results suggest that both preclinical testing of BRD4354 and structure-activity relationship studies based on BRD4354 are warranted to develop more effective anti-COVID therapeutics.

Discovery of Pyrano[2,3-c]pyrazole Derivatives as Novel Potential Human Coronavirus Inhibitors: Design, Synthesis, In Silico, In Vitro, and ADME Studies

The SARS-CoV-2 pandemic at the end of 2019 had major worldwide health and economic consequences. Until effective vaccination approaches were created, the healthcare sectors endured a shortage of operative treatments that might prevent the infection's spread. As a result, academia and the pharmaceutical industry prioritized the development of SARS-CoV2 antiviral medication. Pyranopyrazoles have been shown to play a prominent function in pharmaceutical chemistry and drug sighting because of their significant bioactive properties. We provide herein a novel sequence of pyranopyrazoles and their annulated systems whose antiviral efficacy and cytotoxicity were explored versus human coronavirus 229E (HCoV-229E) Vero-E6 cell lines as a model for the Coronaviridae family. Fifteen synthetic congeners pointed out miscellaneous antiviral efficacies against HCoV-229E with variable inhibition degrees. Compound 18 showed a high selectivity index (SI = 12.6) that established spectacular inhibitory capacity against human coronavirus 229E. Compounds 6, 7, and 14 exposed moderate efficacies. Compounds 6, 7, 14, and 18 exhibited substantial antiviral action through the replication phase with reduction percentages extending from 53.6%, 60.7%, and 55% to 82.2%, correspondingly. Likewise, when assessed to the positive control tipranavir (88.6%), the inhibitory efficiency of compounds 6, 7, 14, and 18 versus the SARS-CoV2 Mpro provided high percentages of 80.4%, 73.1%, 81.4% and up to 84.5%, respectively. In silico studies were performed to investigate further the biological activity and the target compounds' physical and chemical features, including molecular dynamic (MD) simulations, protein-ligand docking, ADME studies, and density functional theory (DFT) calculations. These inquiries demonstrated that this series of metabolically stable pyranopyrazoles and their annulated systems are effective human coronavirus inhibitors that inhibit the viral Mpro protein and may have emerged as a novel COVID-19 curative option.

Discovery of α-Ketoamide inhibitors of SARS-CoV-2 main protease derived from quaternized P1 groups

Although the SARS-CoV-2 pandemic has ended, multiple sporadic cases still exist, posing a request for more antivirals. The main protease (Mpro) of SARS-CoV-2, a key enzyme for viral replication, is an attractive target for drug development. Here, we report the discovery of a new potent α-ketoamide-containing Mpro inhibitor, N-((R)-1-cyclohexyl-2-(((R)-3-methoxy-1-oxo-1-((1-(2-oxo-2-((thiazol-2-ylmethyl)amino)acetyl)cyclobutyl)amino)propan-2-yl)amino)-2-oxoethyl)-4,4-difluorocyclohexane-1-carboxamide (20j). This compound presented promising enzymatic inhibitory activity against SARS-CoV-2 Mpro with an IC50 value of 19.0 nM, and an excellent antiviral activity in cell-based assay with an EC50 value of 138.1 nM. This novel covalent inhibitor may be used as a lead compound for subsequent drug discovery against SARS-CoV-2.

Fragment-based screening targeting an open form of the SARS-CoV-2 main protease binding pocket

To identify starting points for therapeutics targeting SARS-CoV-2, the Paul Scherrer Institute and Idorsia decided to collaboratively perform an X-ray crystallographic fragment screen against its main protease. Fragment-based screening was carried out using crystals with a pronounced open conformation of the substrate-binding pocket. Of 631 soaked fragments, a total of 29 hits bound either in the active site (24 hits), a remote binding pocket (three hits) or at crystal-packing interfaces (two hits). Notably, two fragments with a pose that was sterically incompatible with a more occluded crystal form were identified. Two isatin-based electrophilic fragments bound covalently to the catalytic cysteine residue. The structures also revealed a surprisingly strong influence of the crystal form on the binding pose of three published fragments used as positive controls, with implications for fragment screening by crystallography.

Docking and other computing tools in drug design against SARS-CoV-2

The use of computer simulation methods has become an indispensable component in identifying drugs against the SARS-CoV-2 coronavirus. There is a huge body of literature on application of molecular modelling to predict inhibitors against target proteins of SARS-CoV-2. To keep our review clear and readable, we limited ourselves primarily to works that use computational methods to find inhibitors and test the predicted compounds experimentally either in target protein assays or in cell culture with live SARS-CoV-2. Some works containing results of experimental discovery of corresponding inhibitors without using computer modelling are included as examples of a success. Also, some computational works without experimental confirmations are also included if they attract our attention either by simulation methods or by databases used. This review collects studies that use various molecular modelling methods: docking, molecular dynamics, quantum mechanics, machine learning, and others. Most of these studies are based on docking, and other methods are used mainly for post-processing to select the best compounds among those found through docking. Simulation methods are presented concisely, information is also provided on databases of organic compounds that can be useful for virtual screening, and the review itself is structured in accordance with coronavirus target proteins.

In silico study of inhibition activity of boceprevir drug against 2019-nCoV main protease

Boceprevir drug is a ketoamide serine protease inhibitor with a linear peptidomimetic structure that exhibits inhibition activity against 2019-nCoV main protease. This paper reports electronic properties of boceprevir and its molecular docking as well as molecular dynamics simulation analysis with protein receptor. For this, the equilibrium structure of boceprevir has been obtained by DFT at B3LYP and ωB97XD levels with 6-311+G(d,p) basis set in gas and water mediums. HOMO-LUMO and absorption spectrum analysis have been used to evaluate the boceprevir's toxicity and photosensitivity, respectively. Molecular docking simulation has been performed to test the binding affinity of boceprevir with 2019-nCoV MPRO; which rendered a variety of desirable binding locations between the ligand and target protein's residue positions. The optimum binding location has been considered for molecular dynamics simulation. The findings have been addressed to clarify the boceprevir drug efficacy against the 2019-nCoV MPRO.

On the origins of SARS-CoV-2 main protease inhibitors

In order to address the world-wide health challenge caused by the COVID-19 pandemic, the 3CL protease/SARS-CoV-2 main protease (SARS-CoV-2-Mpro) coded by its nsp5 gene became one of the biochemical targets for the design of antiviral drugs. In less than 3 years of research, 4 inhibitors of SARS-CoV-2-Mpro have actually been authorized for COVID-19 treatment (nirmatrelvir, ensitrelvir, leritrelvir and simnotrelvir) and more such as EDP-235, FB-2001 and STI-1558/Olgotrelvir or five undisclosed compounds (CDI-988, ASC11, ALG-097558, QLS1128 and H-10517) are undergoing clinical trials. This review is an attempt to picture this quite unprecedented medicinal chemistry feat and provide insights on how these cysteine protease inhibitors were discovered. Since many series of covalent SARS-CoV-2-Mpro inhibitors owe some of their origins to previous work on other proteases, we first provided a description of various inhibitors of cysteine-bearing human caspase-1 or cathepsin K, as well as inhibitors of serine proteases such as human dipeptidyl peptidase-4 or the hepatitis C protein complex NS3/4A. This is then followed by a description of the results of the approaches adopted (repurposing, structure-based and high throughput screening) to discover coronavirus main protease inhibitors.

Development of de-novo coronavirus 3-chymotrypsin-like protease (3CLpro) inhibitors since COVID-19 outbreak: A strategy to tackle challenges of persistent virus infection

Although no longer a public health emergency of international concern, COVID-19 remains a persistent and critical health concern. The development of effective antiviral drugs could serve as the ultimate piece of the puzzle to curbing this global crisis. 3-chymotrypsin-like protease (3CLpro), with its substrate specificity mirroring that of the main picornavirus 3C protease and conserved across various coronaviruses, emerges as an ideal candidate for broad-spectrum antiviral drug development. Moreover, it holds the potential as a reliable contingency option to combat emerging SARS-CoV-2 variants. In this light, the approved drugs, promising candidates, and de-novo small molecule therapeutics targeting 3CLpro since the COVID-19 outbreak in 2020 are discussed. Emphasizing the significance of diverse structural characteristics in inhibitors, be they peptidomimetic or nonpeptidic, with a shared mission to minimize the risk of cross-resistance. Moreover, the authors propose an innovative optimization strategy for 3CLpro reversible covalent PROTACs, optimizing pharmacodynamics and pharmacokinetics to better prepare for potential future viral outbreaks.

A Review on Molecular Docking As an Interpretative Tool for Molecular Targets in Disease Management

One of the most often utilized methods for drug discovery is molecular docking. With docking, one may discover new therapeutically relevant molecules by targeting the molecule and predicting the target-ligand interactions as well as different conformation of ligand at various positions. The prediction signifies the effectiveness of the molecule or the developed molecule having different affinity with target. Drug discovery plays an important role in the development of a new drug molecule of different moiety attached to it, which leads us in the management of several diseases. In silico approach led us to identification of numerous diseases caused by virus, fungi, bacteria, protozoa, and other microorganisms that affect human health. By means of computational approach, we can categorize disease symptoms and use the drugs available for such types of warning signs. After the docking process, molecular dynamics computational technique helps in the simulation of the physical movement of atoms and molecules for a fixed period of time, giving a view of the dynamic evaluation of the system. This review is an attempt to illustrate the role of molecular docking in drug development.

Chrysin 7-O-β-D-glucuronide, a dual inhibitor of SARS-CoV-2 3CLpro and PLpro, for the prevention and treatment of COVID-19

The emergence of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) resulted in the coronavirus disease 2019 (COVID-19) pandemic. Given the advent of subvariants, there is an urgent need to develop novel drugs. The aim of this study was to find SARS-CoV-2 inhibitors from Scutellaria baicalensis Georgi targeting the proteases 3CLpro and PLpro. After screening 25 flavonoids, chrysin 7-O-β-D-glucuronide was found to be a potent inhibitor of SARS-CoV-2 on Vero E6 cells, with half-maximal effective concentration of 8.72 µM. Surface plasmon resonance assay, site-directed mutagenesis and enzymatic activity measurements indicated that chrysin-7-O-β-D-glucuronide inhibits SARS-CoV-2 by binding to H41 of 3CLpro, and K157 and E167 of PLpro. Hydrogen-deuterium exchange mass spectrometry analysis showed that chrysin-7-O-β-D-glucuronide changes the conformation of PLpro. Finally, chrysin 7-O-β-D-glucuronide was shown to have anti-inflammatory activity, mainly due to reduction of the levels of the pro-inflammatory cytokines interleukin (IL)-1β and IL-6.

Therapeutic cysteine protease inhibitors: a patent review (2018-present)

INTRODUCTION

Cysteine proteases are involved in a broad range of biological functions, ranging from extracellular matrix turnover to immunity. Playing an important role in the onset and progression of several diseases, including cancer, immune-related and neurodegenerative disease, viral and parasitic infections, cysteine proteases represent an attractive drug target for the development of therapeutic tools.

AREAS COVERED

Recent scientific and patent literature focusing on the design and study of cysteine protease inhibitors with potential therapeutic application has been reviewed.

EXPERT OPINION

The discovery of a number of effective structurally diverse cysteine protease inhibitors opened up new challenges and opportunities for the development of therapeutic tools. Mechanistic studies and the availability of X-ray crystal structures of some proteases, alone and in complex with inhibitors, provide crucial information for the rational design and development of efficient and selective cysteine protease inhibitors as preclinical candidates for the treatment of different diseases.

Persistence of an infectious form of SARS-CoV-2 post protease inhibitor treatment of permissive cells in vitro

Reports have described SARS-CoV-2 rebound in COVID-19 patients treated with nirmatrelvir, a 3CL protease inhibitor. The cause remains a mystery, although drug resistance, re-infection, and lack of adequate immune responses have been excluded. We now present virologic findings that provide a clue to the cause of viral rebound, which occurs in ~20% of the treated cases. The persistence of an intermediary form of infectious SARS-CoV-2 was experimentally documented in vitro after treatment with nirmatrelvir or another 3CL protease inhibitor, but not with a polymerase inhibitor, remdesivir. This infectious intermediate decayed slowly with a half-life of ~1 day, suggesting that its persistence could outlive the treatment course to re-ignited SARS-CoV-2 infection as the drug is eliminated. Additional studies are needed to define the nature of this viral intermediate, but our findings point to a particular direction for future investigation and offer a specific treatment recommendation that should be tested clinically.

E3 ubiquitin ligase ZBTB25 suppresses beta coronavirus infection through ubiquitination of the main viral protease MPro

The main protease of severe acute respiratory syndrome coronavirus 2, Mpro, is a key viral protein essential for viral infection and replication. Mpro has been the target of many pharmacological efforts; however, the host-specific regulation of Mpro protein remains unclear. Here, we report the ubiquitin-proteasome-dependent degradation of Mpro protein in human cells, facilitated by the human E3 ubiquitin ligase ZBTB25. We demonstrate that Mpro has a short half-life that is prolonged via proteasomal inhibition, with its Lys-100 residue serving as a potential ubiquitin acceptor. Using in vitro binding assays, we observed ZBTB25 and Mpro bind to each other in vitro, and using progressive deletional mapping, we further uncovered the required domains for this interaction. Finally, we used an orthologous beta-coronavirus infection model and observed that genetic ablation of ZBTB25 resulted in a more highly infective virus, an effect lost upon reconstitution of ZBTB25 to deleted cells. In conclusion, these data suggest a new mechanism of Mpro protein regulation as well as identify ZBTB25 as an anticoronaviral E3 ubiquitin ligase.

Inhibition of SARS-CoV-2 3CL protease by the anti-viral chimeric protein RetroMAD1

COVID-19 results from SARS-CoV-2, which mutates frequently, challenging current treatments. Therefore, it is critical to develop new therapeutic drugs against this disease. This study explores the interaction between SARS-CoV-2 3CLpro and RetroMAD1, a well-characterized coronavirus protein and potential drug target, using in-silico methods. The analysis through the HDOCK server showed stable complex formation with a binding energy of -12.3, the lowest among reference drugs. The RetroMAD1-3CLpro complex underwent a 100 ns molecular dynamics simulation (MDS) in an explicit solvation system, generating various trajectories, including RMSD, RMSF, hydrogen bonding, radius of gyration, and ligand binding energy. MDS results confirmed intact interactions within the RetroMAD1-3CLpro complex during simulations. In vitro experiments validated RetroMAD1's ability to inhibit 3CLpro enzyme activity and prevent SARS-CoV-2 infection in human bronchial cells. RetroMAD1 exhibited antiviral efficacy comparable to Remdesivir without cytotoxicity at effective concentrations. These results suggest RetroMAD1 as a potential drug candidate against SARS-CoV-2, warranting further in vivo and clinical studies to assess its efficiency.

Structure and function of SARS-CoV and SARS-CoV-2 main proteases and their inhibition: A comprehensive review

Severe acute respiratory syndrome-associated coronavirus (SARS-CoV) identified in 2003 infected ∼8000 people in 26 countries with 800 deaths, which was soon contained and eradicated by syndromic surveillance and enhanced quarantine. A closely related coronavirus SARS-CoV-2, the causative agent of COVID-19 identified in 2019, has been dramatically more contagious and catastrophic. It has infected and caused various flu-like symptoms of billions of people in >200 countries, including >6 million people died of or with the virus. Despite the availability of several vaccines and antiviral drugs against SARS-CoV-2, finding new therapeutics is needed because of viral evolution and a possible emerging coronavirus in the future. The main protease (Mpro) of these coronaviruses plays important roles in their life cycle and is essential for the viral replication. This article represents a comprehensive review of the function, structure and inhibition of SARS-CoV and -CoV-2 Mpro, including structure-activity relationships, protein-inhibitor interactions and clinical trial status.

Evaluation of in vitro antiviral activity of SARS-CoV-2 Mpro inhibitor pomotrelvir and cross-resistance to nirmatrelvir resistance substitutions

The unprecedented scale of the COVID-19 pandemic and the rapid evolution of SARS-CoV-2 variants underscore the need for broadly active inhibitors with a high barrier to resistance. The coronavirus main protease (Mpro) is an essential cysteine protease required for viral polyprotein processing and is highly conserved across human coronaviruses. Pomotrelvir is a novel Mpro inhibitor that has recently completed a phase 2 clinical trial. In this report, we demonstrated that pomotrelvir is a potent competitive inhibitor of SARS-CoV-2 Mpro with high selectivity against human proteases. In the enzyme assay, pomotrelvir is also active against Mpro proteins derived from human coronaviruses CoV-229E, CoV-OC43, CoV-HKU1, CoV-NL63, MERS, and SARS-CoV. In cell-based SARS-CoV-2 replicon and SARS-CoV-2 infection assays, pomotrelvir has shown potent inhibitory activity and is broadly active against SARS-CoV-2 clinical isolates including Omicron variants. Many resistance substitutions of the Mpro inhibitor nirmatrelvir confer cross-resistance to pomotrelvir, consistent with the finding from our enzymatic analysis that pomotrelvir and nirmatrelvir compete for the same binding site. In a SARS-CoV-2 infection assay, pomotrelvir is additive when combined with remdesivir or molnupiravir, two nucleoside analogs targeting viral RNA synthesis. In conclusion, our results from the in vitro characterization of pomotrelvir antiviral activity support its further clinical development as an alternative COVID-19 therapeutic option.

An electrophilic fragment screening for the development of small molecules targeting caspase-2

Recent Alzheimer's research has shown increasing interest in the caspase-2 (Casp2) enzyme. However, the available Casp2 inhibitors, which have been pentapeptides or peptidomimetics, face challenges for use as CNS drugs. In this study, we successfully screened a 1920-compound chloroacetamide-based, electrophilic fragment library from Enamine. Our two-point dose screen identified 64 Casp2 hits, which were further evaluated in a ten-point dose-response study to assess selectivity over Casp3. We discovered compounds with inhibition values in the single-digit micromolar and sub-micromolar range, as well as up to 32-fold selectivity for Casp2 over Casp3. Target engagement analysis confirmed the covalent-irreversible binding of the selected fragments to Cys320 at the active site of Casp2. Overall, our findings lay a strong foundation for the future development of small-molecule Casp2 inhibitors.

Enhanced expression and solubility of main protease (Mpro) of SARS-CoV-2 from E. coli

The main protease (Mpro) of SARS-CoV-2 is a essential enzyme that facilitates viral transcription and replication. Furthermore, the conservation of Mpro across different variants and its non-overlapping nature with human proteases make it an appealing target for therapeutic interventions against SARS-CoV-2. Multiple inhibitors specifically target Mpro to mitigate the infection caused by SARS-CoV-2. In the current study, successful cloning and expression of SARS-CoV-2 Mpro were achieved using two E. coli hosts, namely BL21-DE3 and BL21-DE3-RIL. By optimizing the conditions for induction, the expression of Mpro in the soluble fraction of E. coli was improved. Subsequently, Mpro was purified using affinity chromatography, yielding significantly higher quantities from the BL21-DE3-RIL strain compared to the BL21-DE3 strain, with the former producing nearly twice as much as the latter. The purified Mpro was further characterized by mass spectrometry, fluorescence spectroscopy and circular dichroism (CD). Through fluorescence quenching studies, it was discovered that both GC376 and chitosan, which are inhibitors of Mpro, induced structural changes in the purified Mpro protein. This indicates that the protein retained its functional activity even after being expressed in a bacterial host. Further, FRET-based assay highlighted that the enzymatic activity of Mpro was significantly reduced in presence of both GC376 and chitosan. Consequently, the utilization of optimal conditions and the BL21-DE3-RIL bacterial host facilitates the cost-effective production of Mpro on a large scale, enabling high yields. This production approach can be applied for the screening of potent therapeutic drugs, making it a valuable resource for drug development endeavors.