Postinfection treatment with a protease inhibitor increases survival of mice with a fatal SARS-CoV-2 infection.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection continues to be a serious global public health threat. The 3C-like protease (3CLpro) is a virus protease encoded by SARS-CoV-2, which is essential for virus replication. We have previously reported a series of small-molecule 3CLpro inhibitors effective for inhibiting replication of human coronaviruses including SARS-CoV-2 in cell culture and in animal models. Here we generated a series of deuterated variants of a 3CLpro inhibitor, GC376, and evaluated the antiviral effect against SARS-CoV-2. The deuterated GC376 displayed potent inhibitory activity against SARS-CoV-2 in the enzyme- and the cell-based assays. The K18-hACE2 mice develop mild to lethal infection commensurate with SARS-CoV-2 challenge doses and were proposed as a model for efficacy testing of antiviral agents. We treated lethally infected mice with a deuterated derivative of GC376. Treatment of K18-hACE2 mice at 24 h postinfection with a derivative (compound 2) resulted in increased survival of mice compared to vehicle-treated mice. Lung virus titers were decreased, and histopathological changes were ameliorated in compound 2-treated mice compared to vehicle-treated mice. Structural investigation using high-resolution crystallography illuminated binding interactions of 3CLpro of SARS-CoV-2 and SARS-CoV with deuterated variants of GC376. Taken together, deuterated GC376 variants have excellent potential as antiviral agents against SARS-CoV-2.

Interactions of viruses and the humoral innate immune response.

The innate immune response is crucial for defense against virus infections where the complement system, coagulation cascade and natural antibodies play key roles. These immune components are interconnected in an intricate network and are tightly regulated to maintain homeostasis and avoid uncontrolled immune responses. Many viruses in turn have evolved to modulate these interactions through various strategies to evade innate immune activation. This review summarizes the current understanding on viral strategies to inhibit the activation of complement and coagulation cascades, evade natural antibody-mediated clearance and utilize complement regulatory mechanisms to their advantage.

Broad-Spectrum Cyclopropane-Based Inhibitors of Coronavirus 3C-like Proteases: Biochemical, Structural, and Virological Studies.

The advent of SARS-CoV-2, the causative agent of COVID-19, and its worldwide impact on global health, have provided the impetus for the development of effective countermeasures that can be deployed against the virus, including vaccines, monoclonal antibodies, and direct-acting antivirals (DAAs). Despite these efforts, the current paucity of DAAs has created an urgent need for the creation of an enhanced and diversified portfolio of broadly acting agents with different mechanisms of action that can effectively abrogate viral infection. SARS-CoV-2 3C-like protease (3CLpro), an enzyme essential for viral replication, is a validated target for the discovery of SARS-CoV-2 therapeutics. In this report, we describe the structure-guided utilization of the cyclopropane moiety in the design of highly potent inhibitors of SARS-CoV-2 3CLpro, SARS-CoV-1 3CLpro, and MERS-CoV 3CLpro. High-resolution cocrystal structures were used to identify the structural determinants associated with the binding of the inhibitors to the active site of the enzyme and unravel the mechanism of action. Aldehydes 5c and 11c inhibited SARS-CoV-2 replication with EC50 values of 12 and 11 nM, respectively. Furthermore, the corresponding aldehyde bisulfite adducts 5d and 11d were equipotent with EC50 values of 13 and 12 nM, respectively. The safety index (SI) values for compounds 5c / 11c and 5d / 11d ranged between 7692 and 9090. Importantly, aldehydes 5c / 11c and bisulfite adducts 5d / 11d potently inhibited MERS-CoV 3CLpro with IC50 values of 80 and 120 nM, and 70 and 70 nM, respectively. Likewise, compounds 5c / 11c and 5d / 11d inhibited SARS-CoV-1 with IC50 values of 960 and 350 nM and 790 and 240 nM, respectively. Taken together, these studies suggest that the inhibitors described herein have low cytotoxicity and high potency and are promising candidates for further development as broad-spectrum direct-acting antivirals against highly pathogenic coronaviruses.

Potent Protease Inhibitors of Highly Pathogenic Lagoviruses: Rabbit Hemorrhagic Disease Virus and European Brown Hare Syndrome Virus.

Rabbit hemorrhagic disease (RHD) and European brown hare syndrome (EBHS) are highly contagious diseases caused by lagoviruses in the Caliciviridae family. These infectious diseases are associated with high mortality and a serious threat to domesticated and wild rabbits and hares, including endangered species such as riparian brush rabbits (Sylvilagus bachmani riparius). In the United States (U.S.), only isolated cases of RHD had been reported until Spring 2020. However, RHD caused by GI.2/rabbit hemorrhagic disease virus (RHDV)2/b was unexpectedly reported in April 2020 in New Mexico and has subsequently spread to several U.S. states, infecting wild rabbits and hares and making it highly likely that RHD will become endemic in the U.S. Vaccines are available for RHD; however, there is no specific treatment for this disease. Lagoviruses encode a 3C-like protease (3CLpro), which is essential for virus replication and a promising target for antiviral drug development. We have previously generated focused small-molecule libraries of 3CLpro inhibitors and demonstrated the in vitro potency and in vivo efficacy of some protease inhibitors against viruses encoding 3CLpro, including caliciviruses and coronaviruses. Here, we report the development of the enzyme and cell-based assays for the 3CLpro of GI.1c/RHDV, recombinant GI.3P-GI.2 (RHDV2/b), and GII.1/European brown hare syndrome virus (EBHSV) as well as the identification of potent lagovirus 3CLpro inhibitors, including GC376, a protease inhibitor being developed for feline infectious peritonitis. In addition, structure-activity relationship study and homology modeling of the 3CLpro and inhibitors revealed that lagovirus 3CLpro share similar structural requirements for inhibition with other calicivirus 3CLpro. IMPORTANCE Rabbit hemorrhagic disease (RHD) and European brown hare syndrome (EBHS) are viral diseases that affect lagomorphs with significant economic and ecological impacts. RHD vaccines are available, but specific antiviral treatment for these viral infections would be a valuable addition to the current control measures. Lagoviruses encode 3C-like protease (3CLpro), which is essential for virus replication and an attractive target for antiviral drug discovery. We have screened and identified potent small-molecule inhibitors that block lagovirus 3CLpro in the enzyme- and cell-based assays. Our results suggest that these compounds have the potential for further development as antiviral drugs for lagoviruses.

Structure-Guided Design of Potent Spirocyclic Inhibitors of Severe Acute Respiratory Syndrome Coronavirus-2 3C-like Protease.

The worldwide impact of the ongoing COVID-19 pandemic on public health has made imperative the discovery and development of direct-acting antivirals aimed at targeting viral and/or host targets. SARS-CoV-2 3C-like protease (3CLpro) has emerged as a validated target for the discovery of SARS-CoV-2 therapeutics because of the pivotal role it plays in viral replication. We describe herein the structure-guided design of highly potent inhibitors of SARS-CoV-2 3CLpro that incorporate in their structure novel spirocyclic design elements aimed at optimizing potency by accessing new chemical space. Inhibitors of both SARS-CoV-2 3CLpro and MERS-CoV 3CLpro that exhibit nM potency and high safety indices have been identified. The mechanism of action of the inhibitors and the structural determinants associated with binding were established using high-resolution cocrystal structures.

Structure-Guided Design of Potent Inhibitors of SARS-CoV-2 3CL Protease: Structural, Biochemical, and Cell-Based Studies.

The COVID-19 pandemic is having a major impact on public health worldwide, and there is an urgent need for the creation of an armamentarium of effective therapeutics, including vaccines, biologics, and small-molecule therapeutics, to combat SARS-CoV-2 and emerging variants. Inspection of the virus life cycle reveals multiple viral- and host-based choke points that can be exploited to combat the virus. SARS-CoV-2 3C-like protease (3CLpro), an enzyme essential for viral replication, is an attractive target for therapeutic intervention, and the design of inhibitors of the protease may lead to the emergence of effective SARS-CoV-2-specific antivirals. We describe herein the results of our studies related to the application of X-ray crystallography, the Thorpe-Ingold effect, deuteration, and stereochemistry in the design of highly potent and nontoxic inhibitors of SARS-CoV-2 3CLpro.

3C-like protease inhibitors block coronavirus replication in vitro and improve survival in MERS-CoV-infected mice.

Pathogenic coronaviruses are a major threat to global public health, as exemplified by severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and the newly emerged SARS-CoV-2, the causative agent of coronavirus disease 2019 (COVID-19). We describe herein the structure-guided optimization of a series of inhibitors of the coronavirus 3C-like protease (3CLpro), an enzyme essential for viral replication. The optimized compounds were effective against several human coronaviruses including MERS-CoV, SARS-CoV, and SARS-CoV-2 in an enzyme assay and in cell-based assays using Huh-7 and Vero E6 cell lines. Two selected compounds showed antiviral effects against SARS-CoV-2 in cultured primary human airway epithelial cells. In a mouse model of MERS-CoV infection, administration of a lead compound 1 day after virus infection increased survival from 0 to 100% and reduced lung viral titers and lung histopathology. These results suggest that this series of compounds has the potential to be developed further as antiviral drugs against human coronaviruses.

Characterization of amino acid substitutions in feline coronavirus 3C-like protease from a cat with feline infectious peritonitis treated with a protease inhibitor.

Feline infectious peritonitis (FIP) is a highly fatal disease caused by a virulent feline coronavirus in domestic and wild cats. We have previously reported the synthesis of potent coronavirus 3C-like protease (3CLpro) inhibitors and the efficacy of a protease inhibitor, GC376, in client-owned cats with FIP. In this study, we studied the effect of the amino acid changes in 3CLpro of feline coronavirus from a feline patient who received antiviral treatment for prolonged duration. We generated recombinant 3CLpro containing the identified amino acid changes (N25S, A252S or K260 N) and determined their susceptibility to protease inhibitors in the fluorescence resonance energy transfer assay. The assay showed that N25S in 3CLpro confers a small change (up to 1.68-fold increase in the 50% inhibitory concentration) in susceptibility to GC376, but other amino acid changes do not affect susceptibility. Modelling of 3CLpro carrying the amino acid changes was conducted to probe the structural basis for these findings. The results of this study may explain the observed absence of clinical resistance to the long-term antiviral treatment in the patients.

Protease inhibitors broadly effective against feline, ferret and mink coronaviruses.

Ferret and mink coronaviruses typically cause catarrhal diarrhea in ferrets and minks, respectively. In recent years, however, systemic fatal coronavirus infection has emerged in ferrets, which resembles feline infectious peritonitis (FIP) in cats. FIP is a highly fatal systemic disease caused by a virulent feline coronavirus infection in cats. Despite the importance of coronavirus infections in these animals, there are no effective commercial vaccines or antiviral drugs available for these infections. We have previously reported the efficacy of a protease inhibitor in cats with FIP, demonstrating that a virally encoded 3C-like protease (3CLpro) is a valid target for antiviral drug development for coronavirus infections. In this study, we extended our previous work on coronavirus inhibitors and investigated the structure-activity relationships of a focused library of protease inhibitors for ferret and mink 3CLpro. Using the fluorescence resonance energy transfer assay, we identified potent inhibitors broadly effective against feline, ferret and mink coronavirus 3CLpro. Multiple amino acid sequence analysis and modelling of 3CLpro of ferret and mink coronaviruses were conducted to probe the structural basis for these findings. The results of this study provide support for further research to develop broad-spectrum antiviral agents for multiple coronavirus infections. To the best of our knowledge, this is the first report on small molecule inhibitors of ferret and mink coronaviruses.