The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2) is a global health emergency. An attractive drug target among coronaviruses is the main protease (M ^pro , also called 3CL ^pro ) because of its essential role in processing the polyproteins that are translated from the viral RNA. We report the x-ray structures of the unliganded SARS-CoV-2 M ^pro and its complex with an α-ketoamide inhibitor. This was derived from a previously designed inhibitor but with the P3-P2 amide bond incorporated into a pyridone ring to enhance the half-life of the compound in plasma. On the basis of the unliganded structure, we developed the lead compound into a potent inhibitor of the SARS-CoV-2 M ^pro . The pharmacokinetic characterization of the optimized inhibitor reveals a pronounced lung tropism and suitability for administration by the inhalative route.

A novel coronavirus has been identified as the causative agent of severe acute respiratory syndrome (SARS). The viral main proteinase (M ^pro , also called 3CL ^pro ), which controls the activities of the coronavirus replication complex, is an attractive target for therapy. We determined crystal structures for human coronavirus (strain 229E) M ^pro and for an inhibitor complex of porcine coronavirus [transmissible gastroenteritis virus (TGEV)] M ^pro , and we constructed a homology model for SARS coronavirus (SARS-CoV) M ^pro . The structures reveal a remarkable degree of conservation of the substrate-binding sites, which is further supported by recombinant SARS-CoV M ^pro -mediated cleavage of a TGEV M ^pro substrate. Molecular modeling suggests that available rhinovirus 3C ^pro inhibitors may be modified to make them useful for treating SARS.

The coronavirus disease 2019 (COVID-19) pandemic has stimulated tremendous efforts to develop therapeutic strategies that target severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and/or human proteins to control viral infection, encompassing hundreds of potential drugs and thousands of patients in clinical trials. So far, a few small-molecule antiviral drugs (nirmatrelvir–ritonavir, remdesivir and molnupiravir) and 11 monoclonal antibodies have been marketed for the treatment of COVID-19, mostly requiring administration within 10 days of symptom onset. In addition, hospitalized patients with severe or critical COVID-19 may benefit from treatment with previously approved immunomodulatory drugs, including glucocorticoids such as dexamethasone, cytokine antagonists such as tocilizumab and Janus kinase inhibitors such as baricitinib. Here, we summarize progress with COVID-19 drug discovery, based on accumulated findings since the pandemic began and a comprehensive list of clinical and preclinical inhibitors with anti-coronavirus activities. We also discuss the lessons learned from COVID-19 and other infectious diseases with regard to drug repurposing strategies, pan-coronavirus drug targets, in vitro assays and animal models, and platform trial design for the development of therapeutics to tackle COVID-19, long COVID and pathogenic coronaviruses in future outbreaks.

This review focuses on the important contributions that macromolecular crystallography has made over the past 12 years to elucidating structures and mechanisms of the essential proteases of coronaviruses, the main protease (M^pro) and the papain‐like protease (PL^pro). The role of X‐ray crystallography in structure‐assisted drug discovery against these targets is discussed. Aspects dealt with in this review include the emergence of the SARS coronavirus in 2002–2003 and of the MERS coronavirus 10 years later and the origins of these viruses. The crystal structure of the free SARS coronavirus M^pro and its dependence on pH is discussed, as are efforts to design inhibitors on the basis of these structures. The mechanism of maturation of the enzyme from the viral polyprotein is still a matter of debate. The crystal structure of the SARS coronavirus PL^pro and its complex with ubiquitin is also discussed, as is its orthologue from MERS coronavirus. Efforts at predictive structure‐based inhibitor development for bat coronavirus M^pros to increase the preparedness against zoonotic transmission to man are described as well. The paper closes with a brief discussion of structure‐based discovery of antivirals in an academic setting.

The main protease of coronaviruses and the 3C protease of enteroviruses share a similar active-site architecture and a unique requirement for glutamine in the P1 position of the substrate. Because of their unique specificity and essential role in viral polyprotein processing, these proteases are suitable targets for the development of antiviral drugs. In order to obtain near-equipotent, broad-spectrum antivirals against alphacoronaviruses, betacoronaviruses, and enteroviruses, we pursued structure-based design of peptidomimetic α-ketoamides as inhibitors of main and 3C proteases. Six crystal structures of protease:inhibitor complexes were determined as part of this study. Compounds synthesized were tested against the recombinant proteases as well as in viral replicons and virus-infected cell cultures; most of them were not cell-toxic. Optimization of the P2 substituent of the α-ketoamides proved crucial for achieving near-equipotency against the three virus genera. The best near-equipotent inhibitors, 11u (P2 = cyclopentylmethyl) and 11r (P2 = cyclohexylmethyl), display low-micromolar EC50 values against enteroviruses, alphacoronaviruses, and betacoronaviruses in cell cultures. In Huh7 cells, 11r exhibits three-digit picomolar activity against Middle East Respiratory Syndrome coronavirus.