The highly contagious Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV‑2) emerged in late 2019 and led to the severe COVID-19 pandemic. The emergence of novel virus variants emphasises the importance of targeted efforts to identify inhibitors that can halt viral multiplication and treat coronavirus infections.

A team of researchers from Indian institutes, namely, Shiv Nadar Institute of Eminence (SNIoE), Noida, the All India Institute of Medical Sciences (AIIMS), Delhi, the Regional Centre of Biotechnology (RCB), Faridabad, along with Srikara Biologicals, Tirupati, has identified a new molecule, named STL522228. STL522228 is also known as 4-(3,4‑dihydroxyphenyl)-6,7‑dihydroxy-1-isopropyl-1H-benzofuro[3,2‑b]pyrazolo[4,3‑e]pyridin‑3(2H)-one. This molecule can inhibit a crucial protein responsible for the multiplication of the Severe Acute Respiratory Syndrome (SARS) coronavirus. Deepak Sehgal, Professor, SNIoE, believes that the discovery holds promise for this molecule to be used as a drug candidate to combat different coronaviruses, pending validation by clinical trials.

Recently published in the peer-reviewed journal,​‘The FEBS journal,’ the study addresses concerns about recurring coronavirus outbreaks, the severity of the COVID-19 pandemic, and the rapid emergence of novel variants of concern. While vaccinations provide defense against coronavirus infections, the need for antiviral treatments remains to be discovered. 

Marceline Côté, Canada Research Chair in Molecular Virology and Antiviral Therapeutics and Director, University of Ottawa Centre for Infection, Immunity, and Inflammation, says,

The development of antiviral treatments against SARS-CoV‑2 and various coronaviruses is crucial to manage the pandemic and enhance preparedness for future pandemics. 

The study aims to use the major protein of SARS-CoV‑2, known as Mpro, as the drug target, which plays a critical role in viral replication through the processing of the viral polyprotein. The reported inhibitor halts the enzyme activity and subsequently controls virus replication. Since this protein is present in coronaviruses but not in humans, it becomes an attractive target for antiviral drug development. The inhibitor identified in the present study targets Mpro and subsequently halts the production of functional enzymes, arresting virus multiplication and thus controlling the infection.

To discover potential inhibitors against Mpro, the researchers screened compounds from several databases and finally identified STL 522228 as a potent inhibitor using computational docking. This compound exhibited effective interactions with critical residues of the protein, specifically Cysteine and Histidine in the active site of Mpro. They also identified domains of the enzyme where the drug interacts with the protein, stabilizing the complex. Techniques involving computational simulations, binding, and biochemical assays were used to study the inhibitor-target interaction, marking the first step to halt viral multiplication.

The team confirmed how well the Mpro inhibitors worked by testing them in a protease cleavage inhibition experiment. Aditya Trivedi, the first author of the paper, mentions, ​“Since there was no defined assay for studying the activity or inhibition of Mpro, our biggest challenge involved devising an in-house assay to assess the enzyme activity and its inhibition.” 

Further, cell-based antiviral assays against SARS-CoV‑2 were also conducted using the Human Coronavirus 229E (HCoV-229E) surrogate system. Sehgal, Professor, SNIoE, explains, ​“Since we could not culture the virus in our lab due to safety reasons, we used the surrogate HCoV-229E system, as there is a high structural similarity between the Mpro structures from HCoV-229E and SARS-CoV‑2. The validated inhibitors were further studied for their effect in the SARS-CoV‑2 culture system at RCB, Faridabad. Surprisingly, the effect of the inhibitor was comparable to Remdesivir, the reference drug that can stop virus multiplication. The researchers reported a 97% reduction in the viral load when treated with a biologically safe amount of STL522228.”

The research group also found that this compound strongly interacts with the Mpro of viruses across various groups within the Coronaviridae family, suggesting it could be a potential drug for multiple coronaviruses. They observed that STL522228 meets most of the criteria outlined in the Lipinski drug design rules. Sehgal, Professor, SNIoE, states, 

The efficacy of the STL522228 can be further increased by generating analogs of the compound via structural changes, helping in the development of a therapeutic drug against the coronavirus in future.

Côté added, ​“This compound might also work against other coronaviruses.” They’re optimistic about advancing this research toward clinical trials to test its effectiveness in treating various coronavirus infections.