No specific treatment is currently available for human coronaviruses to date, but numerous antiviral agents are being identified through a variety of approaches, according to Thanigaimalai Pillaiyar, PhD, and colleagues in a review published in Drug Discovery Today.
Using the six previously discovered human coronaviruses – human CoV 229E (HCoV-229E), OC43 (HCoV-OC43), NL63 (HCoV-NL63), HKU1 (HCoV-HKU1); severe acute respiratory syndrome (SARS) CoV; and Middle East respiratory syndrome (MERS) CoV – the investigators examined progress in the use and development of therapeutic drugs, focusing on the potential roles of virus inhibitors.
“Research has mainly been focused on SARS- and MERS-CoV infections, because they were responsible for severe illness when compared with other CoVs,” Dr. Pillaiyar, of the department of pharmaceutical and medicinal chemistry at the University of Bonn (Germany), and colleagues wrote.
2019-nCov has been linked genomically as most closely related to SARS, and the Coronavirus Study Group of the International Committee on Virus Taxonomy, which has the responsibility for naming viruses, has designated the new virus SARS-CoV-2.
Examining extant drugs
The first approach to identifying possible antiviral agents reevaluates known, broadly acting antiviral drugs that have been used for other viral infections or other indications. The initial research into coronavirus therapeutics, in particular, has examined current antiviral therapeutics for their effectiveness against both SARS-CoV and MERS-CoV, but with mixed results.
For example, in a search of potential antiviral agents against CoVs, researchers identified four drugs – chloroquine, chlorpromazine, loperamide, and lopinavir – by screening drug libraries approved by the Food and Drug Administration. They were all able to inhibit the replication of MERS-CoV, SARS-CoV, and HCoV-229E in the low-micromolar range, which suggested that they could be used for broad-spectrum antiviral activity, according to Dr. Pillaiyar and colleagues.
Other research groups have also reported the discovery of antiviral drugs using this drug-repurposing approach, which included a number of broad-spectrum inhibitors of HCoVs (lycorine, emetine, monensin sodium, mycophenolate mofetil, mycophenolic acid, phenazopyridine, and pyrvinium pamoate) that showed strong inhibition of replication by four CoVs in vitro at low-micromolar concentrations and suppressed the replication of all CoVs in a dose-dependent manner. Findings from in vivo studies showed lycorine protected mice against lethal HCoV-OC43 infection.
Along with the aforementioned drugs, a number of others have also shown potential usefulness, but, as yet, none has been validated for use in humans.
Developing new antivirals
The second approach for anti-CoV drug discovery involves the development of new therapeutics based on the genomic and biophysical understanding of the individual CoV in order to interfere with the virus itself or to disrupt its direct metabolic requirements. This can take several approaches.
MERS-CoV and SARS-CoV PL protease inhibitors
Of particular interest are antiviral therapies that attack papain-like protease, which is an important target because it is a multifunctional protein involved in proteolytic deubiquitination and viral evasion of the innate immune response. One such potential therapeutic that takes advantage of this target is disulfiram, an FDA-approved drug for use in alcohol-aversion therapy. Disulfiram has been reported as an allosteric inhibitor of MERS-CoV papain-like protease. Numerous other drug categories are being examined, with promising results in targeting the papain-like protease enzymes of both SARS and MERS.