Anti-SARS-CoV-2, antioxidant and immunomodulatory potential of dietary flavonol quercetin: Focus on molecular targets and clinical efficacy

The current coronavirus disease 2019 (COVID-19) outbreak, caused by the infection of SARS-CoV-2, seems to be a tough global challenge because of its highly contagious nature and rapid evolution of the virus strain. As no effective clinical drugs are available for the treatment of this disease in the early phase till to date, scientists are hunting for drugs from natural sources for its treatment. Quercetin, a dietary polyphenolic flavonoid found in fruits and vegetables, has been reported to exhibit various health-benefit pharmacological properties including antioxidant, anti-inflammatory, antimicrobial, antiviral and anticancer properties. In silico and in vitro studies demonstrated that quercetin potently inhibits the infections of SARS-CoV-2 and other coronaviruses by interfering with viral entry and replication via inhibiting the activity of host ACE2 receptor, and viral S-, 3CL-, PL-, RdRp-and Nsp13 -proteases. Moreover, quercetin is able to suppress oxidative stress, cytokine storm, thrombosis, sepsis and lung fibrosis, and thereby very likely is able to mitigate COVID-19 infection. In this review article, we elaborately discussed the antiviral, immunomodulatory, antioxidant properties of quercetin against viral infections and other diseases, as well as on its nanoformulations for enhancement of oral bioavailability and clinical efficacy in COVID-19 infection for its development as clinical drug for COVID-19.