A new study by researchers from the College of Medicine, China Medical University, Taichung-Taiwan and Asia University, Taichung-Taiwan has found that a class of phytochemicals called tannins can be used to treat COVID-19.
Tannins are polyphenols enriched in wood, bark, roots, leaves, seeds and fruits of a variety of plants. Over the last two decades, there has been an increasing interest in understanding the biological functions of tannins and their applications as antioxidants, anticancer drugs, and food additives. Since the outbreak of the COVID-19 pandemic, much effort has been devoted to finding an expedient cure.
Tannins have been put forward as having possible anti-COVID-19 properties; however, owing to the profuse nature of the structurally diverse derivatives of tannins, the tannin species in the family associated with an indication of anti-COVID-19 have been poorly defined, compounded by frequent terminology misnomers.
This article reviews the tannin family in fruits and the current knowledge about the activities of the compounds with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It will aid molecular and cellular biologists in developing natural anti-viral chemicals as means of overcoming the current and future pandemics.
The study findings were published in the peer reviewed International Journal of Biological Sciences. https://www.ijbs.com/v18p4669.htm
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The first silico docking study involving an international collaboration, showed as early as August 2020 that Tannins could be deployed to disrupt the SARS-CoV-2 replication cycles by targeting the 3CLpro viral proteins.
Already as early as December 2020, Taiwanese researchers found that tannic acid could inhibit SARS-CoV-2 replication. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7783773/
A Canadian study in the March of this year also showed that tannins could inhibit SARS-CoV-2 activity through three distinct molecular pathways.
A Japanese study involving hamsters also showed the efficacy of using tannins derived from persimmons to treat COVID-19.
This new review study by Taiwanese scientists focusses on the tannin family in fruits and the current knowledge about the activities of the compounds with regard to severe acute respiratory syndrome coronavirus 2 or SARS-CoV-2.
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The anti-COVID-19 activities of tannins
The two major frontiers in battling the COVID-19 are infection and propagation of the SARS-CoV-2 coronaviruses, and the possible extreme reaction of the body’s immune system in response to pathogenic intrusion . While most research efforts have been devoted to preventing viral infection or eradicating the virus from the body, relatively less attention has been focused on developing methods to cope with the inflammatory responses, the “cytokine storm”, over the course of COVID-19.
Elevations of IL-6, IL-1β, TNF-α, IFN-γ and IL-10, as well as reductions in CD4+ and CD8+ T lymphocytes have been observed in COVID-19 patients, especially in severe cases . Currently, several polyphenol-based  and tannin-based clinical trials (NCT04911777 , NCT04403646 , and IRCT20200418047122N1 ) have been conducted with COVID-19 patients. Oral tannins extracted from quebracho and chestnut in combination with B12 vitamin and standard treatment reduced macrophage inflammatory protein-1α (MIP-1α) and TNF-α levels in hospitalized COVID-19 patients .
In a Syrian hamster model of COVID-19, persimmon-derived tannins pre-administered by oral gavage were able to suppress SARS-CoV-2 titers, reduce the severity of pneumonia and decrease inflammation-related gene expression such as IL-6, TNF-α, and IFN-γ .
The transmembrane protease serine 2 TMPRSS2 is an essential cell surface protease which processes the viral surface spike protein (S protein) to enhance its binding to the cell surface receptor ACE2 (angiotensin converting enzyme 2) followed by cell entry via membrane fusion.
The intracellular viral genomic RNA is then translated into polyproteins which are subject to further proteolytic cleavage by the viral main protease/3-chymotrypsin-like