Canadian researchers from Laval University-Quebec City, Université de Montréal, Université du Québec à Montréal, Université McGill and the Centre Armand-Frappier Santé Biotechnologie have in a new study found that the phytochemical tannic acid is able to inhibit three key pathways involved in SARS-CoV-2 infection and could be used to develop drugs to prevent and treat COVID-19.
Tannic acid is a plant phenol typically extracted from the following plant parts: Tara pods (Caesalpinia spinosa), gallnuts from Rhus semialata or Quercus infectoria or Sicilian sumac leaves (Rhus coriaria).
It is found in practically all aerial plant tissues including various tea leaves and also herbal teas. Tannic acid is also found in lower quantities in red wines and their consumption would not have any protective effects because of their low concentration of tannic acid.
The study findings were published in the peer reviewed International Journal of Molecular Sciences. https://www.mdpi.com/1422-0067/23/5/2643
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a zoonotic coronavirus first identified in China, has led to the worldwide coronavirus disease (COVID-19) pandemic with more than 400 million known cases of infection and 5.8 million deaths as of February 2022.
Mutated SARS-CoV-2 variants are emerging with increased infectivity, facilitating their spread ; e.g., the SARS-CoV-2 B.1.617.2 (Delta) variant possesses a higher replication rate and transmissibility than B.1.1.7 (Alpha) . The B.1.1.529 (Omicron) variant with 30 mutations has also emerged .
Though vaccination campaigns have been implemented in many countries, there is still an urgency to develop effective and accessible therapeutics.
SARS-CoV-2-induced infection involves multiple steps, from extracellular to transmembrane and finally intracellular. In addition to the spike protein receptor angiotensin-converting enzyme 2 (ACE2), transmembrane protease serine 2 (TMPRSS2) and the viral 3-chymotrypsin like protease (3CLpro) (also known as the main protease (Mpro)) are required for cell entry and replication, respectively.
SARS-CoV-2 infection is controlled by the opening of the spike protein receptor binding domain (RBD), where a conformational transition from a ‘down’ to an exposed ‘up’ state, gated by an N-glycan shield at position N343, occurs to bind with ACE2 .
TMPRSS2 located on the membrane of the host cell cleaves and activates the SARS-CoV-2 spike protein, leading to structural rearrangements, membrane fusion, and the release of the viral RNA into the cytoplasm of the host cell [5,6,7]. Once the virus has taken control of the host cell, it uses its viral replication and transcription complex (RTC) as well as the cell’s own translation machinery for its replication. Then, newly replicated virions exit the cell via exocytosis and spread to neighboring cells and throughout the body.
Many strategies to mitigate the mechanisms of action of SARS-CoV-2 have been investigated [8,9,10,11,12,13,14,15,16,17], including the usage of small molecules to inhibit viral entry and replication [18,19]. Natural products or their derivatives make up 49.2% of the 1881 new drugs developed from January 1981 to September 2019 [10,20], and numerous researchers have turned to naturally occurring polyphenols, as their therapeutic potential has been documented for antiviral uses  and as antiviral-drug candidates for SARS-CoV-1 and SARS-CoV-2 [22,23,24,25].
For example, Yang et al.  showed that corilagin dose-dependently blocks SARS-CoV-2 RBD binding and suppresses the infectious property of RBD pseudo-typed lentivirus in HEK293 cells overexpressing hACE2.
Our recent numerical and experimental results showed that two natural polyphenols, corilagin and 1,3,6-tri-O-galloyl-β-D-glucose (TGG), can disrupt the extracellular interactions between ACE2 and SARS-CoV-2 spike wild-type as well as mutated RBD proteins . Experimentally, both molecules bound preferably to the spike protein, whereas only very weak binding was observed with ACE2. Hence, the physiological side effects induced by ACE2 inhibition would likely remain very limited .
Since the early COVID-19 pandemic, most studies have focused on the inhibition of RBD/ACE2 binding , but natural polyphenols could also target two proteolytic enzymes—the transmembrane TMPRSS2 and the intracellular 3CLpro. The monomer of SARS-CoV-2 3CLpro has three structural domains: I (residues 10–96), II (residues 102–180), and III (200–303) .
Tahir ul Qamar et al.  screened a library containing 32,297 potential antiviral phytochemicals and traditional medicinal compounds and showed that one of the binding areas of 3CLpro is located on the active sites of the catalytic dyad residues His41 and Cys145. This dyad located at the interface of His41 in Domain I and Cys145 in Domain II , separated by 3.6 Å, is the optimum distance to initiate H-bonding .
Using molecular dynamics (MD) and in vitro methods, Loschwitz et al.  showed that corilagin could inhibit 88% of 3CLpro activity. Du et al.  showed (−)-epicatechin-3-O-gallate (ECG) to be a potent inhibitor of 3CLpro with an IC50 of 0.847 ± 0.005 μM. Wang et al.  have identified tannic acid (TA) as a potent inhibitor of both TMPRSS2 and 3CLpro.
TA can inhibit 100% of 3CLpro activity with an IC50 value ranging from 2.1 μM [35,36] to 9 μM . An inhibition of 77 ± 1% of 3CLpro activity could be achieved with a mixture of 5 μM TA combined with 20 μM puerarin, 20 μM daidzein, and 20 μM myricetin .
TA (C76H52O46) is a naturally occurring polyphenolic compound found in several plants with similar properties to the two phenolic ligands previously studied, TGG (C27H24O18) and corilagin (C27H22O18) [28,38,39,40], the latter possessing promising medicinal properties  and very low toxicity in mice even at high dosages . For instance, corilagin has been described as a non-steroidal anti-inflammatory and an antioxidant  with antihypertensive properties .
It has two joined phenolic rings (R3-R6) which make it more rigid than TGG. On the other hand, the less studied TGG is closely related to two tetra-O-galloyl-β-D-glucose molecules that were identified as promising therapeutic compounds against SARS-CoV-1 [22,45]. Turning to TA, this molecule exhibits very low toxicity in mice with a LD50 of 3500 to 5000 mg/kg body weight and is flexible and hydrolysable, whereas its potential metabolites may also show inhibitory effects against SARS-CoV-2.
In this study, we investigate experimentally the interactions between natural polyphenolic ligands—particularly TA, TGG, and corilagin—with proteins involved in the relevant steps for cellular entry and replication of the virus—RBD (N501Y) (the most frequent variant at the start of the study), TMPRSS2, and 3CLpro.
We use a combination of experimental methods (biochemical enzyme-linked immunosorbent assay (ELISA), enzymatic assay, surface plasmon resonance (SPR), and quartz crystal microbalance with dissipation monitoring (QCMD)) and numerical tools (molecular docking, molecular dynamics, and molecular mechanics Poisson–Boltzmann surface area (MMPBSA) free energy calculations).
Based on the biochemical data, we then concentrate on TA and its biophysical and numerical interactions with RBD (N501Y), TMPRSS2, and 3CLpro. Overall, our work highlights the potential role of TA in protection against SARS-CoV-2 infection by inhibiting the extracellular RBD/ACE2 interactions, the activities of the transmembrane TMPRSS2, and intracellular 3CLpro enzymes.