The COVID-19 spreads in Pakistan from the initial of two cases, on 26th February 2020, to more than 2 million cases at the end of July 2020 (Ali, Shah, & Siddiqui, 2020; 2020). A total of three strains of SARS-CoV2, MT240479, MT500122, and MT262993, are reported from Gilgit, Karachi, and Manga, respectively. The phylogenetic analysis helped to trace the molecular footprints of these strains, which indicate the closest similarities of Pakistani strains to Japan, China, Israel, and the USA (Fig. 2). These findings suggest that the origin of SARS-CoV2 in Pakistan may be from these countries, probably due to travelers (Shafique, Ihsan, & Liu, 2020). As the recent outbreak of COVID-19 has caused immense human and economic losses, there are increasing demands for effective cure and therapeutics for this disease (C. Wu et al., 2020). Numerous research groups have targeted viral Mpro/3CLpro and S protein for drug development and several inhibitors of these proteins have been reported during the last few months (Calligari, Bobone, Ricci, & Bocedi, 2020; Rani et al., 2020). For Mpro, Yang et al. and Jin et al. separately identified a mechanism-based inhibitor (N3) (Haitao Yang et al., 2005; J. Yang et al., 2020). In another study, Khaerunnisa et al. reported nelfinavir and lopinavir as the potential inhibitor of this protein (Khaerunnisa, Kurniawan, Awaluddin, Suhartati, & Soetjipto, 2020). Adem et al. found natural polyphenols including, hesperidin, rutin, diosmin, apiin, diacetylcurcumin, to be more effective than nelfinavir (Adem, Eyupoglu, Sarfraz, Rasul, & Ali, 2020). Ebselen was found an excellent inhibitor by Jin et al. (Jin et al., 2020). Likewise, multiple studies have reported that the epitopes from S protein, can be used as the peptide vaccines for treating the COVID-19 infections (Bhattacharya et al., 2020; Dagur, Dhakar, & Gupta; Ka et al., 2020; Kalita, Padhi, Zhang, & Tripathi, 2020).
In this study, we have used in silico strategies to evaluate the therapeutic potential of multiple natural and herbal compounds, from Pakistani herbal plants, Hamdard products, and anti-COVID-19 data from the PubChem database. Initially, a total of 579 compounds were selected from three categories, which were then screened for their binding affinities with the selected viral proteins (Mpro and S), followed by the assessment of drug-likeness and ADMET properties of these compounds. As a useful drug molecule must possess significant binding affinities for the drug targets, therefore, the first step was to find the binding affinities of all drug candidates with the viral proteins (Mpro and S) by molecular docking (Additional file 4). The seven compounds for Mpro, and four for S protein, having B.E ≤ -10 kcal/mol and RMSD of ≤ 1, were screened as the best ligands (Tables 1 and 2). The binding affinity of a drug molecule depends on the molecular interactions of these compounds with the amino acid residues of the viral proteins (Homans, 2006). In case of Mpro protein, the active amino acid residues interacting with the selected drug molecules include, Thr-25, His-41, Cys-44, Thr-45, Ser-46, Met-49, Phe-140, Leu-141, Asn-142, Gly-143, Ser-144, Cys-145, His-163, Met-165, Glu-166 and Gln-189. Likewise,Arg-403, Val-445, Gly-447, Asn-448, Tyr-449, Leu-452, Lys-458, Ser-459, Glu-471, Ile-472, Tyr-473, Gln-474, Ala-475, Ser-494, Tyr-495, Gly-496, Phe-497, Gln-498, Asn-501 and Tyr-505 are the active amino acid residues from the receptor-binding domain of the viral S protein, interacting with the selected drug candidate molecules (Figs. 3 and 4).
After assessing binding affinities, the physicochemical properties of selected drug molecules, including molecular weight, number of hydrogen bond donors, hydrogen bond acceptors, number of rotatable bonds (ROTBs), topological polar surface area (TPSA), lipophilicity and aqueous solubility, were predicted using Swiss-ADME web tool (Tables 3 and 4). A likely drug molecule should have TPSA less than 90 Å2, and it should not more than 140 Å2, the highest value at which oral absorption occurs. At higher TPSA values, the compounds tend to be poor at penetrating cell membranes (Palm, Stenberg, Luthman, & Artursson, 1997). Likewise, to penetrate the blood-brain barrier, the TPSA of compounds must be lower than 90 Å2 (Pajouhesh & Lenz, 2005). The drug candidates selected in this study were having TPSA values in the range of 29 to 151 Å2. The lipophilic character of a drug, relates to its biological activity, as it determines the possibility that it can cross the cell membrane. All drug candidates selected in this study were mildly lipophilic (Lobo, 2020). Log S values report the aqueous solubilities of drug molecules, with values of -10 (insoluble), -6 (poorly soluble), -4 (soluble), -2 (very soluble) and 0 (highly soluble) (Jorgensen & Duffy, 2002). A drug should have good aqueous solubility for oral bioavailability and absorption. Almost all drug molecules, selected in this study, were readily soluble with the log S values in the range of -3.71 to -1.56 mol/L.
For determining the drug-likeness of the compounds, most drug designing studies use Lipinski, Veber, Ghose, Egan, and Muegge's rules as the standard drug-like classifiers (Egan, Merz, & Baldwin, 2000; Ghose, Viswanadhan, & Wendoloski, 1999; C. A. Lipinski, Lombardo, Dominy, & Feeney, 1997 o. p. Lipinski & methods, 2000; Muegge, Heald, & Brittelli, 2001). Of the seven compounds selected for the Mpro two compounds, 5281855 and 5281672, violating more than one of these five rules were excluded. After this filter, a total of nine compounds, five for Mpro and four for S protein, were considered as the potential drug molecules (Tables 5 and 6). The pharmacokinetic parameters, including absorption, distribution, metabolism, excretion, and toxicity, of these molecules, were analyzed via ADMET analysis. These properties are the important determinants of the success of the drug designing strategies (Zhong, 2017). The absorption and distribution are crucial factors for the assessment of any pharmacological agent. As most of the drugs are administered orally, therefore, they should efficiently absorb into the intestine. The drug permeability is tested in-vitro on Caco-2 cell lines, which indicates that the drug is easily absorbed in the intestine. The drugs having poor intestinal or oral absorption, should be administered intramuscularly or intravenously (M. P. J. J. o. m. c. Gleeson, 2008). For distribution to the target site, the drug molecules should interact with the surface proteins of the cells. P-glycoprotein and Organic Anion Transporting Polypeptide proteins (OATPs) in the cell membrane aids in transporting many drugs, and their inhibition affects the drug distribution (Abdullahi, Davis, & Ronaldson, 2017). The drug molecules should, therefore, be the non-inhibitors of these proteins. Ideally, drugs are broken down into harmless soluble metabolites that are easily excreted through urine or bile. Most of the drugs are metabolized by the Cytochrome P450 family of enzymes. Inhibition of these enzymes may, therefore, affect the biodegradation of the drugs (Furge, Guengerich, & Education, 2006). The excretion of degraded drug molecules depends on organic cation transporter 2 (OCT2) expressed on the renal tubule cells, which plays a key role in the disposition and clearance of drugs and endogenous compounds. This activity mostly depends on the multidrug and toxin extrusion transporter 1 (MATE1), which facilitates the elimination of degraded compounds into the urine (Motohashi & Inui, 2013). The toxicity assessment of any pharmacological agent depends on carcinogenicity, Ames mutagenesis, hepatotoxicity, hERG inhibition, and acute oral toxicity (Kramer, Sagartz, & Morris, 2007). The drug candidates having poor absorption, distribution, metabolism, excretion, or having toxic effects on the body are not promising therapeutic agents (P. Gleeson, Bravi, Modi, Lowe, & chemistry, 2009). Considering all these parameters, we hypothesize that three compounds; 9064, 85379763, and 6741 may be the reliable drug candidates for the viral Mpro. Similarly, two compounds, 2806372 and 3314, might be the most suitable drugs for S protein (Additional file 5).
The pharmacokinetic analysis has shown that these five compounds are reliable drug candidates having sufficient drug-like properties. Two of these five compounds, 9064 and 85379763, do not absorb orally with the probability score of 0.7 for 9064, and 0.5 for 85379763. Since the probabilities of poor oral absorption of these compounds are quite low, therefore, this may not be considered as a drawback, moreover, the alternative routes of drug administrations, intravenous, and intramuscular may prove more helpful (Additional file 5). Furthermore, the molecular docking analysis has suggested that selected drug candidates develop multiple stable interactions with the amino acid residues of the respective viral proteins. The two compounds selected for S protein, 3314 and 2806372, have binding energies of -10.0284 and − 10.1429 kcal/mol respectively (Table 2). The compound 3314, is stabilized by the carbon-hydrogen bonds with the Asn-448 (2.2 Å), Gly-447 (2.84 Å), Phe-497 (2.19 Å) and Tyr-449 (2.27 Å), pi-pi interaction with Tyr-449 (4.11 Å) and H-bond with Tyr-495 (2.84 Å). The second compound, 2806372 forms three carbon-hydrogen bonds; two with Arg-403 (2.63 Å and 2.75 Å) and one with Tyr-495 (2.8 Å), one pi-sulfur interaction with Tyr-505 (5.76 Å), one pi-alkyl interaction with Tyr-449 (4.2 Å) and three hydrogen bonds; one with Arg-403 (2.14 Å) and two with Gly-496 (2.74 Å and 1.99 Å) (Fig. 5). Similarly, the three potential inhibitors selected for the viral Mpro protein have binding energies of -10.9852 kcal/mol for 9064, -10.6675 kcal/mol for 85379763, and − 11.4281 kcal/mol for 6741 (Table 1). The first compound 9064, forms one pi-anion interaction with Cys-145 (4.78 Å) and three hydrogen bonds with the His-41 (2.15 Å), Asn-142 (2.23 Å) and Glu-166 (2.03 Å). The second compound, 85379763, forms alkyl interactions with three amino acid residues; Leu-27 (4.43 Å), Met-49 (4.57 Å), and Met-165 (4.51 Å), two pi-alkyl interactions with His-41 (4.6 Å and 4.81 Å), one carbon-hydrogen bond with Asn-142 (2.44 Å), four hydrogen-bonds; two with Gly-143 (2.06 Å and 2.76 Å) and remaining two with the Ser-144 (2.43 Å) and Cys-145 (2.44 Å). The third compound, 6741, forms alkyl interactions with Leu-27 (5.04 Å), Cys-44 (5.04 Å) and Met-49 (5.04 Å), two pi-alkyl interactions with His-41 (4.05 Å), two carbon-hydrogen bonds with Asn-142 (2.45 Å) and Ser-144 (2.71 Å), and four hydrogen bonds with Leu-141 (2.62 Å), Gly-143 (2.68 Å), Ser-144 (2.02 Å) and Cys-145 (1.73 Å) (Fig. 6).
Although the docking and ADMET analysis has been extensively used identify the novel drug target but this approach holds some major limitations (Llanos et al., 2021). The one major limitation of the approach related to confirmational changes in protein triggered upon binding of ligands. Therefore, it is very important to couple the docking approach to molecular dynamics simulations to validate the findings. In this study, we performed 50 ns of molecular dynamics simulations to understand the dynamic binding of our selected ligands with Mpro and S protein of SARS-CoV2.
The MD trajectory analysis revealed the higher RMSD values of four (3314, 2806372, 6741, 6741, 83579763) compounds among five after 10 ns. This shows the week binding of the compounds with the proteins. Along with the RMSD values, the interaction energy analysis of these compounds revealed 9064 as the most stable drug candidate with the average Coul-SR interaction energy (-71.53 kJ/mol) and LJ-SR energy (-95.32 kJ/mol) compared to the other two compounds. The selected (Ligand ID: 9064) is a polyphenolic flavonoid (Catechin or Cianidanol), which is previously well characterized for its immunomodulatory, anti-inflammatory, and antioxidant properties (Bae, Kim, Shin, Kim, & Kim, 2020) and according to this study, this compound has a great potential to become COVID-19 main protease Mpro inhibitor. Nevertheless, for their medicinal use, further experimental validation is necessary.