3.5.  Druglikeness and Bioactivity score (BAS) analysis
Biological targets of prospective drug candidates can be classified into ion channels, proteases, kinases, G-protein coupled receptors (GPCRs), nuclear receptors and enzymes. The BAS of WS phytoconstituents was determined using web-based software Molinspiration (www.molinspiration.com). As a general rule, it is known that if the BAS > 0.0, then the drug candidate is physiologically active; if it is in the range −5.0 to 0.0; then the drug candidate is moderately active, and if the BAS< −5.0, then the drug candidate is inactive.
It is evident from Table 13, that most of the WS phytoconstituents had positive values with respect to the following receptors.

3.5.1.  As GPCR ligands
All WS phytoconstituents were active except withanolide E, anaferine and withasomnine which were predicted to be moderatively active. Most of the reference drugs also had positive values for GPCR except procainamide and arbidol which were predicted to be moderately active.

3.5.2.  As ICMs
All WS phytoconstituents had positive values except withasomnine which was found to be moderately active. Standard reference drugs losartan, cinacalcet, hydroxychloroquine, oberadilol and poziotinib were all found to be active whereas procainamide and arbidol were found to be moderately active.

3.5.3.  As KIs
All WS phytoconstituents displayed moderate activity except withasomnine that displayed significant activity. Standard reference drugs losartan, hydroxychloroquine and poziotinib were found to be active whereas procainamide, cinacalcet, arbidol and oberadilol were found to be moderately active.

3.5.4.  As NRLs
Withaferin A, withanolides A, B, D and E, withanone and viscosalactone B possessed significant BAS scores whereas anaferine and withasomnine were found to be moderately active. All standard reference drugs were predicted to have moderate BAS scores as NRLs.

3.5.5.  As PIs
Withaferin A, withanolides A, B, D and E, withanone and viscosalactone B had positive BAS scores indicating their potential as protease inhibitors. On the other hand, anaferine and withasomnine were found to have moderate activity as protease inhibitors. Interestingly, most withanolides especially withanolide B and withanolide A showed potent binding to papain like protease of SARS-CoV-2 (PDB ID: 6W9C), SARS-CoV 3CL-pro main protease (PDB ID: IP9U) and SARS-CoV-2 Nsp10/Nsp-16 complex (PDB ID: 6W75) thus supporting their role as potential viral protease inhibitors. On the other hand, losartan, cinacalcet and hydroxychloroquine also displayed positive values as protease inhibitors whereas procainamide, arbidol, oberadilol and poziotinib displayed moderate potential as protease inhibitors.

3.5.6.  As EIs
Most of the WS phytoconstituents including Withaferin A, withanolides A, B, D and E, withanone, viscosalactone B and anaferine had positive BAS scores indicating their potential as enzyme inhibitors whereas withasomnine displayed moderate potential. This observation was further validated by the fact that most of the phytoconstituents including Withaferin A, withanolides A, B, D and E, viscosalactone B and anaferine showed potent binding to human ACE2 receptor in the nanomolar range which was about 1000× times greater than the binding of known standard reference drugs arbidol and losartan (Table 4). This finding lends support for targeted use of withanolides from WS as SARS-CoV-2 entry blocking agents by virtue of their preferential binding to human ACE2, thereby blocking or inhibiting it. Losartan, cinacalcet, hydroxychloroquine, oberadilol and poziotinib also displayed significant potential as enzyme inhibitors whereas procainamide and arbidol displayed moderate potential.
Druglikeness of a compound can be predicted by comparing its structural features with those of marketed drugs. All WS phytoconstituents showed molar lipophilicity (cLog P) <5 thereby indicating good permeability across cell membranes (Figure 2A). Withaferin A, withanolide D, viscosalactone B and withasomnine had positive values of druglikeness which indicated that these compounds contain fragments that are present in marketed drugs. Out of the standard reference drugs, procainamide, hydroxychloroquine and oberadilol exhibited positive scores for druglikeness (Table 14).
Table 13.  Bioactivity scores and Druglikeness of WS phytoconstituents versus FDA-approved standard reference drugs (Losartan, Procainamide, Cinacalcet, Arbidol, Hydroxychloroquine, Oberadilol, Poziotinib).
S. No.  Ligands  Parameters of bioactivity score
GPCR ligand  Ion channel modulator (ICM)  Kinase inhibitor (KI)  Nuclear receptor ligand (NRL)  Protease inhibitor (PI)  Enzymeinhibitor (EI)   
                         
1.  Withaferin A  0.07  0.14  –0.49  0.76  0.15  0.94   
2.  Withanolide A  0.04  0.32  –0.43  0.71  0.15  0.86   
3.  Withanolide B  0.07  0.24  –0.47  0.79  0.15  0.76   
4.  Withanolide D  0.05  0.30  –0.50  0.73  0.16  1.07   
5.  Withanolide E  –0.70  0.16  –0.50  0.61  0.06  0.89   
6.  Withanone  0.00  0.27  –0.38  0.71  0.12  0.78   
7.  Viscosalactone B  0.03  0.04  –0.51  0.78  0.19  0.84   
8.  Anaferine  –0.08  0.17  –0.60  –0.58  –0.14  0.08   
9.  Withasomnine  –0.49  –0.43  0.58  –0.10  –0.58  –0.17   
10.  Losartan  1.06  0.16  0.03  0.01  0.33  0.44   
11.  Procainamide  –0.09  0.01  –0.10  –0.70  –0.20  –0.04   
12.  Cinacalcet  0.22  0.15  –0.0.8  0.00  0.17  0.02   
13.  Arbidol  –0.19  –0.44  –0.39  –0.34  –0.46  –0.07   
14.  Hydroxychloroquine  0.35  0.30  0.44  –0.12  0.12  0.15   
15.  Oberadilol  0.04  –0.47  –0.43  –0.37  –0.02  0.02   
16.  Poziotinib  0.04  –0.17  0.53  –0.35  –0.27  0.01   
Rule: BAS >0: Active;
BAS –5.0–0.0: Moderately active, moderately active and inactive.
BAS ≤5.0: Inactive;