3.1. Molecular docking analysis
First, to validate our protocol, we redock co-crystalized with 3 pose per ligand as output and again redocking was done to get 9 poses. RMSD was computed after each docking procedure until RMSD of the docked ligand matched (< 1 A) with the ligand in crystalized structure (Fig. 2).
The virtual screening workflow yielded 139 ligands with docking score. Based on the interaction pattern of the test ligands with positive control ligand, docking score and binding free energy, two test ligands (Fig. 3) and a control were selected and further evaluated through molecular dynamics studies (Fig. 4).
Both the test ligands and positive control had similar binding pattern (Table 2 and Supplementary S1). The screened compound-1 had three H-bonds with S283, R292 and Y318 residues of protein. The keto group present in the pyridine nucleus at 2nd position forms an H-bond with R292 and Y318 while the keto group present in ethyl oxo bridge forms an H-bond with S283. The screened compound-2 had three H-bonds with S283, R292 and Y318 residues of protein. Keto group present in benzoxazole nucleus at 2nd position forms R292 and Y318 and keto group present in ethyl oxo bridge forms an H-bond with S283. Cognate ligand also showed three H-bonds with S283, R292 and Y318 residues of protein. Keto group of carboxylic acid group forms two H-bond with S283 and R292 while OH group of carboxylic acid forms H-bond with Y318.
Title | Smiles | Docking score (kcal/mol) | MMGBSA ΔG Bind (kcal/mol) | H bond forming residues | Other interactions | |
6X3L_1625 | c1cccc2c1n(c(= O)o2)CC(= O)N1CCC[C@@H]1Cn1cc(cn1)C | -5.96 | -54.66 | S283, R292, Y318, I320 | Pi-Pi: Y271 | |
P: S272, S283, S319, T365 | |
HP: Y271, F273, L275, F281, A282, I294, F317, Y318, I320, M330 | |
PC: R292, K227 | |
6X3L_cognate | OC(= O)c1n(nc(C(C)(C)C)c1)Cc1ccccc1 | -6.21 | -43.21 | S283, R292, Y318 | Pi-Pi: Y271 | |
SB: R292 | |
P: S272, S283, S319, T365 | |
HP: Y271, F273, F281, A282, I294, F317, Y318, I320, M330 | |
6X3L_127 | C1CC[C@H]2[C@@H]1CCN2C(= O)c1c(= O)[nH]c(C)cc1 | -6.46 | -53.12 | S283, R292, Y318 | Pi-Pi: Y271 | |
P: S272, S283, S319, T365 | |
HP: Y271, F273, F281, A282, I294, F317, Y318, I320, M330 | |
PC: R292 | |
HP- Hydrophobic, PC-Positively Charged, P - Polar, SB - Salt Bridge
Table 2: Docking results of selected test ligand and positive control ligand
3.2. Drug likeliness prediction studies
Compared to other therapeutic drugs, CNS drugs have lower molecular weights. The marketed CNS drugs have average molecular weight around 319 compared to other oral drugs which have avg 330 molecular weight. Lipophilicity is the very crucial parameter for the molecules which act on CNS. As reported by Hansch and leo [23], CNS penetration is optimal when Log P values are in the range between 1.5 to 2.7. The log P value of screened compounds suggest that they may have good CNS penetration. Hydrogen bonding (O + N atom) also has an impact on CNS drugs. Marketed CNS drugs have an average 2.12 hydrogen bond acceptor and 1.5 hydrogen bond donors. The hetero atoms (O + N) should not be more than five. Overall ideal CNS active molecules should be less polar. The polar surface area of CNS active molecules is around 60–70 A. Drug likeliness prediction studies showed that our selected test ligands had almost all the properties of a CNS drug (Table 3).
Parameter | Range $ | Test ligand 1625 | Test ligand 127 | Positive control |
MW | 170–350 | 340.38 | 246.31 | 258.32 |
ClogP | -0.5–3.0 | 1.27 | 1.12 | 2.25 |
Hb Donors | < 2 | 0 | 1 | 1 |
TPSA | 70 A 2 | 86.3 | 67.3 | 59.6 |
Amide groups | ≤ 1 | 1 | 0 | 0 |
Total H-bonding | < 8 | 4 | 3 | 3 |
Carboxylic acids | ≤ 1 | 0 | 0 | 1 |
CNS MIPO | ≥ 4 | 4 | 4 | 4 |
RotBonds | ≤ 4 | 4 | 1 | 4 |
Aromatic rings | 1–3 | 2 | 0 | 2 |
FSP 3 | 0.15–0.8 | 8 | 8 | 6 |
QPPCaco-2 | > 500 | 598 | 1035 | 347 |
Basic N | ≤ 2 | 0 | 0 | 0 |
QPlogBB | -1 .0-0.8 | -3.06 | -2.06 | -3.90 |
$ https://enamine.net/compound-libraries/targeted-libraries/cns-library |
Table 3: CNS properties of the test and control ligands
3.3. ADMET Analysis
When designing novel medicines, the ADMET qualities are of utmost importance. One of the important methods that reduces the likelihood that drug compounds will fail in subsequent pre-clinical and clinical investigations is the ADMET analysis. We computed PkCSM server to predict the ADMET properties (Table 4).
An oral drug's capacity to cross the intestinal epithelial barrier, which controls the rate and degree of human absorption and ultimately influences its bioavailability, is one of the most significant issues addressing ADMET characteristics. HIA and Caco-2 were therefore chosen to assess the absorption properties. Our test compounds showed excellent intestinal absorption and hence can be considered suitable for oral formulations.
About 90% of oxidative metabolic processes are carried out by the CYP enzymes, especially isoforms 1A2, 2C9, 2C19, 2D6, and 3A4 [24]. A given small molecule is more likely to be implicated in DDI with many other medications if it inhibits more CYP isoforms than others [25]. Test ligand 1625 and control ligand were found to be substrate of CYP3A4, hence likely to show drug interactions with CYP3A4 inducers and inhibitors, while ligand 127 didn’t undergo metabolism with reported CYPs and hence less likely to show significant drug interactions at the level of metabolism. Ligand 1625 was also an inhibitor of CYP1A2, thus likely to be involved in interactions with its substrate. P-Glycoprotein (P-gp or ABCB1) is an ABC transporter protein involved in numerous crucial procedures including gastrointestinal absorption, medication metabolism, distribution, and excretion [26]. P-gp inhibition or substrate is utilised as a significant index to assess the medications from many aspects. P-gp in the intestines decreases the absorption of drugs in the blood while in the brain, P-gp decrease CNS bioavailability. None of our drugs was a substrate or inhibitor of P-gp.
Total Clearance is a crucial PK parameter because it affects a drug's half-life, bioavailability, and oral absorption, all of which have an impact on the dosage schedule (how frequently) and dose size (how much). Its forecast gives us a framework for the starting dose for the first human investigations and aids in assessing the viability of clinical dosing [27]. Total clearance of our test compounds showed while 1625 had better clearance and less likely to be accumulated in the body, 127 showed poor clearance compared with the positive control. Many cationic medications are first secreted by the kidney through the OCT2, and its inhibitors may modify how drugs accumulate in the kidney, causing nephrotoxicity [28]. Both our test compounds are less likely to be nephrotoxic as none of them was an inhibitor of OCT2.
One of the main causes of failure in late-stage drug development is toxicity, which is the extent to which a chemical might harm an organism or organs of the organism, such as cells and tissues. Thus, early toxicity detection would be extremely beneficial [29]. The human ether-a-go-go-related gene (hERG) encodes a potassium ion (K+) channel that is linked to prolonged QT intervals and prolonged ventricular repolarization, which can result in arrhythmia and more severe heart failure [30]. Drugs that are structurally and functionally unrelated to hERG channels have been demonstrated to block them, and some of these have been taken off the market. In the early stages of drug discovery, Ames’s mutagenicity is utilized to assess possible teratogenicity and genotoxicity. Additionally, the toxicological endpoints with the highest potential for harm to human health are acute oral toxicity. A chemical with a lower dose is more lethal than a compound with a higher LD50 when the LD50 doses are compared. Our test ligands have almost same LD50 values as compared to the control ligand. Here also in the toxicity profile, our test compounds showed comparative profile as that of the control and were found to be safer. Other toxicity profile is similar.
Table 4
ADMET properties of the selected ligands
Property | Model Name | Test ligand 1625 | Test ligand 127 | Positive control | Unit |
Absorption | Water solubility | -3.475 | -3.324 | -3.728 | Numeric (log mol/L) |
Absorption | Caco2 permeability | 0.948 | 1.241 | 1.369 | Numeric (log Papp in 10 cm/s) |
Absorption | Intestinal absorption (human) | 96.757 | 95.717 | 92.278 | Numeric (% Absorbed) |
Absorption | Skin Permeability | -3.092 | -3.212 | -2.615 | Numeric (log Kp) |
Absorption | P-glycoprotein substrate | No | No | No | Categorical (Yes/No) |
Absorption | P-glycoprotein I inhibitor | No | No | No | Categorical (Yes/No) |
Absorption | P-glycoprotein II inhibitor | No | No | No | Categorical (Yes/No) |
Distribution | VDss (human) | -0.454 | 0.224 | -0.505 | Numeric (log L/kg) |
Distribution | Fraction unbound (human) | 0.2 | 0.463 | 0.097 | Numeric (Fu) |
Distribution | BBB permeability | -0.568 | 0.266 | 0.25 | Numeric (log BB) |
Distribution | CNS permeability | -2.891 | -2.959 | -2.143 | Numeric (log PS) |
Metabolism | CYP2D6 substrate | No | No | No | Categorical (Yes/No) |
Metabolism | CYP3A4 substrate | Yes | No | Yes | Categorical (Yes/No) |
Metabolism | CYP1A2 inhibitior | Yes | No | No | Categorical (Yes/No) |
Metabolism | CYP2C19 inhibitior | No | No | No | Categorical (Yes/No) |
Metabolism | CYP2C9 inhibitior | No | No | No | Categorical (Yes/No) |
Metabolism | CYP2D6 inhibitior | No | No | No | Categorical (Yes/No) |
Metabolism | CYP3A4 inhibitior | No | No | No | Categorical (Yes/No) |
Excretion | Total Clearance | 0.81 | 0.353 | 0.693 | Numeric (log ml/min/kg) |
Excretion | Renal OCT2 substrate | No | No | No | Categorical (Yes/No) |
Toxicity | AMES toxicity | No | No | No | Categorical (Yes/No) |
Toxicity | Max. tolerated dose (human) | -0.455 | 0.429 | 0.645 | Numeric (log mg/kg/day) |
Toxicity | hERG I inhibitor | No | No | No | Categorical (Yes/No) |
Toxicity | hERG II inhibitor | No | No | No | Categorical (Yes/No) |
Toxicity | Oral Rat Acute Toxicity (LD50) | 2.572 | 2.135 | 2.134 | Numeric (mol/kg) |
Toxicity | Oral Rat Chronic Toxicity (LOAEL) | 1.347 | 2.465 | 1.964 | Numeric (log mg/kg_bw/day) |
Toxicity | Hepatotoxicity | Yes | Yes | Yes | Categorical (Yes/No) |
Toxicity | Skin Sensitisation | No | No | No | Categorical (Yes/No) |
Toxicity | T.Pyriformis toxicity | 0.667 | 0.428 | 0.784 | Numeric (log ug/L) |
Toxicity | Minnow toxicity | 1.348 | 1.739 | 0.708 | Numeric (log mM) |
3.4. Comparative analysis of MD simulation
To comprehend the stability of the protein-ligand complex and the binding interactions between them with respect to time in a hypothetical physiological condition, MD experiments are carried out. We ran 100 ns for apo form and each of our protein-ligand complexes. RMSD, percentage of contact of binding residues during 100ns simulation and protein-ligand contact mapping was computed.
Apo form of the protein underwent 2–3 conformational changes before reaching to equilibrium around 40-45ns. In complex with co-crystalized ligand, the protein went conformational changes till 20ns after that RMSD keeps on increasing till 50ns, around which it stabilized and showing minor fluctuations as in apo form. In presence of test ligand 1625, protein quickly attained equilibrium by around 30ns. RMSD kept on increasing till 40ns in protein complex with test ligand 127 after which the protein showed 2 conformational changes with a little higher fluctuation towards the end of simulation (Fig. 5). Mean RMSD was least for 6X3L-1625 complex indicating that the complex became more stabilized with test ligand 1625 (Table 5).
Table 5
RMSD results after 100 ns Molecular Dynamics
Target | Mean (Å) ± SD |
Apo | 2.83 ± 0.42 |
6X3L_cognate | 2.51 ± 0.32 |
6X3L_1625 | 2.31 ± 0.23 |
6X3L_127 | 2.42 ± 0.27 |
Interaction pattern during the 100 ns simulation showed that both the test ligands and positive control ligand confirmed the H bonding pattern of the docking results. While the positive control showing H bonds with S272, S283, R292 and Y318, it also formed water bridges with Y271, S272, P273, Y318 and I320. Hydrophobic interactions were mainly shown by Y271, P281, R292, I294, P317, I320 and M330. Only H bond remained consistent throughout the study duration. Test ligand 1625 formed H bonds with S272, S283, R292, Y318 and I320 and hydrophobic interactions with L248, Y271, P281, I294, P317 and I320. Here also H bonds remained stable throughout the simulation. Ligand 127 was involved in making H bonds and water bridges for appreciable duration of the simulation. Its key residues in forming H bonds were S283, R292 and Y318, while for forming water bridges, Y271, S283, R292 took part (Fig. 6).