Pass Analysis
The activity spectra prediction results for the substances (PASS) were registered for the activity biological probability (Pa) greater than 0.7 (Table S1) and distributed in the graph of Fig. 2a for the relevant activities for this study. The results showed an activity probability (Pa) in the value of 0.571 of Castanol B act as the free radicals scavenger due to the antioxidant effect is denominated by the action of combat free radicals [42]. Besides, the high probability estimated at 0.96 of the compound inhibits Chlordecone reductase, available as Aldo-Keto Reductase (AKR1C2) with code PDB ID:4JQ1. Therefore, assuming that PASS analyzes may lead the molecular docking simulations, the docking protocol was established against the AKR1C2 target associated with prostate cancer [43, 44]. Furthermore, the teste showed a degree of probability of 0.522 that Castanol B may act as an anti-inflammatory within the spectrum of activity reported for the substance (Fig. 2a).
Virtual Screening Of Target Classes
To conduct the AKR1C2 target for the molecular docking simulations is necessary to apply screening virtual based on chemical structure aided by machine learning functions. These functions had the main goal to realize similarity tests with the ligand or not ligand structure for the series of biological targets since those targets are inside the library in a database [45]. From the similarity test with the chemical structures deposited in ChEMBL [46] with reported activity to the several biological targets of Homo sapiens organism, it was possible to observe that Castanol B tends to perform around 32% of intermolecular interactions with enzymes, highlighting the similarity with at least 12 two-dimensional structures that were interacting with the AKR1C1 (ID: CHEMBL5905) and AKR1C2 (ID: CHEMBL5847) target, and with at least six structures three-dimensional to interact with the AKR1C3 (ID: CHEMBL4681), as showed the graph of Fig. 2b. In addition, the substance showed an affinity with others targets, such as lyases (14%), proteases (10%), ion channels (8%) and kinases (8%).
Estimate Of Major Microspecies By The Pka Calculation
The pKa of Castanol B is unknown due to the be molecule newly discovered. In the graph of Fig. 2c is possible to observe the microspecies distribution of Castanol B, where the pKa estimate value in the order of 3.77 associated with the hydroxyl H-bond donor of group carboxyl (COO–H), suggests the presence of 50% of specie neutral and 50% of deprotonated species in pH of 3.77. Besides, the analysis suggests that the equilibrium chemical of Castanol B moves in the direction of formation of the carboxylate group (COO⁻) with the increase of pH, where a relative concentration of 97.25% is noted conjugated base in pH 7.4, as the majority microspecies.
Structural Optimization Of The Castanol B
Regarding the calculation of radicals, it is of fundamental importance to pay attention to the multiplicity of the molecule since, in the process, it will lose one unpaired electron in its highest energy occupied orbital (HOMO), thus forming the semi-occupied orbital (SOMO), generating the antioxidant radical. Thus, the total spin check (S²) for the neutral molecule should be equal to 1 (singlet), and for the radicals, it should be around 0.75 (duplet) [47].
Castanol B presents in its structure a biphenyl, with a carboxyl group in one of its rings. When treating its geometry in classical mechanics, its structure generated a twist between the rings, differing from the initial structure designed entirely planar. With the twist of -140° on the carbons connecting the rings, the Marvin software identified the most stable conformations for the neutral (Fig. 3a) and anionic (Fig. 3b) chemical structures. The next step was to perform the optimization via DFT using the software ORCA 4.1.1. The optimization via DFT provided us not only with the optimized geometry by quantum mechanics but also the thermochemical indices (internal energy, enthalpy, entropy, Gibbs energy), the energies of HOMO and LUMO boundary orbitals, dipole moment and, among others, shown in Fig. 3c for the neutral species and Fig. 3d for the conjugate base.
Table 1
Values were calculated by the ORCA software, for Castanol B (neutral and carboxylate anion), in gaseous form.
Property
|
COOH
|
COO⁻
|
Internal Energy
|
-876.845299 Eh
|
-876.291030 Eh
|
Enthalpy
|
-876.844354 Eh
|
-876.29008582 Eh
|
Entropy
|
0.000193 Eh
|
0.000144 Eh
|
Gibbs free energy
|
-876.901964 Eh
|
-876.347607 Eh
|
HOMO
|
-0.202103 Eh
|
-0.032883 Eh
|
LUMO
|
-0.046139 Eh
|
0.089418 Eh
|
Dipole moment
|
2.05273 Debye
|
15.87698 Debye
|
Antioxidant Potential
The calculations were initially performed in a vacuum, to study the antioxidant potential. Furthermore, to obtain the BDE of all sites, the hydrogens are removed from the site one wants to calculate. Then the same procedure is performed in water solvent (ε = 80.4). It is worth noting that the solvation factor is a crucial interfering factor in molecular chemical properties. BDE is a simple mechanism of the three discussed, consisting only of the direct abstraction of the radical hydrogen from the antioxidant molecule. Because Castanol B has three possible hydroxyls for the mechanism to occur, following data will compare which site is more favorable for it to occur, comparing simultaneously with the Castanol B carboxylate anion sites.
According to the plot in Fig. 4a and Fig. 4b, the BDE of the oxidizing site molecule 4'-OH has the lowest value. Considering the 4'-OH site and the resonance factor, common in aromatic compounds for radical stabilization, the radical formed forms an intramolecular hydrogen bond at the site in question, further stabilizing the radical formed. According to the calculated BDE, whether in the gas phase or in aqueous media, the HAT mechanism shows better efficiency when the molecule is a carboxylate anion, resulting in lower BDE ratios in gas and water 63 and 69.7 kcal/mol, respectively.
The leading indicator to be analyzed in the SET-PT mechanism is the IP. According to the plot in Fig. 4c, the best ionization potential is of the carboxylate anion molecule 66.07 kcal/mol in the gas phase, increasing with the solvent factor. This can be explained by the solvation of the water around the molecule. As a result, stabilization of the energy of the initial carboxylate anion occurs. When forming the "radical cation", the molecule becomes neutral. However, the IP value is higher than the parameter (BDE), which can say that there is less possibility for SET-PT to occur.
The PDE values are given in the data in Fig. 5a and Fig. 5b. The 4'-OH site presents the lowest values, corresponding that the SET-PT mechanism favors it if it happens. On the other hand, the carboxylate anion in an aqueous medium presented the best results. However, adding to the previous step (IP), the SET-PT becomes quite energetic, reaching 128.39 kcal/mol (value referring to IP + PDE) when compared to the BDE (69.72 kcal/mol, in the same conditions) of the HAT mechanism.
SPLET is the third important mechanism studied, and previous studies of phenolic compounds have confirmed the possible use in the presence of polar solvents [48, 49]. Furthermore, the values of the 4'-OH site are also favored in the SPLET mechanism, standing out in the aqueous medium, where there is a drastic reduction of energies for the PA parameter (Fig. 6) corroborating with the studies already cited.
Performing a final comparison between the three mechanisms, highlighting the 4'-OH site (having the lowest values among the three phenolic hydroxyls of Castanol B), we arrive at the graphs in Fig. 7a and Fig. 7b. The HAT mechanism presents the most thermodynamically favorable data. The deprotonated castanol molecule is favored for free radical oxidation, whose values are comparable to traditional antioxidant compounds such as gallic acid, Quercetin, and others (Fig. 7c) [50].
Molecular docking
The molecular docking simulations were realized to understand the possible mechanism of action of the molecules analyzed in the AKR1C2 receptor. Figure 8 showed the affinity energy values, Root Mean Square Deviation (RMSD) values, and binding site of AKR1C2-NPS (re-docking), AKR1C2-CBN, and AKR1C2-CBA complexes. The AKR1C2-NPS complex (Fig. 8) registered the affinity energy value − 8.7 kcal/mol and RMSD value of 0.9030 Å. Besides, this complex showed one hydrogen bond with Ser 217 amino acid and eight hydrophobic interactions with the Tyr 24, Tyr 55, Tyr 216, Ser 217, Leu 268, and Leu 306 residues. The CBN ligand (Fig. 8) showed the affinity energy value of -9.0 kcal/mol, RMSD value of 1.2800 Å and interacted with the AKR1C2 receptor through five hydrogen bonds (Asp 366, His 433, Ser 482, Asn 483, and Gln 506 amino acids), two hydrophobic interactions (Asp 366 and Leu 622 residues), and one π-stacking (Tyr 532). Furthermore, this ligand showed the charge center with the His 538 amino acid. The AKR1C2-CBA complex (Fig. 8) registered an -9.1 kcal/mol affinity energy value and an RMSD value of 1.2890 Å. Besides, This complex presented six hydrogen bonds (Ser 482, Asn 483, Tyr 532, Ser 533, and Gln 506 amino acids), one hydrophobic interaction (Tyr 532 residue), one π-stacking (Leu 622 amino acid), one charge center (His 538 residue).
The molecular docking results indicated that all ligands showed a high potential interaction with the AKR1C2 receptor due to better affinity energy values than the affinity energy standard value of -6.0 kcal/mol [51]. Furthermore, all complexes registered RMSD values below 2.0 Å [52]. However, only the AKR1C2-CBN and AKR1C2-CBA complexes were used in the molecular dynamics simulation due to the affinity energy values similars.
Molecular dynamics
The MD simulations analyzed the stability of complexes formed by the best-pose from molecular docking calculations. The Root Mean Square Deviation (RMSD) was performed using the C-α of the receptor as the reference for the AKR1C2-CBN (Fig. 9a) and AKR1C2-CBA (Fig. 9b) complexes. The AKR1C2-CBN complex showed the average RMSD value of 1.53 Å and reached the equilibrium from 30 ns in the two MD simulations. However, in the third MD simulation was registered average RMSD in the value of 2.76 Å considered above the standard value (RMSD < 2.0 Å) [52]. The AKR1C2-CBA complex registered the average RMSD in the value of 1.44 Å and reached the equilibrium from 30 ns in the three replicates. Therefore, the RMSD results indicated that the CBA ligand showed interactions more stable with the AKR1C2 receptor than the CBN ligand. The Interaction Potential Energy (IPE) analysis was realized only from the interval time that the complexes reached equilibrium.
The IPE analysis was performed by summing short-range energies Coulomb and Lennard-Jones between the AKR1C2 receptor with the CBN and CBA ligands. Table 2 showed that the AKR1C2-CBN and AKR1C2-CBA complexes registered IPE values of -198.4660 kJ mol− 1 (± 33.4447) and − 278.0350 kJ mol− 1 (± 32.6707), respectively. Besides, the IPE analysis showed the contribution of each residue in the AKR1C2 receptor with a maximum distance of 5.0 Å of ligand analyzed. The AKR1C2-CBN complex registered IPE values of -4.3478 kJ mol− 1 (± 2.8673), -15.5050 kJ mol− 1 (± 14.6606), -1.1295 kJ mol− 1 (± 1.4342), -19.3830 kJ mol− 1 (± 2.8340), -16.8722 kJ mol− 1 (± 3.6573), -2.2047 kJ mol− 1 (± 1.8664), -2.4042 kJ mol− 1 (± 1.0490), -6.8459 kJ mol− 1 (± 5.8047), -0.7878 kJ mol− 1 (± 1.0076), -10.3324 kJ mol− 1 (± 2.6303), -6.8529 kJ mol− 1 (± 2.3461), and − 3.5326 kJ mol− 1 (± 1.9567) with the Gly 22, Asp 50, His 117, Tyr 216, Ser 217, Ala 218, Leu 219, Ser 221, His 222, Leu 268, Ala 269, and Leu 306 amino acids.
The AKR1C2-CBA complex showed the IPE values of -6.6544 kJ mol-1 (± 1.3855), -76.5398 kJ mol-1 (± 7.1892), -11.8891 kJ mol-1 (± 4.8265), -34.3912 kJ mol-1 (± 5.6294), -18.5110 kJ mol-1 (± 9.0857), -4.5020 kJ mol-1 (± 1.6759), -3.6080 kJ mol-1 (± 4.1628), -18.6121 kJ mol-1 (± 16.5207), -8.1960 kJ mol-1 (± 10.0727), -4.9431 kJ mol-1 (± 3.1764), -5.7373 kJ mol-1 (± 2.007), and − 2.8957 kJ mol-1 (± 1.6507) with the Gly 22, Asp 50, His 117, Tyr 216, Ser 217, Ala 218, Leu 219, Ser 221, His 222, Leu 268, Ala 269, and Leu 306 amino acids. Therefore, the IPE results indicated that the CBN and CBA ligands interacted most strongly with the Asp 50, His 117, Tyr 216, Ser 217, Ser 221, and Leu 268 residues, highlighted especially the CBA ligand.
Table 2
Interaction Potential Energy (IPE) (in kJ mol− 1) between the residues of the AKR1C2 receptor with the CBN and CBA ligands, with standard deviation.
Residue
|
IPE (kJ mol− 1)
|
CBN
|
CBA
|
Gly 22
|
-4.3478 (± 2.8673)
|
-6.6544 (± 1.3855)
|
Asp 50
|
-15.5050 (± 14.6606)
|
-76.5398 (± 7.1892)
|
His 117
|
-1.1295 (± 1.4342)
|
-11.8891 (± 4.8265)
|
Tyr 216
|
-19.3830 (± 2.8340)
|
-34.3912 (± 5.6294)
|
Ser 217
|
-16.8722 (± 3.6573)
|
-18.5110 (± 9.0857)
|
Ala 218
|
-2.2047 (± 1.8664)
|
-4.5020 (± 1.6759)
|
Leu 219
|
-2.4042 (± 1.0490)
|
-3.6080 (± 4.1628)
|
Ser 221
|
-6.8459 (± 5.8047)
|
-18.6121 (± 16.5207)
|
His 222
|
-0.7878 (± 1.0076)
|
-8.1960 (± 10.0727)
|
Leu 268
|
-10.3324 (± 2.6303)
|
-4.9431 (± 3.1764)
|
Ala 269
|
-6.8529 (± 2.3461)
|
-5.7373 (± 2.007)
|
Leu 306
|
-3.5326 (± 1.9567)
|
-2.8957 (± 1.6507)
|
Physicochemical Properties And Drug-likeness
The QED (Eq. 8) application resulted in a drug-likeness score in the value of 0.61, indicating that Castanol B satisfies the most physicochemical conditions of drug-similarity due to the weight molecular is of 246.22 g/mol and the logP of 2.37 showed that the substance is small and petite lipophilic, while the total of 2 rotatable bonds (Rotb) characterize its semi-rigid structure, passing through the filter that matches the criteria if drug-likeness of Lipinski [34] and Veber [35]: logP ≤ 5, MW ≤ 500 g/mol, H-bond acceptors (HBA) ≤ 10, H-bond donors (HBD) ≤ 5, polarity (TPSA) ≤ 140 Ų, and Rotb ≤ 10 (Table 3). However, the PAINS filter detected a structural alert associated with the Catechol fragment, revealing the high polarity of Castanol B.
When applicated in the Pf-MPO algorithm is observed that the quantity of H-bond donors associated with the 3'-OH, 4'-OH, 5-OH, and COOH groups, not within the spectrum formed by the physicochemical limits, evidenced in the radar of Fig. 10a by contributed heavily for the high polarity topological of Castanol B available in 97.99 Ų. However, the substance is in a low lipophilicity spectrum, besides being bigger and more polar than the active substances in CNS of Pfizer database (MW ≤ 360 g/mol with TPSA > 40 Ų) [38]. Therefore, the Pf-MPO score of 4.73 suggests that the alignment between the physicochemical properties leads to an optimization of pharmacokinetic parameters of Castanol B (Table 3).
Table 3
Physicochemical properties are calculated and applied to drug-likeness filters and estimates.
Parameter
|
Property
|
Value
|
Physicochemical properties
|
pKa (most acid)
|
3.77
|
|
pKa (most basic)
|
-6.03
|
|
logP
|
2.37
|
|
logD
|
-0.92
|
|
MW
|
246.22 g/mol
|
|
HBA
|
5
|
|
HBD
|
4
|
|
TPSA
|
97.99 Ų
|
|
Rotb
|
2
|
|
NAR
|
2
|
Medicinal chemistry
|
PAINS
|
1 alert; Catechol_A
|
|
Lipinski
|
Accepted
|
|
Veber
|
Accepted
|
|
QED
|
0.61
|
|
Pf-MPO
|
4.73
|
Note: MW (Molecular weight); HBA (H-bond acceptors); HBD (H-bond donors); TPSA (Topological Polar Surface Area); Rotb (Rotatable bonds); NAR (Number of aromatic rings); PAINS (Pan Assay Interference Structures); QED (Quantitative estimate of drug-likeness); Pf-MPO (Pfizer's Multiparameter optimization). |
In Silico Adme-tox Properties
Estimate oral absorption and metabolic stability
To Johnson et al., [41] (Pfizer, Inc.), properties such as MW and logD replace other physicochemical properties, such as TPSA, H-bond donor/acceptor, and ionization, for being strongly used associated with the permeability and metabolic clearance of drugs. Therefore, it is possible to note that the formation of base conjugate causes a decrease in relative lipophilicity of Castanol B, available at -0.92 by the logD at pH 7.4, about intrinsic lipophilicity (logP 2.37), which aligns with the weight molecular of 246.22 g/mol, configured an optimization in the absorption and in the metabolic stability of the substance, according to the physical-chemical space of the Pfizer database (Fig. 10b), formed by the limits of -2 < logD ≤ 5 and 200 < MW ≤ 500 g/mol.
Estimate Of Permeability And Bioavailability
The results of the ADME test between the ADMETlab 2.0, AdmetSAR 2.0, and SwissADME servers showed the correlation between the empirical decisions and pharmacokinetic descriptors of Castanol B and are inspected in Fig. 10c and Table 4. The permeability value of the Madin-Darby canine kidney cell (Papp MDCK) model in the value of 6.5×10⁻⁶ cm/s indicates moderate permeability (2.5 < Papp ≤ 10, in 10⁻⁶ cm/s), according to with the Pfizer's rating system [38]. The prediction corroborates with an empirical decision of the graph of BOILED-Egg [53], where the TPSA ensures good passive intestinal permeability; however, it prevents penetration into the CNS for being in an interval of difficult penetration in the BBB (79 < TPSA ≤ 142 Ų) (Fig. 10c).
Therefore, intestinal absorption becomes a fundamental criterion for selecting a candidate for the local action drug related to the absorbed oral dose with the molecular fraction bioavailable in the systemic circulation [54]. Although ionizable compounds showed a decrease in the oral bioavailable, as a general rule for the drugs [55], an experimental observation made by Martin [56] showed that the fraction of 56% of anionic compounds with the polarity topological of 75 ~ 150 Ų showed selective permeability in Caco-2 cells, of a test database in mice, estimating a fraction of oral bioavailable (F) of 0.56 for the substances with high polarity. However, according to the experimental analysis of Veber [35], the permeability attributes and drug transport decrease as the TPSA approaches 140 Ų. Then, the statistical regressions of QSAR of the ADMET Prediction Service tool estimate a relative oral bioavailability of 55.37% (Fig. 11a) in the function of physicochemical space occupied with the Castanol B (75 < TPSA < 150 Ų), corroborating with the F of bioavailable of empirical decisions of the consensual test around of 0.55–0.56 (Table 4). At the same time, the low volume of distribution available at 0.402 L/kg indicates that the substance tends to be more distributed in blood plasma fluid than in tissues, including the BBB, where the permeability value blood→brain (log BB) in the value of -1.13 reflects the low penetrability of the compounds in the brain (Fig. 11b).
Site Of Metabolism, Clearance And Toxic Effects
The prediction of metabolism sites search reduces the selection of substrate of the isoform of CYP450 able to form secondary metabolites reactive and subjects to induce liver damage (DILI) [57]. Although the lipophilicity estimated (elogD) is a model very associated with solubility and permeability and may also estimate the metabolic stability and the resistance of the biotransformed substance [41]. The low lipophilicity estimated of Castanol B leads a pharmacologic model little likely to form secondary metabolites in the metabolism of phase I, where the value of intrinsic debugging in the value of 10.036 mL/min/kg suggests that the substance is easily excreted (Table 4). In the map of probability of Fig. 11c, the aromatic carbon at position 4 has a low probability of being hydroxylated by the CYP2A6 isoform (phase I). However, constitute a chemical entity most susceptible to undergoing aromatic hydroxylation. Besides, it is also observed that the 3'-OH, 4'-OH, and 5-OH sites presented susceptible sites to form metabolites conjugate active from glucuronida reactions by the UGT enzyme (phase II). However, the biotransformation is barely viable (P 0.5), as shown in Fig. 11c. Therefore, the low susceptibility of Castanol B in being subtracted from the majority isoforms of CYP450 (2C9, 2D6, and 3A4) makes it a non-inhibitor to those isoenzymes with a low probability of inducing lesions to the liver (Table 4).
Table 4
Results of the consensual prediction of ADME between ADMETlab 2.0, AdmetSAR 2.0 and SwissADME databases.
Prediction
|
ADMETlab 2.0
|
AdmetSAR 2.0
|
SwissADME
|
Papp*
|
6.5×10⁻⁶ cm/s
|
+
|
+a
|
P-gp substrate
|
–
|
–
|
–
|
HIA
|
High
|
+
|
Highb
|
Bioavailability, F
|
Low
|
+ (0.557)
|
0.56a
|
Volume of distribution
|
0.402 L/kg
|
n.a.
|
n.a.
|
BBB permeation
|
–
|
–
|
–b
|
CYP2C9 inhibitor
|
–
|
–
|
–
|
CYP2D6 inhibitor
|
–
|
–
|
–
|
CYP3A4 inhibitor
|
–
|
–
|
+
|
*Papp prediction of the Madin-Darby canine kidney cell (MDCK) model from the ADMETlab 2.0 database and adenocarcinoma colorectal cells (Caco-2) from the AdmetSAR 2.0 and SwissADME databases. |
Note: HIA (Human intestinal absorption); BBB (Blood-brain barrier) CYP (cytochrome P450); a = calculated by the method of Martin (2005); b = predicted by the BOILED-Egg graphical model. n.a. = not reported. |
Prediction Of The Herg-liability
Another prediction dependent on structural analysis is the identity of fragments molecular potentially inhibitors of hERG channels (the human Ether-a-go-go-Related Gene), a conveyor of K + ions for the cardiorespiratory system. This inhibition occasioned side effects associated with cardiotoxicity, such as cardiac arrhythmia [58, 59]. In this work, the statical regressions based on QSAR reveal the low potential cardiotoxic of Castanol B. In the map of probability 2D of Fig. 12, the contributions structural of Castanol B are susceptible to the inhibition, or not, of the channel of transport of hERG (the human Ether-a-go-go-Related Gene) ions are observed. In this channel, despite the negative effect of phenolic hydroxyls and carboxylate group (magenta color), the sum of contribution positive associated with the aromatic ring (green color) reflects the low blocking potential hERG, where the pKi evaluated in 4.78, behind 90% of compounds of the database with the pKi values above ideal, suggests that the substance have a low affinity with the channels, resulting in a low incidence of cardiotoxicity.
Evaluation of acute toxicity and environmental toxicity
Recently, the QSAR models constitute the toxic structure database evolved into a physicochemical space that negatively encompasses molecular fragments in species environmental species such as the fish (Fathead Minnow) and the Tetrahymena pyriformis protozoan [60]. The analysis realized in this work suggests that Castanol B has a low toxicity incidence by ingestion and may also act as a cell antiproliferative agent. When evaluating the acute toxicity models is observed that the estimated value of lethal dose of 50% in mice (LD50) in the value of 2044 mg/kg classified the substance as low toxicity class, according to reference [61], corroborating with the predicted concentration is around of 2.0 mol/kg (Table 5). Concerning the lethal concentration (LC50) in environment species is observed noted that the estimated value of 1.374 mM indicates the low lethality in a tested fish population, corroborating with the values inside of domain of applicability of the QSAR model of the GUSAR online tool, while the values of IGC50 between the 0.3–0.6 µg/L, close to the estimated spectrum in -log10(mol/L), indicates that the Castanol B tends to inhibit the growth of the Tetrahymena pyriformis protozoan, conduces the antiproliferative model of substance (Table 5).
Table 5
Results of consensus prediction of organic toxicity and environmental toxicity models between ADMETlab 2.0, AdmetSAR 2.0 and GUSAR Online databases.
Prediction
|
GUSAR Online
|
AdmetSAR 2.0
|
pkCSM
|
LD50 Oral in rat
|
2044 mg/kg
|
2.032 mol/kg
|
1.931 mol/kg
|
LC50 Minnow*
|
-1.833 log10(mmol/L)
|
n.a.
|
1.374 mM
|
IGC50 T. pyriformis
|
1.565 -log10(mol/L)
|
0.622 µg/L
|
0.377 µg/L
|
*Prediction of positive log of lethal concentration (log10LC50 in mmol/L) from GUSAR Online server and negative log of lethal concentration (-log10LC50 in mM) from pkCSM server. |