3.1 Molecular docking
De novo drug design and development have encountered several serious challenges in recent years as a result of its expense and duration. While investment in the pharmaceutical industry has increased, the number of new therapeutic agents that have been approved has remained constant; as a result, computer-aided drug repurposing is an effective and motivating tool for coming up with new applications for therapeutic agents that have already been developed. There are numerous instances of medications that have been repurposed after being found by a computer model and used to treat various ailments. An excellent illustration is the use of Raltegravir, once an HIV-1 integrase inhibitor, as adjuvant therapy in cancer [28], and the use of Valsartan, formerly an angiotensin receptor blocker, for Alzheimer's disease [29].
However, the main focus of this computational study was to identify specific therapeutic agent(s) from FDA-approved neuropsychiatric/psychotic drugs that could serve as promising agents in the treatment of juvenile Parkinson’s disease.
Eight FDA-approved drugs from the named class were screened at the known binding site of the PARKIN crystal structure in order to find promising therapeutic agents with high binding affinities against the active site of PARKIN. The binding energies of these drugs were calculated using extra precision (XP) docking. Excellent binding affinity is indicated by a low docking score. In the specific binding pocket of PARKIN, the molecular docking scores of the tested ligands vary from − 5.845 kcal/mol to -2.658 kcal/mol (Fig. 1). Additionally, the co-crystallized ligand's redocking with an RMSD value of 0.90 supports the validity of the docking approach. In the docking result, six of the screened ligands were shown to be effective namely Lumateperone (CID: 21302490), Anisoperidone (CID: 19104), Melperone (CID: 15387), Bromperidol (CID: 2448), Azabuperone (CID: 18484), Deutetrabenazine (CID: 73437646), could be manifest as an excellent putative and selective inhibitor of PARKIN than reference ligand (L-DOPA; CID: 6047) as determined by their relatively high binding energy score. Following the visualization of PARKIN active site with the co-crystalized ligand, the following essential amino acid residue SER 167, ARG 170, MET 192, ARG 191, ASM 190, PRO 189, ILE 188, LEU 187, VAL 186, ALA 206, GLU 207, PHE 208, ASP185, TRP 183, PHE 209, PHE 210, THR 222, SER 223, VAL 224, ALA225, LEU 226, MET, 227, GLU 300, HIS 302 were revealed to play a pivotal role in PARKIN-ligand interaction. These amino acid residues play a fundamental role in forecasting PARKIN binding sites and their mechanism of catalysis. The docked compounds interact with GLU 207, PHE 210, PHE 208, ALA225, LEU 187, ASN 190, SER 223, ARG 170. Practically, the ligand-PARKIN interaction results in inter/intramolecular forces of interaction such as hydrogen bonding, pi-pi stacking, pi-cation, and salt bridge through hydrogen bond formation with the nitrogen and oxygen atom of the numerous ring and Van der Waals interactions (Table 1). In this study, Fig. 1 represent the docking score where Lumateperone is the best molecule against PARKIN, with the highest binding score of − 5.845kcal/mol, followed by Anisoperidone − 5.517Kcal/mol.
Remarkably, all the docked complexes, as well as the reference ligand (L-DOPA), were observed for additional molecular interaction profiling, including hydrogen, hydrophobic, polar, charged positive and negative, and glycine interactions suggesting the prime role of intermolecular interaction in the stability and better binding orientation of the respective docked complexes.
3.2 Quantum Chemical Calculations
Density functional theory (DFT) is the most widely and popular quantum theory used for the calculation of the electronic structure of molecules. In drug design, DFT is employed to study the electronic parameters of isolated drug molecules, provide an understanding of chemical reactivity and investigate drug-enzyme interactions. The results of all the chemical reactivity properties of the 8 FDA-approved drugs are shown in Table 2. Frontier molecular orbitals (FMOs) are employed to explain many reactions system. The FMOs locate the area of chemical bonds that are chemically reactive. This has been used for describing the chemical reactivity and stability of small molecules [30–32]. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are the two most important molecular orbitals in a molecule. The energy of HOMO (EHOMO) and LUMO (ELUMO) explains the type of donor-acceptor interaction of electrons in a molecular system. The EHOMO and ELUMO are used to study the potential of organic compounds against target receptors. ELUMO describes the electron-donating ability of a molecule while ELUMO explains the electron-accepting ability of a molecule. High EHOMO and low ELUMO values signify the great potential of a molecule to donate and accept electrons readily [30–32]. From Table 2, it was observed that lumateperone had a higher value of EHOMO (-4.47 eV) than other compounds. These predictions from EHOMO suggest that lumateperone will have greater facility towards electron donation therefore highest inhibition potential with the target enzyme than other studied compounds. Also, tafamidis had the lowest ELUMO indicating the compound that can accept electrons readily among others. Furthermore, the energy bang gap (Eg), which is the difference between ELUMO and EHOMO, plays an important role in explicating the chemical reactivity and stability of a molecule. High band gap energy usually signifies greater stability while low band gap energy signifies high reactivity. As shown in Table 2, Lumateperone had the lowest band gap energy (3.13 eV). This indicates the most chemically reactive molecule among all others. lumateperone will have the highest inhibition efficiency with the target enzyme than others.
Ionization energy (I) is the energy required to remove the most loosely bonded electrons from their orbital in an atom or molecule. The higher ionization energy of a molecule indicates that the energy needed to remove its valence electron will be high and hence, high stability of the molecule. Lower the ionization energy, the higher the reactivity and vice versa [33]. Table 2 shows the pattern of increase in ionization potential, which follows the pattern of increase of EHOMO. Lumateperone displayed the lowest ionization energy and therefore, the most reactive among others.
Electronegativity (χ) is a property that explains the ability of an atom, group of atoms or functional group to attract an electron toward itself [34]. Tafamidis displayed the highest electronegativity value (4.50 eV). The high electronegativity of this compound can be related to the presence of 2-chlorine atoms attached to phenyl moiety on the ring.
The electronic chemical potential (µ) measures the releasing propensity of an electron from the equilibrium system. Lumateperone had the highest chemical potential (-2.90 eV)
Global hardness (η) measures the resistance to charge transfer and global softness (δ) measures the molecule’s capability to charge transfer. These properties have been used in the establishment of chemical processes [35]. Lumateperone had the lowest hardness (1.57 eV) and the highest softness (0.64 eV) indicating the best compound susceptible to charge transfer and therefore the most reactive compound.
The electrophilicity index (ω) measures the potential of a molecule to take up electrons. It explains the stabilization energy of the molecule when saturated by electrons coming near the environment [34]. Electron-donating power (ω−) and electron-accepting power (ω+) are used to measure the donor-acceptor interactions. Highly effective electron donors have lower values of electron-donating ability and vice versa. Also, highly effective electron acceptors have a higher value of electron-accepting power. Lumateperone had the lowest electron-donating power (5.82 eV) indicating the best electron donor while tafamidis had the highest electron accepting power.
3.3 HOMO and LUMO orbital surfaces
The HOMO and LUMO orbital surfaces are shown in Table 3. Lumateperone had HOMO orbitals spread over benzopyrazine ring which can be attributed to the presence of a pyrazine ring containing nitrogen atoms that can enter donor-acceptor interactions and release an electron from its lone pair. The LUMO orbital surface spread over the flourophenylpropanone. Anisoperidone had HOMO surface disperse over the phenylpyridnyl ring while the LUMO orbital disperse over the methoxylphenyl ring. In melperone, the LUMO orbital spread over the nitrogen atom in piperidinyl ring attached to the structure. The electron-withdrawing potential of fluorine attached to the structure enables LUMO orbital to shift and disperse over flourophenyl ring in the structure. Bromperidol, Azabuperone and pimozide had HOMO orbital spread over the nitrogen-containing phenyl ring while the LUMO orbital spread over the fluorine-containing phenyl ring. The HOMO and LUMO orbital of tafamidis spread over the ring.
Table 2
Reactivity descriptors of the studied compounds computed
Comp | EHOMO | ELUMO | Eg | I | A | χ | µ | η | δ | ω | ω− | ω+ |
Lumateperone | -4.47 | -1.34 | 3.13 | 4.47 | 1.34 | 2.90 | -2.90 | 1.57 | 0.64 | 2.70 | 5.82 | 2.88 |
Anisoperidone | -5.58 | -1 | 4.58 | 5.58 | 1 | 3.29 | -3.29 | 2.30 | 0.44 | 2.37 | 6.77 | 2.01 |
Melperone | -6.02 | -1.41 | 4.61 | 6.02 | 1.41 | 3.71 | -3.71 | 2.31 | 0.43 | 3.00 | 7.52 | 2.85 |
Bromperidol | -5.91 | -1.49 | 4.42 | 5.91 | 1.49 | 3.70 | -3.70 | 2.21 | 0.45 | 3.10 | 7.45 | 3.05 |
Azabuperone | -5.89 | -1.46 | 4.43 | 5.89 | 1.46 | 3.67 | -3.67 | 2.22 | 0.45 | 3.05 | 7.41 | 2.98 |
Deutetrabenazine | -6.01 | -0.42 | 5.59 | 6.01 | 0.42 | 3.21 | -3.21 | 2.80 | 0.38 | 1.85 | 6.93 | 1.18 |
Pimozide | -5.59 | -0.45 | 5.14 | 5.59 | 0.45 | 3.02 | -3.02 | 2.57 | 0.39 | 1.77 | 6.47 | 1.17 |
Tafamidis | -6.69 | -2.32 | 4.37 | 6.69 | 2.32 | 4.50 | -4.50 | 2.19 | 0.48 | 4.64 | 9.01 | 5.33 |
3.4 Molecular Electrostatic Potential Analysis
Molecular electrostatic potentials (MEPs) provide well information on the chemical/biological reactivity of a molecule. The 3D spatial distribution of the electrostatic potential is responsible for the binding of a ligand to the active site of an enzyme. The MEP map displays the most likely site for nucleophilic and electrophilic attacks. The MEPs are represented by color scheme. Red and blue regions represent partially negative charge (electron-rich) and partially positive charge (electron deficient) while yellow, light blue and green regions represent slightly electron-rich, slightly electron-deficient and neutral. The MEP map for all the compounds are shown in Fig. 2. As seen from the table N, O, F, Cl and carbon atoms in aromatic phenyl ring in all the compounds display the most negative and partial negative electrostatic potential indicating the potential site for electrophilic attacks. Also, all hydrogen atoms display positive electrostatic potential.
3.5 Mulliken charge distribution (MCD) analysis
The electronic charges play an important role in determining the bonding ability of a molecule. It is useful in understanding the charge distribution in a molecule. Mulliken charge values for the constituent atoms of the studied compounds are presented in Table 4. The carbon atom directly attached to the electronegative element displayed the maximum positive atomic charges in all the compounds. In Lumateperone, O1, N1, N2 and N3 displayed the maximum negative atomic charge while C13, C22 and C28 showed the maximum positive atomic charge. Anisoperidone had the highest negative atomic charge at O0, O1 and N2 while the maximum positive atomic charges are located at C12 and C23. C9, O1 and N2 had the maximum negative atomic charge in Melperone and the maximum positive charges are found at C12 and C18. Furthermore, Bromperidol had the highest negative atomic charge at O3 and N4 and the positive atomic charges can be seen at C18 and C25. Azabuperone had maximum positive and negative atomic charges at O1, C5 and C18 In Deutetrabenazine, s maximum negative atomic charges can be found at O0, O1 and 02 while the highest positive atomic charge is centered around C10, C19 and C20. Pimozide and tafamidis had the highest negative atomic charges at 02 and N5.
Table 4
Selected Mulliken Charge Distribution
Atomic charges | Mulliken Lumate- Perone | Mulliken Anisop- eridone | Mulliken Melperone | Mulliken Bromp- eridol | Mulliken Azabu- perone | Mulliken Deutetr- abenazine | Mulliken Pimozide | Mulliken Tafamidis |
F0 | -0.291 | - | -0.292 | - | -0.291 | - | -0.3 | |
O0 | | -0.506 | - | - | - | -0.452 | - | |
O1 | -0.498 | -0.503 | -0.473 | - | -0.473 | -0.53 | - | |
F1 | - | - | - | -0.288 | - | - | 9-0.298 | |
Cl0 | | | | | | | 99 | 0.005 |
Cl1 | | | | | | | | + 0.003 |
Bro | - | - | - | -0.088 | - | - | - | |
O2 | - | - | - | -0.65 | - | -0.531 | -0.538 | -0.528 |
O3 | - | - | - | -0.498 | - | - | - | -0.584 |
04 | - | - | - | - | | - | | -0.473 |
N2 | -0.511 | -0.407 | -0.399 | - | -0.391 | - | - | - |
N3 | -0.413 | - | - | - | -0.408 | -0.432 | -0.4 | - |
N4 | -0.505 | - | - | -0.415 | | - | -0.459 | - |
N5 | - | - | - | - | | | -0.769 | -0.541 |
C3 | - | -0.133 | -0.077 | - | | - | - | - |
C4 | - | -0.108 | -0.264 | - | -0.291 | -0.017 | - | - |
C5 | + 0.042 | -0.337 | -0.26 | + 0.283 | -0.473 | -0.176 | - | - |
C6 | -0.208 | -0.156 | -0.123 | -0.266 | -0.391 | -0.127 | + 0.011 | + 0.341 |
C7 | -0.281 | + 0.123 | -0.128 | -0.302 | -0.408 | -0.338 | -0.271 | + 0.267 |
C8 | -0.127 | -0.275 | -0.118 | -0.121 | + 0.023 | -0.133 | -0.274 | + 0.503 |
C9 | + 0.088 | -0.195 | -0.444 | -0.127 | -0.293 | + 0.1 | -0.125 | + 0.064 |
C10 | + 0.245 | -0.339 | -0.286 | -0.108 | -0.138 | + 0.437 | -0.132 | -0.218 |
C11 | -0.117 | + 0.111 | -0.332 | 0.136 | -0.139 | -0.252 | -0.115 | -0.173 |
C12 | -0.138 | + 0.437 | + 0.386 | -0.278 | -0.292 | -0.334 | -0.288 | + 0.047 |
C13 | 0.358 | -0.19 | + 0.058 | -0.186 | -0.131 | + 0.124 | + 0.34 | -0.173 |
C14 | -0.135 | -0.179 | -0.189 | -0.187 | -0.13 | -0.073 | -0.251 | -0.155 |
C15 | -0.108 | + 0.084 | -0.155 | -0.341 | -0.117 | -0.247 | + 0.771 | -0.148 |
C16 | -0.225 | -0.136 | -0.213 | -0.154 | -0.287 | -0.259 | -0.267 | -0.076 |
C17 | -0.216 | -0.134 | -0.2 | -0.145 | -0.334 | -0.448 | + 0.349 | -0.076 |
C18 | -0.142 | -0.129 | + 0.391 | + 0.443 | + 0.387 | -0.445 | -0.186 | -0.12 |
C19 | -0.277 | -0.188 | - | + 0.012 | 0.058 | + 0.324 | + 0.167 | + 0.547 |
C20 | -0.306 | -0.163 | - | + 0.072 | -0.191 | + 0.335 | + 0.172 | - |
C21 | -0.341 | -0.182 | - | -0.173 | - | -0.216 | -0.182 | - |
C22 | + 0.445 | -0.2 | - | -0.165 | - | -0.217 | -0.148 | - |
C23 | + 0.073 | + 0.382 | - | -0.205 | - | - | -0.146 | - |
C24 | -0.16 | -0.221 | - | -0.204 | - | - | -0.18 | - |
C25 | -0.175 | - | - | +0.391 | - | - | -0.185 | - |
C26 | -0.199 | - | - | - | - | - | -0.191 | - |
C27 | -0.207 | - | - | - | - | - | -0.193 | - |
C28 | + 0.389 | - | - | - | - | - | -0.199 | - |
C29 | - | - | - | - | - | - | -0.193 | - |
C30 | - | - | - | - | - | - | -0.197 | - |
C31 | - | - | - | - | - | - | -0.2 | - |
C32 | - | - | - | - | - | - | 0.377 | - |
C33 | - | - | - | - | - | - | 0.377 | - |
3.6 Drug-likeness Properties
swiessADME webserver the drug-likeness properties of the screened compounds were predicted based on Lipinski’s rule of five to examine the features of the compounds as drug or non-drug like and the result is provided in Table 5 below. The pharmacological and pharmacodynamics model of the compounds were assayed to predict their biological role. The rule of five (ROF) proposed by Christopher Lipinski was employed to determine their pharmacological potency via molecular weight < 500, number of HB acceptors < 10, number of HB donors < 5, and Lipohilicity (iLog p < 5). From the result obtained (Table 5) all the screened compounds were observed to have a molecular weight ranging from 197.19 to 461.55, hydrogen bond acceptor and donor ranging from 3 to 5 and 0 to 4, respectively. Topological polar surface area ranges from 103.78 to 20.31 and Lipohilicity from 0.78 to 4.23. In this regard, all the screened compounds obey the rule of five without violation of any of Lipinski’s parameters.
Table 5
Drug-likeness properties of the screened compounds
Ligands | MW | H-Acceptor | H-donor | TPSA | iLogP | Violation |
Tafamids | 308.12 | 4 | 1 | 63.33 | 2.65 | 0 of 5 |
Leumateprone | 393.5 | 3 | 0 | 26.79 | 3.68 | 0 of 5 |
Bromperidol | 420.32 | 4 | 1 | 40.54 | 3.7 | 0 of 5 |
L-DOPA | 197.19 | 5 | 4 | 103.78 | 0.78 | 0 of 5 |
Meloperone | 263.35 | 3 | 0 | 20.31 | 3.18 | 0 of 5 |
Pimozide | 461.55 | 4 | 1 | 41.3 | 4.23 | 0 of 5 |
Azabuperone | 290.38 | 4 | 0 | 23.55 | 3.23 | 0 of 5 |
Anisolpirol | 335.44 | 3 | 0 | 29.54 | 3.78 | 0 of 5 |
Deuterabenazine | 323.46 | 4 | 0 | 38.77 | 3.47 | 0 of 5 |
3.7 Pharmacokinetics
Pharmacokinetics is determined by the drug candidate's molecular description. Prediction of absorption, distribution, metabolism, excretion, and toxicity (ADMET) features in silico has become significant in drug selection and determining its success for human therapeutic usage. As a result, these physiochemical descriptors were tested in order to establish the ADMET characteristics of the compounds utilizing the admetSAR sever. Tafamids, Bromperidol, L-DOPA, and pimozide were found to be effective and show low absorption while Leumateprone, meloperone, azabuperone, anisolpirol, and deuterabenazine show high absorption in the intestine via Caco-2 permeability, probably admissible by their molecular size. However, all the screened ligands displayed high intestinal absorption (Table 6). The result of the ADMET properties revealed that all tested ligands had a blood-brain barrier (BBB) permeability except L-DOPA. Tafamids, Bromperidol, L-DOPA, and anisolpirol are non-substrate of p-glycoprotein (P-GB) permeability while on the other hand Leumateprone, meloperone, azabuperone, pimozide, and deuterabenazine are substrates of P-glycoprotein permeability. Plasma binding protein is a biomarker for determining the binding of drugs to the proteins within the blood [36]. A drug's efficiency is primarily determined by the rate at which it binds. A low plasma protein binding rate is associated with greater efficiency and ease of diffusion [37]. Because all of the drugs have a high plasma protein binding rate, their migration to the site of action where they exert pharmacological effects may be hampered. Although, L-DOPA and meloperone displayed a moderate plasma protein binding rate.
Cytochrome P450s (CYPs) are an enzyme superfamily that plays an important role in drug metabolism [38]. According to the drug metabolism interaction, investigated compounds and L-DOPA are non-inhibitors of CYP2C19, CYP2C9, and CYP3A4, Bromperidol and pimozide are inhibitors of CYP3A4. Additionally, Tafamids, Leumateprone, meloperone, pimozide are inhibitors of CYP1A2 while all compounds except Tafamids and L-DOPA are inhibitors of CYP2D6. None of the compounds are substrates of CYP2C9. Tafamids and L-DOPA are non-substrates of CYP2D6 and CYP3A4 while Leumateprone, meloperone, azabuperone, pimozide, and deuterabenazine, Bromperidol, and anisolpirol are found potential substrate for CYP2D6 and CYP3A4.
Acute oral toxicity refers to the potential side effects of medication delivery by mouth [39]. All of the chemicals examined had low oral toxicity. The AMES test (Salmonella typhimurium reverse mutation assay) is a pharmacological screening technique that uses genetic mutation induction to assess the carcinogenicity of a medicinal medication [40]. In the AMES test, all chemicals except L-DOPA were shown to be non-toxic. Tafamids and deuterabenazine are nephrotoxic while all compounds except deuterabenazine are non-hepatotoxic. Also, all the compounds were observed to be toxic to reproductive organ and only Tafamids is toxic to the respiratory organ (Table 6). To obtain better pharmacological molecules with a good biosafety profile, the compounds may be subjected to functional group alteration. Medication solubility has been regarded as an ultimate advantage in the drug development process because it aids in determining the drug concentration in the systemic circle, resulting in a maximal optimum response [41]. All the compounds had high aqueous solubility, which might be attributable to their high hydroxyl group count. Human Ether-a-go-go Related Gene (hERG) is a potassium channel that regulates cardiac excitability and maintains appropriate cardiac rhythm [42]. Leumateprone, anisolpirol, pimozide, and deuterabenazine, are inhibitors of hERG gene. However, Tafamids, L-DOPA, Bromperidol, azabuperone, and meloperone are non-inhibitors of the hERG gene, which confirms they would not contribute to drug-induced proarrhythmia.
Table 6
Pharmacokinetics properties of the screened ligand
Models | Tafamids | Leumateprone | Bromperidol | L-DOPA | Meloperone | Pimozide | Azabuperone | Anisolpirol | Deuterabenazine |
Ames mutagenesis | - | - | - | + | - | - | - | - | - |
Acute Oral Toxicity (c) | II | III | II | III | III | III | III | III | III |
Blood Brain Barrier | + | + | + | - | + | + | + | + | + |
Biodegradation | - | - | - | - | - | - | - | - | - |
Caco-2 | - | + | - | - | + | - | + | + | + |
Carcinogenicity (binary) | - | - | - | - | - | - | - | - | - |
CYP1A2 inhibition | + | + | - | - | + | + | - | - | - |
CYP2C19 inhibition | - | - | - | - | - | - | - | - | - |
CYP2C9 inhibition | - | - | - | - | - | - | - | - | - |
CYP2C9 substrate | - | - | - | - | - | - | - | - | - |
CYP2D6 inhibition | - | + | + | - | + | + | + | + | + |
CYP2D6 substrate | - | + | + | - | + | + | + | + | + |
CYP3A4 inhibition | - | - | + | - | - | + | - | - | - |
CYP3A4 substrate | - | + | + | - | + | + | + | + | + |
CYP inhibitory promiscuity | - | + | - | - | + | + | + | + | - |
Hepatotoxicity | + | - | - | - | - | - | - | - | - |
Human Ether-a-go-go-Related Gene inhibition | - | + | - | - | - | + | - | + | + |
Human Intestinal Absorption | + | + | + | + | + | + | + | + | + |
Human oral bioavailability | + | - | + | - | + | - | - | + | - |
Mitochondrial toxicity | - | + | + | + | + | + | + | + | + |
Nephrotoxicity | + | - | - | - | - | - | - | - | + |
Acute Oral Toxicity | 1.927911 | 2.682666 | 2.564216 | 1.584643 | 2.853549 | 2.025461 | 2.574924 | 2.573651 | 0.934101 |
P-glycoprotein inhibitior | - | + | + | - | - | + | - | + | - |
P-glycoprotein substrate | - | + | - | - | + | + | + | - | + |
Plasma protein binding | 0.90996 | 0.761109 | 0.921986 | 0.419808 | 0.529639 | 0.956002 | 0.720404 | 0.830053 | 0.828292 |
Reproductive toxicity | + | + | + | + | + | + | + | + | + |
Respiratory toxicity | - | + | + | + | + | + | + | + | + |
Subcellular localzation | Plasma membrane | Mitochondria | Mitochondria | Nucleus | Mitochondria | Mitochondria | Mitochondria | Mitochondria | Mitochondria |
UGT catelyzed | - | - | - | + | - | - | - | - | - |
Water solubility | -4.55183 | -3.38666 | -4.00928 | -1.66278 | -2.79501 | -3.70658 | -3.40819 | -2.24233 | -3.19399 |