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30
Drug Repurposing and Computational Drug Discovery: Strategies and Advances
purpose.13 In the structure-based approach the investigational compound is tested toward a set of biological targets to determine its binding affinity. Several software tools and databases are available to the researchers for the purpose, namely, AUTODOCK, GOLD, GLIDE, and a few more.
14
FIGURE 2.1 Computational approaches in drug discovery.
2.2.1 LIGAND-BASED APPROACHES
Ligand-based drug design (LBDD) plays an approachable role in the drug discovery process when no information is available regarding the three-
15
dimensional structure of potential drug targets.
Ligand-based approaches evaluate ligands/molecular scaffolds that are known to interact with the targets of interest. Basically, strategies employed by this method are assort­ment of chemical species having chemical resemblance to known ligands with a couple of similarity measures and constructing QSAR models predicting biological activity from the chemical makeup. Of note, this method is primarily used for in silico design and screening of novel ligands as potential drug candidates. The approach is a valuable tool for optimizing the ADMET properties of drug likely candidates. Three-dimensional quantitative structure activity relationship (3D QSAR) and pharmacophore modeling are the most commonly used techniques in ligand-based approaches for giving predictive
31 Computational Drug Discovery and Drug Repurposing
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models for lead generation and optimization.
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Target fishing and reverse docking methods are also being used in the ligand-based drug discovery process.
17
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QSAR analysis is the most efficient tool in establishing a statistically signifi­cant correlation between chemical structures and their pharmacological
18
activities. parameters known as molecular descriptors.
The structural information is defined in terms of a series of
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These molecular descriptors
are classified as topological, geometrical, thermodynamic, electronic, and
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constitutional types.
Table 2.1 lists out the type of software programs available for computation of molecular descriptors. Linear regression, multiple regressions, partial least squares, and principal component analysis or regression are the statistical approaches covered under linear methods of QSAR analysis. Artificial neural networks (ANN), k-nearest neighbors (kNN), and Bayesian neural networks are the nonlinear methods of QSAR
21
modeling.
QSAR model construction involves a sequential preprocessing and transformation of data sets division, feature/descriptor selection, model development and validation, and finally followed by interpretation and domain analysis.
19
To generate a perfect QSAR model, proper selection of compounds (minimum 20 with reliable and comparable bioactivity data), molecular descriptors for ligands with no autocorrelation to avoid over fitting, and use of internal and external model validation methods for deter­mining applicability and productivity is too crucial.
22
TABLE 2.1 Software for Computation of Molecular Descriptors.
Software Description Website Availability
ADAPT Geometrical, topological, http://research.chem.psu.edu/ Free
physicochemical, electronic pcjgroup/adapt.html
ADMET Constitutional, functional www.simulations-plus.com Commercial Predictor group counts, topological,
E-state, 3D descriptors, molecular patterns, acid–base ionization, empirical estimates of quantum
ALOGPS2.1 log P, log S www.vcclab.org Free
status
32
TABLE 2.1 (Continued)
Software Description Website Availability
ADRIANA. Topological, constitutional, www.molecularnetworks.com Commercial Code functional group counts,
ACD/logP log S, log P, log D, pKa www.acdlabs.com Commercial CDK Topological, geometrical, http://cdk.github.io Free
Dragon Topological, constitutional, www.talete.mi.it Commercial
JOElib Topological, counting, www.ra.cs.uni-tuebingen.de Free
MOE Topological, structural keys, www.chemcomp.com Commercial
MOLD2 1D, 2D www.fda.gov Free MOLGEN- Constitutional, topological, www.molgen.molgenqspr. Commercial
QSPR geometrical, etc. html Open 166-bit, MOLPRINT2D www.openbabel.org Free
BABEL MACCS, structural key
PADEL Molecular fingerprints, 1D, www.padel.nus.edu.sg Free
PowerMV Constitutional, atom pairs, www.niss.org/PowerMV Free
Drug Repurposing and Computational Drug Discovery: Strategies and Advances
status
E-state, meylanflags, moriguchi molecular patterns, 3D descriptors, etc.
electronic, constitutional
2dautocorrelations, geometrical, GETAWAY, WHIM, RDF, functional groups, etc.
geometrical properties, etc.
physical properties, etc.
fingerprints, daylight fingerprint (FP2)
2D, 3D descriptors
fingerprints, BCUT

Data quarrying and analysis methodologies aid in speeding up the process
23,24
of target assessment.
Big data analysis and artificial intelligence methods (deep neural networks) are embroidering to the computational approaches, which has resulted in advanced and fruitful drug development.25 Free sources such as Open-Targets,26 UniProt, and ChEMBL27 offer useful starting point to cover areas of disease association, protein annotation, and potential ligands,
33 Computational Drug Discovery and Drug Repurposing
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respectively. In recent times, machine learning algorithms are applied for
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target identification and drug discover
y.
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Polypharmacological studies indicate that most drug molecules are not target
29
specific and hit multiple targets.
This possibility has led to the concept of the “chemical space” to entitle and consider all ensembled organic molecules in the quest for finding new drugs.30 Information regarding all the organic molecules can be extracted from various databases such as Pubchem,
32
ZINC,
Chemspider,
CTD,37 HMDB,38 SMPDB,39 Drugbank.
33
ChemDB,
34
BindingDB,
40
35
ChEMBL,
27
NCI open,
31
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Chemioinformatics deals with collection, storage, analysis, and manipula-
41
tion of large quantities of chemical data.
Computer-aided drug design, structural representation, and chemmetrics are the primary elements of the cheminformatics.
42
The chemical structure representations can be linear,
2D or in 3D format, or as SMILES (simplified molecular input line entry
43
specification).
CAS Draw, DIVA (diverse information, visualization, and analysis), Structure Checker Accord, MarvinSketch, PowerMV, ArgusLab, Babel, Chimera, CLIFF, Dragon, Grace, JOELib, ORTEP, Packmol, Polar, Biosoft, Q-chem, KOWWIN, etc., are some commonly used chemioinfor­matic software tool packages.
44
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The atomic and electronic groups in a molecule are transformed into pharmacophoric features and designated as hydrogen bond donors or acceptors, cationic, anionic, aromatic, or hydrophobic attributes called as
45
pharmacophore fingerprints.
The pharmacophore model is generated first by translating protein–ligand interactions. Ligand-based 3D pharmacophore models can be developed when no 3D structure of the macromolecule target is available. and optimization.
46
3D pharmacophore models are used for hit and lead discovery
47
Accelrys’ (CA, USA), Discovery Studio Schrödinger’s
(NY, USA) PHASE, Chemical Computing Group’s (QC, Canada), Molecular
34
Drug Repurposing and Computational Drug Discovery: Strategies and Advances
Operating Environment (MOE), and Inte:Ligand’s (Maria Enzersdorf, Austria) LigandScout are some frequently utilized pharmacophore modeling software. Advanced approaches employing 3D pharmacophore concept
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such as AutoDock Bias, mapper
52
are popular among researchers.
PyRod,49 SILCS-Pharm,50 PharmIF,51 and Pharm-
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The concept of “drug-likeness” riddles out molecules with unsolicited prop­erties from screening libraries and reduces attrition risk at the later stages of drug discovery. A variety of machine learning techniques are available to the modeling of drug-likeness like artificial neural networks (ANN), support
53
vector machine (SVM), and recursive partitioning. of drug-likeness (QED),
54
relative drug likelihood (RDL),55 and Gaussian
Quantitative estimate
scoring function (GAU)56 are few quantitative approaches used for defining druggability. Recently, artificial intelligence (AI) approaches are applied for model construction for specific ADMET endpoint evaluation,53 like Naïve
57
Bayesian Classifiers (NBC) approach approaches.
58
ADMET studies are indispensable steps in any drug discovery
and recursive partitioning (RP)
program as it provides pharmacokinetic and pharmacodynamic, that is, toxicity (T) properties of lead molecules and reduce the risk of experimental cost, time, and drug failure. Progresses in combinatorial chemistry and high­throughput screening have comprehensively amplified the number of small molecules for which early data on ADMET are available as reference.
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Table 2.2 depicts some software application tools for ADMET screening. Figure 2.2 describes sequential steps in toxicity prediction modeling.
TABLE 2.2 Software Application Tools for ADMET Screening.
Software tools Feature application Web address
ADMET lab Systematic ADMET screening http://admet.scbdd.com/ ADMET ADMET property screening https://www.simulations-plus.com/
predictor
ADVERpred Prediction of adverse effects of http://www.way2drug.com/
drugs adverpred/ eMOLTOX Prediction of molecular toxicity http://xundrug.cn/moltox LIVERTOX Screening of hepatotoxicity https://livertox.nih.gov/ Molinspiration Screening of molecular properties http://www.molinspiration.com/
software/admetpredictor/
TABLE 2.2 (Continued)
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Software tools Feature application Web address
Mousetox Small molecule cytotoxicity
prediction
SOM prediction
QikProp Schrodinger tool for ADMET
SwissADME Assessment of ADMET http://www.swissadme.ch/
Knowledge based prediction of
ADMET properties
prediction
parameters
http://enalos.insilicotox.com/ MouseTox/ http://www.scfbio-iitd.res.in/
software/ drugdesign/som.jsp https://www.schrodinger.com/
qikprop
35 Computational Drug Discovery and Drug Repurposing
FIGURE 2.2 Sequential toxicity modeling.
2.2.2 STRUCTURE-BASED APPROACHES
Structure-based drug design (SBDD) methods are a prominent component
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of modern medicinal chemistry.
Molecular docking, structure-based virtual screening (SBVS), and molecular dynamics (MD) are frequently used SBDD strategies for analysis of molecular recognition events of binding energetic, molecular interactions, and induced conformational changes.
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based approaches use structural data obtained experimentally or through computational homology modeling. The accessibility of 3D macromolecular targets allows a meticulous inspection of the binding site topology along
Structure-
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Drug Repurposing and Computational Drug Discovery: Strategies and Advances
with electrostatic properties, such as charge distribution as well. Current methods allow for the design of ligands containing the necessary features for efficient modulation of the target receptor.
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The advances in computational biology have made molecular modeling a
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reliable method in the drug discovery pipeline.
Comparative modeling or homology modeling uses target sequence information for the prediction of three-dimensional structures. sents thoughtful applications of algorithms of protein structure prediction.
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The approach of molecular modeling repre-
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The computational methods for protein structure prediction include compara­tive modeling, fold recognition, first-principles methods with database infor­mation, and first-principles methods without database information.66 CASP (critical structure prediction assessment, a biennial collective project) plays a key role in protein structure prediction.67 RasMol, PyMOL, Chimera, etc., are some visualization tools for visualizing and analyzing predicted models defining the protein structure.
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Docking precisely fits ligand into the specific receptor-binding site and evalu-
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ates their binding strength.
Molecular docking can be flexible or rigid. Flex­ible docking is a well-thought-out good approach with better prediction than conventional docking.70 Molecular docking is divided into three sections; ligand–receptor preparation based on force field estimations followed by applying flexible or rigid docking methods and setting search strategies for ligand ratification.71 Mapping of binding site is done following ligand and receptor preparation by GRID calculations.
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Various algorithms are applied in the docking process such as fragment-based, Monte Carlo and molecular dynamic based. empirical-based, and knowledge-based scoring functions are usually applied for docking studies.
73,74
Some frequently used software tools for docking studies are depicted in Table 2.3. Analyzing docking outputs is essential as it explains the number and nature of bond present in the ligand protein complex.75 LigPLOT (2D format),
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format)
are highly used software tools for analyzing docking results.
76,77
PyMOL,78 and Chimera (3D
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TABLE 2.3 Some Frequently Used Software Tools for Docking Studies.
Software Algorithms License
AUTODOCK Genetic Lamarckian Open, GNU, GPL SWISSDOCK Evolution based optimization Open, academic AUTODOCKVINA Local optimization Apache GOLD Genetic algorithms Commercial GLIDE Hybrid optimization Commercial CDOCKER Hybrid optimization and shape mapping Commercial iGEMDOCK Genetic Commercial Arguslab Hybrid optimizations Open MOE-DOCK Hybrid optimizations Commercial
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Molecular dynamics simulations are useful tools for the drug discovery process. It is advantageous over molecular docking studies as it facilitates the evalua­tion of the binding energetic and kinetics of the receptor–ligand interactions
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at atomic levels. changes in the biomolecules exhausting the molecular dynamic theories. GROMACS,82 AMBER,83 CHARMM-GUI,
Numerous tools are available to explore the atomic level
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84
NAMD,85 and LAMMPS,
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etc.
are commonly used software packages for molecular dynamic simulations.
2.2.3 SYSTEMS-BASED APPROACHES
Merging of genomics, proteomics, and metabolomics databases with system pharmacology models aims at generating a disease-specific network that would build confidence in particular target identification that is the
87
crucial step in drug discovery and identification process.
Systems-based approaches connect protein targets to their physiological arena, seeing a much wider systemic viewpoint of their environment in conjunction of their molecular details.88 Drug targeting for diseases with complex patho­physiology involving complicated signaling transduction cascades mostly benefits from a system-based approach.
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Quantitative systems pharmacology or network pharmacology is an emerging discipline that aims to integrate methods of systems biology with
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Drug Repurposing and Computational Drug Discovery: Strategies and Advances
pharmacodynamic concepts to foster advanced small-molecule and macro­molecule drug discovery and development.89 40% of current drug discov­eries are based on the concept of network pharmacology.90 Establishing drug target disease network by utilizing high-throughput omics technologies is
91
the major goal of network pharmacology.
Through unbiased investiga­tions on potential target spaces network pharmacology endeavors to discover newer leads and targets and also repurpose prevailing drug molecules for diverse therapeutic conditions.
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The area of network pharmacology has been broadly divided into the experimental and computational approaches. Graph theory, statistical methods, data mining, modeling, and information visualization methods are the computational approaches. The experimental approaches include integrated clinical trials and high-throughput omics techniques. Data mining, big data analytics, network construction, interac­tions prediction, and network analysis are key methodologies in the network
93
pharmacology approach. done through public databases and further experimental analysis.
Data mining in network pharmacology can be
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In contrast to QSAR modeling proteochemometric (PCM) modeling is based
94
on the similarity of a group of ligands and a group of targets.
Commonly PCM has mainly been applied to data sets of G protein-coupled receptors (GPCRs), in particular the rhodopsin-like class A receptors, dopamine, histamine, adrenergic, and melanocortin receptors.
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This versatile approach is deployed for hit identification for orphan targets, as well as modeling of orthosteric and allosteric ligands.
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Table 2.4 summarizes a few more PCM­applied studies. Repurposing of mebendazole and celecoxib was done using a proteochemometric modeling known as TMFS (train-match-fit-streamline) method applying a variety of descriptors.
TABLE 2.4 Summary of Few PCM Applied Studies.
Target protein Method applied No. of
Aromatase Bayesian linear regression,
random forest, support vector machine
Carbonic anhydrases
k-nearest neighbor 9 549 [99]
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No. of ligand
proteins
studied
1 10 [98]
molecules
screened
Reference
TABLE 2.4 (Continued)
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Target protein Method applied No. of No. of ligand Reference
proteins molecules
studied screened
Cyclooxygenases Gradient boosting machine, 11 3228 [100]
elastic net, random forest
Dengue virus Partial least squares 4 45 [101] NS2B-NS3 proteases
HIV-reverse Support vector machine 2 48,221 [95] transcriptases and proteases
Kinases Deep neural network, 284 19.9 million [102]
random forest, gradient boosting machine
Nuclear receptors Decision tree, logistic 11 7267 [103]
regression, ridge classifier, random forest
Serine proteases Partial least squares, 24 5863 [104]
random forest
39 Computational Drug Discovery and Drug Repurposing
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Pathway analysis is also known as enrichment analysis which examines data acquired by high-throughput technologies for correlating physiological mechanism to macromolecular targets and comprises input phase, analysis
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phase, and output phase. in the input phase.
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Null hypothesis generation is the important step
Analysis phase comprises mathematical computations defined by a specific set of algorithms that are used by widely available open-source software programs such as BioConductor
107
and GitHub
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projects. Finally, the output phase comprises visualization and analysis of results. The relevant pathways are ranked arranged in a hierarchal manner by applying statistical attributes.
2.2.4 CASE STUDY
Rath et al. (2021) used the computational approach and designed the novel mesalamine–coumarin conjugate. The macromolecular targets chosen for docking (using AUTODOCKVINA) were COX-2(cyclooxygenase-2),