**5. Methodologies of drug repurposing**

The methodologies adopted in DR can be divided into three broad groups depending on the quantity and quality of the pharmacological, toxicological and biological activity information available. These are mainly (i) drug-oriented, (ii) target-oriented, and (iii) disease/therapy-oriented.

In the drug-oriented methodology, the structural characteristics of drug molecules, biological activities, adverse effects and toxicities are evaluated. This strategy is meant for identifying molecules with biological effects based on cell/

**9**

**Figure 4.**

*Methodologies and steps involved in drug repositioning.*

*Drug Repurposing (DR): An Emerging Approach in Drug Discovery*

animal assays. This type of repositioning methodology is based on traditional pharmacology and drug discovery principles, where studies are usually conducted to determine the biological efficacy of drug molecules without really knowing about the biological targets. Significant successes in DR have been achieved with this orientation profile, through serendipity or clinical observation, such as discoveries

Target-based methodology comprise *in silico* screening or virtual high-through-

The application of disease/therapy-oriented methodology in DR is relevant when there is more information on the disease model is available. In this case, DR can be guided by the disease and/or treatment based upon availability of information given by proteomics (disease specific target proteins), genomics (disease specific genetic data), metabolomics (disease specific metabolic pathways/profile) and phenotypic data (off-target mechanism, pharmacological targets, disease pathways, pathological conditions, adverse and side effects etc.) concerning the disease process. It, therefore, requires construction of specific disease networks, recognizing genetic expression, considering key targets, identifying disease causing protein molecules

**Figure 4** delineates the methodologies and steps involved in drug repositioning. Drug-based phenotypic screening and target-based methods account for more than 50% of the FDA approved small drug molecules and biologics. Phenotypic drug screening methods identify drug candidates from small molecule libraries

put screening (vHTS) of drugs or compounds from drug libraries/compound databases such as ligand-based screening or molecular docking followed by *in vitro* and *in vivo* high-throughput and/or high-content screening (HTS/HCS) of drugs against a selective protein molecule or a biomarker of interest. In this method, there is a significant success rate in drug discovery as compared to drug-oriented method, because most biological targets directly represent the disease pathways/

related to cell and metabolic pathways of interest in the disease model [23].

*DOI: http://dx.doi.org/10.5772/intechopen.93193*

with sildenafil [21].

mechanisms [22].

#### *Drug Repurposing (DR): An Emerging Approach in Drug Discovery DOI: http://dx.doi.org/10.5772/intechopen.93193*

*Drug Repurposing - Hypothesis, Molecular Aspects and Therapeutic Applications*

precise structural information about drug targets and in case, the protein target is not available, disease specific phenotypic or genotypic profiles of drugs are required

The methodologies adopted in DR can be divided into three broad groups depending on the quantity and quality of the pharmacological, toxicological and biological activity information available. These are mainly (i) drug-oriented,

In the drug-oriented methodology, the structural characteristics of drug molecules, biological activities, adverse effects and toxicities are evaluated. This strategy is meant for identifying molecules with biological effects based on cell/

In recent years, discovery scientists and researchers have combined *in silico* and experimental approaches to identify new therapeutic indications for existing drugs, called mixed approach. In the mixed approach, the result of computational methods is validated by pre-clinical biological experiments (*in vitro* and *in vivo* tests) and clinical studies. The simultaneous application of computational and experimental methodologies in a systematic manner offer a robust and logical approach to the discovery of new indications, demonstrating a greater efficiency than the discovery based on serendipity. Further, mixed approach offers opportunities for developing repositioned drugs more effectively and rapidly. This approach is credible and yet,

[19]. **Figure 3** represents the approaches of drug repositioning.

**5. Methodologies of drug repurposing**

(ii) target-oriented, and (iii) disease/therapy-oriented.

**8**

reliable [20].

**Figure 3.**

*Approaches of drug repositioning.*

animal assays. This type of repositioning methodology is based on traditional pharmacology and drug discovery principles, where studies are usually conducted to determine the biological efficacy of drug molecules without really knowing about the biological targets. Significant successes in DR have been achieved with this orientation profile, through serendipity or clinical observation, such as discoveries with sildenafil [21].

Target-based methodology comprise *in silico* screening or virtual high-throughput screening (vHTS) of drugs or compounds from drug libraries/compound databases such as ligand-based screening or molecular docking followed by *in vitro* and *in vivo* high-throughput and/or high-content screening (HTS/HCS) of drugs against a selective protein molecule or a biomarker of interest. In this method, there is a significant success rate in drug discovery as compared to drug-oriented method, because most biological targets directly represent the disease pathways/ mechanisms [22].

The application of disease/therapy-oriented methodology in DR is relevant when there is more information on the disease model is available. In this case, DR can be guided by the disease and/or treatment based upon availability of information given by proteomics (disease specific target proteins), genomics (disease specific genetic data), metabolomics (disease specific metabolic pathways/profile) and phenotypic data (off-target mechanism, pharmacological targets, disease pathways, pathological conditions, adverse and side effects etc.) concerning the disease process. It, therefore, requires construction of specific disease networks, recognizing genetic expression, considering key targets, identifying disease causing protein molecules related to cell and metabolic pathways of interest in the disease model [23].

**Figure 4** delineates the methodologies and steps involved in drug repositioning.

Drug-based phenotypic screening and target-based methods account for more than 50% of the FDA approved small drug molecules and biologics. Phenotypic drug screening methods identify drug candidates from small molecule libraries

**Figure 4.** *Methodologies and steps involved in drug repositioning.*

by serendipitous observations. Target-based methods discover drugs based upon known target molecules. The treatment/therapy-based repositioning methodology is similar to disease-based methodology [23].

A detailed enumeration of various methodologies employed in drug repositioning along with suitable examples is given in **Table 2**.


**11**

*Drug Repurposing (DR): An Emerging Approach in Drug Discovery*

drug–drug similarities, new biomarkers for diseases etc.

changes in expression of genes by using computational approaches.

Several available repositioning methods depicted above in **Table 2** are described

**Blinded search or screening methods** involve serendipitous identification from biological tests/experimental screens aimed at specific disease models and drugs. The advantage of these methods is that they possess higher flexibility for screening

**Target-based methods** carry out *in vitro* and *in vivo* high-throughput and/ or high-content screening (HTS/HCS) of drug molecules for a protein target or a biomarker of interest and *in silico* screening of compounds or drugs from large compound libraries, such as ligand-based screening or molecular docking. In these methods, there is a higher possibility of finding useful drugs/drug leads as compared to blinded search methods. It also requires less time for the entire screening

**Knowledge-based methods** utilize bioinformatics or cheminformatics approaches to gather the available information of drug profile, chemical structures of targets and drugs, drug-target networks, clinical trial information including adverse effects, signaling or metabolic pathways. This information content of knowledge-based methods is rich enough as compared to blinded or target-based methods. The known information can be used to predict therefore, be used to predict the unknown new mechanisms, such as unknown targets for drugs, unknown

**Signature-based methods** use gene signatures derived from disease omics data (genomics data) with or without treatments to discover unknown off-targets or unknown disease mechanisms. Genomics data are publicly available as databases. The advantage of these methods is that they are useful to explore unknown mechanisms of action of drugs. In comparison to knowledge-based methods, signaturebased methods investigate drug mechanisms at more molecular-level, such as

**Pathway- or network-based methods** make use of disease omics data, available signaling or metabolic pathways, and protein interaction networks to reconstruct disease-specific path- ways that provide the key targets for repositioned drugs. The advantage of these methods is that they can narrow down general signaling networks from a large number of proteins to a specific network with a few proteins

**Targeted mechanism-based methods** integrate treatment omics data, available signaling pathway information and protein interaction networks to describe the unknown mechanisms of action of drugs. The advantage of these methods is that they are not only used to discover the mechanisms related to diseases or drugs, but also to identify those directly related to treatments of drugs to specific

Drug repositioning is an alternative approach to traditional drug discovery. With increasing market demand many pharmaceutical companies are developing new drugs or new therapeutic uses from existing/old/available drugs by drug repositioning approaches in less time, yet at low cost. In drug discovery program, the repositioning is usually essentially carried out in two stages as described follows. In the first stage, the *in silico* screening of approved drugs against a particular disease target is carried out, which is followed by the second step, in which the selected identified molecules are further experimentally investigated both *in vitro* and *in vivo* in specific disease models of interest. After successful preclinical studies in the

*DOI: http://dx.doi.org/10.5772/intechopen.93193*

a large number of drugs or diseases.

briefly as follows:

process to complete.

(or target molecules).

diseases [23–25].

**6. Repositioned drugs**

**Table 2.**

*Some available methods of drug repositioning [23, 24].*

*Drug Repurposing (DR): An Emerging Approach in Drug Discovery DOI: http://dx.doi.org/10.5772/intechopen.93193*

*Drug Repurposing - Hypothesis, Molecular Aspects and Therapeutic Applications*

is similar to disease-based methodology [23].

**Methodology Type of method,** 

**Drug-oriented** Phenotypic screening

Target 3D structure, chemical structure, information of drugs and ligands

Drug-target information, chemical structure, information of targets and drugs

Clinical trial information and adverse effects

FDA approval labels

**Disease-oriented** Available Pathway information

Disease omics/ genetics data

Disease omics data, available pathway information, and protein interaction network

**Therapy-oriented**

Disease omics and drug omics data

Drug omics data, disease pathway and protein interaction network

Drug omics data Signature- based or

*Some available methods of drug repositioning [23, 24].*

ing along with suitable examples is given in **Table 2**.

**category**

Screening

Target-based, Cheminformatics

Knowledge-based, Bioinformatics/ Chem-informatics

Knowledge-based, Bioinformatics

Knowledge-based, Bioinformatics

Knowledge-based, Bioinformatics

Signature-based Bioinformatics

Pathway or networkbased, Network biology

Signature- and networkbased, Bioinformatics and/or Network biology

Signature based, Bioinformatics

Targeted- mechanism based, Network biology and Systems biology

Blinded/ Target-based,

by serendipitous observations. Target-based methods discover drugs based upon known target molecules. The treatment/therapy-based repositioning methodology

A detailed enumeration of various methodologies employed in drug reposition-

**Method/specific approach**

*In vitro* and *in vivo* HTS/HCS screening

*In silico* screening, ligand-based screening and molecular docking, fragment-based screening

Drug–target prediction Simvastatin,

Drug similarity studies —

Drug similarity studies —

Discovery of disease mechanism and address of key targets

Studying gene signatures/ genomics to identify key targets

Studying gene signatures

Similarities between drugs and diseases

Elucidating targeted

pathways

Analysis of diseasespecific pathways and networks to identify key targets

**Example(s)**

Sildenafil (erectile dysfunction), rituximab (breast cancer)

Fluorouracil (lung cancer), etoposide (bladder cancer)

ketoconazole (breast

Vismodegib (skin cancer)

Sunitinib, dasatinib (breast cancer, brain

Sirolimus (acute lymphoblastic leukemia), Fasudil (neurodegenerative disorders)

Cimetidine (lung cancer), topiramate (inflammatory bowel

Daunorubicin, clomifene (breast cancer)

disease)

—

tumor)

cancer)

**10**

**Table 2.**

Several available repositioning methods depicted above in **Table 2** are described briefly as follows:

**Blinded search or screening methods** involve serendipitous identification from biological tests/experimental screens aimed at specific disease models and drugs. The advantage of these methods is that they possess higher flexibility for screening a large number of drugs or diseases.

**Target-based methods** carry out *in vitro* and *in vivo* high-throughput and/ or high-content screening (HTS/HCS) of drug molecules for a protein target or a biomarker of interest and *in silico* screening of compounds or drugs from large compound libraries, such as ligand-based screening or molecular docking. In these methods, there is a higher possibility of finding useful drugs/drug leads as compared to blinded search methods. It also requires less time for the entire screening process to complete.

**Knowledge-based methods** utilize bioinformatics or cheminformatics approaches to gather the available information of drug profile, chemical structures of targets and drugs, drug-target networks, clinical trial information including adverse effects, signaling or metabolic pathways. This information content of knowledge-based methods is rich enough as compared to blinded or target-based methods. The known information can be used to predict therefore, be used to predict the unknown new mechanisms, such as unknown targets for drugs, unknown drug–drug similarities, new biomarkers for diseases etc.

**Signature-based methods** use gene signatures derived from disease omics data (genomics data) with or without treatments to discover unknown off-targets or unknown disease mechanisms. Genomics data are publicly available as databases. The advantage of these methods is that they are useful to explore unknown mechanisms of action of drugs. In comparison to knowledge-based methods, signaturebased methods investigate drug mechanisms at more molecular-level, such as changes in expression of genes by using computational approaches.

**Pathway- or network-based methods** make use of disease omics data, available signaling or metabolic pathways, and protein interaction networks to reconstruct disease-specific path- ways that provide the key targets for repositioned drugs. The advantage of these methods is that they can narrow down general signaling networks from a large number of proteins to a specific network with a few proteins (or target molecules).

**Targeted mechanism-based methods** integrate treatment omics data, available signaling pathway information and protein interaction networks to describe the unknown mechanisms of action of drugs. The advantage of these methods is that they are not only used to discover the mechanisms related to diseases or drugs, but also to identify those directly related to treatments of drugs to specific diseases [23–25].
