Drug repositioning: Re-investigating existing drugs for new therapeutic indicationsBM Padhy1, YK Gupta2
1 Department of Pharmacology, Hamdard Institute of Medical Sciences and Research, New Delhi, India
2 Department of Pharmacology, All India Institute of Medical Sciences, New Delhi, India
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0022-3859.81870
Source of Support: None, Conflict of Interest: None
Drug discovery and development is an expensive, time-consuming, and risky enterprise. In order to accelerate the drug development process with reduced risk of failure and relatively lower costs, pharmaceutical companies have adopted drug repositioning as an alternative. This strategy involves exploration of drugs that have already been approved for treatment of other diseases and/or whose targets have already been discovered. Various techniques including data mining, bioinformatics, and usage of novel screening platforms have been used for identification and screening of potential repositioning candidates. However, challenges in clinical trials and intellectual property issues may be encountered during the repositioning process. Nevertheless, such initiatives not only add value to the portfolio of pharmaceutical companies but also provide an opportunity for academia and government laboratories to develop new and innovative uses of existing drugs for infectious and neglected diseases, especially in emerging countries like India.
Keywords: Data mining, drug development, repositioning
Drug discovery and development process proceeds through three broad stages, namely discovery, where new compounds are screened and identified; the preclinical stage, where the compounds are tested in vitro and in animal models; and clinical development, where the drug candidates are tested in human beings as clinical trials.  The discovery process involves target discovery and validation, lead identification by high-throughput screening, and lead optimization (development of the most druggable compounds from the lead compounds). The preclinical studies comprises of analysis of pharmacological efficacy and toxicological and drug interaction studies.  Even with a promising compound, there still remains some amount of uncertainty regarding its usefulness in human beings, as preclinical studies cannot always account for the physiological differences between human beings and animals. Consequently, development of serious adverse events and decreased efficacy in human beings during clinical trials are two common reasons for a compound failing to reach the market.  Therefore, the high-risk reward trade-off in drug discovery and development is a major issue in new drug approval and marketability.
An alternative drug development strategy is exploration of drugs that have already been approved for treatment of other diseases and/or whose targets have already been discovered.  The process of finding new uses of existing drugs (marketed drugs and failed or idle compounds) outside the scope of the original indication is variously referred as repositioning, redirecting, repurposing, and reprofiling.  Nowadays, as more and more pharmaceutical companies are exploring the existing pharmacopoeia for repositioning candidates, the number of repositioning success stories is steadily increasing. This review describes the general advantages of drug repositioning over de novo drug discovery and development; strategies adopted for identifying repositioning candidates; issues typically encountered during the repositioning process; and finally outlines the repositioning initiatives in India.
There has been an increasing interest in drug repositioning in recent years, which is motivated by multiple factors. The average cost of introducing one new drug to the market in developed countries, including the cost of failures, has been estimated to be USD1.24 billion.  In addition, the time required to develop a new drug de novo varies between 10 to 17 years due to regulatory requirements regarding safety, efficacy, and quality, in both, animal studies and clinical trials.  Moreover, high attrition rates are also a major concern for pharmaceutical companies. A Tufts Centre for the Study of Drug Development study has shown that for candidate drugs which entered the clinical development phase during 1999-2004, the likelihood of a compound eventually reaching the marketplace was only 16%. The current pharmaceutical research and development (RandD) productivity is clearly insufficient, with the regulatory authorities in developed markets approving 18 to 20 new drugs annually, despite current pharmaceutical industry research outlays of more than USD50 billion per year. , It has been estimated that only 3 in 10 new products generate revenues equal to or greater than average industry RandD costs. In order to recover RandD costs as early as possible, sponsors are increasingly focusing on drugs to treat chronic and complex indications, such as cardiovascular, endocrine, psychiatric, and neurological disorders and cancers.  Consequently, the current paradigm of drug discovery is ill-equipped to combat rapidly emerging and re-emerging infectious diseases, such as mutated strains of influenza, drug-resistant microorganisms, and neglected tropical diseases (NTDs) that might be perceived as having relatively smaller financial market but certainly are of high public health importance.  This productivity problem, along with worldwide pressure on prices, competition from generics, and ever-increasing regulatory challenges, has driven many drug companies to become more innovative in finding new uses for existing drugs. 
Repositioned drugs have the advantage of decreased development costs and decreased time to market than traditional discovery efforts, due to availability of previously collected pharmacokinetic, toxicology, and safety data.  The recent regulatory environment has also become more restrictive, leading to newer and more stringent regulations that a new drug must meet in order to enter the market. Stricter regulations have also led to significant increase in the time and cost of new drug development.  It has been estimated that the time required for development of a repositioned drug varies between 3 and 12 years with substantially lower costs, thereby ensuring the repositioning company significant savings in terms of time and capital.  For repositioned drugs, as the clinical safety data, pharmacokinetics, and viable dose range are available at the start of a development project, the risks associated with clinical development are significantly reduced with fewer failures in the later stages.  This enables both pharmaceutical companies, noncommercial agencies (academic centers, public sector laboratories, and patient advocacy groups in developed and developing countries), and regulators to quickly and efficiently address medical needs that have continued to be unmet despite de novo drug discovery efforts. ,,,
Repositioned drugs and potential candidates under investigation
Some of the repositioning success stories are very well known. Thalidomide which was prescribed to pregnant women for preventing morning sickness and discredited for development of skeletal defects in the newborn was repositioned for use in erythema nodosum leprosum and multiple myeloma.  Minoxidil (a potassium channel opener), developed for treatment of hypertension, was approved by United States Food and Drug Administration (USFDA) for the repositioned indication of male pattern baldness in1998. Similarly, sildenafil (a phosphodiesterase 5 inhibitor), initially studied for angina, was switched to treating erectile dysfunction in 1998 following observations in phase 1 trials, and raloxifene (a selective estrogen receptor modulator) was approved in 2007 by USFDA for osteoporosis after early clinical trials for breast cancer.  Recent examples include plerixafor, studied as an inhibitor of HIV but subsequently launched in 2009 for mobilization of hematopoietic stem cells in the treatment of multiple myeloma, and milnacipran, initially developed and launched outside the US as an antidepressant and later approved in the US for the treatment of fibromyalgia in 2009. , So far, despite various repositioning initiatives, only a few drugs have been successfully approved for new indications [Table 1]. ,,,,,,,,,,,, Nevertheless, there is considerable interest in drug repositioning and many potential candidates have been investigated for newer indications [Table 2]. ,,,,,,,,,,,,,,,
Broadly, drug repositioning identifies new indications for drugs/compounds which fall into the following categories:
Identification and screening of repositioning candidates
Ideas for repositioning can come from serendipitous observations, from informed insights, or from novel platforms established to identify repositioning opportunities. The range of technologies applied for repositioning includes high-throughput cell-based screening methods, in vivo and ex vivo bioanalytics, and database-driven bioinformatics approaches.  Once the repositioning idea has been generated, and the proposed approach is scientifically validated, a commercially viable target profile for the candidate is created and a search to identify and screen compounds with the desired characteristics is conducted. 
Bioinformatics and data mining for identification of potential targets and repositioning candidates
One of the major challenges in drug repositioning lies in choosing the therapeutic target to prospectively test a drug of interest. There is mounting evidence that a large number of drugs interact promiscuously with biological elements outside of their targets (off target effects). Understanding the potential off-target interactions of existing drugs is of major interest in pharmaceutical research, not only for providing insight into drug side effects, but also for discovering novel therapeutic uses of drugs. However, the challenge remains to identify these targets in a context that also provides basic information on medical exploitability, and to do this with reasonable efficiency.  Rapid advances in genomic, proteomic, structural, functional, and systems studies of the known targets and other disease proteins have enabled the discovery of drugs, multitarget agents, combination therapies, and analysis of on-target and off-target toxicity and pharmacogenetic responses. To facilitate the access of information about therapeutic targets, publicly accessible databases such as DrugBank, Potential Drug Target Database, Therapeutic Target Database, and SuperTarget have been developed. These databases complement each other to provide target and drug profiles.  A common theme among the approaches for target identification is that they are not hypothesis driven and do not focus on single targets. They cover many targets and often involve in vivo or pathway-based methods.  Druggable targets are identified using methods like systems biology and network analysis which integrate biochemistry and cell biology with genetics and physiology, as well as bioinformatics and computational biology to obtain holistic descriptions of biological systems at the cellular, tissue/organ, and organism level.  Various algorithms can be used to identify hidden traits shared by successful targets, which can then be applied across emerging target landscape to predict potential chances of success.  Some notable ligand knowledge bases widely used in drug discovery include World drug index, MDL Drug Data Report, WOMBAT, AurSCOPE (Aureus Pharma, France), ChemBioBase (Jubilant Biosys, India), and GVKBIO database (GVKBio, India) to name a few. 
Once the targets have been identified, the search for promising compounds often involves a review of the public and subscription-based information sources such as company websites, intellectual property (IP), and scientific databases to identify candidates within the generic and branded pharmacopoeia and in the development pipelines of different pharmaceutical companies.  Various databases that track clinical programs from the primary source of target identification through the development phase are available. Online sources such as Prous ( http://www.prous.com ), the Investigational Drugs Database ( http://www.iddb.com ), Adis Insight ( http://www.adisinsight.com ), and TrialTrove ( http://www.citeline.com ) also provide large amounts of information gathered from patents, conferences, websites, and other materials.  Potential candidates are also identified using USFDA's electronic Orange Book, querying its "Disc" (Discontinued Drug Products) list, which contains thousands of drugs that made it through Phase I testing, only to be withdrawn for reasons other than safety. , Recently, a java-based software called IDMap, capable of repositioning marketed drugs to novel targets and vice versa, has been described. The software enables researchers to map commercial chemicals to possible drug targets and prevents wastage of time with querying multiple databases.  Another interesting approach described by Chiang and Butte to identify potential candidates exploited the concept of "Guilt by Association" using a Drug-Disease Knowledge Base to capture the 3 517 USFDA-approved drug indications and 8 130 off-label uses of 2 022 distinct drugs used in the treatment of 726 diseases. It was based on the idea that if two diseases share some similar therapies, then other drugs that are currently used for only one of the two may also be therapeutic for the other.  Other approaches based on the concept of "inverse docking," that is, computer-simulated molecular docking of an existing drug to a panel of known therapeutic targets have also been used for identification of repositioning candidates. ,
Gaining access to the repositioning candidates might be challenging or at worst impossible. However, special initiatives by government and private agencies in the developed countries have been aimed at creating chemical libraries of existing drugs and providing them for screening to interested researchers worldwide. It has been suggested that a comprehensive library should also comprise of major drug metabolites that often have distinct pharmacological properties. Once the clinical compound library is complete, it should become a public resource available to the scientific community for screening on any disease target at minimal cost to investigators in both academia and the industry.
The largest publicly accessible collection of existing drugs is the Johns Hopkins Clinical Compound Screening Initiative.  It was launched in 2002 as a joint collaboration between Johns Hopkins Pharmacology and the Malaria Research Institute at the Johns Hopkins Bloomberg School of Public Health. Currently, the collection of 3 100 existing drugs, many of which are USFDA-approved and off patent, are being screened by several collaborators at Hopkins and elsewhere on a variety of diseases including malaria, angiogenesis, cancers, and HIV. In the long term, the initiative aims to acquire each of the 11 000 drugs ever used in medicine.  A notable achievement of this initiative involves the discovery of in vitro activity of veterinary anthelmintic drug, closantel, previously used in the treatment of sheep and cattle infected with liver fluke against Onchocerca volvulus, the causative organism of river blindness.  In addition, other compounds of this library that are in clinical trials following screening activities include itraconazole in patients with metastatic prostate cancer and pemetrexed in combination with itraconazole in nonsquamous non-small-cell lung cancer.  Another library, the National Institutes of Health (NIH) Clinical Collection, provides a plated array of approximately 450 small molecules that have a history of use in human clinical trials. The collection has been assembled by the NIH through the Molecular Libraries Roadmap Initiative.  Both these initiatives are aimed to supply the researchers with the drugs at a low cost to cover only the expense of production and distribution without profit. Other such existing collections, such as the National institute of neurological disorders and stroke and Prestwick libraries, also provide access to mostly FDA-approved and marketed drugs.  In addition, as a novel public private partnership initiative, the Clinical and Translational Science Awards Pharmaceutical Assets Portal, which is led by the National Centre for Research Resources, provides access to Pfizer Indications Discovery Unit, a division of Pfizer vested with finding new uses for old or discontinued drugs. ,
Novel screening platforms
Once the compounds are obtained, they are screened for their usefulness using in vitro and in vivo systems. Various techniques have been described to hasten the identification of the most promising candidates. Therefore, usage of special screening platforms capable of identifying the best candidate among the library of compounds is a critical step in the drug repositioning process.  The platforms can be novel medium or high-throughput systems using in vitro binding assays, cell-based assays, proprietary multiplexing in vivo assays, or small organism (the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the zebrafish, Danio rerio)-based screening systems.  Additionally, assays cell lines which have been genetically engineered to mimic various aspects of disease biology or to monitor activation of disease-relevant pathways and mechanisms have also been in used for screening. Using reporter proteins/enzymes Q11 or specific dyes, the effect of a particular compound on a specific protein target can be evaluated with a certain degree of accuracy in such assay systems. 
In contrast to conventional repositioning process, where a library of compounds is screened against a single identified target, O'Connor and Roth proposed a phenotypic approach in which multiple drugs are simultaneously screened against multiple targets using several in vitro and in vivo model systems.  The pioneering approach used the concept of "receptoromics" and "kinomics" in which the entire repertoire of "receptors" and "kinases" in the human genome is screened in a parallel fashion against all the potential candidates. As receptors and kinases are considered "most druggable" targets, representing 5% and 2.8% of the genome, respectively, screens focused on these segments of the genome are most likely to yield new "hits." , Drug combinations can also be tested using novel platforms like CombinatoRx which uses a high-throughput combination screening system in conjunction with cell-based phenotypic assays to identify combinations of existing compounds that are able to interact with multiple disease pathways.  Recently, the WISDOM project carried out to screen compounds against Plasmodium falciparum glutathione S-transferase, P. falciparum dihydrofolate reductase, Plasmodium vivax dihydrofolate reductase, and other different plasmepsins implicated in malaria described the use of virtual high-throughput screening and grid computing to significantly enhance the chance of finding successful "hits." This combination has been proposed as an alternative or complementary approach to experimental high-throughput screening at a relatively rapid pace at lower costs. Such virtual screening projects have also been developed for anthrax and cancers.  Novel screening platforms used by various drug repositioning companies are listed in [Table 3]. ,,,,,,,
The identification of a new target indication for an existing drug poses a major challenge in repositioning. However, other issues unrelated to indication discovery but related to clinical trials and/or IP may also be encountered during the process.
Challenges during clinical trials
Significant roadblocks may be encountered during clinical trials of the repositioning candidate. New phase 1 trials may be required to complete or supplement the data package for the candidate if initial clinical trials do not meet current regulatory requirements, adding cost, time and risk of regulatory disapproval. Moreover, other issues that may be encountered during clinical trials include the possibility that proof-of-concept studies in the new indication may fail, particularly if the new target is clinically unprecedented, or if serious safety concerns emerge during clinical trials. 
Intellectual property-related issues
Discovering and validating the repositioning idea and identifying the actual repositioning candidate are only the initial parts of the repositioning process. Market analyses, IP and regulatory diligence, and formulation of new development plans are carried out subsequently during the repositioning process.  Unique challenges pertaining to IP issues are associated with drug repositioning. The repositioned indication can create substantial value for the repositioning company, especially if marketing approval for the candidate has never been received. However, as the candidate is usually known to the scientific community, the existing prior art might make it unpatentable. Pre-existing patents could also hinder commercialization of the repositioned drug. Nevertheless, repositioning companies can exploit a number of strategies to add value such as obtaining composition-of-matter (COM) and use patents. Companies developing drugs in combination can also obtain new COM patent. , Obtaining exclusive marketing approval for different time periods in new geographic markets might also be effective in keeping out competition, especially in case of new indications in a pediatric population, or for an orphan disease. 
The epidemics of tuberculosis (TB) and HIV/AIDS, separately and in synergy, contribute in large degree to prevalent morbidity and mortality in India and other developing countries.  In addition, malaria and NTDs such as leprosy, lymphatic filariasis, visceral leishmaniasis (kala-azar), and yaws also lead to significant morbidity and mortality in these countries.  These diseases receive little attention from policy-makers and are characterized by lack of priority within health strategies, inadequate research, limited resource allocation, and few interventions.  In recent years, in addition to the infectious and tropical diseases, there has been a surge in the incidence of chronic noncommunicable "lifestyle" diseases such as diabetes, hypertension, ischemic heart disease, and cancers in middle class population of India and other Southeast Asian countries, largely driven by the emerging economy and subsequent adoption of western lifestyle. 
Major pharmaceutical companies display considerable interest in development of drugs for chronic lifestyle diseases for both developed and developing markets. However, they are often disinclined to invest significantly in RandD of drugs for infectious diseases and NTDs which are perceived as diseases predominantly of the "developing world" and on whom profitability would be lesser.  Recognizing this unmet medical need, several global initiatives based on public-private partnership models such as the WHO Special Programme for Research and Training in Tropical Diseases (WHO/TDR), Medicines for Malaria Venture, Global Alliance for TB Drug Development, Drugs for Neglected Diseases Initiative have been initiated to carry out pioneering research work on these diseases. ,, In such initiatives, drug repositioning has been adopted as an attractive cost-effective method for providing faster access to drugs to the large patient populations of the developing world. , India being a major pharmaceutical market with a huge patient load will naturally benefit from such repositioning initiatives. Specific instances of the utility of such initiatives include paromomycin and miltefosine that were repositioned for kala-azar following clinical trials in India, and ultimately benefited a large number of kala-azar patients in the country. ,
Council of scientific and industrial research open source drug discovery initiative
Open Source Drug Discovery (OSDD) is a Council of scientific and industrial research-led global initiative launched in September 2008 with the vision to provide affordable healthcare to the developing world. It aims to discover drugs for tropical infectious diseases such like malaria, TB, and leishmaniasis. The initiative enables researchers across the globe to work together to solve key challenges in drug discovery, thus keeping the cost of discovery and making drugs affordable. The OSDD provides access to the computational resources for drug discovery portal and other specific resources such as the SysBorgTB and TBrowse, which is one of the largest integrative genomics resources on Mycobacterium tuberculosis., A major success of the project has been to demonstrate the links between the 4 000 genes of M. tuberculosis and the proteins for which they code. These data have been shared with Jubilant Chemsys, TCG Lifesciences, Sugen Life Sciences, PREMAS Biotech, and Vimta Labs to further the drug development process by especially focusing on repositioning of patented or off-patent drugs. 
Other national laboratories
Recently, scientists at two prominent national laboratories, the National chemical laboratory and National centre for cell sciences, have discovered the antitubercular drug, rifampicin's potent antiglycating property, and have suggested its potential use in preventing diabetic complications.  These laboratories are also actively engaged in identifying other repositioning candidates.  In addition, Central drug research institute that carries out discovery and development initiatives for synthetic and natural products has also been involved in designing repositioning strategies for such compounds.  Researchers at this institute have reported the potent anti-HIV property of thiazolidinones, a group of compounds that have been previously evaluated for their antibacterial activity. ,
Several Indian biopharmaceutical companies and venture capitalist backed start-ups have set up repositioning strategies as an important business model. Notable among them are Connexios (Bangalore, India), Jubilant Biosys (Bangalore, India), and GVK Bio (Hyderabad, India). These companies use specific proprietary databases and translational biology network-based screening systems to identify possible repositioning candidates. ,, However, under the patent act of India, unlike the US, use of patent for a new indication is not permissible for an already patented drug, thus rendering in-country repositioning commercially less appealing. 
The skyrocketing drug discovery and development costs combined with high failure rate of de novo discovery methods and constant pressure to develop blockbuster drugs has currently driven the pharmaceutical industry toward exploration of drug-repositioning strategies with renewed interest.  This interest is also evident from the active industry participation in the annual drug repositioning summits.  Drug repositioning is expected to not only add value to the product portfolio of the drug companies, but also to enhance the ability of nonindustrial entities (academic and governmental) to bring "new" and affordable treatment options forward for a number of serious and neglected diseases.  Therefore, wide-scale implementation of repositioning strategies in the future will provide an opportunity to unleash the potential of the pharmacopeia and usher a win-win situation for the public and private sectors and patients worldwide.
[Table 1], [Table 2], [Table 3]