Drug rechanneling: a novel paradigm for cancer treatment

Abstract

Cancer continues to be one of the leading contributors towards global disease burden. According to NIH, cancer incidence rate per year will increase to 23.6 million by 2030. Even though cancer continues to be a major proportion of the disease burden worldwide, it has the lowest clinical trial success rate amongst other diseases. Hence, there is an unmet need for novel, affordable and effective anti-neoplastic medications. As a result, a growing interest has sparkled amongst researchers towards drug repurposing. Drug repurposing follows the principle of polypharmacology, which states, “any drug with multiple targets or off targets can present several modes of action”. Drug repurposing also known as drug rechanneling, or drug repositioning is an economic and reliable approach that identifies new disease treatment of already approved drugs. Repurposing guarantees expedited access of drugs to the patients as these drugs are already FDA approved and their safety and toxicity profile is completely established. Epidemiological studies have identified the decreased occurrence of oncological or non-oncological conditions in patients undergoing treatment with FDA approved drugs. Data from multiple experimental studies and clinical observations have depicted that several non-neoplastic drugs have potential anticancer activity. In this review, we have summarized the potential anti-cancer effects of anti-psychotic, anti-malarial, anti-viral and anti-emetic drugs with a brief overview on their mechanism and pathways in different cancer types. This review highlights promising evidences for the repurposing of drugs in oncology.

1. Introduction

Cancer is one of the leading global health problem and second major cause of death in the United States. According to a recent statistical analysis, approximately 1 million new cancer cases are expected to be diagnosed and 0.6 million cancer-related deaths can occur this year in the United States [1]. Reports from International Agency for Research on Cancer (IARC), suggest that the global burden of new cancer incidences is expected to increase up to 27.5 million by 2040. Developing countries are expected to be at an enhanced risk of cancer development due to limitation in research, treatment, cancer control and prevention as compared to the developed countries [2]. Therefore, it is important to identify novel therapeutic approach that can combat the current and future challenges associated with cancer development, identification, progression and treatment.

Traditional drug discovery process involves extensive non-clinical and clinical studies followed by characterization of the pharmacological and toxicity prolife of the new drug. This intense process is expensive and time consuming [3]. It has been estimated that an average time of 13 years of time and approximately 1.8 billion USD investment is required to transition a drug from bench to clinical use [4, 5]. Currently there are 10,000 drugs registered for clinical trials. However, less than 1% drugs are expected to progress to clinical trials. In cancer drug discovery, only 5% of newly identified agents pass the phase I clinical trial [6, 7]. Another challenge associated with cancer treatment, is the cost of currently FDA approved drugs [8, 9]. According to American Cancer Society (ACS), about 87.8 billion USDs were invested in 2014 for cancer related health care. Hence, there is an urgent need to fast-track the drug development process for cancer treatment. In order to address these challenges, there is an unmet need of effective, inexpensive and safe cancer drugs with minimal toxicity. Considering the hindrances involved in development of cancer therapeutics, drug repurposing could be one of the innovative strategies to identify novel and effective agents for cancer treatment.

Drug repurposing, repositioning, re-profiling or re-directing defines the process of identifying a novel application of already approved drugs. Drug repurposing has the potential to surpass several challenges associated with de-novo drug discovery [10, 11]. Repurposing of an existing drug guarantees quick clinical trials due to already established pharmacokinetics, tolerability and toxicity profiles [12]. Repurposing can be on-target, i.e validation of already identified drug target for its potential anti-cancer activity or off-target effects that identifies novel molecular targets for already FDA approved drugs [13]. Several clinical and non-clinical studies have revealed that among 200 off target drugs, 50% drugs have efficacy against cancer [14].

Multiple strategies can be implemented to identify novel non-neoplastic drugs for their potential anti-cancer activity. Repurposing involves eight major steps: Knowledge mining, in silico approach, in vitro assays, animal experiments, treatment in companion animals (pets diagnosed with cancer), clinical observations, epidemiological and post-hoc analysis, and two-way drug development rationale (Figure 1) [15]. Knowledge mining involves assessing known drug’s off-target effects using scientific database and analyzing the role of these targets in cancer progression. One of the examples of knowledge mining is detecting the anti-cancer effects of imipramine and clomipramine for non-small cell lung cancer (NSCLC) treatment [16, 17]. In vitro and in silico techniques involve preliminary experiments to identify potential drug candidates in cancer drug repurposing. In in silico method, existing drugs are tested for their novel molecular targets using algorithms involved in data mining technique. This approach utilizes virtual screening of molecular targets for hit identification [18]. Another technique to evaluate the potency of a drug for repurposing involves in vitro assays. In this method, existing drugs are tested for their cytotoxicity against cancer cells, which enables high-throughput screening for established drugs for their anti-neoplastic ability [19]. In vivo experiments and treatment in companion animals can be performed to provide significant scientific validation that the proposed drug could be a potential candidate for drug repurposing [20, 21]. Similarly, epidemiological and post-hoc analysis can be performed on a large cohort of population. For example, epidemiological study in a case-control study suggested reduced risk of cancer in diabetes patients taking metformin [22]. Similarly, population based studies indicating the reduced mortality associated with aspirin intake was due to reduction in cancer [23]. Two-way drug development approach involves the identification of specific mutation, amplification or deletion in the DNA that helps in the screening of potential cancer drugs. One critical example of the two-way drug development was the genetic mapping of patients with Ph-like (Philadelphia chromosome-like) acute lymphoblastic leukemia, which revealed the mutations in the receptor tyrosine kinase of these patients [24]. Therefore, current drug repurposing research follow the above mentioned steps.

Figure 1:

Major steps involved in drug re-purposing

Recent studies indicated that several FDA approved drugs including antipsychotic, antimalarial, antiviral, and antidiabetic could be promising candidates for drug repurposing [25]. In this review, we will discuss drugs that have been tested in the past and their potential future use for cancer treatment. This review will focus on the molecular mechanism targeted by these non-neoplastic drugs for their anti-oncogenic effects (Table 1).

Table 1:

Repurposed drugs and their targeted pathways as anti-neoplastic agents

Drug classification	Drug name	Molecular targets in repurposing	References
Anti-psychotic drugs	Chlorpromazine	Akt/mTOR pathway, p53, JNK pathway	[36,37–42]
	Haloperidol	MEK-ERK signaling,	[45–50]
	Trifluoperazine	Anti and Pro apoptotic pathways, p-Akt, G0/G1 cell cycle arrest	[51–54]
	Penfluridol	Akt-Gli1, Her2/Beta catenin,	[56–68]
	Pimozide	Wnt/Beta catenin, PI3K/Akt/mTOR, RAS/RAF/MEK/ERK, Cell cycle arrest	[70–82]
Anti-malarial drugs	Chloroquine	Autophagy, cell cycle arrest, PI3K/Akt, EGFR, NOTCH signaling, Antiapaoptotic proteins	[88–103]
	Pyrimithamine	DHFR, STAT3 signaling,	[104–109]
	Amodiaquine	p53, p-RB1, CDKN1A	[110–113]
	Atovaquone	Wnt/beta catenin	[115–120]
Anti-diabetic drugs	Metformin	AMPK/mTOR pathway, CD8+ T cells	[124–128]
	Thiazolidinediones	PPARy, Anti-apoptotic proteins	[133–142]
Anti-viral drugs	Abacavir	LINE-1 ORF1, ORF2, DNA damage	[156–158]
	Acyclovir	Apoptosis, cell cycle arrest, caspase-3	[160,161]
	Amantadine	Cyclin E, D1, Cdk1, Bcl-2, Bax	[162]
	Amprenavir	ERK2, BimEL	[163]
	Cidofovir	P53, Rb, DNA damage, MDM2	[164]
	Efavirenz	ERK1/2, p38 MAPK, DNA damage, PSA	[168–170]
	Idoxuridine	DNA damage	[175]
	Lamivudine	DNA damage	[158,171]
	Maraviroc	CCR5, IL-10 receptor B, MET, FAT1; Nm23-H1; and lymphotoxin β receptor	[171–174]
	Nelfinavir	P-gp, autophagy, AMPK, mTOR, DR5, PI3K, ERK, DNA damage, Akt, ROS, eIF2α	[176–184]

2. Anti-psychotic drugs:

Anti-psychotic drugs are prescribed for the treatment of psychotic conditions such as bipolar disorder, acute mania, chronic psychosis (delusions, hallucinations and paranoia), Tourette syndrome and schizophrenia [26]. First generation anti-psychotics were developed in the 1950’s and are termed as typical antipsychotic drugs whereas recently developed neuroleptic drugs are known as atypical antipsychotics [27]. Typical antipsychotic drugs such as chlorpromazine (CPZ), haloperidol, trifluperazine, fluphenazine, thioridazine, and perphenazine mediate their anti-psychotic effects by blocking dopamine D₂ receptor [28]. Epidemiological studies on a large cohort of schizophrenic patient population have shown reduced incidence of cancer in patients undergoing psychosis treatment [29–31]. A population based study in US, Israel, Europe, Asia and Africa showed significantly reduced cancer standardized incidence ratios in schizophrenic patients undergoing psychosis treatment as compared to non-schizophrenic cancer patients [32–34]. These preliminary studies highlighted the anti-neoplastic properties of neuroleptic drugs.

Chlorpromazine (CPZ), which belongs to the phenothiazine class of anti-psychotic drugs were used in late sixties and early seventies. CPZ was prescribed as an anti-psychotic drug for several decades, however, in early nineties; CPZ was identified as a potential candidate for drug repurposing [29, 35]. These observations were supported by a study indicating the decreased occurrence of prostate cancer in schizophrenic men taking CPZ [36]. Moreover, CPZ has been reported to exert its anti-proliferative effects by altering the expression of proteins that regulate the cell cycle and the mitochondrial functions within a cell. In glioma cell lines, CPZ mediate its anti-proliferative effects by inducing autophagy through inhibition of Akt/mTOR pathway [37]. CPZ has also been shown to induce apoptosis by increasing the expression of CDK inhibitor p21, thus causing cell cycle arrest in glioma cell lines [38]. In colorectal cancer and lung cancer cell lines, CPZ inhibits the mitotic kinesin KSP/Eg-5 followed by mitotic arrest and subsequent cell cycle inhibition [39]. Interestingly, CPZ had no effect on the viability of normal lymphocytes however; it suppressed the proliferative potential of leukemia cells by reducing mitochondrial ATP production and DNA polymerase enzyme activity [40]. Genetic mutations in p53, a tumor-suppressor gene plays a critical role in the uncontrolled progression of colorectal cancer [41]. CPZ induces apoptosis in colorectal cancer cells by activating c-Jun N-terminal kinase (JNK) and subsequently activating p53. Additionally, CPZ treatment also inhibits SIRT1 protein, a class III histone deacetylase that plays a significant role in the inactivation of p53 [42]. These studies indicated that CPZ has potential anti-tumor effects. Subsequently, other anti-psychotic drugs belonging to the same class were identified to inhibit tumor growth in several cancers including hepatocellular carcinoma, glioma, melanoma and leukemia [37, 38, 43]. Hence, neuroleptic drugs belonging to the same or different class were assessed for their potential anti-cancer properties.

Haloperidol, a typical antipsychotic medication mediates its effects by antagonizing dopamine D₂ receptor [44]. As an antipsychotic drug, haloperidol affects the ERK1/2 pathway and mediates its anti-psychotic effects in schizophrenic patients. In psychosis treatment, haloperidol can transcriptionally affect a cell by increasing the MEK-ERK-p90RSK phosphorylation in the ERK signaling pathway [45]. ERK (extracellular signaling regulated kinase pathway) plays a critical role in establishing the malignant phenotype of a cancer cell. It also plays a major role in cell proliferation and survival. Therefore, blockade of ERK signaling pathway can inhibit the angiogenic, metastatic and proliferative properties of cancer cells [46]. Repurposing of haloperidol for pancreatic cancer treatment has shown to exhibit its anti-tumor effects in MiaPaCa-2 and PANC-1 pancreatic cancer cells by promoting methylation of ERK1/2 selective Dual-specificity phosphatases (DUSP6) [47]. One of the major challenges associated with the failure of drug development in cancer treatment is acquired resistance to current chemotherapy drugs. Increased function of P-glycoprotein, an efflux protein is one of the accountable factors for this characteristic in cancer cells [48, 49]. Haloperidol was tested against P-glycoprotein overexpressing cancer cells that mediates the multidrug resistance (MDR) in vinblastine resistant human leukemia cells. Haloperidol treatment increased the accumulation of vinblastine with significant decreased expression of P-glycoprotein in a concentration dependent manner [50]. Hence, haloperidol proved to be a suitable candidate in reversing MDR.

Trifluoperazine is a typical antipsychotic drugs used to treat schizophrenia. Pharmacologically, trifluoperazine blocks the dopamine receptors (D₁ and D₂) in the mesolimbic and mesocortical regions of schizophrenic patients [51]. One of the important characteristics of cancer cells is their self-renewal ability. Researchers identify drug targets mainly by using connectivity map software (Cmap). Cmap identified trifluoperazine as one of the promising drug candidates that characterize difference in the features between embryonic stem cells and cancer stem cells [52]. Cancer cells acquire the property of stemness by possessing stem cell like characteristics either by increasing the expression of cell surface markers or by modifying the cellular microenvironment [53]. Trifloperazine when tested against NSCLC (Non-small cell lung cancer) cells had the potential to suppress stem cell like characteristics and induce apoptosis as evaluated by increased expression of Bax, Bak, Cleaved-caspase9, Cleaved PARP and decreased expression of anti-apoptotic protein BCL-2. Additionally, trifluoperazine was able to overcome NSCLC cells drug resistance when combined with gefitinib and cisplatin [52]. Breast cancer metastasis to brain is one of the major challenges in developing a treatment strategy for metastatic breast cancers. Neuroleptic drugs have to cross the blood brain barrier in order to initiate their anti-psychotic effects. Hence, repurposing neuroleptic drugs for brain tumors or metastatic tumors has recently gained tremendous attention. Trifluoperazine exhibited significant anti-proliferative effects in multiple triple negative breast cancer cell lines. In vitro analysis of breast cancer cells showed that trifluoperazine induces apoptosis through mitochondria-mediated apoptotic pathway. These observations were further validated by decreased expression of p-AKT, p-IKB, p-NF-Kb and cell cycle arrest at G0/G1 phase. Inhibition of the growth of breast cancer cells in a subcutaneous xenograft model and intra-carotid brain metastasis model established trifluoperazine as a potential candidate for metastatic breast cancer management [54].

The first generation anti-psychotic drug penfluridol, belongs to the diphenylbutylpiperidine class of anti-psychotics [55]. Penfluridol’s anti- proliferative activity was identified in 1994 when all anti-psychotic drugs with anti-calmodulin property were tested for cytotoxic effects in C6 glioma cells [56]. Since then, penfluridol has been extensively studied for its potential anti-cancer activity. Penfluridol mediates its in vitro and in vivo anti-neoplastic effects by modulating integrin signaling. Previous studies have demonstrated the anti-cancer effects of penfluridol in various cancer models including breast cancer, pancreatic cancer and glioblastoma [57–59]. Penfluridol suppresses the growth of metastatic triple negative breast cancer. In this study, tumor-bearing mice displayed no general signs of toxicity with the chronic administration of penfluridol for 65 days. In another study, low concentrations of penfluridol have shown significant inhibition of VEGEF- induced angiogenesis in breast cancer (manuscript under revision). Several studies have shown that autophagy plays a dual role in cancer progression. It can act as a tumor suppressor or tumor promoter. Therefore, cancer-related drugs could either promote tumor growth or inhibit its growth by affecting autophagy in cancer cells [60]. In pancreatic tumors, penfluridol induced autophagy mediated apoptosis in a concentration and time dependent manner. In vitro and in vivo results supported the hypothesis that penfluridol can induce apoptosis in pancreatic tumors by inducing autophagy [58, 61]. Penfluridol also displayed its anti-cancer effects in glioblastoma cells by Akt-mediated inhibition of Gli1. In this study, penfluridol suppressed the growth of intracranially implanted brain tumors by 72% [57]. Recent studies have reported that penfluridol can overcome paclitaxel resistance in breast cancer. Paclitaxel is approved as a chemotherapeutic agent for the treatment of breast cancer. However, acquired resistance to paclitaxel is observed in metastatic breast cancers. Penfluridol was able to overcome resistance to paclitaxel in resistant and metastatic breast cancer cell lines by inhibiting Her2/β-catenin signaling [62]. Penfluridol was able to inhibit the growth of paclitaxel resistant breast cancer cells in an orthotopic tumor model in this study. Mechanistic studies in the tumors indicated that penfluridol significantly reduced the expression of Her2, β-catenin, Cyclin D1 in paclitaxel resistant cell lines [62]. Several studies have also mentioned that the anti-cancer properties of antipsychotic drugs can be attributed to their cationic amphiphilic nature [63, 64]. This property of anti-psychotic drugs might potentially modify their plasma membrane properties leading to dysregulation in cell cholesterol homeostasis [65, 66]. The hypothesis of cholesterol homeostasis was further supported by the decrease in cholesterol specific genes and proteins post penfluridol treatment. Penfluridol treatment resulted in the abnormal accumulation of un-esterified cholesterol in cancer cells in vitro. Similar results were obtained when subcutaneously inject B16/F10 and 4T1 tumor cells were treated with penfluridol. In vivo results showed decreased cholesterol content in a dose dependent manner [67]. In another study, penfluridol treatment caused a 72% suppression in the MDSC’s along with decreased activation of Treg cells that play a significant role in immune system suppression in glioblastoma progression. Penfluridol treatment also resulted in the increased expression of M1 macrophages by 57%. Taken together these results indicate that penfluridol treatment have a significant impact on the immune system regulation [68].

The diphenylbutylpiperidine (DPBPs) class of drugs include pimozide, fluspirilene, penfluridol and clopimozide [69]. Pimozide was developed by Janssen pharmaceutica in 1963. It is an antipsychotic drug which is also prescribed for other neurological conditions such as tourette syndrome and resistant tics [70]. Pimozide acts as an antagonist of D₂, D₃ and D₄ dopamine receptors as well as 5-HT7 serotonin receptor. Similar to other antipsychotic drugs, pimozide also has more selectivity towards dopamine receptor as compared to serotonin receptor [71, 72]. In 1979, pimozide was first reported to exhibit anti-proliferative activity against melanoma [73]. Pimozide treatment resulted in a remarkable decrease in the metastatic melanoma progression in lungs, with complete disappearance of lung lesions after 8 weeks of therapy. Pimozide was tested for Phase II clinical trial however, the results obtained were not promising. These observations indicated that an extensive research on anti-cancer effects of pimozide is mandated [74]. In 1990, pimozide was reported to exhibit anti-proliferative activity against human breast cancer cell line MCF-7. In this study, pimozide inhibited the growth of both ER (estrogen receptor) positive and negative cell lines in a concentration dependent manner. Inhibition of breast cancer cells by pimozide treatment was mediated through the inhibition of calmodulin dependent enzyme activation. Pimozide has also been studied as one of the most potent calmodulin antagonist amongst several clinically approved neuroleptic drugs [69, 75]. Studies have shown that pimozide is effective against the treatment of carcinomas such as chronic and acute myelogenic leukaemia, melanoma, breast cancer and hepatocellular carcinoma [73, 75–77]. In our study, pimozide treatment induced apoptosis in brain cancer cells U-87 MG, U-251 MG, Daoy and GBM28 by inducing autophagy. Treatment with pimozide resulted in a concentration and time dependent decrease in the major players of STAT3 signaling and its downstream anti-apoptotic proteins such as Mcl-1, Bcl-2 and c-myc. Autophagy induction was observed by increased in the expression of LC3B, Beclin-1 and ATG5. These observations were further verified using acridine orange staining for the formation of auto-phagolysosome. These results confirmed that pimozide can induce autophagy in brain cancer cells (manuscript under preparation). Hepatcocellular carcinoma (HCC) is characterized with poor prognosis along with developed resistance against current chemotherapeutic drugs [77]. The increased expression of tumor initiating cells and cancer stem cells in the tumor microenvironment of HCC is accountable for developed resistance in HCC. Pimozide was able to inhibit the proliferative properties of CD133⁺ HCC cells. Furthermore, pimozide treatment downregulated the expression of cancer stem cell markers Bmi-1, c-Myc and Nanog [78]. Several signaling pathways regulate cancer cells stem cell like characteristics. Wnt/β-catenin, PI3K/Akt/mTOR and RAS/RAF/MEK/ERK signaling pathways contribute majorly to the stem cell like properties of cancer cells. Dysregulation of these signaling pathways play a significant role in the proliferation, epithelial to mesenchymal transition, metastasis and acquired stem cell like characteristics of cancer cells [79, 80]. Wnt/β-Catenin-signaling pathway is aberrantly activated in colorectal cancer (CRC) and conducts the important mechanisms in its progression and metastasis. Pimozide treatment significantly inhibited the expression of mesenchymal characteristics of CRC cells while increasing the expression of epithelial characteristics. Pimozide treatment also significantly downregulated the expression of major players of Wnt/β-catenin signaling such as c-Myc, cyclin D1, Axin2, Survivin in vitro and in vivo [81]. Pimozide when screened against normal cells had the most potent effects on the viability of cancer cells as compared to the normal cells [82].

Fluspirilene is an anti-psychotic drug that belongs to diphenylbutylpiperidine class of neuroleptics. Computational analysis determined that fluspirilene could bind to MDM2 receptor and mediate its effects by inhibiting the interaction between p53 and MDM2. As an anti-cancer drug candidate, fluspirilene altered the expression of several cell cycle markers such as CDK2, Cyclin E and Rb. Fluspirilene, inhibited the cell cycle progression of hepatocellular carcinoma in vitro and in vivo by accumulation of cells in G1 phase [83]. Experimental studies have verified p53 mediated inhibition of cancer cells by fluspirilene treatment [84].

3. Anti-malarial drugs:

Malaria is a parasitic disease caused by the protozoans of genus Plasmodium (P. falciparum, P. Malariae, P.ovale and P. vivax) [85]. The bite of female Anopheles mosquito transmits the saliva into the person’s blood. P. falciparum is the most deadly amongst other species of Plasmodium as it travels to the liver of the patient where it matures and reproduce [86]. In this life cycle of a malaria parasite, several biological properties manifests that serves as a target for many anti-malarial drugs [87]. The currently available antimalarial drugs fall into three major categories: Aryl aminoalcohol compounds that include quinine, quinidine, chloroquine, tafenoquine, halofantrine, mefloquine, piperaquine. The antifolate class of anti-malarial drugs include pyrimethamine, proguanil, chlorproguanil, trimethoprim. Artemisinin class of compounds include artemisinin, dihydroartemisinin, artesunate, artemether [87].

Chloroquine (CHQ) and hydrochloroquine belong to the 4-aminoquinoline class of compounds and are well known as antimalarial agents [88]. In the early phases of drug development for malaria treatment, CHQ was the primary choice of treatment for several decades. However, CHQ’s role as an anti-malarial drug deteriorated when several CHQ resistant malarial strains were identified. The first evidence of CHQ as an anti-neoplastic drug was identified when WHO launched an anti-malaria trial in 1970’s and observed decreased incidence of Burkitt’s lymphoma in CHQ treated population [89, 90]. Since then, a series of experimental observations have reported the anti-neoplastic ability of CHQ [91]. Approximately, seventeen clinical studies have been initiated to investigate the effect of CHQ in the treatment of several neoplasms including GBM [92–94]. As an anti-malarial drug, CHQ has the ability to concentrate itself inside the acidic vacuole in the RBC’s of parasite [95]. Scientific evidence suggests that due to low pH in the lysosome, CHQ gets protonated and the accumulation of protonated CHQ inside the lysosome results in decreased lysosomal function. This characteristic of CHQ to aggregate in the lysosomes of RBC was linked with autophagy. The accumulation of CHQ in the lysosome causes autolysosome degradation and inhibition of autophagy mediated energy production [96]. In cancer cells, autophagy can play a dual role of a suppressor of cancer cells proliferation or promoter of carcinogenesis. Autophagy, as a promoter of cancer cell survival can inhibit the degradation of degraded proteins and promotes cell proliferation. Several drugs targeting autophagy have been repurposed for cancer [60, 97, 98]. To date, inhibition of autophagy in neoplastic cells has so far been the most studied mechanism of CHQ. Experimental evidences have suggested that autophagy upregulation is one of the underlying mechanisms of radiation resistance in glioblastoma progression. Radio-sensitization of glioblastoma cells with CHQ treatment has been attribute to autophagy inhibition [96]. CHQ has been studied to mediate its anti-cancer effects by autophagy dependent and independent mechanisms. In autophagy independent mechanisms, CHQ initiates its effects by inhibition of PI3K/Akt pathway or EGFR signaling in glioblastoma cells [99, 100]. The evidence of anti-neoplastic effects of CHQ were also observed in leukemia, however, until recently, no significant observations were made for leukemia treatment using CHQ. The current therapeutic options available for leukemia include γ-secretase inhibitors (GSI’s). Unfortunately, GSI’s have shown limited success in the treatment of T-cell acute lymphoblastic leukemia (T-ALL). Recently, a published study has demonstrated that CHQ mediates synergistic effects when combined with GSI. CHQ inhibits T-ALL tumor growth by inhibition of the intracellular trafficking of NOTCH1 [101]. The major side effects associated with the current chemotherapeutic drugs include development of resistance and drug associated side effects upon prolonged treatment. Doxorubicin is amongst one of the treatment options approved for hepatocellular carcinoma. However, doxorubicin is associated with increased incidence of cardio toxicity. When doxorubicin was tested in combination with CHQ in hepatocellular carcinoma, CHQ induced apoptosis in hepatocellular carcinoma cells by upregulating the expression of TRAIL/TRAIL2, caspase 3 and caspase 8, along with the down regulation of BCl-2 an anti-apoptotic protein. Moreover, combining CHQ with doxorubicin decreased the cardio toxicity associated with doxorubicin [102]. A neoplastic drug can mediate its effects by two major mechanisms 1) inhibition of tumor promotor genes or 2) initiation of tumor suppressor genes. CHQ has shown to induce the expression of Par-4 (Prostate apoptosis response-4) in various normal cells. Interestingly, CHQ induces the expression of PAR-4 in normal cells by inducing the expression of p53 a tumor suppressor gene. This paracrine secretion of PAR-4 by normal cells triggers the apoptosis in cancer cells as well as inhibits metastatic tumor growth in vivo [103].

Pyrimethamine (Pyr) is an FDA approved drug for the treatment of protozoan parasites. Pyr mediates its anti-protozoan effects of inhibiting dihydrofolate reductase (DHFR). DHFR is an enzyme that is required for the synthesis of folic acid. Clinically approved drugs such as methotrexate administered for the treatment of cancer mediate itsanti-cancer effects by inhibiting DHFR [104]. Therefore, it was interesting to investigate if Pyr can be a potential candidate for cancer therapy. It was observed that, Pyr exhibits accountable anti-cancer activity and induces apoptosis in melanoma cells as evidenced by the cleavage of caspase 3 and caspase9. In melanoma cells, Pyr inhibited the growth of cancer cells by promoting cell cycle arrest in S phase. Similar to CHQ, Pyr also inhibited the induction of autophagy. These results suggests that Pyr mediated its anti-proliferative effects by inhibition of autophagy and promoting cell cycle arrest in the S phase [105]. Similar effects of Pyr were observed in acute myeloid leukemia via inhibition of DHFR. The inhibition of DHFR in AML had significant cytotoxic effects. Oral administration of Pyr resulted in increased expression of folic acid in two xenograft models suggesting the significance of DHFR in the AML progression [106]. STAT3 (Signal transducer and activator of transcriptions) is one of the ubiquitously expressed gene in the cells [107]. STAT3 amongst other STATs plays a significant role in the cell progression, proliferation and homeostasis [108]. Chemical screening has verified that Pyr is a potent STAT3 inhibitor with concentrations minimally toxic to humans. Pyr has shown to inhibit the proliferation of metastatic breast cancer cells in two independent mouse models. Mechanistic studies indicated that decreased STAT3 activation by Pyr treatment resulted in reduced tumor burden as well as reduced tumor-associated inflammation. Immunological effects of Pyr through STAT3 inhibition were also verified by increased expression of Lamp1 by CD8⁺ T cells in tumor bearing mice indicated the increased release of cytotoxic granules [109]. Similar effects of Pyr were observed in the progression of metastatic melanoma. Pyr mediated its neoplastic effects in melanoma by inhibiting DHFR along with increased activation of cathepsin B and enhanced activation of caspase cascade. These effects subsequently increased the mitochondrial depolarization. Therefore, inhibition of tumor growth by Pyr treatment can be considered as a multifaceted strategy involving the critical mechanisms of cell growth, mitochondrial depolarization and DHFR secretion [109].

Amodiaquine (AQ) belongs to the lysosomotropic 4-aminoquinoline class of antimalarial drugs. AQ has been used in combination with other artemisinin drugs and displays its effects by plasmodium directed activity. Repurposing of non-oncological drugs that target autophagy have gained attention in recent years. AQ has shown to surpass CHQ’s ability in inhibiting autophagy. AQ when screened against several other anti-malarial drugs indicated that it possesses comparatively higher anti-autophagy property. AQ caused anti-proliferative and autophagic-lysosomal blockade in melanoma cells. Early ATP depletion along with increased protein expression of TP53, p-RB1, CDKN1A and CCND1 were responsible for the growth inhibitory effects of AQ [110]. In another study, AQ exerted its anti-oncogenic effects by targeting ribosomal biogenesis. Ribosome biogenesis is one of the important characteristics of fast dividing cancer cells as they require continuous protein synthesis to elevate their proliferative and metabolic requirements [111, 112]. Amodiaquine treatment in several cancer cells lines including colorectal cancer, breast cancer, hepatic cancer, melanoma and sarcoma initiated the degradation of RNA polymerase 1 and stabilization of p53. Along with the inhibition of ribosomal biogenesis, amodiaquine also inhibited autophagy. This property of amodaquine is comparatively different from other members of the 4-aminoquinoline family [113].

Atovaquone (ATQ) is an FDA approved drug used for the treatment of pneumocystis pneumonia and primary malaria [114]. ATQ belongs to the class of naphthoquinones. ATQ is a competitive inhibitor of mitochondrial co-enzyme Q10 and it mediates its anti-protozoan effects by inhibiting mitochondrial electron transport chain (ETC) complex III [115, 116]. ATQ has been extensively studied for its anti-cancer effects. ATQ treatment inhibited the proliferation of breast cancer associated cancer stem cells. In breast cancer, inhibition of ETC by ATQ reduces the oxygen consumption and mitigated the tumor growth by reducing tumor hypoxia [117]. ATQ inhibits the growth of hepatocellular carcinoma by upregulating the expression of tumor suppressor gene p53 and p21, and in turn inhibiting the cell cycle progression in S phase. ATQ also activates the extrinsic and intrinsic apoptotic pathways in hepatocellular carcinoma [118]. In another study, ATQ treatment in breast cancer cell line MCF7 induced aerobic glycolysis and reduced the oxygen consumption rate. In MCF7 cell line, ATQ exerted its anti-proliferative and apoptotic effects by inhibiting the expression of cancer stem cell markers CD44⁺/CD24⁻ [53]. Furthermore, ATQ has shown to have no significant effects on the human fibroblast cells [117]. Extensive genome analysis studies have demonstrated that ATQ, sold as Mepron, is a potent STAT3 inhibitor. It significantly down-regulates the expression of glycoprotein 130, which is one of the required factors for STAT3 activation. Oral administration of ATQ in mice significantly reduced the tumor growth in multiple myeloma mice model by targeting STAT3 activation [119]. In a recently published study, ATQ has inhibited the growth of both primary and paclitaxel resistant breast cancer cell lines in a concentration and time dependent manner. ATQ mediated its effects by inhibiting HER2/β-catenin pathway. This study validated the effects of ATQ in several cancer cell lines including SKBR3, HCC1806, MCF-7, 4T1, T47D, CI66, TX-BR-237 cells and TX-BR-290 primary breast cancer and patient-derived breast cancer cells. Treatment with ATQ in these cell lines inhibited the downstream players of β-catenin pathway such as Cyclin D1, c-myc GSK3b and TCF-4. These observations were consistent in CI66 and 4T1 in vivo studies as well with no general signs of toxicity [120]. The studies on antimalarial drugs suggest that these drugs can be potential candidates for drug repurposing for cancer.

4. Anti-diabetic drugs:

Diabetes also referred to as diabetes mellitus is a metabolic disorder with high blood sugar levels maintained over a prolonged period of time [121]. Diabetes can be classified into three subtypes: Type 1, Type 2 and gestational diabetes. Type 1 occurs when pancreas fail to produce enough beta cells resulting in decreased insulin production. Type 2 originates when cells fail to respond to the insulin also referred as insulin resistance. Gestational diabetes is specifically associated with pregnant women with no history of high blood sugar levels [122, 123]. The FDA approved insulin and symlin for type 1 diabetes. Metformin is the first line of treatment for type 2 diabetes. It was discovered in 1922 but French physician Jean Sterne conducted the human studies using metformin in 1950s (Metformin is repurposed for the treatment of polycystic ovarian syndrome.

Metformin belongs to the biguanide class of anti-diabetics. This class of drugs basically include metformin, phenformin and buformin. As an anti-diabetic drug, metformin inhibits the complex 1 of mitochondrial respiratory chain and subsequent blockade of oxidative phosphorylation [124]. This in turn results in the increase of AMP/ATP ratio. Increased AMP increases the activation of several cyclases and kinases such as fructose 1,6-bisphosphate and AMP-activated protein kinases (AMPK) therefore increasing cellular sensitivity to glucose. Tumor cells possess increased expression of mTOR, a kinase downstream of AMPK that plays a significant role in tumor progression [125]. Therefore, metformin could be a promising candidate for treating mTOR overexpressing cancer cells. There is considerable amount of preclinical data that suggests that metformin possess anti-cancer properties. In vitro studies have verified that metformin can mediate the inhibition of cancer cell progression by inhibiting intracellular signaling pathways [126, 127]. In addition, metformin can significantly increase the T-cell mediated immune response [128]. Meta-analysis of data and epidemiological studies have shown decreased incidences of cancer in type 2 diabetes patients prescribed for metformin [129]. As a potential anti-cancer agent, metformin inhibits the signaling mechanisms in cancer cells. The key signaling molecule targeted by metformin is mTOR Metformin has shown to mediate its effects by either inhibiting mTOR, which may be AMPK dependent or independent. In pancreatic cancer, metformin inhibits the insulin like growth factor-1 (IGF-1). Inhibition of IGF-1 and mTOR in pancreatic cancer cell lines was associated with significant increase in the AMPK phosphorylation [126]. The increased phosphorylation of AMPK is associated with enhanced activation of tuberous sclerosis complex (TSC1, TSC2). TSC2 can directly inactivate mTOR once its activity is enhanced by AMPK mediated phosphorylation [130, 131]. In prostate cancer cells, metformin can inhibit the activation of mTOR independent of AMPK. In this cancer type, metformin inhibits the phosphorylation of mTOR by enhancing the activity of tumor suppressor gene p53 [132]. The immune modulatory effects of metformin were identified by reduced apoptosis of CD8+ TIL (tumor-induced lymphocytes). This effect was also accompanied by the shift in the phenotype of CD8+ T cells from memory TILs to effector TILs [128]. In a study evaluating the potential anti-neoplastic effects of metformin, animal models treated with metformin showed increased CD8+ TILs which confirmed the induction of protective immunity in treated mice [128].

Thiazolidinediones (TZD’s) such as troglitazone, rosiglitazone and pioglitazone are known insulin sensitizers. TZD’s are agonists of peroxisome proliferator-activated receptors γ (PPAR γ) and are used as Type 2 anti-diabetic medication for hyperglycemic patients. PPARγ belongs to the hormone receptor nuclear family and is responsible for insulin sensitivity, lipid homeostasis, cell proliferation, differentiation and apoptosis [133]. Cancer cells overexpress PPARγ and play a significant role in the anti-apoptotic properties of cancer cells [134, 135]. A growing scientific evidence suggests that inhibition of PPAR γ in cancer cells can inhibit proliferation and induce apoptosis in several cancer cell lines [136–139]. TZD repurposing for the treatment of several cancer types has been well studied and significant data has been published regarding its anti-proliferative and anti-neoplastic ability. In breast cancer cell lines, TZD increases the sensitivity to doxorubicin by inhibiting the efflux protein P-glycoprotein [140]. TZD can also mediate its anti-proliferative effects in breast cancer cells by decreasing the activity of histone deacetylase (HDAC) [141]. Treatment of renal cell carcinoma with TZD showed increase in apoptosis by decreased expression of anti-apoptotic proteins BcL-2 and increased expression of pro-apoptotic protein Bax. TZD’s also induced cell cycle arrest of renal cell carcinoma cells in G0/G1 phase of cell cycle [142]. These data suggests that anti-diabetic drugs are promising candidates for drug repurposing however, intense mechanistic studies and target profiling is critical in bringing the drugs into the clinical trials.

5. Antiviral drugs

The anti-viral drugs are used to treat a group of viruses that includes herpes virus, hepatitis and influenza virus. Most viral diseases are self-limited illnesses other than human immune deficiency virus (HIV) [143]. The potential anti-neoplastic effects of antiviral drugs gained attention when incidences of HIV-related cancers such as Kaposi’s sarcoma and lymphoma was significantly decreased in patients undergoing HIV treatment [144]. Currently several antiviral drugs are repurposed for cancer treatment. These antiviral drugs exhibit anticancer activities in a number of cancer cells through diverse molecular mechanisms. Ritonavir, a widely used drug against HIV, has shown efficacy against various cancer including renal cancer, pancreatic cancer, breast cancer, ovarian cancer, bladder cancer and prostate cancer [145–150]. Experimental studies have depicted that ritonavir inhibits growth and proliferation of prostate cancer cells by inhibiting cytochrome P450 3A4 enzyme. In animal model, ritonavir in combination with docetaxel inhibit prostate tumor growth by inhibiting NF-kappaB DNA binding activity [150]. Ritonavir has also shown to inhibit breast cancer growth both in vitro and animal model by inhibiting substrates of Hsp90 and Akt. It also depletes ER-alpha levels in ER-positive cell lines and cause G1 cell cycle arrest by suppressing cyclin-dependent kinases 2, 4, and 6 and cyclin D1 [147]. Co-treatment of ritonavir with docetaxel has shown decreased docetaxel metabolism through CYP3A inhibition, which results in increased antitumor activity of docetaxel [151]. Ritonavir alone or in combination with other drugs such as delanzomib, belinostat, panobinostat, bortezomib and suberoylanilide hydroxamic acid inhibit growth and induce apoptosis in renal cell carcinoma. The anticancer effect of these drugs was mediated through various cellular mechanisms including induction of ER stress, inhibition of mTOR pathway, and suppression of HDACs [145, 152–155]. In ovarian cancer, ritonavir decrease the phosphorylation of AKT and inhibits the survival of ovarian cancer cells [148].

Abacavir, a reverse transcriptase inhibitor is used for the treatment of HIV. As an antiviral drug, abacavir incorporates itself in the viral DNA and inhibits the reverse transcriptase activity [156]. Abacavir has been repurposed for cancer treatment due to its potential anticancer activity. It has shown to reduce cancer cell growth, migration and invasion processes as well as senescence and cell death induction in prostate cancer cells. This anticancer activity of abacavir was mediated through up-regulation of LINE-1 ORF1 and ORF2 mRNA levels [157]. Abcavir also sensitizes esophageal squamous cell carcinoma cells to radiation through an increase in radiation-induced DNA damage, apoptosis, and deregulation of telomerase activity [158].

Acyclovir, used for chicken pox, inhibits growth of ovarian cancer cells [159] and enhances chemotherapeutic drug cytotoxicity in leukemia cells by cell cycle arrest and apoptosis [160]. Antiviral drug Adefovir, which is commonly used for the treatment of chronic hepatitis B, induces G2 phase arrest in the cell cycle. This leads to sensitization of vemurafenib-resistant colon cancer cells and tumor xenografts to vemurafenib [161]. Amantadine, used against influenza virus, inhibits the proliferation of hepatocellular carcinoma cells and arrests the cell cycle in G0/G1 phase by inhibiting cyclin D1, cyclin E and CDK2 proteins. It also induces apoptosis by reduction of Bcl2 and induction of Bax protein [162]. Amprenavir, a protease inhibitor used for treatment of HIV, has shown efficacy against breast cancer. It inhibits ERK2 and phosphorylation of BimEL at Ser69 that contribute to its anti-proliferative and apoptosis-inducing activity in MCF-7 cells and in mouse xenograft breast cancer model [163].

Cidofovir is used for the treatment of cytomegalovirus (CMV) retinitis in patients with Acquired Immune Deficiency Syndrome (AIDS). Subsequently cidofovir has shown to be effective against cervical cancer. In cervical cancer xenografts nude mouse model, administration of cidofovir decreased tumor growth and enhanced p53 and p-pRb protein expression. Decreased PCNA index also attributed to the enhanced anti-cancer effects of cidofovir [164]. Recently, specific inhibitors have gained tremendous attention in cancer treatment. Cetuximab, is an EGFR (epidermal growth factor receptor) inhibitor used for the treatment of metastatic colorectal cancer [165]. Cidofovir has shown to enhance the antitumor effects of cetuximab on HPV-positive cervical cancer cell lines both in vitro and in vivo xenografts model [166]. In phase I clinical trial, cidofovir at a dose of 5mg/kg combined with chemo radiotherapy appeared tolerable and displayed significant tumor regressions. It showed 89% complete response to the patients, and the 2-year overall and progression-free survival rates were 93% and 76%, respectively [167].

Efavirenz, an antiretroviral medication used to treat and prevent HIV/AIDS, has shown significant cytotoxicity in a variety of cancer cells [168]. It induced cytotoxicity in pancreatic cancer cells with IC₅₀ of 31.5 μmol/L. Moreover, combined treatment of efavirenz and radiation synergistically inhibited survival and induced cell death of pancreatic cancer cells. At molecular level, efavirenz caused oxidative stress and mitochondrial membrane depolarization along with phosphorylation of ERK1/2 and p38 MAPK in cancer cell lines [169]. However, efavirenz at a dose of 600 mg failed to improve the PSA level in patients with metastatic castration-resistant prostate cancer [170].

Lamivudine is an antiretroviral medication used to prevent and treat HIV/AIDS. However, recent findings showed that lamivudine not only exhibits anticancer activity but also increases radiosensitivity of human esophageal squamous cancer cells. It sensitizes esophageal squamous cell carcinoma (ESCC) cell lines to radiation through an increase in radiation-induced DNA damage and apoptosis [158]. In addition, prophylactic lamivudine significantly reduces the incidence of HBV reactivation and the overall morbidity of breast cancer patients undergoing chemotherapy [171]. Maraviroc is another antiretroviral drug that belongs to the class of CCR5 receptor antagonist. Maraviroc is used for the treatment of HIV infections; however, it has shown anticancer potential in various cancer cells. Maraviroc inhibits the proliferation of gastric cancer cells [172] and induces cytotoxicity and apoptosis by profound G1 phase cell cycle arrest in colorectal cancer cells [173]. Recently, maraviroc is found to inhibit bone metastasis in nude rats implanted with MDA-MB-231 breast cancer cells [174]. Idoxuridine is another antiviral drug used to treat herpes virus. This drug has shown inhibition of proliferation of bladder cancer and colon cancer cells alone or in combination with D1694, a folate-based thymidylate synthase (TS) inhibitor through the mechanism of DNA single-strand breaks paralleled cytotoxicity [175].

Nelfinavir is another antiretroviral drug used in the treatment of HIV. It induces apoptosis independent of caspase activation and inhibits proliferation of various cancer cells. Nelfinavir causes endoplasmic reticulum (ER) stress and autophagy as well as blocks growth factor receptor activation and Akt signaling. Nelfinavir reduces growth of tumor in animal model [176]. This drug also induces cell death in carboplatin-sensitive and carboplatin-resistant ovarian cancer cell lines as well as sensitizes the cells to treatment with an apoptosis-inducing TRAIL receptor antibody and chemotherapeutic drugs [177–179]. Combination of nelfinavir with metformin induces autophagosome formation in human cervical cancer cells leading to apoptosis. Moreover, this drug combination markedly induced autophagy in SiHa xenografts in nude mice [180]. In Phase I/II clinical trials, nelfinavir in combination with chemoradiotherapy showed acceptable toxicity and promising activity in patients with pancreatic cancer [181, 182]. Nevirapine, a reverse transcriptase inhibitor used to treat and prevent HIV/AIDS (specially HIV-1), induces reversible growth arrest and produces premature senescence in human cervical carcinoma cells [183]. Nevirapine also increases susceptibility of prostate tumor cells to docetaxel by inducing apoptosis. It suppresses the growth of prostate carcinoma xenografts in athymic mice and induces a differentiated phenotype in vivo with increased K18 expression [184].

Oseltamivir, used to treat flu virus (influenza), also has anticancer properties. It has shown that oseltamivir phosphate inhibits pancreatic cancer cells and also overcome chemoresistance to cisplatin and gemcitabine. It reverses the epithelial-mesenchymal transition(EMT) in pancreatic cancer cells [185]. Ribavirin, an antiviral drug used for the treatment of RSV infection, hepatitis C, and viral hemorrhagic fever, has shown anticancer effects against ovarian, squamous cell carcinoma, thyroid, acute myeloid leukemia, breast, and lung cancer. In a phase II clinical trial, ribavirin showed substantial clinical benefit to patients with acute myeloid leukemia. Further study revealed that it targets eukaryotic translation initiation factor eIF4E and other key enzymes of guanosine biosynthetic pathway [186]. In breast and thyroid cancer cells, ribavirin suppresses growth and proliferation through eIF4E, as this effect was found to be suppressed by knockdown of eIF4E [187, 188]. It also restores estrogen receptor alpha gene expression in breast cancer [189]. Ribavirin is found to be effective in both in vitro and in vivo xenograft mouse model ovarian cancer alone or in combination with cisplatin. Besides eIF4E, ribavirin also suppresses Akt/mTOR signaling pathways in ovarian cancer cells [190]. Ribavirin hydrazone derivatives has also shown to inhibit the growth of A549 lung cancer cells [191].

Antiretroviral drug ritonavir (a protease inhibitor), used for the treatment of the HIV/AIDS, has also been shown to exhibit anticancer properties. It inhibits both ER-positive and ER-negative breast cancer cell growth by causing G1 arrest, depleting cyclin-dependent kinases 2, 4, and 6 and cyclin D1, and phosphorylation of Rb as well as by inhibiting Hsp90 and Akt [147]. It also induces cell cycle arrest and apoptosis in ovarian and pancreatic cancer cell lines mediated by suppressing of phosphorylation of RB and Akt [146, 148]. Along with cell cycle arrest, ritonavir induces apoptosis by reducing survivin and inhibiting phosphorylation of c-Src and STAT3 [192]. Later Hendrikx et al [151] showed that ritonavir has no direct antitumor effect but it enhances antitumor efficacy of docetaxel in mouse model of breast cancer. Ritonavir in combination with SAHA or bortezomib also found to inhibit proliferation of renal cancer cell carcinoma [154, 155]. Furthermore, the combination of ritonavir and ixazomib reported to induce apoptosis and inhibit growth synergistically in bladder cancer cells. The combination have been found to cause ubiquitinated protein accumulation and endoplasmic reticulum (ER) stress [193].

Saquinavir is another antiretroviral drug used with other medications such as ritonavir or lopinavir/ritonavir to treat or prevent HIV/AIDS. Besides its antiviral effects, anticancer properties of saquinavir has also been demonstrated. Saquinavir has been shown to induce apoptosis in multiple cancer cells including cervical cancer, prostate cancer, glioblastoma, and leukemia cells [194, 195]. It also sensitizes prostate cancer cells to ionizing radiation. The anticancer and radio sensitizing effects of saquinavir is due to inhibition of proteasome activity [195]. However, in ovarian cancer cells, saquinavir induces endoplasmic reticulum stress, autophagy and apoptosis, and further promotes apoptosis and chemo sensitization [196]. Telaprevir, an antiviral drug used for the treatment of hepatitis C, has shown potent anticancer effects but it is found to inhibit P-glycoprotein (P-gp) and BCRP, and P-gp-mediated transport in breast cancer cells [197]. Thus, it can be speculated that telaprevir may have potential in overcoming drug resistance.

Tenofovir, antiviral drug used to treat chronic hepatitis B, and to prevent and treat HIV/AIDS, has been shown to exhibits anticancer activity. This drug reportedly inhibits growth of ovarian cancer cells by inducing DNA damage and cell cycle arrest. Tenofovir in combination with emtricitabine/tenofovir has been found to enhance the cytotoxic effect of doxorubicin in ovarian, breast and cervical cancer cells [198]. Valaciclovir, an antiviral medication used to treat outbreaks of herpes simplex or herpes zoster, and to prevent cytomegalovirus, was studied to determine its effect in prostate cancer patients. A remarkable drop in serum prostate-specific antigen (PSA) was observed in castration-resistant prostate cancer patients treated with valaciclovir [199]. Zidovudine is an antiretroviral medication used to prevent and treat HIV/AIDS. Besides its antiviral effects, cancer therapeutic properties of zidovudine have been investigated. It has been found that this drug inhibits growth of human colon, head and neck, pancreatic, esophageal, breast cancer, and rat mammary tumors [200]. Zidovudine also increases the anticancer effects of therapeutic drugs including cisplatin, 5-FU, gemcitabine and radiation therapy [158, 201–203]. The sensitizing effect of this drug was found to be associated with suppression of Akt-GSK3β-Snail1 pathway [203], mitochondrial dysfunction [201], and occurrence of DNA strand breaks [200]. In phase I/II trial of zidovudine with 5-Fluorouracil, leucovorin, and dipyridamole in colorectal cancer, renal cell carcinoma and malignant melanoma patients, partial responses in colorectal cancer patients (22%) was observed but not in renal cell carcinoma and melanoma patients [204]. However, in later study, zidovudine (8000 mg/m2 i.v. two-hour infusion) in combination with 5-FU and leucovorin has shown potent activity in metastatic colorectal cancer [202].

Even though drug repurposing is a promising and potential therapeutic technique, the side effects associated with these drugs cannot be ruled out before these drugs can be repurposed as potential therapy candidates. The side effects associated with anti-psychotic drugs include but are not limiting to weight gain, type II diabetes, mellitus, hyperlipidemia, myocarditis and extrapyramidal side effects [205]. Similarly, anti-malarial drugs can have neurological and psychiatric side effects [206]. Side effects associated with repurposed anti-diabetic drugs might include physical weakness, diarrhea, muscle pain, abdominal pain, and low blood levels of vitamin B-12 [207]. Therefore, identification and consideration of these outcomes is critical before repurposed drugs are transitioned to the clinical trials.

6. Pitfalls of drug repurposing

Drug repurposing has several advantages over the conventional drug development strategy. These benefits are including but not limiting to the cost and time of drug development. The traditional drug discovery involves a cost of ~2.5 billion US dollars, which represents a loss of about 85% as compared to drug repositioning which costs ~ 300 million US dollars. Additionally, recent reports have identified that repurposing of existing drugs leads to a faster and higher success rate from Phase II to the launch of the newly repurposed drug in the market [1, 2, 3]. Several repurposed drugs have been classified as ‘high potential’ drugs for cancer therapy. These drugs include mebendazole, cimetidine, diclofenac, nitroglycerine, propranolol and, clarithromycin [208]. Even though several advantages are associated with drug repurposing, repositioning of existing drugs come with certain challenges that need to be addressed before they can be successfully used for clinical purposes. Due to the current interests in targeted therapy, very few repurposed drugs have been identified to directly target the cancer cells. On the contrary, the majority of potential repurposed anti-cancer drugs target the tumor microenvironment [209]. Another drawback associated with the repurposing of drugs is that these drugs may have several off target and immunomodulatory effects. Since these drugs have several targets, it is highly unlikely that these drugs can be utilized as monotherapy for cancer treatment [210].

Off patent repurposed drugs, face several challenges during the trial phase due to lack of industry involvement. Therefore, the trial cost of a cheaper repurposed drug is comparatively higher than more expensive high-cost drugs. The other challenge associated with repurposing includes legal implications. These encompass patents and intellectual property rights, product liability and pricing or reimbursement charges [211, 212]. In case of repurposing an off-patent drug, there is a lack of prospect for return on the investment therefore; industries are very less inclined to fund a trial. Hence, where on one hand commercial trials receive the trial drug at no cost, it is likely for a repurposed drug to pay for its trials [12]. Recently, pharmaceutical companies have identified the potential of drug repurposing and initiated avenues to promote external collaboration. For example, AstraZeneca has Open Innovation Platform similarly; Pfizer and GlaxoSmithKline have started Center for Therapeutics Innovation and Center for Excellence for External Drug Discovery. Even though the pharmaceutical company has initiated these efforts, industries face organizational hurdles. These include but are not limited to the company’s core disease area, lack of personnel in the company to work on drug repurposing, limited funding, source of drug supply and so on [208].

Finally, for a successful repurposed drug several measures and incentives have to be initialized that focus on patent and regulatory barriers as discussed above. Conclusively, drug repurposing can be a complex process that might include expertise in several fields.

7. Conclusion

In this review, we have summarized the potential of drug repurposing and identified the foreseen pitfalls associated with it. Drug repurposing is a promising way to identify and develop drugs against cancer. It surpasses several challenges identified in the development of a new anticancer drug. Drug repurposing obviates complexity of new drug discovery, and makes it efficient and relatively rapid. In this review, we have highlighted the antineoplastic properties of various antipsychotic drugs, anti-diabetic drugs, anti-malarial drugs and antiviral medications against several tumors. In pre-clinical studies, these non-neoplastic drugs induce apoptosis, inhibit proliferation, and exhibit anti-metastatic effects through various signaling mechanisms. Some of the repurposed drugs that possess anti-neoplastic effects have shown promising results in clinical studies (Table 2). Drugs undergoing clinical trials have been effective as mono and combination therapy. Even though drug repurposing has shown promising results so far there are several challenges that need to be addressed, one of which is intellectual property right. In certain cases, even though the drug has displayed promising outcomes it is restrained from entering the market due to conflicts associated with intellectual property rights. Another major challenge faced by repurposed drugs is the lack of funding opportunities and interests from pharmaceutical companies. One possible solution to this could be to utilize the support from federal funding sources that have more interest on patient and public health than on the commercialization of the product.

Table 2:

Repurposed drugs under clinical trials (sourced by http://clinicaltrials.gov)

Drugs	Primary Use	Proposed anti-cancer mechanism	Identifier number	Phase	Status
Digoxin	Anti- arrhythmic agent	Kaposi sarcoma (KS): HIF-1 α mediated inhibition of HHV-8 replication in KS	NCT02212639	Phase 2	Recruiting
		Prostate cancer: Inhibition of TNF-α, NF-κB pathway	NCT01162135	Phase 2	Completed
		Breast Cancer: Treatment of metastatic ErB2 positive breast cancer	NCT00650910	Phase 1	Completed
		Melanoma: Unresectable or wt BRAF	NCT02138292	Phase 1	Completed
Disulfiram	Alcohol deterrent agent	Metastatic breast cancer: introducing disulfiram and copper as an active therapy for metastatic cancer	NCT03323346	Phase 2	Recruiting
		Glioblastoma: Investigate disulfiram and copper supplement as add-on treatment in glioblastoma with recurrence receiving alkylating chemotherapy	NCT02678975	Phase 3	Recruiting
		Metastatic melanoma: Identify response rate and evaluate toxicity of disulfiram in stage IV melanoma, DNMT1 inhibitor, induction of metallothionein expression	NCT00256230 NCT02963051	Phase 2 Phase 1	Completed Recruiting
Metformin	Anti-diabetic agent	Colon cancer: non diabetic patients with Ki67 positive colon cancer are treated 10 days after surgery	NCT03359681	Phase 2	Recruiting
		Pancreatic cancer: inhibition of enzymes involved in tumor cell progression	NCT02005419	Phase 2	Completed
Statins	HMG-CoA reductase inhibitor	Breast cancer: inhibition of AKT and MAPK signaling kinase	NCT03454529	Phase 2	Recruiting
Minocyclin	Anti-bacterial agent	Pancreatic cancer: reduction of side effects of chemotherapy in pancreatic patients	NCT01693523	Phase 2	Completed
		NSCLC: Reduction of side effects in chemo-radiation treated NSCLC patients	NCT01636934	Phase 2	Completed
Diclofenac	Anti-inflammatory agent	Prostate cancer: inhibition of C0X-2 and prostaglandin E2 Inhibition of MYC	NCT01939743	Phase 2	Completed
Itraconazole	Anti-fungal agent	NSCLC: Anti-angiogenic factors, tumor specific HIF1 α expression reduction and reduced tumor micro-vessel load	NCT03664115	Phase 2	Recruiting
		Skin basal cell carcinoma: Topical application of Itraconazole for the inhibition of Gli1, target gene of Hedgehog pathway	NCT02735356	Phase 1	Completed
		Prostate cancer: Hedgehog pathway inhibition and autophagy induction	NCT00887458	Phase 2	Completed
Celecoxib	Anti-inflammatory agent	Lung cancer: COX-2 inhibition	NCT00653250	Phase 2	Completed
		Prostate cancer: Selective COX-2 inhibition. Inhibition of NF-κB.	NTC00136487	Phase2/3	Completed

Taken together, these findings suggest that drug-repurposing strategies are very promising for generating new chemotherapeutic drugs. However, recommendations for the use of these drugs can only be made when directed, randomized, and controlled clinical trials are completed. In addition, property rights, pricing and liability issues must be considered in the early stages of drug development process, especially in the cases of diseases that have higher treatment costs.