Clinical trials on combination of repurposed drugs and anticancer therapies

The way of drug repurposing studies

Drug repositioning opportunities evolve from observations, discussions, and other collaborations, including the purposeful development of platforms for drug identification, which identify the potential targets and allow the accessing of compounds. Drug repurposing candidates can be obtained from drugs in clinical development that have failed to demonstrate efficacy for a particular indication during Phase II or III trials but have no major safety concerns and drugs that have been discontinued for commercial reasons and drugs patents are close to expiry in market, and from nonreleased drug candidates. Three main approaches based on computational, biological experimental, and mixed are generally used for drug repurposing studies. Several strategies involved the drug repurposing studies that include various in silico methods, genomic, high-throughput screening technologies, and literature mining. These screening methods also provide opportunities for the exploitation of the open-source model ( Table 15.1 ). DrugBank, the Potential Drug Target Database, Therapeutic Target Database, and Super Target represents the open sources that provide targets and drugs, including protein and active-site structures, association with related diseases, biological functions, and signaling pathways. Compound-specific databases are provided by PubChem, ChEMBL, and ChemSpider, the US FDA's Electronic Orange Book's Discontinued Drug Products List, and ID (Investigational Drug) Map [ ].

Several open-source models permit sharing the data, resources, compounds, clinical molecules, and small libraries, as well as screening platforms in the search for new indications of old drugs or failed candidates. As a result of successful findings of drug repurposing, new data banks have been obtained to the creation of open sources in this field. An online Drug Repurposing Hub ( www.broadinstitute.org/repurposing ) is created, which contains the detailed annotation for each compounds and is designed to rapidly identify drugs for evaluation in disease models [ ].

Conventional drug discovery and development process generally takes 10–15 years that includes five stages: discovery and preclinical, safety review, clinical research, FDA review, and FDA postmarket safety monitoring [ ]. Drug repurposing process takes 9–10 years, and it has four stage as compound identification, compound acquisition, development, and FDA postmarket safety monitoring ( Fig. 15.1 ).

Approaches of drug repurposing

Several rational approaches to identification of drug repurposing candidates are used. Drug repurposing approaches include various relationships such as drug–disease, drug–drug, or drug–target relationships ( Fig. 15.2 ). Experimental data related to disease (e.g., omics data collected from patients) or knowledge on how drugs modulate phenotypes related to disease (e.g., known from their side effects) are utilized in disease focused approaches. Drug focus is based on the single drug that interacts with multiple targets and related with approved molecules for particular indication that can help to identify active compounds that were originally developed for different indications. When primary and/or secondary targets of compounds are known and often involved in several biological processes that targets relevant to one disease or biological process, it is used to find new indications [ ].

Identification of interaction with multiple targets related with several diseases can arise serendipitously during screening studies and observation of some side effects [ ]. Deep understanding of genomics and molecular pathways involved in human diseases and drug activity and using this information are important to finding new mechanisms, dosing levels, routes of administration, or innovative targets [ ].

Source for drug repurposing

Mechanism action of disease, clinical development for new indication, and the original indication can be carried out simultaneously to identify drug for repurposing aim [ ].

At the beginning, generic drugs were preferred as sources of repositioning target. These drugs are safer, easy, and cheap to obtain for clinical trials because their original patents have been expired. Another source of drug repurposing studies are off-label uses of drugs by physicians for the treatment of diseases. FDA-approved drugs can be repurposed for their potential new therapeutic applications. Failed drugs in clinical studies is the another source for drug repurposing studies that are safe in humans and can be used for some other activities. All publicly available data have been constituted from the information of a social network that allows to obtain all relationships between drugs, molecular pathways, genes, and other biological suitable entities [ ]. The other sources of drug repurposing are shown in Fig. 15.3 .

Advantages and disadvantages of drug repurposing

Drug repurposing discovery has some advantages that lowers time-consuming efforts and paper works for licensing and eliminates scientific barriers for proving its safety and efficacy ( Table 15.2 ). Furthermore, when it is approved, their integration into healthcare will be very fast. Effective and ideal candidate for drug repurposing should not need further chemical optimization, and some part of clinical studies should have been done earlier. Their ADMET properties, safety, and clinical efficacy should also be appropriate, and they should be effective for the new indication in the same concentration act for the original disease. If reformulation is needed for new clinical use, safety and ADME studies should not be necessary [ ]. Since the safety, efficacy, and toxicity of an existing drug have been extensively studied and passed all clinical tests in Phase I, Phase II, and Phase III, safety of repurposed drugs has already been confirmed. All data for repurposed drugs are already exist, this saves time and money that provides hope to patients whose treatment conditions are costly for conventional drug development. The most favorite side of drug repurposing is reducing the time for research and development. Moreover, the approval rate of these drugs on market takes approximately 30%, while this rate is ∼10% for new drug applications. As a result, repurposed drugs can enter the pipeline at the efficacy stage, thus significantly decreasing the failure rate probability and increasing the chances for a successful launch [ , ]. On the other hand, the chemistry, manufacturing process, quality profile, and pharmacokinetic and pharmacodynamic profile of repurposed drugs are already proved. Repurposing drugs have already a wealth of accessible data that provide a high level of safety for the pharmacokinetics, toxicities, bioavailability, dosing, and protocols of the agents. Repurposed drugs bring an advantage of a reduced failure rate as they have already been tested for safety [ ]. As well as finding new treatment options for orphan, rare and neglected diseases, providing therapeutic efficacy which did not exist previously is also an advantage for drug repurposing studies.

Drug repurposing has some risks because it still needs a drug development phase. The biological activity, pharmacological parameters, and clinical observations need to be fully analyzed to limit failure in late-stage clinical trials or after marketing. For the generic compounds, there is less option to define IP, current formulations or delivery method may not be appropriate, and drug can be taken by patients outside of clinical studies. If on-label compounds are tried to be repurposed, it may require a much higher level of evidence, which could be even higher to secure reimbursement. It may also need longer and more costly clinical trials. Moreover, advanced level of industry experience is required. When off-label compounds are tried to be repurposed, efficacy may be lower, and monitoring the safety is difficult [ , ]. In various cases, a drug may show potent activity for a new indication, but if higher dosage is applied than first approved one, this results in possible adverse effects relating to toxicity. Patent exclusivity is another problem in limiting the potential of repurposing a drug. In the United States and the European Patent Convention, a patent protection time is determined as 20years. It is normal that a pharmaceutical company will apply for a patent prior to commencing clinical trials. When repurposing on-patent drug, current regulations for drug repurposing are very strict. This can be a deterrent for repurposing drugs for some research groups. On-patent drugs can be screened for new therapeutic use without the need of chemistry and manufacturing applications. However, approval of the original owners providing that the product is used in agreement with the approved product label is necessary. In the case of off-patent or generic drugs, the new indication should be found and it should have clinical benefits. These indications and new claim must not have been previously published. Another difficulty for research is the requirement of a new formulation to have market priority with the proposed new indication [ ].

Advantages and disadvantages of drug repurposing studies are summarized in Table 15.2 .

Anticancer drug repurposing

Repositioned anticancer drugs have a potential to be an excellent strategy for future anticancer drug development [ ]. Recent modern anticancer drugs show better response in clinical studies, but the side effects and poor quality of life still obstacle and cause discontinuation of drugs, dose reduction, and emergence of drug resistance [ ]. For instance; kinase inhibitors target oncogenes or oncoproteins that show high selectivity for tumor cells and prevent tumor progression and reduce side effects. However, most kinase inhibitors often have some pitfalls and lead to drug resistance [ ].

The starting point for development of cancer therapeutics from noncancer drugs is that different diseases share with common molecular pathways and targets in the cell. This fact has been discovered through progress in genomics, proteomics, and informatics technologies. Advances of analytical tools allow researchers to screen large numbers of existing drugs against a particular disease target at the same time [ ].

Drug repositioning has been applied in anticancer drug discovery due to the combination of great need for new anticancer drugs and the availability of a wide variety of cell- and target-based screening assays. With recent successful clinical introduction of noncancer drugs for cancer treatment, drug repositioning became a powerful choice to discover and develop novel anticancer drug candidates from the existing drug data. With the increasing knowledge about the pharmacological properties and identification of new targets, the chemical structure of the old drugs emerges as a great treasure for new cancer drug discovery. Although the several repurposed drugs are being on the market, the therapeutic potential of a large number of noncancer drugs is limited during their repositioning due to various problems including the lack of efficacy and intellectual property. Increasing knowledge about the pharmacological properties of old drugs may have further applications to utilizing them as a drug or using their scaffold for repurposing as cancer therapeutics. Since repurposing drugs may have potential applications with other promising biological targets, it is believed that the scaffold of these drugs will be beneficial for new drug design. The scaffold repurposing may be better tools to develop novel logical properties through the generation of novel chemotypes. The previous SARs and the known pharmacokinetics parameters of old drugs may provide to obtain more efficient drugs instead of a random chemical classification. The scaffold can be optimized by rational design to eliminate the known unwanted properties for increasing the success rate of further development [ ].

By using high-throughput technologies, a large number of genetic and genomic data were obtained by using next-generation sequencing (NGS) and were generated a large volume of genetic and genomic data from several national and international cancer genome projects. These international or national cancer genome projects have identified many cancer-driven genes. Moreover, the cancer genomics data pioneer for the revolution of a novel oncology drug discovery from candidate target or gene studies toward targeting clinically relevant driver alterations or molecular features for the development of precision cancer therapy. For example, among the several selective kinases, axitinib inhibits BRC-ABL1 and it is more effective for treating tumors that have the clinically relevant driver mutations. However, millions of cancer patients are administrating various anticancer medications without their benefits due to both somatically acquired and inherited mutations. Although recent advances in NGS technologies enable the identification of various genetic or epigenetic alterations (e.g., clinically relevant driver mutations) with clinical practice, it is still necessary to develop more efficient and selective target for particularly genetic profiles or molecular features. Ideal candidates for repositioning can contribute to the therapeutically unmet need for more efficient anticancer agents, including drugs that selectively target cancer stem cells [ ].

In this review, the recent advances in the development of novel small molecules for cancer therapy by their structure to repurposing with highlighted examples are summarized. The relevant strategies, advantages, challenges, and future research directions associated with this approach are also discussed. Clinical trials on combination of repurposed drugs and anticancer therapies are explained in pharmacological classification.

Central nervous system drugs

Studies on antipsychotic drugs such as valproic acid (VPA), phenothiazines (PTZs), selective serotonin reuptake inhibitors, tricyclic antidepressants, and monoamine oxidase inhibitors demonstrated several anticancer activities. The details of these studies were reported in some recent literatures [ ].

The first example of these studies is the reporting of sedative thalidomide as an oncology drug ( Fig. 15.4 ). Thalidomide was first used to treat for morning sickness in pregnant women. It was quickly withdrawn from the market due its teratogenic properties. Later studies showed that thalidomide, especially its R -isomer, exhibited potent antiangiogenic properties, and it would considered to use it as an anticancer agent [ ]. Thalidomide shows its mechanism of action in six different way; it affects DNA replication, transcription, synthesis, and function of growth factors and integrins, angiogenesis, chondrogenesis, and cell death [ ]. Understanding the adverse effects of thalidomide prompted scientist to investigate the therapeutic efficacy in multiple myeloma (MM). The response rate of treatment with thalidomide alone in MM and combination with dexamethasone resulted a 32% and >60%, respectively, which was quite effective for the treatment of MM. Structurally similar compound lenalidomide combination with dexamethasone was another promising treatment option in MM. Lenalidomide launches growth arrest and apoptosis of drug-resistant MM cells. It also inhibits to binding of MM cells to bone marrow and stromal cells and angiogenesis and modulates cytokine secretion. Combination of lenalidomide with dexamethasone enhances the response of the survival and prolongs the time. It was approved by FDA for therapy of MM patients in 2006 [ ]. Another study showed that triple combination of lenalidomide–dexamethasone and the proteasome inhibitor bortezomib ( Fig. 15.4 ) extended the life of MM patients by few years [ ]. This combination was also found highly tolerable and is highly effective therapeutic against newly diagnosed MM [ ].

Investigation of mechanisms of action of thalidomide and other structurally similar drugs such as lenalidomide, pomalidomide, and temozolomide ( Fig. 15.4 ) demonstrated that this mechanism was associated with the upregulation of p21 and induction of apoptosis and direct affection in proliferation of MM cell lines and patient's specimens [ ]. Pomalidomide alone was not very effective against MM. However, it showed synergistic effects when combined with dexamethasone. This combination was found safe and effective in relapsing and refractory of patients [ ]. It was shown that temozolomide was highly effective in combination regimen for the treatment of metastatic melanoma [ ]. A combination of thalidomide and temozolomide is currently under clinical Phase I/II studies for brain metastatic melanoma [ ]. It was also found that thalidomide exhibited antiangiogenic effects and inhibits the processing of mRNA encoding peptide molecules including tumor necrosis factor-alpha (TNF-α) and the angiogenic factor, vascular endothelial growth factor (VEGF). As a result of these findings, thalidomide was investigated against several cancer types such as renal carcinoma and prostate cancer (PCa) [ , ]. Nevertheless, additional clinical studies are necessary to clarify the role of thalidomide in the therapy of PCa [ ]. It was also shown that thalidomide inhibits nuclear factor kappa light chain of activated B cells (NF-kB), which is the reason of inflammation, survival, proliferation, invasion, and metastasis of tumors [ ].

Some clinical studies of thalidomide combination therapy with anticancer agents have been done for the treatment of PCa patients. The aim of one of the studies was to evaluate clinical efficacy of thalidomide combination with capecitabine and cyclophosphamide against advanced castrate-resistant prostate cancer (CRPC). The results were found promising with excellent safety profile for patients with advances CRPC [ ]. In other Phase I study, the potential efficacy and safety profile of thalidomide and cyclophosphamide in patients with hormone refractory prostate cancer (HRPC), previously treated with docetaxel-based regimens, was shown. It was suggested that Phase II clinical trials were needed to evaluate clinical efficacy of this regimen in HRPC [ ]. In addition, a Phase I study was conducted to determine the feasibility and safety of combination regimen of gemcitabine with interferon, thalidomide, and capecitabine in patient with metastatic renal cell carcinoma (MRCC) in order to establish a Phase II trial to evaluate antitumor activity. The main aim of this study was to determine the effectiveness of adding gemcitabine to this combination. Weak responses obtained from this regimen demonstrated that adding gemcitabine to the three-drug regimen was not successful. Further investigation on this combination without the gemcitabine was recommended as a reasonable treatment option [ ].

The clinical potency of the combination of weekly intravenous (IV) gemcitabine with continuous infusion fluorouracil (5-FU) and daily oral thalidomide in patients with MRCC was studied. During the application of therapy, several vascular toxicities such as deep venous thromboembolism (VTE), vein thrombosis (DTV), and pulmonary embolization were observed. In this study, the addition of thalidomide to gemcitabine and 5-FU was found ineffective, and it was recommended that further efforts need to be directed toward improving our knowledge of the incidence and pathophysiology of this potentially lethal adverse event [ ].

The efficacy and safety of zoledronate ( Fig. 15.4 ), thalidomide, and interferon treatment for renal carcinoma and bone metastases was studied in 15 patients. The results show that the therapeutic benefit of this combination was minimally enhanced. Nevertheless, the clinical efficacy of bone-targeted therapy might be improved by selecting appropriate patients with bone disease [ ].

Fluspirilene ( Fig. 15.4 ) is currently used as antipsychotic drug and it also demonstrated antiproliferative effect in human hepatocellular. The studies showed that oral fluspirilene was used to treat human hepatocellular carcinoma (HCC) compared to 5-fluorouracil. It was shown that it exhibited anticancer effect in human hepatoma HepG2 and Huh7 cells by inhibition of cyclin-dependent kinase 2 (CDK2). Combination therapy with fluspirilene and 5-fluorouracil produced the highest therapeutic effect. Since fluspirilene has been used safely for a long time as therapeutic agents and valuable results received proving it a potential anti-HCC agent, it may immediately enter in clinical therapy. Having being used with HCC could also be combined with other chemotherapeutic drugs for treatment [ ].

It was determined that penfluridol ( Fig. 15.4 ), which is clinically used for schizophrenia, had antitumor activities in non-small cell lung cancer (NSCLC) and acute myeloid leukemia (AML) cell lines. It was shown that penfluridol inhibited the viability and motility of NSCLC cells in vitro and in vivo. It induced nonapoptotic cell death by blocking autophagic flux and accumulation of autophagosome-related protein, light chain 3 (LC3) B-II, in HCC827 and A549 NSCLC cells. It was also observed that penfluridol used in patients with lung tumors expressing high LC3B had longer overall and disease-free survival times. Mechanistically, it was determined that penfluridol acted as inducer of endoplasmic reticulum (ER) stress and mitogen-activated protein kinase (MAPK) p38 activation [ ]. In another study, it was shown that penfluridol suppressed the cell viability of AML cells with FLT3-WT and FLT3-ITD depending on its concentration. It was found that penfluridol induced apoptosis by activation of protein phosphatase 2A (PP2A) that suppresses AKt and MAPK activities and also augmented the reactive oxygen species (ROS) levels. In clinical studies, it was observed that patients with AML expressing high PP2A had essential prognoses and combination with penfluridol with an autophagy inhibitor that may become a favorable treatment for AML patient [ ].

PTZs are old drugs used for the treatment of psychotic diseases as well as chemotherapy-induced emesis. Recently, they have been intensively studied as anticancer agents to targeting various signaling pathways. It has been shown that PTZs possess cytotoxic activities in tumor cell lines. However, there are some confusion about the cell death mechanism of PTZs [ ]. Other PTZ derivatives with different chemical structures such as chlorpromazine (CPZ), levomepromazine, promethazine, trifluoperazine, and thioridazine (TRDZ) were studied against several leukemic cell lines. These drugs showed strong cytotoxic effect and antiproliferative activity against leukemic cells in clinically relevant doses (up to 20 μM). Among them, TRDZ was found the strongest apoptotic agent. It is considered that the activity mechanism of PTZs is associated with inhibition of mitochondrial DNA polymerase in treated leukemic cells and decreasing of ATP production, which are crucial events for the viability of cancer cells [ ].

PTZs were studied for potential therapy for lung cancer (LC), which is the most prevalent and deadly malignancies worldwide. It was found that PTZs decreased cell viability and induced cell death preferentially in small cell lung carcinoma over NSCLC cell lines [ ].

Antipsychotic drug CPZ ( Fig. 15.4 ) was identified as a potential treatment agent for colorectal cancer (CRC) that effectively inhibited tumor growth and induced apoptosis in CRC cell in a p53-dependent manner. In addition, it was reported that CPZ induced the degradation of sirtuin 1 (SIRT1), which is associated with downstream of JNK and JNK which is suppressed by SIRT1 downregulation. In clinical studies, it was found that high SIRT1 expression and poor outcome in CRC patient was significant. Therefore, it was concluded that SIRT1 was an attractive therapeutic target and treatment agent for CRC [ ]. Moreover, CPZ was reported as anticancer agent for the treatment of breast cancer by suppression of stemness properties including mammosphere formation and stemness-related gene expressions in breast cancer cells. Since CPZ targets the breast cancer stem cells (SSCs), it can be used for breast cancer associated with therapeutic resistance and metastasis [ ].

Antipsychotic aripiprazole ( Fig. 15.4 ) was reported to have a partial dopamine agonist effect alone or in combination with other chemotherapeutic agents to inhibit the growth in serum-cultured cancer cells and cancer stem cells. Furthermore, it was found that it inhibited sphere formation, as well as stem cell marker expression of cancer stem cells. The results were considered that aripiprazole might be used as an anticancer stem cell drug [ ].

Antidepressant drug fluoxetine ( Fig. 15.4 ) was studied for the treatment of colon cancer. It was shown that fluoxetine induced p53-independent apoptosis by altering mitochondrial membrane potential that leaded to DNA fragmentation, depending on concentration used. Furthermore, it induced the cell death with high selectivity for colon, breast, and ovarian carcinoma cells compared with normal cells. These results provided new findings about the cytotoxic effects of fluoxetine and demonstrated a new potential use for cancer treatment [ ].