Therapeutic Potential and Challenges of Pioglitazone in Cancer Treatment

1. Introduction

Thiazolidinediones (TZDs) are a class of oral hypoglycemic agents for the management of diabetes. TZDs are ligands for PPAR α and γ, a member of the nuclear receptor family which regulates glucose and lipid metabolism, inflammation and cell proliferation [1,2,3]. PPARs are classified into three subtypes: PPARα, PPARβ/δ, and PPARγ. Specifically, PPARα receptors are expressed in the heart, liver, skeletal muscle, while PPARγ receptors are expressed in adipose tissue. The former regulate fatty acid oxidation followed by decreased low-density lipoprotein cholesterol (LDL-C) and in-creased high-density lipoprotein cholesterol (HDL-C) levels, while the latter regulate cell differentiation, leptin expression and insulin sensitivity [4,5]. As a result, TZDs find therapeutic application in the treatment of type 2 diabetes mellitus (T2DM) as well as non-alcoholic fatty liver disease (NASH) and multiple types of cancer [6,7].

Initially approved by the FDA in 1999, pioglitazone is marketed under the brand name ACTOS by Takeda Pharmaceuticals. It has been established as a second or third-line treatment for the management of T2DM, when first-line treatment with metformin is inadequate [8,9]. In this scenario metformin is combined with sulfonylureas, glinides, alpha-glucosidase inhibitors, insulin, glucagon-like peptide-1 receptor agonist, dipeptidylpeptidase 4 inhibitors, sodium-glucose co-transporter 2 inhibitors, and thiazolidinediones, such as pioglitazone [10]. Pioglitazone is typically administered orally, with the optimal dose customized based on clinical status. According to the European Medicines Agency (EMA) recommendations, initial dose ranges from 15 to 45 mg once daily. The dose may be increased if inadequate glycemic control is achieved. Alternatively, it may need to be decreased if the patient is receiving pioglitazone along with other diabetes medications (insulin, chlorpropamide, glibenclamide, gliclazide, tolbutamide). The tablets should be administered once a day with water, either with or without food. After three to six months, patients should be evaluated and treatment discontinued if considered non-beneficial [11].

However, despite its effectiveness in enhancing glycemic control, there have been concerns raised about its safety profile, one of which is regarding increased bladder can-cer risk. Notably, indications of a higher incidence of bladder cancer among pioglitazone users prompted the French and German pharmaceutical regulators to withdraw the medication from the market in May 2011. Nevertheless, the medicine remained available throughout the rest of Europe [12].

The aim of this literature review is to thoroughly evaluate pioglitazone's safety profile and efficacy, with an emphasis on its association with bladder cancer. This re-view summarizes clinical data, epidemiological studies, and regulatory recommendations in an effort to evaluate the risk-benefit ratio of pioglitazone. Its evaluation will shed light into current clinical practice and direct future studies to explore the multiple therapeutic applications of pioglitazone.

2. Overview of Pioglitazone

Pioglitazone, also known as 5-{4-[2-(5-Éthyl-2-pyridinyl)éthoxy]benzyl}-1,3-thiazolidine-2,4-dione based on IU-PAC, is a TZD compound derived from a pyridine derivative through nucleophilic aromatic substitution reaction with 72% yield [13]. Pioglitazone appears as needles from dimethylformamide and water, while pioglitazone hydrochloride (HCl) appears as colorless prisms from ethanol with a melting point of 183–184°C and 193–194°C, respectively [14]. The solubility of pioglitazone HCl increases with increasing temperature [15]. Low aqueous solubility and oral bioavailability can be overcome through temperature increase, cosolvents and formulation strategies such as cyclodextrins inclusion complexes or nanostructured lipid carriers (NLCs). More specifically, polyeth-ylene glycols (PEGs) are safe cosolvents for oral or parenteral pioglitazone HCl formulations. The additions of PEGs 200, 400 or 600 and temperature increase, significantly enhanced the solubility profile of pioglitazone [15]. Positive results were achieved through the addition of cyclodextrins in ethanol solution or encapsulation of pioglitazone HCl. Cyclodextrins caused a 6-fold increase in dissolution rate of pioglitazone HCl-β-cyclodextrin when compared to the pure drug, while lipid carriers achieved controlled release up to 24 h [16,17]. The latter was also observed during the administration of a newly discovered racemic compound which serves as a more thermodynamically stable and less soluble form of pioglitazone HCl [18].

The molecular structure and properties of pioglitazone have been well-characterized, providing a solid foundation for its pharmacological profile. The pharmacological activity of pioglitazone has been attributed to its hydroxy and ketone derivatives during in vivo studies on rats and dogs [13]. Similarly to other thiazolidinediones, pioglitazone activates the PPARγ receptor, increasing insulin sensitivity, without inducing insulin release from pancreatic cells. It is typically administered once a day with or without food, metabolized by the cytochrome P450 hepatic enzymes with a half-life of approximately 9 hours, and excreted in the urine. The pharmacokinetics of pioglitazone are dose and time dependent, leading to non-linear changes in plasma concentration and potential toxicity. According to clinical evidence, pioglitazone follows the same pharmacokinetic profile in healthy participants, diabetic patients and patients with renal failure, irrespective of age, gender and race. However, dose adjustment is required for patients with hepatic failure due to decreased metabolic function and by extension decreased plasma concentration [19].

Figure 1. Chemical structure of pioglitazone.

Figure 1. Chemical structure of pioglitazone.

3. Therapeutic Use of Pioglitazone

3.1. Diabetes Mellitus Type 2 (T2DM)

Thiazolidinediones (TZDs) are biguanide compounds that have been well-established for the treatment of type 2 diabetes mellitus (T2DM) [20]. Given the pathophysiology of T2DM, targeting mechanisms regulating lipid metabolism is a viable option [21]. TZDs are peroxisome-proliferator activated receptor γ (PPARγ) agonists which regulate gene expression related to the uptake, distribution and metabolism of lipids, resulting in reduced lipid levels and sensitization to insulin. However, the use of TZDs has been limited during the past years due to major safety and toxicity concerns, such as edema, cardiovascular disease, weight gain and carcinogenesis reports, as analyzed below [22].

3.2. Nonalcoholic Fatty Liver Disease (NAFLD) and Nonalcoholic Steatohepatitis (NASH)

NASH is characterized as one of the most extreme forms of NAFLD, having the potential to lead to severe and often terminal hepatic disorders such as fibrosis, cirrhosis and hepatocellular cancer [23]. Considering the association of NASH with T2DM and dyslipidemia, pioglitazone is a reasonable option against NASH in both diabetic and non-diabetic patients [24]. According to the study conducted by Della Pepa et al. (2021), pioglitazone has demonstrated positive effects against NASH even in relatively small doses. This is achieved through the increase of the peripheral and adipose tissue sensitivity to insulin, leading to the reduction of hepatic inflammation and steatosis in a glucose independent manner [25]. It is important to note several investigations of combination therapies against NASH, with the most significant one being pioglitazone combined with vitamin E [26,27].

3.3. Cardiovascular Protection

By lowering insulin resistance, TZDs can have cardioprotective and anti-atherosclerotic activity and prevent ischemic events to an extent in patients with prior history of insulin resistance [28,29,30]. Additionally, it has been noted that in murine models and in vitro studies, pioglitazone has demonstrated the capability of limiting cardiac hypertrophy in addition to arresting cardiac fibrosis and providing protection against oxidative stress [31,32].

3.4. Neurodegenerative Diseases (NDs)

There have been promising indications that PPAR-γ agonists, such as pioglitazone, possess the capability of reducing neuroinflammation as well as the concentration phosphorylated tau protein and synaptophysin, This is complemented by the elevated permeability of the blood-brain barrier [34]. Focusing on Alzheimer’s Disease (AD), given that PPARγ regulates the expression of key enzymes related to the production and metabolism of amyloid plates, the inhibition of its phosphorylation, and therefore deactivation by pioglitazone suggests favorable results [35]. The neuroprotective properties of pioglitazone that mainly stem from the anti-inflammatory effects and the mitochondrial enhancement can also be applied for Parkinson’s Disease (PD) as well as amyotrophic lateral sclerosis (ALS) in which case the effects on the muscular tissue are also of significance [36,37]. As such, based on the results of studies contacted in rodents and humans, it is evident that pioglitazone can have a positive impact in the cognition and other neurological functions of patients with NDSs [38,39].

Figure 2. Pharmacological effect of pioglitazone on the cardiovascular and nervous system, as well as hepatic and metabolic function.

Figure 2. Pharmacological effect of pioglitazone on the cardiovascular and nervous system, as well as hepatic and metabolic function.

3.5. Anticancer Properties

3.5.1. Overview

As established, pioglitazone has been associated with elevated risk of bladder cancer, especially in the Caucasian population to such extend that it has catalyzed its withdrawal from the French, German and Indian markets and led the Food and Drug Administration (FDA) to introduce a mandate for the providing of information regarding potential risk [40,41]. However, there have been several reports and studies that point out the antineoplastic properties of pioglitazone and the related biochemical pathways that PPARγ is implicated in. Given the pleiotropic effects of PPARγ and its agonists, we consider that investigating the exact molecular interactions will contribute to shedding some light on the mechanism of action and toxicity as well as the interactions and effects of pioglitazone and potentially suggest new applications and optimalization/personalization of interventions involving TZDs.

3.5.2. Molecular Pharmacology

PPARγ is classified as a nuclear receptor, which after binding with its ligand, translocates to the nucleus, heterodimerizes the retinoid X receptor (RXR) and acts as gene regulator via its interaction with the peroxisome proliferator response element of selected genes. The latter is related to metabolism in physiological as well as cancer cells [42]. As such, the receptor possesses the capability of affecting key cancer cell processes related to proliferation, progression, differentiation, metastasis, apoptosis etc. Activation of PPARγ receptors suppresses tumorgenicity through their interaction with insulin growth factor. Additional molecular mechanisms of therapeutic significance are summarized in Table 1.

3.6. Therapeutic Effect on Different Types of Cancer

3.6.1. Breast Cancer

According to retrospective cohort studies, there is insignificant to almost no elevated breast cancer (BC) risk related to the use of pioglitazone [64,65]. It is important to note that PPARγ levels are reduced and methylation of PPARγ promoter is in-creased in BC cells, leading to the indication of its unfavorable role in cancer proliferation. In the example of triple negative breast cancer (TNBC), pioglitazone has proven its value as an adjuvant in the combined use with doxorubicin as it decreases the inva-siveness and tendency for migration, an issue commonly associated with the use of the latter. Similar findings were also reported for the apoptosis-inducing capabilities of cisplatin during in vitro testing [49]. PPARγ facilitates major regulatory processes such as the promotion of CXCR4 and CXCR7 genes, inhibition of eIF2α, and intracellular accumulation of glucose-affecting related migration pathways [66].

3.6.2. Lung Cancer

According to clinical evidence, PPARγ agonists play a major role lung cancer progression. In fact, PPARγ agonists have been associated with differentiation, tumor size, BCL2/BAX ratio and c-MYC expression levels, in cases of non-small cell lung cancer (NSCLC), lung adenocarcinoma and squamous cell lung cancer (SCLC) [67]. Pioglitazone has demonstrated the ability to reduce NSCLC proliferation and invasiveness through downregulation and altering pathways of major significance in proliferation, apoptosis, angiogenesis and metastatic potential. Indeed, rodent studies have shown that pioglitazone has the potential to reduce tumor formation and volume [68]. Additionally, inflammation is also affected as pioglitazone inhibits prostaglandin 2 (PGE2) production while bypassing cyclooxygenase 2 (COX-2) [59]. As a chemopreventive medication, pioglitazone has proven little value considering the double-blind clinical trial conducted by Keith et al. (2019), that reported no significant alterations in the endpoints were noted other than some minor effect on lesions [69]. Chemopreventive studies were also carried out by Seabloom et al. (2017) who noted significant re-duction in adenoma formation but no noteworthy alteration to the effect of metformin when combined with it and, by a team of similar composure, who evaluated the effectiveness of pioglitazone in an aerosol form [70]. They concluded from animal tests that a dose of 150-450 μg/kg was tolerable, with limited to no adverse effects, and effective, given the reduction in adenoma formation after biochemical and histological assessments [71]. As with BC, pioglitazone has also been considered for combination therapy along with more established anticarcinogenic agents, as studied in the following cases:

• The use with EGFR Tyrosine Kinase Inhibitors such as geftinib for NSCLC as it was found that PPARγ mediated upregulation of Phosphate and Tensin homolog (PTEN) downregulated the PI3K/Akt pathway that is correlated to resistance to said TKIs [72].
• Combination of pioglitazone with clarithromycin and a relatively small dose of chemotherapeutic agent was compared against nivolumab in the ModuLung trial by Heudobler et al. (2021) [73]. This trial was terminated early due to the approval of checkpoint inhibitors as first line treatment, with the conclusion that nivolumab was superior to this combination therapy however with difference of the overall survival rate and quality of life between the two regimens being similar, the latter seems to be a viable alternative to be assessed in future trials and be considered in cases that there are few other options [73].

Granted that inactivation of PPARγ combined with increased presence of prosta-glandins is a dangerous sign as it leads to loss of control over Ras/Raf/Mek activation and NF-Κβ mediated proliferation, Kiran et al. (2022) tested pioglitazone along with COX-2 specific antagonist celecoxib on rodents. The results were coherent with the hypothesis as decrease in tumor size, alterations in tumor architecture and increase of lifespan as well as improvement of its quality were noted [74].

However, PPARγ also possesses tumor promoting effects regarding lung cancer [75], such as the case of increased tumor progression in orthotropic mice after systemic PPARγ activation in the cancer cells as well as the tumor microenvironment [59], and, as such, additional studies regarding the exact effects of pioglitazone in a clinical set-ting and risk/benefit assessment in the given context are needed.

3.6.3. Renal Cancer

It has been noted that the antidiabetic use of pioglitazone is not associated with elevation in the risk of kidney cancer [76]. Pioglitazone itself induced apoptosis in Caki cells, a model used for the study of clear cell renal cell carcinoma, as shown in Table 1 [54]. Such effects are expected considering the aforementioned as well as the elevated presence of PPARγ receptors in renal carcinoma cells. Another cell line, 769-P cells, was heavily affected in a dose-dependent manner with imminent apoptosis as the ap-plication of the molecule led to significant morphological alterations, including cell density. The suggested mechanism of apoptosis, with limited proof, in this case, was mitochondrial with membrane irregularities and release of cytochrome c. This cell line also exhibited increased sensitivity to methotrexate after treatment with pioglitazone, something that was not evident in the Vero cell line [77]. Finally, there is a lot of interest in the combination of pioglitazone with cisplatin as it showcased nephroprotective effects in rodent models by preventing tissue damage and oxidative stress and it potentiated the latter’s effects on renal adenocarcinoma cells [78].

3.6.4. Hepatocellular Carcinoma

Assessing the preventative potential of pioglitazone, metanalytical results indicate that the use of TZDs in patients with T2DM is associated with protection against liver cancer [79]. Complimentary, rodent studies indicated prevention of liver fibrosis and carcinogenesis following cirrhosis [80]. Results originating from preclinical studies showcase pioglitazone’s anticancer properties. The increased presence of Receptor for Advanced Glycation End products (RAGE) is deemed as a negative biomarker for the progression of Hepatocellular Carcinoma which possibly indicates invasion. Pioglitazone decreased the expression of RAGE along with HMGB1, NF-κβ, and p38MAPK resulting in reduced proliferation, invasion, and metastatic potential [81]. Additionally, it was reported that metabolomic alterations triggered by pioglitazone, mostly regarding the metabolism of lipids, leading to the death of chemically induced hypoxic HepG2 cells from oxidative stress [82].

3.6.5. Colorectal Cancer

Similar to hepatocellular carcinoma, it has been indicated that TZDs reduce the risk of colorectal cancer occurrence in patients with T2DM after metanalytical assessment of about 2.5 million T2DM patients [83]. Direct effects were reported during in vitro experiments with pioglitazone and, its analog, Δ2-pioglitazone treated HCT116 and HT29 cells. There were similarities in the endpoints such as cell growth arrest and differentiation in addition to key differences in the ways those were achieved, namely arrest in the S phase when treated with the former and in the G0/G1 phase when treated with the latter [84]. Additionally, xenograft studies on rodents using HT29 and SW480 cell lines indicated antiproliferative and antimetastatic activity, towards the liver activity [85]. As an adjuvant, pioglitazone can be used in immunotherapy due to the autophagic degradation of PD-L1 mediated by PPARγ whose binding with the immunomodulatory ligand is similarly enhanced by said xenobiotic agent in the case of lung cancer [86]. Investigations in the field of pharmaceutics addressing pioglitazone’s poor solubility have shown that its combination with capecitabine, an established chemotherapeutic molecule against colorectal cancer, in the form of nanoparticles enhances its bioavailability and overall proapoptotic effect on HT29 and HCT119 cells [87].

3.6.6. Thyroid Cancer

The use of pioglitazone has been correlated with neither increased nor decreased risk of thyroid cancer according to a study comprising mostly Eastern Asian individuals [88]. A widely studied finding is the PAX8-PPARγ Fusion Protein (PPFP) and how pioglitazone alters its function. PPFP is a result of a fused gene containing the significant majority of the Paired Box Gene 8 (PAX8) transcription factor, related to thyroid development and function, gene, and the entirety of the PPARγ gene and, as such, a binding site for TZDs [89]. As evident by the imminent intracellular lipid accumulation and the expression of adipocyte-related biomarkers upon treatment, pioglitazone induces the differentiation of thyroid cancer cells into adipocytes. Moreover, it is accepted that pioglitazone blocks the interaction between PPFP and Thyroid Transcription Factor 1 (TTF-1) which otherwise leads to the severe limiting of the differentiation induction by the former via suspension of recruitment of synergistic molecules such as coactivators and corepressors [90]. Considering immunity, pioglitazone also advanced the infiltration of T cells and macrophages in the tumor microenvironment when test-ed on mice and exhibited favorable properties in a clinical setting [91]. Unfortunately, Giordano et al. (2018) in their examination of 40 thyroid cancer patients found only one tested positive for PPFP, potentially indicating that it is a rare phenomenon in these cases [92]. Finally, studies have suggested that pioglitazone and metformin have a synergistic effect against thyroid cancer, especially in the induction of apoptosis [93].

3.6.7. Glioma

As mentioned before, pioglitazone can cross the blood-brain barrier, a capability that can be utilized in the case of glioma, as it is one of the most aggressive brain malignancies. Cytotoxic activity has been confirmed as it has been reported to reduce glioma cell proliferation in vitro in the U87MG, T98G and U251MG cell lines, as well as LN-229 cells in a xenograft model [94,95]. However in another xenograft study using Gl261 cells, pioglitazone seemed to significantly alter the fate of the test subjects only when injected into the cerebrum [96]. Antineoplastic action can be by conventional, PPARγ dependent and independent pathways previously mentioned, namely alterations in the levels of cyclin D1, MMP9, N-Cadhenin and caspase 3, in addition to β-catenin inhibition as revealed by knockout studies [97,98]. Invasiveness is also affected as it was limited to C6 rat glioma cells [99]. Complimentary to this is the evident increase in the expression of the Excitatory Amino Acid Transporter 2 (EAAT2) in the same cell lines, except T98G. Lack of this transporter is commonly found in glioma cells as the resulting elevation of extracellular glutamate concentration leads to excitotoxicity, assisting in the thriving of malignant cells, as an excitatory neurotransmitter, and sets setting the field for tumor associated epilepsy (TAE). As such, by increasing EAAT2 levels in glioma cells, pioglitazone worsens the conditions for the development of glioma and potentially the associated symptoms such as TAE [100]. Glioma Stem Cells (GSC) do not seem to be immune from the effects of pioglitazone either. In vitro tests by Cilibrasi et al. (2016) involving treatment of GSCs revealed that G166, GliNS2, GBM2 and G144 (with latent effects) cells reduced their metabolic rate and alteration in the proteomic profile regarding the expression of markers related to dif-ferentiation and stemness. These changes in the behavior of the cells were not accom-panied by any alterations in their morphology [101]. Data originating from the clinic reveal a preventative relationship between pioglitazone and glioma [94]. A synergy between TZDs, with pioglitazone being the most potent of them, and statins has been suggested as there was observed a significant reduction in the population, in in vitro tests by Tapia-Perez et al. (2010) using U87 cells, even in hypoxic conditions, using the U87 and RG II cell lines [102]. Follow-up rodent model tests, also by Tapia Perez et al. (2016), revealed no significant differences in overall survival rate compared to the controls and that the only combination displaying reduction in tumor size was lovastatin/atorvastatin + pioglitazone [103]. Related to cancer care, pioglitazone has also been found to protect against radiation induced cognitive decline in patients with brain malignancy and is relatively safe for use in radiation treatment [104].

3.6.8. Hematological Malignancies

There is a great amount of interest in the potential of pioglitazone in blood cancers. Studies conducted on U937 cells, modeling Acute Myeloid Leukemia (AML) cancer cells, by Esmaeili et al. (2021) have shown in their results that pioglitazone damp-ened their survival and development capabilities, led to accumulation of cells in the G1 and pre-G1 cell cycle phases and induction of apoptosis, as indicated by the in-crease in the population of cells that test positive for Annexin-V and PI [105]. Similar targeted antiproliferative effects were also observed on HL60 (modeling Acute Pro-myelocytic Leukemia-APL), K562 (modeling Chronic Myeloid Leukemia) and Jurkat (modeling T-cell lymphoma) cells and the cell cycle of the HL60 line also demonstrated accumulation in the G1 phase and decrease of the G2 and Metaphasis populations [106]. Jurkat cells were also assessed with the combination of pioglitazone and valproic acid with optimistic results as cell cycle deregulation was noted [107]. According to in vitro studies in different types of leukemia ─AML in U937 cells, APL in Jurkat cells, ALL in SD-1, IM-9, Sup-B15, and NALM-6 cells─ the primary mode of action for pioglitazone is cell cycle arrest [108]. Adding to the above are the tests on Philadelphia chromosome positive leukemic cells by Okabe et al. (2017) that reported the antineoplastic effects of pioglitazone on cells bearing the T315I mutation while sparing nor-mal CD34 expressing cells and inducing the phosphorylation of the AMP-activated protein kinase (AMPK) [109]. The combination of pioglitazone with PI3K inhibitors can potentiate its cytotoxic effects as was observed on NB4 cells (APL) when they were treated with pioglitazone and PI3K inhibitors CAL-101 and BKM120. The suggested mechanism of synergy is the cell cycle arrest, as analyzed before, mediated by p21 [110]. However, in NALM-6 cell line experiments, where similar synergy was noted, it was found that said effects are interfered with by NF-κβ signaling and autophagy [111]. Potential synergy was also noticed in more conventional chemotherapeutics with a statistically insignificant remission rate being reported for cytarabine and daunorubicin in the clinical study conducted by Ghadiany et al. (2019) [112]. Additionally, as noted in Table 1, pioglitazone is correlated with elevated effectiveness of arsenic trioxide [113]. Adjuvant use of pioglitazone has been proven to positively in-fluence interventions utilizing BCR-ABL1 tyrosine kinase inhibitors (TKIs) in Chronic Myeloid Leukemia (CML). In vitro experiments carried out on K562 cells by Glodkowska-Mrowka et al. (2016) resulted in the eradication of leukemia stem cells, progenitors, and differentiated CML cells [114,115]. A small phase II clinical trial by Rousselot et al. (2016) involving 24 subjects treated with imatinib who were administered pioglitazone at a dosage ranging from 30mg/d to 45mg/d exhibited that about half of the patients showed response within one year, with no significant adverse effects, other than edema, higher than the rate of 23% in monotherapy with imatinib, thus suggesting correlation between the synergy of pioglitazone and favorable outcomes [116]. Additional discontinuation trials such as EDI-PIO in Brazil have raised the issue of alterations in the lipid metabolism and metabolomic profile of the patients, given the effects of pioglitazone on the adipose tissue, and the aspect of mitochondrial dysfunction that influences the proliferation of CML cells [107]. However, results, also originating from the same study, suggest that pioglitazone does not affect STAT5 expression and, therefore, the levels of downstream molecules to it such as HIF-2α, contrary to the statements of previous reports [117].

3.6.9. Pancreatic Cancer

There has been contradictory evidence regarding the association between the antidiabetic use of pioglitazone and the elevation of the risk of pancreatic cancer. As such, the effect the molecule has on pancreatic cancer occurrence, and the extent of it, is yet to be determined [118,119,120]. Nevertheless, antitumor activity has been observed in cell lines modeling pancreatic cancer including Capan-1, Apsc-1 BxPC-3, PANC-1, and MIApaCa-2. Xenograft studies involving BxPC-3 revealed that antiproliferative and antimetastatic properties as tumor size was limited along with lymph metastasis [121]. Finally, pioglitazone has been reported to enhance the anticancer effects of gemcitabine through inhibition of the NF-κβ, induction of apoptosis, and setting of the platform to include HDAC inhibitors that have proven to be helpful in the activity of PPARγ [122].

3.7. Immunotherapy

PPARγ immunomodulatory effects have long been known and have been multiple studies seeking to assess the significance of said effects in cancer. A study conducted on mice hosting solid Ehrlich carcinoma cells by El-Sisi et al. (2014) concluded that pioglitazone administration led to a statistically important reduction in tumor size and TNF-α amount, increase of peripheral neutrophil and splenic T- lymphocyte populations, increase of splenic T-lymphocyte activity, CD4+/CD8+ ratio and Immunoglobin G (IgG) levels, indicating elevated immune response. Similar results, although to a varying extent and except peripheral neutrophil counts, were also obtained from co-administration with doxorubicin [123]. Immunomodulation is a property that can be exploited in the context of immunotherapy. An example of it is the use of PPARγ agonists to reprogram T cells, however there were no specific reports on pioglitazone [124].

4. Discussion

Concerns have been raised regarding the elevated risk of bladder cancer which has been reported since 2012. This can be attributed to overexpression of PPARγ which are present in the urinary bladder and several other tissues. This adverse event is prevalent especially in cases of prolonged pioglitazone administration, indicating a time-dependent and dose-dependent pharmacological profile. However, analysis does not suggest a strong dose-dependence relationship, since bladder cancer risk can be at-tributed to comorbidities and administration of antidiabetic drugs, such as metformin. Existing studies suggest a slightly but significantly elevated risk of bladder cancer as-sociated with pioglitazone administration in a time- and dose-dependent manner [40]. Due to the structure of these studies, there is no conclusive data regarding the risk-benefit ratio of pioglitazone. Up to date, there is no study that proves the direct cause-effect relationship between pioglitazone and bladder cancer [41].

Similar concerns have been raised regarding the association of metformin with risk of bladder cancer. While metformin does not decrease bladder cancer risk, it im-proves clinical parameters such as recurrence-free survival (RFS), progression-free survival (PFS) and cancer-specific survival (CSS) [125]. Although no effect was reported between overall survival (OS) and bladder cancer, studies show a positive outlook [126]. Another meta-analysis conducted in bladder cancer patients showed neither a protective nor a harmful effect of metformin exposure [127]. Few reports indicate decreased bladder cancer risk in metformin users versus never-users. However, the link between metformin and bladder cancer risk is not clear due to co-administration of other drugs and small proportion of metformin users versus never-users [128]. Overall, meta-analyses show no association of metformin with bladder cancer risk. Considering that these studies are limited to T2DM or bladder cancer patients, a consensus cannot be drawn regarding the risk of bladder cancer in the general population.

5. Conclusions

Based on the aforementioned, it is safe to assume that pioglitazone, as well as other TZDs, possess antineoplastic agents properties. PPARγ receptors, being implicated in multiple pathways, lead to diverse results in tumors. Although in vitro studies have been carried out with positive results, the adverse events presented indicate that additional studies are required in order to fully assess the risk to benefit ratio in an evidence based manner.