Nanotherapy and Reactive Oxygen Species (ROS) in Cancer: A Novel Perspective

The incidence of numerous types of cancer has been increasing over recent years, representing the second-most frequent cause of death after cardiovascular diseases. Even though, the number of effective anticancer drugs is increasing as well, a large number of patients suffer from severe side effects (e.g., cardiomyopathies) caused by these drugs. This adversely affects the patients’ well-being and quality of life. On the molecular level, tumor cells that survive treatment modalities can become chemotherapy-resistant. In addition, adverse impacts on normal (healthy, stromal) cells occur concomitantly. Strategies that minimize these negative impacts on normal cells and which at the same time target tumor cells efficiently are needed. Recent studies suggest that redox-based combinational nanotherapies may represent one option in this direction. Here, we discuss recent advances in the application of nanoparticles, alone or in combination with other drugs, as a promising anticancer tool. Such novel strategies could well minimize harmful side effects and improve patients’ health prognoses.

developed and partly already tested in clinical trials [44]. One of the promising applications in nanomedicine is the use of nanoparticles (<100 nm in size) as carrier system for drug delivery to specific cellular targets or organs, for example the transport to mitochondria [45] or to the brain [46]. Here are just four of the many examples that could be mentioned, bringing to mind the tremendous potential of nanocarriers in anticancer therapies. In an interesting approach using a rat glioma model, the acidic tumor microenvironment was exploited to transport pH-sensitive doxorubicin-loaded PEGylated-gold nanoparticles to the tumor site, resulting in rapid release of doxorubicin at low pH and in an increase of ROS level [47]. Recently, doxorubicin-loaded lipid nanoparticles were tested for topical treatment of skin cancer. Cytotoxicity data with murine melanoma cells and the histological analysis of melanoma induced Balb/C mice showed promising results with those nanoparticles [48]. Furthermore, encapsulation of the platinum(IV) prodrug mitaplatin in block copolymer nanoparticles resulted in an increase in drug circulation time in blood and in a controlled drug release [49]. Due to their nontoxic nature, carbon-based nanoparticles (carbon dots, carbon nanotubes) are used as platform to deliver drugs such as doxorubicin and gemcitabine to the tumor site [50]. However, most of these nanoparticles still face tremendous challenges regarding their limited biocompatibility and subsequent low cytotoxicity in some types of tumor cells [45,51]. Meanwhile, the application of nanoparticles, which themselves act directly as pharmacological substances (nanopharmaceutical), appears to be another therapeutic possibility for the treatment of some cancer types. In this context, especially the lanthanide cerium in the form of polymer-coated or polyacrylate-stabilized cerium oxide (CeO 2 /Ce 2 O 3 ) nanoparticles (CNP, nanoceria), shows promising properties regarding a redox-modulatory and enzyme-like activity. Due to oxygen vacancies on the surface of CNP and the ability to autocatalytically switch between the oxidation states IV (Ce 4+ ) and III (Ce 3+ ), CNP function as a pro-or antioxidant based on the microenvironment [52][53][54][55]. Such nanopharmaceuticals can be used in a redox-based chemotherapy as "stand-alone drugs", or in combination with classical chemotherapeutical agents, like the successful approaches with doxorubicin (see below). In recent years, an increasing number of articles have dealt with nanoceria as an antioxidant-detoxifying superoxide, hydrogen peroxide, and peroxynitrite and, therefore, as having a beneficial effect on different cells and tissues in vitro [56][57][58][59][60]. For example, CNP protect human dermal fibroblasts from the redox cycler paraquat (PQ) initiated increase of superoxide (O 2 − ) level [56]. The uptake of nanoceria results in a subcellular distribution ( Figure 1). Interestingly, it seems that CNP do not enter the nucleus and, thus, exert no genotoxicity which is beneficial for normal, healthy cells [11]. On the other hand, CNP were shown by others to be present in the nucleus and, notably, also to be co-localized with ROS-producing mitochondria [61].
Antioxidants 2018, 7, x FOR PEER REVIEW 3 of 8 approaches are currently developed and partly already tested in clinical trials [44]. One of the promising applications in nanomedicine is the use of nanoparticles (<100 nm in size) as carrier system for drug delivery to specific cellular targets or organs, for example the transport to mitochondria [45] or to the brain [46]. Here are just four of the many examples that could be mentioned, bringing to mind the tremendous potential of nanocarriers in anticancer therapies. In an interesting approach using a rat glioma model, the acidic tumor microenvironment was exploited to transport pHsensitive doxorubicin-loaded PEGylated-gold nanoparticles to the tumor site, resulting in rapid release of doxorubicin at low pH and in an increase of ROS level [47]. Recently, doxorubicin-loaded lipid nanoparticles were tested for topical treatment of skin cancer. Cytotoxicity data with murine melanoma cells and the histological analysis of melanoma induced Balb/C mice showed promising results with those nanoparticles [48]. Furthermore, encapsulation of the platinum(IV) prodrug mitaplatin in block copolymer nanoparticles resulted in an increase in drug circulation time in blood and in a controlled drug release [49]. Due to their nontoxic nature, carbon-based nanoparticles (carbon dots, carbon nanotubes) are used as platform to deliver drugs such as doxorubicin and gemcitabine to the tumor site [50]. However, most of these nanoparticles still face tremendous challenges regarding their limited biocompatibility and subsequent low cytotoxicity in some types of tumor cells [45,51]. Meanwhile, the application of nanoparticles, which themselves act directly as pharmacological substances (nanopharmaceutical), appears to be another therapeutic possibility for the treatment of some cancer types. In this context, especially the lanthanide cerium in the form of polymer-coated or polyacrylate-stabilized cerium oxide (CeO2/Ce2O3) nanoparticles (CNP, nanoceria), shows promising properties regarding a redox-modulatory and enzyme-like activity. Due to oxygen vacancies on the surface of CNP and the ability to autocatalytically switch between the oxidation states IV (Ce 4+ ) and III (Ce 3+ ), CNP function as a pro-or antioxidant based on the microenvironment [52][53][54][55]. Such nanopharmaceuticals can be used in a redox-based chemotherapy as "stand-alone drugs", or in combination with classical chemotherapeutical agents, like the successful approaches with doxorubicin (see below). In recent years, an increasing number of articles have dealt with nanoceria as an antioxidant-detoxifying superoxide, hydrogen peroxide, and peroxynitrite and, therefore, as having a beneficial effect on different cells and tissues in vitro [56][57][58][59][60]. For example, CNP protect human dermal fibroblasts from the redox cycler paraquat (PQ) initiated increase of superoxide (O2 − ) level [56]. The uptake of nanoceria results in a subcellular distribution ( Figure 1). Interestingly, it seems that CNP do not enter the nucleus and, thus, exert no genotoxicity which is beneficial for normal, healthy cells [11].
On the other hand, CNP were shown by others to be present in the nucleus and, notably, also to be colocalized with ROS-producing mitochondria [61]. Hirst and coworker detected CNP around the mitochondria which subsequently scavenge mitochondria-generated ROS in normal murine macrophages [62]. Furthermore, nanoceria were  Hirst and coworker detected CNP around the mitochondria which subsequently scavenge mitochondria-generated ROS in normal murine macrophages [62]. Furthermore, nanoceria were shown to protect mitochondria from mitochondrial fragmentation (fission) in primary skin fibroblasts [63]. Nanoceria caused an increased release of cytochrome c from mitochondria into the cytosol in tumor cells [64]. Overall, CNP appear to act inter alia via modulating mitochondrial function and/or mitochondrial ROS generation. In skin cancer and glioma cells, exogenously added CNP with a defined size have been shown to act as a cell-killing and anti-invasive agent via increasing the level of distinct reactive oxygen species like hydrogen peroxide. In contrast, the same concentration of CNP is nontoxic in stromal (healthy) fibroblasts or endothelial cells [64][65][66], suggesting a bifunctional role of nanoceria in tumor-stroma (Scheme 1). In this context, in a combinational approach with the chemotherapeutical drug doxorubicin, CNP enhance the ROS-mediated toxicity of doxorubicin in different cancer cells, while CNP exhibit some protection against doxorubicin mediated cell death in human dermal fibroblasts and rat cardiomyocytes [11,67]. In addition to the increasing number of publications with in vitro data, more and more in vivo data are available today. To quote only three, the infiltration of immune cells and the expression of proinflammatory cytokines, which goes hand-in-hand with ventricular dysfunction and dilatation, can be significantly lowered by the use of cerium oxide nanoparticles, studied on a murine model of cardiomyopathy [68]. Our studies on a xenograft mouse model show that CNP significantly lower tumor growth and invasion and, furthermore, inhibit some processes of neoangiogenesis [64]. Recently, folic acid-tagged cerium oxide nanoparticles have been shown to increase the cellular nanoparticle internalization and inhibit cell proliferation of an ovarian cancer cell line and significantly lower the tumor burden of such mouse xenografts [69]. Aside from the use in cancer research, it was shown in animal models that mitochondria-targeted cerium oxide nanoparticles protect against amyloid beta (Aβ) induced mitochondrial fragmentation and cell death in a model for Alzheimer's disease [70,71]. shown to protect mitochondria from mitochondrial fragmentation (fission) in primary skin fibroblasts [63]. Nanoceria caused an increased release of cytochrome c from mitochondria into the cytosol in tumor cells [64]. Overall, CNP appear to act inter alia via modulating mitochondrial function and/or mitochondrial ROS generation. In skin cancer and glioma cells, exogenously added CNP with a defined size have been shown to act as a cell-killing and anti-invasive agent via increasing the level of distinct reactive oxygen species like hydrogen peroxide. In contrast, the same concentration of CNP is nontoxic in stromal (healthy) fibroblasts or endothelial cells [64][65][66], suggesting a bifunctional role of nanoceria in tumor-stroma (Scheme 1). In this context, in a combinational approach with the chemotherapeutical drug doxorubicin, CNP enhance the ROSmediated toxicity of doxorubicin in different cancer cells, while CNP exhibit some protection against doxorubicin mediated cell death in human dermal fibroblasts and rat cardiomyocytes [11,67].
In addition to the increasing number of publications with in vitro data, more and more in vivo data are available today. To quote only three, the infiltration of immune cells and the expression of proinflammatory cytokines, which goes hand-in-hand with ventricular dysfunction and dilatation, can be significantly lowered by the use of cerium oxide nanoparticles, studied on a murine model of cardiomyopathy [68]. Our studies on a xenograft mouse model show that CNP significantly lower tumor growth and invasion and, furthermore, inhibit some processes of neoangiogenesis [64].
Recently, folic acid-tagged cerium oxide nanoparticles have been shown to increase the cellular nanoparticle internalization and inhibit cell proliferation of an ovarian cancer cell line and significantly lower the tumor burden of such mouse xenografts [69]. Aside from the use in cancer research, it was shown in animal models that mitochondria-targeted cerium oxide nanoparticles protect against amyloid beta (Aβ) induced mitochondrial fragmentation and cell death in a model for Alzheimer's disease [70,71]. Aside from the fact that a planned personalized (anticancer) medicine in the future still presents many questions and challenges, it appears that other possible anticancer strategies are already available, dealing with major challenges such as killing the tumor cells without damaging normal (healthy, stromal) cells, and lowering harmful effects to improve prognosis and quality of life of the patient. In particular, a ROS-modulating nanotherapy could play an important role in the 21st century. A nanoparticle-based anticancer therapy with either specific nanocarriers or nanopharmaceuticals, which have tremendous potential in a combination therapy, could support classical anticancer strategies such as the use of ROS-producing anthracyclins. Such a nanoparticle-based combination therapy could fulfill the above-mentioned anticancer guideline of killing tumor cells and leaving normal cells intact with less side effects. Certainly, other combinational anticancer approaches, which are independent of the use of nanoparticles, but also depend on modulation of the ROS level, will be discussed to be promising for an anticancer therapy [72].

Conclusions
In conclusion, special nanoparticles such as cerium oxide nanoparticles (nanoceria) appear to be a promising and powerful tool for the development of a nanoparticle-based redox-directed combinational anticancer therapy in the (near) future that is accessible to all classes of society. Such an approach may combine the high efficacy of chemotherapeutical drugs, such as the anthracyclines, with the drug supporting effect (on tumor cells) and protecting potential (on normal cells) of bifunctionally active nanoparticles. However, more data from animal models and current and future clinical trials will show whether the nanomedicine will meet the expectations generated by the recent promising results in cancer research as well as in other fields including neurodegeneration.
Author Contributions: Peter Brenneisen is working on ROS effects and the effect of redox-active nanoparticles in tumor-stroma interactions. He is mainly responsible for the idea and the content of that article. Andreas S. Reichert is working on mitochondrial morphology and mitochondrial dynamics in health and disease. He provided complementary aspects which strengthen the proposed opinion.

Conflicts of Interest:
The authors declare no conflict of interest.