Nanomedicine targets iron metabolism for cancer therapy

Abstract Iron is an essential element for cell proliferation and homeostasis by engaging in cell metabolism including DNA synthesis, cell cycle, and redox cycling; however, iron overload could contribute to tumor initiation, proliferation, metastasis, and angiogenesis. Therefore, manipulating iron metabolisms, such as using iron chelators, transferrin receptor 1 (TFR1) Abs, and cytotoxic ligands conjugated to transferrin, has become a considerable strategy for cancer therapy. However, there remain major limitations for potential translation to the clinic based on the regulation of iron metabolism in cancer treatment. Nanotechnology has made great advances for cancer treatment by improving the therapeutic potential and lowering the side‐effects of the proved drugs and those under various stages of development. Early studies that combined nanotechnology with therapeutic means for the regulation of iron metabolism have shown certain promise for developing specific treatment options based on the intervention of cancer iron acquisition, transportation, and utilization. In this review, we summarize the current understanding of iron metabolism involved in cancer and review the recent advances in iron‐regulatory nanotherapeutics for improved cancer therapy. We also envision the future development of nanotherapeutics for improved treatment for certain types of cancers.

with the development of cancer in humans. Cancer cells reshape the pathways of iron metabolism to meet the high iron demand during tumor growth and malignancy. 3 The vigorous aerobic glycolysis process of tumor cells is closely related to iron metabolism. However, it is still debatable whether iron dysregulation is the cause or the consequence of tumor development, although iron metabolism is well recognized to be perturbed in cancer cells. 4,5 Iron also plays a vital role in tumor progression and metastasis due to its role in tumor cell survival and tumor microenvironment reprogramming. 6 Thus, iron metabolism-targeted therapy, such as iron depletion, has been explored as an effective strategy for intervening in oncogenesis and tumor metastasis. 7,8 Iron chelators have been widely used to treat iron overload diseases and have attracted increasing attention for cancer therapy.
Previous studies have reported that iron chelators, such as deferoxamine (DFO), deferiprone, ciclopirox, or deferasirox, could significantly inhibit cancer cell proliferation by inducing cell cycle arrest and apoptosis. 9 Although iron chelators have shown great potential in preclinical cancer models, small-molecule iron chelators could cause adverse side-effects, such as infection, gastrointestinal bleeding, renal failure, and liver fibrosis. 10 In addition, the lack of tumor cell specificity and poor pharmacokinetics also limit their therapeutic potential and further clinical application. 11 Therefore, overcoming these challenges is demanded to improve the therapeutical efficacy of iron chelators for tumor treatment.
In addition to iron depletion therapy mediated by iron chelators, ferroptosis has attracted considerable interest due to its involvement in immunity, development, and various pathological scenarios as a novel iron-dependent nonapoptotic cell death. 12 The activity of ferroptosis is mainly associated with the bioavailable Fe 2+ . Emerging evidence indicates that the iron-mediated Fenton reaction plays an important role in ferroptosis induction.
Fe 2+ is reduced from imported ferric iron (Fe 3+ ) by duodenal cytochrome B and transported by the divalent metal transporter 1 in the endosome. Free Fe 2+ might participate in the Fenton reaction together with hydrogen peroxide (H 2 O 2 ), resulting in the production of lethal ROS and regeneration of Fe 3+ . Therefore, the investigation of ferroptosis that specifically targets cancer cells highlights the promising role of ferroptosis induction in cancer treatment. Ferroptosis-based cancer therapy has been reported to bypass the drawbacks of well-established cancer therapeutics (eg, chemotherapy or radiotherapy), as well as emerging cancer immunotherapy. Therefore, targeting iron metabolism and inducing ferroptosis have both been regarded as promising strategies for cancer therapy. However, some ferroptosis inducers, such as ferroptosis-suppressing cystine/glutamate antiporter system inhibitors, hardly reach sufficient concentrations in targeted tumor tissues, resulting in minimal therapeutic effect. 13 Additionally, it is possible that systemic inhibition of iron metabolism would generate off-target effects, which leads to undesirable toxicities.
Due to its multifunctional capacity and diverse biological activities, nanoenabled drug encapsulation and delivery technology have been explored to provide an innovative therapeutic regime in the treatment of cancer. [14][15][16][17] By increasing efficacy and reducing adverse side-effects of cytotoxic drugs, some liposomes and albumin-based therapies have been approved by the US FDA and successfully improved the clinical performance of chemotherapeutics in cancer treatment, such as Doxorubicin hydrochloride and Paclitaxel. 18 With the growing understanding of human physiology and pathology, many multifunctional nanomaterials have been designed and synthesized for the delivery of different types of drugs, including nucleic acid drugs, to enhance tissue and cell targeting and improve drug stability, pharmacokinetics, and biodistribution in cancer therapy. In addition, other novel therapeutics, for example, chemodynamic therapy, can induce tumor cell death and encourages attention for tumor-specific therapy through decomposing intertumoral ROS that rely on iron-mediated Fenton or Fenton-like reactions, which utilize Fe 2+ and Fe 3+ in this process. 19 Therefore, iron metabolism-based nanomedicine and the corresponding combination therapy could provide a novel paradigm for cancer treatment. In this review, we summarize the recent progress in iron metabolism-based cancer therapeutic strategies with a focus on iron chelator-based nanostructures and ferroptosisinducing nanotherapeutics (Table 1).

| SUMMARY OF THE MOLECUL AR MECHANIS M OF IRON PERTURBATION IN TUMORIG ENE S IS AND TUMOR PROLIFER ATI ON
Recent studies have indicated that iron overload is associated with the pathogenesis and progression of many human diseases, including cancer. [20][21][22][23] Increasing evidence reveals that iron overload contributes to tumor initiation and is associated with an increased risk of tumor metastasis. [24][25][26] The accumulation of iron, as well as active oxygen/nitrogen and aldehydes catalyzed by iron, leads to DNA chain scission for tumorigenesis. 27 Patients with serous epithelial ovarian cancer have shown increased hemosiderin in the fallopian tubes along with elevated iron overload-mediated oxidative stress and frequent DNA mutations. 28,29 Persistent iron overload also inhibits the activity of a classical tumor suppressor, p53 protein, to promote oncogenesis and tumor metastasis. 30 Iron also regulates key signaling pathways in tumors, including hypoxia-inducible factor (HIF) and Wnt signaling pathways that promote tumor survival, progression, and invasion.
Compared with normal cells, rapid tumor cell growth is largely dependent on iron. The rates of iron uptake and iron consumption are both accelerated simultaneously, leading to a high metabolic level of iron. 31 The disorder of iron metabolism in cancer cells is related to the upregulation of key genes responsible for cellular iron absorption and the downregulation of genes related to iron efflux. 4

| Iron nanochelating agents
Nanotechnology-based iron chelators have been extensively studied in cancer therapy. 9 Novel iron nanochelators based on di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT) have shown promise for the treatment of highly aggressive malignant tumors, for example, brain gliomas. 37 Deferoxamine, the most commonly used iron chelator approved by the FDA, 38 has recently been proposed as a potential therapeutic drug for patients with neuroblastoma, leukemia, prostate cancer, and hepatocellular carcinoma through specifically chelating iron in tumor cells. 39 However, DFO has an extremely short half-life of approximately 20-30 minutes in human plasma so that it must be infused continuously for 8-24 h per day for several days each week, which hampers its potential applications due to its arduous regimen for patients and low compliance. Systemically administered DFO can hardly reach therapeutic concentration and target tumors at sufficient dosage, and local DFO can induce overexpression of HIF1α for tumor survival, which limits its use as an effective antitumor agent in clinical therapy. 40

| Nanotherapeutics harnessing Fenton reaction for ferroptosis induction
Ferroptosis, discovered in 2012, is a type of programmed cell death different from apoptosis, autophagy, and necrosis. It is an irondependent type of nonapoptotic cell death driven by cell metabolism and iron-dependent lipid peroxidation 42 and plays an important role in the pathogenesis and progression of cancer. [43][44][45] Circulating Fe 3+ is introduced into cells through TFR1 and then converted into Fe 2+ in the endosome. Excessive Fe 2+ will produce ROS and cause lipid peroxidation through the Fenton reaction, which will lead to ferroptosis. 46 Iron oxide nanoparticles have been found to inhibit the pro-

| Ferroptosis-inducing cancer nanomedicine combined with chemotherapy
The antitumor mechanism of chemotherapeutic drugs depends on apoptosis induction. Therefore, the combination of chemotherapy with ferroptosis, a different type of programmed cell death, will increase the therapeutic efficacy against cancer growth through synergistic killing effects on cancer cells. A nanolongan delivery system, one core (upconversion nanoparticles, UCNP) in one gel particle

| Ferroptosis-inducing cancer nanomedicine combined with immune regulation
Immunotherapy has revolutionized cancer treatment and rejuvenated the field of tumor immunology through restoring and  A study has shown that combining high-temperature chemotherapy using iron nanoparticles with magnetic guidance represented a powerful method for cancer treatment. 69 In particular, biomimetic nanoparticles, due to their high biocompatibility and low toxicity, have received increasing attention in the field of cancer treatment, including iron-targeting cancer therapy. Exosomes secreted by cancer-associated fibroblasts have been found to be involved in the regulation of ferroptosis in cancer cells through microRNA-522, which provides a novel idea for improving the sensitivity of gastric cancer chemotherapy. 70 Functional ferritin nanoparticles that assembled by original or modified (ie, iron-deleted) ferritin subunits could target TFR1 overexpressed in the tumor microenvironment while delivering chemodrugs or MRI agents for tumor therapy or imaging purposes. 71 Although achievements have been made using nanoenabled iron therapy, the majority of iron-regulatory nanoparticles remain in the preclinical stage. Currently, thermal ablation of prostate cancer using iron nanoparticles, called Magnablate (developed by University College London Hospitals), is being trialed in the clinic (NCT02033447, ClinicalTrials.gov), but it has not yet been approved by the FDA. 72 Challenges are lining up to be overcome, such as improving the surface functionalization to enhance their active targeting, preventing drug leakage during circulation, optimizing the amount and speed of drug release in the lesion area, reducing the aggregation of metal nanoparticles, and reducing the adverse effects.

CO N FLI C T O F I NTE R E S T
The authors have no conflict of interest to declare.

E TH I C A L A PPROVA L
This work did not involve investigations on human subjects, nor experiments involving animals.