Iron is of great physiological and biological functions such as the transport and storage of oxygen and energy metabolism. Normally, the intracellular iron maintains a balance within a narrow range. However, the disordered distribution and content of iron perturb physiological processes and biological survival. Excessive iron can provoke subsequent lipid peroxidation either by the iron-mediated Fenton reaction to produce reactive oxygen species (ROS, a group of molecules derived from molecular oxygen, mainly including hydrogen peroxide (H₂O₂), singlet oxygen (¹O₂), superoxide anion (O₂ ^•–) and hydroxyl radicals (^•OH)), or the activation of enzymes containing iron (for example, lipoxygenases) (Xie et al., 2016a; Dixon and Stockwell, 2014). In general, the iron atom (Fe³⁺) binds to transferrin (TF) and binds with transferrin receptor (TFRC) through blood circulation to form the double iron-TF-TFRC complex, which is then endocytosed into the nucleus and reduced to ferrous (Fe²⁺) by endosomal six-transmembrane epithelial antigen of prostate 3 (STEAP3). Subsequently, Fe²⁺ is released from the nuclear endosome to the labile iron pool (LIP) in the cytoplasm via divalent metal transporter 1 (DMT1, SLC11A2). Excessive iron is stored in ferritin (the primary iron storage protein) which consists of ferritin light chain (FTL) and ferritin heavy chain 1 (FTH1). Finally, iron is exported from cells to blood through ferroportin (FPN, encoded by SLC40A1, the only known iron-efflux protein) in the cell membrane, and Fe²⁺ can be reoxidized by ferroxidases (such as ceruloplasmin or hephaestin) to Fe³⁺ (Torti and Torti, 2013). Alternatively, Fe²⁺ can be exported as ferritin through exosomes. Variable iron levels on account of interventions at multiple levels (such as iron absorption, storage, utilization, export and iron chelators) affect ferroptosis via an integrated signaling pathway (Figure 1). At the post-transcriptional level, iron regulatory protein 1 and 2 (IRP1; also known as ACO1, IRP2; also known as IREB2) can regulate the expression of DMT1, TFRC, ferritin and FPN (Torti and Torti, 2013) and thereby influence ferroptotic activity. Initiation of IREB2 promotes erastin-induced cytotoxicity, while inhibition of F-box and leucine-rich repeats protein 5 (FBXL5), an endogenous IREB2 antagonist, can sensitize cells to erastin (Reed and Pellecchia, 2012). Iron chelators, such as deferoxamine (DFO) (Dixon et al., 2012; Chen X. et al., 2020), dexrazoxane (DXZ) (Fang et al., 2019) and deferiprone (DFP) (Wu et al., 2020), can directly chelate iron and have been used in clinical treatment to reduce iron availability. As a result, these iron chelators are capable of relieving ferroptosis intrinsically or caused by proferroptotic reagents. Ferroptosis was originally identified as an autophagy-independent RCD (Xie et al., 2016a; Najafov et al., 2017). According to current studies, autophagy plays an important role in ferroptosis through the regulation of cellular iron levels. Autophagy recognizes and degrades ferritin into autophagosomes for lysosome-dependent degradation by using nuclear receptor coactivator 4 (NCOA4) as a selective cargo receptor. This process called “ferritinophagy”, which leads to increased iron accumulation and subsequent oxidative damage and ferroptosis. On the contrary, abrogating ferritinophagy by knockdown of autophagy-related genes or NCOA4 or inhibition of lysosomal function by specific inhibitors suppresses the availability of labile iron and decreases ferroptotic sensitivity (Mancias et al., 2014; Hou et al., 2016). In addition to NCOA4-mediated ferritinophagy, other forms of selective autophagy, including lipophagy, clockophagy, and chaperone-mediated autophagy, have been demonstrated to trigger ferroptosis in cells (Gao et al., 2016). Thus, intracellular iron homeostasis directly or indirectly targets ferroptosis, and the crosstalk of ferroptosis and autophagy provides new perspectives and contexts for the regulation of RCD.

Lipid peroxidation has been regarded as the hallmark in the context of ferroptosis. As materials of lipid synthesis, polyunsaturated fatty acids (PUFAs), especially arachidonic acid (AA) and adrenic acid (AdA), are vulnerable to the peroxidation process and incorporated into membrane phospholipids (PLs), which destroy the lipid bilayer to trigger the ferroptotic process (Stockwell et al., 2017; Kagan et al., 2017; Conrad and Pratt, 2019). Acyl coenzyme A synthase long-chain member 4 (ACSL4) connects coenzyme A (CoA) to long-chain PUFAs, preferentially AA or AdA, to form arachidonic acid-CoA (AA-CoA) or adrenic acid-CoA (AdA-CoA), respectively. Then PUFA-CoAs are esterified by lysophosphatidylcholine acyltransferase 3 (LPCAT3) to produce PUFA-containing phospholipids (PUFA-PLs). Particularly, phosphatidylethanolamines (PEs) with both AA and AdA (AA/AdA-PEs) are the significant PUFA-PLs for ferroptosis, and are susceptible to free radical trigged oxidation catalyzed by lipoxygenases (ALOXs). ALOXs are nonheme iron-containing dioxygenases that catalyze the oxidation of PUFAs to produce fatty acid hydroperoxides, with a context-dependent role in mediating lipid peroxidation (Kagan et al., 2017; Doll et al., 2017) (Figure 1). Based on this, many reports have uncovered the implications of ACSL4, LPCAT3 and ALOXs during ferroptosis and exploited some candidate compounds with ferroptotic activities (Ou et al., 2016; Colakoglu et al., 2018). ACSL4 knockdown or inhibition can significantly lower the levels of AA-CoA and AdA-CoA to prevent the destruction of membrane structure, and confer ferroptosis resistance. Conversely, supplementation with AA or other PUFAs can block the original ACSL4-mediated reaction, which promotes the occurrence of ferroptosis or sensitizes cells to ferroptosis induced by erastin or RSL3 (Yang et al., 2016; Shintoku et al., 2017). However, the actual driver of lipid peroxidation and subsequent ferroptosis is still controversial, as it has been shown that ALOX15 is not essential for ferroptosis in a mouse model of GPX4-mediated acute renal failure (Friedmann Angeli et al., 2014).

As a master repressor of ferroptosis, selenoenzyme glutathione peroxidase 4, GPX4, consumes two molecules of reductive GSH after being combined with lipids to reduce toxic phospholipid hydroperoxide (LOOH) to nontoxic phospholipid lipid alcohol (LOH), which simultaneously produces oxidative glutathione (GSSG) as a byproduct, and ultimately inhibits phospholipid lipid peroxidation. Once they enter the membrane, PUFAs which are peroxidized and not cleared up by GPX4, eventually drive a large amount of lipid peroxidation and ferroptosis (Yang et al., 2016; Kagan et al., 2017). Further studies have shown that GPX4 inactivation is observed in the ferroptotic death process in vitro and in vivo, whereas deletion of GPX4 with pharmacological or genetic approaches can induce ferroptosis execution (Hangauer et al., 2017; Guo et al., 2018a). Besides western blot or RT-PCR evidence, many studies have confirmed the function and activity of GPX4 in the regulation of ferroptosis using cell or mouse models (e.g. GPX4 knockout or overexpression) (Alvarez et al., 2017; Hangauer et al., 2017; Viswanathan et al., 2017). For example, GSH-depleting reagents (erastin and buthionine sulfoximine (BSO)) inhibits GPX activity to induce ferroptosis and the total activity of GPXs in cells is examined using tert-butylhydroperoxide (tBuOOH) as a substrate. Furthermore, 7a-cholesterol-OOH is a specific substrate for GPX4; no other GPX enzyme can catalyze the reduction of 7a-cholesterol-OOH (1S, 3R)-RSL3, a ferroptosis inducer, has been demonstrated to inhibit enzyme activity of GPX4, indicated by unchanged level of 7a-cholesterol-OOH (Yang et al., 2014). Additionally, it is shown that FINO₂ indirectly inhibits GPX4 enzymatic function while FIN56 directly depletes GPX4 enzymatic function using ¹H-¹⁵N heteronuclear single quantum coherence (HSQC) NMR spectroscopy, ultimately causing widespread lipid peroxidation and ferroptosis (Gaschler et al., 2018a). It should be noted that the GPX4 defense system is also linked to other non-ferroptotic RCDs such as apoptosis and necroptosis, suggesting that lipid peroxidation lies at the crossroads of these pathways with diverse functions and different downstream effectors (Ran et al., 2007; Canli et al., 2016). Additionally, GPX4 is necessary for tumor recurrence, indicating that eradicating drug-resistant cancer cells by targeting GPX4 to induce cellular ferroptosis is a promising clinical strategy (Rennekamp, 2017).

The cystine-glutamate antiporter system x_c ^--mediated GSH synthesis, representative of the capacity of antioxidation, plays a significant role in the accumulation of lipid peroxides and the occurrence of ferroptosis. The cystine-glutamate antiporter, a transmembrane transport protein, composed of SLC7A11 (xCT) and SLC3A2 (4F2hc), takes up cystine from outside the cell and correspondingly outputs glutamate for exchange at a 1:1 ratio (Dixon et al., 2014). Usually, cysteine under reductive extracellular conditions is transported directly into cells through ASC (a neutral amino acid transport system), while cystine under oxidative extracellular conditions is reduced to cysteine after entering cells through system x_c ^-. Subsequently, cysteine, glutamate, and glycine are catalyzed by γ-glutamylcysteine synthetase (GCLC) and glutathione synthase (GSS) respectively to form GSH, the major intracellular antioxidant (Doll and Conrad, 2017; Liang et al., 2019). The functional activity of GPX4 to reduce lipid hydroperoxides is dependent on GSH biosynthesis, because GSH acts as a cofactor for GPX4. GSH depletion leads to GPX4 inactivation, thereby triggering intracellular lipid peroxidation and ferroptosis. Therefore, system x_c ^- (SLC7A11) is a transportation hub that regulates intracellular redox balance and ferroptotic damage (Yang and Stockwell, 2016; Stockwell et al., 2017). The expression and activity of SLC7A11 is further positively modulated by nuclear factor erythroid 2-related factor 2 (NRF2), and negatively modulated by tumor suppressor genes, such as p53 and BAP1 (Carbone et al., 2013; Zhang Y. et al., 2018). This dual regulation constitutes a fine-tuning mechanism to delicately control and maintain GSH levels in ferroptosis. Consequently, ferroptosis is driven by GSH depletion either by restricting GSH biosynthesis (e.g., BSO inhibits GCLC) or by blocking cystine uptake from extracellular environment (e.g., erastin or sulfasalazine (SAS) inhibits system x_c ^-). As an alternative way apart from system x_c ^-, the transsulfuration pathway can supply cysteine as a compensation system under cystine deprivation, which is negatively regulated by cysteinyl-tRNA synthase (CARS). Loss of CARS has been proven to upregulate the transsulfuration pathway and inhibit erastin-induced ferroptosis (McBean, 2012; Hayano et al., 2016). Taken together, any link of GSH synthesis, representative of antioxidant capacity, can be a mediator of the ferroptotic cascade.

Cancer is a multifactorial and complicated disease that contributes to a major health burden worldwide, despite substantial efforts and considerable improvement. In general, there are two key challenges in cancer treatment. One is how to kill cancer cells effectively without affecting healthy cells. Second, cancer cells in advanced tumors often exhibit multiple genetic variations with high oxidative stress and enhanced antioxidant capacity, which confer drug resistance (Trachootham et al., 2009). Ferroptosis, as a novel form of cell death, offers unique possibilities efficiently against a variety of cancers. A higher iron requirement than their non-malignant counterparts has been observed to support sustained proliferation and immortalized replication in certain cancer cells, along with FPN downregulation and TFRC upregulation. The strong dependency on iron makes cancer cells more susceptible to iron overload and ROS-mediated lipid peroxidation than noncancerous cells. Epidemiological investigations have also revealed that high dietary iron intake increases the risk of several cancer types (such as hepatocellular carcinoma (HCC) and breast cancer) (Fonseca-Nunes et al., 2014). Ferroptosis inducers have been proven effective in suppressing the occurrence and development of cancers derived from iron-rich tissues, such as HCC, breast cancer and pancreatic ductal adenocarcinoma (PDAC) (Gao et al., 2015; Friedmann Angeli et al., 2019). According to a sensitivity profiling in 177 cancer cell lines, renal cell carcinomas (RCCs) and diffuse large B cell lymphomas (DLBCLs) are particularly susceptible to GPX4-regulated ferroptotic cell death (Yang et al., 2014). From another perspective, it is well acknowledged that the activation of the apoptosis cascade is the most prevalent way to eliminate cancer cells. Unfortunately, the apoptotic pathway of most mutant cancer cells is frequently blocked, which confers robust drug resistance (Amable, 2016). Cancer cells resistant to some forms of death may still be vulnerable to the induction of other forms of cell death, suggesting bypass of apoptotic resistance is a considerable approach to combating resistant cancers. A large amount of evidence has confirmed that ferroptosis induction plays a predominant role against certain chemoresistant cancer cells that are in a high mesenchymal state or escaping drug treatment (Holohan et al., 2013). Recent research has showed that cisplatin can induce both ferroptosis and apoptosis in the NSCLC A549 cells and colon cancer HCT116 cells, while drug resistance is observed in cisplatin-induced apoptosis, rather than ferroptosis (Guo et al., 2018a). Additionally, some drug-resistant cancer cells with activated EMT signaling (upregulation of mesenchymal markers and downregulation of epithelial markers) become specifically sensitive to ferroptosis-based therapies (Viswanathan et al., 2017; Müller et al., 2020). Beyond acting as a monotherapy, ferroptosis induction also participates in combination with conventional therapies. For instance, erastin, a classical ferroptosis inducer, has been shown to enhance the chemotherapeutic efficacy of traditional anticancer drugs, such as temozolomide and cisplatin in specific cancer cell lines (Yamaguchi et al., 2013; Chen et al., 2015). Accordingly, deficiencies in a host of NRF2 targets, including GPX4, SLC7A11 and FTH1/FTL, are also capable of predisposing cells to succumb to proferroptotic agents in many cancer types (Hassannia et al., 2019).

Currently, discovered proferroptotic compounds mainly spotlight on key elements of redox homeostasis (e.g., system x_c ^- and GPX4) to disrupt the existing redox balance and trigger the ferroptotic cascade in cancers whose growth and survival are highly dependent on the uptake of amino acids. Lack of SLC7A11 and knockdown of GPX4 have been demonstrated to increase ROS-mediated lipid peroxidation and induce subsequent ferroptosis in even resistant cancer cells, indicating that deficiency in antiferroptotic function enhances cancer cell death (Yang et al., 2014; Roh et al., 2016). Furthermore, increased expression of SLC7A11, GPX4 or their upstream regulatory gene NRF2 not only promotes ferroptosis resistance, but is also correlated with a poorer prognosis and lower survival rate in many types of cancers, such as liver, lung and ovarian cancers (Fan et al., 2017; Daher et al., 2019; Lim et al., 2019). However, the role of ferroptosis in tumorigenesis and progression remains elusive, as ferroptosis appears to have a dual role. There has been evidence that deficiency in NRF2 favors early tumorigenesis in several cancers (Rojo de la Vega et al., 2018). Another critical mediator of the proferroptotic cascade, ACSL4, is downregulated in bladder, brain, breast, leukemia and lung cancer but upregulated in colorectal, head and neck, kidney, myeloma and liver cancer. Prognostic analysis shows that colorectal cancer patients with high ACSL4 expression have a low survival rate. In contrast, brain, breast and lung cancer patients with low ACSL4 expression have a poor survival, suggesting that ACSL4 plays different roles in different cancer types (Chen et al., 2016).

With a comprehensive understanding, ferroptosis has also been implicated in cancers undergoing immunotherapy and radiotherapy. Immunotherapy with immune checkpoint blockade, which activates an effective cytotoxic T cell-triggered antitumor immune-response, has a revolutionary impact on clinical cancer treatment. Mechanistically, interferon gamma (IFNγ) released from CD8⁺ T cells can impair cystine uptake by cancer cells upon downregulating the expression of SLC3A2 and SLC7A11 (two subunits of the cystine-glutamate antiporter system x_c ^-) and consequently promote lipid peroxidation and ferroptosis in ovarian cancer, melanoma and fibrosarcoma (Tsoi et al., 2018; Wang W. et al., 2019; Lang et al., 2019). Radiotherapy acts as frontline therapy for approximately half of patients with many cancers. Recently, ferroptosis has also been proven to be interrelated with cancer radiotherapy, which induces ACSL4 expression and enhances oxidative damage in a variety of cancers, including NSCLC, melanoma and esophageal cancer (Lang et al., 2019; Lei et al., 2020). Indeed, immunotherapy and radiotherapy synergistically activate tumoral lipid oxidation and ferroptosis.

However, ferroptosis also triggers severe damage and toxicity in organisms, including bone marrow injury and cisplatin-induced acute kidney injury. How to modulate ferroptosis pathway to obtain the highest clinical benefit in clinical oncology is a critical question that deserves further investigation.

A large body of evidence suggests that low levels of GSH/GSSG, elevated iron levels and lipid peroxidation are implicated in a variety of neurodegenerative diseases, along with abnormal levels of ferroptosis-associated modulators (Praticò and Sung, 2004; Belaidi and Bush, 2016). Alzheimer’s disease (AD) is the most common neurodegenerative disease and is characterized by synaptic loss and neuronal death. In AD patients, there is abnormally elevated iron in the brain tissue, and ferritin levels are negatively correlated with cognitive ability (Ayton et al., 2015). Excessive ROS generation triggers lipid peroxidation, which contributes to severe oxidative stress damage and is observed in GPX4-deficient AD mouse models, while ferroptosis inhibitors ameliorate neurodegeneration and behavioral dysfunction by blocking iron overload, lipid peroxidation and inflammation (Hambright et al., 2017; Zhang Y.-H. et al., 2018). Parkinson’s disease (PD) is characterized by progressive dopaminergic neuronal loss in the substantia nigra (SN) pars compacta. Epidemiological investigations have shown that high iron levels lead to a high risk of PD, associated with enhanced basal lipid peroxidation and elevated malondialdehyde (MDA) levels, a toxic byproduct of lipid peroxidation (Pyatigorskaya et al., 2015). Given the implication of iron in PD, preclinical experiments and clinical trials have revealed that the U.S. Food and Drug Administration (FDA)-approved iron chelators improve dopamine activity and prevent iron-mediated oxidative damage, by exerting a certain protective effect in PD patients (Devos et al., 2014; Chen X. et al., 2020). Several features of Huntington’s disease (HD) physiopathology are consistent with the ferroptosis pathway, including a lower GSH level, a higher lipid oxidation level and abnormal iron metabolism. Accordingly, administration of DFO significantly improves striatal pathology and motor phenotype in an R6/2 HD mouse model (Klepac et al., 2007; Lee et al., 2011; Chen et al., 2013). Additionally, ferrostatin-1 (Fer-1, classical ferroptosis inhibitor) and its analogs inhibit lipid peroxidation damage and increase the number of healthy spinous process neurons in the HD model. Apart from common neurodegeneration, other neurodegenerative disorders are also characterized by perturbations in iron homeostasis and accumulation, such as periventricular leukomalacia (PVL) (Skouta et al., 2014), amyotrophic lateral sclerosis (ALS) (Southon et al., 2020), multiple sclerosis (MS) (Jhelum et al., 2020), Friedreich’s ataxia (FRDA) (Cotticelli et al., 2019). Affected tissue regions share features of elevated iron, low GSH, decreased GPX4, as well as increased lipid peroxidation, consistent with the salient features of ferroptosis. Although there are few relevant studies, some ferroptosis-associated factors appear to be potential markers and therapeutic targets in these disorders (Hu et al., 2019; Seco-Cervera et al., 2020).

Besides leading to neurodegenerative diseases, the activation of the of ferroptotic cascade has also been determined in a wide variety of neuronal injuries. Neonatal brain injury, as well as traumatic brain injury (TBI), is usually accompanied by a series of biochemical processes, such as dysfunctional iron metabolism, decreased GPX activity, ROS accumulation and disordered ferroptosis-related mediators in vitro and in vivo (Lu et al., 2015; Kenny et al., 2019). Oxidative stress damage induced by specific GPX4 deletion in the brain is observed during the acute phase of intracerebral hemorrhage (ICH) and subsequent secondary brain injury (SBI) and finally leads to hemorrhagic stroke which can be relieved by an increasing GPX4 levels. The application of N-acetylcysteine (NAC, a ROS scavenger) or Fer-1 has been documented to prevent neuronal mortality after ICH and improve the prognosis of patients, by decreasing iron accumulation and lipid ROS (Li et al., 2017; Zhang Z. et al., 2018). Dysfunction of tau protein contributes to age-dependent, iron-mediated neurotoxicity in ischemic stroke tissue. Tau is capable of blocking ferroptosis by promoting iron export against middle cerebral artery occlusion (MCAO)-induced focal cerebral ischemia-reperfusion injury. Ferroptosis has been recognized as a main cause of neuronal death after ischemic stroke, and ferroptosis inhibitors significantly improve the prognosis of patients (Tuo et al., 2017). Actually, ferroptosis inhibitors have been successfully applied in animal models of I/R-associated tissue injuries including heart (Fang et al., 2019), lung (Xu et al., 2020), liver (Yamada et al., 2020a), intestine (Li Y. et al., 2019) and kidney (Friedmann Angeli et al., 2014).

Overall, these studies suggest that ferroptosis irreversibly drives neuronal loss and tissue damage linked with central and/or the peripheral neuronal pathologies. The effectiveness of ferroptosis inhibition as a therapeutic approach has been mostly validated by cellular and animal experiments. However, there seems to be a lack of strong evidence in clinical trials. Providing comprehensive treatments for neurological diseases remains a largely insurmountable challenge.

As “iron-induced heart disease” was first reported, iron-induced ferroptosis has been gradually identified as playing a detrimental role in the occurrence and aggravation of cardiovascular conditions in both animal models and human patients (Sullivan, 1981). Abnormal myocardial iron accumulation plays a pathogenic role in the myocardium which leads to cardiotoxicity and cardiac dysfunction. Studies have shown that ferroptosis mediates myocardial damage caused by doxorubicin (DOX) or I/R mainly by HO-1 upregulation, and subsequently degrading heme to release free iron and generate mitochondrial membrane oxidized lipids. Iron deposition in the myocardium of FPN knockout mice which results in severe damage to cardiac function has been recognized as a risk factor for coronary artery diseases (Bagheri et al., 2013; Lakhal-Littleton et al., 2015). Iron chelators, such as DFO and DFP have been shown to counteract the pathogenic effects of iron overload, indicating a promising therapeutic strategy against ferroptosis-driven cardiac abnormalities (Gao et al., 2015; Fang et al., 2019). Nonetheless, cardiomyocyte iron deficiency can also lead to fatal contractile and metabolic dysregulation as a consequence (Lakhal-Littleton et al., 2016). This findings supports compelling evidence that the influence of ferroptosis on cardiac diseases depends on proper iron homeostasis, rather than absolutely low iron levels. Ferroptosis has also been involved in inflammatory responses after heart transplantation, which promotes the adhesion of neutrophils to coronary endothelial cells and myocardial I/R damage. Fer-1 can alleviate the loss of cardiomyocytes and limit the recruitment of neutrophils by the TLR4/TRIF signaling pathway (Li W. et al., 2019). As the master regulator of systemic iron homeostasis, hepcidin is derived primarily from the liver, and it inhibits the iron exporter FPN to decrease iron availability in blood (Lakhal-Littleton et al., 2016). Due to an inherited genetic disorder or receiving multiple blood transfusions, the depletion of hepcidin induces ferroptosis as a result of iron overload in the body. As an iron overload disease that blocks hepcidin biosynthesis, hemochromatosis can be ameliorated by either iron chelators or genetic regulation of ferroptosis, such as targeting SLC7A11 (Wang H. et al., 2017).

Although the link with cardiovascular diseases still requires further evaluation by clinical evidence, iron-induced ferroptosis has been well-established as a master trigger of several cardiovascular pathologies. Chelation therapy has been considered as an efficient preventative measure and treatment approach and the key is how to maintain relatively precise iron homeostasis.

In addition to common diseases mentioned above, more pathologies associated with ferroptosis are summarized in Table 1 and Figure 3.

Currently, numerous chemical reagents with ferroptotic properties have been continuously exploited and demonstrated in preclinical and clinical experiments. Although the underlying mechanism needs more investigation, ferroptosis inducers are reckoned as a novel therapeutic strategy against various cancers, especially those resistant to conventional drugs. Indeed, ferroptosis was first discovered by studying reaction mechanisms of two small RAS-selective molecule compounds, erastin and RSL3, including their derivatives, that have been confirmed to suppress tumor growth in many types of cancers in vitro and in vivo (Dixon et al., 2012; Yang et al., 2014; Viswanathan et al., 2017; Zhang Y. et al., 2019). Originally, ferroptosis-inducing reagents were divided into two mechanistic classes: 1) inhibitors of cystine via system x_c ^- (e.g., erastin, sorafenib, SAS and glutamate), which subsequently drive GSH depletion, and 2) direct inhibitors of GPX4 (e.g., RSL3 (1S, 3R)-RSL3). More recently, the small molecule FIN56 has been reported to degrade GPX4 and to deplete CoQ10, an endogenous inhibitor of lipid peroxidation, through the mevalonate pathway. Moreover, FINO₂ can either oxidize ferrous iron directly, or deactivate GPX4 indirectly (Shimada et al., 2016a; Gaschler et al., 2018a). On the contrary, ferroptosis also participates in other nonneoplastic lesions and a variety of compounds can pharmacologically inhibit ferroptosis, most of which are iron chelators or antioxidants. Iron chelators (e.g., DFO and DXZ) or regulators of iron metabolism suppress ferroptosis through lowering free iron levels (Fang et al., 2019; Chen X. et al., 2020). In addition, inhibitors of lipid peroxidation, including ACSL4 inhibitors (e.g., thiazolidinediones (TZDs)), lipophilic antioxidants (e.g., α-tocopherol), and agonists of the antioxidant system (e.g., ROS scavengers (NAC) and system xc^- inducers (β-mercaptoethanol)) inhibit ferroptosis via single or complex mechanisms (Dixon et al., 2012; Karuppagounder et al., 2018).

Fortunately, some of these pro- and anti-ferroptotic compounds are clinical drugs approved by the FDA, and are expected to be novel therapeutic applications based on ferroptotic mechanisms. Mechanistically, SAS, sorafenib and lanperisone have been broadly documented to inhibit cysteine-glutamate antiporter and cystine absorption, leading to the attenuation of GSH, and ultimately resulting in the death of certain types of cancer cells, including HCC, lymphoma and prostate cancer (Lachaier et al., 2014; Sun et al., 2016a; Sun et al., 2016b). Siramesine, lapatinib and salinomycin (ironomycin) can trigger ferroptosis by elevating iron levels in breast cancer cells and leukemia cells (Hamaï et al., 2017; Ma et al., 2017; Mai et al., 2017). Iron chelators are usually used as antidotes for iron poisoning in clinical practice, and new indications seem to be available based on their antiferroptotic properties. TZDs, a class of typical antidiabetic compounds, have been proven to act as ACSL4 inhibitors to restore GPX4 expression and reduce COX2 expression, and finally prevent lipid peroxidation and ferroptosis to ameliorate intestinal and pulmonary I/R injuries in vitro and in vivo (Doll et al., 2017; Li Y. et al., 2019; Xu et al., 2020). Furthermore, chelation therapy targeting iron dyshomeostasis has shown promise against neurodegenerative disorders in preclinical studies and clinical trials. Especially, DXZ is currently the only FDA-approved drug for preventing and treating DOX-induced cardiotoxicity in cancer patients (Fang et al., 2019). DFP has been recently used clinically for systemic iron overload dysfunctions and investigated as a conventional or modified treatment in several clinical trials against PD. It has shown reduced iron deposition in the SN and an improvement in motor function, as well as slowed disease progression and relieved neurological symptoms (Devos et al., 2014). Currently, many studies on ferroptosis relevance in neurodegenerative diseases are attempting to use ferroptosis inhibitors, especially iron chelators, to slow disease progression in preclinical animal models, most of which have achieved an potent efficacy (Guo et al., 2016; Rao et al., 2021). Nevertheless, there are still many restrictions and problems associated with chelation therapy. When applied in human clinical trials, iron chelators appears to be far from satisfactory (Grolez et al., 2015). DFP is well tolerated, achieved target engagement (lowering of iron) in patients with neurodegeneration. It seems to somewhat slow disease progression and improve quality of life, but they have not reach significance (Martin-Bastida et al., 2017; Klopstock et al., 2019). Although some of iron chelators, such as DFO, DFP and deferasirox (DFS) are recommended as iron overloaded first-line treatments, they all have their own definite side effects, such as gastrointestinal disorders, hepatotoxicity, nephrotoxicity and visual and auditory neurotoxicity (Vichinsky et al., 2011; Aydinok et al., 2015; Maggio et al., 2020). Furthermore, poor pharmacokinetic properties of DFO and DFS limit their availabilities in the central nervous system, as they are prevented from crossing the blood-brain barrier (BBB) (Sripetchwandee et al., 2016; Crielaard et al., 2017). More appropriate iron chelators that regulate the ferroptotic process need to be further explored and developed for their potential to treat neurodegenerative diseases in preclinical and clinical studies.

Therefore, it is worth noting that nanotechnology has emerged to achieve anticancer effects by improving the pharmacokinetic properties of existing proferroptotic agents or developing novel nano-inducers of ferroptosis. The FDA-approved iron-supplementing agent ferumoxytol (Feraheme) has been proven to have an anti-leukaemia effect in vitro and in vivo via increasing intracellular iron and ROS production, leading to enhanced oxidative stress and ferroptotic cell death (Trujillo-Alonso et al., 2019). Cisplatin (CDDP)-loaded Fe₃O₄/Gd₂O₃ hybrid nanoparticles with the conjugation of lactoferrin and RGD dimer (RGD2) (FeGd-HN@Pt@LF/RGD2), which are able to cross the BBB, have been developed for therapy of orthotopic brain tumors based on ferroptosis mechanisms (Shen et al., 2018). In view of the oxidative stress regulation ability of p53 and inducibility of the metal-organic network (MON) on the Fenton reaction, synthesized MON encapsulated with p53 plasmid (MON-p53) can trigger both ferroptosis and apoptosis pathways to synergistically kill cancer cells (Zheng et al., 2017).

Collectively, we have listed FDA-approved synthetic drugs that have been reported to modulate ferroptosis in Table 2 and experimental chemical compounds in Supplementary Tables S1 and Supplementary Tables S2.

Nowadays, natural medicines are usually used alone or, in most cases, in combination with conventional drugs in clinical in the hope of enhancing the efficacies and/or neutralizing the toxicities of chemical drugs. In addition to clinical (adjuvant) medicines, natural products are widely consumed as supplements in alternative strategies (Shu-Feng Zhou et al., 2007; Kibble et al., 2015). Growing evidence has uncovered the solid regulatory properties of ingredients with bioactive functions on ferroptosis (Figure 4). Hence, we focus on natural compounds that show a pharmacological potential for ferroptosis-associated (adjuvant) therapy.