Introduction

Cell death (especially cell suicide) plays a fundamental role in maintaining physiological homeostasis by removing damaged cells, and it may also be an aberrant pathological reaction to damaging stimuli [1]. The Nomenclature of Cell Death Committee has developed guidelines to divide cell death modes into accidental cell death and regulated cell death (RCD) according to morphology, biochemistry, and function [2]. Accidental cell death is a biologically uncontrolled process of cell death in response to accidental injury stimuli [3]. However, RCD is characterized by controlled signaling pathways that play key roles in organismal development or tissue renewal [4]. Previously, apoptosis was thought to be the major form of RCD, but with more in-depth study on tumor cell biology and thorough examination of cancer therapy mechanisms, more and more subtypes of RCD are progressively emerging. The novel RCD types we are going to introduce include: necroptosis, pyroptosis, ferroptosis, and cuproptosis, which can occur with or without exogenous environmental or intracellular perturbations [5,6,7]. Malignant cells, on the other hand, continue to evade the RCD routes through evolving a variety of mechanisms [8]. Additionally, RCD pathways have also been reported to be crucial for the prognosis of cancer patients, cancer progression and metastasis, and cancer immune surveillance [9,10,11,12,13,14]. Based on accumulating evidence, distinct forms of RCD might change the tumor microenvironment (TME) by releasing pathogen- or damage-associated molecular patterns (PAMPs or DAMPs), which affect the benefits of anticancer therapy [15,16,17].

Our review outlines the molecular mechanisms and processes of four different types of RCD, necroptosis, pyroptosis, ferroptosis, and the newly discovered cuproptosis, as well as their different roles in the initiation and progression of cancer. We specifically focus on RCD processes that influence the TME and the latest advancements in targeting necroptosis, pyroptosis, ferroptosis, and cuproptosis for cancer therapy. We describe the mechanisms of the various cancer therapies currently available, showing that they mainly depend on different RCD modalities. A reasonable assumption is that these novel RCD modalities constitute a mechanism of defense against tumor progression and migration. Furthermore, the significance and prevalence of RCDs in combating cancer drug resistance have been included, demonstrating that the use of conventional therapy in conjunction with RCD modulators might hold significant potential for cancer treatment. Hopefully, this information will lead to improved guidance for approaches to tumor therapy.

Molecular mechanisms of different cell death pathways

The most extensively studied RCD modality is apoptosis, which leads to immunogenicity or induces no immunogenic response in different contexts [18, 19]. The morphological changes observed during apoptotic cell death include cell shrinkage, externalization of phosphatidylserine on the plasma membrane, and nuclear pyknosis and karyorrhexis; notably, the plasma membrane remains intact [2]. This pathway is believed to function as a natural barrier against malignancy, but the primary hallmark of cancer cells and the emergence of chemotherapy resistance during cancer therapy are limiting or causing cells to resist apoptosis [20, 21]. Therefore, while tackling apoptosis resistance, discover methods that induce nonapoptotic forms of RCD must be discovered as alternative cancer therapies. Excitingly, new forms of RCD have been extensively studied in the past decade; these modalities include necroptosis [22], pyroptosis [64]. Ferroptosis is also activated by components of the autophagy machinery, such ATG3, ATG5, ATG4B, ATG7, ATG13, and BECN1 [65]. Additionally, knockout or knockdown of the main genes governing autophagy reduces the effects of erastin on ferroptosis because intracellular ferrous iron levels are reduced [Cell fates in the TME

A 2016 study revealed that alanine released from stroma-associated pancreatic stellate cells by autophagy was a substitute carbon source that fueled the TCA cycle in pancreatic ductal adenocarcinoma. This change in fuel source reduced the tumor cell reliance on glucose and nutrients obtained from serum, which are limited in the pancreatic TME [210]. Based on this finding, we logically suspect that a novel RCD pathway of noncancer cells in the TME may affect cancer cell survival. Therefore, we investigated this hypothesis in detail.

Interestingly, novel forms of RCD for cells that compose the TME profoundly influence the tumor fate. For instance, RIPK3 downregulation in TAMs induces fatty acid oxidation and M2 MΦ polarization in the TME, facilitating HCC tumorigenesis [211]. In addition, Huanrong Lan and colleagues revealed that oxaliplatin resistance in CRC results from the necroptotic evasion of M2 MΦs. Mechanistically, the expression of the methyltransferase METTL3 is increased in oxaliplatin-resistant CRC tissues, and METTL3-mediated N6-adenosine methylation significantly inhibits TRAF5-induced necroptosis both in vitro and in vivo [212]. Thus, the necroptosis of MΦs tends to exert a positive antitumorigenic effect.

The discovery that the serine protease inhibitor Val-boroPro (also called talabostat or PT-100) cleaves the substrate at proline has generated significant interest in this compound as a potential anticancer drug. Val-boroPro achieves its anticancer effects by activating pro-caspase-1, which is subsequently cleaved to activate GSDMD and induces the pyroptosis of monocytes and MΦs [213].

As shown in the study by Hage et al., sorafenib induces pyroptosis in MΦs to stimulate HCC cell killing [214]. Specifically, sorafenib robustly increases the activity of caspase-1, activating GSDM and inducing MΦ pyroptosis. Subsequently, NK cells are activated when cocultured with sorafenib-treated MΦs, and the interplay of MΦs and NK cells induces HCC cell death. Moreover, various cytokines are released from pyroptotic immune cells, including IL-18, which shows established anticancer activity by enhancing the type 1 immune response and can thus be utilized in cancer immunotherapy [215, 216].

MΦs engulf red blood cells and digest them to generate hemoglobin, which is further degraded into heme. Heme is catabolized into iron, which either promotes ROS generation or lipid peroxidation. Through ferroportin, the iron produced by heme is discharged into the environment, increasing the iron level in the TME (Fig. 3A) [217]. Then, iron promotes the Fenton reaction and generates hydroxyl radicals, which cause tumor cells to undergo ferroptosis [218]. Therefore, macrophages increase the content of iron in the TME through their own ferroptosis and promote the subsequent ferroptosis of tumor cells, thus showing a powerful antitumor effect. Ferroptosis of MDSCs was demonstrated to be crucial in fighting malignancies, but Zhu et al. found that N-acyl-sphingosine amidohydrolase (ASAH2) is expressed at high levels in MDSCs in colon carcinoma. ASAH2 reduces MDSC ferroptosis by reducing p53 stability, upregulating Hmox1 expression, and inhibiting lipid ROS production in the TME. The ASAH2 inhibitor NC06 induces ferroptosis in MDSCs by inhibiting ceramidase activity. Animal models confirmed that NC06 inhibits the infiltration of MDSCs into transplanted tumors by promoting MDSC ferroptosis and thus inhibits tumor growth [219]. Furthermore, ferroptosis mediated by tumor-infiltrating lymphocytes significantly enhances the efficacy of ICIs [220, 221].

Since cuproptosis is a novel RCD, determining whether it occurs among noncancerous TME cells is a challenge. However, upon Cu stimulation, exosomes secreted by MΦs increase angiogenesis mediated by endothelial cells in vitro and in vivo [222]. Ryuhei Takemoto and colleagues also found that overexpression of lysyl oxidase, a Cu-containing enzyme, in human leukemic THP-1-cell-derived M2 MΦs promotes tumor metastasis [223]. Therefore, immune cell cuproptosis may have a multifaceted role in TME, and we are awaiting rational animal and cellular investigations to elucidate this role.

Current and future therapeutics targeting different cell death pathways

As we previously discussed, tumor cells and other cells in the TME that undergo necroptosis, pyroptosis, ferroptosis, or cuproptosis possibly contribute to strong antitumor immunity. Additionally, mechanisms for bypassing the apoptosis signaling pathways that cause the death of cancer cells have attracted considerable attention for their use in anticancer therapy [224]. Therefore, we describe small-molecule compounds and other agents targeting novel mechanisms of cell death that might be employed in cancer therapy (Table 1), and we emphasize the therapeutic approaches that have been tested in clinical trials to date (Table 2).

Agents targeting novel cell death pathways

Targeting necroptosis, pyroptosis, and ferroptosis to develop new anticancer medications for therapeutic use has been a long process, and recently, compounds inducing cuproptosis have shown promise as anticancer strategies [225, 226]. In Table 1, we summarize 85 types of therapeutic agents that exert an effect on the mechanisms of newly discovered RCD modalities that have been tested in vivo and/or in vitro.

Table 1 Summary of agents targeting novel RCDs in cancer-related preclinical studies
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Agents inducing novel cell death pathways

Approved and investigational drugs inducing novel RCD pathways

According to recent investigations, many clinically approved medications exert potent antitumor effects by inducing (or inhibiting) inflammatory RCD modalities in preclinical studies [169]. CQ has been shown to upregulate endogenous RIPK3 in CRC cell lines, and Hou et al. reported that necroptosis mediates this process, which is not affected by apoptosis inhibitors [235]. Interestingly, shikonin, a naphthoquinone product synthesized from the roots of a Chinese medicinal herb, induces nasopharyngeal carcinoma cell necroptosis in a dose-dependent manner [227]. Mechanistically, shikonin increases ROS production and upregulates the expression levels of RIPK1, RIPK3, and MLKL, which prompts necroptosis in apoptosis-resistant tumor cells [307]. However, the activation of necroptosis can also be mediated by the modulation of the upstream signaling pathways. For instance, the sphingosine analog FTY720, also called fingolimod, induces necroptosis in human lung cancer cells by binding to inhibitor 2 of PP2A (I2PP2A/SET oncoprotein), thus activating the PP2A/RIPK1 pathway [228].

In addition, metformin inhibits cancer cell proliferation by inducing mitochondrial dysfunction to cause pyroptotic cell death [241]. Specifically, metformin is a sensitization agent that enhances AMPK/SIRT1/NF-κB signaling to trigger the activation of caspase-3 and the generation of GSDME-PFD. Lu Wang and colleagues documented that metformin causes pyroptotic death of esophageal squamous cell carcinoma cells by targeting the miR-497/PELP1 axis [242]. Further, chemotherapeutic medications, including actinomycin-D, doxorubicin, topotecan, and bleomycin, stimulate the pyroptotic death of GSDME-expressing cells [38]. Teng et al. also found that the induction of ROS/ NLRP3/GSDMD signal axis via using polyphyllin VI practically leads to pyroptotic death of NSCLC cells [246]. Our Fig. 6 summarizes other methods of action of pyroptosis inducers.

Fig. 6
figure 6

Summary of the modulators of novel RCDs in cancer treatment

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Sorafenib is an FDA-approved anticancer drug for the treatment of HCC, RCC, and thyroid cancer [308]. Sorafenib inhibits system XC−, thus promoting ferroptosis by inhibiting GSH production [281]. Additionally, sorafenib and sulfasalazine may synergize to prevent the activation of branched-chain amino acid aminotransferase, a principal enzyme involved in sulfur-based amino acid metabolism. This therapeutic approach induced ferroptosis in HCC cell lines both in vitro and in vivo [309]. Additionally, cisplatin triggers ferroptosis via GSH depletion and inactivation of GPX4 in NSCLC and colon cancer [197]. Etoposide is a phenolic antitumor drug that efficiently removes GSH in myeloperoxidase–rich myelogenous leukemia cells, thus decreasing GPX4 levels and leading to ferroptosis [279]. In the study by Ma et al., combining the lysosome disruptor siramesine with lapatinib, a tyrosine kinase inhibitor, induced ferroptotic death of breast cancer cells by inhibiting iron transportation and induction of lipid peroxidation [282].

Peter Tsvetkov and colleagues identified that the Cu ionophore elesclomol induces cuproptosis by inducing lethal proteotoxic stress in various types of cancer cells (as shown in Table 1). However, as indicated by Gao Wei and colleagues, elesclomol causes CRC cells to undergo Cu-dependent ferroptosis by promoting the degradation of Cu-transporting ATPase 1 and subsequently inducing ROS accumulation, which promotes the degradation of SLC7A11 [75]. Since current experimental study on cuproptosis is still in its infancy, more research is needed to support its potential for cancer treatment.

Nanoparticles (NPs) targeting RCD pathways

Advantages of NPs include easy cell barrier penetration, preferential accumulation in specific organelles and cells, and an increased likelihood of effective fine-tuning, endowing them with great potential as anticancer therapies [310]. As we mentioned above, shikonin shows great potential as an antitumor treatment by inducing necroptosis. However, the clinical application of shikonin has been restricted due to its poor tumor-specific accumulation, low water solubility, short duration in circulating blood, and a high risk for hazardous side effects on normal tissues [311]. Therefore, Feng et al. constructed an FSSN based on the metal-polyphenol coordination of Fe(III) and shikonin, and FSSN showed not only greater water solubility and lower cytotoxicity than shikonin in normal cells but was also integrated with the function of Fe ions. FSSNs effectively reduced the GSH level and induced ferroptosis and necroptosis in mouse breast cancer cell lines [238]. Additionally, the use of graphene oxide NPs in CT26 colon cancer cells successfully induced necroptosis by enhancing the function of RIPK1, RIPK3, and HMGB1 [229]. Similarly, the group of Praveen Sonkusre reported that when treating prostate adenocarcinoma cells with selenium NPs, necroptosis was induced through increased ROS production and TNF and interferon regulatory factor 1 expression [230].

Furthermore, NPs have been used to induce pyroptosis in malignant cells. For example, the biomimetic NP designed by Pengfei Zhao and colleagues consisted of a hydrophobic nucleus composed of indocyanine green and decitabine and a cell membrane shell. Biomimetic NPs induced the accumulation of calcium in the cytoplasm, leading to mitochondrial damage and caspase-3 activation and subsequently inducing GSDME-mediated pyroptosis in 4T1 cell lines [250]. In addition, Kataoka et al. constructed an ROS-responsive nanoreactor based on polyion complex-forming vesicles by introducing thioketal linkers into a covalently cross-linked membrane network. These ROS-responsive NPs shielded glucose oxidase to induce pyroptosis by generating oxidative stress and inducing glucose deprivation [255].

A recent study described the use of an efficient ferroptosis agent, an FePt@MoS2 NP, which induced the release of more than 30% Fe(II) in the TME within 72 h of treatment to accelerate the Fenton reaction and efficiently induce ferroptosis in various cancer cell lines [280]. Analogously, another study showed that zero-valent iron NPs converted Fe(II) to promote the Fenton reaction, which induced mitochondrial lipid peroxidation in oral cancer cells [266]. Furthermore, a positively charged lipophilic nanocarrier (Fe-CO@Mito-PNBE) targeted the negatively charged mitochondrial membrane, and the subsequent release of Fe(III)/Fe(II) ions effectively facilitated the Fenton reaction and ultimately led to cell ferroptosis [267]. More NPs that induce ferroptosis in tumor cells are listed in Table 1.

The administration of NPs reverses cisplatin resistance in cancer cells by inducing cuproptosis. Exogenous platinum is widely presumed to cause drug resistance induced by high concentrations of GSH in cancer cells. According to Lu et al., the diethyldithiocarbamate-Cu complex effectively induces cuproptosis in A549/DDP cell lines by downregulating FDX1 expression. Most of the administered diethyldithiocarbamate-Cu complex maintained a stable chemical structure when mixed with GSH in solvent, suggesting that it potentially combats cisplatin-resistant cancer cells [306]. Accordingly, research into nanomaterials that induce recently discovered RCD pathways is ongoing, and we expect more and better NPs to be clinically used for cancer treatment in the near future.

Small molecules targeting novel RCD pathways

An increasing number of small compounds are being tested to target the necroptotic cell death pathway. For instance, Zhou et al. revealed that emodin, an anthraquinone compound purified from various Chinese medicinal herbs, induces necroptosis in glioma cell lines by enhancing TNF/RIPK1/RIPK3 pathway activation and thus inhibits U251 cell proliferation [231]. Additionally, ophiopogonin D′ induces robust necroptosis in prostate cancer cells through RIPK1 activation [232]. Resibufogenin, a small molecule derived from the bufadienolide family of compounds, significantly inhibits the proliferation of CRC cell lines by upregulating RIPK3 expression [104]. These small-molecule compounds still hold a lot of promise to be applied as clinical medicines because of their remarkable ability to cause tumor cells to undergo necroptosis.

Dobrin et al. found that treatment of triple-negative breast cancer cells with ivermectin induces pyroptosis by activating the P2X4/P2X7-gated pannexin-1 channel [259]. Also, based on accumulating evidence, DHA reduces cancer cell viability and proliferation by modulating different cellular responses [312, 313]. For example, Dumont and colleagues proposed that DHA inhibits NLRP3 inflammasome assembly and the JNK signaling pathway in MDSCs, reducing the 5-fluorouracil-induced generation of IL-1 and increasing the anticancer effectiveness of 5-fluorouracil [314]. Yi-Fan Tan and colleagues also revealed that inhibition of BRD4, either through genetic knockdown or the use of the bromodomain inhibitor JQ1, significantly slows the EMT and the cell proliferation rate and leads to caspase-1/GSDMD-mediated pyroptosis in RCC cells [258]. BRD4 is a member of the BET protein family that is involved in the control of epigenetic modifications [315]. Additionally, the thiopyran derivative L61H10 exhibits great antitumor activity by switching apoptosis to pyroptosis in lung cancer cells [316].

Recently, an increasing number of investigations have demonstrated that small-molecule compounds play essential roles in inducing ferroptosis in tumor cells. Zhang et al. discovered that the benzopyran derivative IMCA significantly downregulates SLC7A11 expression and reduces the contents of cysteine and GSH in cells, resulting in lipid ROS accumulation and ferroptosis in human CRC cell lines [268]. In addition, trigonelline is a plant alkaloid that significantly reduces GSH levels, thus induces ferroptosis in head and neck cancer cells [276]. Furthermore, dihydroartemisinin also exerts a robust effect on inhibiting the proliferation and inducing the ferroptosis of leukemia cells [295]. Similarly, Chang et al. found that a marine terpenoid, heteronemin, induces ferroptosis in HCC cells by initiating lipid peroxidation [317]. As shown in the study by Li et al., the small-molecule MMRi62, which targets MDM2-MDM4, induces ferroptosis by degrading mutant p53 and the heavy chain of ferritin and successfully inhibits the metastasis of pancreatic cancer [298]. As we continue our research, we have gradually discovered much promise in the field of pharmaceuticals that induce ferroptosis in tumor cells, and we are eager to see how these medications will be applied in clinical settings.

Other methods to target novel cell death mechanisms

Wan et al. documented that radiation therapy (RT) causes tumor cells to release microparticles with broad antitumor effects and thus abrogates immunogenicity primarily via ferroptotic cell death [318]. Mechanistically, radiation causes lipid peroxidation and ferroptotic cell death through three parallel mechanisms at least [198, 319, 320]. First, RT causes lipid peroxidation by producing excess ROS. RT-generated ROS remove electrons from PUFAs, resulting in the formation of PUFA radicals (PUFAs-OH). Then, these unstable carbon-centered radicals quickly react with oxygen molecules to generate lipid peroxyl radicals (PUFA-OO·), which remove H· from other molecules via the Fenton reaction and ultimately generate lipid hydroperoxides (PUFAs-OOH). Second, radiation increases the expression of ACSL4 to support PUFA-phospholipid biosynthesis, although the precise mechanism by which RT increases ACSL4 levels is still unknown [198]. Third, RT induces GSH depletion, which impairs GPX4-mediated ferroptosis defenses and subsequently promotes ferroptosis [63, 320]. Furthermore, disulfiram, a medicine approved to treat alcoholism, was shown to cause lysosomal membrane permeabilization via a ROS-dependent process, leading to ferroptosis and increasing cellular susceptibility to radiation [288].

Additionally, human umbilical cord mesenchymal stem cells (hUCMSCs) were recently identified as a viable cancer therapy option. For example, these cells prevent NSCLC and HCC cells from migrating [321]. Additionally, hUCMSCs show some advantages over other MSCs because they exhibit minimal immunogenicity and can be produced in large numbers. Following the overexpression of NLRP1 and caspase-4, hUCMSCs cause pyroptosis of the MCF-7 breast cancer cell line; however, hUCMSC treatment has little to no effect on the cell cycle [322].

The growth of schwannomas is proposed to be inhibited via a unique approach based on both the introduction of an adeno-associated virus (AAV-1) and treatment with the GSDMD PFD. This combination was created using an AAV-1-based vector encoding the mouse GSDMD N-terminus under the control of the promoter P0, which is unique to Schwann cells. This gene did not cause neurotoxicity to surrounding tissues following an intratumor injection and inhibited the development of the NF2 and HEI-193 schwannoma cell lines through GSDMD-mediated pyroptosis [323]. The intratumor delivery of GSDMD PFD via AAV-1 offers a better level of protection for the nearby normal tissue since it is more selective than typical medication therapy.

Agents inhibiting novel cell death pathways

Necroptosis occurs in cancer cells, and the TME is partially protumorigenic because the inflammation underlying necroptosis may trigger tumor development by promoting cell proliferation, genomic instability, angiogenesis, and metastasis [31]. Liu et al. harnessed the MLKL inhibitor NSA to treat a mouse xenograft model, which significantly delayed tumor growth, providing strong evidence of the protumorigenic role of necroptosis [110]. The necroptosis inhibitor necrostatin-1 also helps reduce colitis-associated tumorigenesis in mice [240]. RIPA-56 is a highly potent and metabolically stable inhibitor of RIPK1 that has been employed to treat a mouse model of inflammatory disease and has shown very high selectivity [324]. Another novel RIPK1 inhibitor PK68 which possesses high efficacy and conserved potency among human, mouse, and rat has been reported to effectively inhibit necroptosis and suppress metastasis of both melanoma and lung carcinoma cells in mice [239]. Although the above-mentioned necroptosis inhibitor has not been implemented in cancer patients, the RIPK1 inhibitor, GSK2982772, is currently being tested in phase 2a clinical studies for patients with inflammatory disease [325].

The utilization of pyroptosis inhibitors has significant research promise because of the dual role that pyroptosis plays in cancer. The study in 2019 revealed that delivering the specific anti-GSDMB antibody in biocompatible nanocapsules significantly inhibited the metastasis and drug resistance of HER2 breast cancer cells [264]. In addition, dimethyl fumarate is an inhibitor of pyroptotic cell death that functions by inactivating GSDMD [265]. Recent research by Jun Jacob Hu and colleagues suggests that the use of disulfiram also prevents pyroptosis by preventing the creation of GSDMD pores in a mouse model of inflammation [326]. In addition, Zhang et al. documented that the MLKL inhibitor NSA reverses pyroptosis by suppressing GSDMD oligomerization [327]. The use of these pyroptosis inhibitors in cellular and animal experiments offers great potential for treating patients with certain type of refractory cancers.

Current evidence suggests that ferroptosis induced by doxorubicin (DOX) was proved to contribute to the side effect of cancer therapy, including cardiotoxicity [328]. The DOX cardiomyopathy is caused by the excess free iron released from heme degradation which accumulates on mitochondria. Inhibition of ferroptosis through using ferrostatin-1 and HO-1 antagonist exerts some protective effect against myocardial injury [329]. In addition, the overexpression of ASCL4 also contributes to intestinal injury induced by irradiation therapy. Ji et al. have shown that troglitazone successfully suppresses lipid peroxidation in intestine through inhibiting ASCL4 and inhibited subsequent tissue damage [330]. Further, the novel findings from Soňa Jantová and colleagues demonstrated that the combination of 2,2,6,6, tetramethylpiperidine-N-oxyl (a ferroptosis inhibitor) capped TiO2 nanorods with UV-A light irradiation not only killed MCF-7 cell lines significantly, but also overcame the multidrug resistance [304]. We currently speculate that ferroptosis inhibition might have played a role in this process, but the mechanisms behind it are still blur and need further study. In addition, Dai et al. have found that the DNA damage caused by ferroptosis could facilitate pancreatic tumorigenesis through 8-hydroxy-2′-deoxyguanosine (a major product of oxidative DNA damage)-STING-dependent pathway. And the administration of ferroptosis inhibitor liproxstatin-1 effectively inhibits the pro-tumorigenesis of ferroptosis process [305].

Finally, the use of cuproptosis inhibitors, the most recent kind of cell death, in cancer has yet to be revealed. GSH was found to inhibit cuproptosis in cells, but this can lead to cisplatin resistance in tumor cells [306]. Furthermore, to help visualize the multiple modes of action, we displayed the modulators involved in four RCDs in Fig. 6.

Clinical trials targeting novel RCD modalities

Despite the fact that a variety of reports on novel RCD activators and inhibitors have been published lately, clinical trials evaluating the effects of modulators of novel RCDs are still in their infancy. In this section, we summarize the clinical trials to date in primary outcome measures or interventions that have involved the investigation of relevant biomarkers of novel RCD measurement and list them in Table 2.

Table 2 Summary of published clinical trials involving modulators of novel forms of RCDs
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One clinical study aimed to investigate whether the immediate necroptosis induced by the nonablative cryosurgical freezing could be beneficial to the subsequent injection of immunotherapeutic drugs (NCT04739618). This study recruited 32 participants with metastatic solid cancer who are first treated by nonablative cryosurgical freezing and then receive multiplex immunotherapy (including pembrolizumab, ipilimumab, and GM-CSF) and evaluate overall response rate of radiographic changes. In addition, another study posted in 2018 sought for the efficacy of RIPK1 inhibitor GSK3145095 alone and in combination with pembrolizumab included 8 participants. The serious adverse event rate of this study is 50% and it was terminated following an internal review of the company (NCT03681951).

Another phase II clinical study aims to evaluate the efficacy and safety of ferroptosis inhibitor MIT-001 for the prevention of oral mucositis in patients with lymphoma or multiple myeloma receiving conditioning chemotherapy with autologous hematopoietic stem cell transplantation (NCT05493800). This research was launched on August 9, 2022, and we shall keep track of its progress and other messages about the relationship between ferroptosis and inflammatory side effect of cancer therapy.

So far, we have found only the modulators of these two forms of RCDs, necroptosis and ferroptosis, in clinical trials and the results of these cancer therapy methods remain to be discovered. As high-quality articles on cell death modalities continue to emerge, more clinical trials will be conducted with the research purpose stated as understanding these four cell death modalities; therefore, we believe that in the near future, better use of necroptosis, pyroptosis, ferroptosis, pyroptosis, and other mechanisms will optimize anticancer treatments.

RCD: is it a potential approach to reverse drug resistance in cancer?

RCD modulation and chemoresistance

The data from Wang et al. have revealed that the epigenetic repression of RIPK3 allows NSCLC cell lines to escape from necroptosis, which subsequently increases resistance to chemotherapy [331]. Xu Zhao and colleagues successfully used trichothecin to induce necroptosis in chemotherapy resistant cancers. Mechanistically, the expression of RIPK3 was significantly upregulated by the natural secondary metabolite, trichothecin, and then RIPK3 enhanced the phosphorylation of MLKL and also activated the mitochondria energy metabolism and ROS production, leading to a novel strategy to sensitize cancer cells to cisplatin therapy [303]. Thus, it is suggested that the necroptosis pathways and lipid peroxidation can act synergistically and both play crucial roles in overcoming chemoresistance. Intriguingly, the combination of DHA with cisplatin can synergistically induce cytotoxicity against pancreatic ductal adenocarcinoma because DHA induces ferroptosis via promoting GPX4 degradation, ROS production, and ferritin degradation mediated by NCOA4 [332]. In addition, Ophiopogonin B, a bioactive component of traditional Chinese medicine, was reported to have significant impact on inducing pyroptotic cell death of A549 cells, which helps to alleviate the cisplatin resistance [260]. Further, **g Guo and colleagues also revealed that adding GW4064, a synthetic FXR agonist, to oxaliplatin can significantly limit tumor cell proliferation in vitro, and slow tumor growth in xenograft mouse models. GW4064 effectively enhanced caspase-3/GSDME-mediated pyroptosis of HT-29 and SW620 cells, which increased the chemosensitivity of cells to oxaliplatin [261]. Cuproptosis was also demonstrated to fight against platinum-based chemotherapy resistance. Lu et al. revealed that the killing effect of cisplatin was detoxified by GSH in A549 cells, while the nanomedicine based on Cu(II) (CuET) exhibited GSH-resistant cytotoxicity and efficiently reversed cisplatin resistance [306].

RCD modulation and immunotherapy resistance

Nowadays, immunotherapy represented by ICIs has become a major breakthrough in cancer treatment and has achieved considerable success in clinical treatment of some solid tumors [333,334,335]. However, the use of ICIs is restricted by the lack of tumor-associated antigens, which results in more than two thirds of the patients to not react to ICI-based monotherapy [169]. However, due to the intricate role of the novel RCD modes in TME, we may anticipate that manipulating RCDs may affect the efficacy of ICIs in cancer patients. Emerging evidence has demonstrated that CD8 + T cells inhibit tumor cells via induction of necroptosis, pyroptosis, ferroptosis [169, 336, 337], and possibly cuproptosis [338]. Like what we mentioned, the novel RCDs in TME seriously activate proinflammatory cytokines, infiltration of cytotoxic T cells, and other lymphocytes, which are significant for the sensitivity of various tumors to ICIs [176]. In addition, the release of Gzm B from CAR-T cells activates caspase-3/GSDME-dependent pyroptosis of target cells, which enhances the efficacy of CAR-T cell therapy [181]. Thus, similar to chemotherapy, the immunotherapy may partially function as the inducers of the novel RCD mechanisms, which might provide an immune-based underpinnings for some novel combination therapies.

By creating vaccine viruses that loaded MLKL expression, Hoecke et al. directly delivered the necroptosis mediator MLKL to tumor cells, which successfully promoted necroptotic death and enhanced antitumor immunity. The potent antitumor immunity is attributed to the increased immunity directly against neo-epitopes [339]. Additionally, the RNA editing enzyme ADAR1 has long been known to be a major repressor of Z-type dsRNA (a substrate of ZBP1), and this suppression mechanism results in resistance and poor reactivity to ICIs, while the use of small-molecule drug, CBL0137, directly induces Z-type dsDNA formation in cells and results in activation of ZBP1-depenent necroptosis, which significantly reverses ICIs unresponsiveness of melanoma mouse models [236]. Similarly, the RIPK1-dependent necroptosis is inhibited by the cIAPs that can be antagonized by Smac mimetics and activate necroptotic death pathway in Burkitt’s lymphoma cell lines [237]. Also, in melanoma, the use of Smac mimetics enhances the response to ICIs via directly controlling immune cell (including B cells, MDSCs, DCs, and cytotoxic T cells) [340]. The evidence demonstrates that we might harness necroptotic mechanism in modify the TME to be more prepared for immunotherapy.

Pyroptosis is the main host defensive mechanism, and it boosts the tumor-killing activity of immune cells [181]. Wang et al. found that in the presence of pyroptosis, ICI-based therapies were effective in killing cold tumor cells, which is attributed to the fired up TME caused by pyroptosis-induced inflammation [341]. Analogously, the engineering of multienzyme-mimicking covalent organic frameworks induces pyroptosis and remodel the TME to trigger durable antitumor immunity for αPD-1 checkpoint blockade therapy [342]. However, the potent proinflammatory role of pyroptosis may cause undesirable side effect in immunotherapy. We previously mentioned that pyroptotic cell death that mediated via CAR-T cell therapy can positively enhance the efficacy, but it also counteracts the effectiveness of CAR-T therapy via initiating cytokine release syndrome [343]. The cytokine release syndrome is a severe side effect brought on by an amplified inflammatory reaction mediated by pyroptosis. In detail, IL-1β and IL-18 are released through the first activated Gzm B/caspase-3/GSDME pathway in target tumor cells, which later amplifies the inflammatory response by activating the caspase-1/GSDMD axis in MΦs [343].

It has been reported that lipid peroxides caused by ferroptosis can be utilized as a signal to facilitate the recognition and processing of tumor antigens by DCs and present them to CD8 + T cells, activating cytotoxic T lymphocytes to enhance tumor immunotherapy [344]. Thus, the combination of ferroptosis inducers with ICIs might be an excellent choice for sensitizing malignant cells to immunotherapy. Indeed, the research from Weimin Wang and colleagues has shown that the combination of GPX4 inhibitor, cyst(e)inase with PD-L1 blockade, can improve T cell-induced antitumor immunity and ferroptotic death of cancer cells synergistically [345]. Similarly, an innovative NRF2 nanomodulator, ZVI-NP, which both inhibits the antiferroptotic function of NRF2 and generates massive ROS via Fenton reaction, can potently augment antitumor immune response by reprograming the TME [194]. However, similar to the consequence of macrophage pyroptosis, the ferroptotic death of nontumor cells is associated with impaired antitumor ability because of reduced cytotoxic cytokine production. And harnessing ferroptosis inhibitor ferrostatin-1 significantly prevents CD8 + T lymphocytes ferroptosis via suppressing lipid peroxidation; consequently, cytokine production is increased, resulting in tumor clearance. More importantly, ferroptosis inhibition therapy obtains greater antitumor efficacy when in combination with anti-PD-1 antibodies [346].

Since cuproptosis is a novel mode of cell death reported this year, its role in immunotherapy is more focused on research in bioinformatics. The cuproptosis-related modification patterns developed by Zhiyong Cai and colleagues were demonstrated to be employed in prediction of immune cell infiltration in TME and evaluation of an individual’s sensitivity to ICIs [347]. It is highly likely that cuproptosis can also play a role in tumor immunotherapy, so we look forward to more experimental studies on the aspect of cuproptosis. Based on these findings, we assume that the combination therapy strategies might possess great potential to alleviate the challenge of monotherapy resistance, such as 1. combination of RCD modulators with conventional drug therapy; 2. combination of chemotherapy drugs and immunotherapy; and 3. combination of immunotherapy with radiotherapy.

Conclusions

Approaches targeting novel RCD modalities hold promise as novel treatments for cancer, and considerable efforts are devoted to translating novel regulators to the clinic. Thus, we complemented the review of approved drugs that modulate novel RCD pathways with descriptions of some newly developed beneficial small-molecule compounds and nanomaterials and clinical trials which intend to explore the changes in the expression levels of four novel biomarkers of RCD. Finally, RCDs can also make tumors more responsive to immunotherapy by regulating tumor immunogenicity and enhancing lymphocyte infiltration in the TME.

However, in spite of the discovery of many compounds and agents that induce or modulate novel RCD programming and that exert strong antitumor effects, many studies reported opposite outcomes. For instance, Chao-Chieh Lin and colleagues discovered that the expression of the key necroptosis mediator RIPK3 in recurrent tumor cells contributed to clonogenic cell growth, causing p53 destabilization and promoting the activities of the YAP/TAZ pathways [348]. Yee et al. also found that ferroptosis induced by neutrophils played a significant role in promoting the aggressiveness of glioblastoma [349]. Therefore, accurate identification of the role of RCD in different types of cancer allows for better utilization of RCD modulators. Greater knowledge of the role played by the TME in controlling tumor cell death may also facilitate the development of cancer eradication therapy. In conclusion, our review postulates that strategies for the pharmacological modulation of novel tumor cell death pathways may be very helpful in cancer treatments, and we encourage future studies using animal models to identify additional outcomes. Additionally, we hope that more clinical trials investigating the use of novel cell death modulations in cancer patients will be conducted.