Teng Liu ,Ki Yng ,Zhung Liu

a State Key Laboratory of Radiation Medicine and Protection,School of Radiation Medicine and Protection&School for Radiological and Interdisciplinary Sciences(RADX),Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions,Soochow University,Suzhou,Jiangsu,215123,China

b Institute of Functional Nano &Soft Materials (FUNSOM),Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices,Soochow University,Suzhou,Jiangsu,215123,China

Keywords:X-ray irradiation Nanomaterials Cancer therapy Radiation therapy Radiodynamic therapy

ABSTRACT X-ray is a form of ionizing radiation with high energy and strong tissue-penetrability,which has been widely used for cancer therapy,including radiation therapy (RT) as one of the mainstream cancer treatments and radiodynamic therapy (RDT) as an emerging treatment modality.However,the nonspecific absorption of X-ray and peculiar tumor microenvironment would lead to the unsatisfied therapeutic effect and severe damage to normal tissues.The fast progress of nanotechnology has provided great opportunities for the development of functional nanoparticles and strategies to solve these problems.In this review,we will summarize the recent advances in functional nanomaterials and the related strategies,discuss how they solve the challenges in order to improve the therapeutic efficacy and safety of RT and RDT,and moreover comment on the challenges and prospects for efficient X-ray triggered cancer therapy based on nanomedicine.

1.Introduction

Since the discovery in 1895 by Roentgen,X-ray has received a lot of research interests due to its strong energy and high penetration depth.The interaction of X-ray with matter would lead to the attenuation of energy and the production of electrons and photons.During the last century,the specific physical processes of X-ray when encountered with atoms (Fig.1) have been systematically studied:Rayleigh scattering of X-ray provides no energy transfer due to the elastic nature;Meanwhile,Compton scattering(inelastic scattering)of X-ray would transfer partial energy of the incident proton to an ejected electron;Moreover,the photoelectric effect of X-ray would eject electrons and generate excess energy that released in the form of Auger electrons (short-range secondary electrons) or fluorescent photons [1].The above clarified mechanism has promoted the applications of X-ray in various fields including biomedical disease treatment.

Cancer is one of the leading diseases that threaten human health worldwide.X-ray computed tomography (CT) was developed to obtain three-dimensional information of human by the differential X-ray attenuation (or absorption),which has been applied for tumor diagnosis with the help of contrast agents [2].X-ray triggered radiation therapy(or radiotherapy,RT) has been developed as one of the mainstream methods for clinical cancer treatments [3].During RT,the emissive photoelectrons and Compton electrons of X-ray can directly interact with DNA to cause damage,while the Auger electrons can indirectly react with the surrounding water to produce reactive oxygen species(ROS)to destroy DNA or other cellular components,all of which would induce cellular apoptosis and necrosis to inhibit tumor growth [4,5].Although RT has been widely used to treat more than half of the cancer patients using X-ray with focused energy and strong tissue-penetrability,this cancer treatment modality still has several shortcomings including the compromised therapeutic effect due to the insufficient Xray absorption of tumors,the severe side effects to normal tissues under high-dose radiation,the tumor hypoxia-related RT resistance,and the incapability of treating metastatic cancer cells.

Most recently,the fluorescent photons emitted from X-ray radiation has been studied for X-ray excited optical luminescence (XEOL) imaging and cancer treatment (X-ray induced photodynamic therapy,or radiodynamic therapy,RDT) [6,7].In general,RDT utilize X-ray induced fluorescence to trigger ROS generation via the photodynamic effect of photosensitizers,resulting in the death of cancer cells [8,9].Compared to conventional photodynamic therapy,RDT has advantages of high penetration-depth and feasible clinical facilities.Meanwhile,RDT is superior to RT treatment because RDT required relatively lower dose of X-ray irradiation.However,RDT is still in the infant stage,asking for more efforts devoting to the study of multifunctional agents and concrete mechanism to expand its wide applications.

Fig.1.Schematic illustration of the physical interaction of X-rays with nanomaterials.

The development of nanomedicine has evoked many interests in exploiting various functional nanomaterials to improve the therapeutic outcome of cancer [10,11].The nanosized dimension of nanomaterials facilitate their blood circulation and passive or active targeting for tumor accumulation.On the other hand,nanomaterials with devisable physiochemical properties are beneficial for exploring novel strategies to enhance the therapeutic effects.Recently,various functional nanomaterials have been developed as nanomedicine for novel therapy strategies to enhance the therapeutic effect of X-ray triggered cancer therapy.Herein,we will summarize the recent progress of nanomaterials for RT and RDT and discuss their functional roles in enhancing Xray triggered cancer treatment.The emerging nanomaterial-based strategies for RT therapy will be categorized and discussed.In addition,we will review the different types of nanomaterials for RDT and the corresponding mechanisms.

2.Nanomaterials for radiation therapy

RT has been widely applied for more than half of cancer patients either by itself or combined with other therapy modality.However,the therapeutic effect of RT is limited due to the low radiation absorption of tumors while RT may cause injury to the surrounding normal tissues at large dose of radiation.The tumor microenvironment (TME) with lowlevel of oxygen and high-level of reductant (glutathione,GSH) would cause radioresistance.Moreover,most of the RT treatment could only inhibit the growth of solid tumors and hardly control metastasis.To combat these challenges of classical RT,several strategies based on nanotechnology(Table 1)have been developed in order to enhance the therapeutic effect and avoid the side effect.

Table 1 Summary of representative nanomaterial-based strategies to enhance the therapeutic effect of radiation therapy.

2.1.Nanomaterials for radiosensitization

Since the photoelectric effect of X-ray radiation is positively correlated to(Z/E)3,nanomaterials with high-atomic number(Z)could act as potent radiosensitizers to increase the effectiveness of a given radiation dose.Gold nanostructures(Z=79)with high chemical inertness are the most representative radiosensitizers that have been widely explored to locally enhance the radiation deposition at the tumor site[12—14].It is worth noting that taking advantage of the strong X-ray attenuation ability of high-Z elements,CT imaging based on the identical nanomaterials is often performed to mark the tumor before RT treatment.Many other nanomaterials with high-Z elements have also been documented to adopt their strong photoelectric absorption capabilities for RT treatment [5].

For example,upconversion nanoparticles have been reported to enable effective radiosensitization by concentrating higher dose in specific focus [15].In the meanwhile,their biodistribution can be timely tracked by CT,magnetic resonance (MR),or upconversion fluorescence imaging which are beneficial for the selection of the optimized treatment window.Considering that bismuth has the highest atomic number(Z=83)among all nonradiative elements and selenide has anticancer activity,Zhang et al.developed Bi2Se3nanoplates with the protection of poly(vinylpyrrolidone)(PVP)as a radiosensitizer[16].They discovered that the Bi2Se3nanoplates could accumulate into tumor,efficiently delayed the tumor growth after ionizing radiation,and be cleared after 90 days via oxygenation.This work has inspired more studies on the toxicity,biodistribution,and clearance behavior of nanosensitizers.

Recently,Shen et al.prepared a renal-clearable nano radiosensitizer by coordinating tungsten ions (WVI) and gallic acid (GA),which could be labeled with64Cu in a chelator-free manner (Fig.2) [17].As monitored by positron emission tomography (PET) imaging,the W-GA nanodots could efficiently accumulate in the tumor to greatly improve the RT efficacy in inhibiting tumor growth.The W-GA nanodots could be rapidly excreted after the treatment,which excludes the concerns regarding long-term toxicity of heavy metals and shows great promise for clinical applications.

In addition to conventional nanomaterials with high-Z elements which can enhance the X-ray deposition in tumor,other nanomaterials with distinctive chemical properties have recently emerged as novel radiosensitizers to enhance RT through other mechanisms.For example,it has been reported that the silver ions released from Ag nanostructures could sensitize RT to kill cancer cells in vitro [18,19].In addition,iron based-nanomaterials have been reported to facilitate the production of ROS under X-ray irradiation due to the iron-catalyzed Haber-Weiss cycle and Fenton reaction [20].Moreover,Bu et al.discovered that the continuous Fenton reaction induced by iron ions in nanomaterials could reassort cell cycle to radiosensitive G2/M phase and down regulate DNA repair-related proteins,thus enhancing the cancer cell killing effect of X-ray irradiation [21].

Another kind of radiosensitizer is nanomaterials releasing nitric oxide (NO)under stimuli.NO has been well recognized as a biological molecule involved in physiological process,showing anticancer effects without side effect at the concentration of 1 μM-1 mM [22].Recent researches have revealed that the on-demand generation of NO could sensitize RT by inhibiting the DNA repair after high-energy irradiation as well as continually reacting with ROS to yield more toxic peroxynitrite radicals[23].For example,Shi's group designed a polyethylene glycol modified UCNP@silica nanostructure with the loading of S-nitrosothiol(PEG-USMSs-SNO),which could cleave S—N bond upon X-ray irradiation and release electrophilic NO to secure the damage of cellular DNA and enhance RT treatment[24].Klein et al.functionalized nitrosyl tetrafluoroborate(NOBF4)onto snowman-shaped Au-Fe3O4nanoheterodimers (NHDs) [25].The efficient electron transfer across the NHDs facilitated the simultaneous production of NO radicals and oxygen radicals under X-ray irradiation.Moreover,the surfaceconfined reaction between them would lead to the production of highly reactive peroxynitrite,resulting in rapid cell death.

2.2.Nanomaterials for combination therapy

Besides using nanomaterials with high-Z elements to focus radiation energy in tumors,it is also common to design multifunctional nanocarriers loaded with therapeutic agents to combine RT with other therapeutic modalities,such as chemotherapy,photodynamic therapy(PDT),photothermal therapy (PTT) and immunotherapy.

Liu's group has demonstrated the synergistic effect of RT and chemotherapy or PDT on cancer therapy by tantalum oxide nanomaterials and biodegradable nanoscale metal organic frameworks [26—28].Recently,Yi and his coworkers studied the mechanism behind the synergistic effect of RT and chemotherapy based on CuS@Melanin-PEG/DOX [29].This work showed that ionizing radiation would be able to induce cell cycle change and Caveolin-1 upregulation,and thusobviously increase the uptake and reduce the efflux of drug-loaded nanomaterials by tumor cells,resulting in significant synergistic effect for antitumor therapy.

Fig.2.Renal-clearable W-GA nanodots for chelatorfree 64Cu-labeling and imaging-guided radiation therapy.(a) Schematic illustration of labeling radioisotope 64Cu onto W-GA nanodots and their applications for RT/PET guided radiation therapy of tumor.(b) In vivo PET images of mice after the injection of 64Cu—W-GA nanodots.(c)Quantification of the uptake dose of nanodots in the liver,kidney,tumor,bladder and muscle at various time points.Reprinted with permission from Refs.[17].Copyright 2017 American Chemical Society.

The integration of hyperthermia and RT based on multifunctional nanomaterials could also enhance the therapeutic effect of tumor.For example,Cheng et al.has developed a two-dimensional FeSe2—Bi2Se3nanostructure with high-Z elements and high optical absorbance,achieving synergistic tumor ablation by combined PTT and radiation therapy [30].Meanwhile,the tetra-modal imaging capability of the64Cu labeled FeSe2—Bi2Se3nanostructure helped to optimize the treatment planning and monitor the therapeutic results.To address the relatively weak penetration ability of NIR-I light,a few nanocomposites with high absorbance in the NIR-II region (1000—1500 nm) as well as strong X-ray attenuation capability has been designed,such as Ti3C2@Au core-shell nanocomposites [31] and Cu3BiSe3nanoparticles[32].It has been revealed that the hyperthermia induced by laser would increase the blood flow and elevate the oxygen level in tumor,which could enhance the sensitivity of cancer cells to ionizing radiation.

Fig.3.Inhibition of tumor metastasis by combing radiation therapy with PLGA-R837@Cat and αCTLA4 immunotherapy.(a)Schematic illustration of the combined radiation therapy and immunotherapy based on PLGA-R837@Cat and the mechanism of the antitumor immune responses.(b) Schematic illustration of the experiments designed for inhibiting tumor metastasis.(c) Morbidity free survival of different groups of mice after elimination of their primary tumors by various treatments.Reprinted with permission from Ref.[34].Copyright 2019 John Wiley &Sons,Inc.

Since localized RT could only eliminate the primary solid tumor,the metastasis and recurrence of tumor still threaten the health of patients.It has been proposed that local radiation may produce systemic immune responses and,potentially,antimetastatic effects [33].Chen et al.designed a core-shell nanoparticle (PLGA-R837@Cat) to combine the local radiotherapy and immune-modulation for cancer treatment(Fig.3) [34].The PLGA-R837@Cat nanoparticles could not only directly enhance RT therapeutic efficacy by modulating the immunesuppressive tumor microenvironment,but also induce immunogenic cell death to trigger strong immune responses with the help of Toll likereceptor-7 agonist R837.The RT treatment based on PLGA-R837@Cat nanoparticles,together with CTLA-4 checkpoint blockade,would result in effective abscopal effect for metastasis inhibition and long-term immunogenic memory for tumor challenging protection.

2.3.Nanomaterials for TME modulation

The rapid growth and disordered blood vessels of tumor would result in the unique tumor microenvironment (TME),which is characterized with insufficient oxygen (hypoxia),acidic pH(acidosis) and elevated hydrogen peroxide concentration.During radiation therapy,oxygen plays an important role by preventing the radiation-induced DNA breaks from being repaired by cells [35].Hence,the oxygen deficiency in solid tumors is one of the major reasons for radiation resistance,leading to unsatisfied DNA damage and therapeutic failure[36].Moreover,the tumoral hypoxia as well as RT treatment would cause the upregulation of hypoxia inducible factor 1 (HIF1) activity,which may protect tumor and endothelial cells from damage by the cytotoxic therapy and further compromise RT efficacy [37].Various nanotechnology-based therapeutic strategies,including oxygen delivery,oxygen generation,regulation of GSH concentration and oxygen-independent therapy strategy,have been proposed in order to modulate the TME for optimized RT cancer treatment.

In addition to the above hyperthermia-mediated hypoxia relief method in combination therapy,nanocarriers for oxygen delivery have been developed to alleviate the tumor hypoxia and improve the RT effectiveness.Perfluorocarbon (PFC) has been widely applied as artificial blood substitute due to their high oxygen solubility.Many groups including ours have applied liposome[38],human serum albumin[39],polymer and erythrocyte membrane [40] or hollow inorganic nanomaterials [41,42] to stabilize PFC in nanoscale for prolonged blood circulation,which have been effectively relieve the hypoxia in tumor for enhanced RT.Among them,hollow Bi2Se3nanoparticles loaded with PFC (Bi2Se3@PFC) have combined the RT sensitization by high-Z elements and oxygen delivery capability of PFC [41].Moreover,NIR laser irradiation could externally trigger the burst release of oxygen from Bi2Se3@PFC,resulting in synergistic improvement in radio-sensitization effect.

Besides forthright oxygen delivery into tumor,the tumoral acidic pH and elevated hydrogen peroxide concentration has been adopted to produce oxygen in situ and alleviate the radiation resistance.Nanomaterials containing MnO2[43—45] or catalase [46] have been designed to specifically decompose the endogenic hydrogen peroxide inside tumor and release tremendous oxygen to overcome the hypoxic microenvironment to enhance RT efficacy.For example,Prasad et al.engineered polyelectrolyte—albumin complex and MnO2nanoparticles(A-MnO2NPs) for simultaneous oxygen generation and pH increase.The treatment of A-MnO2NPs would lead to the downregulation ofHIF1αand vascular endothelial growth factor(VEGF)regulators in the tumor,which in combination with X-ray radiation,resulting in massive DNA double strand breaks and cancer cell death [47].Furthermore,biomineralized BSA-Au-MnO2nanoparticles were prepared with biodegradability,imaging functionality,and TME modulation capability,which are multifunctional radiosensitizers promising for optimized cancer radiotherapy [48].

Fig.4.Amplification of tumor oxidative stresses by BSO/GA-Fe(II)@liposome for enhanced radiation therapy.(a) Schematic illustration of experimental design for enhanced radiation therapy by BSO/GA-Fe(II)@liposome.(b) Tumor growth curves of mice after various treatments.(c) H&E stained tumor slices of each group at 24 h post after various treatments.(d) Schematic illustration of the mechanism for efficient disruption of redox homeostasis and enhanced radiation therapy.Reprinted with permission from Ref.[49].Copyright 2018 American Chemical Society.

Recently,Dong et al.designed a biocompatible BSO/GA-Fe(II)@liposome to amplify intracellular oxidative stress by simultaneous ROS generation and GSH depletion (Fig.4) [49].Among this liposome,GAFe(II)with Fenton catalytic activity could persistently convert peroxide into highly cytotoxic hydroxyl radicals,L-buthionine sulfoximine(BSO)could efficiently inhibit the intracellular GSH synthesis,and the feasible chelation of99mTc4+radioisotope could favor in vivo tracking of liposome by SPECT imaging.With the increased hydroxyl radicals and GSH depletion,BSO/GA-Fe(II)@liposome efficiently increased the tumor oxidative stresses and disrupted the intratumoral redox hemostasis,leading to the therapeutic enhancement of X-ray induced radiotherapy.

Moreover,Zhao's group proposed an oxygen-independent therapy strategy named “X-ray-induced catalytic reaction” to combat the oxygen deficiency in TME [50].They discovered that Au—Bi2S3heterostructure could not only sensitize RT efficacy by high-Z elements,but also catalyze the decomposition of H2O2into highly toxic hydroxyl radicals (·OH) under X-ray irradiation.The simultaneous enhancement of X-ray energy deposition and radio catalysis capability of this Au—Bi2S3heterostructure,which combined with its intrinsic photothermal effect could completely ablate the tumors in mice.The therapeutic effect of this strategy has also been confirmed by Cu(OH)PO4nanocrystals [51] and Bi2WO6nanoplates [52] in tumor-bearing mice.

2.4.Nanomaterials for radioprotection

During RT treatment of tumor,the nonspecific ionizing radiation would cause serious damage to the adjacent healthy tissues.Hence,nanomaterials with ROS-scavenging capability have been designed as radio protectors to selectively protect normal tissues from X-ray induced damage and increase RT efficacy under high dose of ionizing radiation.

It has been reported that nanoceria (CeO2nanoparticles) can scavenge free radicals by changing the charge stage of Ce(Ce3+and Ce4+)in neutral environment of normal cells,while their catalase catalytic property is inactive in acidic cancer cells [53,54].The unique selective anti-oxidative property of CeO2nanoparticles in cells has inspired extensive exploration for more radio protective nanoagents.

Fig.5.MoS2 nanodots with renal clearance for radiation protection.(a)Scheme illustration of the mechanism for MoS2 nanodots to function as radioprotectors.(b)The TEM images showing the ultrasmall size of MoS2 nanodots and the corresponding photos of MoS2 nanodot solution.(c) Dose-dependent radiation protection in vitro.(d)Survival curves of mice treated with or without cysteine protected MoS2 dots.Reprinted with permission from Ref.[55].Copyright 2016 American Chemical Society.

Zhang et al.has designed highly catalytic MoS2nanodots as radioprotectants against ionizing radiation (Fig.5) [55].The MoS2nanodots with ultrasmall size and cysteine protection could clean up the accumulated radicals within body and subsequently be excreted via urine rapidly.With the injection of MoS2nanodots in advance,the surviving fraction of radiation-treated mice was appreciably increased because that the injected MoS2nanodots could repair DNA damage and recover all vital chemical and biochemical indicators.Similar to MoS2nanodots,other transition metal dichalcogenide(TMDC)nanomaterials[56,57],noble-metal nanomaterials [58,59] and carbon-based nanomaterials[60—62]have also been reported as radioprotectors to reduce Xray-induced DNA damage of normal cells.

In addition to adopting inorganic nanoparticles as radioprotectors themselves,nanomaterials have also been explored to serve as nanocarriers for the delivery of radioprotective molecules including curcumin,methylproamine,sesamol and amifostine [63].The bioavailability of molecular radioprotective drugs could be enhanced by loading onto nanocarriers,leading to the improvement of their radioprotective efficacy and ultimately promising for potent and safe RT therapeutic efficiency.

3.Nanomaterials for radiodynamic therapy

Radiation therapy triggered by ionizing radiation are mostly dependent on the Compton and Auger electrons produced by the Compton scattering and photoelectric effect of X-ray.Recently,the fluorescence emitted by X-ray stimulation has also been adopted for cancer therapy,which is commonly denoted as radiodynamic therapy (RDT) or X-ray induced photodynamic therapy(X-PDT).In general,RDT utilized the Xray triggered fluorescence to stimulate the ROS generation in the presence of photosensitizers (PSs) and oxygen.RDT possesses the advantages of deep-penetration capability compared to PDT and low-radiation dose requirement compared to RT.However,RDT still faces challenges including insufficient energy transfer,photodestruction of PSs and strong oxygen dependence.Various kinds of nanoplatforms and strategies (Table 2) have been designed to solve these problems and enhance the RDT therapeutic efficacy of cancer.

Table 2 Summary of representative nanomaterials for radiodynamic therapy.

3.1.Scintillating nanoparticles loaded with PSs for RDT

Scintillators including rare-earth-element-based nanomaterials and heavy metal-based nanoparticles have been adopted to introduce X-ray as the external source to stimulate RDT of tumor in depth [7].These scintillating nanoparticles with wide band gap would absorb X-rays and transmit the energy to UV/visible photons,emitting luminescence to activate nearby photosensitizers for ROS generation.To achieve the efficient RDT treatment,the scintillating nanoparticles should meet the requirements including excellent luminescence efficacy,high loading capacity of PSs,the emission spectrum well-overlapped with PSs absorption,and last but not least the nontoxic nature in physiological environment.

In 2006,Chen et al.first reported the scintillating nanoparticles attached with PSs(porphyrin)for RDT to kill cancer cells[6],leading to the fast and prosperous development of various scintillating nanoparticles for RDT.Interestingly,Zhang et al.designed a nanosystem composed of CeIII-doped LiYF4as the scintillating nanoparticles and ZnO as the photosensitizer,producing highly reactive ·OH radicals upon X-ray irradiation via an oxygen-independent manner,which could avoid the oxygen deficiency-related restriction to RDT efficiency [64].In another work,SrAl2O4:Eu2+(SAO) possessing strong luminescent and hydrolytic nature was developed to lower the required X-ray dose for RDT stimulation and minimize long-term toxicity [65].After the coating of silica and the loading of PSs MC540,the SAO-based nanosystems achieved potent production of cytotoxic singlet oxygen (1O2)upon X-ray irradiation at low dose(0.5 Gy).Moreover,these SAO-based nanosystems could not only efficiently cause tumor shrinkage,but most of which were degraded and excreted by day 16.

Persistent luminescence nanoparticles(PLNPs)emitting long-lasting phosphorescence without continuous excitation have also been proposed for the depth-independent and repeatable RDT [66].Song et al.synthesized W(VI)-doped ZnGa2O4:Cr PLNPs,showing stronger persistent luminescence intensity and longer persistent luminescence time(40 min) compared to traditional ZnGa2O4:Cr PLNPs [67].After modification and loading with photosensitizer molecule ZnPcS4,the nanoplatform could be activated by a particularly low-dose of X-ray irradiation (0.18 Gy) and significantly control the growth of tumor.

Beyond rare-earth-element-based nanomaterials,nanoparticles composed of heavy metals have also been reported as promising scintillators for RDT treatment.Sun et al.conjugated heterogeneous gold clustoluminogens (AIE-Au) with photosensitizer molecule rose bengal(RB) [68].Taking advantage of the high-Z elements and aggregation induced emission (AIE) of gold clustoluminogens,this RB-AIE-Au nanoplatform could simultaneously generate hydroxyl radicals to enhance RT effect and convert X-rays to excite the conjugated photosensitizers for RDT,allowing for deep-seated tumor treatment at relatively low dose of ionizing radiation(1 Gy).The same group also prepared Zn-and Mn-doped silicate(ZSM)nanoscintillators with uniform size and strong X-ray excited optical luminescence(XEOL)in a large scale(Fig.6)[69].The RGD-ZSM-RB nanosensitizers was achieved by conjugating with photosensitizer RB and targeting peptides arginylglycylaspartic acid(RGD).The fluorescence and CT images demonstrated the effective tumor accumulation of the RGD-ZSM-RB,which is beneficial for the inhibition of the radioresistant brain tumor under low-dose X-ray irradiation (1 Gy).Moreover,1O2-detection,lipid peroxidation assay,COX-2 assay,γH2AX assay and comet assay revealed that the efficient damage to cancer cells was attributed to the synergistic effect of RTRDT combination therapy.Most recently,polyoxomolybdate (POMo)nanocluster loaded with RB was reported and they achieved efficient cancer therapy by combining RT and RDT [70].

3.2.Nanomaterials without extra PSs for RDT

Scintillating nanoparticles loaded with PSs is the mainstream for RDT cancer treatment.However,the efficiency of RDT may be impeded by the instability of organic photosensitizer molecules,the complex synthesis procedure of inorganic PSs onto scintillators,as well as the inefficient energy transmission transformation from X-ray to PSs.Hence,RDT based on individual nanomaterials without introducing extra PSs provides a novel design aspect to improve the RDT therapeutic efficiency.

Lin's group has designed a series of nanoscale metal-organic frameworks (NMOFs) for X-ray induced RDT and combination therapy[71—74].Different from traditional scintillating nanoparticles loaded with PSs,the photosensitizing ligands in NMOFs were directly stimulated by photoelectrons produced from X-ray irradiation instead of visible light generated from scintillators.Generally,the heavy Hf atoms within these NMOFs in the secondary building units (SBUs) would efficiently absorb X-rays and transfer to the coordinated ligands to generate ROS for effective RDT.Among them,the MOF layers with twodimensional structure would facilitate the free diffusion of ROS,further contributing to the superb anticancer efficacy of RDT[71,72].Hf-DBBRu was synthesized as a cationic NMOF with intrinsic mitochondriatargeting property,which could generate hydroxyl radicals from its Hf6SBUs and produce singlet oxygen from the DBB-Ru ligands upon X-ray irradiation,enabling the mitochondria-targeted RT-RDT therapy of colorectal tumors at very low X-ray doses and with negligible side effects [73].

To achieve both systemic tumor eradication,Lu et al.combined the NMOF-based RT-RDT therapy with checkpoint blockade immunotherapy [74].After the intratumoral injection of DBP-Hf or TBPHf and low-dose X-ray irradiation,the local tumors of the breast and colorectal cancer models could be effectively eradicated through RTRDT therapy.Moreover,the X-ray treatment based on NMOFs would induce immunogenic cell death and in-situ vaccination,which in combination with indoleamine 2,3-dioxygenase inhibitor (IDOi) being loaded on the pores and channels of NMOFs,would achieve synergistic immunological responses to eradicate distant tumors via consistent abscopal effect as well as inhibit tumor-challenging via antitumor immune memory.

Fig.6.Zn-and Mn-doped silicate (ZSM)nanoscintillators for low-dose X-ray induced radiodynamic therapy.(a) A scheme illustrating the synthesis procedure of ZSM nanoscintillators.(b) Schematic illustration of the experimental treatment of tumor in vivo.(c) Photographs of representative tumors taken from each group.Reprinted with permission from Refs.[69].Copyright 2019 John Wiley &Sons,Inc.

Fig.7.NaCeF4:Gd,Tb scintillator for multimodal imaging-guided synchronous radio/radiodynamic therapy.(a)Schematic illustration of NaCeF4:Gd,Tb scintillator for combined radio/radiodynamic therapy guided by XEF,CT,and T2-weighted MR imaging.(b) Schematic illustration of the scintillator mediated XEF and RDT excited by X-ray.(c)XEF imaging of tumor-bearing mice with different irradiation voltages and time.(d)MR imaging of tumor before and after intratumoral injection of NaCeF4:Gd,Tb scintillator.(e) CT imaging of mice before and after intratumoral injection of NaCeF4:Gd,Tb scintillator.(f) The relative tumor growth curves of mice after each treatment.Reprinted with permission from Ref.[75].Copyright 2019 American Chemical Society.

In a recent report,Zhong et al.designed a theranostic NaCeF4:Gd,Tb scintillating nanoparticle which can be directly triggered by X-ray for synchronous RT-RDT cancer therapy (Fig.7) [75].Upon X-ray irradiation,the NaCeF4:Gd,Tb nanoparticles with the sensitization of Ce3+ions can emit fluorescence (XEF) and meanwhile absorb the secondary electrons to generate ROS for RDT.Meanwhile,the NaCeF4:Gd,Tb consisted of high-Z lanthanide elements have strong X-ray absorption,which would enhance CT imaging and sensitize RT efficacy.Moreover,the interaction between Ce3+,Tb3+,and Gd3+ions leads to the shortened transverse relaxation time of Gd3+ions,which is beneficial for T2-weighted MR imaging.Taken together,theNaCeF4:Gd,Tb scintillating nanoparticle could serve as an effective theranostic agent for XEF,CT,and T2-weightedMR imaging,as well as RT-RDT combination therapy.In addition,copper-cysteamine (Cu-Cy) nanoparticles have also been reported as a promising nanoagent that can be directly activated by X-rays to generate ROS for efficient treatment of deepseated tumor [76].

4.Conclusions and prospects

The therapeutic efficiency of radiotherapy triggered by X-ray is affected by multiple parameters,such as the X-ray energy deposition,tumoral radiosensitivity and the DNA repair tendency.In order to optimize the therapeutic regimen for X-ray triggered cancer therapy,herein,we have summarized and discussed the recent progress of nanomaterials for RT and RDT,their functional roles in cancer treatment as well as the related therapy strategies.

For traditional RT therapy with the problems of insufficient tumor absorption of X-ray,nonspecific damage to normal cells,dependence on oxygen,incapable of treating metastatic cancer cells,functional nanomaterials have inspired a number of strategies including high-Z nanomaterials,combination therapy,TME modulation and radiation protection.

(I) Various classical nanomaterials with high-Z elements have been developed to specifically concentrate X-ray deposition in tumor.During the adoption of this strategy,the ideal biodistribution and high tumor accumulation of nanomaterials are especially crucial to effectively sensitize the RT effect for cancer treatment and,in the meanwhile,alleviate side effects in normal tissues and organs.

(II) The combination of RT with other therapy modality such as chemotherapy,PDT,PTT and immunotherapy could enhance the therapeutic effect through synergistic effect.Especially,the combination with immunotherapy would bring opportunities to treat metastatic cancer and prevent cancer recurrence.During combination therapy,the synergistic mechanism should be thoroughly studied to enhance the synergistic effect,and meanwhile the therapeutic procedure including the sequence,time and dosage,should be carefully tailored to avoid additional toxicity to normal tissues.

(III) The modulation of TME by O2delivery,in-situ O2production,reducing GSH concentration and direct decomposition of peroxide could elevate the ROS generation to induce DNA damages and impede the repairment,leading to the potent cancer cell killing capability.During TME modulation,it should be noticed that the chemical reactivity of nanomaterials in tumor may not be the same as in vitro.Hence,the comprehensive understanding and simultaneous regulation of several parameters in tumor are of great importance to effectively implement the TME-modulation function of nanomaterials.

(IV) The application of nanomaterials as radioprotectors themselves or as nanocarrier for radioprotective molecules could scavenge ROS in normal tissues and eliminate the side effect concern.With the help of radioprotectors,RT treatment could be conducted using a relatively potent X-ray irradiation.To achieve enhanced RT effi-cacy and prolonged survival time for the patients,it is challengeable to realize the selective and effective protection of normal tissues under X-ray irradiation.

On the other hand,the latest X-ray triggered therapeutic modality RDT has achieved fast progress by the development of (i) nanoscintillators loaded with PSs (rare-earth-element-based NPs and metal-based NPs) and (ii) unitary nanomaterials without extra PSs(MOFs and others).These functional nanomaterials and therapy strategies have achieved success in improving the therapeutic efficacy and decreasing side effects in X-ray triggered cancer therapy.However,on the way to pursuing effective cancer therapy promising for clinical translation,there are still many challenges regarding the design of nanomaterials,the mechanism of RT and RDT,and the studies of oncology.

1) The optimization of the biocompatibility and utilization of nanomaterials.Great efforts have been devoted to design various functional nanomaterials for X-ray triggered cancer therapy.However,one of the critical obstacles for clinical translation is the toxic issues of nanomaterials.Although appropriate modification could endow the nanomaterials with biocompatibility and active-targeting capability,the long-term retention of nanomaterials in the whole body would lead to concerns of long-term toxic.Hence,it is of great importance to design ultrasmall nanomaterials with renal-clearable nature or biodegradable nanomaterials with biocompatible components,and at the meantime maintain their functions for X-ray triggered cancer therapy.It is also highly desired to design simultaneous imaging functions on nanomaterials to monitor their biodistribution and tumor accumulation for optimized treatment window,as well as their metabolism behavior and clearance routes for long-term toxicity evaluation.

2) The mechanism study of ROS generation either dependent or independent of oxygen.Taking advantage of the physical interaction of X-ray with nanomaterials,RT could directly or indirectly damage the DNA of cancer cells by electrons,while RDT would utilize the fluorescence or photons produced by X-ray to produce ROS for cancer treatment in a lower dose.However,most of them are highly dependent on oxygen,which is deficient in solid tumors.There are several reports declaring an oxygen-independent manner for RDT,while their energy conversion efficiency is still limited.Hence,great efforts should be devoted to understanding the mechanism for X-ray induced ROS generation assisted by NPs and resolve the limitations of oxygen deficiency and inefficient energy transformation.

3) Comprehensive studies on all of the TME parameters.By excavating the characteristic of tumor microenvironment,the therapeutic effi-ciency of RT treatment has been improved by strategies including elevating oxygen concentration,decomposition of hydroperoxide and limiting the production of GSH.These strategies are also promising for enhancing the efficacy of RDT.On the other hand,further investigations are required for oncology studies to modulate TME by other methods such as neutralize the acidic pH,normalize the vasculature,decomposition of the extracellular matrix.

4) Thorough evaluation of the immune combination therapy and related mechanism.Several pioneer studies have shown the combination of X-ray triggered therapy and immunotherapy (especially checkpoint blockage immunotherapy),showing efficient eradication of both local and systemic cancer cells.Hence,the development of novel nanomaterials to combine X-ray triggered therapy with other immunotherapy modalities and more studies on their mechanisms to enhance antitumor immunity would inspire strategies for comprehensive cancer therapy and clinical translation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21927803,51525203,51761145041,31822022 and U1932208),the National Science Foundation of the Jiangsu Higher Education Institutions of China (18KJB430024),the Natural Science Foundation of Jiangsu Province (BK20190946),and the Collaborative Innovation Center of Suzhou Nano Science and Technology.