Metal nanoparticles are novel radiosensitizers due to high absorption coefficient, biocompatibility, synthetic versatility and unique chemical, electronic and optical properties. The relationship between the X-ray absorption phenomenon (E) and the atomic number (Z) is as follows: µ = ρZ4/(AE3), where ρ is the density, A is the atomic mass of the element and μ is the X-ray absorption coefficient (μ) (10). Therefore, the X-ray absorption coefficient (μ) varies with the atomic number (Z). Nanoparticles with high atomic number have a dose-enhancing effect (8, 11, 12). By investigation of the biological mechanisms of high atomic number nanomaterials, theories such as cell cycle effects, DNA repair inhibition and mitochondrial dysfunction have been proposed to explain the biological process of radiation sensitisation of nanomaterials. The cell cycle is an important factor affecting radiosensitivity, with most cells exhibiting radioresistance in late S-phase and radiosensitivity in late G2 and mitotic phases; therefore, metal nanoparticles can improve the radiosensitivity of tumor cells by altering the cell cycle. IR treatment leads to mitochondrial dysfunction and induces the action of related molecules that increase the production of intracellular reactive oxygen species (ROS). Metal nanomaterials promote IR-induced oxidative stress, another important mechanism for their radiosensitization. Chow et al. (13) used Monte Carlo simulation algorithms to obtain how to maximize DER for nanoparticle-enhanced radiotherapy and demonstrated that gold nanoparticles are the most effective material in nanoparticle-enhanced radiation therapy. In addition, lower photon beam energy (6 MV), FFF photon beam, and higher nanoparticle concentration improve the DER of radiation therapy. Understanding the mechanisms of action can help to improve nanoparticle-enhanced radiotherapy and achieve better treatment outcomes.

The atomic number of metal nanoparticles is high, and has been shown to have a dose-enhancing effect on radiotherapy. Among them, gold nanoparticles have been the most extensively studied. AuNPs, with diameters of 1 to 100 nm, are widely used in materials, bioanalytical chemistry, industrial catalysis and medicine due to their high electron density, dielectric properties and catalytic properties, and their ability to bind to various biomolecules without affecting their biological activity (14). In the field of radiotherapy sensitization, there have been many reports in the literature both at home and abroad confirming the role of radiotherapy sensitization. As early as 2000, Herold et al. (15) conducted a sensitization study on AuNPs. Since then, Hainfeld et al. have investigated the sensitization of AuNPs in mammary tumor mice (16), mice with squamous cell carcinoma of the head and neck (17) and mice with malignant glioma in the brain (18) during radiotherapy. Bobyk et al. (19) actually evaluated the therapeutic efficiency of synchrotron stereotactic radiotherapy combined with loco-regional administration of gold nanoparticles for the treatment of orthotopic F98 gliomas in rats. The median survival time reached 41 days in the rats treated with the combination therapy, while the median overall survival time was 35 days in the rats irradiated alone, which equated to a significant 58% increase in life span. In addition, Zhang et al. (20) used a Monte Carlo simulation algorithm to find that the addition of 10¹³ gold nanospheres per cubic centimeter increased the absorbed radiation dose by 60%, mathematically demonstrating that AuNPs could enhance radiosensitivity. The exact mechanism of radiosensitization by gold nanoparticles is still under debate. The mainstream view is that their radiosensitizing effect is due to the increased photoelectron absorption of high atomic number materials when irradiated (21). Wang et al. (22) suggested that gold nanoparticles could achieve radiosensitization by modulating the cell cycle. Cells are the most radiosensitized in the G2/M phase (15). Zheng et al. (23) conducted a series of experiments to investigate the mechanism of AuNPs sensitization, and found that AuNPs could bind to DNA strands with and without AuNPs by electrostatic forces and adsorb a monolayer of DNA. This demonstrated that AuNPs could induce DNA breakage in tumor cells. Charnay Cunningham et al. (24) confirmed the radiosensitizing potential of AuNPs with proton radiotherapy by evaluating the radiosensitizing effect of AuNPs in combination with a 200 MeV proton beam.

Similar to gold nanomaterials, other metals such as silver, platinum, gadolinium and titanium have similar sensitizing effects on radiotherapy. Silver nanoparticles (AgNPs) combined with radiotherapy can prolong the survival time of glioma mice, which proves that they can enhance the radiosensitivity of human glioma cells in vitro ( 25, 26). Liu et al. (27) found that in rats with malignant gliomas treated with silver nanoparticles, the mean survival time was significantly higher than the rest of the controls. Synergistic anti-proliferative and pro-apoptotic effects were also obtained. Porcel et al. (28) concluded that platinum nanoparticles (PtNPs) caused nearly 2-fold lethal DNA damage after radiation exposure. Hossain et al. (29) found that the radiosensitization effect of bismuth nanoparticles was stronger than that of gold and platinum nanoparticles under the same physicochemical conditions. Gadolinium (Z=64)-based nanoparticles are another commonly used radiosensitizers. One study found that in SQ20B tumor-bearing mouse model, combining the Gd-based nanoparticles with 10 Gy irradiation significantly delayed tumor growth (30). Zhang Li et al. (31) showed that tail vein injection of hyaluronic acid-functionalized gadolinium oxide nanoparticles (HA-Gd2O3) in combination with radiotherapy significantly inhibited the growth of mouse hepatocellular carcinoma cells compared with radiotherapy alone. The proliferation and cloning efficiency of HepG-2 cells were significantly inhibited by radiotherapy after intravenous tail injection of HA-Gd2O3 compared with radiotherapy alone. Titanium nanoparticles also have a sensitizing effect on radiotherapy, and Townley et al. (32) used human rhabdomyosarcoma cell lines RH30 and RD, and thymic carcinoma cell line MCF7 as targets. After radiotherapy, radiation energy was transferred to the TiO2 crystal structure, resulting in a high production of ROS that destroy cancer cells. Jiang et al. (33) found that palladium nanoparticles alone did not decrease the cell viability, indicating the excellent cytocompatibility of the nanoagents. Treatment of cancer cells with both X-rays and Pd NSs resulted in lower survival rates than cells treated with X-ray irradiation alone, indicating the radiosensitizing effect of Pd NSs. Alloying is an effective chemical method to adjust the properties of metal clusters. Recently, it has been demonstrated that Pt2 Au4 clusters exhibit peroxidase-like activity, which regulates tumor hypoxia and enhances the efficacy of radiotherapy through the sustained production of O2 by endogenous H2O2 decomposition (34). In addition to the above nanomaterials, metallic magnetic nanomaterial Zn/Fe2O4can also enhance radiotherapy sensitivity (35).

The inorganic non-metallic nanomaterials also have the function of sensitizing radiotherapy. Carbon nanomaterials, which are isomers of carbon, are promising in the study of tumor radiosensitization due to their unique properties. Kleinauskas et al. (36) treated human prostate cancer cell lines LNCaP, Du145 and immortalized fibroblasts F11-hTERT with Carbon-core silver-shell nanodots, and the survival rate of normal cells F11-hTERT was significantly above than that of cancer cells LNCaP and Du145 after the same dose of radiation. The intrinsic mechanism may be due to the production of large amounts of ROS, which rupture lysosomes and release histone proteases, causing apoptosis. Ni et al. (37) found that C60 fullerenes induced cell membrane disruption and synergistic DNA damage after γ-ray irradiation in a study of mouse melanoma cell line B16 and human liver cancer cell line SMMU-7721.

Selenium (Se) nanoparticles have also been found to have a sensitizing effect on radiotherapy. Selenium nanoparticles both function as chemotherapeutic agents (38) and enhance the anti-tumor effects of X-rays by activating signaling pathways associated with ROS production, DNA cleavage, caspase-3 activation, mitochondrial damage and other apoptosis-inducing pathways. Yu et al. (39) co-cultured PEG decorated selenium nanoparticles (PEG-SENPs) with human cervical cancer Hela cells and mouse embryonic fibroblasts NIH3T3. After X-ray irradiation, 20 μM PEG-SENPs reduced the survival rate of Hela cells to 39%, while NIH3T3 cells were still 85% active even after the addition of 80 μM PEG-SENPs. In a retrospective study by Li et al. (40), it was found that among 18 patients with stage III/IV non-small cell lung cancer with local recurrence or metastasis who received multiple iodine particle implantation combined with external radiotherapy, the 1-year and 2-year survival rates were 62.5% and 32.7%, respectively, with a median survival of 31 months, compared with 6-9 months for radiotherapy and chemotherapy alone.

Nanodiamonds can also act as active nanoparticles that weaken the resistance of tumor cells to radiotherapy by promoting ROS production, damaging DNA and regulating the cell cycle (41). Hydrogenated nanodiamonds (H-NDs) have negative electron affinity that makes it highly reactive and positively charged with oxygen species and a positive charge in aqueous solutions. It can emit electrons after photon irradiation and may therefore enhance the effects of radiation on cancer cells. These studies suggest that the deleterious effects of DNA DSBs produced by NCS or ionizing irradiation can be amplified by H-NDs. H-NDs is not only expected to improve the treatment of radioresistant tumors, but may also reduce side effects by lowering the dose of radiotherapy for radiosensitive tumors. In summary, H-NDs is undoubtedly a valuable candidate radiosensitizer possibly associated with antisense molecular therapy.

Chitosan nanomaterials have a bidirectional regulatory effect on tumor cells and normal cells. Pan et al. (42) used heavy ion radiation alone as a control and found that at a nano-chitosan concentration of 500-1,000 mg/L, it significantly increased the radiosensitivity of human nasopharyngeal carcinoma KB cells, while increasing the tolerance of murine osteoblasts MC3T3⁃E1 to radiation damage. Chitosan can increase oxygen supply and improve intracellular oxygen levels, especially in hypoxic cells, while normal cells are well oxygenated. Chitosan can antagonize the damage caused by ROS-induced lipid peroxidation and enhance the radiation tolerance of normal tissues.

Dai et al. (43) developed a ruthenium-based metal-organic nanostructured radiotherapy sensitizer (ZrRuMn-MONs@mem) for the combined treatment of ROS and CO by increasing the direct absorption of radiation dose and facilitating the deposition of photons and electrons. In the present study, ruthenium metal-organic nanostructures show unique advantages in radiotherapy sensitization. Firstly, the presence of high Z elements enhances the absorption of X-rays and improves the production of ROS. Secondly, the special metal-organic nanostructures are able to enhance the efficiency of radiokinetic therapy by enabling energy and electron transfer through organic ligands. In addition, the domain-limited spatial structure plays an important role in confinement and conduction, increasing the chance of electron collisions with ground state electrons in the excited state and reducing electron losses.

The abundant blood flow, wide gap and poor structural integrity of blood vessel wall in solid tumor tissue, and lack of lymphatic reflux lead to the phenomenon of selective high permeability and retention of macromolecules and nanoparticles in some tissues, which is called ERP effect. Due to the EPR effect, nanomaterials of the right size can accumulate in tumor tissues and improve the sensitivity of tumors to radiotherapy through passive targeting (44, 45).

Currently, many small molecule compounds or chemotherapeutic agents also have a sensitizing effect on radiotherapy (46) such as catechin (47),adriamyci, paclitaxel (48–50), docetaxel (51–54), cyclopamine (55), Cisplatin and other platinum-based drugs (56–59), mitomycin C (60), selenocysteine (61), topotecan (62), camptothecin (63), histone deacetylase inhibitors (64), curcumin (65–67), tirapazamine (46, 68, 69), etanidazole (46), Arsenic trioxide (70), derivatives of selenium (71), NO (72–74). These chemotherapeutic agents with radiosensitizing effects are coupled with liposomes, proteins, polymers, dendrimers, exosomes, etc. to produce nanomedicines with radiosensitizing effects. In a study by Werner et al. (48) paclitaxel nanopolymeric micelles (Genexol-PM) were produced by polymerization of paclitaxel and measured (23.91±0.41) nm. After treatment with Genexol-PM, the sensitivity enhancement ratios (SER) of non-small cell lung cancer cell lines A549 and H460 were 1.12, 1.23, 1.03 and 1.12 at the small molecule and nano levels of paclitaxel, respectively, after X-ray irradiation. This indicates that paclitaxel nanoparticles have a better sensitization effect. At the same time, these nano-polymeric micelles can slowly release paclitaxel and enhance the synergistic effect of paclitaxel and radiotherapy. Cui et al. (75) made docetaxel⁃loaded nanoparticles (DOC⁃NPs) with an average particle size of 85 nm and used them to study gastric cancer cell lines BGC823, SGC7901, MKN45 and gastric mucosa cell line GES⁃1. After radiation treatment, the SER increased by 24%, 18% and 9% in the three gastric cancer cells, respectively, but only by 2% in GES⁃1 cells.

In addition, the relatively high concentrations of NO can also be used as an effective oxygen-depleted radiosensitizer. Fan et al. (72) modified NO donor (S-nitrosothiol) on upconverted nanoparticles to inhibit the growth of deep oxygen-depleted solid tumors by controlled release of NO under X-ray excitation.

Nano drug delivery systems are drug delivery systems that use nano technology as a carrier to disperse, encapsulate and adsorb drugs onto them, and are made with a particle size of 1 to 100 nm, which can increase drug absorption, improve drug targeting and slow release, increase the permeability of biological membranes, and reduce the toxic side effects of drugs. Nanodrug delivery systems can be loaded with Chinese and natural drugs, chemically synthesized drugs, peptide proteins and nucleotides (76, 77). More importantly, radioactive particles such as 223Ac(releasing A-particles), 131I, and 125I can be delivered precisely to tumor sites (9). The mechanism of controlled drug release using different types of nanocarriers is shown in Figure 1C . With the development of nanotechnology, nano based delivery systems show great potential for the delivery of radiosensitizers. Tian et al. (78) loaded the DNA double-strand repair inhibitor KU55933 into a high molecular polymer to produce nano-KU55933 and treated human lung cancer cell lines H460, A549, H23 and non-obese diabetic severe combined immunodeficient mice (NOD SCID white mice). After X-ray irradiation, this nanoparticle-loaded KU55933 was found to inhibit the DNA repair signaling pathway for a longer period of time than regular KU55933, while exhibiting lower skin toxicity. The presence of a large number of hypoxic cells in tumor cells is an important factor in the efficacy of tumor radiation therapy, and the more cells in the hypoxic region, the greater the likelihood of local postoperative recurrence. In a study of breast cancer cell lines MDA-MB-231 and of ZR-75-1, the number of oxygen-depleted cells in the tumor cells is a major factor affecting the effectiveness of radiation therapy. Jia et al. (79) used carbon nanotubes (CNTs) as carriers to load oxygen and modified them with folic acid to increase the dispersion and targeting of the drug delivery system to treat breast cancer cells in a hypoxic environment. Compared to the irradiated group alone, apoptosis-related proteins Bcl-2 and Survivin, hypoxia-inducible factor HIF-1α and radiation-related proteins Rad51 and Ku80 were all down-regulated, suggesting that folic acid-modified rare earth-modified oxygen-loaded carbon nanotubes could enhance the radiosensitivity of breast cancer cells.

Specific active targeting is the most striking feature of nanodrug delivery systems. Nanotechnology has been used to construct nanodrug delivery systems that are coupled to antigens or antibodies and target genes specifically expressed by tumor cells, with specific targeting to that tumor cell. Khoshgard et al. (80) coupled folic acid with gold nanoparticles to form a nano-drug delivery system that was targeted to Hela cells and significantly improved the sensitivity of radiotherapy. Deng Wen et al. (81) prepared polyethyleneimine (PEI)-Fe3O4 magnetic nanoparticles (PEI-Fe3O4) encapsulated with miRNA-Survivin plasmids and used this novel nanotransfection vector to transfect the Survivin gene in CNE-2 cells of nasopharyngeal carcinoma to target and regulate the sensitivity of CNE-2 cells to radiation. This novel nano-transfection vector was found to be low in toxicity, non-immunogenicity, and it can significantly increase the apoptosis rate of CNE-2 cells compared with radiotherapy alone. Menon et al. (82) coupled the prostate cancer cell-permeable condensate R11 and the radiosensitizer NU7441 with poly (lactic-co-glycolic acid) (PLGA) into nanoparticles ((274 ± 80) nm) with sustained and slow release of radiosensitizer, which demonstrated good targeting and radiosensitization of prostate cancer PC3 cells. EVs loaded with a variety of therapeutic components such as tumor suppressor drugs, siRNAs, proteins, peptides, and conjugates exhibit significantly enhanced anti-tumor effects, so EVs can be used as an advanced and promising nanocavitor (83) ( Figure 2A ). In addition, the design and preparation of tumor-targeted modified EVs have greatly enhanced the specificity and effectiveness of tumor therapy, providing new ideas for tumor precision medicine.

In the physical sensitization, the strong X-ray absorption capacity of the material is mainly used to improve the ray absorption cross section, deposit high-energy ray energy, promote the generation of free radicals, directly enhance DNA damage, so as to improve the effect of radiotherapy.

DNA is the main target molecule in radiation therapy, and ionizing radiation usually damages DNA in two ways ( Figure 2B ). Firstly, it can directly ionize DNA molecules, resulting in various types of damage such as single or double strand breaks and cross-linking of bases and sugars. The second is the indirect reaction of high-energy radiation with water in the tissue to produce free radicals that bind to DNA, causing oxidation of the target molecule by electron transfer and inducing cellular DNA damage, leading to cellular damage or apoptosis. Many scholars believe that the localization of high-Z substances in the nucleus can produce more efficient DNA damage and radiobiotic effects, and cytoplasmic events are also important mechanisms leading to cell death (84, 85). Chun et al. (86) studied gold nanoparticles (GNPs) as radiation sensitizers leading to DNA damage during radiotherapy and concluded that the dose enhancement ratio (DER) was significantly correlated with GNP size, distance to DNA, and the photon energy. Under normal conditions, cells automatically repair the damaged DNA molecules to restore normal physiological function. In the presence of the reducing agent Glutathione (GSH), free radicals can be scavenged by hydrogen atom transfer, which acts as a radioprotector and prevents tumor cells from being killed. In the presence of an electrophilic sensitizer, on the one hand, it can take electrons from the target molecule DNA to oxidise it and prevent it from reabsorbing electrons for repair, resulting in potentially lethal chemical damage to the cell. On the other hand, it can also inhibit sulfhydryl compounds such as GSH in the cell, depleting the protective sulfhydryl groups in the cell and sensitizing it, thus increasing the sensitivity of the cell to radiation and improving the effectiveness of radiotherapy.

Radiotherapy sensitizers can enhance the sensitivity of tumor cells to radiation by promoting reactive oxygen species (ROS) generation, regulating cell pathways and cell cycle, improving endogenous hypoxia in cells, inhibiting DNA repair, inhibiting tumor angiogenesis, and inhibiting autophagy. ROS mainly includes superoxide anion(O2•-), hydrogen peroxide (H2O2), singlet oxygen (1O2), hydroxyl radical (•OH) and so on. The DNA or proteins in cancer cells were damaged by the generated free radicals (87). Among them, H2O2 is a significant signaling molecule in tumor cells, which can promote the growth and development of tumor, and the increased H2O2 level is conducive to the stability of HIF. Since H2O2 is the most stable ROS in cells, the high concentration of cellular H2O2 can accelerates the production of various highly reactive oxygen species (HROS), such as •OH, hypochlorous acid (HOCl) and peroxynitrite (ONOO-). Next these HROS can promote the oxidative damage of proteins, membrane lipids and DNA. The tumor microenvironment (e. g. intratumoral oxygen content) has an important influence on the outcome of radiotherapy for tumors. 10%-50% oxygen-depleted cells are present in tumor tissue. Studies have shown that the radiosensitivity of cells irradiated in the presence of oxygen is about three times higher than in the absence of oxygen (88, 89). This means that the radiation dose required to kill hypoxic cells is much higher than that required to kill fully oxygenated cells. These hypoxic tumor cells are less likely to be killed because they are insensitive and resistant to radiation, resulting in metastasis or recurrence of tumor cells and ultimately failure of radiotherapy. Other factors may also affect the effectiveness of radiotherapy, for example, free radicals such as ROS generated during ionization are easily scavenged by reduced glutathione, reducing the killing effect of radiotherapy. Reduced effectiveness due to repair of potentially lethal damage to damaged cells is also a factor that affects cellular radiosensitivity. The radiation sensitivity of cells in different phases of the division cycle varies considerably ( Figure 2C ), with cells in the M and late G2 phases being the most sensitive and cells in the G1/S phase being three times more radioresistant than those in the G2/M phase (89); autophagy has a cytoprotective effect and reduces cytotoxicity and radiotherapy damage. Therefore, the biochemical perspective is to target the characteristics of the tumor microenvironment and to achieve sensitization radiotherapy through bioreduction, inhibition of damage repair, depletion of endogenous sulfhydryl groups, modulation of the cell cycle, radiosensitization of cell membranes, alteration of cellular oxygenation, inhibition of energy metabolism, inhibition of autophagy, induction of apoptosis, and radiation-induced gene therapy.