Cancer has severely threatened human life worldwide, and malignant tumors are still a serious problem that needs to be solved urgently (Siegel et al., 2020). Traditional tumor treatment methods mainly include surgery, chemotherapy and radiotherapy, while some limitations are also manifested, such as the damage to surrounding tissues, poor effect on hypoxic tumors, possible wound complications, and inconvenience of treatment (Kaur and Asea, 2012; Sundaram et al., 2020). Research on development of tumor therapies is aimed to better eradicate tumors while minimizing side effects.

The application of nanomaterials has further optimized traditional tumor therapies and provided more options for emerging tumor therapies such as phototherapy, immunotherapy, sonodynamic therapy, chemodynamic therapy and RNA interference (RNAi) therapeutics (Song et al., 2017; Hou et al., 2018). The unique properties of nanomaterials made them applicative for tumor diagnose and treatment (He et al., 2015). The nano-scale size facilitates the penetration cross biological barriers, increases the drug delivery efficiency and controls the release behavior, thus achieving better therapeutic effects (Sun et al., 2014). In addition, appropriate nanomaterials can be effectively targeted to the tumor site and reduce systemic toxicity; Modified nanomaterials have been widely applied for both traditional and novel tumor therapies due to the specific physicochemical properties and targeting ability (Sun et al., 2014; Aikins et al., 2020). Nanocarriers have also been constructed to overcome hypoxia of the tumor microenvironment to obtain more efficient tumor killing effects (Abbasi et al., 2016; Meng et al., 2018). Nevertheless, although monotherapy strategies such as radiotherapy and phototherapy are proven effective methods for tumor treatment, there may be a potential risk of tumor recurrence or the refractory of deep-seated tumors. Based on this, tumor combination therapy has been proposed to eradicate tumors synergistically and effectively in a complementary manner, minimizing systemic toxicity and side effects as much as possible. Studies have demonstrated that radiotherapy can break through the limitation of insufficient tissue penetration of phototherapy. In turn, photothermal therapy can promote oxygen perfusion to relieve the hypoxic environment of tumors, which is beneficial to the efficiency of oxygen-dependent therapies such as radiotherapy, photodynamic therapy and sonodynamic therapy. In addition, a large amount of evidence has shown that the therapies can induce the release of tumor-associated antigens locally to activate the immune response, therefore, combination therapies with immunotherapy are a smart approach to achieve synergistic tumor treatment effects (Hou et al., 2018; Chen et al., 2019; Galon and Bruni, 2019). On this basis, recent research has also focused on the development of nanomaterials for the construction of carriers for combination therapy.

The review is aim to outline the recent advances in nanomaterial-mediated radiotherapy, phototherapy, and immunotherapy, and some other therapies (sonodynamic therapy, chemodynamic therapy, and RNAi therapeutics). Besides, we will present innovate strategies for combined tumor treatment based on nanomaterials in the past 5 years.

Radiotherapy utilizes high-dose radiation to induce DNA destruction and generate free radicals to kill tumor cells, including internal and external radiotherapy, usually in conjunction with chemotherapy or surgery (Sundaram et al., 2020; Zeng et al., 2021). However, traditional radiotherapy causes irreversible damage to surrounding normal tissues due to the large amount of radiation. Besides, the hypoxic environment limits oxygen-dependent DNA damage, which in turn leads to tumor resistance to radiation (Jarosz-Biej et al., 2019). Hypoxia also protects dormant cancer stem cells, allowing them to retain the potential for proliferation and differentiation, thus leading to a potential risk of recurrence (Wang et al., 2019a). Therefore, it is necessary to improve the sensitivity of tumor cells to irradiation, overcome the hypoxic environment at the tumor site and increase oxidative stress, strengthen the killing effect on tumor cells and alleviate its side effects on normal tissues (Figure 1).

High-Z materials such as gold (Dong et al., 2020), silver (Liu Z. et al., 2018), platinum (Li et al., 2019) and gadolinium (Du and Sun, 2020) have been widely reported to be used as radiosensitizers and accelerate the generation of reactive oxygen species (ROS) through photoelectric effect, Auger electronics and Compton effect (Sundaram et al., 2020). Lan et al. developed a metal-organic frameworks (MOF) hierarchically composed of Hf-based secondary building units and Ir-based bridging ligands, encapsulated (P₂W₁₈O₆₂)^6– (W₁₈) polyoxometalates through electrostatic adsorption to further enhance the sensitivity of radiotherapy (Lan et al., 2019). The hierarchical structure facilitated the generation of hydroxyl radical (^•OH), singlet oxygen (¹O₂) and superoxide (O₂ ⁻), which is an excellent radio-enhancer for the treatment of MC38 and CT26 tumor bearing mice (Lan et al., 2019).

Among high-Z materials, gold-based nanoparticles (AuNPs) have received extensive attention due to the good biocompatibility and surface modifiability. Studies have further enhanced its superiority in radiotherapy sensitization by changing the size, structure or surface properties (Yang et al., 2014; Dong et al., 2020). Dong et al. prepared gold sub-nanoparticles encapsulating cell cycle regulator α-difluoromethylornithine, and modified them with arginine-glycine-aspartic acid (RGD) penetrating peptide to promote its penetration of the blood-brain barrier. The results showed enhanced sensitivity to radiation and significant therapeutic effects in low-dose radiation (Dong et al., 2021). AuNPs can also be modified to improve targeting ability. A conjugated complex composed of AuNPs and a plectin-1 targeting peptide showed more aggregation at the tumor site, and induced tumor cell apoptosis with better biosafety (Dong et al., 2020).

Hypoxia of the tumor microenvironment hinders radiation therapy sensitivity and the formation of ROS. Hypoxia-inducible factor 1 (HIF-1) is stably expressed under hypoxic conditions, which has been shown to participate in the proliferation and metastasis of tumor cells by regulating its downstream signaling molecules (Meng et al., 2018; Sundaram et al., 2020). Based on this, studies utilized nanocarriers combined with tumor oxygenation agents such as catalase and MnO₂ to decompose the over-expressed H₂O₂ into oxygen, thereby alleviating the tumor hypoxia and enhancing the sensitivity of radiotherapy (Abbasi et al., 2016; Song et al., 2016). Meng et al. encapsulated the HIF-1 inhibitor acriflavine into MnO₂ nanoparticles to generate oxygen in tumor tissues and improve the effect of radiotherapy. On the other hand, acriflavine inhibited the expression of HIF-1 and down-regulated its downstream signaling molecules such as vascular endothelial growth factor and glucose transporter, showing an 84.70% tumor inhibition rate in abscopal tumors (Meng et al., 2018).

Photothermal therapy and photodynamic therapy are currently the most common phototherapy for tumor treatment. Photothermal therapy is to gather materials with high light-to-heat conversion efficiency in the tumor tissue and heat it to ablate cancer cells under the irradiation of external light source (Hou et al., 2018). And photodynamic therapy utilizes a specific wavelength of light to irradiate a photosensitizer at the tumor site and generate ROS to kill tumor cells. Compared with traditional tumor therapy, both photothermal and photodynamic therapy can be applied accurately and effectively with less side effects and greatly reduce the patient’s pain in a noninvasive way (Figure 2) (Li et al., 2020).

Photothermal agents are materials with good light-to-heat conversion capabilities, mainly including noble metals (gold (Wang et al., 2013), silver (Park et al., 2020), platinum (Ma et al., 2020), etc.), two-dimensional materials (graphene (Chen et al., 2016), black phosphorus (BP) (Geng et al., 2019), transition metal-based materials (Xie et al., 2019), etc.), and some organic small molecule photothermal agents (porphyrin, phthalocyanine, cyanine, etc.) (Lv et al., 2020). The current research aims to develop materials with excellent biocompatibility in addition to photothermal performance. Most metal-free 2D materials such as graphene oxide and BP have great biocompatibility and biodegradability, and have attracted more attention as candidate materials for photothermal therapy in recent years (Liu S. et al, 2020). Graphene has been a hotspot of scientific research since its discovery. Recently, graphene derivatives have been favored due to its better drug loading efficiency and photothermal conversion capacity (Chen et al., 2016). It is proved that folic acid coupled chitosan functionalized graphene oxide (FA-CS-GO) can completely destroy cancer cells under near-infrared (NIR) light irradiation in vitro, and showed excellent anti-tumor effect in vivo with no recurrence in 20 days (Jun et al., 2020). Besides, BP has been considered as a promising photothermal material, but its rapid degradation in the physiological environment limits its application. Based on this, Geng et al. assembled NIR-II responsive carbon dots on BP nanosheets to construct a hybrid photothermal agent, which improved the stability of BP and showed synergistic photothermal effect, attained higher light-to-heat conversion efficiency in the NIR-I and NIR-II window (77.3 and 61.4%, respectively), and the deep tumors were eradicated under the 1,064 nm laser (Geng et al., 2019). Shao et al. prepared a core-shell structured nanospheres with black phosphorous quantum dots (BPQDs) encapsulated in poly (lactic-co-glycolic acid) (PLGA), and the nanospheres achieved a better passive targeting effect, retained good photothermal performance with excellent stability (Shao et al., 2016).

As for photodynamic therapy, there have been many studies on nanocarriers loaded photosensitizers such as photofrin (Kano et al., 2013), 5-aminolevulinic acid (Zhang et al., 2015) and chlorine e6 (Ce6) (Yang et al., 2019) to improve delivery efficiency and enhance tumor killing effects, but the hypoxia microenvironment greatly limits the therapeutic efficiency. In addition to the strategies of delivering MnO₂ and catalase to increase the production of oxygen, studies have indicated that nano-heterostructure can promote electron-hole separation and the accumulation of ROS, which is considered an effective material for photodynamic therapy (Cheng et al., 2020). Zhang et al. designed a bismuth sulfide (Bi₂S₃)-bismuth (Bi) Z-type heterostructure with a good electron-hole separation ability, simultaneously generated ROS and O₂ under NIR irradiation, improved photodynamic therapy efficiency with excellent biocompatibility and biodegradability (Cheng et al., 2020). Qiu et al. also prepared bismuth/bismuth oxide (Bi/BiOx) lateral nano-heterostructures based on the regioselective oxidation strategy. Under 660 nm laser irradiation, ¹O₂ can be effectively generated under normal oxygen conditions, and cytotoxic ⋅OH and H₂ can be generated under hypoxic conditions, showing synergetic photodynamic performance on tumor elimination (Qiu et al., 2021).

Furthermore, studies used up-conversion nanomaterials (UCNPs) to emit visible light under NIR excitation and stimulate photosensitizers to effectively produce ROS, overcome the limitation of weak tissue penetration of visible light, and improve the efficacy of photodynamic therapy (Liang et al., 2020a; Liu et al., 2020b). Liu et al. constructed a Nd³⁺ ion-doped UCNPs surface-modified porphyrin-like metal-organic framework and functionalized it to target the mitochondria to achieve enhanced photodynamic therapy under the trigger of NIR irradiation (Liu et al., 2020c). Liang et al. developed UCNPs modified with oleic acid-polyamide dendrimers to form hydrophobic and hydrophilic pockets on the surface of nanoparticles through click reaction, loaded with photosensitizer Ce6 and catalase, which can effectively overcome tumor hypoxia and improve photodynamic therapy efficiency, indicating significant anti-tumor effect through synergistic mitochondrial targeting modification (Liang et al., 2020b).

In addition, photodynamic and photothermal therapy are usually applied in combination due to their synergistic tumor killing effect (Figure 2). Photothermal therapy generates heat locally can not only ablate tumor cells, but also accelerate blood circulation and accumulate oxygen in the tumor’s hypoxic microenvironment to provide suitable conditions for photodynamic therapy (Yang et al., 2021). Guan et al. prepared covalent organic framework nanoparticles with a particle size of ∼140 nm that co-loaded the photosensitizer porphyrin and the photothermal agent naphthalocyanine. The temperature increase led by the photothermal effect significantly enhanced the sensitivity of photodynamic therapy by destroying lysosomes and mitochondria (Guan et al., 2019). Yang et al. synthesized TiO₂@RP nanorods with red phosphorus (RP) as the shell and TiO₂ as the core. After being irradiated with 808 nm NIR light for 5 min, the nanorods can reach 39.3–44.1°C and induce the production of ¹O₂, effectively killing cancer cells in deep tissues of renal cell carcinoma (Yang et al., 2021).

Immunotherapy is based on the regulation of the immune system, activating a long-term immune response against malignant tumor cells, thereby eradicating tumors and inhibiting tumor metastasis (Riley et al., 2019). Tumor associate antigens derived from necrotic or apoptotic tumor cells can be taken up and processed by antigen-presenting cells, presented to T cells in lymph nodes, inducing the activation of immature T cells to effector T cells. T cell-mediated cellular immune response is the key to tumor immunity (Song et al., 2017).

Tumor vaccines can effectively trigger potent immune responses for tumor prevention and treatment specifically. Nanomaterials are applied to co-deliver antigens and adjuvants (Zhu et al., 2018), increase the uptake of antigen-presenting cells, improve the ability of cross presentation, or directly target lymph nodes to induce strong effective immune response (Li H. et al, 2017; Zhong et al., 2019a). Zhu et al. co-encapsulated B16 melanoma-derived tyrosinase-related protein 2 (Trp2) peptides and toll-like receptor agonists (CpG oligonucleotides and monophosphoryl lipid A) in a mesoporous silica vector, which improved the efficiency of antigen delivery to dendritic cells, effectively activated T cell-specific immune responses and prolonged the survival period of tumor-bearing mice significantly (Zhu et al., 2018). Li et al. prepared micelles composed of polycaprolactone-polyethyleneimine and polycaprolactone-polyethylene glycol, and co-loaded Trp2 and the adjuvant CpG oligodeoxynucleotide, which showed higher activity of cytotoxic T lymphocytes (CTLs) and stronger anti-tumor ability in B16F10 melanoma mice compared with the mixture of free Trp2 and CpG (Li M. et al, 2017).

The antigen captured by dendritic cells are presented to the major histocompatibility complex class I (MHCI) or major histocompatibility complex class II (MHCII) molecules extracellularly (Song et al., 2017). For tumor immunotherapy, it is vital that exogenous antigens escape from lysosomes and activate CD8⁺T cells through the MHCI pathway through cross presentation (Du et al., 2020). Suitable nanomaterials are applied according to the unique acidic environment of the lysosome to promote the escape of antigen from the lysosome and enhance cross presentation (Li H. et al, 2017; Du et al., 2020). The most commonly used methods are cationic polymer modifications such as polyethylenimine (Li M. et al, 2017), 1,2-dioleoyl-3-trimethylammonium-propane (Zhang et al., 2019) and dimethyl dioctadecylammonium bromide (Xia et al., 2018), inducing the proton sponge effect to cause the destruction of the endosomal membrane, thereby increasing the endosomal escape of the antigen (Du et al., 2020). Recent studies have shown that pH-sensitive materials can also promote cross presentation by inducing endosomal membrane fusion and transferring antigens through the cytosol (Zhong et al., 2019b; Du et al., 2020). MOFs have attracted attention due to their excellent drug-carrying capacity and pH-sensitive properties. Zhong et al. used zeolite imidazole skeleton (ZIF-8), a pH-sensitive degradable MOF, co-loaded antigen and CpG for subcutaneous injection, which decomposed in the lysosome to release the antigen into the cytoplasm and enhanced cross presentation, remarkably inhibiting EG7-OVA tumor growth (Zhong et al., 2019a). In another study, the MOF composed of guanine monophosphate and lanthanide ions can be actively internalized by antigen presenting cells, release the antigen and promote its escape from the lysosome, significantly improving the CTL response (Duan et al., 2017).

Although tumor vaccines have shown excellent anti-tumor effects, the immunosuppressive microenvironment presents a major challenge for tumor immunotherapy, and immune checkpoint inhibitors have been considered as a promising strategy in recent years (Song et al., 2017). Blocking the immune checkpoints facilitates the recognition of T cells and initiates an effective anti-tumor immune response, thus preventing tumor immune escape and improving the efficiency of anti-tumor immunotherapy (Song et al., 2017; Chen et al., 2018). The most studied inhibitory receptors are cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed death 1 (PD-1). CTLA-4 is expressed on activated T cells and interacts with costimulatory molecules on antigen presenting cells to impede T cell activation (Ruan et al., 2019). And PD-1 is up-regulated on activated T cells, interacts with the corresponding ligand PD-L1 on tumor cells to inhibit the function of T cells. Anti-CTLA-4 monoclonal antibody ipilimumab and anti-PD-1/PD-L1 antibodies such as pembrolizumab and avelumab have been approved by the United States Food and Drug Administration (FDA) for clinical cancer treatment (Chen et al., 2018; Ruan et al., 2019). Studies have shown that combining tumor vaccines with immune checkpoint inhibitors can effectively combat tumors and prevent the recurrence (Kuai et al., 2017; Kim et al., 2020). Kim et al. used biocompatible phospholipid nanoparticles to co-encapsulate tumor antigens and CpG adjuvants, which showed strong anti-tumor effects in the prevention and treatment of EG7 tumor models, but the vaccine induced T-cell exhaustion by increasing PD-L1 expression, leading to tumor recurrence. Combining anti-PD-1 therapy with nano-vaccine suppressed tumor recurrence and showed a synergistic anti-tumor effect (Kim et al., 2020).

Nanomaterials have exhibited great potential in tumor treatment and obtained remarkable results in both traditional and emerging therapies. However, there are still some limitations in monotherapy such as the inability to completely eliminate the tumor or the potential risk of tumor recurrence. The application of nanomaterials in combination therapy is emphasized to achieve synergistic effects for tumor treatment (Figure 3).

Here we reviewed the progress of nanomaterials used in radiotherapy, phototherapy, immunotherapy and some other therapies such as sonodynamic therapy, chemodynamic therapy and RNAi therapeutics in the past 5 years, listed some creative designs of nanocarriers and summarized the major development direction of tumor combination therapies. The application of nanomaterials has been a promising strategy in both traditional and emerging therapies, and a variety of suitable nanomaterial-based carriers with specific properties have been gradually developed to effectively accumulate at the tumor site and overcome the adverse tumor microenvironment, which have shown great potential for improving the efficiency of tumor treatment. Recent studies have been focused on the development of the combination of multiple therapies in order to maximize the therapeutic effect. The combination of radiotherapy and phototherapy can effectively combat deep-seated tumors with minimal side effects, and the oxygen perfusion lead by photothermal therapy is conducive to oxygen-dependent therapies such as radiotherapy and photodynamic therapy. In addition, various therapies have been proved to activate the immune response to a certain extent in the process of tumor treatment. Therefore, combining immunotherapy can synergistically strengthen the therapeutic outcome and dramatically prevent tumor metastasis and recurrence.

Despite satisfactory progress in the development and application of nanomaterials, some challenges remain to be addressed. The clinical translation of nanomaterials is the most critical issue for tumor treatment, so the materials should possess excellent biocompatibility and biodegradability, and FDA-approved materials ought to be prioritized for consideration. Similarly, the scale-up process from the laboratory to the clinic usually requires optimization or changes of the preparation method, so the efforts should be made to establish nanocarriers that facilitate the scale-up production while ensuring the effectiveness of tumor treatment. Besides, the toxicity and safety of nanomaterials are still pressing issues, and research should give more consideration to the long-term and potential toxicity of nanomaterials. In addition, it is of great importance to develop nanomaterials with specific physicochemical properties to meet the needs of different tumor therapies. Currently combination therapy is favored due to its synergistic anti-tumor efficacy, accordingly, nanomaterials with multiple functional properties are gradually developed to simplify the design of the carriers and achieve better therapeutic effect. It is vital to explore multifunctional nanomaterials to trigger multiple therapeutic effects in further research. The complexity of the tumor microenvironment presents a great challenge for the design of nanocarriers. With the continuous deepening of current research, the relationship between tumor therapies and its further mechanism has gradually become clear. According to the actual tumor conditions, suitable carriers can be designed to reverse unfavorable microenvironment to enhance the therapeutic outcome through monotherapies or combination therapies. The development of nanomaterials with suitable properties and biosafety to achieve remarkable therapeutic effect is the vital topic for tumor treatment in the future.