Advances in Novel Tumor Therapeutics Based on Nanomaterial Technologies

Cancer is the leading cause of death in the world, throughout the global researches of cancer treatment, people have a deeper understanding of cancer, and the treatment methods are constantly breakthrough. Conventional surgery, chemotherapy and radiotherapy have serious adverse effects and patients' quality of life is not significantly improved. Now, photodynamic therapy, photothermal therapy and thermodynamic therapy based on nanotechnology and materials technology are booming, and the development of these novel cancer therapies and their combination therapies brings more possibilities for cancer treatment. This review summarizes the research progress of novel cancer therapies based on nano and material technology from the aspects of mechanism of action and therapeutic methods, hoping to provide reference for their clinical application.


Introduction
Cancer, as one of the leading causes of death in the world, seriously threatens people's lives. The global incidence of malignant tumors is increasing year by year. Worldwide, an estimated 19.3 million new cancer cases and about 9.95 million cancer deaths occurred in 2020. The global cancer burden is estimated to be 28.4 million cases in 2040, a 47% rise from 2020 1, 2 . Many types of cancer are characterized by high mortality and recurrence rates. Traditional treatments include surgery, chemotherapy, and radiation therapy, however, many contraindications, serious adverse reactions and other limitations can not significantly improve the quality of life of patients. Moreover, most tumors are occult and metastases have already occurred when diagnosed, and some tumors are not sensitive to radiotherapy and chemotherapy. Therefore, surgery is still the main treatment at present. The process of complete excision of cancer tissue by surgery is limited due to the unclear boundary of cancer tissue. Therefore, there is an urgent need for extensive therapeutic modalities with minimal adverse reactions 3,4 . New therapeutic modalities such as gene therapy, epigenetic therapy and immunotherapy are all gradually developing. Surprisingly, although there are many modalities such as gene therapy, RNA interference therapy and cell therapy, the overall survival time of tumor patients does not seem to be prolonged due to intratumoral heterogeneity and tumor competitive metastasis, so that, new tumor treatment modalities need to be established. For the past few years, photodynamic therapy (PDT), photothermal therapy (PTT) and thermodynamic therapy (TDT) have gradually come into people's vision with the continuous efforts of researchers.
The emergence of nanotechnology and material technology has promoted the continuous development of new forms of anti-tumor drugs, which realized the targeted therapy that can accurately deliver drugs to tumor tissues. These new drug forms, with their stronger specificity, deeper delivery range and other characteristics, provide the possibility for humans to deepen the understanding of the anti-tumor regulatory mechanism. Moreover, with the rapid development of nanotechnology, the applications of PDT, PTT and TDT have gained a broader platform.

The mechanism of photodynamic therapy (PDT)
PDT is a new form of cancer treatment with noninvasive and highly selective. Although still emerging, it is already a successful and clinically approved therapeutic modality used for the management of neoplastic and nonmalignant diseases. PDT consists of 3 essential components: photosensitizer (PS), light (harmless visible or near-infrared light of the appropriate wavelength corresponding to the absorption range of PS), and oxygen. The specific light stimulates PS to an excited state, excited photosensitizer delivers energy to surrounding oxygen, reactive oxygen species (ROS) including singlet oxygen are produced. The latter can rapidly cause significant toxicity on surrounding tissues and cells, leading to death via apoptosis or necrosis ( Figure 1) [5][6][7] . It is worth noting that hydrogen peroxide (H2O2), a component of ROS, can either induce autophagy in cells or transform into hydroxyl group with stronger oxidative activity, which is also more cytotoxic. The laser power required for this process is low, therefore it does not cause thermal damage to cells and tissues 6 . In addition, it is found that none of the clinically approved PSs can enter the nucleus of target cells, limiting DNA damage which can lead to mutations that make cells resistant to treatment, therefore the occurrence of PDT resistance can be prevented to a certain extent. PDT has been widely used to treat tumors such as esophageal cancer and skin cancer. However, the application of PDT in the treatment of deep tissue diseases such as deep tumors is limited due to the low solubility of most photosensitizers, 3 the lack of targeting, and the insufficient penetration of excitation light source to skin and tissue. There are two approaches may increase the antitumor effectiveness of PDT: one is sensitization of tumor cells to PDT and another is interference with cytoprotective molecular responses induced by PDT in surviving tumor or stromal cells 7 .

Figure 1. Photosensitization processes 8 .
A photosensitizer, is irradiated with a light wavelength and is converted from the singlet basic energy state S0 into the excited singlet state S1 because of the photon absorption. The molecule in S1 may undergo intersystem crossing to an excited triplet state (T1) and then either form radicals or, more likely, transfers its energy to molecular oxygen ( 3 O2) and form singlet oxygen ( 1 O2), which is the major cytotoxic agent involved in PDT.

Photosensitizers
Photosensitizers are catalysts for the generation of ROS in the process of PDT, which is an important factor determining the effect of PDT. The choice of photosensitizer is related to the type of disease and the route of administration 9 . Currently, photosensitizers can be divided into three generations. The first generation is represented by hematoporphyrin derivatives, and the active components are mainly dihematoporphyrin ethers or esters. The second generation includes phthalocyanines, porphyrins derivatives and gecycloquinones which uses longer wavelength excitation light, achieve a stronger tissue penetration, deeper treatment, higher ROS production rate and better therapeutic effect compared with the first generation of photosensitizers ( Table 1). The third generation is formed by the combination of the second generation photosensitizers and targeting ligands, including photosensitizers of immune targeting, photosensitizers of epidermal growth factor receptor targeting, photosensitizers of mRNA targeting and so on, which further improve the targeting of photosensitizers and the efficiency and safety of PDT [10][11][12][13][14][15] .
Tetrapyrrole absorption spectrum showing porphyrins, chlorins and bacteriochlorins. Photosensitizers, as an important part of PDT, are limited in application due to their own defects, for example, most photosensitizers are fat-soluble, poorly targeted, and have potential phototoxicity. While the use of nano drug delivery system to deliver photosensitizers can foster strengths and circumvent weaknesses, reduce the nonspecific accumulation of photosensitizers in normal tissues, reduce phototoxicity and so on. Moreover, the surface modification with functional or targeting groups can further strengthen the targeting, reduce the dosage and adverse reactions, so as to improve the efficacy of PDT 9 . As an excellent photosensitizer of PDT, porphyrin MOFs material has high efficiency PDT performance, which is mainly attributed to the advantages of the material itself, such as high load capacity of porphyrin, good biocompatibility and so on. In a recent study, researchers synthesized porphyrin MOFs nanodots (MOF QDs) by a simple method and used them as renal scavable nanoreagents to enhance PDT. MOF QDs has high renal clearance rate and abundant tumor accumulation, which can greatly overcome the long-term toxicity caused by residual drugs after systemic administration. Compared with nanometallic organic framework (NMOF), MOF QDs showed better PDT treatment effect than NMOF under the same treatment conditions 29 . Meanwhile, a novel intelligent oxygen (O2) releasing PDT nanoparticles (Mn NPs) are synthesized by coating manganese dioxide (MnO2) nanosheets on porphyrin MOFs materials for tumor targeted therapy and dual-5 mode imaging, it provides a new idea for cancer cell imaging and PDT therapy 30 .

PDT therapy in tumor
In modern oncology, it is a common idea to combine several different therapeutic methods in malignant tumor therapy. Since the potentiated toxicity of the combinations is localized, so the combination of PDT and other therapies also has no significant systemic toxicity. This is especially important for elderly or debilitated patients who may tolerate more intensive therapeutic regimes poorly. Furthermore, the combination of PDT and other therapies is not likely to induce cross-resistance in the body because PDT is mainly dependent on the cytotoxic effects of relatively specific reactive oxygen species 31 . Now, there have been many reports on the treatment of tumor by PDT combined with other therapies at home and abroad. PDT is commonly used for highly selective treatment of tumors combined with surgical therapy, chemoradiotherapy, chemoradiotherapy, and especially fluorescence endoscopy 32,33 . On the one hand, the delivery efficiency of the photosensitizer to the target tissue can be enhanced to improve the therapeutic efficiency of PDT, for example, by coupling monoclonal antibody to the photosensitizer to enhance the tumor targeting of the photosensitizer. On the other hand, PDT in combination with drugs that inhibit the excretion of photosensitizers can greatly enhance the sensitivity of tumor cells to PDT, which is limited to those photosensitizers that have clearly acted on an transport protein (such as ABCG2) as a substrate 34 . Recently, it has been reported that cyclooxygenase (COX) inhibitors, anti-angiogenic drugs, and monoclonal antibodies against neovascular growth factors (such as VEGF) can significantly enhance the inhibition effect of PDT on the growth of tumor cells 35,36 . The combination of PDT and antibodies related to signal transduction pathway such as epidermal growth factor antibody cetuximab can also significantly enhance the therapeutic effect of PDT 37 . Geldanamycin, an Hsp-90 protein inhibitor, can increase the sensitivity of tumor cells to PDT by regulating the function of Hsp-90 38 . The proteasome inhibitor bortezomib, a drug used for hematopoietic disorders, is found to increase the cytotoxic effect of PDT by further enhancing ER stress 39 . Researches have shown that rapamycin, Bcl-2 antagonists, urdeoxycholic acid, ceramide analogues, and inhibitors of enzymes involved in ROS scavenging (such as superoxide dismutase SOD, HO-1, or NOS) are found to enhance the antitumor effect of PDT [40][41][42] .
PDT has many advantages compared with chemoradiotherapy, such as less systemic adverse reactions, resistant to drug resistance, repeatable application and can combine with other therapies to improve the efficacy further. In addition, in recent years, the research on the photosensitizer, light source and oxygen, three factors of PDT, has become more and more in-depth, and the three factors have been continuously optimized. Hence, more and more PDT combination therapies have been continuously researched and put into clinical application.

The mechanism of photothermal therapy (PTT)
PTT is a new noninvasive tumor treatment method that uses photothermal agent (PTA) to convert light energy into heat energy to kill tumor cells under the irradiation of near-infrared (NIR) and other external light sources. PTA absorb energy from photons under NIR irradiation and are transformed from their ground singlet state to an excited singlet state. The electronic excitation energy then undergoes vibrational relaxation, a non-radiative form of decay, and returns to the ground state is mediated by collisions between the excited PTA and their surrounding molecules. Consequently, increased kinetic energy will heat up the surrounding microenvironment, resulting in thermal effects (Figure 2) 43 . It has shown significant advantages owing to noninvasive, targeted and high-efficiency compared with most traditional cancer therapy 44 . PTT can be divided into traditional photothermal therapy (≥45℃) 6 and mild photothermal therapy (MPTT) (<45℃). The temperature of traditional PTT is relatively higher, and the killing ability to tumor tissue is stronger, but also to normal tissue. In contrast, MPTT has less damage to normal tissues, while it also has lower killing capacity. It is mostly used as a regulatory mechanism, rather than directly used to kill tumor tissues at present. The PTA (small green circles) is administered intravenously to the patient, and then subsequently distributed around the body. Accumulation of PTA in tumour tissues (indicated by previously grey ovals, representing the tumour, turning green) can be achieved through active and/or passive targeting strategies and optional molecular activation exploiting, for example, proteases or hypoxia in the tumour microenvironment. While, local application of light of a specific wavelength to the tumour tissues can stimulate PTA from a ground single state to an excited singlet state (indicated by red oval). Finally, tumour ablation following excitation of the PTA results predominantly from thermal and chemical damage, respectively.

The core of PTT is PTA
PTA, can transform light energy into heat energy under the irradiation of NIR and other external light sources to kill tumor cells. At present, there are a lot of researches on photothermal converter, and a variety of new materials are frequently produced which mainly concentrated on nanometer level material roughly divided into inorganic PTA and organic PTA ( Table 2). Inorganic PTA has the advantages of high stability and strong photothermal conversion efficiency, and mainly includes precious metals (Au, Pd, etc.), carbon-based (carbon nanotubes (CNT), graphene (GE), graphene oxide (GO), carbon quantum dots (CQDs), etc.) and transition metals (metal sulfides and metal oxides). In recent research, a kind of relatively small sized (35 nm) and slightly-hollow degree gold-silver nanocages (GSNCs) with an absorbance locked at 532 nm has been successfully synthesized which can enhance the intrinsic photoacoustic imaging performances for blood vessels around tumor sites, and it may assist PAM-based tumor diagnosis and induce a tumor targeted PTT effect 45 .

PTT treat tumor
PTT, as a new tumor therapy, has the advantages of non-invasive, small adverse reactions and high targeting, which has great potential in the development of tumor therapy. Whereas the single use of PTT also faces such problems as not being able to completely kill the whole tumor tissue, damage the surrounding normal tissue and 7 biological safety of photothermal materials. Hence, a series of combination therapies based on PTT have been extensively studied to address the limitations of PTT monotherapy, including chemotherapy, PDT, gene therapy and immunotherapy.
Chemotherapy, as one of the traditional ways of tumor treatment, has been widely used in clinical practice, but it is limited by problems such as insufficient local drug concentration, serious adverse reactions and drug resistance. Nanomaterials loaded with chemotherapy drugs could effectively solve these problems by passive targeting enhanced permeability and retention effect or active targeting by surface-to-binding molecules. In addition, local heating during PTT can also improve the permeability of cell membrane and cytotoxicity of drugs, achieving the effect of "1+1>2" 52 . Synergetic treatments that combine chemotherapy with PTT/PDT have been developed as promising new strategies for cancer therapy, especially for drug-resistant cancers. Polycaprolactone (PCL) nanoparticles loaded with IR780 and paclitaxel (PTX) and modified with luteinizing hormone releasing hormone (LHRH) peptide are prepared by emulsion method using bovine serum albumin (BSA) as stabilizer, and used for combinational phototherapy and chemotherapy. The PCL-LHRH NPs can efficiently hinder the growth of drugresistant xenografts in vivo with the assistance of an 808 nm NIR laser (100%) and the effect is stronger than that without NIR (17.8%) 53 . Similarly, the research of Lee also proved that the single use of chemotherapy and PTT both had certain defects, and the combination of the two could maximize the efficacy 54 .
It has been reported that PTT could enhance the efficacy of PDT by increasing the cell absorption of Chlorin e6 (Ce6) 55 . Au for PPT and Ce6 nanocomposite for PDT are successfully synthesized, and the researches in vitro and in vivo both proved that PPT combined with PDT has greater efficacy than the single use 56,57 . A nanocomplex composed of the thermosensitizers Fe3O4 and the PTA curcumin is prepared and used in a BALB/c mice triplenegative breast cancer model, resulting in a 27% reduction in tumor volume in the last day of treatment compared to the initial volume, significantly superior to PTT and PDT treatment alone 58 . Similarly, a nanoplatform (PDA-ICG-TPZ NPs) integrated with thermal sensitizer polydopamine (PDA), photosensitizer ICG and chemotherapeutic agent tirapazamine is constructed by loading TPZ, a hypoxic-activated chemotherapy drug, using the hypoxic microenvironment in the treatment of PTT combined with PDT. The synergistic treatment of PTT, PDT and chemotherapy is realized, and remarkable inhibitory effect is obtained against subcutaneous tumor cell line U87MG and in situ tumor cell line B16F10 59 .
Gene therapy aims to treat disease at its root by correcting or compensating for faulty and abnormal genes, rather than simply alleviating its symptoms. In the early stage, the development of gene therapy has been limited due to the lack of ideal vector, whereas the rapid development of nanomaterials has also driven the rapid progress of gene therapy. While killing tumor cells, the photothermal effect can promote gene release by breaking chemical bonds between genes and nanomaterials 60 . For the past few years, PTT combined with gene therapy has shown remarkable therapeutic effect in the treatment of hepatoma and pancreatic cancer 61,62 .
Tumor immunotherapy can activate the body's own defense system to recognize, attack and destroy tumor cells, which is a research hotspot in recent years. It has been reported that tumor associated antigens (TAAs) can be released from the thermogenic effects of tumor, while the TAAs are recognized by dendritic cells (DCs) and presented to T cell receptors with the help of immune adjuvants to activate the immune response 63 . As a branch of PTT, MPTT plays a significant role in tumor immunity and microenvironment. Although it does not directly reduce tumor size like PTT, it can inhibit the progression of tumor cells by changing the "soil" of tumor cell growth and metastasis, providing a new strategy for tumor immunotherapy 64,65 .
As an emerging tumor therapy, PTT has shown great potential in the clinical application of cancer. In recent years, the application of PTT based on different PTAs is also more efficient and extensive with the rapid development of nanomedicine. However, although different types of PTA show unique advantages, a photothermal material with 8 low cost, good water solubility, strong biosafety and high photothermal conversion efficiency is still to be explored.
In addition, compared with traditional therapy, PTT has advantages such as small adverse effects and strong targeting, but, it also has shortcoming such as incomplete elimination of tumors, high recurrence rate, and ineffective treatment of distant lesions, and thermal damage to normal tissue has always been a difficult problem to solve. The problem has long puzzled researchers that how to balance killing efficiency with normal tissue protection. To some extent, the combined application of other therapies can solve the above problems faced by a single PTT, but the synergistic effect of these therapies should be fully explored firstly, rather than the simple combination of different therapies. For example, more drugs and other ingredients can be loaded into nanosystems while it may affect the therapeutic effect of PTA, so the internal mechanism of the combination also needs to be improved.

The mechanism of thermodynamic therapy (TDT)
TDT has recently emerged as an attractive approach for cancer therapy. It uses heat to activate thermosensitizers and produce reactive active chemicals, including ROS and other free radicals unrelated to oxygen 66,67 . In TDT, ROS, free radicals and other active species can be obtained directly from the thermosensitizers during heating, showing a property unrelated to oxygen, and not limited to the anoxic environment in the tumor, which could overcome the hypoxia limitation of the clinical tumor therapy (Figure 3) 68,69 . In consequence, TDT has incomparable advantages over PTT and PDT, and is considered as a novel, flexible and promising cancer treatment method, which has recently received special attention from researchers 66,68,69 . In contrast to PTT, the heat source of TDT can be obtained in a variety of ways, such as chemical reactions, light, ultrasound, and microwave 70 . However, TDT also has some problems and limitations, for instance the need to rely on external energy, the availability of fewer thermosensitizers. When obtaining heat source from light, TDT also has similar problems as PTT, such as limited penetrating power of NIR laser, uneven distribution of heat generated by photothermal action, poor accumulation of photothermal conversion materials at tumor sites. Moreover, cells adapt to the living environment of oxidative stress to produce the mechanism of resistance to high levels of ROS and the body's own oxidative stress response, which will weaken the effect of TDT on tumor killing 71 . These problems and limitations make it difficult to eliminate the tumor completely with TDT alone, which inevitably leads to tumor recurrence and metastasis.
Fortunately, combining TDT with other drugs or therapies to construct a multi-modal synergistic therapy system can compensate for this deficiency, even achieve a better synergistic therapeutic effect. 9 Figure 3. Schematic illustration synthesis and action of thermosensitizers 68,69 . a, the preparation of AIPH@Nb2C@mSiO2 NPs; b, the thermal decomposition of AIPH for radical production; c, the systematic delivery of AIPH@Nb2C@mSiO2 NPs as a photo-sensitive nanomedicine for multifunctional imagingguided synergistic photothermal thermodynamic therapy and thermodynamic therapy; d, the schematic diagram of the formation of the hydrogels composed of HA-GMA and NIPAM, loaded with AIPH and AuNRs, and the combination of the photothermal and thermodynamic therapy for hypoxic tumor therapy. CTAC is abbreviated for cetyltrimethylammonium chloride, TEOS is abbreviated for tetraethylorthosilicate, AIPH is abbreviated for 2, 2′azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, A@Nb2C@Si NPs are abbreviated for AIPH@Nb2C@mSiO2 NPs and Nb2C@Si NPs are abbreviated for Nb2C@mSiO2 NPs in the Figure 3 (a-c).

The key of TDT-Thermosensitizers
Thermosensitizers, can absorb heat and decompose to produce ROS or other free radicals unrelated to oxygen. These active chemicals can cause the damage of DNA and mitochondrial, lipid peroxidation, and then activate apoptosis pathways, thus killing tumor cells and inhibiting tumor growth. To date, there are not many thermosensitizers reported in the literature. A few diazo (isobutyronitrile) derivatives, which are not heat-resistant, can decompose easily to produce free radicals during thermal activation, and can be used as thermal sensitive agents of TDT 72,73 . Among them, 2,2' -azadidi (2-imidazoline) dihydrochloride (AIPH or AIBI) has been used in the application study of TDT 67, 69 .

The application of TDT
The combination of TDT and PTT is also the most studied in tumor therapy 68,74,75 . It is found that artesunate (ARS) could produce ROS upon heating. Therefore, a pH-sensitive liposomal nanoplatform (ICG-ARS@NPs) composed of indocyanine green (ICG) and ARS is prepared for photoinduced TDT as well as PTT. ICG-ARS@NPs exhibited highly efficient anti-cancer therapeutic efficacy in H22 tumor-bearing mice owing to their specific tumor targeting and synergistic photothermal and thermodynamic effects 76 . A NIR-triggered thermo-responsive longacting hydrogels, composing of HA-GMA and NIPAM, loading with AuNRs and AIPH, could combine PTT induced by AuNRs and TDT induced by free radicals. And this new thermodynamic therapeutic strategy is an oxygenindependent and showed great advantages to the hypoxia tumor therapy. Furthermore, the carrier has good safety 10 and biocompatibility due to the biodegradable of hydrogel 69 .
The nanocomposites BMT@AIBI NCs are prepared by using black mesoporous titanium dioxide (BMT) with large pore size as the support of AIBI. In BMT@AIBI NCs, the BMT, as a light transducer, can convert NIR light energy into thermal energy and chemical energy (·OH), contributing to PTT and PDT respectively. Meanwhile, the heat generated by BMT triggers the decomposition of AIBI to produce a large number of alkyl radicals (·R) for TDT. High levels of free radicals produced by BMT@AIBI NCs under NIR light cause DNA double strand breaks, and ultimately inducing apoptosis of cancer cells. Since free radical production is independent of oxygen, BMT@AIBI NCs exhibit excellent anti-cancer effects in vitro and in vivo under hypoxic conditions. PTT/PDT/TDT combined therapy can overcome the problem of tumor hypoxia and enhance the anti-cancer effect 67 . There have also shown that MCN-AR@PCM combined with PTT/TDT/CT therapy can cause large area of HeLa cell death under both normal and hypoxia conditions, and can completely eliminate the tumor of HeLa cervical cancer bearing mice without recurrence, which has a significant anti-cancer effect 77 .
TDT combined with other therapies has achieved good therapeutic effect in animal researches, but there are still a lot of problems to be solved. We should develop other ways to capture heat energy, find and develop more available heat-sensitive agents. And we can take advantage of the flexibility in the design of nanomaterials to further explore to introduce other anti-tumor drugs or molecules for therapeutic purposes into the nanomaterial structure, so as to realize the combination of TDT with tumor therapy techniques such as PTT, PDT, radiotherapy, microwave therapy, ultrasound therapy, immunotherapy, gene therapy and so on, further improve the therapeutic effect of tumor and provide technical support and experimental basis for the clinical application of TDT.

Conclusions
As an emerging cancer therapy, PDT, PTT and TDT have shown great potential in the clinical application of cancer. The rapid development of nanotechnology and material technology in recent years has also made these applications more efficient and extensive. These novel treatment modes combined with other therapies still lack the accumulation of researches and the support of a large number of research data, as well as relatively immature technology and uncertain biological safety and other problems. Moreover, the solubility and biosafety of PTA, thermosensitizers and photosensitizers used in these novel therapies also need to be addressed. In the future, it should be focused on solving the existing problems of PTT, PDT and TDT to remove the obstacles, so as to further improve the therapeutic effect of tumor, and provide technical support and experimental basis for their clinical application.

Funding statement
This research received no external funding.

Conflict of interest
All the authors claim that the manuscript is completely original. The authors also declare no conflict of interest.

Author contributions
Conceptualization: Chen Li, Yi Liu; Investigation: Chen Li; Summarization: Chen Li; Writing-original draft: Chen Li; Writing -review & editing: Yi Liu. All authors contributed to the article and approved the final manuscript.