Progress in Nanocarriers Codelivery System to Enhance the Anticancer Effect of Photodynamic Therapy

Photodynamic therapy (PDT) is a promising anticancer noninvasive method and has great potential for clinical applications. Unfortunately, PDT still has many limitations, such as metastatic tumor at unknown sites, inadequate light delivery and a lack of sufficient oxygen. Recent studies have demonstrated that photodynamic therapy in combination with other therapies can enhance anticancer effects. The development of new nanomaterials provides a platform for the codelivery of two or more therapeutic drugs, which is a promising cancer treatment method. The use of multifunctional nanocarriers for the codelivery of two or more drugs can improve physical and chemical properties, increase tumor site aggregation, and enhance the antitumor effect through synergistic actions, which is worthy of further study. This review focuses on the latest research progress on the synergistic enhancement of PDT by simultaneous multidrug administration using codelivery nanocarriers. We introduce the design of codelivery nanocarriers and discuss the mechanism of PDT combined with other antitumor methods. The combination of PDT and chemotherapy, gene therapy, immunotherapy, photothermal therapy, hyperthermia, radiotherapy, sonodynamic therapy and even multidrug therapy are discussed to provide a comprehensive understanding.


Photodynamic Therapy
Photodynamic therapy (PDT) is a modern noninvasive antitumor technique that has great clinical application potential due to its advantages of simple operation and low systemic toxicity. PDT has been clinically approved for the treatment of some tumors, such as advanced esophageal cancer and advanced lung cancer [1]. Local or systemic photosensitizers (PSs) can be activated to produce cytotoxic reactive oxygen species (ROS) after absorbing light from an appropriate wavelength laser, which can induce tumor cell necrosis and apoptosis or cause angiotoxicity to block tumor cell nutrient supply. In addition, acute inflammatory responses and immunogenic cell death (ICD) induced by PDT have been shown to activate the body's immune system and result in the reconstruction of the tumor microenvironment [2,3].
There are two main mechanisms of the photodynamic reaction. When PSs enter cells, light at a wavelength coinciding with the PS absorption spectrum irradiates the issue, and PSs will be converted from the singlet basic energy state into the excited singlet state because of photon absorption. Part of the energy directs a PS molecule to the excited triplet state. In type I reactions, the PSs in their excited triplet state react with biomolecules and ROS can be generated from radical oxygen species formed by electron transfer reactions such as superoxide ions (O 2 •− ) and hydroxyl radical (OH • ). In type II reactions, the energy of PSs in their excited triplet state is transferred directly to the oxygen molecule in the basic energetic state, resulting in the generation of singlet oxygen, which is highly The upregulation of vascular endothelial growth factor (VEGF) and heat shock protein 70  are the main causes of PDT tolerance. To obtain a more ideal PDT treatment effect, PS can be delivered together with gene therapy drugs or small molecule inhibitors targeting VEGF or HSP-70 [58]. In addition, NO produced by photostressed tumor cells can induce anti-PDT effect and enhance tumor aggressiveness. This can be addressed by codelivery with iNOS inhibitors. These would be described in detail in the section on codelivery treatment [59][60][61].

Codelivery of PSs and Anticancer Drugs with Nanoparticles
There are many factors that contribute to decline in photodynamic therapy, including the metastatic tumor at unknown sites, inadequate light delivery, lack of sufficient oxygen and induction of an anti-PDT effect [1]. To overcome these limitations and to improve efficiency, the researchers have combined PDT with other treatments. Codelivery is one of the most common methods to achieve combination therapy and has been widely studied. Currently, common treatment schemes that are amenable to PS codelivery can be divided into the following types: chemotherapy, gene therapy, immunotherapy, photothermal therapy (PTT), hyperthermia (HT), radiotherapy, sonodynamic therapy and multiple drugs codelivery [62][63][64][65][66][67][68][69][70][71] (Figure 1). Here, we will mainly introduce the nanocarriers used for the codelivery of PSs and antitumor drugs and discuss how the drugs enhance each other.

Chemotherapy
Chemotherapy is a commonly used method for tumor treatment, but systemic toxicity and side effects cause patients to face great pain when receiving treatment, and the efficacy Pharmaceutics 2021, 13, 1951 5 of 41 is limited by stability, targeting and multidrug resistance [72]. Many reports have proven that the application of targeted delivery vectors can improve the physical and chemical properties of chemotherapy drugs, while combination with PDT can help to improve multidrug resistance and other problems [73,74]. Mechanistically, chemotherapeutic drugs bind to the DNA of tumor cells and block DNA replication, which leads to suppressed cell division and ultimately death [75]. It has also been found that many chemotherapeutic drugs can increase intracellular ROS levels and oxidation-reduction homeostasis of cancer cells, thus enhancing the sensitivity of tumor cells to PDT [76]. On the other hand, light-independent chemotherapy can kill deep-level tumor cells that PDT cannot, while increased tumor vascular permeability induced by PDT can enhance the accumulation of nanomaterials in tumors, thus enhancing the efficacy of chemotherapy [77,78].
Cationic liposomes are widely used in the codelivery system of PSs and chemotherapeutic drugs due to their preferential accumulation in the vascular endothelium [79]. The introduction of porphyrin-phospholipid (PoP), which is a kind of PS-coupled lipid, enables photoprogrammed controlled release of cationic liposomes. ROS produced by irradiation can oxidize unsaturated lipids and accelerates the release of chemotherapeutic drugs [80,81] (Figure 2A). Doxorubicin (DOX) was loaded into PoP liposomes for intravenous injection. Tumor vascular permeability and drug accumulation were significantly increased under near-infrared light, while empty PoP liposomes without drug loading also showed antitumor effects, suggesting that conjugation with phospholipids did not affect the photodynamic effect of PS [82,83]. In the synthesis of PoP liposomes, the selection of PoP and the proportion of other raw materials are crucial to the morphology and serum stability of the liposomes. Generally, a lower content of PoP in the lipid bilayer leads to higher serum stability [84]. Moreover, the dosage needs to be considered. When DOX is loaded in excess, the bilayer of PoP liposomes becomes elliptical due to instability, while loading irinotecan (a camptothecin-derived anticancer drug) does not affect the morphology of the liposomes [62]. In addition, carrier modification can achieve multiple functions; for example, 64 Cu-labeled POP liposomes can be used for imaging simultaneously with treatment and have been shown to not cause drug leakage [85]. The timing of illumination after POP liposome administration is also important, and studies have shown that the drug accumulates more in tumors at short drug-light intervals [86].
Hypoxia is the cause of drug resistance in both chemotherapy and PDT. The effect of photochemotherapy can be enhanced by codelivery of oxygen carriers [87] or oxygen generators [88] loaded with therapeutic drugs. There are also substances that can serve as both chemotherapeutic agents and oxygen donors. For example, nanoplatinum (Pt), when encapsulated in liposomes with PSs, can provide oxygen as a catalase-like nanoenzyme, while Pt ions leached separately can also exert cytotoxic effects [89]. Although the application of nanocarriers can reduce the toxicity and side effects of chemotherapy drugs, it cannot improve the antitumor tolerance caused by the upregulation of heat shock protein HSP-70 after PDT. Hailong Tian [90] combined quercetin (Qu), a chemotherapy drug with the dual effects of anticancer and heat shock protein inhibition, with IR780. IR780 modified with hydrophilic biotin and Qu were assembled into a delivery carrier in solution, which successfully enhanced the therapeutic effect of PDT by inhibiting the expression of HSP-70. PDT and some chemotherapeutic agents, such as DOX and Pt, can mediate ICD. For example, the combined action of oxaliplatin and PDT was shown to expose calreticulin (CRT) on tumor cells, successfully activating host immunity and creating a suitable microenvironment for subsequent immunotherapy combination [91]. More examples of recently published studies on the codelivery of photosensitizers and chemotherapy drugs based on nanocarriers are illustrated in Table 1.

Gene Therapy
The rapid development of nanodelivery systems has improved the penetration ability of cells and tissues and stability of exogenous genes under physiological conditions, allowing gene therapy technology to break through previous bottlenecks and enable clinical application [102,103]. The combination of gene therapy and PDT offers a high degree of precision based on complementary base pairs, which most other therapeutic combinations lack. Exogenous genes are required to efficiently enter cells and successfully escape intracellular bodies before they can play a therapeutic role, which poses great challenges to the material properties of delivery vectors [104]. Programmable vectors have been proposed to solve this problem to some extent and are commonly released under internal stimuli such as enzymes under special pH conditions. Photochemical internalization (PCI) is an intracellular delivery technique using an exogenous light as a stimulus. After activation of PS, ROS can be rapidly produced within a short time to destroy the membrane of the intracellular body and release therapeutic agents such as nucleic acid drugs in the cytoplasm [105,106] ( Figure 2B). This external light-dependent regulation is more controllable and stable than the internal response-dependent system and can achieve higher spatially controlled and targeted gene delivery [107]. Another type of photoprogrammed gene regulation uses PDT active nanomaterials as gene delivery vectors, such as PPBP, the black phosphorus (BP) nanosheets prepared with PEG and PEI modification, a black scale nanomaterial that shows PDT activity under light and then specifically degrades to release siRNA in a high ROS and acidic tumor environment to achieve targeted delivery [108].
Hypoxia influences PDT, while upregulation of VEGF and HSP-70 leads to PDT tolerance. Changing the expression of these proteins by gene therapy can help resolve these challenges to some extent and enhance the sensitivity of tumor cells to PDT. HIF1α is a hypoxia-inducible factor that plays a key role in tumor cell proliferation and angiogenesis [109]. Zheng WH [110] used anisamide-targeted lipid-calcium-phosphate (LCP) nanoparticles to achieve codelivery of protoporphyrin IX (PpIX) and HIF-1α siRNA. The results showed that HIF1α downregulation not only directly inhibited tumor cell generation but also promoted ROS production in the tumor environment, thereby enhancing PDTmediated apoptosis. Nrf2 is a key antioxidation regulator that prevents ROS accumulation and promotes angiogenesis. Deng S [111] codelivered CRISPR-Cas9 ribonucleoprotein (RNP) with Ce6, ROS generated by the latter caused release of Cas9/sgRNA into cytoplasm, resulting in Nrf2 interference and preventing tumor cells escaping from ROS-mediated killing. In terms of improving PDT-induced tolerance, Jang Y [112] prepared a DOX-siVEGF-NPS/Ce6-MBS complex coloaded with VEGF siRNA and Ce6 for the treatment of squamous cell carcinoma. Under the action of VEGF siRNA, tumor angiogenesis was significantly reduced, and the antitumor effect was improved. Similar results were reported in Cao Y [113], the MnO 2 nanosheet was first surface decorated with Cu 2−x S and then loaded with HSP-70 siRNA to form MnO 2 /Cu 2−X S-HSP-70-siRNA, which mediated the heat shock response and showed superior synergistic antitumor ability. In addition, the use of therapeutic genes to regulate the expression of proteins related to the growth, development, differentiation and metabolism of tumor cells can enhance the effect of PDT or supplement the limitations of PDT. For example, gene therapy can activate the body's immune system to treat metastatic cancer [114] and inhibit epithelial-mesenchymal transition (EMT) to avoid tumor recurrence [115]. Overall, the diversity of cancer-causing genes and the complexity of pathogenesis, as well as the innate differences of individuals, give gene therapy and PDT to infinite possibilities in combination. More examples of recently published studies on the codelivery of photosensitizers and gene therapy drugs based on nanocarriers are illustrated in Table 2. • The vector effectively self-assembled with siRNA and mediated effective gene silencing (35% and 52% gene silencing efficiencies, respectively).

•
After successful delivery of siRNA, the photoactivity of the vector was restored, which could mediate further PDT.

Immunotherapy
PDT can activate the body's immune response in two ways: one is to induce an acute inflammatory response of the host and release various proinflammatory signals; the other is to trigger ICD by injured or dead tumor cells to release damage-associated molecular patterns (DAMPs) and neoantigens as danger signals [3,124]. Although the immune response mediated by PDT is not enough to kill tumor cells, it can create a reconstructed immune microenvironment for further antitumor immunotherapy [63].
Immune adjuvants are a class of immune-stimulating molecules that activate tumorspecific immune responses by interacting with Toll-like receptors (TLRs) on antigenpresenting cells (APCs) [125]. The combination of immune adjuvant and PDT has the dual ability to activate the immune system. A nanometal organic framework (nMOF) formed by direct self-assembly of metal ions and PSs is often used as a codelivery carrier and is characterized by a high loading efficiency and good biocompatibility [126]. Cai Z [127] prepared PCN-ACF-CpG@HA NPs loaded with the immune adjuvant CPG by combining PS tetrakis (4-carboxyphenyl) porphyrin (H 2 TCPP) with zirconium ions to target tumors with high expression of the CD44 receptor. CPG and PDT together mediated a strong antitumor immune response, with significantly higher CD8+ and CD4+ T cell infiltration at the tumor site than that in the control group. Similarly, the cationic W-TBP designed by Ni K [128] based on nMOF can directly adsorb negatively charged CPG through electrostatic action, promote its internalization and DC maturation, and enhance antigen presentation in coordination with PDT-induced CRT exposure. In addition to treating tumors in situ, this codelivery combination kills distant metastatic cancer cells. Xia Y [129] evaluated the efficacy of CPG combined with the PS verteporfin in the treatment of 4T1 metastatic breast cancer; the results showed that the activation of DC cells was significantly increased, and the tumor volume of tumor-bearing mice was smaller than that of other control groups. Xu C [130] also achieved similar results in the treatment of local and metastatic B160F10 melanoma, which was enhanced by promoting DC recruitment and cytotoxic T cell infiltration at the tumor site. Exogenous antigens can also activate immune responses. Ovalbumin (OVA), a commonly used model antigen, can be used as a supplement to immune stimulation induced by PDT. The effect of such codelivery was evaluated in the studies of Huang R [131] and Ding B [132], both of which showed synergistic immune enhancement.
Immune checkpoint therapy has made significant breakthroughs in recent years and can effectively improve the immune system's response to tumors. At present, several immune checkpoint inhibitors have been approved by the FDA for clinical treatment [133]. Tumor vascular abnormalities and the immunosuppressive tumor microenvironment (TME) caused by indoleamine 2,3-dioxygenase 1 (IDO1) seriously affect the efficacy of PDTmediated immunotherapy. Codelivery of PSs with IDO1 inhibitors is beneficial for amplifying the effects of photodynamic immunotherapy. Combinations that have been reported include ferritin and polyethylene glycol-PLGA (PEG-PLGA) coloaded with ZnF 16 Pc and IDO inhibitor NLG919 [134] or Ce6, tyrosinase inhibitor axitinib (AXT), IDO1 inhibitor dextro-1-methyltryptophan (1MT) and human serum albumin self-assembled NPs [135]. These combinations can improve the tumor microenvironment by normalizing tumor blood vessels, improving hypoxia levels, promoting the invasion of immune effector CD8+ T cells in tumors, and reducing the immunosuppressive properties of tumors, which represents a promising tumor treatment strategy. Cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) and programmed death-ligand 1 (PD-L1) are the most common immune checkpoints and are closely related to the immune function of T cells. Xu J [136] used the self-assembly property of Ce6 and immunoglobulin G (IgG) in the nanoscale affinity range to bind Ce6 to aPD-L1 or double bind to αPD-L1 and αCTLA-4 in the immunocheckpoint blocking treatment of glioma in situ and colon cancer. This combination therapy successfully prolonged the survival of tumor-bearing mice and produced a long-term memory response, avoiding tumor recurrence. In addition, zinc phthalocyanine and aCTLA4 have been coadded into microneedles prepared by hyaluronic acid and dextran for skin cancer delivery. This mini-mally invasive percutaneous drug delivery platform can also effectively induce an antitumor immune response and avoid the systemic distribution of drugs to reduce toxicity and side effects [137]. Without affecting coloaded drugs, nanodelivery systems targeting important organelles such as mitochondria [138] and the endoplasmic reticulum [139] can be designed to enhance PDT-triggered ICD, thereby enhancing immune activation (Figure 3). aCTLA4 have been coadded into microneedles prepared by hyaluronic acid and dextran for skin cancer delivery. This minimally invasive percutaneous drug delivery platform can also effectively induce an antitumor immune response and avoid the systemic distribution of drugs to reduce toxicity and side effects [137]. Without affecting coloaded drugs, nanodelivery systems targeting important organelles such as mitochondria [138] and the endoplasmic reticulum [139] can be designed to enhance PDT-triggered ICD, thereby enhancing immune activation ( Figure 3). Photoimmunotherapy is a tumor-targeting therapy using specific antibodies to tumor-associated receptors chemically coupled with PSs, which is more accurate than conventional PDT and is suitable for tumors at sensitive anatomical sites. Hasan T [140] coupled epidermal growth factor receptor (EGFR) monoclonal antibody Cetuximab with benzoporphyrin derivative for pancreatic ductal adenocarcinoma treatment. The results showed that the photoimmune nanoconjugate (PIN) had a high binding specificity and could rapidly penetrate heteromorphic organoids, providing approximately 16-fold enhancement in molecular targeted NIR photodestruction. Nevertheless, single-receptor targeted therapy may cause tumor subsets with low receptor expression to evade treatment and thus fail to completely ablate tumors. Hasan T [141] further constructs a triple recep- When photosensitizers in tumor sites are activated, they can cause acute inflammation and induce cell apoptosis or necrosis. Dendritic cells mature when stimulated by cytokines released at the site of inflammation and provide antigens to T lymphocytes in regional lymph nodes. Activated T lymphocytes become effector T cells, which are attracted to chemokines, migrate to the tumor and kill tumor cells. Different types of immunotherapy drugs play a role in different steps of the complete antitumor immune cycle. Codelivery of a photosensitizer with an immune adjuvant or tumor antigen can synergistically enhance the activation of the host immune system and improve the immunosuppressive microenvironment. The codelivery of a photosensitizer with CTLA-4 and PD-L1 monoclonal antibodies can enhance the antitumor immunity effect of T cells. The activation of these immune cells also plays a role in preventing metastasis and recurrence.
Photoimmunotherapy is a tumor-targeting therapy using specific antibodies to tumorassociated receptors chemically coupled with PSs, which is more accurate than conventional PDT and is suitable for tumors at sensitive anatomical sites. Hasan T [140] coupled epidermal growth factor receptor (EGFR) monoclonal antibody Cetuximab with benzoporphyrin derivative for pancreatic ductal adenocarcinoma treatment. The results showed that the photoimmune nanoconjugate (PIN) had a high binding specificity and could rapidly penetrate heteromorphic organoids, providing approximately 16-fold enhancement in molecular targeted NIR photodestruction. Nevertheless, single-receptor targeted therapy may cause tumor subsets with low receptor expression to evade treatment and thus fail to completely ablate tumors. Hasan T [141] further constructs a triple receptor-targeted PIN (TR-PIN), cetuximab, holo-transferrin, and trastuzumab conferred specificity for EGFR, transferrin receptor (TfR), and human epidermal growth factor receptor 2 (HER-2). Researchers compared the binding ability of TR-PIN to tumor cells with different levels of receptor expression (EGFR, TfR or HER-2), and found that TR-PIN has the ability to recognize multiple tumor targets, effectively photodynamically eradicating different tumor subsets and reducing escape. More examples of recently published studies on the codelivery of photosensitizers and immunotherapy drugs based on nanocarriers are illustrated in Table 3.

Photothermal Therapy (PTT)
Additionally, as a minimally invasive treatment, PTT has certain similarities with PDT. Photothermal agents concentrated at the tumor site absorb laser radiation energy and convert light energy into hyperthermia, resulting in the thermal ablation of adjacent cells. PTT enhances the effect of PDT, mainly by improving blood flow and increasing oxygen content in tumors [146] (Figure 4A).

Photothermal Therapy (PTT)
Additionally, as a minimally invasive treatment, PTT has certain similarities with PDT. Photothermal agents concentrated at the tumor site absorb laser radiation energy and convert light energy into hyperthermia, resulting in the thermal ablation of adjacent cells. PTT enhances the effect of PDT, mainly by improving blood flow and increasing oxygen content in tumors [146] (Figure 4A).  [69]. Gold nanocages [147] show strong absorption in the NIR region, and their empty interior and porous walls are suitable for encapsulating PSs. Bo Tian et al. [148] coupled Ce6 on PEG-functionalized or alternating magnetic fields are applied to magnetic nanoparticles, heat is generated at the tumor site, increasing blood flow and synergistically enhancing PDTSome photothermal agents of inorganic materials themselves can be used as nanocarriers, which makes the combination of PTT and PDT convenient. Gold nanorods have been used as carriers with photothermal conversion capability, and the joint delivery of DOX and PS ICG can simultaneously achieve chemo-PTT-PDT triple therapy [69]. Gold nanocages [147] show strong absorption in the NIR region, and their empty interior and porous walls are suitable for encapsulating PSs. Bo Tian et al. [148] coupled Ce6 on PEG-functionalized GO (GO-PEG-CE6). The photothermal effect of GO promoted the transfer of Ce6, and its destruction effect on cancer cells was significantly better than that of free Ce6. The photothermal agent plasma copper sulfide (Cu 2 -XS) has also been reported to have photodynamic properties [149], allowing the combination of PTT and PDT to be achieved simultaneously, and has been verified in in vitro cultured melanoma cells and mouse melanoma models. The poor photostability and potential long-term toxicity of inorganic nanomaterials limit their clinical application, so more materials with good biocompatibility and high stability have been developed as alternatives. The conductive polymer polypyrrole is a material with relatively high biocompatible photostability and photothermal conversion efficiency, using polyacrylamide (PAH), polyacrylic acid (PAA) and AlPCS 4 . Further modification of polypyrrole resulted in a more stable AlPCS 4 @PPyCONH-PAH-PAA nanoneedle complex in a physiological environment. The results showed an enhanced synergistic effect, with tumor ablation in mice 14 days after treatment and no recurrence within 30 days [150].
Another more conventional codelivery method is to simultaneously load the photothermal agent and PS or prodrug of PS in nanodelivery systems such as micelles, vesicles, and liposomes. Gang Chen et al. [151] developed CS NPs as codelivery carriers of photothermal agents (IR780) and prodrug of protoporphyrin IX (5-aminolevulinic acid, 5-ALA) for oral administration in the treatment of subcutaneous colon cancer in mice. CS keeps the drug stable even under acidic conditions in the stomach, allowing the drug to successfully accumulate at the tumor site. Mechanistic studies have shown that the oxidative stress response at the tumor site is enhanced, producing more ROS, superoxide and 1 O 2 . The enhanced effect of this synergistic administration of light and heat on cancer treatment was also reported in the research of Xiaodong Liu [65]. The high singlet oxygen generation capacity and photothermal conversion efficiency make this treatment strategy more severely phototoxic to both superficial and deep tumor cells.
When the excitation wavelengths of the PS and photothermal agent were different, the complexity of the treatment was greatly increased. To solve this problem, Jing Lin [152] designed a gold vesicle with a strong plasma coupling effect. This gold vesicle was densely packed with monolayer gold NPs, and Ce6 was encapsulated inside. The enhancement of plasma coupling between gold NPs in close proximity lead to a redshift in extinction spectra. Therefore, the photothermal and photodynamic effects could be stimulated simultaneously by a single 671 nm laser irradiation. Some special materials, such as nano GO, themselves have the dual nature of promoting both PDT and PTT, and their development for PTT can reduce the complexity of material preparation [153]. More examples of recently published studies on the codelivery of photosensitizers and photothermal agents based on nanocarriers are illustrated in Table 4.

Hyperthermia Therapy (HT) and Magnetic Hyperthermia Therapy (MH)
As one of the emerging noninvasive treatment options, HT may enhance PDT by alleviating hypoxia [168,169]. The mechanism is that as temperature increases, blood flow at the tumor site increases and microcirculation improves, thereby increasing tumor oxygenation [170]. MH, also known as magnetic therapy, is based on the heat generated by magnetic NPs (MNPs) under the action of an alternating magnetic field (AMF) to target and kill tumors without harming surrounding healthy tissue [171]. The emergence of MH has made the combination of HT and PDT a possibility, and the generated heat affects deeper tumors, which is beneficial for enhancing PDT [172] (Figure 4B). In terms of codelivery, magnetic nanomaterials can be directly used as carriers or loaded with PS in other carriers. Hongwei Gu [66] conjugated magnetite Fe 3 O 4 with porphyrin derivatives, which is highly efficient and has a low systemic toxicity. Huang WC [173] used monocytes derived from bone marrow as carriers to jointly transport Ce6 and oxygen superparamagnetic iron oxide NPs for combined therapy. This is an interesting combination strategy, but the relevant literature is still limited, and researchers need to conduct more in-depth research and discussions. More examples of recently published studies on the codelivery of photosensitizers and hyperthermia agents based on nanocarriers are illustrated in Table 5.

Radiotherapy
Band gap materials convert X-rays into light photons in the UV-vis region, which can be used to activate PSs. This is the basis of a combination of PDT and radiation therapy known as X-ray PDT (XPDT) [177,178]. The use of light is one of the limitations of PDT, and the strong tissue penetration of X-rays can compensate for this shortcoming. The method of operation delivers scintillators and PSs together to the tumor site. Under the action of X-rays, scintillators emit persistent light to activate PSs. This strategy can also produce good therapeutic effects for deep tumors [179], and the therapeutic effect depends to some extent on the energy transfer efficiency of the scintillator. The primary choices of scintillators are rare earth materials [180][181][182] with high photon conversion rates and some metallic materials [183], in addition to a few nonmetallic materials [184] and even quantum dots [185].
Another factor affecting the therapeutic effect is the distance between the PS and the scintillator. One approach is to physically package PS in a coating bound to the scintillator by electrostatic or hydrophobic means or to load both the scintillator and PS into NPs [186,187]; for example, the combination of protoporphyrin IX (PpIX) and scintillator LaF 3 :Ce 3+ coloaded in PEG-PLGA was shown to exhibit highly efficient loading and high stability under physiological conditions [188]. Physical combination has a certain risk of missing the target, whereas chemical bonding based on covalent bonds or conjugation is complex but more stable. K. K. Popovich [67] coated CeF 3 :Tb 3+ with PpIX-coupled SiO 2 . The monodisperse SiO 2 coating had high energy transfer efficiency and protected the scintillator encapsulated within it [189].
The combination of a scintillator and PS solves the problem of light sources, but it is still subject to the strong oxygen dependence of PDT. Zhang C [190] was inspired to integrate scintillators and semiconductor quantum dots with unique optical properties into ionizing radiation-induced PDT synchronous radiotherapy. The addition of a PS without traditional effects reduces the oxygen dependence of the complex, which can subsequently produce a process similar to type I PDT under ionizing radiation. Chuang YC [191] introduced the annealing process to achieve PS-free PDT. The yttrium oxide nanoscintillation complex coated with a silica shell (Y 2 O 3 :Eu@SiO 2 ) was subjected to X-ray irradiation and subsequent annealing, and then, photodynamic effects were promoted to mediate tumor cell damage in conjunction with radiotherapy.
Radionuclide-mediated Cerenkov luminescence is a phenomenon produced by the interaction of high-speed charged particles with the surrounding medium. The codelivery of radionuclides with PSs has been reported in several studies to have antitumor effects and prolong the survival of tumor-bearing mice. In addition to mediating Cerenkov luminescence, radionuclides can directly kill tumor cells [192,193]. Although this combination has achieved some success in the treatment of deep tumors, there are still many shortcomings to be overcome, such as the blueshift of the low-energy radiation emission spectrum to the ultraviolet region. More examples of recently published studies on the codelivery of photosensitizers and scintillators/radionuclides based on nanocarriers are illustrated in Table 6. • Under an external magnetic field, the NPs were highly concentrated in the tumor.

•
Multimodal imaging of fluorescence, Cerenkov luminescence and Cerenkov resonance energy transfer could be implemented to detect the treatment process. • The NPs effectively killed HCT116 cells in the presence 6 MeV X-ray radiation.

•
The 6 MeV X-ray radiation from LINAC produced energetic secondary electrons and Cerenkov radiation in the samples, which in turn excited the VP molecules. [197]  • The combination of X-ray guided photodynamic therapy and anaerobic oncolytic bacteria killed hypoxic and aerobic tumor tissue.

Sonodynamic Therapy (SDT)
Sonodynamic therapy (SDT) uses ultrasound and acoustic sensitizers to treat cancer. Ultrasound, on the one hand, activates ultrasonic-sensitive species gathered in tumor cells through transient sonoluminescence to produce ROS. Hot spots from ultrasonic radiation, on the other hand, release energy high enough to cause bubbles to form and oscillate, resulting in permanent cell damage. The latter phenomenon is also defined as sonographyinduced cavitation effects that together lead to tumor cell apoptosis or necrosis [200][201][202][203]. The extent of tumor cell death depends on the intensity and frequency of ultrasound and the duration of exposure, so it is controllable, safe and noninvasive, similar to PDT.
The strong penetration ability and low tissue attenuation of ultrasound make sonophotodynamic therapy (SPDT) have a wider range of indications. PDT is suitable for superficial cancer types such as skin cancer [204] and esophageal cancer [205], but the treatment effect for melanoma skin cancer has not been ideal, which is caused by the absorption of visible light by melanin to remove ROS [206]. In contrast, SDT was significantly more effective for melanoma than PDT, although there was no differences between the two treatments for common skin cancer [207].
The ultrasonic activation ability of PSs has also been verified to some extent. Some PSs have both SPDT activity, which are referred to as photoacoustic agents. We only need to use a specific wavelength of light and a specific frequency of sound to activate the photoacoustic agent so that the photoacoustic combination can be achieved without considering the coordination of physical and chemical properties in the process of multidrug combination. Currently, porphyrins as well as Ce6, Photolon™/Fotolon™, and Sonoflora 1 Sonnelux-1 have been verified [208][209][210]; several studies have reported their therapeutic efficacy and safety, and there are many potential PSs whose ultrasonic activation ability is being verified, which will also find applications in photoacoustic combination in the near future [211].
The modification of codelivery vectors helps to achieve multifunctional and integrated photoacoustic therapy with high spatial activation and preferential treatment of deep penetration. Hong L et al. [68] prepared Ce6 and the high oxygen carrying capacity carrier perfluoropolyether into a nanoemulsion and tested the ROS generation ability at different tissue depths after the application of ultrasound or light. The results showed that PDT had a high degree of spatial selectivity for surface and endoscopically accessible areas, which made it more suitable for the treatment of tumors in vital organs, such as brain cancer. SDT, on the other hand, had the same ROS production efficiency for tissues at different depths. SPDT was conducive to targeting different depths and meeting specific spatial accuracy requirements. Accordingly, sensitive drugs can be combined with tissueengineered scaffolds to kill tumor cells after surgery. The results showed that SPDT had fewer toxic side effects than conventional chemotherapy, so it has great potential [212]. More examples of recently published studies on the codelivery of photosensitizers and sonophotosensitizers based on nanocarriers are illustrated in Table 7.

Multidrug Codelivery
Cumbersome drug delivery processes, limited efficiency, slow release and low efficacy are common characteristics of multidrug codelivery. Therefore, the main research direction is still focused on solving the problem of coloading and controlled release.
Chen Y [70] designed an NIR-sensitive nanocomposite DLA-UCNPs@SiO 2 -C/HA@ mSiO 2 -DOX@NB. This complex breaks the chemical bond and releases paclitaxel (PTX) when irradiated by NIR light at 980 nm, which is then activated by visible light at 450-480 nm to exert synergistic photodynamic and photothermal therapeutic effects. Similarly, the Fe 3 O 4 /g-C 3 N 4 @Ppy-DOX nanocomposite prepared by Cheng HL [4] and graphitelike carbon oxide (G-C 3 N 4 ) can not only generate O 2 through photocatalytic degradation to improve the hypoxic state of solid tumors but also enable the loading of PTX into its mesoporous structure. Polypyrrole, as a photothermal agent, was shown to enhance the antitumor effect of chemo-PDT.
CuS NPs with both PTT and PDT activities were used to achieve the triple combination of PTT-PDT-HT [216]. Curcio A [217] prepared a maghemite (γ-Fe 2 O 3 ) nanoflower-like multicore nanoparticle conceived for MH and coated it with a spiky copper sulfide shell (IONF@CuS) for PTT and PDT. This combination showed a good therapeutic effect and reduced the dose of maghemite, which meant lower toxicity. Simultaneous activation of PDT and PTT by X-ray radiation-induced scintillator luminescence can be triply magnified when combined with radiotherapy. Luo L [218] conjugated a scintillator complex and a gold nanorod nanosensitizer. Lanthanide complexes can provide excellent luminescence under X-ray excitation, while gold nanorods can be used not only as PSs for PTT but also as radiosensitizers for enhanced radiotherapy due to their strong near-infrared light and X-ray absorption capacity. As expected, the group treated with both laser and X-ray irradiation showed the best synergistic effect, with significantly more effective tumor ablation than that in the other monotherapy groups.
Low-dose docetaxel has been reported to mediate ICD-activated immune effects [219], and this function of PDT is also mentioned above. Therefore, folic acid-modified mesoporous CuS NPs coloaded with DOX, polyethylenimide-PpIX (PEI-PpIX) and CPG for the treatment of cold tumor breast cancer can effectively rebuild the tumor microenvironment and promote the invasion of cytotoxic T lymphocytes (CTLs) [220]. In addition, it was reported that a MOF was used to integrate chemotherapy-phototherapy-photothermoimmunotherapy. Cus PpIX and DOX were loaded into the core and shell of the metalorganic skeleton ZIF-8, respectively, while CPG was adsorbed outside the shell. This design effectively realized the organic unification of antitumor and antirecurrence/metastasis [221]. More examples of recently published studies on the codelivery of multidrugs based on nanocarriers are illustrated in Table 8.

Outlook and Discussion
Complex microenvironments, abnormal growth rates and drug resistance during treatment make cancer treatment difficult. PDT, as a new treatment scheme, has the advantages of simple operation, noninvasiveness, safety and few side effects. The emergence of nano-carriers has made great contributions to the development of photodynamic therapy, and the limitation of physical and chemical properties of PS in PDT was solved. In addition to therapeutic effects, PS-mediated fluorescence can also be used for optical imaging to detect tumors, or as image guidance for surgery [230]. The major issues contributing to the failure of PDT are metastatic tumor at unknown sites, inadequate light delivery and lack of sufficient oxygen. This limitation can be addressed by multidrug combination. The combination of PDT and other treatment schemes, such as chemotherapy and radiotherapy, immunotherapy, gene therapy and PTT, has obvious advantages over the single application of one treatment scheme. With the development of nanodelivery vectors, codelivery has become a common method of multidrug combination. In this paper, we review the research progress of PDT combined with other therapies to improve the therapeutic effect on tumors based on the codelivery of nanodelivery carriers.
The combination of PDT and chemotherapy can significantly reduce the dose of chemotherapy drugs, overcome the deep-seated tumors that PDT cannot kill by multidrug resistant chemotherapy, and increase the level of ROS in the tumor microenvironment to enhance the sensitivity of cancer cells to PDT. In combination with immunotherapy, the PDT-mediated inflammatory response and ICD activate the body's immune response, which can be further enhanced by codelivering an immune adjuvant CPG or tumor antigen, providing a reconstructed immune microenvironment for tumor immunotherapy. PDT can be codelivered with therapeutic genes that target the regulation of hypoxia, angiogenesis, and heat shock proteins, alleviating the tolerance of long-term PDT therapy. Combined with therapeutic genes that target the regulation of other key proteins in tumor growth and metabolism, multimechanism antitumor effects can be achieved. The combination of PDT and PTT can synergistically increase the oxidative stress response at the tumor site, and the PTT-mediated increase in reactive oxygen species levels at the tumor site enhances the sensitivity of tumor cells to PDT. Studies on the combination of PDT and HT are still scarce. Although the discovery of MH is of great help to this codelivery strategy, more studies are still needed to confirm any synergistic effects. PDT combined with radiotherapy can overcome the limitation of PDT in light and can significantly improve the therapeutic effect of PDT on deep tumors. Combined sonodynamic therapy is beneficial to achieve accurate treatment, while satisfying different depths and is suitable for tumor treatment of fine organs. Multidrug codelivery can realize the organic unity of antitumor and antimetastatic effects, as well as recurrence through different antitumor mechanisms.
Although these different antitumor combinations have shown good results, they have their own limitations. PDT has been reported many times to induce sustained systemic immunosuppression, but the current mechanism has not been clarified. The speculated reasons are as follows: (1) ROS produced during PDT inactivate DAMPs released by apoptotic tumor cells, thus failing to stimulate immunity; and (2) apoptotic tumor cells release IL-10, TGF-beta and other immunosuppressive cytokines that affect the generation of CD8+ T cells [231]. This immunosuppression will affect not only the subsequent treatment effect of PDT but also the combination of PDT and immunotherapy. The combination of PDT and PTT is dependent on laser irradiation, which has a poor effect on deep tumors. The combination of PDT and radiotherapy is also subject to the strong oxygen dependence of PDT. Multidrug combination delivery is limited by the development of delivery vectors, drug loading and release. These problems are one of the reasons why codelivery systems based on nanocarriers have not been applied in clinical applications, while other reasons include incomplete safety assessment of the preparation protocol and difficulty in large-scale clinical applications of the preparation.
In summary, this paper focuses on the synergistic enhancement of antitumor therapeutic effects by codelivery combinations of PDT and different treatment regimens. This type of approach is definitely a potential therapeutic strategy, and the existing problems mentioned above need more research to resolve.

Conflicts of Interest:
The authors declare no conflict of interest.