Recent progress in synergistic chemotherapy and phototherapy by targeted drug delivery systems for cancer treatment

Abstract

Although it’s pharmacological effect for cancer therapy, conventional chemotherapy has been compromised by a series of shortcomings such as limited stability, nonspecific tumour targeting ability and severe toxic side effects. To overcome these limitations, multifunctional targeted drug delivery systems for combinatorial therapeutics have been widely explored as novel cancer therapy strategies, showing encouraging results in many pre-clinical animal experiments. Among them, synergistic phototherapy and chemotherapy have demonstrated their abilities to enhance therapeutic efficacies and reduce unwanted side effects via a variety of mechanisms. In this review, we will summarize the latest progress in the development of targeted drug delivery systems with combinations of phototherapy and chemotherapy and discuss the important roles of phototherapy agents involved in those non-conventional therapeutic strategies.

Keywords:

• Targeted drug delivery system
• phototherapy
• combination therapy
• phototherapy agents

Introduction

The malignant tumour surpassing the cardiovascular disease has become the most lethal disease and seriously threatens human’s health and life [1]. At present, surgery, radiotherapy and chemotherapy are the main means of cancer treatment. However, there are some limitations of each method. For example, although surgical treatment of cancer is one of the most primitive methods and can achieve the effect of healing for most of the non-diffused tumours, it is not applicable to the treatment of metastasis tumours or tumours growing in some sensitive and important part of the body. Radiotherapy can produce a series of systemic and local toxic side effects, including it can damage the normal tissue around the radiotherapy site, produce scar tissue and lead to systemic immune suppression. Traditional chemotherapy is the use of chemical drugs to control and kill tumour cells to achieve the purpose of treatment. Chemotherapy drugs can be circulated throughout the body with the vast majority of tissues and organs, which is an effective way for systemic treatment [2]. However, chemotherapy drugs have poor targeting and dose-limiting toxicity problems. Therefore, it is important to seek new generations of cancer treatment methods with high selectivity for tumour cells.

In recent years, more and more researchers have paid attention to the tumour targeted combination therapies, especially in the field of nanomedicine. Combination therapy with multiple treatments is more effective than single therapy because it produces synergistic anti-cancer effects, reduces drug-related toxicity and inhibits multidrug resistance through different mechanisms [3]. Therefore, the use of two methods or a variety of methods of comprehensive treatment or a method to assist another method of treatment has become a new trend in cancer treatment. Excitingly, synergistic phototherapy and chemotherapy have demonstrated their abilities to enhance therapeutic efficacies and reduce unwanted side effects in many pre-clinical animal studies, and shown many unique advantages compared with conventional chemotherapy. Phototherapy is one of the promising non-invasive clinical approaches to eradicate cancer owing to its high efficiency and minimal side effects, which are achieved by using radiation energy to kill cancer [4]. Phototherapy will convert light energy into either chemical energy or heat energy in the body, resulting in a series of chemical reactions in the body to reach the therapeutic effect [5]. When represented by photodynamic (PDT) [4] or photothermal (PTT) [6] therapies, phototherapy requires light and photoactive agents to generate reactive oxygen species (ROS) or heat, respectively. PDT involves the administration of photosensitizer (PS) and then localizing the lesion using a specific wavelength of light to activate the PS. A series of photochemical reactions initiated by PS results in the death of cancer cells [4]. Unlike PDT, PTT does not require oxygen accessibility to damage targeted tissues and generates locally elevated temperature to destruct tumour cells under near-infrared (NIR) light irradiation [6]. As PDT, PTT and chemotherapy act via different mechanisms, their combination into a single therapeutic system provides a highly efficient approach to treat neoplastic tissues. The combinatorial phototherapy and chemotherapy can be an efficient treatment for chemo- and radio-resistant cancer cells by involving cytotoxic mechanisms distinct from chemo and radiotherapies.

Why targeted drug delivery systems for phototherapy combined with chemotherapy

Targeting drug delivery system (TDDS), as a new means of selectively administering a drug to a target organ, a target tissue or a target cell without affecting healthy normal cells, showed good results in the anti-tumour drug delivery. Researchers in the field of pharmaceutical preparations explore and practice a variety of targeted approaches and methods. At the same time, the emergence of new targeting drug carriers has been an amazing popular concept in the past decades, including liposomes (80–300 nm), silicon particles (10–300 nm), magnetic nanoparticles, solids lipid nanoparticles (80–300 nm), polymer nanoparticles (10–80 nm), dendrimers (1–10 nm), carbon nanoparticles (1–5 nm) and targeted preparations such as monoclonal antibodies and prodrugs [7–9]. Nano-drug carriers can significantly increase the drug concentration of antitumor drugs in tumour tissue and cells by the ERR effect [10]; the targeting effect on the tumour group is more pronounced, and to a large extent overcome or compensate for the traditional tumour therapy methods (e.g. chemotherapy). In addition, if a specific targeting ligand is administered to the drug delivery system to form a proactive targeting, it can better mediate its distribution and release to the target cells, which can significantly improve the therapeutic effect of the tumour. The overexpression of some receptors on the outer membrane of tumour cells facilitates recognition of drug delivery systems functionalized with bio-molecules such as folic acid (FA), Arg-Gly-Asp (RGD) peptide or some other specific peptides or antibodies. This strategy has been broadly validated in in vitro and in vivo experiments [7–9].

In combination therapies, especially in synergistic chemotherapy and phototherapy, either anticancer drugs or photoactive agents are lack of tumour selectivity, thus increase potential toxicity in normal tissues. The use of nano-targeted delivery system to transport photosensitizer and chemotherapy drugs can enhance the drug concentration at the target site, significantly reduces the side effects and improves the effectiveness of light therapy and chemotherapy. This is due to the structural flexibility and adaptability of the combined treatment, and the well-designed multicomponent nanomaterials have shown superior prospects in conducting multidisciplinary treatment, including targeted drug delivery, sustained chemotherapeutic drug release, photosensitizer and photothermal transducers [11]. Therefore, the study of targeted delivery system for the use of chemotherapy drugs and photoactive agents to fight the tumour has a very important significance, and it provides new ideas for the development of anti-tumour drugs. Taking those into consideration, tremendous efforts have been devoted to the development of nanoscale targeted drug delivery systems which are effective for tumour therapies. In this review article, we will thus summarize recent progresses in the research of targeted formulations with combinations of phototherapy and chemotherapy, and try to address future prospective in this field.

Targeted drug delivery systems for photodynamic therapy combined with chemotherapy

PDT is of considerable concern as a novel and promising phototherapy approach of treating cancer. Unlike traditional cancer treatments, PDT is a 2-stage procedure, which usually depends on three important and non-toxic components: photosensitizer, light and oxygen. In PDT, photosensitizers would generate ROS to disrupt the target tissue and cells under appropriate light irradiation (Figure 1) [12]. Various photosensitizers (PS) such as Photofrin^®, methylene blue, 5-aminolaevulinic and chlorin e6, have been widely used in antitumor applications [13]. The two most important aspects of PDT are the processes of light absorption and energy transfer. A ground state PS has two electrons with opposite spins in a low energy molecular orbital, which is known as the singlet state. Following the absorption of light, one of these electrons is boosted into a high-energy orbit, but keeps its spin from the first excited singlet state. When a PS is in its excited state, it can interact with molecular triplet oxygen (³O₂) and produce radicals and reactive oxygen species (ROS), crucial to the Type II mechanism. These species include singlet oxygen (¹O₂), hydroxyl radicals (•OH) and superoxide (O₂⁻) ions. They can interact with cellular components including unsaturated lipids, amino acid residues and nucleic acids. If sufficient oxidative damage ensues, this will result in target-cell death [12,13]. However, many of those photosensitizers are hydrophobic and have limited tumour targeting ability. Thus, various nano-sized drug delivery systems have been developed to delivery photosensitizers to improve PDT efficacy. Organic nanoparticles, such as liposomes, micelles, dendrimers and hollow polymer microcapsules, have been extensively used in the delivery of photosensitizers [14–18]. In addition, a wide variety of inorganic nanoparticles including zinc oxide, graphene, carbon nanomaterials or quantum dots has been employed to delivery photosensitizers or used as PDT agents themselves [19–21]. During clinical cancer therapy, although chemotherapy is effective, it generally has more serious side effects. One of the approaches to overcome limitations of chemotherapy drugs is to combine conventional chemotherapy with photodynamic therapy. PDT can generally reduce the systemic toxic side effects of the chemotherapeutic drug, due to its local targeting and potentially synergistic effects that can reduce the required dose of the chemotherapeutic drug, and has the potential to overcome drug resistance via generating new pathways to kill tumour cells. Previous clinical animal trials have already demonstrated that combinations of two or more drugs were more effective in the cancer treatment, especially sequential photodynamic design combing with sequential chemotherapy [16–18,22–33].

Figure 1. The principles of photodynamic therapy (PDT). A photosensitizer (PS) is administered systemically or topically. After a period of systemic PS distribution, it selectively accumulates in the tumour. Irradiation activates the PS and in the presence of molecular oxygen triggers a photochemical reaction that culminates in the production of singlet oxygen (1O₂). Irreparable damage to cellular macromolecules leads to tumour cell death via an apoptotic, necrotic or autophagic mechanism, accompanied by induction of an acute local inflammatory reaction that participates in the removal of dead cells, restoration of normal tissue homeostasis and, sometimes, in the development of systemic immunity. Copyright from ACS, 2011 (Ref. [12]).

Common used PDT agents for combined PDT and chemotherapy

As a typical example, PDT agents, such as methylene blue (MB), 5-aminolaevulinic, chlorin e6 (Ce6) and many others, could be encapsulated into the centre of nanomaterials, with chemotherapeutic molecules are uploaded or absorbed onto the surface of the carriers. Fan et al. proposed a ternary cocktail sequential release system by introducing big and small self-decomposable NPs (NP_{big & small}) as the drug carrier. (Figure 2) [16,17]. Photosensitizer methylene blue (MB) was encapsulated into the centre of the nanocarriers, with three different drug release profiles, which could realize multi-level programmable PDT treatment. And the antitumor drug gemcitabine hydrochloride (GM) was absorbed onto the carrier’s surface. This delivery system could passively targeted to the tumour site and realizes synergistic chemo-photodynamic therapy through double loading chemo-drugs and multi-level programmable PDT treatment. In the combination of photodynamic therapy (PDT) and chemotherapy, the major challenge is how to achieve high co-loading capacity for both photosensitizer and chemo-drugs. Feng et al. prepared a nanocarrier based on amphiphilic copolymer PPa-PLA-PEG-PLA-PPa for codelivery of photosensitizer (pyropheophorbide-a [PPa]) and chemo-drugs (paclitaxel [PTX]) to tumour site [18]. To enhance the tumour-targeting therapy, a tumour homing and penetrating peptide F3 was modified on the surface of nanocarriers. The obtained nanoparticles (PP NP) exhibited a satisfactory high drug-loading capacity for both drugs and exhibited higher cellular association and a more preferential enrichment at the tumour site than PP NP. In vivo anticancer evaluation demonstrated that a longer survival time was achieved by combination PDT and chemotherapy than those treated with chemotherapy or PDT only.

Figure 2. Illustration of NP_{big & small} sequential release system and its sequential release function. Copyright from Elsevier, 2017 (Ref [16]).

Inorganic nanoparticles were also designed for combination of PDT and chemotherapy. To efficiently apply these nanocarriers in biological systems, precise chemical modification is necessary, which provides the possibility to endow them with specific targeting functions and sufficient biocompatibility [22,23]. Liu et al. developed an amino-functionalized metal − organic framework (MOF), with anticancer drug camptothecine encapsulated for chemotherapy, folic acid (FA) as the targeted element and chlorine e6 (Ce6)-labelled CaB substrate peptide as the recognition moiety and signal switch [22]. FR-positive HeLa and MCF-7 cells and FR-negative HaCaT and A549 cells were used to validate the targeted delivery of the designed MOF nanocarrier. Confocal fluorescence imaging assays and flow cytometric analysis confirmed the specificity of the drug delivery system to cancer cells by the recognition of FA receptor. Meanwhile, in vitro and in vivo experiments demonstrated the designed MOF nanocarrier can be conveniently used for selective cathepsin B responsive imaging and the synergistic chemo-photodynamic therapy against tumour issues with excellent specificity and sufficient efficiency. Yang et al. developed an intelligent biodegradable hollow manganese dioxide (H-MnO₂) nanoplatform, with H-MnO₂ nanoshells post modification with PEG, and the inner core co-loaded with a photodynamic agent chlorine e6 (Ce6) and a chemotherapy drug doxorubicin (DOX) (Figure 3) [23]. This nanoplatform would be dissociated under reduced pH within TME to release loaded therapeutic molecules, and in the meantime, induce decomposition of tumour endogenous H₂O₂ to relieve tumour hypoxia. As a result, a remarkable in vivo synergistic therapeutic effect is achieved through the combined chemo-photodynamic therapy, which simultaneously triggers comprehensive effects favouring anti-tumour immune responses.

Figure 3. (A) A Scheme indicating the step-by-step synthesis of H-MnO₂-PEG nanoparticles and the subsequent dual-drug loading. (B) The proposed mechanism of anti-tumour immune responses induced by hollow manganese dioxide (H-MnO₂) nano-platform. Copyright from Nature, 2017 (Ref [23]).

UCNPs as PDT agents for combined PDT and chemotherapy

Most currently used photosensitizers are activated by visible light (400–700 nm), which are not suitable for the treatment of solid tumours that are at a thickness greater than a few millimetres, thus has limited tissue penetration. For deep penetration of light into tissues, wavelength of the irradiation should be in the near-infrared (NIR) window (650–900 nm). Recently, the use of upconversion nanoparticles (UCNPs) has opened a new version to realize NIR-triggered PDT, especially in tumour combination therapy field. UCNPs usually containing rare-earth elements, can convert NIR irradiation to short-wavelength emissions. Interestingly, the emission wavelength can cover from the UV to visible light region by changing the amount of doped lanthanide ions, the crystallite size, phase, and associated defect state, thus benefiting the photosensitizers to absorb these emissions to generate cytotoxic singlet oxygen species by resonance energy transfer [34]. Up to now, many research groups have made great efforts to synergistic UCNPs-mediated PDT and chemotherapy, and achieved promising results [24–30]. Dong et al. [24] developed a combined therapy system of DOX-loaded UCN/ZnPc@FABSA-PCL, by co-encapsulation of UCNs, photosensitizer ZnPc and anticancer drug DOX into the hydrophobic core of the self-assembled nanostructures of BSA-PCL. The hydrophilic protein BSA as the head of the tadpole offers excellent biocompatibility, low immunogenicity, low nonspecific protein adsorption and inherent biofunctional properties, while the polymer component PCL as its tail imparts amphiphilic self-assembly property, diversity, good stability and some other fascinating properties. By combining folic acid with BSA-PCL, this multifunctional drug delivery system could actively target to FA-overexpressed cancer cells. Under 980 nm light irradiation, in vitro combination PDT and chemotherapy of this UCNs system demonstrated significantly enhanced tumour cell killing efficiency.

Since UCNPs are irradiated by NIR light, in the combination therapy, the targeted anticancer drug delivery systems were often designed with on-demand drug release regulated by NIR light [25–30]. Despite an indicator of cancer, intracellular ROS always showed low concentration in cells as well as short lifetime (<0.1 ms), thus it should be more effective to generate ROS in situ using a ROS-sensitive drug-delivery system. One typical example is to introducing a ROS-cleavable linker between the PS and the drug; upon illumination, the generated ROS causes drug release through the cleavage of the linker. Yuan et al. report a novel theranostic platform based on a conjugated-polyelectrolyte (CPE) polyprodrug for chemo and PDT, and on-demand drug release upon light irradiation (Figure 4(A)) [25]. cRGD was modified on the surface of the nanocarrier to enhance the cellular uptake via α_vβ₃ integrin receptor mediation. The PEGylated CPE not only serves as a carrier, but also as a PDT agent which can generate ROS under irradiation with light. It is covalently conjugated to doxorubicin through a thioketal linker (TL) that can be cleaved by ROS for on-demand drug release and chemotherapy. In vitro cytotoxicity studies showed enhanced cell-viability inhibition for the combined therapy as compared to PDT or chemotherapy alone. Yue et al. developed a thioketal linker-based ROS responsive drug TL-CPT, which conjugated anticancer drug camptothecin with thioketal linker that could be cleaved by ROS (Figure 4(B)) [26]. To achieve cancer synergistic therapy, the photosensitizer Chlorin e6(Ce6), TL-CPT and carboxyl-mPEG were loaded on the upconversion nanoparticles (UCNPs). This nanocarrier could target in vivo lung cancer via EPR effects (enhanced permeability and retention). Upon 980 nm laser irradiation, Ce6 could absorb the light to produce ROS, which was both used for PDT and to cleave the thioketal linker in the resulted drug delivery system to release camptothecin for chemotherapy.

Figure 4. (A) Chemical structure of the PEGylated polyprodrug and (B) schematic illustration of the light-regulated ROS-activated on-demand drug release and the combined chemo–photodynamic therapy. PEG: poly (ethylene glycol). (C) Schematic illustration of the preparation of Ce6-CPT-UCNPs and concept of the light-regulated ROS-activated Ce6-CPT-UCNPs, OM: oleylamine. Copyright from Wiley, 2014 (Ref [25]), Ivyspring International, 2016 (Ref [26]).

Except for ROS-responsive drug delivery system, the use of UCNPs-mediated PDT could also combine with other stimulus-responsive targeted nanocarriers (pH, temperature, GSH, enzymes, etc.) [27–30]. Recently, Liu’s group developed a multipurpose liposome by encapsulating hydrophilic hypoxia-activated prodrug AQ4N, and hydrophobic hexadecylamine-conjugated photosensitizer Ce6, into its aqueous cavity and hydrophobic bilayer, respectively (Figure 5(A)) [28]. As AQ4N only shows toxicity to cancer cells under hypoxic environment, those liposomes show severe tumour hypoxia after with 660 nm light irradiation, which in turn would trigger activation of AQ4N, and finally contributes to remarkably improved cancer treatment outcomes via sequential PDT and hypoxia-activated chemotherapy. This work highlights a passive homing of liposome-based drug delivery system that could utilize tumour hypoxia, a side effect of PDT, to trigger chemotherapy, resulting in greatly improved efficacy compared to conventional cancer PDT or chemotherapy. Feng constructed a multifunctional nanosystem by loading chemotherapy agent DOX and photosensitive drug Ce6 into the channels of mesoporous zirconium dioxide (ZrO₂) layer which coats on Nd_3t-doped UCNPs [29]. A temperature sensitive phase change material (PCM), tetradecanol (melting point: 39–40 °C) was utilized to seal the porous channel of ZrO₂ and served as switch for control release of DOX and reactive oxygen species (ROS) in the condition of enhanced temperature triggered by the near-infrared (NIR) light irradiation, to markedly enhanced cancer therapeutic efficacy. Zhao et al. developed novel caspase-3 responsive functionalized nanoparticles (CFUNs), consisting of caspases-3 cleavable DOX prodrug tethered with DEVD peptide (DEVD-DOX), UCNP, photosensitizer pyropheophorbide-a methyl ester (MPPa) and tumour-targeting cRGD-PEG-DSPE (Figure 5(B)) [30]. Confocal imaging assays and time-dependent flow cytometry analysis confirmed the targeting ability of CFUN on integrin avb3 overexpressed 4T1 cells. Upon NIR irradiation, MPPa absorbed the emitted visible light from UCNP to afford ROS, and concurrently activated caspase-3 to specifically cleave the peptide sequence within DOX prodrug and release activated DOX at tumour sites, thus arousing cascade chemotherapy to combat remaining tumour cells after previous PDT. This proposed NIR-initiated two-step combination treatment is promising to overcome tumour heterogeneity and multidrug resistance in future precision tumour therapy.

Figure 5. (A) Scheme illustrating the liposome can serve as a multifunctional theranostic agent for PDT-induced, hypoxia-activated cancer therapy. (B) Schematic illustration and proposed mechanism of NIR-triggered high-efficient photodynamic and chemo-cascade therapy of caspase-3 responsive functionalized upconversion nanoparticles tethered with anticancer doxorubicin (DOX) (CFUNs, MPPa/UCNP-DEVD-DOX/cRGD). Copyright from ACS, 2017 (Ref [28]), Elsevier, 2017 (Ref [30]).

Stimuli-responsive combination therapy

Beyond UCNPs-mediated stimulus-responsive combinations, internal or external stimulus PDT/chemo therapies have been increasingly explored that demonstrated a number of unique advantages: (1) Combination stimulus-responsive therapies can spatially control the therapeutic effect only in the tumour region without causing much damage to normal tissues; (2) Many physical stimuli may be able to control the drug internalization or drug release in the tumour, and greatly enhance the efficacy of chemotherapy drugs and reduce their systemic toxicity; (3) Combination stimulus-responsive therapies not only can be used to directly kill cancer cells, but is also used to trigger or enhance other different cancer therapies to achieve desired synergistic therapeutic effects via different mechanisms (e.g. by changing the tumour microenvironment or activating certain enzymes) [31–34].

Targeted drug delivery systems for photothermal therapy combined with chemotherapy

PTT is another promising phototherapy because its advantages of non-invasive, harmless and high selectivity. In PTT, NIR light is absorbed by photothermal agents and converted into heat to kill cancer cells. This approach is an extension of PDT, in which a photosensitizer is excited with specific band light. This activation brings the sensitizer to an excited state where it then releases vibrational energy (heat), which is what kills the targeted cells. Unlike PDT, PTT does not require oxygen accessibility to damage targeted tissues Current studies also show that photothermal therapy is able to use longer wavelength light, which is less energetic and therefore less harmful to other cells and tissues [6]. In combination therapy, “chemotherapy and PTT synergy” has become a promising method for efficient tumour ablation and minimally invasive treatment, thereby enhancing the sensitivity of chemotherapy and synergistically develop the therapeutic effects [35,36]. Especially, targeted drug delivery system based combination therapy has been proven to be an effective strategy to enhance the efficiency of cancer therapy and to reduce the drug resistance. There have been several different mechanisms when combining PTT with chemotherapy [37]. (1) While the temperatures triggered by PTT reach over 50 °C, which would be able to effectively kill tumour cells, may be able to change the tumour microenvironment, such as increase the blood flow, oxygen levels in the tumour, the perfusion and permeability of the tumour vasculature, then improving the accumulation of nanocarriers at the tumour site. (2) It has been well demonstrated that the NIR-induced photothermal effect is able to trigger drug release from the mesoporous silica shell, or the surface of NIR absorbing nano-carriers (Figure 6(A)), enabling enhanced efficacy for such combined PTT and chemotherapy [37]. (3) In addition to triggering drug release, it has been found that the hyperthermia caused by PTT agent could improve the membrane penetrability to enhance the cellular uptake ability of those nano-carriers (Figure 6(B)) [37]. Such effect could be utilized to promote the intracellular delivery of chemotherapeutic agents, for photothermally enhanced chemotherapy.

Figure 6. The schemes showing two mechanisms in combined photothermal and chemotherapy. (A) NIR-triggered drug release from nano-carriers inside tumour cells. (B) Enhanced cellular uptake of drug-loading nanoparticles under a mild photothermal heating in the combination therapy. Copyright from Elsevier, 2015 (Ref. [37]).

Organic PTT agents for combined PTT and chemotherapy

To realize effective and safe PTT, biocompatible PTT agents, which are non-toxic in dark, exhibit high absorbance in the tissue-transparent NIR window and show great tumour homing abilities, are required. To date, various PTT agents have been explored, as shown in Figure 7 [37], including organic dyes such as cyanine dyes, polyaniline and polypyrrole, and inorganic nanomaterials, such as different nanostructures of noble metals (Au, Pt and Ag), semiconductor nanoparticles and carbon nanomaterials [38]. Although with an appropriate surface coating, the inorganic PTT agents have demonstrated their high effectiveness in animal tumour models, future clinical applications have been a challenge because of their potential long-term toxicity, as many of them are not biodegradable and would retain in the body. Organic PTT agents, such as NIR dyes, could incorporate into nano-assemblies, such as micelles, liposomes, porphysomes and protein, have been explored in recent years to achieve safe and effective PTT. Those agents, which are engineered with biodegradable/biocompatible components, may encounter less obstacles in terms of future clinical translation. Among the NIR dyes, indocyanine green (ICG), which has been approved by US FDA for various clinical applications [39], exhibiting advantages of biocompatibility, ideal absorbance in the NIR region and multifunctional characteristics is the most attractive one. Combination of ICG-mediated PTT and chemotherapy could not only show a strong synergistic anticancer effect, but also allows the identification of microscopic cancer tumours for a more precise and highly efficient treatment [40–45]. Chen et al. developed novel designed nanoparticles consisting of arylboronic ester and cholesterol modified hyaluronic acid (PPE-Chol1-HA), with both ICG and DOX-loaded within the nanoparticles [40]. Hyaluronic acid (HA) was a natural ligand of CD44 that overexpressed by various tumour cells, which was employed as both hydrophilic skeleton and active targeting constituent. Due to the ROS and heat production capability of ICG and ROS-sensitivity of arylboronic ester, the nanocarriers exhibited remarkable photothermal effect and light-triggered faster release of DOX with NIR laser irradiation. In vitro and in vivo imaging and antitumor activity certificated the outstanding targeting ability and antitumor activity of this targeted drug delivery system. Li et al. developed zwitterionic temperature/redox-sensitive nanogels loaded with ICG and anticancer drug doxorubicin (I/D@NG), which exhibits enhanced tumour accumulation, synergistic anticancer effect [41]. Furthermore, I/D@NG can effectively escape from lysosomes by singlet oxygen-induced lysosomal disruption, and DOX is then sufficiently released from the nanogels to the nucleus in response to high intracellular GSH and photothermal effects. Such nanoplatform provides new insights into designing nanoplatforms for synergistic cancer therapy. Liu’s group developed a new “Abraxane-like” nano-theranostic formulation self-assembled from three clinically approved agents, human serum albumin (HSA), paclitaxel (PTX) and indocyanine green (ICG) via a simple one-step method [42]. The obtained HSA-PTX-ICG nanoparticles could be utilized for fluorescence imaging guided combined photothermal and chemotherapy, realizing synergistic therapeutic effect not only to treat subcutaneous tumours, but also in a lung metastasis tumour model.

Figure 7. A scheme showing commonly explored photothermal agents. Copyright from Elsevier (Ref [37]).

Except for ICG, many ICG derivate also is explored as potential PTT agents, among which cypate was commonly studied [43–45]. Our group developed a NIR light-triggered biodegradable chitosan-based amphiphilic block copolymer micelles (SNSC) with tumour targeting ligand c(RGDyK) on the surface and light-sensitive 2-nitrobenzyl alcohol on the hydrophobic block. NIR dye cypate as PTT agent and antitumor drug paclitaxel (PTX) were co-loaded into the inner core of the nanoplatform (Figure 8(A,B)) [43,44]. In vitro and in vivo targeting and antitumor studies demonstrated enhanced targeting properties in tumour site, high-temperature response for PTT on cancer cells and two-photon photolysis for fast release of anticancer drugs under NIR irradiation, and a spatial-temporal synchronism of chemo- and photo-thermal therapy. Recently, Li et al. designed light-triggered nanoparticles with multipronged physicochemical and biological features to overcome cisplatin resistance via the assembly of Pt(IV) prodrug and Cypate within the copolymer for efficient ablation of cisplatin-resistant tumour (Figure 8(C)) [45]. NIR light not only triggers the PTT of the micelles, but also leads to the reduction-activatable Pt(IV) prodrug into cytoplasm through the lysosomal disruption for chemotherapy, as well as the remarkable inhibition on the expression of a drug-efflux transporter, multidrug resistance-associated protein 1 (MRP1) for further reversal of drug resistance of A549R cells.

Figure 8. (A) Synthetic scheme of targeted NIR light sensitive nanoplatform c(RGDyK)-SNSC and αvβ3-mediated binding of tumour cells; (B) schematic illustration of using NIR light excitation of cypate to trigger dissociation of SNSC micelles; (C) Schematic illustration of the micelles encapsulating Pt(IV) prodrug and Cypate (P/C-Micelles) as a multipronged nanoparticle platform for efficient ablation of resistant tumour. Copyright from Elsevier, 2013 (Ref [43]), ACS, 2015 (Ref [45]).

Inorganic PTT agents for combined PTT and chemotherapy

For inorganic PTT agents, such as gold nanorods, graphene, carbon nanotubes, CuS nanoparticles and many others, many NIR-absorbing nano-carriers have been engineered to load chemotherapeutic agents to realize combined PTT and chemotherapy [46–51]. On one hand, these PTT agents could be coated with mesoporous silica nanoparticles (MSN), in which chemotherapeutic agent compounds are encapsulated [46–49]. As a typical example, Shen et al. firstly developed mesoporous silica-encapsulated gold nanorods (GNRs@mSiO₂) as a synergistic therapy tool for delivery heat and drug to the tumorigenic region [46]. In order to target the tumour and increase the therapeutic efficacy, they conjugated RGD peptides on the terminal groups of poly (ethylene glycol) (PEG) on GNRs@mSiO₂ (namely pGNRs@mSiO₂-RGD) and encapsulated anticancer drug DOX into the nanomaterials. In vitro TEM clearly demonstrated that the uptake amount of pGNRs@mSiO₂-RGD in A549 cells were significantly more than that of pGNRs@mSiO₂ because the RGD peptide could target α_vβ₃ integrin receptors overexpressed on several cancer cell lines including the A549 cells. Furthermore, they studied the ablation of tumour both in vitro and in vivo by the combination of photothermal therapy and chemotherapy using doxorubicin (DOX)-loaded GNRs@mSiO₂. Results showed significantly greater cell killing upon NIR irradiated, due to both GNRs@mSiO₂-mediated photothermal ablation and cytotoxicity of light-triggered DOX release. This design would be extended to the other mesoporous silica-encapsulated photothermal or magnetic-thermal nanomaterials (e.g. mesoporous silica-encapsulated carbon nanotubes or mesoporous silica-encapsulated magnetite nanoparticles) for synergistic destruction of tumours. Liu et al. developed a multifunctional nanomaterial for chemo-PTT combination therapy based on silica and graphene core/shell structure (SiO₂@GN) [47]. The coated GN as a shell material simultaneously processes photothermal conversion ability and drug loading capacity. Most importantly, serum protein was modified onto the surface to improve the solubility and stability of the GN-based core/shell NPs. Meanwhile, the modified protein can be used as a multifunction platform to load the anticancer drug DOX and immobilize the targeting molecules of folic acid that is a fully characterized ligand whose receptor is overexpressed in various tumour cells. The developed nanocomposites show low cytotoxicity, high photothermal stability, pH and heat-responsive drug release, as well as enhanced synergistic anticancer efficacy. On the other hand, many photothermal agents, such as carbon nanomaterials, CuS nanoparticles, magnetic iron oxide nanoparticles and conjugated polymers could serve as drug loading carriers by themselves. To increase the anticancer drug release rate, Park et al. developed DOX-loaded poly(lactic-co-glycolic acid)-Au half-shell nanoparticles (PLGA-Au H-S NPs) for tumour-specific delivery of heat and drugs in human cervical cancer (HeLa) cell line [50]. The drug is encapsulated within biocompatible and biodegradable PLGA NPs, which could be degraded more rapidly at elevated temperatures, and Au layer is deposited on these NPs as PTT agent. Compared with chemotherapy or photothermal treatment alone, the combined treatment demonstrated a synergistic effect, resulting in higher therapeutic efficacy and shorter treatment times. Xia et al. developed biodegradable polyaniline/porous silicon hybrid nanocarriers for combined chemo-photothermal therapy of cancer [51]. These nanocomposites are not only biodegradable, which could be completely cleared in body, but also exhibited a robust photothermal effect and an efficient loading and dual pH/NIR light-triggered release of DOX. Under NIR laser irradiation, this nanocarrier showed a remarkable synergistic anticancer effect combining chemotherapy with photothermal therapy, whether in vitro or in vivo.

Targeted drug delivery systems for combination of PDT/PTT/chemotherapy

To represent a powerful anti-cancer therapy by attacking the cancer in a diverse and cooperative manner at an early stage, the design, construction and operation of a multifunctional nanocarrier for triple combinatorial anti-cancer therapy is a research goal. Recently, several groups have made great efforts to develop novel and biodegradable targeted drug delivery systems for combination of PDT/PTT/chemotherapy [52–54]. Liu’s group has developed novel organic NIR-absorbing nanoparticles based on PEG-coating poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate), as a drug carrier to load various types of therapeutic agents, including chemotherapy drugs DOX and SN38, as well as a photodynamic agent Ce6 [52]. This design showed several unique benefits, including rather high passive tumour accumulation owing to the EPR effect of cancerous tumours, improving the anticancer drug’s water-solubility, accelerating cellular uptake of the photosensitizer for enhanced efficacy in PDT, enabling synergistic cancer cell killing, as well as allowing for co-loading and co-delivery of multiple therapeutic molecules simultaneously. This work highlights the great potential of NIR-absorbing polymeric nanoparticles as multifunctional drug carriers for potential cancer combination therapy with high efficacy. Feng et al. proposed an intelligent drug delivery system by capping DOX-loaded hollow mesoporous CuS nanoparticles (HMCuS NPs) with multifunctional FDA-approved superparamagnetic iron oxide nanoparticles (IONPs) to integrate programmed functions including enhanced PTT effect, sophisticated controlled drug release, magnetic targeting property and MR imaging. (Figure 9) [53]. Under near infrared (NIR) light irradiation, this system could exploit the merits of both PTT and PDT simultaneously. In vitro and in vivo results showed significantly enhanced anti-tumour therapeutic efficacy due to the synergistic combination of chemo-phototherapy. By this delicate design, such smart and extreme versatile all-in-one drug delivery platform could arouse broad interests in the fields of biomaterials, nanotechnology and drug delivery system.

Figure 9. Schematic representation of the synthesis of the drug delivery system (HMCuS/DOX@IONP-PEG) for combining MR imaging with chemo-phototherapy. Copyright from Elsevier, 2017 (Ref [53]).

In Kim’s group, a functional DNA-decorated dynamic gold (Au) nanomachine as a therapeutic agent for triple combinatorial anti-cancer therapy are revealed (Figure 10) [54]. Taking advantage of the DNA as a cargo loading vehicle, they used a G-quadruplex structure for the stable loading and intracellular delivery of a photosensitizer to achieve effective photodynamic therapy under light illumination. Then, the pH-responsive cytosine-rich i-motif DNA structure is employed for anticancer drug release and photothermal ablation upon NIR irradiation with infrared light. This DNA-operated Au hybrid nanocarrier supplied new insights for combinational cancer therapy.

Figure 10. Schematic illustration of the Au-GI nanomachine. (A) Design of the Au-GI nanomachine and its operation mechanism depending on pH. (B) Intracellular dynamic operation of the Au-GI nanomachine facilitating triple combination of photothermal, photodynamic, and chemotherapy. AuNP: gold nanoparticle; GI, DNA containing both the G-quadruplex and the i-motif sequence; Au-GI, GI-decorated AuNP; ZnPc, zinc phthalocyanine; DOX, doxorubicin Copyright from PMC, 2018 (Ref [54]).

Conclusion and outlook

In summary, in contrast to traditional single phototherapy or chemotherapy, targeted delivery systems for combination therapies showed many advantages, such as improved tumour targeting properties, reduced toxic side effects, more flexible controlled drug release and a synergistic effect for enhanced therapeutic efficacy. Although these nanocarriers have demonstrated exciting results in pre-clinical animal studies, they are still facing many challenges, such as potential cytotoxicity, compensating each of their respective performances, etc. Moreover, some new light-sensitive materials in vivo and intracellular metabolic pathways and cell phagocytosis mechanism have not yet been cleared. For the clinical and translational applications of combinational therapy nanomaterials and successful bench-to-bedside transition, further studies regarding safety/biocompatible, facile inexpensive synthesis and highly reproducible methods, much higher efficiency and lower side-effects are urgently required. We believe in the near future, many innovative breakthroughs and discoveries will continue to be brought to fruition in the combination cancer therapies by targeted drug delivery systems.

Abbreviation
PDT	=	photodynamic therapy
PTT	=	photothermal therapy
ROS	=	reactive oxygen species
PS	=	photosensitizer
NIR	=	near-infrared
TDDS	=	targeting drug delivery system
FA	=	folic acid
RGD	=	Arg-Gly-Asp
MB	=	methylene blue
Ce6	=	chlorin e6
PTX	=	paclitaxel
MOF	=	metal − organic framework
DOX	=	doxorubicin
UCNPs	=	upconversion nanoparticles
PE	=	polyelectrolyte
TL	=	thioketal linker
EPR	=	enhanced permeability and retention
CPT	=	camptothecin
PEG	=	poly (ethylene glycol)
ZrO2	=	zirconium dioxide
ICG	=	indocyanine green
HA	=	hyaluronic acid
HSA	=	human serum albumin
MSN	=	mesoporous silica nanoparticles
PLGA	=	poly(lacticco-glycolic acid)
IONPs	=	iron oxide nanoparticles

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This review article was partially supported by the National Natural Science Foundation of China [No. 81673360], Shandong Provincial Natural Science Foundation [No. ZR2016HM45 and ZR2017BH006], China Postdoctoral Natural Science Foundation (2017M612210) and Scientific Research Foundation for Youth Scholars from Qingdao University (No. 41117010026).