As a safe type of stimulus, magnetic fields are prevalent in nano-drug delivery in the therapy of tumors
. Besides treatment, magnetic field-sensitive nanoparticles were also used in diagnostic imaging
. A recent study used targeted magnetic carriers to deliver the DOX into the bladder wall, which caused more significant accumulation and provided site-specific delivery of drugs
. One recent study revealed that biomimetic magnetic Fe
-SAS@PLT nanoparticles were effective in curing non-inflamed tumors for ferroptosis with immunotherapy
. Magnetic field-induced hyperthermia could be used in the nano-drug treatment of bladder cancer in a hyperthermia-responsive manner
Light-controlled drug delivery was first reported in 2010
[73]. The UV light irradiation could induce the burst release of hydrophobic NR molecules from the nanoparticle
[74]. In another study, a copolymer system consisting of poly(ethylene oxide) (PEO) and poly(2-nitrobenzyl methacrylate) (PNBM) was sensitive to UV-light and led to the light-controlled release of payload drugs
[73]. Compared with UV, infrared ranges from 600–900 nm, has deeper penetration, and is more effective in treating cancer in the deep part of the tissue. NIR-responsive systems are another main light-responsive system
[75]. As an FDA-approved fluorescent dye, ICG is widely used in cancer therapy, which could absorb NIR light into heat
[76]. Additionally, doxorubicin and ICG loaded in PLGA-based and dual-modality imaging guided chemo-photothermal nanoparticles showed faster release under NIR irradiation; this nanoparticle had a diameter of around 200 nm with good fluorescence stability. Doxorubicin release was stimulated by heat, and nanoparticle penetration of the tumor was improved after NIR irradiation, this research provided a promising strategy for early diagnosis and therapy for cancer
[77].
2.2. Surface Conjugation with Targeting Ligands
The active target of cancer cells based on monoclonal antibodies, peptides, and aptamer has aroused significant interest in the field of targeting ligands
[4].
2.2.1. Monoclonal Antibodies
Antibody-drug conjugates are monoclonal antibodies conjugated to cytotoxic agents
[78]. Monoclonal antibodies (MAbs) are macromolecules widely used as targeting ligands to various types of nanoparticles, such as SPIONs
[79], QDs
[79], liposomes
[80], and Au nanocages
[81]. However, their bulky size and constant redundant region may limit their use. As reported in a previous study, nanoparticles formed by 2-methoxy-estradiol (2-ME) based on anti-human epidermal growth factor receptor 2 (HER2) antibody-modified BSA were validated in targeted cancer therapy; this system was prepared using the desolution method and was proven effective in retaining the immunospecificity of the anti-HER2 antibody for the targeted cancer therapy
[82]. Furthermore, in a previous study, PTX was absorbed on graphene oxide nanosheets and then conjugated with vascular endothelial growth factor (VEGF) to form the targeted nanoparticles, which showed remarkable potential in photothermal controllable tumor treatment
[83].
2.2.2. Peptides
Due to the bulky size of MAbs, peptides represent a viable targeting moiety with relative flexibility and overcome the disadvantages of using MAbs
[84]. Peptide−drug conjugates (PDCs) are increasingly recognized in targeted drug delivery
[85]. For example, RGD peptides were intimately connected with integrin
[86]. Nanoparticles composed of RGD conjugation with superparamagnetic iron oxide nanoparticles (SPIONs) possessed better targeting affinity and specificity
[87]. In one previous study, a biocompatible conjugate consisting of a fatty acid-substituted dextran decorated with cyclo[RGDfK(C-6-aminocaproic acid)] cRGDfK peptide could be used as a candidate for conventional PEG
[88]. Recently, gemcitabine (GEM) and amphiphilic peptide conjugation were effective in breast cancer therapy, this system was formed via self-assembled behaviors, and the stability of GEM could be maintained during the circulation and accumulated in the tumor site, which broadened the application of GEM for breast cancer therapy
[89]. Tumor vascular endothelial cells (tVECs) were targeted with cRGD-functionalized polyplex micelle loading anti-angiogenic protein encoding pDNA for anti-tumor activity in human pancreatic adenocarcinoma tumor-bearing mice by noting that tVECs abundantly express αvβ3 and αvβ3 integrin receptors
[90][91]. Moreover, the application of cell-penetrating peptides (CPPs) based on nanoplatforms in cancer treatment has also been worth attention since their identification 25 years ago
[92]. Nanoparticles could be functionally attached to CCPs to achieve good therapeutic efficacy. For example, a nanoparticle against CapG to a series of CPPs showed a good potential for metastasis in breast cancer
[93].
2.2.3. Aptamers
Aptamers are complex three-dimensional structures composed of short, single-stranded, synthetic nucleic acid oligomers, DNA or RNA, with some good characteristics for their high affinity and specificity, easy synthesis, low molecular weight, and lack immunogenicity
[94]. As ligands, conjugation of aptamers to nanoparticles has been reported in a series of previous studies
[95]. The combined use of cisplatin in liposomes conjugated with aptamers has been reported where the nucleolins (NCL) were the target of aptamers. Aptamer-conjugated liposomes were formed by cholesterol incorporation and hydration, which showed strong anti-proliferative activity in breast cancer cells overexpressing NCL
[96].
2.3. Large Molecules-Based Therapy
2.3.1. Nucleic Acid-Based Therapy
Nucleic acid-based therapy is a technology that transfers nucleic acids, including plasmid DNA, mini vector DNA, siRNA, etc., to the nucleus of diseased cells or tissues for the gene therapy of cancers
[97][98][99]. Furthermore, gene therapy focuses on the mutated genome of the tumor cells
[100], aiming to restore instead of kill cells
[101].
Poly(
N-isopropylacrylamide) (PNIPAM) is the most extensively used in gene delivery systems
[102]. The polyplex micelles consisting of PNIPAAm and therapeutic plasmid DNA (pDNA) could prolong blood circulation and suppress tumor growth in H22 tumor-bearing mice
[103]. Nanoparticle-based delivery of small interfering RNAs (siRNAs) has also been reported to possess an anti-proliferative effect
[104]. A biodegradable and redox-sensitive nanocarrier consisting of solid poly (disulfide amide) (PDSA)/cationic lipid core and a lipid-PEG shell for siRNA delivery was proven to have a good therapeutic effect
[105]. Liposomes targeting the interleukin 12 (IL-12) gene in a non-viral manner could induce an immune response and achieve good therapeutic efficacy
[106].
Additionally, thermosensitive nanocarriers have also been used in gene transfection.
2.3.2. Protein-Based Drug Delivery
Natural biological molecules usually form protein-based nanoparticles with biocompatible, biodegradable, and non-antigenic properties and can be functionalized with cell-targeting groups or ligands
[107][108][109]. Gelatin is a protein obtained from the hydrolysis of collagen
[110]. One previous study in dogs with bladder cancer revealed that gelatin nanoparticles loaded with paclitaxel (PTX) had a good therapeutic efficacy
[111]. Drug molecules conjugated with proteins were used for cancer therapy
[112]. Hollow mesoporous silica capsules have garnered significant attention as protein delivery vehicles
[113]. For example, Fluorescein isothiocyanate (FITC)-labeled proteins were loaded into the nanoparticles to achieve efficient therapeutic efficacy
[114], evidenced by several examples of protein delivery in liposomes
[115]. Moreover, magnetic field-responsive protein conjugation nanoparticles were also reported to be an efficient system for brain tumors
[116].
2.4. Others—Hydrogel
Hydrogel is a polymer with a three-dimensional, hydrophilic, and cross-linking network, capable of retaining a large amount of water or physiological fluids
[117]. Injectable biodegradable hydrogels have broadened novel methods of cancer treatment
[118][119].
In cancer therapy, hydrogels provide a platform for drug combinations. For example, an injectable DNA hydrogel assembled by chemo drug-grafted DNA in a previous study showed excellent anti-tumor efficacy and represented a promising adjuvant therapy in cancer treatment
[120]. Additionally, thermosensitive PPZ hydrogel loaded with PEGylated cobalt ferrite nanoparticles showed a fantastic therapeutic efficacy in the breast cancer mice model
[121]. Furthermore, a unique “Jekyll and Hyde” nanoparticle–hydrogel (NP-gel) hybrid platform was designed to load DOX, leading to a good anti-recurrence efficiency and low toxicity
[122]. Based on the hyperthermia caused by ultrasound, a magnetic hyperthermia responsive hydrogel comprised of silk fibroin and iron oxide nanocubes was effective in the 4T1tumor-bearing mice model
[123].