Lung cancer is one of the most common malignancies and the leading cause of cancer-related deaths, with an estimated 2.09 million newly diagnosed cases in both genders and 1.76 million deaths worldwide ^[1]. Small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC) are the two main pathological types of lung cancer, and NSCLC accounts for approximately 80% to 85% of all cases. NSCLC has three main subtypes, including adenocarcinoma, squamous cell carcinoma, and large cell carcinoma ^[2]. Treatment of lung cancer varies greatly depending on the type and stage of the disease, including traditional chemotherapy, radiotherapy, targeted therapy, and immunotherapy. In recent years, targeted therapy has emerged as an important therapeutic strategy for the management of NSCLC. For example, epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitors (TKIs) and anaplastic lymphoma kinase (ALK)/c-ros oncogene 1 (ROS1) inhibitors have replaced chemotherapy as the first-line treatment of lung cancer ^[3]. Although targeted therapies are initially effective, acquired drug resistance is usually inevitable due to cancer-driven genetic alterations, epigenetic alterations, and tumor heterogeneity ^[4]. Immunotherapy, such as immune checkpoint blockade (ICB), is a new treatment strategy that may improve the survival of lung cancer. However, the clinical benefits of ICB in the treatment of advanced lung cancer have proven to be limited and unsatisfactory, with an overall response rate (ORR) of approximately 10–20% ^[5]. Therefore, there is an urgent need to identify alternative strategies to improve existing treatments or to provide new treatments.

Signal transducer and activator of transcription 3 (STAT3), a key component of the Janus kinase (JAK)–STAT pathway, is found aberrantly activated in the majority of NSCLC patients ^[6]. STAT3 can be phosphorylated by multiple cytokines, interferons, and growth factors, and the activated STAT3 can be transferred to the nucleus and bind to the promoter and enhancer regions of target genes to modulate gene transcription ^[7]. Multiple studies have proven that increased phosphorylated STAT3 (pSTAT3) is usually associated with cell proliferation, invasion, and angiogenesis, leading to tumor progression, metastasis, drug resistance, and immune escape ^[8]. Moreover, the existence of a subpopulation of cells in tumors called cancer stem cells (CSCs) or tumor initiating cells (TICs), responsible for drug resistance, tumor metastasis, and relapse, is also reported to correlate with increasing levels of STAT3 activation ^[9]. Thus, targeting the STAT3 signaling pathway has emerged as a promising therapeutic strategy for lung cancers.

A variety of nanocarriers have been recently developed to deliver poorly water-soluble drugs, thus improving their bioavailability and targeting capability. Nanoparticles (NPs) with particle sizes of 10–100 nm can increase drug enrichment in tumor tissue and reduce its distribution in normal tissue through enhanced permeability and retention (EPR) effect and active targeting strategies (e.g., decoration of specific ligands) ^[10]. NPs-based drug delivery systems, such as inorganic NPs, polymeric NPs, micelles, dendrimers, and liposomes, have shown great potential in the diagnosis, imaging, and treatment of cancer. Some of these nanocarriers have been successfully used in the clinic for drug delivery, such as Abraxane^® (albumin-bound paclitaxel) and Doxil^® (liposomal doxorubicin).

2. LNPs/Liposomes

LNPs, typically surrounded and stabilized by lipid bilayers with an aqueous (liposomes), oil, solid, or amorphous core (for nucleic acid delivery), are one of the most common nanoparticle formulations used to deliver anti-cancer drugs or genes because of their stable drug encapsulation and enhanced delivery efficiency [73]^[11]. The initial success of several liposome/LNPs-based drugs has fuelled further clinical investigations [74]^[12]. The improved therapeutic outcomes of Doxil^® (a liposome formulation of doxorubicin) and the recent approval by the U.S. Food and Drug Administration (FDA, Silver Spring, MD, USA) for LNP-loaded mRNA vaccines used to prevent COVID-19 have made LNPs a promising drug delivery system for various diseases [75]^[13]. Due to uniform particle size distribution, LNPs have exhibited superior biodistribution in vivo, which has established them as outstanding drug carriers for lung cancer treatment [76]^[14]. Moreover, since the main components of LNPs (e.g., phospholipids and cholesterol) are very similar to pulmonary surfactants in mammals, several highly biocompatible and biodegradable inhaled LNPs are promising candidates for pulmonary drug delivery [77]^[15]. Studies indicated that inhaled LNPs could increase the therapeutic effect of the drug and decrease the systemic toxicity simultaneously because they are able to restrict the drug effect in the pulmonary system for a prolonged time [78]^[16]. Villanueva and colleagues reported the potential of STAT3 decoy-loaded cationic lipid microbubbles (STAT3-MB) combined with ultrasound-targeted microbubble cavitation (UTMC) in the treatment of head and neck squamous cell carcinoma (HNSCC). The STAT3 decoy was loaded via charge–charge interaction. The formed STAT3-MB combined with UTMC treatment promoted the delivery of cell-impermeant oligonucleotides exclusively to sites exposed to the ultrasound beam, significantly inhibiting tumor growth and prolonging the survival in CAL33 tumor-bearing mice compared to the negative control groups, which was associated with the downregulation of the expression of target genes Bcl-xL and Cyclin-D1 at the RNA transcription and protein levels [79]^[17]. In order to achieve better tumor targeting ability and efficient cell transfection, surface modification of liposomes/LNPs by covalently conjugating specific ligands has gained much attention. A study reported that α5β1 integrin receptor selective liposomes prepared via conventional thin-film hydration method containing RGDK-lipopeptide simultaneously delivered a small molecule STAT3 inhibitor (WP1066) and STAT3 siRNA to brain tumors. It was found that WP1066/STAT3-siRNA-loaded liposomes were internalized in glioblastoma cells via integrin α5β1 receptors and selectively accumulated in brain tissues of glioblastoma-bearing mice, thus significantly improving the overall survival of orthotopically-established glioblastoma-bearing mice [80]^[18]. Solid lipid nanoparticles (SLNs) are another promising delivery system for small molecules and genes due to their good biocompatibility and physical stability. A recent study developed and evaluated the use of cationic SLNs for delivery of RNA interference (RNAi)-mediating plasmid DNA to downregulate STAT3 in cisplatin-resistant lung cancer cells. Cationic SLNs were prepared by a modified hot microemulsion method, and these cSLN:plasmid DNA complexes successfully encoded anti-STAT3 short hairpin RNA, reduced STAT3 expression, and improved the sensitivity of the cisplatin-resistant Calu1 cell line to cisplatin [81]^[19]. Since STAT3 activation is associated with immune suppression, inhibition of STAT3 activation by different strategies has shown promising results in cancer immunotherapy. For example, ablation of STAT3 in mice could induce potent anti-tumor immunity by increasing the production of IL-12 and tumor necrosis factor α (TNFα), reducing the production of IL-10, and inducing M1-like reprogramming of murine macrophages [82]^[20]. To this end, Møller et al. reported a type of long-circulating liposomes (CA-LCL-αCD163), which were passively inserted lipidated CD163 (markers of M2-polarized macrophages) into the liposome lipid bilayer and packed with STAT3 inhibitor corosolic acid (CA). The CA-LCL-αCD163 liposomes were able to target macrophages with high CD163 expression, inhibit IL-6-induced STAT3 activation, and induce the production of pro-inflammatory cytokines, resulting in reprogramming tumor-associated macrophages (TAMs) from a tumor-supporting (M2-like) phenotype towards a tumoricidal (M1-like) phenotype [82]^[20]. Another study prepared doxorubicin-loaded, cholesterol-free CA liposomes (DOX/CALP) based on PEGylated liposomal doxorubicin (DOXIL^Ⓡ) by replacing its cholesterol with CA. They found that DOX/CALP displayed higher in vitro cellular uptake and tumor spheroid permeation, as well as stronger anti-tumor cytotoxicity, compared to doxorubicin-loaded cholesterol liposomes (DOX/LP). In addition, the pSTAT3 level in the DOX/CALP group was significantly suppressed, and fewer intratumoral macrophages were observed in the DOX/CALP group, further suggesting that CALP as a functional delivery nanocarrier has some advantages over classic liposomes, and hence could enhance the efficacy of chemotherapeutic drugs [83]^[21]. In addition to STAT3 inhibitory drug delivery, targeting TAMs with STAT3 siRNA-loaded LNPs to modify their function responsible for M2 polarization could also be used to reverse the tumor-promoting function of TAMs. Harashima et al. fabricated a type of pH-sensitive LNPs (CL4H6-LNPs) for targeted delivery of STAT3 siRNA to TAMs. The silencing of STAT3 and hypoxia-inducing factor 1α (HIF-1α) led to an increase in levels of infiltrated macrophages (CD11b⁺ cells) and M1 macrophages (CD169⁺ cells) in the tumor microenvironment (TME), achieving novel macrophage-based cancer immunotherapy [84]^[22]. Studies also revealed that the activation of STAT3 was strongly associated with the expression of PD-L1 in multiple cancers, and inhibition of STAT3 can reduce the expression of PD-L1, resulting in the improved therapeutic effect of checkpoint inhibitors [26]^[23]. Li et al. synthesized a novel IL-20 receptor subunit alpha (IL20RA)-targeted liposomal NP that encapsulates the STAT3 inhibitor stattic (NP-Stattic-IL20RA) to inhibit breast cancer. They demonstrated that IL20RA could promote the stemness of breast cancer cells via the JAK1-STAT3-SOX2 signaling pathway and regulate the expression of PD-L1 to modulate the immune microenvironment. NPs-Stattic-IL20RA combined with anti-PD-L1 antibody effectively inhibited the stemness of cancer cells and improved the tumor immune microenvironment, resulting in an increase in the efficacy of chemotherapy [85]^[24]. Recently, a pH-responsive liposome (Liposome-PEO, LP) loaded with apatinib (AP) and cinobufagin (CS-1) and coated with a hybrid membrane (R/C) (LP-R/C@AC NPs) was prepared for combined treatment of gastric cancer. LP-R/C@AC efficiently killed tumor cells by inhibiting the vascular endothelial growth factor receptor 2 (VEGFR2)/STAT3 pathway and reverse tumor immunosuppression by inhibiting the expression of PD-L1 and matrix metalloproteinase 9 (MMP-9), showing the dual advantages of targeting tumor cells and immune escape [86]^[25].

3. Inorganic Nanoparticles

Inorganic NPs, including gold/silver NPs, mesoporous silica NPs, and magnetic NPs, have been extensively explored in cancer theranostics over the past two decades due to their advantages of facile preparation, excellent biocompatibility, and wide surface conjugation chemistry [87]^[26]. In addition, these inorganic NPs, including gold NPs and magnetic NPs with minimal toxicity, good stability, and powerful imaging properties, are widely used in lung cancer diagnosis, acting as nanoprobes in computed tomography (CT) or magnetic resonance imaging (MRI) for molecular imaging of lung cancer in the clinic [88]^[27]. However, the potential cellular toxicity and adverse effect of magnetic NPs should not be ignored; thus, the size, concentration, and exposure time must be carefully understood [89]^[28]. Gold NPs are widely used for drug delivery because of their easy synthesis, high surface volume, and functionalization [90]^[29]. A study reported the synthesis of curcumin-loaded or curcumin/paclitaxel co-loaded gold NPs for the treatment of triple-negative breast cancer [90]^[29]. The results demonstrated that gold NPs loaded with curcumin with/without paclitaxel exhibited anti-cancer and anti-metastatic properties by downregulating the expression of STAT3 and downstream genes (MMPs, VEGF, and Cyclin D) [90]^[29]. Another study reported the development of layer-by-layer assembled gold NPs (LbL-AuNP) containing anti-STAT3 siRNA and imatinib mesylate (IM) to treat melanoma. Notably, LbL-AuNP prepared using sequential adsorption of natural polyelectrolytes, chitosan, and sodium alginate resulted in a positive charged surface, which could be utilized for iontophoresis therapy to enhance skin penetration in the local treatment of melanoma at an early stage. The topical iontophoretic application of dual-drug loaded LbL-AuNP significantly inhibited tumor growth and STAT3 expression in mouse melanoma models, compared with the control treatments [91]^[30]. Iron oxide is another commonly used material for the synthesis of inorganic NPs. Superparamagnetic iron oxide nanoparticles (SPIONs) composed of superparamagnetic magnetite (Fe₃O₄) or maghemite (Fe₂O₃) at certain sizes are considered to be highly efficient nanocarriers for anti-cancer therapeutics [92]^[31]. However, non-specific binding to serum proteins and rapid clearance from the bloodstream are major challenges for the application of SPIONs. Niaragh and colleagues coated SPIONs with positively charged chitosan derivatives such as trimethyl chitosan (TMC) and thiolated chitosan (ChT) to improve the stability of SPIONs and siRNA loading potential. In addition, Hyaluronate (HA) and TAT peptide were conjugated on the surface of SPIONs to facilitate their tumor tissue penetration and tumor cellular uptake. These HA-conjugated TAT-chitosan-SPION (SPION-TMC-ChT-TAT-H) NPs successfully co-delivered STAT3/HIF-1α siRNA, and significantly inhibited STAT3/HIF-1α gene-driven tumor proliferation, migration, and metastasis [93]^[32]. In addition to metal NPs, inorganic, nonmetallic materials have attracted considerable attention in the field of drug delivery. For example, silica NPs have been successfully used for gene and drug delivery, owing to their ability to improve the stability of protected substances in their cores without interfering with their chemical and physical properties [94]^[33]. One study developed a type of SiO₂ NPs (ZnAs@SiO₂), which encapsulated arsenic trioxide (ATO) by a “one-pot” reverse emulsification approach [95]^[34]. The ZnAs@SiO2 NPs reduced the expression of stemness markers (CD133, Sox-2, and Oct-4) and EMT markers (E-cadherin, Vimentin, and Slug) by inhibiting the STAT3 signaling pathway and thus inhibited tumor spheroid formation in vitro and tumor initiation and metastasis in vivo [95]^[34]. Calcium phosphate NPs (CaP) are also utilized for gene delivery with negligible cytotoxicity and superior biodegradability. Furthermore, CaP dissolves in acidic endosomes and helps the cargo be released into the cytosol through the endosome rupture [96]^[35]. Li et al. proposed a novel hybrid vesicle with inorganic CaP as the kernel and with HA modification on the surface (CaP@HA), for targeted delivery of STAT3-decoy ODNs. They demonstrated that the STAT3-decoy ODNs-loaded CaP@HA vesicles effectively suppressed the expression of STAT3 and its downstream target gene mucin 4 (MUC4), which could interfere with the interaction of Trastuzumab (TRAZ) and human epidermal growth factor receptor 2 (HER2), thereby efficiently reversing TRAZ resistance in anti-HER2 therapy [96]^[35]. Similarly, Ke et al. proposed another inorganic kernel of CaP as the core of reconstituted low-density lipoprotein (LDL) nanovehicles (CaP@LDL) for targeted delivery of STAT3-decoy ODNs to reverse the resistance of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). The results showed that CaP@LDL nanovehicles possessed LDL-mimicking pharmacokinetics, which enabled them to efficiently deliver STAT3 decoy-ODNs to overcome TRAIL resistance by blocking the expression of STAT3 and downstream anti-apoptotic target genes (Bcl-2, Bcl-xl, and Mcl-1) [97]^[36].

4. Polymeric Micelles

Polymeric micelles with core–shell structures have been extensively applied for the delivery of small molecules, therapeutic genes, antibodies (Abs), and RNA-based therapeutics due to their stable structure, good biocompatibility, high drug loading, outstanding pharmacokinetics, and preferential tumor accumulation [98]^[37]. Despite the passive targeting ability of polymeric micelles under the EPR effect, facile methods were used to modify their surfaces with specifically targeting ligands, which could target overexpressed receptors on lung cancer cells (e.g., EGFR and CD44 receptors) in order to achieve improved tumor-specific targeting [99,100]^[38][39]. Moreover, these targeting ligands could partially inhibit the function of overexpressed receptors and regulate over-activated pathways, thus improving drug action in tumor-specific lung tissues [99]^[38]. Micellar formulations containing cationic polymers, such as poly-L-lysine (PLL) and poly(ethylenimine) (PEI), were able to deliver siRNA via electrostatic interaction, thus reducing RNA degradation and enhancing intracellular accumulation [101]^[40]. For example, researchers developed cholesterol-modified dicer-substrate siRNA (Chol-DsiRNA) Polyplexes, which were formed by the encapsulation of STAT3 siRNA with 50 poly-L-lysine residues and 5 kDa polyethylene glycol. Chol-DsiRNA Polyplexes demonstrated improved anti-tumor efficacy with good tolerance by efficiently inhibiting STAT3 [102]^[41]. Another study prepared the dual-targeting system by electronic self-assembly, which was composed of folic acid-conjugated carboxymethyl chitosan for targeting and cationic chitosan derivatives for STAT3 siRNA package (FA-OCMCS/ N-2-HACC/siSTAT3). The NPs dramatically reduced STAT3 expression in M2 macrophages and Lewis lung cancer cells and shifted the phenotype of macrophages from M2 to M1, resulting in the suppression of tumor growth [103]^[42]. The unique core–shell structure of micelles is able to co-deliver two or more therapeutic agents through self-assembly, which is considered an effective combination therapy strategy to overcome drug resistance, tumor metastasis, and immunosuppression. A study developed a micellar delivery system (PEG-PLA NPs) based on FDA-approved poly(ethylene glycol) (PEG)-poly(lactic acid) (PLA) for the co-delivery of Erlotinib (ELTN, EGFR-TKI) and fedratinib (FDTN, JAK2 inhibitor). A synergistic anti-cancer effect was achieved by PEG-PLA NPs in ELTN-resistant NSCLC by downregulating the expression levels of proteins in the JAK2/STAT3 signaling pathway, including pEGFR, pJAK2, pSTAT3, and survivin [104]^[43]. PEG-PLA was also utilized to co-deliver gefitinib (Gef) and Cyclosporin A (CsA), and the results showed that CsA formulated in NPs sensitized Gef-resistant NSCLC to Gef treatment by inactivating the STAT3/Bcl-2 signaling pathway [105]^[44]. Li and colleagues developed a multifunctional nanocomplex to simultaneously deliver paclitaxel (PTX) and STAT3 siRNA (siSTAT3) to inhibit tumor growth and prevent metastasis. PTX and siSTAT3 were encapsulated into the synthesized polyethyleneimine-polylactic acid-lipoic acid (PPL) micelles through hydrophobic or electrostatic interaction, respectively. Furthermore, the negatively charged HA was coated on the surface of the drug-loaded nanocomplex (HA/^siSTAT3PPL_PTX) in order to effectively enter CD44-overexpressed 4T1 cells via an active targeting mechanism. HA/^siSTAT3PPL_PTX exhibited superior anti-tumor efficacy and effectively reduced the lung metastasis of 4T1 cells by silencing the expression of STAT3 and pSTAT3 [106]^[45]. Another study reported the development of a pulmonary delivery system (FM@PFC/siRNA) based on perfluorocarbon (PFC) nanoemulsions for co-delivery of C-X-C motif chemokine receptor 4 antagonist (FM) and anti-STAT3 siRNA. The FM@PFC/siRNA nanoemulsions inhibited both CXCR4 and STAT3 signaling, induced apoptosis and anti-invasive activity, and overcame the immunosuppressive TME, achieving good efficacy in lung metastatic tumor models [107]^[46]. Studies have shown that siRNA delivery by polymeric micelles though electrostatic interaction is not very stable, resulting in inefficient transfection efficiency and high variability [108]^[47]. In addition, cationic NPs are toxic and potentially capable of inducing immunogenicity in animals and humans. Furthermore, siRNAs are susceptible to being cleared by the kidneys because excess cationic components could make NPs easy to disassemble at the glomerular basement membrane [109]^[48]. To solve these problems, researchers used dithiothreitol (DTT) to reduce the disulfide-protecting groups of siRNA at the 3′ end of the sense strand, and the obtained siRNA was reacted with Pluronic F108 functionalized with pyridyl disulfide groups. This covalent conjugation of siRNA with Pluronic F108 provides a stable nanoparticle formulation with efficient siRNA loading, achieving consistent target-specific gene knockdown [108]^[47]. Shi et al. reported a new class of cation-free polymeric micellar spherical nucleic acid (SNA), which can deliver both STAT3 siRNA and temozolomide (TMZ) in a controlled release manner. The siRNA-disulfide-poly (N-isopropylacrylamide) (siRNA-SS-PNIPAM) diblock copolymer could self-assemble to form SNAs, cross the blood–brain barrier, and enter brain tumor cells through a scavenger receptor-mediated mechanism, achieving a remarkable synergistic effect against TMZ-resistant tumors [109]^[48]. Micelles for targeted STAT3 delivery may potentially modulate immunosuppressive TME, leading to improvements in immunotherapy. Jiang and colleagues synthesized a pH-responsive copolymer PEG-poly(lysine-DMMA)-poly(phenylalanine) to co-encapsulate two prodrugs, Gemcitabine-C18 and NI-HJC0152 (STAT3 inhibitor). NI-HJC0152 responded to the progressively intensive hypoxia in tumor tissue to yield parental HJC0152 that inhibits STAT3, leading to the reversal of tumor immunosuppression through modulating TAM polarization, recruiting cytotoxic T lymphocytes, and reducing regulatory T cells. In addition, inhibition of STAT3 also downregulated the expression of cytidine deaminase (CDA) and α-smooth muscle actin (α-SMA), thus relieving the resistance of gemcitabine [110]^[49]. Another study constructed a self-assembling vehicle-free multi-component anti-tumor nanovaccine (SVMAV) using an unsaturated fatty acid docosahexaenoic acid (DHA)-conjugated antigen and R848 (a Toll-like receptor 7/8 agonist) to encapsulate stattic (STAT3 inhibitor). The obtained SVMAV efficiently migrated into lymph nodes and primed CD8⁺ T cells to exert neoantigen-specific killing by promoting antigen uptake of dendritic cells (DCs), stimulating DCs maturation, and enhancing antigen cross-presentation, and finally achieved a robust anti-tumor effect in primary and lung metastasis models of melanoma [111]^[50].

5. Extracellular Vesicles

Studies have demonstrated that multiple cell types can excrete extracellular vesicles (EVs) with phospholipid bilayer membrane-bound structures, and EVs have shown great potential as drug delivery vehicles due to their nano-sized structure and ability to transport bioactive cargos between cells or tissues [112]^[51]. According to formation mechanism and typical size, Evs are mainly classified into three categories: exosomes (30–150 nm), microvesicles (MVs), or microparticles (MPs) (100–1000 nm) and apoptotic bodies (500–2000 nm) [113]^[52]. Compared with other nanocarriers, EVs have some advantages in lung cancer treatment, such as low toxicity, low immunogenicity, the ability to cross biological barriers, and the realization of multifunction through chemical or genetic modifications [112]^[51]. For example, exosomes modified with CD47 protein on the surface were able to evade phagocytosis by macrophages, leading to prolonged circulation time [114]^[53]. Furthermore, exosomes delivered from brain endothelial cells had the ability to cross the blood-brain barrier, exhibiting the potential ability to treat lung cancer with brain metastasis [115]^[54]. However, the acquirement of abundant exosomes with high quality is quite costly, and the lack of standardized methods to isolate, purify, and store exosomes has limited large-scale production and clinic translation [116]^[55]. Therefore, there is an urgent need to establish standard protocols to ensure the mass and consistent production of exosomes. EVs from various origins hold great promise in cell-free anti-cancer treatment. For example, mesenchymal stem cell (MSCs)-derived EVs have shown unique advantages as carriers for anti-cancer drugs due to their lower immunogenicity and tumor migration capacity [117]^[56]. Qian et al. prepared EVs from human umbilical cord MSCs (huc-MSCs) transfected with adenovirus encoding Lipocalin-type prostaglandin D2 synthase (L-PGDS). EVs-L-PGDS inhibited the phosphorylation of STAT3 and the expression of downstream stem cell markers (Oct4, Nanog, and SOX2), thus inhibiting in vitro cancer cell proliferation and in vivo tumor growth [117]^[56]. Neural stem cell (NSCs)-derived exosomes have also been reported as vehicles for delivering oligonucleotide therapeutics (CPG-STAT3 antisense oligonucleotides, CpG-STAT3 ASO) to the glioma microenvironment, as NSCs have been shown to traffic into hypoxic areas of gliomas and secondary brain metastases. The results demonstrated that CpG-STAT3 ASO encapsulated NSCs/EV significantly activated glioma-associated myeloid cells and inhibited tumor progression in mice [118]^[57]. In addition, plant-derived nanovesicles for drug delivery have been discussed in recent years due to their safe and cost-efficient characteristics. Chen et al. obtained cucumber-derived nanovesicles (CsDNVs) at high yield and low cost, which may be natural nanocarriers that contain Cucurbitacin B (CuB, STAT3 inhibitor). They demonstrated that these CsDNVs enhanced the anti-cancer effects of CuB by improving its bioavailability [119]^[58]. Notably, owing to the remarkable capability for penetration of the BBB, exosomes might improve the prognosis of glioblastoma (GBM). Ye and colleagues prepared Angiopep-2 (An2)-conjugated (STAT3) siRNA-loaded exosomes (Exo-An2-siRNA) derived from human M1 macrophages. Exo-An2-siRNA could boost BBB permeation and GBM targeting by exploiting the tumor-homing characteristic of M1 macrophages and specifically targeting (low-density lipoprotein receptor-related protein 1) LRP-1 ligands at the surfaces of both GBM cells and BBB endothelial cells, resulting in the favorable inhibition of the proliferation of orthotopic U87MG xenografts [120]^[59].

6. Challenges in Lung Cancer-Targeted Drug Delivery

Although intravenous injection of nanomedicine is commonly used in other cancers, inhalation of nano-delivery systems is an additional administration route for lung cancer due to the unique anatomical and physiological characteristics of the lungs. However, due to the complex molecular and biochemical composition of the lung tissue, different biological barriers to drug delivery in lung cancer should be taken into consideration when designing efficient strategies for nanotechnology in lung cancer. The presence of mucus in the respiratory system is the key mechanical barrier of the pulmonary region [121]^[60]. Mucins could form complex mesh by interacting with other mucin molecules and glycans in the mucins could provide negative charge, allowing NPs of different sizes and positive charges to be deposited in the mucus layer [121,122,123]^[60][61][62]. The pulmonary surfactant is one of the crucial chemical barriers for NPs to overcome before reaching the pneumocytes. Studies indicate that proteins in pulmonary surfactants prefer to bind magnetic NPs [124]^[63]. Moreover, hydrophilic NPs are easily trapped by the surfactant layer [125]^[64]. Proteolytic enzymes (e.g., cathepsin H) are another chemical barrier as they are responsible for the hydrolysis of protein and peptides of the NPs [126,127]^[65][66]. In addition, the complex tumor microenvironment (TME) is another biological barrier in the drug delivery of lung cancer. One of the most critical stromal cells in TME of lung cancer, namely cancer-associated fibroblasts (CAFs), has been demonstrated to play a key role in remodeling the tumor stroma and increasing the stiffness of the extracellular matrix (ECM), which might restrict the diffusional movement of NPs in tumor cells [128,129]^[67][68]. Macrophages in the TME, part of the clearance system, were found to be able to engulf, degrade, and remove NPs, thus affecting the number of NPs entering the tumor site [121]^[60]. Furthermore, the engulfment of NPs might induce the release of pro-inflammatory cytokines by macrophages, resulting in the activation of the immune system [130]^[69].