It is generally believed that the anatomy and pathophysiology of solid tumors differ from that of normal tissue. For example, solid tumors have a high blood vessel density to meet the nutrition and oxygen required by tumor cells growth. Furthermore, the tumor lacks functional lymphatic vessels and the gap between tumor endothelial cells is relatively large, which can extravasate or retain macromolecular drugs. The phenomenon that makes nanoparticles accumulate in tumor cells is called the enhanced permeability and retention (EPR) effect ^[14][15][16][14,15,16].

The discovery of this phenomenon has promoted the emergence of passive targeting nanoparticles, which can target tumors through the EPR effect. In research, oxaliplatin was encapsulated into N, O-carboxymethyl chitosan nanoparticles by ionic crosslinking, and resveratrol was encapsulated into them by emulsification crosslinking. The particle size of the former was about 190.0 nm and that of the latter was about 164.2 nm. The nanoparticles enhanced the solubility, stability, and EPR effect of oxaliplatin and resveratrol, showing stronger anti-CRC activity in subcutaneous tumor-bearing mice compared to the free drug [17].

However, according to research findings, nanoparticles smaller than 10 nm are filtered out by the kidneys, and particles larger than 100 nm are captured by the liver, so the ideal nanoparticles should be between 10 nm and 100 nm in size, which can circulate for a long time to increase the possibility of uptake by tumors [18]. Moreover, Anitha et al. [19] mentioned that the degree of vascularization in CRC is low. It can be seen that there are controversies about the EPR effect in CRC. In recent years, scientists have gradually realized that the tumor-targeting mediated by EPR is highly heterogeneous. Hence, it is necessary to enhance the targeting ability of nanoparticles based on EPR by combining them with other targeting mechanisms ^[20][21][20,21].

Ligand-modified nanoparticles can actively accumulate in the tumor through a ligand–receptor binding mechanism, thus delivering the drug to the target. These nanoparticles are called active targeting nanoparticles [22]. Cancer cells overexpress some types of receptors and secrete biomolecules that can promote cell proliferation of cancer and surrounding tissues through paracrine or autocrine pathways [23]. In studies, there are two main methods to combine ligands with nanoparticles: one is to modify them chemically during the synthesis of nanoparticles, and the other is to bind ligands with polymers before the synthesis of nanoparticles ^[24][25][24,25]. Ligand-modified nanoparticles can accumulate in the tumor through passive targeting, and then enter tumor cells through active targeting, resulting in a more selective and enhanced therapeutic effect.

In the recent three years, the active targeting design idea of nano-drug targeted delivery systems for CRC mainly adopt the receptor–ligand binding strategy, which involved many highly expressed receptors in CRC, such as folate receptor [26], EGFR [27], CD44, epithelial cell adhesion molecule (EpCAM) [28], CD133, αvβ3 integrin receptor, carcinoembryonic antigen [29], nucleolin [30], mannose receptor [31], hyaluronic acid receptor [32], N-acetyl-d-glucosamine [33], transferrin receptor [34], checkpoint kinase 2 [35], CXCR4+ [36], lipoprotein receptor-related protein-1 [37], MUC1 [38], NRP-1 [39], P-selectin [40], sigma-2 receptors [41], SSTRs [42], and glucocorticoid receptor [43].

Epithelial cell adhesion molecule (EpCAM) is a 314 amino acid and 39 kDa transmembrane glycoprotein that almost only exists on epithelial cells playing a role in adhering cells in normal cells ^[44][45][44,45]. As early as 1979, it has been reported as the main surface antigen of CRC [46], Later, scholars have found that overexpression of EpCAM also existed in human adenocarcinoma cells, and it could also carry out intercellular signal transduction [47].

In many kinds of research, to make nanoparticles target CRC after systemic administration, a DNA EpCAM aptamer (SYL3C) having a strong affinity with EpCAM protein and cancer cells expressing EpCAM was modified on nanoparticles. To reduce drug toxicity and improve therapeutic effect, Ge et al. [28] synthesized biological conjugates loaded with celastrol, which could be captured by CRC overexpressed with EpCAM. EpCAM aptamer, PEG, and dendrimers constituted the conjugates. The results showed that SW620 would undergo extensive apoptosis when exposed to biological conjugates. Moreover, in the xenograft mice and zebrafish models, the biological conjugate showed low toxicity. The dendrimer used in the study is composed of biocompatible components and has an excellent multivalent effect, which can improve the safety of chemotherapeutic drugs.

In another study, the author prepared a cationic liposome to deliver miR-139-5p. The main lipids contained in it are HSPC, DOTAP, Chol, and DSPE-PEG2000-COOH. Moreover, to make the nanoparticles target CRC, the surface of it was modified with EpCAM aptamer. It is found that nanoparticles had inhibitory effects on HCT cells, consistent with the experimental results of animal pharmacodynamics experiments. They also restrained the tumor growth of CRC mice injected subcutaneously with HCT8 [48].

In addition to the specific targets of CRC, some targets of other tumors are also expressed in CRC, which can be used as the targets of nano-drug delivery systems in the targeted therapy of CRC.

The folate receptor is a cell membrane-anchored protein, which is slightly expressed or absent in normal cells, but overexpressed in various malignant tumor cells, especially in CRC [22]. Therefore, it can be used as a target. Natural polymers chitosan and chondroitin sulfate have the advantages of biocompatibility and biodegradability. Soe et al. [26] prepared self-assembled nanoparticles using these two materials to incorporate the hydrophobic drug, bortezomib, and modified the nanoparticles to actively target folate receptors.

Folate was attached to DSPE-PEG and then coupled with nanoparticles. It was found that DSPE-PEG can not only promote blood circulation, but also improve core drug loading and stability. In the pharmacodynamic experiment of animal models, researchers found that nanoparticles had a strong inhibitory effect on the transplanted tumor and were almost non-toxic to other parts outside the tumor.

The overexpression of epidermal growth factor receptor (EGFR) occurs in about one-third of epithelial malignancies. It can stimulate tumor growth, proliferation, promote angiogenesis, and promote cell invasion and metastasis. Therefore, the poor prognosis of CRC may have a great relationship with the overexpression of EGFR in CRC [49]. In order to solve this problem, targeted therapy using monoclonal antibodies to block biological pathways has been used to treat CRC.

The commonly used monoclonal antibody is cetuximab, which is used for CRC under FDA approval [50]. It can selectively target the EGFR to block signal transduction in vivo [49]. This characteristic also makes cetuximab widely used to modify nano-drug delivery systems, actively target CRC, and increase the accumulation in CRC [51].

Sankha Bhattacharya [27] formulated PLGA-PEG-coated nanoparticles to deliver anti-epidermal growth factor receptor-5-fluorouracil (5-FU), which could improve the pharmacodynamics and distribution of the drug in vivo. The Anti-EGFR mAb was bound to the polymeric nanoparticles by using m-maleimidobenzoyl-N hydroxysuccinimide ester to increase target specificity. On the other hand, polymeric nanoparticles consisting of PLGA and PEG can block opsonic action through the reticular epithelial system. These nanoparticles have great clinical value because their preparation methods are solvent emulsification and evaporation, which are simple and rapid [52].

Another attractive carrier is gold nanoparticles, which are commercially available and easily functionalized. Hallal et al. [53] found that AuNPs loaded with cetuximab had better EGFR endocytosis and could inhibit downriver signaling pathways compared with cetuximab and gold nanoparticles alone, which could inhibit cell proliferation and accelerate cell apoptosis. This finding is of great help in solving the problem of drug resistance in the treatment of EGFR monoclonal antibodies.

Moreover, many studies found that the therapeutic effect of monoclonal antibodies could be improved by combining McAb with chemotherapeutic drugs ^[54][55][54,55]. Chen et al. [56] constructed a hybrid nano delivery system to deliver 5-FU, including mesoporous silica nanoparticles (MSN), a supported lipid bilayer (SLB), and cetuximab. Furthermore, the SLB-MSN was functionalized with hydrophilic PEG, which made the nanoparticles more stable, circulate longer in the blood, and more likely to accumulate in the tumor through the EPR effect. Then, to enable the nanoparticles to actively target CRC, cetuximab was coupled to the PEG terminal in SLB-MSN. The SLB-MSN has the advantages of both MSN and liposome, showing significant biocompatibility. It can also encapsulate much drug, achieve controlled release, and its stability can be greatly improved.

2.2.4. Nanoparticles Targeting CD44

CD44 is a transmembraneous glycoprotein that exists as an adhesion molecule on the cell surface and takes part in intercellular and cytomatrix interactions, as well as cell adhesion and migration [57]. The overexpression of CD44 in CRC cells is closely related to tumor metastasis ^[58][59][58,59]. Therefore, it is also a biomarker for CRC as well as a target. Hyaluronic acid (HA) is the main ligand of CD44, consisting of units of D-glucuronic acid and N-acetyl-D-glucosamine alternately. Especially, it is very popular with researchers, because of its non-toxic, excellent biocompatibility, and biodegradability [60]. Wang et al. [58] synthesized HA-decorated polydopamine nanoparticles to deliver chlorin e6 to the CRC. By coupling the HA with nanoparticles, the nanoparticles were easily internalized via endocytosis under the guidance of CD44. Moreover, chlorin e6 [61], the second-generation PS approved by FDA, and polydopamine, the photothermal agent with excellent light-thermal conversion efficiency were conjugated to promote the synergistic effect of photodynamic therapy and photothermal therapy by avoiding the toxicity induced by mental-ion. The result showed that the tumor growth was inhibited strongly in HCT-116 tumor mice, which also confirmed that the antitumor efficiency could be enhanced by combining the two treatments.

2.2.5. Biomimetic Nano Delivery Systems

In addition to the ligand–receptor binding strategy, the nanoparticles can also actively target the tumor by bionic technology, which mainly uses biofilms to coat nanoparticles. This strategy can not only prevent nanoparticles from being identified by the immune system, but also make use of membrane proteins, glycoproteins, and homologous adhesion to make nanoparticles specifically accumulate in the tumor. Recently, the bionic cell membranes used in the reported nanoparticles are mainly erythrocyte membrane [62], cancer cell membrane [63], leukocyte membrane [64], and so on. Furthermore, to make the nanoparticles endowed with more properties, it is better to camouflage the nanoparticles with hybrid cell membranes. Wang et al. [63] prepared Zn_1.25Ga_1.5Ge_0.25O₄: Cr³⁺, Yb³⁺, Er³⁺ persistent luminescence nanoparticles coated with mesoporous silica to deliver IR825 as well as irinotecan and encapsulated the nanoparticles with a cancer cell-macrophage hybrid membrane. The membrane made the nanoparticles actively target the tumor, then the nanoparticles continued to glow at the tumor, which could provide an accurate position for subsequent photothermal therapy. Moreover, after the CT26 tumor-bearing mice were treated with three months of nanoparticles and laser irradiation, the relative tumor volume of mice decreased significantly. In a study, hollow long persistence luminescence nanomaterials loaded with cisplatin were synthesized and coated with a hybrid cell membrane, which was mainly composed of a customized erythrocyte membrane that could prevent nanoparticles from being recognized by the immune system and biofilm expressing PD-1 that gave nanoparticles targeting ability. Then, this team evaluated their efficacy through in vitro anti-cancer experiments and concluded that these nanoplatforms, which played the role of immunotherapy and chemotherapy, were efficient in CT26 tumor-bearing mice [65]. These studies indicated that bionic technology may be an effective method for tumor-targeted therapy.