2). In colorectal cancer liver metastasis (CRLM), for example, CRC cells need to change the tumor microenvironment (TME) first, and in the initiation phase of the invasion-metastasis cascade reaction, colorectal cancer (CRC) cells inhibit the expression of E-cadherin and disrupt the junctional structure between epithelial cells to detach from the primary site [4]. Subsequently, secretion of matrix metalloproteinases (MMPs) and activation of urokinase-type plasminogen activator (u-PA) lead to degradation of the basement membrane. At the same time, pro-angiogenic factors (e.g., VEGF) released by ECM degradation can induce neovascularisation and lymphangiogenesis, providing a gateway for CTCs to enter the circulatory system [5].

Fig. 2.

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Upon entering the circulation, CTCs enhance their own resistance to blood flow shear forces by forming adhesion complexes with other blood cells and inhibit loss-of-nest apoptosis. Notably, only a small number of circulating tumor cells are able to adapt to the microenvironmental pressure of the target organ and evade immune clearance, seeking out the hostile environment of the circulation and eventually extravasating to the liver, where they successfully colonise pre-metastatic ecological niches that have been reprogrammed in advance [6]. Moreover, CTCs arriving at the target organ may enter a dormant state with cell cycle arrest and activation of pro-survival signalling pathways.

How CTCs originated

Cellular infiltration of tumor cells and intravasation into adjacent blood vessels is the first step in the development of metastasis [7] (Table 1).

Table 1.

Key mechanisms and pathways of CTC-mediated tumor metastasis

Invasion-metastasis cascade	Mechanisms	Key molecules/pathways	References
How CTCs originated	EMT - EMT enhances invasiveness and metastatic potential; - TAMs promote EMT and liver metastasis; - Exosomes carry EMT-promoting factors.	SNAIL family, TWIST family, ZEB family, MMPs, IL-6/STAT3-CCL2, TGF-β, lncRNA PCAT1, etc.	[8–27]
	Vascular damage - Vascular barrier disruption increases CTC release	miR-27b-3p, VE-Cad/p120-catenin, ADAM17, VE-Cad, etc.	[28, 29]
How CTCs survive in the bloodstream	Surviving Shear Stress	Integrin β1, CD44, Talin-1, etc.	[30–39]
	Resisting attacks from immune system	TGF-β, Smad/NF-κB, FAK/JNK/c-Jun, CD155, NETs, etc.	[40–44]
	Preventing anoikis	CPT1A, ROS, TCF7L2, PLAUR, etc.	[45, 46]
	Hypoxic microenvironment	HIF, OCT4, NANOG, etc.	[47–59]
How CTCs form metastases	Extravasation - Enhances endothelial adhesion; - Enhances vascular permeability	NETs, protease, ROS, VEGFR2, etc.	[60–64]
	Colonization - PMN formation - Metabolic reprogramming - Hibernation and reactivation	TGF-β, IL-10, miR-934, miR-203a-3p, GDH/STAT3, ACLY/CTNNB1, uPAR/ERK/p38, etc.	[65–80]
			[81–84]
			[85]

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Epithelial-mesenchymal transition (EMT)

Epithelial-mesenchymal transition (EMT) is the initiating step of metastasis [8–10] and refers to the reversible process of transient transdifferentiation of epithelial cells into motile mesenchymal cells [11] (Fig. 3).Tumor cells are endowed with enhanced invasiveness and metastatic potential after the EMT process, which in turn induces tumor cell intravasation and detachment [7, 12]. This process occurs by the orchestration of EMT-inducible transcription factors (EMT-TFs). core EMT-TFs such as SNAIL family members (Snail, Slug), TWIST family (TWIST1 and TWIST2), and E-box-binding (ZEB) transcription factors [13, 14]in a variety of combinations, lead to epithelial phenotypic repression and mesenchymal phenotypic activation of gene expression changes [15–18]. Among them, matrix metalloproteinases (MMP) of the zinc-dependent endopeptidase family can be activated by Snail and Zeb2 to degrade ECM components [11, 19] for tumor cell invasion. In addition to this, it has been shown that ZEB1 is positively correlated with cancer invasiveness, that mutations in the Transcriptional repressor GATA binding 1 (TRPS1) can directly bind to the ZEB1 promoter to activate the onset of transcription, and that the TRPS1 R544Q/ZEB1 axis is closely associated with CRC metastasis, which ultimately leads to a poor prognosis for CRC patients [20]. However, another study suggests that ZEB1 can paradoxically inhibit the EMT process in BRAF-mutated CRC, presenting a role as a tumor suppressor [21].

Fig. 3.

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Tumor-associated macrophages (TAMs) also play a facilitating role in EMT formation. Both HCC cells and pancreatic cancer cells stimulate TAMs to undergo M2 polarization, which promotes the progression of EMT [22, 23]. In the process of colorectal cancer liver metastasis, tumor-associated macrophages (TAMs) and mesenchymal phenotypic tumor cells form a vicious circle through the bidirectional regulation mechanism of IL-6/STAT3-CCL2 axis, which ultimately promotes the formation and progression of liver metastases [24]. Besides, TAMs are involved in almost the whole process of tumor metastasis. Recent studies have shown that through comparative analysis of primary foci and paired liver metastatic tissues from patients with primary colorectal cancer, researchers identified a Subpopulation of TAMs characterized by high expression of secreted phosphoprotein 1 (SPP1), which was significantly correlated with enhanced tumor aggressiveness and reduced patient survival [25] (Table 2).

Table 2.

Roles of immune cells in regulating CTCs across the metastatic cascade

Cell type	Interactions	Key molecules/pathways	References
Neutrophils/NETs	- Induces angiogenesis and intravasation - Promotes extravasation and metastasis formation - Enhances endothelial adhesion - Inhibits immune cell attacks on tumor cells	MMP9, ICAM-1/Mac-1, β1-integrin, IL-17, IL-1β, matrix metalloproteinases, etc.	[61, 86–88]
Macrophages/TAMs	- Induces EMT to enhance migration and invasion capabilities - Promotes intravasation	IL-6/STAT3, CSF-1, EGF, etc.	[24, 89]
MDSCs	- Promotes CTC dissemination and metastatic potential	ROS/Notch/Nodal, CD11b, etc.	[90, 91]
NK cells	- Induction of cell apoptosis - Activation of caspase-dependent apoptosis	FasL, Trail, PD-1/PD-L1, NKG2D/NKG2DL, NCRs/NCRL, etc.	[92, 93]
Lymphocytes (T/B Cells)	- Immune evasion	PD-L1/PD-1, Fas/FasL, CD95/CD95L, etc.	[94, 95]

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Similarly, exosomes are also an important part. EVs can enhance the shedding ability of CTCs by carrying EMT-promoting factors (e.g., TGF-β) and regulating key genes in the EMT pathway [26]. It has been suggested that exosomal long chain non-coding RNA (lncRNA) PCAT1 can induce EMT, which in turn promotes CTC migration leading to colorectal cancer liver metastasis [27].

In addition, research results indicate that gut microbiota can also influence the EMT process. Research findings indicate that F. nucleatum reduces E-cadherin expression in colorectal cancer cells and increases vimentin expression, suggesting that F. nucleatum may promote colorectal cancer metastasis by influencing the epithelial-mesenchymal transition (EMT) process [96].

Interestingly, EMT does not result in a single mesenchymal state; cells are usually in “epithelial-mesenchymal plasticity” (EMP). In this case, the cells are in various intermediate states with varying degrees of epithelial and mesenchymal characteristics, rather than a simple binary switching pattern. The epithelial state is retained both because the high proliferation rate associated with the epithelial phenotype is essential for colonization of distant organs and because a complete transition to a mesenchymal phenotype would limit essential protein interactions [97]. Among them, E- and E/m-type cells have enhanced adhesion and extravasation capacity, which facilitates proliferation and metastasis, while M/e and M-type cells are associated with tumor recurrence [98]. One study showed that the ratio of m-CTCs/e-CTCs was significantly higher in patients with postoperative recurrence of pancreatic cancer than in patients without recurrence (0.23:0.1) [99]. In addition, the m-CTCs to ratio was a negative factor for postoperative HCC recurrence [100].

Vascular damage

During colorectal cancer metastasis, tumor cells undergoing EMT can target and inhibit the expression of VE-calmodulin (VE-Cad) and p120-catenin in vascular endothelial cells through the release of exosomes rich in miR-27b-3p, disrupting endothelial cell tight junctions and leading to the loss of vascular barrier integrity [28]. This abnormal elevation of vascular permeability aims to significantly increase the generation and dissemination of circulating tumor cells by remodeling the microenvironment at the tumor-vessel interface, forming a cascade of “EMT-exosomes-vascular leakage-CTC release” to promote metastasis.

Data from another study showed a significant positive correlation between the level of ADAM17-positive exosomes and the number of CTCs in the peripheral blood of colorectal cancer patients. Explained at the mechanistic level, ADAM17 specifically shears VE-calmodulin in vascular endothelial cells, leading to disruption of endothelial barrier integrity and increased vascular permeability [29]. This alteration provides the microenvironmental basis for the formation of pre-metastatic niche and ultimately leads to distal organ colonization of colorectal cancer via the blood pathway.

How CTCs survive in the bloodstream

In the circulation, CTCs will face many challenges such as shear stress, immune system attack, and so on, but the formation of CTC clusters and hypoxic microenvironment can protect some CTCs from surviving in the circulation for extravasation and eventual metastasis [101] (Table 1; Fig. 4).

Fig. 4.

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Surviving shear stress

Some of the CTCs entering the arteries may fragment and die due to high shear stress [102]thus regions of low shear stress are more susceptible to attack by tumor cells and thus intravasation. During circulation, integrins contribute to CTC-dependent adhesion to low shear stress regions of the vessel wall. It has been shown that β1 integrin and CD44 can help CTC resist shear forces and save them from detachment from the endothelial cell layer [30–32]. It has been shown that Talin-1, an adhesion plaque protein present in integrin-associated complexes, helps to activate integrin β1, which promotes transendothelial migration and subsequent formation of liver metastases in colon cancer cells [33] (Table 2).

Ion mobility is also thought to be associated with the resistance of CTC to shear stress. CTC protects the plasma membrane from damage in the face of shear stress by increasing cellular stiffness, which is associated with Ca²⁺ and Na⁺/H⁺ exchange [34].

The unique structural characteristics of CTC clusters result in a much lower hydrodynamic resistance to their movement in the bloodstream. It has been shown that although CTC clusters are less numerous than individual CTCs in the circulation, their metastatic potential is 23–50 times higher than the latter [98]which may be related to the fact that CTC clusters up-regulate cellular connectivity components, such as spotted bead proteins, and the associated blood cell crosstalk [35]. It has been shown that tumor cells undergo membrane damage due to shear stress in the absence of physical protection from platelets [36]. When CTCs express tissue factor on their surface, platelets are activated and recruited to form a “platelet microthrombus” around CTCs, which physically protects CTCs under the threat of shear stress. In addition, this protective clot also attracts monocytes/macrophages to recruit to the CTC [37, 38]helping the CTC to adhere to the endothelium and undergo further extravasation (Table 2). Similarly, CAF is resistant to high shear. In addition, it has been shown that intra-CTC bacteria can regulate the cytoskeleton to undergo rearrangement and enhance the viability of CTCs under mechanical stress in order to improve the survival of CTCs in circulation, correlating with the mechanism by which cytoplasmic bacteria inhibit RhoA and ROCK activation [39].

Resisting attacks from immune system

The blood environment is not friendly to CTC [103]but at the same time a number of components are used by CTC for their metastatic purposes (Table 2).

Platelets are involved in almost the entire process of tumor metastasis as “accomplices” [104]. TGF-β secreted by platelets can activate Smad and NF-κB signaling pathways in tumor cells [40] in order to induce the EMT state of CTC [41] and enhance the invasive ability of CTC. In addition, platelets can activate the FAK/JNK/c-Jun signaling axis in CTC through physical contact and transcriptionally up-regulate the immunosuppressive checkpoint molecule CD155, which confers the ability of CTC to escape NK cell killing and promotes distant metastasis of hepatocellular carcinoma [42]. Platelets may also exhibit the ability to promote CTC extravasation at later stages of tumor cell metastasis. The resistance of platelets to immune surveillance might serve as a future chemotherapeutic target for tumor metastasis [105]. Meanwhile, with the breakthrough of nanomedicine, the development prospect of platelet membrane for drug delivery in cancer therapy is also very promising [106].

NK cells could have mediated lysis and thus recognition and elimination of CTCs to prevent further progression of tumor metastasis, but the interaction of NK cell receptor D with the MICA/MICB of CTCs prevents this process from occurring, thus giving CTCs an opportunity to take advantage of it [105].

For assisting CTC metastasis, the credit of neutrophils should not be underestimated as well. It has been shown that neutrophils can account for about 90% of the total WBCs that escort CTCs to metastasis in mouse models and up to 75% in patients [43]. Neutrophils can assist CTC in immune escape and significantly enhance their metastasis-related biological properties through two modes of interactions: (i) CTCs directly adhere to the top of neutrophils and hitchhike, and (ii) they indirectly bind to neutrophils through platelet-constructed molecular bridges. This dynamic interaction network not only stabilizes the mechanical integrity of CTC, but also enhances its vascular wall anchoring and transendothelial migration by activating adhesion signaling pathways. Notably, CTC-neutrophil clusters are significantly superior to isolated CTCs in terms of metastatic potential, with a significant positive correlation between their blood detection rate and the invasive progression of malignant tumors such as breast, gastric, and hepatocellular cancers, and the presence of CTC-neutrophil clusters is associated with a worse prognosis for patients [44]. Therefore CTC-neutrophil cluster has the potential to be an important biomarker for assessing the risk of tumor metastasis. Neutrophils form adhesive meshwork structures called neutrophil extracellular traps (NETs), which can protect CTCs from NK cell-mediated cytotoxicity by forming physical barriers and secreting immunosuppressive cytokines.

Preventing anoikis

Loss-of-nest apoptosis is a protective barrier against metastatic cell dissemination. After detaching from extracellular matrix (ECM) anchorage, circulating tumor cells need to overcome loss-of-nest apoptosis (anoikis) - a specific process of programmed death directly induced by the disruption of intercellular adhesion or extracellular matrix attachment [107]. Normal epithelial cells activate adhesion spot kinase (FAK) via integrins and initiate the PI3K/AKT and RAS-RAF-MEK-ERK signaling pathways [108]. When anchorage deficiency occurs, metabolic disorders such as activation of the mitochondrial apoptotic pathway [109] or ATP depletion can trigger cell death [110]. However, tumor cells remodel the survival network through multidimensional mechanisms such as endogenous signaling reprogramming, microenvironmental adaptive remodeling, and metabolic stress compensation to compensate for integrin signaling deficits, inhibit mitochondrial translocation and phosphorylation of pro-apoptotic proteins, establish a pre-metastatic survival microenvironment, and scavenge reactive oxygen species to maintain energy homeostasis.

It has been shown that CPT1A-mediated activation of fatty acid oxidation (FAO) induces ROS scavenging and thus the ability of CRC cells to resist nest loss apoptosis [45]. In addition, TCF7L2 is highly expressed in GC as an important transcriptional regulator of urokinase-type plasminogen activator receptor (PLAUR) and promotes loss-of-nest apoptosis resistance and metastasis in GC cells [46]. It is noteworthy that viruses can also modulate anoikis resistance (AR) related pathways to maintain infected cell survival and promote metastasis [111]. For example, HBx proteins can drive hepatocellular carcinoma cells to bypass resistance to anoikis apoptosis [112] and promote hepatocellular carcinoma metastasis through multiple pathways synergistically (PI3K/AKT, EMT, PAK1, NF-κB and miRNA network).

Hypoxic microenvironment

A hypoxic environment can place tumors under a powerful selective pressure that not only drives malignant cell proliferation but also diminishes their susceptibility to apoptosis [47]. HIF signaling makes tumor cells more aggressive in a hypoxic environment [35]. It has been shown that overexpression of HIFs can promote tumor cells to undergo metastasis [48]and it has been suggested that it may be related to the ROS-resistant phenotype [35]. In addition to this, the aggregation of tumor cells can in turn induce HIF-1α-mediated mitochondrial autophagy and scavenge ROS [49]. Elevated levels of HIF-1α have been shown to correlate with poor prognosis in patients with gastrointestinal tumors such as colorectal cancer, gastric cancer, and hepatocellular carcinoma [50–52].

CTC clusters and microemboli

CTC clusters, although fewer in circulation than individual CTCs, have 23–50 times higher metastatic potential [53, 54] and better survival than individual CTCs [55, 56]. This is due to the own characteristics of CTC clusters. First, CTCs traveling in clusters seem to prefer to be released from hypoxic regions of the tumor [57]and their own cluster structure provides a hypoxic microenvironment [55]which facilitates their survival. Second, compared with individual CTCs, CTC clusters exhibit specific hypomethylation patterns associated with stem cell characteristics [58]such as OCT4, NANOG, SOX2, and SIN3A [59]which establishes CTC clusters to be more invasive and is closely associated with poorer prognosis. In addition, the residence time of CTC clusters in the circulation is much less than that of individual CTCs, which is conducive to the dissemination and colonization of tumor cells to distant organs, thanks to the characteristics of CTC clusters, whose own larger structure leads to slower mobility in the circulation and easier adhesion to the vessel wall [113].

In addition to homotypic CTC clusters formed between CTCs, CTCs can form heterotypic CTC clusters with other cell types such as neutrophils, myeloid-derived suppressor cells, platelets, and CAFs [114] or circulating tumor microemboli (CTM), which have the ability to resist shear stress, avoid anoikis [115] and immune escape [40]. It has been shown that patients with detectable CTC-neutrophil clusters have a poor prognosis [114]. Another study showed that MDSCs can help CTCs to be invisible in immune surveillance [116] and thus protected from the immune system (Table 2). In addition to this, tumor microemboli formed by aggregation of multiple cells are more likely to linger in the microvasculature of distal organs and form a pre-metastatic microenvironment by invading the vessel wall, as compared to single circulating tumor cells [117]. In addition to this, CD44-mediated signaling pathways are also responsible for the high efficiency of CTC cluster transfer [118].

Notably, medical manipulations such as puncture biopsy may also result in passive release of CTC from the primary tumor and portend poorer progression-free survival [119].

How CTCs form metastases

CTCs that have survived multiple challenges in the circulation next face extravasation and colonization. Even if they reach distant organs successfully, some CTCs may become dormant rather than forming metastases immediately (Table 1).

Extravasation

After overcoming multiple obstacles, some of the surviving colorectal cancer CTCs will be specifically captured into the hepatic microvascular system, and subsequently successfully infiltrate into the hepatic parenchymal microenvironment by interacting with endothelial cells and immune cells in the local capillaries, which ultimately mediate their transendothelial migration into the hepatic parenchyma (i.e., extravasation).

The main mechanisms of extravasation are paracellular migration (dominant) and transcellular migration (rare). It is noteworthy that host blood cells often play an “accomplice” role in the extravasation process. Platelets can enhance tumor cell invasiveness [40]and can also help CTCs to achieve extravasation in other direct or indirect ways, such as by inducing the opening of the endothelial barrier [60]. Neutrophils capture CTCs and enhance their endothelial adhesion by releasing NET [61]and can also promote CTC extravasation by releasing proteases or ROS to degrade ECM and increase vascular permeability. For colorectal cancer cells such as HT-29, CTC capture can be mediated by NET-associated CEACAM1 [62]. VEGF released by metastasis-associated macrophages (MAMs) can induce an increase in vascular permeability [63]which promotes CTCs extravasation (Table 2).

It has been shown that colorectal cancer cells can derive miR-25-3p after uptake by liver sinusoidal endothelial cells (LSEC), which leads to upregulation of VEGFR2 and enhanced vascular permeability [64]. Similar expression changes promote endothelial windowing, which further promotes extravasation.

Colonization

It is worth mentioning that CTC does not randomly colonize any organ and metastasize but shows organ tropism. When CTC leaves the primary tumor, its destination has already been predetermined and a supportive pre-metastatic niche (PMN) has already been formed.

Pre-metastatic niche formation

There is increasing evidence that primary tumors have released relevant factors including tumor-derived secretory factors (TDSF) and extracellular vesicles (EV) to the designated sites before CTCs reach the metastatic foci [65, 66]and these factors subsequently act on bone marrow-derived dendritic cells (BMDCs) or tissue-resident cells to reprogram the microenvironment of the distant target organ to make it suitable for CTC survival and colonization. Suitable “soil” prepared before the arrival of the “seed” seems to suggest that the treatment of tumor metastasis should focus on both the “seed” and the “soil” [67].

Tumor-derived secreted factors (TDSF): Previous studies have shown that TGF-β promotes colorectal cancer metastasis by recruiting tumor-associated macrophages (TAMs) and regulatory T cells (Tregs) and inducing the release of S100A8/S100A9 to drive epithelial-mesenchymal transition [68, 69]while IL-10 inhibits CD8 + T cells by up-regulating PD-L1 expression on monocytes infiltration and its mediated anti-tumor immune response, synergistically enhancing the process of colorectal cancer metastasis [70] (Table 2).

Extracellular vesicles (EV): EV can determine the site of CTC stagnation in the vascular system and can also assist in the development of extravasation [71]. Studies have shown that miR-934 and miR-203a-3p promote colorectal cancer metastasis by inducing M2 macrophage polarization to promote CXCL13 secretion and activation of the CXCL12/CXCR4/NF-κB pathway to drive the formation of pre-metastatic ecological niches, respectively [72, 73]; miR-181a-5p, on the other hand, activates a positive feedback loop through CCL20 secreted by hepatic stellate cells, thereby inducing tumor microenvironment (TME) reprogramming [74]; miR-135a-5p can initiate the LATS2-YAP-MMP7 signaling axis [75]; ADAM17 induces leaky angiogenesis by targeting vascular endothelial cells and accelerates the pre-metastatic ecological niche establishment [29]. Another study showed that CRC-derived extracellular vesicles can alter their gene expression through specific fusion with hepatic KCs, which in turn promotes pre-metastatic ecotope formation [6]. Similar processes exist in the metastasis of other digestive system tumors. miR-494, IL-1β and hGF promote hepatocellular carcinoma metastasis [76–78]; IL-8 and miR-21 promote gastric cancer metastasis [31, 79, 80];

Metabolic reprogramming

The organ tropism observed during tumor metastasis is closely associated with metabolic reprogramming in tumor cells, including alterations in glucose metabolism, lipid metabolism, and amino acid metabolism.

During liver metastasis in colorectal cancer, tumor cells exhibit enhanced dependence on amino acid metabolism. The glutamine metabolic pathway is significantly activated, with glutamine dehydrogenase (GDH) promoting metastasis and invasion of colorectal cancer cells through STAT3-mediated epithelial-mesenchymal transition (EMT) [81]. Additionally, lysine metabolism is involved in the colonization process, with its metabolic product acetyl-CoA activating the Wnt signaling pathway, while the generated glutamate helps maintain redox balance, thereby promoting liver colonization [82]. Furthermore, liver metastasis of colorectal cancer is associated with creatine levels. When creatine levels increase due to exogenous supplementation or endogenous synthesis, tumor metastatic capacity is enhanced [83].

In contrast, tumor cells in lung metastasis of colorectal cancer tend to activate lipid metabolic pathways. Related studies have shown that the expression of ATP-citrate lyase (ACLY) and stearoyl-CoA desaturase 1 (SCD1) is significantly upregulated in colorectal cancer cells. ACLY interacts with the CTNNB1 protein to stabilize it, and this complex may promote the transport of CTNNB1 from the cytoplasm to the nucleus, thereby enhancing the transcriptional activity of CTNNB1 and the migration and invasion capabilities of colon cancer cells, accelerating the process of colorectal cancer metastasis [84]. In addition, glycometabolic reprogramming plays a crucial role in the metastasis process in both liver and lung organs.

These findings not only reveal the key role of metabolic reprogramming in tumor metastasis organ selection but also provide a robust theoretical basis for future research into targeted therapeutic strategies tailored to different metastatic sites.

Immune system regulation

The immune system also plays an important role in organ tropism. Among these, neutrophils can systematically regulate the reprogramming of the pre-metastatic microenvironment, ultimately forming a pathological microenvironment that supports the colonization and proliferation of circulating tumor cells. Studies have shown that neutrophils can (i) degrade the extracellular matrix to promote tumor invasion, (ii) induce neovascularization to provide nutritional support, (iii) establish a pro-inflammatory environment and recruit immunomodulatory cells such as myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages, etc., through secretion of bioactive molecules, such as matrix metalloproteinases (MMPs), vascular endothelial growth factors (VEGFs), transforming growth factor β (TGF-βs), and interleukin 17 (IL-17s). In addition, MDSCs, tumor-associated macrophages, and other immunomodulatory cell populations play important biological roles, shaping multiple mechanisms for primary tumors and host factors to synergistically create suitable conditions for tumor metastasis (Table 2).

Anatomical and therapeutic factors

For colorectal cancer liver metastasis, the special anatomy of the portal vein also provides a shortcut for CTC convergent delivery from the intestine to the liver [6]which allows rapid encapsulation of CTC in the liver [120]. Interestingly, newer studies suggest that the effects produced by chemotherapeutic agents on the pre-metastatic microenvironment may promote the development of tumor metastasis. Significant reductions in the number of macrophages and T cells were observed in the livers of colorectal cancer patients treated with oxaliplatin chemotherapy. The number of macrophages and T cells was significantly reduced in the liver tissue of colorectal cancer patients who had received oxaliplatin, but the residual macrophages showed an immunosuppressive phenotype, an alteration that may play a role in synergistically promoting liver metastasis [121].

When CTCs leave the circulation, they may become disseminated tumor cells (DTCs) homing to new metastatic sites [7]. Interestingly, most CTCs enter a dormant state upon arrival at the target organ and, upon receiving metastatic site signals, reactivate uPAR and mitogenic signaling (ERBB2 or EGFR) to induce ERK activation and p38 inactivation, and ultimately switch to a value-added mode [85].

122]is constructed by enriching patients’ CTCs in peripheral blood and inoculating them into immune-deficient mice, which can expand primary tumor cells and generate clinically relevant xenograft tumors [49].

The first successful construction of the CDX model was achieved by researchers using the technique of immunodeficient mice undergoing intracardiac or tail vein injection of BC CTC-derived cell lines [123]. Currently, CDX has achieved relevant research results in breast cancer, prostate cancer, small-cell lung cancer, non-small-cell lung cancer, and melanoma [124]and has proven to be a reliable model for disease modeling and drug testing. However, the establishment and specific application of CDX models in digestive system tumors such as colorectal cancer and hepatocellular carcinoma still have a long way to go.

With technological advances in CTC isolation and enrichment strategies, we can expect higher CDX generation efficiency and wider clinical applications [125].

CTC-derived organoids(CDO)

CTC-derived organoids (CDO) can reflect the heterogeneity of primary tumors to a certain extent, and thus can be used as a prospective tool to dynamically monitor the evolutionary spectrum of tumor mutations [126]. It is safer to invade than patient-derived organoids (PDOs), exhibits a higher propensity to metastasize than primary-derived organoids [127]and may be able to better mimic tumor behavior and metastatic mechanisms [128]. CTC-derived colorectal cancer organoids have been successfully established using suspension culture methods, and for the first time, it has been demonstrated that CTCs isolated from CRC patients have a phenotype that expresses cancer stem cells, which can be used to determine drug sensitivity [129]providing a new idea for personalized medicine for patients with metastatic CRC.