2.1 Origins and tumour vasculature

The endothelium of arteries, veins and capillaries in humans is made up of flat, single-layered cells called ECs that are mostly found on the inner walls of blood vessels and come into close touch with the blood's constituent cells and components.⁶ It is now widely accepted that ECs originate from the mesoderm during the embryonic period.⁷ Research has indicated that the transcription factor families FOX and ETS play important roles in the specialisation and establishment of ECs.⁸ Under normal conditions, ECs are typically in a quiescent state, with a relatively low proliferation rate. Under physiological conditions, ECs proliferate approximately once every 150 days, contributing to the blood vessels' stability and integrity.⁹

However, during tumourigenesis, the proliferation rate of ECs significantly increases. Owing to the hypoxic conditions in the tissue, tumour cells (TCs) secrete various angiogenic factors, including vascular endothelial growth factor (VEGF), which cause ECs in the adjacent normal tissue to proliferate and migrate. Thus, new blood vessels are formed. TCs grow and proliferate when oxygen and nutrients are delivered by blood arteries.¹⁰ In contrast to normal blood vessels, tumour vasculature exhibits characteristics such as tortuosity, intertwining, dilation and disorganisation. They also have loose pericyte covering and leaky neovasculature, which raises vascular permeability and, in turn, interstitial pressure. High interstitial pressure further causes vessel collapse and reduced TC perfusion, creating a hypoxic and acidic tumour immune microenvironment. Vascular abnormalities and compromised perfusion hinder the infiltration of immune cells and antitumour agents from the circulatory system into the tumour, suppressing antitumour activity.¹¹

Typically, tumour angiogenesis manifests through various patterns. Sprouting angiogenesis, which is brought on by tip cell migration and proliferation, is the most characteristic process of both normal and pathological angiogenesis, resulting in new branches forming within existing blood vessels. An existing vessel is divided into two vessels during intussusceptive angiogenesis (IA), which are used by TCs to expand and invade. Endothelial progenitor cells (EPCs) derived from bone marrow, as well as cancer stem-like cells, can also differentiate into ECs, forming new vasculature. Additionally, in order to enter the tumour, TCs can approach and take over preexisting blood arteries. Ultimately, TCs can specify ‘avascular’ channels to facilitate the transportation of oxygen and essential nutrients. Various patterns of angiogenesis may be associated with tumour resistance to therapy. Tumour endothelial cells (TECs) have unique metabolic pathways increased fructose metabolism activation, fatty acid oxidation and glycolysis, and the origin of TECs can differ from that of normal ECs^12,13 (Figure 1).

2.2 Senescence

3.1 Interaction between TECs and TCs

The interaction between TCs and ECs drives tumour angiogenesis and vascular dysfunction through multiple mechanisms. Tumour angiogenesis is a multifaceted process that entails HIF-1 activation, angiogenic agent release and remodelling of the TME. Under hypoxic conditions, TCs activate HIF-1 to induce bFGF and VEGF expression, enhancing EC proliferation and vascular permeability. HIF-1 also upregulates the expression of matrix metalloproteinases (MMPs), which degrade the extracellular matrix (ECM), facilitating EC migration.⁵ Furthermore, TCs modulate EC phenotypes to favour angiogenesis. For example, TCs induce ECs to express specific integrins (e.g., αvβ3 and αvβ5), which are critical for angiogenic processes.³⁷

Vascular dysfunction manifests as hyperpermeability, hypoxia, an acidic TME, and impaired drug delivery. Multiple vascular-disrupting factors contribute to tumour progression. Leucine-rich α-2 glycoprotein 1 (LRG), a key vascular-disrupting factor, drives vascular dysfunction, tumour metastasis and resistance to checkpoint inhibitors in melanoma.^38-42 Tumour-derived PAK4 phosphorylates VE-cadherin, disrupting EC junctions, increasing vascular permeability and promoting TC extravasation, a process notably observed in aggressive tumours such as glioblastoma.⁴³ Loss of CD93 and CX3CL1 overexpression mediate aberrant EC‒ECM adhesion, resulting in the formation of disorganised vascular networks.^{44, 45} Additionally, PHGDH hyperactivation in the TME reprograms serine metabolism to provide biosynthetic precursors for ECs, supporting their abnormal proliferation and fostering vascular heterogeneity.^{46, 47} These mechanisms collectively lead to structurally and functionally compromised neovasculature (Figure 2).

3.2 TECs and tumour metastasis

ECs play pivotal roles in multiple phases of tumour metastasis.^48-50 During the invasion phase, EC promotes tumour invasion by modulating the microenvironment and activating Notch signalling via CXCL12 and transforming growth factor Beta (TGF-β).

Vascular hyperpermeability is crucial for tumour metastasis. During extravasation, ECs interact with circulating tumour cells (CTCs) to facilitate their transmigration across the vascular wall into new tissues. This process involves adhesion and signalling between TCs and TECs. Tumour-derived molecules such as ADAM17, miR-27b-3p and miR-25-3p compromise the integrity of the endothelial barrier by targeting ZO-1, occludin, claudin and VE-cadherin, thereby promoting vascular permeability.^51-53 Recent studies have revealed that CCR2 expression and pyroptosis-related proteins in ECs induce endothelial retraction and CTC extravasation.^{54, 55}

During circulation, endothelial surface adhesion molecules are crucial for immune evasion and metastatic migration. One mechanism involves the unresponsive state of TECs to inflammatory signals, termed endothelial anergy.⁵⁶ VEGF and FGF2 induce endothelial anergy by suppressing vascular cell adhesion molecule 1 (VCAM-1) and intercellular adhesion molecule 1 (ICAM-1) expression, impairing leukocyte extravasation as well as tumour immune surveillance.⁵⁷ Additionally, proinflammatory signals upregulate PD-L1 and FasL expression in ECs, reducing the efficacy of immunotherapy.^{58, 59} Moreover, the maturation of dendritic cells is inhibited, leading to compromised antigen presentation and a subsequent inability to activate T cells.⁶⁰ Growing evidence suggests that antiangiogenic drugs can reverse endothelial anergy and improve immunotherapy outcomes, highlighting the need for future research on the immunotherapeutic potential of these agents.^{61, 62} On the other hand, tumour metastasis may potentially be impacted by adhesion molecule expression. For example, E-selectin binds to certain TCs, facilitating CTC migration across the endothelium.⁶³

During the colonisation phase, ECs support TC growth and proliferation by forming new vascular networks. Some TCs can maintain a dormant state through endothelium-derived thrombospondin-1 and are activated during future metastasis.^{64, 65}

3.3 Heterogeneity of TECs from a single-cell perspective

The differences in the efficacy and resistance of AATs among various patients highlight the heterogeneity of TECs. Zeng et al. provided an overview of the heterogeneity observed in human TECs as examined through single-cell methodologies across various tumour types. In malignancies such as colorectal cancer and hepatocellular carcinoma (HCC), TECs exhibit high heterogeneity. Of particular note, the three most commonly seen TEC markers are IGFBP3, ACKR1 and PLVAP, with widespread expression across multiple cancer types.⁴ Li et al. amalgamated single-cell RNA sequencing data from 575 patients with cancer to establish a comprehensive pan cancer EC atlas and discovered two notable subpopulations: SELE+ veins and CXCR4+ tip cells. These subpopulations may be linked to response rates to AATs and immunotherapies across different cancer types. In most cancers, the abundance of the proinflammatory SELE+ vein subpopulation decreases as the tumour progresses, whereas the CXCR4+ proangiogenic tip cell subpopulation increases.⁶⁶ Pan et al. conducted the research that produced a detailed atlas of tumour vasculature at the single-cell level, encompassing almost 200 000 cells across 31 cancer types, elucidating the intricacies of tumour vasculature. Pan defined the stages of angiogenesis as SI, SII and SIII, noting that poor overall survival and progression-free survival are predicted by significant infiltration of APLN+ TipSI cells. However, treatment with bevacizumab significantly reduces their infiltration, enhancing patient responses to AAT. Moreover, APLN+ TipSI cells can induce T-cell dysfunction, contributing to an immunosuppressive environment that promotes angiogenesis.⁶⁷ Zheng et al. identified subpopulations of tip cells expressing insulin receptors and placental growth factor. INSR+ tip cells, which highly express vascular endothelial growth factor receptor 1/2 (VEGFR), are key drivers of angiogenesis. Zheng's work positions the insulin receptor as a novel target for AAT. The above results further emphasise the significance of EC subpopulations in immunological modulation, with important implications for immunotherapeutic strategies within the TME.⁶⁸

Despite the clear advantages of single-cell technologies in revealing heterogeneity, challenges remain owing to the minimal sample sizes, restricted variety of cancer types examined, and the scarce presence of ECs (Figure 3).

4.1 EndMT and tumour angiogenesis

Under pathological conditions, EndMT leads to vascular abnormalities and tumour metastasis.^71-74 Although TECs undergoing EndMT exhibit reduced intrinsic angiogenic capacity, they promote angiogenesis via paracrine signalling and strengthen TC invasion and metastasis.^75-77 Research has shown that neutrophil extracellular trap-induced EndMT impairs angiogenesis and delays diabetic wound healing.⁷⁸ Tumour necrosis factor-alpha (TNF-α) robustly triggers EndMT to drive angiogenesis and stromal development in pancreatic cancer.^{79, 80} Furthermore, EndMT markers correlate positively with microvessel density in breast cancer, suggesting a link between EndMT and poor tumour prognosis. Notably, partial EndMT that produces cells exhibiting both endothelium and mesenchymal characteristics is characterised as a transient and reversible phenomenon^{81, 82} (Figure 4a).

4.2 EndMT and tumour proliferation

EndMT enhances tumour invasiveness by promoting the mesenchymal transition of ECs. TAMs promote EndMT by releasing osteopontin (OPN), whereas low shear stress and interstitial fluid flow have been proven to upregulate EndMT as well.^{77, 83, 84} In addition, high cholesterol metabolites, as well as radiotherapy and chemotherapy, play important roles in the regulation of EndMT.^85-88 Notably, the induction mechanisms of EndMT include the Notch signalling pathway, TGF-β, and other factors, as well as exosomes, and TGF-β works in synergy with TNF-α, IL-1β and Notch to regulate EndMT.^{75, 76, 80, 89-91} EndMT is a significant source of cancer-associated fibroblasts (CAFs) (more than 50% in gliomas and over 40% in melanomas) and CAFs differentiated from EndMT secrete HSP90α to promote the polarisation of M2 macrophages^92-94 (Figure 4b).

4.3 EndMT and drug resistance

It has been found that the reversal of EndMT in lung cancer helps to mitigate chemoresistance, independent of soluble TGF-β levels.⁹⁵ EndMT appears to facilitate the formation of a perivascular niche that supports drug resistance in multiple ways: promoting hypoxia, drug delivery failure, disappearance of therapeutic targets and enlistment of protumour immune cells. For example, inhibiting EndMT can significantly increase glioma cells' susceptibility to temozolomide, which is undoubtedly related to drug delivery failure and hypoxia.⁹² In TECs that undergo EndMT, VEGFR2 expression is reduced, resulting in resistance of glioblastoma ECs to VEGF treatment.⁷⁴ Moreover, EndMT promotes resistance to radiotherapy in colorectal cancer by awakening cancer stem cells and polarising M2 TAMs⁸⁶ (Figure 4c).

4.4 EndMT and PMNs

PMNs have attracted much more attention in targeted cancer therapy, representing an abnormal, tumour favouring and cancer cell-free microenvironment that is essential for the metastasis of TCs to remote organs.^{96, 97} EndMT in PMNs can promote the extravasation and metastasis of TCs. Extracellular vesicles released from breast cancer cells induce EndMT in liver sinusoidal ECs,⁹⁸ indicating that TCs tend to induce EndMT at metastatic sites.⁹⁸ Therefore, we conclude that EndMT has a specific function in the formation of PMNs. Future elucidation of the sequence of events in which EndMT occurs in tumour metastasis is anticipated to offer a novel approach to cancer therapy (Figure 4d).

5.1 TAMs

Macrophages exhibit significant plasticity in regulating tumour angiogenesis. TAMs are currently believed to alter the transcriptional programs in reaction to cues from TME, forming a continuous spectrum, with the extremes being the M1 and M2 phenotypes.⁹⁹ M1 TAMs suppress tumour angiogenesis, whereas M2 TAMs enhance angiogenesis.¹⁰⁰ Cytokines such as IL-4 and IL-13 can polarise macrophages towards the M2 phenotype, leading to the secretion of proangiogenic mediators, including VEGF, TGF-β and MMPs.^101-103 In contrast, M1 TAMs induced by lipopolysaccharides and interferon-γ (IFN-γ) suppress angiogenesis by releasing proinflammatory factors, including IL-6 and TNF-α. Conversely, the TEM represents a unique subset of macrophages. Tie-2 on the surface of the TME promotes tumour angiogenesis by binding with ANGPT2. A recently identified subset of proangiogenic macrophages (PraM) is widely distributed across multiple tissues and promotes angiogenesis through the expression of high levels of angiogenesis-promoting genes.¹⁰⁴ Studies have also shown a strong interaction between SPP1+ TAMs and TECs, with spatial transcriptomics revealing the colocalisation of APLN+ TipSI cells with SPP1+ TAMs.⁶⁷ In conclusion, the polarisation state and subtype diversity of macrophages in the TME determine their complex effects on angiogenesis (Figure 5a).

5.2 CAFs

The tumour stroma comprises various cell subtypes, with fibroblasts and ECs being among the most abundant. During tissue injury, these two cell types coordinate their activities to promote the regeneration of tissue. In cancer, however, such regenerative pathway is usurped by malignant cells that manipulate stromal cell interactions to facilitate tumour progression.¹⁰⁶ Within the TME, CAFs support angiogenesis by secreting large quantities of VEGF-A and other proangiogenic factors. Additionally, CAFs release platelet-derived growth factor (PDGF-C), which not only enhances EC resistance to anti-VEGF therapies but also promotes further angiogenesis by inducing the release of other proangiogenic growth factors.^106-108 PDGF-C-induced FGF2 activates ECs, fostering tumour growth and invasion while linking different types of tumour parenchyma and stroma.¹⁰⁸ In addition to inducing VEGF expression, OPN also recruits proangiogenic monocytes into the TME.¹⁰⁷ In metastatic breast and prostate cancers, TCs facilitate angiogenesis at metastatic sites by downregulating CAF-derived thrombospondin-1.¹⁰⁹ Moreover, CAFs release enzymes and MMPs that degrade and restructure the ECM, establishing a supportive ECM for EC migration and neovascularisation. CAFs can also indirectly influence ECs and angiogenesis by regulating the levels of lactate and other metabolites within the TME (Figure 5b).

5.3 MDSCs

The immunosuppressive characteristics of MDSCs are essential in the TME.¹¹⁰ MDSCs are divided into two major subtypes: Monocytic MDSCs (M-MDSCs) resemble monocytes, while polymorphonuclear MDSCs (PMN-MDSCs) resemble neutrophils, both in terms of their phenotype and physical characteristics.¹¹¹ Within the TME, MDSCs exhibit low phagocytic activity but promote angiogenesis, tumour invasion and immune tolerance via secreting nitric oxide, ROS and anti-inflammatory cytokines.¹¹² MDSCs facilitate neovascularisation through two primary mechanisms: by releasing growth factors such as bFGF VEGF, PDGF and prokineticin 2 (Bv8) and by remodelling the ECM through MMPs while reprogramming other cells towards a protumour phenotype.^113-116 As a protease, MMP9 promotes EC migration and accelerates angiogenesis. Besides promoting angiogenesis, VEGF possesses immunosuppressive functions, increasing the recruitment of Tregs and MDSCs.¹¹⁴ Furthermore, MDSCs can mimic TCs by promoting angiogenesis through exosomes containing proangiogenic factors such as VEGF-A and miRNA-126α.^117-119 The angiogenic activity of MDSCs is driven primarily by the VEGF/JAK/STAT signalling pathway. Although direct interactions between ECs and MDSCs have not yet been explicitly described, their roles within the TME suggest a potential reciprocal influence, which future research may elucidate^{114, 115, 120, 121} (Figure 5c).

5.4 NK cells

Natural killer (NK) cells perform essential immunomodulatory functions within the TME.¹²² TECs in the TME can activate NKG2D by expressing RAE-1ε, resulting in the internalisation of NKG2D from the surface of NK cells and the conveyance of inhibitory signals, resulting in NK cell dysfunction.¹²³ Conversely, in poliomyelitis, NK cells activated by IL-15 can proficiently eliminate ECs that have elevated expression of the nectin cell adhesion molecule 2 and poliovirus receptor.¹²⁴ Additionally, cytokines including IL-1β and TNF-α, which are released by CD11b+ cells, can stimulate ECs to produce CCL7 and CCL2, thereby recruiting NK cells. This procedure additionally promotes the generation of VCAM-1 and ICAM-1, which facilitates stable contacts between ECs and NK cells¹²⁵ (Figure 5d).

5.5 PCs

In the TME, abnormal interactions between pericytes and ECs can lead to tumour vascular malformation, neogenesis and dysfunction.¹²⁶ After the formation of vascular sprouts, ECs secrete hepatocyte growth factor (HB-EGF) and TGFβ1/2 to promote pericyte proliferation and recruitment.¹²⁷ Subsequently, TCs activate ECs to produce Platelet-Derived Growth Factor Subunit B (PDGF-BB), which recruits PCs expressing Platelet-Derived Growth Factor Receptor Beta (PDGFR-β). These PCs, in turn, release Ang-1 and VEGF-A to promote EC sprouting and neovascularisation. Moreover, ECs secrete ANGPT1 and ANGPT2, which effectively bind to the TIE2-R, disrupting the adhesion complex between PCs and ECs and thereby regulating angiogenesis within tumours.¹²⁸ The PDGF-BB/PDGFR-β signalling pathway is crucial to this process.¹²⁹ Studies have shown that inhibiting the expression of PDGFB reduces the recruitment of pericytes and impairs the integrity of blood vessels, thereby accelerating the process of tumour metastasis.¹³⁰ Additionally, NG2 expressed by pericytes stabilises the contact between ECs and recruited PCs via a β1 integrin-dependent manner and participates in pericyte recruitment and vascular maturation during angiogenesis by promoting collagen deposition.¹³¹ Conversely, PDGF-BB derived from tumours can further induce ECs to attract PCs through the SDF-1α/CXCR4 axis and increase the barrier coverage of tumour vessels.¹³² The high barrier coverage of PCs further protects TCs from drug attack and immune cell surveillance, correlating with unfavourable clinical prognosis.¹³³ Notably, however, the barrier function of PCs also reduces the risk of tumour invasion¹³⁴ (Figure 5e).

5.6 Neutrophils

Neutrophils, the predominant granulocytes in the bloodstream, are essential in promoting angiogenesis throughout the initial phases of cancer development, with the generation and expansion regulated by CSF3 (G-CSF).^{135, 136} CXCL chemokines guide neutrophil migration to tumours through the CXCR1 and CXCR2 receptors.^{137, 138} Similar to TAMs, neutrophils constitute a significant source of proteases and proangiogenic factors. In mouse models, STAT3 signalling activates the transcription of VEGF-A, FGF2 and MMP9, regulating the angiogenic functions of myeloid cells, including neutrophils.¹³⁹ However, IFN signalling can inhibit STAT3, limiting neutrophil production of VEGF-A and MMP9, thereby suppressing their angiogenic capabilities. Research has shown that IFN-deficient mice exhibit accelerated tumour vascularisation and increased neutrophil infiltration than wild-type mice.^{137, 140-142} Human neutrophils can release substantial amounts of VEGF under TNF stimulation.¹⁴² CSF3 enhances EC proliferation and angiogenesis by activating STAT3 to upregulate BV8 expression.^{143, 144} Consequently, blocking CSF3, CSF3R or BV8 can reduce neutrophil numbers and, in turn, inhibit angiogenesis.¹³⁵ Additionally, activated neutrophils secrete proMMP-9 devoid of tissue inhibitors of metalloproteinases, which induces angiogenesis by releasing fibroblast growth factor 2 (FGF-2) and VEGF from the ECM. Neutrophils contribute significantly more to pro-MMP9 production than TAMs^{145, 146} (Figure 5f).

5.7 Mast cells

Mast cells (MCs), as innate immune cells, influence the TME by modulating TC proliferation, angiogenesis, invasion and metastasis.¹⁴⁷ MCs produce and secrete angiogenic factors including FGF and TGF-β, along with chymase and tryptase, which influence ECs and TCs to promote their proliferation and metastasis.¹⁴⁸ Several angiogenesis inhibitors that target tryptase, such as gabexate mesilate, may become new targets in future cancer treatments.¹⁴⁹ The proangiogenic functions of MCs have been demonstrated in multiple malignancies, including skin cancer and gastric cancer.^150-154 Furthermore, a favourable association exists between microvessel density and MC density^{150, 154} (Figure 5g).

5.8 T cells

In the TME, T cells influence tumour development and treatment by modulating immune responses. They can directly recognise and attack TCs or interact with other cells to collectively regulate the immune status.¹⁵⁵ Studies indicate that TECs lacking PD-L1 show a reduced ability to induce T-cell apoptosis, ultimately suppressing tumour growth in vivo models.^{156, 157} Additionally, APLN+ TipSI cells induce T-cell dysfunction and promote Treg infiltration, forming an immune-suppressive microenvironment that supports angiogenesis. T cells can affect tumour growth via regulating angiogenesis. CD4+ Th1 cells secrete IFN-γ, which inhibits EC proliferation and hinders angiogenesis¹⁵⁸; CD4+ Th2 cells promote angiogenesis indirectly by secreting IL-4, which activates TAMs.¹⁵⁹ CD8+ cytotoxic T lymphocytes (CTLs) secrete IFN-γ to enhance the expression of angiogenesis inhibitors in TAMs and use cytotoxic actions to kill tumours and ECs, disrupting the vascular network.^{160, 161} Conversely, Treg cells may promote angiogenesis through a variety of mechanisms, such as suppressing the overconsumption of VRGFA-expressing Tregs and inhibiting IFN-γ-expressing T cells. Tregs can also inhibit other immune cells, such as CTLs, through cell contact-dependent mechanisms, indirectly promoting tumour angiogenesis.¹⁶² Overall, the connections between T cells and ECs coregulate tumour growth and metastasis, highlighting their complex roles in tumour immunity (Figure 5h).

5.9 B cells

B cells are essential adaptive immune cells and are crucial in antitumour immunity.¹⁶³ By releasing proangiogenic factors, including MMP9 and VEGF-A, B cells facilitate tumour angiogenesis and support tumour invasion and metastasis.^{164, 165} Additionally, B cells activate cytokines through antigen‒antibody complexes, further enhancing TEC-mediated angiogenesis.^166-168 However, IgG1 antibodies can inhibit angiogenesis via the FcγRI pathway.¹⁶⁹ Tumour-secreted chemokines also regulate B-cell activity, impacting angiogenesis.^{170, 171} The STAT3 signalling pathway is crucial in B-cell-mediated angiogenesis, with the HMGB1 protein in esophageal squamous cell carcinoma enhancing angiogenesis via altering the B-cell signalling balance.^{164, 172-174} In chronic lymphocytic leukaemia, B cells abnormally express CD40L, causing ECs to secrete the B-cell activation factors B-cell-activating factor and APRIL¹⁷⁵ (Figure 5i).

6.1 Strategies targeting angiogenesis

Angiogenesis, a fundamental hallmark of cancer growth and survival, is a crucial factor for tumour growth and metastasis. Currently, various strategies targeting angiogenesis within the TME have been formulated, including methods to suppress angiogenesis, disrupt vascular, normalise the vasculature and promote angiogenesis (Table 1). By combining these diverse therapeutic strategies, more comprehensive treatments for tumours may be achieved.¹⁷⁶ Here, we primarily discuss these four distinctive strategies that target angiogenesis.

6.2 Antiangiogenic immunotherapy

Antiangiogenic immunotherapy has become a prominent area of research in cancer treatment in recent years (Table 2). Its core principle lies in combining immune checkpoint inhibitors (ICIs) with antiangiogenic drugs to counteract the immunosuppressive state of the TME and promote vascular normalisation, thereby achieving synergistic antitumour effects.^{189, 190}

Vascular abnormalities contribute to the immune evasion of tumours, while the use of antiangiogenic drugs to normalise the vasculature simultaneously opens the door for T-cell infiltration into tumours. Preclinical evidence suggests that antiangiogenic immunotherapy has significant potential for enhancing cancer treatment outcomes.^191-198 The Ang2 and TIE pathways are critical therapeutic targets in antiangiogenic immunotherapy.^191-193 Studies have shown that conditional deletion of Tie1 reduces microvascular density, improves vascular normalisation, and increases intratumoural necrosis. In transgenic and transplanted tumour models, the blockade of VEGF-A and ANGPT2 by bispecific antibody (A2V) significantly enhances antitumour immunity and promotes the accumulation of CD8+ CTLs.¹⁹⁴ On the other hand, the modulation of tumour lymphatics and the induction of tertiary lymphoid structures via LIGHT–VTP substantially augmented the quantity of memory T cells and intratumoural effector T cells, increasing survival rates.¹⁹⁵ Additionally, research has shown that combined anti-PD-L1 and anti-VEGFR2 antibody therapy induced high levels of endothelial venules in mouse models, stimulating tumour immunity.¹⁹⁶ Yang's research indicates that STING regulates tumour blood vessels and has synergistic effects with ICIs and anti-VEGFR2 agents.¹⁹⁷ Interestingly, ivonescimab (A2V blockade of VEGF and PD-1) monotherapy showed significant effectiveness in patients with metastatic NSCLC.¹⁹⁸ Clinically, research has demonstrated that combined VEGF2 and PD-1 inhibitor therapy provides greater clinical benefits in HCC and RCC than monotherapy does.^199-203 In treating NSCLC and RCC, bevacizumab in combination with immunotherapy has demonstrated superior survival advantages over single-agent therapy. Recently, Deng et al. proposed a novel method using nanoparticles of gambogenic acid for the combined application of antiangiogenic and immunotherapeutic strategies.²⁰⁴ Nonetheless, numerous obstacles remain in optimising the therapeutic efficacy of these combination therapies. First, the current lack of robust predictive biomarkers hinders the precise identification of patients, necessitating urgent efforts to identify predictive biomarkers for screening potential beneficiaries. Second, determining the optimal combination regimen is critical. This includes shifting the focus from conventional anti-VEGF/R agents to other candidates (e.g., FGF/R inhibitors) and evaluating whether the combination exerts synergistic rather than only additive effects. Third, the organ-specific heterogeneity of angiogenic phenotypes complicates therapeutic interventions, as a universal approach is unlikely to suit all tumour types. Thus, in-depth analyses are required to develop organ-specific personalised strategies. Finally, variations in treatment sequencing and dosing intervals may significantly impact outcomes, underscoring the need to explore optimal therapeutic schedules.²⁰⁵

6.3 Application of nanodrug delivery systems

In recent years, the application of nanotechnology in AAT has demonstrated immense potential, with tumour-targeted nanodrug delivery systems gaining significant attention in cancer treatment because of their tumour specificity, efficient drug delivery and resistance to drug resistance mechanisms.²⁰⁶

Targeted drug delivery can be classified into ‘passive’ and ‘active’ types.²⁰⁷ ‘Passive’ targeting utilises the unique features of the tumour vasculature, improving nanodrug delivery efficiency through enhanced permeability and retention effects, which has shown promising results in preclinical models.²⁰⁸ Increased tumour vascular permeability is a key factor in the enhanced permeability and retention effect and is closely related to the degree of vascularisation. Studies have shown that, in lung injuries, nanocarriers can interact with the pulmonary endothelium via passive targeting to restore homeostasis, demonstrating considerable therapeutic activity.²⁰⁹ Additionally, inorganic nanoparticles can induce endothelial leakage, enhancing the ability of drugs to cross vascular barriers.²¹⁰ ‘Active’ targeting, on the other hand, involves ligand modifications on the nanocarriers, allowing their binding to particular sites and improving the accuracy of drug delivery.²¹¹

Combining angiogenesis inhibitors or VDAs with nanodrug technology to modulate tumour vascular normalisation has demonstrated efficacy as an approach for improving cancer therapy.^{212, 213} Chauhan et al. found in breast tumour trials that inhibiting VEGFR-2 could ‘normalise’ the tumour vasculature, enhancing the administration of tiny nanodrugs, while larger nanoparticles have difficulty penetrating the tumour.²¹⁴ Jiang et al. reported that restoring tumour vasculature enhanced the administration of medium-sized nanoparticles up to 40 nm in size.²¹⁵ Cun et al. developed multifunctional size-switchable nanoparticles that showed promising effects in vascular normalisation.²¹⁶ Additionally, inhibiting the Nogo-B receptor (NgBR) can reduce EC migration, thereby suppressing angiogenesis.²¹⁷ Jiang et al. designed jet-lagged nanoparticles combined with vascular normalisation and metabolic therapy using apatinib-loaded nanomedicines to block VEGF/VEGFR2 and effectively inhibit tumour vascular proliferation.²¹⁸ Interestingly, a DNA nanorobot loaded with thrombin and specifically delivered to the tumour vasculature can trigger intratumoural thrombosis, inducing vascular infarction and ultimately leading to tumour necrosis.²¹³ Nanodrug-based strategies demonstrate substantial potential in cancer therapy; however, they confront multiple challenges in drug release control, biocompatibility, safety evaluation and cost-effectiveness.²¹⁹ First, imprecise control over drug release often leads to off-target accumulation, elevating the risk of systemic toxicity. Second, biocompatibility challenges arise from the suboptimal biodistribution and metabolism of nanoparticles, which are swiftly eliminated by the mononuclear phagocyte system. This results in an excessive buildup in organs such as the spleen and liver, compromises drug availability at tumour sites, and triggers immune activation. Furthermore, the prolonged in vivo accumulation as well as the possible toxicity of nanoparticles remain incompletely characterised, posing safety concerns for clinical translation. Finally, the complex fabrication processes and high production costs associated with nanodrugs—involving diverse materials and intricate techniques—hinder large-scale manufacturing and broad clinical implementation. Addressing these critical barriers through innovative research is critical to realising the full therapeutic promise of nanomedicine in oncology.²²⁰

6.4 Other promising targeting strategies

Antiangiogenic strategies have also been explored for methods to influence tumour metabolism and disrupt the TME. The metabolism of ECs, as a new and promising target within the TME, is becoming a hot research direction in clinic.²²¹ Recently, Fang et al. demonstrated that focusing on fructose metabolism could serve as an effective strategies to diminish angiogenesis and inhibit tumour proliferation. Furthermore, targeting PFKFB3 or CPT1a has been shown to result in a reduction of pathological angiogenesis as well as the normalisation of tumour vasculature, signifying a prospective novel therapeutic target for cancer therapy.¹³ Angiocrine factors are associated with multiple facets of cancer progression, including proliferation, stemness, invasion, epithelial-mesenchymal transition and immunosuppression. For instance, studies have demonstrated that focal adhesion kinase in ECs acts as a pivotal regulator of chemotherapy sensitivity. Inhibition of FAK expression disrupts EC-derived paracrine signals (also termed angiocrine signals), thereby suppressing tumour growth.^{222, 223} EndMT in ECs can result in organ fibrosis and cancer development, involving the regulation of cytoskeletal components.^{70, 224} A key strategy for targeting EndMT involves the development of antibody‒drug conjugates that inhibit microtubule protein polymerisation and target TGF-β signalling, thereby delaying the progression of diseases such as tumours.^225-231 In the past few years, with the growing volume of research on tumour-derived exosomes (TDEs) targeting ECs to promote tumour metastasis, the strategy of targeting TDEs has shown broad prospects in inhibiting tumour metastasis. Olejarz et al. explored the application of exosomes in angiogenesis and suggested that controlling the exosomal composition could be a promising method to increase the long-term effectiveness of AAT.²³² Currently, it has been found that cannabidiol, Chloramidine/bisindolylmaleimide-I, and traditional Chinese medicine can inhibit the release of TDEs to impede cancer progression. However, the targeted delivery of TDEs remains a challenge.^233-236 Recently, strategies aimed at altering vascular phenotypes to optimise cancer treatment have gained rapid momentum, primarily related to the induction of high endothelial venules within tumours, which promote the extravasation of lymphocytes and correlate with improved prognoses in multiple cancers. In fibrosarcoma models, the absence of Tregs results in the emergence of high endothelial venules, enabling T-cell recruitment.^{176, 237} The glycocalyx (GCX) is essential for the interaction between ECs and TCs. Targeting endothelial GCX can inhibit tumour metastasis by regulating vascular permeability. Study shows that TCs enhance the establishment of an adhesive vascular niche by depositing GCX components along ECs, thereby facilitating their extravasation. Targeting the hyaluronic acid-CD44 GCX complex significantly reduces the extravasation of TCs, providing new evidence for developing therapeutic strategies against metastasis.²³⁸ The exploration and development of novel targets are conducive to more precisely interfering with the tumour vasculature. Zhang et al. discovered that high expression of DGKG promotes angiogenesis as well as immune evasion in HCC, suggesting that targeting endothelial DGKG may be a viable approach for precision therapy.²³⁹ A deeper comprehension of the biological properties of tumour vasculature in the future will allow for more effective cancer interventions and the formulation of novel approaches to therapy.

6.5 Resistance to AAT and countermeasures

The mechanisms of resistance to AAT have historically been a considerable challenge in cancer treatment. These resistance mechanisms are complex and diverse, involving TCs, ECs and various factors within the TME. First, tumours escape treatment pressure by activating alternative angiogenic modes, such as IA, vasculogenic mimicry and vessel co-option.^240-242 For example, Saravanan et al. found that the IA mechanism can bypass VEGF inhibition to continuously drive tumour angiogenesis.²⁴³ Teuwen et al. revealed that anti-VEGF treatment-induced vessel co-option depends on the synergistic action of invasive cancer cell subtypes and matrix-remodelling macrophages.²⁴⁴ Cannell et al. confirmed that the Foxc2-driven vasculogenic mimicry pathway is a key mechanism of resistance in gliomas and other solid tumours.²⁴⁵ Second, persistent vascular dysfunction prompts ECs to undergo metabolic reprogramming and activate compensatory signalling pathways, thereby significantly promoting drug resistance. After VEGF inhibition, ECs maintain survival through alternative signalling pathways such as Angiopoietin/Tie2, Notch, PDGF and FGF.^246-251 For example, the LRG1‒TGF-β axis can activate the BMP9/ALK1 signalling pathway in ECs.²⁵² In addition, during AAT, TCs undergo phenotypic changes. For example, the stimulation of the HGF/c-MET pathway enhances tumour invasiveness. In patients with glioblastoma, the expression of c-MET is significantly elevated after recurrence following bevacizumab therapy.^{113, 253} Meanwhile, tumour heterogeneity also leads to differences in sensitivity to antiangiogenic drugs. Finally, immune-suppressive cells (e.g., Tregs and MDSCs) weaken the effects of AAT by forming immunosuppressive niches. These mechanisms collectively lead to resistance to AAT.

Breakthroughs in overcoming resistance to AAT will rely on the coordinated advancement of multiple strategies. In terms of biomarker development, blood flow and volume may serve as a potential biomarker. For example, Chen et al. successfully predicted differences in bevacizumab response among patients (colorectal cancer liver metastasis) by quantitatively assessing vessel co-option and angiogenic features through liver CT scans, providing a model for the clinical application of imaging biomarkers.²⁵⁴ In endometrial cancer, TP53 mutations and p53 overexpression have been shown to be significantly associated with bevacizumab sensitivity.²⁵⁵ Regarding combination therapy, clinical and preclinical investigations have shown that the combination of antiangiogenic drugs with ICIs significantly enhances treatment efficacy, representing an important direction for future combination therapies. Moreover, precisely grasping the ‘therapeutic window’ is a hidden lever for improving efficacy: in a glioblastoma model, the CXCR4 inhibitor AMD3100 effectively inhibited angiogenesis only when administered early after radiotherapy to block SDF-1-mediated recruitment of vascular precursors, but was ineffective at later time points.^{256, 257} This indicates that in treatments aimed at preventing post-treatment vascular regeneration and tumour recurrence, strict adherence to the therapeutic window is necessary to ensure effective intervention. In summary, These advances not only chart a path towards overcoming AAT resistance but also lay the groundwork for personalised therapeutic paradigms.