Implications of flavonoids as potential modulators of cancer neovascularity

Abstract

Purpose

The formation of new blood vessels from previous ones, angiogenesis, is critical in tissue repair, expansion or remodeling in physiological processes and in various pathologies including cancer. Despite that, the development of anti-angiogenic drugs has great potential as the treatment of cancer faces many problems such as development of the resistance to treatment or an improperly selected therapy approach. An evaluation of predictive markers in personalized medicine could significantly improve treatment outcomes in many patients.

Methods

This comprehensive review emphasizes the anticancer potential of flavonoids mediated by their anti-angiogenic efficacy evaluated in current preclinical and clinical cancer research.

Results and conclusion

Flavonoids are important groups of phytochemicals present in common diet. Flavonoids show significant anticancer effects. The anti-angiogenic effects of flavonoids are currently a widely discussed topic of preclinical cancer research. Flavonoids are able to regulate the process of tumor angiogenesis through modulation of signaling molecules such as VEGF, MMPs, ILs, HIF or others. However, the evaluation of the anti-angiogenic potential of flavonoids within the clinical studies is not frequently discussed and is still of significant scientific interest.

Introduction

The formation of new blood vessels from previous ones, a process known as angiogenesis, occurs throughout the whole life beginning in utero and continuing to the old age in both physiologic and pathological state (Adair and Montani 2010; Qin et al. 2019). Requirements of tumor cells for nutrients, oxygen and removal of metabolites lead to an activation of angiogenesis (Bhat and Singh 2008) that is essential for the tumor progression and formation of metastases (Loizzi et al. 2017). Tumor angiogenesis, which is considered to be one of ten hallmarks of cancer (Qin et al. 2019), is associated with an imbalance between angiogenic stimulators and inhibitors in favor of excessive angiogenic signaling (Darweesh et al. 2019).

Therapeutics approved for therapy of cancer are relatively commonly associated with therapy failure (Maeda and Khatami 2018) due to the acquisition of resistance (Bueno et al. 2017). Targeting different components of tumor angiogenesis is therefore an integral part of current cancer therapy for growing number of cancer types (Qin et al. 2019; Abdollahi and Folkman 2010). The identification of predictive biomarkers for anti-angiogenic therapy is essential for the proper selection of patients to prevent toxicity or resistance to therapy, to propose alternative therapy as well as to avoid the high cost of the therapy (Pircher et al. 2011). Moreover, better clinical outcome may be associated with selective targeting considering also the complexity of tumor microenvironment involved in tumor angiogenesis. Therefore, targeting angiogenesis through novel compounds is highly required for the improvement of therapeutic efficacy (Jászai and Schmidt 2019).

Consequently, potent anti-cancer properties of phytochemicals gain more attention (Avram et al. 2017) also in tumor angiogenesis-targeted approach of cancer management (Rajasekar et al. 2019). Despite an important role of angiogenesis in cancer treatment, a growing body of evidence demonstrate an important role of angiogenesis inhibition in cancer prevention (Li et al. 2012) in which chemopreventive agents of plant origin can be easily included in the daily diet (Bhat and Singh 2008). Flavonoids, consumed at a regular base, are associated with the reduction of risk of number of diseases including cancer (Kozłowska and Szostak-Wegierek 2014). A number of clinical studies evaluated potential effects of flavonoids in cancer management or chemoprevention (Lazarevic et al. 2012; Gee et al. 2017; Ko et al. 2013; Nechuta et al. 2012). Interestingly, daily high consumption of soy isoflavones (25–50 mg) led to the reduction of breast cancer (BC) risk in Asian countries when compared to U. S. or Europe characterized by daily intake of soya lower than 8 mg (Messina et al. 2006; Horn-Ross et al. 2001).

Despite significant progress in cancer management, the treatment is constantly facing multiple challenges. In order to achieve the greatest possible success, it is highly necessary to personalize therapeutic strategy in accordance with the evaluation of predictive markers (Golubnitschaja et al. 2013, 2014, 2016; Cheng and Zhan 2017; Uramova et al. 2018). Based on this, we point out the need for the development of new selective-targeting anti-cancer molecules. As is discussed in the proposed review, flavonoids are agents with potent anticancer properties and observed to be great modulators of tumor angiogenesis functioning via regulation of multiple signaling pathways. The benefits associated with the application of flavonoids in cancer prevention represent a promising modality that, in combination with conventional therapy, defines a perspective path for future progression in the field of oncology.

Angiogenesis: a driver of cancer sprout

The growth of solid tumors is significantly affected by formation of blood vessels [3-26] (Bhat and Singh 2008; Loizzi et al. 2017; Qin et al. 2019; Darweesh et al. 2019; Maeda and Khatami 2018; Bueno et al. 2017; Abdollahi and Folkman 2010; Pircher et al. 2011; Jászai and Schmidt 2019; Avram et al. 2017; Rajasekar et al. 2019; Li et al. 2012; Kozłowska and Szostak-Wegierek 2014; Lazarevic et al. 2012; Gee et al. 2017; Ko et al. 2013; Nechuta et al. 2012; Messina et al. 2006; Horn-Ross et al. 2001; Golubnitschaja et al. 2013, 2014, 2016; Cheng and Zhan 2017; Uramova et al. 2018; Keith and Simon 2015).

While the avascular state of the tumor is defined by a balance between proliferation and apoptosis (Zuazo-Gaztelu and Casanovas 2018), the blood supply of cancer cells is a key requirement for the further tumor growth and sustaining of unlimited proliferation. The initiation of the process leading to the angiogenic state of the tumor (Bhat and Singh 2008; Zuazo-Gaztelu and Casanovas 2018) is also known as “angiogenic switch” (Loizzi et al. 2017).

Due to the rapidly proliferating tumor cells, hypoxia arises in the early process of tumor development. However, the persistent hypoxia has a positive effect on tumor growth because it provides angiogenic and metastatic stimuli (Tonini et al. 2003) with enhanced and chaotic formation of blood vessels. Hypoxia-inducible factors (HIFs), with HIF-1α being the main mediator of transcriptional response, are key regulators of adaptation of cells to hypoxia. Stabilization of HIF-1α in a response to hypoxia leads to the upregulation of pro-angiogenic factors such as VEGF, PDGF or FGF (Lugano et al. 2020). A shift toward angiogenesis can be affected not only by hypoxia, but also by metabolic or mechanical stress, genetic mutations or alterations in the expression of tumor-suppressor genes or oncogenes (Loizzi et al. 2017; Tonini et al. 2003). Moreover, epigenetic changes have also significant role in the regulation of tumor angiogenesis as was demonstrated in microRNA (miRNA) expression, such as the master hypoxia-induced miRNA-210 that is upregulated in hepatocellular carcinoma (Yang et al. 2016a, b) or miRNA-140-5p inhibiting angiogenesis through targeting VEGF in BC (Lu et al. 2017).

Pro-angiogenic stimuli activating angiogenesis in tumors are associated mainly with tumor cells. However, cells of the tumor microenvironment, such as cancer-associated fibroblasts (CAFs), immune cells or vascular-associated components also contribute to tumor angiogenesis (Zuazo-Gaztelu and Casanovas 2018).

The process of tumor angiogenesis is shown in Fig. 1 (Tonini et al. 2003; Darweesh et al. 2019; Gupta and Qin 2003; Hillen and Griffioen 2007). The growth of new capillaries from pre-existing ones involves interactions of multiple factors with pro-angiogenic or anti-angiogenic properties (Barron et al. 2017) such as VEGF family of proteins and their receptors (Katayama et al. 2019). Further pro-angiogenic factors are summarized in Table 1.

Fig. 1.

Complex process of tumor angiogenesis. ANG angiopoietin, FGF fibroblast growth factor, MMP matrix metallopeptidase, PDGF platelet-derived growth factor, VEGF, vascular endothelial growth factor. An activation of endothelial cells (growth factors-receptors binding) → local degradation of basement membrane and extracellular matrix and migration of endothelial cells (proteases, MMPs, angiopoietins, chymases, heparanases) → organization of endothelial cells into tubes → new basement membranes → lumen formation → blood flow

Table 1.

Main angiogenic factors and their receptors

Pro-angiogenic factors	Receptors	References
VEGF (VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, PIGF)	VEGFR1, VEGFR2, VEGFR3	(Loizzi et al. 2017; Teleanu et al. 2020)
PDGF (PDGF-A, PDGF-B, PDGF-C, PDGF-D)	PDGFRα, PDGFRβ	(Loizzi et al. 2017; Teleanu et al. 2020)
FGF (FGF-1, FGF-2)	FGFR1, FGFR2, FGFR3, FGFR4, integrins	(Loizzi et al. 2017; Teleanu et al. 2020)
EGF	EGFR	(Loizzi et al. 2017)
TGF	TGF receptors type I and type II	(Loizzi et al. 2017)
MMPs	LRP	(Loizzi et al. 2017)
TNF	TNFR1 and TNFR2	(Loizzi et al. 2017; Teleanu et al. 2020)
Ang (Ang1, Ang2, Ang3, Ang4)	Tie-1 and Tie-2	(Loizzi et al. 2017; Teleanu et al. 2020)
HGF	c-Met	(Teleanu et al. 2020)
HIFs (HIF-1α, HIF-1β)	Arnt	(Teleanu et al. 2020)
IGF (IGF-1, IGF-2)	IGF1R, IGF2R	(Teleanu et al. 2020)

Ang angiopoietin, Arnt Aryl hydrocarbon receptor nuclear translocator, c-Met tyrosine-protein kinase Met, EGF epidermal growth factor, EGFR epidermal growth factor receptor, HGF hepatocyte growth factor, HIF hypoxia-inducible factor, IGF insulin-like growth factor, IGF1R insulin-like growth factor 1/2 receptor, LRP lipoprotein receptor-related protein, MMP matrix metallopeptidase, PDGF platelet-derived growth factor, PDGFRα/β platelet-derived growth factor receptor alpha/beta, PIGF placental growth factor, TG, transforming growth factor, Tie-1/2 tyrosine-protein kinase receptor 1/2, TNF tumor necrosis factor, TNFR1 tumor necrosis factor receptor 1/2, VEGF vascular endothelial growth factor, VEGFR1/2/3 vascular endothelial growth factor receptor 1/2/3

On the contrary, thrombospondins, endostatin or angiostatin represent primary anti-angiogenic factors (Qin et al. 2019; Huang et al. 2015). Taking into account the complexity of cancer angiogeneesis, Fig. 2a shows a brief overview of selected factors involved in the tumor angiogenesis. Due to the current discussion of the improvements of anti-angiogenic therapy with immunotherapy and vice versa (Khan and Kerbel 2018) as well as important role of tumor microenvironment including immune cells in the tumor angiogenesis (Zuazo-Gaztelu and Casanovas 2018), Fig. 2b summarizes the involvement of immune cells of tumor microenvironment in the process of angiogenesis.

Fig. 2.

a Selected signaling involved in tumor angiogenesis (based on Qin et al. 2019; Lugano et al. 2020; Hillen and Griffioen 2007; Huang 2004; Khan and Kerbel 2018; Siamakpour-Reihani et al. 2011; Mancini and Toker 2009; Narayanan et al. 2013; Lee et al. 2014; Natori et al. 2002; Garcia and Kandel 2012; Yu et al. 2019; Tang et al. 2018; Zhao et al. 2016; Schlüter et al. 2018; Minder et al. 2015). AKT serine/threonine protein kinase B, Ang1 angiopoietin1, Ang2 angiopoietin2, ATOX copper chaperone antioxidant, bFGF basic fibroblast growth factor, Ca²⁺ calcium ion, CD31 cluster of differentiation 31, COX-2 cyclooxygenase-2, CTR1 copper transporter 1, EGF epidermal growth factor, EGFR epidermal growth factor receptor, EphA ephrin protein A, erbB receptor tyrosine kinases, ERK extracellular-signal-regulated kinase, ETS-1 protein C-Ets-1, G-CSF granulocyte colony-stimulating factor, HIF-1α hypoxia-inducible factor 1-alpha, HRE hypoxia-regulated element, HSPGs heparan sulfate proteoglycans, IL-8 interleukin-8, MAPK the mitogen-activated protein kinase, Mdm2 murine double minute 2, MEK member of MAPK, MMP matrix metallopeptidase, MMP-9 matrix metallopeptidase 9, NFAT nuclear factor of activated T-cells, NF-κB nuclear factor kappa B, PDGF platelet-derived growth factor, PDGFR platelet-derived growth factor receptor, PDK-1 pyruvate dehydrogenase kinase 1, PI3K phosphoinositide 3-kinase, PI3K phosphoinositide 3-kinase, PLCγ phospholipase Cγ, PTN pleiotrophin, RAF RAF kinases, RLIP76 oncoprotein Ral-interacting protein of 76 kDa, TGF-β transforming growth factor beta, Tie-2 tyrosine-protein kinase receptor 2, TNF-α tumor necrosis factor alpha, TSP1 thrombospondin 1, Ub ubiquitination, VEGF vascular endothelial growth factor, VEGFR-2 vascular endothelial growth factor receptor 2, vHL Hippel-Lindau protein. b Contribution of immune cells in the process of tumor angiogenesis (based on Keith and Simon 2015; Lugano et al., 2020; Ren et al. 2006). Ang1 angiopoietin-1, Ang2 angiopoietin-2, bFGF basic fibroblast growth factor, COX-2 cyclooxygenase-2, CXCL10 C-X-C Motif Chemokine Ligand 10, CXCL11 C-X-C Motif Chemokine Ligand 11, CXCL2 C-X-C Motif Chemokine Ligand 2, CXCL8 C-X-C Motif Chemokine Ligand 8, CXCL9 C-X-C Motif Chemokine Ligand 9, EGF epidermal growth factor, GM-CSF granulocyte–macrophage colony-stimulating factor, HB-EGF heparin-binding EGF-like growth factor, IL-1β interleukin 1 beta, iNOS inducible nitric oxide synthase, MCP-1 monocyte chemoattractant protein-1, MDSC myeloid-derived suppressor cell, MMP-9 matrix metallopeptidase 9, MMPs matrix metallopeptidases, TGFβ transforming growth factor beta, TNF tumor necrosis factor, TNFα tumor necrosis factor alpha, VEGF vascular endothelial growth factor, VEGFC vascular endothelial growth factor C, VEGFD vascular endothelial growth factor D. The contribution of immune cells in tumor angiogenesis: a Macrophages (tumor-associated macrophages) resemble M2 macrophages and release number of molecules that modulate angiogenesis such as VEGFs, EGF, FGF2, CXCL6, CXCL12, TNFα, MCP-1, semaphorin 4D, adrenomedullin, thymidine phosphorylase, MMPs. b Myeloid-derived suppressor cells (MDSC) regulate tumor angiogenesis primarily by secretion of MMPs; in the presence of VEGF, MDSCs secrete CCL2, CXCL8, CXCL2, IL-1β, Ang1, Ang2, GM-CSF. c STAT3 activation in neutrophils triggers angiogenic switch through signaling molecules such as VEGF, TNFα or MMPs. d Lymphocytes: T-cells promote angiogenesis by secretion of FGF-2, HB-EGF. Most prominent angiogenic factors derived from T cells including TNF, TGFβ, interferons have anti-angiogenic potential. Interferon-induced CXC family chemokines inhibit endothelial cell proliferation, promote Th1 type T cell, natural killer cells and dendritic cell infiltration (inhibition of tumor growth). CXCL9, CXCL10 and CXCL11 can directly inhibit angiogenesis (binding to CXCR3 on endothelial cells)

Additionally, other mechanisms are involved in the promotion of tumor angiogenesis including the vessel co-option, the intussusceptive microvascular growth, the glomeruloid angiogenesis or vasculogenic mimicry (Loizzi et al. 2017).

Angiogenesis: a target of cancer management and the role of phytochemicals in the regulation of tumor neovascularization

Many synthetic anti-angiogenic drugs approved by U.S. Food and Drug Administration (FDA) are used for cancer treatment. Bevacizumab, a monoclonal antibody designed against VEGF-A, was the first anti-angiogenic drug approved by FDA (Ferrara et al. 2004). Other FDA approved anti-angiogenic drugs such as tyrosine-kinase inhibitors (TKIs) (lapatinib, neratinib, pazopanib) (Ríos-Luci et al. 2020; Amiri-Kordestani et al. 2012), mTOR inhibitors (fulvestrant, palbociclib) (Herrscher et al. 2020) or growth factor inhibitors (sorafenib, axitinib) (Bronte et al. 2017; Ma et al. 2019) are known. Despite that majority of cancer types responds to anti-angiogenic therapy, rapid escape from the treatment eventually occurs. Initial efforts primarily based on VEGF-targeting agents did not produce enduring clinical outcomes (Giuliano and Pagès 2013). Due to the complex and complicated nature of tumor angiogenesis, if one signal molecule is blocked, tumor may switch to another (Huang 2004). Eventually, therapeutic resistance represents a limitation to the development of anti-cancer drugs (Giuliano and Pagès 2013). The resistance to current anti-angiogenesis agents may be a result of activation of many signaling pathways (Samuel et al. 2019a, b; Giuliano and Pagès 2013). Interestingly, an activation of FGF signaling is a mechanism proposed for an escape of tumor cells from VEGF-targeted therapies (Lugano et al. 2020). Complementary approaches inhibiting angiogenesis indirectly have been developed with an aim to abrogate acquired resistance (Bueno et al. 2017). Despite the advances in anti-angiogenic drugs, advances in progression-free or overall survival are modest, and failure in the treatment ultimately occurs in every patient with metastatic cancer (Jayson et al. 2016). Therefore, the identification of patients who benefit from anti-angiogenic therapeutic approach is even more significant (Hegde et al. 2018). An evaluation of predictive markers play an important role in the individually selected therapy (Rykala et al. 2011), monitoring of angiogenesis, response rate, definition of optimal dose, improvements in chosen therapy or alternatives in case of treatment failure and identification of resistance to treatment (Pircher et al. 2011; Sessa et al. 2008). Major biomarkers used for monitoring angiogenesis in preclinical and clinical studies include circulating angiogenic factors (VEGF, FGF-2, MMP-9, IL-8 etc.), endothelial cells-derived molecules (sVEGFR1, sVEGFR2, sTie-2, etc.), circulating proteins or peptides (endostatin etc.), tissue-based markers (phosphorylation of ERK and AKT) or other biological as well as functional markers expression (Sessa et al. 2008).

Adaptive response of cancer to anti-angiogenic drugs is closely linked to the interaction between tumor and non-cancerous cells within the tumor microenvironment. After all, improvements in patients outcome are suggested to be associated with complex approach including modulation of signaling pathways as well as dynamic changes in the microenvironment of tumor and stromal interactions (Bueno et al. 2017). Over the past 10 years, the natural compounds contained in plant-derived food, also known as phytochemicals, demonstrated complex anticancer potential with an ability to modulate various processes of cancer initiation, promotion and progression. Recently, flavonoids are constantly gaining interest as anti-cancer agents of plant origin with complex ability to modulate signaling pathways involved in processes of carcinogenesis such as angiogenesis (Mirossay et al. 2017). Moreover, the improvement of the overall anti-angiogenic and anticancer activity of synthetic agents was also associated with the use of flavonoids as was demonstrated in metformin in combination with quercetin (Sun et al. 2018a, b) or irinotecan/SN-38 with quercetin in treatment of gastric cancer (Lei et al. 2018). Consequently, natural compounds that are capable to modulate various signaling pathways exhibit a great potential in anticancer strategy developed to target different angiogenesis pathways (Bhat and Singh 2008; Varghese et al. 2020, 2018).

Flavonoids: natural inhibitors of tumor vascularization

Flavonoids are defined as the most abundant phenolics of plant food with more than four thousands of different flavonoid aglycones and glycosides (Birt and Jeffery 2013) found in fruit, vegetable, grains, barks, roots, stems, and flowers of medicinal plants (Panche et al. 2016; Abotaleb et al. 2019). In preclinical cancer models, flavonoids showed anti-angiogenic effect mediated by an inhibition of various pro-angiogenic factors and changes in the regulation of many signaling pathways including VEGF, bFGF, HIF-1, and MMPs (Mirossay et al. 2017).

An intake of flavonoids by human is estimated at 20–200 mg/day, but the intake of regular tea drinkers may reach more than 1000 mg/day. While toxicity is not associated with flavonoids consumed at normal amounts, the potential risk can be associated with extraordinary amounts of flavonoids in the form of high-potency supplements. Flavonoids are organized into different classes (flavones, flavonols, flavanones, anthocyanidins, isoflavones, flavanols or catechins) based on chemical structure and various level of oxidation, pattern of substitution of the heterocyclic pyrane ring (ring C), and individual compounds within a class differ also in the substitution of the two benzene rings (rings A and B) (Kumar and Pandey 2013; Rodríguez-García et al. 2019). Moreover, chalcones are considered to be biosynthetic precursors of flavonoids (Mirossay et al. 2017) that are together with flavanones referred as minor flavonoids (Ninomiya et al. 2013). Different classes of flavonoids with selected individual compounds as well as their food sources are shown in Table 2.

Table 2.

Classification of well-known flavonoids and their food sources

Class	Compound	Food source	References
Flavan-3-ols (catechins)	Epigallocatechin-3-gallate, catechin, epicatechin, epigallocatechin	Green tea, bananas, apples, blueberries, peaches, pears, medicinal plants	(Kozłowska and Szostak-Wegierek, 2014; Birt and Jeffery, 2013; Panche et al. 2016; Kumar and Pandey, 2013)
Flavones	Apigenin, tangeretin, luteolin, baicelein, chrysin, morusin, eupatilin, wogonin	Celery, parsley, red peppers, chamomile, mint, Ginkgo biloba, fruit skin, tomato skin, red wine, buckwheat, medicinal plants	(Birt and Jeffery, 2013; Panche et al. 2016; Kumar and Pandey, 2013; Yin et al. 2018; Jegal et al. 2016; Wang et al. 2018a, b)
Flavonols	Quercetin, kaempferol, myricetin, fisetin, rutin, morin, tamarixetin	Red onion, apples, kale, lettuce, tomatoes, grapes, berries, tea, red wine, olive oil, medicinal plants	(Birt and Jeffery, 2013; Panche et al. 2016; Kumar and Pandey, 2013; Kim, 2017)
Flavanones	Hesperitin, naringenin, eriodictyol, hesperidin, naringin, taxifolin	Citruses (oranges, lemons, grapes), medicinal plants	(Birt and Jeffery, 2013; Panche et al. 2016; Kumar and Pandey, 2013)
Anthocyanidins	Cyanidin, delphinidin, malvidin, pelargonidin, peonidin, apigenidin	Cranberries, bilberries, blackberries, raspberries, strawberries, blueberries, black currants, red grapes, merlot grapes, cherry, elderberries, nuts, medicinal plants	(Birt and Jeffery, 2013; Panche et al. 2016; Kumar and Pandey, 2013)
Isoflavonoids	Genistein, daidzein, glycitein	Soya beans, other leguminous plants, medicinal plants	(Birt and Jeffery, 2013; Panche et al. 2016; Kumar and Pandey, 2013)
Chalcones	Isoliquiritigenin, xanthohumol	Medicinal plants	(Zhao et al. 2019; Ramalingam et al. 2018; Jiang et al. 2018)

Flavonoids occur commonly as O-glycosylated substances; however, the sugar unit can also be connected directly to carbon of flavonoid skeleton. The presence of sugar-free form of flavonoid, referred as flavonoid aglycone is also possible. Glycosides of the flavonoids are relatively hydrophilic, but the aglycones are lipophilic, moreover, their methylation, prenylation or geranylation can enhance their solubility in relatively hydrophobic solvents. The prenyl or geranyl moiety may also be modified in different ways (oxidation, reduction, cyclization, etc.) (Šmejkal 2014; Brezani et al. 2018). Therefore, flavonoid glycosides are soluble in water and alcohols, but sometimes sparingly (e.g., rutin, hesperidin). Aglycones are usually well soluble in non-polar solvents. Flavonoids can be after administration into organism relatively quickly metabolized and excreted. They undergo both phase 1 and phase 2 of metabolism, including the sulphation and glucuronidation. Furthermore, they can be metabolized by microbiota of the gut (Treml and Šmejkal 2016).

Importantly, several studies of cancer management focused on inhibition of angiogenesis mediated by flavonoids. Next chapters are summarizing information about the selected substances (Figs. 3, 4), displaying the anti-angiogenic potential.

Fig. 3.

Selected dietary flavonoids

Fig. 4.

Selected flavonoids obtained from medicinal plants

Epigallocatechin-3-gallate

Epigallocatechin-3-gallate (EGCG), the most abundant flavonoid found in green tea, inhibits various signaling pathways that are associated with the initiation or progression of many cancer types. In 4T1 BC cells, EGCG significantly reduced the expression of pro-angiogenic hypoxia inducible factor-1 (HIF1α) and a glucose transporter 1 (GLUT1) that are both critical players in the regulation of glycolysis. Due to the aerobic glycolysis or the Warburg effect, cancer cells generate many glycolytic intermediates, which are crucial for cell proliferation and cancer progression. Furthermore, EGCG reduced the expression of VEGF from vehicle-treated to 20 mg kg⁻¹ in EGCG-treated Balb/c mice injected with 4T1 cells (Wei et al. 2018). Moreover, EGCG inhibited cell proliferation under hypoxia through the reduction of HIF-1α and its downstream target gene VEGF levels in human SGC7901 gastric cancer (GC) cells (Fu et al. 2019). Additionally, EGCG, silibinin and their combination downregulated VEGF and VEGFR2 as well as pro-angiogenic members of MiR-17-92 cluster (miR-18a, miR-20a and miR-92a) in human umbilical vein endothelial cells (HUVECs) co-cultured with A549 lung cancer (LC) cells (Mirzaaghaei et al. 2019). Similarly, a study focusing on EGCG prodrug (Pro-EGCG) as a novel angiogenic inhibitor demonstrated that Pro-EGCG inhibited VEGFA and HIF-α level through phosphatidylinositol-4,5-biphosphate 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR)/HIF1α pathway and reduced tumor-associated macrophages (TAM)-secreted VEGFA in AN3CA and RL95-2 endometrial cancer cells and AN3CA and RL95-2 xenografts, respectively (Wang et al. 2018a, b).

Genistein

Soybeans (Glycine max) contain many isoflavones such as genistein, daidzein, and glycitein that structurally resemble human 17β-estradiol and have many benefits for human health including anti-cancer properties (Messina and Hilakivi-Clarke 2009). Anti-angiogenic effect of genistein was mediated by the decreased level of HIF-1α with subsequent inhibition of VEGF expression in MDA-MB-231 BC cells. Genistein was bound to the FIH-1 (factor inhibiting HIF-1) binding site of HIF-1α protein suggesting that the antagonists could be a potential treatment for BC (Mukund et al. 2019). Genistein treatment also downregulated signaling pathways, which play important role in growth of cancer cells, tumor angiogenesis and invasion such as PI3K/AKT/HIF-1α and nuclear factor-kappa B (NF-κB)/cyclooxygenase-2 (COX-2) resulting in an increased reactive oxygen species (ROS), reduced proliferation and induced apoptosis in A549 LC cells (Zhang et al. 2017). Additionally, genistein and thymoquinone treatment dose-dependently decreased the mRNA level of VEGF-A in anaplastic CAL-62 (2.5 or 10 μM thymoquinone alone and 25 or 50 μM genistein alone; 18-fold decrease after 10 μM thymoquinone + 25 μM genistein) and follicular CGTH-W1 (80 μM thymoquinone alone and 25 or 50 μM genistein alone; about tenfold decrease after 80 μM thymoquinone + 25 μM genistein treatment for 72 h) cells of human thyroid cancer (Ozturk et al. 2018).

Fisetin

Fisetin is the flavonoid with inhibitory effect on multiple signaling pathways, including angiogenesis, inflammation, cell division, metastasis, and oxidative stress (Syed et al. 2016; Mukhtar et al. 2015; Murtaza et al. 2009; Lall et al. 2016). Noh et al. (2015) demonstrated that fisetin decreased 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced MMP-9 expression connected with blockage of the NF-κB activation via the PKCα/ROS/MAPK signaling pathways, which resulted in the inhibition of tumor metastasis in MCF-7 BC cells. In addition, 48 h treatment with fisetin at a concentration of 5–10 μM in human osteosarcoma (OS) U-2 cells demonstrated that fisetin significantly decreased levels of VEGF and other factors contributing to the cancer progression such as MMP-7, MMP-9, MMP-13, p-EGFR^Tyr1068, NF-ĸB, Ras or p-P38FAK (Chen et al. 2019).

Quercetin

Antitumor and anti-angiogenic effects of quercetin were assessed via inactivation of the enzyme activity of calcineurin, accompanied with the decreased protein levels of VEGF, VEGFR2, and nuclear factor of activated T cells-3c (NFATc3), and subsequent reduction in the micro-vessel density in MCF-7 xenografts (Zhao et al. 2016). Similarly, decreased level of VEGFR by targeting VEGF in a dose-dependent manner was observed in human retinoblastoma Y79 cell line (Song et al. 2017). Moreover, quercetin upregulated endogenous anti-angiogenic factor thrombospondin-1 (TSP-1) in prostate cancer (PC) PC-3 cells and mice PC-3 xenografts, resulting in an inhibition of tumor growth and angiogenesis mediated via reduction of micro-vessel density (Yang et al. 2016a). Interestingly, anticancer drug irinotecan, an inhibitor of DNA topoisomerase I, and its metabolite SN-38, in a combination with quercetin demonstrated an anti-angiogenic effect through the inhibition of VEGF and VEGFR levels and the percentage of Tie2-expressing monocytes in AGS cells and AGS xenograft mouse model of GC. Results indicated that the combinations of irinotecan/SN-38 and quercetin were more effective than treatment alone (Lei et al. 2018). Similarly, metformin in combination with Qu inhibited VEGF/PI3K/Akt signaling pathway in prostate cancer cells indicating that this combination would represent a promising therapeutic strategy (Sun et al. 2018a, b).

Apigenin

Apigenin is a flavonoid associated with beneficiary properties in cancer treatment mediated by several mechanisms, including regulation of cell cycle, apoptosis, metastasis, and angiogenesis (Kashyap et al. 2018). Apigenin decreased the expression of IL-6 and inhibited angiogenesis through the reduced VEGF expression in human esophagus cancer Eca-109 and Kyse-30 cells. Moreover, tumor growth inhibition was detected in Eca-109 nude mice xenografts (Qiu et al. 2019). An anti-angiogenic efficacy of apigenin was evaluated in human melanoma (HM) A375 cells using in vivo chorioallantoic membrane assay. Interestingly, the results of the study revealed that a relatively low concentration (30 µM) of apigenin reduced the number of capillaries inside the application area more effectively when compared with apigenin at a concentration of 60 µM. Moreover, apigenin treatment led to the complete blockage of IL-6 and IL-10 secretion in A375 cells while the secretion of TNF-ɑ was reduced by about 60% (Ghițu et al. 2019).

Luteolin

Luteolin belongs to a group of flavonoids commonly found in many fruits and green plants with positive effects on human health (Choi et al. 2016; Hirano et al. 2006; Zang et al. 2017a, b). Luteolin treatment significantly decreased secretion of VEGF and expression of Notch1 in Hs-746 T GC cells and HUVECs with subsequent inhibition of cell migration, proliferation, and vasculogenic mimicry tubes. Results suggested that the inhibition of VEGF secretion by luteolin treatment in GC was dependent on the suppression of Notch1 expression (Zang et al. 2017a, b). Moreover, luteolin significantly inhibited the migration, invasiveness, adhesiveness, and tube‐formation in highly metastatic A375 and B16‐F10 HM cells or HUVECs without cytotoxicity at sub‐IC₅₀ concentrations. Luteolin inhibited protein expression of HIF‐1α, VEGF‐A, p‐VEGFR‐2, MMP‐2, MMP‐9, and p‐Akt and suppressed the epithelial–mesenchymal transition (EMT). Results suggested that HIF‐1α/VEGF signaling‐mediated EMT and angiogenesis played a critical role in anti-metastatic effect of luteolin in HM cancer (Li et al. 2019).

Kaempferol

Kaempferol is a flavonoid with positive biological activities such as anti-inflammatory, antioxidant, cardioprotective and antitumor (Hun et al. 2017; Duan et al. 2017; Zhuang et al. 2017). Kaempferol inhibited cell growth in androgen-sensitive LNCaP PC cells in dose-dependent manner but showed limited effect on PC-3 cells and nonmalignant RWPE-1 cells. Kaempferol also reduced vasculogenic mimicry formation and invasion in PC-3 cells. Overall, kaempferol is suggested to be a therapeutic candidate for PC treatment in which the androgen signaling pathway as well as vasculogenic mimicry are involved (Da et al. 2019). Additionally, kaempferol inhibited VEGFR-2 and regulated its downstream signaling cascades such as PI3K/AKT, MEK, and ERK in VEGF-stimulated HUVECs, which was associated with its anti-angiogenic activity (Chin et al. 2018).

Myricetin

Myricetin is a flavonoid similar to quercetin with additional hydroxyl group (pyrogallol arrangement at ring A) (Kim 2017). Myricetin was found to dose-dependently down-regulate the activity of MMP-2 and -9 and suppress the mRNA levels of MMP-2 and -9 in MDA-Mb-231Br BC cells (Ci et al. 2018). In addition, myricetin and galangin (flavonoid commonly found in propolis) exhibited anti-angiogenic effects via an inhibition of VEGF by Akt/p70S6K/HIF-1α pathway in A2780/CP70 and OVCAR-3 ovarian cancer cells while p21/HIF-1α/VEGF pathways may be also involved in the anti-angiogenic potential of myricetin (Huang 2004).

Catechin

Catechin interacts with copper that is essential for angiogenesis. Because catechin is quickly metabolized, its clinical applications are limited. Some attempts to improve the bioavailability were carried out, and the conjugation of catechin with dextran (C-D) represents a potent way to improve its antineoplastic activity against neuroblastoma (NB) mediated via targeting of copper homeostasis. C-D generated ROS that was followed by depletion of copper importer CTR-1 and copper trafficking ATOX-1 protein thus disrupted the copper homeostasis leading to the inhibition of angiogenesis in endothelial cells both in vitro (HMEC-1 cells) and in vivo (human NB IMR-32 cells xenograft model in BALB/c-Fox1nu/Ausb mice) (Yee et al. 2017).

2′-Hydroxyflavanone

Another study concerning the protective effects of flavonoids in cancer treatment announced that citrus flavonoid 2′-hydroxyflavanone suppressed the expression of essential protein RLIP76 (Singhal et al. 2017) (oncoprotein Ral-interacting protein of 76 kDa), an essential mediator of angiogenesis (Lee and Goldfinger 2014), along with the inhibition of VEGF in all treated BC cells (MCF-7, MDA-MB-231, T47D). Additionally, the administration of 2′-hydroxyflavanone to triple-negative MDA-MB-231 BC xenografts in nude mice significantly inhibited tumor progression and level of angiogenesis marker, cluster of differentiation 31 (CD31) acquired by immunohistochemical analysis (Singhal et al. 2017).

Mixtures of flavonoids

Scutellaria flavonoids, including baicalin, baicalein and wogonin, exhibited potent anti-cancer effects in A549 and H1299 LC cells and A549 subcutaneous tumor model in vivo via modulation of several cancer-associated pathways including the downregulation of VEGF-A. Moreover, micro-vessel density of tumors was also inhibited by Scutellaria flavonoids (Zhao et al. 2019).

Quingdu granule (QDG) is a traditional Chinese herbal prescription prepared from eleven herbs. The main components of QDG are rutin, hesperidin or quercetin. QDG was found to inhibit migration and tube formation in HUVECs as well as downregulate NFATc3 gene expression, NFATc3 protein amount and reduced the ratio of NFATc3 nuclear translocation. Moreover, QDG inhibited release of VEGF, VEGFR2, NFATc3 and decreased micro-vessel density in BC xenograft model. Significant anti-angiogenic potential of QDG in BC in vitro as well in vivo is at least partially affected by acting on NFAT signaling that plays an important role in the regulation of tumor angiogenesis (Zhao et al. 2018).

Similarly, Murraya koenigii leaf extract, a rich source of bioactive natural compounds including flavonoids quercetin, apigenin, kaempferol, and rutin, decreased angiogenesis in in vivo BC xenograft mouse model (Noolu et al. 2016).

A recent study confirmed the content of flavonoid in the okra (Abelmoschuc esculentus (L.) Moench) seed extract (OSE). Subsequently, OSE inhibited VEGF in MCF-7 BC cells, HEPG2 hepatocellular carcinoma (HC) cells, and HeLa cervical cancer cells, significant results were observed after 48 h of treatment (Chaemsawang et al. 2019).

Other flavonoids

Recent studies evaluating anticancer efficacy of natural substances focus also on generally less abundant flavonoids. Morusin is a prenylated flavone isolated for example from root bark of Morus alba L. Morusin significantly downregulated the expression levels of VEGF and COX-2 in A549 LC cells (Yin et al. 2018). Eupatilin is another lipophilic flavone obtained from Artemisia asiatica (Jegal et al. 2016) that was observed to suppress the transcription of MMP-2 and -9 in SNU182 HC cells and to block the tube formation. Eupatilin also downregulated HIF-1α, VEGF and phosphorylated Akt expression in HUVECs (Park et al. 2018). Isoliquiritigenin is a chalcone-type compound exhibiting potent anti-angiogenic activities via remodeling of COX-2, mPGES-1 and CYP4A mediated-AA metabolism in glioma (U87 glioma cells, C6 glioma models) thus inhibiting the angiogenic Akt- FGF-2/TGF-β/VEGF signaling through ceRNA effect of miR-194-5p and lncRNA NEAT1 (Wang et al. 2019). Similarly, xanthohumol, a prenylated chalcone from Humulus lupulus L., inhibited angiogenesis in pancreatic cancer cell lines with the mechanisms mediated by blocking the activation of NF‐κB in BxPC-3, AsPC-1, MIA pAcA-2, by suppression of VEGF and IL-8 at mRNA and protein level in BxPC-3 and AsPC-1. Regarding in vivo BxPC-3 subcutaneous xenograft model, xanthohumol inhibited CD31 positive micro-vessel density, NF‐κB p65 expression, and VEGF and IL‐8 levels (Saito et al. 2018). 4′-hydroxywogonin is a potent compound found for example in Scutellaria barbata or Verbena littoralis. 4′-hydroxywogonin inhibited colorectal cancer angiogenesis by decreasing the mRNA and protein expression of VEGF-A mediated by disruption of PI3K/AKT signaling in SW620 cells (Sun et al. 2018a, b). Moreover, vitexicarpine, a flavonoid extracted from fruit of Vitex rotundifolia attenuated gene expression of VEGF and p-Akt in thioacetamide-induced HC in Sprague Dawley rats (Hassoun et al. 2017). Amentoflavone is a biflavonoid isolated besides other plants from Selaginella tamariscina that diminished angiogenesis-related proteins such as VEGF, MMP-2 and -9 in TSGH8301 human bladder cancer cell line (Chiang et al. 2019). The study evaluating protective effects of hesperidin, a flavone glycoside abundantly present in the peel and membranous part of citruses, on DEN-initiated and Fe-NTA-promoted renal carcinogenesis in Wistar rats revealed an ability of HES to downregulate VEGF and COX-2 (Siddiqi et al. 2018).

Table 3 shows overview of preclinical studies evaluating anti-angiogenic potential of flavonoids.

Table 3.

Flavonoids in cancer angiogenesis—preclinical evidence

Flavonoid	Cancer type	Study design	Mechanism of action	References
EGCG	BC	4T1 cells, Balb/c mice injected with 4T1 cells	↓ HIF-1ɑ, ↓ VEGF	(Wei et al. 2018)
	GC	SGC7901 cells	↓ HIF-1ɑ, ↓ VEGF	(Fu et al. 2019)
EGCG, silibinin	LC	HUVECs co-cultured with A549	↓ VEGF, ↓ VEGFR2, ↓ miR-17–92 cluster members	(Mirzaaghaei et al. 2019)
Pro-EGCG	EnC	AN3CA, RL95-2 cells, AN3CA, RL95-2 xenografts	↓ HIF-1ɑ, ↓ VEGFA	(Wang et al. 2018a, b)
Genistein	BC	MDA-MB-231 cells	↓ HIF-1ɑ, ↓ VEGF	(Mukund et al. 2019)
	LC	A549 cells	↓ PI3K/AKT/HIF-1α, ↓ NF-κB/COX-2	(Zhang et al. 2017)
Genistein + thymoquinone	TC	CAL-62, CGTH-W1 cells	↓ VEGF-A	(Ozturk et al. 2018)
Fisetin	BC	MCF-7 cells	↓ MMP-9, ↓ NF-κB, ↓ PKCα/ROS/MAPK	(Noh et al. 2015)
	OS	U-2 cells	↓ VEGF, ↓ MMP-7, ↓ MMP-9, ↓ MMP-13, ↓ p-EGFRTyr1068, ↓ NF-ĸB, ↓ Ras, ↓ p-ERK1/2, ↓ p-JNK, ↓ p-P38, ↓ PI3K, ↓ p-AktThr308, ↓ p-AktSer473	(Chen et al. 2019)
Quercetin	BC	MCF-7 xenografts	↓ calcineurin, ↓ VEGF, ↓VEGFR2, ↓ NFATc3, ↓ microvessel density	(Zhao et al. 2016)
	RB	Y79 cells	↓ VEGF, ↓VEGFR	(Song et al. 2017)
	PC	PC-3 cells, PC-3 xenografts	↓ TSP-1, ↓ microvessel density	(Yang et al. 2016a, b)
Quercetin + irinotecan/SN-38	GC	AGS cells, AGS xenograft mouse model	↓ VEGF, ↓ VEGFR, ↓ Tie2-expressing monocytes	(Lei et al. 2018)
Apigenin	EC	Eca-109, Kyse-30 cells, Eca-109 nude mice xenografts	↓ IL-6, ↓ VEGF	(Qiu et al. 2019)
	HM	A375 cells	↓ number of capillaries ↓ IL-6, ↓ IL-10, ↓ TNF-ɑ	(Ghițu et al. 2019)
Lutelin	GC	Hs-746 T, HUVEC cells	↓ VEGF, ↓ Notch1, ↓ vasculogenic mimicry tubes	(Zang et al. 2017b)
	HM	A375, B16‐F10, HUVEC cells	↓ HIF‐1α, ↓ VEGF‐A, ↓ p‐ VEGFR‐2, ↓ MMP‐2, ↓ MMP‐9, ↓ p‐Akt	(Li et al. 2019)
Kaempferol	PC	PC-3, LNCaP, RWPE-1 cells	↓ vasculogenic mimicry formation	(Da et al. 2012)
		HUVECs	↓ VEGFR-2	(Chin et al. 2018)
Myricetin	BC	MDA-Mb-231Br	↓ MMP-2, ↓ MMP-9	(Ci et al. 2018)
Myricetin + galangin	OC	A2780/CP70 and OVCAR-3 cells	↓ VEGF	(Huang et al. 2015)
C-D	NB	HMEC-1 cells, IMR-32 xenografts	↑ ROS, ↓ CTR-1, ↓ ATOX-1, ↓ copper homeostasis	(Yee et al. 2017)
2´-Hydroxyflavanone	BC	MCF-7, MDA-MB-231, T47D cells, MDA-MB-231 xenografts	↓ RLIP76, ↓ VEGF, ↓ CD31	(Singhal et al. 2017)
SF	LC	A549, H1299 cells A549 tumor model	↓ VEGF-A ↓ micro-vessel density	(Zhao et al. 2019)
QDG	BC	HUVEC BC xenografts	↓ NFATc3, ↓ VEGF, ↓ VEGFR2 ↓ microvessel density	(Zhao et al. 2018)
MK	BC	xenografts	↓ angiogenesis	(Noolu et al. 2016)
OSE	BC, HC, CC	MCF-7, HepG2, HeLa	↓ VEGF	(Chaemsawang et al. 2019)
Morusin	LC	A549	↓ VEGF, ↓ COX-2	(Yin et al. 2018)
Eupatilin	LiC	SNU182, HUVECs	↓ MMP-2, ↓ MMP-9 ↓ HIF-1α, ↓VEGF, ↓ pAkt	(Park et al. 2018)
Isoliquiritigenin	Glioma	U87, C6 cells	↓ Akt- FGF-2/TGF-β/VEGF	(Wang et al. 2019)
Xanthohumol	PaC	BxPC-3, AsPC-1, MIA pAcA-2	↓ NF‐κB, ↓ VEGF, ↓ IL-8, ↓ microvessel density	(Saito et al. 2018)
4-hydroxywogonin	CoC	SW620	↓ VEGF-A (PI3K/Akt disruption)	(Sun et al. 2018a, b)
Vitexicarpin	HC	Sprague Dawley rats	↓ VEGF, ↓ p-Akt	(Hassoun et al. 2017)
Amentoflavon	BlC	TSGH8301	↓ VEGF, ↓ MMP-2, ↓ MMP-9	(Chiang et al. 2019)
Hesperidin	RC	Wistar rats	↓ VEGF, ↓ COX-2	(Siddiqi et al. 2018)

Explanatory notes: ↓ decrease, reduction; ↑ increase

3βmWi-A 2,3-dihydro-3β-methoxy analogue of Wi-A, EGCG epigallocatechin-3-gallate, MK Murraya koenigii leaf extract, OSE Abelmoschuc esculentus (L.) Moench seed extract (okra), Pro-EGCG EGCG prodrug, QDG Quingdu granule, SF Scutellaria flavonoids, BC breast cancer, BlC bladder cancer, CC cervical carcinoma, CoC colorectal cancer, EC esophageal cancer, EnC endometrial cancer, GC gastric cancer, HM human melanoma, HUVECs human umbilical vein endothelial cells, LC lung cancer, LiC liver cancer, NB neuroblastoma, OC ovarian cancer, OS osteosarcoma, PaC pancreatic cancer, PC prostate cancer, RB retinoblastoma, RC renal cancer, TC thyroid cancer, AKT protein kinase B, ATOX-1 copper chaperone antioxidant, CD 31 cluster of differentiation 31, COX-2 cyclooxygenase-2, COX-2 cyclooxygenase-2, CTR-1 copper transporter 1, EGFR epidermal growth factor receptor, FGF-2 basic fibroblast growth factor, GLUT1 glucose transporter 1, HIF-1ɑ hypoxia-inducible factor 1-alpha, IL interleukin, JNK c-Jun N-terminal kinase, MMP matrix metallopeptidase, mTOR mammalian target of rapamycin, NFATc3 nuclear factor of activated T cells-3c, NF-κB nuclear factor-kappa B, p-ERK extracellular-signal-regulated kinase, PI3K phosphoinositide 3-kinase, PKCα protein kinase C alpha, RLIP76 oncoprotein Ral-interacting protein of 76 kDa, ROS reactive oxygen species, TAM tumor-associated macrophages, TGF-β transforming growth factor beta, Tie2 tyrosine-protein kinase receptor, TSP-1 thrombospondin-1, VEGFR vascular endothelial growth factor receptor

Anti-angiogenic efficacy of flavonoids in clinical studies

A number of anti-angiogenic agents has been developed and evaluated in clinical cancer research (Qin et al. 2019). Phytochemicals represent an interesting alternative in the management of cancer therapy. Phytochemicals are relatively easily accessible, they commonly show lower number of side effects, and they are able to affect multiple signaling pathways associated with cancer (Hosseini and Ghorbani 2015). Anticancer activity of natural substances is intensively evaluated in preclinical cancer research. However, only poor evidence is available for the effectiveness of natural compounds, including flavonoids, in clinical sphere.

Green tea catechins are able to modulate various carcinogenesis-related signaling pathways including angiogenesis. Polyphenon E (Pol E) is a formulation from green tea leaves primarily containing EGCG (Gee et al. 2017), but other catechins such as (−)-epicatechin, (−)-epigallocatechin, and (−)-epicatechin-3-gallate are present at lower levels (McLarty et al. 2009). Retention of health beneficiary efficacy of black and green tea is suggested to be obtained via green tea extract supplements without possible side effects connected with consumption of black or green tea beverages containing caffeine (Henning et al. 2004). The effect of short-term supplementation with the Pol E was evaluated in a phase II clinical trial conducted on 26 patients with positive biopsies and scheduled for radical prostatectomy. The results of the study revealed significant decrease of prostate-specific antigen (PSA), VEGF, and HGF in subjects after the Pol E treatment. Eventually, these results provide an evidence of a potential positive role of Pol E in the PC treatment or prevention (McLarty et al. 2009). Similarly, the preventive potential of Pol E was evaluated in phase II pilot study with 29 patients prior to bladder cancer surgery, who received the formulation containing 800 or 1200 mg of EGCG or placebo for 14 to 28 days. Results of the primary objectives of study showed tissue accumulation of EGCG in non-malignant bladder urothelium. However, an evaluation of tissue biomarkers in study participants taking the formulation showed only dose-dependent downregulation of PCNA (a marker of cellular proliferation) and clusterin, but no significant changes in VEGF or MMP-2 (Gee et al. 2017). Furthermore, the effects of Pol E on markers of angiogenesis as well as other pathways of BC progression were analyzed in phase II clinical trial. Pol E was administered during the time period between breast biopsy and surgery in women with diagnosed BC in a dose of 4 capsules daily while 19 out of 32 women completed the trial. Primary outcome of this study with Pol E treatment revealed an observed change in serum VEGF from baseline to post treatment (NCT00676793).

Chemopreventive efficacy of quercetin in a dose of 30 mg/day incorporated into a blackcurrant beverage was evaluated in a study conducted on healthy men receiving either quercetin supplementation or placebo. Quercetin administered in healthy subjects did not change the gene transcription of MMP-2 and TIMP-2 or their corresponding level of protein in plasma, but it was associated with significant decrease in TIMP-1 gene transcription and plasma protein level. However, the function of TIMP-1 in carcinogenesis is not fully clear and there is an evidence of an association between its increased level and protection against as well as support of the tumor growth. The effects of TIMP-1 may depend on its concentration in cell or ECM or tissue type (Morrow et al. 2001).

Effects of supplementation with fisetin on the inflammatory status and MMP levels was evaluated in a double-blinded, randomized placebo controlled trial conducted on 37 colorectal cancer patients undergoing chemotherapy. Reduction in plasma level of IL-8 and decrease in MMP-7 level was associated with the group of patients supplemented with fisetin (Farsad-Naeimi et al. 2018).

Figure 5 shows an overview of effects of flavonoids used as anti-angiogenic agents in preclinical and clinical cancer research.

Fig. 5.

The role of flavonoids in anti-angiogenic strategy of cancer management. 2HF 2′-hydroxyflavanone, 3βmWi-A 2,3-dihydro-3β-methoxy analogue of Wi-A, 4HW 4′-hydroxywogonin, AMT amentoflavone, API apigenin, CAT catechin, EGCG epigallocatechin-3-gallate, EU eupatilin, FIS fisetin, GEN genistein, HES hesperidin, ISL isoliquiritigenin, KAEM kaempferol, LUT luteolin, MK Murraya koenigii leaf extract, MOR morusin, MYR myricetin, OSE Abelmoschuc esculentus (L.) Moench seed extract (okra), Pol E Polyphenon E, Pro-EGCG EGCG prodrug, QDG Quingdu granule, Qu quercetin, SF Scutellaria flavonoids, SIL silibinin, TQ thymoquinone, VIT vitexicarpine, XAN xantohumol

Nanotechnology as a tool of improved bioavailability of flavonoids in the organism

An increasing interest in the utilization of phytochemicals in the management of cancer therapy is complicated by several factors. For example, flavonoids are associated with relatively rapid metabolic degradation, poor solubility in water, and also poor absorption in the gastrointestinal tract. However, some advanced methods, a nanotechnology and its utilization in formulation of advanced drug forms can improve the bioavailability of flavonoids (Aiello et al. 2019) as well as enhance the efficacy and targeted delivery of drugs and reduce side effects (Saifullah et al. 2018). Multidrug therapeutic regiments are related to the reduction of toxicity and enhancement of conventional therapeutic efficacy. Therefore, some attempts were carried out in this area. The liposomal nanocarrier system enabling co-delivery of API and 5-fluorouracil decreased cell proliferation, increased apoptotic potential and enhanced suppression of angiogenesis in preclinical model of colorectal cancer (Sen et al. 2019). Similarly, encapsulated nanoEGCG and elemental selenium nanoparticles enhanced chemopreventive effects via advantages in inhibition of cancer growth and metastasis as well as induction of apoptosis (Wang et al. 2012). Moreover, folic acid modified poly(ethylene glycol)-poly(ecaprolactone) (Fa-PEG-PCL) nano-micelles were used to encapsulate luteolin (Luteolin/Fa-PEG-PCL micelles) and enhanced its antiglioma efficacy with mechanisms involving suppression of neovascularization (Wu et al. 2019). This all shows that nanotechnology represents a promising way of improvements in the solubility, bioavailability, efficacy, and targeted delivery of flavonoids as well as the delivery of synthetic drugs (Aiello et al. 2019; Bunkar et al. 2019).

Conclusion and outlook

Anti-angiogenic approach is considered to be a key component of anticancer therapy causing suppression of tumor growth due to a deficiency of supplies by oxygen and nutrients in cancer cells. However, in some cases, such treatment does not meet all clinical expectations. Certain patients do not respond at all to anti-angiogenic therapy and others acquire the resistance. Therefore, the development of novel therapeutics able either to directly target the tumor vessels or to optimize their function is urgently needed. Flavonoids, as a large group of phenolic compounds, have been clinically documented as agents with chemopreventive potential in a variety of cancer types (Lazarevic et al. 2012; Gee et al. 2017; Ko et al. 2013; Nechuta et al. 2012).

Moreover, flavonoids have been shown to inhibit cancer angiogenesis in numerous preclinical studies (Fu et al. 2019; Mirzaaghaei et al. 2019; Wang et al. 2018a, b; Messina and Hilakivi-Clarke 2009; Mukund et al. 2019; Zhao et al. 2016; Da et al. 2019; Chin et al. 2018; Noolu et al. 2016; Chaemsawang et al. 2019; Saito et al. 2018; Hassoun et al. 2017; Wang et al. 2018a, b).

Based on this comprehensive data, flavonoids and potentially their derivatives are proposed among the most promising anti-angiogenic agents. A discovery of non-toxic and cost-effective molecules based or derived from flavonoid structure is highly imperative for the progress in the management of cancer angiogenesis. Moreover, improved bioavailability of flavonoids in human organism (e.g., using advanced drug formulations utilizing nanotechnologies) represents a big challenge for research and development of teams involving pharmacists, pharmacologists, technologists, and oncologists. For the understanding of full anti-angiogenic potential of flavonoids and their efficient clinical use, fundamental advances in clinical research are crucial.

It is already well understood that effective medicinal management in cancer diseases consists not only from high quality health service but also from healthy lifestyle patterns and well balanced diet that can halt carcinogenesis at the earliest microscopic stages. Flavonoids, which are present in many dietary sources (especially vegetables and fruits), possess an ability to affect wide spectrum of molecular mechanisms that can suppress angiogenesis and consequently the growth of these microscopic tumors. Importantly, potential inhibitors of angiogenesis with chemical structure involving flavonoid skeleton described in this review are common components of herbs, vegetables and fruits with culinary tradition (i.e., Mediterranean, Asian, Mexican cousine). Such “anti-angiogenic diet” represents practical and easily accessible clinical approach that has the potential to substantially reduce not only the risk of development of cancer, but also other civilization diseases and thus enhances the overall quality of life. It is obvious that mentioned strategy could play an important chemopreventive role in populations that recognized the real value of health-promoting dietary factors.

In summary, tumor angiogenesis represents a critical target in cancer management within clinical practice. For the substantial progress in anti-angiogenic treatment of cancer, it is necessary to understand targeted angiogenic mechanisms associated with individual characteristics with an aim to develop personalized medical approach. In this regard, effective anti-vascular treatment and chemoprevention strategies in cancer disease need intensive research focused on specific predictive biomarkers, with the aim to support an introduction of the novel generation of optimized anti-angiogenic drugs that will be chemically inspired by flavonoid compounds.

Funding

The Qatar National Library funded the publication of this article. This work is supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic under the contracts no. VEGA 1/0136/19 and the Slovak Research and Development Agency under the contract no. APVV-16-0021. This publication is the result of the project implementation: "CENTER OF EXCELLENCE FOR RESEARCH IN PERSONALIZED THERAPY (CEVYPET)", ITMS: 26220120053 supported by the Operational Programme Research and Innovation funded by the ERDF.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Research involving human participants and/or animals

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

Informed consent

Informed consent was obtained from all individual participants included in the study.