Normalization of the vasculature for treatment of cancer and other diseases

Anti-angiogenic drug discovery: lessons from the past and thoughts for the future

INTRODUCTION

Since the pioneering work of Judah Folkman, the discovery of bevacizumab has introduced the use of anti-angiogenic agents as a new modality for the treatment of cancer. Currently, hundreds of clinical trials involving anti-angiogenic agents, targeting different elements of the tumour angiogenesis pathway, are underway. However, thus far, the benefits of anti-angiogenic therapy in unselected patient populations are often marginal with harmful side effects.

AREAS COVERED

This article presents a detailed discussion of the lessons learnt from the use of bevacizumab and other VEGF pathway inhibitors in the clinical setting. Specifically, this article provides a review of the literature on anti-VEGF agents and other angiogenesis inhibitors used in pre-clinical and clinical trials for cancer treatment.

EXPERT OPINION

Future anti-angiogenic drug design centres on multiple protein targets and combinations including: growth factors, hypoxia-inducible factor and tumour endothelial cell markers unique to the tumour vasculature. Furthermore, treatment dosing, scheduling and combination with radiation and chemotherapy require further investigation, as does the potential of treating early disease, and the development of biomarkers which accurately predict response to therapy. These are essential for the future development of these drugs with individualised therapy likely to be the ultimate goal.

Endostatin gene therapy stimulates upregulation of ICAM-1 and VCAM-1 in a metastatic renal cell carcinoma model

One of the greatest challenges in urological oncology is renal cell carcinoma (RCC), which is the third leading cause of death in genitourinary cancers. RCCs are highly vascularized and respond positively to antiangiogenic therapy. Endostatin (ES) is a fragment of collagen XVIII that possesses antiangiogenic activity. In this study, we examined the potential of ES-based antiangiogenic therapy to activate tumor-associated endothelial cells in metastatic RCC (mRCC). Balb/c-bearing Renca cells were treated with NIH/3T3-LendSN or, as a control, with NIH/3T3-LXSN cells. The T-cell subsets and lymphocyte populations of tumors, mediastinal lymph nodes and the spleen were assessed by flow cytometry. The expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) was assessed by real-time PCR, flow cytometry and immunohistochemistry analysis. ES gene therapy led to an increase in the percentage of infiltrating CD4-interferon (IFN)-γ cells (P<0.05), CD8-IFN-γ cells (P<0.01) and CD49b-tumor necrosis factor-α cells (P<0.01). In addition, ES therapy caused an increase at the mRNA level of ICAM-1 (1.4-fold; P<0.01) and VCAM-1 (1.5-fold) (control vs treated group; P<0.001). Through flow cytometry, we found a significant increase in the CD34/ICAM-1 cells (8.1-fold; P<0.001) and CD34/VCAM-1 cells (1.6-fold; P<0.05). ES gene therapy induced a significant increase in both T CD4 and CD8 cells in the lymph nodes and the spleen, suggesting that ES therapy may facilitate cell survival or clonal expansion. CD49b cells were also present in increased quantities in all of these organs. In this study, we demonstrate an antitumor inflammatory effect of ES in an mRCC model, and this effect is mediated by an increase in ICAM-1 and VCAM-1 expression in tumor-associated endothelial cells.

Evolutionary dynamics of carcinogenesis and why targeted therapy does not work

All malignant cancers, whether inherited or sporadic, are fundamentally governed by Darwinian dynamics. The process of carcinogenesis requires genetic instability and highly selective local microenvironments, the combination of which promotes somatic evolution. These microenvironmental forces, specifically hypoxia, acidosis and reactive oxygen species, are not only highly selective, but are also able to induce genetic instability. As a result, malignant cancers are dynamically evolving clades of cells living in distinct microhabitats that almost certainly ensure the emergence of therapy-resistant populations. Cytotoxic cancer therapies also impose intense evolutionary selection pressures on the surviving cells and thus increase the evolutionary rate. Importantly, the principles of Darwinian dynamics also embody fundamental principles that can illuminate strategies for the successful management of cancer.

Knockdown of aberrantly expressed nuclear localized decorin attenuates tumour angiogenesis related mediators in oral cancer progression model in vitro

Background

Oral cancer accounts for roughly 3% of cancer cases in the world with about 350,000 newly reported cases annually and a 5-year survival rate of only 50%. Majority of oral cancers are squamous cell carcinomas that originate in the oral mucosal epithelial linings. We have previously shown that in human malignant squamous cells carcinoma (SCC-25) as well as in dysplastic oral keratinocytes (DOK), a small leucine-rich multifunctional proteoglycan decorin is aberrantly expressed and localized in the nucleus where it interacts with nuclear epidermal growth factor receptor (EGFR). Post-transcriptional silencing of nuclear decorin significantly reduced IL-8 and IL8-dependent migration and invasion in these dysplastic and malignant oral epithelia. The objective of this study was to further examine the effects of nuclear decorin silencing on angiogenesis and angiogenesis related mediators in this oral cancer progression cell line model.

Methods

We have used multiplex PCR, western blotting, and in vitro endothelial tube formation assay to study angiogenesis and related pathways in nuclear decorin silenced (stable knockdown) DOK and SCC-25 cells.

Results

Nuclear decorin knockdown resulted in significant down regulation of IL-8 expression, however IL-10, and TGF-β expression was not affected in either DOK or SCC25 cells as measured by multiplex RT PCR. IL-8 receptor CXCR 1 and 2 expression was slightly lower in nuclear decorin silenced cells indicating a contributing mechanism in previously shown reduced IL-8 mediated migration and invasion phenotype in these cells. IL-8 is known to induce Matrix metalloproteinase 9 (MMP9) which not only plays a role in tumour migration and invasion but also induces angiogenic switch. We found MMP9 to be significantly reduced in nuclear decorin silenced dysplastic and malignant oral epithelia. Other potent angiogenic mediators, VEGF₁₈₉ and ANG-1 were either significantly reduced or completely abrogated in these cells. Angiogenesis as measured by endothelial tube-like formations of HUVEC cells was reduced by almost 50 percent when HUVECs were incubated in the presence of conditioned medium form nuclear decorin silenced dysplastic and malignant cell lines as compared to respective controls.

Conclusions

Together these results indicate that aberrantly expressed nuclear localized decorin strongly influences angiogenic potential of dysplastic and malignant oral epithelial cells.

A flavonoid component from Docynia delavayi (Franch.) Schneid represses transplanted H22 hepatoma growth and exhibits low toxic effect on tumor-bearing mice

The fruit of Docynia delavayi (Franch.) Schneid is a kind of popular food in southwestern areas of China. Additionally, its rhizome has been long used as a folk medicine in the treatment of liver cancer by local people. Chrysin is a kind of flavonoid which induces cancer cell death in vitro. However, its anti-tumor activity in vivo and toxicological effects on the tumor-bearing animals still remain poorly understood. In this study, we obtained four flavonoids from this herb. Among them, chrysin showed the strongest cytotoxic effect on an array of cultured tumor cells. Further investigations revealed that it significantly repressed transplanted H22 ascitic hepatic tumor cell growth in vivo. Moreover, this compound displayed little toxic effects. Additionally, we demonstrated that in transplanted tumor tissues, chrysin not only activated caspase-3 and induced apoptosis, but also inhibited the production of vascular endothelial growth factor (VEGF) and suppressed angiogenesis. These data showed that chrysin exhibited prominent anti-tumor activities and low toxic effects in vivo.

Changes in tumor blood flow as measured by Dynamic Contrast-Enhanced Magnetic Resonance Imaging (DCE-MRI) may predict activity of single agent bevacizumab in recurrent epithelial ovarian (EOC) and primary peritoneal cancer (PPC) patients: an exploratory analysis of a Gynecologic Oncology Group Phase II study

OBJECTIVE

To explore feasibility of measuring tumor blood flow as marker for antiangiogenic activity using DCE-MRI (Dynamic Contrast-Enhanced Magnetic Resonance Imaging) in women with recurrent EOC/PPC treated with bevacizumab.

METHODS

In a phase II study, 62 patients with recurrent/persistent EOC/PPC were treated with bevacizumab (15 mg/kg IV q21 days) until disease progression. DCE-MRI was performed pre-cycle 1 and 4 of bevacizumab. Images were analyzed retrospectively by a single experienced blinded radiologist. Tumor and muscle contrast enhancement was measured by region of interest signal intensity within the same DCE-MRI images. Flow rates were obtained with concentration of dye as a function of time. Relative blood flow (RBF) was calculated as a ratio of average blood flow into tumor to muscle tissue. Associations between RBF and characteristics/outcomes were explored.

RESULTS

Sixty-two patients were eligible for study. Unfortunately, only 14 (23%) patients had imaging data available for analysis at baseline and 13 of those same patients (21%) had imaging data available for analysis pre-cycle 4. The RBF distribution was similar from pre-cycle 1 to 4. RBF remained stable for the majority of the cases (median change -0.21). Baseline RBF was not significantly associated with being progression-free at 6 months, microvessel density, 17 month overall survival, tumor response, or platinum sensitivity. However, increases in blood flow rates were associated with likelihood to be progression-free at 6 months.

CONCLUSION

Functional imaging of tumor blood flow is a potential research endpoint that may be explored further. Consideration should be given to timing of endpoint and standardizing the technique.

Imaging key biomarkers of tumor angiogenesis

Angiogenesis is a fundamental requirement for tumor growth and therefore it is a primary target for anti-cancer therapy. Molecular imaging of angiogenesis may provide novel opportunities for early diagnostic and for image-guided optimization and management of therapeutic regimens. Here we reviewed the advances in targeted imaging of key biomarkers of tumor angiogenesis, integrins and receptors for vascular endothelial growth factor (VEGF). Tracers for targeted imaging of these biomarkers in different imaging modalities are now reasonably well-developed and PET tracers for integrin imaging are currently in clinical trials. Molecular imaging of longitudinal responses to anti-angiogenic therapy in model tumor systems revealed a complex pattern of changes in targeted tracer accumulation in tumor, which reflects drug-induced tumor regression followed by vascular rebound. Further work will define the competitiveness of targeted imaging of key angiogenesis markers for early diagnostic and image-guided therapy.

Imaging key biomarkers of tumor angiogenesis

Angiogenesis is a fundamental requirement for tumor growth and therefore it is a primary target for anti-cancer therapy. Molecular imaging of angiogenesis may provide novel opportunities for early diagnostic and for image-guided optimization and management of therapeutic regimens. Here we reviewed the advances in targeted imaging of key biomarkers of tumor angiogenesis, integrins and receptors for vascular endothelial growth factor (VEGF). Tracers for targeted imaging of these biomarkers in different imaging modalities are now reasonably well-developed and PET tracers for integrin imaging are currently in clinical trials. Molecular imaging of longitudinal responses to anti-angiogenic therapy in model tumor systems revealed a complex pattern of changes in targeted tracer accumulation in tumor, which reflects drug-induced tumor regression followed by vascular rebound. Further work will define the competitiveness of targeted imaging of key angiogenesis markers for early diagnostic and image-guided therapy.

Diverse roles for VEGF-A in the nervous system

Vascular endothelial growth factor A (VEGF-A) is best known for its essential roles in blood vessel growth. However, evidence has emerged that VEGF-A also promotes a wide range of neuronal functions, both in vitro and in vivo, including neurogenesis, neuronal migration, neuronal survival and axon guidance. Recent studies have employed mouse models to distinguish the direct effects of VEGF on neurons from its indirect, vessel-mediated effects. Ultimately, refining our knowledge of VEGF signalling pathways in neurons should help us to understand how the current use of therapeutics targeting the VEGF pathway in cancer and eye disease might be expanded to promote neuronal health and nerve repair.

Rapid angiogenesis onset after discontinuation of sunitinib treatment of renal cell carcinoma patients

PURPOSE

To investigate the angiogenic changes in primary tumor tissue of renal cell carcinoma (RCC) patients treated with VEGF-targeted therapy.

EXPERIMENTAL DESIGN

Phase II trials of VEGF pathway-targeted therapy given before cytoreductive surgery were carried out with metastatic RCC patients with the primary tumor in situ to investigate the necessity of nephrectomy. Primary tumor tissues were obtained and assessed for angiogenesis parameters. Results were compared with similar analyses on untreated tumors.

RESULTS

Sunitinib or bevacizumab pretreatment resulted in a significant reduction of microvessel density in the primary tumor. Also, an increase in vascular pericyte coverage was found in sunitinib-pretreated tumors, consistent with efficient angiogenesis inhibition. Expression of several key regulators of angiogenesis was found to be suppressed in pretreated tissues, among which VEGFR-1 and VEGFR-2, angiopoietin-1 and angiopoietin-2 and platelet-derived growth factor-B. In addition, apoptosis in tumor and endothelial cells was induced. Interestingly, in sunitinib-pretreated tissues a dramatic increase of the number of proliferating endothelial cells was observed, which was not the case in bevacizumab-pretreated tumors. A positive correlation with the interval between halting the therapy and surgery was found, suggesting a compensatory angiogenic response caused by the discontinuation of sunitinib treatment.

CONCLUSION

This study describes, for the first time, the angiostatic response in human primary renal cancers at the tissue level upon treatment with VEGF-targeted therapy. Discontinuation of treatment with tyrosine kinase inhibitors leads to accelerated endothelial cell proliferation. The results of this study contribute important data to the ongoing discussion on the discontinuation of treatment with kinase inhibitors.

High interstitial fluid pressure is associated with low tumour penetration of diagnostic monoclonal antibodies applied for molecular imaging purposes

The human epithelial cell adhesion molecule (EpCAM) is highly expressed in a variety of clinical tumour entities. Although an antibody against EpCAM has successfully been used as an adjuvant therapy in colon cancer, this therapy has never gained wide-spread use. We have therefore investigated the possibilities and limitations for EpCAM as possible molecular imaging target using a panel of preclinical cancer models. Twelve human cancer cell lines representing six tumour entities were tested for their EpCAM expression by qPCR, flow cytometry analysis and immunocytochemistry. In addition, EpCAM expression was analyzed in vivo in xenograft models for tumours derived from these cells. Except for melanoma, all cell lines expressed EpCAM mRNA and protein when grown in vitro. Although they exhibited different mRNA levels, all cell lines showed similar EpCAM protein levels upon detection with monoclonal antibodies. When grown in vivo, the EpCAM expression was unaffected compared to in vitro except for the pancreatic carcinoma cell line 5072 which lost its EpCAM expression in vivo. Intravenously applied radio-labelled anti EpCAM MOC31 antibody was enriched in HT29 primary tumour xenografts indicating that EpCAM binding sites are accessible in vivo. However, bound antibody could only be immunohistochemically detected in the vicinity of perfused blood vessels. Investigation of the fine structure of the HT29 tumour blood vessels showed that they were immature and prone for higher fluid flux into the interstitial space. Consistent with this hypothesis, a higher interstitial fluid pressure of about 12 mbar was measured in the HT29 primary tumour via "wick-in-needle" technique which could explain the limited diffusion of the antibody into the tumour observed by immunohistochemistry.

Angiogenesis is a link between atherosclerosis and tumorigenesis: role of LOX-1

Angiogenesis is defined as the formation of new blood vessels sprouting from pre-existing vessels. It plays an important role not only in physiological situations such as embryonic vascular development and wound healing, but also in pathological conditions including atherogenesis and evolution and spread of certain tumors. Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), a receptor for oxidized low density lipoprotein (ox-LDL), is mainly expressed in endothelial cells. It has diverse physiological functions and it could be a link between atherogenesis and tumorigenesis. The risk factors for atherosclerosis like hypertension, diabetes mellitus and hyperlipidemia are associated with LOX-1. Dyslipidemia and obesity are also being recognized as risk factor for certain tumors. LOX-1 is also found to be important for maintaining the transformed state in developmentally diverse cancer cell lines and for tumor growth. There is emerging evidence that LOX-1 plays an important role in the angiogenesis process. In this review, we outline the roles of angiogenesis in atherogenesis and tumorigenesis, and describe the role of LOX-1 as a potential molecular target for blocking angiogenesis.

The expression of sonic hedgehog in diabetic wounds following treatment with poly(methacrylic acid-co-methyl methacrylate) beads

The expression of native sonic hedgehog (Shh) was significantly increased in poly(methacrylic acid-co-methyl methacrylate) bead (MAA) treated wounds at day 4 compared to both poly(methyl methacrylate) bead (PMMA) treated and untreated wounds in diabetic db/db mice. MAA beads also increased the expression of the Shh transcription factor Gli3 at day 4. Previously, topical application of MAA beads (45 mol % methacrylic acid) improved wound closure and blood vessel density in excisional wounds in these mice, while PMMA beads did not. Gene expression within the granulation tissue of healing wounds was studied to provide insight into the mechanism of vessel formation and wound healing in the presence of MAA beads. In addition to the increased expression of Shh, MAA-treated wounds had increased expression of osteopontin (OPN), IL-1β and TNF-α, (at day 7) similar to the previously reported MAA response of macrophage-like and endothelial cells in vitro.

Overcoming disappointing results with antiangiogenic therapy by targeting hypoxia

Cancer cells rely on angiogenesis to fulfil their need for oxygen and nutrients; hence, agents targeting angiogenic pathways and mediators have been investigated as potential cancer drugs. Although this strategy has demonstrated delayed tumour progression--leading to progression-free survival and overall survival benefits compared with standard therapy--in some patients, the results are more modest than predicted. A significant number of patients either do not respond to antiangiogenic agents or fairly rapidly develop resistance to them, which raises questions about how resistance develops and how it can be overcome. Furthermore, whether cancers, once they develop resistance, become more invasive or lead to metastatic disease remains unclear. Several mechanisms of resistance have been recently proposed and emerging evidence indicates that, under certain experimental conditions, antiangiogenic agents increase intratumour hypoxia by promoting vessel pruning and inhibiting neoangiogenesis. Indeed, several studies have highlighted the possibility that inhibitors of VEGF (and its receptors) can promote an invasive metastatic switch, in part by creating an increasingly hypoxic tumour microenvironment. As a potential remedy, a number of therapeutic approaches have been investigated that target the hypoxic tumour compartment to improve the clinical outcome of antiangiogenic therapy.

Antiangiogenic and anticancer molecules in cartilage

Cartilage is one of the very few naturally occurring avascular tissues where lack of angiogenesis is the guiding principle for its structure and function. This has attracted investigators who have sought to understand the biochemical basis for its avascular nature, hypothesising that it could be used in designing therapies for treating cancer and related malignancies in humans through antiangiogenic applications. Cartilage encompasses primarily a specialised extracellular matrix synthesised by chondrocytes that is both complex and unique as a result of the myriad molecules of which it is composed. Of these components, a few such as thrombospondin-1, chondromodulin-1, the type XVIII-derived endostatin, SPARC (secreted protein acidic and rich in cysteine) and the type II collagen-derived N-terminal propeptide (PIIBNP) have demonstrated antiangiogenic or antitumour properties in vitro and in vivo preclinical trials that involve several complicated mechanisms that are not completely understood. Thrombospondin-1, endostatin and the shark-cartilage-derived Neovastat preparation have also been investigated in human clinical trials to treat several different kinds of cancers, where, despite the tremendous success seen in preclinical trials, these molecules are yet to show success as anticancer agents. This review summarises the current state-of-the-art antiangiogenic characterisation of these molecules, highlights their most promising aspects and evaluates the future of these molecules in antiangiogenic applications.

Intra-tumour heterogeneity: a looking glass for cancer?

Populations of tumour cells display remarkable variability in almost every discernable phenotypic trait, including clinically important phenotypes such as ability to seed metastases and to survive therapy. This phenotypic diversity results from the integration of both genetic and non-genetic influences. Recent technological advances have improved the molecular understanding of cancers and the identification of targets for therapeutic interventions. However, it has become exceedingly apparent that the utility of profiles based on the analysis of tumours en masse is limited by intra-tumour genetic and epigenetic heterogeneity, as characteristics of the most abundant cell type might not necessarily predict the properties of mixed populations. In this Review, we discuss both genetic and non-genetic causes of phenotypic heterogeneity of tumour cells, with an emphasis on heritable phenotypes that serve as a substrate for clonal selection. We discuss the implications of intra-tumour heterogeneity in diagnostics and the development of therapeutic resistance.

Imaging tumor-stroma interactions during chemotherapy reveals contributions of the microenvironment to resistance

Little is known about the dynamics of cancer cell death in response to therapy in the tumor microenvironment. Intravital microscopy of chemotherapy-treated mouse mammary carcinomas allowed us to follow drug distribution, cell death, and tumor-stroma interactions. We observed associations between vascular leakage and response to doxorubicin, including improved response in matrix metalloproteinase-9 null mice that had increased vascular leakage. Furthermore, we observed CCR2-dependent infiltration of myeloid cells after treatment and that Ccr2 null host mice responded better to treatment with doxorubicin or cisplatin. These data show that the microenvironment contributes critically to drug response via regulation of vascular permeability and innate immune cell infiltration. Thus, live imaging can be used to gain insights into drug responses in situ.

The dynamics of developmental and tumor angiogenesis-a comparison

The blood vasculature in cancers has been the subject of intense interest during the past four decades. Since the original ideas of targeting angiogenesis to treat cancer were proposed in the 1970s, it has become evident that more knowledge about the role of vessels in tumor biology is needed to fully take advantage of such strategies. The vasculature serves the surrounding tissue in a multitude of ways that all must be taken into consideration in therapeutic manipulation. Aspects of delivery of conventional cytostatic drugs, induction of hypoxia affecting treatment by radiotherapy, changes in tumor cell metabolism, vascular leak and trafficking of leukocytes are affected by interventions on vascular function. Many tumors constitute a highly interchangeable milieu undergoing proliferation, apoptosis, and necrosis with abundance of growth factors, enzymes and metabolites. These aspects are reflected by the abnormal tortuous, leaky vascular bed with detached mural cells (pericytes). The vascular bed of tumors is known to be unstable and undergoing remodeling, but it is not until recently that this has been dynamically demonstrated at high resolution, facilitated by technical advances in intravital microscopy. In this review we discuss developmental genetic loss-of-function experiments in the light of tumor angiogenesis. We find this a valid comparison since many studies phenocopy the vasculature in development and tumors.

Semaphorin 3A overcomes cancer hypoxia and metastatic dissemination induced by antiangiogenic treatment in mice

Cancer development, progression, and metastasis are highly dependent on angiogenesis. The use of antiangiogenic drugs has been proposed as a novel strategy to interfere with tumor growth, but cancer cells respond by developing strategies to escape these treatments. In particular, animal models show that antiangiogenic drugs currently used in clinical settings reduce tumor tissue oxygenation and trigger molecular events that foster cancer resistance to therapy. Here, we show that semaphorin 3A (Sema3A) expression overcomes the proinvasive and prometastatic resistance observed upon angiogenesis reduction by the small-molecule tyrosine inhibitor sunitinib in both pancreatic neuroendocrine tumors (PNETs) in RIP-Tag2 mice and cervical carcinomas in HPV16/E2 mice. By improving cancer tissue oxygenation and extending the normalization window, Sema3A counteracted sunitinib-induced activation of HIF-1α, Met tyrosine kinase receptor, epithelial-mesenchymal transition (EMT), and other hypoxia-dependent signaling pathways. Sema3A also reduced tumor hypoxia and halted cancer dissemination induced by DC101, a specific inhibitor of the VEGF pathway. As a result, reexpressing Sema3A in cancer cells converts metastatic PNETs and cervical carcinomas into benign lesions. We therefore suggest that this strategy could be developed to safely harnesses the therapeutic potential of the antiangiogenic treatment.

Normalization of tumour blood vessels improves the delivery of nanomedicines in a size-dependent manner

The blood vessels of cancerous tumours are leaky and poorly organized. This can increase the interstitial fluid pressure inside tumours and reduce blood supply to them, which impairs drug delivery. Anti-angiogenic therapies--which 'normalize' the abnormal blood vessels in tumours by making them less leaky--have been shown to improve the delivery and effectiveness of chemotherapeutics with low molecular weights, but it remains unclear whether normalizing tumour vessels can improve the delivery of nanomedicines. Here, we show that repairing the abnormal vessels in mammary tumours, by blocking vascular endothelial growth factor receptor-2, improves the delivery of smaller nanoparticles (diameter, 12 nm) while hindering the delivery of larger nanoparticles (diameter, 125 nm). Using a mathematical model, we show that reducing the sizes of pores in the walls of vessels through normalization decreases the interstitial fluid pressure in tumours, thus allowing small nanoparticles to enter them more rapidly. However, increased steric and hydrodynamic hindrances, also associated with smaller pores, make it more difficult for large nanoparticles to enter tumours. Our results further suggest that smaller (∼12 nm) nanomedicines are ideal for cancer therapy due to their superior tumour penetration.

Semaphorin 4D cooperates with VEGF to promote angiogenesis and tumor progression

The semaphorins and plexins comprise a family of cysteine-rich proteins implicated in control of nerve growth and development and regulation of the immune response. Our group and others have found that Semaphorin 4D (SEMA4D) and its receptor, Plexin-B1, play an important role in tumor-induced angiogenesis, with some neoplasms producing SEMA4D in a manner analogous to vascular endothelial growth factor (VEGF) in order to attract Plexin-B1-expressing endothelial cells into the tumor for the purpose of promoting growth and vascularity. While anti-VEGF strategies have been the focus of most angiogenesis inhibition research, such treatment can lead to upregulation of pro-angiogenic factors that can compensate for the loss of VEGF, eventually leading to failure of therapy. Here, we demonstrate that SEMA4D cooperates with VEGF to promote angiogenesis in malignancies and can perform the same function in a setting of VEGF blockade. We also show the potential value of inhibiting SEMA4D/Plexin-B1 signaling as a complementary mechanism to anti-VEGF treatment, particularly in VEGF inhibitor-resistant tumors, suggesting that this may represent a novel treatment for some cancers.

Key Features of the Intragraft Microenvironment that Determine Long-Term Survival Following Transplantation

In this review, we discuss how changes in the intragraft microenvironment serve to promote or sustain the development of chronic allograft rejection. We propose two key elements within the microenvironment that contribute to the rejection process. The first is endothelial cell proliferation and angiogenesis that serve to create abnormal microvascular blood flow patterns as well as local tissue hypoxia, and precedes endothelial-to-mesenchymal transition. The second is the overexpression of local cytokines and growth factors that serve to sustain inflammation and, in turn, function to promote a leukocyte-induced angiogenesis reaction. Central to both events is overexpression of vascular endothelial growth factor (VEGF), which is both pro-inflammatory and pro-angiogenic, and thus drives progression of the chronic rejection microenvironment. In our discussion, we focus on how inflammation results in angiogenesis and how leukocyte-induced angiogenesis is pathological. We also discuss how VEGF is a master control factor that fosters the development of the chronic rejection microenvironment. Overall, this review provides insight into the intragraft microenvironment as an important paradigm for future direction in the field.

Towards the biomarker-guided rational use of antiangiogenic agents in the treatment of metastatic colorectal cancer

SUMMARY Clinical oncology experience with recently marketed antiangiogenic agents, which inhibit proteins important for tumor angiogenesis, has exposed significant limitations to their efficacy. Bevacizumab, a humanized neutralizing anti-VEGF-A monoclonal antibody, used in combination with cytotoxic chemotherapy for the treatment of metastatic colorectal cancer, represents the best-studied clinical example of targeted antiangiogenic therapy. In this context, bevacizumab provides modestly improved progression-free and overall survival in unselected patient populations via poorly understood mechanisms. Here we review concepts central to the identification and development of biomarkers in order to refine clinical use of bevacizumab in treating colorectal cancer and outline a phenotype-driven strategy for the discovery of high-value candidate biomarkers based on large-scale screening by molecular perturbation.

The biology of personalized cancer medicine: facing individual complexities underlying hallmark capabilities

It is a time of great promise and expectation for the applications of knowledge about mechanisms of cancer toward more effective and enduring therapies for human disease. Conceptualizations such as the hallmarks of cancer are providing an organizing principle with which to distill and rationalize the abject complexities of cancer phenotypes and genotypes across the spectrum of the human disease. A countervailing reality, however, involves the variable and often transitory responses to most mechanism-based targeted therapies, returning full circle to the complexity, arguing that the unique biology and genetics of a patient's tumor will in the future necessarily need to be incorporated into the decisions about optimal treatment strategies, the frontier of personalized cancer medicine. This perspective highlights considerations, metrics, and methods that may prove instrumental in charting the landscape of evaluating individual tumors so to better inform diagnosis, prognosis, and therapy. Integral to the consideration is remarkable heterogeneity and variability, evidently embedded in cancer cells, but likely also in the cell types composing the supportive and interactive stroma of the tumor microenvironment (e.g., leukocytes and fibroblasts), whose diversity in form, regulation, function, and abundance may prove to rival that of the cancer cells themselves. By comprehensively interrogating both parenchyma and stroma of patients' cancers with a suite of parametric tools, the promise of mechanism-based therapy may truly be realized.

Pericytes on the tumor vasculature: jekyll or hyde?

The induction of tumor vasculature, known as the 'angiogenic switch', is a rate-limiting step in tumor progression. Normal blood vessels are composed of two distinct cell types: endothelial cells which form the channel through which blood flows, and mural cells, the pericytes and smooth muscle cells which serve to support and stabilize the endothelium. Most functional studies have focused on the responses of endothelial cells to pro-angiogenic stimuli; however, there is mounting evidence that the supporting mural cells, particularly pericytes, may play key regulatory roles in both promoting vessel growth as well as terminating vessel growth to generate a mature, quiescent vasculature. Tumor vessels are characterized by numerous structural and functional abnormalities, including altered association between endothelial cells and pericytes. These dysfunctional, unstable vessels contribute to hypoxia, interstitial fluid pressure, and enhanced susceptibility to metastatic invasion. Increasing evidence points to the pericyte as a critical regulator of endothelial activation and subsequent vessel development, stability, and function. Here we discuss both the stimulatory and inhibitory effects of pericytes on the vasculature and the possible utilization of vessel normalization as a therapeutic strategy to combat cancer.

Accessories to the crime: functions of cells recruited to the tumor microenvironment

Mutationally corrupted cancer (stem) cells are the driving force of tumor development and progression. Yet, these transformed cells cannot do it alone. Assemblages of ostensibly normal tissue and bone marrow-derived (stromal) cells are recruited to constitute tumorigenic microenvironments. Most of the hallmarks of cancer are enabled and sustained to varying degrees through contributions from repertoires of stromal cell types and distinctive subcell types. Their contributory functions to hallmark capabilities are increasingly well understood, as are the reciprocal communications with neoplastic cancer cells that mediate their recruitment, activation, programming, and persistence. This enhanced understanding presents interesting new targets for anticancer therapy.

A vaccine targeting angiomotin induces an antibody response which alters tumor vessel permeability and hampers the growth of established tumors

Angiomotin (Amot) is one of several identified angiostatin receptors expressed by the endothelia of angiogenic tissues. We have shown that a DNA vaccine targeting Amot overcome immune tolerance and induce an antibody response that hampers the progression of incipient tumors. Following our observation of increased Amot expression on tumor endothelia concomitant with the progression from pre-neoplastic lesions to full-fledged carcinoma, we evaluated the effect of anti-Amot vaccination on clinically evident tumors. Electroporation of plasmid coding for the human Amot (pAmot) significantly delayed the progression both of autochthonous tumors in cancer prone BALB-neuT and PyMT genetically engineered mice and transplantable TUBO tumor in wild-type BALB/c mice. The intensity of the inhibition directly correlated with the titer of anti-Amot antibodies induced by the vaccine. Tumor inhibition was associated with an increase of vessels diameter with the formation of lacunar spaces, increase in vessel permeability, massive tumor perivascular necrosis and an effective epitope spreading that induces an immune response against other tumor associated antigens. Greater tumor vessel permeability also markedly enhances the antitumor effect of doxorubicin. These data provide a rationale for the development of novel anticancer treatments based on anti-Amot vaccination in conjunction with chemotherapy regimens.

Hypoxia control to normalize pathologic angiogenesis: potential role for endothelial precursor cells and miRNAs regulation

Tumor microenvironment is a complex and highly dynamic milieu that provides very important clues on tumor development and progression mechanisms. Tumor-associated endothelial cells play a key role in stroma organization. They achieve tumor angiogenesis, a formation of tumor-associated (angiogenic) vessels mainly through sprouting from locally preexisting vessels and/or recruitment of bone marrow-derived endothelial progenitor cells. This process participates to supply nutritional support and oxygen to the growing tumor. Endothelial cells constitute the interface between circulating blood cells, tumor cells and the extracellular matrix, thereby controlling leukocyte recruitment, tumor cell behavior and metastasis formation. Hypoxia, a critical parameter of the tumor microenvironment, controls endothelial/tumor cell interactions and is the key to tumor angiogenesis development. Under hypoxic stress, tumor cells produce factors that promote angiogenesis, vasculogenesis, tumor cell motility, metastasis and cancer stem cell selection. Targeting tumor vessels is a therapeutic strategy that has lately been fast evolving from antiangiogenesis to vessel normalization as discussed in this review. We shall focus on the pivotal role of endothelial cells within the tumor microenvironment, the specific features and the part played by circulating endothelial precursors cells. Attention is stressed on their recruitment to the tumor site and their role in tumor angiogenesis where they are submitted to miRNAs-mediated de/regulation. Here the compensation of the tumor deregulated angiogenic miRNAs - angiomiRs - is emphasized as a potential therapeutic approach. The strategy is to over express anti-angiomiRs in the tumor angiogenesis site upon selective delivery by precursor endothelial cells as miRs carriers.

Progress in tumor vascular normalization for anticancer therapy: challenges and perspectives

Antitumor angiogenic therapy has been shown promising in the treatment of several advanced cancers since the approval of the first antiangiogenic drug Avastin in 2004. Although the current antiangiogenic drugs reduce the density of tumor blood vessels and result in tumor shrinkage at the early stage of treatment, recent studies have shown that antiangiogenic therapy has transient and insufficient efficacy, resulting in tumor recurrence in patients after several months of treatment. Blockage of blood and oxygen supplies creates a hypoxic and acidic microenvironment in the tumor tissues, which fosters tumor cells to become more aggressive and metastatic. In 2001, Jain proposed tumor vascular normalization as an alternative approach to treating cancers based on the pioneering work on tumor blood vessels by several other researchers. At present, normalizing the disorganized tumor vasculature, rather than disrupting or blocking them, has emerged as a new option for anticancer therapy. Preclinical and clinical data have shown that tumor vascular normalization using monoclonal antibodies, proteins, peptides, small molecules, and pericytes resulted in decreased tumor size and reduced metastasis. However, current tumor vascular normalizing drugs display moderate anticancer efficacy. Accumulated data have shown that a variety of vasculogenic/angiogenic tumor cells and genes play important roles in tumor neovascularization, growth, and metastasis. Therefore, multiple-targeting of vasculogenic tumor cells and genes may improve the efficacy of tumor vascular normalization. To this end, the combination of antiangiogenic drugs with tumor vascular normalizing therapeutics, as well as the integration of Western medicine with traditional Chinese medicine, may provide a good opportunity for discovering novel tumor vascular normalizing drugs for an effective anticancer therapy.

CTA combined with CT perfusion for assessing the efficacy of anti-angiogenic therapy in rabbit VX2 tumors

RATIONALE AND OBJECTIVES

The aim of this study was to validate the feasibility of assessing the efficacy of antiangiogenic therapy on VX2 tumors using three-dimensional computed tomographic (CT) angiography (CTA) combined with CT perfusion.

MATERIALS AND METHODS

Forty rabbits with VX2 tumors were randomly assigned to four groups according to different doses of antiangiogenic drug, which were administered intraperitoneally daily for 14 days. In each group, 10 animals were scanned using three-dimensional CTA and CT perfusion on days 1 and 2 after the latest administration of the drug. Tumor masses were sectioned, stained by immunohistochemistry, and processed for correlation between CT imaging and histology.

RESULTS

The numbers of new tumor vessels from CTA were significantly different among the four groups (P < .001). As the dose of the drug increased, blood flow and blood volume on CT perfusion increased linearly, but the mean transit time and permeability surface-area product decreased linearly (P < .001). Immunohistochemical analyses showed that microvascular density decreased, while both luminal vascular number and mature vessel number increased linearly as the drug dose increased (P < .001). CT manifestations were correlated well with histologic findings (P < .05).

CONCLUSIONS

It is feasible to assess the efficacy of antiangiogenic therapy on VX2 tumors using three-dimensional CTA combined with CT perfusion. Three-dimensional CTA can display the morphologic changes of tumor vessels, while CT perfusion can predict the functional changes of tumor vessels after antiangiogenic therapy.

Tumor vascular microenvironment determines responsiveness to photodynamic therapy

The efficacy of photodynamic therapy (PDT) depends upon the delivery of both photosensitizing drug and oxygen. In this study, we hypothesized that local vascular microenvironment is a determinant of tumor response to PDT. Tumor vascularization and its basement membrane (collagen) were studied as a function of supplementation with basement membrane matrix (Matrigel) at the time of tumor cell inoculation. Effects on vascular composition with consequences to tumor hypoxia, photosensitizer uptake, and PDT response were measured. Matrigel-supplemented tumors developed more normalized vasculature, composed of smaller and more uniformly spaced blood vessels than their unsupplemented counterparts, but these changes did not affect tumor oxygenation or PDT-mediated direct cytotoxicity. However, PDT-induced vascular damage increased in Matrigel-supplemented tumors, following an affinity of the photosensitizer Photofrin for collagen-containing vascular basement membrane coupled with increased collagen content in these tumors. The more highly collagenated tumors showed more vascular congestion and ischemia after PDT, along with a higher probability of curative outcome that was collagen dependent. In the presence of photosensitizer-collagen localization, PDT effects on collagen were evidenced by a decrease in its association with vessels. Together, our findings show that photosensitizer localization to collagen increases vascular damage and improves treatment efficacy in tumors with greater collagen content. The vascular basement membrane is thus identified to be a determinant of therapeutic outcome in PDT of tumors.

Vascular normalization by loss of Siah2 results in increased chemotherapeutic efficacy

Tumor hypoxia is associated with resistance to antiangiogenic therapy and poor prognosis. The Siah E3 ubiquitin ligases regulate the hypoxic response pathway by modulating the turnover of the master proangiogenic transcription factor hypoxia-inducible factor-1α (Hif-1α). In this study, we show that genetic deficiency in the Siah family member Siah2 results in vascular normalization and delayed tumor growth in an established transgenic model of aggressive breast cancer. Tumors arising in a Siah2(-/-) genetic background showed increased perfusion and pericyte-associated vasculature, similar to that occurring with antiangiogenic therapy. In support of the role of Siah2 in regulating levels of Hif-1α, expression of angiogenic factors was decreased in Siah2(-/-) tumors. Blood vessel normalization in Siah2(-/-) tumors resulted in an increased response to chemotherapy and prolonged survival. Together, our findings offer a preclinical proof of concept that targeting Siah2 is sufficient to attenuate Hif-1α-mediated angiogenesis and hypoxia signaling, thereby improving responses to chemotherapy.

Abnormal tumor vasculatures and bone marrow-derived pro-angiogenic cells in cancer

Tumor-derived factors affect the stroma of cancer tissue by activating pro-angiogenic signals. One of the key components of this response is the mobilization of the pro-angiogenic cells from bone marrow (BM), which contribute to the development of abnormal tumor vasculature. Evidence is accumulating that the pro-angiogenic cells derived from BM are involved in the physiological processes of tissue repair and wound healing. However, vascular structure in cancer tissue is impaired, resulting in the formation of chaotic neo-vessels and hypoxic microenvironments. Ultimately, these structural and functional abnormalities result in the limited delivery of chemotherapeutic agents and create regions of metabolic derangement, both of which enhance resistance to chemotherapy. In spite of recent advances in targeted therapy using anti-vascular agents, clinical results from studies using individual agents have unsatisfactory, necessitating the combinatorial use of anti-cancer drugs and a targeting agent. We suggest the possibility of a new therapeutic approach in which aberrant tumor vessels are normalized by BM-derived pro-angiogenic cells, and the delivery of anti-cancer drugs is maximized. In this review, we focus on the current understanding of the structure and function of tumor vessels, and an alternative approach to the repair of abnormal tumor vasculature by the use of BM-derived pro-angiogenic cells. This approach may improve both the delivery and the efficacy of anti-cancer drugs by restoring aberrant tumor vascularization and hypoxia.

Jumping the barrier: VE-cadherin, VEGF and other angiogenic modifiers in cancer

The endothelial barrier controls the passage of fluids, nutrients and cells through the vascular wall. This physiological function is closely related to developmental and adult angiogenesis, blood pressure control, as well as immune responses. Moreover, cancer progression is frequently characterized by disorganized and leaky blood vessels. In this context, vascular permeability drives tumour-induced angiogenesis, blood flow disturbances, inflammatory cell infiltration and tumour cell extravasation. Although various molecules have been implicated, the vascular endothelial adhesion molecule, VE-cadherin (vascular endothelial cadherin), has emerged as a critical player involved in maintaining endothelial barrier integrity and homoeostasis. Indeed, VE-cadherin coordinates the endothelial cell-cell junctions through its adhesive and signalling properties. Of note, many angiogenic and inflammatory mediators released into the tumour microenvironment influence VE-cadherin behaviour. Therefore restoring VE-cadherin function could be one very promising target for vascular normalization in cancer therapies. In this review, we will mainly focus on recent discoveries concerning the molecular mechanisms involved in modulating VE-cadherin plasticity in cancer.

Glucose-independent glutamine metabolism via TCA cycling for proliferation and survival in B cells

Because MYC plays a causal role in many human cancers, including those with hypoxic and nutrient-poor tumor microenvironments, we have determined the metabolic responses of a MYC-inducible human Burkitt lymphoma model P493 cell line to aerobic and hypoxic conditions, and to glucose deprivation, using stable isotope-resolved metabolomics. Using [U-(13)C]-glucose as the tracer, both glucose consumption and lactate production were increased by MYC expression and hypoxia. Using [U-(13)C,(15)N]-glutamine as the tracer, glutamine import and metabolism through the TCA cycle persisted under hypoxia, and glutamine contributed significantly to citrate carbons. Under glucose deprivation, glutamine-derived fumarate, malate, and citrate were significantly increased. Their (13)C-labeling patterns demonstrate an alternative energy-generating glutaminolysis pathway involving a glucose-independent TCA cycle. The essential role of glutamine metabolism in cell survival and proliferation under hypoxia and glucose deficiency makes them susceptible to the glutaminase inhibitor BPTES and hence could be targeted for cancer therapy.

Targeting the metabolic microenvironment of tumors

The observation of aerobic glycolysis by tumor cells in 1924 by Otto Warburg, and subsequent innovation of imaging glucose uptake by tumors in patients with PET-CT, has incited a renewed interest in the altered metabolism of tumors. As tumors grow in situ, a fraction of it is further away from their blood supply, leading to decreased oxygen concentrations (hypoxia), which induces the hypoxia response pathways of HIF1α, mTOR, and UPR. In normal tissues, these responses mitigate hypoxic stress and induce neoangiogenesis. In tumors, these pathways are dysregulated and lead to decreased perfusion and exacerbation of hypoxia as a result of immature and chaotic blood vessels. Hypoxia selects for a glycolytic phenotype and resultant acidification of the tumor microenvironment, facilitated by upregulation of proton transporters. Acidification selects for enhanced metastatic potential and reduced drug efficacy through ion trapping. In this review, we provide a comprehensive summary of preclinical and clinical drugs under development for targeting aerobic glycolysis, acidosis, hypoxia and hypoxia response pathways. Hypoxia and acidosis can be manipulated, providing further therapeutic benefit for cancers that feature these common phenotypes.

Mechanisms of vessel regression: toward an understanding of the resolution of angiogenesis

Physiological angiogenesis refers to a naturally occurring process of blood vessel growth and regression, and it occurs as an integral component of tissue repair and regeneration. During wound healing, sprouting and branching results in an extensive yet immature and leaky neovascular network that ultimately resolves by systematic pruning of extraneous vessels to yield a stable, well-perfused vascular network ideally suited to maintain tissue homeostasis. While the molecular mechanisms of blood vessel growth have been explored in numerous cell and animal models in remarkable detail, the endogenous factors that prevent further angiogenesis and control vessel regression have not received much attention and are largely unknown. In this review, we introduce the relevant literature from various disciplines to fill the gaps in the current limited understanding of the major molecular and biomechanical inducers of vascular regression. The processes are described in the context of endothelial cell biology during wound healing: hypoxia-driven activation and sprouting followed by apoptosis or maturation of cells comprising the vasculature. We discuss and integrate the likely roles of a variety of endogenous factors, including oxygen availability, vessel perfusion and shear stress, intracellular negative feedback mechanisms (Spry2, vasohibin), soluble cytokines (CXCL10), matrix-binding proteins (TSP, PEDF), protein cleavage products (angiostatin, vasostatin), matrix-derived anti-angiogenic peptides (endostatin, arresten, canstatin, tumstatin), and the biomechanical properties of remodeling the extra-cellular matrix itself. These factors aid in the spatio-temporal control of blood vessel pruning by inducing specific anti-angiogenic signaling pathways in activated endothelial cells, pathways which compete with pro-angiogenic and maturation signals in the resolving wound. Gaining more insight into these mechanisms is bound to shed light on unresolved questions regarding scar formation, tissue regeneration, and increase our understanding of the many diseases with angiogenic phenotypes, especially cancer.

Low-dose metronomic oral dosing of a prodrug of gemcitabine (LY2334737) causes antitumor effects in the absence of inhibition of systemic vasculogenesis

Metronomic chemotherapy refers to the close, regular administration of conventional chemotherapy drugs at relatively low, minimally toxic doses, with no prolonged break periods; it is now showing encouraging results in various phase II clinical trials and is currently undergoing phase III trial evaluation. It is thought to cause antitumor effects primarily by antiangiogenic mechanisms, both locally by targeting endothelial cells of the tumor neovasculature and systemically by effects on bone marrow-derived cells, including circulating endothelial progenitor cells (CEP). Previous studies have shown reduction of CEPs by metronomic administration of a number of different chemotherapeutic drugs, including vinblastine, cyclophosphamide, paclitaxel, topotecan, and tegafur plus uracil (UFT). However in addition to, or even instead of, antiangiogenic effects, metronomic chemotherapy may cause suppression of tumor growth by other mechanisms such as stimulating cytotoxic T-cell responses or by direct antitumor effects. Here we report results evaluating the properties of metronomic administration of an oral prodrug of gemcitabine LY2334737 in nontumor-bearing mice and in preclinical models of human ovarian (SKOV3-13) and breast cancer (LM2-4) xenografts. Through daily gavage (at 6 mg/kg/d), the schedules tested were devoid of toxicity and caused antitumor effects; however, a suppressive effect on CEPs was not detected. Unexpectedly, metronomic LY2334737 administration caused increased blood flow in luciferase-tagged LM2-4 tumor xenografts, and this effect, readily measured using contrast micro-ultrasound, coincided with a relative increase in tumor bioluminescence. These results highlight the possibility of significant antitumor effects mediated by metronomic administration of some chemotherapy drugs without a concomitant inhibition of systemic angiogenesis.

Increased survival of glioblastoma patients who respond to antiangiogenic therapy with elevated blood perfusion

The abnormal vasculature of the tumor microenvironment supports progression and resistance to treatment. Judicious application of antiangiogenic therapy may normalize the structure and function of the tumor vasculature, promoting improved blood perfusion. However, direct clinical evidence is lacking for improvements in blood perfusion after antiangiogenic therapy. In this study, we used MRI to assess tumor blood perfusion in 30 recurrent glioblastoma patients who were undergoing treatment with cediranib, a pan-VEGF receptor tyrosine kinase inhibitor. Tumor blood perfusion increased durably for more than 1 month in 7 of 30 patients, in whom it was associated with longer survival. Together, our findings offer direct clinical evidence in support of the hypothesis that vascular normalization can increase tumor perfusion and help improve patient survival.

Focal adhesion kinase and tumour angiogenesis

Angiogenesis, the formation of new blood vessels from pre-existing ones, is essential for tumour development. It is initiated and regulated by growth factors via their surface receptors, which activate several intracellular signalling pathways in endothelial cells. Cell adhesion molecules, such as integrins, also regulate angiogenesis. Despite these facts, inhibitors of endothelial cell growth factor receptors or integrins have not been as effective as initially hoped in the long-term inhibition of angiogenesis in cancer patients. Signalling downstream of growth factor receptors and integrins converge on the ubiquitously expressed non-receptor tyrosine kinase focal adhesion kinase (FAK). FAK is involved in endothelial cell proliferation, migration and survival, is up-regulated in many cancers and has recently been shown to control tumour angiogenesis. Indeed, FAK inhibitors are presently being developed for the treatment of cancer. However, recent studies have indicated the complexities of understanding the precise role for FAK in angiogenesis. Here we have summarized some of the key features of FAK, addressed some of the apparently contradictory roles of this molecule in angiogenesis and provided some perspectives for future studies.

Pharmacokinetics and pharmacodynamics of VEGF-neutralizing antibodies

Background

Vascular endothelial growth factor (VEGF) is a potent regulator of angiogenesis, and its role in cancer biology has been widely studied. Many cancer therapies target angiogenesis, with a focus being on VEGF-mediated signaling such as antibodies to VEGF. However, it is difficult to predict the effects of VEGF-neutralizing agents. We have developed a whole-body model of VEGF kinetics and transport under pathological conditions (in the presence of breast tumor). The model includes two major VEGF isoforms VEGF₁₂₁ and VEGF₁₆₅, receptors VEGFR1, VEGFR2 and co-receptors Neuropilin-1 and Neuropilin-2. We have added receptors on parenchymal cells (muscle fibers and tumor cells), and incorporated experimental data for the cell surface density of receptors on the endothelial cells, myocytes, and tumor cells. The model is applied to investigate the action of VEGF-neutralizing agents (called "anti-VEGF") in the treatment of cancer.

Results

Through a sensitivity study, we examine how model parameters influence the level of free VEGF in the tumor, a measure of the response to VEGF-neutralizing drugs. We investigate the effects of systemic properties such as microvascular permeability and lymphatic flow, and of drug characteristics such as the clearance rate and binding affinity. We predict that increasing microvascular permeability in the tumor above 10^-5 cm/s elicits the undesired effect of increasing tumor interstitial VEGF concentration beyond even the baseline level. We also examine the impact of the tumor microenvironment, including receptor expression and internalization, as well as VEGF secretion. We find that following anti-VEGF treatment, the concentration of free VEGF in the tumor can vary between 7 and 233 pM, with a dependence on both the density of VEGF receptors and co-receptors and the rate of neuropilin internalization on tumor cells. Finally, we predict that free VEGF in the tumor is reduced following anti-VEGF treatment when VEGF₁₂₁ comprises at least 25% of the VEGF secreted by tumor cells.

Conclusions

This study explores the optimal drug characteristics required for an anti-VEGF agent to have a therapeutic effect and the tumor-specific properties that influence the response to therapy. Our model provides a framework for investigating the use of VEGF-neutralizing drugs for personalized medicine treatment strategies.

Basic and therapeutic aspects of angiogenesis

Blood vessels form extensive networks that nurture all tissues in the body. Abnormal vessel growth and function are hallmarks of cancer and ischemic and inflammatory diseases, and they contribute to disease progression. Therapeutic approaches to block vascular supply have reached the clinic, but limited efficacy and resistance pose unresolved challenges. Recent insights establish how endothelial cells communicate with each other and with their environment to form a branched vascular network. The emerging principles of vascular growth provide exciting new perspectives, the translation of which might overcome the current limitations of pro- and antiangiogenic medicine.

Microvessel Density Is Associated with VEGF and α-SMA Expression in Different Regions of Human Gastrointestinal Carcinomas

Tumor angiogenesis is known to be regulated by growth factors secreted by host and tumor cells. Despite the importance of tumor vasculature and angiogenic heterogeneity in solid tumors, few studies have compared the vasculature in different regions of human cancer. Blood vessels from different regions of carcinomas might have morphofunctional implications in tumor angiogenesis. In the present study, therefore, we have examined the relationship between microvascular density (MVD) and vascular endothelial growth factor (VEGF) expression and alpha smooth muscle actin (α-SMA) expression in the center of the tumor (CT), periphery (P) and metastasis (M) regions from gastrointestinal carcinomas (GITC), as well as the association of MVD with clinicopathological factors. Surgically resected specimens corresponding to the CT, P and M from 27 patients were examined for FVIII, VEGF and α-SMA by immunohistochemistry. The MVD was not significantly different in the CT, P and M regions from GITC. The MVD in the VEGF positive group was significantly higher than in the VEGF negative group (CT, p = 0.034; P, p = 0.030; M, p = 0.032). The MVD as a function of α-SMA expression was also significantly higher in the CT and P region compared to the M region (p = 0.0008). In conclusion, the MVD association with VEGF and α-SMA expression, might indicate an increase of the number of neoformed and preexisting blood vessels uniformly or partially covered by pericytes in different regions of GITC, suggesting that not only MVD and VEGF are important parameters to the tumor vasculature, but also blood vessels maturation is a crucial factor for gastrointestinal tumor angiogenesis regulation and possible target of vascular therapy.