VEGF-targeted therapy: mechanisms of anti-tumour activity

The effects of vandetanib on paclitaxel tumor distribution and antitumor activity in a xenograft model of human ovarian carcinoma

This study was designed to determine the effects of vandetanib, a small-molecule receptor tyrosine kinase inhibitor of vascular endothelial growth factor and epidermal growth factor receptor, on paclitaxel (PTX) tumor distribution and antitumor activity in xenograft models of human ovarian carcinoma. Nude mice bearing A2780-1A9 xenografts received daily (5, 10, or 15 days) doses of vandetanib (50 mg/kg per os), combined with PTX (20 mg/kg intravenously). Morphologic and functional modifications associated with the tumor vasculature (CD31 and alpha-smooth muscle actin staining and Hoechst 33342 perfusion) and PTX concentrations in plasma and tumor tissues were analyzed. Activity was evaluated as inhibition of tumor growth subcutaneously and spreading into the peritoneal cavity. Vandetanib treatment produced no significant change in tumor vessel density, although a reduced number of large vessels, an increased percentage of mature vessels, and diminished tumor perfusion were evident. Pretreatment with vandetanib led to decreased tumor PTX levels within 1 hour of PTX injection, although 24 hours later, tumor PTX levels were comparable with controls. In efficacy studies, the combination of vandetanib plus PTX improved antitumor activity compared with vandetanib or PTX alone, with greater effects being obtained when PTX was administered before vandetanib. The combination of PTX plus vandetanib reduced tumor burden in the peritoneal cavity of mice and significantly increased their survival. Analysis of vascular changes and PTX tumor uptake in vandetanib-treated tumors may help to guide the scheduling of vandetanib plus PTX combinations and may have implications for the design of clinical trials with these drugs.

Promising molecular targets in ovarian cancer

PURPOSE OF REVIEW

Clinically and on a molecular level, ovarian cancer is a unique and complex disease. The explosion in potential molecular targets over the last decade has led to the arrival of many novel therapies into oncology. In the present article, we review the most promising of these agents in ovarian cancer.

RECENT FINDINGS

Targeted therapies, such as epidermal growth factor receptor inhibitors, that have worked well in other cancers have shown only moderate success in ovarian cancer, whereas other treatment approaches have yielded surprisingly positive outcomes. An example is anti-vascular endothelial growth factor and proapoptotic strategies, which are effective in both primary and relapsed ovarian cancer. Use of poly (ADP-ribose)-polymerase inhibitors has shown that targeting one form of DNA repair profoundly affects cell survival in those with a hereditary failure to mend DNA damage using another mechanism. This can be extrapolated to patients with sporadic ovarian cancers, with or without the 'BRCAness' phenotype.

SUMMARY

Using targeted agents in ovarian cancer, we are discovering not only how these novel therapies work but are also unveiling the complex 'wiring' of the disease itself, and the interconnections between what were previously believed to be distinct molecular pathways. The addition of targeted agents to our therapeutic armoury is likely to significantly and positively impact on patient survival.

Strategies for targeting tumors and tumor vasculature for cancer therapy

Effective cancer therapy remains a challenge despite recent advances in the identification of novel targets. A major limitation of most chemotherapeutic drugs is their systemic toxicity and the efficacy of cancer treatments is, by and large, determined by the ability to balance their benefits against their toxicity. Targeted treatments for cancer, especially those that target the tumor vasculature, have provided promising antitumor results with minimal systemic toxicity. To date significant progress has been made in developing a variety of delivery systems to target cancer and its vasculature ranging from isolated limb and organ perfusion to tumor targeted biological and nonbiological vectors.

Oxygen sensing: a common crossroad in cancer and neurodegeneration

Prolyl hydroxylase domain (PHD) proteins are cellular oxygen sensors that orchestrate an adaptive response to hypoxia and oxidative stress, executed by hypoxia-inducible factors (HIFs). By increasing oxygen supply, reducing oxygen consumption, and reprogramming metabolism, the PHD/HIF pathway confers tolerance towards hypoxic and oxidative stress. This review discusses the involvement of the PHD/HIF response in two, at first sight, entirely distinct pathologies with opposite outcome, i.e. cancer leading to cellular growth and neurodegeneration resulting in cell death. However, these disorders share common mechanisms of sensing oxygen and oxidative stress. We will focus on how PHD/HIF signaling is pathogenetically implicated in metabolic and vessel alterations in these diseases and how manipulation of this pathway might offer novel treatment opportunities.

ERK1/2-dependent vascular endothelial growth factor signaling sustains cyst growth in polycystin-2 defective mice

BACKGROUND & AIMS

Severe polycystic liver disease can complicate adult dominant polycystic kidney disease, a genetic disease caused by defects in polycystin-1 (Pkd1) or polycystin-2 (Pkd2). Liver cyst epithelial cells (LCECs) express vascular endothelial growth factor (VEGF) and its receptor, VEGFR-2. We investigated the effects of VEGF on liver cyst growth and autocrine VEGF signaling in mice with Pkd1 and Pkd2 conditional knockouts.

METHODS

We studied mice in which Pkd1 or Pkd2 were conditionally inactivated following exposure to tamoxifen; these mice were called Pkd1(flox/-):pCxCreER (Pkd1KO) and Pkd2(flox/-):pCxCreER (Pkd2KO).

RESULTS

Pkd1KO and Pkd2KO mice developed liver defects; their LCECs expressed VEGF, VEGFR-2, hypoxia-inducible factor (HIF)-1alpha, phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2), and proliferating cell nuclear antigen (PCNA). In Pkd2KO but not Pkd1KO mice, exposure to the VEGFR-2 inhibitor SU5416 significantly reduced liver cyst development, liver/body weight ratio, and expression of pERK and PCNA. VEGF secretion and phosphorylation of ERK1/2 and VEGFR-2 were significantly increased in cultured LCECs from Pkd2KO compared with Pkd1KO mice. Inhibition of protein kinase A (PKA) reduced VEGF secretion and pERK1/2 expression. Addition of VEGF to LCECs from Pkd2KO mice increased phosphorylated VEGFR-2 and phosphorylated mitogen signal-regulated kinase (MEK) expression and induced phosphorylation of ERK1/2; this was inhibited by SU5416. Expression of HIF-1alpha increased in parallel with secretion of VEGF following LCEC stimulation. VEGF-induced cell proliferation was inhibited by the MEK inhibitor U1026 and by ERK1/2 small interfering RNA.

CONCLUSIONS

The PKA-ERK1/2-VEGF signaling pathway promotes growth of liver cysts in mice. In Pkd2-defective LCECs, PKA-dependent ERK1/2 signaling controls HIF-1alpha-dependent VEGF secretion and VEGFR-2 signaling. Autocrine and paracrine VEGF signaling promotes the growth of liver cysts in Pkd2KO mice. VEGF inhibitors might be used to treat patients with polycystic liver disease.

Molecular-targeted therapy for cancer and nanotechnology

In order to stop malignant tumor growth, >90% of a critical biochemical pathway needs to be blocked. Due to extraordinary advances in molecular biology, there is an increased understanding of rationale and relevant molecular targets in cancer. However, due to the heterogeneity of the molecular abnormalities in multiple tumor types, strategies designed to interfere with multiple molecular abnormalities will be necessary to impact survival. Nanoparticles have the potential to provide therapies not possible with other drug modalities. Researchers and clinicians must take advantage of these opportunities in order for nanotechnology to make an impact in the diagnosis and treatment of malignancy. A discussion of relevant targets either on the cell surface or the cytoplasm and strategies to achieve optimal drug targeting are the focus of this chapter.

Forkhead box M1 transcription factor: a novel target for cancer therapy

FoxM1 signaling has been reported to be associated with carcinogenesis. Therefore, the FoxM1 may represent a novel therapeutic target, and thus the development of agents that will target FoxM1 is likely to have significant therapeutic impact on human cancer. This review describes the mechanisms of signal transduction associated with FoxM1 and provides emerging evidence in support of its role in the carcinogenesis. Further, we summarize data on several FoxM1 inhibitors especially "chemopreventive agents" and these agents could be useful for targeted inactivation of FoxM1, which indeed could become a novel approach for the prevention and/or treatment of human cancer.

Inhibition of Vascular Endothelial Growth Factor Receptor-1/Wnt/{beta}-catenin Crosstalk Leads to Tumor Cell Death

Two genes are considered synthetic lethal if mutation of either alone allows cell viability, whereas simultaneous mutation leads to cell death. A synthetic lethal screen unveiled the dependency of Wnt/beta-catenin-addicted colorectal cancer cells on vascular endothelial growth factor receptor-1 kinase activity and suggested a novel therapeutic approach for this malignancy. (Clin Cancer Res 2009;15(24):7453-5).

Resistance to Targeted Therapies: Refining Anticancer Therapy in the Era of Molecular Oncology

The advent of targeted therapy for treatment of human cancers has added significantly to our armamentarium as we strive to prolong patient survival while minimizing toxicity. In cancers driven by a dominant oncogene, targeted therapies have led to remarkable improvements in response and survival, whereas in others the outcome has been more modest. One key aspect toward realizing the potential of targeted therapies is a better understanding of the intrinsic or acquired resistance mechanisms that limit their efficacy. The articles in this CCR Focus provide insights into molecular mechanisms of resistance to targeted therapy. Recent discoveries of the molecular pathways that mediate intrinsic resistance to targeted therapy have led to the identification of predictive biomarkers that allow for better patient selection for front line treatment. Equally important, the identification of mechanisms of acquired resistance following front line therapy has led to the discovery of novel agents that overcome these resistance mechanisms. Improving the efficacy of targeted therapies in the future will require expanding our understanding of resistance mechanisms, the development of new generations of rationally designed targeted agents, and translating this information to the clinic to select patients for appropriate therapy. (Clin Cancer Res 2009;15(24):7471-8).

FOXO transcription factors and VEGF neutralizing antibody enhance antiangiogenic effects of resveratrol

Resveratrol (trans-3,5,4'-trihydroxystilbene), a compound found largely in the skins of red grapes and wines, possesses anti-cancer and anti-angiogenic properties and protects the cardiovascular system. However, the molecular mechanisms by which resveratrol inhibits angiogenesis are currently subjects of intense investigation. The purpose of this study was to examine whether FOXO transcription factors mediate anti-angiogenic effects of resveratrol, and whether vascular endothelial growth factor (VEGF) neutralizing antibody can enhance these effects of resveratrol. Inhibition of PI3 kinase (PI3K)/AKT and MEK/ERK pathways synergistically inhibited migration and capillary tube formation of Human Umbilical Vein Endothelial Cells (HUVECs) and further enhanced the anti-angiogenic effects of resveratrol. Inhibitors of AKT and MEK kinase synergistically inhibited cytoplasmic FOXO3a phosphorylation, which was accompanied by its nuclear translocation in HUVECs. Interestingly, inhibition of PI3K/AKT and MEK/ERK pathways synergistically induced FOXO transcriptional activity and inhibited cell migration and capillary tube formation. Antiangiogenic effects of resveratrol were enhanced by inhibitors of AKT and MEK. Phosphorylation-deficient mutants of FOXOs induced FOXO transcriptional activity, inhibited HUVEC cell migration, and capillary tube formation, and also enhanced antiangiogenic effects of resveratrol. Finally, VEGF neutralizing antibody enhanced the anti-proliferative and anti-angiogenic effects of resveratrol. In conclusion, regulation of FOXO transcription factors by resveratrol may play an important role in angiogenesis which is critical for cancer, diabetic retinopathy, rheumatoid arthritis, psoriasis, and cardiovascular disorders.

Pathways mediating VEGF-independent tumor angiogenesis

FDA approval of several inhibitors of the VEGF pathway has enabled significant advances in the therapy of cancer and neovascular age-related macular degeneration. However, similar to other therapies, inherent/acquired resistance to anti-angiogenic drugs may occur in patients, leading to disease progression. So far the lack of predictive biomarkers has precluded identification of patients most likely to respond to such treatments. Recent suggest that both tumor and non-tumor (stromal) cell types are involved in the reduced responsiveness to the treatments. The present review examines the role of tumor- as well as stromal cell-derived pathways involved in tumor growth and in refractoriness to anti-VEGF therapies.

Anti-angiogenic tyrosine kinase inhibitors: what is their mechanism of action?

Tyrosine kinases are important cellular signaling proteins that have a variety of biological activities including cell proliferation and migration. Multiple kinases are involved in angiogenesis, including receptor tyrosine kinases such as the vascular endothelial growth factor receptor. Inhibition of angiogenic tyrosine kinases has been developed as a systemic treatment strategy for cancer. Three anti-angiogenic tyrosine kinase inhibitors (TKIs), sunitinib, sorafenib and pazopanib, with differential binding capacities to angiogenic kinases were recently approved for treatment of patients with advanced cancer (renal cell cancer, gastro-intestinal stromal tumors, and hepatocellular cancer). Many other anti-angiogenic TKIs are being studied in phase I-III clinical trials. In addition to their beneficial anti-tumor activity, clinical resistance and toxicities have also been observed with these agents. In this manuscript, we will give an overview of the design and development of anti-angiogenic TKIs. We describe their molecular structure and classification, their mechanism of action, and their inhibitory activity against specific kinase signaling pathways. In addition, we provide insight into what extent selective targeting of angiogenic kinases by TKIs may contribute to the clinically observed anti-tumor activity, resistance, and toxicity. We feel that it is of crucial importance to increase our understanding of the clinical mechanism of action of anti-angiogenic TKIs in order to further optimize their clinical efficacy.

Role of hypoxia in the hallmarks of human cancer

Hypoxia has been recognized as one of the fundamentally important features of solid tumors and plays a critical role in various cellular and physiologic events, including cell proliferation, survival, angiogenesis, immunosurveillance, metabolism, as well as tumor invasion and metastasis. These responses to hypoxia are at least partially orchestrated by activation of the hypoxia-inducible factors (HIFs). HIF-1 is a key regulator of the response of mammalian cells to oxygen deprivation and plays critical roles in the adaptation of tumor cells to a hypoxic microenvironment. Hypoxia and overexpression of HIF-1 have been associated with radiation therapy and chemotherapy resistance, an increased risk of invasion and metastasis, and a poor clinical prognosis of solid tumors. The discovery of HIF-1 signaling has led to a rapidly increasing understanding of the complex mechanisms involved in tumor hypoxia and has helped greatly in screening novel anticancer agents. In this review, we will first introduce the cellular responses to hypoxia and HIF-1 signaling pathway in hypoxia, and then summarize the multifaceted role of hypoxia in the hallmarks of human cancers.

Human cancers converge at the HIF-2alpha oncogenic axis

Cancer development is a multistep process, driven by a series of genetic and environmental alterations, that endows cells with a set of hallmark traits required for tumorigenesis. It is broadly accepted that growth signal autonomy, the first hallmark of malignancies, can be acquired through multiple genetic mutations that activate an array of complex, cancer-specific growth circuits [Hanahan D, Weinberg RA (2000) The hallmarks of cancer. Cell 100:57-70; Vogelstein B, Kinzler KW (2004) Cancer genes and the pathways they control. Nat Med 10:789-799]. The superfluous nature of these pathways is thought to severely limit therapeutic approaches targeting tumor proliferation, and it has been suggested that this strategy be abandoned in favor of inhibiting more systemic hallmarks, including angiogenesis (Ellis LM, Hicklin DJ (2008) VEGF-targeted therapy: Mechanisms of anti-tumor activity. Nat Rev Cancer 8:579-591; Stommel JM, et al. (2007) Coactivation of receptor tyrosine kinases affects the response of tumor cells to targeted therapies. Science 318:287-290; Kerbel R, Folkman J (2002) Clinical translation of angiogenesis inhibitors. Nat Rev Cancer 2:727-739; Kaiser J (2008) Cancer genetics: A detailed genetic portrait of the deadliest human cancers. Science 321:1280-1281]. Here, we report the unexpected observation that genetically diverse cancers converge at a common and obligatory growth axis instigated by HIF-2alpha, an element of the oxygen-sensing machinery. Inhibition of HIF-2alpha prevents the in vivo growth and tumorigenesis of highly aggressive glioblastoma, colorectal, and non-small-cell lung carcinomas and the in vitro autonomous proliferation of several others, regardless of their mutational status and tissue of origin. The concomitant deactivation of select receptor tyrosine kinases, including the EGFR and IGF1R, as well as downstream ERK/Akt signaling, suggests that HIF-2alpha exerts its proliferative effects by endorsing these major pathways. Consistently, silencing these receptors phenocopies the loss of HIF-2alpha oncogenic activity, abrogating the serum-independent growth of human cancer cells in culture. Based on these data, we propose an alternative to the predominant view that cancers exploit independent autonomous growth pathways and reveal HIF-2alpha as a potentially universal culprit in promoting the persistent proliferation of neoplastic cells.

Immature blood vessels in rheumatoid synovium are selectively depleted in response to anti-TNF therapy

Background

Angiogenesis is considered an important factor in the pathogenesis of Rheumatoid Arthritis (RA) where it has been proposed as a therapeutic target. In other settings, active angiogenesis is characterized by pathologic, immature vessels that lack periendothelial cells. We searched for the presence of immature vessels in RA synovium and analyzed the dynamics of synovial vasculature along the course of the disease, particularly after therapeutic response to TNF antagonists.

Methodology/Principal Findings

Synovial arthroscopic biopsies from RA, osteoarthritis (OA) and normal controls were analyzed by double labeling of endothelium and pericytes/smooth muscle mural cells to identify and quantify mature/immature blood vessels. To analyze clinicopathological correlations, a cross-sectional study on 82 synovial biopsies from RA patients with variable disease duration and severity was performed. A longitudinal analysis was performed in 25 patients with active disease rebiopsied after anti-TNF-α therapy. We found that most RA synovial tissues contained a significant fraction of immature blood vessels lacking periendothelial coverage, whereas they were rare in OA, and inexistent in normal synovial tissues. Immature vessels were observed from the earliest phases of the disease but their presence or density was significantly increased in patients with longer disease duration, higher activity and severity, and stronger inflammatory cell infiltration. In patients that responded to anti-TNF-α therapy, immature vessels were selectively depleted. The mature vasculature was similarly expanded in early or late disease and unchanged by therapy.

Conclusion/Significance

RA synovium contains a significant fraction of neoangiogenic, immature blood vessels. Progression of the disease increases the presence and density of immature but not mature vessels and only immature vessels are depleted in response to anti-TNFα therapy. The different dynamics of the mature and immature vascular fractions has important implications for the development of anti-angiogenic interventions in RA.

Peptide hormone regulation of angiogenesis

It is now apparent that regulation of blood vessel growth contributes to the classical actions of hormones on development, growth, and reproduction. Endothelial cells are ideally positioned to respond to hormones, which act in concert with locally produced chemical mediators to regulate their growth, motility, function, and survival. Hormones affect angiogenesis either directly through actions on endothelial cells or indirectly by regulating proangiogenic factors like vascular endothelial growth factor. Importantly, the local microenvironment of endothelial cells can determine the outcome of hormone action on angiogenesis. Members of the growth hormone/prolactin/placental lactogen, the renin-angiotensin, and the kallikrein-kinin systems that exert stimulatory effects on angiogenesis can acquire antiangiogenic properties after undergoing proteolytic cleavage. In view of the opposing effects of hormonal fragments and precursor molecules, the regulation of the proteases responsible for specific protein cleavage represents an efficient mechanism for balancing angiogenesis. This review presents an overview of the actions on angiogenesis of the above-mentioned peptide hormonal families and addresses how specific proteolysis alters the final outcome of these actions in the context of health and disease.

Selective inhibition of retinal angiogenesis by targeting PI3 kinase

Ocular neovascularisation is a pathological hallmark of some forms of debilitating blindness including diabetic retinopathy, age related macular degeneration and retinopathy of prematurity. Current therapies for delaying unwanted ocular angiogenesis include laser surgery or molecular inhibition of the pro-angiogenic factor VEGF. However, targeting of angiogenic pathways other than, or in combination to VEGF, may lead to more effective and safer inhibitors of intraocular angiogenesis. In a small chemical screen using zebrafish, we identify LY294002 as an effective and selective inhibitor of both developmental and ectopic hyaloid angiogenesis in the eye. LY294002, a PI3 kinase inhibitor, exerts its anti-angiogenic effect in a dose-dependent manner, without perturbing existing vessels. Significantly, LY294002 delivered by intraocular injection, significantly inhibits ocular angiogenesis without systemic side-effects and without diminishing visual function. Thus, targeting of PI3 kinase pathways has the potential to effectively and safely treat neovascularisation in eye disease.

Antiangiogenic arming of an oncolytic vaccinia virus enhances antitumor efficacy in renal cell cancer models

Oncolytic vaccinia viruses have shown compelling results in preclinical cancer models and promising preliminary safety and antitumor activity in early clinical trials. However, to facilitate systemic application it would be useful to improve tumor targeting and antitumor efficacy further. Here we report the generation of vvdd-VEGFR-1-Ig, a targeted and armed oncolytic vaccinia virus. Tumor targeting was achieved by deletion of genes for thymidine kinase and vaccinia virus growth factor, which are necessary for replication in normal but not in cancer cells. Given the high vascularization typical of kidney cancers, we armed the virus with the soluble vascular endothelial growth factor (VEGF) receptor 1 protein for an antiangiogenic effect. Systemic application of high doses of vvdd-VEGFR-1-Ig resulted in cytokine induction in an immunocompromised mouse model. Upon histopathological analysis, splenic extramedullary hematopoiesis was seen in all virus-injected mice and was more pronounced in the vvdd-VEGFR-1-Ig group. Analysis of the innate immune response after intravenous virus injection revealed high transient and dose-dependent cytokine elevations. When medium and low doses were used for intratumoral or intravenous injection, vvdd-VEGFR-1-Ig exhibited a stronger antitumor effect than the unarmed control. Furthermore, expression of VEGFR-1-Ig was confirmed, and a concurrent antiangiogenic effect was seen. In an immunocompetent model, systemic vvdd-VEGFR-1-Ig exhibited superior antitumor efficacy compared to the unarmed control virus. In conclusion, the targeted and armed vvdd-VEGFR-1-Ig has promising anticancer activity in renal cell cancer models. Extramedullary hematopoiesis may be a sensitive indicator of vaccinia virus effects in mice.

Role of class 3 semaphorins and their receptors in tumor growth and angiogenesis

Class 3 semaphorins (SEMA3) were first identified as glycoproteins that negatively mediate neuronal guidance by binding to neuropilin and repelling neurons away from the source of SEMA3. However, studies have shown that SEMA3s are also secreted by other cell types, including tumor cells, where they play an inhibitory role in tumor growth and angiogenesis (specifically SEMA3B and SEMA3F). SEMA3s primarily inhibit the cell motility and migration of tumor and endothelial cells by inducing collapse of the actin cytoskeleton via neuropilins and plexins. Besides binding to SEMA3s, neuropilin also binds the protumorigenic and proangiogenic ligand vascular endothelial growth factor (VEGF). Although some studies attribute the antitumorigenic and antiangiogenic properties of SEMA3s to competition between SEMA3s and VEGF for binding to neuropilin receptors, several others have shown that SEMA3s display growth-inhibitory activity independent of competition with VEGF. A better understanding of these molecular interactions and the role and signaling of SEMA3s in tumor biology will help determine whether SEMA3s represent potential therapeutic agents. Herein, we briefly review (a) the role of SEMA3s in mediating tumor growth, (b) the SEMA3 receptors neuropilins and plexins, and (c) the potential competition between SEMA3s and VEGF family members for neuropilin binding.

Cytokine levels correlate with immune cell infiltration after anti-VEGF therapy in preclinical mouse models of breast cancer

The effect of blocking VEGF activity in solid tumors extends beyond inhibition of angiogenesis. However, no studies have compared the effectiveness of mechanistically different anti-VEGF inhibitors with respect to changes in tumor growth and alterations in the tumor microenvironment. In this study we use three distinct breast cancer models, a MDA-MB-231 xenograft model, a 4T1 syngenic model, and a transgenic model using MMTV-PyMT mice, to explore the effects of various anti-VEGF therapies on tumor vasculature, immune cell infiltration, and cytokine levels. Tumor vasculature and immune cell infiltration were evaluated using immunohistochemistry. Cytokine levels were evaluated using ELISA and electrochemiluminescence. We found that blocking the activation of VEGF receptor resulted in changes in intra-tumoral cytokine levels, specifically IL-1β, IL-6 and CXCL1. Modulation of the level these cytokines is important for controlling immune cell infiltration and ultimately tumor growth. Furthermore, we demonstrate that selective inhibition of VEGF binding to VEGFR2 with r84 is more effective at controlling tumor growth and inhibiting the infiltration of suppressive immune cells (MDSC, Treg, macrophages) while increasing the mature dendritic cell fraction than other anti-VEGF strategies. In addition, we found that changes in serum IL-1β and IL-6 levels correlated with response to therapy, identifying two possible biomarkers for assessing the effectiveness of anti-VEGF therapy in breast cancer patients.

Cytokine levels correlate with immune cell infiltration after anti-VEGF therapy in preclinical mouse models of breast cancer

The effect of blocking VEGF activity in solid tumors extends beyond inhibition of angiogenesis. However, no studies have compared the effectiveness of mechanistically different anti-VEGF inhibitors with respect to changes in tumor growth and alterations in the tumor microenvironment. In this study we use three distinct breast cancer models, a MDA-MB-231 xenograft model, a 4T1 syngenic model, and a transgenic model using MMTV-PyMT mice, to explore the effects of various anti-VEGF therapies on tumor vasculature, immune cell infiltration, and cytokine levels. Tumor vasculature and immune cell infiltration were evaluated using immunohistochemistry. Cytokine levels were evaluated using ELISA and electrochemiluminescence. We found that blocking the activation of VEGF receptor resulted in changes in intra-tumoral cytokine levels, specifically IL-1beta, IL-6 and CXCL1. Modulation of the level these cytokines is important for controlling immune cell infiltration and ultimately tumor growth. Furthermore, we demonstrate that selective inhibition of VEGF binding to VEGFR2 with r84 is more effective at controlling tumor growth and inhibiting the infiltration of suppressive immune cells (MDSC, Treg, macrophages) while increasing the mature dendritic cell fraction than other anti-VEGF strategies. In addition, we found that changes in serum IL-1beta and IL-6 levels correlated with response to therapy, identifying two possible biomarkers for assessing the effectiveness of anti-VEGF therapy in breast cancer patients.

Stroma in breast development and disease

It is increasingly apparent that normal and malignant breast tissues require complex local and systemic stromal interactions for development and progression. During development, mammary cell fate specification and differentiation require highly regulated contextual signals derived from the stroma. Likewise, during breast carcinoma development, the tissue stroma can provide tumor suppressing and tumor-promoting environments that serve to regulate neoplastic growth of the epithelium. This review focuses on the role of the stroma as a mediator of normal mammary development, as well as a critical regulator of malignant conversion and progression in breast cancer. Recognition of the important role of the stroma during the progression of breast cancers leads to the possibility of new targets for treatment of the initial breast cancer lesion as well as prevention of recurrence.

Evaluation of metastatic and angiogenic potentials of human colon carcinoma cells in chick embryo model systems

Increased metastatic and angiogenic potentials of aggressive human colon carcinoma cells were verified in independent chick embryo models by comparing in vivo highly metastatic SW620 colon carcinoma cell line with its isogenic, non-metastatic SW480 cell variant. In the experimental metastasis model, both cell types rapidly arrested in the chorioallantoic membrane (CAM) vasculature as demonstrated by quantitative PCR and immunohistochemistry. Live cell imaging also indicated that both SW620 and SW480 cells efficiently extravasated from the CAM capillary system. However, only few SW480 cells were present in the CAM tissue after 24-48 h. In contrast, the numbers of SW620 cells increased exponentially, indicating proliferative and survival advantages of metastatic colon carcinoma cells in vivo. Multicellular SW620 foci were identified in close proximity to CAM blood vessels. A positive correlation between increased metastatic ability and VEGF-expression of colon carcinoma SW620 cells was demonstrated by the substantial inhibitory effects of anti-VEGF treatment on the levels of metastatic colonization and density of blood vessels adjacent to tumor cell foci. Furthermore, the chick embryo angiogenesis model confirmed high levels of VEGF-dependent angiogenesis induced by SW620 cells, but not SW480 cells. Thus, chick embryo experimental metastasis and CAM angiogenesis models appear to coordinately reflect critical features of advanced colon carcinomas, i.e., the acquisition of enhanced survival and increased angiogenic potentials, both constituting critical determinants of colon cancer progression. The use of rapid and quantitative chick embryo models might provide alternative approaches to conventional mammalian model systems for screening anti-cancer agents.

Contrasting actions of selective inhibitors of angiopoietin-1 and angiopoietin-2 on the normalization of tumor blood vessels

Angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2) have complex actions in angiogenesis and vascular remodeling due to their effects on Tie2 receptor signaling. Ang2 blocks Ang1-mediated activation of Tie2 in endothelial cells under certain conditions but is a Tie2 receptor agonist in others. We examined the effects of selective inhibitors of Ang1 (mL4-3) or Ang2 (L1-7[N]), alone or in combination, on the vasculature of human Colo205 tumors in mice. The Ang2 inhibitor decreased the overall abundance of tumor blood vessels by reducing tumor growth and keeping vascular density constant. After inhibition of Ang2, tumor vessels had many features of normal blood vessels (normalization), as evidenced by junctional accumulation of vascular endothelial-cadherin, junctional adhesion molecule-A, and platelet/endothelial cell adhesion molecule-1 in endothelial cells, increased pericyte coverage, reduced endothelial sprouting, and remodeling into smaller, more uniform vessels. The Ang1 inhibitor by itself had little noticeable effect on the tumor vasculature. However, when administered with the Ang2 inhibitor, the Ang1 inhibitor prevented tumor vessel normalization, but not the reduction in tumor vascularity produced by the Ang2 inhibitor. These findings are consistent with a model whereby inhibition of Ang2 leads to normalization of tumor blood vessels by permitting the unopposed action of Ang1, but decreases tumor vascularity primarily by blocking Ang2 actions.

[Renal cell carcinoma and antiangiogenic agents: ongoing controversies are seeking answers for improvement of therapeutic management]

The recent development of agents targeting VHL-related growth factors as therapy for metastatic renal cell carcinoma has generated a revolution in the management of these patients, with a clear improvement of survival. However, there are still many questions related to these antiangiogenic agents and upcoming studies are strongly warranted to provide answers. The controversy is ongoing regarding several specific questions as the use of antiangiogenic treatments remains a moot point in certain cases. Notably, the role for radical nephrectomy in metastatic cases, the interest of adjuvant treatment in high-risk recurrence cases, the place for neoadjuvant treatment in locally advanced renal cell carcinoma, the choice of the best agent at first-line treatment, the various therapeutic options and their consequences on the quality of life and the potential contribution of dynamic imaging to assess antiangiogenic treatment's efficiency.

Overexpression and interactions of interleukin-10, transforming growth factor beta, and vascular endothelial growth factor in esophageal squamous cell carcinoma

BACKGROUND

Sharing the role of immune suppression, interleukin-10 (IL-10), transforming growth factor beta (TGF-beta), and vascular endothelial growth factor (VEGF) are critical genes in several aspects of tumorigenesis. To elucidate the role of these cytokines in esophageal squamous cell carcinoma (ESCC), their relative mRNA expression in tumoral tissue compared with corresponding tumor-free tissue was evaluated.

METHODS

A total of 49 patients with histologically confirmed ESCC were included in the study prior to any therapeutic interventions. Quantitative analysis of the mRNA expression was performed by real-time reverse transcription-polymerase chain reaction and the clinicopathologic associations were assessed.

RESULTS

The mRNA of IL-10, VEGF, and TGF-beta was frequently overexpressed in 53.2%, 44.9%, and 37.5% of ESCC patients, respectively. TGF-beta was significantly co-expressed with IL-10 and with VEGF. Although VEGF was not independently associated with increased tumor size (p = 0.065), concomitant overexpression of VEGF with TGF-beta was significantly correlated with increased size of the tumor (p < 0.05).

CONCLUSIONS

Overexpression of IL-10, TGF-beta, and VEGF plays an important role in ESCC and consequently leads to the frequent event of immune evasion in ESCC. TGF-beta is concomitantly overexpressed with IL-10 and with VEGF in ESCC. A stimulatory signal from TGF-beta to VEGF is necessary for VEGF to promote tumor progression.

Pleiotropic roles of matrix metalloproteinases in tumor angiogenesis: contrasting, overlapping and compensatory functions

A number of extensive reviews are available discussing the roles of MMPs in various aspects of cancer progression from benign tumor formation to overt cancer present with deadly metastases. This review will focus specifically on the evidence functionally linking the MMPs and tumor-induced angiogenesis in various in vivo models. Emphasis has been placed on the cellular origin of the MMPs in tumor tissue, the requirement of proMMP activation and the resulting proteolytic activity for the induction and progression of tumor angiogenesis, and the pleiotropic roles for some of the MMPs. The functional mechanisms of the angiogenic MMPs are discussed as well as their catalytic detection in complex biological systems. In addition, the contribution of active MMPs to metastatic spread and establishment of secondary metastasis will be discussed in view of the findings indicating that MMPs are involved in the preparation of pre-metastatic niches. Finally, the most recent evidence, indicating the pro-metastatic consequences of anti-angiogenic therapies employing MMP inhibitors will be presented as examples highlighting possible outcomes of interfering with the pleiotropic nature of the MMP functionality.

Molecular imaging and therapy of cancer with radiolabeled nanoparticles

This review summarizes the current state-of-the-art of radiolabeled nanoparticles for molecular imaging and internal radiotherapy applications targeting cancer. With the capacity to provide enormous flexibility, radiolabeled nanoparticles have the potential to profoundly impact disease diagnosis and patient management in the near future. Currently, the major challenges facing the research on radiolabeled nanoparticles are desirable (tumor) targeting efficacy, robust chemistry for both radionuclide encapsulation/incorporation and targeting ligand conjugation, favorable safety profile, as well as certain commercial and regulatory hurdles.

(-)-Epigallocatechin gallate suppresses the growth of human hepatocellular carcinoma cells by inhibiting activation of the vascular endothelial growth factor-vascular endothelial growth factor receptor axis

The receptor tyrosine kinase vascular endothelial growth factor (VEGF) receptor (VEGFR) plays an important role in tumor angiogenesis of hepatocellular carcinoma (HCC). (-)-Epigallocatechin gallate (EGCG), the major biologically active component of green tea, inhibits growth in a variety of human cancer cells by inhibiting the activation of several types of receptor tyrosine kinases. In this study, we examined the effects of EGCG on the activity of the VEGF-VEGFR axis in human HCC cells. The levels of total and phosphorylated (i.e. activated) form of VEGFR-2 protein (p-VEGFR-2) were observed to increase in a series of human HCC cell lines in comparison to the Hc normal human hepatocytes. EGCG preferentially inhibited the growth of HuH7 HCC cells, which express constitutive activation of the VEGF-VEGFR axis, in comparison to Hc cells. Treatment of HuH7 cells with EGCG caused a time- and dose-dependent decrease in the expression of VEGFR-2 and p-VEGFR-2 proteins. The production of VEGF from HuH7 cells was reduced by treatment with EGCG. Drinking of EGCG significantly inhibited the growth of HuH7 xenografts in nude mice and this was associated with inhibition of the activation of VEGFR-2 and its related downstream signaling molecules, including ERK and Akt. EGCG drinking also decreased the expression of Bcl-x(L) protein and VEGF mRNA in the xenografts. These findings suggest that EGCG can exert, at least in part, its growth-inhibitive effect on HCC cells by inhibiting the VEGF-VEGFR axis. EGCG might therefore be useful in the treatment of HCC.

Malignant cell-derived PlGF promotes normalization and remodeling of the tumor vasculature

Vascular functions of PlGF remain poorly understood and controversial. Here, we show that tumor cell-derived PlGF-1 and PlGF-2 displayed significant remodeling effects on the tumor vasculature, leading to a normalized vascular phenotype and improved functions against leakage. In two murine tumor models, that is, T241 fibrosarcoma and Lewis lung carcinoma, stable expression of PlGF-1 and PlGF-2 in tumor cells resulted in significant reduction of tumor microvascular density and branch formation. Markedly, the vasculature in PlGF-expressing tumors consisted of relatively large-diameter microvessels with substantial improvement of pericyte coverage. Similarly, PlGF-induced vascular normalization and remodeling were also observed in a spontaneous human choriocarcinoma that expressed endogenous PlGF. Our findings shed light on functions of PlGF as a vascular remodeling factor that normalizes the tumor vasculature and thus may have conceptual implications of cancer therapy.

A novel peptide (GX1) homing to gastric cancer vasculature inhibits angiogenesis and cooperates with TNF alpha in anti-tumor therapy

Background

The discovery of the importance of angiogenesis in tumor growth has emphasized the need to find specific vascular targets for tumor-targeted therapies. Previously, using phage display technology, we identified the peptide GX1 as having the ability to target the gastric cancer vasculature. The present study investigated the bioactivities of GX1, as well as its potential ability to cooperate with recombinant mutant human tumor necrosis factor alpha (rmhTNFα), in gastric cancer therapy.

Results

Tetrazolium salt (MTT) assay showed that GX1 could inhibit cell proliferation of both human umbilical vein endothelial cells (HUVEC) (44%) and HUVEC with tumor endothelium characteristics, generated by culturing in tumor-conditioned medium (co-HUVEC) (62%). Flow-cytometry (FCM) and western blot assays showed that GX1 increased the rate of apoptosis from 11% to 31% (p < 0.01) by up-regulating caspase 3 expression level. A chorioallantoic membrane assay indicated that GX1 could suppress neovascularization in vivo, with the microvessel count decreasing from 21 to 11 (p < 0.05). When GX1 was fused to rmhTNFα, GX1-rmhTNFα selectively concentrated in the gastric cancer vasculature, as shown by enzyme-linked immunosorbent assay, immunofluorescence and emission-computed tomography. In vitro MTT and FCM assays showed that, compared to rmhTNFα alone, GX1-rmhTNFα was more effective at suppressing co-HUVEC proliferation (45% vs. 61%, p < 0.05) and inducing apoptosis (11% vs. 23%, p < 0.05). In a tumor formation test, GX1-rmhTNFα more effectively inhibited tumor growth than rmhTNFα (tumor volume: 271 mm³ vs. 134 mm³, p < 0.05), with less systemic toxicity as measured by body weight (20.57 g vs. 19.30 g, p < 0.05). These therapeutic effects may be mediated by selectively enhanced tumor vascular permeability, as indicated by Evan's blue assay.

Conclusion

GX1 had both homing activity and the ability to inhibit vascular endothelial cell proliferation in vitro and neovascularization in vivo. Furthermore, when GX1 was conjugated to rmhTNFα, the fusion protein was selectively delivered to targeted tumor sites, significantly improving the anti-tumor activity of rmhTNFα and decreasing systemic toxicity. These results demonstrate the potential of GX1 as a homing peptide in vascular targeted therapy for gastric cancer.

An overview of small-molecule inhibitors of VEGFR signaling

VEGFR inhibitors are in broad use for the treatment of metastatic renal-cell carcinoma, gastrointestinal stromal tumors and hepatocellular carcinoma and in development in a number of other oncology indications, including colorectal cancer, non-small-cell lung cancer, pancreatic cancer, thyroid malignancies, ovarian cancer, breast cancer and sarcomas. This Review outlines the structure-activity relationships of the 44 VEGFR inhibitors currently in development. An overview of the pharmacokinetic profile of each molecule and its stage in development is provided. Phase III clinical trials being conducted for licensing of these agents for specific indications and phase III developmental efficacy trials are described in detailed tables that include the disease studied, trial design including combination therapy, study end points, and projected or final accrual. The relative frequency of on-target and off-target adverse events observed in 3,060 patients is described for a subset of agents in development in clinical trials sponsored by the National Cancer Institute. No interagent comparisons were undertaken and no data from pharmaceutical pharmacovigilance databases were used. The on-target effects seem to be mechanistically based and predicted by VEGFR inhibition. Small-molecule inhibitors of angiogenesis are active in a wide variety of malignancies and fill a unique niche for cancer therapeutics.

Aflibercept in epithelial ovarian carcinoma

Angiogenesis is a hallmark of malignant transformation. With improved understanding of angiogenic signaling in both the normal and malignant state, there have been a number of agents developed that target VEGF signaling. These targeted agents can affect downstream VEGF signal transduction via unique mechanisms at different cellular and extracellular locations. The aflibercept, or VEGF-Trap, molecule is the subject of this article. Its molecular structure, pharmacokinetic and pharmacodynamic profile, and preclinical and early clinical data in epithelial ovarian carcinoma is reviewed. For comparison, other anti-angiogenic agents that have been or are currently being studied in epithelial ovarian carcinoma are also summarized. Finally, the anticipated role of aflibercept in the treatment of epithelial ovarian carcinoma is also discussed.

Angiogenesis and lymphangiogenesis are downregulated in primary breast cancer

BACKGROUND

Angiogenesis and lymphangiogenesis are considered to play key roles in tumour growth, progression and metastasis. However, targeting tumour angiogenesis in clinical trials showed only modest efficacy. We therefore scrutinised the concept of tumour angiogenesis and lymphangiogenesis by analysing the expression of crucial markers involved in these processes in primary breast cancer.

METHODS

We analysed the expression of angiogenic, lymphangiogenic or antiangiogenic factors, their respective receptors and specific markers for endothelial and lymphendothelial cells by quantitative real-time RT-PCR in primary breast cancer and compared the expression profiles to non-cancerous, tumour-adjacent tissues and breast tissues from healthy women.

RESULTS

We found decreased mRNA amounts of major angiogenic and lymphangiogenic factors in tumour compared to healthy tissues, whereas antiangiogenic factors were upregulated. Concomitantly, angiogenic and lymphangiogenic receptors were downregulated in breast tumours. This antiangiogenic, antilymphangiogenic microenvironment was even more pronounced in aggressive tumours and accompanied by reduced amounts of endothelial and lymphatic endothelial cell markers.

CONCLUSION

Primary breast tumours are not a site of highly active angiogenesis and lymphangiogenesis. Selection for tumour cells that survive with minimal vascular supply may account for this observation in clinical apparent tumours.

The role of bone-marrow-derived cells in tumor growth, metastasis initiation and progression

Emerging evidence from murine models suggests that tumor-specific endocrine factors systemically stimulate the quiescent bone marrow (BM) compartment, resulting in the expansion, mobilization and recruitment of BM progenitor cells. Discrete subsets of tumor-instigated BM-derived progenitor cells support tumor progression and metastasis by regulating angiogenesis, inflammation and immune suppression. Notably, clinical studies have begun to reveal that increased BM recruitment in tumors is associated with poor prognosis. Thus, the BM-derived tumor microenvironment is an attractive therapeutic target, and drugs targeting the components of the microenvironment are currently in clinical trials. Here, we focus on recent advances and emerging concepts regarding the intriguing role of BM-derived cells in tumor growth, metastasis initiation and progression, and we discuss future directions in the context of novel diagnostic and therapeutic opportunities.

Characterizing the effects of the juxtamembrane domain on vascular endothelial growth factor receptor-2 enzymatic activity, autophosphorylation, and inhibition by axitinib

The catalytic domains of protein kinases are commonly treated as independent modular units with distinct biological functions. Here, the interactions between the catalytic and juxtamembrane domains of VEGFR2 are studied. Highly purified preparations of the receptor tyrosine kinase VEGFR2 catalytic domain without (VEGFR2-CD) and with (VEGFR2-CD/JM) the juxtamembrane (JM) domain were characterized by kinetic, biophysical, and structural methods. Although the catalytic parameters for both constructs were similar, the autophosphorylation rate of VEGFR2-CD/JM was substantially faster than VEGFR2-CD. The first event in the autophosphorylation reaction was phosphorylation of JM residue Y801 followed by phosphorylation of activation loop residues in the CD. The rates of activation loop autophosphorylation for the two constructs were determined to be similar. The autophosphorylation rate of Y801 was invariant on enzyme concentration, which is consistent with an intramolecular reaction. In addition, the first biochemical characterization of the advanced clinical compound axitinib is reported. Axitinib was found to have 40-fold enhanced biochemical potency toward VEGFR2-CD/JM (K(i) = 28 pM) compared to VEGFR2-CD, which correlates better with cellular potency. Calorimetric studies, including a novel ITC compound displacement method, confirmed the potency and provided insight into the thermodynamic origin of the potency differences. A structural model for the VEGFR2-CD/JM is proposed based on the experimental findings reported here and on the JM position in c-Kit, FLT3, and CSF1/cFMS. The described studies identify potential functions of the VEGFR2 JM domain with implications to both receptor biology and inhibitor design.

Paracrine Hedgehog signaling in cancer

Ligand-dependent and ligand-independent activation of the Hedgehog (Hh) signaling pathway is involved in tumorigenesis. The importance of mutations in Hh pathway components leading to constitutive signaling has been well established in basal cell carcinoma and medulloblastoma. However, the role of ligand-driven Hh pathway activation in cancer remains to be established. Three recent articles support a model in which, in the absence of mutations in the Hh pathway, Hh ligands expressed by a subset of epithelial cancers, including colon, pancreatic, and ovarian cancer, promote tumor growth indirectly by activating Hh signaling in the surrounding stroma, which, in turn, provides a more favorable environment for tumor growth. These data have important implications for the use of Hh pathway inhibitors currently in development and for selection of tumors likely to respond to such inhibitors.

Changing the clinical picture of challenging tumors: tales becoming reality?

The treatment of neoplastic diseases has become increasingly dependent on tumor biology and is focused on targeted therapy. Understanding complex networks of intracellular signaling pathways, blockades of specific targets and a myriad of other approaches has brought new fuel to the battle against many types of cancer. Unfortunately, the degree of benefit achieved in this new era of cancer treatment has not been distributed homogeneously among the different disease types. Neoplasms with lower incidence rates, but that are also highly challenging, are not consistently given due attention by research leaders. This article aims to evaluate new insights and potential gains obtained with new therapies in a particular group of tumors: those rarely debated in clinical practice, but which still pose a considerable challenge to clinical oncology.

Spheroid-based human endothelial cell microvessel formation in vivo

The study of angiogenic endothelial cells (ECs) has in recent years greatly stimulated multiple fields of vascular biology research. A number of cellular models and numerous complex developmental, manipulatory and tumor animal models have been developed to study angiogenesis in vitro and in vivo. To connect the versatility of cellular assays with the complexity of readouts of in vivo experimentation, we have developed an endothelial transplantation assay. This assay is based on grafting ex vivo generated EC spheroids (2 d) in a suitable matrix in immunocompromised mice, to give rise to a 3D network of capillaries (20 d). This vasculature connects to the mouse vasculature, is perfused and matures by recruiting mouse mural cells. Here we describe the detailed protocol for this assay, including generation of spheroids, injection into mice, excision and processing of resulting plugs, and quantification by immunohistochemical analysis of the resulting vasculature.

Antiproliferative and antiangiogenic effects of the benzophenanthridine alkaloid sanguinarine in melanoma

This study was aimed at evaluating the potential application of benzophenanthridine alkaloids, sanguinarine and cheleritrine, in the therapy of melanoma cancer. In vitro antiproliferative activity of sanguinarine was higher than that of cheleritrine against the B16 melanoma 4A5 cells. Both agents were able to produce DNA breaks, and the DNA unwinding assay showed that they act as DNA intercalating agents. Sanguinarine was selected for determination of its in vivo preclinical efficacy. Oral treatment with sanguinarine reduced the tumor burden in a transplantable murine tumor grown in a syngeneic host (B16 melanoma 4A5 in C57BL/6 mice), and in a human tumor xenograft grown in immunodeficient mice (A375 human melanoma in athymic nude mice). In A375 tumors a significant decrease in the proliferation marker Ki67, and a reduction in the activated mitogen-activated protein kinases (p-p44/42 MAPK), and in protein kinase B (pAKT) were also observed. Three out of eleven A375-bearing treated mice were tumor-free at the end of treatment, and did not develop any tumor after a further, treatment-free, observation period of 60 days. Sanguinarine also showed a striking antiangiogenic activity in mice. Data from the present study support the concept that sanguinarine can be effective in melanoma skin cancer.

Peptide-targeted diagnostics and radiotherapeutics

Radiotherapeutic drugs and medical imaging agents, although used for different purposes, both benefit from precise targeting. When systemically administered, either would be most useful if designed to find and bind only a tumor, single type of cell, or unique molecular assembly thereon. In this Account, we examine the use of small peptides, natural and synthetic, to create biochemically specific molecular imaging agents and radiotherapeutic pharmaceuticals, discussing three distinct examples. In one project, a small natural peptide known to target members of the bombesin family of receptors was chemically attached to a strong, versatile metal chelator, DO3A, through a series of small-molecule linkers. The linkers powerfully affected not only binding strength for the bombesin receptors, tissue distribution, and tumor uptake in vivo but also receptor subtype specificity. When the assembly is combined with an active metal ion for human trials, the versatility of the DO3A (dodecanetriacetate) chelate affords choices in selecting the metal ion for different purposes: lutetium for a combination radiotherapeutic and diagnostic agent, (177)Lu-AMBA, and gallium for a positron emission tomography (PET) imaging agent, (68)Ga-AMBA. We also created small (approximately 5-kDa) bivalent peptides, each composed of different chemically linked peptides derived from phage display. The monomer peptides bound to the same target protein, VEGF-R2, a primary target of vascular endothelial growth factor (VEGF), the angiogenesis-stimulating protein. Several families of the monomer peptides did not compete with one another for the binding site on VEGF-R2. Their combination into fully synthetic hetero-bivalent molecules yielded subnanomolar K(d) values and greater than 100-fold improvements over homo-bivalent molecules. Biological activity was evident in the hetero-bivalents, whereas none or very little existed in homo-bivalents, monomers, and monomer mixtures. In ultrasound imaging, tiny bubbles (2 microm in diameter) filled with inert gas can be used as effective contrast agents. By coating the shell of such bubbles with the peptide TKPPR (a tuftsin antagonist), we created contrast agents that bound unexpectedly to cultured endothelial cells expressing angiogenesis targets; the binding was attributable to a previously unnoticed and powerful multivalency effect. TKPPR binds specifically to neuropilin-1 (NP-1), a VEGF co-receptor, but only when multimerized is it avid. Tuftsin, a small peptide derived from immunoglobulin G (IgG) that binds to macrophages during inflammation, has been studied for over 30 years; the receptor has never been cloned. Our results led to new conclusions about tuftsin, NP-1, and the purpose, heretofore unknown, of exon 8 in VEGF, which appears to be involved in NP-1 binding. Our disparate projects demonstrate that small-peptide targeted molecules can be very versatile in drug discovery in combination with classical medicinal chemistry. In particular, multivalent interactions can lead to unpredictable and useful biochemical information, as well as new drug candidates.

Targeting tumor angiogenesis

Our understanding of the process of tumor angiogenesis has changed significantly since the late 1970s, when vascular endothelial growth factor (VEGF) was first identified as vascular permeability factor and later found to be the major mediator of physiologic and pathologic angiogenesis. Since then, several additional VEGF-related ligands, VEGF receptors (VEGFRs), and complementary/alternative pathways that regulate tumor angiogenesis have been identified. Over the last decade, several antiangiogenic agents have been developed with the aim to inhibit new blood vessel growth, and we have learned that VEGF inhibition does far more than simply block new blood vessel growth. Clinical studies have demonstrated an improvement of progression-free and overall survivals with anti-VEGF therapy (with or without chemotherapy) in patients with advanced-stage malignancies. Unfortunately, even when anti-VEGF therapy is effective, the benefit of therapy is short-lived, with the development of tumor growth. We now recognize the presence of numerous complementary and redundant pathways that regulate tumor vasculature. For example, VEGF/VEGFR and angiopoietin/Tie-2 axes are two redundant, complementary components regulating tumor angiogenesis and vascular maintenance. The current clinical challenge is to identify: (1) factors that predict efficacy, and (2) markers of tumor response to anti-VEGF therapy, which can be achieved only by developing a thorough understanding of the biology of the VEGF system and the role of complementary pathways that may mediate resistance to anti-VEGF therapy.