Benefits of targeting both pericytes and endothelial cells in the tumor vasculature with kinase inhibitors

Targeting the PDGF signaling pathway in tumor treatment

Platelet-derived growth factor (PDGF) isoforms and PDGF receptors have important functions in the regulation of growth and survival of certain cell types during embryonal development and e.g. tissue repair in the adult. Overactivity of PDGF receptor signaling, by overexpression or mutational events, may drive tumor cell growth. In addition, pericytes of the vasculature and fibroblasts and myofibroblasts of the stroma of solid tumors express PDGF receptors, and PDGF stimulation of such cells promotes tumorigenesis. Inhibition of PDGF receptor signaling has proven to useful for the treatment of patients with certain rare tumors. Whether treatment with PDGF/PDGF receptor antagonists will be beneficial for more common malignancies is the subject for ongoing studies.

Blocking SDF-1α/CXCR4 downregulates PDGF-B and inhibits bone marrow-derived pericyte differentiation and tumor vascular expansion in Ewing tumors

Bone marrow cells (BMC) are critical to the expansion of the tumor vessel network that supports Ewing sarcoma growth. BMCs migrate to the tumor and differentiate into endothelial cells and pericytes. We recently demonstrated that stromal-derived growth factor 1α (SDF-1α) regulates platelet-derived growth factor B (PDGF-B) and that this pathway plays a critical role in bone marrow-derived pericyte differentiation in vitro. We investigated the role of SDF-1α/PDGF-B in the tumor microenvironment in vivo in promoting bone marrow-derived pericyte differentiation in Ewing tumors. The CXCR4 antagonist AMD 3100 was used to disrupt the SDF-1α/CXCR4 axis in vivo in two xenograft Ewing tumor models. BMCs from GFP(+) transgenic mice were transplanted into lethally irradiated nude mice to track BMC migration to the tumor site. Following BMC engraftment, tumor-bearing mice received daily subcutaneous injections of either PBS or AMD 3100 for 3 weeks. Tumors were resected and tumor sections were analyzed by immunohistochemistry. AMD 3100 inhibited BMC differentiation into desmin(+) and NG2(+) pericytes, affected the morphology of the tumor vasculature, decreased perfusion, and increased tumor cell apoptosis. We observed smaller vessels with tiny lumens and a decrease in the microvessel density. AMD 3100 also inhibited PDGF-B protein expression in vitro and in vivo. SDF-1α in the tumor microenvironment plays a critical role in promoting pericyte formation and Ewing sarcoma tumor neovascularization by regulating PDGF-B expression. Interfering with this pathway affects tumor vascular morphology and expansion.

Sustained delivery of a HIF-1 antagonist for ocular neovascularization

Doxorubicin (DXR) and daunorubicin (DNR) inhibit hypoxia-inducible factor-1 (HIF-1) transcriptional activity by blocking its binding to DNA. Intraocular injections of DXR or DNR suppressed choroidal and retinal neovascularization (NV), but also perturbed retinal function as demonstrated by electroretinograms (ERGs). DXR was conjugated to novel copolymers of branched polyethylene glycol and poly(sebacic acid) (DXR-PSA-PEG3) and formulated into nanoparticles that when placed in aqueous buffer, slowly released small DXR-conjugates. Intraocular injection of DXR-PSA-PEG3 nanoparticles (1 or 10 μg DXR content) reduced HIF-1-responsive gene products, strongly suppressed choroidal and retinal NV, and did not cause retinal toxicity. In transgenic mice that express VEGF in photoreceptors, intraocular injection of DXR-PSA-PEG3 nanoparticles (10 μg DXR content) suppressed NV for at least 35 days. Intraocular injection of DXR-PSA-PEG3 nanoparticles (2.7 mg DXR content) in rabbits resulted in sustained DXR-conjugate release with detectable levels in aqueous humor and vitreous for at least 105 days. This study demonstrates a novel HIF-1-inhibitor-polymer conjugate formulated into controlled-release particles that maximizes efficacy and duration of activity, minimizes toxicity, and provides a promising new chemical entity for treatment of ocular NV.

Integrin α(v)β₃-targeted radiotracer (99m)Tc-3P-RGD₂ useful for noninvasive monitoring of breast tumor response to antiangiogenic linifanib therapy but not anti-integrin α(v)β₃ RGD₂ therapy

Purpose: ^99mTc-3P-RGD₂ is a ^99mTc-labeled dimeric cyclic RGD peptide that binds to integrin α_vβ₃ with high affinity and specificity. The purpose of this study was to demonstrate the utility of ^99mTc-3P-RGD₂ SPECT/CT (single photon emission computed tomography/computed tomography) as a molecular imaging tool for noninvasive monitoring breast tumor early response to antiangiogenesis therapy with linifanib, and to illustrate its limitations in monitoring the efficacy of anti-α_vβ₃ treatment.

Methods: To support SPECT/CT imaging, biodistribution and therapy studies, the xenografted breast cancer model was established by subcutaneous injection of 5 × 10⁶ MDA-MB-435 cells into the fat pad of each athymic nude mouse. Linifanib (ABT-869) was used as antiangiogenesis agent. The tumor volume was 180 ± 90 mm³ on the day (-1 day) before baseline SPECT/CT. Each animal was treated twice daily with vehicle or 12.5 mg/kg linifanib. Longitudinal ^99mTc-3P-RGD₂ SPECT/CT imaging was performed on days -1, 1, 4 and 11. Tumors were harvested at each time point for pathological analysis of hematoxylin and eosin (H&E) and immunohistochemistry (IHC). Tumor uptake of ^99mTc-3P-RGD₂ was calculated from SPECT/CT quantification. When cyclic peptide E[c(RGDfK)]₂ (RGD₂) was used as the anti-α_vβ₃ agent, SPECT/CT images were obtained only at 7 and 21 days after last RGD₂ dose.

Results: The tumor uptake of ^99mTc-3P-RGD₂ from SPECT/CT quantification was almost identical to that from biodistribution. There was a dramatic reduction in both %ID and %ID/cm³ tumor uptake of ^99mTc-3P-RGD₂ during the first 24 hours of linifanib therapy. The therapeutic effect of linifanib was on both tumor cells and vasculature, as determined by IHC analysis of integrin α_vβ₃ and CD31. Changes in tumor vasculature were further confirmed by pathological H&E analysis of tumor tissues. While its %ID tumor uptake increased steadily in vehicle-treated group, the %ID tumor uptake of ^99mTc-3P-RGD₂ decreased in linifanib-treated group slowly over the 11-day study period. The degree of tumor response to linifanib therapy correlated well to the integrin α_vβ₃ expression levels before linifanib therapy.

Conclusion: ^99mTc-3P-RGD₂ is an excellent radiotracer for monitoring integrin α_vβ₃ expression during and after linifanib therapy. ^99mTc-3P-RGD₂ SPECT/CT is an useful molecular imaging tool for patient selection before antiangiogenic and anti-α_vβ₃ therapy; but it would be difficult to use ^99mTc-3P-RGD₂ for accurate and noninvasive monitoring of early tumor response to anti-α_vβ₃ therapy.

The Semaphorin 4D-Plexin-B1-RhoA signaling axis recruits pericytes and regulates vascular permeability through endothelial production of PDGF-B and ANGPTL4

Semaphorin 4D (SEMA4D) is a member of a family of transmembrane and secreted proteins that have been shown to act through its receptor Plexin-B1 to regulate axon growth cone guidance, lymphocyte activation, and bone density. SEMA4D is also overexpressed by some malignancies and plays a role in tumor-induced angiogenesis similar to vascular endothelial growth factor (VEGF), a protein that has been targeted as part of some cancer therapies. In an attempt to examine the different effects on tumor growth and vascularity for these two pro-angiogenic factors, we previously noted that while inhibition of both VEGF and SEMA4D restricted tumor vascularity and size, vessels forming under conditions of VEGF blockade retained their association with pericytes while those arising in a background of SEMA4D/Plexin-B1 deficiency did not, an intriguing finding considering that alteration in pericyte association with endothelial cells is an emerging aspect of anti-angiogenic intervention in the treatment of cancer. Here we show through array analysis, immunoblots, migration and co-culture assays and VE-cadherin immunohistochemistry that SEMA4D production by head and neck carcinoma tumor cells induces expression of platelet-derived growth factor-B and angiopoietin-like protein 4 from endothelial cells in a Plexin-B1/Rho-dependent manner, thereby influencing proliferation and differentiation of pericytes and vascular permeability, whereas VEGF lacks these effects. These results partly explain the differences observed between SEMA4D and VEGF in pathological angiogenesis and suggest that targeting SEMA4D function along with VEGF could represent a novel anti-angiogenic therapeutic strategy for the treatment of solid tumors.

Endothelial cell junctional adhesion molecule C plays a key role in the development of tumors in a murine model of ovarian cancer

Junctional adhesion molecule C (JAM-C) is a transmembrane protein with significant roles in regulation of endothelial cell (EC) functions, including immune cell recruitment and angiogenesis. As these responses are important in promoting tumor growth, the role of EC JAM-C in tumor development was investigated using the ID8 syngeneic model of ovarian cancer. Within 10-15 wk, intraperitoneally injected ID8 cells form multiple tumor deposits and ascites that resemble human high-grade serous ovarian cancer. Compared to wild-type mice, survival in this model was increased in EC JAM-C knockouts (KOs; 88 vs. 96 d, P=0.04) and reduced in EC JAM-C transgenics (88 vs. 78.5 d, P=0.03), mice deficient in or overexpressing EC JAM-C, respectively. While tumor growth was significantly reduced in EC JAM-C KOs (87% inhibition at 10 wk, P<0.0005), this was not associated with alterations in tumor vessel density or immune cell infiltration. However, tumor microvessels from EC JAM-C-deficient mice exhibited reduced pericyte coverage and increased vascular leakage, suggesting a role for EC JAM-C in the development of functional tumor vessels. These findings provide evidence for a role for EC JAM-C in tumor growth and aggressiveness as well as recruitment of pericytes to newly formed blood vessels in a model of ovarian cancer.

Phase I/II trial of capecitabine and oxaliplatin in combination with bevacizumab and imatinib in patients with metastatic colorectal cancer: AIO KRK 0205

Background:

Combined inhibition of platelet-derived growth factor receptor beta signalling and vascular endothelial growth factor promotes vascular normalisation in preclinical models and may lead to increased delivery of chemotherapy to tumour tissue. This phase I/II trial assessed the safety and efficacy of capecitabine plus oxaliplatin (XELOX) plus bevacizumab and imatinib in the first-line treatment of patients with metastatic colorectal cancer.

Methods:

Two dose levels (I/II) were defined: capecitabine 850/1000 mg m⁻² twice daily on days 1–14; oxaliplatin 100/130 mg m⁻² on day 1; bevacizumab 7.5 mg kg⁻¹ on day 1; imatinib 300 mg day⁻¹ on days 1–21 every 21 days. The primary study endpoint was safety. The phase II secondary endpoint was 6-month progression-free survival (PFS).

Results:

Dose level I was chosen for phase II testing because, even though further dose escalation was permitted by the protocol, gastrointestinal toxicities were considered to be clinically significant. A total of 49 patients were evaluated. The 6-month PFS rate was 76%, median PFS was 10.6 months and median overall survival was 23.2 months. Haematological toxicities were generally mild. Sensory neuropathy and diarrhoea were the most common grade 3 toxicities.

Conclusion:

The combination of XELOX with bevacizumab and imatinib is tolerable and has promising efficacy.

Effectivity of pazopanib treatment in orthotopic models of human testicular germ cell tumors

Background

Cisplatin (CDDP) resistance in testicular germ cell tumors (GCTs) is still a clinical challenge, and one associated with poor prognosis. The purpose of this work was to test pazopanib, an anti-tumoral and anti-angiogenic multikinase inhibitor, and its combination with lapatinib (an anti-ErbB inhibitor) in mouse orthotopic models of human testicular GCTs.

Methods

We used two different models of human testicular GCTs orthotopically grown in nude mice; a CDDP-sensitive choriocarcinoma (TGT38) and a new orthotopic model generated from a metastatic GCT refractory to first-line CDDP chemotherapy (TGT44). Nude mice implanted with these orthotopic tumors were treated with the inhibitors and the effect on tumoral growth and angiogenesis was evaluated.

Results

TGT44 refractory tumor had an immunohistochemical profile similar to the original metastasis, with characteristics of yolk sac tumor. TGT44 did not respond when treated with cisplatin. In contrast, pazopanib had an anti-angiogenic effect and anti-tumor efficacy in this model. Pazopanib in combination with lapatinib in TGT38, an orthotopic model of choriocarcinoma had an additive effect blocking tumor growth.

Conclusions

We present pazopanib as a possible agent for the alternative treatment of CDDP-sensitive and CDDP-refractory GCT patients, alone or in combination with anti-ErbB therapies.

Recruitment and retention: factors that affect pericyte migration

Pericytes are critical for vascular morphogenesis and contribute to several pathologies, including cancer development and progression. The mechanisms governing pericyte migration and differentiation are complex and have not been fully established. Current literature suggests that platelet-derived growth factor/platelet-derived growth factor receptor-β, sphingosine 1-phosphate/endothelial differentiation gene-1, angiopoietin-1/tyrosine kinase with immunoglobulin-like and EGF-like domains 2, angiopoietin-2/tyrosine kinase with immunoglobulin-like and EGF-like domains 2, transforming growth factor β/activin receptor-like kinase 1, transforming growth factor β/activin receptor-like kinase 5, Semaphorin-3A/Neuropilin, and matrix metalloproteinase activity regulate the recruitment of pericytes to nascent vessels. Interestingly, many of these pathways are directly affected by secreted protein acidic and rich in cysteine (SPARC). Here, we summarize the function of these factors in pericyte migration and discuss if and how SPARC might influence these activities and thus provide an additional layer of control for the recruitment of vascular support cells. Additionally, the consequences of targeted inhibition of pericytes in tumors and the current understanding of pericyte recruitment in pathological environments are discussed.

Angiogenic endothelial cell invasion into fibrin is stimulated by proliferating smooth muscle cells

These studies aimed to determine the effect of smooth muscle cells (SMCs) on angiogenic behavior of endothelial cells (ECs) within fibrin hydrogels, an extracellular matrix (ECM) commonly used in tissue engineering. We developed a 3-D, fibrin-based co-culture assay of angiogenesis consisting of aggregates of SMCs with ECs seeded onto the aggregates' surface. Using digital fluorescence micrography, EC matrix invasion was quantified by average length of sprouts (ALS) and density of sprout formation (DSF). We demonstrated that ECs and SMCs co-invade into the ECM in close proximity to one another. ECs that were co-cultured with SMCs demonstrated increased invasion compared to ECs that were cultured alone at all time points. At Day 19, the ALS of ECs in co-culture was 327+/-58μm versus 70+/-11μm of ECs cultured alone (p=.01). The DSF of co-cultured ECs was also significantly greater than that of ECs cultured alone (p=.007 on Day 19). This appeared to be a function of both increased EC invasion as well as improved persistence of EC sprout networks. At 7days, ECs in co-culture with proliferation-inhibited SMCs previously treated with Mitomycin-C (MMC) demonstrated significantly attenuated sprouting compared to ECs co-cultured with SMCs that were untreated with MMC (82+/-14μm versus 205+/-32μm; p<.05). In assays in which multiple co-culture aggregates were cultured within a single hydrogel, we observed directional invasion of sprouts preferentially towards the other aggregates within the hydrogel. In co-culture assays without early EC/SMC contact, the ALS of ECs cultured in the presence of SMCs was significantly greater than those cultured in the absence of SMCs by Day 3 (320+/-21μm versus 187+/-16μm; p<.005). We conclude that SMCs augment EC matrix invasion into 3-D fibrin hydrogels, at least in part resulting from SMC proliferative and invasive activities. Directed invasion between co-culture aggregates and augmented angiogenesis in the absence of early contact suggests a paracrine mechanism for the observed results.

The functionalized human serine protease granzyme B/VEGF₁₂₁ targets tumor vasculature and ablates tumor growth

The serine protease granzyme B (GrB) induces apoptosis through both caspase-dependent and -independent multiple-cascade mechanisms. VEGF₁₂₁ binds to both VEGF receptor (VEGFR)-1 and VEGFR-2 receptors. We engineered a unique GrB/VEGF₁₂₁ fusion protein and characterized its properties in vitro and in vivo. Endothelial and tumor cell lines showed varying levels of sensitivity to GrB/VEGF₁₂₁ that correlated closely to total VEGFR-2 expression. GrB/VEGF₁₂₁ localized efficiently into VEGFR-2-expressing cells, whereas the internalization into VEGFR-1-expressing cells was significantly reduced. Treatment of VEGFR-2(+) cells caused mitochondrial depolarization in 48% of cells by 48 hours. Exposure to GrB/VEGF₁₂₁ induced apoptosis in VEGFR-2(+), but not in VEGFR-1(+), cells and rapid caspase activation was observed that could not be inhibited by treatment with a pan-caspase inhibitor. In vivo, GrB/VEGF₁₂₁ localized in perivascular tumor areas adjacent to microvessels and in other areas in the tumor less well vascularized, whereas free GrB did not specifically localize to tumor tissue. Administration (intravenous) of GrB/VEGF₁₂₁ to mice at doses up to 40 mg/kg showed no toxicity. Treatment of mice bearing established PC-3 tumor xenografts with GrB/VEGF₁₂₁ showed significant antitumor effect versus treatment with GrB or saline. Treatment with GrB/VEGF₁₂₁ at 27 mg/kg resulted in the regression of four of five tumors in this group. Tumors showed a two-fold lower Ki-67-labeling index compared with controls. Our results show that targeted delivery of GrB to tumor vascular endothelial cells or to tumor cells activates apoptotic cascades and this completely human construct may have significant therapeutic potential.

Novel Phenotypic Fluorescent Three-Dimensional Platforms for High-throughput Drug Screening and Personalized Chemotherapy

We have developed novel phenotypic fluorescent three-dimensional co-culture platforms that efficiently and economically screen anti-angiogenic/anti-metastatic drugs on a high-throughput scale. Individual cell populations can be identified and isolated for protein/gene expression profiling studies and cellular movement/interactions can be tracked by time-lapse cinematography. More importantly, these platforms closely parallel the in vivo angiogenic and metastatic outcomes of a given tumor xenograft in the nude mouse model but, unlike in vivo models, our co-culture platforms produce comparable results in five to nine days. Potentially, by incorporating cancer patient biopsies, the co-culture platforms should greatly improve the effectiveness and efficiency of personalized chemotherapy.

Tivozanib in the treatment of renal cell carcinoma

Renal cell carcinoma (RCC) is an aggressive malignancy compared to other urological malignancies and has been associated with poor responses to conventional cytotoxic chemotherapy. Interferon-α and interleukin-2 were previously utilized in a limited number of patients with good performance status due to toxicity and safety issues. Over the last decade, through advances in the understanding of the biology and pathology of RCC, the important role of vascular endothelial growth factor (VEGF) in RCC has been identified. Data from randomized trials have led to the approval of first-generation tyrosine kinase inhibitors (TKIs) sorafenib, sunitinib, and pazopanib; however, these agents inhibit a wide variety of kinase targets and are associated with a range of adverse effects. More recently, a new generation TKI, axitinib, has been approved by the US Food and Drug Administration. Tivozanib is a novel TKI, which is a potent inhibitor of VEGF-1, VEGF-2, VEGF-3, c-kit, and PDGR kinases, with a more restricted target spectrum. Phase II and III studies have demonstrated significant activity and a favorable safety profile as an initial targeted treatment for advanced RCC. This review examines the emerging data with tivozanib for the treatment of advanced RCC. Preclinical investigations as well as Phase I, II, and III data are examined; data on the comparative benefits of tivozanib are reviewed. Finally, we discuss the future potential of tivozanib in combination, biomarkers associated with tivozanib response, and acquisition of resistance and nonkidney cancer indications.

Monitoring tumor response to linifanib therapy with SPECT/CT using the integrin αvβ3-targeted radiotracer 99mTc-3P-RGD2

The objective of this study was to determine the utility of (99m)Tc-3P-Arg-Gly-Asp (RGD2) single photon emission computed tomography (SPECT)/computed tomography (CT) for noninvasive monitoring of integrin αvβ3-expression response to antiangiogenic treatment with linifanib. Linifanib or vehicle therapy was carried out in female athymic nu/nu mice bearing U87MG glioma (high αvβ3 expression) or PC-3 prostate (low αvβ3 expression) tumors at 12.5 mg/kg twice daily. The average tumor volume was 180 ± 90 mm(3) the day prior to baseline SPECT/CT. Longitudinal (99m)Tc-3P-RGD2 SPECT/CT imaging was performed at baseline (-1 day) and days 1, 4, 11, and 18. Tumors were harvested at all imaging time points for histopathological analysis with H&E and immunohistochemistry. A significant difference in tumor volumes between vehicle- and linifanib-treated groups was observed after 4 days of linifanib therapy in the U87MG model. The percent injected dose (%ID) tumor uptake of (99m)Tc-3P-RGD2 peaked in the vehicle-treated group at day 11, while the %ID/cm(3) tumor uptake decreased slowly over the whole study period. During the first 2 days of linifanib treatment, a rapid decrease in both %ID/cm(3) tumor uptake and tumor/muscle ratios of (99m)Tc-3P-RGD2 was observed, followed by a slow decrease until day 18. No decrease in tumor uptake of (99m)Tc-3P-RGD2 or tumor volume was observed for either treatment group in the PC-3 model. Changes in tumor vasculature were confirmed by histopathological H&E analysis and immunohistochemistry. Longitudinal imaging using (99m)Tc-3P-RGD2 SPECT/CT may be a useful tool for monitoring the downstream biologic effects of linifanib therapy.

The Drosophila platelet-derived growth factor and vascular endothelial growth factor-receptor related (Pvr) protein ligands Pvf2 and Pvf3 control hemocyte viability and invasive migration

Vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) family members are essential and evolutionary conserved determinants of blood cell development and dispersal. In addition, VEGFs are integral to vascular growth and permeability with detrimental contributions to ischemic diseases and metastatic cancers. The PDGF/VEGF-receptor related (Pvr) protein is implicated in the migration and trophic maintenance of macrophage-like hemocytes in Drosophila melanogaster embryos. pvr mutants have a depleted hemocyte population and a breakdown in hemocyte distribution. Previous studies suggested redundant functions for the Pvr ligands, Pvf2 and Pvf3 in the regulation of hemocyte migration, proliferation, and size. However, the precise roles that Pvf2 and Pvf3 play in hematopoiesis remain unclear due to the lack of available mutants. To determine Pvf2 and Pvf3 functions in vivo, we generated a genomic deletion that simultaneously disrupts Pvf2 and Pvf3. From our studies, we identified contributions of Pvf2 and Pvf3 to the Pvr trophic maintenance of hemocytes. Furthermore, we uncovered a novel role for Pvfs in invasive migrations. We showed that Pvf2 and Pvf3 are not required for the directed migration of hemocytes, but act locally in epithelial cells to coordinate trans-epithelial migration of hemocytes. Our findings redefine Pvf roles in hemocyte migration and highlight novel Pvf roles in hemocyte invasive migration. These new parallels between the Pvr and PDGF/VEGF pathways extend the utility of the Drosophila embryonic system to dissect physiological and pathological roles of PDGF/VEGF-like growth factors.

BIBF 1120 (nintedanib), a triple angiokinase inhibitor, induces hypoxia but not EMT and blocks progression of preclinical models of lung and pancreatic cancer

Signaling from other angiokinases may underlie resistance to VEGF-directed therapy. We evaluated the antitumor and biologic effects of BIBF 1120 (nintedanib), a tyrosine kinase inhibitor that targets VEGF receptor, platelet-derived growth factor receptor, and fibroblast growth factor receptor in preclinical models of lung and pancreatic cancer, including models resistant to VEGF-targeted treatments. In vitro, BIBF 1120 did not show antiproliferative effects, nor did it sensitize tumor cells to chemotherapy. However, in vivo BIBF 1120 inhibited primary tumor growth in all models as a single agent and in combination with standard chemotherapy. Analysis of tumor tissue posttreatment revealed that BIBF 1120 reduced proliferation (phospho-histone 3) and elevated apoptosis (cleaved caspase-3) to a greater extent than chemotherapy alone. Furthermore, BIBF 1120 showed potent antiangiogenic effects, including decreases in microvessel density (CD31), pericyte coverage (NG2), vessel permeability, and perfusion, while increasing hypoxia. Despite the induction of hypoxia, markers of epithelial-to-mesenchymal transition (EMT) were not elevated in BIBF 1120-treated tumors. In summary, BIBF 1120 showed potent antitumor and antiangiogenic activity in preclinical models of lung and pancreatic cancer where it induced hypoxia but not EMT. The absence of EMT induction, which has been implicated in resistance to antiangiogenic therapies, is noteworthy. Together, these results warrant further clinical studies of BIBF 1120.

Glioblastoma stem cells generate vascular pericytes to support vessel function and tumor growth

Glioblastomas (GBMs) are highly vascular and lethal brain tumors that display cellular hierarchies containing self-renewing tumorigenic glioma stem cells (GSCs). Because GSCs often reside in perivascular niches and may undergo mesenchymal differentiation, we interrogated GSC potential to generate vascular pericytes. Here, we show that GSCs give rise to pericytes to support vessel function and tumor growth. In vivo cell lineage tracing with constitutive and lineage-specific fluorescent reporters demonstrated that GSCs generate the majority of vascular pericytes. Selective elimination of GSC-derived pericytes disrupts the neovasculature and potently inhibits tumor growth. Analysis of human GBM specimens showed that most pericytes are derived from neoplastic cells. GSCs are recruited toward endothelial cells via the SDF-1/CXCR4 axis and are induced to become pericytes predominantly by transforming growth factor β. Thus, GSCs contribute to vascular pericytes that may actively remodel perivascular niches. Therapeutic targeting of GSC-derived pericytes may effectively block tumor progression and improve antiangiogenic therapy.

Tumor-derived mural-like cells coordinate with endothelial cells: role of YKL-40 in mural cell-mediated angiogenesis

Tumor neo-vasculature is characterized by spatial coordination of endothelial cells with mural cells, which delivers oxygen and nutrients. Here, we explored a key role of the secreted glycoprotein YKL-40, a mesenchymal marker, in the interaction between endothelial cells and mesenchymal mural-like cells for tumor angiogenesis. Xenotransplantation of tumor-derived mural-like cells (GSDCs) expressing YKL-40 in mice developed extensive and stable blood vessels covered with more GSDCs than those in YKL-40 gene knockdown tumors. YKL-40 expressed by GSDCs was associated with increased interaction of neural cadherin/β-catenin/smooth muscle alpha actin; thus, mediating cell-cell adhesion and permeability. YKL-40 also induced the interaction of vascular endothelial cadherin/β-catenin/actin in endothelial cells (HMVECs). In cell co-culture systems, YKL-40 enhanced both GSDC and HMVEC contacts, restricted vascular leakage, and stabilized vascular networks. Collectively, the data inform new mechanistic insights into the cooperation of mural cells with endothelial cells induced by YKL-40 during tumor angiogenesis, and also enhance our understanding of YKL-40 in both mural and endothelial cell biology.

Imatinib disrupts lymphoma angiogenesis by targeting vascular pericytes

Pericytes and vascular smooth muscle cells (VSMCs), which are recruited to developing blood vessels by platelet-derived growth factor BB, support endothelial cell survival and vascular stability. Here, we report that imatinib, a tyrosine kinase inhibitor of platelet-derived growth factor receptor β (PDGFRβ), impaired growth of lymphoma in both human xenograft and murine allograft models. Lymphoma cells themselves neither expressed PDGFRβ nor were growth inhibited by imatinib. Tumor growth inhibition was associated with decreased microvascular density and increased vascular leakage. In vivo, imatinib induced apoptosis of tumor-associated PDGFRβ(+) pericytes and loss of perivascular integrity. In vitro, imatinib inhibited PDGFRβ(+) VSMC proliferation and PDGF-BB signaling, whereas small interfering RNA knockdown of PDGFRβ in pericytes protected them against imatinib-mediated growth inhibition. Fluorescence-activated cell sorter analysis of tumor tissue revealed depletion of pericytes, endothelial cells, and their progenitors following imatinib treatment. Compared with imatinib, treatment with an anti-PDGFRβ monoclonal antibody partially inhibited lymphoma growth. Last, microarray analysis (Gene Expression Omnibus database accession number GSE30752) of PDGFRβ(+) VSMCs following imatinib treatment showed down-regulation of genes implicated in vascular cell proliferation, survival, and assembly, including those representing multiple pathways downstream of PDGFRβ. Taken together, these data indicate that PDGFRβ(+) pericytes may represent a novel, nonendothelial, antiangiogenic target for lymphoma therapy.

Development and characterization of a high-throughput in vitro cord formation model insensitive to VEGF inhibition

Background

Anti-VEGF therapy reduces tumor blood vessels, however, some vessels always remain. These VEGF insensitive vessels may help support continued tumor growth and metastases. Many in vitro assays examining multiple steps of the angiogenic process have been described, but the majority of these assays are sensitive to VEGF inhibition. There has been little focus on the development of high-throughput, in vitro assays to model the vessels that are insensitive to VEGF inhibition.

Methods

Here, we describe a fixed end-point and kinetic, high-throughput stem cell co-culture model of cord formation.

Results

In this system, cords develop within 24 hours, at which point they begin to lose sensitivity to VEGF inhibitors, bevacizumab, and ramucirumab. Consistent with the hypothesis that other angiogenic factors maintain VEGF-independent vessels, pharmacologic intervention with a broad spectrum anti-angiogenic antagonist (suramin), a vascular disrupting agent (combretastatin), or a combination of VEGF and Notch pathway inhibitors reduced the established networks. In addition, we used our in vitro approach to develop an in vivo co-implant vasculogenesis model that connects with the endogenous vasculature to form functional blood vessels. Similar to the in vitro system, over time these vessels become insensitive to VEGF inhibition.

Conclusion

Together, these models may be used to identify novel drugs targeting tumor vessels that are not sensitive to VEGF inhibition.

Critical appraisal of the use of regorafenib in the management of colorectal cancer

The lack of valid clinical management options for patients affected by metastatic colorectal cancer, which has progressed after all approved standard treatments, has lead to research into new active molecules. Regorafenib is an oral small-molecule multi kinase inhibitor, binding to several intracellular kinases, with powerful inhibitory activity against vascular endothelial growth factor receptors (VEGFR-1,VEGFR-2, and VEGFR-3), platelet-derived growth factor receptor, fibroblast growth factor receptor 1, Raf, TIE-2, and the kinases KIT, RET, and BRAF. The antitumor activity of regorafenib has been tested in vitro and in vivo, and inhibition of tumor growth has been observed in several cancer models, particularly colorectal cancer and gastrointestinal stromal tumors. The most frequent adverse events of grade 3 or higher related to regorafenib were hand-foot skin reaction, fatigue, diarrhea, hypertension, and rash or desquamation. Only a few Phase I–II trials, and most recently a Phase III trial in pretreated colorectal cancer, have been carried out to date. Several ongoing trials are testing the efficacy of regorafenib in combination with chemotherapy. At this point in time, regorafenib is the first small-molecule tyrosine kinase inhibitor to gain approval by the US Food and Drug Administration for pretreated metastatic colorectal cancer patients.

Mouse models for studying angiogenesis and lymphangiogenesis in cancer

The formation of new blood vessels (angiogenesis) is required for the growth of most tumors. The tumor microenvironment also induces lymphangiogenic factors that promote metastatic spread. Anti-angiogenic therapy targets the mechanisms behind the growth of the tumor vasculature. During the past two decades, several strategies targeting blood and lymphatic vessels in tumors have been developed. The blocking of vascular endothelial growth factor (VEGF)/VEGF receptor-2 (VEGFR-2) signaling has proven effective for inhibition of tumor angiogenesis and growth, and inhibitors of VEGF-C/VEGFR-3 involved in lymphangiogenesis have recently entered clinical trials. However, thus far anti-angiogenic treatments have been less effective in humans than predicted on the basis of pre-clinical tests in mice. Intrinsic and induced resistance against anti-angiogenesis occurs in patients, and thus far the clinical benefit of the treatments has been limited to modest improvements in overall survival in selected tumor types. Our current knowledge of tumor angiogenesis is based mainly on experiments performed in tumor-transplanted mice, and it has become evident that these models are not representative of human cancer. For an improved understanding, angiogenesis research needs models that better recapitulate the multistep tumorigenesis of human cancers, from the initial genetic insults in single cells to malignant progression in a proper tissue environment. To improve anti-angiogenic therapies in cancer patients, it is necessary to identify additional molecular targets important for tumor angiogenesis, and to get mechanistic insight into their interactions for eventual combinatorial targeting. The recent development of techniques for manipulating the mammalian genome in a precise and predictable manner has opened up new possibilities for the generation of more reliable models of human cancer that are essential for the testing of new therapeutic strategies. In addition, new imaging modalities that permit visualization of the entire mouse tumor vasculature down to the resolution of single capillaries have been developed in pre-clinical models and will likely benefit clinical imaging.

Imatinib and Dasatinib Inhibit Hemangiosarcoma and Implicate PDGFR-β and Src in Tumor Growth

Hemangiosarcoma, a natural model of human angiosarcoma, is an aggressive vascular tumor diagnosed commonly in dogs. The documented expression of several receptor tyrosine kinases (RTKs) by these tumors makes them attractive targets for therapeutic intervention using tyrosine kinase inhibitors (TKIs). However, we possess limited knowledge of the effects of TKIs on hemangiosarcoma as well as other soft tissue sarcomas. We report here on the use of the TKIs imatinib and dasatinib in canine hemangiosarcoma and their effects on platelet-derived growth factor receptor β (PDGFR-β) and Src inhibition. Both TKIs reduced cell viability, but dasatinib was markedly more potent in this regard, mediating cytotoxic effects orders of magnitude greater than imatinib. Dasatinib also inhibited the phosphorylation of the shared PDGFR-β target at a concentration approximately 1000 times less than that needed by imatinib and effectively blocked Src phosphorylation. Both inhibitors augmented the response to doxorubicin, suggesting that clinical responses likely will be improved using both drugs in combination; however, dasatinib was significantly (P < .05) more effective in this context. Despite the higher concentrations needed in cell-based assays, imatinib significantly inhibited tumor growth (P < .05) in a tumor xenograft model, highlighting that disruption of PDGFR-β/PDGF signaling may be important in targeting the angiogenic nature of these tumors. Treatment of a dog with spontaneously occurring hemangiosarcoma established that clinically achievable doses of dasatinib may be realized in dogs and provides a means to investigate the effect of TKIs on soft tissue sarcomas in a large animal model.

Targeted treatment for advanced soft tissue sarcoma: profile of pazopanib

Soft tissue sarcomas comprise approximately 1% of all adult solid malignancies. While chemotherapy is the mainstay of treatment for patients with metastatic or inoperable disease, overall survival for these patients is approximately 12 months, highlighting the need for novel agents. Both laboratory and clinical data have suggested that antiangiogenic agents may have a role in the treatment of soft tissue sarcomas. Pazopanib is a multitargeted receptor tyrosine kinase inhibitor with antiangiogenic activity. The randomized, double-blind, placebo-controlled, Phase III PALETTE (pazopanib for metastatic soft-tissue sarcoma) study demonstrated improved progression-free survival in patients receiving pazopanib compared with placebo. In this review, we discuss the rationale and clinical evidence for the use of pazopanib in the treatment of metastatic and inoperable soft tissue sarcomas.

Anti-angiogenic and anti-tumor effects of TAK-593, a potent and selective inhibitor of vascular endothelial growth factor and platelet-derived growth factor receptor tyrosine kinase

We recently reported that TAK-593, a novel imidazo[1,2-b]pyridazine derivative, is a highly potent and selective inhibitor of the vascular endothelial growth factor (VEGF) and platelet derived growth factor (PDGF) receptor tyrosine kinase families. Moreover, TAK-593 exhibits a uniquely long-acting inhibitory profile towards VEGF receptor 2 (VEGFR2) and PDGF receptor β (PDGFRβ). In this study, we demonstrated that TAK-593 potently inhibits VEGF- and PDGF-stimulated cellular phosphorylation and proliferation of human umbilical vein endothelial cells and human coronary artery smooth muscle cells. TAK-593 also potently inhibits VEGF-induced tube formation of endothelial cells co-cultured with fibroblasts. Oral administration of TAK-593 exhibited strong anti-tumor effects against various human cancer xenografts along with good tolerability despite a low level of plasma exposure. Even after the blood and tissue concentrations of TAK-593 decreased below the detectable limit, a pharmacodynamic marker (phospho VEGFR2) was almost completely suppressed, indicating that its long duration of enzyme inhibition might contribute to the potent activity of TAK-593. Immunohistochemical staining indicated that TAK-593 showed anti-proliferative and pro-apoptotic effects on tumors along with a decrease of vessel density and inhibition of pericyte recruitment to microvessels in vivo. Furthermore, dynamic contrast-enhanced magnetic resonance imaging revealed that TAK-593 reduced tumor vessel permeability prior to the onset of anti-tumor activity. In conclusion, TAK-593 is an extremely potent VEGFR/PDGFR kinase inhibitor whose potent anti-angiogenic activity suggests therapeutic potential for the treatment of solid tumors.

Investigational agents in development for the treatment of ovarian cancer

Although significant success has been achieved in the treatment of advanced and recurrent ovarian cancer, there is clearly room for improvement. The use of targeted agents in this patient population has the promise to provide improved survival and quality of life. There are a myriad of relevant pathways under exploration in all settings of ovarian cancer. Clinical trial data are accumulating for antiangiogenic therapy, including vascular endothelial growth factor (VEGF)-specific inhibitors and multiple angiogenic signaling target inhibitors, as well as poly-ADP-ribose polymerase (PARP) inhibitors. Other types of tumorigenic pathway inhibitors, including those that target phosphatidylinositol-3-kinase (PI3K), mammalian target of rapamycin (mTOR), protein kinase B (AKT), Src, folate receptor alpha, and insulin-like growth factor-1 receptor (IGF-1R) pathways are in earlier phases of development for ovarian cancer. Attempts to target the epidermal growth factor receptor (EGFR) of ovarian tumors have been met with limited success; however, newer agents that inhibit this pathway show promise. Finally, with recognition of the role of Wee-1 in p53-deficient tumors, an inhibitor of this tyrosine kinase is being evaluated in recurrent ovarian cancer. The logistical challenge is to determine the optimal timing and proper combinations of novel agents independently as well as concomitantly with conventional chemotherapeutics. Reported results have been modest; however, our growing understanding of these pathways will be potentially reflected in greater impact on response and survival.

P-Rex1 cooperates with PDGFRβ to drive cellular migration in 3D microenvironments

Expression of the Rac-guanine nucleotide exchange factor (RacGEF), P-Rex1 is a key determinant of progression to metastasis in a number of human cancers. In accordance with this proposed role in cancer cell invasion and metastasis, we find that ectopic expression of P-Rex1 in an immortalised human fibroblast cell line is sufficient to drive multiple migratory and invasive phenotypes. The invasive phenotype is greatly enhanced by the presence of a gradient of serum or platelet-derived growth factor, and is dependent upon the expression of functional PDGF receptor β. Consistently, the invasiveness of WM852 melanoma cells, which endogenously express P-Rex1 and PDGFRβ, is opposed by siRNA of either of these proteins. Furthermore, the current model of P-Rex1 activation is advanced through demonstration of P-Rex1 and PDGFRβ as components of the same macromolecular complex. These data suggest that P-Rex1 has an influence on physiological migratory processes, such as invasion of cancer cells, both through effects upon classical Rac1-driven motility and a novel association with RTK signalling complexes.

Spindle shaped human mesenchymal stem/stromal cells from amniotic fluid promote neovascularization

Human amniotic fluid obtained at amniocentesis, when cultured, generates at least two morphologically distinct mesenchymal stem/stromal cell (MSC) subsets. Of these, the spindle shaped amniotic fluid MSCs (SS-AF-MSCs) contain multipotent cells with enhanced adipogenic, osteogenic and chondrogenic capacity. Here, we demonstrate, for the first time, the capacity of these SS-AF-MSCs to support neovascularization by umbilical cord blood (UCB) endothelial colony forming cell (ECFC) derived cells in both in vitro and in vivo models. Interestingly, although the kinetics of vascular tubule formation in vitro were similar when the supporting SS-AF-MSCs were compared with the best vasculogenic supportive batches of bone marrow MSCs (BMSCs) or human dermal fibroblasts (hDFs), SS-AF-MSCs supported vascular tubule formation in vivo more effectively than BMSCs. In NOD/SCID mice, the human vessels inosculated with murine vessels demonstrating their functionality. Proteome profiler array analyses revealed both common and distinct secretion profiles of angiogenic factors by the SS-AF-MSCs as opposed to the hDFs and BMSCs. Thus, SS-AF-MSCs, which are considered to be less mature developmentally than adult BMSCs, and intermediate between adult and embryonic stem cells in their potentiality, have the additional and very interesting potential of supporting increased neovascularisation, further enhancing their promise as vehicles for tissue repair and regeneration.

Safety and efficacy of the multitargeted receptor kinase inhibitor pazopanib in the treatment of corneal neovascularization

PURPOSE

To evaluate the safety and efficacy of topical pazopanib in the treatment of corneal neovascularization (CNV).

METHODS

Twenty eyes of 20 patients with stable CNV were enrolled in a prospective, open label, noncomparative study and treated with topical pazopanib 0.5% for 3 weeks, and followed for 12 weeks. The primary endpoint was to determine the tolerability and safety of topical pazopanib in the treatment of CNV defined by the occurrence of ocular and systemic adverse events during the study. The secondary endpoint was to evaluate the effect of topical pazopanib on the reduction of (1) neovascular area (NA), defined as the area of the corneal vessels themselves, (2) invasion area (IA), defined as the fraction of the total cornea into which the vessels extend, (3) vessel length (VL), defined as the mean measurement of the extent of vessels from end to end, and (4) vessel caliber (VC), defined as the mean diameter of the corneal vessels.

RESULTS

There were no severe adverse events following the use of topical pazopanib. Compared with the baseline visit, NA and VL showed a statistically significant decrease at week 3 (P = 0.02 and 0.01, respectively); and NA, IA, and VL statistically significantly decreased at week 12 (P = 0.03, 0.04, and <0.01, respectively). Visual acuity maintained without changes after the 12 week follow-up.

CONCLUSIONS

This preliminary study suggests that topical treatment with pazopanib 0.5% is safe, well tolerated, and may have a role as an alternative for the treatment of CNV (ClinicalTrials.gov number, NCT01257750).

Odyssey of a cancer nanoparticle: from injection site to site of action

No chemotherapeutic drug can be effective until it is delivered to its target site. Nano-sized drug carriers are designed to transport therapeutic or diagnostic materials from the point of administration to the drug's site of action. This task requires the nanoparticle carrying the drug to complete a journey from the injection site to the site of action. The journey begins with the injection of the drug carrier into the bloodstream and continues through stages of circulation, extravasation, accumulation, distribution, endocytosis, endosomal escape, intracellular localization and-finally-action. Effective nanoparticle design should consider all of these stages to maximize drug delivery to the entire tumor and effectiveness of the treatment.

Tanshinone IIA inhibits metastasis after palliative resection of hepatocellular carcinoma and prolongs survival in part via vascular normalization

Background

Promotion of endothelial normalization restores tumor oxygenation and obstructs tumor cells invasion, intravasation, and metastasis. We therefore investigated whether a vasoactive drug, tanshinone IIA, could inhibit metastasis by inducing vascular normalization after palliative resection (PR) of hepatocellular carcinoma (HCC).

Methods

A liver orthotopic double-tumor xenograft model in nude mouse was established by implantation of HCCLM3 (high metastatic potential) and HepG2 tumor cells. After removal of one tumor by PR, the effects of tanshinone IIA administration on metastasis, tumor vascularization, and survival were evaluated. Tube formation was examined in mouse tumor-derived endothelial cells (TECs) treated with tanshinone IIA.

Results

PR significantly accelerated residual hepatoma metastases. Tanshinone IIA did not inhibit growth of single-xenotransplanted tumors, but it did reduce the occurrence of metastases. Moreover, it inhibited PR-enhanced metastases and, more importantly, prolonged host survival. Tanshinone IIA alleviated residual tumor hypoxia and suppressed epithelial-mesenchymal transition (EMT) in vivo; however, it did not downregulate hypoxia-inducible factor 1α (HIF-1α) or reverse EMT of tumor cells under hypoxic conditions in vitro. Tanshinone IIA directly strengthened tube formation of TECs, associated with vascular endothelial cell growth factor receptor 1/platelet derived growth factor receptor (VEGFR1/PDGFR) upregulation. Although the microvessel density (MVD) of residual tumor tissue increased after PR, the microvessel integrity (MVI) was still low. While tanshinone IIA did not inhibit MVD, it did dramatically increase MVI, leading to vascular normalization.

Conclusions

Our results demonstrate that tanshinone IIA can inhibit the enhanced HCC metastasis associated with PR. Inhibition results from promoting VEGFR1/PDGFR-related vascular normalization. This application demonstrates the potential clinical benefit of preventing postsurgical recurrence.

Glioblastoma resistance to anti-VEGF therapy is associated with myeloid cell infiltration, stem cell accumulation, and a mesenchymal phenotype

Vascular endothelial growth factor (VEGF) is a critical regulator of angiogenesis. Inhibiting the VEGF-VEGF receptor (R) signal transduction pathway in glioblastoma has recently been shown to delay progression, but the relative benefit and mechanisms of response and failure of anti-VEGF therapy and VEGFR inhibitors are not well understood. The purpose of our study was to evaluate the relative effectiveness of VEGF sequestration and/or VEGFR inhibition on orthotopic tumor growth and the mechanism(s) of treatment resistance. We evaluated, not only, the effects of anti-VEGF therapy (bevacizumab), anti-VEGFR therapy (sunitinib), and the combination on the survival of mice bearing orthotopic gliomas, but also the differential effects of the treatments on tumor vascularity, cellular proliferation, mesenchymal and stem cell markers, and myeloid cell infiltration using flow cytometry and immunohistochemistry. Bevacizumab significantly prolonged survival compared with the control or sunitinib alone. Both antiangiogenic agents initially reduced infiltration of macrophages and tumor vascularity. However, multitargeted VEGFR inhibition, but not VEGF sequestration, rapidly created a vascular gradient and more rapidly induced tumor hypoxia. Re-infiltration of macrophages was associated with the induction of hypoxia. Combination treatment with bevacizumab and sunitinib improved animal survival compared with bevacizumab therapy alone. However, at the time of tumor progression, a significant increase in CD11b(+)/Gr1(+) granulocyte infiltration was observed, and tumors developed aggressive mesenchymal features and increased stem cell marker expression. Collectively, our results demonstrate a more prolonged decrease in tumor vascularity with bevacizumab than with sunitinib, associated with a delay in the development of hypoxia and sustained reduction of infiltrated myeloid cells.

Antiangiogenic agents in the management of non-small cell lung cancer: where do we stand now and where are we headed?

Several therapies targeting angiogenesis are currently in development for non-small cell lung cancer (NSCLC). This review discusses results of recent clinical trials evaluating chemotherapy plus antiangiogenic therapy for NSCLC. Bevacizumab, an anti-VEGF antibody, is currently approved for the treatment of advanced NSCLC in combination with carboplatin and paclitaxel. Completed phase III trials evaluating bevacizumab plus chemotherapy have shown prolonged progression-free survival; however, not all trials showed significant improvement in overall survival (OS). Phase III trials of the tyrosine kinase inhibitors (TKIs) vandetanib and sorafenib and the vascular disrupting agent ASA404 also failed to improve OS compared with chemotherapy alone. Clinical trials are ongoing involving several new antiangiogenic therapies, including ramucirumab, aflibercept, cediranib, BIBF 1120, sunitinib, pazopanib, brivanib, ABT-869, axitinib, ABT-751, and NPI-2358; several of these agents have shown promising phase I/II results. Results from recently completed and ongoing phase III trials will determine if these newer antiangiogenic agents will be incorporated into clinical practice.

Gene expression analysis in human breast cancer associated blood vessels

Angiogenesis is essential for solid tumour growth, whilst the molecular profiles of tumour blood vessels have been reported to be different between cancer types. Although presently available anti-angiogenic strategies are providing some promise for the treatment of some cancers it is perhaps not surprisingly that, none of the anti-angiogenic agents available work on all tumours. Thus, the discovery of novel anti-angiogenic targets, relevant to individual cancer types, is required. Using Affymetrix microarray analysis of laser-captured, CD31-positive blood vessels we have identified 63 genes that are upregulated significantly (5-72 fold) in angiogenic blood vessels associated with human invasive ductal carcinoma (IDC) of the breast as compared with blood vessels in normal human breast. We tested the angiogenic capacity of a subset of these genes. Genes were selected based on either their known cellular functions, their enriched expression in endothelial cells and/or their sensitivity to anti-VEGF treatment; all features implicating their involvement in angiogenesis. For example, RRM2, a ribonucleotide reductase involved in DNA synthesis, was upregulated 32-fold in IDC-associated blood vessels; ATF1, a nuclear activating transcription factor involved in cellular growth and survival was upregulated 23-fold in IDC-associated blood vessels and HEX-B, a hexosaminidase involved in the breakdown of GM2 gangliosides, was upregulated 8-fold in IDC-associated blood vessels. Furthermore, in silico analysis confirmed that AFT1 and HEX-B also were enriched in endothelial cells when compared with non-endothelial cells. None of these genes have been reported previously to be involved in neovascularisation. However, our data establish that siRNA depletion of Rrm2, Atf1 or Hex-B had significant anti-angiogenic effects in VEGF-stimulated ex vivo mouse aortic ring assays. Overall, our results provide proof-of-principle that our approach can identify a cohort of potentially novel anti-angiogenic targets that are likley to be, but not exclusivley, relevant to breast cancer.

Resistance and escape from antiangiogenesis therapy: clinical implications and future strategies

Angiogenesis has long been considered an important target for cancer therapy. Initial efforts have primarily focused on targeting of endothelial and tumor-derived vascular endothelial growth factor signaling. As evidence emerges that angiogenesis has significant mechanistic complexity, therapeutic resistance and escape have become practical limitations to drug development. Here, we review the mechanisms by which dynamic changes occur in the tumor microenvironment in response to antiangiogenic therapy, leading to drug resistance. These mechanisms include direct selection of clonal cell populations with the capacity to rapidly upregulate alternative proangiogenic pathways, increased invasive capacity, and intrinsic resistance to hypoxia. The implications of normalization of vasculature with subsequently improved vascular function as a result of antiangiogenic therapy are explored, as are the implications of the ability to incorporate and co-opt otherwise normal vasculature. Finally, we consider the extent to which a better understanding of the biology of hypoxia and reoxygenation, as well as the depth and breadth of systems invested in angiogenesis, may offer putative biomarkers and novel therapeutic targets. Insights gained through this work may offer solutions for personalizing antiangiogenesis approaches and improving the outcome of patients with cancer.

Essential roles of EphB receptors and EphrinB ligands in endothelial cell function and angiogenesis

Eph receptor tyrosine kinases and their Ephrin ligands represent an important signaling system with widespread roles in cell physiology and disease. Receptors and ligands in this family are anchored to the cell surface; thus Eph/Ephrin interactions mainly occur at sites of cell-to-cell contact. EphB4 and EphrinB2 are the Eph/Ephrin molecules that play essential roles in vascular development and postnatal angiogenesis. Analysis of expression patterns and function has linked EphB4/EphrinB2 to endothelial cell growth, survival, migration, assembly, and angiogenesis. Signaling from these molecules is complex, with the potential for being bidirectional, emanating both from the Eph receptors (forward signaling) and from the Ephrin ligands (reverse signaling). In this review, we describe recent advances on the roles of EphB/EphrinB protein family in endothelial cell function and outline potential approaches to inhibit pathological angiogenesis based on this understanding.

FDA approval summary: sunitinib for the treatment of progressive well-differentiated locally advanced or metastatic pancreatic neuroendocrine tumors

On May 20, 2011, the U.S. Food and Drug Administration (FDA) approved sunitinib malate capsules (Sutent®; Pfizer, Inc., New York) for the treatment of progressive, well-differentiated pancreatic neuroendocrine tumors (pNETs) in patients with unresectable locally advanced or metastatic disease. In a phase III randomized trial, 171 patients received either sunitinib (37.5 mg) or placebo once daily. The progression-free survival (PFS) interval was the primary efficacy endpoint. Secondary endpoints included the overall survival (OS) time, objective response rate (ORR), patient-reported outcomes, and safety. Based on early results favoring sunitinib, the independent data monitoring committee recommended trial termination prior to the prespecified interim analysis. This premature analysis may have led to an overestimate of the treatment effect. In the FDA analysis of investigator-assessed PFS times, the median values for the sunitinib and placebo arms were 10.2 months and 5.4 months, respectively. The ORRs were 9.3% and 0% in the sunitinib and placebo arms, respectively. The OS data were not mature at the time of approval and were confounded by 69% crossover. Common adverse reactions in patients receiving sunitinib included diarrhea, nausea, asthenia, fatigue, neutropenia, hypertension, and palmar-plantar erythrodysesthesia syndrome. Two patients on sunitinib died as a result of cardiac failure. The Oncologic Drugs Advisory Committee voted eight to two that, despite residual uncertainty about the magnitude of the PFS effect because of early trial termination, sunitinib demonstrated a favorable benefit-risk profile in pNET patients. The FDA concurred with the committee's assessment and granted sunitinib regular approval for this rare malignancy with few available therapies.

Combining Bevacizumab with Radiation or Chemoradiation for Solid Tumors: A Review of the Scientific Rationale, and Clinical Trials

Radiation therapy or the combination of radiation and chemotherapy is an important component in the local control of many tumor types including glioblastoma, rectal cancer, and pancreatic cancer. The addition of anti-angiogenic agents to chemotherapy is now standard treatment for a variety of metastatic cancers including colorectal cancer and non-squamous cell lung cancer. Anti-angiogenic agents can increase the efficacy of radiation or chemoradiation for primary tumors through mechanisms such as vascular normalization and augmentation of endothelial cell injury. The most commonly used anti-angiogenic drug, bevacizumab, is a humanized monoclonal antibody that binds and neutralizes vascular endothelial growth factor A (VEGF-A). Dozens of preclinical studies nearly uniformly demonstrate that inhibition of VEGF-A or its receptors potentiates the effects of radiation therapy against solid tumors, and this potentiation is generally independent of the type or schedule of radiation and timing of VEGF-A inhibitor delivery. There are now several clinical trials combining bevacizumab with radiation or chemoradiation for the local control of various primary, recurrent, and metastatic tumors, and many of these early trials show encouraging results. Some added toxicities occur with the delivery of bevacizumab but common toxicities such as hypertension and proteinuria are generally easily managed while severe toxicities are rare. In the future, bevacizumab and other anti-angiogenic agents may become common additions to radiation and chemoradiation regimens for tumors that are difficult to locally control.

Intrinsic regulation of hemangioma involution by platelet-derived growth factor

Infantile hemangioma is a vascular tumor that exhibits a unique natural cycle of rapid growth followed by involution. Previously, we have shown that hemangiomas arise from CD133+ stem cells that differentiate into endothelial cells when implanted in immunodeficient mice. The same clonally expanded stem cells also produced adipocytes, thus recapitulating the involuting phase of hemangioma. In the present study, we have elucidated the intrinsic mechanisms of adipocyte differentiation using hemangioma-derived stem cells (hemSCs). We found that platelet-derived growth factor (PDGF) is elevated during the proliferating phase and may inhibit adipocyte differentiation. hemSCs expressed high levels of PDGF-B and showed sustained tyrosine phosphorylation of PDGF receptors under basal (unstimulated) conditions. Inhibition of PDGF receptor signaling caused enhanced adipogenesis in hemSCs. Furthermore, exposure of hemSCs to exogenous PDGF-BB reduced the fat content and the expression of adipocyte-specific transcription factors. We also show that these autogenous inhibitory effects are mediated by PDGF receptor-β signaling. In summary, this study identifies PDGF signaling as an intrinsic negative regulator of hemangioma involution and highlights the therapeutic potential of disrupting PDGF signaling for the treatment of hemangiomas.

Retinoblastoma treatment: impact of the glycolytic inhibitor 2-deoxy-d-glucose on molecular genomics expression in LH(BETA)T(AG) retinal tumors

PURPOSE

The purpose of this study was to evaluate the effect of 2-deoxy-D-glucose (2-DG) on the spatial distribution of the genetic expression of key elements involved in angiogenesis, hypoxia, cellular metabolism, and apoptosis in LH(BETA)T(AG) retinal tumors.

METHODS

The right eye of each LH(BETA)T(AG) transgenic mouse (n = 24) was treated with either two or six subconjunctival injections of 2-DG (500 mg/kg) or saline control at 16 weeks of age. A gene expression array analysis was performed on five different intratumoral regions (apex, center, base, anterior-lateral, and posterior-lateral) using Affymetrix GeneChip Mouse Gene 1.0 ST arrays. To test for treatment effects of each probe within each region, a two-way analysis of variance was used.

RESULTS

Significant differences between treatment groups (ie, 0, 2, and 6 injections) were found as well as differences among the five retinal tumor regions evaluated (P < 0.01). More than 100 genes were observed to be dysregulated by ≥2-fold difference in expression between the three treatment groups, and their dysregulation varied across the five regions assayed. Several genes involved in pathways important for tumor cell growth (ie, angiogenesis, hypoxia, cellular metabolism, and apoptosis) were identified.

CONCLUSIONS

2-DG was found to significantly alter the gene expression in LH(BETA)T(AG) retinal tumor cells according to their location within the tumor as well as the treatment schedule. 2-DG's effects on genetic expression found here correlate with previous reported results on varied processes involved in its in vitro and in vivo activity in inhibiting tumor cell growth.

The VHL/HIF axis in clear cell renal carcinoma

Inactivation of the VHL tumor suppressor protein (pVHL) is a common event in clear cell renal carcinoma, which is the most common form of kidney cancer. pVHL performs many functions, including serving as the substrate recognition module of an ubiquitin ligase complex that targets the alpha subunits of the heterodimeric HIF transcription factor for proteasomal degradation. Deregulation of HIF2α appears to be a driving force in pVHL-defective clear cell renal carcinomas. In contrast, genetic and functional studies suggest that HIF1α serves as a tumor suppressor and is a likely target of the 14q deletions that are characteristic of this tumor type. Drugs that inhibit HIF2α, or its downstream targets such as VEGF, are in various stages of clinical testing. Indeed, clear cell renal carcinomas are exquisitely sensitive to VEGF deprivation and four VEGF inhibitors have now been approved for the treatment of this disease.

Heterogeneity of the tumor vasculature: the need for new tumor blood vessel type-specific targets

Therapies directed against VEGF-A and its receptors are effective in treating many mouse tumors but have been less so in treating human cancer patients. To elucidate the reasons that might be responsible for this difference in response, we investigated the nature of the blood vessels that appear in human and mouse cancers and the tumor “surrogate” blood vessels that develop in immunodeficient mice in response to an adenovirus expressing VEGF-A¹⁶⁴. Both tumor and tumor surrogate blood vessels are heterogeneous and form by two distinct processes, angiogenesis and arterio-venogenesis. The first new angiogenic blood vessels to form are mother vessels (MV); MV arise from preexisting venules and capillaries and evolve over time into glomeruloid microvascular proliferations (GMP) and subsequently into capillaries and vascular malformations (VM). Arterio-venogenesis results from the remodeling and enlargement of preexisting arteries and veins, leading to the formation of feeder arteries (FA) and draining veins (DV) that supply and drain angiogenic vessels. Of these different blood vessel types, only the two that form first, MV and GMP, were highly responsive to anti-VEGF therapy, whereas “late”-formed capillaries, VM, FA and DV were relatively unresponsive. This finding may explain, at least in part, the relatively poor response of human cancers to anti-VEGF/VEGFR therapies, because human cancers, present for months or years prior to discovery, are expected to contain a large proportion of late-formed blood vessels. The future of anti-vascular cancer therapy may depend on finding new targets on “late” vessels, apart from those associated with the VEGF/VEGFR axis.

Liquiritigenin inhibits tumor growth and vascularization in a mouse model of HeLa cells

Angiogenesis is one of the crucial steps in the transition of a tumor from a small, harmless cluster of mutated cells to a large, malignant growth, capable of spreading to other organs throughout the body. Vascular endothelial growth factor (VEGF) that stimulates vasculogenesis and angiogenesis is thought to be as an anti-angiogenic target for cancer therapy. Liquiritigenin (LQ), a flavanone existing in Radix glycyrrhiza, shows extensive biological activities, such as anti-inflammatory and anti-cancer properties. In our studies, liquiritigenin effectively inhibited the growth of tumors xenografted in nude mice from human cervical cancer cell line HeLa cells, and microvascular density (MVD) of the tumor exposed to liquiritigenin was reduced in a dose dependent manner, especially in the high dose group. Moreover, the expression and secretion of VEGF were down-regulated by the drug in vivo and in vitro. Therefore, liquiritigenin can be further studied on cancer and other diseases associated with VEGF up-regulation.

Anti-VEGF/VEGFR therapy for cancer: reassessing the target

Judah Folkman recognized that new blood vessel formation is important for tumor growth and proposed antiangiogenesis as a novel approach to cancer therapy. Discovery of vascular permeability factor VEGF-A as the primary tumor angiogenesis factor prompted the development of a number of drugs that targeted it or its receptors. These agents have often been successful in halting tumor angiogenesis and in regressing rapidly growing mouse tumors. However, results in human cancer have been less impressive. A number of reasons have been offered for the lack of greater success, and, here, we call attention to the heterogeneity of the tumor vasculature as an important issue. Human and mouse tumors are supplied by at least 6 well-defined blood vessel types that arise by both angiogenesis and arterio-venogenesis. All 6 types can be generated in mouse tissues by an adenoviral vector expressing VEGF-A(164). Once formed, 4 of the 6 types lose their VEGF-A dependency, and so their responsiveness to anti-VEGF/VEGF receptor therapy. If therapies directed against the vasculature are to have a greater impact on human cancer, targets other than VEGF and its receptors will need to be identified on these resistant tumor vessels.

Vascular endothelial growth factor receptor 2 inhibition in-vivo affects tumor vasculature in a tumor type-dependent way and downregulates vascular endothelial growth factor receptor 2 protein without a prominent role for miR-296

The precise molecular effects that antiangiogenic drugs exert on tumor vasculature remain to be poorly understood. We therefore set out to investigate the molecular and architectural changes that occur in the vasculature of two different tumor types that both respond to vascular endothelial growth factor receptor 2 (VEGFR2) inhibitor therapy. Mice bearing Lewis lung carcinoma (LLC) or B16.F10 melanoma were treated with vandetanib (ZD6474), a VEGFR2/epidermal growth factor receptor (EGFR)/REarranged during Transfection (RET) kinase inhibitor, resulting in a significant 80% reduction in tumor outgrowth. Although in LLC the vascular density was not affected by vandetanib treatment, it was significantly decreased in B16.F10. In LLC, vandetanib treatment induced a shift in vascular gene expression toward stabilization, as demonstrated by upregulation of Tie2 and N-cadherin and downregulation of Ang2 and integrin β3. In contrast, only eNOS and P-selectin responded to vandetanib treatment in B16.F10 vasculature. Strikingly, vandetanib reduced protein expression of VEGFR2 in both models, whereas mRNA remained unaffected. Analysis of miR-296 expression allowed us to exclude a role for the recently proposed microRNA-296 in VEGFR2 posttranslational control in LLC and B16.F10 in vivo. Our data demonstrate that VEGFR2/EGFR inhibition through vandetanib slows down both LLC and B16.F10 tumor growth. Yet, the underlying molecular changes in the vasculature that orchestrate the antitumor effect differ between tumor types. Importantly, in both models, vandetanib treatment induced loss of its pharmacological target, which was not directly related to miR-296 expression. Validation of our observations in tumor biopsies from VEGFR2 inhibitor-treated patients will be essential to unravel the effects of VEGFR2 inhibitor therapy on tumor vasculature in relation to therapeutic efficacy.

Chemical proteomics: terra incognita for novel drug target profiling

The growing demand for new therapeutic strategies in the medical and pharmaceutic fields has resulted in a pressing need for novel druggable targets. Paradoxically, however, the targets of certain drugs that are already widely used in clinical practice have largely not been annotated. Because the pharmacologic effects of a drug can only be appreciated when its interactions with cellular components are clearly delineated, an integrated deconvolution of drug-target interactions for each drug is necessary. The emerging field of chemical proteomics represents a powerful mass spectrometry (MS)-based affinity chromatography approach for identifying proteome-wide small molecule-protein interactions and mapping these interactions to signaling and metabolic pathways. This technique could comprehensively characterize drug targets, profile the toxicity of known drugs, and identify possible off-target activities. With the use of this technique, candidate drug molecules could be optimized, and predictable side effects might consequently be avoided. Herein, we provide a holistic overview of the major chemical proteomic approaches and highlight recent advances in this area as well as its potential applications in drug discovery.

Relevance of angiogenesis in neuroendocrine tumors

While traditional cytotoxic drugs have shown limited efficacy in neuroendocrine tumors (NETs), their biological features have been characterized and can be exploited therapeutically. Their most prominent trait is an extraordinary vascularization in low-grade NETs and an hypoxia-dependent angiogenesis in high-grade NETs, which is associated to a significant expression of many proangiogenic molecules. Therefore, several antiangiogenic compounds have been tested in these malignancies, and among these, sunitinib has demonstrated activity in pancreatic NET patients by dually targeting the VEGFR and PDGFR pathways. In spite of these efficacious clinical results, apparent resistance to antiangiogenic therapies has been described in NET animal models and in clinical trials. Therefore, overcoming antiangiogenic resistance is a crucial step in the subsequent development of antiangiogenic therapies. Several strategies have been postulated to fight resistance, but preclinical studies and clinical trials will investigate and address these therapeutic approaches in the coming years in order to overcome resistance to antiangiogenic therapies in NETs.

Blood vessels as targets in tumor therapy

The landmark papers published by Judah Folkman in the early 1970s on tumor angiogenesis and therapeutic implications promoted the rapid development of a very dynamic field where basic scientists, oncologists, and pharmaceutical industry joined forces to determine the molecular mechanisms in blood vessel formation and find means to exploit this knowledge in suppressing tumor vascularization and growth. A wealth of information has been collected on angiogenic growth factors, and in 2004 the first specific blood vessel-targeted cancer therapy was introduced: a neutralizing antibody against vascular endothelial growth factor (VEGF). Now (2011) we know that suppression of tumor angiogenesis may be a double-edged sword and that the therapy needs to be further refined and individualized. This review describes the hallmarks of tumor vessels, how different angiogenic growth factors exert their function, and the perspectives for future development of anti-angiogenic therapy.

Early vascular deficits are correlated with delayed mammary tumorigenesis in the MMTV-PyMT transgenic mouse following genetic ablation of the NG2 proteoglycan

INTRODUCTION

The neuron-glial antigen 2 (NG2) proteoglycan promotes pericyte recruitment and mediates pericyte interaction with endothelial cells. In the absence of NG2, blood vessel development is negatively impacted in several pathological models. Our goal in this study was to determine the effect of NG2 ablation on the early development and function of blood vessels in mammary tumors in the mammary tumor virus-driven polyoma middle T (MMTV-PyMT) transgenic mouse, and to correlate these vascular changes with alterations in mammary tumor growth.

METHODS

Three different tumor paradigms (spontaneous tumors, transplanted tumors, and orthotopic allografts of tumor cell lines) were used to investigate the effects of NG2 ablation on breast cancer progression in the MMTV-PyMT transgenic mouse. In addition to examining effects of NG2 ablation on mammary tumor growth, we also investigated effects on the structure and function of tumor vasculature.

RESULTS

Ablation of NG2 led to reduced early progression of spontaneous, transplanted, and orthotopic allograft mammary tumors. NG2 was not expressed by the mammary tumor cells themselves, but instead was found on three components of the tumor stroma. Microvascular pericytes, myeloid cells, and adipocytes were NG2-positive in both mouse and human mammary tumor stroma. The effect of NG2 on tumor progression therefore must be stromal in nature. Ablation of NG2 had several negative effects on early development of the mammary tumor vasculature. In the absence of NG2, pericyte ensheathment of endothelial cells was reduced, along with reduced pericyte maturation, reduced sprouting of endothelial cells, reduced assembly of the vascular basal lamina, and reduced tumor vessel diameter. These early deficits in vessel structure are accompanied by increased vessel leakiness, increased tumor hypoxia, and decreased tumor growth. NG2 ablation also diminishes the number of tumor-associated and TEK tyrosine kinase endothelial (Tie2) expressing macrophages in mammary tumors, providing another possible mechanism for reducing tumor vascularization and growth.

CONCLUSIONS

These results emphasize the importance of NG2 in mediating pericyte/endothelial cell communication that is required for proper vessel maturation and function. In the absence of normal pericyte/endothelial cell interaction, poor vascular function results in diminished early progression of mammary tumors.

Role of bone marrow-derived cells in angiogenesis: focus on macrophages and pericytes

Tumor growth relies on the formation of new blood vessels to receive an adequate supply of oxygen and nutrient. This process is facilitated by both the remodeling of the pre-existing vasculatures and the recruitment of the progenitor/stem cells originated from bone marrow-derived cells (BMDCs). Evidences from both animal studies and human trials have reported that these tumor-associated BMDCs differentiate into a series of stromal cells including macrophages and pericytes, and regulate tumor angiogenesis in various aspects. Macrophages constitute a large portion of the BMDCs infiltrated in the tumor microenvironment, and have been shown to disrupt the balance of pro- and anti-angiogenic signalings by the secretion of various cytokines. Pericytes, mainly derived from the subpopulation of PDGFRβ(+) BMDCs, can provide both pro-survival signaling and mechanical support to maintain the newly formed endothelium via the direct interactions with endothelial cells. In the current review, we summarize the recruitment mechanisms of BMDC-derived macrophages and pericytes within tumor microenvironment, and also review the contribution of these cells to the different aspects of angiogenesis, with particular emphasis on their therapeutic implications as potential targets for anti-tumor strategies.