Inhibition of choroidal neovascularization by blocking vascular endothelial growth factor receptor tyrosine kinase

Synergistic Anti-Angiogenic Effects Using Peptide-Based Combinatorial Delivery of siRNAs Targeting VEGFA, VEGFR1, and Endoglin Genes

Angiogenesis is a process of new blood vessel formation, which plays a significant role in carcinogenesis and the development of diseases associated with pathological neovascularization. An important role in the regulation of angiogenesis belongs to several key pathways such as VEGF-pathways, TGF-β-pathways, and some others. Introduction of small interfering RNA (siRNA) against genes of pro-angogenic factors is a promising strategy for the therapeutic suppression of angiogenesis. These siRNA molecules need to be specifically delivered into endothelial cells, and non-viral carriers modified with cellular receptor ligands can be proposed as perspective delivery systems for anti-angiogenic therapy purposes. Here we used modular peptide carrier L1, containing a ligand for the CXCR4 receptor, for the delivery of siRNAs targeting expression of VEGFA, VEGFR1 and endoglin genes. Transfection properties of siRNA/L1 polyplexes were studied in CXCR4-positive breast cancer cells MDA-MB-231 and endothelial cells EA.Hy926. We have demonstrated the efficient down-regulation of endothelial cells migration and proliferation by anti-VEGFA, anti-VEGFR1, and anti-endoglin siRNA-induced silencing. It was found that the efficiency of anti-angiogenic treatment can be synergistically improved via the combinatorial delivery of anti-VEGFA and anti-VEGFR1 siRNAs. Thus, this approach can be useful for the development of therapeutic angiogenesis inhibition.

Photothermal Optical Coherence Tomography of Anti-Angiogenic Treatment in the Mouse Retina Using Gold Nanorods as Contrast Agents

Purpose

Optical coherence tomography (OCT) is widely used for ocular imaging in clinical and research settings. OCT natively provides structural information based on the reflectivity of the tissues it images. We demonstrate the utility of photothermal OCT (PTOCT) imaging of gold nanorods (GNR) in the mouse retina in vivo in the laser-induced choroidal neovascularization (LCNV) model to provide additional image contrast within the lesion.

Methods

Wild-type C57BL/6 mice were imaged following the intravenous injection of ICAM2-targeted or untargeted GNR. Mice were also imaged following the injection of ICAM2-targeted GNR with or without the additional ocular delivery of a neutralizing monoclonal anti-vascular endothelial growth factor (anti-VEGF) antibody.

Results

Mice cohorts injected with untargeted or ICAM2-targeted GNR demonstrated increased lesion-associated photothermal signal during subsequent imaging relative to phosphate-buffered saline (PBS)-injected controls. Additionally, intravitreal injection of anti-VEGF antibody caused a detectable reduction in the extent of anatomic laser damage and lesion-associated photothermal signal density in mice treated in the LCNV model and injected with ICAM2-targeted GNR.

Conclusions

These experiments demonstrate the ability of PTOCT imaging of GNR to detect anti-VEGF-induced changes in the mouse retina using the LCNV model.

Translational Relevance

This study shows that PTOCT imaging of GNR in the LCNV model can be used to detect clinically relevant, anti-VEGF-induced changes that are not visible using standard OCT systems. In the future this technology could be used to aid in early detection of disease, monitoring disease progress, and assessing its response to therapies.

Ultrastructural analysis of submacular choriocapillaris and its transport systems in AMD and aged control eyes

The choriocapillaris is the source of nutrients and oxygen for photoreceptors, which consume more oxygen per gram of tissue than any other cell in the body. The purpose of this study was to evaluate and compare the ultrastructure of the choriocapillaris and its transport systems in patients with and without age-related macular degeneration (AMD). Ultrastructural changes were also evaluated in subjects that were homozygous for polymorphisms in high risk CFH alleles (Pure 1) only or homozygous only for high risk ARMS2/HTRA1 (Pure 10) alleles. Tissue samples were obtained from the macular region of forty male (n = 24) and female (n = 16) donor eyes and prepared for ultrastructural studies with transmission electron microscopy (TEM). The average age of the aged donors was 74 ± 7.2 (n = 30) and the young donors 31.7 ± 11.25 (n = 10). There was no significant difference in average ages between the adult groups. TEM images of the capillaries in the choriocapillaris (CC) were taken at 4,000X and 25,000X and used to measure the area of endothelial cell somas, the number of fenestrations, and area of caveolae within the endothelial cells per length of Bruchs membrane (BrMb). The Student t-test and Wilcoxon sum rank test were used to determine significant differences. There was no significant difference between young subjects and aged controls in any of the morphological criteria assessed. There was a significant decrease in the number of fenestrations/mm of BrMb in atrophic areas of GA eyes (p = 0.007) when compared with aged control eyes. A significant increase was found in the caveolae area as a percent of the endothelial cell soma of capillaries from GA subjects as compared with the controls (p = 0.03). Loss of capillary segments in choriocapillaris was also evident, especially in areas of geographic atrophy and CNV. In eyes from patients with sequence variations, the capillary endothelial cells often appeared degenerative and exhibited atypical fenestrations and pericytes covering the blood vessels. Subjects that were homozygous for polymorphisms in high risk CFH alleles only had more fenestrations/mm of BrMb than subjects that were homozygous only for high risk ARMS2/HTRA1 alleles (p = 0.04), while the latter had greater caveolae area/endothelial cell area than the former (p = 0.007). This study demonstrated an attenuation of CC and a significant decline in the two major transport systems in CC endothelial cells in AMD. This may contribute to drusen deposition, nutrient transport, and vision loss in AMD subjects.

Correlation of in vivo and in vitro methods in measuring choroidal vascularization volumes using a subretinal injection induced choroidal neovascularization model

BACKGROUND

In vivo quantification of choroidal neovascularization (CNV) based on noninvasive optical coherence tomography (OCT) examination and in vitro choroidal flatmount immunohistochemistry stained of CNV currently were used to evaluate the process and severity of age-related macular degeneration (AMD) both in human and animal studies. This study aimed to investigate the correlation between these two methods in murine CNV models induced by subretinal injection.

METHODS

CNV was developed in 20 C57BL6/j mice by subretinal injection of adeno-associated viral delivery of a short hairpin RNA targeting sFLT-1 (AAV.shRNA.sFLT-1), as reported previously. After 4 weeks, CNV was imaged by OCT and fluorescence angiography. The scaling factors for each dimension, x, y, and z (μm/pixel) were recorded, and the corneal curvature standard was adjusted from human (7.7) to mice (1.4). The volume of each OCT image stack was calculated and then normalized by multiplying the number of voxels by the scaling factors for each dimension in Seg3D software (University of Utah Scientific Computing and Imaging Institute, available at http://www.sci.utah.edu/cibc-software/seg3d.html). Eighteen mice were prepared for choroidal flatmounts and stained by CD31. The CNV volumes were calculated using scanning laser confocal microscopy after immunohistochemistry staining. Two mice were stained by Hematoxylin and Eosin for observing the CNV morphology.

RESULTS

The CNV volume calculated using OCT was, on average, 2.6 times larger than the volume calculated using the laser confocal microscopy. The correlation statistical analysis showed OCT measuring of CNV correlated significantly with the in vitro method (R 2 =0.448, P = 0.001, n = 18). The correlation coefficient for CNV quantification using OCT and confocal microscopy was 0.693 (n = 18, P = 0.001).

CONCLUSIONS

There is a fair linear correlation on CNV volumes between in vivo and in vitro methods in CNV models induced by subretinal injection. The result might provide a useful evaluation of CNV both for the studies using CNV models induced by subretinal injection and human AMD studies.

Powerful anti-tumor and anti-angiogenic activity of a new anti-vascular endothelial growth factor receptor 1 peptide in colorectal cancer models

To assess the therapeutic outcome of selective block of VEGFR1, we have evaluated the activity of a new specific antagonist of VEGFR1, named iVR1 (inhibitor of VEGFR1), in syngenic and xenograft colorectal cancer models, in an artificial model of metastatization, and in laser-induced choroid neovascularization. iVR1 inhibited tumor growth and neoangiogenesis in both models of colorectal cancer, with an extent similar to that of bevacizumab, a monoclonal antibody anti-VEGF-A. It potently inhibited VEGFR1 phosphorylation in vivo, determining a strong inhibition of the recruitment of monocyte-macrophages and of mural cells as confirmed, in vitro, by the ability to inhibit macrophages migration. iVR1 was able to synergize with irinotecan determining a shrinkage of tumors that became undetectable after three weeks of combined treatment. Such treatment induced a significant prolongation of survival similar to that observed with bevacizumab and irinotecan combination. iVR1 also fully prevented lung invasion by HCT-116 cells injected in mouse tail vein. Also, iVR1 impressively inhibited choroid neovascularization after a single intravitreal injection. Collectively, data showed the strong potential of iVR1 peptide as a new anti-tumor and anti-metastatic agent and demonstrate the high flexibility of VEGFR1 antagonists as therapeutic anti-angiogenic agents in different pathological contexts.

Human IgG1 antibodies suppress angiogenesis in a target-independent manner

Aberrant angiogenesis is implicated in diseases affecting nearly 10% of the world’s population. The most widely used anti-angiogenic drug is bevacizumab, a humanized IgG1 monoclonal antibody that targets human VEGFA. Although bevacizumab does not recognize mouse Vegfa, it inhibits angiogenesis in mice. Here we show bevacizumab suppressed angiogenesis in three mouse models not via Vegfa blockade but rather Fc-mediated signaling through FcγRI (CD64) and c-Cbl, impairing macrophage migration. Other approved humanized or human IgG1 antibodies without mouse targets (adalimumab, alemtuzumab, ofatumumab, omalizumab, palivizumab and tocilizumab), mouse IgG2a, and overexpression of human IgG1-Fc or mouse IgG2a-Fc, also inhibited angiogenesis in wild-type and FcγR humanized mice. This anti-angiogenic effect was abolished by Fcgr1 ablation or knockdown, Fc cleavage, IgG-Fc inhibition, disruption of Fc-FcγR interaction, or elimination of FcRγ-initated signaling. Furthermore, bevacizumab’s Fc region potentiated its anti-angiogenic activity in humanized VEGFA mice. Finally, mice deficient in FcγRI exhibited increased developmental and pathological angiogenesis. These findings reveal an unexpected anti-angiogenic function for FcγRI and a potentially concerning off-target effect of hIgG1 therapies.

Safe dose of intravitreal imatinib and its effect on laser-induced choroidal neovascularization: a rat-model experiment

Background

This two-phase experimental study was conducted to determine the maximum safe dose of intravitreal imatinib (IVI) and its inhibitory effect on a rat model of choroidal neovascularization (CNV).

Methods

In phase I, 60 rats were divided into six groups (A to F); five of which received IVI with concentrations of 330 (A), 250 (B), 165 (C), 80 (D), and 40 (E) µg/5 µl, and the control group (F) received balanced salt solution (BSS). In addition to electroretinography (ERG), routine histopathological analysis and immunohistochemistry for glial fibrillary acidic protein were performed. In phase II, CNV was induced by laser photocoagulation in 25 rats and the animals were divided into two groups. One group received the maximum safe dose of IVI, determined in phase I, and the other received intravitreal BSS. After 4 weeks, the groups were compared in terms of mean scores of fluorescein leakage in fluorescein angiography and the mean CNV areas in histopathological sections.

Results

In phase I, ERG and the histopathological findings revealed retinal toxicity in groups A to D and A to C, respectively; therefore, a dose of 40 µg/5 µl imatinib was specified as the maximum safe dose for phase II. In phase II, late phase fluorescein leakage and the CNV areas were not significantly different between the imatinib-treated eyes and the controls (p = 0.62 and p = 0.5, respectively).

Conclusions

Despite the safety of IVI with a dose of 40 µg/5 µl, no inhibitory effect on laser-induced CNV was observed. Further studies are required to investigate the possible synergistic effects of Imatinib with conventional anti-CNV drugs.

Intravitreal tanibirumab, a fully human monoclonal antibody against vascular endothelial growth factor receptor 2, partially suppresses and regresses laser-induced choroidal neovascularization in a rat model

PURPOSE

The study investigated the effect of intravitreally administered tanibirumab, a fully human monoclonal antibody against vascular endothelial growth factor receptor 2, in a rat model of laser-induced choroidal neovascularization (CNV).

METHODS

CNV was induced by laser photocoagulation on day 0 in the eyes of Brown Norway rats. Intravitreal injection of tanibirumab or phosphate-buffered saline (PBS) was done on day 0 (prevention arm) or day 7 (treatment arm). Seven days after injection, the eyes were enucleated and retinal pigment epithelium-choroid-sclera flat mounts were prepared. Areas of CNV were determined in the flat mounts using tetramethylrhodamine isothiocyanate Bandeiraea simplicifolia (BS) isolectin labeling and intravenously administered fluorescein isothiocyanate-dextran and quantified using an image analysis program.

RESULTS

In the prevention arm, the mean area of CNV measured by BS isolectin labeling was reduced by 28.2% and 53.9% in tanibirumab-treated eyes (20 and 60 μg, respectively) compared with PBS-treated control eyes on day 7 (P=0.038 and P<0.001, respectively). In the treatment arm, the mean area of CNV measured by BS isolectin labeling was reduced by 28.7% and 46.0% in tanibirumab-treated eyes (20 and 60 μg, respectively) compared with PBS-treated control eyes on day 14 (P=0.048 and P<0.001, respectively).

CONCLUSIONS

Intravitreally administered tanibirumab partially suppressed the formation of new CNV and partially regressed preformed laser-induced CNV in the rat model. Tanibirumab may be a feasible treatment for CNV associated with age-related macular degeneration or other causes.

Monocyte function and trafficking in cardiovascular disease

Monocytes are key effectors of the immune homeostasis and play a crucial role in (vascular) injury repair. Despite their role in immune defense and tissue repair mechanisms, monocytes are also involved in several pathological conditions such as autoimmune and cardiovascular diseases as well as cancer. This suggests that monocytes can be used as diagnostic and as therapeutic targets. A better understanding and characterisation of monocytes and their function in both physiological and pathological situations is thus of great interest. This review focuses on recent advances on the role of monocytes in cardiovascular diseases and describes the value of monocytes as either disease marker or therapeutic target for (cardio)vascular diseases.

Soluble fms-like tyrosine kinase 1 and soluble endoglin are elevated circulating anti-angiogenic factors in pre-eclampsia

Pre-eclampsia, characterized by hypertension and proteinuria, affects approximately 3-5% of all pregnancies worldwide and is a major cause of maternal and fetal morbidity and mortality. Maternal endothelial dysfunction is associated with disease pathogenesis. Recently, reports have shown that elevated levels of circulating soluble fms-like tyrosine kinase 1 [sFlt1] and soluble endoglin [sEng] are associated with pre-eclampsia. Flt1 is a receptor for vascular endothelial growth factor receptor [VEGF], whereas endoglin [Eng] is an auxiliary receptor for transforming growth factor-β [TGF-β] super-family members. Both signaling pathways modulate angiogenesis and are involved in vascular homeostasis. Increased levels of sFlt1 and sEng dysregulate VEGF and TGF-β signaling respectively, resulting in endothelial dysfunction of maternal blood vessels. This review summarizes our current knowledge of Flt1 and endoglin and soluble forms in pre-eclampsia. Furthermore, it highlights the predictive and early-screening value of circulating levels of sFlt1 and sEng for the risk of developing pre-eclampsia.

Inhibition of placenta growth factor with TB-403: a novel antiangiogenic cancer therapy

INTRODUCTION

There is clinical evidence that therapies targeting the vascular endothelial growth factor pathway are effective in delaying cancer progression. However, tumors may be either intrinsically resistant or evolve resistance to such therapies. Hence, there is a need for new therapies targeting angiogenesis.

AREAS COVERED

The data are obtained by searching in the PubMed database. The search terms used included antiangiogenic therapy, TB-403 (RO5323441), placenta growth factor (PlGF) and VEGFR-1 (Flt-1). We review preclinical data concerning the function and inhibition of PlGF and summarize data on expression of PlGF in cancer patients. Data from early-phase clinical trials of TB-403 (RO5323441), a monoclonal antibody inhibiting PlGF, are discussed. Future development strategies, therapeutic potentials and limitations of TB-403 are further evaluated.

EXPERT OPINION

There are some conflicting data on the function of PlGF and the importance of its role in primary tumor growth. Data from some preclinical models of PlGF inhibition and early-phase clinical trials with TB-403 are, however, promising, although the true potential of the drug is yet to be determined. Further clinical development should be preceded by molecular studies in the context of well-designed preclinical models and/or small translational studies. Future challenges involve identifying predictive biomarkers.

Forkhead box transcription factor FoxC1 preserves corneal transparency by regulating vascular growth

Normal vision requires the precise control of vascular growth to maintain corneal transparency. Here we provide evidence for a unique mechanism by which the Forkhead box transcription factor FoxC1 regulates corneal vascular development. Murine Foxc1 is essential for development of the ocular anterior segment, and in humans, mutations have been identified in Axenfeld-Rieger syndrome, a disorder characterized by anterior segment dysgenesis. We show that FOXC1 mutations also lead to corneal angiogenesis, and that mice homozygous for either a global (Foxc1(-/-)) or neural crest (NC)-specific (NC-Foxc1(-/-)) null mutation display excessive growth of corneal blood and lymphatic vessels. This is associated with disorganization of the extracellular matrix and increased expression of multiple matrix metalloproteinases. Heterozygous mutants (Foxc1(+/-) and NC-Foxc1(+/-)) exhibit milder phenotypes, such as disrupted limbal vasculature. Moreover, environmental exposure to corneal injury significantly increases growth of both blood and lymphatic vessels in both Foxc1(+/-) and NC-Foxc1(+/-) mice compared with controls. Notably, this amplification of the angiogenic response is abolished by inhibition of VEGF receptor 2. Collectively, these findings identify a role for FoxC1 in inhibiting corneal angiogenesis, thereby maintaining corneal transparency by regulating VEGF signaling.

VEGFR-1 mediates endothelial differentiation and formation of blood vessels in a murine model of infantile hemangioma

Vascular endothelial growth factor receptor-1 (VEGFR-1) is a member of the VEGFR family, and binds to VEGF-A, VEGF-B, and placental growth factor. VEGFR-1 contributes to tumor growth and metastasis, but its role in the pathological formation of blood vessels is still poorly understood. Herein, we used infantile hemangioma (IH), the most common tumor of infancy, as a means to study VEGFR-1 activation in pathological vasculogenesis. IH arises from stem cells (HemSCs) that can form the three most prominent cell types in the tumor: endothelial cells, pericytes, and adipocytes. HemSCs can recapitulate the IH life cycle when injected in immuncompromised mice, and are targeted by corticosteroids, the traditional treatment for IH. We report here that VEGF-A or VEGF-B induces VEGFR-1-mediated ERK1/2 phosphorylation in HemSCs and promotes differentiation of HemSCs to endothelial cells. Studies of VEGFR-2 phosphorylation status and down-regulation of neuropilin-1 in the HemSCs demonstrate that VEGFR-2 and NRP1 are not needed for VEGF-A- or VEGF-B-induced ERK1/2 activation. U0216-mediated blockade of ERK1/2 phosphorylation or shRNA-mediated suppression of VEGFR-1 prevents HemSC-to-EC differentiation. Furthermore, the down-regulation of VEGFR-1 in the HemSCs results in decreased formation of blood vessels in vivo and reduced ERK1/2 activation. Thus, our study reveals a critical role for VEGFR-1 in the HemSC-to-EC differentiation that underpins pathological vasculogenesis in IH.

Signal transduction by vascular endothelial growth factor receptors

VEGFs (vascular endothelial growth factors) control vascular development during embryogenesis and the function of blood vessels and lymphatic vessels in the adult. There are five related mammalian ligands, which act through three receptor tyrosine kinases. Signalling is modulated through neuropilins, which act as VEGF co-receptors. Heparan sulfate and integrins are also important modulators of VEGF signalling. Therapeutic agents that interfere with VEGF signalling have been developed with the aim of decreasing angiogenesis in diseases that involve tissue growth and inflammation, such as cancer. The present review will outline the current understanding and consequent biology of VEGF receptor signalling.

Inhibitory effects of SU5416, a selective vascular endothelial growth factor receptor tyrosine kinase inhibitor, on experimental corneal neovascularization

PURPOSE

Treatment of neovascularization in ocular diseases with vascular endothelial growth factor (VEGF) inhibition shows promising results. SU5416 is a low-molecular-weight tyrosine kinase inhibitor. It selectively inhibits the membrane-bound tyrosine kinase activity of VEGF-2 receptor (Flk-1/KDR) and blocks the intracellular signaling process. The aim of this study was to evaluate the effect of SU5416 on corneal neovascularization.

METHODS

Corneas were cauterized with silver nitrate/potassium nitrate sticks in 20 eyes of 20 BALB/C mice. In the study group (n = 10), SU5416 (25 mg/kg) dissolved in dimethyl sulfoxide was given as an intraperitoneal injection in a single daily dose for 7 days. The other group of 10 mice given intraperitoneal dimethyl sulfoxide alone served as a control group. After 7 days, corneal neovascularization was evaluated using photographs captured by fluorescein angiography. Colored photographs were taken by a biomicroscope with a digital camera. Data were expressed as mean neovascular length and mean number of new vessels for each animal. The values were computed and compared between the groups.

RESULTS

The mean burn stimulus intensities were not different between the groups. In the study group, the mean length of the vessels and the mean number of vessels were 0.49 ± 0.05 and 11.20 ± 1.69 mm, respectively. In the control group, the mean length of the vessels and the mean number of the vessels were 0.89 ± 0.11 and 17.80 ± 1.03 mm, respectively. There is a statistically significant difference in the mean length and the mean number of new vessels between the study and control groups (p < 0.001).

CONCLUSION

Selective inhibition of VEGFR-2 (Flk-1/KDR) tyrosine kinase with SU5416 was shown to have an inhibitory effect on corneal neovascularization in this animal model. VEGFR-2 (Flk-1/KDR) tyrosine kinase inhibition may represent a different pathway for treatment of the neovascularization process in ocular pathologies. Fluorescein angiography photographs of new vessels on the cornea may provide a better evaluation of neovascularization than colored images in animal models.

Vascular endothelial growth factors and receptors: anti-angiogenic therapy in the treatment of cancer

Vascular endothelial growth factors (VEGFs) are critical regulators of vascular and lymphatic function during development, in health and in disease. There are five mammalian VEGF ligands and three VEGF receptor tyrosine kinases. In addition, several VEGF co-receptors that lack intrinsic catalytic activity, but that indirectly modulate the responsiveness to VEGF contribute to the final biological effect. This review describes the molecular features of VEGFs, VEGFRs and co-receptors with focus on their role in the treatment of cancer.

Involvement of Flt-1 (VEGF receptor-1) in cancer and preeclampsia

We previously isolated a novel tyrosine kinase receptor, Flt-1, now known as VEGF-receptor (VEGFR)-1. The VEGF-VEGFR system plays a pivotal role in not only physiological but also pathological angiogenesis. We examined the role of Flt-1 in carcinogenesis using Flt-1-signal-deficient (Flt-1 TK-/-) mice, and found that this receptor stimulates tumor growth and metastasis most likely via macrophages, making it an important potential target in the treatment of cancer. In addition to the full-length receptor, the Flt-1 gene produces a soluble protein, sFlt-1, an endogenous VEGF-inhibitor. sFlt-1 is expressed in trophoblasts of the placenta between fetal and maternal blood vessels, suggesting it to be a barrier against extreme VEGF-signaling. Abnormally high expression of sFlt-1 occurs in most preeclampsia patients, whose main symptoms are hypertension and proteinurea. In cancer patients, strong suppression of VEGF-VEGFR by drugs induces similar side effects including hypertension. These results indicate a close relationship between abnormal VEGF-block and hypertension/proteinurea. sFlt-1 is an attractive target for the control of preeclampsia.

Notch regulates the angiogenic response via induction of VEGFR-1

Notch is a critical regulator of angiogenesis and arterial specification. We show that ectopic expression of activated Notch1 induces endothelial morphogenesis in human umbilical vein endothelial cells (HUVEC) in a VEGFR-1-dependent manner. Notch1-mediated upregulation of VEGFR-1 in HUVEC increased their responsiveness to the VEGFR-1 specific ligand, Placental Growth Factor (PlGF). In mice and human endothelial cells, inhibition of Notch signaling resulted in decreased VEGFR-1 expression during VEGF-A-induced neovascularization. In summary, we show that Notch1 plays a role in endothelial cells by regulating VEGFR-1, a function that may be important for physiological and pathological angiogenesis.

Targeting retinal and choroid neovascularization using the small molecule inhibitor carboxyamidotriazole

Neovascular ocular diseases as exemplified by proliferative diabetic retinopathy (PDR), exudative age-related macular degeneration (AMD), and retinopathy of prematurity (ROP) are severe diseases affecting all age groups in the US. We asked whether a small molecule, carboxyamidotriazole (CAI) known for its anti-angiogenic and anti-tumor effects and its ability to be administered orally in humans, could have anti-angiogenic effects in ocular in vitro and in vivo angiogenesis models. The anti-proliferative effects of CAI were examined by BrdU incorporation using human retinal and dermal endothelial cells and human pigment epithelial cells. The effect of CAI was determined using the Matrigel tube formation assay. The mouse model of choroidal neovascularization (CNV) initiated by laser rupture of Bruch's membrane was used to quantify in vivo effects of aqueous beta-hydroxypropyl cyclodextrin (bHPCD) formulations of CAI on neovascularization. The pharmacokinetics (PK) of CAI after intravitreal administration of bHPCD-CAI was studied in rabbit. The intravitreal toxicology of bHPCD-CAI was also examined in rat ocular tissue. We observed that CAI treatment of human endothelial cells decreased cell proliferation in a dose-dependent manner. In the in vivo tests bHPCD-CAI treatment reduced choroidal neovascular lesion volume, also in a dose-dependent manner. The intravitreal PK of bHPCD-CAI demonstrated that highly efficacious concentrations of CAI are reached in the vitreous compartment. No ocular toxicology was observed with intravitreous injection of CAI. These studies support the potential of developing intravitreal CAI in an bHPCD ocular formulation for treatment of proliferative retinopathies in humans.

Reduction in endothelial tip cell filopodia corresponds to reduced intravitreous but not intraretinal vascularization in a model of ROP

To determine the effect of a vascular endothelial growth factor receptor 2 tyrosine kinase (VEGFR2) inhibitor on intravitreous neovascularization (IVNV), endothelial tip cell filopodia, and intraretinal vascularization in a rat model of retinopathy of prematurity (ROP). Within 4h of birth, newborn Sprague-Dawley rat pups and their mothers were cycled between 50% and 10% oxygen daily until postnatal day (p)12. Pups were given intravitreous injections of VEGFR2 inhibitor, SU5416, or control (dimethyl sulfoxide, DMSO) and returned to oxygen cycling until p14, then placed into room air. Intravitreous neovascularization (IVNV), avascular/total retinal areas, and endothelial tip cell filopodial number and length were determined in lectin-labeled neurosensory retinal flat mounts. Cryosections or fresh tissue were analyzed for phospho-VEGFR1, phospho-VEGFR2, activated caspase-3, or phospho-beta3 integrin. Human umbilical venous (HUVECs) and human choroidal endothelial cells (ECs) were treated with VEGFR2 inhibitor to determine effect on VEGFR2 phosphorylation and on directed EC migration toward a VEGF gradient. Filopodial length and number of migrated ECs were also measured. Compared to control, the VEGFR2 inhibitor reduced VEGFR2 phosphorylation in HUVECs in vitro and clock hours and areas of IVNV but not percent avascular retina in vivo. Filopodial length and number of filopodia/EC tip cell were reduced in retinal flat mounts at doses that inhibited IVNV, whereas at lower doses, only a reduction in filopodial length/EC tip cell was found. There was no difference in phosphorylated beta3 integrin and cleaved caspase-3 labeling in VEGFR2 inhibitor-treated compared to control in vivo. Doses of the VEGFR2 inhibitor that reduced filopodial length and number of filopodia/migrating EC corresponded to reduced EC migration in in vitro models. VEGFR2 inhibitor reduced IVNV and filopodial number and length/EC tip cell without interfering with intraretinal vascularization. Reducing the number and length of filopodia/endothelial tip cell may reduce guidance cues for endothelial cells to migrate into the vitreous without interfering with migration into the retina toward a VEGF gradient.

A2E, a component of lipofuscin, is pro-angiogenic in vivo

A recent study in vitro demonstrated that a major lipofuscin component, A2E, serves as a retinoic acid receptor ligand. The current study investigated the effects of A2E on retinal pigment epithelial (RPE) cells in vivo and was performed to extend the understanding of the effects of A2E. Firstly, subretinal injection of A2E was performed and 3 weeks after the injection, and it was demonstrated that subretinal injection of A2E induced RPE cell death, and concomitant upregulation of vascular endothelial growth factor (VEGF) in the RPE and choroid. The upregulation of VEGF was attenuated by an RARalpha antagonist. Next we performed laser photocoagulation in mice that accumulated A2E either after subretinal injection, by Ccl2 gene knockout or by aging demonstrated that mice that accumulated A2E in the RPE, which showed higher rates of choroidal neobascularization (CNV) formation after weak laser injury than the controls and the formation of CNV was inhibited by an RARalpha antagonist in all models tested. The data suggest that A2E accumulation induces RPE cell death, and concomitant increase of VEGF. Accumulation of A2E alone is not sufficient to induce CNV in vivo, but induces the expression of VEGF in RPE and choroid. The mice that accumulated A2E in RPE cells are vulnerable to CNV development via RAR activation, at least in part.

Antiangiogenic immunotherapy targeting Flk-1, DNA vaccine and adoptive T cell transfer, inhibits ocular neovascularization

Ocular neovascularization (NV) is the primary cause of blindness in a wide range of ocular diseases. The exact mechanism underlying the pathogenesis of ocular NV is not yet well understood, and so there is no satisfactory therapy for ocular NV. Here, we describe a strategy targeting Flk-1, a self-antigen overexpressed on proliferating endothelial cells in ocular NV, by antiangiogenic immunotherapy-DNA vaccine and adoptive T cell therapy. An oral DNA vaccine encoding Flk-1 carried by attenuated Salmonella typhimurium markedly suppressed development of laser-induced choroidal NV. We further demonstrated that adoptive transfer of vaccine-induced CD8+ T cells reduced pathological preretinal NV, with a concomitant facilitation of physiological revascularization after oxygen-induced retinal vessel obliteration. However, physiological retinal vascular development was unaffected in neonatal mice transferred with vaccine-induced CD8+ T cells. These findings suggested that antiangiogenic immunotherapy targeting Flk-1 such as vaccination and adoptive immunotherapy may contribute to future therapies for ocular NV.

Positive and negative modulation of angiogenesis by VEGFR1 ligands

Vascular endothelial growth factor-A (VEGF-A) is a key target for new antiangiogenic drugs for the treatment of both malignant and nonmalignant human diseases. Vascular effects of VEGF family members are mainly mediated by VEGF receptor 2 (VEGFR2). Conversely, the function and signaling of VEGFR1, which is present on endothelial and nonendothelial cells, are poorly understood. Intriguingly, two of five members in the VEGF family--VEGF-B and placental growth factor (PlGF)--are exclusive ligands for VEGFR1 and do not interact with the other VEGFRs, VEGFR2 and VEGFR3. These VEGFR1-specific ligands may be important therapeutic targets for the treatment of cancer. This Review discusses the distinctive roles of VEGFR1 and its ligands PlGF and VEGF-B in the mediation of angiogenic signaling and considers the therapeutic potential of targeting these particular vascular factors.

FLT1 and its ligands VEGFB and PlGF: drug targets for anti-angiogenic therapy?

Less than 5 years ago, it was still not clear whether anti-angiogenic drugs would prove successful in the clinic. After numerous patients with cancer or age-related macular degeneration have been treated with these drugs, they have now become part of the standard range of therapeutic tools. Despite this milestone, anti-angiogenic therapy still faces a number of clinical hurdles, such as improving efficacy, avoiding escape and resistance, and minimizing toxicity. Hopefully, other agents with complementary mechanisms, such as those that target placental growth factor, will offer novel opportunities for improved treatment.