Synthesis and biological profile of the pan-vascular endothelial growth factor receptor/tyrosine kinase with immunoglobulin and epidermal growth factor-like homology domains 2 (VEGF-R/TIE-2) inhibitor 11-(2-methylpropyl)-12,13-dihydro-2-methyl-8-(pyrimidin-2-ylamino)-4H-indazolo[5,4-a]pyrrolo[3,4-c]carbazol-4-one (CEP-11981): a novel oncology therapeutic agent

Selective Termination of Autophagy-Dependent Cancers

The goal of cancer research is to identify characteristics of cancer cells that allow them to be selectively eliminated without harming the host. One such characteristic is autophagy dependence. Cancer cells survive, proliferate, and metastasize under conditions where normal cells do not. Thus, the requirement in cancer cells for more energy and macromolecular biosynthesis can evolve into a dependence on autophagy for recycling cellular components. Recent studies have revealed that autophagy, as well as different forms of cellular trafficking, is regulated by five phosphoinositides associated with eukaryotic cellular membranes and that the enzymes that synthesize them are prime targets for cancer therapy. For example, PIKFYVE inhibitors rapidly disrupt lysosome homeostasis and suppress proliferation in all cells. However, these inhibitors selectively terminate PIKFYVE-dependent cancer cells and cancer stem cells with not having adverse effect on normal cells. Here, we describe the biochemical distinctions between PIKFYVE-sensitive and -insensitive cells, categorize PIKFYVE inhibitors into four groups that differ in chemical structure, target specificity and efficacy on cancer cells and normal cells, identify the mechanisms by which they selectively terminate autophagy-dependent cancer cells, note their paradoxical effects in cancer immunotherapy, and describe their therapeutic applications against cancers.

Prostate cancer immunotherapy: Improving clinical outcomes with a multi-pronged approach

Cancer immunotherapy has gained traction in recent years owing to remarkable tumor clearance in some patients. Despite the notable success of immune checkpoint blockade (ICB) in multiple malignancies, engagement of the immune system for targeted prostate cancer (PCa) therapy is still in its infancy. Multiple factors contribute to limited response, including the heterogeneity of PCa, the cold tumor microenvironment, and a low number of neoantigens. Significant effort is being invested in improving immune-based PCa therapies. This review is a summary of the status of immunotherapy in treating PCa, with a discussion of multiple immune modalities, including vaccines, adoptively transferred T cells, and bispecific T cell engagers, some of which are undergoing clinical trials. In addition, this review also focuses on emerging mechanism-based small-molecule tyrosine kinase inhibitors with immune modulatory properties that, either as single agents or in combination with other immunotherapies, have the potential to improve clinical outcomes.

Overfit deep neural network for predicting drug-target interactions

Drug-target interactions (DTIs) prediction is an important step in drug discovery. As traditional biological experiments or high-throughput screening are high cost and time-consuming, many deep learning models have been developed. Overfitting must be avoided when training deep learning models. We propose a simple framework, called OverfitDTI, for DTI prediction. In OverfitDTI, a deep neural network (DNN) model is overfit to sufficiently learn the features of the chemical space of drugs and the biological space of targets. The weights of trained DNN model form an implicit representation of the nonlinear relationship between drugs and targets. Performance of OverfitDTI on three public datasets showed that the overfit DNN models fit the nonlinear relationship with high accuracy. We identified fifteen compounds that interacted with TEK, a receptor tyrosine kinase contributing to vascular homeostasis, and the predicted AT9283 and dorsomorphin were experimentally demonstrated as inhibitors of TEK in human umbilical vein endothelial cells (HUVECs).

Beyond PI3Ks: targeting phosphoinositide kinases in disease

Lipid phosphoinositides are master regulators of almost all aspects of a cell's life and death and are generated by the tightly regulated activity of phosphoinositide kinases. Although extensive efforts have focused on drugging class I phosphoinositide 3-kinases (PI3Ks), recent years have revealed opportunities for targeting almost all phosphoinositide kinases in human diseases, including cancer, immunodeficiencies, viral infection and neurodegenerative disease. This has led to widespread efforts in the clinical development of potent and selective inhibitors of phosphoinositide kinases. This Review summarizes our current understanding of the molecular basis for the involvement of phosphoinositide kinases in disease and assesses the preclinical and clinical development of phosphoinositide kinase inhibitors.

The Immunotherapy and Immunosuppressive Signaling in Therapy-Resistant Prostate Cancer

Prostate cancer is one of the most common malignant tumors in men. Initially, it is androgen-dependent, but it eventually develops into castration-resistant prostate cancer (CRPC), which is incurable with current androgen receptor signaling target therapy and chemotherapy. Immunotherapy, specifically with immune checkpoint inhibitors, has brought hope for the treatment of this type of prostate cancer. Approaches such as vaccines, adoptive chimeric antigen receptor-T (CAR-T) cells, and immune checkpoint inhibitors have been employed to activate innate and adaptive immune responses to treat prostate cancer, but with limited success. Only Sipuleucel-T and the immune checkpoint inhibitor pembrolizumab are approved by the US FDA for the treatment of limited prostate cancer patients. Prostate cancer has a complex tumor microenvironment (TME) in which various immunosuppressive molecules and mechanisms coexist and interact. Additionally, prostate cancer is considered a “cold” tumor with low levels of tumor mutational burden, low amounts of antigen-presenting and cytotoxic T-cell activation, and high levels of immunosuppressive molecules including cytokines/chemokines. Thus, understanding the mechanisms of immunosuppressive signaling activation and immune evasion will help develop more effective treatments for prostate cancer. The purpose of this review is to summarize emerging advances in prostate cancer immunotherapy, with a particular focus on the molecular mechanisms that lead to immune evasion in prostate cancer. At the same time, we also highlight some potential therapeutic targets to provide a theoretical basis for the treatment of prostate cancer.

Synthesis and biological evaluation of marine natural product, Cryptoechinuline D derivatives as novel antiangiogenic agents

Tumor angiogenesis is an important biological process involved in the proliferation and migration of endothelial cells, regulated by Ang/Tie-2 signaling pathways, which is essential for tumor growth and metastasis. Therefore, blocking Ang/Tie-2 signaling pathways is a promising anti-angiogenic strategy for tumor treatment. 2,5-Diketopiperazines (DKPs) are a kind of bioactive compounds derived from marine fungi and they present a wide spectrum of pharmacological properties, particularly in the field of cancer treatment. Herein, a DKP marine natural product, Cryptoechinuline D (Cry D) was applied to structural modification and twelve derivatives were synthesized. Among which, compound 5 showed significant inhibitory activity against HUVECs with an IC₅₀ value of 12.6 μmol/L, which weakened the proliferation, migration and invasion of HUVECs by inhibiting the Ang2/Tie-2 signaling pathway. The results of these evaluations indicated that compound 5 might be a promising anti-angiogeneic agent and worth further optimization and development for cancer therapy.

Autophagy Inhibition by Targeting PIKfyve Potentiates Response to Immune Checkpoint Blockade in Prostate Cancer

Multi-tyrosine kinase inhibitors (MTKIs) have thus far had limited success in the treatment of castration-resistant prostate cancer (CRPC). Here, we report a phase I-cleared orally bioavailable MTKI, ESK981, with a novel autophagy inhibitory property that decreased tumor growth in diverse preclinical models of CRPC. The anti-tumor activity of ESK981 was maximized in immunocompetent tumor environments where it upregulated CXCL10 expression through the interferon gamma pathway and promoted functional T cell infiltration, which resulted in enhanced therapeutic response to immune checkpoint blockade. Mechanistically, we identify the lipid kinase PIKfyve as the direct target of ESK981. PIKfyve-knockdown recapitulated ESK981's anti-tumor activity and enhanced the therapeutic benefit of immune checkpoint blockade. Our study reveals that targeting PIKfyve via ESK981 turns tumors from cold into hot through inhibition of autophagy, which may prime the tumor immune microenvironment in advanced prostate cancer patients and be an effective treatment strategy alone or in combination with immunotherapies.

Exploration of carbamide derived pyrimidine-thioindole conjugates as potential VEGFR-2 inhibitors with anti-angiogenesis effect

The development of new small molecules from known structural motifs through molecular hybridization is one of the trends in drug discovery. In this connection, we have combined the two pharmacophoric units (pyrimidine and thioindole) in a single entity via molecular hybridization strategy along with introduction of urea functionality at C2 position of pyrimidine to increase the efficiency of H-bonding interactions. Among the synthesized conjugates 12a-aa, compound 12k was found to exhibit significant IC₅₀ values 5.85, 7.87, 6.41 and 10.43 μM against MDA-MB-231 (breast), HepG2 (liver), A549 (lung) and PC-3 (prostate) cancer cell lines, respectively. All these compounds were further evaluated for their inhibitory activities against VEGFR-2 protein. The results specified that among the tested compounds, 12d, 12e, 12k, 12l, 12p, 12q, 12t and 12u prominently suppressed VEGFR-2, with IC₅₀ values of 310-920 nM in association to the positive control (210 nM). Angiogenesis inhibition was evident by tube formation assay in HUVECs and cell-invasion by transwell assay. The mechanism of cellular toxicity on MDA-MB-231 was found through depolarisation of mitochondrial membrane potential, increased ROS production and subsequent DNA damage resulting in apoptosis induction. Moreover, clonogenic and wound healing assays designated the inhibition of colony formation and cell migration by 12k in a dose-dependent manner. Molecular docking studies also shown that compound 12k capably intermingled with catalytically active residues GLU-885, ASP-1046 of the VEGFR-2 through hydrogen-bonding interactions.

New Insights Into Implementation of Mesenchymal Stem Cells in Cancer Therapy: Prospects for Anti-angiogenesis Treatment

Tumor microenvironment interacts with tumor cells, establishing an atmosphere to contribute or suppress the tumor development. Among the cells which play a role in the tumor microenvironment, mesenchymal stem cells (MSCs) have been demonstrated to possess the ability to orchestrate the fate of tumor cells, drawing the attention to the field. MSCs have been considered as cells with double-bladed effects, implicating either tumorigenic or anti-tumor activity. On the other side, the promising potential of MSCs in treating human cancer cells has been observed from the clinical studies. Among the beneficial characteristics of MSCs is the natural tumor-trophic migration ability, providing facility for drug delivery and, therefore, targeted treatment to detach tumor and metastatic cells. Moreover, these cells have been the target of engineering approaches, due to their easily implemented traits, in order to obtain the desired expression of anti-angiogenic, anti-proliferative, and pro-apoptotic properties, according to the tumor type. Tumor angiogenesis is the key characteristic of tumor progression and metastasis. Manipulation of angiogenesis has become an attractive approach for cancer therapy since the introduction of the first angiogenesis inhibitor, namely bevacizumab, for metastatic colorectal cancer therapy. This review tries to conclude the approaches, with focus on anti-angiogenesis approach, in implementing the MSCs to combat against tumor cell progression.

Carbazole scaffolds in cancer therapy: a review from 2012 to 2018

For over half a century, the carbazole skeleton has been the key structural motif of many biologically active compounds including natural and synthetic products. Carbazoles have taken an important part in all the existing anti-cancer drugs because of their discovery from a large variety of organisms, including bacteria, fungi, plants, and animals. In this article, we specifically explored the literature from 2012 to 2018 on the anti-tumour activities reported to carbazole derivatives and we have critically collected the most significant data. The most described carbazole anti-tumour agents were classified according to their structure, starting from the tricyclic–carbazole motif to fused tetra-, penta-, hexa- and heptacyclic carbazoles. To date, three derivatives are available on the market and approved in cancer therapy.

Receptor tyrosine kinases in PI3K signaling: The therapeutic targets in cancer

The phosphoinositide 3-kinase (PI3K) pathway, one of the most commonly activated signaling pathways in human cancers, plays a crucial role in the regulation of cell proliferation, differentiation, and survival. This pathway is usually activated by receptor tyrosine kinases (RTKs), whose constitutive and aberrant activation is via gain-of-function mutations, chromosomal rearrangement, gene amplification and autocrine. Blockage of PI3K pathway by targeted therapy on RTKs with tyrosine kinases inhibitors (TKIs) and monoclonal antibodies (mAbs) has achieved great progress in past decades; however, there still remain big challenges during their clinical application. In this review, we provide an overview about the most frequently encountered alterations in RTKs and focus on current therapeutic agents developed to counteract their aberrant functions, accompanied with discussions of two major challenges to the RTKs-targeted therapy in cancer - resistance and toxicity.

In Silico Exploration for Novel Type-I Inhibitors of Tie-2/TEK: The Performance of Different Selection Strategy in Selecting Virtual Screening Candidates

The receptor tyrosine kinase Tie-2 is involved in vessel remodeling and maturation, and has been regarded as a potential target for the treatment of various solid tumors. The absence of novel, potent and selective inhibitors severely hampers the understanding of the therapeutic potential of Tie-2. In the present work, we describe the discovery of novel type-I inhibitors of Tie-2 by structure-based virtual screening. Preliminary SAR was also performed based on one active compound, and several novel inhibitors with low micro-molar affinity were discovered. To directly compare the efficiency between different filtering strategies in selecting VS candidates, two methods were separately carried out to screen the same chemical library, and the selected VS candidates were then experimentally assessed by in vitro enzymatic assays. The results demonstrate that the hit rate is improved when stricter drug-likeness criteria and less number of molecules for clustering analysis are used, and meanwhile, the molecular diversity of the compounds still maintains. As a case study of TIE-2, the information presented in this work underscores the importance of selecting an appropriate selection strategy in VS campaign, and the novel inhibitors identified and the detailed binding modes of action provide a starting point for further hit-to-lead optimization process.

A Phase I, First-in-Human Study of AMG 780, an Angiopoietin-1 and -2 Inhibitor, in Patients with Advanced Solid Tumors

PURPOSE

To assess the toxicity, pharmacokinetics, tumor vascular response, tumor response, and pharmacodynamics of AMG 780, a mAb designed to inhibit the interaction between angiopoietin-1 and -2 and the Tie2 receptor.

EXPERIMENTAL DESIGN

This was a phase I dose-escalation study of patients with advanced solid tumors refractory to standard treatment without previous antiangiogenic treatment. AMG 780 was administered by intravenous infusion every 2 weeks in doses from 0.1 to 30 mg/kg. The primary endpoints were incidences of dose-limiting toxicity (DLT) and adverse events (AE), and pharmacokinetics. Secondary endpoints included tumor response, changes in tumor volume and vascularity, and anti-AMG 780 antibody formation.

RESULTS

Forty-five patients were enrolled across nine dose cohorts. Three patients had DLTs (0.6, 10, and 30 mg/kg), none of which prevented dose escalation. At 30 mg/kg, no MTD was reached. Pharmacokinetics of AMG 780 were dose proportional; median terminal elimination half-life was 8 to 13 days. No anti-AMG 780 antibodies were detected. At week 5, 6 of 16 evaluable patients had a >20% decrease in volume transfer constant (K(trans)), suggesting reduced capillary blood flow/permeability. The most frequent AEs were hypoalbuminemia (33%), peripheral edema (29%), decreased appetite (27%), and fatigue (27%). Among 35 evaluable patients, none had an objective response; 8 achieved stable disease.

CONCLUSIONS

AMG 780 could be administered at doses up to 30 mg/kg every 2 weeks in patients with advanced solid tumors. AMG 780 treatment resulted in tumor vascular effects in some patients. AEs were in line with toxicity associated with antiangiopoietin treatment. Clin Cancer Res; 22(18); 4574-84. ©2016 AACR.

Roles of signaling and transcriptional networks in pathological lymphangiogenesis

Lymphangiogenesis, the generation of new lymphatic vessels, plays important roles in cancer metastasis. Outstanding progress during the past decade has dramatically increased the novel knowledge and insights of the mechanisms underlying the generation of new lymphatic vessels, the roles of transcription factors and lymphangiogenic growth factors during physiological development and pathological processes such as cancer and inflammation. Furthermore, an understanding of the molecular consequences during tumor lymphangiogenesis has provided chances to develop better diagnostic and therapeutic approaches that aim to limit the progression of cancer. In this article, we will explain the current knowledge of how lymphatic function is altered in various pathological conditions including cancer progression.

Angiogenesis in malignant melanoma

Despite the development of novel therapies, the therapy of malignant melanoma remains challenging. Various studies have shown the vascular system to be pivotal for metastasis in melanoma. Consequently, the effect of various antiangiogenic therapies has been and is being investigated in preclinical and clinical trials. While most studies focus on inhibition of vascular endothelial growth factor (VEGF) signaling, others are aimed at determining the effect of multikinase inhibitors or the inhibition of angiogenic integrin activity. However, overall survival rates have not significantly improved in clinical trials with antiangiogenic agents. Resistance to anti-VEGF monotherapy has been observed in several studies, especially in malignant melanoma. Angiopoietin-2 (Ang-2) represents a promising candidate molecule for antiangiogenic therapy and the effect of Ang-2 inhibitors is currently being explored in first trials. In melanoma, Ang-2 has been shown to be a marker for metastasis formation and represents an interesting therapeutic target molecule. Future studies are required to analyze the effect of a combined approach, using anti-VEGF and anti-Ang-2, as therapy for malignant melanoma.

Angiogenesis in melanoma: an update with a focus on current targeted therapies

Angiogenesis plays a crucial role in melanoma metastasis and progression. In recent years, numerous studies have investigated the prognostic and clinical significance of this phenomenon, and the development of molecular techniques has enabled us to achieve a better understanding of angiogenesis in melanoma. Herein, we review the current state of knowledge regarding angiogenesis in melanoma, including the pathophysiological, histological and immunohistochemical aspects of this phenomenon. We also review the molecular pathways involved in angiogenesis and the interplay between different components that might be manipulated in the future development of efficient targeted therapies. Recently developed targeted antiangiogenic therapies in clinical trials and included in the treatment of advanced-stage melanoma are also reviewed.

(Lymph)angiogenic influences on hematopoietic cells in acute myeloid leukemia

The purpose of this review is to provide an overview of the effect of (lymph)angiogenic cytokines on hematopoietic cells involved in acute myeloid leukemia (AML). Like angiogenesis, lymphangiogenesis occurs in pathophysiological conditions but not in healthy adults. AML is closely associated with the vasculature system, and the interplay between lymphangiogenic cytokines maintains leukemic blast survival in the bone marrow (BM). Once AML is induced, proangiogenic cytokines function as angiogenic or lymphangiogenic factors and affect hematopoietic cells, including BM-derived immune cells. Simultaneously, the representative cytokines, VEGFs and their receptors are expressed on AML blasts in vascular and osteoblast niches in both the BM and the peripheral circulation. After exposure to (lymph)angiogenic cytokines in leukemogenesis and infiltration, immune cell phenotypes and functions are affected. These dynamic behaviors in the BM reflect the clinical features of AML. In this review, we note the importance of lymphangiogenic factors and their receptors in hematopoietic cells in AML. Understanding the functional characterization of (lymph)angiogenic factors in the BM niche in AML will also be helpful in interrupting the engraftment of leukemic stem cells and for enhancing immune cell function by modulating the tumor microenvironment.

An open-label study to determine the maximum tolerated dose of the multitargeted tyrosine kinase inhibitor CEP-11981 in patients with advanced cancer

Background This phase I study evaluated the pharmacokinetics and pharmacodynamics of CEP-11981, an oral vascular endothelial growth factor (VEGF) tyrosine kinase inhibitor, in patients with advanced, relapsed, or refractory solid tumors. Methods Oral CEP-11981 dose escalations followed a modified Fibonacci sequence (from 3.0 to 4.2, 5.9, 11.8, 19.7, 29.6, 41.4, 55.0, 73.0, 97.4, and 126.6 mg/m²). The maximum-tolerated dose (MTD), dose-limiting toxicities (DLTs), tumor response, and safety were evaluated. Results CEP-11981 was tolerated at doses between 3.0 and 97.4 mg/m². The MTD of CEP-11981 was determined to be 97.4 mg/m², with DLTs observed at the 126.6 mg/m² dose. The DLTs were grade 4 neutropenia in 1 patient and grade 3 T-wave inversion with chest heaviness and fatigue in 1 patient. All 3 events resolved on stopping CEP-11981. The most frequently reported adverse events of any grade were fatigue, nausea, diarrhea, decreased appetite, abdominal pain, back pain, vomiting, constipation, headache, dizziness, and dyspnea. Treatment-related grade 3/4 neutropenia was observed in the highest-dose cohorts (2 patients at 97.4 mg/m² and 1 patient at 126.6 mg/m²), indicating some off-target inhibition. VEGF inhibition was greatest in the higher-dose groups. Although no patient experienced complete or partial response, 44 % patients achieved stable disease when measured at ≥ 6 weeks, which occurred more frequently in cohorts receiving ≥ 73.0 mg/m². Conclusions In patients with recurrent or refractory solid tumors, disease stabilization was achieved. Despite acceptable tolerability of CEP-11981 at the MTD, further development by the sponsor has ceased.

Lymphangiogenesis and lymphatic vessel remodelling in cancer

The generation of new lymphatic vessels through lymphangiogenesis and the remodelling of existing lymphatics are thought to be important steps in cancer metastasis. The past decade has been exciting in terms of research into the molecular and cellular biology of lymphatic vessels in cancer, and it has been shown that the molecular control of tumour lymphangiogenesis has similarities to that of tumour angiogenesis. Nevertheless, there are significant mechanistic differences between these biological processes. We are now developing a greater understanding of the specific roles of distinct lymphatic vessel subtypes in cancer, and this provides opportunities to improve diagnostic and therapeutic approaches that aim to restrict the progression of cancer.

Receptor tyrosine kinase-mediated angiogenesis

The endothelial cell is the essential cell type forming the inner layer of the vasculature. Two families of receptor tyrosine kinases (RTKs) are almost completely endothelial cell specific: the vascular endothelial growth factor (VEGF) receptors (VEGFR1-3) and the Tie receptors (Tie1 and Tie2). Both are key players governing the generation of blood and lymphatic vessels during embryonic development. Because the growth of new blood and lymphatic vessels (or the lack thereof) is a central element in many diseases, the VEGF and the Tie receptors provide attractive therapeutic targets in various diseases. Indeed, several drugs directed to these RTK signaling pathways are already on the market, whereas many are in clinical trials. Here we review the VEGFR and Tie families, their involvement in developmental and pathological angiogenesis, and the different possibilities for targeting them to either block or enhance angiogenesis and lymphangiogenesis.

Targets, structures, and recent approaches in malignant melanoma chemotherapy

Malignant metastatic melanoma is one of the oncologic diseases with the worst clinical prognosis, due primarily to resistance phenomena against chemotherapeutic agents in current use. However, over the last few years, characterization of the molecular mechanisms involved in the development and progression of the disease has contributed to elucidation of the main pathways by which tissue invasion and metastasis can occur. More importantly, the identification of abnormalities in signaling cascades in melanoma cells has facilitated new therapeutic approaches against malignant melanoma through the design of highly potent and selective drugs with low associated toxicity. Ultimately, recognition of the restricted applicability of new chemotherapies in certain genetic contexts has led to significant improvements in the results of clinical trials, anticipating the existing need for investment in personalized therapies, and taking into account the molecular alterations observed in tumors. Although significant advances have been made in terms of extending the median overall survival rate and improving the quality of life for patients, the mechanisms that compromise in vivo drug efficacy remain poorly understood, particularly those concerning therapeutic resistance phenomena. This review summarizes recently validated targets from the perspective of the medicinal chemistry carried out in the design of the most promising structures.

Novel approaches to glioma drug design and drug screening

INTRODUCTION

Gliomas are considered the most malignant form of brain tumors, and ranked among the most aggressive human cancers. Despite advance standard therapy the prognosis for patients with gliomas remains poor. Chemotherapy has played an important role as an adjuvant in treating gliomas. The efficacy of the chemotherapeutic drug is limited due to poor drug delivery and the inherent chemo- and radio-resistance. Challenges of the brain cancer therapy in clinical settings are; i) to overcome the chemo- and radio-resistance, ii) to improve drug delivery to tumors and iii) the development of effective drug screening procedures.

AREAS COVERED

In this review, the authors discuss clinically important chemotherapeutic agents used for treating malignant gliomas along with novel drug design approaches. The authors, furthermore, discuss the in vitro and in vivo drug screening procedures for the development of novel drug candidates.

EXPERT OPINION

The development of novel and highly potent chemotherapeutic agents for both glioma and glioma stem cells (GSCs) is highly important for future brain cancer research. Thus, research efforts should be directed towards developing innovative molecularly targeted antiglioma agents in order to reduce the toxicity and drug resistance which are associated with current forms of therapy. Development of novel pre-clinical drug screening procedures is also very critical for the overall success of brain cancer therapies in clinical settings.

Connection of pericyte-angiopoietin-Tie-2 system in diabetic retinopathy: friend or foe?

Pericytes are distinctive regulators of vascular morphologenesis and function during vascular development and homeostasis. Pericytes have recently come into focus as implications of aberrant interactions between pericytes and endothelial cells in number of pathological angiogenesis conditions, including diabetic retinopathy and tumor angiogenesis. Pericyte dropout is a hallmark of early diabetic retinopathy. Abnormal angiopoietin (Ang)-Tie-2 signaling is one principal system participating in pericyte/endothelial cell dissociation during early stages of diabetic retinopathy. Angiopoietin 2 (Ang-2) is among the relevant growth factors induced by hypoxia and plays an important role in the initiation of retinal neovascularization and cause pericyte loss. Furthermore, high levels of VEGF synergize Ang-Tie-2 signaling during the development of diabetic retinopathy. An accelerated rate of clinical development Ang-Tie-2-manipulating drugs requests a better mechanistic understanding the connection between pericytes and Ang-Tie-2 systems both under normal and disease conditions. We summarize recent advances in pericyte study in conjunction with Ang-Tie-2 signaling and also discuss possible therapeutic strategies for diabetic retinopathy by targeting pericytes through manipulating Ang-Tie-2 signaling.

Angiopoietin-2: an attractive target for improved antiangiogenic tumor therapy

Anti-VEGF pathway therapies primarily target immature blood vessels in tumors. However, emerging approaches to combine with targeted therapies impacting the later stages of remodeling and vessel maturation are expected to improve clinical efficacy by expanding the target vessel population. The angiopoietin/Tie ligand/receptor system is a prototypic regulator of vessel remodeling and maturation. Angiopoietin-2 (Ang2) appears to be a particularly attractive therapeutic target. In fact, the experimental proof-of-concept showing improved efficacy when VEGF and Ang2-targeting therapies are combined has been solidly established in preclinical models, and several Ang2-targeting drugs are in clinical trials. However, rational development of these second-generation combination therapies is hampered by a limited understanding of the biological complexity that is generated from agonistic and antagonistic Ang/Tie signaling. This review discusses recent mechanistic advances in angiopoietin signaling, particularly in light of the recent study published on REGN910 and summarizes the status quo of Ang2-targeting therapies. In light of the clarified partial agonist function of Ang2, we propose that clarity on the expression profile of the angiopoietin ligands and Tie1 and Tie2 receptors in subsets of cancer vessels and cancer cells will provide clearer hypotheses for more focused rational clinical trials to exploit this seminal pathway and improve current antiangiogenic therapies.

Development and strategies of VEGFR-2/KDR inhibitors

VEGF is an important signaling protein involved in both vasculogenesis and angiogenesis. As an essential receptor protein tyrosine kinase propagating cellular signal transduction processes, VEGFR-2 is a central target for drug discovery against tumor-associated angiogenesis. Since the autophosphorylation of VEGFR-2 represents a key step in this signal pathway that contributes to angiogenesis, the discovery of small molecule inhibitors that block this reaction has attracted great interest for novel drugs research and development. Advances in the understanding of catalytic cleft and the conformational changes of DFG motif have resulted in the development of small molecule inhibitors known as type I and type II. High-resolution crystal structures of various inhibitors in complex with the receptor offer an insight into the relationship among binding modes, inhibition mechanisms, activity, selectivity and resistance. To control selectivity, improve activity and introduce intellectual property novelty, the strategies for the further development are discussed through structural and conformational analysis in this review.

Vascular endothelial growth factor (VEGF) receptors: drugs and new inhibitors

The recent launch onto the market of five VEGFR inhibitors indicates the therapeutic value of these agents and the importance of the research in the field of angiogenesis inhibitors for future oncologic therapy. In this Perspective we briefly report the inhibitors that are in clinical use, while we dedicate two wider sections to the compounds that are in clinical trials and to the new derivatives appearing in the literature. We especially consider the medicinal chemistry aspect of the topic and report the structure-activity relationship studies and the binding mode of some inhibitors as well as the biological data of the compounds discovered in the past 5 years.