Receptor tyrosine kinases as targets for inhibition of angiogenesis

Recent Advances in Multi-target Drugs Targeting Protein Kinases and Histone Deacetylases in Cancer Therapy

Protein Kinase Inhibitors (PKIs) and Histone Deacetylase Inhibitors (HDACIs) are two important classes of anticancer agents and have provided a variety of small molecule drugs for the treatment of various types of human cancers. However, malignant tumors are of a multifactorial nature that can hardly be "cured" by targeting a single target, and treatment of cancers hence requires modulation of multiple biological targets to restore the physiological balance and generate sufficient therapeutic efficacy. Multi-target drugs have attracted great interest because of their advantages in the treatment of complex cancers by simultaneously targeting multiple signaling pathways and possibly leading to synergistic effects. Synergistic effects have been observed in the combination of kinase inhibitors, such as imatinib, dasatinib, or sorafenib, with an array of HDACIs including vorinostat, romidepsin, or panobinostat. A considerable number of multi-target agents based on PKIs and HDACIs have been developed. In this review, we summarize the recent literature on the development of multi-target kinase-HDAC inhibitors and provide our view on the challenges and future directions on this topic.

Novel piperazine-chalcone hybrids and related pyrazoline analogues targeting VEGFR-2 kinase; design, synthesis, molecular docking studies, and anticancer evaluation

New piperazine–chalcone hybrids and related pyrazoline derivatives have been designed and synthesised as potential vascular endothelial growth factor receptor-2 (VEGFR-2) inhibitors. The National Cancer Institute (NCI) has selected six compounds to evaluate their antiproliferative activity in vitro against 60 human cancer cells lines. Preliminary screening of the examined compounds indicated promising anticancer activity against number of cell lines. The enzyme inhibitory activity against VEGFR-2 was evaluated and IC₅₀ of the tested compounds ranged from 0.57 µM to 1.48 µM. The most potent derivatives Vd and Ve were subjected to further investigations. A cell cycle analysis showed that both compounds mainly arrest HCT-116 cell cycle in the G2/M phase. Annexin V-FITC apoptosis assay showed that Vd and Ve induced an approximately 18.7-fold and 21.2-fold total increase in apoptosis compared to the control. Additionally, molecular docking study was performed against VEGFR (PDB ID: 4ASD) using MOE 2015.10 software and Sorafenib as a reference ligand.

1,3,5-Triazine Analogs: A Potent Anticancer Scaffold

Background:

This review presents the exhaustive exploration of 1,3,5-triazine scaffold for development of analogs of anticancer drugs, over the last century. In the recent years, striazine moiety has been one of the most studied moiety, showing broad-spectrum pharmacological activities such as antibacterial, antifungal, analgesic, anti-HIV, antileishmanial, antitrypanosomal, antimalarial and antiviral. Nowadays, many boffins are have become interested in novel synthesis of s-triazine derivatives because of low cost and ease of availability.

Methods:

This scaffold has been extensively investigated mainly in the past decade. Many products have been synthesized from different starting materials and these synthetic products possess anticancer potential against various cell lines.

Results:

Many 1,3,5-triazine analogs exhibited significant anticancer activity in various models and cell lines exhibiting different mechanisms. Some analogs have also shown good pharmacokinetic parameters with less IC50 values.

Conclusion:

Various 1,3,5-triazine analogs have shown potent activities and may be regarded as clinical candidates for future anticancer formulations. This review may be helpful to those researchers seeking required information with regard to the drug design and medicinal properties of 1,3,5-triazine derivatives for selected targets. This review may also offer help to find and improve clinically viable anticancer molecules.

Herbal compounds for rheumatoid arthritis: Literatures review and cheminformatics prediction

Rheumatoid arthritis (RA) is a systemic disease characterized by autoimmunity, joint inflammation, and cartilage destruction, which affects 0.5-1% of the population. Many compounds from herbal medicines show the potentials to treat RA. On this basis, the compounds with good pharmacokinetic behaviors and drug-likeness properties will be further studied and developed. Therefore, the herbal compounds with anti-RA activities were reviewed in this paper, and the cheminformatics tools were used to predict their drug-likeness properties and pharmacokinetic parameters. A total of 90 herbal compounds were analyzed, which were reported to be effective on RA models through anti-inflammation, chondroprotection, immunoregulation, antiangiogenesis, and antioxidation. Most of the herbal compounds have good drug-likeness properties. Most of the compounds can be an alternative and valuable source for anti-RA drug discovery.

Synthesis and biological evaluation of novel oxazolo[5,4-d]pyrimidines as potent VEGFR-2 inhibitors

Tumor angiogenesis is mediated by vascular endothelial growth factor receptor (VEGFR) and other protein kinases. Inhibition of these kinases presents an attractive approach for developing anticancer therapeutics. In this work, a series of 2,5,7-trisubstituted oxazolo[5,4-d]pyrimidines were synthesized, and their inhibitory activities were investigated against VEGFR-2 and human umbilical vein endothelial cells (HUVEC) in vitro. Compound 9n exhibited the most potent inhibitory activity with IC50 values of 0.33 and 0.29 μM for VEGFR-2 kinase and HUVEC, respectively. A further kinase selectivity assay revealed that these compounds exhibit good VEGFR and moderate EGFR inhibitory activities. Docking analysis suggested a common mode of interaction at the ATP-binding site of VEGFR-2.

The immunopharmacologic potential of Semaxanib and new generation directed therapeutic drugs: Receptor tyrosine kinase regulation with anti-tumorigenensis/angiogenesis properties

Molecular signaling of messages emanating from cellular membranes through receptor tyrosine kinases (RTKs) is a major mechanism for intercellular communication and transduction during development and metabolism, as well as in disease-associated processes. The phosphorylation status and signaling activity of RTKs are determined by a dynamic equilibrium of the activity of both RTKs and protein tyrosine phosphatases (PTPs). RTKs are essentially a class of cell-surface receptors for growth factors and other extracellular ligands, the most conspicuous perhaps are members of the vascular endothelial growth factor (VEGF) gene family, which plays a fundamental role in the growth and differentiation of vascular, as well as lymphatic endothelial cells. In particular, VEGF is a major regulator of normal (physiologic) and abnormal (cancerous) angiogenesis, including that associated with tumors and cancer. Blockers/inhibitors and regulators of RTKs are indeed promising cancer interventions, their specific mechanisms are yet to be unraveled. In this cutting-edge synopsis, I elaborate on breakthroughs/advances and current concepts of RTK regulation, further shedding light on exploring the role of potential regulators, particularly the RTK inhibitor Semaxanib, and the mechanisms associated with tumorigenesis in an effort to understand a potentially alleviating pharmacologic therapeutic intervention. This survey also tackles the loopholes and shortcomings of the aforementioned inhibitory role of Semaxanib, especially its inefficacy and ultimate discontinuation of relevant clinical trials.

Liposomal nanomedicine for breast cancer therapy

Liposomes are well-established nanocarriers for improving the therapeutic index of anticancer agents. A remarkable understanding in the pathophysiology of breast cancer progression has emerged with information on the involved specific biomolecules, which may serve as molecular targets for its therapy. Hormonal and nonhormonal receptors can both be exploited for targeting to breast cancer cells. Targeted delivery of cytotoxic drugs using liposomes is a novel approach for breast cancer therapy. In the present article, we summarize molecular targets present on the breast cancer cells. Recent developments in liposome-based delivery of bioactives for selective treatments of breast cancer are discussed. In addition, utilization of bioenvironmental conditions of tumor for liposome-based targeted delivery is also summed up.

Traceless synthesis of diketopiperazine fused tetrahydro-β-carbolines on soluble polymer support

The Pictet-Spengler reaction, using polyethylene glycol immobilized tryptophan ester with a variety of ketones, was achieved by refluxing condition in acidic chloroform. The linear as well as cyclic ketones were employed. All the ketones were reacted within 6-8 h to furnish soluble polymer-supported tetrahydro-β-carboline in good yields. Further expansion at N-terminus of tetrahydro-β-carbolines was achieved through a reaction with chloroacetyl chloride. Finally, the 2,5-diketopiperazine skeleton was constructed over a β-carboline by amination of the resulting N-chloroacetamides and subsequent intramolecular cyclization leading to cleavage of the polymer; constitutes a traceless synthesis of tetracyclic molecular architecture. Significantly, this strategy affords a straightforward and efficient approach for the construction of biological promising molecules with high purity and good yields.

Pharmacophore modeling and virtual screening studies for new VEGFR-2 kinase inhibitors

Virtual screening was performed to determine potent vascular endothelial growth factor receptor (VEGFR)-2 kinase inhibitors. A database of approximately 820,000 commercial compounds was used for screening, and 100 compounds were chosen as candidate VEGFR-2 inhibitors through pharmacophore modeling and docking studies. These 100 compounds were purchased to test their biological activities: 10 compounds were found to inhibit the enzyme, with IC(50) values ranging from 10 to 1 μM. Compound 1, which has a triazinoindole ring, inhibited the enzymatic activity of VEGFR-2, with an IC(50) value of about 1.6 μM, making it the most potent inhibitor of this enzyme. The triazinoindole derivative may therefore serve as the starting point in the design of new VEGFR-2 kinase inhibitors.

Molecular modeling studies of vascular endothelial growth factor receptor tyrosine kinase inhibitors using QSAR and docking

The vascular endothelial growth factor (VEGF) and its receptor tyrosine kinases VEGFR-2 or kinase insert domain receptor (KDR) are attractive targets for the development of novel anticancer agents. In the present work, comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) were performed on a series of selective inhibitors of KDR. Docking studies were performed to explore the binding mode between all of the inhibitors and the KDR and produce the bioactive conformation of each compound in the whole dataset. Two conformer-based alignment strategies were employed to construct reliable 3D-QSAR models. The docked conformer-based alignment strategy gave the best 3D-QSAR models. The best CoMFA and CoMSIA models gave a cross-validated coefficient q(2) of 0.546 and 0.715, non-cross-validated r(2) values of 0.936 and 0.961, predicted r(2) values of 0.673 and 0.797, respectively. The information obtained from molecular modeling studies were very helpful to design some novel selective inhibitors of KDR with desired activity.

Discovery and preclinical studies of 5-isopropyl-6-(5-methyl-1,3,4-oxadiazol-2-yl)-N-(2-methyl-1H-pyrrolo[2,3-b]pyridin-5-yl)pyrrolo[2,1-f][1,2,4]triazin-4-amine (BMS-645737), an in vivo active potent VEGFR-2 inhibitor

We report herein a series of substituted N-(1H-pyrrolo[2,3-b]pyridin-5-yl)pyrrolo[2,1-f][1,2,4]triazin-4-amines as inhibitors of vascular endothelial growth factor receptor-2 tyrosine kinase. Through structure-activity relationship studies, biochemical potency, pharmacokinetics, and kinase selectivity were optimized to afford BMS-645737 (13), a compound with good preclinical in vivo activity against human tumor xenograft models.

Anti-angiogenic activity of the methanol extract and its fractions of Ulmus davidiana var. japonica

This study aimed to elucidate anti-angiogenic activity of Ulmus davidiana var. japonica that has been widely used in folk medicine. The methanol extract (UDE) of Ulmus davidiana var. japonica concentration-dependently displayed a strong inhibition in the chick chorioallantoic membrane (CAM) angiogenesis. The n-butanol fraction of UDE and subsequent 30% MeOH subfraction were identified to be most responsible for the anti-angiogenic activity.

Evolution of a Highly Selective and Potent 2-(Pyridin-2-yl)-1,3,5-triazine Tie-2 Kinase Inhibitor

Inhibition of angiogenesis is a promising and clinically validated approach for limiting tumor growth and survival. The receptor tyrosine kinase Tie-2 is expressed almost exclusively in the vascular endothelium and is required for developmental angiogenesis and vessel maturation. However, the significance of Tie-2 signaling in tumor angiogenesis is not well understood. In order to evaluate the therapeutic utility of inhibiting Tie-2 signaling, we developed a series of potent and orally bioavailable small molecule Tie-2 kinase inhibitors with selectivity over other kinases, especially those that are believed to be important for tumor angiogenesis. Our earlier work provided pyridinyl pyrimidine 6 as a potent, nonselective Tie-2 inhibitor that was designed on the basis of X-ray cocrystal structures of KDR inhibitors 34 (triazine) and 35 (nicotinamide). Lead optimization resulted in pyridinyl triazine 63, which exhibited >30-fold selectivity over a panel of kinases, good oral exposure, and in vivo inhibition of Tie-2 phosphorylation.

Alkynylpyrimidine amide derivatives as potent, selective, and orally active inhibitors of Tie-2 kinase

The recognition that aberrant angiogenesis contributes to the pathology of inflammatory diseases, cancer, and myocardial ischemia has generated considerable interest in the molecular mechanisms that regulate blood vessel growth. The receptor tyrosine kinase Tie-2 is expressed primarily by vascular endothelial cells and is critical for embryonic vasculogenesis. Interference with the Tie-2 pathway by diverse blocking agents such as soluble Tie-2 receptors, anti-Tie-2 intrabodies, anti-Ang-2 antibodies, and peptide-Fc conjugates has been shown to suppress tumor growth in xenograft studies. An alternative strategy for interfering with the Tie-2 signaling pathway involves direct inhibition of the kinase functions of the Tie-2 receptor. Herein we describe the development of alkynylpyrimidine amide derivatives as potent, selective, and orally available ATP-competitive inhibitors of Tie-2 autophosphorylation.

Design and effective synthesis of novel templates, 3,7-diphenyl-4-amino-thieno and furo-[3,2-c]pyridines as protein kinase inhibitors and in vitro evaluation targeting angiogenetic kinases

A novel class of 3,7-diphenyl-4-amino-thieno and furo[3,2-c]pyridine has been designed based on pharmacophore models of ATP competitive kinase inhibitors. Versatile synthetic methods via double Suzuki coupling to explore SAR have been established and potent inhibitors against angiogenetic targets, VEGFR2, Tie-2, and EphB4, have been successfully discovered.

Discovery and evaluation of N-cyclopropyl- 2,4-difluoro-5-((2-(pyridin-2-ylamino)thiazol-5- ylmethyl)amino)benzamide (BMS-605541), a selective and orally efficacious inhibitor of vascular endothelial growth factor receptor-2

Substituted 3-((2-(pyridin-2-ylamino)thiazol-5-ylmethyl)amino)benzamides were identified as potent and selective inhibitors of vascular endothelial growth factor receptor-2 (VEGFR-2) kinase activity. The enzyme kinetics associated with the VEGFR-2 inhibition of 14 (Ki=49+/-9 nM) confirmed that the aminothiazole-based analogues are competitive with ATP. Analogue 14 demonstrated excellent kinase selectivity, favorable pharmacokinetic properties in multiple species, and robust in vivo efficacy in human lung and colon carcinoma xenograft models.

Discovery and preclinical studies of (R)-1-(4-(4-fluoro-2-methyl-1H-indol-5-yloxy)-5- methylpyrrolo[2,1-f][1,2,4]triazin-6-yloxy)propan- 2-ol (BMS-540215), an in vivo active potent VEGFR-2 inhibitor

A series of substituted 4-(4-fluoro-1H-indol-5-yloxy)pyrrolo[2,1-f][1,2,4]triazine-based inhibitors of vascular endothelial growth factor receptor-2 kinase is reported. Structure-activity relationship studies revealed that a methyl group at the 5-position and a substituted alkoxy group at the 6-position of the pyrrolo[2,1-f][1,2,4]triazine core gave potent compounds. Biochemical potency, kinase selectivity, and pharmacokinetics of the series were optimized and in vitro safety liabilities were minimized to afford BMS-540215 (12), which demonstrated robust preclinical in vivo activity in human tumor xenograft models. The l-alanine prodrug of 12, BMS-582664 (21), is currently under evaluation in clinical trials for the treatment of solid tumors.

(6,7-Dimethoxy-2,4-dihydroindeno[1,2-c]pyrazol-3-yl)phenylamines: platelet-derived growth factor receptor tyrosine kinase inhibitors with broad antiproliferative activity against tumor cells

A series of (6,7-dimethoxy-2,4-dihydroindeno[1,2-c]pyrazol-3-yl)phenylamines has been optimized to preserve both potent kinase inhibition activity against the angiogenesis target, the receptor tyrosine kinase of Platelet-Derived Growth Factor-BB (PDGF-BB), and to improve the broad tumor cell antiproliferative activity of these compounds. This series culminates in the discovery of 17 (JNJ-10198409), a compound with anti-PDGFR-beta kinase activity (IC(50)=0.0042 microM) and potent antiproliferative activity in six of eight human tumor cell lines (IC(50) < 0.033 microM).

Synthesis and Structure−Activity Relationships of Soluble 7-Substituted 3-(3,5-Dimethoxyphenyl)-1,6-naphthyridin-2-amines and Related Ureas as Dual Inhibitors of the Fibroblast Growth Factor Receptor-1 and Vascular Endothelial Growth Factor Receptor-2 Tyrosine Kinases

7-Substituted 3-aryl-1,6-naphthyridine-2,7-diamines and related 2-ureas are inhibitors of fibroblast growth factor receptor-1 (FGFR-1) and vascular endothelial growth factor receptor-2 (VEGFR-2). 3-(3,5-Dimethoxyphenyl) and 3-phenyl analogues were prepared from 7-acetamido-2-tert-butylureas by alkylation with benzyl omega-iodoalkyl ethers, debenzylation, and amination, followed by selective cleavage of the 7-N-acetamide. 3-(2,6-Dichlorophenyl) analogues were prepared from the 7-fluoro-2-amine by displacement with substituted alkylamines, followed by selective acylation of the resulting substituted naphthyridine-2,7-diamines with alkyl isocyanates. The 3-(3,5-dimethoxyphenyl) derivatives were low nanomolar inhibitors of both FGFR and VEGFR and were highly selective (>100-fold) over PDGFR and c-Src. Variations in the base strength or spatial position of the 7-side chain base had only small effects on the potency (<5-fold) or selectivity (<20-fold). The 3-(2,6-dichlorophenyl)-2-urea derivatives were slightly less active against VEGFR and less selective, being more effective against PDGFR (ca. 10-fold) and c-Src (ca. 500-fold). The 3-(3,5-dimethoxyphenyl)-1,6-naphthyridines were generally more potent than the corresponding pyrido[2,3-d]pyrimidines against both VEGFR and FGFR (2- to 20-fold), with only slightly increased PDGFR and c-Src activity. The 3-(3,5-dimethoxyphenyl)-1,6-naphthyridine 2-ureas were also low nanomolar inhibitors of the growth of human umbilical vein endothelial cells (HUVECs) stimulated by serum, FGF, or VEGF, at concentrations that did not affect the growth of representative tumor cell lines, and were more (3- to 65-fold) potent than the corresponding pyrido[2,3-d]pyrimidines.

Design, synthesis, and evaluation of orally active 4-(2,4-difluoro-5-(methoxycarbamoyl)phenylamino)pyrrolo[2,1-f][1,2,4]triazines as dual vascular endothelial growth factor receptor-2 and fibroblast growth factor receptor-1 inhibitors

A series of substituted 4-(2,4-difluoro-5-(methoxycarbamoyl)phenylamino)pyrrolo[2,1-f][1,2,4]triazines was identified as potent and selective inhibitors of the tyrosine kinase activity of the growth factor receptors VEGFR-2 (Flk-1, KDR) and FGFR-1. The enzyme kinetics associated with the VEGFR-2 inhibition of compound 50 (K(i) = 52 +/- 3 nM) confirmed that the pyrrolo[2,1-f][1,2,4]triazine analogues are competitive with ATP. Several analogues demonstrated low-nanomolar inhibition of VEGF- and FGF-dependent human umbilical vein endothelial cell (HUVEC) proliferation. Replacement of the C6-ester substituent of the pyrrolo[2,1-f][1,2,4]triazine core with heterocyclic bioisosteres, such as substituted 1,3,5-oxadiazoles, afforded compounds with excellent oral bioavailability in mice (i.e., 50 F(po) = 79%). Significant antitumor efficacy was observed with compounds 44, 49, and 50 against established L2987 human lung carcinoma xenografts implanted in athymic mice. A full account of the synthesis, structure-activity relationships, pharmacology, and pharmacokinetic properties of analogues within the series is presented.

Philinopside A, a novel marine-derived compound possessing dual anti-angiogenic and anti-tumor effects

Philinopside A is a novel sulfated saponin isolated from the sea cucumber, Pentacta quadrangulari. The effects of philinopside A on angiogenesis and tumor growth were assessed in a series of models in vitro and in vivo. Our results demonstrated that philinopside A significantly inhibited the proliferation, migration and tube formation of human microvascular endothelial cells (HMECs) in a dose-dependent manner, with average IC(50) values of 1.4 +/- 0.17, 0.89 +/- 0.23 and 0.98 +/- 0.19 microM, respectively. Rat aortas culture assay provides a close imitation of in vivo angiogenesis process and 2-10 microM philinopside A suppressed the formation of new microvessels in cultured rat aortas. Philinopside A 2-10 nmol/egg obviously inhibited angiogenesis in chick embryo chorioallantoic membrane assay. In addition, philinopside A manifested strong anti-tumor activities both in vitro and in vivo. Through immunofluorescent analysis, we found the compound reduced mouse sarcoma 180 tumor volume by inducing apoptosis of tumor and tumor-associated endothelial cells. An examination of the effects of philinopside A on the angiogenesis-related receptor tyrosine kinases (RTKs) showed that philinopside A broadly inhibited all tested RTKs, including vascular endothelial growth factor (VEGF) receptor, fibroblast growth factor (FGF) receptor-1, platelet-derived growth factor (PDGF) receptor-beta and epithelial growth factor (EGF) receptor, with IC(50) values ranging from 2.6-4.9 microM. These results suggest that philinopside A is a promising anti-cancer agent that possesses dual cytotoxic and anti-angiogenic effects that were at least partly due to its inhibitory effects on RTKs.

The selective class III/V receptor tyrosine kinase inhibitor SU11657 inhibits tumor growth and angiogenesis in experimental neuroblastomas grown in mice

Vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF) and their cognate receptor tyrosine kinases are strongly implicated in angiogenesis associated with solid tumors. SU11657 (SUGEN) is a selective multitargeted tyrosine kinase inhibitor with antitumor and antiangiogenic activity exerted by targeting PDGF receptors (PDGFR), VEGF receptors (VEGFR), stem cell factor receptor (c-KIT), and FMS-related tyrosine kinase 3. Oral administration of SU11657 at 40 mg x kg(-1) x d(-1) to athymic mice resulted in significant growth inhibition of a panel of s.c. human neuroblastoma xenografts, namely, fast-growing SK-N-AS, MYCN- amplified IMR-32, and SH-SY5Y, by 90, 93.8, and 88%, respectively, and was well tolerated. All of the cell lines expressed VEGFR-2, PDGFR-beta, and c-KIT protein in the tumor cell and endothelial cell compartment by immunohistochemistry, and the expression decreased during therapy. Plasma concentrations of VEGF-A, PDGF-BB, and stem cell factor increased per milliliter of tumor volume at days 10, 18, and 20 of therapy. Furthermore, SU11657 reduced tumor angiogenesis by 63-96%. Our experimental data suggest that the angiogenesis inhibitor SU11657 may be beneficial in the treatment of rapidly growing and highly vascularized solid tumors of childhood, such as neuroblastoma. In summary, the class III/V receptor tyrosine kinases and their ligands are implicated in angiogenesis, tumor cell proliferation, and cell survival, and it seems reasonable to determine whether interference with these pathways can suppress neuroblastoma growth or not.

Post-transcriptional control of expression of sFlt-1, an endogenous inhibitor of vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is a major modulator of angiogenesis. Biological effects of VEGF are mediated by endothelial cell-surface receptors, KDR and Flt-1. Alternative Flt-1 RNA processing, involving retention of intron 13 and the use of intronic cleavage-polyadenylation signals, produces a secreted form of Flt-1, "sFlt-1," that binds VEGF with high affinity and can inhibit VEGF signaling. To probe mechanisms controlling sFlt-1 expression, we have cloned and sequenced Flt-1 intron 13 from a mouse genomic library and located RNA processing signals potentially involved in sFlt-1 mRNA formation. A minigene construct containing Flt-1 intron 13 directed the expression of both secreted (i.e., cleaved/polyadenylated) and transmembrane (i.e., spliced) forms of Flt-1 mRNA and protein. Using rapid amplification of 3' cDNA ends (3'-RACE) and quantitative PCR (QPCR) analysis to test the activity of intronic cleavage-polyadenylation signals, we observed that multiple sites were utilized for sFlt-1 mRNA processing in both native tissues and Flt-1 minigene transfectants. In transfectants, the most distal signal was utilized preferentially. The potential for interaction between pathways leading to sFlt-1 or full-length Flt-1 was evaluated using QPCR to measure relevant mRNAs after transfection with signal mutants. Decreased expression of sFlt-1 mRNA in cleavage-polyadenylation mutants was accompanied by reciprocal increases in full-length Flt-1 mRNA. Multiple sFlt-1 mRNA species are formed that differ by up to 3.9 kb in their 3'-untranslated regions (UTRs), which contain sites of potential regulatory importance. The reciprocity between sFlt-1 and Flt-1 mRNA expression suggests a novel post-transcriptional mechanism by which sFlt-1 protein production and, thereby, responsiveness to VEGF, may be modulated.

Breast Cancer Highlights: Key Findings from the San Antonio Breast Cancer Symposium: A U.S. Perspective

The San Antonio Breast Cancer Symposium has become one of the leading forums for communication of important discoveries in breast cancer research. Over the past couple of years, seminal, practice-changing results have been presented at this meeting. The aromatase inhibitors represent the most effective endocrine interventions for postmenopausal women with hormone receptor-positive breast cancer. Their introduction into the adjuvant therapy of primary breast cancer was prompted by evidence from the ATAC trial. Progress in adjuvant chemotherapy included the introduction to taxanes, and more recently, the demonstration that the dose-dense administration of paclitaxel in association with doxorubicin and cyclophosphamide resulted in significant improvements in relapse-free and overall survival rates. Molecular targets have become accepted as rational targets, and targeted therapies are proceeding through clinical trials. The success of trastuzumab elicited much excitement, but a number of theoretical and practical hurdles must be overcome before other molecularly targeted agents are incorporated into standard therapy of primary and metastatic breast cancer.

Anti-angiogenic effects of Shiraiachrome A, a compound isolated from a Chinese folk medicine used to treat rheumatoid arthritis

The Chinese folk medicine Shiraia bambusicola has long been utilized in the treatment of rheumatoid arthritis, a disease in which angiogenesis plays an important role. We report here the isolation of the compound Shiraiachrome A from S. bambusicola and the demonstration of its anti-angiogenic properties. We found that Shiraiachrome A significantly inhibited the proliferation, migration, and tube formation of human microvascular endothelial cells (HMEC) in a dose-dependent manner, with average IC(50) values of 2.1+/-0.36, 1.97+/-0.44, and 1.65+/-0.59 microM, respectively. In addition, Shiraiachrome A inhibited the formation of new microvessels in a rat aorta culture model as well as in the chick embryo chorioallantoic membrane (CAM) assay. Investigation of the mechanism of action of Shiraiachrome A demonstrated that this compound suppressed the autophosphorylation of four receptor tyrosine kinases (RTKs), with IC(50) values ranging from 2.2 to 4.3 microM. These results suggest that Shiraiachrome A inhibits angiogenesis by blocking growth factor-stimulated autophosphorylation of RTKs. These findings also indicate that Shiraiachrome A may be a potent therapeutic agent for angiogenesis-related diseases such as cancer, rheumatoid arthritis, and diabetic retinopathy.

Complete radiological and metabolic response of metastatic renal cell carcinoma to SU5416 (semaxanib) in a patient with probable von Hippel-Lindau syndrome

We report a case of a patient with probable von Hippel-Lindau (VHL) syndrome and metastatic renal cell cancer (RCC) who had a complete radiological and metabolic response to SU5416 (semaxanib). The patient was enrolled on a clinical study examining the efficacy of SU5416 in patients with metastatic cancer. Treatment with SU5416 was given at a dose of 145 mg/m2 intravenously twice-weekly for 11 doses. The patient achieved an early metabolic response on an F-18 fluorodeoxyglucose (FDG) Positron Emission Tomographic (PET) scan within 2 weeks of therapy. Subsequent computerized tomography (CT) and PET scans (9 and 12 months after treatment, respectively) confirmed ongoing complete radiological and metabolic response. He remains tumor-free 18 months after treatment. This is the first documented report of metastatic RCC in the setting of presumed VHL syndrome responding to treatment with SU5416. While vascular endothelial growth factor (VEGF) inhibitors have been shown to produce a modest response in sporadic metastatic RCC, further studies utilizing VEGF inhibitors in patients with VHL syndrome and RCC warrants exploration.

Vascular endothelial growth factor-mediated angiogenesis inhibition and postoperative wound healing in rats

BACKGROUND

Vascular endothelial growth factor (VEGF) is an angiogenic factor that acts by binding to specific high-affinity tyrosine kinase receptors. SU5416 is an antiangiogenic agent that acts as a potent and selective inhibitor of the VEGF Flk-1/KDR receptor tyrosine kinase. SU5416 has been shown to inhibit VEGF-dependent mitogenesis of human endothelial cells and to decrease the growth of xenografts of melanoma, lung carcinoma, ovarian carcinoma, and gliomas. The effect of pre- or perioperative use of this drug on angiogenesis and wound healing in the postoperative setting has not been shown. We sought to analyze the efficacy and safety with respect to functional dosing of SU5416 in the setting of wound healing. This represents an important step forward in the use of this and similar drugs in the perioperative setting of treatment for multiple types of cancers. The use of an inhibitor of VEGF receptors such as SU5416 is distinct and it is likely complementary to other agents in the treatment of such cancers.

METHODS

We injected 8-week-old male Sprague-Dawley rats with SU5416 (8 or 12 mg/kg) or dimethyl sulfoxide intraperitoneally, daily for 14 days. We then performed a right pulmonary lobectomy and 6-mm full-thickness punch biopsies of the back. Tissue perfusion measured via laser Doppler on Postoperative Day 2 was 1.65, 1.22, and 1.14 perfusion units (P < 0.0004) for control, 8 mg/kg, and 12 mg/kg groups, respectively.

RESULTS

We successfully treated a murine model with functional doses of the anti-VEGF drug SU5416 so as to achieve decreased vascularity and blood flow in postoperative wounds. There was no effect on gross wound healing or infection in either control or treatment groups. Also, no drug-related impairment of histologic healing or decrease in wound tensile strength was demonstrated at either 6 or 14 days.

CONCLUSION

Preoperative therapy with functional dosing of SU5416 does not appear to have any major effect on postoperative morbidity or mortality in rats. We additionally conclude that preoperative therapy with SU5416 should be investigated further with careful attention to wound integrity.

Novel 4-anilinoquinazolines with C-7 basic side chains: design and structure activity relationship of a series of potent, orally active, VEGF receptor tyrosine kinase inhibitors

We have previously shown that 4-anilinoquinazolines can be potent inhibitors of vascular endothelial growth factor (VEGF) receptor (Flt-1 and KDR) tyrosine kinase activity. A novel subseries of 4-anilinoquinazolines that possess basic side chains at the C-7 position of the quinazoline nucleus have been synthesized. This subseries contains potent, nanomolar inhibitors of KDR (median IC(50) 0.02 microM, range 0.001-0.04 microM), which are comparatively less potent vs Flt-1 tyrosine kinase (median IC(50) 0.55 microM, range 0.02-1.6 microM). The compounds also retain some inhibitory activity against the tyrosine kinase associated to the endothelial growth factor receptor (EGFR) (median IC(50) 0.2 microM, range 0.075-0.8 microM) but demonstrate selectivity vs that associated to the FGF receptor 1 (median IC(50) 2.5 microM, range 0.9-19 microM). This selectivity profile is also evident in a growth factor-stimulated human endothelial cell (HUVEC) proliferation assay (i.e., inhibition of VEGF > EGF > FGF), with inhibition of VEGF-induced proliferation being achieved at nanomolar concentrations (median IC(50) 0.06 microM). Further examination of compound 2 (ZD6474) in recombinant enzyme assays revealed excellent selectivity for the inhibition of KDR tyrosine kinase (IC(50) 0.04 microM) vs the kinase activity of erbB2, MEK, CDK-2, Tie-2, IGFR-1R, PDK, PDGFRbeta, and AKT (IC(50) range: 1.1 to >100 microM). Anilinoquinazolines possessing basic C-7 side chains exhibited markedly improved aqueous solubility over previously described anilinoquinazolines possessing neutral C-7 side chains (up to 500-fold improvement at pH 7.4). In addition, aqueous solubility of the neutral fraction present at pH 7.4 of the basic subseries of anilinoquinazoline proved to be higher than that of the neutral analogue 1 (ZD4190). Oral administration of representative compounds to mice (50 mg/kg) produced plasma levels between 0.2 and 3 microM at 24 h after dosing. Our development candidate 2 demonstrated a very attractive in vitro profile combined with excellent solubility (330 microM at pH 7.4) and good oral bioavailability in rat and dog (> 80 and > 50%, respectively). This compound demonstrated highly significant, dose-dependent, antitumor activity in athymic mice. Once daily oral administration of 100 mg/kg of compound 2 for 21 days inhibited the growth of established Calu-6 lung carcinoma xenografts by 79% (P < 0.001, Mann Whitney rank sum test), and substantial inhibition (36%, P < 0.02) was evident with 12.5 mg/kg/day.

Pharmacokinetics and interspecies scaling of a novel VEGF receptor inhibitor, SU5416

The pharmacokinetics and allometric relationships of SU5416, a novel small anti-angiogenesis agent, were studied. The pharmacokinetics of SU5416 were examined in mice, rats, dogs, and cancer patients. The in-vitro intrinsic clearance (CLint) was estimated from the in-vitro metabolism study in mouse, rat, dog, monkey and human liver microsomes. The parameters of interest were correlated across species as a function of bodyweight using an allometric approach. The steady-state volume of distribution (Vd(ss)), plasma clearance (CLs), and CLint of SU5416 were well correlated across species. The exponent of the allometric relationship (b) of the corresponding parameters was 0.92, 0.80 and 0.66, respectively. The elimination half-life (t1/2) was consistent across species and independent of bodyweight. The prediction of CLs, Vd(ss), CLint, and t1/2 in humans using the data from mouse, rat, and dog, and monkey (for CLint) was reasonably good (within 4-fold of the observed values). However, an improved prediction (within 2-fold of the observed values) of the corresponding parameters in humans was obtained when extrapolation from only the rodent data was performed, suggesting that the rodent data are sufficient for the scale-up of SU5416 pharmacokinetic parameters in humans. Using allometry, it was possible to achieve reasonable predictions of the pharmacokinetic parameters of SU5416 in cancer patients with various solid tumours.

Soluble 2-substituted aminopyrido[2,3-d]pyrimidin-7-yl ureas. Structure-activity relationships against selected tyrosine kinases and exploration of in vitro and in vivo anticancer activity

In continuing our search for medicinal agents to treat proliferative diseases, we have discovered 2-substituted aminopyrido[2,3-d]pyrimidin-7-yl ureas as a novel class of soluble, potent, broadly active tyrosine kinase (TK) inhibitors. An efficient route was developed that enabled the synthesis of a wide variety of analogues with substitution on several positions of the template. From the lead structure 1, several series of analogues were made that examined the C-6 aryl substituent, a variety of water solublizing substitutents at the C-2 position, and urea or other acyl functionality at the N-7 position. Compounds of this series were competitive with ATP and displayed submicromolar to low nanomolar potency against a panel of TKs, including receptor (platelet-derived growth factor, PDGFr; fibroblast growth factor, FGFr;) and nonreceptor (c-Src) classes. Several of the most potent compounds displayed submicromolar inhibition of PDGF-mediated receptor autophosphorylation in rat aortic vascular smooth muscle cells and low micromolar inhibition of cellular growth in five human tumor cell lines. One of the more thoroughly evaluated members, 32, with IC50 values of 0.21 microM (PDGFr), 0.049 microM (bFGFr), and 0.018 microM (c-Src), was evaluated in in vivo studies against a panel of five human tumor xenografts, with known and/or inferred dependence on the EGFr, PDGFr, and c-Src TKs. Compound 32 produced a tumor growth delay of 14 days against the Colo-205 colon xenograft model.

New paradigms for the treatment of cancer: the role of anti-angiogenesis agents

Angiogenesis, the sprouting of new blood vessels, plays a role in diverse disease states including cancer, diabetic retinopathy, age-related macular degeneration, rheumatoid arthritis, psoriasis, atherosclerosis, and restenosis. With regard to cancer, the clinical association of tumor vascularity with tumor aggressiveness has been clearly demonstrated in numerous tumor types. The observation of increased microvessel density in tumors not only serves as an independent prognostic indicator, but also suggests that anti-angiogenic therapy may be an important component of treatment regimens for cancer patients. The complexity of the angiogenic process, which involves both positive and negative regulators, provides a number of targets for therapy. Many positive regulators, including growth factor receptors, matrix metalloproteinases, and integrins, have been correlated with increased vascularity of tumors and poor prognosis for patient survival. Thus, these serve as ideal targets for anti-angiogenesis therapy. Many inhibitors of these targets are currently undergoing clinical evaluation as potential anti-cancer agents. In this article, we discuss the role of positive regulators in angiogenesis and tumor growth and describe the anti-angiogenic agents under development.

Inhibition of tumor angiogenesis by synthetic receptor tyrosine kinase inhibitors

Protein tyrosine kinases have emerged as crucial targets for therapeutic intervention in cancer. More recently, growth factor ligands and their respective receptor tyrosine kinases (RTKs) have been shown to be required for tumor cell growth. This latter aspect includes tumor angiogenesis where the growth of tumors leads to compensatory effects on host cells in the tumor microenvironment leading to the growth of microvessels. The purpose of this review is to focus on synthetic chemical approaches to block RTKs associated with tumor angiogenesis as a means to limit the growth and spread of human tumors.

Identification of substituted 3-[(4,5,6, 7-tetrahydro-1H-indol-2-yl)methylene]-1,3-dihydroindol-2-ones as growth factor receptor inhibitors for VEGF-R2 (Flk-1/KDR), FGF-R1, and PDGF-Rbeta tyrosine kinases

A series of new 3-substituted indolin-2-ones containing a tetrahydroindole moiety was developed as specific inhibitors of receptor tyrosine kinases associated with VEGF-R, FGF-R, and PDGF-R growth factor receptors. These compounds were evaluated for their inhibitory properties toward VEGF-R2 (Flk-1/KDR), FGF-R1, PDGF-Rbeta, p60(c)()(-)()(Src)(), and EGF-R tyrosine kinases and their ability to inhibit growth factor-dependent cell proliferation. Structure-activity relationships of this new pharmacophore have been determined at the level of kinase inhibition. Compounds containing a propionic acid moiety at the C-3' position of the tetrahydroindole ring represented the most potent indolin-2-ones to inactivate the VEGF, FGF, and PDGF receptor kinases. The inhibitory activities of 9d against VEGF-R2 (Flk-1), 9h against FGF-R1, and 9b against PDGF-Rbeta were 4, 80, and 4 nM, respectively. However, all of these compounds were inactive when tested against the EGF-R tyrosine kinase. Compounds 9a and 9b represented the most potent inhibitors of these classes to inhibit both biochemical kinase and growth factor-dependent cell proliferation for these three targets. In addition, compound 9a was cocrystallized with the catalytic domain of FGF-R1 providing evidence to explain the structure-activity relationship results. This study has provided evidence to support the potential of these new tyrosine kinase inhibitors for the treatment of angiogenesis and other growth factor-related diseases including human cancers.

New anilinophthalazines as potent and orally well absorbed inhibitors of the VEGF receptor tyrosine kinases useful as antagonists of tumor-driven angiogenesis

The sprouting of new blood vessels, or angiogenesis, is necessary for any solid tumor to grow large enough to cause life-threatening disease. Vascular endothelial growth factor (VEGF) is one of the key promoters of tumor induced angiogenesis. VEGF receptors, the tyrosine kinases Flt-1 and KDR, are expressed on vascular endothelial cells and initiate angiogenesis upon activation by VEGF. 1-Anilino-(4-pyridylmethyl)-phthalazines, such as CGP 79787D (or PTK787 / ZK222584), reversibly inhibit Flt-1 and KDR with IC(50) values < 0.1 microM. CGP 79787D also blocks the VEGF-induced receptor autophosphorylation in CHO cells ectopically expressing the KDR receptor (ED(50) = 34 nM). Modification of the 1-anilino moiety afforded derivatives with higher selectivity for the VEGF receptor tyrosine kinases Flt-1 and KDR compared to the related receptor tyrosine kinases PDGF-R and c-Kit. Since these 1-anilino-(4-pyridylmethyl)phthalazines are orally well absorbed, these compounds qualify for further profiling and as candidates for clinical evaluation.

Tumor angiogenesis: past, present and the near future

The concept of treating solid tumors by inhibiting tumor angiogenesis was first articulated almost 30 years ago. For the next 10 years it attracted little scientific interest. This situation changed, relatively slowly, over the succeeding decade with the discovery of the first pro-angiogenic molecules such as basic fibroblast growth factor and vascular endothelial growth factor (VEGF), and the development of methods of successfully growing vascular endothelial cells in culture as well as in vivo assays of angiogenesis. However, the 1990s have witnessed a striking change in both attitude and interest in tumor angiogenesis and anti-angiogenic drug development, to the point where a remarkably diverse group of over 24 such drugs is currently undergoing evaluation in phase I, II or III clinical trials. In this review I will discuss the many reasons for this. These features, together with other recent discoveries have created intense interest in initiating and expanding anti-angiogenic drug discovery programs in both academia and industry, and the testing of such newly developed drugs, either alone, or in various combinations with conventional cytotoxic therapeutics. However, significant problems remain in the clinical application of angiogenesis inhibitors such as the need for surrogate markers to monitor the effects of such drugs when they do not cause tumor regressions, and the design of clinical trials. Also of concern is that the expected need to use anti-angiogenic drugs chronically will lead to delayed toxic side effects in humans, which do not appear in rodents, especially in short-term studies.

Design and structure-activity relationship of a new class of potent VEGF receptor tyrosine kinase inhibitors

A series of substituted 4-anilinoquinazolines and related compounds were synthesized as potential inhibitors of vascular endothelial growth factor (VEGF) receptor (Flt and KDR) tyrosine kinase activity. Enzyme screening indicated that a narrow structure-activity relationship (SAR) existed for the bicyclic ring system, with quinazolines, quinolines, and cinnolines having activity and with quinazolines and quinolines generally being preferred. Substitution of the aniline was investigated and clearly indicated that small lipophilic substituents such as halogens or methyl were preferred at the C-4' position. Small substituents such as hydrogen and fluorine are preferred at the C-2' position. Introduction of a hydroxyl group at the meta position of the aniline produced the most potent inhibitors of Flt and KDR tyrosine kinases activity with IC(50) values in the nanomolar range (e.g. 10, 12, 13, 16, and 18). Investigation of the quinazoline C-6 and C-7 positions indicates that a large range of substituents are tolerated at C-7, whereas variation at the C-6 is more restricted. At C-7, neutral, basic, and heteroaromatic side chains led to very potent compounds, as illustrated by the methoxyethoxy derivative 13 (IC(50) < 2 nM). Our inhibitors proved to be very selective inhibitors of Flt and KDR tyrosine kinase activity when compared to that associated with the FGF receptor (50- to 3800-fold). Observed enzyme profiles translated well with respect to potency and selectivity for inhibition of growth factor stimulated proliferation of human umbilical vein endothelial cells (HUVECs). Oral administration of selected compounds to mice produced total plasma levels 6 h after dosing of between 3 and 49 microM. In vivo efficacy was demonstrated in a rat uterine oedema assay where significant activity was achieved at 60 mg/kg with the meta hydroxy anilinoquinazoline 10. Inhibition of growth of human tumors in athymic mice has also been demonstrated: compound 34 inhibited the growth of established Calu-6 lung carcinoma xenograft by 75% (P < 0.001, one tailed t-test) following daily oral administration of 100 mg/kg for 21 days.

Design, synthesis, and evaluations of substituted 3-[(3- or 4-carboxyethylpyrrol-2-yl)methylidenyl]indolin-2-ones as inhibitors of VEGF, FGF, and PDGF receptor tyrosine kinases

Receptor tyrosine kinases (RTKs) have been implicated as therapeutic targets for the treatment of human diseases including cancers, inflammatory diseases, cardiovascular diseases including arterial restenosis, and fibrotic diseases of the lung, liver, and kidney. Three classes of 3-substituted indolin-2-ones containing propionic acid functionality attached to the pyrrole ring at the C-3 position of the core have been identified as catalytic inhibitors of the vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF) RTKs. Some of the compounds were found to inhibit the tyrosine kinase activity associated with isolated vascular endothelial growth factor receptor 2 (VEGF-R2) [fetal liver tyrosine kinase 1 (Flk-1)/kinase insert domain-containing receptor (KDR)], fibroblast growth factor receptor (FGF-R), and platelet-derived growth factor receptor (PDGF-R) tyrosine kinase with IC(50) values at nanomolar level. Thus, compound 1 showed inhibition against VEGF-R2 (Flk-1/KDR) and FGF-R1 tyrosine kinase activity with IC(50) values of 20 and 30 nM, respectively, while compound 16f inhibited the PDGF-R tyrosine kinase activity with IC(50) value of 10 nM. Structural models and structure-activity relationship analysis of these compounds for the target receptors are discussed. The cellular activities of these compounds were profiled using cellular proliferation assays as measured by bromodeoxyuridine (BrdU) incorporation. Specific and potent inhibition of cell growth was observed for some of these compounds. These data provide evidence that these compounds can be used to inhibit the function of these target receptors.