Unfulfilled promise of endostatin in a gene therapy-xenotransplant model of human acute lymphocytic leukemia

Endostatin and endorepellin: A common route of action for similar angiostatic cancer avengers

Traditional cancer therapy typically targets the tumor proper. However, newly-formed vasculature exerts a major role in cancer development and progression. Autophagy, as a biological mechanism for clearing damaged proteins and oxidative stress products released in the tumor milieu, could help in tumor resolution by rescuing cells undergoing modifications or inducing autophagic-cell death of tumor blood vessels. Cleaved fragments of extracellular matrix proteoglycans are emerging as key players in the modulation of angiogenesis and endothelial cell autophagy. An essential characteristic of cancer progression is the remodeling of the basement membrane and the release of processed forms of its constituents. Endostatin, generated from collagen XVIII, and endorepellin, the C-terminal segment of the large proteoglycan perlecan, possess a dual activity as modifiers of both angiogenesis and endothelial cell autophagy. Manipulation of these endogenously-processed forms, located in the basement membrane within tumors, could represent new therapeutic approaches for cancer eradication.

Evaluation of endostatin and EGF serum levels in patients with gastric cancer

THE AIM OF THE STUDY was to assess angiogenesis markers - endostatin and endothelial growth factor (EGF) as markers of detection of gastric carcinoma. MATERIAL AND METHODS. The study involved 20 patients with colorectal cancer (10 women, 10 men) aged 35 - 75 years, mean age = 55 years ± 11.2 who referred to the 2nd Department of General Surgery, Medical University in Lublin between June 2008 and June 2009. The control group comprised 10 volunteers (6 women, 4 men) who underwent upper gastrointestinal (GI) endoscopy due to the reflux disease and in whom gastric cancer was not diagnosed. RESULTS. The mean endostatin concentration in controls was 5.21 ng/mL ± 1.37. Mean concentrations in patients with gastric cancer were higher than those in controls - 5.91 ng/mL ± 1.5. The difference was not statistically significant (p= 0.714). The EGF concentration in the control group was 28.19 pg/mL ± 12.94. EGF concentrations in patients with gastric cancer were higher compared to the control group - 28.8 pg/mL ± 12.63. The difference was not statistically significant (p= 0.85). The mean concentration of endostatin before the operation was 5.91 ng/mL ± 1.5 and after surgery was 5.33 ng/mL ± 2.01, the difference was not statistically significant. CONCLUSIONS. Blood endostatin and EGF quantitative determinations probably is not useful for detection of gastric carcinoma and effectiveness of treatment.

Selenium-Bifidobacterium longum as a delivery system of endostatin for inhibition of pathogenic bacteria and selective regression of solid tumor

Bifidobacterium longum (B. longum) as a delivery system for endostatin was shown to have definite antitumor effects. Moreover, it was found that the enrichment of selenium was able to enhance the immunity of mice. In order to further evaluate the safety and efficacy of B. longum carrying pBV22210-endostatin (B. longum-En) enriched with selenium (Se-B. longum-En), we determined the biochemical characteristics of Se-B. longum-En. We then investigated its effect on macrophage activity, as well as its inhibitory effect on the multiplication of pathogenic bacteria in vitro and the antitumor effects on murine hepatic (H22) tumor-bearing mice. The results showed that Se-B. longum-En exhibited similar biochemical characteristics to that of wild-type B. longum, i.e., Se-B. longum-En strongly enhanced macrophage phagocytosis in rats and inhibited the growth of pathogenic bacteria. In addition, Se-B. longum-En showed a definite inhibitory effect of tumor growth when H22 tumor-bearing mice were fed through oral or tail vein delivery. These results suggested that Se-B. longum is able to retain the advantages of wild-type B. longum and be used as a novel gene delivery system for liver cancer gene therapy.

Fibrinogen facilitates the anti-tumor effect of nonnative endostatin

Endostatin is a potent inhibitor of tumor angiogenesis. Interestingly, nonnative endostatin also exhibits an anti-tumor effect, which remains a mystery so far. Here, we show that intravenous injection of nonnative endostatin results in tumor inhibition effect. Soluble and active endostatin is isolated from human blood after the addition of nonnative endostatin in vitro. By fractionation of the whole blood, we surprisingly identify fibrinogen specifically binding to and inhibiting the aggregation of nonnative endostatin. Moreover, the anti-tumor activity of nonnative endostatin is substantially impaired in fibrinogen-deficient mice. Our studies demonstrate that fibrinogen facilitates the anti-tumor effect of nonnative endostatin, which also provides new insights into the novel physiological function of fibrinogen.

Non-viral endostatin plasmid transfection of mesenchymal stem cells via collagen scaffolds

Angiogenesis is critical in the early stage of reparative processes and tissue regeneration, but the persistence of a vascular network may interfere with later transformation/maturation in naturally avascular tissues such as articular cartilage. Our supposition is that the timed delivery of an anti-angiogenic factor in cartilage tissue engineering may facilitate the formation of hyaline cartilage by inducing the regression of vascularization. To this end our overall goal is to prepare an off-the-shelf scaffold containing the gene for a potent anti-angiogenic factor. The objective of this study was to investigate the use of a type I/III collagen scaffold for the non-viral transfection of marrow stromal cells (MSCs, also referred to as mesenchymal stem cells) with the plasmid encoding endostatin. Caprine MSCs were transfected by the naked plasmid alone and plasmid incorporated into a cationic lipid complex in three experiments: 1) cells were transfected in monolayer; 2) monolayer-transfected cells were grown in a collagen sponge-like scaffold; and 3) non-transfected cells were grown in a collagen scaffold containing the naked plasmid and endostatin lipoplex. Independent variables were the passage number of the cells and the plasmid loading. The amount of endostatin released by the cells into the medium was measured using an ELISA. The results demonstrated the overexpression of endostatin by MSCs growing in the endostatin lipoplex-supplemented collagen scaffolds. Endostatin released by the cell-seeded scaffolds reached a peak of 13ng/ml for scaffolds incorporating as little as 20mug of plasmid, at the 3-day collection period ending 5 days post-seeding. The accumulated endostatin synthesis over a 2-week period began to achieve what may be a therapeutic level. MSCs transfected with the endostatin gene in monolayer continued to express the gene when grown in the collagen scaffolds. The results demonstrate the promise of the non-viral delivery of the gene for this potent anti-angiogenic protein to MSCs via a collagen scaffold.

Hormesis and medicine

Evidence is presented which supports the conclusion that the hormetic dose-response model is the most common and fundamental in the biological and biomedical sciences, being highly generalizable across biological model, endpoint measured and chemical class and physical agent. The paper provides a broad spectrum of applications of the hormesis concept for clinical medicine including anxiety, seizure, memory, stroke, cancer chemotherapy, dermatological processes such as hair growth, osteoporosis, ocular diseases, including retinal detachment, statin effects on cardiovascular function and tumour development, benign prostate enlargement, male sexual behaviours/dysfunctions, and prion diseases.

Endostatin gene therapy enhances the efficacy of paclitaxel to suppress breast cancers and metastases in mice

Chemotherapy combined with antiangiogenic therapy is more effective than chemotherapy alone. The aim of this study was to investigate whether endostatin, a potent anti-angiogenic agent, could enhance the efficacy of paclitaxel to combat breast cancer. An expression plasmid encoding mouse endostatin (End-pcDNA3.1) was constructed, which produced intense expression of endostatin and inhibited angiogenesis in the chorioallantoic membrane assay. 4T1 breast tumors were established in BALB/c mice by subcutaneous injection of 1 x 10(5) 4T1 cells. The End-pcDNA3.1 plasmid diluted in the transfection reagent FuGENE was injected into the tumors (around 100 mm(2)), and paclitaxel was injected i.p. into the mice. Endostatin gene therapy synergized with paclitaxel in suppressing the growth of 4T1 tumors and their metastasis to the lung and liver. Both endostatin and paclitaxel inhibited tumor angiogenesis and induced cell apoptosis. Despite the finding that endostatin was superior to paclitaxel at inhibiting tumor angiogenesis, paclitaxel was nevertheless more effective at inducing tumor apoptosis. The combination of paclitaxel and endostatin was more effective in suppressing tumor growth, metastases, angiogenesis, and inducing apoptosis than the respective monotherapies. The combinational therapy with endostatin and paclitaxel warrants future investigation as a therapeutic strategy to combat breast cancer.

Gene transfer of endostatin enhances the efficacy of doxorubicin to suppress human hepatocellular carcinomas in mice

Hepatocellular carcinoma (HCC) is one of the most common cancer-related causes of death, and is chemoresistant to anticancer drugs. Anti-angiogenic therapy has been shown to enhance the efficacy of chemotherapy to treat solid tumors. The aim of the present study was to determine whether endostatin, a potent antiangiogenic agent, could enhance the efficacy of doxorubicin to combat HCC. An endostatin expression plasmid was constructed and its expression in vitro and in vivo was detected after gene transfer. Recombinant endostatin inhibited angiogenesis in the chorioallantoic membrane assay, and showed synergistic effects with doxorubicin in inhibiting the in vitro proliferation of endothelial cells, but not that of tumor cells. Both endostatin gene therapy and doxorubicin suppressed the growth of subcutaneous human HepG2 tumors established in BALB/c nude mice, and tumor angiogenesis. Combination therapy with endostatin gene therapy and doxorubicin showed a stronger effect in suppressing tumor growth, and tumor angiogenesis, than the respective monotherapies. Gene transfer of endostatin down-regulated the expression of both hypoxia-inducible factor-1alpha and vascular endothelial growth factor (VEGF), whereas doxorubicin only down-regulated VEGF expression. Endostatin and doxorubicin synergized to down-regulate VEGF expression. Endostatin and doxorubicin combination therapy warrants investigation as a therapeutic strategy to combat HCC.

Construction of gene engineered hE/293 cells continuously releasing human endostatin

AIM: To express the recombinant eukaryotic expression vector containing human endostatin gene in human embryonic kidney HEK293 cells, and construct a cell line continuously secreting human endostatin (hES).

METHODS: Human endostatin cDNA containing interleukin-2 (IL-2) secreting peptide was cloned into eukaryotic expression plasmid pSNA2 to construct the recombinant plasmid pSNA2/endostatin. The plasmid pSNA2/endostatin was transfected into HEK293 cells by cationic liposome. The positive cell clones were selected by G418 and then named hE/293 cells. The expression of endostatin protein was analyzed by Western blot. The release of biologically active endostatin was confirmed using assays of ECV304 proliferation and the angiogenesis experiment of chicken chorioallantoic membrane (CAM).

RESULTS: Enzyme digestion and sequence analysis confirmed that the eukaryotic expression vector pSNA2/endostatin had been successfully constructed. After G418 selection, 3 strains of cells stably expressing hES were obtained, named as hE/293 cells. Western blot showed that ES protein with a molecular weight of 20 kDa existed in the supernatant of hE/293 cells. No endostatin expression was found in the control cells. The assays of ECV304 proliferation showed that the supernatant of hE/293 cells inhibited ECV304 cell proliferation induced by basic fibroblast growth factor in comparison with that of HEK29 cells (48 h: 0.125 ± 0.007 vs 0.159 ± 0.020, P < 0.01; 72 h: 0.088 ± 0.016 vs 0.249 ± 0.070, P < 0.01). There were fewer blood vessels in the CAM treated with ES protein.

CONCLUSION: Target cell line hE/293 is successfully constructed which it can stably secret hES protein, inhibit the ECV304 cell proliferation and CAM angiogenesis.

Endostatin: Current concepts about its biological role and mechanisms of action

Endogenous inhibitors of angiogenesis are proved to be a major factor preventing the emergence of clinically manifested stages of human cancer. The protein endostatin, a 20-kD proteolytic fragment of type XVIII collagen, is one of the most active natural inhibitors of angiogenesis. Endostatin specifically inhibits the in vitro and in vivo proliferation of endothelial cells, inducing their apoptosis through inhibition of cyclin D1. On the surface of endothelial cells, endostatin binds with the integrin alpha(5)beta(1) that activates the Src-kinase pathway. The binding of endostatin with integrins also down-regulates the activity of RhoA GTPase and inhibits signaling pathways mediated by small kinases of the Ras and Raf families. All these events promote disassembly of the actin cytoskeleton, disorders in cell-matrix interactions, and decrease in endotheliocyte mobility, i.e., promote the suppression of angiogenesis. Endostatin displays a high antitumor activity in vivo: it inhibits the progression of more than 60 types of tumors. This review summarizes results of numerous studies concerning the biological activity and action mechanism of endostatin.

Anti-angiogenic gene therapy of cancer: current status and future prospects

The discovery of endogenous inhibitors of angiogenesis has made it possible to test the hypothesis that blocking the angiogenic switch may keep tumor growth in check, and has added a new investigational arm to the field of cancer gene therapy. Angiogenesis inhibitors are heterogeneous in origin and potency, and their growing list includes proteolysis products of larger molecules with a different function, such as angiostatin, endostatin and vasostatin, modulators of vascular endothelial growth factor activity, such as sFLT-1, and some cytokines/chemokines with marked anti-endothelial activity, such as IL-12, IFN-alpha, and CXCL10. Pre-clinical studies have clearly indicated that these factors are essentially cytostatic and that they need long-term administration in order to obtain prolonged anti-tumor effects, representing a rational basis for their delivery by a gene therapy approach. The experimental approaches attempted to date, reviewed herein, indicate overall that anti-angiogenic gene therapy has efficacy mainly as an early intervention strategy and that a better understanding of the biological mechanisms underlying resistance to angiogenesis inhibition, as well as appropriate combined treatments, are required to generate a conceptual advancement which could drive the field towards successful management of established tumors.

Subepithelial myofibroblasts are novel nonprofessional APCs in the human colonic mucosa

The human gastrointestinal mucosa is exposed to a diverse normal microflora and dietary Ags and is a common site of entry for pathogens. The mucosal immune system must respond to these diverse signals with either the initiation of immunity or tolerance. APCs are important accessory cells that modulate T cell responses which initiate and maintain adaptive immunity. The ability of APCs to communicate with CD4+ T cells is largely dependent on the expression of class II MHC molecules by the APCs. Using immunohistochemistry, confocal microscopy, and flow cytometry, we demonstrate that alpha-smooth muscle actin(+), CD90+ subepithelial myofibroblasts (stromal cells) constitutively express class II MHC molecules in normal colonic mucosa and that they are distinct from professional APCs such as macrophages and dendritic cells. Primary isolates of human colonic myofibroblasts (CMFs) cultured in vitro were able to stimulate allogeneic CD4+ T cell proliferation. This process was dependent on class II MHC and CD80/86 costimulatory molecule expression by the myofibroblasts. We also demonstrate that CMFs, engineered to express a specific DR4 allele, can process and present human serum albumin to a human serum albumin-specific and DR4 allele-restricted T cell hybridoma. These studies characterize a novel cell phenotype which, due to its strategic location and class II MHC expression, may be involved in capture of Ags that cross the epithelial barrier and present them to lamina propria CD4+ T cells. Thus, human CMFs may be important in regulating local immunity in the colon.

Angiogenesis

Angiogenesis inhibitors for the treatment of cancer have now been approved by the Food and Drug Administration in the United States, and in 28 other countries including China. Clinical application of this new class of drugs is informed by certain principles from angiogenesis research. Oncogenic mutations initiate tumorigenesis, but angiogenesis is necessary for expansion of tumor mass. Two angiogenesis inhibitors have been developed that have a broad spectrum of anticancer activity, yet virtually no side effects. Endogenous angiogenesis inhibitors act as tumor suppressor proteins. The angiogenic response in vivo is based on the genetic background of the host. Several types of angiogenesis inhibitors reveal a biphasic, U-shaped curve of efficacy. "Antiangiogenic chemotherapy" is a novel approach to the treatment of drug resistance.

Antitumor efficacy improved by local delivery of species-specific endostatin

OBJECT

Conflicting results have been reported concerning the antitumor efficacy of the angiogenesis inhibitor endostatin. This may be due to differences in the biological distribution of endostatin between studies or to the varying biological efficacies of the different protein forms that were examined. To address this issue, the authors used a local delivery approach in which each tumor cell secreted endostatin, providing uniform endostatin levels throughout the tumors. This allowed a direct assessment of the biological efficacy of soluble endostatin in vivo.

METHODS

The authors genetically engineered BT4C gliosarcoma cells so that they would stably express and secrete either the human or murine form of endostatin. Endostatin-producing cells or mock-infected cells were implanted intracerebrally in syngeneic BD-IX rats. The antitumor efficacy of endostatin was evaluated on the basis of survival data and tumor volume comparisons. In addition, microvascular parameters were assessed. The authors confirmed the continuous release of endostatin by the BT4C cells. A magnetic resonance imaging-assisted comparison of tumor volumes revealed that local production of murine endostatin significantly inhibited tumor growth. Notably, 40% of the animals in this treatment group experienced long-term survival without histologically verifiable tumors 7 months after cell implantation. After local treatment with murine endostatin, tumor blood plasma volumes were reduced by 71%, microvessel density counts by 84%, and vascular area fractions by 75%. In contrast, human endostatin did not inhibit tumor growth significantly in this model. Centrally located regions of necrosis were present in tumors secreting both the human and the murine species-specific form of endostatin.

CONCLUSIONS

The results suggest that endostatin inhibits tumor angiogenesis in vivo in a species-specific manner.

Angiogenic inhibitors: a new therapeutic strategy in oncology

Angiogenesis is a multistep, complex and tightly regulated process that is necessary for tumor growth and metastasis. Based on data of preclinical models, several antiangiogenic compounds has been shown to modify activated tumor endothelium, which suggests that these compounds can improve cytotoxic drug delivery. Such agents have entered clinical trials as single agents or in combination with cytotoxic drugs, and have shown promising antitumor activity. The pharmacodynamic and pharmacokinetic characteristics of antiangiogenic drugs are reviewed here. Most of the early clinical testing of these agents was conducted in patients with advanced disease resistant to standard therapies. Phase III trials compared the efficacy of standard chemotherapy alone with standard chemotherapy in combination with an experimental angiogenesis inhibitor. Although some of these studies were negative or controversial, recent studies validated in large clinical trials with an anti-vascular endothelial growth factor antibody demonstrated significant clinical benefit and renewed enthusiasm for this therapeutic strategy. This review describes the clinical studies of antiangiogenic agents and highlights the challenges related to choosing appropriate strategies for the selection of patients, study design and choice of appropriate endpoints for the studies' development.

Therapeutic Efficacy of Endostatin Exhibits a Biphasic Dose-Response Curve

We show here that recombinant endostatin protein has a biphasic effect on the inhibition of endothelial cell migration in vitro. In tumor-bearing animals, there is a similar biphasic effect on the inhibition of tumor growth and on circulating endothelial cells after once-daily s.c. injections. This biphasic effect is revealed as a U-shaped curve in which efficacy is optimal between very low and very high doses depending on the tumor type. This result may be applicable to other inhibitors of endothelial growth and to angiogenesis. Furthermore, these results have important implications for clinicians who administer angiogenesis inhibitors for cancer or other angiogenesis-dependent diseases. When these results are taken together with two previous reports of angiogenesis inhibitors with a U-shaped dose-response, they suggest that other regulators of endothelial growth may display a similar pattern.

Matrix changes induced by transglutaminase 2 lead to inhibition of angiogenesis and tumor growth

Administration of active TG2 to two different in vitro angiogenesis assays resulted in the accumulation of a complex extracellular matrix (ECM) leading to the suppression of endothelial tube formation without causing cell death. Matrix accumulation was accompanied by a decreased rate of ECM turnover, with increased resistance to matrix metalloproteinase-1. Intratumor injection of TG2 into mice bearing CT26 colon carcinoma tumors demonstrated a reduction in tumor growth, and in some cases tumor regression. In TG2 knockout mice, tumor progression was increased and survival rate reduced compared to wild-type mice. In wild-type mice, an increased presence of TG2 was detectable in the host tissue around the tumor. Analysis of CT26 tumors injected with TG2 revealed fibrotic-like tissue containing increased collagen, TG2-mediated crosslink and reduced organized vasculature. TG2-mediated modulation of cell behavior via changes in the ECM may provide a new approach to solid tumor therapy.

Enhanced inhibition of murine tumor and human breast tumor xenografts using targeted delivery of an antibody-endostatin fusion protein

Endostatin can inhibit angiogenesis and tumor growth in mice. A potential limitation of endostatin as an antitumor agent in humans is the short serum half-life of the protein that may decrease effective concentration at the site of tumor and necessitate frequent dosing. In an effort to improve antitumor activity, endostatin was fused to an antibody specific for the tumor-selective HER2 antigen to create an antibody-endostatin fusion protein (anti-HER2 IgG3-endostatin). Normal endostatin rapidly cleared from serum in mice (T(1/2)(2), = 0.6-3.8 hours), whereas anti-HER2 IgG3-endostatin had a prolonged half-life (90% intact; T(1/2)(2), 40.2-44.0 hours). Antigen-specific targeting of anti-HER2 IgG3-endostatin was evaluated in BALB/c mice implanted with CT26 tumors or CT26 tumors engineered to express the HER2 antigen (CT26-HER2). Radio-iodinated anti-HER2 IgG3-endostatin preferentially localized to CT26-HER2 tumors relative to CT26 tumors. Administration of anti-HER2 IgG3-endostatin to mice showed preferential inhibition of CT26-HER2 tumor growth compared with CT26. Anti-HER2 IgG3-endostatin also markedly inhibited the growth of human breast cancer SK-BR-3 xenografts in severe combined immunodeficient mice. Anti-HER2 IgG3-endostatin inhibited tumor growth significantly more effectively than endostatin, anti-HER2 IgG3 antibody, or the combination of antibody and endostatin. CT26-HER2 tumors treated with the endostatin fusion protein had decreased blood vessel density and branching compared with untreated CT26-HER2 or CT26 treated with the fusion protein. The enhanced effectiveness of anti-HER2 IgG3-endostatin may be due to a longer half-life, improved serum stability, and selective targeting of endostatin to tumors, resulting in decreased angiogenesis. Linking of an antiangiogenic protein, such as endostatin, to a targeting antibody represents a promising and versatile approach to antitumor therapy.

Antiangiogenic treatment with endostatin inhibits progression of AML in vivo

Increased vessel density in the bone marrow of patients with acute myeloid leukemia as well as elevated expression of proangiogenic factors by leukemic cells implies a central role of angiogenesis in hematological malignancies. Endostatin (ES), a fragment of collagen XVIII, is an endogenous inhibitor of angiogenesis that has shown therapeutic activity in solid tumors in various preclinical models. Using microencapsulation technology, we studied the therapeutic effect of ES in AML. While ES had no effect on proliferation of M1 murine leukemic cells in vitro, ES producing microbeads significantly inhibited growth of subcutaneous chloromas in SCID mice as compared to controls. In a leukemia model using M1 cells the concomitant treatment of mice with ES microbeads prolonged median survival significantly. Histological analysis revealed a decreased microvessel density and a reduced number of CD31-positive single cells, putatively endothelial progenitor cells, in the bone marrow of treated animals. Taken together, ES has inhibitory effects on neo-angiogenesis in the bone marrow and on progression of leukemia in vivo. These experiments suggest a possible therapeutic role of antiangiogenic gene therapy with ES in AML.

Endostatin signaling and regulation of endothelial cell-matrix interactions

The growth and survival of a malignant tumor are dependent on the formation and maintenance of its own microvasculature, a process termed angiogenesis. Inhibition of this phenomenon is an emerging strategy in cancer therapy. The extracellular matrix surrounding the vascular endothelial cells contains cryptic protein domains, which are exposed by changes in the proteolytic homeostasis of the tumor microenvironment. These fragments transmit local signals, which regulate vascular endothelial cell proliferation and migration. Endostatin, the proteolytic fragment of collagen type XVIII, is a potent inhibitor of tumor angiogenesis in various mouse models and is currently in clinical trials for therapeutic use in human cancer. Multiple cell surface receptors have been described for endostatin, but the signals transmitted by these receptors resulting in the inhibition of angiogenesis have so far been poorly characterized. Studies on the effects of endostatin on cultured endothelial cells suggest that the antimigratory and antiproliferative properties of this molecule are the major mechanisms underlying its antiangiogenic potential. These effects may be a consequence of endostatin modulation of endothelial cell-matrix interactions and pericellular proteolysis.

Endostatin cDNA/cationic liposome complexes as a promising therapy to prevent lung metastases in osteosarcoma: study in a human-like rat orthotopic tumor

Antiangiogenesis or destruction of tumor neovessels is an effective strategy to prevent tumor growth. Endostatin, one of the many inhibitors of angiogenesis that have been discovered, has shown conflicting results in preclinical assays. We studied the therapeutic potential of lipid/DNA complexes consisting of cationic liposomes and an endostatin-coding plasmid (Endo cDNA/CLP) in an orthotopic osteosarcoma model in rats. Empty plasmid without the endostatin gene complexed with cationic liposomes served as control. Animals were treated intravenously three times a week starting on the day tumors were detectable by (18)FDG tomoscintigraphy. During treatment, tumor progression was followed by PET scan and angioscintigraphy, and the effects of antivascular therapy on primary tumor, metastases, and tumor vascular density were confirmed by histologic analysis. Our results demonstrate that therapy using Endo cDNA/CLP is associated with pronounced delay in tumor growth. Moreover, it effectively prevented the occurrence of lung metastases, the major reason for bad prognosis and death in osteosarcoma patients. This approach could be used as an adjuvant therapy for osteosarcoma.

Combination of antiangiogenic therapy with other anticancer therapies: results, challenges, and open questions

Angiogenesis is necessary for tumor growth. Drug discovery efforts have identified several potential therapeutic targets on endothelial cells and selective inhibitors capable of slowing tumor growth or producing tumor regression by blocking angiogenesis in in vivo tumor models. Certain antiangiogenic therapeutics increase the activity of cytotoxic anticancer treatments in preclinical models. More than 75 antiangiogenic compounds have entered clinical trials. Most of the early clinical testing was conducted in patients with advanced disease resistant to standard therapies. Several phase III trials have been undertaken to compare the efficacy of standard chemotherapy versus the same in combination with an experimental angiogenesis inhibitor. Preliminary results of the clinical studies suggest that single-agent antiangiogenic therapy is poorly active in advanced tumors. Although some of the results of combination trials are controversial, recent positive outcomes with an antivascular endothelial growth factor antibody combined with chemotherapy as front-line therapy of metastatic colorectal cancer have renewed enthusiasm for this therapeutic strategy. This article presents an overview of experimental and clinical studies of combined therapy with antiangiogenic agents and highlights the challenges related to the appropriate strategies for selection of the patients, study design, and choice of proper end points for preclinical and clinical studies using these agents.

Different subsets of primary chronic myeloid leukemia stem cells engraft immunodeficient mice and produce a model of the human disease

Xenograft models of chronic phase human chronic myeloid leukemia (CML) have been difficult to develop because of the persistence of normal hematopoietic stem cells in most chronic phase CML patients and the lack of methods to selectively isolate the rarer CML stem cells. To circumvent this problem, we first identified nine patients' samples in which the long-term culture-initiating cells were predominantly leukemic and then transplanted cells from these samples into sublethally irradiated NOD/SCID and NOD/SCID-beta2microglobulin-/- mice. This resulted in the consistent and durable (>5 months) repopulation of both host genotypes with similar numbers of BCR-ABL+/Ph+ cells. The regenerated leukemic cells included an initial, transient population derived from CD34+CD38+ cells as well as more sustained populations derived from CD34+CD38- progenitors, indicative of a hierarchy of transplantable leukemic cells. Analysis of the phenotypes produced revealed a reduced output of B-lineage cells, enhanced myelopoiesis with excessive production of erythroid and megakaropoietic cells and the generation of primitive (CD34+) leukemic cells displaying an autocrine IL-3 and G-CSF phenotype, all characteristics of primary CML cells. These findings demonstrate the validity of this xenograft model of chronic phase human CML, which should enable future investigation of disease pathogenesis and new approaches to therapy.

Anti-angiogenic therapy subsequent to adeno-associated-virus-mediated immunotherapy eradicates lymphomas that disseminate to the liver

Liver cancer has a very poor prognosis and lacks effective therapy. We have previously demonstrated that intraportal injection of adeno-associated-viral (AAV) particles that express angiostatin lead to long-term expression of angiostatin capable of suppressing the outgrowth of EL-4 tumors in the liver. Here we combine AAV-mediated angiostatin therapy with immunotherapy by employing an AAV vector encoding the T-cell costimulator B7.1. Incubation of EL-4 cells with AAV-B7.1 viruses resulted in the rapid expression of B7.1 on the surface of 80% of EL-4 cells. Mice that were vaccinated with B7.1-engineered tumor cells rejected the tumor cells and resisted a secondary challenge with unmodified parental cells. Splenocytes from the vaccinated mice were highly cytotoxic towards parental EL-4 cells in vitro. However, the vaccinated mice failed to resist the challenge of a heavy burden of EL-4 cells. Intraportal injection of AAV particles that express angiostatin into mice that had been vaccinated 1 month earlier with B7.1-engineered tumor cells protected mice against the challenge of a heavy burden of EL-4 cells and eradicated tumors that had disseminated to the liver. The combinational therapy increased the survival rate of mice with advanced liver cancer. These encouraging results warrant investigation of the employment of anti-angiogenic therapy subsequent to cancer immunotherapy for targeting unresectable disseminated liver metastases.

Antiangiogenic gene therapy of myeloproliferative disease developed in transgenic mice expressing P230 bcr/abl

Antiangiogenic gene therapy offers an attractive approach to the treatment of a variety of malignancies, including those of the hematological system. However, evaluation of this approach has been hampered by the lack of appropriate animal models. We have recently produced transgenic mice expressing P230 bcr/abl that develop myeloproliferative disease (MPD) closely resembling human chronic myelogenous leukemia. Using this MPD murine model, we examined the feasibility of systemic antiangiogenic gene therapy for hematological malignancy. An adenoviral vector containing the secretable endostatin gene was injected into the right quadriceps muscle of the MPD mice. The increased endostatin level was detected for at least 6 months. Hematological parameters including platelet counts, granulocyte counts, and the hemoglobin concentration were improved by this gene therapy. Infiltration of megakaryocytes was also significantly inhibited in treated MPD mice. Reduction of the microvessel density was confirmed by histological examination. These results demonstrated, for the first time, that antiangiogenic gene therapy is effective to inhibit leukemogenesis caused by expression of the chimeric bcr/abl gene.

Intramuscular delivery of antiangiogenic genes suppresses secondary metastases after removal of primary tumors

The success of surgery to remove primary tumors can be compromised by the subsequent outgrowth of metastases. It is recognized that primary tumors secrete antiangiogenic factors that suppress the outgrowth of their daughter metastases. In accord we show here that surgical removal of primary EL-4 lymphomas led to a marked decrease in the levels of circulating angiostatin and endostatin, and promoted the growth of distant nodular tumors. Expression vectors encoding angiostatin and endostatin, formulated with poly-N-vinyl pyrrolidone (PVP), were injected into the tibialis and gastrocnemia muscles, leading to expression of angiostatin and endostatin in muscle fibers. High levels of biologically active exogenous proteins were secreted into the circulation. Intramuscular gene therapy with angiostatin and endostatin plasmids significantly inhibited tumor vascularity and induced tumor cell apoptosis, and thereby suppressed the growth of secondary subcutaneous and disseminated metastatic tumors in the lung and liver. Simultaneous intramuscular delivery of both angiostatin and endostatin plasmids significantly prolonged the survival of mice after removal of primary tumors. These results suggest that intramuscular gene transfer of angiostatin and endostatin might serve as a prophylactic cancer-prevention strategy to combat the recurrence of cancer after surgical resection of primary tumors.

Do antiangiogenic protein fragments have amyloid properties?

Tumor growth requires proteolytic activity. As a consequence, protein breakdown products are present in the circulation of patients with cancer. Within the past decade a large number of proteolytic fragments have been identified that inhibit angiogenesis and tumor growth. The mechanism of action of these inhibitors is still poorly understood. We recently found that the effects of the angiogenesis inhibitor endostatin on endothelial cells is critically dependent on the presence of cross-beta structure, a structure also present in amyloidogenic polypeptides in plaques of patients with amyloidosis, such as Alzheimer disease. We also showed that cross-beta structure containing endostatin is a ligand for tissue-type plasminogen activator (tPA). We noted that many angiogenesis inhibitors stimulate tPA-mediated plasminogen activation. Because the presence of cross-beta structure is the common denominator in tPA-binding ligands, we hypothesize that these endogenous antiangiogenic proteolytic fragments share features with amyloidogenic polypeptides. We postulate that the cross-beta structural fold is present in these antiangiogenic polypeptide fragments and that this structure mediates the inhibitory effects. The hypothesis provides new insights in the potential mechanisms of these angiogenesis inhibitors and offers opportunities to improve their use.

Adeno-Associated Virus 2-Mediated Antiangiogenic Cancer Gene Therapy

Angiogenesis is characteristic of solid tumor growth and a surrogate marker for metastasis in many human cancers. Inhibition of tumor angiogenesis using antiangiogenic drugs and gene transfer approaches has suggested the potential of this form of therapy in controlling tumor growth. However, for long-term tumor-free survival by antiangiogenic therapy, the factors controlling tumor neovasculature need to be systemically maintained at stable therapeutic levels. Here we show sustained expression of the antiangiogenic factors angiostatin and endostatin as secretory proteins by recombinant adeno-associated virus 2 (rAAV)-mediated gene transfer. Both vectors provided significant protective efficacy in a mouse tumor xenograft model. Stable transgene persistence and systemic levels of both angiostatin and endostatin were confirmed by in situ hybridization of the vector-injected tissues and by serum ELISA measurements, respectively. Whereas treatment with rAAV containing either endostatin or angiostatin alone resulted in moderate to significant protection, the combination of endostatin and angiostatin gene transfer from a single vector resulted in a complete protection. These data suggest that AAV-mediated long-term expression of both endostatin and angiostatin may have clinical utility against recurrence of cancers after primary therapies and may represent rational adjuvant therapies in combination with radiation or chemotherapy.

Anti-Tumor Efficacy of Human Angiostatin Using Liver-Mediated Adeno-Associated Virus Gene Therapy

Angiostatin is a potent endogenous inhibitor of angiogenesis and tumor growth in vivo. The therapeutic potential of adeno-associated viral (AAV) gene delivery of angiostatin in modulating tumor growth in vivo was evaluated. Sustained levels of angiostatin were detected in the sera of mice for up to 6 months after they received a single injection of AAV-angiostatin. AAV-mediated stable expression of angiostatin inhibited tumor burden in the highly aggressive B16F10 melanoma and Lewis lung carcinoma (LLC) models of experimental metastasis. Moreover, AAV-angiostatin prolonged survival in B16F10 and LLC tumor-bearing mice compared to control groups. Anti-tumor efficacy was consistently observed when angiostatin serum levels of 15-50 ng/ml were detected following gene transfer, but the effect was minimal when the levels were lower or higher than this range. The combination of AAV-angiostatin gene therapy with chemotherapy was also shown to extend marginally the survival of mice bearing preestablished human tumors; however, the effect was evident only within a narrow dose of circulating angiostatin. These studies demonstrate the feasibility of using AAV anti-angiogenic gene therapy as a cancer treatment modality and suggest that the optimal anti-tumor efficacy of angiostatin following gene transfer may be limited to a narrow dose range.

Low dose carboplatin combined with angiostatic agents prevents metastasis in human testicular germ cell tumor xenografts

PURPOSE

Low dose chemotherapy combined with angiogenesis inhibitors has been shown to be more effective for experimental tumor treatment than chemotherapy alone. To our knowledge whether germ cell tumors could benefit from this treatment strategy remains to be evaluated. We examined the efficacy of angiostatic thrombospondin-1 (TSP-1), endostatin and combined angiostatic/low dose carboplatin in mice xenografted with human nonseminomatous germ cell tumor.

MATERIALS AND METHODS

We monitored tumor progression and angiogenesis in the established model of human nonseminomatous germ cell tumor xenograft in 120 SCID mice using intravital video microscopy, immunocytochemistry and real-time polymerase chain reaction. Mice received TSP-1 (20 mg/kg daily) or endostatin (10 mg/kg daily) subcutaneously (via osmotic mini pumps) for 2 weeks starting 15 days after cancer cell grafting, carboplatin cycled twice (30 mg/kg intraperitoneally days 14 and 21 after cancer cell grafting), or a combination of carboplatin with TSP-1 or endostatin. Untreated, sham and tumor bearing mice treated with Ringer's solution served as controls.

RESULTS

Primary tumor development was not affected in mice treated with TSP-1, endostatin or carboplatin alone. All animals had metastases at 6 months, while metastasis did not develop following the combination of carboplatin with TSP-1 or endostatin. This combined therapy suppressed tumor angiogenesis, enhanced apoptosis in tumor cells and decreased vascular endothelial growth factor-A tissue mRNA expression vs controls (p <0.05).

CONCLUSIONS

These data indicate that angiostatic agents added to low dose carboplatin have the ability to suppress the progression of human germ cell tumor xenografts toward a metastatic phenotype. Therefore, this treatment strategy might be beneficial to prevent metastasis in germ cell tumors.

An experimental platform for studying growth and invasiveness of tumor cells within teratomas derived from human embryonic stem cells

There is currently no available experimental system wherein human cancer cells can be grown in the context of a mixed population of normal differentiated human cells for testing biological aspects of cancer cell growth (e.g., tumor cell invasion and angiogenesis) or response to anti-cancer therapies. When implanted into immunocompromised mice, human embryonic stem cells develop teratomas containing complex structures comprising differentiated cell types representing the major germ line-derived lineages. We sought to determine whether human cancer cells would grow within such teratomas and display properties associated with malignancy, such as invasiveness and recruitment of blood vessels. HEY ovarian cancer cells stably expressing an H2A-GFP fusion protein (HEY-GFP) injected into mature teratomas developed into tumors, which allowed tracking of tumor cell invasion and recruitment of human teratoma-derived blood vessels. This provides a straightforward and powerful approach to studying the biological properties of cancer cells within the microenvironment of normal differentiated human cells.

Coelectrotransfer to skeletal muscle of three plasmids coding for antiangiogenic factors and regulatory factors of the tetracycline-inducible system: tightly regulated expression, inhibition of transplanted tumor growth, and antimetastatic effect

We describe an approach employing intramuscular plasmid electrotransfer to deliver secretable forms of K1-5 and K1-3-HSA (a fusion of K1-3 with human serum albumin), which span, respectively, five and three of the five kringle domains of plasminogen. A tetracycline-inducible system (Tet-On) composed of three plasmids coding, respectively, for the transgene, the tetracycline transcriptional activator rtTA, and the silencer tTS was employed. K1-3-HSA and K1-5, produced from C2C12 muscle cells, were found to inhibit endothelial cell (HMEC-1) proliferation by 30 and 51%, respectively. In vivo, the expression of the transgene upon doxycycline stimulation was rapid, stable, and tightly regulated (no background expression) and could be maintained for at least 3 months. Blood half-lives of 2.1 and 3.7 days were found for K1-5 and K1-3-HSA, respectively. The K1-5 protein was secreted from muscle into blood at a level of 45 ng/ml, which was sufficient to inhibit MDA-MB-231 tumor growth by 81% in nude mice and B16-F10 melanoma cell lung invasion in C57BL/6 mice by 73%. PECAM-1 immunostaining studies revealed modest tumor vasculature in mice expressing K1-5. In contrast, K1-3-HSA, although secreted into blood at much higher level (250 ng/ml) than K1-5, had no effect on tumor growth.

The extracellular matrix: a new turn-of-the-screw for anti-angiogenic strategies

Anti-angiogenic therapy is currently one of most active fields in cancer research. The initial strategies, which were aimed at inhibiting tumor vascularization, included upregulation of endogenous inhibitors and blocking of the signals delivered by angiogenic factors. However, interactions between endothelial cells and their surrounding extracellular matrix also play a crucial role in modulation of the angiogenic process. Compounds that target either the integrins implicated in these interactions or the proteases responsible for matrix remodeling have been shown to halt tumor growth in murine models and are now in clinical trials. However, little attention has been paid to integrin ligands, the extracellular matrix components that support endothelial cell survival, movement and reorganization. Here, we summarize the current knowledge about these angiogenesis inhibitors and propose a novel therapeutic approach based on the blocking of crucial binding sites present in the extracellular matrix.

Long-term expression of angiostatin suppresses metastatic liver cancer in mice

Metastatic liver cancer has a very poor prognosis and lacks effective therapy. Anti-angiogenic therapies, which starve tumors of blood supply, have proven to be effective in preclinical models because tumor growth is angiogenesis dependent. However, long-term, high-level, and sustained expression of angiogenesis inhibitors, such as angiostatin, is necessary to prevent dormant tumors from becoming active again. To achieve this objective, we engineered a recombinant adeno-associated virus (AAV) vector encoding mouse angiostatin, an endogenous inhibitor of tumor vascularization. After intraportal delivery of this vector, high-level, stable transgene expression of angiostatin lasting for at least 6 months was observed locally in hepatocytes. Gene transfer of AAV-angiostatin via the portal vein led to significant suppression of the growth of both nodular and metastatic EL-4 lymphoma tumors established in the liver and prolonged the survival time of the mice. The growth of neovessels was inhibited significantly, and extensive apoptosis of tumor cells was observed. The anti-angiogenic activity of angiostatin was independent of vascular endothelial growth factor (VEGF). The AAV-angiostatin viruses did not appear to be toxic to mice, and there was no detectable apoptosis of hepatocytes. In conclusion, these encouraging results warrant future investigation of the use of AAV-mediated anti-angiogenic gene therapy for targeting unresectable liver metastases, especially after surgical removal of primary tumors.

Current and future strategies for the treatment of malignant brain tumors

Glioblastoma (GB) is the most common subtype of primary brain tumor in adults. These tumors are highly invasive, very aggressive, and often infiltrate critical neurological areas within the brain. The mean survival time after diagnosis of GB has remained unchanged during the last few decades, in spite of advances in surgical techniques, radiotherapy, and also chemotherapy; patients' survival ranges from 9 to 12 months after initial diagnosis. In the same time frame, with our increasing understanding and knowledge of the physiopathology of several cancers, meaningful advances have been made in the treatment and control of several cancers, such as breast, prostate, and hematopoietic malignancies. Although a number of the genetic lesions present in GB have been elucidated and our understanding of the progressions of this cancer has increased dramatically over the last few years, it has not yet been possible to harness this information towards developing effective cures. In this review, we will focus on the classical ways in which GB is currently being treated, and will introduce a novel therapeutic modality, i.e., gene therapy, which we believe will be used in combination with classical treatment strategies to prolong the life-span of patients and to ultimately be able to control and/or cure these brain tumors. We will discuss the use of several vector systems that are needed to introduce the therapeutic genes within either the tumor mass, if these are not resectable, or the tumor bed, after successful tumor resection. We also discuss different therapeutic modalities that could be exploited using gene therapy, i.e., conditional cytotoxic approach, direct cytotoxicity, immunotherapy, inhibition of angiogenesis, and the use of pro-apoptotic genes. The advantages and disadvantages of each of the current vector systems available to transfer genes into the CNS are also discussed. With the advances in molecular techniques, both towards the elucidation of the physiopathology of GB and the development of novel, more efficient and less toxic vectors to deliver putative therapeutic genes into the CNS, it should be possible to develop new rationale and effective therapeutic approaches to treat this devastating cancer.

Angiogenesis and apoptosis

This review assembles the laboratory and clinical evidence that cytotoxic chemotherapy and antiangiogenic therapy are each dependent on endothelial cell apoptosis. During cytotoxic chemotherapy, apoptosis of endothelial cells in the vascular bed of tumors precedes apoptosis of tumor cells, even when the tumor has been made drug resistant. Administration of an angiogenesis inhibitor which is not directly cytotoxic to tumor cells can increase tumor cell apoptosis and inhibit tumor growth by inhibiting endothelial proliferation and migration and/or by inducing endothelial apoptosis. Furthermore, oncogene expression and loss of tumor suppressor gene activity can at once protect tumor cells against apoptosis and increase their angiogenic output. Both of these survival advantages conferred on the tumor can be overcome by antiangiogenic therapy. They can also be overcome by cytotoxic chemotherapy administered on a low dose 'antiangiogenic schedule' which continuously exposes endothelial cells in the tumor bed to the drug. As a result, endothelial apoptosis can be demonstrated to precede tumor cell apoptosis, and tumors regress or are inhibited, whether or not the tumor cells are resistant to the drug, and with little or no host toxicity. In contrast, cytotoxic chemotherapy administered on a 'conventional schedule' of maximal tolerated dose followed by an off-therapy interval, becomes ineffective after drug resistance is acquired. On the basis of these experimental findings, chemotherapy of cancer may possibly be improved-i.e. decreased drug resistance and decreased toxic side-effects-by changing dose and schedule to maximize apoptosis of endothelial cells in the vascular bed of tumors. Further improvement may be achieved by combining angiogenesis inhibitors with 'antiangiogenic chemotherapy'.

Recombinant endostatin forms amyloid fibrils that bind and are cytotoxic to murine neuroblastoma cells in vitro

Endostatin is a fragment of collagen XVIII that acts as an endogenous inhibitor of tumor angiogenesis and tumor growth. Anti-tumor effects have been described using both soluble and insoluble recombinant endostatin. However, differences in endostatin structure are likely to cause differences in bioactivity. In the present study we have investigated the structure and cellular effects of insoluble endostatin. We found that insoluble endostatin shows all the hallmarks of amyloid aggregates. Firstly, it binds Congo red and shows the characteristic apple-green birefringe when examined under polarized light. Secondly, electron microscopy shows that endostatin forms short unbranched fibrils. Thirdly, X-ray analysis shows the abundant presence of cross-beta sheets, the tertiary structure that underlies fibrillogenesis. None of these properties was observed when examining soluble endostatin. Soluble endostatin can be triggered to form cross-beta sheets following denaturation, indicating that endostatin is a protein fragment with an inherent propensity to form amyloid deposits. Like beta-amyloid, found in the brains of patients with Alzheimer's disease, amyloid endostatin binds to and is toxic to neuronal cells, whereas soluble endostatin has no effect on cell viability. Our results demonstrate a previously unrecognized functional difference between soluble and insoluble endostatin, only the latter acting as a cytotoxic amyloid substance.

The combination of HSV-tk and endostatin gene therapy eradicates orthotopic human renal cell carcinomas in nude mice

BACKGROUND AND METHODS

Gene therapy may offer a new tool for the treatment of renal cell carcinoma (RCC). We have tested a combination of cytotoxic and antiangiogenic gene therapy for wild-type orthotopic human RCC xenografts in nude mice using intratumoral adenovirus-mediated herpes simplex virus thymidine kinase (HSV-tk) and endostatin (ES) gene therapy. In vivo magnetic resonance imaging, morphometry, immunocytochemistry, and survival were used to evaluate the treatment effect. Adenovirus-mediated marker gene transfers (GFP) were used as controls.

RESULTS

In vivo transduction efficiency, measured using GFP gene transfer, was 27+/-7%. The combination gene therapy with HSV-tk and ES adenoviruses resulted in a significant antitumor effect (P<.01) compared to single HSV-tk (n.s.) or ES (n.s.). In the survival study, all tumors with single gene therapy using HSV-tk, ES, and marker gene adenoviruses showed progression in magnetic resonance imaging. In contrast, the majority of the tumors in the combination treatment group remained dormant or were eradicated (57%). Survival of these mice equaled healthy nude mice, and was significantly prolonged (P<.0001) compared to HSV-tk (P<.028) and ES (n.s.) groups.

CONCLUSIONS

It is concluded that the inhibition of angiogenesis using ES gene transfer together with the cytotoxic HSV-tk gene therapy results in a significantly improved treatment effect in RCC compared to the single gene treatments.

Clinical translation of angiogenesis inhibitors

Angiogenesis inhibitors are a new class of drugs, for which the general rules involving conventional chemotherapy might not apply. The successful translation of angiogenesis inhibitors to clinical application depends partly on the transfer of expertise from scientists who are familiar with the biology of angiogenesis to clinicians. What are the most common questions that clinicians ask as they begin to test angiogenesis inhibitors in cancer clinical trials?

Antiangiogenic and antitumor effects of endostatin on follicular thyroid carcinoma

Tumor growth and metastasis depend on blood supply and blood vessel formation. Angiogenesis, therefore, represents a promising target for cancer therapy. Endostatin is one of the most potent antiangiogenic factors and has been shown to effectively inhibit angiogenesis and tumor growth in a variety of in vivo models. In this study, we tested the effects of endostatin on xenografted human follicular thyroid carcinoma (FTC) in nude mice. Our result demonstrated that recombinant endostatin significantly inhibited the growth of FTC xenografts. Furthermore, we established an endostatin-expressing FTC cell line (FTC-BmEndo) using retrovirus-mediated gene transfer approach. We found that the in vivo growth of FTC-BmEndo cells was significantly inhibited, compared with the parental FTC cells, whereas both lines grew at the same rate in vitro. High-level expression of endostatin within the FTC-BmEndo tumors was evidenced by immunohistochemical staining, paralleled with a reduced microvessel density. The systemic level of vascular endothelial growth factor was significantly lower in mice bearing the FTC-BmEndo tumors than in those bearing parental FTC tumors. By using two different approaches, namely the recombinant endostatin protein and the gene therapy strategy, our study demonstrated that endostatin could be effective in suppressing the growth of human FTC in immunodeficient mice.

Continuous intravascular secretion of endostatin in mice from transduced hematopoietic stem cells

Endostatin, a 20-kDa carboxy-terminal fragment of collagen XVIII, is the leading member of a class of physiologic inhibitors of angiogenesis with potent antitumor activity. Repeated subcutaneous administration of recombinant endostatin in mice led to permanent regression of established tumors to a microscopic dormant state and prompted the initiation of human clinical trials. However, a discrepancy remained unresolved: sustained tumor regression has only been observed with a non-soluble, precipitated form of recombinant endostatin produced in bacteria. To shed light on this question and establish a model of systemic anti-angiogenic gene therapy of cancer that may surmount obstacles in protein production and delivery, we transduced murine hematopoietic stem cells with a retrovirus encoding a secretable form of endostatin. Despite continuous, high-level secretion of endostatin in the vasculature of all transplanted mice, we detected neither inhibition of in vivo neoangiogenesis nor antitumor activity. Resolution of this paradox may come from human trials of endostatin now underway.