Platelets and platelet adhesion support angiogenesis while preventing excessive hemorrhage

Platelets contain both pro- and antiangiogenic factors, but their regulatory role in angiogenesis is poorly understood. Although previous studies showed that platelets stimulate angiogenesis in vitro, the role of platelets in angiogenesis in vivo is largely uncharacterized. To address this topic, we used two in vivo approaches, the cornea micropocket assay and the Matrigel model, in four animal models: thrombocytopenic, Lyst(bg) (platelet storage pool deficiency), glycoprotein (GP) Ibalpha/IL4R transgenic (lacking extracellular GPIbalpha, the receptor for von Willebrand factor as well as other adhesive and procoagulant proteins), and FcgammaR(-/-) (lacking functional GPVI, the collagen receptor) mice. Adult mice were rendered thrombocytopenic by i.p. administration of an antiplatelet antibody. The number of growing vessels in the thrombocytopenic mice was lower in the cornea assay, and they showed significantly increased appearance of hemorrhage compared with mice treated with control IgG. The thrombocytopenic mice also showed more protein leakage and developed hematomas in the Matrigel model. GPIbalpha/IL4R transgenic mice presented increased hemorrhage in both assays, but it was less severe than in the platelet-depleted mice. FcgammaR(-/-) and Lyst(bg) mice showed no defect in experimental angiogenesis. Intravital microscopy revealed a >3-fold increase in platelet adhesion to angiogenic vessels of Matrigel compared with mature quiescent skin vessels. Our results suggest that the presence of platelets not only stimulates angiogenic vessel growth but also plays a critical role in preventing hemorrhage from the angiogenic vessels. The adhesion function of platelets, as mediated by GPIbalpha, significantly contributes to the process.

Endostatin inhibits the growth of endometriotic lesions but does not affect fertility

OBJECTIVE

To determine whether endometriosis can be treated with the angiogenesis inhibitor endostatin and the effect of this treatment on fertility and reproduction.

DESIGN

Pharmacologic intervention in a surgically induced model of endometriosis and in female mice undergoing mating.

SETTING

Animal research facility.

ANIMAL(S)

Eight-week-old, female C57BL/6 and SCID mice.

INTERVENTION(S)

After implantation of autologous endometrium, mice received endostatin or the vehicle-matched control for 4 weeks. For the reproductive function study, mice receiving endostatin or vehicle were mated and reproductive functions were observed.

MAIN OUTCOME MEASURE(S)

Growth of endometriotic lesions after 4 weeks of treatment; estrous cycling, corpus luteum formation, serum hormone levels, and mating time as fertility measures; and pregnancy rates, length of pregnancy, fetal vitality, number, and outcome of litter as reproductive measures.

RESULT(S)

Endostatin suppressed the growth of endometriotic lesions by 47% compared with controls. Estrous cycling and corpus luteum formation were normal in both groups. Female mice receiving endostatin were as fertile as mice receiving vehicle, had normal pregnancies, and delivered the same number of pups. The offspring were healthy without teratogenic stigmata and reproduced normally themselves.

CONCLUSION(S)

Antiangiogenic therapy with endostatin may present a promising novel, nontoxic therapeutic option for patients with endometriosis.

Short synthetic endostatin peptides inhibit endothelial migration in vitro and endometriosis in a mouse model

OBJECTIVE

To determine the active peptide regions inside the angiogenesis inhibitor endostatin that can inhibit endothelial migration in vitro and also inhibit endometriosis in a mouse model.

DESIGN

Pharmacologic intervention in a surgically induced mouse model of endometriosis and endothelial migration assay.

SETTING

Animal research and laboratory facility.

SUBJECT(S)

Eight-week-old, female C57BL/6 mice and human microvascular endothelial cells.

INTERVENTION(S)

Eight overlapping synthetic peptides were tested for inhibitory potential on endothelial migration in vitro. The peptides with significant activity then were given for 4 weeks to mice after implantation of autologous endometrium.

MAIN OUTCOME MEASURE(S)

Inhibition of vascular endothelial growth factor-induced endothelial migration for in vitro studies. In vivo studies examined the growth rate of endometriotic lesions after 4 weeks of treatment, as well as the effect on estrous cycling and ovulation as assessed by corpus luteum formation.

RESULT(S)

The N-terminal mP-1 peptide and the internal mP-6 peptide inhibited endothelial migration in a dose-dependent manner. Additionally, both synthetic peptides suppressed growth of endometriotic lesions significantly in vivo. However, estrous cycling and corpus luteum formation were normal in both groups.

CONCLUSION(S)

Short endostatin fragments may be promising as a new, nontoxic therapeutic strategy for the treatment of endometriosis without inhibition of normal estrous cycles.

Evidence by molecular profiling for a placental origin of infantile hemangioma

The origin of the pathogenic endothelial cells in common infantile hemangioma is unknown. We show here that the transcriptomes of human placenta and infantile hemangioma are sufficiently similar to suggest a placental origin for this tumor, expanding on recent immunophenotypical studies that have suggested this possibility [North, P. E., et al. (2001) Arch. Dermatol. 137, 559-570]. The transcriptomes of placenta, hemangioma, and eight normal and diseased tissues were compared by hierarchical and nonhierarchical clustering analysis of >7,800 genes. We found that the level of transcriptome similarity between placenta and hemangioma exceeded that of any other tissue compared and paralleled that observed between a given tissue and its derived tumor, such as normal and cancerous lung. The degree of similarity was even greater when a subset of endothelial cell-specific genes was analyzed. Genes preferentially expressed in both placenta and hemangiomas were identified, including 17-beta hydroxysteroid dehydrogenase type 2 and tissue factor pathway inhibitor 2. These data demonstrate the value of global molecular profiling of tissues as a tool for hypothesis-driven research. Furthermore, it suggests that the unique self-limited growth of infantile hemangioma may, in fact, mirror the lifetime of placental endothelium.

Antiangiogenesis in cancer therapy--endostatin and its mechanisms of action

The first angiogenesis inhibitors for cancer have now been approved by the F.D.A. in the U.S. and in 28 other countries, including China. The majority of these are monotherapies that block VEGF. However, mutant tumor cells may over time produce redundant angiogenic factors. Therefore, for long-term use in cancer, combinations of angiogenesis inhibitors or broad spectrum angiogenesis inhibitors will be needed. The two most broad spectrum and least toxic angiogenesis inhibitors are Caplostatin and endostatin. Endostatin inhibits 65 different tumor types and modifies 12% of the human genome to downregulate pathological angiogenesis without side-effects. The recent discovery that small increases in circulating endostatin can suppress tumor growth and that orally available small molecules can increase endostatin in the plasma suggests the possible development of a new pharmaceutical field.

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.

A feasibility trial of antiangiogenic (metronomic) chemotherapy in pediatric patients with recurrent or progressive cancer

Standard chemotherapeutic drugs, when modified by the frequency and dose of administration, can target angiogenesis. This approach is referred to as antiangiogenic chemotherapy, low-dose chemotherapy, or metronomic chemotherapy. This study evaluated the feasibility of 6 months of metronomic chemotherapy, its toxicity and tolerability, surrogate markers of activity, and preliminary evidence of activity in children with recurrent or progressive cancer. Twenty consecutive children were enrolled and received continuous oral thalidomide and celecoxib with alternating oral etoposide and cyclophosphamide every 21 days for a planned duration of 6 months using antiangiogenic doses of all four drugs. Surrogate markers including bFGF, VEGF, endostatin, and thrombospondin were also evaluated. Therapy was well tolerated in this heavily pretreated population. Toxicities (predominantly reversible bone marrow suppression) responded to dose modifications. Sixty percent of the patients received less than the prescribed 6 months of therapy due to toxicity (one case of deep vein thrombosis), personal choice (1 patient), or disease progression (10 patients). Forty percent of the patients completed the 6 months of therapy, resulting in prolonged or persistent disease-free status. One quarter of all patients continue to be progression free more than 123 weeks from starting therapy. Sixteen percent of patients showed a radiographic partial response. Only elevated thrombospondin-1 levels appeared to correlate with prolonged response. This oral antiangiogenic chemotherapy regimen was well tolerated in this heavily pretreated pediatric population, which showed prolonged or persistent disease-free status, supporting the continued study of antiangiogenic/metronomic chemotherapy in human clinical trials.

The route of lipid administration affects parenteral nutrition-induced hepatic steatosis in a mouse model

BACKGROUND

The etiology of parenteral nutrition (PN)-associated hepatic injury remains unresolved. Recent studies have suggested that the intravenous (IV) lipid emulsion administered with PN may contribute to PN-associated hepatic injury. We therefore examined whether the route of lipid administration would affect the development of PN-associated liver injury in a previously established animal model of PN-induced hepatic steatosis.

METHODS

Mice were fed ad libitum PN solution as their only nutritional source for 19 days with lipid supplementation by either the enteral or the IV route. Control mice received chow alone, and a final group received enteral PN solution without lipid supplementation.

RESULTS

All mice gained equivalent weight during the study. Mice receiving PN alone or PN with IV lipid developed severe histologic liver damage that was not seen in control mice or in mice receiving PN with enteral lipid. Liver fat content as measured by magnetic resonance spectroscopy was significantly lower in the control and enteral lipid groups when compared with mice receiving PN alone or with IV lipid. Mice receiving enteral lipid had significantly lower levels of serum aspartate aminotransferase and alanine aminotransferase compared with animals receiving PN alone.

CONCLUSIONS

These data provide preliminary evidence that lipid administered through the enteral route protects against PN-associated hepatic injury in an animal model.

Prevention of intra-abdominal adhesions using the antiangiogenic COX-2 inhibitor celecoxib

OBJECTIVE

To determine the effects of COX-2 specific inhibitors on postoperative adhesion formation.

SUMMARY AND BACKGROUND DATA

Intra-abdominal adhesions are the major cause of intestinal obstruction and secondary infertility after surgical procedures. Because adhesion synthesis requires angiogenesis, and cyclooxygenase-2 enzyme (COX-2) inhibitors have antiendothelial activity, we tested COX-2 inhibitors in a murine model of intra-abdominal adhesion formation.

METHODS

A silicone patch was secured to the lateral abdominal wall of groups of C57BL/6 mice, followed by cecal abrasion to promote adhesion formation. Beginning on the day of surgery, mice were treated with the selective COX-2 agents, celecoxib or rofecoxib, and the nonspecific COX inhibitors, aspirin, naproxen, ibuprofen, or indomethacin. Animals were treated for 10 days and killed. A second group (celecoxib, rofecoxib, aspirin) was treated for 10 days and observed for an additional 25 days. After treatment, intra-abdominal adhesions were scored using a standard method. The patch was subjected to immunohistochemistry with the endothelial-specific marker, CD31.

RESULTS

Animals treated with selective and nonselective COX-2 inhibitors, except aspirin, had significantly fewer adhesions than control animals. Celecoxib produced a maximal reduction in adhesion formation compared with rofecoxib and the nonselective COX-2 inhibitors at 10 days. After 25 days, celecoxib and rofecoxib, but not aspirin, had fewer adhesions than control mice. Adhesions from mice treated with celecoxib had reduced microvessel density compared with rofecoxib, the nonselective COX inhibitors, and control animals.

CONCLUSIONS

Selective COX-2 inhibitors, in particular celecoxib, provide durable inhibition of intra-abdominal adhesions through an antiangiogenic mechanism.

Differential effects of vascular endothelial growth factor receptor-2 inhibitor ZD6474 on circulating endothelial progenitors and mature circulating endothelial cells: implications for use as a surrogate marker of antiangiogenic activity

PURPOSE

Circulating endothelial cells (CEC) comprise at least two distinct populations: bone marrow-derived circulating endothelial progenitors (CEP) and mature CECs derived from existing vasculature. We hypothesized that antiangiogenic agents may have differential effects on CEPs and mature CECs and that these changes may serve as a marker of biological activity.

EXPERIMENTAL DESIGN

The effect of angiogenesis inhibitors on CECs was evaluated by flow cytometry after vascular endothelial growth factor (VEGF)-induced mobilization and in mice bearing Lewis lung carcinoma (LLC). Tumor angiogenesis was evaluated in parallel by immunohistochemistry.

RESULTS

In nontumor-bearing mice, VEGF administration increased both mature CECs and CEPs. This increase was inhibited by the VEGF receptor 2 inhibitor ZD6474 as well as the VEGF inhibitor-soluble Flt-1. ZD6474 had no significant effect on CECs in the absence of exogenous VEGF stimulation. In contrast, LLC-bearing mice had an increase in mature CECs but not CEPs after 3 days of treatment with ZD6474. The increase in mature CECs was dose-dependent, accompanied by a decrease in tumor microvessel density, and preceded reduction in tumor volume. Treatment of LLC-bearing mice with the vascular targeting agent ZD6126 also increased mature CECs.

CONCLUSIONS

VEGF inhibitors can have differential effects on mature CECs and CEPs, and agents inhibiting tumor angiogenesis may cause a concomitant increase in mature CECs. This increase occurs in tumor-bearing but not in nontumor-bearing mice, suggesting that tumor endothelium is a potential source of mature CECs. Therefore, assessing both mature CECs and CEPs may provide insights into the mechanism of antiangiogenic agents and serve as an early surrogate marker of biological activity.

Increased expression of urinary matrix metalloproteinases parallels the extent and activity of vascular anomalies

OBJECTIVE

Matrix metalloproteinases (MMPs) and the angiogenic proteins basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) have been implicated in mechanisms of human cancer and metastasis. Assays were conducted on the urine of patients with vascular anomalies (tumors and malformations), relatively common and occasionally life-threatening disorders for which few therapies exist. We sought to determine whether these angiogenesis modulators are present in the urine and whether their expression is associated with the extent and clinical course of the vascular lesion.

METHODS

A total of 217 patients with vascular anomalies and 74 age-matched control subjects participated. Urinary MMP expression was determined by substrate gel electrophoresis. Urinary bFGF and VEGF levels were measured by enzyme-linked immunosorbent assay. Each patient was assigned to 1 of 2 categories (tumor or malformation) and 1 of 9 specific groups. Extent of the vascular lesion and activity were scored by a blinded clinician.

RESULTS

Urinary high molecular weight (hMW) MMPs and bFGF were significantly increased in patients with vascular tumors (53%) and vascular malformations (41%), compared with control subjects (22%). These percentages increased as a function of extent of the lesion and disease activity. hMW MMPs were increased in 4 groups: infantile hemangioma, other vascular neoplasms, lymphatic malformation and capillary-lymphaticovenous malformations, and extensive and unremitting capillary malformation and arteriovenous malformation. No significant differences among the groups were detected for low molecular weight MMPs or VEGF.

CONCLUSIONS

Expression patterns of hMW MMPs and bFGF in the urine of patients with tumors and malformations are consistent with their different clinical behavior. These data represent the first evidence that MMPs are elevated in the urine of children with vascular anomalies. These data also suggest that the increased expression of urinary MMPs parallels the extent and activity of vascular anomalies in children. In addition to tumors, vascular malformations are angiogenesis dependent, suggesting that progression of a vascular malformation might be suppressed by angiogenic inhibitors, which would target bFGF and MMPs.

A 27-Amino-Acid Synthetic Peptide Corresponding to the NH2-Terminal Zinc-Binding Domain of Endostatin Is Responsible for Its Antitumor Activity

The first recombinant endostatin that elicited strong antitumor activity was expressed in Escherichia coli and administered as a suspension. Under these conditions, the protein retained its full antiangiogenic activity. Lack of requirement for a folded structure prompted us to investigate antitumor properties of synthetic peptides corresponding to different regions of endostatin. Here, we show that the entire antitumor, antimigration, and antipermeability activities of endostatin are mimicked by a 27-amino-acid peptide corresponding to the NH2-terminal domain of endostatin. This peptide contains three histidines that are responsible for zinc binding. Mutations of the zinc-binding histidines abolished its antitumor and antimigration activities, but not antipermeability properties.

CDK2 translational down-regulation during endothelial senescence

Here we report for the first time that loss of CDK2 activity, by translational inhibition and through CDK2 inhibition by p21(Cip1/Waf1), may be responsible for endothelial senescence. We show that expression of dominant-negative p53 extends human umbilical vein endothelial cell (HUVEC) lifespan past senescence. HUVEC expressing telomerase can completely bypass senescence and become immortal (i-HUVEC). Surprisingly, early passage i-HUVEC, like senescent HUVEC, express high levels of the CDK inhibitors p16(INK4a) and p21(Cip1/Waf1). Expression of p16(INK4a) can persist for over 280 population doublings, while p21(Cip1/Waf1) expression was eventually lost in five of six i-HUVEC lines. Senescent HUVEC contain undetectable CDK2 activity, which results from a dramatic reduction of CDK2 protein levels and inhibition of remaining CDK2 by p21(Cip1/Waf1). The decreased CDK2 levels in senescent HUVEC are not due to decreased transcription or protein stability; rather, CDK2 translation declines during senescence. Bypass of endothelial senescence by telomerase entails the restoration of CDK2 translation and activity. These results suggest that p16(INK4a) does not play a role in endothelial senescence. Rather, CDK2 translational down-regulation may be a key regulatory event in replicative senescence of endothelial cells. Understanding the mechanisms regulating endothelial senescence will be critical in determining the role of endothelial senescence in tumor growth.

Blue rubber bleb nevus syndrome: surgical eradication of gastrointestinal bleeding

OBJECTIVE

We report the largest clinical experience to date of surgically treated patients with blue rubber bleb nevus syndrome (BRBNS).

SUMMARY BACKGROUND DATA

BRBNS is a rare congenital disorder presenting with multifocal venous malformations of the skin, soft tissues, and gastrointestinal (GI) tract. Patients with BRBNS develop anemia from chronic GI bleeding, and require lifelong treatment with iron and blood transfusions. An aggressive surgical approach to treat the GI venous malformations of BRBNS has been considered unlikely to be successful because of the large number of lesions, their position throughout the GI tract, and the likelihood of recurrence. Based on our belief that eradicated lesions would not recur, we undertook the removal of all GI tract lesions in an effort to eliminate bleeding.

METHODS

Ten patients with BRBNS were treated from 1993 to 2002. Lesions were identified using complete GI endoscopy. The multiple venous malformations were removed by a combination of wedge resection, polypectomy, suture-ligation, segmental bowel resection, and band ligation.

RESULTS

Patient ages ranged from 2 to 36 years, and patients received an average of 53 prior blood transfusions. A mean of 137 focal GI venous malformations per patient were resected at operation (range 4-557), with a mean operative duration of 14 hours (range 7-23 hours). Only 1 patient who had a less extensive procedure developed recurrent GI bleeding. The mean follow-up period was 5.0 years (range 2.9-10.3 years).

CONCLUSIONS

We believe that an aggressive excisional approach is indicated for the venous anomalies that cause GI bleeding in BRBNS.

Omega-3 fatty acid supplementation prevents hepatic steatosis in a murine model of nonalcoholic fatty liver disease

Prolonged use of total parenteral nutrition can lead to nonalcoholic fatty liver disease, ranging from hepatic steatosis to cirrhosis and liver failure. It has been demonstrated that omega-3 fatty acids are negative regulators of hepatic lipogenesis and that they can also modulate the inflammatory response in mice. Furthermore, they may attenuate hepatic steatosis even in leptin-deficient ob/ob mice. We hypothesized that omega-3 fatty acid supplementation may protect the liver against hepatic steatosis in a murine model of parenteral nutrition in which all animals develop steatosis and liver enzyme disturbances. For testing this hypothesis, groups of mice received a fat-free, high-carbohydrate liquid diet ad libitum for 19 d with enteral or i.v. supplementation of an omega-3 fatty acid emulsion or a standard i.v. lipid emulsion. Control mice received food alone or the fat-free, high-carbohydrate diet without lipid supplementation. Mice that received the fat-free, high-carbohydrate diet only or supplemented with a standard i.v. lipid emulsion developed severe liver damage as determined by histology and magnetic resonance spectroscopy as well as elevation of serum liver function tests. Animals that received an i.v. omega-3 fatty acid emulsion, however, showed only mild deposits of fat in the liver, whereas enteral omega-3 fatty acids prevented hepatic pathology and led to normalization of liver function tests. In conclusion, whereas standard i.v. lipid emulsions fail to improve dietary-induced steatotic injury to the liver, i.v. supplementation of omega-3 fatty acids partially and enteral supplementation completely protects the liver against such injury.

Inhibition of vessel permeability by TNP-470 and its polymer conjugate, caplostatin

Angiogenesis inhibitors, such as TNP-470 and the nontoxic HPMA copolymer-TNP-470 (caplostatin), are emerging as a class of anticancer drugs. We report that TNP-470 and caplostatin inhibit vascular hyperpermeability of tumor blood vessels as well as that induced in mouse skin by different mediators. Treatment with TNP-470 or angiostatin for 3 days was sufficient to reduce permeability of tumor blood vessels, delayed-type hypersensitivity, and pulmonary edema induced by IL-2. TNP-470 also inhibited VPF/VEGF-induced phosphorylation of VEGFR-2, calcium influx, and RhoA activation in endothelial cells. These results identify an activity of TNP-470, that of inhibiting vessel hyperpermeability. This activity likely contributes to TNP-470's antiangiogenic effect and suggests that caplostatin can be used in the treatment of cancer and inflammation.

Nonsteroidal antiinflammatory drugs differentially suppress endometriosis in a murine model

OBJECTIVE

To determine whether nonsteroidal antiinflammatory drugs (NSAIDs) affect the establishment and progression of endometriotic lesions in a murine model.

DESIGN

Pharmacologic intervention in a surgically induced murine model of abdominal/peritoneal endometriosis.

SETTING

Animal research facility.

PATIENT(S)

Eight-week-old, female C57BL/6 mice.

INTERVENTION(S)

After implantation of autologous endometrium, mice were randomized into groups and treated with one of several NSAIDs or the vehicle-matched control for 4 weeks.

MAIN OUTCOME MEASURE(S)

Establishment, growth, and total burden of endometriotic lesions.

RESULT(S)

The NSAIDs differentially inhibited lesion establishment and growth, resulting in significantly reduced disease burden. Compared with controls (5.7 +/- 2.3 mm(2)), lesion burden was reduced by celecoxib (1.3 +/- 1.2 mm(2)), indomethacin (1.4 +/- 1.4 mm(2)), naproxen (2.7 +/- 1.2 mm(2)), sulindac (3.1 +/- 1.5 mm(2)), rofecoxib (3.4 +/- 3.0 mm(2)), and ibuprofen (4.1 +/- 1.4 mm(2)). In contrast, aspirin (5.9 +/- 1.2 mm(2)) had no statistically significant effect. Uninterrupted estrus cycling was confirmed by vaginal exams and smears in celecoxib-treated mice.

CONCLUSION(S)

Chronic administration of certain NSAIDs limits the progression of endometriosis in this murine model. The data suggest that NSAID selection in the treatment of endometriosis should be extended beyond pain management to maximize the inhibitory effect on disease burden.

Phase II study of the antiangiogenic agent SU5416 in patients with advanced soft tissue sarcomas

PURPOSE

SU5416 (semaxanib) is a small molecule inhibitor of the vascular endothelial growth factor (VEGF) receptor-2 and KIT receptor tyrosine kinases. This Phase II study was conducted to investigate the safety and efficacy of SU5416 for patients with soft tissue sarcomas.

EXPERIMENTAL DESIGN

Thirteen patients with locally advanced or metastatic soft tissue sarcomas were treated with SU5416 via intravenous infusion at a dose of 145 mg/m(2) twice weekly. In selected cases tumor biopsies were taken before and after 2 months of treatment.

RESULTS

The median progression-free survival was 1.8 months. Median overall survival was 22.8 months. No objective tumor responses were observed. There was evidence of shorter survival among patients with high baseline urine VEGF levels (P = 0.04). No grade 4 toxicities were observed. The most common grade 3 toxicities were headache and thrombosis. Other less serious toxicities included fatigue, nausea, and abdominal pain. The median systolic blood pressure increased from 118 mmHg at baseline to 133 after 1 month of treatment (P = 0.01). Post-treatment tumor biopsies showed no significant decreases in VEGF receptor phosphorylation compared with baseline in 3 evaluable patients. One patient with gastrointestinal stromal tumor who had rapid progression during SU5416 treatment was subsequently treated with another KIT inhibitor, imatinib mesylate, and had a partial response lasting >36 months.

CONCLUSIONS

SU5416 was relatively well tolerated but did not demonstrate significant antitumor activity against advanced soft tissue sarcoma. Correlative studies suggest that VEGF receptor or KIT inhibition was incomplete in at least some cases, providing a possible explanation for the observed lack of activity.

Endogenous angiogenesis inhibitors

When the FDA commissioner announced in February 2004 the approval of Avastin for the treatment of patients with colon cancer, he called angiogenesis inhibitors a fourth modality of anti-cancer therapy. Because angiogenesis inhibitors are relatively less toxic than conventional chemotherapy and have a lower risk of drug resistance, they may also represent a new class of anti-cancer agents, some of which have sufficiently reduced toxicity that they may be safely used long term. These include immunotherapy, vaccines, telomerase inhibitors, apoptosis inducers, low dose metronomic chemotherapy, novel hormonal therapies, gene therapy and others. However, at least 16 endogenous angiogenesis inhibitors have been discovered in the circulation, and/or in the extracellular matrix. These may become the safest and least toxic of anti-cancer therapies. Four are already being administered by injection in clinical trials for cancer. Recently, it has been reported that at least two endogenous angiogenesis inhibitors can be significantly increased in humans (endostatin), and in mice (thrombospondin), by oral administration of small molecules which themselves are already FDA approved for other uses. This finding suggests several new clinical applications for the future, including the possibility of guiding the use of angiogenesis inhibitors by blood or urinary biomarkers, currently being developed, that may detect the presence of cancer before it is symptomatic, or before it can be located by conventional methods.

Is oncogene addiction angiogenesis-dependent?

Does an activated oncogene that initiates tumor growth need to remain activated to maintain the cancer phenotype? This question has been answered affirmatively by experiments in which doxycycline-regulated oncogene activation induces growth of large tumors that regress completely upon oncogene inactivation--a phenomenon called oncogene addiction. We assemble here the evidence that oncogene addiction is angiogenesis-dependent. Although activated oncogenes increase tumor cell proliferation and decrease their apoptosis, these activities are not sufficient to expand tumor mass beyond a microscopic size. Oncogenes must also induce tumor angiogenesis for expansion of tumor mass. We propose experiments to validate the "endothelial centric" hypothesis of oncogene addiction.