Murine endothelial cell lines as models of tumor endothelial cells.

Identification of appropriate models for in vivo and in vitro preclinical testing of inhibitors of tumor angiogenesis and progression is vital to the successful development of anticancer therapeutics. Although the focus is on human molecular targets, most preclinical in vivo efficacy testing occurs in mice. The goal of the current studies was to identify a murine endothelial cell line to model tumor endothelium for studying the antiangiogenic activity of therapeutic compounds in vitro. In situ hybridization was performed on three s.c. grown syngeneic murine tumors (B16 melanoma, Lewis lung carcinoma, and CT26 colon carcinoma) to assess expression of murine homologs of human tumor endothelial cell markers in the vasculature of these tumor models. Seven murine endothelial cell lines were characterized for expression of the murine homologs of recognized endothelial cell surface markers as well as for tumor endothelial cell surface markers. The seven murine endothelial cell lines had similar generation times and five of the seven lines were able to form tubes on Matrigel. Real-time-PCR and flow cytometry analysis were used to evaluate relative mRNA and protein expression of murine homologs of several recognized endothelial cell surface markers in the seven cell lines. The expression of the mRNA for the murine homologs of five tumor endothelial cell surface markers was also evaluated. The 2H11 cell line expressed all five of the tumor endothelial cell surface markers as well as several well-recognized endothelial cells markers. The 2H11 cell line responds to known and novel antiangiogenic agents by inhibition of proliferation and tube formation. These cells can be used in in vitro angiogenesis assays for evaluating the potential antiangiogenic properties and interspecies cross-reactivity of novel compounds.     

Itraconazole inhibits angiogenesis and tumor growth in non-small cell lung cancer

The antiangiogenic agent bevacizumab has been approved for the treatment of non-small cell lung cancer (NSCLC), although the survival benefit associated with this agent is marginal, and toxicities and cost are substantial. A recent screen for selective inhibitors of endothelial cell proliferation identified the oral antifungal drug itraconazole as a novel agent with potential antiangiogenic activity. In this article, we define and characterize the antiangiogenic and anticancer activities of itraconazole in relevant preclinical models of angiogenesis and lung cancer. Itraconazole consistently showed potent, specific, and dose-dependent inhibition of endothelial cell proliferation, migration, and tube formation in response to both VEGF- and basic fibroblast growth factor-mediated angiogenic stimulation. In vivo, using primary xenograft models of human NSCLC, oral itraconazole showed single-agent growth-inhibitory activity associated with induction of tumor hypoxia-inducible factor 1 alpha expression and marked inhibition of tumor vascularity. Itraconazole significantly enhanced the antitumor efficacy of the chemotherapeutic agent cisplatin in the same model systems. Taken together, these data suggest that itraconazole has potent and selective inhibitory activity against multiple key aspects of tumor-associated angiogenesis in vitro and in vivo, and strongly support clinical translation of its use. Based on these observations, we have initiated a randomized phase II study comparing the efficacy of standard cytotoxic therapy with or without daily oral itraconazole in patients with recurrent metastatic NSCLC.     

Inhibitory effect of the salmosin gene transferred by cationic liposomes on the progression of B16BL6 tumors

Salmosin is a novel disintegrin containing the Arg-Gly-Asp sequence that significantly inhibits platelet aggregation, basic fibroblast growth factor-induced endothelial cell proliferation, and tumor progression by antagonizing integrin-mediated cell interactions. Previously, it was shown that daily administration of salmosin was able to inhibit tumor-derived angiogenesis and adherence and proliferation of tumor cells, resulting in suppression of tumor progression. However, it is very difficult to maintain a therapeutic level of salmosin in the blood by systemic administration of the protein. Hence, an alternative strategy for antiangiogenic cancer therapy, based on the in vivo expression of the salmosin gene administered with cationic liposomes, was investigated. The salmosin peptides expressed in vitro inhibited the proliferation of bovine capillary endothelial cells in a dose-dependent manner, presumably as a result of inhibition of cell adhesion mediated via alpha(v)beta(3) integrin. Subcutaneous administration of the salmosin gene resulted in systemic expression of the gene product and concomitant inhibition of the growth of B16BL6 melanoma cells. Suppression of pulmonary metastases, verified by experimental and spontaneous metastasis models in mice, also resulted from salmosin gene treatment. These results suggest that administration of the salmosin gene complexed to cationic liposomes is effective in maintaining antiangiogenic salmosin at an effective therapeutic level and may be clinically applicable to anticancer gene therapy.     

Approaches to preclinical screening of antiangiogenic agents.

Angiogenesis, or new blood vessel growth, is essential for the growth, invasion, and metastasis of solid tumors. The inhibition of this process, or antiangiogenesis, is a promising new therapeutic anticancer strategy. Several antiangiogenic compounds are currently in preclinical or clinical development for the treatment of cancer. However, the challenge for the discovery and characterization of antiangiogenic targets remains in developing efficient in vitro or in vivo preclinical angiogenesis screening assays to assess and compare antiangiogenic activity. Several semiquantitative or quantitative angiogenesis assays exist, including in vitro endothelial cell systems and ex vivo or in vivo neovascularization models utilizing mouse, rat, or human tissues. We describe the more common and cost-effective angiogenesis assays currently in use, summarizing their unique advantages and disadvantages. Since angiogenesis inhibition is a novel therapeutic modality towards controlling solid tumors, antiangiogenic drug development underlines the importance in describing, standardizing, and developing quantitative screening assays for the next generation of antiangiogenic agents.     

rAAV-mediated long-term liver-generated expression of an angiogenesis inhibitor can restrict renal tumor growth in mice

It is now well established that tumor growth is angiogenesis dependent. Inhibition of angiogenesis, therefore, is likely to be an effective anticancer approach. A gene therapy-mediated approach to the delivery of antiangiogenic agents using adeno-associated virus (AAV) vectors has a number of advantages, including the potential for sustained expression. We have constructed a rAAV vector in which the expression of a soluble, truncated form of the vascular endothelial growth factor receptor-2 (Flk-1), a known inhibitor of endothelial cell activation, is driven by a composite beta-actin-based promoter. After intraportal injection of this vector, high-level, stable transgene expression was generated in mice. This established a systemic state of angiogenesis inhibition; sera from these mice inhibited endothelial cell activation in vitro and Matrigel plug neovascularization in vivo. Significant antitumor efficacy was observed in two murine models of pediatric kidney tumors. Tumor development was prevented in 10 of 15 (67%) mice, with significant growth restriction of tumors in the remaining mice. For the first time, long-term, in vivo expression of a functional angiogenesis inhibitor has been established using rAAV, with resultant anticancer efficacy in a relevant, orthotopic tumor model. These findings establish the feasibility of using rAAV vectors in antiangiogenic gene therapy.     

A systems approach to cancer therapy

Conclusion The molecules described herein as antiangiogenic agents and antimetastatic agents represent a wide variety of molecular structures with a wide variety of biological effects and targets. Most often these agents have been generally classified as antiangiogenic or antimetastatic by their effects in an in vitro bio-assay system. The diversity in this group of molecules gives strength to the potential of this approach in therapeutic applications. The biological and biochemical pathways involved in angiogenesis are numerous and redundant. It is likely that there are many angiogenic factors and many pathways of invasion, therefore it is likely that blockade of more than one pathway related to angiogenesis and/or invasion will be necessary to impact on the natural progress of a malignant disease.The vasculature forms the first barrier to penetration of molecules into tumors. Although the antiangiogenic agent treatments administered in this study did not inhibit angiogenesis in these tumors completely, the vasculature present in the treated tumors may be impaired compared to control tumors. Overall, therefore, the best speculation is that the main targets for the antiangiogenic agents are extracellular matrix processes and/or tumor endothelial cells and that inhibition and/or impairment of these non-malignant functions can improve therapeutic responses when used in combination with cytotoxic therapies. The incorporation of antiangiogenic agents and/or antimetastatic agents into therapeutic regimens represents an important challenge. The successful treatment of cancer requires the eradication of all malignant cells and therefore treatment with cytotoxic therapies. The compatibility of antiangiogenic therapy and/or anti-invasion agents with cytotoxic chemotherapeutic agents is not obvious [316].The goal of the addition of any non-cytotoxic potentiator to a therapeutic regimen is to take a good therapy and, without additional toxicity, push it to cure.Cyclophosphamide is a good drug against the Lewis lung carcinoma although no long-term survivors of animals bearing Lewis lung carcinoma are achieved with cyclophosphamide treatment alone. Adding antiangiogenic agents to treatment of this tumor with cyclophosphamide produced a cure rate of 40–50%, meaning that both the primary and metastatic disease has been eradicated in these animals. Cures were achieved only when the antiangiogenic treatments extended from days 4–18 post Lewis lung tumor implantation. The results obtained with the addition of antiangiogenic agents to cytotoxic anticancer therapies in in vivo models of established solid tumors have been very positive and provide direction for future clinical trials including these antiangiogenic agents. Two conclusions may be drawn. First, combinations of antiangiogenic and/or antimetastatic agents evoke a greater effect on tumor response to therapy than does treatment with single agents of these classes. Second, treatment with antiangiogenic agents and/or antimetastatic agents can interact in a positive way with cytotoxic therapies.     

Low density lipoprotein receptor mediates anti-VEGF effect of lymphocyte T-derived microparticles in Lewis lung carcinoma cells

Nonstop proliferation and vigorous neovascularization are two prominent characteristics of cancer. Antiangiogenic therapy has emerged as an important modality in treatment of solid tumors. Our previous work demonstrated that microparticles derived from apoptotic T-lymphocytes (LMPs) not only reduced the viabilities of high-proliferating cells, but also exhibited potent antiangiogenic effects through inhibition of the vascular endothelial growth factor (VEGF)/VEGF receptor 2 signalling pathway. In the present study, we extended these studies to explore the anticancer potential of LMPs using a murine model of Lewis lung carcinoma (LLC). Results show that intratumoral injection of LMPs (2.5 mg/kg) decreased tumor size by more than 50% relative to control. Tumor microvessel density and VEGF-A levels were also markedly reduced upon LMPs treatment. To elucidate the underlying mechanisms of LMPs-mediated antitumor activity, LLC cells were utilized in in vitro experiments. LMPs suppressed VEGF-A protein levels in LLC cells and led to inhibition of LLC cell viability and proliferation. In addition, knockdown of the low-density lipoprotein receptor (LDLR) expression reduced the uptake of LMPs into LLC cells and attenuated the inhibitory effects of LMPs on cell growth and VEGF-A expression. Our findings demonstrate that LMPs exert antiangiogenic and proapoptotic effects that lead to inhibition of lung carcinoma by reducing VEGF-A levels and LDLR mediates the anti-VEGF effect of LMPs through translocating LMPs into LLC cells. These results suggest that LMPs are promising antiangiogenic therapeutic agent and represent a new therapeutic strategy for treating lung carcinomas.Key words: lewis lung carcinoma, lymphocyte-derived microparticles, VEGF-A, low density lipoprotein receptor, angiogenesis, cell viability     

抗血管生成化疗的研究进展

肿瘤血管生成是肿瘤生长和转移的关键步骤,抑制肿瘤血管生成能有效达到抗肿瘤的目的。近年来,抗血管生成分子靶向治疗是研究热点。除此之外,传统细胞毒化疗药物以低剂量、频繁或持续、短间歇的方式给药亦可产生抗血管生成作用,此时,药物的靶点是肿瘤血管内皮细胞而非肿瘤细胞,这种治疗模式称为“抗血管生成化疗”。本文就近年来抗血管生成化疗的研究进展作一综述。     

Antiangiogenic mechanisms of PJ-8, a novel inhibitor of vascular endothelial growth factor receptor signaling

Angiogenesis occurs not only during tissue growth and development but also during wound healing and tumor progression. Angiogenesis is a balanced process controlled by proangiogenic and antiangiogenic molecules. As a critical factor in the induction of angiogenesis, vascular endothelial growth factor (VEGF) has become an attractive target for antiangiogenic and cancer therapeutic agents. In an effort to develop novel inhibitors to block VEGF signaling, we selected Pj-8, a benzimidazole derivative, and investigated its inhibitory mechanisms in human umbilical vascular endothelial cells (HUVECs). Pj-8 concentration-dependently inhibited VEGF-induced proliferation, migration and tube formation of HUVECs. Pj-8 also suppressed VEGF-induced microvessel sprouting from aortic rings ex vivo and suppressed neovascularization of implanted matrigel plugs in vivo. Pj-8 inhibited VEGF-induced phosphorylation of VEGF receptor (VEGFR) 2 and the downstream protein kinases, including Akt, focal adhesion kinase, extracellular signal-regulated kinases and Src. Results from in vitro kinase assay further demonstrated that Pj-8 suppressed the kinase activity of 3-phosphoinositide-dependent kinase 1 (PDK1). Using xenograft tumor angiogenesis model, Pj-8 markedly eliminated tumor-associated angiogenesis. Taken together, our findings suggest that Pj-8 inhibits VEGF and tumor cells MDA-MB-231-induced angiogenesis, and it may be a potential drug candidate in anticancer therapy. Downregulation of VEGFR2-mediated signaling may contribute to its antiangiogenic actions.     

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.     

The antiangiogenic property of docetaxel is synergistic with a recombinant humanized monoclonal antibody against vascular endothelial growth factor or 2-methoxyestradiol but antagonized by endothelial growth factors

Numerous chemotherapeutic agents have been shown to have an inhibitory effect on endothelial cell proliferation and migration, and tubule formation. In this study, we examined the antiangiogenic activity of docetaxel. Docetaxel inhibited endothelial cell proliferation and tubule formation in vitro in a dose-dependent fashion. Docetaxel treatment also inhibited angiogenesis in an in vivo Matrigel plug assay. The endothelial stimulating factors, vascular endothelial cell growth factor (VEGF) and basic fibroblast growth factor are able to protect endothelial cells from the antiangiogenic properties of docetaxel. This protective effect can be overcome by a recombinant humanized monoclonal antibody directed against VEGF in both in vitro and in vivo models. Similarly, combination of docetaxel with the antiangiogenic agent 2-methoxyestradiol also overcomes the protective effect of VEGF in both in vitro and in vivo models. These data suggest that microenvironmental factors (e.g., local release of VEGF and basic fibroblast growth factor) could play a role in decreasing the antiangiogenic effects of docetaxel, whereas agents such as 2- methoxyestradiol and recombinant humanized monoclonal antibody directed against VEGF may reverse this protective effect.     

Influence of chemotherapeutic-agents on the proliferation of capillary endothelial-cells and tumor angiogenesis in murine bladder-cancer

The current studies investigated the potential of conventional chemotherapeutic agents to inhibit the proliferation of capillary endothelial cells and to affect tumor angiogenesis using a murine bladder cancer model. Endothelial cell lines were established from the skin and used for an in vitro target. The anti-angiogenesis activity was assessed by an i.d. assay. Among the eight agents examined, adriamycin, 4-hydroperoxycyclophosphamide, and methotrexate preferentially inhibited the cell proliferation. However, these agents showed no influence on tumor angiogenesis. In contrast, mitomycin C, which was less cytotoxic to endothelial cells in vitro, showed a significant anti-angiogenesis activity. The data suggest that certain chemotherapeutic agents had anti-angiogenesis activity distinct from cytotoxicity.     

Dopamine increases the efficacy of anticancer drugs in breast and colon cancer preclinical models.

PURPOSE: Because neurotransmitter dopamine inhibits vascular permeability factor/vascular endothelial growth factor (VEGF)-induced angiogenesis and as anti-VEGF agents act synergistically with anticancer drugs, we therefore investigated whether dopamine can increase the efficacies of these drugs. EXPERIMENTAL DESIGN: The effect of dopamine was investigated in human breast cancer-(MCF-7) and colon (HT29) cancer-bearing mice. Experimental groups received either dopamine or doxorubicin or dopamine plus doxorubicin in MCF-7 tumor-bearing mice, and either dopamine or 5-fluorouracil or dopamine plus 5-fluorouracil in HT29-bearing mice. Thereafter, tumor growth, angiogenesis, tumor cell apoptosis, life span, and the effect of dopamine on the growth and survival of tumor cells in vitro were determined. Finally, the effects of dopamine on tumor vascular permeability; on VEGF receptor-2, mitogen-activated protein kinase, and focal adhesion kinase phosphorylation; and also on the proliferation and migration of tumor endothelial cells were investigated. RESULTS: Dopamine, in combination with anticancer drugs, significantly inhibited tumor growth and increased the life span when compared with treatment with dopamine or anticancer drugs alone. Dopamine had no direct effects on the growth and survival of tumor cells. The antiangiogenic action of dopamine was mediated by inhibiting proliferation and migration of tumor endothelial cells through suppression of VEGF receptor-2, mitogen-activated protein kinase, and focal adhesion kinase phosphorylation. CONCLUSION: Our study shows that dopamine significantly enhances the efficacies of commonly used anticancer drugs and also indicates that an inexpensive drug like dopamine, which is being extensively used in the clinics, might have a role as an antiangiogenic agent for the treatment of breast and colon cancer.     

Suppression of angiogenesis and tumor growth by selective inhibition of angiopoietin-2

Angiopoietin-2 (Ang2) exhibits broad expression in the remodeling vasculature of human tumors but very limited expression in normal tissues, making it an attractive candidate target for antiangiogenic cancer therapy. To investigate the functional consequences of blocking Ang2 activity, we generated antibodies and peptide-Fc fusion proteins that potently and selectively neutralize the interaction between Ang2 and its receptor, Tie2. Systemic treatment of tumor-bearing mice with these Ang2-blocking agents resulted in tumor stasis, followed by elimination of all measurable tumor in a subset of animals. These effects were accompanied by reduced endothelial cell proliferation, consistent with an antiangiogenic therapeutic mechanism. Anti-Ang2 therapy also prevented VEGF-stimulated neovascularization in a rat corneal model of angiogenesis. These results imply that specific Ang2 inhibition may represent an effective antiangiogenic strategy for treating patients with solid tumors.     

Role of mTOR in solid tumor systems: a therapeutical target against primary tumor growth, metastases, and angiogenesis

The mammalian target of rapamycin (mTOR) is a controller of cell growth with multiple effects on cancer development and progression. Being closely linked to key oncogenic pathways that regulate tumor cell growth and cell cycle progression, mTOR integrates the cellular response to mitogenic and growth stimuli. Rapamycin and its analogs temsirolimus and everolimus are specific inhibitors of mTOR that exert suppressive effects on proliferation, invasion, and metastasis and induce apoptosis of tumor cells. Apart from the direct effects of mTOR inhibitors on tumor cells, rapamycin and its analogs have potent antiangiogenic properties related to the suppression of vascular endothelial growth factor signal transduction. While the use of mTOR inhibitors as a monotherapy seems to be insufficient to effectively control tumor progression in most tumor entities, combination with tyrosine kinase inhibitors or cytotoxic agents might potentiate the antitumoral effects of mTOR inhibition. In a clinical setting, mTOR inhibitors show an acceptable safety profile over a wide dose range. Currently, mTOR inhibitors are tested in multiple trials in combination with other agents in various cancer entities in intermittent schedules to avoid immunosuppression. However, lacking adequate surrogate and response parameters, the most effective biological dosing schedules remain to be defined. Considering these apparent limitations, the full clinical potential of this promising class of drugs is at risk to be missed by applying them inadequately.     

Mechanisms and future directions for angiogenesis-based cancer therapies.

Targeting angiogenesis represents a new strategy for the development of anticancer therapies. New targets derived from proliferating endothelial cells may be useful in developing anticancer drugs that prolong or stabilize the progression of tumors with minimal systemic toxicities. These drugs may also be used as novel imaging and radiommunotherapeutic agents in cancer therapy. In this review, the mechanisms and control of angiogenesis are discussed. Genetic and proteomic approaches to defining new potential targets on tumor vasculature are then summarized, followed by discussion of possible antiangiogenic treatments that may be derived from these targets and current clinical trials. Such strategies involve the use of endogenous antiangiogenic agents, chemotherapy, gene therapy, antiangiogenic radioligands, immunotherapy, and endothelial cell-based therapies. The potential biologic end points, toxicities, and resistance mechanisms to antiangiogenic agents must be considered as these therapies enter clinical trials.     

Metronomic chemotherapy: an antiangiogenic scheduling

Conventional cytotoxic anticancer chemotherapeutic drugs were developed with the intent of treating cancer by direct killing or inhibition of growth of cycling tumour cells. Recently, however, there has been considerable interest in the notion of exploiting such drugs as angiogenesis inhibitors. The rationale is based on the fact that virtually all classes of cancer chemotherapeutic drugs are designed to damage DNA or disrupt microtubules of dividing cells, and endothelial cell division takes place during new blood vessel formation, including tumour angiogenesis. The results of recent experimental studies have suggested that frequent administration of certain cytotoxic agents at low doses, known as "metronomic chemotherapy", increases the putative antiangiogenic activity of certain drugs. Metronomic chemotherapy refers to the chronic administration of comparatively low doses of cytotoxic drugs at close, regular intervals, with no prolonged drug-free interruptions. The advantage of this strategy is lower toxicity and risk of emergence of drug-resistant tumour cells than conventional administration. This review describes the possible antiangiogenesis basis of this therapeutic strategy, the experimental studies published and the recent clinical studies that explore this less toxic schedule.     

TEM8/ANTXR1 blockade inhibits pathological angiogenesis and potentiates tumoricidal responses against multiple cancer types

Current antiangiogenic agents used to treat cancer only partially inhibit neovascularization and cause normal tissue toxicities, fueling the need to identify therapeutic agents that are more selective for pathological angiogenesis. Tumor endothelial marker 8 (TEM8), also known as anthrax toxin receptor 1 (ANTXR1), is a highly conserved cell-surface protein overexpressed on tumor-infiltrating vasculature. Here we show that genetic disruption of Tem8 results in impaired growth of human tumor xenografts of diverse origin including melanoma, breast, colon, and lung cancer. Furthermore, antibodies developed against the TEM8 extracellular domain blocked anthrax intoxication, inhibited tumor-induced angiogenesis, displayed broad antitumor activity, and augmented the activity of clinically approved anticancer agents without added toxicity. Thus, TEM8 targeting may allow selective inhibition of pathological angiogenesis.     

Angiogenesis and lung cancer: prognostic and therapeutic implications.

Lung cancer is the most common cause of cancer death worldwide, with most patients dying with metastatic disease. The prognosis for the majority of patients remains poor. It is evident that advances in the treatment of this and other tumor types will require new approaches, and recent research has focused on molecular-targeted therapies. A key therapeutic strategy is inhibition of specific processes essential for tumor vascular development (a concept known to be beneficial in colorectal cancer) and a range of such antiangiogenic agents are currently in development. The most promising of these target the proangiogenic vascular endothelial growth factor (VEGF), either by preventing VEGF-receptor binding or inhibiting downstream receptor signaling. However, other more direct approaches against tumor vasculature are also in development. Since antiangiogenic agents often exert an indirect, cytostatic effect, many are being evaluated in combination with conventional chemotherapies in order to optimize the anticancer effects of both strategies. Additionally, the combination of several antiangiogenic agents is also being explored. This has become possible given the large number of agents currently available. As part of this evaluation process, the assessment of surrogate markers of target inhibition and treatment effect is ongoing in the hope of identifying reliable surrogate markers to aid the development of this new generation of anticancer agents.     

Hypoxia-inducible factor: Achilles' heel of antiangiogenic cancer therapy (review).

Could a rational, hypothesis-driven and well-tolerated therapy drive tumor progression? This scenario can be foreseen for antiangiogenic therapy, despite it is one of the most elegant anticancer strategies. Antiangiogenic agents inhibit growth of endothelial cells resulting in tumor hypoxia and starvation which in turn inhibit tumor growth. On the other hand, it is known that hypoxia selects for a highly aggressive and metastatic cancer and is associated with unfavorable prognosis. This review attempts to reconcile these opposite notions and to revisit the thesis that antiangiogenic therapy is "resistant to resistance". The latter logical paradigm is based on the notion that endothelial cells cannot become drug resistant. Although endothelial cells may not acquire drug-resistance, cancer cells can acquire hypoxia-resistance which is also associated with the resistance to growth arrest and apoptosis as well as high metastatic potentials. Hypoxia-inducible factor (HIF-1) renders cells capable of surviving hypoxia and stimulating endothelial growth. Disruption of the HIF-1 pathway inhibits tumor growth, indicating HIF-1 as a potential anticancer target. Furthermore, inhibition of HIF-1 is a mechanism-based antiangiogenic strategy because it is the HIF-mediated response that drives tumor angiogenesis. Pharmacological approaches to HIF-1 inhibition are discussed.