Possible Mechanisms of Acquired Resistance to Anti-angiogenic Drugs: Implications for the Use of Combination Therapy Approaches

The ultimate target of anti-angiogenic drugs is the genetically stable, activated endothelial cell of a newly forming tumor blood vessel, rather than the genetically unstable tumor cell population per se. This led to the notion that acquired resistance to such drugs may not develop as readily, if at all. While there is some evidence that this lack of resistance development may be the case for some direct-acting angiogenesis inhibitors, it is becoming apparent that resistance can develop over time to many types of angiogenesis inhibitors including, possibly, some direct inhibitors, especially when used as monotherapies. Possible mechanisms for such acquired or induced resistance include: (i) redundancy of pro-angiogenic growth factors when the drug used targets a single such growth factor or its cognate endothelial cell-associated receptor tyrosine kinase; (ii) the anti-apoptotic/pro-survival function of growth factors such as VEGF, which, in high local concentrations, can antagonize the pro-apoptotic effects of various angiogenesis inhibitors; (iii) epigenetic, transient upregulation, or induction, of various anti-apoptotic effector molecules in host-endothelial cells; and (iv) heterogeneous vascular dependence of tumor cell populations. It is suggested that long-term disease control with anti-angiogenic drugs can be best achieved by judicious combination therapy. In this regard, the great molecular diversity of anti-angiogenic drug targets, in contrast to chemotherapy, makes this a particularly attractive therapeutic option, especially when approved, commercially available drugs considered to have anti-angiogenic effects are used in such combination treatment strategies.     

Endogenous Regulators of Angiogenesis – Emphasis on Proteins with Thrombospondin – Type I Motifs

Angiogenesis has been acknowledged as an important requirement for growth and metastasis of tumors. Complete or partial suppression of vascular growth by a number of different strategies has been consistently associated with suppression of tumor expansion and even reduction of tumor burden. Consequently, identification of the molecular pathways of the angiogenic response has been a major focus of interest in academia and industry. The development of tumor-specific anti-angiogenic therapy was also catalyzed by the finding that inhibitors of angiogenesis appeared immune to the development of drug resistance by the tumor cells, a major restrain in current chemotherapy. Although the full identification of players and their cross-talk is still at its infancy, it appears that partial blockade of one of the steps in the angiogenesis cascade, is sufficient to affect capillary morphogenesis. Thus, suppression of specific integrin pathways or vascular endothelial growth factor signaling have been shown effective in the suppression of tumor-mediated angiogenesis and have led to subsequent initiation of clinical trials.In addition to the generation of antibodies or chemical mimetics to interfere with particular steps during vascular organization, several endogenous (or physiological) molecules have also been identified. The list of endogenous modulators of angiogenesis is growing and can offer additional and important tool for the generation of therapies to restrain tumor vascularization. This review will focus one group of such molecules which include the thrombospondins and metallospondins, two families of proteins linked by the presence of a conserved anti-angiogenic functional domain.     

Interferon-mediated anti-angiogenic therapy for neuroblastoma

Angiogenesis appears to be a fundamental requirement for tumor growth, invasion and metastasis. Evidence also exists to suggest that inhibition of tumor-associated angiogenesis can retard tumor growth and prevent tumor spread. Several naturally occurring angiogenesis inhibitors have been identified, including type I interferons (alpha/beta). These proteins are potent inhibitors of angiogenesis and may also have direct anti-tumor and immunomodulatory effects. Because anti-angiogenic therapy is likely cytostatic, long-term delivery of angiogenesis inhibitors may be required for the successful treatment of cancer. We have, therefore, explored the utility of a gene therapy-mediated approach for the delivery of interferon-beta and tested this approach, both alone and in combination with conventional chemotherapy, in murine models of neuroblastoma.     

Molecular diagnosis of tumor angiogenesis and anti-angiogenic cancer therapy

Angiogenesis is regulated by the balance of pro-angiogenic factors and angiogenesis inhibitors, and the imbalance of these regulators is the cause of pathological angiogenesis, including tumor angiogenesis. Angiogenesis is required for tumor growth and metastasis, and thus constitutes an important target for the control of tumor progression. While the benefit of anti-angiogenic therapy is potentially profound, limitations have also been recognized by the results obtained thus far by clinical trials. Precise understanding of the process of angiogenesis should lead us to new regimens for more efficient anti-angiogenic therapy. This review focuses on our current understanding of the molecular mechanism of tumor angiogenic and the status of the anti-angiogenesis approach for cancer treatment.     

ANGPT-TIE信号通路与肿瘤关系的研究进展

目的:总结ANGPT-TIE信号通路与肿瘤之间关系的进展.方法:应用PubMed、Science Direct、Springer及CNKI期刊全文数据库,以“Angiopoietin、Tie2、实体瘤、炎症和转移”为检索词,检索2000-01-2011-10的相关文献,共检索到英文文献287篇,中文文献122篇.文献纳入标准:1)Angiopoietin及Tie的生物学特征及作用机制;2)Angiopoietin、Tie与血管发生、炎症及肿瘤转移、预后之间的关系.根据纳入标准,将符合条件的24篇文献纳入分析,进行综述.结果:ANGPT-TIE信号系统在血管形成中起关键作用,它不仅对血管内环境稳定和血管成熟至关重要,而且也是血管发生和炎症通路中的一个主要环节,参与肿瘤的发生发展.结论:ANGPT-TIE信号系统在肿瘤诱导的血管发生中起重要作用,并且通过阻断该信号通路也可达到抑制肿瘤生长的作用,是抗血管生成治疗最有效的靶点之一.     

Recent advances in anti-angiogenic therapy of cancer

Since angiogenesis is critical for tumor growth and metastasis, anti-angiogenic treatment is a highly promising therapeutic approach. Thus, for over last couple of decades, there has been a robust activity aimed towards the discovery of angiogenesis inhibitors. More than forty anti-angiogenic drugs are being tested in clinical trials all over the world. This review discusses agents that have approved by the FDA and are currently in use for treating patients either as single-agents or in combination with other chemotherapeutic agents.     

小分子抗血管生成药物在非小细胞肺癌中的研究进展

肺癌的发病率和死亡率均是全球恶性肿瘤之最,目前对于晚期非小细胞肺癌的主要治疗方法有化疗、靶向治疗、免疫治疗、抗血管生成治疗及放疗。新生血管形成不仅可以促进肿瘤生长,还可协助转移、侵袭,因此目前抗血管生成药物治疗成为晚期肺癌的研究热点。VEGFR是诱导血管形成的关键因子,在包括肺癌的多种肿瘤中过度表达,抑制VEGFR不仅可以使肿瘤血管正常化还可以抑制新生血管形成,从而更好的发挥作用。VEGF抑制剂包括:抗VEGF抗体,可溶性VEGFR,抗VEGF受体抗体,小分子酪氨酸激酶抑制剂（TKIs）。目前主要用于晚期非小细胞肺癌研究的小分子药物有安罗替尼、阿帕替尼、呋喹替尼、尼达尼布、法米替尼,而且有研究表明抗血管生成药物与化疗、靶向药物、免疫抑制剂等药物联用可以增强治疗效果。因此本文主要对抗血管生成药物的作用机制,与其他药物联用的理论基础及小分子抗血管生成药物在晚期非小细胞肺癌中的研究进展进行综述。     

Cabozantinib: an Active Novel Multikinase Inhibitor in Renal Cell Carcinoma

Clear cell renal cell carcinoma (RCC) is characterized by inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene. VHL loss drives tumor angiogenesis and accounts for the clinical activity of VEGF receptor (VEGFR) tyrosine kinase inhibitors (TKIs), the first-line standard of care for advanced RCC. Within the last year, three new second-line treatments have received FDA approval for use after anti-angiogenic therapy: the immune checkpoint inhibitor nivolumab, the TKI cabozantinib, and the combination of the TKI lenvatinib and the mTOR inhibitor everolimus. Cabozantinib inhibits VEGFRs, MET, and AXL, kinases that promote tumorigenesis, angiogenesis, metastasis, and drug resistance. Compared with everolimus, cabozantinib has shown statistically significant improvements in the three key efficacy endpoints of overall survival, progression-free survival, and objective response rate in patients with RCC who were previously treated with a VEGFR TKI. Herein, we summarize the translational research and clinical development that led to approval of cabozantinib as second-line therapy in RCC.     

Anti-angiogenic strategies for cancer therapy (Review)

Acquired drug resistance to chemotherapy is a major problem in the treatment of cancer. After primary surgical intervention, followed by chemotherapy treatments, the majority of patients develop disease recurrence. This is due to tumor cell heterogeneity and genetic instability. In contrast to tumor cells, proliferating host endothelial cells (ECs) are genetically stable and have a low mutational rate. Furthermore, tumor angiogenesis plays an important role in tumor development, vascular invasion and hematogenous metastasis. Thus, anti-angiogenic therapy directed against tumor ECs should, in principle, improve the efficacy of antitumor therapy by inducing little or no drug resistance. We review different therapeutic approaches directed against tumor angiogenesis, showing potent antitumor activity in vitro and in vivo. These strategies involve the inhibition of the expression of proangiogenic molecules, as well as the overexpression of anti-angiogenic molecules by either injection of recombinant proteins or transfer of genes encoding anti-angiogenic molecules. The gene therapy approach based on the gene-directed enzyme prodrug therapy (GDEPT) system, as well as the use of endothelial progenitor cells (EPCs) as angiogenesis-selective gene-targeting vectors, will be further discussed.     

Characterization of the effects of antiangiogenic agents on tumor pathophysiology

A variety of strategies have been proposed to control tumor growth and metastasis by inhibiting tumor angiogenesis. To optimally combine such antiangiogenic approaches with conventional therapy, improved methods are needed to characterize the underlying pathophysiologic changes. The objective of the current work was to demonstrate the utility of a combination of recently developed immunohistochemical and image analysis techniques in quantitating changes in tumor vasculature and hypoxia. Murine MCa-35 mammary carcinomas were frozen after administration of two COX-2 inhibitors: meloxicam and celecoxib (Celebrex). Total blood vessels were visualized using anti-CD31 staining, perfused vessels by intravenous injection of DiOC7, and tumor hypoxia by EF5 uptake. Although both agents produced similar reductions in tumor volume compared with untreated tumors, varied effects on tumor vasculature and hypoxia were noted. Meloxicam reduced total vessel numbers significantly, whereas celecoxib had no effect. Both drugs substantially increased perfused vessel densities. Although mean hypoxic marker uptake was unchanged from matched controls, intratumor EF5 heterogeneities were significantly different between drugs. The results suggest that COX-2 inhibitors can have varying effects on tumor pathophysiology. Successful use of these drugs to enhance radiation response will likely require optimization of drug choice, dose schedule, and direct physiologic monitoring.     

Inhibition of Tie-2 signaling induces endothelial cell apoptosis, decreases Akt signaling, and induces endothelial cell expression of the endogenous anti-angiogenic molecule, thrombospondin-1

Small molecule inhibitors of endothelial cell specific tyrosine kinases are currently under investigation as potential means to block tumor angiogenesis. We have investigated the utility of blocking Tie-2 signaling in endothelial cells as a potential anti-angiogenic strategy. We have found that interruption of Tie-2 signaling either via RNAi or overexpression of a kinase-dead Tie-2 led to loss of endothelial cell viability, even in the presence of serum. Mechanistically, this is linked to a block in Akt signaling and increased thrombospondin expression. Thrombospondins are endogenous anti-angiogenic matricellular proteins known to regulate tumor growth and angiogenesis. We observed that both Tie-2 and subsequent PI3Kinase signaling regulates thrombospondin-1 expression. These data have lead to the model that Angiopoietin signaling through Tie-2 activates PI3Kinase/Akt, which represses thrombospondin expression. Thus, targeting Tie-2 with small molecules maybe efficacious as an anti-angiogenic therapy.     

Angiogenesis in cutaneous malignant melanoma and potential therapeutic strategies

Metastatic melanoma (MM) carries a dismal prognosis, as it is largely resistant to conventional cytotoxic chemotherapy, biochemotherapy and immunotherapy. There is, therefore, a pressing need to identify new, effective treatments to improve outcomes from MM. Innovative approaches in oncology drug development include anti-angiogenic strategies, in the form of monoclonal antibodies and small-molecule kinase inhibitors. In this review we aim to present current concepts and controversies surrounding the role of angiogenesis and anti-angiogenic therapies in MM, alluding to other tumor types in which increasing knowledge may supply avenues for future directions in melanoma research and management. An overview of angiogenesis and its importance in melanoma progression is presented, highlighting the key molecules that represent potential therapeutic targets. The results of using anti-angiogenic strategies in preclinical and clinical trials are discussed and future perspectives for anti-angiogenic therapies in MM are considered.     

Endostatin and anastellin inhibit distinct aspects of the angiogenic process

Background

Endostatin and anastellin, fragments of collagen type XVIII and fibronectin, respectively, belong to a family of endogenous inhibitors of angiogenesis which inhibit tumor growth and metastasis in a number of mouse models of human cancer. The mechanism of action of these inhibitors is not well understood, but they have great potential usefulness as non-toxic long-term therapy for cancer treatment.

Methods

In this study, we compare the anti-angiogenic properties of endostatin and anastellin using cell proliferation and transwell migration assays.

Results

Anastellin but not endostatin completely inhibited human dermal microvessel endothelial cell proliferation in response to serum stimulation. Both anastellin and endostatin additively inhibited endothelial cell migration in response to VEGF. Anastellin but not endostatin lowered basal levels of active ERK.

Conclusion

These data indicate that anastellin and endostatin exert their anti-angiogenic effects by modulating distinct steps in the angiogenic pathway and suggest that matrix-derived inhibitors of angiogenesis may exhibit higher efficacy when used in combination.     

Angiogenesis and anti-angiogenesis in hepatocellular carcinoma

Experimental and clinical data indicate that in human hepatocellular carcinoma (HCC) tumor progression is associated with angiogenesis and that an increase in microvascular density is associated with a poor prognosis. This review summarizes the literature concerning the relationship between angiogenesis and progression in HCC. It is becoming increasingly evident that agents which interfere with blood vessel formation also block tumor progression. Accordingly, anti-angiogenic tumor therapy has gained much interest in preclinical and clinical assessments. The recent applications of anti-angiogenic agents which interfere or block HCC progression are reviewed.     

Tumor inflammatory angiogenesis and its chemoprevention

The importance of angiogenesis for the growth of tumors is widely recognized. Drugs that successfully target the endothelium, such as antivascular endothelial growth factor antibodies, are beginning to have an effect on the life expectancy of cancer patients. However, the endothelial cell is not the only possible target for antiangiogenic therapy or prevention of vascularization (angioprevention). It is evident from the literature that native immune cells recruited into tumors in turn stimulate the endothelium and are responsible for an indirect pathway of tumor vascularization. Inflammation-dependent angiogenesis seems to be a central force in tumor growth and expansion, a concept supported by the observation that the use of "classic" anti-inflammatory drugs, such as nonsteroidal anti-inflammatory drugs, leads to angiogenesis inhibition. The mechanisms of inflammatory angiogenesis provide new approaches to target, cure, or even better, prevent tumor angiogenesis by treatment with synthetic or natural agents with anti-inflammatory properties. We propose chemoprevention of inflammatory angiogenesis as a way of checking the cancer before it progresses.     

Molecular targeted therapies in hepatocellular carcinoma

In vivo tumor progression requires the supply of oxygen and nutrition by neovasculature. Hepatocellular carcinoma (HCC) is one of the typical tumors with neovascularization, and the dramatic alteration in the arterial vascularity may lead to acquisition of the potential for vascular invasiveness and metastasis. In 2008, phase III clinical trials revealed anti-angiogenic agent "sorafenib" as the first drug that demonstrated an improved overall survival in patients with advanced HCC. A new era of HCC treatment had arrived, but there has been limited further improvement in survival benefits. This review summarizes molecular targeted therapy with a focus on angiogenesis, growth signals, and mitotic abnormalities, as well as the promising concepts of "cancer stemness" and "synthetic lethality" for the strategy of targeted therapy.     

Angiogenesis,Metastasis, and Endogenous Inhibition

Angiogenesis and the development of metastases are intrinsically connected. Experimental data suggest that establishment and growth of metastases are influenced by soluble factors secreted from the originating solid tumor. Among these factors are so-called endogenous inhibitors of angiogenesis which keep metastasis in a non-proliferating quiescent state. For a number of tumors it has been shown that this dormant state is mediated through inhibition of angiogenesis. This dormant state is characterized by normal proliferation, increased apoptosis, and insufficient neovascularization. Removal of inhibiting anti-angiogenic factors led to growth of dormant metastases. A number of endogenous inhibitors have been identified and have shown success in experimental therapeutic trials. This might be of special interest for the treatment of cerebral metastases which are the most common type of malignant brain tumors. Similar to the spread of metastases, it is known that single glioma cells can be found in distant parts of the brain. While local recurrence is a common phenomenon in glioma, formation of clinical apparent distant metastasis occurs rarely. Several lines of evidence suggest that growth inhibition of remote glioma cells may be mediated by an endogenous inhibitory mechanism.     

Anti-invasion drugs

Summary Most of the pharmaceuticals in clinical practice today for treatment of breast and other cancers are cytotoxic or cytostatic inhibitors of tumor growth. While this type of drug has found its place, along with surgery and radiotherapy, in treatment of disease, the breast cancer death rate has not decreased. This appears to be the result of rising incidence, resistance to therapy, and metastasis of the disease. Since distant metastasis (usually indicated by lymph node involvement) of breast cancer is related only indirectly to tumor size, it would appear that a concerted effort should be made to discover drugs which directly interfere with this complex process. Metastasis appears to depend upon tumor cell motility, dedifferen-tiation, local invasion, and angiogenesis. Significant progress has been recently made in the creation of new animal models of metastasis and in identifying several new drugs which may be suitable for clinical inhibition of this process. This article reviews current findings on anti-invasion/metastasis drugs with a focus on breast cancer.Presented at the symposium "New Approaches in the Therapy of Breast Cancer", Georgetown University Medical Center, Washington DC, October 1994, generously supported by an education grant from Bristol-Myers Squibb.     

Anti-angiogenic therapies in cancer: achievements and open questions

The approval in 2004 of bevacizumab (Avastin), a neutralizing monoclonal antibody directed against vascular endothelial growth factor (VEGF) as the first anti-angiogenic systemic drug to treat cancer patients validated the notion introduced 33 years earlier by Dr. Judah Folkman, that inhibition of tumor angiogenesis might be a valid approach to control tumor growth. Anti-angiogenic therapy was greeted in the clinic a major step forward in cancer treatment. At the same time this success recently boosted the field to the quest for new anti-angiogenic targets and drugs. In spite of this success, however, some old questions in the field have remained unanswered and new ones have emerged. They include the identification for surrogate markers of angiogenesis and anti-angiogenesis, the understanding about how anti-angiogenic therapy and chemotherapy synergize, the characterization of the biological consequences of sustained suppression of angiogenesis on tumor biology and normal tissue homeostasis, and the mechanisms of tumor escape from anti-angiogenesis. In this review we summarize some of these outstanding questions, and highlight future challenges in clinical, translational and experimental research in anti-angiogenic therapy that need to be addressed in order to improve current treatments and to design new drugs.     

血管正常化提高肿瘤治疗疗效

抗血管生成治疗作为肿瘤靶向治疗方式已得到临床广泛应用。抗血管生成治疗诱导的血管正常化效应机制,可以逆转肿瘤内部结构和功能异常的新生血管,改善肿瘤微环境的组织间隙高压、低氧和酸中毒,提高放化疗及免疫治疗的疗效。