Synopsis of Angiogenesis Inhibitors in Oncology

OncologyONCOLOGY Vol 16 No 5
Volume 16
Issue 5

Angiogenesis is a dynamic process essential for primary tumor growth and metastases. New insights into the basic understanding of the biologic processes responsible for angiogenesis have led to the characterization of potential therapeutic targets. Several strategies for the development of antiangiogenic therapeutic modalities have been employed, including agents that (1) decrease the activity of specific angiogenic factors, (2) decrease the activity of endothelial survival factors, (3) increase the activity of naturally occurring antiangiogenic agents, or (4) indirectly downregulate angiogenic and survival factor activity.

ABSTRACT: Angiogenesis is a dynamic process essential for primary tumor growth and metastases. New insights into the basic understanding of the biologic processes responsible for angiogenesis have led to the characterization of potential therapeutic targets. Several strategies for the development of antiangiogenic therapeutic modalities have been employed, including agents that (1) decrease the activity of specific angiogenic factors, (2) decrease the activity of endothelial survival factors, (3) increase the activity of naturally occurring antiangiogenic agents, or (4) indirectly downregulate angiogenic and survival factor activity. Because antiangiogenic therapy is unlikely to induce tumor regression, the criteria for efficacy must be evaluated by means other than the standard response criteria used to evaluate cytotoxic chemotherapy. Further, the redundancy of molecules responsible for the angiogenic process suggests it is unlikely that a single antiangiogenic agent will provide prolonged inhibition of angiogenesis. Nevertheless, the understanding of the basic principles that drive tumor angiogenesis will lead to the development of therapies that will likely prolong survival without the toxicity associated with standard chemotherapy. [ONCOLOGY 16(Suppl 4):14-22, 2002]

By definition, angiogenesis is the establishment of aneovascular blood supply derived from preexisting blood vessels, whereasvasculogenesis is the embryonic establishment of a blood supply from mesodermalprecursors such as angioblasts or hemangioblasts. Tumor angiogenesis moreaccurately refers to a combination of angiogenesis and vasculogenesis in whichthe main blood supply to a tumor is derived from preexisting blood vessels,although circulating endothelial cell precursors may contribute to the growingendothelial cell mass.

Numerous investigators have established the association of tumor angiogenesiswith metastasis.[1] Indeed, it is thought that tumor angiogenesis is essentialfor the growth of both primary and metastatic tumors,[2,3] and provides bothnutrients and oxygen to the growing tumor mass. A neovascular blood supply isalso essential for increasing the chance that tumor cells will gain access tothe circulation and subsequently begin the process of forming metastases atdifferent sites. Once a tumor establishes an invasive phenotype in the organ ofmetastasis, it must then establish its own neovascular blood supply in order togrow.

This process, more complex than was previously thought, requires thecoordinated activities of multiple factors and cell types. For tumors to developa neovascular blood supply, tumor and host cells must secrete proangiogenicfactors that offset the activities of inhibitory angiogenic factors. Inaddition, the newly derived tumor endothelium must respond to and survive in arelatively caustic microenvironment; thus, endothelial cell-survival factors areessential in the maintenance of this neovasculature. Nevertheless, because theprocess of angiogenesis is regulated by redundant factors and pathways,inhibition of any single pathway is unlikely to lead to prolonged response inmost patients with solid malignancies.

More than 1,700 papers were published on aspects of tumor angiogenesis in2001. This field of research is closely scrutinized by scientists, clinicians,patients, and the media. However, data from phase I and II antiangiogenic trialshave only been reported in abstract form; most of the data is too preliminary todraw meaningful conclusions. Further, phase III trials, even if they havereached their target accrual, are several years away from maturity withappropriate follow-up. The published reports available on clinical trials havethus far produced little more than information on the toxicity and tolerabilityof angiogenesis inhibitors.

Given the complexity of angiogenesis, the basic biology of this process mustbe better understood before effective antiangiogenic therapy can be developed.Herein, we review recent advances in the basic understanding of angiogenesis andthe role of angiogenic factors in tumorigenesis. Further, we will discussoverall strategies, expectations, and future directions of antiangiogenesistherapy.

The Angiogenic Switch in Tumor Progression

Under normal physiologic conditions, the activity of endogenouspro-angiogenic factors equals that of antiangiogenic factors, leading to ahomeostatic balance that prevents the uncontrolled growth of tissues. Pathologicangiogenesis occurs when the effect of stimulatory molecules outweighs theeffect of inhibitory molecules (Table 1).[4] Intensive study of the angiogenicprocess led to the realization that this process involves more than simpleproliferation of endothelial cells. This process also requires endothelial cellsto divide, invade the basement membrane, migrate, and undergo differentiationand capillary-tube formation (Figure 1).[4] This process is driven not only byangiogenic molecules, but also by other factors, such as degradative enzymes,that mediate the above processes. Interestingly, the processes of tumorangiogenesis (as noted above) and the processes of tumor-cell invasion are verysimilar.

Vascular Endothelial Growth Factor

The best characterized of the stimulatory angiogenic factors is vascularendothelial growth factor (VEGF), which has also been associated with anaggressive phenotype in numerous solid malignancies.[5-10] Vascular endothelialgrowth factor is a 32- to 44-kDa protein secreted by nearly all cells.[4] Atleast four isoforms of VEGF, derived from alternate splicing of the mRNA, havebeen characterized.[4,11] The smaller isoforms, VEGF-121 and VEGF-165 (thenumbers denote the number of amino acids), are secreted from cells. The largerisoforms, VEGF-189 and VEGF-205, are cell associated, and their functions arecurrently being investigated.

One distinguishing factor of VEGF is its ability to induce vascularpermeability. In fact, this factor was originally named vascular permeabilityfactor (VPF) and was subsequently found to be homologous to VEGF.[12-14] Theextent of vascular permeability induced by VEGF is 50,000 times that ofhistamine, which was historically the gold standard for induction ofpermeability. This action by VEGF allows proteins to diffuse into theinterstitium and to form the lattice network onto which endothelial cellsmigrate.

In the past, it was believed that receptors for VEGF were expressedpredominantly on endothelial cells. Recently, the VEGF receptors have also beenfound on cells of neural origin, Kaposi’s sarcoma cells, hematopoieticprecursor cells, certain leukemias, and selected epithelial tumors.[15,16] Thecurrent nomenclature for the three known VEGF receptors is VEGFR-1(Flt-1),VEGFR-2 (KDR/Flk-1), and VEGFR-3 (Flt-4). These tyrosine kinase receptorsrequire dimerization to induce intracellular signaling following specific ligandbinding. The receptors for VEGF may mediate distinct functions within theendothelial cell. For example, VEGFR-1 may be important in migration, whereasVEGFR-2 may be important in the induction of permeability, endothelial cellproliferation, and survival. Neuropilin, a receptor involved in neuronalguidance, has been identified as a coreceptor for VEGF-165 and may enhanceangiogenesis.

Recently, the angiopoietin family of ligands has been found to play animportant role in the homeostasis of the tumor vasculature. The angiopoietinsare proteins involved in angiogenesis that bind to the endothelial-cell-specifictyrosine kinase receptor Tie-2. Angiopoietin-1 (Ang-1) acts as an agonist and isinvolved in endothelial-cell differentiation and stabilization.[17] In contrast,Ang-2 binds to Tie-2 and blocks the binding of Ang-1 to this receptor.[18,19]This blockade leads to endothelial-cell destabilization and vascularregression.[20]

Angiogeneis Hypotheses

It has been hypothesized that tumor angiogenesis involves the co-option ofpreexisting blood vessels in addition to vascular regression and subsequentneovascularization.[20] This theory suggests that tumors initially co-optexisting blood vessels within an organ for their nutrient blood supply. Shortlythereafter, the existing vasculature becomes destabilized, most likely throughthe release of Ang-2 by endothelial cells. This loss of vascular integrity leadsto relative hypoxia within the tumor, which in turn leads to upregulation ofVEGF in the tumor cells. These events then lead to a robust angiogenic response.At that stage, the newly developed endothelial cells require stabilization,which is achieved through release of Ang-1 by endothelial cells and possiblythrough continued response to VEGF. Thus, the process of angiogenesis depends onthe temporal coordination of factors that regulate pathways in the establishmentof stable conduits that provide a nutrient blood supply to the tumor.

In vitro, Ang-1 has been shown to be angiogenic, inducing tube formation ofendothelial cells growing on extracellular matrix components. However, recent invivo studies have demonstrated that Ang-1 may in fact be antiangiogenic. We haveshown that overexpression of Ang-1 in human colon cancer cells leads todecreased angiogenesis and tumor growth, whereas overexpression of Ang-2 leadsto an increase in tumor growth and angiogenesis.[21] This finding is consistentwith immunohistochemical studies that demonstrate that colon cancers expressAng-2 but do not express Ang-1. This suggests that the imbalance of Ang-2 overAng-1 may be an initiating factor in tumor angiogenesis. Others have alsoconfirmed the above findings in breast and gastric cancer tumor cells and celllines.[22,23]

Numerous nonspecific angiogenic factors affect the growth of cell types otherthan endothelial cells. These factors include the fibroblast growth factors(acidic and basic), transforming growth factor-alpha, and epidermal growthfactor (EGF), both of which bind to the EGF receptor; platelet-derived growthfactor (PDGF); platelet-derived endothelial-cell growth factor (PD-ECGF);angiogenin; and the CXC chemokines interleukin-8, macrophage inflammatoryprotein 1, platelet factor 4, and growth-related oncogene alpha (Table1).[24]

These factors are known to be angiogenic in in vivo models but are notspecific for endothelial cells. However, as noted earlier, a single molecule orfamily of molecules does not drive angiogenesis; rather it depends on thecooperation and integration of various factors leading to endothelial cellproliferation, migration, invasion, differentiation, and capillary-tubeformation. It has yet to be determined whether inhibiting the activity of asingle angiogenic factor will lead to vascular compromise of significantduration. More likely, the redundancy in the angiogenic process will select forother angiogenic factors when a specific angiogenic factor is targeted.

Upstream Regulation of Angiogenic Factor Expression in Tumors

Tumors may constitutively express high levels of angiogenic factors or mayexpress high levels of angiogenic factors in response to the tumormicroenvironment. Signals that upregulate angiogenic factors includeextracellular signals, intrinsic upregulation of signal transduction activity,and loss of tumor suppressor genes (Table 2).

Extracellular Signals

Extracellular signals that lead to the induction of angiogenic factorexpression include environmental stimuli such as hypoxia or a decrease inpH.[25-27] In fact, hypoxia is the most potent stimulus for inducing angiogenicfactors, especially VEGF. Hypoxic induction of VEGF is probably mediated throughSrc kinase activity, which then leads to downstream induction of signalingcascades and eventually to an increase in the activity of hypoxia-induciblefactor-1(HIF-1) alpha.[28,29] This factor then increases the transcription ofthe VEGF gene, which in turn leads to the induction of angiogenesis. Recentevidence suggests that activation of growth factor receptors may also increaseHIF-1 alpha activity.[30]

Cyclooxygenase-2 is an enzyme constitutively overexpressed in colon cancerand other solid malignancies.[31] Its overexpression may play a role inmalignant cell survival. In addition, elegant studies from Dubois and othershave demonstrated that COX-2 can regulate VEGF expression andangiogenesis.[31-33] Thus, COX-2 inhibitors may provide a means of indirectlyinhibiting angiogenesis with minimal toxicity.

Several studies have shown that activation of the EGF receptor (EGF-R) canlead to induction of angiogenic factors in tumor cells.[34-36] In orthotopicmodels of bladder and pancreatic cancers, treatment of mice with an anti-EGF-Rantibody led to a decrease in VEGF and interleukin-8 expression that wasassociated with a decrease in tumor growth and vascularity.[36,37] Othercytokines and growth factors such as insulin growth factors (IGF)-I and -II,hepatocyte growth factor, interleukin-1, and platelet-derived growth factor haveall been shown to upregulate VEGF. Thus, antiangiogenic therapy could involvedownregulation of upstream mediators of the angiogenic factors rather thantargeting the angiogenic factors themselves.[28,38]

Intrinsic Upregulation of Signal Transduction

Once a growth factor or a cytokine binds to its receptor, a cascade ofintracellular signaling events is initiated. Two specific signal transductionpathways are well known to mediate the upregulation of angiogenic factors: thephosphatidylinositol 3 (PI3)-kinase/Akt signal transduction pathway, whicheventually leads to stabilization of HIF-1 alpha,[39,40] and themitogen-activated protein kinase (MAPK) pathway, in which phosphorylation ofErk-1/2 activates factors that increase transcription of the VEGF gene.[41]Activated ras and Src have also been shown in in vivo models to be associatedwith increased VEGF production and angiogenesis.[42] Again, therapeuticstrategies that target the upstream effector molecules in angiogenesis may be arational means of preventing angiogenesis. Indeed, inhibitors of signaltransduction molecules have been shown to inhibit angiogenesis in in vivo tumormodels.[28]

Loss of Tumor Suppressor Genes

Protein products of tumor suppressor genes such as the von Hippel-Lindau (VHL)or p53 genes also regulate angiogenesis. The wild-type VHL protein repressestranscriptional regulation of the VEGF gene by facilitating degradation ofHIF-1.[43-45] A loss of heterozygosity with a mutation in the remaining VHLallele leads to loss of transcriptional control of the VEGF gene andoverexpression of VEGF. Mutant p53 has also been associated with an increase inangiogenesis.[46] Reinsertion of the wild-type p53 gene into cells with mutantp53 can downregulate VEGF expression and angiogenesis. Thus, the process ofangiogenesis is driven by external forces (including environmental stimuli),aberrations in internal signaling, and alterations in tumor suppressor genefunction.

Effective Antiangiogenic Therapy

Overall Expectations

The knowledge that angiogenesis is essential for tumor growth and theformation of metastases has led to a large research effort in an attempt todiscover effective antiangiogenesis compounds. However, angiogenesis not only isa pathologic process but also is essential for homeostasis. Physiologicangiogenesis is important in reproduction, wound healing, and menses, as well asa compensatory response to ischemia in coronary-artery and peripheral vasculardiseases. Thus, therapeutic efficacy of antiangiogenic therapy requires abalance where angiogenesis in tumors is inhibited without disrupting physiologicangiogenesis.

For example, controversy exists regarding the effects of antiangiogenictherapy and wound healing.[47-50] Because of the need for neovascularization inwound healing, one would expect that an effective antiangiogenic agent wouldinhibit healing similar to its antiangiogenic effect on tumor growth. However,treatment with endostatin did not significantly decrease the breaking strengthof cutaneous wounds in mice,[49] and although it decreased functional bloodvessels and matrix density in granulation tissue in another mouse model,endostatin did not significantly affect overall wound healing.[50]Interestingly, wound angiogenesis is being used as a surrogate marker of drugactivity.

In addition to potential effects of antiangiogenic therapy on homeostasis,duration of antiangiogenic therapy and criteria for efficacy are other issues tobe considered. Because most antiangiogenic therapies are intended to decreasethe development of new blood vessels, the traditional end points for tumortreatment success or failure must be redefined. For example, a desirableresponse for standard chemotherapy is a 50% decrease in the cross-sectional areaof a tumor; however, the desired end point after antiangiogenic therapy might beinhibition of further tumor growth (ie, tumor stabilization or prolongation oftime to progression). Thus, the criteria for the effectiveness of antiangiogenictherapy (whether in the clinic or in the laboratory) must be considered from anew perspective relative to conventional therapies.

Although some reports exist of tumor regression in experimental models ofangiogenesis,[51,52] such findings are rare; the vast majority of studies inthis field demonstrate that antiangiogenic therapy leads to an inhibition oftumor growth rather than a regression of established tumors.[53,54] Therefore,the ability to appropriately interpret the results from experimental models iscritical to ensure that extrapolations to the clinical setting are not fraughtwith unrealistic expectations.

For example, a typical growth curve for a subcutaneously implanted tumor maydemonstrate that antiangiogenic therapy significantly decreases tumor growthrate. In this preclinical model, this "positive" result may lead toclinical trials of that same agent. In the clinic, however, inhibition of tumorgrowth can be interpreted as "progressive disease" and the therapythus considered a failure, particularly if tumor-imaging studies are done atshort intervals. Therefore, longer periods of antiangiogenic therapyadministration may be required to fully characterize the efficacy ofantiangiogenic therapy (assessed by the inhibition of tumor growth rate andreduced metastases) compared with chemotherapy (assessed by decreases in tumorsize).

Effective antiangiogenic therapy will probably need to be delivered on achronic basis. Chronic administration will require that the agent be deliveredeasily (perhaps by the oral route) and have few cumulative long-term effects. Aspreviously noted, the effect of antiangiogenic therapy may require longerevaluation intervals. One must also consider that the goal of standardantiangiogenic therapy is intended to decrease blood vessel formation andprevent further tumor growth, not cause tumor regression. Therefore, uniformresponse criteria should be developed for determining the effectiveness ofantiangiogenic therapy (eg, time to progression, survival, quality of life);these criteria will probably differ from current criteria for tumor response tocytotoxic agents that include reductions in tumor size.

Overall Strategies

Despite the simplified view that antiangiogenic therapy simply interfereswith the blood supply to a tumor, the strategies in the development ofantiangiogenic therapies are quite diverse and distinct. Antiangiogenicstrategies can be classified under four major categories: (1) those thatdecrease the activity of specific angiogenic factors; (2) those that decreasethe activity of endothelial survival factors; (3) those that increase theactivity of naturally occurring antiangiogenic agents, such as angiostatin,endostatin, thrombospondin; and (4) those that indirectly downregulate activityof angiogenic and survival factors.

Decreased Activity of Angiogenic Factors

In the following discussion, VEGF will be the prototype molecule used todescribe strategies to decrease the activity of angiogenic factors because ithas been linked to the angiogenesis and aggressiveness of many disease types.Anti-VEGF strategies include the use of neutralizing antibodies to VEGF or itsreceptors, ribozymes targeted to receptor mRNA, and tyrosine kinase inhibitorsthat block downstream signaling. All the above-mentioned strategies have shownpromise in preclinical trials and are now in clinical development.

One of the earliest strategies used to inhibit VEGF activity involved the useof a neutralizing antibody to VEGF where the antibody is a hybrid of a variableregion that recognizes the epitope and a humanized Fc region that is notrecognized as foreign by the human host. This latter region should also interactwith human Fc-receptor-bearing effector cells and/or human complement. Asimilar strategy is utilized for anti-VEGF receptor antibodies. Antibodies mustbe delivered intravenously, although the long half-life may allow administrationonce every 2 or 3 weeks.

The other commonly used strategy for inhibiting VEGF activity is the use oftyrosine kinase inhibitors.[55] These are small molecules that prevent kinaseactivation on binding of the ligand to a tyrosine kinase receptor. Althoughthese compounds are claimed to be relatively selective for their specifictargets, these tyrosine kinase inhibitors actually do have some cross-reactivitywith other receptors, albeit requiring a much higher dose to achieve an effect.These inhibitors are delivered intravenously or orally.

Anti-VEGF and Increased Apoptosis

Studies from our laboratory have examined anti-VEGF receptor antibodies andtyrosine kinase inhibitors in mouse models of colon cancer and livermetastasis.[54,56] Interestingly, these agents demonstrated similar efficacy,leading to a decrease in hepatic tumor burden, vessel count, and proliferativeindex of the tumor cells. Surprisingly, we found an increase in the number oftumor cells undergoing apoptosis. We further investigated this phenomenon todetermine if endothelial cell apoptosis was the preemptive cause of tumor cellapoptosis. We established a technique of double-staining whereby we could firstidentify endothelial cells and then identify those endothelial cells undergoingapoptosis using the terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick-end labeling (TUNEL) assay.[54]

We found that a wave of endothelial cell apoptosis preceded a wave of tumorcells undergoing apoptosis.[56] This suggests that endothelial cell apoptosisoccurs prior to tumor cell apoptosis, demonstrating that VEGF is a survivalfactor for tumor endothelial cells and further supporting the hypothesis thatmaintenance of the integrity of the tumor vasculature is required for tumorsurvival.

Because anti-VEGF therapy leads to an increase in tumor and endothelial cellapoptosis, one would surmise that this therapy could lead to a decrease in tumorsize. There are reports of studies in subcutaneous xenograft models wheretyrosine kinase inhibitors to the VEGF receptor and other angiogenic factorreceptors can cause regression of established tumors.[55] However, in our modelof colon cancer liver metastasis, while tumor growth was inhibited, the growingcancer, albeit at a slower rate, eventually led to the demise of the animals.This is likely due to the fact that there are redundant mechanisms forangiogenesis within tumors and that antiangiogenic therapy directed at aspecific factor may lead to selection of cells whose angiogenesis is driven by adifferent factor.[3,57]

Decreased Activity of Endothelial Survival Factors

A second antiangiogenic strategy involves agents that decrease the activityof endothelial cell survival factors (Figure2).[58] Typically, angiogenesis issimply thought of as the development of a new vasculature within tumors whereendothelial cells migrate, proliferate, invade the basement membrane, anddifferentiate to form a primitive capillary network. However, the tumormicroenvironment is a caustic one with low pH and low oxygen tension. Therefore,for these fragile endothelial cells to survive, they must be exposed toendothelial cell survival factors that prevent apoptosis in these adverseconditions.

Endothelial cell survival factors include pericytes that may stabilizeendothelium, either by cell-to-cell contact or by secretion of endothelial cellsurvival factors such as VEGF or Ang-1. Vascular endothelial growth factor andAng-1 are two endothelial cell survival factors that are necessary forendothelial cell survival in the absence of pericytes.[20] These factors can besecreted by endothelial cells, tumor cells, or nonmalignant cells within themicroenvironment. Vascular endothelial growth factor has been shown to inhibitendothelial cell apoptosis by activation of various intracellular signalingproteins, including the Akt pathway, IAP, A1, and the MAPK pathway.[59]Angiopoietin-1 binds to the specific endothelial cell receptor, Tie-2, andactivates the Akt pathway, a pathway that mediates survival in many celltypes.[60]

Another very important mechanism for endothelial cell survival is the bindingof integrins located on the endothelial cell surface to the extracellularmatrix. At first, integrins were thought to be important only in cell-to-cellcontact and binding to the extracellular matrix, but it is now known thatintegrins may mediate intracellular signaling, either alone or in combinationwith other receptors.[61] The integrins alphav beta 3, alphav beta 5, alphavbeta 1 have been shown to act as survival factors for endothelial cells, anddisruption of the binding between the integrins and the extracellular matrix maylead to endothelial cell death (Figure 3).[61-63] It is likely that integrinengagement with the extracellular matrix leads to integrin aggregation andactivation of focal adhesion kinase. As a result, downstream signaling isactivated, initiating endothelial cell survival mechanisms.[64]

Specific small molecules have been developed that may inhibit integrinactivation, and antibodies have been synthesized that block integrin binding tothe extracellular matrix.[65,66] Numerous agents are in preclinical evaluationor early clinical testing.

Increased Activity of Naturally Occurring Antiangiogenic Agents

Another antiangiogenic strategy is one that increases the activity ofnaturally occurring antiangiogenic agents. These agents include thrombospondin,angiostatin, and endostatin. A great deal of publicity has surrounded thediscovery of angiostatin and endostatin, as these agents were first discoveredas fragments of larger molecules (angiostatin is a fragment of plasminogen, andendostatin is a fragment of collagen XVIII).[51,67,68] The exact mechanism bywhich these two compounds lead to a decrease in angiogenesis is not clearlyunderstood. Thrombospondin, a naturally occurring angiogenic antagonist, is alsobeing evaluated in preclinical trials.

The interferon family of proteins, although better known for otheractivities, also has antiangiogenic properties.[69-76] These cytokines,specifically interferon-alpha, were shown to cause regression oflife-threatening childhood hemangiomas in a study published in the early1990s.[72] Further investigation has demonstrated that interferon-alpha andinterferon-beta can downregulate basic fibroblast growth factor levels invarious tumor systems.[76] More recently, reports have demonstrated the efficacyof interferon-alpha in regression of tumors in children.[65] The efficacy ofinterferon therapy may be dependent on chronic low-dose therapy because higherdose therapy is often associated with intolerable side effects.

Indirect Downregulation of Angiogenic and Survival Factor Activity

The last antiangiogenic strategy is one that indirectly downregulates theactivity of angiogenic factors. Vascular endothelial growth factor and otherangiogenic factors are oftentimes unregulated in response to stress, such ashypoxia, low pH, or cytokines. Strategies that downregulate the upstreamsignaling pathways to VEGF and other angiogenic factors may indirectlydownregulate VEGF activity and angiogenesis. Our laboratory, as well as others,has demonstrated that several growth factor receptors are involved in inductionof VEGF on binding of its ligand to its receptor (EGF-R, IGF-receptor I).[36,77]Strategies to inhibit the activity of these receptors may lead to a decrease ofin vivo VEGF production and angiogenesis, which in turn leads to a decrease intumor growth.

It is also known that tumor suppressor genes, such as p53 and VHL, represstranscription of VEGF. We have shown that in a colon cancer cell line with amutated p53 gene, infection of a wild-type p53 gene can lead to downregulationof VEGF and decrease angiogenesis in vivo.[38] It is possible that anti-VEGFtherapy may be beneficial in patients with the VHL syndrome, which is almostcertainly due to overexpression of VEGF in the formation of multiple vasculartumors.[43]

Future Directions in Antiangiogenic Therapy

Most local tumors can be adequately treated by surgery and/or radiation.However, the true challenge in oncology lies in treating metastatic cancers. Thehost microenvironment plays a major role in modulating gene expression in tumorsgrowing at different sites, and this holds true for angiogenic factor expressionas well. In our laboratory, we have found that VEGF expression is actuallyhigher in primary tumors than it is in liver metastases. Therefore, it would benaive for oncologists to think that antiangiogenic activity would be equallyefficacious in different tumors growing at different sites. In addition, theendothelium is phenotypically distinct at different sites and, therefore, eachtumor may not only express different angiogenic factors, but the endothelium mayhave different angiogenic factor receptors.[78]

Selection of Appropriate Therapy

It is foreseeable that in the future we will need to obtain biopsies oftumors growing at various metastatic sites and analyze expression of variousgenes within these biopsies. The revolution of microarray technology may allowus to rapidly identify angiogenic factors that may be driving angiogenesis inspecific tumors at specific sites. At that point, we may then be able to directappropriate antiangiogenic therapy toward specific targets. It is also possiblethat continued antiangiogenic therapy against a specific target may lead toselection of clones whose angiogenesis is driven by a different tumor.Therefore, it may be important to "restage" patients with repeatbiopsy of these tumors to adequately determine the angiogenic profile of tumorswithin the course of their growth and, hopefully, response to antiangiogenicregimens. Clearly, inhibition of angiogenesis will play a substantial role inthe future of oncology.

Supported in part by National Institutes of Health training grant T3209599-08 (SA), the Gillson Longenbaugh Foundation (LME), the Jon and Suzie HallFund for Colon Cancer Research (LME), National Institutes of Health grantCA74821 (LME), the RGK Foundation (LME), the University Cancer Foundation (LME),and the National Institutes of Health Core Grant CCSG CA16672.


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