Mini Review

Current Strategies and Future Directions
of Antiangiogenic Tumor Therapy

Zhang Zi-Lai, Wang Jin-Hui, Liu Xin-Yuan*

( Institute
of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences,
the Chinese Academy of Sciences, Shanghai 200031, China)

Abstract        Neovascularization
is a prerequisite for progressive growth of most tumors and their metastases.
Therefore, inhibition of angiogenesis could be one of the most promising
strategies that might lead to the development of novel anticancer therapy. New
blood vessels forming in tumors can be avoided by interfering the process of
angiogenesis through suppressing the proangiogenic signal or augmenting the
antiangiogenic factors. Concentrated efforts in this area have lead to the
discovery of many proangiogenic factors as well as their inhibitors, and
antiangiogenic factors. For the established tumor vasculature, tumor
vasculature-targeted delivery of antiangiogenic substances and endothelial cell
vaccines has been explored. Although some antiangiogenic drugs are currently in
clinical development, for future reason, more efficient therapeutic methods,
including antiangiogenic gene therapy strategy, targeted drug delivery system,
and the combination of antiangiogenic agents with immunotherapy, chemotherapy
or radiotherapy are being explored. With the development of tumor model
assessment system, clinical use of the above antiangiogenic tumor therapy
methods could be achieved.

Key words     angiogenesis;
tumor; vasculature; antiangiogenesis tumor therapy

Angiogenesis is
a critical process required by solid tumors to support their growth. This
concept is supported by a large number of evidences, demonstrating that once
solid tumors grow to more than 1－2 mm³, further growth must be preceded by neovascularization
of the tumor. There is also increasing evidence to support the concept that
metastasis from solid tumors is facilitated by angiogenesis of the primary
tumor[1]. Consequently, a major effort has been focused on understanding the
biology of angiogenesis, the identification of mediators that normally dampen
angiogenesis, and the evaluation of strategies that employ antiangiogenesis
agents to inhibit the growth of tumors[2].

Antiangiogenic
therapies have already proved to be very powerful in a number of experimental
animal tumor models, which have shown that angiogenesis inhibitors can prevent
metastasis and shrink established experimental tumors to small dormant
microtumors. The first substances have entered clinical trials, highlighting
the fact that angiogenesis research has advanced from basic science to the
bedside[3].

In this paper,
we overview current strategies in angiogenesis-based cancer therapies field
which either target the process of forming new blood vessels or directly target
the immature neovasculature, and prospect the future of antiangiogenic tumor
therapies.

1    Current
Strategies of Inhibit Tumor Angiogenesis

The fact that
tumors are dependent on blood supply has inspired many researchers to search for
anti-angiogenic molecules and design antiangiogenic strategies for cancer
treatment. It is usual to divide therapeutic approaches based on tumor
angiogenesis into two major classes: (1) interfere with the process of forming
new blood vessels; (2) directly destroy immature neovasculature.

1.1   Interfere
with the process of forming new blood vessels

Angiogenesis, a
complex process that includes the activation, proliferation, and migration of
endothelial cells, disruption of vascular basement membranes, formation of
vascular tubes and networks, and linkage to the pre-existing vascular
networks[1], is tightly controlled by the balance of stimulators to inhibitors.
When the ratio is low, tumor angiogenesis is blocked or modest in magnitude; in
contrast, when the ratio is high, the switch is turned to the ‘on’ position[4].
So, one strategy for antiangiogenic therapy is to interfere with the process of
angiogenesis through suppressing the proangiogenic signal or augmenting the
effect of antiangiogenic factors.

1.1.1       Suppression
of the proangiogenic signal       Suppressing
proangiogenic signal by decreasing the amounts of the angiogenic mediator
available to induce tumor neovascularization or by interfering with the process
of the angiogenic mediator signaling within the endothelial cell should
potently disrupt the angiogenesis cascade[1]. There are various moleculars in
different phases of vascular growth that could act as the target.

(1) Target proteolytic enzymes involved in
basement membrane breakdown             To
initiate the formation of new capillaries, endothelial cells of existing blood
vessels must degrade the underlying basement membrane and invade into the
stroma of the neighboring tissue. These processes of endothelial cell invasion
and migration require the cooperative activity of the PA(plasminogen activator)
system and the MMPs(matrix metalloproteinases)[5].

The
uPAs(urokinase-type plasminogen activators) and tPAs(tissue plasminogen
activators) are serine proteases that convert plasminogen into plasmin. The
fibrinolytic activity in blood is mainly regulated by tPA, whereas the
activation of plasminogen in tissues is regulated by uPA. The MMPs family
consists of at least 16 members, which are expressed as latent enzymes with a
similar domain structure[6]. In preclinical studies, both PA and MMP inhibitors
have been shown to inhibit proliferation in tumor cell lines and EC(endothelial
cell) lines as well as in animal models. BAY 12-9566 and BMS-275291 are
examples of MMP inhibitors, which act selectively against MMP-2, -3 and -9[7].

(2) Target angiogenic factors involved in
endothelial cell migration and proliferation         Following
proteo-lytic degradation of the ECM, “leader” endothelial
cells start to migrate into the degraded matrix. They are followed by
proliferating endothelial cells, which are stimulated by a variety of growth
factors that can be divided into three classes[6]. The first class consists of
the vascular endthelial growth factor (VEGF) family[8] and the angiopoietins[9]
which specifically act on endothelial cells. The second class contains most
direct-acting molecules, including several cytokines, chemokines and angiogenic
enzymes that activate a broad range of target cells besides endothelial cells.
The prototype member of this group, FGF-2 (fibroblast growth factor-2)[10], was
one of the first angiogenic proteins to be characterized. The third group of
angiogenic molecules includes the indirect-acting factors whose effect on
angiogenesis results from the release of direct-acting factors from macrophages,
endothelial or tumor cells. The most extensively studied are TNF-α[11] and
TGF-β[12]. All these angiogenesis inducers have provided good targets for
antiangiogenesis therapy. Here, only the characteristics of the most prominent
angiogenic factors, such as VEGF and FGF, will be addressed.

VEGF     As one of the most
important regulators of angiogenesis, the biological effects of VEGF on vessels
are mediated by two specific receptors, KDR and flt-1. These receptors are
mainly expressed on endothelial cells in proliferating. Several VEGF
antagonists have been developed, which have lead to very promising results in
vitro and in animal models. The VEGF-antibody HuMV and the low molecular-weight
inhibitors of the VEGF2 (KDR) receptor tyrosine kinase, PTK787/ZK22254 and SU
5416, belong to the most effective substances[13,14]. The newer substances,
like SU 6668 and ZD6474, are VEGF, FGF and PDGF (platelet derived growth
factor) receptor tyrosine kinase inhibitors[15,16].

FGF        It’s
family includes more than 20 different structurally related polypeptides
characterized by high affinity binding to cellular heparan sulphates. Its first
identified two members, FGF-1 (acidic FGF) and FGF-2 (basic FGF), have been
extensively studied and have significant effects on the processes involved in
angiogenesis. Inhibition of FGFs mitogenic activity has been suggested as a
crucial target for the development of antiangiogenic cancer treatment. For
example, suramin, suradistas and their derivatives and analogues have been
discovered and currently tested in clinical trials[17].

(3) Target adhesion molecules involving in
cell-cell and cell-matrix interactionsTo initiate the angiogenic process,
endothelial cells have to dissociate from neighboring cells before they can
invade the underlying tissue. The final phases of the angiogenic process
required for lumen formation, including the construction of capillary loops and
the determination of the polarity of the endothelial cells, involve cell-cell
contacts and cell-ECM interactions[18].

Cell adhesion
molecules can be classified into four families depending on their biochemical
and structural characteristics. These families include the selectins, the
immunoglobulin supergene family, the cadherins, and the integrins. Members of
each family are implicated in neovascularization. We use integrins as an
example.

Integrins              A group of
more than 22 cell-surface glycoproteins, which are composed of α and β chains.
These mediate specific molecular interactions between vascular cells and the
extracellular matrix and are capable of recognizing the so-called RGD sequence
present in their ligands, the extracellular matrix proteins. The α_vβ₃
and α_vβ₅ integrins play a critical role in angiogenesis.
Several inhibitors are known as following. Vitaxin(r) (LM609) is a monoclonal
anti-αvβ5 antibody, Sch221153 is a small-molecular-weight peptide which
antagonizes αvβ3 and αvβ5, and EMD121974 is an αvβ3 inhibitor^[19–21].
All of these substances have shown an antiangiogenic effect in vitro and in
animal models.

1.1.2       Augmentation
the effect of antiangiogenic factors             Since
angiogenesis is the result of a dynamic balance between the proangiogenic and
antiangiogenic factors in the extracellular microenvironment of the tumor,
increasing the local concentrations of endogenous inhibitors of angiogenesis
will be another strategy of interfering with the angiogenic cascade.

(1) Target endogenous inhibitors of
angiogenesis        With
the concept of tumor suppressor genes being accepted, a large growing and
structurally diverse family of endogenous protein inhibitors of angiogenesis
has been discovered, e.g. thrombospondin-1^[22], interferon α/β^[23],
the 16 kd fragment of prolactin^[24], angiostatin^[25],
endostatin^[26], vascular endothelial cell growth inhibitor (VEGI)^[27],
vasostatin^[28], Meth-1 and Meth-2^[29], cleavage products
of platelet factor 4^[30], or anti-thrombin III^[31].

Angiostatin and endostatin     Angiostatin and
endostatin are two secreted proteins inhibiting ATP synthase and play a role in
maintaining the quiescent state of normal endothelial cells. Angiostatin is a
38-kD protein with an identical sequence to the first four kringle structures
of plasminogen[25] and it acts specifically on endothelial cells without
affecting tumor cells directly. Administration of angiostatin to tumor-bearing
mice leads to an inhibition of angiogenesis and an increasing apoptotic rate in
the tumor cells, which result in a state of tumor dormancy. Analogous to
angiostatin, endostatin is an 18-kD protein cleaved enzymatically from collagen
XVIII^[26] and has antiangiogenesis activity as angiostatin.

(2) Target oncogenes associated with tumor
angiogenesis          There
was an increasing awareness of the importance of the relationship between
cancer causing genetic changes and angiogenesis. Bouck et al.^[22] found
it was of considerable interest that loss of wide-type p53 gene function
resulted in a loss of thrombospondin expression. Not only did this finding
establish a possible critical link between the genetic basis of cancer and
tumor angiogenesis, but also it opened up the now flourishing field of
endogenous angiogenesis inhibitors. Many oncogenes, such as mutant ras^[32],
VHL and p16^[33], may also contribute to the tumor angiogenesis by
influencing the production of proangiogenic molecules such as VEGF. Thus,
treatment using drugs such as Ras farenesyltransferase inhibitors (Ras FTIs)
which block oncoprotein function could contribute to the their potential
ability of blocking angiogenesis^[34].

1.2   Destroy
immature neovasculature

In contrast to
the former antiangiogenic tumor approach which specifically interfere with the
angiogenic cascade and decrease the formation of new vessels, anti-vascular
approaches aim to cause a rapid and extensive shut-down of the established
tumor vasculature, leading to secondary tumor cell death. Tumor
vasculature-targeted therapy and endothelial cell-based therapies are two
approaches that are receiving considerable attention.

1.2.1       Tumor
vasculature-targeted therapy      The
goal of vascular targeting is to utilize specific molecular determinants of
angiogenic endothelial cells to deliver substances or activities that destroy
the vasculature. A fundamental principle of this approach is that tumor
vasculature is different from that in normal tissues, with altered endothelial
cell morphology and a decreased number of perivascular cells that help to
maintain the blood vessel integrity.

Direct
cytotoxicity against tumor vascular endothelial cells may be induced using
hybrid molecules able to functionally cross-link immune effector cells and
tumor endothelial cells. For example, BsMAbs recognizing both tumor vascular
specific epitopes and CTLs as effector cells are under development[35]. One
potential advantage of such an approach may be the possibility to create an
inflammatory response at the site of the tumor upon CTL activation and
endothelial cell killing. Blockade of the tumor blood supply and induction of
inflammatory reactions may act synergistically in reducing the tumor load.
Growth inhibiting substances may be delivered at the tumor vascular endothelial
cells as well.

The vasculature
of individual normal tissues is also highly specialized. It follows that tumor
metastasis into preferred organs is likely to be dependent on adhesive
interactions between tumor cells and organ-specific endothelial markers[36].
The tissue specific vascular markers provide new opportunities for the
targeting of therapeutic compounds, such as genes and drugs, in order to avoid
unwanted toxicity towards other tissues. For example, coupling of doxorubicin to
an av integrin-binding RGD peptide or NGF peptide yielded compounds that homed
specifically to tumors and thus had reduced side effects[37].

1.22        Endothelial
cell vaccines       Endothelial
cells have also been evaluated in vivo as a source of immunogens that could
potentially induce host responses against angiogenic endothelial cells within a
tumor. Wei et al have shown that primary xenogeneic endothelial cells can be
administrated to mice with subsequent immunoprotective effects on tumor growth
and therapy of established tumors[38].

In addition,
several reports have indicated the presence of a circulating bone marrow
derived population of CD34+ endothelial cell precursors. The isolation of CD34+
stem cells from bone marrow followed by genetic manipulation may be exploited
to deliver suicide genes or antiangiogenic factors to shrink tumors. A recent
study using herps simplex virus thymidine kinase transduced CD34+ cells
administered intravenously to primates undergoing skin grafting has shown
rejection of the graft after administration of gancyclovir[39].

2    Future
Directions

In current
clinical development, a number of antiangiogenic drugs highlight the obvious
fact that we shall soon have some kind of indication of the potential value of
this new therapeutic approach to treat cancer. This is an exciting time, but
there are a number of problems to be overcome: existence of significant intra-
and inter-species variations in the tumor cell and tumor blood vessel
microenvironment; delayed toxicity associated with long-term antiangiogenic
therapy; the difficulties associated with clinical evaluation of antiangiogenic
drug efficacy and so on. Tackling these problems presents some challenging
opportunities, and further study could even help foster a new era of cancer therapeutics
in the clinic.

2.1   Gene
therapy strategy

Gene therapy for
angiogenesis means transferring the gene encoding angiogenic protein into a
host cell to overexpressing it. It represents an interesting alternative for
the effective delivery of antiangiogenic therapy. Advantages of gene therapy
over the direct administration of the inhibitors include the localized delivery
and sustained expression of the antiangiogenic molecules, the ability to
inhibit multiple angiogenic pathways with the delivery of more than one
transgene, the generation of properly folded inhibitor molecules, and the
potential for decreased cost[40]. Moreover, the expression of an
antiangiogenesis transgene should be regulatable by using an inducible
promoter. A variety of viral (including retroviruses, lentiviruses,
adenoviruses, adeno-associated viruses) or nonviral (cationic liposomes,
microencapsulation and naked DNA) methods are being assessed, and many studies
have demonstrated that a gene therapy-based antiangiogenesis approach is an
effective means of reducing tumor growth in animal models.

However, as the
goal of antiangiogenic therapy of cancer is the long-term suppression of
neovascularization and tumor growth, there is an important challenge for
current gene transfer vectors, which either inherently provide only transient
expression (e.g., non viral vectors) or elicit host responses that conspire to
eliminate the genetically modified cells (e.g., adenovirus vectors). Some
progresses have been made to this challenge: (1) designing vectors to be more
efficient in entering the target cell and transferring genes to the nucleus,
such as modify the adenovirus fiber protein by the addition of an
integrin-binding RGD peptide[41]; (2) permanently incorporating the transgene
into the target cell genome. In this regard, the use of lentiviruses and
adeno-associated viruses might prove advantageous, especially when planning for
delivery into nondividing cells such as muscles and liver[42]; (3) designing
the vector to be stealthy with regard to detection by the hosts’ innate and
adaptive immune systems[1]; (4) using specific promoters which can lead to the
accumulation of antiangiogenic proteins in specific organs or inducible
promoters to up- and down-regulate the transgene expression.

Another problem
associated with the antiangiogenesis gene therapy is the low gene transduction
efficiencies as with common cancer gene therapy. Replication-competent virus
therapy that could overcome these limitations was developed. Consequently, it
is time to consider inhibitors of angiogenesis in the context of armed
therapeutic oncolytic viruses. Such experiments were carried out in our lab,
restricted-replication adenovirus containing antiangiogenesis gene (such as
endostatin, angiostatin, soluble VEGF receptor and so on) was successfully
constructed and showed good results in human cancer xenografted mice models.

2.2   Targeted
delivery strategy

An ideal
antiangiogenesis strategy should be targeted to only the organs that contain
the tumors and should not interfere with normal angiogenesis; it must achieve
high ratio of regional-to-systemic concentrations to minimize systemic
toxicity. There are two advances in increasing the specificity of
antiangiogenesis: (1) novel low-molecular-weight drugs selectively toxic to
tumor vasculature are developed; (2) either direct analysis of tumor
vasculature or bioinformatic approaches to identify novel targets are
available[43].

Low-molecular-weight
compounds that target the vasculature include the combretastatins. They are
selectively toxic to tumor vasculature by disrupting the tubulin cytoskeleton
and show no peripheral neuropathy or neuro-toxicity following chronic
dosing[44].

Many approaches
have been taken to discover novel endothelial surface markers with organ specificity.
Phage display peptide libraries are commonly used to obtain defined peptide
sequences interacting with a particular molecule. In this way, motifs including
RGD, NGR or GSL were found frequently in the screenings with various types of
tumors[45]. In order to discover a whole range of genes and proteins that are
specifically expressed in endothelial cells undergoing rapid growth in tumors,
other strategies such as microarray[46], SAGE[47] (serial analysis of gene
expression) and proteomic technologies[48] have been developed. However,
genetic screens with functional end points may be more insightful in
determining molecules that may be potential in cancer therapy. Technologies
such as retroviral cDNA and peptide library systems[49] will enable investigators
to incite distinct markers in endothelial cells and subsequently discover novel
genes and peptides that could counteract or promote physiologic changes.

These methods
have identified several new endothelial-specific genes as exciting targets, such
as magic roundabout (robo4)[50], which is endothelial specific and appears to
be a developmental gene because it is not expressed in adult tissue. Another
such gene is Delta4, which is also endothelial specific and only found on tumor
endothelium in the adult[51].

2.3   Combination
therapeutic strategy

Recently there
are accumulating pre-clinical and clinical evidences that a lot of
antiangiogenic drugs will lose their activity over time. This could be caused
by several mechanisms, e.g. an over-expression of other proangiogenic factors
that may antagonize the function of the antiangiogenic agents, especially if
only one such factor is the target of an antiangiogenic therapy. For this
reason the combined application of angiogenesis inhibitors might be a promising
strategy[52]. Inhibition of various receptor/ligand systems could be effective
due to their different mechanisms of action within the angiogenesis process.
Preliminary in vitro data describe an increased effect when two angiogenesis
inhibitors are applied in combination. For example, one study has shown that
delivery of a combination of genes encoding the antiangiogenic proteins,
angiogenesis and endostatin, can yield inhibitory effects on tumor growth in
mice bearing B16F19 melanoma and L1210 leukemia[53].

In addition,
application of antiangiogenic drugs in an adjuvant setting might be another
option. Studies have indicated the advantages of combining antiangiogenic
agents with immunotherapy, chemotherapy or radiotherapy, and so on. The reason
for these combinations is the assumption that the various mechanisms of action
and various targets could lead to additive antitumoral effects. For example,
with regard to combining angiogenesis inhibitors with radiation, angiogenesis
inhibitors may work as radiation-sensitizers. Gorski et al.[54] showed that
radiation therapy delivered to tumors in mice was enhanced when mice were
treated with angiostatin. On the other hand, radiation also has been shown to
increase the production of various angiogenic molecules as endostatin in
tumors[55].

An exciting
result obtained from our lab recently that all mice treated with Ad.K5RC (an
restricted-replication adenovirus armed with the kringle 5 of plasminogen gene)
plus Ad.TrailRC (an restricted-replication adenovirus armed with Trail gene)
were tumor free. It is another good example to show the efficacy of combine
therapy as it using a three-pronged approach to kill cancer cells selectively:
concomitant viral, antiangiogenesis gene and cytotoxicity gene.

2.4   From
mice to men

It has always
been a challenge to extrapolate animal data into the clinical setting[56]. The
result of the preclinical studies with antiangiogenic agents had been very
promising, often showing part or complete tumor regression without any drug
resistance. However, these results could not be confirmed in all clinical
trials. For example, in clinical the antitumoral effects of
endostatin/angiostatin in doses equivalent to those used in animal models has
been disappointing.

So several
things should be done to apply the results of experimental studies to novel,
clinically applicable therapies.

2.4.1       Establish
a better animal tumor therapy model    Traditional
xenotransplant models involve culturing tumor cells that are often
subcutaneously inoculated into different sites, where they assemble into
nodules and grow. Then human cancers arise de novo, originating out of
once-normal cells in natural tissue microenvironments. Such spontaneous tumor
models in which normal cells become malignant via a multi-step pathway may be
preferable for these studies[57]. This tumor can well recapitulate the human
situation as it induce or enhance not only the incidence of metastases but also
the response of a tumor mass growing ectopically that may be abnormal as the
one growing in a physiologically relevant site.

These models can be used to study the
impact of angiogenesis inhibitors on blocking the angiogenic switch in early
pre-malignant lesions or reducing small or bulky late stage tumors. So far,
these models have been of predictive value. For example, the RipTag model
confirmed recent clinical trials showing that broad-spectrum MMP inhibitors are
not effective in treating end-stage disease, whereas combinatorial strategies
involving low dose chemotherapy make effect[58]. Therefore, there is reason to
be optimistic that genetically engineered mouse models of organ-specific
carcinogenesis will indeed provide new insight into the potential efficacy of
targeted antiangiogenic therapies for different kinds of cancer[56].

2.4.2       Develop
techniques towards an individual quantitative assessment of angiogenesis          The
assessment of an active angiogenesis within a tumor may be of prognostic
relevance and a possible therapeutic target. The endothelial cell markers used
so far in most MVD studies (vWF, CD31, CD34) are useful for assessing the
vascular status of a tissue, but do not reflect active angiogenesis[59].
However, marker molecules of angiogenic endothelial cells are being further
identified. MVD counting techniques using angiogenic endothelial cell markers
have been developed might help a more realistic assessment of angiogenesis in
tumors. Surrogate markers like dceMRI and soluble angiogenesis markers even
turn out to be more predictive for biological activity, defining optimal doses
below the MTD[52]. Moreover, imaging techniques to assess tumor vascularization
and perfusion applying colour Doppler ultrasound[60] and dynamic
contrast-enhanced magnetic resonance imaging[61] have been improved. Imaging
techniques in combination with MVD analysis should allow the reliable
non-invasive and histological biopsy-based assessment of the angiogenesis
status of an individual tumor patient. This will become a prerequisite for the
identification of tumor patients who will benefit most from antiangiogenic
therapies.

At the same
time, many approaches were being developed to evaluate the potential efficacy
of antiangiogenic drugs in early phase clinical trials. In the experimental
animal situation, tumors can be removed and examined for such as the extent of
vascularization, endothelial cell viability or apoptosis. But this may not be a
particularly practical or desirable approach in the clinical situation. So
reliable surrogate markers of tumor angiogenesis that could be detected in
serum or urine may be necessary. At present, few, if any, such reliable markers
exist. As various non-invasive medical imaging strategies [e.g. MRI, Doppler
ultrasound and positron emission tomography (PET)[62]] to monitor change in
tumor blood flow, vascular structure and permeability may be the most fruitful
approaches, there are indeed considerable research efforts (and some successes)
need to be made in this area[63].

3    Conclusion

Antiangiogenic
tumor therapies are on the verge of clinical application and bring great promise
for the treatment of solid tumors in the near future. They will undoubtedly
have the potential to change traditional chemotherapeutic tumor therapies
because of the many advantages that it may offer such as the easy access to
targets within the vasculature, independence of tumor cell resistance
mechanisms, and the broad applicability of this therapy to many tumor
types[54].

What’s more, although the antiangiogenesis
process strategy is exciting, many problems still need to be considered because
probably no single strategy itself could be successful in eradicating solid
tumors in cancer patients. First, a cocktail of antibodies/inhibitors may be
required. Second, important characteristics of targets epitope expression
(heterogeneity, normal physiology and frequency of expression in animal and
human tumors) should be taken into account in an approach targeting tumor
vasculature[35]. In addition, it will be necessary to develop techniques for
the assessment of the angiogenesis status of individual tumor patient to
identify those patients most suitable for antiangiogenic therapies.
Furthermore, the long-term side effects of many antiangiogenic therapies on
normal tissues and physiological angiogenesis are not known yet. However, these
considerations do not in any way diminish the interest on antiangiogenic
therapy, but only remind one that clinical cancer therapy usually does not
progress by quantum leaps but by incremental, yet immensely useful, steps[64].

In conclusion,
the inhibition of angiogenesis presents an attractive possibility for treating
cancer, but further preclinical and clinical studies are necessary in order to
define an effective application for this form of therapy. Studies that involve
gene therapy, targeting therapy, combination therapy and the use of more
realistic animal models will improve clinical-trial design and help to identify
drugs that are most effective in treating and preventing human cancers.

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_________________________________________________

Received: July 4, 2003          Accepted:
September 1, 2003

This study was supported by the grants from
the Major Programs of the Chinese Academy of Sciences during the 9th Five-Year
Plan Period (No. KSCX2-3-06), the National Natural Science Foundation of China
(No. 30120160823) and the National High Technology R&D Program(863 Program)
of China (No. 2001AA217031)

*Corresponding author: Tel, 86-21-64746127;
Fax, 86-21-64746127; e-mail, [email protected]

Updated at: 2003-10-09