Targeting angiogenesis in pancreatic cancer: Rationale and pitfalls

Article (PDF Available)inLangenbeck s Archives of Surgery 393(6):901-10 · February 2008with27 Reads
DOI: 10.1007/s00423-008-0280-z · Source: PubMed
Abstract
Introduction Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive cancer responsible for over 20% of deaths due to gastrointestinal malignancies. PDAC is usually diagnosed at an advanced stage which, in part, helps to explain its high resistance to chemotherapy and radiotherapy. In addition, the cancer cells in PDAC have a high propensity to metastasize and to aberrantly express several key regulators of angiogenesis and invasion. Chemotherapy has only provided a modest impact on mean survival and often induces side effects. Targeting angiogenesis alone or in combination with other modalities should be investigated to determine if it may provide for increased survival. Materials and methods This review summarizes the alterations in PDAC that play a critical role in angiogenesis and provides an overview of current and therapeutic strategies that may be useful for targeting angiogenesis in this malignancy.

Figures

ORIGINAL ARTICLE
Targeting angiogenesis in pancreatic cancer: rationale
and pitfalls
Chery Whipple & Murray Korc
Received: 6 December 2007 / Accepted: 21 December 2007 / Published online: 22 January 2008
#
Springer-Verlag 2008
Abstract
Introduction Pancreatic ductal adenocarcinoma (PDAC) is
a highly aggressive cancer responsible for over 20% of
deaths due to gastrointestinal malignancies. PDAC is
usually diagnosed at an advanced stage which, in part,
helps to explain its high resistance to chemothera py and
radiotherapy. In addition, the cancer cells in PDAC have a
high propensity to metastasize and to aberrantly express
several key re gulators of angiogenesis and inv asion.
Chemotherapy has only provided a modest impact on mean
survival and often induces side effects. Targeting angiogenesis
alone or in combination with other modalities should be
investigated to determine if it may provide for increased
survival.
Materials and methods This review summarizes the
alterations in PDAC that play a critical role in angiogenesis
and provides an overview of current and therapeutic
strategies that may be useful for targeting angiogenesis in
this malignancy.
Keywords Pancreatic ductal adenocarcinoma (PDAC)
.
Angiogenesis
.
Invasion
.
Metastasis
.
Angiogenesis inhibitors
Introduction
Pancreatic ducta l adenocarcinoma (PDA C) is a highly
metastatic and biologically aggressive malignancy that is
the fourth leading cause of cancer death in the USA with an
overall 5-year survival rate of less than 5% and a median
survival of less than 6 months [13]. The only potential
cure is surgery. However, since the diagnosis of PDAC is
often established at an advanced stage, less than 20% of the
patients diagnosed with the disease are suitable candidates
for surgical resection. Late detection of PDAC is the result of
indolent tumor growth, difficulty in visualization of early
lesions, and lack of specific and sensitive diagnostic serum
markers [3]. Treatment failure is due, in part, to the fact that
PDAC displays a wide range of genetic and epigenetic
alterations, a resistance to chemotherapy and radiotherapy,
and a high propensity to metastasize even when small [3, 4].
Genetic alterations in PDAC include a greater than 90%
frequency of mutation in the KRAS oncogene, a high
frequency of mutation in the p16 gene, alterations in
the tumor suppressors p53, SMAD4/DPC4, BRCA2,
CDKN2A, as well as mutations in the DNA mismatch
repair gene MLH1 [5, 6]. Activating KRAS mutations are
often the first genetic muta tion to be detected whic h
suggests an initiating role in tumorigenesis [7]. Downstream
effector pathways of KRAS that have been shown to be
excessively a ctivated in PDAC include RAF-mitogen-
Langenbecks Arch Surg (2008) 393:901910
DOI 10.1007/s00423-008-0280-z
This study was supported by grants from the National Cancer Institute
(CA-101306 and CA-102687) awarded to MK.
C. Whipple
:
M. Korc
Departments of Medicine, Pharmacology & Toxicology,
Dartmouth Hitchcock Medical Center
and Dartmouth Medical School,
Hanover, NH, USA
C. Whipple
:
M. Korc
Norris Cotton Comprehensive Cancer Center,
Dartmouth Hitchcock Medical Center
and Dartmouth Medical School,
Lebanon, NH, USA
M. Korc (*)
Department of Medicine, Dartmouth Hitchcock Medical Center,
One Medical Center Drive,
Lebanon, NH 03756, USA
e-mail: murray.korc@dartmouth.edu
activated kinase (MAPK), PI3K, and NFκB. Together, all of
these mutations suppress apoptosis and promote cellular
proliferation, differentiation, and genomic instability.
Essential developmental signaling pathways, such as
hedgehog, Not ch, and Wnt have also been shown to be
aberrantly expressed in approximately 6070% of PDACs
[811]. Overexpression of sonic hedgehog enhances tumor
initiation and growth through the activation of the glioma-
associated oncogene homolog 1 (GLI-1) transcription factor
which plays a role in regulating proliferation, differentiation,
and extracellular mat rix interactions [12, 13]. Activat ion of
hedgehog signaling also upregulates Wnt activity, which in
turn, has been shown to induce tumor development and
progression [ 11]. In a ddition, Notc h signaling has an
essential role in maintaining balance between cell prolifer-
ation, differentiati on, and apoptosis, which affect the
development and function of many organs [14, 15]. Of
the four Notch family members, Notch-1 has been shown to
induce the nuclear factor κB (NF-κB) apoptotic regulatory
pathway, expression of matrix metalloproteinase-9 (MMP-9),
and expression of vascular endothelial growth factor (VEGF),
all of which are critically involved in tumor-cell invasion,
metastasis, and angiogenesis [14, 15].
In addition to the above abnormalities within the cancer
cells, certain features of thetumormicroenvironment
(TME) also contribute to the highly aggressive nature of
PDAC. Thus, PDAC is a highly desmoplastic cancer,
exhibiting a dense stroma that is comprised of fibroblasts,
inflammatory cells, activated pancreatic stellat e cells
(PSCs), pericytes, macrophages, and aberrant endothelial
cells. This complex matrix is rich in fibronectin, collagens I
and III, cytokines and growth facto rs (which are both
supportive and mitogenic to fibroblasts), and PSCs [ 3, 16,
17]. Collectively, these components promote cancer-cell
proliferation and invasiveness.
PDAC is also characterized by the abundant expression
in the cancer cells of multiple mitogenic growth factors and
their receptors in cluding the epidermal growth fa ctor
receptor (EGFR), heparin-binding EGF-like growth factor
(HB-EGF), fibroblast growth factors (FGFs), platelet-
derived growth factor (PDGF), and the heparin-binding
vascular permeability factor, VEGF-A [18, 19]. These
growth factors are pro-angiogenic and exert a critical role
in the proliferation of endothelial cells and their subsequent
invasion of the basement membrane [20].
Angiogenesis, the process by which new blood supply is
established from pre-existing blood vessels, is required for
tumor growth beyond the distance that oxygen can diffuse
(12mm) [20, 21]. More specifically, angiogenesis entails
the release of angiogenic factors and proteolytic enzymes
which initiate and support the proliferation of endothelial
cells from pre-existing blood vessels and facilitate the
breakdown of the basement membrane and the extracelluar
matrix (ECM) [22, 23]. The resulting increased vascularity
enables primary tumor growth, assures adequate oxygen
and nutrient supplies, and increases the likelihood of
metastatic spread as well as the growth of the metastatic
foci. The resulting blood vessels are often dilated and
irregularly shaped and may be leaky. Additionally, there is
evidence that cancer cells may have integrated into the
vessel walls in some tumors and that angiogenesis is
enhanced by the recruitment of endothelial precursor cells
from the bone marr ow [24,
25].
Since pancrea tic tumors have a high p ropensity to
metastasize when the tumor is still small and undetectable,
a better understanding of the mechanism s that contribute to
angiogenesis and metastasis may aid in the elucidation of
more effective treatment options. This review will briefly
cover the major contributors to angiogenesis as it relates to
PDAC as well as current and poten tial targets for
therapeutic intervention.
Key regulators of angiogenesis in pancreatic cancer
Angiogenesis is essential for tumor growth and metastasis
and is initiated by cytokines, growth factors, hypoxia, the
activation of oncogenes, and loss of function of certain
tumor suppressor genes [20, 26 ]. Targeting angiogenesis to
reduce tumor progression and metastasis may yield novel
strategies for combination therapy. However, to develop
innovative therapies, it is necessary to understand the key
signaling molecules and pathways involved in angiogenesis.
Angiogenesis and hypoxia
Angiogenesis is often driven by the tumors response to
hypoxia, and resistance to hypoxia may be another
important part of tumor growth. The transcription factor,
hypoxia-inducible factor (HIF-1), plays a critical role in
oxygen homeostasis and in pancreatic cancer and has been
shown to be overexpressed in several tumors [20, 27, 28].
HIF-1 controls the expression of numerous genes that
mediate developmental and physiological pathways that
either deliver oxygen to cells or allow cells to survive
hypoxic conditions [27]. HI F-1 i s com prise d of two
subunits, HIF-1α and HIF-1β. Under normoxic conditions,
HIF-1β is constitutively expressed, whereas HIF-1α is
continuously synthesized, hydroxylated by proline hydrox-
ylase, ubiquitinylated by the von HippelLindau (VHL)
protein, and degraded in the proteosome [27, 29]. However,
since oxygen is required for proline hydroxylase to target
HIF-1α, under hypoxic conditions, HIF-1α is stabilized
and allowed to accumulate. HIF-1α then binds to HIF-1β
which leads to the upregulation of several hypoxia-response
proteins such as lactate dehydrogenase isoenzyme-5 (LDH-5),
902 Langenbecks Arch Surg (2008) 393:901910
carbonic anhydrase 9 (CA9), and glucose transporter-1
(GLUT-1) [2931]. HIF-1α is a pro-angiogenic factor that
acts upstream of VEGF-A (both increasing its expression
and stabilizing the mRNA transcript) which renders pancreatic
cells resistant to hypoxia-induced apoptosis while promoting
angiogenesis [20, 3234].
Growth factors, their receptors, and their role in PDAC
Many growth factors and their receptors are overexpressed
in PDAC, including the platelet-derived growth factors,
transforming growth factor-β (T GF-β), fibroblast growth
factors, and vascular endothelial growth factor. These
growth factors have been shown to be critical to the
development and progression of pancreatic cancer and
function as stimulators and/or inhibitors of cell proliferation,
metastasis, and angiogenesis [17, 35]. Additionally, several
signaling pathways, such as those activated by PDGF and
VEGF, may act synergistically to stimulate endothelial
motility and induce angiogenesis [20, 36].
One of the most complex of the growth factor families,
TGF-β is a multifunctional cytokine that has been shown to
both enhance the growth of cells of mese nchymal origin
and to inhibit the growth of epithelial cells through
interaction with the Smads, cyclin-dependent kinases
(CDK), and CDK inhibitors such as p21 and p27 [37].
The growth inhibitory effects of TGF-β signaling is exerted
through the binding of TGF-β type I and type II serine/
threonine receptors, subsequent phosphorylation of several
Smad family members including Smads 2, 3, and 4, and
activation of TGF-β-responsive genes [35, 38, 39]. In
contrast, inhibit ory Sma ds, Smad 6 and 7, act in an
opposing manne r to antagonize this signaling. In PDAC,
it has been observed that c ancer cells have lost their ability
to respond to the growth-suppressive effects of TGF-β.
Moreover, all three TGF-β isoforms are overexpressed in
PDAC, as are inhibitory Smad 6 and 7, whereas Smad 4 is
often mutated [39, 40]. Together, these observations imply
that cancer-cell-derived TGF-β may act in a paracrine
manner to enhance tumor angiogenesis and promote cancer-
cell metastasis.
A critical regulator of angiogenesis is represented by the
VEGF family of growth factors. The frequent overexpression
of VEGF in PDAC correlates with a high microvessel density,
disease progression, increased risk of metastatic spread, and a
poor prognosis [4144]. Although the VEGF family consists
of six members (VEGF-A, -B, -C, -D, -E, and placenta
growth factor); the principle form of VEGF is the homo-
dimeric glycoprotein VEGF-A. VEGF-A consists of five
major isoforms, all of which act as anti-apoptotic agents,
possess vasodilatory abilities, and promote endothelial cell
migration and proliferation via binding with its respective
tyrosine kinase receptors, VEGFR-1 (flt-1) and VEGFR-2
(flk-1/KDR) [19, 20
, 4550]. An additional co-receptor for
VEGF-A is neuropilin-1 (Np-1), a neuronal guidance
molecule for axons in the developing nervous system, whose
overexpression in transgenic mice is associated with excess
capillary and blood-vessel formation [19]. VEGF-A expres-
sion is regulated by multiple mechanisms, including mutant
KRAS and p53, transcription factors such as HIF-1α and
SP1, the VHL protein, and by growth factors such as
fibroblast growth factor-2 (FGF-2), and TGF-β [19, 5153].
FGF-2 is another potent stimulator of angiogenesis and
has been shown to interact synergistically with VEGF in
tumor development and growth [54]. Overexpression of
FGFs and their receptors has been correlated with tumor
invasiveness, angiogenesis, and lymph-node metastasis [55,
56]. The two most predominant factors of the 23-member
FGF family are FGF-1 and FGF-2. These FGFs signal
through transmembrane receptor tyrosine kinases, FGFRs.
Of the four known FGFRs, FGFRI is comm only expressed
on endothelial cells. Stable binding of FGF to FGFR
requires the presence of heparan sulfate proteoglycans
(HSPGs) which leads to an increased cellular response
and induce s endothelial cell proliferation, migration, and
tubulogenesis [54, 5759].
Interaction of cytokines with VEGF and their role
in angiogenesis
Small peptide hormones, called cytokines, have also been
implicated in increased tumor growth, adhesion, invasion,
and angiogenesis. In particular, interleukin-6 (IL-6) and
interleukin-8 (IL-8) are over-expressed in PDAC and have
been associated with increased VEGF expression, especially
under hypoxic condition s [6063]. IL-6 upregulates the
expression of several cytokines that act together to create a
tumor environment which favors tumor growth and
suppresses cancer-directed immunity [60 ]. For example, in
gastric cancer cell lines, transfection with IL-8 resulted in
rapidly growing highly vascular tumors [20, 64]. Moreover,
neutralizing antibodies against IL-8 and/or VEGF have been
shown to attenuate the growth and metastasis of human
pancreatic cancer in an orthotopic mouse model [44, 65].
Heparin sulfate proteoglycans and growth-factor signa ling
pathways
The heparan sulfa te proteoglyc an, glyp ican-1 (GPC1)
exerts a pivotal role in angiogenesis by modulating growth
factor signaling. GPC1 is overexpressed in PDAC, whereas
its expression is low in both the normal pancreas and in
chronic pancreatitis [18]. GPC1 is a cell-membrane-associated
co-receptor that enhances the signaling pathways activated
by a variety of heparin binding growth factors (HBGFs),
including FGF2, VEGF, PDGF, TGF-β, and heregulins
Langenbecks Arch Surg (2008) 393:901910 903
(HRG) [50]. HBGFs play critical roles in cell recognition,
growth, differentiation, and cell-cell adhesion [66]. GPC1
modulates the biological activity of these growth factors often
by acting as a co-receptor for more efficient signaling [18, 50,
54, 67, 68], and stable expression of GPC1 antisense mRNA
in pancreatic cancer cells results in reduced GPC1 protein
expression and decreased tumor growth, angiogenesis and
metastasis in vivo [67, 69], underscoring the importance of
cancer-cell-derived GPC1 for tumor angiogenesis. Moreover,
endothelial cells isolated form GPC1 knockout mice exhibit
an attenuated mitogenic response to VEGF-A [69], empha-
sizing the importance of host-derived GPC1 for tumor
angiogenesis. The important role of host endothelial cell
heparan sulfates in angiogenesis was also demonstrated in
mice carrying an endothelial-targeted deletion of N-acetyl-
gluocsamine N-deacetylasr/N-sulfotransferase (Ndst1) which
exhibit decreased N- sulfation of glusoamine residues and
attenuated tumor angiogenesis [70].
The angiopoietins and the angiogenic shift
Although V EGF and FGF play major roles in the
progression of a tumor beyond its avascular or quiescent
state, angiopoietins also have a critical role in activating
the angiogenic switch. There are four angiopoietins,
encoded by four distinct genes. Angiopoietin-1 (Ang-1)
and angiopoietin-2 (Ang-2) have been implicated in tumor
angiogenesis. Both interact with the tyrosine kinase
receptor Tie-2. However, Ang-2 is an antagonist to Ang-1
and prevents Tie-2 activation by competitively binding
Tie-2 [26, 71, 72]. The binding of Ang-1 to Tie-2 results in
downstream activation of the phosphatidylinositol 3-kinase
(PI3-K)/Akt survival pathway which ultimately leads to
endothelial cell migration, tube formati on, sprouting, and
survival [47, 71]. Overall, Ang-1 acts as a maturation factor
promoting endothelial cell migration and adhesion, whereas
Ang-2 is expressed at the site of vascular remodeling and
promotes vessel destabilization [47, 73]. These destabilized
vessels may undergo regression unless VEGF is present to
promote angiogenesis [26]. Thus, the dynamic balance
between vessel regression and growth is mediated by
VEGF, Ang- 1, and Ang-2. It is noteworthy therefore, that
Ang-1, Ang-2, and Tie-2 are overexpressed in a variety
of tumor types. However, Ang-2 is more frequently
upregulated than Ang-1 and Tie-2, especially in highly
vascular tumors [73], and relatively high levels of both
Ang-2 and VEGF in hepatocellular carcinoma (HCC) and
PDAC have been correlated with more advanced disease
and a worse prognosis [47, 73]. Thus, while a critical
determinant of their angiogenic potential is dependent on
the ratio of Ang-1 to Ang-2 levels, both ligands stimulate
the sprouting of new blood vessels in the presence of high
VEGF-A levels.
Src may play a pivotal role in VEGF expression
and angiogenesis
The oncogene Src is overexpressed in about 70% of
pancreatic tumors and has also been strongly implicated in
the development, progression, and metastas is of pancreatic,
colon, breast, brain , and lung cancer [74
76]. Src kinase
belongs to a family of non-receptor intracellular tyrosine
kinases that are involved with numerous signaling path-
ways involved in cellcell adhesion, cell proliferation,
migration, and angiogenesis [77]. Src is activated by
several growth factors including EGF, PDGF, and HER2/
Neu but has been shown to be critical to hypox ia-induced
expression of VEGF [78, 79]. Ischenko et al. [78] report
that inhibition of Src with the Src kinase inhibitor,
AZM475271, in an orthotopic human pancreatic carcinoma
mouse model results in downregulation of pancreatic tumor
growth and metastasis partially due to a reduction in
angiogenesis and a decrease in VEGF levels [77, 80].
Additional modulators of angiogenesis
Tumor angiogenesis is also enhanced by excessive production
of matrix metalloproteases and heparanase [22, 81] by the
expression of specific integrins such as αVβ3 and αVβ5
and by loss of suppressors of angiogenesis such as
angiostatin, endostatin, interferons, platelet factor 4, throm-
bosponding, and the 16-kDa fragment of prolactin. For
example, the MMPs, a multi-gene family of related enzymes,
promote angiogenesis and facilitate tumor cell invasion and
metastasis by degrading the basement membrane and the
stromal extracellular matrix [15, 22, 82]. Heparanase is
another enzyme that plays a role in the proteolysis of the
extracellular matrix by degrading heparan sulfate found on
HSPGs, thereby releasing stored growth factors which can
then stimulate angiogenesis [83, 84]. In contrast, angiostatin,
a cleaved product of plasminogen, and endostatin, the
C-terminal fragment of collagen, both suppress angiogenesis
by specifically inhibiting endothelial cell proliferation and
migration [8587]. Attenuated expression of angiogenesis
suppressors can lead to enhanced tumor angiogenesis by
allowing the actions of pro-angiogenic factors to proceed
without interference by signaling pathways that attenuate
angiogenesis.
Current therapiesadvances and difficulties
Agents curren tly in use
As mentioned pr eviously, since PDAC is resistant to
chemotherapy and radiotherapy, surgical resection is the
only potentially curative treatment. Drug targets thus far
904 Langenbecks Arch Surg (2008) 393:901910
have only provided a modest impact on mean survival.
Until relatively recently, 5-fluorouracil (5-FU) was the
predominant chemotherapy, with response rates rarely
exceeding 10% [88]. Gemcitabine, which has become the
standard form of chemotherapy for advanced and metastatic
PDAC, is a pyrimidine analog that inhibits DNA synthesis
and that has been shown to double post-surgery survival
(from 7.5 to 14.2 months) [88]. However, gemcitabine
therapy alone (without surgery) exerts only modest results
on survival (46months).Thus,severalstudiesare
combining novel treatments either alone or in combination
with gemcitabine. For example, clinical trials have shown
that the combination of gemcitabine and cisplatin (a
platinum-based drug that introduces DNA damage and
induces apoptosis) increases median survival from 5 to
8.2 months [89]. Moreover, several new novel agents that
have displayed a potential for decreasing angiogenesis,
reducing tumor growth and met astasis, and/or increasing
overall survi val, are now available (Table 1).
Small-molecule inhibitors targeting growth factors
and/or their receptors
Since angiogenesis is required for solid tumor growth and
metastasis, a great deal of effort has been directed at targets
that are the key players in angiogenesis, namely, the growth
factors VEGF and FGF and their high-affinity receptors.
The firs t VEGFR-2 inhibitors to be evaluated, SU5416 and
SU6668, are highly selective competitive inhibitors of the
ATP-binding site in the VEGFR-2 kinase domain [9093].
Although significant inhibition of tumor-vessel density and
growth as well as increased apoptosis was observed with
SU6668 in preclinical trials, neither drug produced significant
clinical activity in single-agent trials [93].
Another small-molecule inhibitor which targets both
VEGFR-2 and FGFR1 is PD1 73074. By targeting both
receptors, PD17304 should be both anti-angiogenic
and anti-mitogenic, thereby potentially abrogating two
critical pathways in tumor growth and metastasis [94].
Preclinical trials in orthotopic mouse models showed
significant reduction in tumor growth, a lower incidence
of metastatic spread, and a lower volume of ascites in
PD173074-treated animals [95]. These data suggest that
PD17304 m ay be a promising new treatment option for
PDAC.
Yet another potential treatmen t that targets VEGF-A is a
modified soluble VEGFR chimeric protein that consists of
two immunoglobulin-like (Ig) domains (second Ig domain
of VEGFR-1 and third Ig domain of VEGFR-2) termed
VEGF-Trap [48]. VEGF-Trap has been shown to attenuate
intra-pancreatic tumor growth and distant metastasis in an
orthotopic nude mouse model [48], raising the possibil ity
that it may have a therapeutic role in PDAC.
Monoclonal antibodies against growth factors
and/or their receptors
Several monoclonal anti bodies have also been used to
target VEGF-A and/or EGFR expression (for a complete
review, see [88]). For examp le, cetuximab is a monoclonal
antibody that competitively binds to t he EGFR thus
preventing stimulation of the receptor and in turn inhibiting
cell proliferation, invasion, and metastasis [88, 96]. Pre-
clinical studies have shown that cetuximab reduces chemo-
Table 1 Summary of current and potential therapies
Therapeutic target Type of chemical Target pathway
5-Fluorouracil Thymidine synthase inhibitor Inhibits DNA replication
Gemcitabine Pyrimidine analog Inhibits DNA replication
Cisplatin Platinum based Introduces DNA damage
SU5416 Small molecule inhibitor VEGFR-2 inhibitor
SU6668 Small molecule inhibitor VEGFR-2 inhibitor
PDI73074 Small molecule inhibitor VEGF-RII & FGF-RI
VEGF-Trap Modified soluble VEGFR VEGF-A
Cetuximab Monoclonal antibody EGFR
Bevacizumab Monoclonal antibody VEGF-A
Sorafenib Oral bi-aryl urea (kinase inhibitor) VEGFR-1, -2, -3, and PDGFR
Sunitinib Tyrosine kinase inhibitor VEGFR-2, PDGFR, and FLT-3
IMC-2C6 Monoclonal antibody PDGFR and VEGFR-2
Erlotinib Oral quinazoline EGFR
Vatalanib Small molecule aminophthalazine VEGFR inhibitor
Genistein Isoflavone Several growth factors (VEGF)
and tyrosine kinases (EGFR)
Langenbecks Arch Surg (2008) 393:901910 905
therapy and radiotherapy resistance but that cetuximab may
need to be combined with gemcitabine for effective activity
in PDAC [88, 96].
Bevacizumab is a recombinant humanized monoclonal
antibody against VEGF-A. It is the first angiogenesis
inhibitor to demonstrate efficacy in tumor growth and
metastasis and has resulte dinimprovedsurvivalin
randomized phase-III trials of metastatic lung and breast
cancer [97]. Early studies designed to test the effectiveness
of a combination of Bevacizumab and gemcitabine in the
treatment of advanced PDAC have shown promising results
(an increase of median survival from 5.8 to 9 months).
However, a subsequent phase-III trial was closed in June of
2006 due to the failure to document efficacy [97], under-
scoring the difficulties in translating success in phase-II
trials to success in phase-III trials and beyond. Such
discrepancies may be due to the smaller number of patients
in phase-II studies and to the inherent selection bias when
studies are not fully randomized.
Novel and combinational treatments
There are several promising VEGFR inhibitors, including
Vatal a n i b, Va n d e tani b , Axitinib, S o r a feni b , Sunintini b ,
AEE-788, and AZD-2171 which are either in preclinical
development or currently being tested in early phase-I/II
studies [ 97 ]. Of particular interest, combination therapy
with gemcitabine and vatalanib in an orthotopic pancreatic
cancer model suppressed angiogenesis and metastasis of
pancreatic tumo rs by 81% and significantly increased
animal survival [ 97]. Unfortunately, these impressive
preclinical results do not always translate into a significant
response in preliminary phase-I trials. All too often, dose-
limiting toxicity can reduce the potential impact of the
drug.
Erlotinib is another tyrosine kinase inhibitor that has
been used in conjunction with gemcitabine. Erlotinib is an
oral HER1/EGFR inhibitor that, when combined with
gemcitabine, showed a modest increase in survival rate
(6.24 vs 5.91 months and a 1-year survival increase from
17% t o 23%) com pared to g emcitabine alone [98].
Although routine use of erlotinib and gemcitabine is not
recommended for patients with advanced pancreatic cancer
due to the high cost of erlotinib and limited efficacy [99],
erlotinib may prove beneficial in combination with other
anti-angiogenic compounds.
Another potential combinational treatment that has
shown strong anti-angiogenic activity is Genistein, a
naturally occurring isoflavone present in soybeans [100,
101]. Although the underlying mechanism of inhibition
remains unclear, Genistein ha s been shown to indu ce
apoptosis, chemosensitize cells, and inhibit both tyrosine
kinases as well as hypoxic activation of HIF-1 which is an
important regulator of VEGF expression [101103]. Since
Genistein reduces tumor growth and angiogenesis through
the downregulation of several growth-signaling pathways,
especially VEGF and EGFR, it may be another effective
treatment option [100, 101, 104].
Fig. 1 Schematic representation of VEGFR-dependent s ignaling
pathways in a hypothetical pancreatic cancer cell. Binding of VEGF-
A to its rec eptors ( VEGFR -1 and VEGFR-2)isenhancedby
interacti ons with co-rece ptors (Np-1 and Np-2), leading to the
initiation of signaling cascades which are transmitted through the
Shc/Grb2/SOS complex and the activation of the canonical Raf/MEK/
ERK signaling cascade. In addition, there is activation of the
phospholipase C gamma (PLC-γ) pathway. Once PLC-γ is activated,
it hydrolyzes phosphatidylinositol (PIP2) into two important second-
messenger molecules: inositol triphosphate (IP
3
) and diacylglycerol
(DAG). IP
3
mobilizes intracellular calcium, whereas DAG activates
protein kinase C (PKC), which in turn phosphorylates the protein
kinase Raf. Both pathwa ys lead to enhanced mitogenesis in cancer
cells and endothelial cells. In addition, there is activation of the PI3K/
Akt pathway, which promotes the survival of both cancer cells and
endothelial cells. VEGFR-1 may also promote cancer cell metastasis,
whereas VEGFR-3 may promote cancer spread to local lymph nodes.
Thus, these pathways may be targeted in the future in a manner that
directly suppresses angiogenesis, cancer cell proliferation and
metastasis
906 Langenbecks Arch Surg (2008) 393:901910
Potential pitfalls of anti-angiogenic therapies
Novel therapies that target angiogenesis are often expen-
sive and may only be effective in subgroups of PDAC
patients yet expose all patients to a multiplicity of side
effects, some of which may be life-threatening. Examples
of side effects include fatigue, nausea, vomiting, diarrhea,
weight loss, depression, pain, or skin rash. More severe
complications that arise from targeting angiogenesis in-
clude hypertension, proteinuria, thrombotic events that may
be associated with angina or stroke, bleeding, impaired
wound healing, severe allergic reactions, and gastrointes-
tinal perforation. Complications that occur as a result of
targeting angiogene sis may increase in frequency and
severity when used in combination with chemotherapy or
with other forms of targeted therapies, underscoring the
need to maintai n clinical vigilance.
The multiplicity of mechanisms that have the potential to
promote tumor angiogenesis in PDAC [19]andthe
redundancies that exist in the signaling pathw ays that
promote angiogenesis suggest that a multi-pronged approach
will be required to effectively suppress the angiogenic
process in PDAC. To avoid excessive side effects that may
arise as a result of targeting multiple pathways and to
effectively suppress tumor angiogenesis, it may be necessary
to target the common signaling pathways that are down-
stream of VEGF rece ptors and other pro-angiogenic
receptors. Another advantage of targeting shared down-
stream signaling pathways is th at the same signaling
cascades that are active in proliferating endothelial cells are
utilized in pancreatic cancer cells to promote their prolifera-
tion and metastasis, and these cancer cells also express VEGF
receptors as well as the neuropilin co-receptors (Fig. 1). For
example, the PI3-K/AKT and RAS/RAF/MAPK and phos-
pholipase C-γ (PLC-γ) signaling pathways are active in
proliferating endothelial and cancer cells [105107]. More-
over, VEGF-C, which binds to VEGFR-3, is overexpressed
in PDAC and has been associated with lymph-node
metastasis [108110]. VEGF-A and VEGF-C may interact
with neuropilins (NP-1 and -2), which act as coreceptors and
are important for the development of vessels and capillaries
[105, 111, 112]. In pancreatic cancer cells, the interactions of
neuropilins with VEGFRs may act to enhance their survival,
mitogenesis, and metastasis (Fig. 1).
Conclusion and future directions
Although many new drugs are being developed, attempts to
improve upon gemcitabine therapy and to increase the
survival rate (beyond a few months) have met with limited
success. Often, encouraging in vitro and in vivo data
obtained from animal models do not translate wi th a
significant response in clinical trials. Success is hindered
by several factors, including dose-limiting toxicity, tumor
heterogeneity, a dvanced disease stage at the time of
diagnosis, and acquisition of drug resistance. It is also
probable that combination therapy, despite an increased side
effect profile, may be required to overcome redundancies in
deregulated signaling cascades [20].
Given the importance of angiogenesis to tumor growth
and metastasis, anti-angiogenic therapy may greatly en-
hance the efficacy and long-term survival of PDAC
patients. Tumor-associated endothelial cells are ideal targets
because they are genetically more stable than the cancer
cells, their cell turnover is higher than that of endothelial
cells in normal tissues, and destroying a few tumor-
associated endothelial cells may disrupt the growth of
many cancer cells [44, 78]. However, to be effective and to
minimize side effects, anti-angiogenic therapy may need to
be used in conjunction with individualized treatment plans
that are based on the gene expression profile of a patients
tumor [113] and that also target the cancer cells and the
stroma, thereby opening the door to increased patient
survival.
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910 Langenbecks Arch Surg (2008) 393:901910
    • "Of these factors, VEGF-A is the best characterized one and is recognized as a major pro-angiogenic cytokine released by TAM [67]. VEGF-A stimulates angiogenesis by promoting endothelial cell migration and proliferation via binding with its corresponding tyrosine kinase receptors, VEGFR-1 and VEGFR-2 [68]. Additionally, TAM also involved in angiogenic processes by producing several angiogenesis-modulating enzymes such as MMP-2, MMP-7, MMP-9, MMP-12 and cyclooxygenase-2 (Cox2), and chemokines such as CXCL12, CCL2, CXCL8, CXCL1, CXCL13 and CCL5 (Figure 2). "
    [Show abstract] [Hide abstract] ABSTRACT: The tumor microenvironment is replete with cells that evolve with and provide support to tumor cells during the transition to malignancy. The hijacking of the immune system in the pancreatic tumor microenvironment is suggested to contribute to the failure to date to produce significant improvements in pancreatic cancer survival by various chemotherapeutics. Regulatory T cells, myeloid derived suppressor cells, and fibroblasts, all of which constitute a complex ecology microenvironment, can suppress CD8+ T cells and NK cells, thus inhibiting effector immune responses. Tumor-associated macrophages (TAM) are versatile immune cells that can express different functional programs in response to stimuli in tumor microenvironment at different stages of pancreatic cancer development. TAM have been implicated in suppression of anti-tumorigenic immune responses, promotion of cancer cell proliferation, stimulation of tumor angiogenesis and extracellular matrix breakdown, and subsequent enhancement of tumor invasion and metastasis. Many emerging agents that have demonstrated efficacy in combating other types of tumors via modulation of macrophages in tumor microenvironments are, however, only marginally studied for pancreatic cancer prevention and treatment. A better understanding of the paradoxical roles of TAM in pancreatic cancer may pave the way to novel preventive and therapeutic approaches. Here we give an overview of the recruitment and differentiation of macrophages, TAM and pancreatic cancer progression and prognosis, as well as the potential preventive and therapeutic targets that interact with TAM for pancreatic cancer prevention and treatment.
    Full-text · Article · May 2016
    • "Although PDAC is generally a hypovascular tumor, there was significant interest in using anti-VEGF agents in PDAC in the early 2000s because VEGF expression was thought to protect the tumor endothelium and tumor cells from cytotoxic agents and radiation [12]. Indeed, angiogenesis is a fundamental phenomenon associated with the development and progression of almost every type of cancer including pancreatic cancer13141516 . Neovascularization promotes the growth of tumor cells by nourishing them with oxygen and nutrients. "
    [Show abstract] [Hide abstract] ABSTRACT: Pancreatic ductal adenocarcinoma (PDAC) has a high mortality rate and outcomes have not improved substantially for decades. Significant attention has focused on the biological drivers of the disease, and preclinical work has pointed to multiple biomarker candidates and therapeutic avenues. However, translation of these promising biomarkers and treatment strategies to patients has not been overwhelmingly successful. New strategies to account for the significant heterogeneity of the disease are needed so that rational treatments can be administered. Here, we focus on how physical sciences-based approaches may play a role in stratifying patients for clinical trials, and how this view of PDAC may reinvigorate treatment strategies that have been abandoned after "failing" to fulfill their potential in unselected patient populations. By complementing biological approaches, the development of physical biomarkers of PDAC may help deliver on the promise of personalized medicine for this devastating disease.
    Full-text · Article · Jan 2016
    • "Thus, to block angiogenesis effectively, we need to target multiple molecules simultaneously. Because many pro-angiogenic growth factors such as VEGF-A, FGF2, PDGFs, TGF-β, and heregulin (gene: NRG1) (HRG) bind to HSPGs to facilitate their signaling, another targetable common denominator would be these proteoglycans [73,81]. The validity of this strategy has been shown with Kras LSL-G12D/+ , Cdkn2a LoxP/LoxP , Pdx-1-Cre (KIC) mice that were null for glypican-1 (Gpc1), one of the HSPGs. "
    [Show abstract] [Hide abstract] ABSTRACT: The importance of angiogenesis in Pancreatic Ductal Adenocarcinoma (PDAC) and its therapeutic potential have been explored in both pre-clinical and clinical studies. Human PDACs overexpress a number of angiogenic factors and their cognate high-affinity receptors, and anti-angiogenic agents reduce tumor volume, metastasis, and microvessel density (MVD), and improve survival in subcutaneous and orthotopic pre-clinical models. Nonetheless, clinical trials using anti-angiogenic therapy have been overwhelmingly unsuccessful. This review will focus on these pre-clinical and clinical studies, the potential reasons for failure in the clinical setting, and ways these shortcomings could be addressed in future investigations of angiogenic mechanisms in PDAC.
    Full-text · Article · Jan 2016
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