Vol. 9, 1957–1971, June 2003
Clinical Cancer Research 1957
Review
Tumor Response to Ionizing Radiation Combined with
Antiangiogenesis or Vascular Targeting Agents:
Exploring Mechanisms of Interaction1
Phyllis Wachsberger,2 Randy Burd, and
Adam P. Dicker
Division of Experimental Radiation Oncology, Department of
Radiation Oncology, Kimmel Cancer Center, Jefferson Medical
College of Thomas Jefferson University, Philadelphia, Pennsylvania
19107
Abstract
Recent preclinical studies have suggested that radiotherapy in combination with antiangiogenic/vasculature targeting agents enhances the therapeutic ratio of ionizing
radiation alone. Because radiotherapy is one of the most
widely used treatments for cancer, it is important to understand how best to use these two modalities to aid in the
design of rational patient protocols. The mechanisms of
interaction between antiangiogenic/vasculature targeting
agents and ionizing radiation are complex and involve interactions between the tumor stroma and vasculature and
the tumor cells themselves. Vascular targeting agents are
aimed specifically at the existing tumor vasculature. Antiangiogenic agents target angiogenesis or the new growth of
tumor vessels. These agents can decrease overall tumor
resistance to radiation by affecting both tumor cells and
tumor vasculature, thereby breaking the codependent cycle
of tumor growth and angiogenesis. The hypoxic microenvironment of the tumor also contributes to the mechanisms of
interactions between antiangiogenic/vasculature targeting
agents and ionizing radiation. Hypoxia stimulates up-regulation of angiogenic and tumor cell survival factors, giving
rise to tumor proliferation, radioresistance, and angiogenesis. Preclinical evidence suggests that antiangiogenic agents
reduce tumor hypoxia and provides a rationale for combin-
Received 12/18/02; revised 2/5/03; accepted 2/11/03.
The costs of publication of this article were defrayed in part by the
payment of page charges. This article must therefore be hereby marked
advertisement in accordance with 18 U.S.C. Section 1734 solely to
indicate this fact.
1
Supported in part by National Cancer Institute Grant P30CA56036-03;
a Bridge Grant (to A. P. D.) from the Office of the Dean, Thomas
Jefferson University; seed grants (to P. W. and A. P. D.) from the
Radiation Therapy Oncology Group (RTOG); and a grant to the RTOG
[RTOG from the Commonwealth of Pennsylvania, Tobacco Settlement
Act (to R. B. and A. P. D.)]. A. P. D. has unrestricted grants from
Merck, Pharmacia, AstraZeneca Pharmaceuticals, and Entremed Pharmaceuticals. P. W. has an unrestricted grant from AstraZeneca Pharmaceuticals. R. B. has unrestricted grants from Aventis and AstraZeneca
Pharmaceuticals.
2
To whom requests for reprints should be addressed, at Thomas Jefferson University, Department of Radiation Oncology, 111 South 11th
Street, Philadelphia, PA 19107. Phone: (215) 955-5238; Fax: (215) 9555825; E-mail: [email protected].
ing these agents with ionizing radiation. Optimal scheduling
of combined treatment with these agents and ionizing radiation will ultimately depend on understanding how tumor
oxygenation changes as tumors regress and regrow during
exposure to these agents. This review article explores the
complex interactions between antiangiogenic/vasculature
targeting agents and radiation and offers insight into the
mechanisms of interaction that may be responsible for improved tumor response to radiation.
Introduction
Ionizing radiation is an effective modality for the treatment
of many tumors. It is a widely used treatment for cancer, with
over half of all cancer patients receiving radiation therapy
during their course of treatment (1). Although widely used, a
need remains to improve the cure rate by radiation therapy
alone. The most frequent treatment is to combine cytotoxic
chemotherapeutic agents with radiation (2, 3). The cytotoxicity
of chemotherapeutic agents, however, is not limited to tumor
cells because treatment of tumors with these agents can result in
significant normal tissue toxicity.
There has been a recent rapid development of two new
classes of drugs termed antiangiogenesis and vascular targeting
agents that target the relatively genetically stable tumor-associated vasculature (endothelial cells) rather than the genetically
unstable tumor cell (4). The importance of targeting tumor
vasculature development and function first became apparent in
the 1970s through the seminal studies of Judah Folkman (5),
who demonstrated that angiogenesis is important for the growth
and survival of tumor cells. The relationship between angiogenesis and tumor growth suggests that both tumor cells and their
supporting endothelial cells are potential targets for cell killing
and should be considered when planning cancer treatment (6).
At least four theoretical advantages exist for considering tumor
endothelial cells as targets for cancer therapy: (a) endothelial
cells are more easily accessed by antiangiogenic/vascular targeting agents compared with drugs that act on tumor cells
directly and have to penetrate large bulky masses; (b) antiangiogenic/vascular targeting agents may avoid tumor drug resistance mechanisms because they are directly cytotoxic to endothelial cells; (c) angiogenesis occurs in very limited
circumstances in adults (wound healing and ovulation), thus
antiangiogenic therapies targeting specific receptors on proliferating tumor endothelium potentially are safe and should avoid
normal tissue toxicities; and (d) because each tumor capillary
potentially supplies hundreds of tumor cells, targeting of the
tumor vasculature should lead to a potentiation of the antitumorigenic effect (7).
Investigations of antiangiogenic/vascular targeting agents
that have been conducted in preclinical and clinical trials indicate that tumor cures are limited when these agents are used as
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1958 Antivascular Therapy and Ionizing Radiation
the sole method of treatment (8). Some recent preclinical studies
suggest that the combination of radiotherapy and angiogenic
blockade enhances the therapeutic ratio of ionizing radiation by
targeting both tumor cells and tumor vessels (9 –12). At present,
there is great interest in combining antiangiogenic/vascular targeting strategies with conventional cytotoxic therapies such as
radiotherapy to improve therapeutic gain. The mechanisms by
which tumor response to radiation is enhanced by these new
agents, however, are not currently understood. In light of the
known fact that oxygen is a potent radiosensitizer, the combination of ionizing radiation and antiangiogenic/vascular targeting agents would appear to be a counterintuitive approach to
tumor cure because a reduction in tumor vasculature would be
expected to reduce tumor blood perfusion and reduce oxygen
concentration in the tumor. Some recent studies, however, indicate that oxygen levels may actually increase after treatment
with antiangiogenic agents and ionizing radiation (13–16).
Because the mechanisms of interaction between ionizing
radiation and antiangiogenic/vascular targeting agents are not
fully understood, the ideal way to use this potentially powerful
combination for tumor cure has yet to be determined. The
purpose of this review is to examine possible mechanisms of
interaction between antiangiogenic/vascular-induced cell death
and cell death resulting from ionizing radiation, in the expectation of aiding in the design of rational protocols for treatment
with antiangiogenic/vascular targeting agents and radiation.
Role of Angiogenesis in Primary Tumor Growth
and Metastasis
A competent and expanding vascular supply is a necessary
component of the progressive growth of solid tumors because
cells in solid tumors, like normal tissue, must receive oxygen
and other nutrients to survive and grow (17). The connection
between oxygen supply and tumor growth was first made in the
1950s by radiobiologists who were aware of oxygen as a radiosensitizer and the fact that hypoxic cells in tumors are resistant
to radiation therapy (18). Histological analyses of human and
rodent tumors performed by Thomlinson and Gray (18) were the
first studies to suggest that regions of viable cells exist close to
tumor blood vessels and that these walls or cords of viable
tumor cells correspond in thickness to the distance that oxygen
can diffuse (1–2 mm3). The “tumor cord” model implied that
hypoxic cells exist in a state of oxygen and nutrient starvation at
the limits of the diffusion range of oxygen, and it was hypothesized that tumor cells could proliferate and grow only if they
were close to a supply of oxygen from tumor stroma. In the
1970s, Folkman (5) corroborated the earlier findings of radiobiologists and proposed the importance of tumor vasculature as
a viable target for anticancer therapy. He reported that a tumor
without an adequate blood supply would grow only to a few
thousand cells in size or around 1–2 mm3, which is the distance
that nutrients can enter tumor cells by passive diffusion (5). To
increase in size beyond this passive diffusion-limited state, the
growing tumor mass must acquire new blood vessels. A switch
to the angiogenic phenotype allows the tumor to expand rapidly.
This so-called “angiogenic switch” (19) is regulated by environmental factors and by genetic alterations that act to either
up-regulate proangiogenic factors (i.e., VEGF3 and bFGF) and
transforming growth factors (TGF-␣ and TGF-␤) and/or downregulate inhibitors of angiogenesis [i.e., angiostatin, endostatin,
thrombospondin, and IFN-␣ (20)]. Tumor angiogenesis is a
multistep process of degradation of the extracellular matrix,
migration and proliferation of endothelial cells from postcapillary venules, and, finally, tube formation (21). More than 20
endogenous activators and inhibitors have been identified in this
process (7). The initial step in the process is the activation of
quiescent endothelial cells by binding of tumor-produced or
stromal-produced growth factors to endothelial receptors. VEGF
is the most potent and specific growth factor for endothelial cell
activation (22). Evidence for the importance of VEGF-induced
angiogenesis in tumor growth was demonstrated by use of
neutralizing antibodies or a dominant-negative soluble receptor
to inhibit VEGF action and growth of primary and metastatic
experimental tumors (10, 23). VEGF also functions as a powerful antiapoptotic factor for endothelial cells in new blood
vessels (24). VEGF is secreted by almost all solid tumors (25).
Leukemias are now also considered to be angiogenesis-dependent malignancies and have been shown to express VEGF
(26 –28). Factors that lead to up-regulation of VEGF expression
and secretion include external stresses such as ionizing radiation
produced in the application of radiation therapy and tumor
microenvironment factors such as hypoxia or a decrease in pH
(29, 30). Hypoxia is the most potent stimulus for induction of
VEGF, which occurs by activation of Src kinase. Src kinase
activation leads to an increase in HIF-1␣ and consequent upregulation of VEGF expression (31, 32). Growth factors stimulating VEGF production include IGF-I and -II, epidermal
growth factor, and PDGFs. Two specific signal pathways are
known to mediate the up-regulation of VEGF: (a) the phosphatidylinositol 3⬘-kinase/Akt (protein kinase B) signal transduction
pathway, which leads to stabilization of HIF-1␣ (33, 34); and
(b) the MAPK pathway, in which activation of extracellular
signal-regulated kinase increases transcription of the VEGF
gene (see Fig. 1 for summary of signaling mechanisms; Ref. 35).
Mutant ras oncogenes and loss or mutation of the tumor suppressor gene, p53, can also result in increased VEGF expression
and angiogenesis (36, 37).
Although the growth of solid tumors depends on angiogenesis for generation of a vascular network, the amount of newly
formed vessels in the tumoral stroma does not necessarily lead
to increased blood flow (38). This inequality exists because
newly formed microvessels in most solid tumors are abnormal
when compared with the morphology of the host tissue vasculature (39). Endothelial cells lining tumor blood vessels differ in
many respects with regard to gene expression from those of
normal vasculature (40). Because of lack of adequate vascular
3
The abbreviations used are: VEGF, vascular endothelial growth factor;
VEGFR, VEGF receptor; bFGF, basic fibroblast growth factor, TGF,
transforming growth factor; IGF, insulin-like growth factor; PDGF,
platelet-derived growth factor; MAPK, mitogen-activated protein kinase; IFP, interstitial fluid pressure; HIF-1␣, hypoxia-inducible factor
1␣; PG, prostaglandin; EBRT, external beam radiation therapy; EGFR,
epidermal growth factor receptor; COX, cyclooxygenase; rhAngiostatin,
recombinant human angiostatin.
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Clinical Cancer Research 1959
Fig. 1 Signaling mechanisms involved in VEGFinduced tumor angiogenesis. Endothelial cell survival and neovascular processes are induced by
VEGF/VEGFR-2 signaling via the phosphatidylinositol 3⬘-kinase/Akt signaling axis. VEGF is upregulated by external environmental stress (i.e.,
ionizing radiation) and internal tumor microenvironmental stress (i.e., hypoxia).
maturation (41), vessels are often dilated and tortuous (42), have
an incomplete or missing endothelial lining, and have interrupted basement membranes that result in an increased vascular
permeability with extravasation of RBCs and blood plasma
expanding the interstitial fluid space (39). Moreover, blood flow
is often erratic, with stasis or even reversal of blood flow within
individual vessels (43). Extravasation of macromolecules and
development of high IFP often results in vascular collapse (42,
44, 45). Consequently, oxygen availability to the tumor cells
shows great variability. Many human tumors exhibit hypoxic
regions that are heterogeneously distributed within the tumor
mass (46) Low perfusion rates and hypoxia may then coexist
with high nonfunctional vascular density, creating hypoxic regions (41). In these regions of hypoxia, endothelial cells may
up-regulate survival factors to maintain their integrity and prevent apoptosis (47). Thus, so-called “angiogenic hot spots” or
localized regions of intense angiogenesis may be created and
may be associated with failure of radiotherapy (48). However, it
is not clearly known whether hypoxia in the tumor produces
angiogenic hot spots or whether these regions of intense angiogenesis are a property of the genomic instability of the tumor
cells themselves, resulting in up-regulation of proangiogenic
molecules.
Ionizing Radiation and Hypoxia
Radiation-induced cell death is usually attributed to DNA
damage to tumor cells, thus triggering cell death by apoptosis
and/or necrosis. Oxygen is known to be a potent radiosensitizer
and, through interaction with the radicals formed by radiation, is
essential for the induction of radiation-induced DNA damage.
Cells irradiated in the presence of air are about three times more
sensitive than cells irradiated under conditions of severe hy-
poxia (49). Hypoxia, then, contributes to radiation resistance. As
already discussed in an earlier section, the “tumor cord” model
of early radiobiologists implied that hypoxic cells exist in a state
of oxygen and nutrient starvation at the limits of the diffusion
range of oxygen, and it was hypothesized that tumor cells could
proliferate and grow only if they were close to a supply of
oxygen from tumor stroma [i.e., 1–2 mm3 (18, 50)]. Today,
however, it is known that the width of viable regions of tumor
cells can vary quite widely, depending on tumor microenvironmental factors such as pO2 and respiratory metabolism of viable
cells (51). Experimental and clinical studies have provided both
direct and indirect evidence for the presence of hypoxic cells in
tumors (52, 53). Regions of viable cells exist in proximity to
tumor blood vessels, whereas regions of necrosis are observed at
increased distances from blood vessels. Viable hypoxic cells, at
very low oxygen tensions, are believed to exist at the edges of
necrotic regions. Hypoxia resulting from diffusion-limited processes is known as chronic hypoxia. It is also known that in
addition to these radiobiologically hypoxic cells, there is a
distribution of tumor cells at intermediate oxygen levels that can
influence response to low-dose fractionated radiotherapy (54,
55). Hypoxia can also be the result of intermittent blood flow
arising from the abnormal tumor microvasculature (56 –58).
This acute hypoxia is distinct from that of chronic hypoxia
because the affected cells would be expected to have intermittently perfused areas that could give rise to tumor regrowth (55).
The effect of hypoxia on tumor control is that the existence of
viable cells in a microenvironment at low oxygen tensions can
give rise to populations of tumor cells that are not only radioresistant but also highly angiogenic and are potentially resistant to
antiangiogenic therapy as well (59, 60).
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1960 Antivascular Therapy and Ionizing Radiation
Table 1
Agent
TNP-40
Angiostatin
VEGF/VEGFR inhibitors
COX-2 inhibitors
EGFR inhibitors
Vascular targeting agents
(i.e. ZD6126)
Antiangiogenic and vascular targeting agents: target and activity
Target
Activity
Protein myristoylation and dependent signaling
pathways in endothelial cells
Endothelial cell membrane receptor (possibly
membrane proton pump)
Neovascularization processes; endothelial cell
survival pathways; tumor cell survival
pathways indirectly and possibly directly
through autocrine signaling loops
Prostaglandin synthesis
VEGF synthesis, apoptotic signaling in tumor
cells and endothelial cells
Microtubular cytoskeleton in endothelial cells
Radiation and Angiogenesis: A Vicious Cycle
Tumor vasculature is abnormal, and the endothelial cells
lining tumor blood vessels have different phenotypic properties
from those of normal vasculature (41). Consequently, increased
tumor angiogenesis, as indicated by increased microvessel density or by increased VEGF expression, does not necessarily
correlate with increased blood flow and oxygen availability.
This situation, together with the existence of heterogeneous
hypoxic regions within tumors, makes it difficult to predict how
tumor angiogenesis will affect response to radiation therapy in a
particular tumor. As already noted, hypoxia leads to radiation
resistance because of lack of oxygen to facilitate DNA damage
by radiation-induced free radicals. Hypoxic conditions also create a microenvironment in which tumor cells become less angiogenesis dependent, more apoptosis resistant, more capable of
existing under hypoxic conditions, and more malignant because
of the development of genomic instability and mutant genotypes
impacting on apoptosis/survival signaling pathways (61). Hypoxic tumor cells are particularly known to up-regulate HIF-1␣,
which increases the expression of VEGF (30, 62). All these
factors suggest diminished sensitivity to antiangiogenic therapy
as well as radiation therapy.
Strong evidence exists that cytotoxic therapy alone, such as
radiation, can result in intensification of angiogenic processes
(48). Direct up-regulation of VEGF after irradiation of various
cancer cell lines has been reported (10). This response is part of
the overall cellular response to stress and is associated with the
induction of a variety of transcription factors that can activate
transcription of cytokine, growth factor, and cell cycle-related
genes. The products of these genes regulate intracellular signaling pathways through tyrosine kinases, MAPKs, stress-activated
protein kinases, and ras-associated kinases. These multiple pathways affect tumor cell survival or alter tumor cell proliferation.
With regard to angiogenesis, radiation exposure can result in
activation of the EGFR, which, in turn, can activate the MAPK
pathway (63). MAPK signaling is linked to increased expression
of growth factors, such as TGF-␣ and VEGF. It is possible that
radiation therapy itself contributes to radioresistance by upregulating and intensifying angiogenic pathways. The increased
tumor cell proliferation that is often seen after radiation may be
the result of up-regulated angiogenic pathways (48) as well as
Up-regulation of apoptotic pathways in endothelial cells
Inhibition of pH regulation and up-regulation of apoptotic
pathways in endothelial cells
Inhibition of endothelial cell migration, tube formation,
etc.; up-regulation of apoptosis in endothelial cells
through down-regulation of key signaling molecules,
i.e. survivin, BCL-2
Inhibition of VEGF synthesis in tumor cells; upregulation of apoptotic signaling pathways in tumor
cells and endothelial cells
Inhibition of tumor angiogenesis; up-regulation of
apoptosis in both tumor cells and endothelial cells
Vasculature shutdown leading to tumor necrosis
increased proliferation in the tumor stem cell compartment (64).
Although many tumors reoxygenate after radiation, in tumors
that are unresponsive to radiation therapy, up-regulated angiogenesis after radiation may lead to factors contributing to radiation resistance such as increased vascular permeability, increased IFP, decreased tumor perfusion, increased oxygen
consumption, increased hypoxia, and up-regulated survival
pathways (14, 45, 65). These factors all contribute to making
radiation therapy less effective in some tumors (62, 66, 67).
Radiation and Antiangiogenic Interactions
The existence of tumor microenvironmental factors, such
as hypoxia, that can promote the up-regulation of angiogenic
and survival pathways and hence resistance to radiation therapy
has prompted studies combining antiangiogenic agents with
radiation in an effort to overcome this resistance. Teicher et al.
(15) were the first to show an increased response to single-dose
radiotherapy with antiangiogenic agents. A number of preclinical studies have since indicated that antiangiogenic agents can
enhance the tumor response to radiation [see Tables 1 and 2
(9 –11, 13, 14, 68 –75)]. However, it is difficult to compare and
interpret relative efficacies of different antiangiogenic agents
with radiation because of the variability in experimental conditions reported in the literature such as tumor models, tumor host
strain, starting tumor size, final tumor volume measured, and
dosing and scheduling (see Table 2). As expected, different
tumor models can vary a great deal in their responses to antiangiogenesis treatment. The differences in tumor response to
antiangiogenic agents may occur because of differences in angiogenic growth patterns arising from differences in the relative
levels of angiogenic growth factors such as VEGF that stimulate
angiogenesis. For example, the U87 human glioblastoma xenograft model that appears in many studies is known to be a
highly vascularized aggressive tumor and exhibits high levels of
VEGF, which would theoretically make it more resistant to
therapy compared with tumors with lower VEGF expression,
and thus it would require higher concentrations of antiangiogenic agents for efficacy (76). Differences in tumor response to
angiogenic agents may also result from the presence of different
patterns of multiple angiogenic factors among different tumors
influencing tumor growth and angiogenesis. This would explain
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Clinical Cancer Research 1961
Table 2
Antiangiogenic agent
Fumagillin analogue
TNP-470
TNP-40
Antiangiogenic agents in combination with radiation therapy (RT)
Tumor model
Mouse mammary
carcinoma
Human glioblastoma
multiforme (U87)
Tumor host
C3H/He mice
4
Athymic nude
mice
7
Angiostatin
Recombinant Adk3 Rat C6 glioma
Athymic nude
(expressing
mice
angiostatin)
Angiostatin
Murine Lewis lung
C57BL/6 mice
carcinoma (LLC)
Human malignant
Athymic nude
glioma (D54)
mice
Human squamous cell Athymic nude
carcinoma (SQ-20B) mice
Human prostate
Athymic nude
adenocarcinoma
mice
(PC3)
VEGFR-2 blockade
DC 101
Human small cell lung Athymic nude
carcinoma (54A)
mice
Human glioblastoma Athymic nude
multiforme (U87)
mice
SU5416
Murine glioblastoma C57BL6 mice
(GL261)
SU6668
Murine mammary
carcinoma (SCK)
PTK787/ZK222548 Human colon
carcinoma
Anti-VEGF antibodies
Anti-vegf 164, 165 Murine Lewis lung
carcinoma (LLC)
Human esophageal
adenocarcinoma
(Seg-1)
Human squamous cell
carcinoma (Sq 20B)
Human glioblastoma
multiforme (U87)
VEGF mAb A.4.6.1 Human glioblastoma
multiforme (U87)
A/J mice
C225 antibody
COX-2 inhibitors
SC⬘-236
Mouse sarcoma
(NFSA)
Reference
no.
10 Gy/5 fractions Additive effect with 2–4 doses of
100 mg/kg drug after RT
10 Gy/single
Greater than additive effect with
fraction
daily dose of 6.7 mg/kg
before RT
75
73
69
12
40 Gy/2 fractions Significant tumor regression
25 mg/kg drug given
30 Gy/5 fractions Significant tumor regression
25 mg/kg drug given
50 Gy/10 fractions Significant tumor regression
25 mg/kg drug given
40 Gy/10 fractions Significant tumor regression
25 mg/kg drug given
12
9
11
11–12
8
8
10–11,
11–12
6–7
C57BL/6 mice
9–10
Athymic nude
mice
9–10
Athymic nude
mice
Athymic nude
mice
Athymic nude
mice
9–10
C3Hf/Kam mice
Effect of RT ⫹ drug
(end point: tumor growth delay)
7.5 Gy/3 fractions Greater than additive with vector
given after RT
7
Human squamous cell
carcinomas
SCC-1
Athymic nude
mice
SCC-6
Athymic nude
mice
Human head and neck Athymic nude
carcinoma (A431)
mice
Total radiation
dose
6–7
Athymic nude
mice
Colon adenocarcinoma Athymic nude
mice
EGFR inhibitors
ZD1839 (Iressa)
Tumor
diameter at
start of
treatment
(mm)
9–10
6
6
⬃5
⬃5
8
6
with
with
with
with
5–24 Gy/5
Additive effect in both models
fractions
with 6 ⫻ 20–40 mg/kg drug
5–24 Gy/5
Given during RT
fractions
24 Gy/8 fractions Greater than additive effect with
0.75 mg/kg drug given during
RT
15 Gy/single
Greater than additive effect with
fraction
100 mg/kg injected daily for 2
days before and after RT
12 Gy/4 fractions Greater than additive with 4 ⫻
100 mg/kg drug injected on 4
consecutive days during RT
40 Gy/2 fractions Greater than additive with 10 ␮g
of drug given before each
fraction for all models
20 Gy/4 fractions Greater than additive with 10 ␮g
of drug given before each
40 Gy/4 fractions Greater than additive with 10 ␮g
of drug given before each
40 Gy/8 fractions Greater than additive with 10 ␮g
of drug given before each
20 Gy, 30 Gy/
Greater than additive with 100 ␮g
single fraction
of drug given 6⫻ on alternate
days prior to RT
20 Gy/30
Additive with 100 ␮g of drug
Gy/single
given 6⫻ on alternate days
fraction
prior to RT
21 Gy/7 fractions Synergistic effect with 0.5 mg
(orally) in 12 doses during RT
21 Gy/7 fractions Additive effect with 0.5 mg drug
(orally) in 12 doses during RT
18 Gy/single
Greater than additive effect with
fraction
1 mg of drug in 3 doses
before and after RT
25–80 Gy/single
fraction
Greater than additive effect with 6
mg/kg drug before and during
and after radiation for 10
consecutive days
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72
68
13
70
10
14
71
84
74
1962 Antivascular Therapy and Ionizing Radiation
the lack of efficacy of the single agent in antiangiogenesis trials.
Differences in tumor response to antiangiogenic agents may also
be influenced by the organ microenvironment in which the
tumor is growing during the study. Studies of ectopic versus
orthotopic tumor implantations have revealed differences in
response to antiangiogenic treatments (73, 77).
The size of transplantable tumors at the start of an experiment is also an important factor to consider when examining
tumor response to radiation in murine systems. Tumor growth
delay studies reported in the literature vary greatly with regard
to tumor size at start of treatment as well as tumor volume that
is permitted to be reached during treatment until a statistically
meaningful effect is seen. Tumor size can affect oxygen tension,
nutrient supply, and pH, which are all factors in determining
radiation response. Radioresistance is directly proportional to
tumor size (Fig. 1). As tumor size increases, oxygen tension and
pH decrease because of a greater demand for oxygen and
nutrients. Many murine tumors, even tumors as small as 6 ⫻ 6
mm, have low oxygen tension (1–10 mm Hg) and are considered
hypoxic. As tumors become increasingly large, oxygen is unavailable, and glycolysis dominates, leading to acidosis. Eventually, the lack of oxygen and nutrients and chronic exposure to
low pH lead to tumor necrosis. When a tumor is allowed to
reach a size beyond 2000 mm3 in volume, a volume large
enough to create hypoxia, necrosis, and consequent radioresistance, the comparison of antiangiogenic treatment plus radiation
with radiation alone under these conditions may result in an
underestimation of the tumor growth delay. In some tumor
models, for example, radiation resistance emerges at ⬃250
mm3. In tumors larger than 500 mm3, tumor oxygen tension is
less than 2 mm Hg, and radiation resistance is significant
(Fig. 2).
Another factor that may influence tumor growth delay is
the tumor bed effect, which can prolong tumor growth delay
indirectly through the late effects of radiation on tumor stroma
[vasculature/connective tissue (78)]. Other factors such as the
type of anesthetic or restraint used to irradiate the animal, as
well as animal and tumor temperature, should also be considered
because they all could affect perfusion. Special attention should
be paid to generalizations about relative efficacies of treatments
based on individual in vivo tumor growth delay studies. With
these caveats in mind, some of the antiangiogenic strategies that
have been used in combination with radiation are reviewed
below.
The list of antiangiogenic agents used in combination with
radiation include angiostatin (9, 12), anti-VEGF or VEGFR
antibodies and tyrosine kinase inhibitors (10, 13, 14, 68, 72),
TNP-470 (15, 73, 79), COX-2 inhibitors (80 – 83), and antiEGFR inhibitors (71, 84). The mechanisms of enhancement are
believed to involve both direct and indirect targeting of tumor
cells and/or endothelial cells (see Table 1). The antiangiogenic
and antitumor effects have been reported to be additive as well
as synergistic. Possible mechanisms of enhanced tumor response include (a) indirect inhibition of vessel formation by
inhibition of vascular growth factors and endothelial receptors
(14), (b) direct radiosensitization of endothelial cells [endothelial cell apoptosis (72, 85, 86], (c) direct radiosensitization of
tumor cells (i.e., tumor cell apoptosis) (87), and (d) decrease in
number of hypoxic cells [improved oxygenation (14, 15, 88)].
Fig. 2 Model of tumor oxygen tension, pH, and radioresistance as a
function of size. As a tumor grows larger, the demand for oxygen
increases. However, the vascular network is unable to supply the tumor
with enough oxygen to maintain an oxygenated state, and the tumor
becomes hypoxic. To produce sufficient energy for cell survival, the rate
of glycolysis and nutrient consumption is increased, and the pH becomes acidic through lactic acidosis. The lack of oxygen and nutrients
and state of chronic low pH eventually lead to necrosis, and pH may
increase. The increase in tumor burden and hypoxic low pH environment result in tumor radioresistance. The theoretical curves are based on
pO2 and pH data obtained in our laboratory using U87 glioma and DB-1
melanoma xenografts.
Radiation and VEGF/VEGFR Signaling Pathway
Inhibitors
Some studies targeting the VEGF/VEGFR signaling pathway in conjunction with radiation have been reported. Gorski et
al. (10) found that antibodies to VEGF, when combined with
ionizing radiation in vitro, resulted in increased endothelial cell
death without affecting tumor cells. Greater than additive antitumor effects were noted in a variety of tumor model systems in
vivo. It was then proposed that enhancement of radiation response occurs through inhibition of VEGF-induced protection
against radiation damage to endothelial cells. This suggestion
was supported by a recent study showing that SU5416, an
inhibitor of VEGFR kinase, increased the radiation-induced
apoptosis in endothelial cells (68). It has also been reported that
SU668, which inhibits VEGF, bFGF, and PDGF receptor kinases, which are angiogenic growth factors, caused apoptosis in
endothelial cells in vivo (89). It was then proposed that VEGF,
fibroblast growth factor, and PDGF may be required not only for
neoangiogenesis but also for survival of existing endothelial
cells and for maintenance of existing tumor vasculature.
In addition to growth factors and their receptors, other
factors associated with endothelial cell survival exist such as
maintenance of association with perivascular cells (pericytes
and vascular smooth muscle cells), integrin interaction with the
extracellular matrix, and antiapoptotic proteins such as survivin
and BCL2 (7, 47, 60). Endothelial cells, like other cells, also
depend on intracellular pH-regulatory mechanisms to survive. If
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they fail, metabolism ceases, and apoptosis ensues (90). It has
been proposed that angiostatin may compromise endothelial
intracellular pH and hence survival by inhibiting a major proton
pump in the cell membrane of endothelial cells (91).
Because most endothelial survival factors interact with
VEGF function in complex pathways, VEGF seems to be the
predominant stabilizing factor (47). More recently, Gupta
et al. (86) demonstrated that genetically engineered VEGFproducing xenografts were more resistant to the cytotoxic
effects of ionizing radiation than xenografts that do not
produce VEGF (86). Because the VEGF-null and the VEGFproducing xenograft cell lines exhibited identical radiosensitivities in vitro, their observations support the hypothesis that
radiation targets the tumor endothelial cells as well as the
tumor cells under conditions in which protective survival
factors are diminished. These observations further confirm
earlier studies that suggest that VEGF mediates tumor radioresistance by directly affecting the radiosensitivity of the
endothelial cells.
Effects of COX-2 Inhibitors on Angiogenesis and
Radiation Response of Tumors
Although the exact mechanisms for improved response
in tumors are not known, the rationale for using COX-2
inhibitors with ionizing radiation is based on the angiogenic
tumor environment and the biochemical responses of tumors
to radiation (92). The COX-2 enzyme is induced in cells after
injury or stress, such as radiation, and catalyzes the synthesis
of PGs from arachidonic acid. PGs serve to protect cells and
tissue from radiation damage (92) and promote angiogenesis
by the up-regulation of factors such as VEGF (93). Therefore,
pretreatment with COX-2 inhibitors before exposure to stress
may inhibit the inflammation response induced by PGs. Furthermore, COX-2 inhibitors at clinically relevant doses, such
as those achieved during the treatment of arthritis, may act
directly on endothelial cells to inhibit proliferation and to
increase their intrinsic sensitivity to radiation (80). The ability of COX-2 inhibitors to enhance radiation therapy has been
demonstrated in murine and human tumor models (83, 74,
94 –96). For example, it was demonstrated that the radiation
response of a murine sarcoma was increased if tumor-bearing
mice were pretreated with a COX-2 inhibitor before radiation
therapy (74). Tumor growth delay by drug plus radiation was
significantly increased over drug or radiation alone, and the
TCD 50 (dose of radiation required for 50% tumor cure) was
reduced by almost half in the drug plus radiation group. To
elucidate the mechanism of sensitization, an intradermal injection of tumor cells was used to visualize and count newly
forming vessels. Neovascularization was shown to precede
tumor growth, and pretreatment with a COX-2 inhibitor effectively reduced neovascularization, resulting in reduced
tumor growth.
There is also direct evidence that down-regulation of PGs
by COX-2 inhibitors reduces angiogenesis and increases radiosensitivity. In another study with a murine fibrosarcoma expressing high levels of PGE2, radiosensitization was again observed by pretreatment of mice with a COX-2 inhibitor (95).
COX-2 expression was not affected by treatment, and no in-
crease in the amount of tumor cell apoptosis was observed, but
PGE2 was significantly decreased. PGE2 is a vasoactive compound that can affect tumor perfusion as well as stimulate tumor
growth and induce angiogenesis. The effect of the COX-2
inhibitor on PGE2-mediated angiogenesis was determined to be
a key mechanism in the increased radiation response.
The concept that COX-2 inhibitors can reduce the radiation
dose required to achieve tumor control caused great excitement
among radiation oncologists and stimulated the writing of many
clinical protocols. However, preclinical studies on human tumor
xenografts showed that, although COX-2 inhibitors increased
the effectiveness of radiation, inhibition of angiogenesis was not
a factor in the improved response. Authors concluded that in
human tumors, the intrinsic sensitivity to radiation was increased (96), possibly via a cell cycle mechanism (97). An
increase in the incidence of apoptosis was also a factor (83).
Experimental evidence in murine tumor systems indicates that
inhibition of angiogenesis by COX-2 inhibitors is a factor in the
increased radiation response. However, in human tumor models,
other factors may play a predominant role in the increased
radiation sensitivity.
The existing approval of these agents for noncancerous
conditions has allowed clinicians to use COX-2 inhibitors in a
large number of clinical trials in combination with radiation and
chemoradiation. To date, these trials are still too immature to
yield meaningful outcome data.
Effect of Radiation and Antiangiogenic Agents on
Tumor Oxygenation
Enhancement of tumor response to radiation plus antiangiogenic agents has also been explained by an increase in tumor
oxygenation after treatment. Tumor oxygenation is a function of
perfusion and consumption of oxygen, including consumption
of oxygen by both tumor and endothelial cells. Antiangiogenic
agents can theoretically improve tumor oxygenation by reducing
the number of oxygen-consuming tumor cells and endothelial
cells, which, in turn, will reduce the overall demand for a given
amount of oxygen. Antiangiogenic agents can also theoretically
increase perfusion by reducing the number of immature, inefficient vessels (41, 98 –100) and by decreasing IFP (38, 101).
However, IFP is related to many factors, including vascular
leaking and vascular resistance. High vascular resistance may
reduce tumor IFP and, at the same time, reduce perfusion. Thus,
a reduction of tumor IFP may not necessarily improve perfusion
or oxygenation (102).
The belief that there is an improved tumor response to
angiogenesis inhibitors and radiation is derived from animal
tumor models. Fig. 3 provides a hypothetical model of how
oxygen tension could be increased by treatment with antiangiogenic drugs and radiation.
Tumor oxygen tension is a function of oxygen consumption (by both tumor cells and endothelial cells) and tumor
perfusion. In nonmalignant tissue, the blood supply is organized and efficient. Perfusion and oxygen consumption are
balanced so that the tissue is in an oxygenated steady state. In
contrast, malignant tissue contains dividing tumor cells that
outgrow their blood supply (Fig. 3a). The overgrowth causes
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1964 Antivascular Therapy and Ionizing Radiation
Fig. 3 Hypothetical model for the rationale of combining antiangiogenic agents with radiotherapy. Nonmalignant tissue has a mature organized blood
supply. Perfusion and oxygen consumption are balanced so that the tissue is in an oxygenated steady state. In normal tissue, the IFP is low. a, in
malignant tissue, an increase in tumor cell number increases oxygen consumption and demand. As oxygen tension falls, angiogenesis is induced by
cytokines to compensate for decreased oxygen tension. However, the newly formed vessels are torturous and inefficient, which may further
compromise perfusion. Production of VEGF increases tumor IFP. b, after irradiation, oxygenated cells are destroyed, leaving the radioresistant
hypoxic cells. The hypoxic cells can reoxygenate during radiation therapy and improve radiation response and tumor control. c, in cases where tumor
control is not achieved, it has been proposed that in response to irradiation, protective cytokines are expressed by the tumor that reinitiate angiogenesis.
Angiogenesis and vascular leakage induced by VEGF will compromise perfusion. d, in theory, pretreatment with antiangiogenic agents reduces the
number of inefficient vessels and increases perfusion. More oxygenated tumor cells are killed by irradiation, which reduces oxygen consumption, and
induction of angiogenesis. No regrowth is observed, and the oxygenated steady state is maintained. The tumor response to fractionated radiotherapy
is then greatly improved.
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Clinical Cancer Research 1965
oxygen tension to fall, and angiogenesis is induced by cytokines (103). The newly formed vessels are torturous and
inefficient, further compromising perfusion and resulting in
hypoxia. Expression of VEGF increases endothelial cell permeability, which can also result in high tumor IFP (21, 104),
which has been correlated with tumor hypoxia (101).
Although many biological factors can contribute to tumor
radiation response, tumor oxygen tension is one of the most
important parameters, and it will be considered here. As shown
in Fig. 3b, oxygenated cells are more sensitive to radiation and
are destroyed by irradiation, leaving the radioresistant hypoxic
cells. During a course of radiotherapy, the hypoxic cells in the
tumor may reoxygenate (105), and continued fractions of radiation can lead to tumor control. Studies in patient tumors and
tumor models indicate that reoxygenation occurs because of
tumor shrinkage (106, 107), decreased oxygen consumption
(108), and increased perfusion (105, 108, 109).
In cases where radiation therapy is ineffective, it has been
proposed that angiogenesis induced by radiation may compromise tumor response (Fig. 3c). After a radiation insult, the
surviving hypoxic cells can produce protective cytokines such
as VEGF (10). Angiogenesis can reinitiate, and the tumor cells
can reoxygenate and proliferate (110). This scenario could theoretically be avoided if treatment with antiangiogenic agents
was used before radiotherapy (Fig. 3d). A reduction in the
number of inefficient vessels and in the number of oxygenconsuming tumor cells and endothelial cells by pretreatment
with antiangiogenic agents would increase perfusion and reduce
consumption, giving rise to more oxygenated tumor cells that
are sensitive to radiation. This would prevent tumor regrowth
and maintain the oxygenated state. This scenario would be ideal
for fractionated radiotherapy because it prevents a reversion
back to the hypoxic state during the course of therapy.
In a number of studies, hypoxia was reduced by angiogenesis inhibitors (13–16). In particular, Lee et al. (14) noted that
vessel regression induced by an anti-VEGF antibody was associated with a decrease in hypoxia in a U87 xenograft. They
suggested that the observed increase in oxygenation, despite a
decrease in vascular density, was related to vascular reorganization after inhibition of VEGF. One possible mechanism by
which VEGF affects vascular organization is its effect on vascular permeability and IFP within a tumor. Its inhibition decreases vascular permeability (111) and IFP, thereby reducing
compression of vessels and allowing for normalization of vascular architecture and perfusion and consequent normalization
of pO2. Improvement in tumor oxygenation may also be explained by decreased oxygen consumption resulting from increased killing of tumor and endothelial cells (14). Lee et al.
(14) concluded that the oxygenation status of a tumor at time of
radiation is probably not the major cause of the increased
radioresponse to anti-VEGF therapy because they observed similar enhanced responses under both normoxic and hypoxic conditions. They concluded that the critical factor in producing
enhanced tumor response to radiation is more likely related to
the reduced vessel density induced by anti-VEGF treatment
rather than the state of oxygenation of the tumor at the time of
radiation.
Effect of Radiation and Antiangiogenic Agents on
Tumor Cell Apoptosis
Enhancement of tumor response to radiation plus antiangiogenesis agents has also been explained by increases in tumor
cell apoptosis relative to tumor cell proliferation (87, 112–114).
It has been observed that, on the tumor cell level, the proliferation index is unaffected, but the apoptotic index is increased
after treatment with antiangiogenic agents such as angiostatin
and TNP-470 (87, 114). Tumor cell apoptosis may occur directly by radiation-induced DNA damage to tumor cells or
indirectly by antiangiogenic and radiation-induced damage to
endothelial cells, upon which tumor cells ultimately depend.
The molecular mechanism by which damage to endothelial cells
results in increased rates of tumor cell apoptosis is not clear. It
has been speculated that angiogenesis inhibition of tumor cellexpressed autocrine growth factors and receptors (115) or loss
of endothelial-derived paracrine factors needed for tumor
growth contributes to tumor cell cytotoxicity via apoptotic
mechanisms (87, 113).
Radiation and Vascular Targeting Agents
Vascular targeting agents differ from antiangiogenic agents
in their mechanism of action. Vascular targeting agents take
advantage of the unique properties of tumor endothelium to
selectively target the tumor vasculature (116). They are directed
at the immature, rapidly proliferating tumor endothelial cells in
existing tumor vasculature rather than the multistep processes
involved in neovascularization. Vascular targeting strategies
include biological agents, such as targeted gene therapy and
antibodies to neovascular antigens, and drug-based agents (117).
The drug-based strategies have progressed the furthest in development (117). These agents include drugs related to flavone
acetic acid (118 –121) and tubulin-binding compounds (122–
127). Flavone acetic acid and its derivatives are believed to
work by inducing tumor necrosis factor ␣ (128 –130), a cytokine
known to induce hemorrhagic necrosis in experimental tumor
models (131). The tubulin-binding compounds are believed to
work by affecting the immature endothelial microtubular cytoskeleton found in tumors, leading to abnormalities in endothelial cell shape, thrombus formation, rapid vasculature shutdown, reduction in tumor blood flow, and secondary induction
of tumor necrosis (132). However, after treatment with vascular
targeting agents, viable tumor cells have been found at the tumor
periphery (122, 123, 125, 132). It has been suggested that
increased blood flow in the adjacent normal tissue, together with
probable rapid up-regulation of angiogenic factors such as
VEGF, directly facilitates rapid growth and expansion of the
remaining rim of viable tissue because of the extensive ischemic
insult (125, 133). Because the cells surviving the vascular targeting treatment are believed to be well oxygenated and consequently present an excellent target for radiation therapy, vascular targeting has been studied in combination with ionizing
radiation. A number of studies have reported an increased antitumor effect with this combined complementary approach.
Combretastatin A-4-disodium phosphate and ZD6126, potent
and selective tubulin-binding agents against tumor vasculature,
enhanced the radiation response of murine mammary carcinoma
and sarcoma (123–125). Scheduling was found to be an impor-
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1966 Antivascular Therapy and Ionizing Radiation
tant factor in the efficacy of ZD6126 when combined with
radiation (125). ZD6126 increased tumor cell killing in KHT
murine sarcoma when administered 24 h before radiation or ⱖ1
h after radiation, but it was found not to be as effective if
administered 1 h before radiation. It was suggested that blood
flow needs to be reestablished in the remaining viable tissue to
obtain maximum radiosensitization of the tumor.
Overall Tissue Response to Radiotherapy
The respective roles of endothelial cells and tumor cells in
the overall tissue response to radiotherapy still remain controversial. In mouse models, Paris et al. (134) showed that a single
large dose of radiation administered to mouse gastrointestinal
mucosa preferentially damaged endothelial cells of the gut microvasculature that lie in close apposition to the epithelial cells
lining the gut. Previous work showed that irradiation of microvascular endothelial cells resulted in the generation of ceramide,
a compound that facilitates endothelial cell apoptosis (135). In
more recent experiments, Paris et al. (134) showed that systemic
administration of bFGF, an endothelial cell survival factor, or
deletion of the acid sphingomyelinase gene (the gene upon
which ceramide generation depends) can override the apoptotic
signal from ceramide, thus protecting gut endothelial cells and
epithelial stem cells from the effects of whole body irradiation.
They concluded that the endothelial cells may represent the
principal targets for radiation and that the death of epithelial
stem cells may be a secondary event dependent on endothelial
cell death. In an analogous manner, it has been speculated that
tumor cell death in response to radiotherapy may represent a
secondary event after death of endothelial cells, on which tumor
cells ultimately depend (85, 135). However, it should be pointed
out that the concept of what is the radiation target in normal
tissue may well depend on the radiation dose per fraction. In this
regard, it is well established that the effects of a single large
dose, i.e., 15 Gy, are not equivalent in terms of normal tissue
side effects to a more clinically relevant low-dose fractionated
radiotherapy regimen of 2 Gy times 8 (136).
Clinical Trials with Antiangiogenic Agents and
Radiation Therapy
To date, only one clinical trial has been completed evaluating an antiangiogenic drug (angiostatin) with radiation therapy. rhAngiostatin is a protein consisting of the first 3 kringles
(amino acids 97–357) of human proplasminogen with a single
amino acid substitution (Asn308 to Glu308) to prevent N-glycosylation. Previous studies using angiostatin in combination with
ionizing radiation have indicated that the antitumor activity of
human angiostatin is potentiated in a number of preclinical
tumor models, resulting in a significant reduction of tumor
volume without increase in toxicity.
A single-center, open-label, dose-escalation, Phase I clinical study at Thomas Jefferson University evaluated the safety
and pharmacodynamics of three dose levels (15, 60, and 240
mg/m2/day) of rhAngiostatin i.v. protein in combination with
EBRT for the treatment of cancer patients with solid tumors
(137). Patients received rhAngiostatin i.v. 5 times/week, 30 min
before EBRT (head and neck, thoracic, or pelvic regions), for a
minimum of 25 EBRT fractions. This study had a unique design
in that it was carried out concurrently with a Phase I drug dose
clinical trial at Thomas Jefferson University (137). As safe
doses were achieved in the drug alone study, these were used for
the radiation study. Twenty-three patients were enrolled and
evaluated for safety. Three patients were not evaluable, and
three who did not complete the minimum EBRT were excluded
from response analysis. The 17 remaining patients with evaluable tumors had advanced head and neck, thoracic, or pelvic
cancers. No added toxicity was observed in normal tissue contained within the radiation portal. Mild rash was noted in three
patients. No clinical thrombotic or bleeding events occurred in
any patient. Tumor responses were demonstrated in 90% of
patients who entered the trial with measurable disease in the
radiation field. The conclusions from this Phase I trial are that
concomitant administration of daily 10-min infusions of
rhAngiostatin and EBRT is safe and does not increase radiationinduced toxicity. Local durable tumor responses (National Cancer Institute Response Evaluation Criteria in Solid Tumors)
have been observed in this Phase I study, although this would be
expected with the use of radiation alone (137).
Summary and Conclusions
Despite the expectation that agents causing tumor vessel
regression should increase hypoxia and thus limit the effectiveness of radiation, there is accumulating experimental evidence
to refute this expectation. Preclinical evidence suggests that
antiangiogenic agents reduce tumor hypoxia and provides a
rationale for combining these agents with ionizing radiation.
The combination of antiangiogenic/vascular targeting agents
and radiation therapy has led to additive or greater than additive
tumor responses. The basis for the enhanced tumor responses
lies in the complex interactions between the tumor microenvironment (tumor stroma and vasculature) and the tumor cells
themselves. Although many factors such as VEGF expression,
survivin down-regulation, endothelial sensitization to radiation,
and many other complex interactions may be affected by antiangiogenic/vascular targeting agents, these molecules and interactions ultimately affect tumor size and pO2, which are primary
factors affecting tumor response to radiation (67).
Most tumors exist in a hypoxic and low pH environment by
virtue of abnormal tumor vasculature giving rise to poor tumor
blood perfusion and high IFP. Hypoxic conditions stimulate
up-regulation of angiogenic and tumor cell survival factors by
tumor and tumor stromal cells that promote tumor and endothelial cell proliferation and high IFP, all of which give rise to
radioresistance. In response to radiation alone, oxic cells are
killed, leaving behind a hypoxic, radioresistant fraction that can
produce protective cytokines giving rise to a second wave of
angiogenesis and tumor cell proliferation. Antiangiogenic
agents, by targeting both tumor cells and endothelial cells, can
reduce tumor proliferation and improve tumor oxygenation by
inhibiting angiogenesis and high IFP through VEGF blockade.
Thus, antiangiogenic agents can decrease overall tumor resistance to radiation by targeting both the tumor stroma and the
tumor cell compartments, thereby eliminating the codependent
cycle of tumor growth and angiogenesis (see Fig. 4 for summary
of possible mechanisms of interaction between radiation and
antiangiogenic agents).
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Clinical Cancer Research 1967
Fig. 4 Possisble mechanisms
for enhanced tumor response to
radiation with antiangiogenesis
and vascular targeting therapy.
Antiangiogenic agents may
work synergistically or additively with ionizing radiation
by improving tumor oxygenation through inhibition of neovascularization processes. Antiangiogenic agents may also
improve radiosensitivity by directly and/or indirectly inhibiting protective cell survival
signaling pathways in both endothelial and tumor cells, resulting in increased apoptosis in
both the tumor cell compartment and the tumor stromal
(i.e., endothelial cell) compartment. Vascular targeting agents
may act in an additive fashion
to improve tumor response to
radiation by decreasing overall
tumor burden through vascular
shutdown and tumor necrosis
and by creating a tumor environment whereby remaining viable, well-oxygenated tumor
cells present an ideal target for
radiation therapy.
A close examination of this area of research indicates there
is great promise for future clinical treatment protocols. Because
angiogenesis is one of the most fundamental processes of development, the complexity and redundancy of the process are
not surprising. Whereas complexity and redundancy may make
treatment and interpretation of the results difficult, they also
provide for many opportunities to interfere with the angiogenic
process. Exploring which compounds and combinations of compounds will work best with radiation and how to optimize their
use will be a long and difficult task. The potential to significantly benefit patients, however, warrants further investigations.
In conclusion:
(a) Antiangiogenic agents appear to target tumor cells and
tumor vasculature by both indirect and direct mechanisms.
(b) Antiangiogenic agents can decrease overall tumor resistance to radiation by targeting both tumor cells and tumor
vasculature.
(c) Tumor growth and angiogenesis are part of a codependent cycle. Antivascular treatments can break the cycle and
prevent revascularization after radiation.
(d) The differential effects observed with combined radiation and antivascular treatments on tumor growth among various
tumor models may arise from differences in the intrinsic radiosensitivities of endothelial cells and tumor cells. The intrinsic
radiosensitivity of the endothelial cell compartment in a tumor
may be the result of differences in expression of survival factors
such as VEGF, survivin, and BCL2 that come about in response
to changes in the tumor microenvironment. The intrinsic radiosensitivity of the tumor cell compartment likewise depends on
microenvironmental factors influencing expression of growth
factors and receptors controlling tumor cell growth, proliferation, and capacity to repair DNA damage.
(e) Many factors are involved in the response to radiation
and antiangiogenic/vascular targeting agents, but ultimately
these factors affect tumor size and pO2, which are primary
factors affecting tumor response to radiation.
(f) Hypoxia not only participates in resistance to radiotherapy but can also affect the efficacy of antivascular therapy by
up-regulating angiogenic and survival pathways. Understanding
how tumor oxygenation changes during tumor regression and
regrowth during exposure to antivascular agents and radiation is
critical for the optimal scheduling of combined treatments.
Acknowledgments
We acknowledge the expert administrative assistance of Stephanie
Lavorgna and Nichol Marrero in preparation of the manuscript.
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Tumor Response to Ionizing Radiation Combined with
Antiangiogenesis or Vascular Targeting Agents: Exploring
Mechanisms of Interaction
Phyllis Wachsberger, Randy Burd and Adam P. Dicker
Clin Cancer Res 2003;9:1957-1971.
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