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Integrin Antagonists and Other Low Molecular Weight Compounds as Inhibitors of Angiogenesis New Drugs in Cancer Therapy.

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HIGHLIGHTS
Integrin Antagonists and Other Low Molecular Weight Compounds
as Inhibitors of Angiogenesis: New Drugs in Cancer Therapy**
Athanassios Giannis* and Frank Riibsam
Angiogenesis (neovascularization), the process by which new
blood capillaries are formed from an already existing blood
vessel, is of fundamental importance for a number of physiological and pathological events including embryogenesis, wound
healing, chronic inflammation. and malignant processes.['] Due
to the observation that an adequate blood supply is necessary
for tumor growth, Folkman postulated in the early 1970s that
inhibitors of angiogenesis are of potential use in cancer therapy.['] Subsequent investigations revealed that not only tumor
growth but also tumor metastasis depends on angiogenesi~.[~]
In
recent years many endogenous regulators of angiogenesis have
been isolated and identified. The most important positive regulators include growth factors like basic fibroblast growth factor
(bFGF), vascular endothelial growth factor (VEGF), and tumor necrosis factor a (TNF-a). An additional component controlling the progress of neovascularization at several stages is a
protein derived from endothelial cells, SPARC (secreted
protein, acidic, cysteine-rich). Its proteolytic degradation generates fragments containing the sequence Gly-His-Lys, which
stimulate angiogenesis both in vitro and in v ~ v o . [The
~ ] negative
regulators include thrombospondin 1, angiostatin, platelet factor4, and interferona. The complex process of angiogenesis
proceeds in the following stages (Figure 1): a) activation of
endothelial cells (EC) and pericytes by the growth factors,
b) degradation of the basal lamina of the bloodvessel by the
action of proteases (collagenases, plasminogen activator),
c) migration (and proliferation) of EC and pericytes towards
Figure 1. Schematic representation of angiogenesis.
An angiogenic stimulator (A) triggers the secretion
of proteases (collagenases and plasminogen activator) by endothelial cells, leading to degredation of
the basal lamina of the blood vessel (B). The endothelial cells proliferate and migrate within the extracellular matrix in the direction of the angiogenic
stimulator (C). The migration within the extracellular matrix is enabled by the continous secretion of
proteases. For further details see text. P: pericyte,
EC: endothelialcell, BM: basal lamina, PEC: proliferating endothelial cell, MEC: migrating endothelial
cell.
[*I
[**I
Priv.-Dor. Dr. A. Giannis, Dr. F. Riibsam
Institut fur Organische Chemie und Biochemie der Universitiit
Gerhard-Domdgk-Strasse 1. D-53121 Bonn (Germany)
Fax: Int. code +(228)737778
e-mail: giannis(u~uni-bonn.de
This work was supported by the Deutsche I~orschungsgemeinschaft(Gi 2041
1-2) and the Fonds der Chemischcn Industrie.
588
VCH Verlugsgesi~llschaftmhH, 0-69451 Weinheim, 1997
the angiogenic stimulus, d) formation of a new capillary vessel
lumen, e) appearance of pericytes around the forming vessel,
f) development of a new basal lamina, and g) anastomosis
of the proximal ends of two forming vessels enabling blood
flow.
In the last few years considerable progress has been made in
tracing the biochemical events of angiogenesis. The mechanism
of endothelial cell migration within the extracellular matrix has
been investigated extensively. The adhesion of endothelial cells
to the extracellular matrix was shown to be mediated by the
interaction of a membrane-bound integrin avP3(vitronectin receptor) with the tripeptide sequence RGD (Arg-Gly-Asp) .[51
The binding of the ligand, which occurs in a multivalent form,
leads to integrin clusteringr6] (Figure 2). This in turn initiates
cytoskeletal proteins
w
k
clustering
.
d
Figure 2. a& Integrins and angiogenesis. The action of different growth factors
stimulates the expression of integrina,p3 on endothelial cells. During the subsequent migration of endothelkal cells in the direction of the angiogenic stimulator
(arrow), integrin%B3 binds to RGD sequences present in multivalent form on the
extracellular matrix (EM). As a consequence, integrin receptors aggregate on the
cell membrane (A) and proteins of the cytoskeleton such as talin, paxilin, a-actinin,
tensin, vinculin, and F-actin accumulate. This in turn results in maintenance of thc
migration process, serves as a signal for the survival of endothelial cells, and finally
leads to the formation of a new blood vessel. Prevention of integrin aggregation by
soluble monovalent RGD analogues (B) leads to programmed cell death (apoptosis)
of the migrating endothelial cells and therefore prevents vessel formation.
an intracellular signal cascade that supports both migration of
endothelial cells and protection of the cells from programmed
cell death (apoptosis). In contrast, interaction of a J 3 integrins
with soluble monovalent RGD ligands or their analogues causes
death of migrating endothelial cells and therefore prevents neovascularization.
The RGD sequence is a partial structure of proteins belonging to the extracellular matrix such as vitronectin, fibronectin,
osteopontin, thrombospondin, and the von Willebrandt factor.
It also occurs in the blood protein fibrinogen, which decisively
contributes to blood coagulation. The RGD group serves as a
ligand not only for the a,P3 integrin, but also for other integrins,
for example a,P1, aJ,,
and a J 6 , as well as-for the platelet
integrin aIIbp3(fibrinogen receptor). The specificity of the
RGD-integrin interaction is generated by a combination of
variations in the RGD conformation in different proteins and
contributions of sequences near the RGD moiety."] Structure-
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Angew. Chem. Int. Ed. Engl. 1997. 36, No. 6
HIGHLIGHTS
activity relationships have shown that within linear RGD-containing peptides the exchange of glycine for alanine or aspartate
for glutamate prevents recognition and binding of the resulting
peptides by the integrins.lsl
Until now the search for R G D analogues was dominated by
the demand for potent, selective and orally available allbB3
antagonists. These efforts culminated in the synthesis of nonpeptidic R G D analogues, which are currently in clinical trials as
anticoagulants.[91They constitute the first clinically valuable
antiintegrins and provide important contributions to the treatment of thromboembolic diseases. These results together with
further insights into the importance of angiogenesis for tumor
growth and metastasis prompted several groups in the pharmaceutical industry and in academia to search intensively for antagonists of the endothelial cell integrina,P3 in order to block
angiogenesis. Inhibitors of angiogenesis are not only of interest
as anticancer drugs but also for the treatment of disorders in
which neovascularization plays a critical role. These conditions
include diabetic retinopathy (leads to blindness) and arthritis
(leads to destruction of the joints).
A severe problem in the design of a,& antagonists is their
selectivity. A suitable antagonist should have a high affinity to
the vitronectin receptor as well as favorable pharmacokinetic
properties. In the long term it is desirable to develop potent,
selective, and orally available nonpeptidic ligands. The oral
availability is of particular importance since angiogenesis
should be blocked for a longer period of time. A decisive contribution to the understanding of the RGD-a,P, interaction and
therefore to the development of such drugs was provided by the
group of H. Kessler at the Technische Universitat Miinchen.
Already in 1991 these researchers incorporated the R G D sequence into different cyclopeptides and investigated their affinity and selectivity towards different integrins.["] The cyclopentapeptide c:l.clo(Arg-Gly-Asp-D-Phe-Val) (c(RGDfV)) 1 turned
out to be a selective c(J3 ligand.
1
The IC,, value for the binding of 1 to both soluble and immobilized cw,@,integrin is 50 nM.["] Therefore its affinity to this
integrin is comparable to the affinity of the endogenous ligand
vitronectin (IC5,, = 25 nM). In compound 1 the R G D sequence
forms a y-turn with glycine in the central position. N M R spectroscopic investigations in solution indicate that a parallel arA n g i w <'hem. I!?[. E d Engl. 1997. 36. N o . 6
::<;
rangement of the side chains of arginine and aspartate results.
As expected, the D-amino acid occupies the i + 1 position of a
pI1'-turn. The distance between the p carbon atoms of the Asp
and Arg side chains within peptide 1 was determined by N M R
spectroscopy to be 0.69 nm, which is considerably shorter than
the distance thought to be optimal for recognition by the fibrinogen receptor (0.75-0.85 nm). These findings were confirmed
by Bach et aI.['*]
Recently, Kessler et al. synthesized all possible stereoisomers
of peptide 1 and of its retro-sequence (32 p e p t i d e ~ ) . ( ' ~
An
] important result of the subsequent biochemical and N M R spectroscopic investigations was that the retro-inverso compound
c(vFdGr) shows a drastically reduced affinity towards the
a,P3 integrin, since its conformation is different from that of
peptide 1. More importantly, another peptide, c(VfdGr),
shows nearly no affinity to the vitronectin receptor although
the orientation of its side chains is identical to that of 1.
This means that not only the side chains but also the peptide
backbone contribute to receptor binding by the formation of at least one hydrogen bond. In the same study a sterically
demanding group in position 4 (D-Phe) was shown to be necessary for the biological activity of cyclopeptide 1. In contrast to
this, the L-valine residue can be replaced by any other amino
acid.
Brooks et al. proved that peptide 1 is suitable as a blocker of
angiogenesis.['] The authors implanted human tumor tissue
(melanoma) on the chorioallantoic membrane (CAM) of 10day-old chick embryos. Implantation is accompanied by angiogenesis caused by the tumor. IntegrinaJ, is expressed on endothelial cells and tumorous tissue is supplied with blood vessels.
After 24 hours a single dose of peptide 1 (300 m p per 100 mL)
was administered intravenously. Two days later neovascularization of the tumor tissue was observed to be interrupted. The
control peptide cyclo(Arg-Ala-Asp-D-Phe-Val) had no effect on
the growth of new blood vessels. Comparable results were
achieved with monoclonal antibodies against xVp3integrin. In
this case also a regression of different tumors implanted on
CAM was observed. The treatment had no influence on already
existing vessels of CAM. These studies showed for the first time
that a,B, antagonists are potentially beneficial in the therapy of
malignant diseases and other disorders characterized by excessive angiogenesis.
In general, antiangiogenic therapy is well tolerated, exhibits
low toxicity, and does not lead to resistance phenomena. In
studies on antiangiogenic therapy platelet factor4 as well as the
fungal metabolite fumagillin (2) (Scheme 1) and its synthetic
derivative AGM-1470 (or TNP-470) 3 were used; the latter is the
most potent low molecular weight inhibitor of angiogenesis
(IC5,, = 10 pg per mL).[15]Theangioinhibitoric action o f AGM1470 depends in part on the inhibition of cyclin-dependent
kinases in endothelial cells.['61
An additional angiogenesis blocking agent is the well-known
drug thalidomide (4), which was removed from the market in
the 1960s because of its teratogenic action. When applied orally
(200 mg per kg) it inhibits bFGF-induced corneal neovascularization in rabbits.["] Recently published data reveal that
thalidomide downregulates the expression of Pintegrins.""
This might explain both the angioinhibitoric and the teratogenic
action of thalidomide.
VCH Verlugsgesellschufr mhH. 0.69451 Weinheim, 1997
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589
HIGHLIGHTS
2
3
4
5
not sufficient for cancer therapy, but they enhance the effect of
traditional chemotherapy. After traditional therapy they can
force metastases that may still be present into a microscopically
dormant state.['b1 The fact that angiogenic therapy is generally
well-tolerated makes it probable that inhibitors of neovascularization will find broad use in the treatment of other diseases
accompanied by excessive angiogenesis, for example rheumatism, arthritis, and diabetic retinopathy. Most of the currently
employed drugs were discovered by screening, but the structure-activity relationships introduced by Kessler et al. constitute a valuable basis for the rational development of low molecular weight aV& antagonists. Future work in this area will
certainly refer to the general principles in the design of peptidom i m e t i c ~ [and
~ ~ Jto the experience in the design of uIIbB3
antagonists.
qC'
German version: Angew. Chem. 1997,109,606-609
CI
Keywords: antitumor agents
ture- activity relationships
YN
6
7
8
Scheme 1. Low molecular weight inhibitors of angiogenesis
Currently several angioinhibitoric drugs are in clinical or preclinical trials.[lbl In addition to AGM-1470 and thalidomide,
orally active protease inhibitors (inhibitors of basal lamina
degradation) are also used. This group of compounds includes
the well-known antibiotic minocyclin (9,
an inhibitor of collagena~e.['~]
The antiulcus drug irsogladin (6) suppresses the
degradation of the basal lamina by inhibition of plasminogen
activator biosynthesis.f201Another drug worth mentioning is the
fungal metabolite eponemycin (7),which was discovered in a
screening study. This natural product inhibits the proliferation
and migration of endothelial cells (IC50=77 nM and 740 nM,
respectively), thereby inhibiting neovascularization.f2'1 The
estradiol metabolite 2-methoxyestradiol(8), which does not display glucocorticoid activity, is the most recently discovered
compound of endogenous origin with angioinhibitoric action
(IC50= 130 nM).r221Its mechanism of action is not known.
In summary, the investigation of the molecular aspects of
angiogenesis has led to a deeper understanding of many pathological processes. Folkman's assumption that angiogenesis inhibitors are of potential use in cancer therapy initiated a search
for such compounds. It proved to be a valuable concept and led
to alternative forms of treatment for malignant diseases. Preclinical trials showed that inhibitors of angiogenesis per se are
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0 VCH Verlugsgesellschufi mbH, D-694Sl
Weinheim, 1997
- biochemistry - peptides
*
struc-
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