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Recent Patents on Anti-Cancer Drug Discovery, 2012, 7, 56-63
Antineoplastic Action of Growth Hormone-Releasing Hormone (GHRH)
Antagonists
Agnieszka Siejka1, Hanna Lawnicka2, Gabriela Melen-Mucha2, Ewelina Motylewska2, Jan
Komorowski1 and Henryk Stepien2,*
1
Department of Clinical Endocrinology, Chair of Endocrinology, Medical University of Lodz, 91-425 Lodz, Dr. Sterling
str 3, Poland, 2Department of Immunoendocrinology, Chair of Endocrinology, Medical University of Lodz, 91-425 Lodz,
Dr. Sterling str 3, Poland
Received: December 22, 2010; Accepted: February 9, 2011; Revised: February 28, 2011
Abstract: Some of the antagonists of growth hormone-releasing hormone (GHRH) are able to inhibit the growth of various experimental human cancers. The antitumor effects of first antagonists seemed to be dependent mainly on the disruption of pituitary secretion of growth hormone (GH), followed by the reduction in the levels of circulating insulin-like
growth factor (IGF)-1, an important growth factor for cancer cells. It seems obvious, that growth hormone deficiency
(GHD) induced by GHRH antagonists with all its complications, could limit the beneficial effects of GHRH antagonists
therapy, and decrease patients’ quality of life. The discovery of local autocrine/paracrine production of GHRH and other
related growth factors in many tumoral tissues, in combination with the wide expression of GHRH receptors on cancer
cells, directed the research to the synthesis of more potent GHRH antagonists. These compounds exert strong inhibitory
effects directly on tumor growth, with scarce endocrine action. The receptor-mediated mechanisms comprise complex and
still not completely understood effects on intracellular signaling pathways that are strictly related to human tumorigenesis.
This review summarizes recent patents and latest observations on the antineoplastic role of GHRH antagonists in human
tumors with emphasis on potential therapeutic applications in clinical oncology.
Keywords: Cancer therapy, GHRH, GHRH antagonists, IGF-I, IGF-II.
INTRODUCTION
The existence of growth hormone releasing factor was
first suggested by Reichlin in 1961, who demonstrated that
bilateral lesions of the ventromedial hypothalamus in rats
resulted in impaired linear growth in growing individuals
[1]. Despite many efforts, the growth hormone releasing factor was not identified until more than 20 years after. In 1982
two groups of researchers isolated and characterized the 40and 44- amino-acid peptides from pancreatic tumors that
induced acromegaly in the absence of a pituitary tumor [2-4].
Discovered later on hypothalamic growth hormone-releasing
hormone (GHRH) was eventually shown to be identical with
the one which was isolated from the tumors one year before
[4-6]. The structure of 44 amino acid peptide was identified
as
Tyr-Ala-Asp-Ala-Ile-Phe-Thr-Asn-Ser-Tyr-Arg-Lys-Val
Leu-Gly-Gln-Leu-Ser-Ala-Arg-Lys-Leu-Leu-Gln-Asp-Ile
Met-Ser-Arg-Gln-Gln-Gly-Glu-Ser-Asn-Gln-Gly-Arg-Gly
Ala-Arg-Ala-Arg-Leu-NH2.
*Address correspondence to this author at the Department of Immunoendocrinology, Chair of Endocrinology, Medical University of Lodz, Dr. Sterling
3 Str., 91-425, Lodz, Poland; Tel: +48426324854; Fax: +48426324854; Email: [email protected]
-9/12 $100.00+.00
The human GHRH gene has been localized in chromosome 20 [7]. It includes five exons and spans about 10-18
kilobases of genomic DNA. The precursor protein for human
GHRH contains 108 amino acids, which can be processed
into the 40- or 44-amino acid GHRH peptide [8]. The full
biological activity is retained by the N-terminal 29 amino
acid fragment [9]. GHRHs isolated from human pancreatic
tumor and from hypothalamus were shown to stimulate the
secretion of growth hormone from pituitary cells [10-12].
GHRH belongs to the pituitary adenylate cyclaseactivating polypeptide (PACAP)/glucagon superfamily of
hormones. This family includes nine hormones: GHRH, glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, glucosedependent insulinotropic polypeptide (GIP), peptide histidine-methionine (PHM), PACAP, secretin, and vasoactive
intestinal polypeptide (VIP). These peptides are related by
structure, distribution (especially the brain and gut), function
(acting often by activation of cAMP), and receptors (a subset
of seven-transmembrane receptors) [13].
GHRH is synthesized in central nervous system (CNS),
mainly in arcuate and ventromedial nuclei of hypothalamus
and then it is released from neuronal projections into the
hypothalamic-pituitary portal vascular system [8]. Upon
binding to its receptors (GHRHR) it stimulates synthesis and
release of growth hormone (GH), and possesses mitogenic
activity for pituitary cells [14]. GHRH deficiency can lead to
pituitary hypoplasia and dwarfism [15]. Extrahypothalamic
distribution of GHRH-containing neurons in the CNS indi© 2012 Bentham Science Publishers
GHRH Antagonists in Cancer
cates that GHRH may play a role in neuromodulation, such
as promoting sleep and food intake [16].
Expression of GHRH has been found in various normal
extra-pituitary tissues: ovary, placenta, testis, pancreas, gastrointestinal tract, prostate and immune cells [8, 13, 17].
Since GHRH is secreted only into the hypothalamichypophyseal portal system, the level of this hormone in the
plasma is very low. The disappearance half-time for GHRH
(1-44)NH2 is only 6 minutes due to degradation by peptidases [8]. The most important role of GHRH is the stimulation of the secretion of GH with resultant increase in the serum concentrations of insulin-like growth factor-1 (IGF-1)
and increase in linear growth in growing individuals and
many other consequences in not growing ones. Excessive
GHRH synthesis results in GH oversecretion and leads to
somatotroph hyperplasia and severe metabolic results together with excessive growth (acromegaly or gigantism).
Transgenic mice ectopically expressing human GHRH show
increased growth during growth phase and pituitary hyperplasia that can often progresses to adenoma in older animals
[18].
GHRH receptor (GHRHR) belongs to the family of Gsprotein-coupled receptors that also includes secretin, VIP,
glucagon, GLP-1, PACAP, and GIP receptors. GHRHR is a
423 amino acids protein and has seven hydrophobic transmembrane domains. The human GHRH receptor gene has
been mapped to chromosome 7. It contains more than 10
exons and spans more than 8 kilobases [19]. The 5-flanking
region of GHRHR gene contains several binding sites for
transcription factors, such as CREB (cAMP-response element), pituitary-specific transcription factor (Pit-1), nuclear
factor-kappa B (NF-B) and other factors such as estrogens
(estrogen responsive elements), glucocorticoids (glucocorticoid responsive element) [16, 19].
GHRHR is expressed predominantly in the anterior pituitary gland (pituitary-type GHRHR, pGHRHR) with lower
expression in several other tissues, such as placenta and kidney [18]. Very low levels of the mRNA for GHRHR and or
protein expression (immunocytochemistry) can be also detected in other tissues, such as gastric mucosa, neuroendocrine cells in colonic mucosa, pancreas, kidney, lung, liver,
prostate [20, 21].
Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 1 57
such as: prostate, breast, ovarian, endometrial, adrenal, lung
(both small-cell lung carcinomas [SCLC] and non-SCLC),
gastric, colorectal, brain and pancreatic cancers, bone sarcomas, lymphomas and renal carcinomas [17, 26, 27]. The expression of mRNA for GHRH was higher in some of the
neoplasms derived from the immune-cells as compared to
normal lymphocytes [28]. GHRH has been shown to act as
potent mitogen in many of human cancers. It has been also
shown to stimulate the proliferation of various cancer cell
lines such as prostate, breast, endometrial, ovarian, gastric,
pancreatic, colorectal, lung, and osteosarcomas [reviewed in
17, 26, 27, 29]. Knocking down GHRH expression in human
prostate, breast and lung cancer cell lines resulted in dramatically decreased cell proliferation, an effect which was
abolished after the addition of exogenous GHRH [30].
Therefore, it should be assumed that GHRH can be considered a potent growth factor and that deregulation of GHRH
gene transcription might contribute to the pathogenesis of
some of human malignancies.
Extensive studies report low expression of pituitary-type
GHRHRs in only some of human cancers, including lymphomas, glioblastoma and SCLC cell lines and in surgical
specimens of human non-small cell lung cancer [17, 21]. The
receptors that are commonly expressed in most of the human
cancers were identified ten years ago and are splice variants
(SVs) of the full length pGHRHR [31, 32]. The major part of
the cDNA sequence of SV1 is identical to the corresponding
sequence of pGHRHR cDNA, but the first 334 nucleotides
of SV1 are different. The protein sequence of SV1 differs
from that of pGHRHR only in the N-terminal extracellular
domain, where a 25-amino-acid sequence replaces the first
89 amino acids of pGHRHR. The mRNA sequence of SV1
and the observed molecular mass of the receptor protein
from western-blot assays are consistent with a seventransmembrane-domain receptor that has the third intracellular loop critical for the interaction with G proteins [17, 31].
Upon binding of GHRH to its receptor, this peptide
stimulates the adenylyl cyclase (AC) with resulting increase
in intracellular cAMP levels and activation of protein kinase
A (PKA). The influx and increased concentration of intracellular calcium causes the release of GH from somatotrophs
[8]. There is also direct evidence on the mitogenic action of
GHRH and activation of mitogen-activated protein kinase
(MAPK) pathway in somatotroph cells [18, 22-24] and in
GH3 cells transfected with GHRH receptor [25].
The expression of mRNA for SVs and the protein has
been demonstrated in several cancers, including prostate,
breast, colorectal, gastric, pancreatic, renal, ovarian and endometrial cancers, adrenal tumors, SCLC and non-SCLC,
bone sarcomas, glioblastomas and lymphomas [17, 27]. Furthermore, the induced expression of pGHRHR or SV1 in
MCF-7 human breast cancer cells which normally do not
produce endogenous GHRH receptors, restores their responsiveness to exogenously given GHRH [33, 34]. Cells transfected with pGHRHR or SV1 responded with increased proliferation rate to GHRH, with SV1 being more potent than
GHRHR [34, 35]. Moreover, the cells transfected with SV1
proliferated more quickly even in the absence of exogenous
GHRH, suggesting ligand-independent activity of SV1 [3436]. Thus, GHRH receptors can be considered as potential
targets for anticancer therapy.
GHRH AND RECEPTORS FOR GHRH IN CANCER
SYNTHESIS OF GHRH ANTAGONISTS
The presence of GHRH and its receptors has been demonstrated in various tumors, suggesting that this neuropeptide may be involved in the pathogenesis of neoplasms as an
autocrine/paracrine growth factor [17]. The expression of
mRNA for GHRH and/or GHRH protein has been demonstrated in surgical specimens or cell lines of human tumors,
Antagonists of growth hormone-releasing hormone
(GHRH) have been initially developed for the treatment of
disorders caused by excessive synthesis of GHRH or growth
hormone (GH). Robberecht et al. demonstrated in 1985 that
replacement of Ala at position 2 of GHRH(1-29)NH2 with
D-arginine produces GHRH antagonists [37]. This first stan-
58 Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 1
dard antagonist was able to suppress GH release in rats [37].
It also produced a 30-40% inhibition of GH levels in patients
with acromegaly from ectopic GHRH synthesis [38].
The first antagonist (Ac-[Tyr1,D-Arg2]GHRH(129)NH2) was used as a standard for the subsequent development of more potent antagonistic analogs of GHRH which
have been developed for the treatment of various cancers
[39]. The clinical need for anticancer applications of GHRH
antagonists was supported by Pollak et al. on the grounds
that somatostatin analogs do not adequately suppress GH and
IGF-I levels in patients with neoplasms potentially dependent on IGF-I [40, 41]. Inhibitory effects of these GHRH antagonists have been evaluated in a variety of human experimental cancer models.
The first potent antagonists (MZ-series) contained hydrophobic moieties at the N-terminus, the enzyme resistant
agmatine at the C-terminus, and other substitutions such as
4-chloro-phenylalanine, 2-aminobutyric acid, and norleucine
that increased the receptor-binding affinity and enhanced the
stability [42, 43 and reviewed in 17, 39]. Some representatives of this class of antagonists also contained lactam bridge
constraints. MZ-4-71 and MZ-5-156 were the most potent
antagonists in this series of analogs both in vitro and in vivo.
The incorporation of positively charged amino acids into
early antagonists led to the JV-series of analogs with further
increased bioactivity [39, 44, 45]. Replacement of the native
serine by arginine or homoarginine led to development of
antagonists with increased activity and binding affinity to the
pituitary GHRH receptors. Analog JV-1-36 induced a strong
and prolonged inhibition of GH release in vivo for at least 30
min [44]. Antagonistic activity of JV-1-38 for in vivo GH
secretion was weak, but still exerted stronger anti-tumor effects as compared to the earlier antagonists. Antagonists JV1-62 and -63 with paraamidinophenylalanine (Amp) at position 10 significantly inhibited GHRH-induced GH release
for about 1h in vivo [45]. GHRH antagonist JV-1-65, containing Amp at position 9, did not significantly inhibited GH
release in vivo but still decreased the growth of human lung
cancer lines xenografted into nude mice more potent than
JV-1-63 [46]. This led to the conclusion that antitumor effects of GHRH antagonists are not always related to the endocrine activities on the release of GH from the pituitary [17,
45].
Acylation of JV-peptides by fatty acids at the N-terminus
(either monocarboxylic or dicarboxylic, containing six to
sixteen carbon atoms) led to development of MZ-J series of
antagonists [47]. The binding affinity of these lipopeptide
antagonists to GHRH receptors was very high, about 100
fold higher than that of the standard antagonist. These compounds had superior inhibitory effects on the growth of various cancers, including prostate and pancreatic cancers and
were demonstrated to cross the blood-brain barrier [48].
Other antagonists containing histidine and ornithine instead
of arginine and lysine in positions 11, 12, 20 and 21 of the
GHRH peptide (including MZ-J-7-118, MZ-J-7-138 and
JMR-132) possessed one of the most potent antitumor effects
reported so far [17]. Antagonists with weaker endocrine effects (ex JMR-132) than JV-1-63, MZ-4-71 or MZ-5-156
exhibited greater anticancer potency. This confirmed previous observations that the oncological activity of GHRH an-
Siejka et al.
tagonists can be based on direct inhibitory effects on cancer
cells. In 2009, researchers at Ipsen Pharma S.A.S. disclosed
novel series of synthetic antagonistic analogues
(WO2009108364). The data revealed that selected peptides
could significantly inhibit the GH release from HEK293
cells transfected with GHRHR. Moreover, when administered subcutaneously for 3 weeks, they significantly inhibited the growth of MX-1 (human mammary carcinoma cells),
and PC-3 (human prostate cancer cells) xenografted into
athymic mice [49]. Data for novel fluorinated synthetic analogs of hGHRH(1-29)NH2 and hGHRH(1-30)NH2 are presented in US20100092539 and US20110066230 [50].
GHRH antagonist MIA-602 (P-12602) studied for 4 weeks
(2g/day s.c.) in male nude mice implanted with PC-3 prostate cancer inhibited tumor growth by 65.92% (p<0.001 versus control). When MIA-610 (P-12610) peptide was studied
in the same experimental model, it inhibited tumor growth
by 77.78% (p<0.001 versus control) [50]. New series of
GHRH antagonists have been developed for oncological
indications, and have already proven effective in various
experimental models (ex PC3 human prostate cancer cells,
glioblastoma and breast cancers) [50-52]. Chemical structure
of most commonly studied GHRH antagonists is presented in
Table 1 and described in detail elsewhere (including
US6057422, US7452865, US5550212, US20100092539)
[39, 42-45, 47, 50, 53-55].
MECHANISMS OF ACTION OF GHRH ANTAGONISTS
The indirect mechanism of inhibition of tumor growth by
GHRH antagonists comprises their effect on the GH-IGF-I
axis and seems to be an important approach for IGF-Idependent cancers (such as prostate, breast and colorectal
cancers). GHRH-antagonists bind to GHRH receptors located on pituitary somatotrophs and block the hypothalamic
GHRH-mediated activation of the intracellular cAMP signal
transduction pathway. This leads to the inhibition of GH
synthesis and release which results in the reduction of IGF-I
production from the liver [17, 56]. The first synthesized series of antagonists possessed strong endocrine activity and
caused significant decrease in the serum concentrations of
IGF-I. However, IGFs (IGF-I and IGF-II) can influence the
growth of various human cancers not only through endocrine, but also through autocrine and/or paracrine mechanisms [56, 57]. At least several neoplasms express receptors
for IGFs and respective ligands, and proliferate in response
to exogenous IGFs in vitro. These include numerous neoplasms, such as prostate, breast, lung and colorectal cancers
[57]. In many human cancer lines in vitro and in human cancers xenografted into nude mice, including prostate, renal,
pancreatic and colorectal cancers, glioblastomas, ovarian
cancers, and non-SCLC, GHRH antagonists inhibited the
production of IGF-I and IGF-II and the expression of IGF-II
mRNA [17, 39, 56]. However, in H-69 SCLC and other cancers, such as MDA-MB-435 breast cancers, LNCaP prostate
cancers, and in HEC-1A endometrial cancers, tumor inhibition observed after GHRH antagonists was not associated
with a suppression of IGF-I and IGF-II but due to the blockade of the stimulatory action of autocrine GHRH [17, 58,
59]. In MXT mouse mammary cancer, tumoral GH and GH
receptor levels were diminished after treatment with GHRH
GHRH Antagonists in Cancer
Table 1.
Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 1 59
Structure of Most Commonly Studied GHRH Antagonists [39, 42-45, 47, 50, 53-55].
Standard antagonist
[Ac-Tyr1, D-Arg2] hGHRH(1-29)NH2
MZ-4-71
[Ibu-Tyr1, D-Arg2 , Phe(4-Cl) 6, Abu15, Nle27] hGHRH(1-28)Agm
MZ-5-156
[PhAc-Tyr1, D-Arg2, Phe(4-Cl)6 , Abu15 , Nle27] hGHRH(1-28)Agm
MZ-4-243
[Nac0-Tyr1, D-Arg2, Phe(4-Cl) 6, Abu15, Nle27] hGHRH(1-28)Agm
JV-1-10
[PhAc-Tyr1, D-Arg2, Phe(4-Cl)6 , Arg9 , Abu15 , Nle27, D-Arg29] hGHRH(1-29)NH2
JV-1-36
[PhAc-Tyr1, D-Arg2, Phe(4-Cl)6 , Arg9 , Abu15 , Nle27, D-Arg28 , Har29] hGHRH(1-29)NH2
JV-1-38
[PhAc-Tyr1, D-Arg2, Phe(4-Cl)6 , Har9 , Tyr(Me)10, Abu15, Nle27 , D-Arg29] hGHRH(1-29)NH2
MZ-J-7-30
[HOOC-(CH2)12-CO-Tyr1, D-Arg2 , Phe(4-Cl) 6, Arg9, Abu15, Nle27, D-Arg28 , Har29 ] hGHRH(1-29)NH2
MZ-J-7-46
[CH3-(CH2)4-CO-Tyr1, D-Arg2, Phe(4-Cl) 6, Arg 9, Abu15 , Nle 27, D-Arg28, Har29] hGHRH(1-29)NH2
MZ-J-7-110
[HOOC-(CH2)12-CO-Tyr1, D-Arg2 , Phe(4-Cl) 6, Amp9 , Tyr(Me)10, Abu15, Nle27, D-Arg28 , Har29] hGHRH(1-29)NH2
MZ-J-7-114
[CH3-(CH2)6-CO-Tyr1, D-Arg2, Phe(4-Cl) 6, Amp9 , Tyr(Me)10, Abu15 , Nle 27, D-Arg28, Har29] hGHRH(1-29)NH2
JMR-132
[PhAc-Tyr1, D-Arg2, Phe(4-Cl)6 , Ala 8, Har 9, Tyr(Me)10, His11, Abu15, His20 , Nle27, D-Arg28 , Har29] hGHRH(1-29)NH2
MIA-602
[(PhAc-Ada)0-Tyr1, D-Arg2 , Fpa56, Ala8 , Har9 , Tyr(Me)10, His11 , Orn12, Abu 15, His20, Orn 21, Nle27, D-Arg28, Har29] hGHRH(129)NH2
MIA-610
[PhAc0-Tyr1, D-Arg 2, Cpa6 , Ala 8, Har 9, Fpa510, His11 , Orn12, Abu15, His20 , Orn21, Nle27 , D-Arg28, Har 29, Ada30] hGHRH(1-30)NH2
Ibu, Isobutyryl; Nac, 1-Naphthylacetyl; PhAc, Phenylacetyl; Cpa, 4-chloro-Phe; Agm, Agmatine, Decarboxy-Arg; Amp, para-Amidino-Phe; Har, Homo-Arg; Abu, 2-Aminobutyric
acid; Tyr(Me), O-Methyl-Tyrosine; Orn, Ornithine; Fpa, Pentafluoro-Phenylalanine; Ada, 12-Aminododecanoyl.
antagonists [60]. GHRH antagonists inhibited the proliferation and secretion of vascular endothelial growth factor
(VEGF) from mouse endothelial cells cultured in vitro [61].
Subsequently, it was shown that the protein and mRNA levels of tumoral angiogenic growth factors: VEGF and its receptor and/or basic fibroblast growth factor (bFGF), were
decreased by GHRH antagonists in human prostate cancer,
SCLC and non-SCLC, and lymphoma models [17, 62-64]. In
the model of ovarian cancer GHRH antagonist JMR-132
significantly decreased the level of epidermal growth factor
receptor (EGFR) and after knocking down of the expression
of EGFR by siRNA the antiproliferative effect of JMR-132
was abolished in SKOV3 and CaOV3 cells [65]. Newly synthesized GHRH antagonists MIA-313 and MIA-602 decreased VEGF secretion of OVCAR-3 cells and reduced
tumor vascularization in in a Matrigel plug assay [66].
The identification of binding sites for GHRH antagonists
confirmed the observations that this group of peptides may
also directly inhibit tumor growth. Splice variant 1 of the full
length pGHRHR was shown to exhibit strong liganddependent and ligand-independent activity in 3T3 mouse
fibroblasts and MCF-7 human breast cancer cells [34, 35]. It
has been recently demonstrated that exogenously supplemented GHRH which can act through SV1 receptor strongly
activates the MAPK, a pathway associated with the stimulation of cellular proliferation [36]. These observations suggested that the expression of this receptor in tumoral cells
may by itself lead to important intracellular changes involving the activation of pro-growth and antiapoptotic pathways.
This also supported the efforts for the development of newer
antagonists which can target tumor cells directly.
The evidence of direct inhibitory effect of GHRH antagonists on the growth of various cancer cell lines, the iden-
tification of SVs, and the synthesis of new more potent analogues, initiated studies aiming at the identification of intracellular pathways involved in oncogenesis that are affected
by GHRH antagonists. GHRH antagonists can inhibit the
PKC-MAPK (protein kinase C-mitogen-activated protein
kinase) [53, 67] and PI3K-AKT (phosphatidylinositol 3kinase-protein kinase B) [52] signaling pathways, reduce the
expression levels of c-jun and c-fos proto-oncogenes, of
wild-type and/or mutant tumor-suppressor protein p53 in
human SCLC, non-SCLC, and prostate cancer models [46,
68-71], and decrease the telomerase activity in glioblastomas
[72]. GHRH antagonists also reduced the levels of the antiapoptotic Bcl-2 protein and increased levels of BAX (a pro
apoptotic protein) and caspases in various models [reviewed
in 17]. They also trigger a calcium-dependent apoptotic
mechanism in the LNCaP prostate cancer cell line [73].
GHRH antagonists were shown to inhibit proliferation and
invasive potential of DBTRG-05 glioblastoma, MDA-MB468 estrogen-independent breast, and ES-2 clear cell ovarian
cancers [51]. After treatment with GHRH antagonist (MIA602), the authors observed an inhibition of the release of
metalloproteinases (MMP), decreased attachment of cancer
cells to fibronectin and matrigel, an upregulation of caveolin-1 and E-cadherin, as well as downregulation of NF-B
and beta-catenin [51]. In LNCaP human prostate cancer cell
line GHRH antagonist possessed antioxidant activity [74],
reducing the levels of lipid and protein oxidative stress
markers (MDA and nitrotyrosine) and generation of intracellular reactive oxygen species (ROS) [68]. The data for suppressing the reactive oxygen species (ROS) of certain cells
by administration of GHRH antagonists are presented in
US20100152114 [74]. GHRH antagonist JMR-132, at the
concentration of 1M, influenced the expression of antioxi-
60 Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 1
dant enzymes, superoxide dismutase (SOD1), which is also a
target for the inhibition of angiogenesis and tumor growth,
quinine oxidoreductase 1 (NQO1), a cytochrome protein that
reduces and detoxifies quinones protecting cells against redox cycling and oxidative stress, GPX1 which is a main glutathione peroxidase and thioredoxin (Trx), a major cytoplasmic antioxidant enzyme [68, 74]. In A549 lung cancer
cell line it also decreased the expression of inducible nitric
oxide synthase (iNOS) [67], an enzyme involved in the generation of reactive nitrogen species (RNS), and the expression of cyclooxygenase 2 (COX-2) [67, 68, 71]. NOS and
COX-2 are enzymes involved in the pathogenesis of inflammatory responses and in oncogenesis as well, linking these
two processes, and suggesting also a non-oncological activity of GHRH analogues in various human pathologies [75,
76]. Furthermore, GHRH antagonists were shown to reduce
the activity of Janus kinase-signal transducer and activator of
transcription (JAK-STAT) pathway in experimental model
of human prostate hyperplasia, and decreased the proliferation of these cells in vitro [77], as well as to counteract the
stimulatory effect of GHRH and the secretion of interleukin17 from human peripheral blood mononuclear cells [78].
Schematic demonstration of selected mechanisms of action
of GHRH antagonists is shown in Fig. (1).
Fig. (1). Mechanism of Action of GHRH Antagonists.
Siejka et al.
CURRENT & FUTURE DEVELOPMENTS
GHRH antagonists were developed for the controlling the
effects of the excessive action of GHRH and GH/IGF-1 that
is seen in acromegaly. Further studies with the use of this
group of possible future drugs concentrated on their effects
on extrapituitary neoplasms. Acromegaly however affects
considerable number of the population and in many cases
cannot be treated effectively with surgery because of local
invasion of the pituitary tumor. Currently, there are three
drug classes available for the treatment of acromegaly:
dopamine agonists, somatostatin analogs, and a GH receptor
antagonist. However, many patients do not respond to this
kind of treatment and the responders very rarely may show
the desensitisation to the action of somatostatin analogs
within weeks or months. Considering various targets for
somatostatin analogs and GHRH antagonists, it would be of
great interest to evaluate the effects of those peptides applied
together in pituitary tumors leading to acromegaly. It is
worth emphasizing that GHRH antagonists have been shown
to cross the blood-brain barrier [48]. Similarly, patients with
neuroendocrine tumors (NETs)/carcinoids whose symptoms
are not completely controlled with somatostatin analogs
(being currently the main treatment option) might benefit
from this type of therapy. The expression of GHRH recep-
GHRH Antagonists in Cancer
Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 1 61
tors predominantly in cancer cells allows GHRH antagonists
to be used as carrier systems linked to radionuclides for
tumor localisation or therapy, or conjugated to chemotherapeutic agents or toxins. So far, only the results of
preclinical studies evaluating the anticancer effects of GHRH
antagonists are available, but noteworthy this group of
synthetic peptides does not cause any significant side effects,
especially those of them not having endocrine effects on
GH-IGF-I axis. Scarce results in humans with the use of first
antagonist did not show any toxicity either. It is intriguiging
to speculate that GHRH antagonists might be used as
prophylactic agents to decrease the cancer risk, mainly
because they can diminish the level of IGFs, but other direct
(also metabolic) effects cannot be excluded.
IGF
=
Insulin-like Growth Factor
iNOS
=
Inducible Nitric Oxide Synthase
JAK
=
Janus Kinase
MAPK
=
Mitogen Activated Protein Kinase
MDA
=
Malondialdehyde
MMP
=
Metalloproteinase
mRNA
=
Messenger Ribonucleic Acid
NF-B
=
Nuclear Factor-kappa B
Nle
=
Norleucine
Orn
=
Ornithine
In conclusion, though studied only in preclinical models
so far, GHRH antagonists create a new potential therapeutic
option for various neoplasms. The growing understanding of
their mechanisms of action gives new directions to their
possible applications. Clinical studies will determine their
efficacy in humans.
p
=
Pituitary
p-Akt
=
Akt/phospho-Akt
PACAP
=
Pituitary Adenylate Cyclase-Activating
Polypeptide
PCR
=
Polymerase Chain Reaction
ABBREVIATIONS
PCNA
=
Proliferating Cell Nuclear Antigen
Abu
=
2-Aminobutyric acid
PhAc
=
Phenylacetyl
Ada
=
12-Aminododecanoyl
PHM
=
Peptide Histidine-Methionine
Agm
=
Agmatine, decarboxy-arginine
PI3K
=
Phosphatidylinositol 3-kinases
Amp
=
Para-Amidino-Phenylalanine
Pit
=
Pituitary-Specific Transcription Factor
ATP
=
Adenosine-Tri-Phosphate
PKA
=
Protein Kinase A
BAX
=
Pro-Apoptotic Protein
PKC
=
Protein Kinase C
Bcl-2
=
Anti-Apoptotic Protein
R
=
Receptor
bFGF
=
Basic Fibroblast Growth Factor
ROS
=
Reactive Oxygen Species
c-AMP
=
Cyclic-Adenosine-Mono-Phosphate
SCLC
=
Small Cell Lung Cancer
CREB
=
cAMP-Response Element
siRNA
=
Silencing Ribonucleic Acid
Cit
=
Citrulline
STAT
=
CNS
=
Central Nervous System
Signal Transducer and Activator of Transcription
Cox
=
Cyclooxygenase
SV
=
Splice Variant
Cpa
=
4-Chloro-Phenylalanine
Tyr(Me)
=
O-Methyl-Tyrosine
DNA
=
Deoxyribonucleic Acid
VEGF
=
Vascular Endothelial Growth Factor
EGFR
=
Epidermal Growth Factor Receptor
VIP
=
Vasoactive Intestinal Peptide
ERK
=
Extracellular Signal-Regulated Kinase
VPAC
=
VIP and Pituitary Adenylate Cyclaseactivating peptide
Fpa5
=
Pentafluoro-Phenylalanine
G-protein
=
Guanine nucleotide binding proteins
GH
=
Growth Hormone
GHRH
=
Growth Hormone-Releasing Hormone
GIP
=
Glucose-Dependent Insulinotropic Polypeptide
GLP
=
Glucagon-like peptide
h
=
Human
Har
=
Homoarginine
REFERENCES
HER
=
Human EGF Receptor
[1]
ACKNOWLEDGEMENTS
This paper was supported by European Cooperation in
the field of Scientific and Technical Research - COST ACTION, Grant No. CM0602 and Polish Ministry of Science
and Higher Education, Grant No. 505-04-001.
CONFLICT OF INTEREST
None.
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