Download Emerging MEK inhibitors

Review
Emerging MEK inhibitors
1.
Background
2.
Medical need
3.
Scientific rationale
4.
Existing therapy and
competitive environment
5.
Current research goals
6.
Conclusions
7.
Expert opinion
James A McCubrey†, Linda S Steelman, Steven L Abrams, William H Chappell,
Suzanne Russo, Roger Ove, Michele Milella, Agostino Tafuri, Paolo Lunghi,
Antonio Bonati, Franca Stivala, Ferdinando Nicoletti, Massimo Libra,
Alberto M Martelli, Giuseppe Montalto & Melchiorre Cervello
†
Brody School of Medicine at East Carolina University, Department of Microbiology & Immunology,
600 Moye Boulevard, Greenville, NC 27858, USA
Importance of the field: The Ras/Raf/MEK/ERK pathway is often activated
by genetic alterations in upstream signaling molecules. Integral components
of this pathway such as Ras and B-Raf are also activated by mutation. The
Ras/Raf/MEK/ERK pathway has profound effects on proliferative, apoptotic
and differentiation pathways. This pathway can often be effectively silenced
by MEK inhibitors.
Areas covered by this review: This review will discuss targeting of MEK which
could lead to novel methods to control abnormal proliferation which arises in
cancer and other proliferative diseases. This review will cover the scientific
literature from 1980-present.
What the reader will gain: By reading this review the reader will understand the
important roles that genetics play in the response of patients to MEK inhibitors,
the potential of combining MEK inhibitors with other types of therapy, the
prevention of cellular aging and the development of cancer stem cells.
Take home message: Targeting MEK has been shown to be effective in
suppressing many important pathways involved in cell growth and the prevention of apoptosis. MEK inhibitors have many potential therapeutic uses in
the suppression of cancer, proliferative diseases and aging.
Keywords: apoptosis, cancer, ERK, kinases, MEK, MEK inhibitors, proliferative disorders,
protein phosphorylation, signal transduction
Expert Opin. Emerging Drugs (2010) 15(1):27-47
1.
Background
This review is a follow on to a review which focused on Emerging Raf Inhibitors
published in this same review series [1]. The Ras/Raf/MEK/ERK signaling pathway
transmits signals from extracellular mitogens, growth factors and cytokines. Cytokines
trigger and activate cell membrane receptors and the Ras/Raf/MEK/ERK pathway
relays these messages from the cytoplasm to the nucleus to control gene expression [2-8].
Furthermore, this pathway also regulates the activity of, and subcellular localization of,
many apoptotic molecules which are normally localized and function in the mitochondrion. An introductory overview of this pathway is presented in Figure 1. Also
indicated in this figure is the site of intervention with MAPK/ERK kinase (MEK)
inhibitors. Many of these inhibitors have been evaluated in various clinical trials and
some are even currently being used to treat patients with specific cancers (see below).
The Raf/MEK/ERK pathway regulates the activity of many proteins involved in
apoptosis. Many of the effects of the Ras/Raf/MEK/ERK pathway on apoptosis
are mediated by extracellular-signal-regulated kinases 1,2 (ERK) phosphorylation
of key apoptotic effector molecules (e.g., Bcl-2, Mcl-1, Bad, Bim, Caspase-9 and
many others) [4]. Targeting MEK could result in, or contribute to, the induction
of apoptosis. An overview of the effects of the Ras/Raf/MEK/ERK pathway on the
phosphorylation of critical proteins involved in regulation of apoptosis is presented
in Figure 2.
10.1517/14728210903282760 © 2010 Informa UK Ltd ISSN 1472-8214
All rights reserved: reproduction in whole or in part not permitted
27
Emerging MEK inhibitors
EGF
Plasma
membrane
EGF-R
Activated
EGF-R
P
Shc
Grb2
SOS
Activated
Ras/Raf/MEK/ERK
pathway
Ras
Raf
P
AZD-6244
PD0325901
XL-518
RDEA119
[Negative
feedback
loops
of ERK]
MEK
MEK
Inh
P
ERK
P
p90Rsk
P
P
P
AP-2
Elk
CREB
Nucleus
AAA
Regulation of gene transcription
(proliferation, transformation, drug resistance, angiogenesis)
Figure 1. Overview of the Raf/MEK/ERK pathway. The Raf/MEK/ERK pathway is regulated by Ras, as well as various upstream growth
factor receptors and non-receptor kinases. In this scenario, we have depicted the activation of the EGFR by EGF. MEK inhibitors which have
been evaluated to suppress this pathway (many in clinical trials, see below) are indicated in open boxes to the left of the pathway. The
downstream transcription factors regulated by this pathway are indicated in diamond shaped outlines.
1.1
Introduction
Raf is responsible for serine/threonine (Ser/Thr, S/T) phosphorylation of MEK1 [4-9] . MEK1 phosphorylates ERK1
and 2 at specific Thr and Tyr (Y) residues [4] . Activated
ERK1 and ERK2 S/T kinases phosphorylate and activate
28
a variety of substrates, including p90Rsk1 [4]. ERK1,2 have
many downstream and even upstream substrates. The
number of ERK1,2 targets is easily in the hundreds. Thus,
suppression of MEK and ERK will have profound effects on
cell growth.
Expert Opin. Emerging Drugs (2010) 15(1)
McCubrey, Steelman, Abrams, et al.
Growth factor
receptor
Ras
Bim and Bad phosphorylation
status influence their ability
to dimerize with Bcl-2, Bcl-XL and
Mcl-1, in turn alter their interactions
with Bax and Bak
PI3K
Raf
PDK
MEK
Akt
ERK
JNK or Gsk3β
phosphorylation of Mcl-1
induces proteosomal
degradation, apoptosis
Akt
P
P
ERK via p90Rsk,
phosphorylates CREB,
stimulates Mcl-1 transcription
ERK phosphorylation
of Mcl-1 results
in Mcl-1 stabilization
MEK
Ihn
P
CREB
Gsk3β
P
P
P
P
P
McI-1
Bim
Bad
P
P
P
P
P
Bcl-XL
ERK and Akt can phosphorylate
Bim and Bad which affects their
subcellular localization and they are
ubiquitinated and targeted to the
proteasome for degradation
JNK phosphorylation
of Bim and Bad may
reverse negative
phosphorylation
by ERK and Akt and
promote pro-apoptotic
effects of Bim and Bad
Bcl-2
Raf-1 can interact with many
apoptotic regulatory
molecules at mitochondrion
JNK
Bax/Bak
ERK can phosphorylate
Bcl-2 which enhances its
anti-apoptotic effects
Regulation of apoptosis
Figure 2. Effects of the Raf/MEK/ERK pathway on post-transcriptional modification of apoptotic regulatory molecules. The Raf/
MEK/ERK and PI3K/PTEN/Akt/mTOR pathways can effect the expression of apoptotic molecules by transcriptional and post-transcriptional
mechanisms. In this figure, we only consider post-transcriptional mechanisms. The effects of the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR
pathways are often counterbalanced by JNK and glycogen synthase kinase-3b which can serve to promote apoptosis. ERK can also
phosphorylate caspase 9 which inhibits activation of the caspase cascade and degradation of Mcl-1. MEK inhibitors would inhibit the activity
of ERK which would result in the altered stability of certain anti-apoptotic molecules. Open circles represent active molecules, whereas closed
circles represent molecules with decreased activity or inactive molecules due to phosphorylation by the Raf/MEK/ERK or PI3K/PTEN/Akt/
mTOR pathways. The MEK inhibitor is in a closed octagon as it serves to inhibit the effects of the Raf/MEK/ERK pathway.
Biochemical details of the Raf/MEK/ERK pathway
The mammalian Raf gene family consists of A-Raf, B-Raf
and Raf-1 (C-Raf). Raf is a S/T kinase and is normally
activated by a complex series of events including: i) recruitment to the plasma membrane mediated by an interaction with
Ras; ii) dimerization of Raf proteins; iii) phosphorylation/
dephosphorylation on different domains; iv) disassociation
with the Raf kinase inhibitory protein (RKIP); and v) association with scaffolding complexes (e.g., kinase suppressor of
Ras) [4]. Raf activity is further modulated by proteins such as
Bag1, 14-3-3 and heat shock protein 90 [1-5].
There are at least 13 regulatory phosphorylation sites on
Raf-1 [4]. Some of these sites (e.g., S43, S259 and S621) are
phosphorylated when Raf-1 is inactive. This allows 14-3-3 to
bind Raf-1 and confer a configuration which is inactive. Upon
cell stimulation, S621 becomes transiently dephosphorylated
by an unidentified phosphatase. Phosphatases such as protein phosphatase 2A dephosphorylate S259 [1-9]. 14-3-3
then disassociates from Raf-1. This allows Raf-1 to be
1.2
phosphorylated at S338, Y340 and Y341, rendering Raf-1
active. A Src family kinase is probably responsible for
phosphorylation at Y340 and Y341 [1-9].
Y340 and Y341, the phosphorylation targets of Src family
kinases, are conserved in A-Raf (Y299 and Y300), but are
replaced with aspartic acid (D) at the corresponding positions
in B-Raf (D492 and D493) [1-9]. The negatively charged
aspartic acid residues mimic activated residues, which makes
B-Raf highly active. Maximal activation of Raf-1 and A-Raf
requires both Ras and Src activity while B-Raf activation is
Src-independent [1-9]. Interestingly, as is discussed later, a
greater number of mutations are detected at B-Raf than either
Raf-1 or A-Raf in human cancer.
The S338 residue present in Raf-1 is conserved among the
three Raf isoforms; however, in B-Raf (S445), this corresponding site is constitutively phosphorylated [4]. S338 phosphorylation on Raf-1 is stimulated by Ras and is dependent on
p21-activated protein kinase [4]. Other phosphorylation sites
in Raf-1 that may modulate its activity include: S43, S339,
Expert Opin. Emerging Drugs (2010) 15(1)
29
Emerging MEK inhibitors
T491, S494, S497, S499, S619 and S621. PKC has been
shown to activate Raf and induce cross-talk between PKC and
Raf/MEK/ERK signaling pathways [4]. S497 and S499 were
identified as the target residues on Raf-1 for PKC phosphorylation. However, other studies suggest that these sites are not
necessary for Raf-1 activation [4].
Raf activity is negatively regulated by phosphorylation
on the conserved region (CR)2 regulatory domain. Akt
and protein kinase A phosphorylate S259 on Raf-1 and
inhibit its activity [1-9]. Furthermore, Akt or the related
serum/glucocorticoid regulated kinase phosphorylate B-Raf
on S364 and S428 and inactivate its kinase activity [1-9].
These S-phosphorylated Rafs associate with 14-3-3 and
become inactive.
The scaffolding protein, RKIP, has been shown to inhibit
Raf-1 activity [4]. RKIP is a member of the phosphatidylethanolamine-binding protein family. This multi-gene family is
evolutionarily conserved and has related members in bacteria,
plants and animals [4]. Interestingly, RKIP can bind either to Raf
or to MEK/ERK but not to Raf, MEK and ERK all together.
Various isoforms of PKC have been shown to phosphorylate
RKIP on S153 which results in the disassociation of Raf
and RKIP.
The importance of Raf-1 in the Raf/MEK/ERK signal
transduction pathway has come into question due to the
discovery that B-Raf was a much more potent activator of
MEK compared to Raf-1 and A-Raf. Many of the ‘functions’
of Raf-1 persist in Raf-1 knockout mice, and are probably
maintained by endogenous B-Raf [4]. Interestingly and controversially, it was recently proposed that B-Raf is not only
the major activator of MEK1, but B-Raf is also involved in
Raf-1 activation. B-Raf may be temporally activated before
Raf-1. However, there may be different subcellular localizations of B-Raf and Raf-1 within the cell that exert different
roles in signaling and apoptotic pathways [4]. In some cases,
B-Raf may transduce its signal through Raf-1 and B-Raf can
form heterodimers with Raf-1. Dimerization is one important
component involved in Raf activation [4]. The reasons for
these added complexities in the Raf/MEK/ERK kinase cascade are not obvious but may represent another layer of finetuning. Alternatively, B-Raf:Raf-1 heterodimers may have
different substrate specificities or affinities than B-Raf:B-Raf
and Raf-1:Raf-1 homodimers. Furthermore, as is discussed
later, some B-Raf mutants which are kinase deficient may
transmit their signals via forming heterodimer with Raf-1 [1-4].
MEK1 is a tyrosine (Y-) and S/T-dual specificity protein
kinase [1-9]. Its activity is positively regulated by Raf phosphorylation on S residues in the catalytic domain. All three
Raf family members are able to phosphorylate and activate
MEK but different biochemical potencies have been observed
(B-Raf > Raf-1 >> A-Raf) [1-9]. Another interesting aspect
regarding MEK1 is that its predominant downstream target is
ERK. In contrast, both upstream Raf and downstream ERK
have multiple targets. Thus, therapeutic targeting of MEK1 is
relatively specific.
30
ERK1,2 are S/T kinases and their activities are positively
regulated by phosphorylation mediated by MEK1 and MEK2.
ERKs can directly phosphorylate many transcription factors
including Ets-1, c-Jun and c-Myc. ERKs can also phosphorylate and activate the 90 kDa ribosomal S6 kinase (p90Rsk).
p90Rsk can then phosphorylate and activate the cAMP
response element binding protein (CREB) transcription factor (Figure 1) [1]. Moreover, through an indirect mechanism,
ERKs can lead to activation of the NF-kB transcription factor
(nuclear factor immunoglobulin k chain enhancer-B cell) by
phosphorylating and activating inhibitor kB kinase [1-9]. ERK1
and ERK2 are differentially regulated. These are only a few
examples of the downstream targets of ERKs 1,2. ERK can enter
the nucleus to phosphorylate many transcription factors [1].
2.
Medical need
In the Western world, cancer has surpassed heart disease and
is now the number one killer of adults. As we discuss below,
there is often an aberrant activation of the Raf/MEK/ERK
pathway in human cancer due to various genetic and epigenetic mechanisms. Furthermore, as we discuss in section 5.4.1,
targeting of MEK may be effective in treating other proliferative diseases including aging. MEK lies in a critical position in
this pathway as it has few direct upstream activators (e.g., Raf)
and few downstream targets (e.g., ERK). Thus, the development of effective MEK inhibitors could prove very useful in
cancer therapy. MEK inhibitors have been some of the first
small molecule, cell membrane permeable inhibitors developed which have been determined to have a high degree of
specificity in extensive preclinical studies with various types of
cancer as well as other diseases and syndromes [2,3]. As we
discuss below, MEK inhibitors have also been evaluated in
combination with other signal transduction pathway inhibitors as well as chemo- and radiotherapeutic approaches. The
vast majority of these studies have indicated that the combination of MEK inhibitors and various other types of therapy
have resulted in improved outcomes. The development and
proper usage of effective MEK inhibitors has the real potential
to advance medical treatment.
Abnormal activation of MEK in human cancer
Perhaps one of the biggest advances in medical science in the
1980s was the confirmation of the proto-oncogene hypothesis,
which predicted that the human genome contains genes related
to viral oncogenes which when mutated could cause human
cancer. Key genetic members of the Ras/Raf/MEK/ERK pathway (e.g., RAS, RAF) and upstream receptors (e.g., EGFR,
ERBB1, ERBB2) were shown to fulfill this hypothesis as they
were sometimes mutated/amplified in specific human cancers [9]. Genetic mutations at these cellular oncogenes often
confer sensitivity to MEK inhibitors. An illustration of some
of the receptors, kinases and phosphatases that are mutated/
amplified in human cancer and how they may impact on MEK
activity is presented in Figure 3.
2.1
Expert Opin. Emerging Drugs (2010) 15(1)
McCubrey, Steelman, Abrams, et al.
Constitutive activation of pathways
can lead to autocrine growth factor
production & transcription
HER2 amplified
in 30% of
breast cancers
EGF-R
HER2
Flt-3
VEGF-R
IGF1-R
Kit
PDGF-R
Mutations at
Flt-3 in ~ 20% AMLs
BCR-ABL
Ras mutations
detected in ~ 35%
human cancers
Mutations at
B-Raf in
~ 70% melanomas
Mutations at
PI3K present in
~ 25% breast cancers
Other mutations/
translocations
EGF-R mutated
in brain cancers
and NSCLC
Ras
BCR-ABL translocation
detected in ~100% of CML
also in some ALLs
Raf
PI3K
PTEN
Kit mutated in
AML, GIST
MEK
Akt
PDGFR mutated in
AML, GIST
ERK
mTOR
PTEN mutations
(loss of activity) present
in many different
tissue types
Effects on gene transcription,
protein translation
and prevention of apoptosis
Cell growth, transformation
Figure 3. Dysregulated expression of upstream receptors and kinases can result in activation of MEK. Sometimes dysregulated
expression of growth factor receptors occurs by either increased expression or genomic amplifications (e.g., VEGFR, EGFR, HER2, IGF-1R).
Mutations have been detected in EGFR, Flt-3, Kit, PDGF receptor, PI3K, PTEN, Ras and B-Raf. Amplification of HER2 and EGFR is detected in
certain cancer types. The BCR-ABL chromosomal translocation is present in virtually all CMLs and some ALLs. Many of these mutations and
chromosomal translocations result in the activation of the Raf/MEK/ERK cascade. The PI3K/PTEN/Akt pathway is also activated in certain
cancer and can regulate the Raf/MEK/ERK cascade. These pathways can also be activated by autocrine growth stimulation, the genetic basis
of which is frequently unknown.
ALL: Acute lymphocytic leukemia; AML: Acute myeloid leukemia; CML: Chronic myeloid leukemia; GIST: Gastrointestinal stromal tumor; PTEN: Phosphatase and
tensin homolog.
Mutations that lead to the expression of constitutivelyactive Ras proteins have been observed in ~ 20 – 30% of
human cancers [5,9,10]. In cholangiocarcinoma, KRAS gene
mutations have been identified in 45% of examined
tumors [9]. The KRAS gene is also mutated at a very high
incidence in pancreatic cancer, as high as 80%. The mutations in KRAS and other genes in pancreatic cancer have
recently been reviewed [10]. These are often point mutations
that alter Ras activity. Genome RAS amplification or overexpression of Ras, perhaps due to altered methylation of
its promoter region, are also detected in some tumors [5,9].
Mutations that result in increased Ras activity may also
perturb MEK activity and may confer sensitivity to MEK
inhibitors. A key event in the activation of the Ras protein
is farnesylation. Inhibitors which target the enzyme farnesyl
transferase (FT) have been developed with the goal of targeting Ras. Clinical testing of FT inhibitors (FTI), unfortunately, has yielded disappointing results [11]. The lack of
usefulness of FTIs may be due to multiple reasons. First, there
are many proteins which are regulated by FT. Second,
although H-Ras is exclusively modified by FT and K-Ras
to a lesser extent, N-Ras can also be modified by geranylgeranyltransferase. This modified N-Ras is still able to support the biological requirement of Ras in the cancer cell.
Geranylgeranylation of K-Ras and N-Ras becomes critical
only when farnesylation is inhibited [5]. The majority of
RAS mutations in humans occur in KRAS, which is followed
by NRAS [9]. The mutation rate at HRAS brings up a distant
third. Hence, it is very possible that the effects which FTIs
had in initial clinical trials were not due to inhibition of
mutant RAS genes present in the cell, but in fact resulted from
nonspecific effects which were related to the first point mentioned. Importantly, some of these tumors with RAS mutations
may be sensitive to MEK inhibitors.
BRAF mutation occurs in ~ 7% of all cancers [8,12,13];
however, this frequency may change as more and diverse
tumors are examined for BRAF mutation (see below). One
study has observed that mutated alleles of CRAF are present
Expert Opin. Emerging Drugs (2010) 15(1)
31
Emerging MEK inhibitors
in therapy-induced acute myelogenous leukemia (t-AML) [14].
The t-AML arose after chemotherapeutic drug treatment of
breast cancer patients. The mutated CRAF genes were transmitted in the germ line; thus, they were not spontaneous
mutations in the leukemia, but may be associated with the
susceptibility to induction of t-AML in the breast cancer
patients studied.
Prior to around 2002, it was believed that the RAF
oncogenes were not frequently mutated in human cancer.
With the advent of improved methods of DNA sequencing,
it was demonstrated that BRAF is frequently mutated in
melanoma (27 – 70%), papillary thyroid cancer (36 – 53%),
colorectal cancer (5 – 22%), cholangiocarcinoma (22%),
ovarian cancer (30%) and a small minority of lung cancer
patients (1 – 3%) [13-15]. Patients with BRAF and CRAF
mutations are predicted to be sensitive to MEK inhibitors.
In many cancers with BRAF mutations, the mutations
are believed to be initiating events, but are not sufficient for
full-blown neoplastic transformation [8,12,16,17]. Mutations
at other genes (e.g., in components of the PI3K/PTEN/
Akt/mTOR pathway, especially PTEN) have been hypothesized to also be necessary for malignant transformation
in some cancers [8].
Most cancers contain multiple mutations [8-23]. Some of
these mutations in cancer can be suppressed by inhibitors
which target kinases such as MEK and additional kinases
which may also be activated in that cancer. For example,
melanoma has mutations in genes encoding components
of the Ras/Raf/MEK/ERK pathway. Mutations at RAS and
BRAF are detected in melanoma and some of these mutant
kinases will be sensitive to B-Raf and MEK inhibitors.
However, if the melanoma has a mutation at KRAS, it may
not be sensitive to the B-Raf and MEK inhibitors as instead
the PI3K/PTEN/Akt/mTOR pathway may be activated by
the mutant KRAS. In addition, as stated earlier, there are
multiple mutations in certain cancers and melanoma is a
good example. BRAF mutations may only be part of the
mutational events necessary for establishment of a melanoma;
additional mutations at genes such as PTEN may also be
required for melanoma. Recently, it has been shown that it
is more effective to treat melanomas with inhibitors which
target both the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR
pathways to enhance melanoma treatment [8]. In addition, it
has been shown that certain basal-type breast cancers are
sensitive to MEK inhibitors and a consequence of treatment
of certain cells with MEK inhibitor is the activation of Akt, by
the MEK inhibitor suppressing a negative feedback loop [23].
Co-treatment of certain breast cancer with a combination of
MEK and PI3K/mTOR inhibitors was shown to be more
effective than treatment with MEK inhibitors by themselves [23]. In the above examples, it was possible to target
two kinase pathways with inhibitors which suppressed kinase
activity. However, in those cases where there are mutations
at tumor suppressor genes, it may not be as easy to target the
affected pathways.
32
Different BRAF mutations have been mapped to various
regions of the B-Raf protein. Mutations at BRAF which result
in low kinase activity may signal through Raf-1 [12]. Heterodimerization between B-Raf and Raf-1 proteins may allow the
impaired B-Raf to activate Raf-1 [19]. These particular BRAF
mutations may be sensitive to MEK inhibitors.
3.
Scientific rationale
Clearly, there is a scientific rationale for the development of
MEK inhibitors as the Raf/MEK/ERK pathway is frequently
aberrantly expressed in human cancer and the targeting of this
pathway might improve cancer therapy.
Altered expression of MEK and sensitivity to
therapy
3.1
It has been reported that a high frequency of acute myeloid
leukemias (AMLs) and acute lymphocytic leukemias
(ALLs; > 50%) display constitutive activation of MEK in
the absence of any obvious genetic mutation [20,21]. While
there may be some unidentified mutations at one component
of the pathway or a phosphatase that regulates the activity of
the pathway, the genetic mechanism responsible for the
constitutive activation of MEK is unknown. Elevated expression of downstream ERK in AMLs and ALLs is associated with
a poor prognosis [21]. Often elevated expression of signaling
pathways is correlated with increased sensitivity to targeted
therapy (e.g., MEK inhibitors) [5,22]. MEK inhibitors may
prove useful in the treatment of a large percentage of AMLs and
ALLs. The observation that there is often constitutive activation of MEK in these leukemias as well as many other cancers
and proliferative disorders (e.g., certain allergies and inflammatory diseases) has propelled the pharmaceutical industry to
develop inhibitors which target MEK.
There can be a genetic basis for the sensitivity of some
NSCLCs to EGFR inhibitors, such as erlotinib and gefitinib [24-28]. Interestingly, NSCLC with mutations at KRAS are
resistant to EGFR inhibitors; these particular NSCLCs may
be sensitive to MEK inhibitors [28]. In addition, some melanoma cells carrying BRAF mutations are sensitive to MEK
inhibitors, while cells lacking these BRAF mutations are resistant [29]. The introduction of activated EGFR mutants into
hematopoietic cells renders them sensitive to MEK inhibitors [30,31]. Furthermore, introduction of activated RAS, RAF
and MEK genes into hematopoietic cells makes them sensitive
to MEK inhibitors [32-37]. These results point to the dependence (oncogene addiction) of the particular cells on the
expression of the mutated kinase or signaling pathway [38-40].
This also occurs in chronic myeloid leukemia (CML) patients,
as cells containing the BCR-ABL chromosomal translocation
are exquisitely sensitive to BCR-ABL inhibitors (e.g., imatinib). BCR-ABL is the chimeric gene product which results
from the translocation between the BCR and ABL genes in
virtually 100% of CML patients [38,39]. BCR-ABL-transformed
cells are a prime example of oncogene-addiction as their
Expert Opin. Emerging Drugs (2010) 15(1)
McCubrey, Steelman, Abrams, et al.
proliferation is dependent upon that particular oncogene
product. Cells with the BCR-ABL chromosomal translocation
are also sensitive to MEK inhibitors; however, in general the
BCR-ABL inhibitors are more effective because they shut off
additional signaling pathways.
Existing therapy and competitive
environment
4.
Existing therapy for many of the cancers which display
aberrant Raf/MEK/ERK pathway expression is not optimal.
For example, the successful therapy of melanoma, pancreatic,
hepatocellular carcinoma (HCC), and advanced breast cancer
are poor, as is the successful therapy of adult patients with
AML. Development of novel methods to treat these and other
cancer patients is essential.
Advantages and strategies of targeting MEK to
enhance cancer therapy
4.1
Small-molecule inhibitors, such as imatinib (Novartis), have
proven effective in the treatment of CML and certain other
leukemias that grow in response to BCR-ABL (e.g., some
ALLs) and other cancers which proliferate in response to
mutant platelet-derived growth factor receptor (PDGFR)
and KIT genes [38,39], such as gastrointestinal stromal tumors,
as imatinib also inhibits these related kinases. As previously
stated, some lung carcinomas that have EGFR mutations are
sensitive to EGFR inhibitors [4]. These cancers represent
special examples of tumors that proliferate in response to a
defined mutation. Unfortunately, the mechanisms responsible for aberrant proliferation of most cancers are unknown.
Many of the mutations in human cancer result in the aberrant
activation of signaling pathways which impinge upon MEK.
Hence, MEK inhibitors have been developed [41-57] and some
have been evaluated in clinical trials (see below).
Other non-mutational-based mechanisms can result in
activation of MEK and contribute to either malignant transformation or drug resistance. A consequence of chemotherapeutic drug treatment of breast, hematopoietic, prostate and
other cancers is the induction of ERK [4]. Eliminating this
deleterious side effect of these therapies using MEK inhibitors
may enhance their ability to kill drug resistant cancer. Use of
these inhibitors may augment the effects of chemo-, radio- and
hormonal-based therapies (see below).
Specific MEK inhibitors
Specific inhibitors of MEK have been developed (e.g.,
PD98059 (Pfizer), U0126 (DuPont), PD184352 (CI-1040)
(Pfizer), PD0325901 (Pfizer), ARRY-142886 (also known as
AZD6244) (Array Biopharama/AstraZeneca), RDEA119
(Ardea Biosciences/Bayer) and XL-518 (Exelixis/Genetech))
(Table 1) [2-4,41-57]. The successful development of MEK
inhibitors may be due to the relatively few phosphorylation
sites on MEK involved in activation/inactivation. MEK
inhibitors differ from most other kinase inhibitors as they
4.2
do not compete with ATP binding (non-ATP competitive),
which confers a high specificity [41,43]. Most MEK inhibitors
are specific and do not inhibit many different protein
kinases [58] although as is discussed below, certain MEK
inhibitors are more specific than others.
The crystal structures of MEK1 and MEK2 have been
solved as ternary complexes with ATP and PD184352, and
have revealed that both MEK1 and MEK2 have unique
inhibitor binding sites located on a hydrophobic pocket
adjacent to, but not overlapping with, the ATP-binding
site [44]. Furthermore, effective targeting of MEK1/MEK2 is
highly specific, as ERK1/ERK2 are the only well-described
downstream targets. A distinct advantage of inhibiting the
Raf/MEK/ERK cascade is that it can be targeted without
knowledge of the precise genetic mutation that results in its
aberrant activation. This is important as the nature of the
critical mutation(s) that leads to the malignant growth of
most cancers is really unknown. An advantage of targeting
MEK is that the Raf/MEK/ERK pathway is a convergence
point where a number of upstream signaling pathways can be
blocked with the inhibition of MEK. For example, MEK
inhibitors, such as ARRY-142886 (AZD-6244), are also being
investigated for the treatment of diverse cancers including
lung, colo-rectal, pancreatic and HCC [45-48].
RDEA119 is a more recently described MEK inhibitor
developed by Ardea Biosciences [55]. It is a highly selective
MEK inhibitor which displays > 100-fold selectivity in kinase
inhibition in a panel of 205 kinases. In contrast, in the same
kinase specificity analysis, other recently developed MEK
inhibitors (e.g., PD0325901) also inhibited the Src and
RON kinases.
MEK inhibitors in clinical studies
ARRY-142886 is an orally-active MEK1 inhibitor that has
undergone Phase II clinical trials [45-49]. ARRY-142886 has
demonstrated significant tumor suppressive activity in preclinical models of cancer, including melanoma, pancreatic,
colon, lung, liver and breast cancers. The effects of ARRY142886 are enhanced significantly if the tumor has a mutation
that activates MEK.
ARRY-142886 inhibits MEK1 in vitro with an IC50 value
of 14.1 ± 0.79 nM [45]; it is specific for MEK1 as it did
not appear to inhibit any of the ~ 40 other kinases in the
panel tested. ARRY-142886 is not competitive with ATP.
Molecular modeling studies indicate that ARRY-142886
binds to an allosteric binding site on MEK1/MEK2. The
binding sites on MEK1/MEK2 are relatively unique to these
kinases and may explain the high specificity of MEK inhibitors. This binding may lock MEK1/2 in an inactivate conformation that enables binding of ATP and substrate, but
prevents the molecular interactions required for catalysis and
access to the ERK activation loop. In basic research studies,
treatment with the MEK inhibitor results in the detection of
activated MEK1/2 when the proteins present on a western
immunoblot are probed with an antibody which recognizes
4.3
Expert Opin. Emerging Drugs (2010) 15(1)
33
Emerging MEK inhibitors
Table 1. Competitive environment.
Ref.
Cancer examined
Clinical trial
Result
Company
MEK inhibitors under evaluation
MEK inhibitor
CI-1040,
PD-184352
(non-competitive
inhibitor)
[41-43]
Advanced colorectal,
NSCLC, pancreatic,
kidney, melanoma,
breast
Phase I/II
Reduced pERK levels,
discontinued clinical
trials due to
pharmacological
problems
Pfizer
PD0325901
(non-competitive
inhibitor)
[29,39,53,54,56]
Breast, colon, NSCLC,
melanoma
Phase I/II
Discontinued
Pfizer
XL518
(non-competitive
inhibitor)
Exelixis (Internet)
Phase I
Ongoing
Exelixis
AZD6244/
ARRY-142886
(non-competitive
inhibitor)
[45-48]
Multiple melanoma,
HCC, advanced solid
cell tumors
Phase I/II
77% pERK reduction
Astra Zeneca/Array
BioPharma
RDEA119
(non-competitive
inhibitor)
[55]
Advanced tumors
Phase I/II
Ongoing
Ardea/Bayer
Combination MEK inhibitor trials
MEK inhibitor + other agent
CI-1040 + ATO
[92-95]
Leukemia
Preclinical
ERK reduction
RDEA119 + sorafenib
Ardea (Internet)
Advanced cancer
patients, HCC
Phase I
Ongoing
ArdeaBayer
AZD6244/
ARRY-142886 +
doxorubicin
[48]
HCC
Phase I
ERK reduction
Astra Zeneca/Array
BioPharma
AZD6244/
ARRY-142886 +
docetaxel
[84]
Various solid tumors
Preclinical
ERK reduction
AZD6244/
ARRY-142886 +
radiation
[98]
Various solid tumors
Preclinical
ERK reduction
ERK: Extracellular-signal-regulated kinase; HCC: Hepatocellular carcinoma.
active MEK1/2, while downstream ERK1/2 will not appear
activated with the activation specific ERK1/2 antibody [51].
ARRY-142886 inhibited downstream ERK1/ERK2 activation
in in vitro cell line assays with stimulated and unstimulated
cells, and also inhibited activation in tumor-transplant models.
In contrast, ARRY-142886 did not prevent the activation of
the related ERK5 that occurs with some older MEK1 inhibitors, which are not being pursued in clinical trials. Inhibition
of ERK1,2 suppresses their ability to phosphorylate and
modulate the activity of MEK1 but not MEK2 as MEK2
lacks the ERK1/ERK2 phosphorylation site. In essence, by
inhibiting ERK1,2 the negative loop of MEK phosphorylation is suppressed and hence there will be an accumulation
of activated MEK. This biochemical feedback loop may
provide a rational for combining Raf and MEK inhibitors
in certain therapeutic situations. The actual concentrations
of ARRY-142886 required to inhibit proliferation in various
34
cancer cell lines may depend on how dependent they are on
the Raf/MEK/ERK pathway for proliferation. Cells with
mutations in Raf or upstream molecules which signal through
this pathway may be highly sensitive to ARRY-142886 and
IC50s of 10 – 50 nM are observed. In contrast, other cell lines
which do not have mutations in neither B-Raf nor upstream
molecules may be resistant to ARRY-142886 and doses
approaching 5000 nM may be required to reach the IC50.
Furthermore, there may be certain mutations in other pathways (e.g., PI3K/PTEN/Akt/mTOR) which confer resistance
to ARRY-142886.
In colon, melanoma, pancreatic, liver and some breast
cancers, ARRY-142886 inhibited the growth of tumors in
tumor xenograph studies performed in mice. The new MEK
inhibitors are also at least 10- to 100-fold more effective
than earlier MEK inhibitors and hence can be used at lower
concentrations [45-48]. ARRY-142886 also inhibits the growth
Expert Opin. Emerging Drugs (2010) 15(1)
McCubrey, Steelman, Abrams, et al.
of human leukemia cells, but does not affect the growth of
normal human cells. ARRY-142886 also suppressed the
growth of pancreatic BxPC3 cells (see below), which do not
have a known mutation in this pathway, suggesting that
this drug may also be useful for treating cancers that lack
definable mutations. However, it is likely that BxPC3 cells
have some type of upstream gene mutation/amplification or
autocrine growth factor loop which results in the activation of
the Raf/MEK/ERK pathway.
ARRY-142886 induced G1/S cell-cycle arrest in colon and
melanoma cancer cell lines and activated caspase-3 and -7 in
some cell lines (Malme3M and SKMEL2); however, caspase
induction was not observed in other melanoma (SKMEL28)
or colon cancer cell lines (HT29), demonstrating that further
research needs to be performed with this inhibitor to determine if it normally induces apoptosis and whether the induction of apoptosis can be increased with other inhibitors or
chemotherapeutic drugs. One report demonstrated that the
treatment of HCC cells with ARRY-142886 plus doxorubicin led to synergistic growth inhibition and apoptosis both
in vitro and in vivo [48] (further discussed below).
ARRY-142886 has undergone several Phase I and II clinical trials [45-49]. A Phase I clinical trial to assess the safety,
tolerability and pharmacokinetics of ARRY-142886 in
patients with various solid malignancies was performed.
Phase II clinical trials have compared: i) the efficacy of
ARRY-142886 versus temozolomide in patients with unresectable stage 3 or 4 malignant melanomas; ii) the efficacy and
safety of ARRY-142886 versus capecitabine in patients with
advanced or metastatic pancreatic cancer who have failed to
respond to gemcitabine therapy; iii) the efficacy and safety of
ARRY-142886 compared with pemetrexed in patients with
NSCLC who have previously failed to respond to one or two
previous chemotherapy regimens; iv) the efficacy and safety
of ARRY-142886 versus capectiabine in patients with colorectal cancer who have failed to respond to one or two previous chemotherapy regimens; and v) advanced or metastatic
HCC [45-49]. Initial results from clinical trials have not yielded
overwhelming support for the use of MEK inhibitors (see
below) as a single therapeutic agent in cancer patients who
are not pre-screened for pre-existing activation of the Raf/
MEK/ERK pathway [52-54]. The proper pre-identification of
cancer patients who display activation of the Raf/MEK/ERK
pathway may be necessary for prescribing MEK inhibitors as
part of their therapy, as MEK inhibitors are cytostatic and
not cytotoxic.
An additional MEK inhibitor is PD-0325901 (Pfizer) [56],
which follows on from the earlier MEK inhibitors PD-98059
and PD-184352, both of which have been extensively examined in preclinical investigations to determine the role of
MEK in various biochemical processes. PD-184352 was the
first MEK inhibitor to enter clinical trials and it demonstrated
inhibition of activated ERK and antitumor activity [42,43]; however, subsequent multi-center, Phase II studies with patients
with diverse solid tumors did not demonstrate encouraging
results [43,53]. This was probably due to low oral bioavailability
and high metabolism, which led to plasma drug levels that were
inadequate to suppress tumor growth.
The newer PD-0325901 MEK inhibitor is an orally-active,
potent, specific, non-ATP competitive inhibitor of MEK.
PD-0325901 demonstrated improved pharmacological and
pharmaceutical properties compared with PD-184352,
including a greater potency for inhibition of MEK, and higher
bioavailability and increased metabolic stability.
PD-0325901 has a Ki value of 1 nM against MEK1 and
MEK2 in in vitro kinase assays. PD-0325901 inhibits the
growth of cell lines which proliferate in response to elevated
signaling of the Raf/MEK/ERK pathways in the low nanomolar levels [29,56]. In general, the concentrations of this
MEK inhibitor required to inhibit proliferation of cells
in vitro ranges from 50 to 5000 nM, cells which are inhibited
at 50 nM are referred to as highly sensitive and cells which
require 5000 nM are referred to as resistant. The highly
sensitive cells often have mutations at key components of
the Raf/MEK/ERK pathway which makes their growth highly
dependent on MEK.
As stated previously, the BRAFV600E mutation is present
in ~ 6 – 8% of human cancers (overall). Interestingly, ~ 5%
of lung cancers have mutations at BRAF which are not at
V600E [58]. The effects of PD-0325901 were examined in
conditional BRAFV600E tumor models where genetically
modified mice express normal B-Raf prior to Cre-mediated
recombination, after which they express B-RafV600E at physiological levels [58]. When B-RafV600E was induced, the
mice developed lung tumors which could be inhibited by
PD-0325901 (25 mg/(kg day) for ~ 2 weeks, followed by
12.5 mg/(kg day) for an additional 2 weeks). In contrast,
mice treated with vehicle alone developed adenomas. This
experimental mouse model suggests the effects of the
PD-0325901 in the treatment of patients with tumors that
proliferate in response to activation of the Raf/MEK/ERK
pathway should be re-evaluated. However, these and other
models also indicate that therapy needs to include a cytotoxic
drug, as the MEK inhibitors are cytostatic and often as soon
as the MEK inhibitors are removed, the tumor will re-emerge
and eventually kill the host.
Clinical trials with PD-0325901 have documented some
successes and some adverse side effects [52-54]. The results of
these clinical trials with PD-0325901 as well as other MEK
inhibitors have recently been extensively and critically
reviewed [52-54]. The use of the PD-0325901 MEK inhibitor
to treat cancer patients is currently in limbo. Pfizer has currently (2009) suspended its evaluation in clinical trials [53].
This may have resulted in part from the design of the clinical
trials as MEK inhibitors may not be appropriate to treat all
types of cancer. MEK inhibitors may be appropriate to treat
only those cancers which proliferate in response to activation
of the Raf/MEK/ERK pathway. Furthermore, it may also be
important to include a chemotherapeutic drug or radiation
treatment to induce death of the cancer cell. The reasons for
Expert Opin. Emerging Drugs (2010) 15(1)
35
Emerging MEK inhibitors
suspension of the clinical trials with PD-0325901 are well
described in [52-54].
Advantages of targeting MEK in cancers where the
pathway components are not specifically mutated: the
hepatocellular cancer model
4.4
In the following section, we discuss the advantages of using
MEK inhibitors to treat a cancer with a poor prognosis, yet
this cancer does not have mutations at MEK. HCC is the fifth
most common cancer worldwide and the third most prevalent
cause of cancer mortality, accounting for ~ 6% of all human
cancers and > 600,000 deaths annually worldwide [59-62].
Although the clinical diagnosis and management of earlystage HCC has improved significantly, HCC prognosis is still
extremely poor [59-62]. Therefore, investigating HCC pathogenesis and finding new diagnostic and treatment strategies
is important.
Signaling via MEK plays a critical role in liver carcinogenesis [63-68]. Although mutations of Ras and Raf occur
infrequently in HCC, a recent study demonstrated that activation of the Ras/Ras/MEK/ERK pathway occurred in 100%
of HCC specimens analyzed when compared with nonneoplastic surrounding tissues and normal livers. This
increased Ras expression coincided with the decreased expression of genes which serve to inhibit Ras expression, namely
the RAS association domain family 1A (RASSF1A) and the
novel Ras effector 1A (NORE1A). These genes may be suppressed due to aberrant methylation of their promoters [68].
Decreased expression of these genes, which normally serve to
suppress Ras, may lead to activation of downstream MEK.
Human HCC tumors have higher expression and enhanced
activity of MEK1/2 and ERK1/2 compared with adjacent
non-neoplastic liver [63]. Overexpression of activated MEK1
in HCC HepG2 cells resulted in enhanced tumor growth
in vivo [64]. On the other hand, preclinical studies have demonstrated the potential of MEK inhibition to suppress hepatoma
cell proliferation and tumorigenicity [47]. Huynh et al.
recently reported that treatment of human HCC xenografts
with AZD6244 (ARRY-142886) blocked ERK1/2 activation, reduced in vivo tumor growth and induced apoptosis [47]. Moreover, targeting MEK with PD-0325901 had
in vivo chemopreventive effects on HCC development in an
animal model using TGF-a-transgenic mice in which liver
cancers were induced by diethylnitrosamine treatment [69].
Therefore, MEK represents a potential therapeutic target
for HCC.
Obesity is another important contributing factor for the
development of HCC. The important role of Ras/Raf/
MEK/ERK signaling has also been suggested for HCC progression in obese patients. A possible explanation for risk
associated between obesity and HCC comes from the study
of Saxena et al. who for the first time demonstrated that leptin,
a key molecule involved in the regulation of energy balance and
body weight control, promotes HCC growth and invasiveness
through activation of Raf/MEK/ERK signaling [70].
36
Other well known risk factors for HCC such as hepatitis
viruses B and C also utilize the Ras/Raf/MEK/ERK pathway
for the control of hepatocyte survival and viral replication [71-77]. Among the four proteins encoded by HBV
genome, HBx is involved in heptocarcinogenesis. HBx activates Ras/Raf/MEK/ERK signaling cascade [71-77]. Among
HCV components, the core protein has been reported to
activate MEK pathway and thereby might contribute to
HCC carcinogenesis [75-77]. Therefore, these studies suggest
that MEK is a novel therapeutic target which could be
exploited for the treatment of HCC resulting from HBV
and HCV infection.
Advantages of targeting MEK in pancreatic cancer
In the following section, we discuss the advantages of using
MEK inhibitors to treat a cancer with a poor prognosis. This
cancer does not have mutations at MEK and yet may have
mutations in upstream RAS. As we stated earlier, a high
percentage of pancreatic cancer have RAS mutations. Certain
MEK inhibitors have shown great promise in the treatment
of pancreatic cancers (see below), which often have mutations
in RAS that can lead to downstream Raf/MEK/ERK pathway
activation. In the US, ~ 37,000 new cases of pancreatic cancer
are diagnosed each year and there are 34,000 deaths. It is
currently the fourth leading cause of cancer deaths in the US
due to the high and rapid mortality rate. The incidence of
pancreatic cancer increases with age and peaks between the
years 60 and 70. The highest incidence of pancreatic cancer
worldwide appears to be in the US, Israel, Sweden and Canada
(http://www.cancer.gov/cancertopics/types/pancreatic). This
may be related to the diets in these various countries. Cigarette
smoking, certain food additives and diabetes may be linked to
pancreatic cancer, as well as exposure to certain industrial
chemicals. Diets low in fruit and vegetables and high in red
meat have been associated with pancreatic cancer. Due to the
asymptomatic nature of pancreatic cancer, it is often detected
when the disease is in an advanced stage; this leads to poor
prognoses for the patients and a short life expectancy (< 1 year).
If MEK inhibitors are ever to be used in the treatment of
certain pancreatic cancer patients, it will be important to
determine if these patients have a RAS mutation which results
in Raf/MEK/ERK or PI3K/PTEN/Akt/mTOR activation or
activation of both pathways as certain RAS mutations will
result in the activation of both pathways. If the PI3K/PTEN/
Akt/mTOR pathway is also activated, then it is important to
treat these pancreatic cancer patients with a PI3K, Akt or
mTOR inhibitor. If both pathways are activated, then treatment with inhibitors which target both pathways is warranted.
Due to the frequent detection of pancreatic cancer at advanced
stages, it may be necessary to combine signal transduction
inhibitor therapy with conventional chemotherapy after
surgical removal of the pancreatic cancer if possible.
ARRY-142886 suppressed the tumor growth of pancreatic
cells, such as BxPC3, in immunocompromised mice more
effectively than conventional chemotherapeutic drugs, such as
4.5
Expert Opin. Emerging Drugs (2010) 15(1)
McCubrey, Steelman, Abrams, et al.
gemcitabine, which is commonly used to treat pancreatic
cancer; however, once treatment with ARRY-142886 was
discontinued, the tumors re-grew [46]. Most likely MEK
inhibitors do not induce apoptosis, but rather, they inhibit
proliferation. That is, MEK inhibitors are cytostatic. As
discussed previously, there is a significant amount of overlap between the Raf/MEK/ERK and PI3K/PTEN/Akt/
mTOR pathways in terms of the regulation of apoptosis
and cell growth. Inhibition of both pathways may more
effectively induce apoptosis. To induce apoptosis and eliminate tumor growth, combination treatments may be required.
This could include co-treatment with MEK and PI3K/Akt/
mTOR inhibitors or co-treatment with MEK inhibitors and
chemotherapeutic drugs.
5.
Current research goals
The obvious goal of current inhibitor development is to
improve the effectiveness of treatment of cancer patients
with small molecule signal transduction inhibitors. This has
proven to be difficult for multiple reasons: first, as previously
discussed, there tends to be a distinct genetic susceptibility
for the success of a signal transduction inhibitor in suppressing cancerous growth; second, many of the small molecule
signal transduction inhibitors are cytostatic as opposed to
being cytotoxic and, therefore, will need to be combined
with a therapeutic modality which induces cell death; and
third, more than one signal transduction pathway may be
activated in the cancer cells.
Dual pathway inhibition: targeting MEK and
PI3K/PTEN/Akt/mTOR
5.1
Previously, we have predominately discussed studies which
used a MEK inhibitor, sometimes in combination with a
chemotherapeutic drug. In the following section, we discuss
the potential of combining inhibitors which target two pathways to more effectively limit cancer growth. We present a
brief overview of the biochemistry of the PI3K/PTEN/Akt/
mTOR pathway to enable the reader to better understand how
targeting both pathways may be more effective than just
targeting MEK.
PI3K is a heterodimeric protein with an 85-kDa regulatory
subunit and a 110-kDa catalytic subunit (PIK3CA). PI3K
serves to phosphorylate a series of membrane phospholipids
including PtdIns(4)P and PtdIns(4,5)P2, catalyzing the transfer of ATP-derived phosphate to the D-3 position of the
inositol ring of membrane phosphoinositides, thereby, forming the second messenger lipids PtdIns(3,4)P2 and PtdIns
(3,4,5)P3. Most often, PI3K is activated via the binding of
a ligand to its cognate receptor, whereby p85 associates
with phosphorylated tyrosine residues on the receptor via an
Src-homology 2 domain. After association with the receptor,
the p110 catalytic subunit then transfers phosphate groups
to the aforementioned membrane phospholipids. It is these
lipids, specifically PtdIns(3,4,5)P3, that attract a series of
kinases to the plasma membrane thereby initiating the signaling cascade [1]. An overview of the Ras/PI3K/PTEN/Akt/
mTOR pathway is presented in Figure 4.
Downstream of PI3K is the primary effector molecule
of the PI3K signaling cascade, Akt/protein kinase B. Akt
was originally discovered as the cellular homologue of the
transforming retrovirus AKT8 and as a kinase with properties
similar to protein kinases A and C [1]. Akt contains an aminoterminal pleckstrin homology domain that serves to target the
protein to the membrane for activation [1]. Within its central
region, Akt has a large kinase domain and is flanked on the
carboxy-terminus by hydrophobic and proline-rich
regions [1]. Akt is activated via phosphorylation of two
residues: T308 and S473.
The phosphotidylinositide-dependent kinases (PDKs) are
responsible for activation of Akt. PDK1 is the kinase responsible for phosphorylation of T308 [1]. Akt is also be phosphorylated by the mTOR complex referred to as mTORC2
(rapamycin-insensitive companion of mTOR/mLST8 complex) (Figure 4). Before its discovery, the activity responsible
for this phosphorylation event was called PDK2. Phosphorylation of Akt is complicated as it is phosphorylated by a
complex which lies downstream of itself. Moreover, it can be
dephosphorylated by events mediated by p70S6K which also
lies downstream of Akt [1]. So, as with the Ras/Raf/MEK/ERK
pathway, there are negative feedback loops which serve to
regulate the Ras/PI3K/PTEN/Akt/mTOR pathway. Once
activated, Akt leaves the cell membrane to phosphorylate
intracellular substrates.
After activation, Akt is able to translocate to the nucleus [1]
where it affects the activity of a number of transcriptional
regulators, CREB, E2F, NF-kB (via Ik-K) and the forkhead
transcription factors [1]. These are all either direct or indirect
substrates of Akt and each can promote either cellular proliferation or survival. Aside from transcription factors, Akt is
able to target a number of other molecules to affect the survival state of the cell including caspase-9, the pro-apoptotic
molecule BAD and glycogen-synthase kinase-3b [1]. When
these targets are phosphorylated by Akt, they may either be
activated or inactivated but the end result is to promote
survival of the cell.
Negative regulation of the PI3K pathway is primarily
accomplished through the action of the phosphatase and
tensin homologue deleted on chromosome ten (PTEN) tumor
suppressor protein. PTEN encodes a lipid and protein phosphatase whose primary lipid substrate is PtdIns(3,4,5)P3.
PTEN has four primary structural domains. On the amino
terminus is the lipid and protein phosphatase domain, which
is flanked by a domain responsible for lipid binding and
membrane localization. Next is the PEST domain which
regulates protein stability. Last, PTEN has a PDZ domain,
which helps facilitate protein–protein interactions. Mutations
within the phosphatase domain have been reported to nullify
the endogenous function of PTEN [1]. Thus, PTEN is a key
therapeutic target, although it is frequently inactivated in
Expert Opin. Emerging Drugs (2010) 15(1)
37
38
P
ERK
P
MEK
P
Expert Opin. Emerging Drugs (2010) 15(1)
S6
Foxo3aP
P P
p70S6K
Protein translation
growth
cell size
eIF4E
4E-BP1
P
P
P
mLST8 mTOR Raptor
mTORC1
Rheb
P
P
P TSC2 P
TSC1 P
Activation
by other
pathways
and stress
GSK-3β
P
P
Growth
factor
CREB
P
AAA
Both ERK and Akt
phosphorylate many transcription
factors which control the expression
of genes involved in regulation of
growth and drug resistance
mTORC2
SIN1 P
mLST8 mTOR Rictor
Figure 4. Conceptual overview of targeting MEK and mTOR to suppress malignant growth. The Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR pathways can interact at many different
levels. In this diagram, we have focused on how they interact to regulate mTOR, p70S6K and protein synthesis. Dotted lines indicate negative feedback loops which may also include other
signaling molecules. Targeting both of MEK and mTOR may be an effective means to regulate cell growth.
mTOR
Inh
SOS
P
Raf
Grb2
Ras
Both ERK and Akt
phosphorylate many proteins
involved in apoptosis regulation
which often result in
prevention of apoptosis
MEK
Inh
Src
Family
Kinase
PH
Akt
Autocrine loops
of cellular transformation
PTEN
P
P
P P P
PIP2
PH
Generic
PIP3
receptor
PDK1
PI3K
PI3K
P
P
p85
p110
P
Shc P
Growth
factor
Growth
factor
Emerging MEK inhibitors
McCubrey, Steelman, Abrams, et al.
human cancer and its inactivation results in elevated Akt
activity and abnormal growth regulation.
Next, we discuss some of the key downstream targets of
Akt which can also contribute to abnormal cellular growth
and are key therapeutic targets. Akt-mediated regulation of
mTOR activity is a complex multi-step phenomenon. Akt
inhibits tuberous sclerosis 2 (TSC2 or hamartin) function
through direct phosphorylation. TSC2 is a GTPase-activating
protein (GAP) that functions in association with the putative
tuberous sclerosis 1 (TSC1 or tuberin) to inactivate the
small G protein Rheb (Ras homolog enriched in brain) [1].
TSC2 phosphorylation by Akt represses GAP activity of the
TSC1/TSC2 complex, allowing Rheb to accumulate in a
GTP-bound state. Rheb-GTP then activates, through a mechanism not yet elucidated, the protein kinase activity of
mTOR when complexed with the Raptor (Regulatory associated protein of mTOR) adaptor protein and mLST8, a
member of the Lethal-with-Sec-Thirteen gene family, first
identified in yeast [1]. The mTOR/Raptor/mLST8 complex
(mTORC1) is sensitive to rapamycin and, importantly, inhibits Akt via a negative feedback loop which involves, at least
in part, p70S6K [1].
The relationship between Akt and mTOR is further complicated by the existence of the mTOR/Rictor (mTORC2),
which displays rapamycin-insensitive activity. The mTORC2
complex has been found to directly phosphorylate Akt on
S473 in vitro and to facilitate T308 phosphorylation. Thus,
mTORC2 complex can function as the elusive PDK-2 which
phosphorylates Akt on S473 in response to growth factor
stimulation [1]. Akt and mTOR are linked to each other via
ill-defined positive and negative regulatory circuits, which
restrain their simultaneous hyperactivation through a mechanism involving p70S6K and PI3K. Assuming that equilibrium
exists between these two complexes, when the mTORC1
complex is formed, it could antagonize the formation of the
mTORC2 complex and reduce Akt activity [1]. Thus, at least
in principle, inhibition of the mTORC1 complex could result
in Akt hyperactivation. This is one problem associated with
therapeutic approaches using rapamycin which block some
actions of mTOR but not all.
mTOR is a 289-kDa S/T kinase. It regulates translation
in response to nutrients/growth factors by phosphorylating components of the protein synthesis machinery, including p70S6K and eukaryotic initiation factor (eIF)-4E binding
protein-1 (4EBP-1), the latter resulting in release of the
eIF-4E, allowing eIF-4E to participate in the assembly of a
translational initiation complex [1]. p70S6K, which can also be
directly activated by PDK-1, phosphorylates the 40S ribosomal protein, S6, leading to active translation of mRNAs [1].
Integration of a variety of signals (mitogens, growth factors,
hormones) by mTOR assures cell cycle entry only if nutrients
and energy are sufficient for cell duplication [1]. Therefore,
mTOR controls multiple steps involved in protein synthesis.
Hence targeting the mTOR pathway could have many effects
on the regulation of cellular growth.
In addition to the BRAF mutations present in melanomas
which we have previously discussed, the PTEN phosphatase
tumor suppressor gene is also deleted in ~ 45% of melanomas
and the downstream Akt gene is amplified in ~ 45% [78]. Both
of these mutations result in increased expression of Akt.
Elevated Akt expression is often associated with a poor prognosis in human cancer. Downstream of Akt is mTOR.
Increased Akt expression will lead to mTOR activation.
Increased mTOR activity can increase the efficiency of protein
translation. The targeting of mTOR has been examined in
melanoma therapy as well as in the treatment options for many
diverse cancers. Treatment of inducible lung cancers containing KRAS and PIK3CA mutations with PI3K/mTOR
(NVP-BEZ235, Novartis) and MEK (ARRY-142886)
inhibitors led to an enhanced response [79].
The combined effects of inhibiting MEK with PD-0329501
and mTOR with rapamycin or its analogue AP-23573
(ARIAD Pharmaceuticals/Merck) were examined in human
NSCLC cell lines, as well as in animal models of human
lung cancer [80]. PD-0325901 and rapamycin demonstrated
synergistic inhibition of proliferation and protein translation.
Suppression of both MEK and mTOR inhibited ribosomal
biogenesis and was associated with a block in the initiation
phase of translation. These results support suppression of
both the MEK and mTOR pathways in lung cancer therapy
and indicate that both pathways converge to regulate the
initiation of protein translation. ERK phosphorylates MAPK
signal integrating kinases (Mnk1/2) and p90 ribosomal
S6 kinase p90Rsk, which regulate the activity of the eukaryotic
translation initiation factor eIF4E [81]. The phosphorylation
of 4EBP1 is altered in cells with BRAF mutations. It should
also be pointed out that the 4EBP1 is also regulated by Akt,
mTOR and p70S6K [1-9,81]. This may result in the efficient
translation of certain mRNAs in BRAF-mutant cells. This
could explain how co-inhibition of MEK and mTOR synergize to inhibit protein translation and growth in certain
lung cancer cells.
Back to the future: the need to combine MEK
inhibitors with chemotherapy and radiotherapy
5.2
Classical chemotherapy often remains the most prescribed
anticancer therapy for many different types of cancer treatment. Drugs such as doxorubicin and taxol are effective in the
treatment of many cancers, even though in some cases drug
resistance develops after prolonged treatment. Doxorubicin
and taxol target cellular events, such as DNA replication and
cell division, which are often downstream of the targets of
MEK. Chemotherapeutic drug treatment can also unfortunately lead to drug resistance. Chemotherapeutic drugs can
activate the Raf/MEK/ERK pathway by diverse mechanisms
(Figure 5). Drugs such as doxorubicin can activate p53 which
can lead to increased expression of the discoidin domain
receptor (DDR), which in turn can result in Raf/MEK/ERK
pathway activation. Activated ERK can phosphorylate p53 and
regulate its activity. Doxorubicin can also activate the calcium
Expert Opin. Emerging Drugs (2010) 15(1)
39
Emerging MEK inhibitors
ATM
P
P
p53
DDR1
Doxorubicin
or irradiation
Docetaxel
CaM-K
Ras
Raf
B-Raf
P
P
MEK
Inh
MEK
ERK regulates the
expression of many proteins
(by phosphorylation) and
genes (by transcription)
involved in control
of apoptosis and cell growth
MEK
P
P
ERK
ERK
Activate MAPK localization
to asters, kinetochores and
actin-microtubule cytoskeleton
during mitosis augmented by
microtubule-stabilizing taxanes
leading to drug resistance
P
GATA-1
AAA
Transcription of
genes involved in DNA repair
resulting in drug resistance
(e.g., XRCC1, ERCC1)
Figure 5. Targeting MEK may prevent drug resistance. Chemotherapeutic drugs such as doxorubicin and docetaxel may also induce the
Raf/MEK/ERK pathway which may contribute to emergence of drug resistant clones. The Raf/MEK/ERK pathway may regulate downstream
transcription factors such as GATA-1 which control the transcription of genes such as XRCC1 and ERCC1 that are involved in DNA repair and
their aberrant expression may contribute to drug resistance. Treatment of drug resistant cells with MEK inhibitors, or combined treatments
consisting of a chemotherapeutic drug and a MEK inhibitor, may be an effective approach to prevent drug resistance.
CaM-K: Calcium/calmodulin dependent kinase; DDR1: Discoidin domain receptor-1.
calmodulin-dependent kinase cascade via reactive oxygen species [4]. Activation of this cascade can also result in MEK
activation. Activation of this cascade can result in the transcription of genes such as XRCC1 and ERCC1 which are
involved in DNA repair and lead to drug resistance [81,82].
Taxols can also stimulate activation of the Raf/MEK/ERK
cascade and lead to their increased association with proteins
involved in cell division [82-84]. Thus, chemotherapeutic drugs
can activate the Raf/MEK/ERK cascade which can promote
drug resistance. Thus, by combining classical chemotherapy
with targeted therapy, it may be possible to enhance toxicity,
while lowering the effective concentrations of classical chemotherapeutics necessary for effective elimination of the
tumor [84]. As we have previously discussed, activation of
the Raf/MEK/ERK cascade can alter the activity and subcellular localization of many proteins which play critical roles
in apoptotic cascades. Also, the Raf/MEK/ERK cascade can
40
regulate the transcription of many critical genes involved in cell
cycle progression, growth and differentiation. Thus, combining chemotherapy with MEK inhibitors may enhance the
therapeutic effectiveness of chemotherapy.
Cells containing the BCR-ABL chromosomal translocation
signal in part through the Raf/MEK/ERK cascade [1] and are
sensitive to MEK inhibitors [2]. MEK inhibitors synergize with
the BCR-ABL inhibitor imatinib in inhibiting the growth
of BCR-ABL positive cells [85]. Furthermore, MEK inhibitors
synergize with histone deacetylase inhibitors to suppress the
growth of BCR-ABL-positive cells [86]. MEK inhibitors have
potentiated the antitumor activity of selective COX-1 and
COX-2 inhibitors in suppressing the growth and inducing
apoptosis in human liver cancer cells [87].
As mentioned previously, a side effect of some chemotherapeutic drugs, such as paclitaxel, is the induction of the
Raf/MEK/ERK pathway. Activation of this pathway can under
Expert Opin. Emerging Drugs (2010) 15(1)
McCubrey, Steelman, Abrams, et al.
certain circumstances promote proliferation, prevent apoptosis
and contribute to drug resistance. Combining paclitaxel treatment with MEK inhibitors synergistically enhances apoptosis
and inhibits tumor growth [88,89]. The synergistic effects of
paclitaxel and MEK inhibitors are complex and have not
been fully elucidated, but may be in part mediated by inhibition of Bad phosphorylation at S112 by ERK [88]. Cisplatin
also induces the Raf/MEK/ERK pathway in certain cancer cell
types such as squamous cell carcinoma [90]. A common problem after treatment of this cancer type with cisplatin is the
development of resistance to cisplatin. Combining cisplatin
with MEK inhibitors increased cell death in cells normally
refractory to cisplatin [90]. Obviously, many other key phosphorylation events mediated by ERK may be suppressed which
play critical roles in cell growth.
The cytotoxic effects of combinations of MEK inhibitors
and paclitaxel may be specific for cells of certain origins
and may depend on the levels of endogenous activated
MEK/ERK present in those cells. A study with NSCLC
cells, which constitutively-expressed activated MEK, revealed
no increase in paclitaxel-induced apoptosis when treated
with a MEK inhibitor [89]. In contrast, addition of a dominant negative (DN) MEK gene to these cells potentiated
paclitaxel-induced apoptosis.
In neuroblastoma cells, cisplatin-induced apoptosis was
associated with increased levels of both p53 and the downstream Bax protein. Activated ERK1/ERK2 levels also increased
earlier in these cells upon cisplatin treatment. Cell cultures
incubated with MEK inhibitors blocked apoptotic cell death,
which prevented the cisplatin-induced accumulation of p53
and Bax proteins [91]. Activation of ERK1/ERK2 also occurs
in HCC cells and in other transformed cell types after exposure to nonselective, NSAIDs or selective COX-1 and COX-2
inhibitors. Inhibition of the Raf/MEK/ERK signaling pathway by a MEK inhibitor potentiates the antitumor activity of
COX inhibitors, suggesting that the Raf/MEK/ERK pathway
does not mediate cytotoxicity induced by COX inhibitors,
but may protect cells from death [87].
MEK inhibitors also synergize with arsenic trioxide (ATO)
to induce apoptosis in APL and AML cells [92-95]. The
p53-related gene p73 is a molecular target of the combined
therapy. ATO modulates the expression of the p73 gene by
inducing the proapoptotic and antiproliferative p73 isoforms.
P53 requires p63 and p73 for the induction of apoptosis in
response to DNA-damaging drugs. P73 exists as multiple
transactivation competent (TA) proapoptotic and antiproliferative p73 COOH-terminal splicing isoforms (a, b, g, d, e
and z), of which the two major forms are p73a and p73b.
DN p73 variants are expressed by a second promoter. These
DNp73 variants lack the amino-terminal transactivation
domain, act as trans-repressors of p53- and p73-dependent
transcription, and have antiapoptotic and proproliferative potential. APL cells treated with the MEK inhibitor
PD-184352 displayed reduced levels of DNp73 and decreased
the ATO-mediated upregulation of DNp73, thus, causing an
increase in the TA:DNp73 ratio of dual-treated cells. High
doses of ATO induced p53 accumulation in 11/21 patients.
Combined treatment resulted in the induction of the proapoptotic p53/p73 target gene p53AIP1 and greatly enhanced
the apoptosis of treated cells [93]. Thus, this study documented
the effectiveness of combining ATO with MEK inhibitors in
the treatment of APL and identified the molecular mechanism
responsible for the observed synergism.
MEK inhibitors synergize with UCN-01 (NCI/Keryx
Biopharmaceuticals, Inc.) and induce apoptosis in multiple
myeloma cells [96]. UCN-01 is a modified staurosporine
and inhibits many kinases. UCN-01 has been evaluated in
some clinical trials. The synergy may be partially caused by
UCN-01 inducing ERK activation, which is suppressed by the
MEK inhibitor.
It should be noted that the combination of MEK inhibitors and chemotherapeutic drugs may not always result in a
positive interaction and in some cases, combination therapy
results in an antagonistic response. For example, combining
MEK inhibitors with betulinic acid, a drug lethal for melanoma cells, antagonized the normal enhancing effects of
betulinic acid on apoptosis [97]. Furthermore, the precise
timing of the addition of two agents is important as they
may differentially affect cell-cycle progression; therefore, the
order of administration may be important for a synergystic
response to be obtained and perhaps to prevent an antagonistic
response [97].
Back to the future again: combining MEK inhibitors
with radiotherapy
5.3
Radiotherapy is a common therapeutic approach for the
treatment of many diverse cancers. A side effect of radiotherapy in some cells is the induction of the Ras/Raf/MEK/ERK
cascade [4]. Recently, various signal transduction inhibitors
have been evaluated as radiosensitizers. The effects of pretreatment of lung, prostate, and pancreatic cancer cells with the
ARRY-142886 MEK inhibitor were evaluated [98]. The MEK
inhibitor treatment was observed to radiosensitize the various
cancer cell lines in vitro and in vivo. The MEK inhibitor
treatment was correlated with decreased Chk1 phosphorylation 1 – 2 h after radiation. The authors noticed the effects
of the MEK inhibitor on the G2 checkpoint activation after
irradiation, as the MEK inhibitor suppressed G2 checkpoint
activation. Because ERK1/ERK2 activity is necessary for
carcinoma cells to arrest at the G2 checkpoint, suppression
of phosphorylated Chk1 was speculated to lead to the abrogated G2 checkpoint, increased mitotic catastrophe and
impaired activation of cell cycle checkpoints. Mitotic catastrophe was increased in cells receiving both the MEK inhibitor and radiation when compared to the solo-treated cells. It
was also postulated in this study that the MEK inhibitor
suppressed the autocrine cascade in DU145 prostate cancer
cells which normally resulted from EGF secretion and EGFR
activation. Suppression of this autocrine cascade by the MEK
inhibitor may have served as a radiosensitizer to the radiation
Expert Opin. Emerging Drugs (2010) 15(1)
41
Emerging MEK inhibitors
therapy. The other two cancer cell lines examined in this study
(A549 and MiaPaCa2) had KRAS mutations and both were
radiosensitized by the MEK inhibitor. Although these studies
document the ability of a MEK inhibitor to radiosensitize
certain cells, clearly other cancer cell lines without activating
mutations in the Ras/Raf/MEK/ERK pathway or autocrine
growth stimulation should be examined for radiosensitization
by the MEK inhibitor.
inhibitors. We have observed that some drug resistant cells
which display properties similar to cancer initiating cells
exhibit elevated activation of the Raf/MEK/ERK signaling
cascade [40]. Hence, these cells could be preferentially sensitive
to MEK inhibitors. Targeting MEK could be very important
in terms of cancer stem cell elimination.
Novel concepts involving MEK inhibitors
In the following two sections, we discuss novel concepts
regarding MEK inhibitors. These scientific topics are in their
infancy but should be considered as they have important
implications regarding the potential uses of these inhibitors.
Various pharmaceutical companies have developed inhibitors
to the MEK pathway which were shown to be both highly
specific and effective. However, these inhibitors may have
limited effectiveness in treating human cancers, unless the
particular cancer proliferates directly in response to the Raf/
MEK/ERK pathway. Moreover, MEK inhibitors are often
cytostatic as opposed to cytotoxic; thus, their ability to function as effective anticancer agents in a monotherapeutic setting
is limited, and they may be more effective when combined
with chemo- or radiotherapy.
Combination therapy with either a traditional drug/
physical treatment or another inhibitor which targets a
specific molecule in a different signal transduction pathway
is also a key approach for improving the effectiveness and
usefulness of MEK inhibitors. This avenue of investigation
has not been pursed as actively often due to the conflicting interests of pharmaceutical companies as different companies will often have competing interests for the different
inhibitors/chemotherapeutic drugs.
5.4
Targeting MEK and PI3K/PTEN/Akt/mTOR
pathways to suppress cellular senescence and aging
5.4.1
A recent series of manuscripts by Blagosklonny and colleagues
have demonstrated that treatment of cells induced to undergo
senescence with MEK, PI3K and mTOR inhibitors will
prevent the induction of cellular senescence and aging [99-109].
These experiments have led to the innovative hypothesis that
cellular senescence results from the hyper-activation of proliferative pathways and that drugs (e.g., metformin used to
treat diabetes) and signal transduction inhibitors (e.g., MEK,
PI3K, mTOR inhibitors) can inhibit cellular proliferation and
aging [105-110]. Similar effects on the prevention of cellular
senescence were observed with reversitol, the active component
contained in the skins of red grapes which was shown to also
inhibit mTOR and cellular senescence [107]. Additional studies
have shown that the commonly-prescribed diabetes drug
metformin will also inhibit mTOR and prevent cellular
aging [110]. Because both the Raf/MEK/ERK and PI3K/
PTEN/Akt/mTOR pathways interact to regulate the activity
of mTOR and downstream components of this pathway which
are critical for both mRNA stability and protein translation, it
is believed that by inhibiting some of these key pathways, it
may be possible to prevent cellular aging.
Targeting Raf/MEK/ERK in cancer initiating cells
An area of intense interest in cancer biology is the cancer stem
cell, more appropriately referred to as the cancer initiating
cell [38]. It is widely believed that after therapy of cancer
patients by various approaches which eliminate the bulk of
the cancer that the cancer may eventually raise its ugly head
again. This re-emergence of the cancer may be due to the
outgrowth of therapy resistant cancer initiating cells. The
abilities of the various MEK inhibitors to target and suppress
the proliferation of the cancer initiating cells (cancer stem cells)
are only beginning to be examined. It is not clear whether
MEK inhibitors will specifically target the cancer initiating
cells. Cancer stem cells have unique properties as they can be
both quiescent and also resistant to chemotherapeutic drugs.
However, under certain conditions, they resume proliferation
and hence should be potentially susceptible to MEK
5.4.2
42
6.
7.
Conclusions
Expert opinion
Over the past 25 years, there has been significant progress
in elucidating the involvement of the Ras/Raf/MEK/ERK
cascade in promoting cell growth, regulating apoptosis, as
well as the induction of chemotherapeutic drug resistance.
Initial seminal studies performed in the late 1970s and early
1980s elucidated that oncogenes were present in the genomes
of avian and murine retroviruses. Many of the viral oncogenes: ErbB, Fms, Ras, PI3K, Akt, Src, Abl, Raf, Fos, Ets
and NF-kB (Rel) were subsequently identified as cellular
genes which in some cases were captured by retroviruses.
Now, we know that these cellular genes are frequently
abnormally regulated in human cancer and many of them
serve to activate the Raf/MEK/ERK and PI3K/Akt pathways which have been discussed as playing critical roles in
cellular proliferation in this review. Biochemical studies continue to elucidate the roles that these cellular and viral oncogenes have in cellular transformation. Mutations at many of
these genes can result in abnormal MEK activation. Hence,
targeting MEK with small-molecule inhibitors may inhibit
cell growth of these cells. Specific MEK inhibitors have
been developed and represent promising therapies for cancer
and other proliferative diseases. The usefulness of these
inhibitors may depend on the mechanism of transformation
of the particular cancer. If the tumor exhibits a dependency
Expert Opin. Emerging Drugs (2010) 15(1)
McCubrey, Steelman, Abrams, et al.
on the Raf/MEK/ERK pathway, then it may be sensitive to
MEK inhibitors. In contrast, tumors which do not display
enhanced expression of the Raf/MEK/ERK pathway may
not be sensitive to MEK inhibitors. Finally, it is likely that
MEK inhibitors will only be effective in inhibiting tumor
growth in combination with cytotoxic chemotherapeutic
drugs or radiation.
Scientists and clinicians often have an intentionally narrow
view of a particular topic. For example, cancer researchers predominately feel that MEK inhibitors will suppress the growth
of malignant cancer cells. Yet, MEK and other inhibitors may
also be useful in the treatment of autoimmune and allergic
disorder where there is abnormal cellular proliferation. Recent
reports have also demonstrated that the suppression of MEK
may prevent the induction of cellular senescence and aging.
Clearly, these last two clinical topics, immune disorders and
aging, greatly enhance the potential clinical uses of these
targeted therapeutic drugs.
Declaration of interest
JA McCubrey and LS Steelman were supported in part
by a grant from the NIH (R01098195). JA McCubrey,
WH Chappell, S Russo and R Ove were supported in part
from the Brody Brothers Endowment Funt (MT7826).
A Bonati, M Milella and A Tafuri were supported in part
from grants from Associazione Italiana Ricerca sul Cancro
(AIRC grants). AM Martelli was supported in part by a grant
from the CARISBO Foundation and the Progetti Strategici
Universita di Bologna EF2006.
Bibliography
targets for anti-cancer and
anti-angiogenesis treatments. Curr Signal
Transduct Ther 2009. In Press
.
A key manuscript documenting the
complex genetics of melanoma in a
mouse model.
6.
Avruch J. MAP kinase pathways: the first
twenty years. Biochem Biophys Acta
2007;1773:1150-61
13.
7.
Leicht DT, Balan V, Kaplan V, et al. Raf
kinases: function, regulation and role in
human cancer. Biochem Biophys Acta
2007;1773:1196-213
.
Davies H, Bignell GR, Cox C, et al.
Mutations of the BRAF gene in human
cancer. Nature 2002;417:949-54
A seminal manuscript documenting
mutation of BRAF in human cancer.
Papers of special note have been highlighted
as either of interest (.) or of considerable
interest (..) to readers.
1.
.
2.
.
3.
.
4.
5.
McCubrey JA, Steelman LS, Abrams SL,
et al. Emerging Raf Inhibitors.
Expert Opin Emerg Drugs
2009;14:633-48
A current review of multiple aspects of the
Raf/MEK/ERK, PI3K/PTEN/Akt/
mTOR and JAK/STAT pathways with an
emphasis on leukemia.
McCubrey JA, Steelman LS, Abrams SL,
et al. Targeting survival cascades
induced by activation of
Raf/Raf/MEK/ERK, PI3K/PTEN/Akt/
mTOR and Jak/STAT pathways for
effective leukemia therapy. Leukemia
2008;22:708-22
A current review on the prospects
of targeting the Raf/MEK/ERK,
PI3K/PTEN/Akt/mTOR and
JAK/STAT pathways with an emphasis
on leukemia.
McCubrey JA, Milella M, Tafuri A, et al.
Targeting the Raf/MEK/ERK pathway
with small-molecule inhibitors. Curr Opin
Invest Drugs 2008;9:614-30
A current review on the prospects of
targeting the Raf/MEK/ERK pathway.
McCubrey JA, Steelman LS, Chappell WH,
et al. Roles of the Raf/MEK/ERK pathway
in cell growth, malignant transformation
and drug resistance. Biochem Biophys Acta
2007;1773:1263-84
Ciuffreda L, McCubrey JA, Milella M.
Cytoplasmic signaling intermediates
(PI3K/PTEN/Akt/mTOR and
Raf/MEK/ERK pathways as therapeutic
8.
.
9.
Hoeflich KP, Herter S, Tien J, et al.
Antitumor efficacy of the novel Raf
inhibitor GDC-0879 is predicted by
BRAFV600E mutational status and
sustained extracellular signal-regulated
kinase/mitogen-activated protein kinase
pathway suppression. Cancer Res
2009;69:3042-51
A key manuscript documenting multiple
mutations in human cancer and the need
for targeting more than one pathway to
inhibit growth.
Rajalingam K, Schreck R, Rapp UR, et al.
S. Ras oncogenes and their downstream
targets. Biochem Biophys Acta
2007;1773:1177-96
10.
Koliopanos A, Avgerinos C, Paraskeva C,
et al. Molecular aspects in pancreatic
cancer. Hepatobilary Pancreat Dis Int
2008;7:345-56
11.
Morgillo F, Lee HY. Development of
farnesyl transferase inhibitors as anti cancer
agents: current status and future.
Cancer Ther 2007;5:11-8
12.
Dhomen N, Reis-Filho JS, da Rocha
Dias S, et al. Oncogenic B-Raf induces
melanocyte senescence and melanoma in
mice. Cancer Cell 2009;15:294-303
Expert Opin. Emerging Drugs (2010) 15(1)
14.
.
Zebisch A, Staber PB, Delavar A, et al.
Two transforming C-RAF germ-line
mutations identified in patients with
therapy-related acute myeloid leukemia.
Cancer Res 2006;66:3401-8
A key paper documenting pre-existing
CRAF mutations in certain humans.
15.
Fransén K, Klintenäs M, Osterström A,
et al. Mutation analysis of the BRAF,
ARAF and RAF-1 genes in human
colorectal adenocarcinomas.
Carcinogenesis 2004;25:527-33
16.
Rajagopalan H, Bordelli A, Lengauer C,
et al. Tumorigenesis: RAF/RAS oncogenes
and mismatch-repair status. Nature
2002;418:934
17.
Yuen ST, Davies H, Chan TL, et al.
Similarity of the phenotypic patterns
associated with BRAF and KRAS
mutations in colorectal neoplasia.
Cancer Res 2002;62:6451-55
18.
Rushworth LK, Hindley AD, O’Neill E,
et al. Regulation and role of Raf-1/B-Raf
heterodimerization. Mol Cell Biol
2006;26:2262-72
A key manuscript documenting ability of
Raf-1 to heterodimerize with B-Raf to
transmit an altered proliferative signal.
.
19.
Garnett MJ, Rana S, Paterson H, et al.
Wild-type and mutant B-RAF activate
43
Emerging MEK inhibitors
.
20.
C-RAF through distinct mechanisms
involving heterodimerization. Mol Cell
2005;20:963-9
A key manuscript documenting ability
of mutant B-Raf to transmit signal
through Raf-1 and preserve the mutant
signaling ability.
Ricciardi MR, McQueen T, Chism D,
et al. Quantitative single cell determination
of ERK phosphorylation and regulation in
relapsed and refractory primary acute
myeloid leukemia. Leukemia
2005;29:1543-9
21.
Kornblau SM, Womble M, Qiu YH, et al.
Simultaneous activation of multiple signal
transduction pathways confers poor
prognosis in acute myelogenous leukemia.
Blood 2006;108:2358-65
22.
Deng X, Kornblau SM, Ruvolo PP, et al.
Regulation of Bcl2 phosphorylation and
potential significance for leukemic cell
chemoresistance. J Natl Cancer
Inst Monogr 2001;28:30-7
A key manuscript demonstrating
involvement of Raf/MEK/ERK pathway
in the phosphorylation of Bcl-2 and
regulation of apoptosis and
chemoresistance of hematopoietic cells.
.
23.
.
24.
25.
44
Mirzoeva OK, Das D, Heiser LM, et al.
Basal subtype and MAPK/ERK kinase
(MEK)-phosphoinositide 3-kinase
signaling determine susceptibility of breast
cancer cells to MEK inhibition. Cancer Res
2009;69:565-72
A key manuscript documenting
differential sensitivity of breast
cancer to MEK inhibitors and
demonstration of a negative feedback
loop suppressed by MEK inhibitors
which results in Akt activation and
resistance to MEK inhibitors. The
manuscript also documents the ability
of MEK inhibitors to synergize with
PI3K inhibitors to suppress breast
cancer growth.
Shelton JG, Steelman LS, Abrams SL,
et al. The epidermal growth factor
receptor gene family as a target for
therapeutic intervention in numerous
cancers: what’s genetics got to do
with it? Expert Opin Ther Targets
2005;9:1009-30
Lynch TJ, Bell DW, Sordella R, et al.
Activating mutations in the epidermal
growth factor receptor underlying
responsiveness of non-small-cell lung
cancer to gefitinib. N Engl J Med
2004;350:2129-39
.
A key manuscript documenting
sensitivity of NSCLC patients to targeted
inhibitor depends on genetic mutations.
26.
Sordella R, Bell DW, Haber DA, et al.
Gefitinib-sensitizing EGFR mutations in
lung cancer activate anti-apoptotic
pathways. Science 2004;305:1163-7
A key manuscript documenting sensitivity
of NSCLC patients to targeted inhibitor
depends on genetic mutations.
.
27.
28.
.
29.
.
Pao W, Miller V, Zakowski M, et al.
EGF receptor gene mutations are
common in lung cancers from
“never smokers” and are associated with
sensitivity of tumors to gefitinib and
erlotinib. Proc Natl Acad Sci USA
2004;101:13306-11
Pao W, Wang TY, Riely GJ, et al. KRAS
mutations and primary resistance of lung
adenocarcinomas to gefitinib or erlotinib.
PLoS Med 2005;2:57-61
A key manuscript documenting how
KRAS mutations alter the sensitivity of
lung cancers to targeted therapy.
Solit DB, Garraway LA, Pratilas CA,
et al. BRAF mutation predicts sensitivity
to MEK inhibition. Nature
2006;439:358-62
A key manuscript documenting that
the sensitivity of melanomas to MEK
inhibitors depends on the mutation
status of BRAF.
30.
Shelton JG, Steelman LS, Abrams SL, et al.
Conditional EGFR promotes cell cycle
progression and prevention of apoptosis in
the absence of autocrine cytokines.
Cell Cycle 2005;4:822-30
31.
McCubrey JA, Shelton JG, Steelman LS,
et al. Effects of a conditionally active
v-ErbB and an EGF-R inhibitor on
transformation of NIH-3T3 cells and
abrogation of cytokine dependency of
hematopoietic cells. Oncogene
2004;23:7810-20
32.
33.
34.
Konopleva M, Shi Y, Steelman LS, et al.
Development of a conditional in vivo
model to evaluate the efficacy of small
molecule inhibitors for the treatment of
Raf-transformed hematopoietic cells.
Cancer Res 2005;65:9962-70
Blalock WL, Pearce M, Steelman LS, et al.
A conditionally-active form of MEK1
results in autocrine transformation of
human and mouse hematopoeitic cells.
Oncogene 2000;19:526-36
Hoyle PE, Moye PW, Steelman LS, et al.
Differential abilities of the Raf family of
Expert Opin. Emerging Drugs (2010) 15(1)
protein kinases to abrogate
cytokine-dependency and prevent
apoptosis in murine hematopoietic cells by
a MEK1-dependent mechanism. Leukemia
2000;14:642-56
35.
McCubrey JA, Steelman LS, Hoyle PE,
et al. Differential abilities of activated Raf
oncoproteins to abrogate
cytokine-dependency, prevent apoptosis
and induce autocrine growth factor
synthesis in human hematopoietic cells.
Leukemia 1998;12:1903-29
36.
Shelton JG, Blalock WL, White ER, et al.
Ability of the activated PI3K/Akt
oncoproteins to synergize with MEK1
and induce cell cycle progression and
abrogate the cytokine-dependence of
hematopoietic cells. Cell Cycle
2004;3:503-12
37.
Steelman LS, Bertrand FE, McCubrey JA.
The complexity of PTEN: mutation,
marker and potential target for therapeutic
intervention. Expert Opin Ther Targets
2004;8:537-50
38.
Misaghian N, Ligresti G, Steelman LS,
et al. Targeting the leukemic stem cell—the
holy grail of leukemia therapy. Leukemia
2009;23:25-42
Current review on leukemia stem cells
and their targeting.
.
39.
Tortora G, Bianco R, Daniele G, et al.
Overcoming resistance to molecularly
targeted anticancer therapies: rational drug
combinations based on EGFR and MAPK
inhibition for solid tumors and
hematological malignancies.
Drug Resist Updat 2007;10:81-100
40.
McCubrey JA, Abrams SL, Ligresti G, et al.
Involvement of p53 and Raf/MEK/ERK
pathways in hematopoietic drug resistance.
Leukemia 2008;22:2080-90
A key manuscript document ability of
Raf/MEK/ERK pathway to interact
with p53 pathway and regulate drug
resistance and sensitivity to small
molecule inhibitors.
.
41.
Delaney AM, Printen JA, Chen H,
et al. Identification of a novel
mitogen-activated protein kinase kinase
activation domain recognized by the
inhibitor PD 184352. Mol Cell Biol
2002;22:7593-602
42.
Allen LF, Sebolt-Leopold J, Meyer MB.
CI-1040 (PD184352), a targeted signal
transduction inhibitor of MEK (MAPKK).
Semin Oncol 2003;30:105-16
McCubrey, Steelman, Abrams, et al.
43.
44.
.
45.
46.
47.
48.
49.
50.
51.
Rinehart J, Adjei AA, LoRusso PM, et al.
Multicenter phase II study of the oral MEK
inhibitor, CI-1040, in patients with
advanced non-small cell lung, breast,
colon, and pancreatic cancer. J Clin Oncol
2004;22:4456-62
Ohren JF, Chen H, Pavlovsky A, et al.
Structures of human MAP kinase kinase 1
(MEK1) and MEK2 describe novel
noncompetitive kinase inhibition.
Nat Struct Mol Biol 2004;11:1192-97
A key manuscript documenting
how certain MEK inhibitors function
using crystallography.
Davie BD, Logie A, McKay JS, et al.
AZD6244 (Arry 142886) a potent
inhibitor of mitogen-activated protein
kinase/extracellular signal-related kinase
kinase 1 /2 kinases: mechanism of action in
vivo, pharmacokinetic/pharmacodynamic
relationship and potential for combination
in preclinical models. Mol Cancer Ther
2007;6:2209-19
Yeh TC, Marsh V, Bernat BA, et al.
Biological characterization of
ARRY-142886 (AZD6244), a potent,
highly selective mitogen-activated protein
kinase kinase 1/ 2 inhibitor.
Clin Cancer Res 2007;13:1576-83
Huynh H, Soo KC, Chow PK, et al.
Targeted inhibition of the extracellular
signal-regulated kinases kinase
pathway with AZD-6244 (ARRY-142886)
in the treatment of hepatocellular
carcinoma. Mol Cancer Ther
2007;6:138-46
Huynh H, Chow PK, Soo KC. AZD-6244
and doxorubicin induce growth
suppression and apoptosis in mouse models
of hepatocellular carcinoma.
Mol Cancer Ther 2007;6:2468-76
52.
.
53.
.
54.
.
55.
Friday BB, Yu C, Dy GK. BRAF
V600E disrupts AZD6244-induced
abrogation of negative feedback pathways
between extracellular signal-regulated
kinase and Raf proteins. Cancer Res
2008;68:6145-53
Sebolt-Leopold JS. Advances in the
development of cancer therapeutics
directed against the Ras-mitogen-activated
protein kinase pathway. Clin Cancer Res
2008;14:3651-56
A key review discussing the lack
of initial success with trials with
inhibitors involving unselected
cancer patients.
Garber K. Trial offers early test case for
personalized medicine. JNCI
2009;101:136-8
A key review discussing the influence
of genetics on successful clinical trials
with inhibitors.
Miner J, Yu C, Iverson C, et al.
Selective MEK inhibitor REA119 exhibits
efficacy in orthotopic hepatoma models
and cytostatic potential in multiple cell
based models of cancer. EJC Suppl
2008;12:181
56.
Ricciardi MR, Milella M, Scerpa MC,
et al. Targeting the Raf/MEK/ERK
signaling pathway by the novel MEK
inhibitor PD0325901: molecular
and functional effects in pre-clinical
leukemia models. Haematologica
2007;92:278
57.
Johnston S. XL518, a potent, selective
orally bioavailable MEK1 inhibitor,
down-regulates the Ras/Raf/MEK/ERK
pathway in vivo, resulting in tumor growth
inhibition and regression in preclinical
models. 19th AACR-NCI-EORTC
International Conference on Molecular
Targets and Cancer Therapeutics
(Abstract C209); 2007
O’Neil BH, Williams-Goff LW,
Kauh J, et al. A phase II study of
AZD6244 in advanced or metastatic
hepatocellular carcinoma. J Clin Oncol
2009;27:e15574
Smalley KSM, Flaherty KT. Integrating
BRAF/MEK inhibitors into combination
therapy for melanoma. Br J Can
2009;100:431-5
Dhomen N, Marais R. BRAF signaling
and targeted therapies in melanoma.
Hematol Oncol Clin North Am
2009;23:529-45
A recent critical review on role of
Raf/MEK/ERK and other signaling
pathways in melanoma as well as
potential mechanisms of targeting
and pitfalls and advantages of current
MEK inhibitors.
58.
.
59.
Dankort D, Filenova E, Collado M, et al.
A new mouse model to explore the
initiation, progression, and therapy of
BRafV600E-induced lung tumors.
Genes Dev 2007;21:379-84
A key paper documenting creation of
conditional B-Raf mutant mouse which
can be used to evaluate Raf and
MEK inhibitors.
Montalto G, Cervello M, Giannitrapani L,
et al. Epidemiology, risk factors, and
Expert Opin. Emerging Drugs (2010) 15(1)
natural history of hepatocellular
carcinoma. Ann NY Acad Sci
2002;963:13-20
60.
Parkin DM, Bary F, Ferlay J, et al. Global
cancer statistics 2002. CA Cancer J Clin
2005;55:74-108
61.
El-Serag HB, Rudolph KL. Hepatocellular
carcinoma: epidemiology and molecular
carcinogenesis. Gastroenterology
2007;132:2557-76
62.
El-Serag HB. Hepatocellular carcinoma:
recent trends in the United States.
Gastroenterology 2004;127:S27-34
63.
Schmidt CM, McKillop IH, Cahill PA,
et al. Increased MAPK expression and
activity in primary human hepatocellular
carcinoma. Biochem Biophys
Res Commun 1997;236:54-8
64.
Huynh H, Nguyen TT, Chow KH, et al.
Over-expression of the mitogen-activated
protein kinase (MAPK) kinase
(MEK)–MAPK in hepatocellular
carcinoma: its role in tumor progression
and apoptosis. BMC Gastroenterol
2003;3:19
65.
Ito Y, Sasaki Y, Horimoto M, et al.
Activation of mitogen-activated protein
kinases/extracellular signal-regulated
kinases in human hepatocellular
carcinoma. Hepatology 1998;27:951-8
66.
Tanimura S, Chatani Y, Hoshino R, et al.
Activation of the 41/43 kDa
mitogen-activated protein kinase signaling
pathway is required for hepatocyte growth
factor-induced cell scattering. Oncogene
1998;17:57-65
67.
Tsuboi Y, Ichida T, Sugitani S, et al.
Overexpression of extracellular
signal–regulated protein kinase and its
correlation with proliferation in human
hepatocellular carcinoma. Liver Int
2004;24:432-6
68.
Calvisi DF, Ladu S, Gorden A, et al.
Ubiquitous activation of Ras and Jak/Stat
pathways in human HCC.
Gastroenterology 2006;130:1117-28
69.
Wentz SC, Wu H, Yip-Schneider MT,
et al. Targeting MEK is effective
chemoprevention of hepatocellular
carcinoma in TGF-alpha-transgenic mice.
J Gastrointest Surg 2008;12:30-7
70.
Saxena NK, Sharma D, Ding X, et al.
Concomitant activation of the
JAK/STAT, PI3K/AKT, and ERK
signaling is involved in leptin-mediated
promotion of invasion and migration of
45
Emerging MEK inhibitors
hepatocellular carcinoma cells. Cancer Res
2007;67:2497-507
71.
72.
73.
74.
75.
76.
Benn J, Schneider RJ. Hepatitis B virus
HBx protein activates Ras-GTP complex
formation and establishes a Ras, Raf, MAP
kinase signaling cascade. Proc Natl Acad
Sci USA 1994;91:10350-4
Yun C, Cho H, Kim SJ, et al. Mitotic
aberration coupled with centrosome
amplification is induced by hepatitis B
virus X oncoprotein via the
Ras-mitogen-activated protein/
extracellular signal-regulated
kinase-mitogen-activated protein pathway.
Mol Cancer Res 2004;2:159-69
Natoli G, Avantaggiati ML, Chirillo P,
et al. Ras- and Raf-dependent activation of
c-jun transcriptional activity by the
hepatitis B virus transactivator pX.
Oncogene 1994;9:2837-43
82.
Andrieux LO, Fautrei A, Bessard A,
et al. GATA-1 is essential in
EGF-meditated induction of nucleotide
excision repair activity and ERCCI
expression through ERK2 in human
hepatoma cells. Cancer Res
2007;67:2114-23
A key manuscript demonstrating
involvement of ERK mediated GATA-1
phosphorylation and activation of
DNA repair genes in drug resistance
of HCC cells.
.
Giambartolomei S, Covone F, Levrero M,
Balsano C. Sustained activation of the
Raf/MEK/Erk pathway in response to EGF
in stable cell lines expressing the hepatitis C
virus (HCV) core protein. Oncogene
2001;20:2606-10
84.
Hayashi J, Aoki H, Kajino K, et al.
Hepatitis C virus core protein activates the
MAPK/ERK cascade synergistically with
tumor promoter TPA, but not with
epidermal growth factor or transforming
growth factor alpha. Hepatology
2000;32:958-61
78.
Goel V, Lazar AJF, Warneke CL, et al.
Examination of mutations in B-Raf, N-Ras
and PTEN in primary cutataneous
melanoma. J Inv Dermatol
2005;126:154-60
46
Waskiewicz AJ, Flynn A, Proud CG, et al.
Mitogen activated kinases active the
serine/threonine kinases Mnk-1 and
Mnk2. EMBO J 1997;16:1909-20
83.
Aoki H, Hayashi J, Moriyama M, et al.
Hepatitis C virus core protein interacts
with 14-3-3 protein and activates the kinase
Raf-1. J Virol 2000;74:1736-41
80.
81.
Chung TW, Lee YC, Kim CH.
Hepatitis B viral HBx induces matrix
metalloproteinase-9 gene expression
through activation of ERK and
PI-3K/AKT pathways: involvement of
invasive potential. FASEB J
2004;18:1123-5
77.
79.
.
Engelman JA, Chen L, Tan X, et al.
Effective use of PI3K and MEK inhibitors
to treat mutant Kras G12D and PIK3CA
H104R murine lung cancers. Nat Med
2008;14:1351-6
Legrier ME, Yang CPH, Yan HG,
et al. McDaid HM: Targeting
protein translation in human non-small
cell lung cancer via combined MEK
and mammalian target of
rapamycin suppression. Cancer Res
2007;67:11300-8
A key manuscript document mechanism
of action of synergy between MEK and
mTOR inhibitors.
85.
McDaid HM, Lopez-Barcons L,
Grossman A, et al. Enhancement of the
therapeutic efficacy of taxol by the
mitogen-activated protein kinase kinase
inhibitor CI-1040 in nude mice bearing
human heterotransplants. Cancer Res
2005;65:2854-60
Haass NK, Sproesser K, Nguyen TK, et al.
The mitogen-activated protein/
extracellular signal-regulated kinase
kinase inhibitor AZD6244
(ARRY 142886) induces growth arrest
in melanoma cells and tumor regression
when combined with docetaxel.
Clin Cancer Res 2008;14:230-9
Yu C, Krystal G, Varticovksi L, et al.
Pharmacologic mitogen-activated
protein/extracellular signal-regulated
kinase kinase/mitogen-activated
protein kinase inhibitors interact
synergistically with STI571 to induce
apoptosis in Bcr/Abl-expressing
human leukemia cells. Cancer Res
2002;62:188-99
86.
Yu C, Dasmahapatra G, Dent P,
et al. Synergistic interactions between
MEK1/2 and histone deacetylase
inhibitors in BCR/ABL+ human
leukemia cells. Leukemia
2005;19:1579-89
87.
Cusimano A, Fodera D, D’Alessandro N,
et al. M. Potentiation of the antitumor
effects of both selective cyclooxygenase-I
and cyclooxygenase-2 inhibitors in human
hepatic cancer cells by inhibition of the
Expert Opin. Emerging Drugs (2010) 15(1)
MEK/ERK pathway. Cancer Biol Ther
2007;6:1461-8
88.
Mabuchi S, Ohmichi M, Kimura A, et al.
Inhibition of phosphorylation of BAD and
Raf-1 by Akt sensitizes human ovarian
cancer cells to paclitaxel. J Biol Chem
2002;277:33490-500
89.
Brognard J, Dennis PA. Variable
apoptotic response of NSCLC cells to
inhibition of the MEK/ERK pathway by
small molecules or dominant negative
mutants. Cell Death Differ
2002;9:893-904
90.
Aoki K, Ogawa T, Ito Y, et al.
Cisplatin activates survival signals
in UM-SCC-23 squamous cell
carcinoma and these signal pathways
are amplified in cisplatin-resistant
squamous cell carcinoma. Oncol Rep
2004;11:375-9
91.
Park SA, Park HJ, Lee BI, et al. Bcl-2
blocks cisplatin-induced apoptosis by
suppression of ERK-mediated p53
accumulation in B104 cells. Brain Res Mol
2001;93:18-26
92.
Lunghi P, Tabilio A, Lo-Coco F, et al.
Arsenic trioxide (ATO) and MEK1
inhibition synergize to induce apoptosis in
acute promyelocytic leukemia cells.
Leukemia 2005;19:234-44
93.
Lunghi P, Costanzo A, Salvatore L, et al.
MEK1 inhibition sensitizes primary acute
myelogenous leukemia to arsenic
trioxide-induced apoptosis. Blood
2006;107:4549-53
94.
Bonati A, Rizzoli V, Lunghi P.
Arsenic trioxide in hematological
malignancies: the new discovery of an
ancient drug. Curr Pharm Biotechnol
2006;7:397-405
95.
Lunghi P, Giuliani N, Mazzera L, et al.
Targeting MEK/MAPK signal
transduction module potentiates
ATO-induced apoptosis in multiple
myeloma cells through multiple signaling
pathways. Blood 2008;112:2450-62
96.
Dai Y, Landowski TH, Rosen ST, et al.
Combined treatment with the checkpoint
abrogator UCN-01 and MEK1/2
inhibitors potently induces apoptosis in
drug-sensitive and -resistant myeloma cells
through an IL-6-independent mechanism.
Blood 2002;100:3333-43
97.
Rieber M, Rieber MS. Signalling
responses linked to betulinic
acid-induced apoptosis are antagonized
by MEK inhibitor U0126 in adherent
McCubrey, Steelman, Abrams, et al.
or 3D spheroid melanoma irrespective
of p53 status. Int J Cancer
2006;118:1135-43
98.
Chung EJ, Brown AP, Asano H, et al.
In vitro and in vivo radiosensitization
with AZD6244 (ARRY-142886), an
inhibitor of mitogen-activated protein
kinase/extracellular signal-regulated
kinases 1 /2 kinase. Clin Cancer Res
2009;15:3050-7
99.
Blagosklonny MV. Paradoxes of aging.
Cell Cycle 2007;6:2997-3003
100.
Blagosklonny MV. Aging: ROS or TOR.
Cell Cycle 2008;7:3344-54
101.
Blagosklonny MV, Campisi J. Cancer and
aging: more puzzles, more promises.
Cell Cycle 2008;7:2615-18
102.
Blagosklonny MV. An anti-aging drug
today: from senescence-promoting genes to
anti-aging pill. Drug Discov Today
2007;12:218-24
103.
.
104.
Blagosklonny MV. Aging, stem cells, and
mammalian target of rapamycin: a prospect
of pharmalogical rejuvenation of aging
stem cells. Rejuvenation Res
2008;11:801-8
A key manuscript discussing the
potential for rapamycin to prevent aging
in stem cells.
105.
Blagosklonny MV. Program-like aging and
mitochondria: instead of random damage
by free radicals. J Cell Biochem
2007;102:1389-99
106.
Demidenko ZN, Shtutman M,
Blagosklonny MV. Pharmacologic
inhibition of MEK and PI-3K converges on
the mTOR/S6 pathway to decelerate
cellular senescence. Cell Cycle
2009;8:1896-900
107.
Demidenko ZN, Blagosklonny MV. At
concentrations that inhibit mTOR,
resveratrol suppresses cellular senescence.
Cell Cycle 2009;8:1901-4
108.
Demidenko ZN, Blagosklonny MV.
Growth stimulation leads to cellular
senescence when the cell cycle is blocked.
Cell Cycle 2008;7:3355-61
109.
Demidenko ZN, Zubova SG,
Bukreeva EI, et al. Rapamycin decelerates
cellular senescence. Cell Cycle
2009;8:1888-95
110.
Blagosklonny MV. Aging-suppressants:
Cellular senescence (hyperactivation) and
its pharmacologic deceleration. Cell Cycle
2009;8:1883-7
Blagosklonny MV. Cancer stem cell and
cancer stemloids: from biology to therapy.
Cancer Biol Ther 2007;6:1684-90
Expert Opin. Emerging Drugs (2010) 15(1)
Affiliation
James A McCubrey†1, Linda S Steelman1,
Steven L Abrams1, William H Chappell1,
Suzanne Russo2, Roger Ove2, Michele Milella3,
Agostino Tafuri4, Paolo Lunghi5,
Antonio Bonati5,6, Franca Stivala7,
Ferdinando Nicoletti7, Massimo Libra7,
Alberto M Martelli8, Giuseppe Montalto9 &
Melchiorre Cervello10
†
Author for correspondence
1
Brody School of Medicine at
East Carolina University,
Department of Microbiology & Immunology,
600 Moye Boulevard,
Greenville, NC 27858, USA
Tel: +1 252 744 2704; Fax: +1 252 744 3104;
E-mail: [email protected]
2
Brody School of Medicine at
East Carolina University,
Leo Jenkins Cancer Center,
Department of Radiation Oncology,
600 Moye Blvd, Greenville, 27834, USA
3
Regina Elena Cancer Center,
Via Elio Chianesi n.53,
Rome 00144, Italy
4
University of Rome, La Sapienza,
Department of Hematology-Oncology,
Via Benevento 6,
Rome 99161, Italy
5
University of Parma,
Department of Clinical Sciences,
Via Gramsci n.14,
Parma 43100, Italy
6
University Hospital of Parma,
Unit of Hematology and Bone-Marrow
Transplantation,
Via Gramsi n.14,
Parma 43100, Italy
7
University of Catania,
Department of Biological Sciences,
Via Androne 83,
Catania 95124, Italy
8
University of Bologna,
Department of Human Anatomical Sciences,
Via Imerio 48,
Bologna, Italy
9
University of Palermo,
Department of Clinical Medicine and
Emerging Pathologies,
141 Via del Vespro,
Palermo 90127, Italy
10
Consiglio Nazionale delle Ricerche,
Istituto di Biomedicina e Immunologia,
Molecolare ‘Alberto Monroy”,
Via Ugo La Malfa 153,
Palermo 90146, Italy
47