| Current
Drug Targets
ISSN: 1389-4501
Current Drug Targets
Volume 8, Number 6, June 2007
Contents
Current Drug Targets in Childhood Acute Leukemia
Guest Editor: R.B. Lock
Editorial Pp. 683
Targeting the Leukemia Microenvironment Pp.
685-701
M. Konopleva and M. Andreeff
[Abstract]
FLT3 as a Therapeutic Target in Childhood Acute Leukemia
Pp. 703-714
M.C. Stubbs and S.A. Armstrong
[Abstract]
Targeting RAS Signaling Pathways in Juvenile Myelomonocytic
Leukemia Pp. 715-725
C. Flotho, C. Kratz and C.M. Niemeyer
[Abstract]
Targeting Transcription Factors in Acute Leukemia
in Children Pp. 727-737
J.N. Berman and A.T. Look
[Abstract]
The Cytoskeleton as a Therapeutic Target in Childhood
Acute Leukemia: Obstacles and Opportunities Pp. 739-749
T.Y.E. Liaw, M.H.Y. Chang and M. Kavallaris
[Abstract]
Simultaneous Interruption of Signal Transduction and
Cell Cycle Regulatory Pathways: Implications for New Approaches
to the Treatment of Childhood Leukemias Pp. 751-759
S. Grant and P. Dent
[Abstract]
Microarray-Based Identification of New Targets for
Specific Therapies in Pediatric Leukemia Pp. 761-764
M.L. den Boer and R. Pieters
[Abstract]
Assessing Combinations of Cytotoxic Agents Using Leukemia
Cell Lines Pp. 765-771
C.P. Reynolds, M.H. Kang, N. Keshelava and B.J. Maurer
[Abstract]
In Vivo Models of Childhood Leukemia for
Preclinical Drug Testing Pp. 773-783
P.S. Bachmann and R.B. Lock
[Abstract]
Abstracts
[Back to top]
Editorial
Pediatric acute lymphoblastic leukemia, the most common
childhood malignancy, represents a profound cancer therapy
success story of the 20th
Century. Well-designed clinical trials, coupled with improvements
in supportive care, have increased the likelihood of cure
from almost nil prior to the 1960s to approaching 80% at the
present day.
Nevertheless, our rejoicing that the most common pediatric
malignancy is also one of the most curable should be tempered
by the realization that a significant number of children still
relapse and die from the disease, that survivors suffer significant
long-term effects of cytotoxic chemotherapy, and that the
cure rates for other less common acute leukemia subtypes remain
at less than 50%.
The identification of novel targets for drug development in
pediatric acute leukemia provides the vision to protect our
most valuable resource, by saving those children who have
relapsed, by developing drugs that more specifically target
leukemia cells (and thereby spare the normal cells of the
body), and by improving the likelihood of survival from the
leukemias currently considered for the most part incurable.
A substantial number of biomedical researchers remain committed
to understanding the molecular basis for pediatric acute leukemogenesis,
its progression, and in too many cases its resistance to conventional
treatment. The current issue highlights exciting developments
in this field of research.
It has long been established that the microenvironment influences
the development and progression of acute leukemia. The chapter
by Konopleva and Andreef summarizes our current understanding,
and explores the possibility of targeting microenvironment/leukemia
interactions in the development of novel therapies. Furthermore,
it has become increasingly evident that dysregulation of tyrosine
kinases is intimately involved in leukemogenesis. Stubbs and
Armstrong describe the role of mutations in the FLT3 receptor
tyrosine kinase in leukemia, and summarize current clinical
development of FLT3 inhibitors.
Inappropriate activation of the RAS signaling pathway is commonly
observed in juvenile myelomonocytic leukemia (JMML), a rapidly
fatal myeloproliferative disorder of early childhood. The
chapter by Flotho and colleagues summarizes current understanding
of JMML, and offers insight into how the RAS signaling pathway
might be exploited in the development of novel therapeutics
to improve the outcome for this devastating disease.
Transcription factors are dysregulated in many childhood acute
leukemias by chromosomal translocations that result in their
repressed activity or inappropriate activation. Berman and
Look review the current state of knowledge in the field, and
focus on recent attempts to target transcription factors in
leukemia, and in particular acute promyelocytic leukemia and
T-lineage acute lymphoblastic leukemia, as well as predictions
for future drug targeting.
Tubulin-binding agents, in particular the Vinca alkaloids
vincristine and vinblastine, have become central components
of combination chemotherapy regimens used in the treatment
of a wide range of malignancies, including pediatric acute
leukemia. Liaw and colleagues review the mechanism action
of tubulin-binding agents, how drug resistance can emerge,
and provide insight into current and future clinical development
of novel tubulin-binding drugs. While tubulin-binding drugs
exert their anti-leukemic effects via induction of apoptosis
(programmed cell death), the chapter by Grant and Dent explores
the rational combination of drugs that simultaneously target
signal transduction and cell cycle regulatory pathways to
specifically induce apoptosis in acute leukemia cells. Den
Boer and Pieters then review the use of microarray analysis
of gene expression to identify new drug targets in acute leukemia
subtypes.
A significant hindrance in the development of new drugs to
treat pediatric acute leukemia is that pharmaceutical companies,
not surprisingly, tend to focus drug development programs
towards the common malignancies, the adult solid tumors. Some
of these new drugs are likely to show activity against pediatric
leukemias, although there are many more new chemotherapeutics
drugs available for clinical trials than there are children
available to be entered into those trials. Therefore, the
issue concludes with chapters that address the requirement
for predictive in vitro (Reynolds and colleagues)
and in vivo (Bachmann and Lock) experimental models
that can be used to prioritize new drugs for clinical trials
in children.
Thus, despite the progress that has been made over the past
40 years in the treatment of pediatric acute lymphoblastic
leukemia, continued efforts must focus on the development
of new approaches for improving therapy for high-risk patients,
for those who relapse, and for those children afflicted with
less curable leukemia subtypes. This issue provides insight
into current and future efforts to identify novel targets
for drug development in pediatric acute leukemia, and offers
optimism that outcomes will continue to improve.
R.B. Lock
[Back to top]
Targeting the Leukemia Microenvironment
M. Konopleva and M. Andreeff
Normal hematopoiesis is maintained by dynamic interactions
between hematopoietic cells and the bone marrow microenvironment.
In hematological malignancies, there are reciprocal interactions
between leukemic cells and cells of the bone marrow microenvironment
such as stroma, osteoblasts and endothelium. In this review,
we will discuss the influence of the microenvironment on the
evolution of the leukemic phenotype. We propose that specific
niches within the bone marrow microenvironment may provide
a sanctuary for subpopulations of leukemic cells to evade
chemotherapy-induced death and allow acquisition of a drug-resistant
phenotype. We will also discuss recent studies that suggest
novel therapeutic interventions targeting the microenvironment/leukemia
interaction. Focus on this stroma-leukemia crosstalk may result
in the development of strategies that alleviate the acquisition
of a chemoresistant phenotype and enhance the efficacy of
therapies in hematological malignancies.
[Back to top]
FLT3 as a Therapeutic Target in Childhood Acute Leukemia
M.C. Stubbs and S.A. Armstrong
During the past few years, a major focus of leukemia research
has centered on tyrosine kinases as potential therapeutic
targets. This is due in large part to the success of imatinib
mesylate (STI571, Gleevec), which has proven effective as
a therapy for chronic myeloid leukemias bearing the t(9;22)
encoding the BCR-ABL kinase. It has become increasingly evident
that mutations producing constitutively active tyrosine kinases
play a role in leukemogenesis. Another kinase that has drawn
significant attention is the FMS-like tyrosine kinase 3 (FLT3).
FLT3 is expressed in most childhood acute leukemias. Select
genetic subgroups possess particularly high-level expression,
with a significant percentage therein harboring activating
mutations. In this review we will discuss FLT3 as a potential
therapeutic target in childhood acute leukemias. We will highlight
the role of FLT3 in hematopoiesis, and how when activated,
it may play a role in the development of acute myeloid or
acute lymphoblastic leukemia. We will examine the successes
in elucidating FLT3 function in acute leukemias, highlight
current FLT3 targeted therapeutics, and discuss how FLT3 inhibitors
might be used in combination therapies in the future.
[Back to top]
Targeting RAS Signaling Pathways in Juvenile Myelomonocytic
Leukemia
C. Flotho, C. Kratz and C.M. Niemeyer
The RAS proteins function as fundamental signaling switches
that control normal cell growth and differentiation. Deregulated
activation of RAS-dependent signaling pathways constitutes
a potent mechanism of malignant cell transformation. Juvenile
myelomonocytic leukemia (JMML) is a rapidly fatal myeloproliferative
disorder of early childhood for which no effective treatment
other than hematopoietic stem cell transplantation is currently
available. Many aspects of JMML pathobiology are linked to
deregulated RAS signaling. Hence, targeting RAS or its interactors
on a molecular level is a promising strategy in the development
of novel rational therapies for this menacing disease. Here
we give an overview of current concepts on the pathogenesis
of JMML, present important aspects of cellular RAS biology
that can be exploited for pharmacologic manipulation, and
discuss mouse models that have greatly advanced our understanding
of the role RAS plays in JMML. In addition, we review recent
approaches to develop agents that interfere with the RAS network
at the level of the granulocyte-macrophage colony-stimulating
factor receptor, posttranslational RAS processing (prenylation
and endoprotease cleavage), RAF serine/threonine kinase, MEK
mitogen-activated protein kinase, and target of rapamycin
activity. Preclinical and clinical data of these pharmaceuticals
in JMML and other myeloid malignancies is discussed.
[Back to top]
Targeting Transcription Factors in Acute Leukemia
in Children
J.N. Berman and A.T. Look
Transcription factors play essential roles in controlling
normal blood development and their alteration leads to abnormalities
in cell proliferation, differentiation and survival. In many
childhood acute leukemias, transcription factors are altered
through chromosomal translocations that change their functional
properties resulting in repressed activity or inappropriate
activation. The development of therapies that specifically
target these molecular abnormalities holds promise for improving
the outcome in diseases that remain challenging to treat,
such as childhood T-cell acute lymphoblastic leukemia and
acute myeloid leukemia, with improved toxicity profiles. All
trans-retinoic acid and arsenic trioxide have already demonstrated
efficacy in acute promyelocytic leukemia in both adults and
children. Newer agents, such as histone deacetylase inhibitors,
drugs targeting the NOTCH pathway, and short interfering RNAs
have shown encouraging results in pre-clinical studies and
are likely to enter the clinical arena in the near future.
Through an improved understanding of the pathways and mechanisms
underlying the malignant transformation induced by altered
transcription factors, new targeted therapies will be designed
that should greatly enhance current available treatments.
[Back to top]
The Cytoskeleton as a Therapeutic Target in Childhood
Acute Leukemia: Obstacles and Opportunities
T.Y.E. Liaw, M.H.Y. Chang and M. Kavallaris
Antimitotic agents that interfere with the tubulin/microtubule
system are important in the treatment of a range of cancers.
Natural product tubulin-binding agents such as the Vinca
alkaloids have proven highly effective in the treatment of
leukemia. Improved understanding of the mechanisms of action
of these and related drugs has led to the identification of
distinct binding sites on tubulin that cause inhibition of
spindle microtubule dynamics, mitotic arrest and cell death.
Despite the efficacy of these agents, treatment failure caused
by the emergence of drug resistant leukemic cells is a significant
clinical problem. Alterations in the cellular target of tubulin-binding
agents have been strongly implicated in resistance to these
agents. This review will focus on the microtubule cytoskeleton
and its role in drug resistance in leukemia. The identification
of novel protein pathways involved in drug response and the
development of new drugs targeted against microtubules, offers
opportunities to treat resistant disease, improve outcome
and potentially reduce toxicity for leukemia patients.
[Back to top]
Simultaneous Interruption of Signal Transduction and
Cell Cycle Regulatory Pathways: Implications for New Approaches
to the Treatment of Childhood Leukemias
S. Grant and P. Dent
The last decade has witnessed the introduction of a large
number of novel, molecularly targeted agents into the therapeutic
armamentarium against diverse forms of cancer, including leukemia.
Such agents include signal transduction, cell cycle, histone
deacetylase, Hsp90, proteasome, and Bcl-2 family member inhibitors,
among others. While most of these agents have been or are
currently being evaluated in adult patients with acute leukemia,
experience in childhood leukemia is very limited. Although
the use of such targeted agents as potentiators of conventional
cytotoxic agent activity represents a logical approach, an
emerging body of evidence suggests that neoplastic cells in
general, and leukemic cells in particular, are highly susceptible
to a therapeutic strategy in which survival signaling and
cell cycle regulatory pathways are simultaneously disrupted.
In in vitro studies, highly synergistic antileukemic
interactions have been reported between CDK and HDAC inhibitors;
HDAC and proteasome inhibitors; Bcl-2 antagonists and CDK
inhibitors; MEK/ERK and Chk1 inhibitors, and proteasome and
CDK inhibitors, among other combinations. Some of these strategies,
including combinations of HDAC and CDK inhibitors, and CDK
and proteasome inhibitors, have now entered the clinical arena
in patients with leukemia and other hematologic malignancies.
Based upon preclinical results to date, there is reason to
suspect that such strategies might prove to be active against
several types of childhood leukemia. Thus, over the next decade,
the introduction of molecularly targeted agents, alone and
in combination, into the therapeutic armamentarium against
childhood leukemia may have significant implications for children
with this disease.
[Back to top]
Microarray-Based Identification of New Targets for
Specific Therapies in Pediatric Leukemia
M.L. den Boer and R. Pieters
The efficacy of current treatment protocols for childhood
cancer is mainly based on empirical studies by adding drugs,
changing drug dosages and changing drug combinations. In pediatric
acute lymphoblastic leukemia (ALL), this approach has resulted
into ~80% 5-year disease-free survival whereas less favorable
results have yet been obtained for acute myeloid leukemia
(AML), i.e. ~50%, and other types of tumors, e.g. ~60% for
medulloblastoma. A further optimization of therapy results
requires more insights into the molecular biology of tumor
cells, including genetic defects and aberrant expression of
genes. This knowledge is needed to rationally develop more
specific therapies in which relapse-risk and side-effects
of therapy are reduced using targeted drugs.
Genome-wide analysis of gene expression levels (mRNA) has
revealed many new insights into the biology of leukemic cells.
In this review we will discuss the recent progress that has
been made in the use of microarrays for identifying new markers
and targets for treatment of acute leukemia in children.
[Back to top]
Assessing Combinations of Cytotoxic Agents Using Leukemia
Cell Lines
C.P. Reynolds, M.H. Kang, N. Keshelava and B.J. Maurer
The mainstay of clinical anti-neoplastic chemotherapy is multi-agent
combinations, most of which were developed empirically. To
speed research and decrease costs, there is increasing interest
in moving new drugs into clinical trials in potentially active
combinations based upon pre-clinical testing data. Because
testing drug combinations in animals is expensive and complex,
defining drug combinations initially in cell culture assays
is essential. For in vitro testing we employ a panel
of well-characterized cell lines and DIMSCAN, a semi-automatic
fluorescence-based digital image microscopy system that quantifies
relative total (using a DNA stain) or viable [using fluorescein
diacetate (FDA)] cell numbers in tissue culture multi-well-plates
ranging from 6 to 384 wells per plate. DIMSCAN is a rapid
and efficient tool for conducting in vitro cytotoxicity
assays across a 4 log dynamic range. The specificity of detecting
viable cells with FDA is achieved by use of digital image
processing and chemical quenching of fluorescence in non-viable
cells with eosin Y. Cytotoxicity measured by DIMSCAN was found
to be comparable to manual trypan blue dye exclusion counts
or colony formation in soft agar, but with a significantly
wider dynamic range, that enables drug combination studies
used to detect synergistic or antagonistic interactions in
cell lines from both solid tumors and leukemias. While different
mathematical models have been proposed for evaluating drug
interactions, which can be classified as synergistic (combinations
demonstrating greater than the additive activity expected
from each agent alone), additive, or antagonistic (drugs showing
less activity in combination than expected from the sum of
each agent alone), we generally find the Combination Index
method (as developed by Chou, et al.) to be the most
suitable for evaluating of drug interactions in cell culture
assays.
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In Vivo Models of Childhood Leukemia for
Preclinical Drug Testing
P.S. Bachmann and R.B. Lock
The number of new anti-cancer drugs emerging for clinical
trials in humans far exceeds the availability of pediatric
acute leukemia patients to be entered into clinical trials.
Therefore, preclinical testing of new agents for the treatment
of childhood acute leukemia is essential to ensure that the
most promising drugs are prioritized to enter clinical trials.
Historically, the murine system has been central to modeling
human leukemia in vivo. A greater knowledge of the
molecular lesions underlying particular subtypes of leukemia
has led to the generation of genetically engineered murine
models, generally involving the knockin or knockout of certain
genes and fusion genes at their normal genetic locus. However,
the most predominant in vivo models for preclinical
drug testing have been human leukemia xenografts. Successful
engraftment of all subtypes of acute lymphoblastic leukemia,
most subtypes of acute myeloid leukemia as well as juvenile
myelomonocytic leukemia, chronic myeloid leukemia and chronic
lymphocytic leukemia have been described in various immune-deficient
murine hosts. Preclinical testing of novel therapeutics in
vivo will likely identify the most promising new agents
to enter clinical trials, and will allow their future use
to be optimized in combination with other novel and conventional
chemotherapeutics.
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