Contents
The Role of Emerging Genomics and Proteomics
Technologies in Cancer Drug Target Discovery Pp.111-124
Patrick
Onyango
Protein Kinase C Isozymes as Potential
Targets for Anticancer Therapy
Pp.125-146
Johann
Hofmann
Changes in the Apoptotic and Survival
Signaling in Cancer Cells and Their Potential Therapeutic Implications Pp.147-163
Albert
F.Kabore ,James B.Johnston , and Spencer B.Gibson
Transforming Growth Factor-b Superfamily:Evaluation as Breast Cancer
Biomarkers and Preventive Agents Pp.165-182
V.Gupta ,D.P.Harkin
,H.Kawakubo and S.Maheswaran
A Rationale for Inhibiting
Progesterone-Related Pathways to Combat Breast Cancer Pp.183-189
Michael
R.Moore
Novel Fluoropyrimidines:Improving the
Efficacy and Tolerability of Cytotoxic Therapy Pp.191-204
Russell D.Petty and J.Cassidy
Targeted Histone Deacetylase Inhibition for
Cancer Therapy Pp.205-218
D.M.Vigushin
and R.C.Coombes
AT Islands – Their Nature and Potential for
Anticancer Strategies
Pp.219-234
Jan M.Woynarowski
Abstracts
[Back to top] The
Role of Emerging Genomics and Proteomics Technologies in Cancer Drug Target
Discovery
Patrick Onyango
Cancer drugs have
traditionally been identified in screens designed to produce broad biological
end points such as cell death. A serious undesired outcome of drugs discovered
in these screens is that the mechanism of drug action is unknown and such drugs
often have adverse side effects. Designing cancer drugs that act on specific
targets offer the advantage that the mechanism of drug action can be understood
and accurately monitored in clinical trials leading to development of better
drugs. The pharmacological industry has recently shifted to a target directed
drug discovery model. However, until recently potential cancer drug targets
comprised of only a small
fraction of the human genome.
The human genome project and high-throughput structural and functional genomics
have dramatically increased the number of cancer drug targets. Deciphering cancer drug targets requires the
understanding of biochemical pathways that are affected in the cancer genome.
It has been suggested that utilization of Single-nucleotide polymorphisms
(SNPs) will aid in identifying individuals at high risk of developing certain
cancers, and will also help in development of tailored medication or identify
genetic profiles of specific drug action and toxicity. Achieving successful new
cancer drug development schemes will require a merger of research disciplines
that include pharmacology, genomics, comparative genomics, functional genomics,
proteomics and bioinformatics. In this review the significance and challenges
of these rapidly evolving technologies in cancer drug target discovery are
discussed.
[Back to top] Protein Kinase C Isozymes as Potential
Targets for Anticancer Therapy
Johann
Hofmann
Protein kinase C
(PKC) comprises a family of isozymes (a, bI, bII, g , d, e, q, h, l/i
[mouse/human], and z) which are involved in
signal transduction from membrane receptors to the nucleus. Activation of PKC
by phorbol esters promotes tumor formation, and from that it was concluded that
inhibitors of PKC might prevent carcinogenesis or inhibit tumor proliferation.
However, the situation is more complicated because the exact function of the
different PKC isozymes is not known at present. They have been shown to be
involved in synaptic transmissions, the activation of ion fluxes, secretion,
cell cycle control, differentiation, proliferation, tumorigenesis,
metastasis and apoptosis. Modulators such as bryostatin-1, phospholipid
analogues, PKC-activating adriamycin derivatives, CGP41251, UCN-01, and
antisense oligonucleotides directed against PKCa,
have shown antitumor activity in cancer patients. PKC inhibitors are not
specific to PKC, but also interact with other signaling molecules, which may
contribute to the antitumor effects. Modulators of PKC have also been shown to
influence non-MDR1-mediated and MDR1-mediated antitumor drug resistance. This
review is focussed on the role of PKC isozymes in human cell proliferation,
apoptosis and antitumor drug resistance, and on the use of PKC modulators as
antitumor agents.
[Back to top] Changes in the
Apoptotic and Survival Signaling in Cancer Cells and Their Potential
Therapeutic Implications
Albert
F.Kabore ,James B.Johnston , and Spencer B.Gibson
In normal healthy
tissues, an equilibrium is established between cell death and survival. This
equilibrium ensures that cells survive in the right milieu, but undergo
programmed cell death (apoptosis) when damaged, or when the environment is no
longer supportive. Diseases may occur with alterations in this homeostasis. For
example, cancer cells may survive in an environment in which they would not
normally exist. This is accomplished by alterations in the expressions or
functions of genes controlling both survival and apoptotic signaling pathways.
Survival signaling pathways involve the activation of cell surface
receptors, serine threonine kinases,
transcription factors as well as other molecules. In breast and ovarian
cancers, the ErbB2 growth factor receptor is overexpressed and this contributes
to the progression of these cancers, in part by constitutively activating
survival signaling pathways. In contrast, apoptotic signal transduction
pathways are often inhibited in cancer. For example, overexpression of Bcl-2
blocks apoptosis and this contributes to the accumulation of cells in
follicular lymphomas and chronic lymphocytic leukemia. Furthermore, alterations
in these signaling pathways in cancer cells may lead to drug resistance. Recent
advances in molecular targeted therapies have taken advantage of alterations in
survival and apoptotic signaling pathways in cancer to specifically target
aberrantly regulated molecules. For example, Herceptinª inhibits ErbB2 function
and anti-sense oligonucleotides against Bcl-2 reduce Bcl-2 expression. These
agents can thus induce apoptosis in the specific cancer cell against which they
have been targeted. In this review, we will discuss alteration in survival and
apoptotic signal transduction pathways in cancer and the development of novel
chemotherapeutic drugs to target these pathways.
[Back to top] Transforming Growth Factor-b Superfamily:Evaluation as Breast Cancer
Biomarkers and Preventive Agents
V.Gupta ,D.P.Harkin
,H.Kawakubo and S.Maheswaran
The Transforming
Growth Factor-b (TGFb)
superfamily of cytokines is comprised of a number of structurally-related,
secreted polypeptides that regulate a multitude of cellular processes including
proliferation, differentiation and neoplastic transformation. These growth
regulatory molecules induce ligand-mediated hetero-oligomerization of distinct
type II and type I serine/threonine kinase receptors that transmit signals
predominantly through receptor-activated Smad proteins but also induce
Smad-independent pathways. Ligands, receptors and intracellular mediators of
signaling initiated by members of the TGFb
family are expressed in the mammary gland and disruption of these pathways may
contribute to the development and progression of human breast cancer. Since
many facets of TGFb and breast cancer have
been recently reviewed in several articles, except for discussion of recent
developments on some aspects of TGFb, the major
focus of this review will be on the role of activins, inhibins, BMPs, nodal and
MIS-signaling in breast cancer with emphasis on their utility as potential
diagnostic, prognostic and therapeutic targets.
[Back to top] A Rationale for Inhibiting
Progesterone-Related Pathways to Combat Breast Cancer
Michael
R.Moore
Inhibitors of
estrogen-related pathways have been used with some success in the treatment of
breast cancer. These include the antiestrogens tamoxifen, more recently
faslodex, and the aromatase inhibitor anastrazole. However, failure and
recurrence rates are substantial with drugs countering the effects of
estrogens. Progestins, unlike estrogens, have generally been considered to
oppose breast cancer and have been used with reasonable efficacy after
antiestrogen failure. However, a building body of evidence, from cell culture,
animal studies, and, most recently, several major clinical studies involving
hormone replacement therapy, strongly supports the notion
that progestins generally stimulate breast cancer. Our
studies and those of others suggest that progestins increase the numbers of
breast cancer cells by both stimulating the rate of proliferation and
inhibiting cell death. These data indicate that progestin-related pathways
might provide effective targets for breast cancer therapy. This review
addresses the rationale for using inhibitors of progestin-related pathways to
treat breast cancer and comments on some possible points of attack.
[Back to top] Novel Fluoropyrimidines:Improving the
Efficacy and Tolerability of Cytotoxic Therapy
Russell
D.Petty and J.Cassidy
The
fluoropyrimidines were first synthesised nearly 50 years ago as rationally
designed anti-cancer agents. Their target was pyrimidine and hence DNA
synthesis. 5-Fluorouracil has been the most extensively used in a wide variety
of malignancies. In more recent years a fuller understanding of the pharmacokinetics
of these agents has lead to their utilisation as more effective and versatile
anti-cancer drugs than might have been initially envisaged. This in part has
occurred due to recognition of the schedule dependency of efficacy of 5-FU and
modulation of its activity by leucovorin. However the development of novel
fluropyrimidines such as cap- cetabine, UFT, and eniluracil which can be
administered orally, has offered equal if not superior efficacy with improved
tolerability and patient acceptance. It is now recognised that enzyme
polymorphism’s and heterogeneity of expression of key molecules are important
determinants of the pharmacokinetic handling and pharmacodynamic effects of
these drugs in individual patients. Further characterisation of such inter-individual
and inter-tumoral variability, for example in enzymes such as DPD and thymidine
phosphorylase is ongoing. This work offers the promise of improvements in
efficacy and tolerability by the process of individualisation of chemotherapy
(for both patient and tumour).
In contrast to the
advances made in the understanding of the pharmacokinetics, less progress has
been made in Fluropyrimidine pharmacodynamics. The inhibition of thymidylate
synthetase by dFUMP and thereby dTMP and DNA synthesis is thought to be the
critical mechanism. The incorporation of FUTP and dFUTP into RNA and DNA are
also postulated to be of importance. While these events have been well defined,
exactly how they lead to cell death is less clearly understood. Similarly, the
mechanism of selective cancer cell cytotoxicity is not well understood.
Pharmacokinetics and cell cycle kinetics provide a partial explanation. There
is some evidence to suggest that the most important factor in determining
cytotoxicity is the cellular response to fluoropyrimidine induced biochemical
abnormalities rather than the lesions themselves. In this hypothesis the
difference in response between normal and cancer cells is of critical
importance. Further improvements in efficacy and tolerability could be made by
elucidation of the molecular mechanisms behind this process. This knowledge in
combination with the advances already made (and ongoing) in pharmacokinetics
may allow the full potential of fluoropyrimidines as anti-cancer agents to be
realised in the future.
[Back to top] Targeted Histone Deacetylase Inhibition for
Cancer Therapy
D.M.Vigushin
and R.C.Coombes
The histone
deacetylase inhibitors are a new class of cytostatic agents that inhibit the
proliferation of tumor cells in culture and in vivo by inducing cell cycle
arrest, differentiation and/or apoptosis. Histone acetylation and deacetylation
play important roles in the modulation of chromatin topology and the regulation
of gene transcription. Histone deacetylase inhibition induces the accumulation
of hyperacetyl-ated nucleosome core histones in most regions of chromatin but
affects the expression of only a small subset of genes, leading to
transcriptional activation of some genes, but repression of an equal or larger
number of other genes. Non-histone proteins such as transcription factors are
also targets for acetylation with varying functional effects. Ace-tylation
enhances the activity of some transcription factors such as the tumor
suppressor p53 and the erythroid
differentiation factor GATA-1 but may repress transcriptional activity of
others including T cell factor and the co-activator ACTR. Recent studies in our
laboratory and others have shown that the estrogen receptor a (ERa)
can be hyperacetylated in response to histone deacetylase inhibition,
suppressing ligand sensitivity and regulating transcriptional activation by
histone deacetylase inhibitors. Conservation of the acetylated ERa motif in other nuclear receptors suggests
that acetylation may play an important regulatory role in diverse nuclear
receptor signaling functions. A number of structurally diverse histone
deacetylase inhibitors have shown potent antitumor efficacy with little
toxicity in vivo in animal models. Several compounds are currently in early
phase clinical development as potential treatments for solid and hematological
cancers both as monotherapy and in combination with cytotoxics and
differentiation agents. This report reviews the biology and clinical
development of histone deacetylase inhibitors for cancer therapy.
[Back to top] AT Islands – Their Nature and Potential for
Anticancer Strategies
Jan
M.Woynarowski
The human genome
contains a unique class of domains, referred to as AT islands, which consist
typically of 200-1000 bp long tracts of up to 100% A/T DNA. The significance of
AT islands as potential targets for chemotherapeutic intervention stems from
two main aspects. First, AT islands are inherently unstable (expandable)
minisatellites that are found in various known loci of genomic instability,
such as AT-rich fragile sites. Second, AT islands are involved in the
organization of the genomic DNA on the nuclear matrix by acting as
scaffold/matrix attachment regions, S/MARs. DNA duplexes of AT islands are
unusually flexible and prone to
base unpairing, which are crucial MAR attributes. Various AT
islands show high binding affinity for isolated nuclear matrices and associate
with the nuclear matrix in the cell. The cellular MAR function of AT islands
may differ in cancer and normal cells. The abnormally expanded AT islands in
the FRA16B fragile site in leukemic CEM cells act as strong, permanent MARs,
while their unexpanded counterparts in normal cells are loop localized. Given
their instability and involvement in the remodeling of the nuclear
architecture, AT islands may be a factor in cancerous phenotypes. AT islands
are preferentially targeted by the extremely potent DNA-alkylating antitumor
drugs, bizelesin and U78100%. High lethality of lesions in AT islands is
consistent with the critical role of MAR domains in DNA replication. The
abnormal structure/function of AT islands, such as their expansion and acquired
strong MAR properties, may sensitize cancer cells to AT island targeting drugs.