Development of Novel Anticancer Agents and
Identification of Mode of Action in Eradicating
Associate Professor Ilana Nathan
Faculty of Health Sciences
Ben-Gurion University of the Negev, Beer Sheva
Cancer in most cases is still considered an incurable
disease and thus there is a critical need to discover
new anticancer drugs. In our efforts to search for novel
anticancer agents or approaches, we have concentrated on
three major groups of compounds, all of which act at
least in part by inducing cancer cell death.
Structure-function relationships of the newly
synthesized compounds, selectivity, mode of action in
killing cancer cells and cellular targets were explored.
The first group consists of compounds whose chemical
structures comprises arylethylene moiety, the second
group consists of iron chelators and the third group
consists of naphthoquinones. Both known and newly
synthesized compounds were studied for their antitumor
efficacy in vitro on cell lines derived from
hematological malignancies and some of the compounds
were tested on cell lines derived from solid tumors.
Based on structure-function relationship studies we
found two active subgroups of compounds among the
arylethylene derivatives namely, substituted
9-arylideneanthrones (BAs) and members of the
triarylvinylic systems and two active groups among the
naphthoquinones, namely, chloro- and
pyrolidino-amino-phenyl-naphthoquinones. The iron
chelators used are known compounds. The arylethylene
derivatives exhibited a high degree of selectivity for
cancer cells. Normal cells were relatively resistant to
the naphthoquinones studied and iron chelators were
active against different cell lines in a range of
concentrations that are safe for human use.
The arylethylene derivatives and iron chelators killed
the cells by apoptosis while the naphthoquinones induced
apoptotic or necrotic cell death depending on time and
concentration conditions. Apoptosis induced by members
of the different groups was accompanied by the
disruption of mitochondrial transmembrane potential and
was followed by cytochrome c release and caspases
Apoptosis induced by compounds from the three groups
involves oxidative stress, which plays a critical role
in mediating cell death. Nevertheless, the source of
reactive oxygen species (ROS) varies. The different
compounds were found to act through different signaling
pathways. Using members of the triarylvinylic systems,
the results suggest involvement of FLAP/5-LOX. However,
the main sources for ROS production using BAs were
NAD(P)H oxidase and mitochondria. The iron chelator
mimosine caused formation of hydrogen peroxide and a
decrease in reduced glutathione levels probably through
its iron-chelating activity via catalase inhibition,
while the results obtained with naphthoquinone
derivatives suggest NAD(P)H-oxidase involvement in
induction of oxidative stress and cell death.
The direct effect of the compounds on mitochondria was
studied. The active naphthoquinones and mimosine induce
mitochondrial swelling in isolated rat liver
mitochondria, via opening of permeability transition
pore. While the arylethylene derivatives tested from
both subgroups, alone, do not cause mitochondria
swelling. However, they protected mitochondria from
Ca2+-induced swelling. It is possible that these
compounds have a direct effect on the mitochondria
causing stabilization of their membrane.
Some specific signaling steps were observed for the
different compounds. PKC-? is specifically activated at
early stage by the arylethylene derivatives. Our finding
suggests a role for this enzyme in apoptosis mediated by
members of the triarylvinyl family but not to those of
the BA family. Treatment with the naphthoquinone
derivatives induced a rapid phosphorylation of p38
mitogen-activated protein kinase (p38MAPK).
Phosphorylation of extracellular signal-regulated
protein kinases (ERK1/2) was observed as well. The
results obtained imply that induction of p38 is involved
in apoptosis mediated by naphthoquinones, whereas ERK1/2
plays role in cell survival and possibly in necrosis.
ICL670 (ExjadeŽ, Deferasirox) a relatively new oral iron
chelator was tested against mantle cell lymphoma cells.
Our results indicate that the mechanism of ICL670 action
includes shortening of cyclin D1 half-life via
enhancement of protein degradation. The results point to
the role of proteasome in this action.
The various groups of compounds affected the ratio of
pro- and anti-apoptotic proteins of the Bcl-2 family.
Differences in behaviour were obtained even within the
same group of compounds. For example, an increase in the
expression of Bax at both transcriptional and
translational levels was seen with two active
naphthoquinone derivatives. Moreover, most intriguing
was the down regulation of Mcl-1 expression, which is
one of the main anti-apoptotic proteins in leukemia, by
one of the compounds while down regulation of Bcl-2, was
seen with other compounds.
The effect of the different compounds was examined ex
vivo on CLL and AML cells and on mononuclear cells
obtained from healthy donors. The results indicated that
the compounds killed the leukemic cells selectively and
exhibited different specificities towards CLL and AML
By virtue of the results obtained with members of the
triarylvinylic systems, the Hematology Institute in
Soroka University Medical Center began a phase I-II
clinical trial to evaluate the potential therapeutic
effect of clomiphene in advanced and refractory chronic
lymphoblastic leukemia and acute myeloid leukemia
patients. Our preliminary results revealed that the
treatment caused stabilization of leukocyte levels
during drug administration and slowed down disease
progression in a few treated AML patients.
In summary, our results present novel and known
compounds that act by the killing of cancer cells
through various mechanisms and thus may provide
effective anti-cancer strategies.