Synthesis of amphiphilic, chalcogen-based redox modulators with in vitro cytotoxic activity against cancer cells, macrophages and microbes

Article abstract of DOI:10.1039/c3md00204g

Several amphiphilic, chalcogen-based redox modulators have been synthesized which exhibit a widespread, yet in some instances also selective, biological activity which is most likely based on their ability to modulate the intracellular redox balance and t

Full text of DOI:10.1039/c3md00204g

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CONCISE ARTICLE
Synthesis of amphiphilic, chalcogen-based redox
modulators with in vitro cytotoxic activity against
cancer cells, macrophages and microbes†
Cite this: Med. Chem. Commun., 2014,
5, 25
Peng Du,ab Uma M. Viswanathan,ac Khairan Khairan,ah Tomislav Buric,a
Nathaniel E. B. Saidu,b Zhanjie Xu,d Benjamin Hanf,e Inga Bazukyan,c
Armen Trchounian,c Frank Hannemann,f Ingolf Bernhardt,e Torsten Burkholz,a
Britta Diesel,g Alexandra K. Kiemer,g Karl-Herbert Schafer,h Mathias Montenarh,b
¨
d
a
*
Gilbert Kirsch and Claus Jacob
Received 17th July 2013
Accepted 7th October 2013
Several amphiphilic, chalcogen-based redox modulators have been synthesized which exhibit
a
widespread, yet in some instances also selective, biological activity which is most likely based on their
ability to modulate the intracellular redox balance and to interact with cellular membranes and specific
proteins.
DOI: 10.1039/c3md00204g
www.rsc.org/medchemcomm
The last decade has witnessed a growing interest in the devel- aqueous media and oen unstable – and hence possess a
opment of redox modulating agents, which are able to effec- comparably poor bioavailability and drug prole, even in cell
tively, yet also selectively, attack cells with a disturbed redox culture. While attempting to circumvent some of the problems
balance, such as diverse cancer cells, macrophages and scle- associated with such unfavourable physico-chemical properties,
rodermic broblasts.1–5 A similar strategy has also been the idea of amphiphilic redox modulators has emerged, as
employed to kill various microorganisms, which are vulnerable detergent-like properties may, at least in theory, bestow such
because of a weak antioxidant defence.1,6–8 In many cases, molecules with certain additional, quite benecial aspects.
catalytic selenium and tellurium agents have been at the fore- Firstly, such amphiphilic compounds should be fairly soluble in
front of these developments, since these compounds exploit the aqueous media. Secondly, amphiphilic structures should cross
efficiency and selectivity of a chemical catalyst for its intracel- cell membranes and hence enter cells more easily when
lular substrate(s) to generate pronounced cytotoxic events in compared to more hydrophilic or lipophilic agents. Thirdly,
specic cells.4–6,9,10 Such compounds are, for instance, able to amphiphilic agents should be able to interact strongly with
induce apoptosis in leukemic B-cells while healthy B-cells of the membranes and (hydrophobic parts of) proteins. And nally,
same patient remain largely unaffected.2,11
such agents may cluster together at certain cellular sites (e.g. at
Unfortunately, many of the most interesting organoselenium membranes, in hydrophobic pockets) and hence may act
and tellurium agents available to date are only poorly soluble in synergistically.
Surprisingly, only few suitable amphiphilic redox-
aDivision of Bioorganic Chemistry, School of Pharmacy, Saarland University, D-66123
modulating agents have been reported in the literature so far,
Saarbruecken, Germany. E-mail: c.jacob@mx.uni-saarland.de; Fax: +49 681 302 and most of these substances have not been studied compre-
hensively in cell culture.12,13 Recently, some relevant chalcogen
compounds have emerged in this context in the literature,
conrming that the synthesis of such structures is realistic.13
3464; Tel: +49 681 302 3129
bDivision of Medical Biochemistry and Molecular Biology, Saarland University, D-
66424 Homburg, Germany
cDivision of Microbiology, Plants and Microbes Biotechnology, Department of Biology,
We have therefore decided to synthesize a series of amphi-
Yerevan State University, 0025 Yerevan, Armenia
philic selenium and tellurium compounds with an anionic head
d
´
´
Laboratoire d’Ingenierie Moleculaire et Biochimie Pharmacologique, SRSMC UMR
group and hydrocarbon ‘tails’ of different tail lengths in order to
´
7565, Universite de Lorraine, 1 Boulevard Arago, 57070 Metz, France
eDivision of Biophysics, Department of Biology, Saarland University, D-66123 explore the various aspects – and possible benets – of such
Saarbruecken, Germany
fDivision of Biochemistry, Saarland University, D-66123 Saarbruecken, Germany
gDepartment of Pharmacy, Saarland University, Pharmaceutical Biology,
agents in the context of biological activity, selectivity and
mode(s) of action. The compounds ultimately selected and
subsequently synthesized as part of this study are shown in
Saarbruecken, Germany
Fig. 1. The design of these compounds combines aspects of
previously studied selenium and tellurium compounds (e.g. aryl
substituents for enhanced stability) with typical features of an
amphiphilic agent.13 As for the synthesis of such compounds,
hDepartment of Biotechnology, University of Applied Sciences Kaiserslautern,
Zweibruecken, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3md00204g
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indeed possess amphiphilic properties, with Critical Micelle
Concentrations (CMCs) in water in the low millimolar range
(Fig. 2a). As expected, the CMC values of these compounds
generally decrease with increasing hydrocarbon chain length,
and compounds DP35 and DP45, respectively, show the lowest
CMC values of the selenium and tellurium compounds
studied. Under these conditions, the selenium-containing
compound DP35 has a CMC of 0.7 mM and the tellurium-
containing compound DP45 of just 0.3 mM, which is compa-
rable to the CMC of sodium dodecylsulfate (SDS) under these
conditions (Table 1). The latter is used here as a benchmark
anionic surfactant and has a CMC of 0.9 mM under the
experimental conditions used. Interestingly, there is no
major difference in CMC values between the selenium and
tellurium analogues.
At the same time, these compounds are redox active and
exhibit an electrochemical behaviour more or less typical of
mono-selenides and tellurides (Fig. 2b). Here, the oxidation
potentials Epa are typically in the range of +472 to +827 mV vs.
the standard silver/silver chloride electrode (SSE) for the sele-
nides and +306 to +386 mV vs. SSE for the tellurides. In contrast
to the CMC values, the Epa values therefore do differ signi-
cantly between the selenium and tellurium analogues, in line
with previous reports pointing towards generally lower Epa
values for tellurium compounds compared to their selenium
analogues. Interestingly, the Epa values within the respective
selenium and tellurium series do seem to increase with
increasing hydrocarbon ‘tail’ length; yet these increases are in
the range of a couple of tens of millivolts only and hence minor
in comparison.
Fig. 1 Chemical structures of compounds selected for this study. The
most successful synthetic route is provided (see text and ESI for
details†).
once the most promising synthetic avenue was selected (see
Fig. 1), the preparation of these molecules has been fairly
straightforward (see the ESI†). In many instances, good yields
(up to 63%) were possible even without further optimization.
Once synthesized and chemically characterized, the two
most interesting physico-chemical properties associated with
these compounds from a biological perspective (i.e. the
amphiphilic and redox properties) have been conrmed using
surface tension measurements and cyclic voltammetry. The
results, illustrated exemplarily for DP33 and DP43 in Fig. 2 and
summarized in Table 1, conrm that the compounds selected
While it is unlikely that such amphiphilic agents form
micelles under physiological conditions – the concentrations
generally employed in cell culture are too low for micellation
and the complexity of the biological ‘buffer’ also interferes with
micelle formation – they should still be able to interact rather
strongly with
– hydrophobic parts of – biomolecules
surrounding the cell or being present therein. Within this
context, and in analogy to the action of SDS, interactions with
cellular membranes and with proteins are of particular
interest.14 We have therefore briey explored the potential
interactions of some of our compounds with the membrane of
red blood cells (RBCs) and/or whole RBCs with a representative
protein, i.e. haemoglobin (Hb) (see also the ESI†).
RBCs were chosen to study compound–membrane interac-
tions as they represent intact cells rather than just liposomes.
RBCs therefore enable the study of direct effects of compounds
on the cell membrane, yet do not possess the kind of signalling
usually associated with dividing cells.14 We know, for instance,
that redox modulators can trigger secondary, indirect responses
in dividing cells, hence complicating investigations (see also
below).9,15,16 Fig. 3 shows the pronounced effect of the selenium
Fig. 2 Biologically relevant physico-chemical properties associated
with the amphiphilic compounds investigated: (a) representative and tellurium containing agents on the integrity of the RBCs.
surface tension measurements of ‘average tail length’ (n ¼ 10) sele-
nium compound DP33 and tellurium compound DP43 in order to
determine the CMC values of these compounds and to compare them
to the benchmark surfactant SDS. (b) Cyclic voltammograms of the
same compounds to demonstrate redox activity and to determine Epa
values.
This effect is concentration dependent (see Fig. S1 in the ESI†).
Rather modest concentrations of compounds such as DP35,
DP44 and DP45 cause signicant lysis of the cell membrane, as
determined by the haemoglobin release assay. Compound DP45
is the most ‘active’ in this assay, with 67.6% lysis at 100 mM of
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Table 1 Overview of the most relevant physico-chemical parameters and biological activities associated with the compounds developed and
evaluated as part of this study. See text and ESI for further details† (Epa values vs. SSE)
Compound
X
n
Yield (%)
CMC (mM)
Epa (mV)
Lysis RBCs (%) at 100 mM
IC50 (mM) HCT116
IC50 (mM) RAW 264.7
SDS
31
32
33
34
35
41
42
43
44
45
—
Se
Se
Se
Se
Se
Te
Te
Te
Te
Te
—
8
9
10
11
12
8
—
64
73
77
67
81
63
80
77
81
82
0.9
2.6
2.5
1.2
1.1
0.7
5.0
4.8
0.9
0.5
0.3
—
9.51 ꢀ 1.31
1.24 ꢀ 0.18
2.73 ꢀ 0.63
7.94 ꢀ 1.81
63.41 ꢀ 4.02
66.80 ꢀ 2.55
3.09 ꢀ 0.07
20.44 ꢀ 0.90
45.35 ꢀ 1.35
53.35 ꢀ 3.35
67.61 ꢀ 2.04
>100
>100
>100
>100
>100
>100
5.99 ꢀ 1.16
4.99 ꢀ 1.46
7.47 ꢀ 0.74
9.73 ꢀ 0.34
19.84 ꢀ 1.26
>100
>100
>100
>100
>100
>100
3.95 ꢀ 1.16
4.67 ꢀ 1.01
5.32 ꢀ 1.20
7.84 ꢀ 1.22
4.96 ꢀ 1.41
472.3
783.8
819.4
822.2
826.5
306.1
363.1
374.5
385.9
385.9
9
10
11
12
In the next step, we have turned our attention to possible
(non-covalent) interactions with proteins. Here, Hb has been
used as the initial choice as this protein occurs commonly in
RBCs, exhibits distinct signals in CD and, as a metalloprotein,
also accounts for possible chalcogen–metal interactions (the
iron–sulfur cluster containing protein Adx has been used as a
further control). Indeed, the studies employing circular
dichroism (CD) in order to determine possible interactions of
the amphiphilic agents with Hb indicate rather pronounced
effects of these compounds on the secondary structure of such
proteins. At 50 mM concentrations, most of the compounds
studied showed some impact on the Hb structure, which was
comparable to that of SDS at the same concentration. The
underlying interaction(s) were not particularly specic and
seemed to be dominated by the amphiphilic character of the
molecules (see Fig. S2 in the ESI†). SDS and DP31 had
pronounced effects on the structure of Hb, while the impact of
DP41 was less apparent. Interactions of such compounds with
Fig.
3 Interactions of amphiphilic compounds with possible
biochemical target molecules. Membrane interactions were recorded
with subsequent lysis of RBCs (at a compound concentration of 100
mM). The chain lengths refer to n, the number of carbon atoms in the
hydrophobic tail, as defined in Fig. 1. Lysis of RBCs increases with
increasing tail length from n ¼ 8 (DP31 and DP41) to n ¼ 12 (DP35 and
DP45). Data are presented as mean ꢀ SD, and one-way ANOVA test
(Dunnett) is used: p < 0.05 (*), p < 0.01 (**) or p < 0.001 (***).
the compound tested. This compound has also the lowest CMC the protein structure could also be observed in the case of the
value (around 300 mM), indicating that the activity on the RBC reference protein Adx, where DP41 was slightly more potent
cell membrane is apparently dominated by the amphiphilic than DP31 (see the ESI†).
character of the compounds and is less dependent on the
Encouraged by these results, which clearly support the
nature of the chalcogen involved. Indeed, there is a dramatic notion of possible multiple interactions of such compounds
increase in lysis with increasing hydrocarbon chain length in with various types of biomolecules, we have turned our atten-
the selenium as well as tellurium series, from a couple of tion towards potential biological targets. In the rst part of the
percents in the case of short tails (DP31, DP41) to almost 70% in evaluation of biological activity, two different epithelial human
the case of the longest tails (DP35, DP45). There is only a slight cell lines have been chosen: human colon cancer cells (HCT116
increase in lysis when switching from selenium to tellurium cells, ATCC® CCL-247™) represent a well established cancer
(66.8% lysis in the case of DP35 and 67.6% in the case of DP45). cell model, while human retinal pigment epithelia cells (ARPE-
Therefore the impact of compounds such as DP34, DP35, DP44 19, ATCC® CRL-2302™) provide a suitable and oen acceptable
and DP45 on the cell membrane is driven primarily by the control as they exhibit many features of non-cancerous, primary
amphiphilic character of these agents, and is possibly slightly cells, yet do not result in the various practical and ethical issues
enhanced by the presence of tellurium. Although speculative at associated with using primary human colon cells obtained, for
this time, one may envisage that the primary interaction of instance, from biopsies. ARPE-19 cells are particularly suited as
these compounds is indeed with the phospholipid bilayer control cells when the activity of redox modulating agents is
directly, while a smaller, secondary, chalcogen-driven effect is investigated, as such epithelial cells are fairly resilient towards
perhaps due to yet to be specied interactions with – probably oxidative stress.15 Hence screening compounds against HCT116
cysteine containing – membrane proteins. A similar behaviour and ARPE-19 cells in tandem oen allows a rst but certainly
apparently dominated by amphiphilicity rather than redox preliminary glance at cytotoxicity against human cells and also
activity has also been found in the case of nematodes, whose a comparison of a cancer cell line with ‘normal’ cells.2,15–17
cuticula or epidermis may be the target of such compounds (see
below).
Fig. 4 shows the results obtained for a set of amphiphilic
compounds against HCT116 and ARPE-19 cells (for IC50 values
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conrms that the compound has crossed the cell membrane
and indeed is ultimately present inside the cell.
Amazingly, the IC50 values of some of these tellurium agents
were consistently lower in HCT116 cells when compared to the
IC50 values of the same compounds in ARPE-19 cells (the latter
were usually around 50 mM), pointing towards a high and, in
this case, also selective activity in the HCT116 cell line. While it
is premature to speculate why such tellurium compounds may
be particularly active in HCT116 cells – and not to the same
extent in ARPE-19 cells, and why the selenium analogues are not
active in HCT116 cells, these ndings are in excellent agree-
ment with previous results obtained for organotellurium
compounds in such cell lines.10 They also conrm once more
the signicantly higher cytotoxicity associated with tellurium
compounds
when
compared
to
their
selenium
analogues.1,3,4,10,17 As for the possible underlying biochemical
causes of such high activity and selectivity (for tellurium as well
as for HCT116 cells), a specic interaction of tellurium
compounds with cellular targets, such as individual compo-
nents of the intracellular thiolstat, or burst of reactive oxygen
species (ROS) is most likely.1,2,4,5,10,15,16,18–22 Indeed, there are
numerous reports that selenium as an antioxidant seems to be
unable to induce a ROS generating chemistry, which may also
explain the rather low activity of the selenium compounds in
our assays.1,3,23,24
Fig. 4 Selective cytotoxicity of tellurium compound DP41. Panel (a):
cytotoxicity of DP41 in HCT116 cells compared to the less cytotoxic
selenium analogue DP31 and SDS. Panel (b): cytotoxicity of DP41 in
HCT116 cells compared to ARPE-19 cells. Data are presented as mean
ꢀ SD, t-test is used, p < 0.05 (*), p < 0.01 (**) or p < 0.001 (***). Panel
(c): cytotoxicity of DP41 in RAW 264.7 cells compared to the less
cytotoxic selenium analogue DP31 and SDS.
see Table 1). While the selenium-containing compounds and
SDS were more or less inactive against HCT116 cells at the
concentrations used, all of the tellurium containing
compounds showed considerable cytotoxic activity against
these cancer cells with IC50 values in the range of 5 to 20 mM.
Interestingly, a small but consistent and statistically signicant,
ultimately around 3–4-fold, increase in IC50 values was observed
when the hydrocarbon tail length was increased, from 6.0 mM in
the case of DP41 to 19.8 mM in the case of DP45. In line with the
notable absence of any observable cytotoxicity associated with
the selenium compounds (DP31 to DP35) up to a concentration
of 100 mM, this trend in IC50 values obviously counts against
(purely) amphiphilic (inter-)actions of such compounds.
Here, one must bear in mind, of course, that the IC50 values
(below or around 20 mM) are considerably lower than the
concentrations employed for lysis of RBCs (100 mM), which in
turn are lower than the CMCs (300 mM in the case of DP45). It is
therefore likely that most of the most dramatic amphiphilic
effects of such compounds only come to bear at concentrations
which are 10-fold or even higher than the ones required to kill a
human cell. In the case of the most cytotoxic compound DP42,
the IC50 value in HCT116 cells is around 1000 times lower than
the CMC of this compound.
In fact, while longer chain lengths may promote interactions
with cell membranes (as seen in the case of RBCs), they may
also prevent compounds from ultimately crossing such
membranes, i.e. from leaving the membrane again and entering
the cytosol. In the case of one of the most active compounds,
DP41, we have therefore used Energy-dispersive X-ray spec-
troscopy (EDX) to conrm the presence of tellurium inside
(washed) HCT116 cells (see Fig. S3 in the ESI†). While it is not
possible to conrm the exact location and chemical state of
tellurium inside these cells using this method, EDX nonetheless
We have therefore investigated this possible ‘redox link’
between a tellurium compound and apoptosis in HCT116 cells
in more detail.15 In the rst instance, uorescent staining of
intracellular superoxide anion radicals (O2cꢁ) was performed
using dihydroethidium (DHE) as a fairly specic probe for this
particular radical.15 As expected, initial results conrm a sharp
increase in intracellular O2cꢁ levels in response to compounds
such as DP41. At a concentration of 50 mM and within 40 min of
application, this tellurium compound causes a signicant
increase of O2cꢁ levels (in our preliminary experiments, the O2cꢁ
concentration measured by this method almost doubles).
Similar increases in intracellular ROS levels have already been
observed for quinone-containing organotellurium agents, and
ultimately may lead to cell death via apoptosis.5,10,15 Indeed, our
subsequent investigation of the effects of DP41 on the mito-
chondrial membrane potential DJM using JC-1 as a dual colour
red/green uorescent reporter dye revealed a signicant, time-
and concentration-dependent decrease of DJM in response to
this tellurium compound (see Fig. 5 for a graphic evaluation of
uorescent data). Such a loss of mitochondrial membrane
potential may form an important part of apoptotic processes
resulting in cell death. It should be emphasized, however, that
any – causal – relationship(s) between O2cꢁ levels on the one
side and DJM, on the other, are not immediately obvious and
require further investigation. Considering the timing of these
events, i.e. less than 1 h for the increase in O2cꢁ levels and
evidently more than 2 h for the decrease in DJM, it appears that
the increase in ROS occurs prior to mitochondrial damage. This
in turn points towards a redox event as the initial cause of
activity and counts against a decisive amphiphilic interaction of
compounds such as DP41 with the mitochondrial membrane.
Indeed, the selenium-analogue of DP41, i.e. compound DP31,
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the low concentrations of this compound required, once more
count against an amphiphilic interaction as the main cause of
(cytotoxic) activity in proliferating human cell lines.
The results obtained in isolated or cultured human cells may
only provide a partial picture of the various biological activities
and biochemical mode(s) of action associated with these
compounds. It is known, for instance, that many chalcogen-
based compounds also exhibit a pronounced activity against
various bacteria and some redox sensitive parasites. Therefore
initial screens for antibiotic activity against Staphylococcus
aureus and Escherichia coli were performed, which point
towards some activity of the tellurium compounds when used at
higher micromolar concentrations, which exceed the activity of
the selenium analogues and SDS. DP41, which was among the
most cytotoxic compounds in the case of cultured human cells,
was also the most active compound against E. coli with an IC50
of around 550 mM. Nonetheless, antibacterial activity was weak
when compared to the cytotoxicity against human cells.
Fig. 5 Time- and dose-dependent impact of DP31 and DP41 on the
mitochondrial membrane potential (DMSO as the negative control).
The fluorescent intensities of the green and red channels from 150
cells were integrated and final ratios of red to green calculated. Panel
(a): DP41 (at 50 mM), but not DP31, causes a significant decrease in
DJM after 6 h of incubation. Panel (b): the impact of DP41 on DJM is
time- and dose-dependent. It manifests itself after around 6 to 8 h and
at concentrations from around 5 mM upwards. Data are presented as
mean ꢀ SD, one-way ANOVA test (Dunnett), p < 0.05 (*), p < 0.01 (**)
or p < 0.001 (***).
When DP31 and DP41 were tested in the context of the
agricultural nematode Steinernema feltiae, a rather different
picture emerged. S. feltiae is a parasitic nematode oen used
in initial toxicity screens, as this organism is easy to cultivate
and also fairly reliable in producing representative and
did not cause any signicant increases in O2cꢁ levels and also reproducible results.25 The nematicidal activities associated
did not cause a major decrease in DJM, hence supporting the with DP31, DP41 and SDS are shown in Fig. S4 in the ESI.†
notion of a tellurium-specic activity which is linked to the SDS is only weakly active against S. feltiae (LD50 > 400 mM),
generation of intracellular ROS. DP41 differs from previously counting against a particular sensitivity of this organism
used quinone-containing organotellurium compounds, and the towards surfactants. Perhaps surprisingly, therefore, DP41
build-up of O2cꢁ occurs in the absence of a radical-generating and especially DP31 are quite active against this organism.
quinone moiety. It may not be caused directly by the redox Interestingly, the selenium compound DP31 (LD50 ¼ 30 mM) is
chemistry of DP41, but by a more indirect, secondary event, even more active when compared to its tellurium analogue,
possibly mediated by cellular processes, such as ER stress.
DP41 (LD50 ¼ 78 mM). The exact causes for this pronounced
While the initial results obtained in human RBCs, HCT116 activity of DP31 – compared to DP41 – are still unclear. One
and ARPE-19 cells are rather instructive, they may be due to may speculate that the selenium compound is taken up and
specic interactions which occur – perhaps solely – in those cell metabolized to toxic metabolites more readily, as most
types. We have therefore also considered the impact of such organisms possess pathways for the metabolism of selenium
selenium and tellurium compounds on cultured RAW 264.7 compounds, but not for tellurium compounds. Indeed, the
macrophages (ATCC® TIB-71™). Macrophages were selected as results obtained for our compounds in the four human cell
an additional cell system because these immune cells naturally models, the bacteria and nematodes conrm that such agents
produce high levels of ROS and hence should provide a suitable are not merely globally toxic but possess some selectivity
target for compound-induced redox modulation.1 The results which may result from their ability to enter cells and to act on
obtained in RAW 264.7 cells are summarized in Table 1 and particular cellular targets, be it on specic proteins or on a
shown for DP31 and DP41 in Fig. 4c. These results conrm the local or global redox state.
considerable cytotoxicity of DP41 (IC50 ¼ 3.95 mM). Indeed, it
Ultimately, our studies have shown that it is possible to
seems that the macrophages are even slightly more sensitive to synthesize a range of selenium- and tellurium-containing
DP41 when compared to the HCT116 cells (IC50 ¼ 5.99 mM), surfactants with comparable ease. As expected, these mole-
which may be due to the high ROS levels present naturally in cules combine redox activity with amphiphilic properties and
and near such immune cells. A similar activity of tellurium- hence exhibit several advantages when compared to traditional
based redox modulators on macrophages has been observed redox modulators or surfactants. The compounds reported here
previously.1 While the precise cause of such activity is specula- are easy to handle, fairly soluble in aqueous solutions and
tive at this time, it should be noted that the selenium analogue endowed with considerable biological activity against a
of DP41, i.e. DP31, as well as SDS, are both considerably less range of important therapeutic targets, such as certain cancer
active against RAW 264.7 cells with IC50 values of over 100 mM. cells, macrophages, bacteria and a representative nematode.
These ndings support further the notion of a tellurium-based, In fact, the activity determined so far compares well with
redox modulating activity of compounds such as DP41, which is that of so-called ‘multifunctional’ redox modulators which
notably absent in the case of the corresponding selenium combine a selenium or tellurium redox moiety with a
compounds or SDS. The specic activity of DP41, together with quinone.1,26 While the latter require the presence of a radical
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generating quinone for adequate activity in the high nano-
molar to low micromolar range, the compounds discussed
here exhibit a similar, slightly lower activity yet do not require
the presence of a cytotoxic quinone moiety. At the same time,
the tellurium compounds, which are oen more active than
their selenium counterparts, show activity not only against
HCT116 cancer cells, but also against RAW 264.7 macro-
phages, S. feltiae and E. coli.
Acknowledgements
The authors acknowledge nancial support from Saarland
University, the Landesforschungsfoerderungsprogramm Saar-
land (T/1-14.2.1.1.-LFFP 12/23) and the BMBF (grant number
01DK12002). The authors would like to thank Dr Josef Zapp
from Saarland University and Ms Veronique Poddig from the
University Lorraine for NMR measurements.
This raises the question, why such compounds are active,
and possibly even show some selectivity. Our initial experi-
ments conducted to explore the underlying biochemical
mode(s) of action point towards a combination of two activities.
On the one hand, there is clearly an amphiphilic, probably non-
covalent and disruptive interaction of these agents with
membranes and proteins; on the other hand, the presence of
tellurium (and less so selenium) seems to directly or indirectly
enable these compounds to affect the cellular redox balance (as
seen for O2cꢁ levels) and maybe also cause a covalent modi-
cation of key proteins of the cellular thiolstat. While the
amphiphilic events seem to occur only at higher concentrations
(50 to 100 mM and above) and do not discriminate signicantly
between the presence of selenium and tellurium, the redox
interactions seem to be more or less specic for tellurium and
occur at lower concentrations (10 to 50 mM in cultured human
cells).
Extensive future studies are obviously required to investigate
further the exact underlying biochemical mode(s) of action and
to identify possible intracellular targets (such as specic
organelles, membranes or proteins). At this point, the intra-
cellular pathways triggered or inuenced by such compounds
also need to be mapped out in more detail. Ultimately, it will
also be necessary to produce a wider range of such compounds,
including some sulfur-containing analogues, and to screen for
further activities and selectivity, also in order to derive reliable
structure–activity relationships. Our initial results point
towards a particularly promising spectrum of activities associ-
ated with the tellurium compounds, especially compound
DP41, which may be considered as a lead compound emerging
from these studies. As the structure of this compound provides
considerable scope for modications, and the synthesis of
derivatives is now straightforward, a wider spectrum of addi-
Notes and references
1 M. Doering, B. Diesel, M. C. H. Gruhlke, U. M. Viswanathan,
D. Manikova, M. Chovanec, T. Burkholz, A. J. Slusarenko,
A. K. Kiemer and C. Jacob, Tetrahedron, 2012, 68, 10577–
10585.
2 N. Lilienthal, C. Prinz, A. A. Peer-Zada, M. Doering, L. A. Ba,
M. Hallek, C. Jacob and M. Herling, Leuk. Lymphoma, 2011,
52, 1407–1411.
3 V. Jamier, L. A. Ba and C. Jacob, Chem.–Eur. J., 2010, 16,
10920–10928.
4 W. K. Marut, N. Kavian, A. Servettaz, C. Nicco, L. A. Ba,
M. Doering, C. Chereau, C. Jacob, B. Weill and F. Batteux,
J. Invest. Dermatol., 2012, 132, 1125–1132.
5 M. Doering, L. A. Ba, N. Lilienthal, C. Nicco, C. Scherer,
M. Abbas, A. A. P. Zada, R. Coriat, T. Burkholz,
L. Wessjohann, M. Diederich, F. Batteux, M. Herling and
C. Jacob, J. Med. Chem., 2010, 53, 6954–6963.
6 S. Mecklenburg, S. Shaaban, L. A. Ba, T. Burkholz,
T. Schneider, B. Diesel, A. K. Kiemer, A. Roseler, K. Becker,
J. Reichrath, A. Stark, W. Tilgen, M. Abbas,
L. A. Wessjohann, F. Sasse and C. Jacob, Org. Biomol.
Chem., 2009, 7, 4753–4762.
7 C. Jacob, Biochem. Soc. Trans., 2011, 39, 1247–1253.
8 T. Schneider, A. Baldauf, L. A. Ba, V. Jamier, K. Khairan,
M. B. Sarakbi, N. Reum, M. Schneider, A. Roseler,
K. Becker, T. Burkholz, P. G. Winyard, M. Kelkel,
M. Diederich and C. Jacob, J. Biomed. Nanotechnol., 2011, 7,
395–405.
9 S. Shaaban, R. Diestel, B. Hinkelmann, Y. Muthukumar,
R. P. Verma, F. Sasse and C. Jacob, Eur. J. Med. Chem.,
2012, 58, 192–205.
tional compounds based on this initial lead appears possible. 10 T. Schneider, Y. Muthukumar, B. Hinkelmann, R. Franke,
Here, it will be interesting to see if the presence of a quinone
helper group will further enhance activity and/or selectivity, as
M. Doring, C. Jacob and F. Sasse, MedChemComm, 2012, 3,
784–787.
has already been observed for a previous generation of such 11 N. Lilienthal, A. A. Peer-Zada, L. A. Ba, H. Liu, C. Jacob,
redox modulating compounds.1,2,4,5,9,26
M. Hallek and M. Herling, Onkologie, 2010, 33, 240–240.
In the future, such compounds will be evaluated extensively 12 L. B. Xing, S. Yu, X. J. Wang, G. X. Wang, B. Chen,
for possible anticancer and antimicrobial activity. Cells and
organisms which naturally produce high amounts of ROS (such
L. P. Zhang, C. H. Tung and L. Z. Wu, Chem. Commun.,
2012, 48, 10886–10888.
as certain cancer or immune cells), or exhibit a weak antioxi- 13 P. Han, N. Ma, H. Ren, H. Xu, Z. Li, Z. Wang and X. Zhang,
dant defence (such as certain parasites, including nematodes
Langmuir, 2010, 26, 14414–14418.
and Plasmodium falciparum), will obviously form the prime 14 T. Schneider, L. A. Ba, K. Khairan, C. Zwergel, N. D. Bach,
targets of such redox modulating agents.6 In any case, our
I. Bernhardt, W. Brandt, L. Wessjohann, M. Diederich and
ndings bode well for the further development of such
C. Jacob, MedChemComm, 2011, 2, 196–200.
amphiphilic redox modulators as lead structures for the treat- 15 N. E. B. Saidu, R. Touma, I. Abu Asali, C. Jacob and
ment of a range of human diseases and for possible agricultural
applications.
M. Montenarh, Biochim. Biophys. Acta, Gen. Subj., 2013,
1830, 2214–2225.
30 | Med. Chem. Commun., 2014, 5, 25–31
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MedChemComm
16 C. Busch, C. Jacob, A. Anwar, T. Burkholz, L. A. Ba, C. Cerella, 22 N. M. Giles, N. J. Gutowski, G. I. Giles and C. Jacob, FEBS
M. Diederich, W. Brandt, L. Wessjohann and M. Montenarh,
Int. J. Oncol., 2010, 36, 743–749.
17 L. A. Ba, M. Doring, V. Jamier and C. Jacob, Org. Biomol.
Chem., 2011, 8, 4203–4216.
Lett., 2003, 535, 179–182.
23 C. A. Collins, F. H. Fry, A. L. Holme, A. Yiakouvaki, A. Al-
Qenaei, C. Pourzand and C. Jacob, Org. Biomol. Chem.,
2005, 3, 1541–1546.
18 K. K. Bhasin, E. Arora, A. S. Grover, Jyoti, H. Singh, 24 G. I. Giles, F. H. Fry, K. M. Tasker, A. L. Holme, C. Peers,
S. K. Mehta, A. K. K. Bhasin and C. Jacob, J. Organomet.
Chem., 2013, 732, 137–141.
K. N. Green, L. O. Klotz, H. Sies and C. Jacob, Org. Biomol.
Chem., 2003, 1, 4317–4322.
19 C. Jacob, E. Battaglia, T. Burkholz, D. Peng, D. Bagrel 25 B. Czepukojc, U. M. Viswanathan, A. Raza, S. Ali, T. Burkholz
and M. Montenarh, Chem. Res. Toxicol., 2012, 25, 588–
604.
and C. Jacob, Phosphorus, Sulfur Silicon Relat. Elem., 2013,
188, 446–453.
20 C. Scherer, C. Jacob, M. Dicato and M. Diederich, Phytochem. 26 F. H. Fry, A. L. Holme, N. M. Giles, G. I. Giles, C. Collins,
Rev., 2009, 8, 349–368.
21 F. H. Fry and C. Jacob, Curr. Pharm. Des., 2006, 12, 4479–
K. Holt, S. Pariagh, T. Gelbrich, M. B. Hursthouse,
N. J. Gutowski and C. Jacob, Org. Biomol. Chem., 2005, 3,
2579–2587.
4499.
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