Synthesis of novel 6-(4-substituted piperazine-1-yl)-9-(β-d- ribofuranosyl)purine derivatives, which lead to senescence-induced cell death in liver cancer cells

Article abstract of DOI:10.1021/jm3001532

Novel purine ribonucleoside analogues (9-13) containing a 4-substituted piperazine in the substituent at N⁶ were synthesized and evaluated for their cytotoxicity on Huh7, HepG2, FOCUS, Mahlavu liver, MCF7 breast, and HCT116 colon carcinoma cell lines. The purine nucleoside analogues were analyzed initially by an anticancer drug-screening method based on a sulforhodamine B assay. Two nucleoside derivatives with promising cytotoxic activities (11 and 12) were further analyzed on the hepatoma cells. The N⁶-(4- Trifluoromethylphenyl)piperazine analogue 11 displayed the best antitumor activity, with IC₅₀ values between 5.2 and 9.2 μM. Similar to previously described nucleoside analogues, compound 11 also interferes with cellular ATP reserves, possibly through influencing cellular kinase activities. Furthermore, the novel nucleoside analogue 11 was shown to induce senescence-associated cell death, as demonstrated by the SAβ-gal assay. The senescence-dependent cytotoxic effect of 11 was also confirmed through phosphorylation of the Rb protein by p15^INK4b overexpression in the presence of this compound.

Full text of DOI:10.1021/jm3001532

Article
pubs.acs.org/jmc
Synthesis of Novel 6-(4-Substituted piperazine-1-yl)-9-(β-D-
ribofuranosyl)purine Derivatives, Which Lead to Senescence-Induced
Cell Death in Liver Cancer Cells
Meral Tuncbilek,*,† Ebru Bilget Guven,‡ Tugce Onder,† and Rengul Cetin Atalay*,‡,§
†Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Ankara University, 06100 Ankara, Turkey
‡Department of Molecular Biology and Genetics, Bilkent University, 06800 Ankara, Turkey
§Bilkent University Genetics and Biotechnology Research Center (BilGen), Bilkent University, 06800, Ankara, Turkey
ABSTRACT: Novel purine ribonucleoside analogues (9−13)
containing a 4-substituted piperazine in the substituent at N6
were synthesized and evaluated for their cytotoxicity on Huh7,
HepG2, FOCUS, Mahlavu liver, MCF7 breast, and HCT116
colon carcinoma cell lines. The purine nucleoside analogues
were analyzed initially by an anticancer drug-screening method
based on a sulforhodamine B assay. Two nucleoside derivatives
with promising cytotoxic activities (11 and 12) were further
analyzed on the hepatoma cells. The N6 -(4-
Trifluoromethylphenyl)piperazine analogue 11 displayed the
best antitumor activity, with IC50 values between 5.2 and 9.2
μM. Similar to previously described nucleoside analogues,
compound 11 also interferes with cellular ATP reserves, possibly through influencing cellular kinase activities. Furthermore, the
novel nucleoside analogue 11 was shown to induce senescence-associated cell death, as demonstrated by the SAβ-gal assay. The
senescence-dependent cytotoxic effect of 11 was also confirmed through phosphorylation of the Rb protein by p15INK4b
overexpression in the presence of this compound.
impaired DNA synthesis.6 For this reason, nucleoside analogues
INTRODUCTION
Nucleobase analogues and nucleoside analogues are significant
■
often cause apoptosis-induced cell death.3 Recently, expression
levels of ribonucleotide reductase subunits have been proposed
drugs used in chemotherapy for the treatment of solid tumors
and hematological malignancies.1 These groups of compounds
as molecular markers for nucleoside analogue-induced cell
death in cancer therapy response; nevertheless, it was
are considered antimetabolites because nucleobases and
previously shown that reduced ribonucleotide reductase and
nucleosides are the metabolic precursors of nucleotides.
altered dNTP pools have been associated with cellular
Nucleotides and their derivatives are involved in a large
senescence in diploid fibroblasts.7−9 Hence, nucleoside
number of cellular processes, including cell growth and division,
analogues too, may induce senescence-associated cell death.
Apoptosis and necrosis are the most-studied chemother-
apeutic-induced cell death mechanisms. However, in the past
decade, senescence, autophagy, and mitotic catastrophe have
been shown to be induced by cytotoxic agents.10 Senescence-
associated growth arrest is a significant cellular event in tumor
development and progression. Initially, replicative senescence
was reported to be due to telomere shortening during
replication.11 Later, Serrano et al. showed that premature
senescence was associated with cancer.12 Studies on the
molecular analysis of senescence in cancer revealed oncogene-
induced senescence (OIS) and tumor-suppressor-dependent
senescence (PICS).13,14 Therefore, senescence-induced cell
death through pro-senescence therapy is currently the target of
small-molecule inhibitors.14
and for this reason nucleobase and nucleoside analogues have
been exploited as anticancer agents.2 Initially, nucleobase
analogues such as fluorinated pyrimidines were investigated as
antimetabolite chemotherapeutic agents on cancer cells. Later,
pyrimidine analogues Ara-C and Gembicitabine were used in
cancer therapy.3 Success with pyrimidine nucleoside analogues
in cancer therapy led to the discovery of purine nucleoside
analogues. For more than six decades, 6-mercaptopurine and 6-
thioguanine have been used as inhibitors of nucleic acid
metabolism in pediatric acute lymphoblastic leukemia.4
Currently, the purine nucleoside analogues fludarabine,
cladribine, and pentostatin are used for treating hematological
malignancies.5 The synthetic nucleoside analogues fludarabine
and cladribine are synthesized into the dATP analogues, and
the natural substance pentostatin leads to an increase in the
dATP levels in the cell. Nucleoside analogues create an
imbalance in the cellular dNTP reserve by inhibiting the
ribonucleotide reductase enzyme, which in turn leads to
Received: October 31, 2011
Published: March 12, 2012
© 2012 American Chemical Society
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a
Recently, studies focusing on evading senescence in murine
premalignant hepatocytes have revealed a mechanism called
senescence surveillance during hepatocarcinogenesis.15 Fur-
thermore, replacement of the tumor suppressor p53 in murine
liver cancer models has led to senescence and therefore to
regression of these tumors. Primary liver cancer, hepatocellular
carcinoma (HCC), is the fourth most common cause of cancer
mortality and the third most common malignancy in human
cancers. Chronic liver injury is due to viral diseases, exposure to
chemicals, and other environmental or autoimmune conditions
that are the risk factors for HCC. These factors induce an
acquired tolerance to genotoxic stress, but ultimately a
cancerous state that does not respond to the cellular death
mechanisms.16 Recently, Sorafenib, a multikinase inhibitor, was
approved by the FDA and the EU for hepatocellular carcinoma
treatment.17 Sorafenib prolongs median survival and the time to
progression by nearly three months in patients with advanced
hepatocellular carcinoma. Therefore, there is a need for new
liver-cancer-specific drugs based on the molecular mechanisms
involved in liver carcinogenesis. In this study, we synthesized
novel purine ribonucleoside analogues (9−13) containing a 4-
substituted piperazine in the substituent at N6 as putative
cytotoxic agents. The newly obtained compounds were then
characterized for their anticancer senescence-inducing activity
in liver cancer cells.
Scheme 1
RESULTS AND DISCUSSION
■
Chemistry. The 6-(4-substituted piperazine-1-yl)-9-(β-D-
ribofuranosyl)purine derivatives (9−13) were synthesized as
shown in Scheme 1. 6-Chloropurine (1) was condensed with
the sugar 1,2,3,5-tetra-O-acetyl-β-D-ribofuranose (2) under
microwave irradiation (30 min) to obtain 6-chloro-9-(2,3,5-
tri-O-acetyl-β-D-ribofuranosyl)-9H-purine (3) as a yellowish
foam in good yield of 75.7%. This reaction gave significantly
higher yields than the previously published method.18 Displace-
ment of the 6-chloro was accomplished by nucleophilic
substitution with appropriate N-substituted piperazines. Re-
moval of the acetyl-protecting groups was performed with
NaOMe in MeOH to produce nucleosides 9−13.
a
Reagents: (i) silica gel 60, EtOAc, microwave irradiation; (ii) the
appropriate piperazine, TEA, EtOH; (iii) NaOMe, MeOH.
Biological Evaluation and Discussion. The newly
synthesized compounds 9−13 were first evaluated for their
antitumor activities against human liver (Huh7), colon
(HCT116), and breast (MCF7) carcinoma cell lines (Figure
1A). The IC50 values were in micromolar concentrations with
N6-(substituted phenyl)piperazine purine nucleoside derivatives
(Figure 1A and Table 1). We then tested the cytotoxic effect of
these molecules on additional hepatocellular carcinoma (HCC)
cell lines: HepG2, Mahlavu, and FOCUS (Figure 1B). We
observed strong cell growth inhibition in the presence of the
novel nucleosides 11 and 12. Time-dependent IC50 values for
each molecule were also calculated in comparison with the
nucleobase analogue 5-fluorouracil (5-FU) and DNA top-
oisomerase inhibitor camptothecin (CPT) (Table 1). N6-(4-
Trifluoromethylphenyl)piperazine derivative 11 displayed the
best cytotoxic activity, with IC50 values of 5.2−9.2 μM (Table
1). The (3,4-dichlorophenyl)piperazine derivative 12 was also
very active (IC50 values in the range of 5.5−9.7 μM) against all
tested cell lines. When there was a larger substituent at the 4-
position of piperazine moiety (diphenylmethyl group, 13),
cytotoxic activity was decreased. On the other hand, compound
9, which has no substitution at the phenyl ring, did not show
any significant cytotoxic activity; the compound 10, with 4-
fluorophenyl, had some cytotoxicity (Table 1). Nucleosides 11
and 12 demonstrated significant cytotoxicity for all the cell lines
tested. When we compared their IC50 values with the known
cell growth inhibitors CPT and 5-FU, we observed that our
compounds 11 and 12 had showed lower values in micromolar
concentrations. Compounds 11 and 12 had a better cytotoxic
activity on Huh7 cells (7.8 and 7.1 vs 30.7 μM for 5-FU).
Considering the cytotoxic activity of our novel nucleosides
9−13 on hepatoma cell lines, we further analyzed the cellular
activity of the most potent inhibitor (11) on these cell lines as a
promising candidate anticancer agent.
Real-Time Cellular Response of Hepatocellular Carci-
noma Cells with Compound 11 Treatment. Real-time cell
electronic sensing (RT-CES) was used to evaluate compound
11's mediated cytotoxicity on Huh7, HepG2, Mahlavu, and
FOCUS hepatoma cells in triplicate (Figure 2). Real-time
dynamic monitoring of the electrode impedance indicates a cell
index (CI) that correlates with cell growth. Compound 11
triggered a time- and dose-dependent decrease in CI cell
growth indexes in all hepatoma cells (Figure 2). A cell growth
index with 30−5 μM of compound 11 treatment clearly
demonstrates the potent inhibitor action of compound 11,
which correlates with our initial observation with the NCI-SRB
assay. The PTEN-deficient cell line Mahlavu was the least
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Figure 1. Percent cell death in the presence of compounds 9−13. Huh7, HCT116, MCF7 (A) and HepG2, Mahlavu and FOCUS (B) cells were
inoculated in 96-well plates. All molecules and their DMSO controls were administered to the cells in triplicate with five different concentrations: 40,
20, 10, 5, and 2.5 μM. After 72 h of incubation, SRB assays were generated and the cell death percentages were calculated in comparison with
DMSO-treated wells.
Table 1. IC50 Values in μM Concentrations for 9−13 with 72
for 72 h. A luminescent ATP-detection assay was then achieved.
The luminescence (measured as relative light units (rlu))
indicated a dose-dependent ATP amount in the presence of 11
similar with MeSAdo (Figure 3) with the same cell count. High
rlu values obtained with the STS-treated Huh7 cells are
consistent with both the principle of the assay and the
molecular mechanism of STS as a multikinase inhibitor.
The Cytotoxic Activity of 11 Is Neither Apoptosis nor
Necrosis. We then characterized the cytotoxic pathways
involved in the molecular action of 11. The apoptotic pathway
activation indicator Poly-ADP-ribosyl-polymerase (PARP)'s
protein cleavage was assessed on Huh7, HepG2, Mahlavu,
and FOCUS cells in the presence of 11. For each cell line, 11
was used as its cell-line-specific IC50 value for 72 h (Table 1).
The endogenous PARP protein has an atomic mass of 113 kDa.
During apoptosis, PARP is cleaved into 89 kDa and 24 kDa
fragments, and when the cytotoxic effect is due to necrosis, the
cleaved PARP is detected as a 50 kDa fragment band in
Western blot analysis.20 Seventy-two hours of treatment with
11 did not induce cleavage of the PARP protein in all treated
liver cancer cell lines (data not shown). This cleavage analysis
demonstrated that the cytotoxic activity of 11 was neither
apoptosis nor necrosis.
a
h of Treatment
9
10
11
12
13
5-FU
CPT
Huh7
>100
>100
>100
>100
>100
>100
49.7
7.8
5.7
9.2
5.2
6.7
7.5
7.1
6.1
7.0
5.5
8.4
9.7
44.4
63.5
92.7
30.7
5.0
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
HepG2
Mahlavu
FOCUS
HCT116
MCF7
>100
>100
>100
>100
>100
10.0
7.6
>100
48.5
40.1
6.0
3.5
a
IC50 values were calculated from the cell growth inhibition
percentages obtained with five different concentrations.
affected by 11 (Mahlavu cells have a hyperactive PI3K/Akt
pathway due to PTEN deficiency).19 Higher nucleoside−11
concentrations were needed for the cytotoxicity on Mahlavu
cells. This observation indicated that compound−11 might be a
putative kinase−protein interfering molecule. For that reason,
we tested a nontargeted broad-spectrum kinase assay with the
aim of detecting cellular ATP levels affected by the presence of
11.
Nucleoside Analogue 11 Possesses Kinase-Inhibitor
Potential. With the aim of elucidating the possible kinase-
interfering activity of 11, we used a luminescent ATP-detection
assay. Luminescence correlates with the amount of ATP in the
milieu, therefore an increase in the luminescence might indicate
the presence of a protein kinase inhibitor. Because of their
established kinase-inhibition potentials, we used staurosporine
(STS), a multikinase inhibitor, and the nucleoside analogue 5′-
deoxy-5′-methylthioadenosine (MeSAdo) as positive controls
for the experiment. Huh7 cells were incubated with IC50 and
IC100 values of 11 (Table 1), 5 μM MeSAdo, and 0.5 μM STS
Compound 11 Induces Cellular Senescence. Replicative
senescence has long been characterized as proliferative arrest
that occurs in normal cells after a limited number of population
doublings. Recently, premature senescence has been associated
with cancer cells. INK4a and INK4b proteins inhibit CyclinD1/
CCDK4, leading to pRB activation and therefore induction of
senescence. For this reason, higher expression of these proteins
is among the premature senescence markers (in addition to
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senescence-associated β-galactosidase (SAβ-gal) activity at pH
6.0 due to increased lysosomal activity).
With the aim of identifying the possible senescence-involved
cytotoxic activity of 11, we performed a SAβ-gal assay and
BrdU incorporation assays in parallel. Huh7 cells were plated in
six-well plates on coverslips at low density for the logarithmic
phase growth. The next day, Huh7 cells were treated with 11 at
its IC50 and IC100 values both for three or six days. Doxorubicin
(25 ng/mL) was used as a positive control for senescence-
inducing agent, and DMSO was used as the negative control.
Twenty-four hours prior to the end of the incubation with the
compound, BrdU was administered to test its incorporation
into the cellular DNA. The large blue-stained senescent (SAβ-
gal-assay-positive) cells were negative for BrdU incorporation
for compound 11 and doxorubicin (Figure 4A,B) when
Figure 2. Real-time cell growth of Huh7, HepG2, Mahlavu, and
FOCUS cells in the presence of compound 11. Cells were inoculated
in triplicate in E-plates. The cell growth index was monitored every 30
min in the presence of the different concentrations of compound 11
(30 μM,red; 20 μM, blue; 15 μM, green; 10 μM, magenta; 7.5 μM,
cyan; 5 μM, coral; DMSO control, black).
Figure 4. SAβ-gal and BrdU assays with compound 11. Huh7 cells
were plated on coverslips in six-well plates (5000 cells/well). (A)
Huh7 cells were incubated with IC50 and IC100 values of 11,
doxorubicin, and DMSO only controls for three and six days.
Doxorubicin was used as a positive control at its senescence-inducing
dose (25 ng/mL). BrdU (30 μM) was administered to the cells 24 h
prior to the end of three and six days of incubation. (B) Cells were
counted and the percent distribution between SAβ-gal and BrdU
positive was presented.
compared to the DMSO control. However, BrdU-positive
proliferating cells were marked visible in DMSO-treated wells
only.
Compound 11-Induced Senescence Is Associated
with the Induction of p15INK4b and a Decrease in Rb
Phosphorylation. In addition to testing for the most widely
used and accepted marker of senescent cells (an increase in
SAβ-gal activity), we tested another senescence-associated
marker (p15INK4b levels) in 11-treated Huh7 cells. Huh7 cells
were treated in the presence of IC50 and IC100 concentrations of
11 both for three and six days, then Western blot analysis was
realized. Indeed, we observed an increase in the protein
Figure 3. Whole cell protein kinase activity in the presence of 11.
Huh7 cells were incubated with IC50 and IC100 values of 11 (Table 1),
staurosporine (STS, 0.5 μM), MeSAdo (0.5 μM), and their
corresponding DMSO controls for 72 h. Then, using the Kinase-Glo
assay kit, 80000 were cells tested for their ATP content by
chemoluminesce.
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Figure 5. Senescence-associated proteins p15INK4bp and pRb proteins in the presence 11. Huh7 cells were treated with IC50 and IC100 values of 11,
doxorubicin, and DMSO for three and six days. Doxorubicin was used as a positive control at its senescence-inducing dose (25 ng/mL). (A) p15INK4b
protein expression by Western blotting after three and six days of treatment in comparison with positive control doxorubicin and negative control
DMSO. Treatment of Huh7 cells with 11 induced the accumulation of p15INK4b in a dose- and time-dependent manner. (B) Rb activity is detected
with anti-Rb and antiphospho-Rb antibodies. (C) Comparative Rb phosphorylation levels confirmed senescence-induced cytotoxicity by compound
11. Calnexin protein was used as an equal loading control.
expression levels of p15INK4b in a dose- and time-dependent
manner (Figure 5A). Next, we determined the downstream
effect of p15INK4b on the phosphorylation of the Rb protein. It
is known that p15INK4b activates the Rb protein by inhibiting
CyclinD1/CCDK4CD and therefore inhibits the phosphor-
ylation of Rb. An observed decrease in the phosphorylated-
form of the Rb protein correlates with the 11-induced
accumulation of p15INK4b (Figure 5A,B). This observation
thus also confirmed the senescence-induced cytotoxic activity
of compound 11.
senescence has also been proposed for mechanism-based drug
discovery. For this reason, induction of irreversible cell cycle
arrest by senescence with novel candidate chemotherapeutic
agents has become an important strategy against cancer.
EXPERIMENTAL SECTION
■
Chemistry. Melting points were recorded with a capillary melting
point apparatus (Electrothermal 9100) and are uncorrected. NMR
spectra were recorded on a VARIAN Mercury 400 FT-NMR
1
spectrometer (400 for H, 100.6 MHz for 13C). TMS was used as
internal standard for the 1H and 13C NMR spectra; values are given in
δ (ppm) and J values are in Hz. High resolution mass spectra data
(HRMS) were collected in-house using a Waters LCT Premier XE
mass spectrometer (high sensitivity orthogonal acceleration time-of-
flight instrument) operating in ESI (+) method, also coupled with an
AQUITY Ultra Performance liquid chromatography system (Waters
Corporation, Milford, MA, USA). All compounds were of >95%
purity. Elemental analyses (C, H, N) were determined on a Leco
CHNS 932 instrument and gave values within 0.4% of the theoretical
values. Microwave reactions were carried out using a domestic
microwave oven (White Westinghouse SG-KM97VL, 50 Hz, 1400 W).
Column chromatography was accomplished on silica gel 60 (40−63
mm particle size). The chemical reagents used in synthesis were
purchased from E. Merck, Fluka, Sigma, and Aldrich.
CONCLUSION
■
We synthesized a novel group of nucleoside analogues (N6-
substituted piperazine derivatives) as putative anticancer agents.
We identified their cytotoxic activity and determined the
minimum required concentration for their action. Two
molecules, 11 and 12, were promising as candidate chemo-
therapeutic agents and had IC50 values less than 10 μM. We
selected the most active compound (11) to pursue further
experiments on with the aim of analyzing its molecular
cytotoxic action on hepatoma cells. Our results indicated that
the novel candidate chemotherapeutic agent 11 induces
senescence-associated cell death through the inhibition of
some kinase proteins (Figures 3 and 4). In addition our analysis
with p15INK4b protein levels in 11-treated cells indicates that the
target kinases could be upstream of this protein; this must be
further investigated in detail.
6-Chloro-9-(2,3,5-tri-O-acetyl-β-D-ribofuranosyl)-9H-purine
(3). 6-Chloropurine (1) (154 mg, 1 mmol) and 1,2,3,5-tetra-O-acetyl-
β-D-ribofuranose (2) were dissolved in EtOAc, and then 500 mg of
silica gel 60 (200−400 mesh) was added. The mixture was
concentrated in vacuo, and the dry residue was irradiated for 30 min
in a White Westinghouse SG-KM97VL domestic microwave oven (50
Hz, 1400 W). The residue was purified by flash chromatography on
silica gel (EtOAc−hexane, 3:1) to yield 3 as yellowish viscous oil
Recent studies on the involvement of senescence-associated
cell death in cancer have established the concept stress or
aberrant signaling-induced senescence (STASIS), which is
telomere independent.21 Reprogramming senescence in cancer
cells was extensively discussed as one of the hallmarks of
cancer.22 Targeting replicative immortality and inducing
1
(312.3 mg, 75.72%). H NMR (CDCl3) δ 2.07 (s, 3H, OAc), 2.10 (s,
3H, OAc), 2.14 (s, 3H, OAc), 4.34−4.48 (m, 3H, H-4′, H-5′), 5.63 (t, J
= 5.2 Hz, 1H, H-3′), 5.93 (t, J = 5.6 Hz, 1H, H-2′), 6.21 (d, J = 5.2 Hz,
1H, H-1′), 8.28 (s, 1H, H-8), 8.76 (s, 1H, H-2). 13C NMR (CDCl3) δ
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20.59, 20.74, 20.97 (3 × CH3), 63.09 (CH2-5′), 70.67 (CH-4′), 73.32
(CH-3′), 80.74 (CH-2′), 87.08 (CH-1′), 132.57 (C-5), 143.79 (C-8),
151.44 (C-6), 151.87 (C-4), 152.54 (C-2), 169.56, 169.78, 170.48 (3
× CO). HRMS (ESI+) m/z calcd for C16H18ClN4O7 (M + H)+
413.0864, found 413.0859. Anal. Calcd for C16H17ClN4O7·0.6EtOAc:
C, 47.46; H, 4.71; N, 12.03. Found C, 47.84; H, 4.59; N, 11.89.
General Procedure for the Synthesis of Compounds 4−8. 6-
Chloro-9-(2,3,5-tri-O-acetyl-β-D-ribofuranosyl)-9H-purine (3) was dis-
solved in 10 mL of absolute EtOH, and then 1-substituted piperazines
and (Et)3N (3 equiv) were added. The mixture was refluxed for 3−8 h.
The reaction mixture was concentrated in vacuo, and the residue was
purified by column chromatography.
6-(4-Phenylpiperazine-1-yl)-9-(2,3,5-tri-O-acetyl-β-D-ribofurano-
syl)-9H-purine (4). The compound was prepared from (3) (174.4 mg,
0.4 mmol) and 1-phenylpiperazine (0.13 mL, 0.8 mmol) at reflux for 4
h according to the general procedure and was purified by column
chromatography (EtOAc−hexane, 1:1) to yield 4 (209 mg; 92%); mp
62−64 °C. 1H NMR (CDCl3) δ 2.08 (s, 3H, OAc), 2.14 (s, 6H, OAc),
3.32 (t, 4H, piperazine CH2), 4.34−4.50 (m, 7H, H-4′, H-5′, piperazine
CH2), 5.66 (t, J = 5.6 Hz, 1H, H-3′), 5.91 (t, J = 5.2 Hz, 1H, H-2′),
6.22 (d, J = 5.6 Hz, 1H, H-1′), 6.91 (t, 1H, J = 7.2 Hz, H-4 in phenyl),
6.98 (d, 2H, J = 7.6 Hz, H-2,6 in phenyl), 7.30 (t, 2H, J = 7.6 Hz, H-
3,5 in phenyl), 7.92 (s, 1H, H-8), 8.37 (s, 1H, H-2). 13C NMR
(CDCl3) δ 20.65, 20.78, 21.03 (3 × CH3), 45.24, 49.85 (CH2 in
piperazine), 63.41 (CH2-5′), 70.95 (CH-3′), 73.32 (CH-2′), 80.40
(CH-4′), 86.12 (CH-1′), 116.78, 120.63, 129.45 (C in phenyl), 136.71
(C-5), 150.99 (C-8), 151.37 (C-6), 152.98 (C-2), 154.01 (C-4),
169.63, 169.85, 170.57 (3 × CO). HRMS (ESI+) m/z calcd for
C26H31N6O7 (M + H)+ 539.2254, found 539.2253. Anal. Calcd for
C26H30N6O7·0.5H2O: C, 57.03; H, 5.70; N, 15.34. Found C, 57.35; H,
5.58; N, 14.97.
(3) (241.5 mg, 0.58 mmol) and 1-(3,4-dichloropheny)piperazine
(135.1 mg, 0.58 mmol) at reflux for 8 h according to the general
procedure and was purified by column chromatography (EtOAc−
1
hexane, 1:1) to yield 7 (166.2 mg; 46.69%); mp 67−69 °C. H NMR
(CDCl3) δ 2.08 (s, 3H, OAc), 2.15 (s, 6H, OAc), 3.29 (t, 4H,
piperazine CH2), 4.34−4.48 (m, 7H, H-4′, H-5′, piperazine CH2), 5.67
(t, J = 4.8 Hz, 1H, H-3′), 5.92 (t, J = 5.2 Hz, 1H, H-2′), 6.22 (d, J = 5.6
Hz, 1H, H-1′), 6.79 (dd, Jo = 8.8 Hz, Jm = 2.8 Hz, 1H, H-6 in phenyl),
7.0 (d, Jm = 2.8 Hz, 1H, H-2 in phenyl), 7.29 (d, Jo = 9.2 Hz, 1H, H-5
in phenyl), 7.93 (s, 1H, H-8), 8.37 (s, 1H, H-2). 13C NMR (CDCl3) δ
20.64, 20.77, 21.02 (3 × CH3), 44.94, 49.21 (CH2 in piperazine),
63.37 (CH2-5′), 70.91 (CH-3′), 73.33 (CH-2′), 80.39 (CH-4′), 86.23
(CH-1′), 115.92 (CH in phenyl), 117.86 (CH in phenyl), 120.66 (CH
in phenyl), 122.98 (C in phenyl), 130.76 (C in phenyl), 133.09 (C in
phenyl), 136.91 (C-5), 150.76 (C-8), 150.99 (C-6), 152.92 (C-2),
153.91 (C-4), 169.62, 169.83, 170.55 (3 × CO). HRMS (ESI+) m/z
calcd for C26H29Cl2N6O7 (M + H)+ 607.1475, found 607.1475. Anal.
Calcd for C26H28Cl2N6O7·0.2EtOAc: C, 51.50; H, 4.77; N, 13.44.
Found C, 51.43; H, 4.65; N, 13.08.
6-(4-(Diphenylmethyl)piperazine-1-yl)-9-(2,3,5-tri-O-acetyl-β-D-ri-
bofuranosyl)-9H-purine (8). The compound was prepared from (3)
(312 mg, 0.7 mmol) and 1-(diphenylmethyl)piperazine (190.9 mg, 0.7
mmol) at reflux for 5 h according to the general procedure and was
purified by column chromatography (EtOAc−hexane, 1:1) to yield 8
1
(166.8 mg; 35.1%); mp 81−83 °C. H NMR (CDCl3) δ 2.06 (s, 3H,
OAc), 2.12 (s, 3H, OAc), 2.13 (s, 3H, OAc), 2.52 (t, 4H, piperazine
CH2), 4.19−4.44 (m, 8H, CH, H-4′, H-5′, piperazine CH2), 5.63 (t, J =
4.8 Hz, 1H, H-3′), 5.87 (t, J = 5.2 Hz, 1H, H-2′), 6.20 (d, J = 5.2 Hz,
1H, H-1′), 7.19 (t, 2H, Jo = 7.6 Hz, H-4 in phenyl), 7.29 (t, 4H, Jo =
7.6 Hz, H-3,5 in phenyl), 7.44 (d, 4H, Jo = 7.2 Hz, H-2,6 in phenyl),
7.84 (s, 1H, H-8), 8.30 (s, 1H, H-2). 13C NMR (CDCl3) δ 20.65,
20.78, 21.02 (3 × CH3), 45.46 (CH2 in piperazine), 52.26 (CH2 in
piperazine), 63.39 (CH), 70.93 (CH2-5′), 73.30 (CH-3′), 76.35 (CH-
2′), 80.34 (CH-4′), 85.96 (CH-1′), 120.52 (CH in phenyl), 127.31
(CH in phenyl), 128.19 (CH in phenyl), 128.78 (C in phenyl), 136.32
(C-5), 142.47 (C-8), 150.88 (C-6), 152.99 (C-2), 154.03 (C-4),
169.61, 169.85, 170.57 (3 × CO). HRMS (ESI+) m/z calcd for
C33H37N6O7 (M + H)+ 629.2724, found 629.2719. Anal. Calcd for
C33H36N6O7·0.7H2O: C, 61.80; H, 5.87; N, 13.10. Found C, 61.57; H,
5.89; N, 13.07.
6-[4-(4-Fluorophenyl)piperazine-1-yl]-9-(2,3,5-tri-O-acetyl-β-D-ri-
bofuranosyl)-9H-purine (5). The compound was prepared from (3)
(140.1 mg, 0.3 mmol) and 1-(4-fluoropheny)piperazine (61.1 mg, 0.3
mmol) at reflux for 7 h according to the general procedure and was
purified by column chromatography (EtOAc−hexane, 1:1) to yield 5
1
(151 mg; 80.3%); mp 57−59 °C. H NMR (CDCl3) δ 2.08 (s, 3H,
OAc), 2.14 (s, 6H, OAc), 3.22 (t, 4H, piperazine CH2), 4.35−4.47 (m,
6H, H-4′, H-5′, piperazine CH2), 5.66 (t, J = 4.8 Hz, 1H, H-3′), 5.91 (t,
J = 5.2 Hz, 1H, H-2′), 6.22 (d, J = 5.6 Hz, 1H, H-1′), 6.91−7.02 (m,
4H, H-2,3,5,6 in phenyl), 7.92 (s, 1H, H-8), 8.37 (s, 1H, H-2). 13C
NMR (CDCl3) δ 20.64, 20.77, 21.02 (3 × CH3), 45.29, 50.88 (CH2 in
piperazine), 63.39 (CH2-5′), 70.92 (CH-3′), 73.32 (CH-2′), 80.39
(CH-4′), 86.16 (CH-1′), 115.77, 115.99, 118.67 (2), 120.63 (C in
phenyl), 136.75 (C-5), 148.02 (C-8), 150.99 (C-6), 152.95 (C-2),
153.98 (C-4), 169.62, 169.84, 170.56 (3 × CO). HRMS (ESI+) m/z
calcd for C26H30FN6O7 (M + H)+ 557.2160, found 557.2163. Anal.
Calcd for C26H29FN6O7·1.3EtOAc: C, 55.84; H, 5.91; N, 12.52. Found
C, 56.10; H, 5.87; N, 12.14.
General Procedure for the Deacetylation of the Protected
Nucleosides 9−13. The protected nucleosides (4−8) were dissolved
in 10 mL of absolute MeOH, and then NaOMe (30% in MeOH) (2
equiv) was added and stirred at room temparature for 1−12 h. The
reaction mixture was concentrated in vacuo. The residue was dissolved
with CH2Cl2:MeOH and purified by column chromatography.
6-(4-Phenylpiperazine-1-yl)-9-(β-D-ribofuranosyl)-9H-purine (9).
The compound was prepared from (4) (209.1 mg, 0.388 mmol) at
room temperature for 1 h according to general procedure and was
purified by column chromatography (EtOAc−hexane, 3:1) to yield 9
(27 mg; 16.3%); mp 99−100 °C. 1H NMR (DMSO-d6) δ 3.26 (t, 4H,
piperazine CH2), 3.53−3.70 and 3.65−3.73 (2m, 2H, CH2-5′), 3.96−
4.16 (m, 2H, H-2′,3′), 4.38 (br s, 4H, piperazine CH2), 4.58 (q, 1H, H-
4′), 5.21 (d, 1H, 3′-OH), 5.33 (t, 1H, 5′-OH), 5.48 (d, 1H, 2′-OH),
5.93 (d, 1H, 1′-H), 6.81 (t, 1H, J = 7.2 Hz, H-4 in phenyl), 7.0 (d, 2H,
J = 8.4 Hz, H-2,6 in phenyl), 7.24 (t, 2H, J = 8.4 Hz, H-3,5 in phenyl),
8.28 (s, 1H, H-8), 8.45 (s, 1H, H-2). 13C NMR (DMSO-d6) δ 49.15
(CH2 in piperazine), 62.14 (CH2-5′), 71.13 (CH-3′), 74.20 (CH-2′),
86.42 (CH-4′), 88.44 (CH-1′), 116.51, 119.97, 120.32, 129.65 (C in
phenyl), 139.66 (C-5), 150.99 (C-8), 151.61 (C-6), 152.47 (C-2),
153.81 (C-4). HRMS (ESI+) m/z calcd for C20H25N6O4 (M + H)+
413.1937, found 413.1938. Anal. Calcd for C20H24N6O4·1.2H2O: C,
55.34; H, 6.13; N, 19.36. Found C, 54.97; H, 6.07; N, 18.98.
6-[4-(4-Fluorophenyl)piperazine-1-yl]-9-(β-D-ribofuranosyl)-9H-
purine (10). The compound was prepared from (5) (151 mg, 0.27
mmol) at room temperature for 5 h according to the general
procedure and was purified by column chromatography (EtOAc−
hexane, 4:1 and then EtOAc) to yield 10 (42.4 mg; 36.5%); mp 189−
6-[4-(4-Trifluoromethylphenyl)piperazine-1-yl]-9-(2,3,5-tri-O-ace-
tyl-β-D-ribofuranosyl)-9H-purine (6). The compound was prepared
from (3) (92.8 mg, 0.23 mmol) and 1-(α,α,α-trifluoro-p-tolyl)-
piperazine (52.1 mg, 0.23 mmol) at reflux for 2.5 h according to the
general procedure and was purified by column chromatography
(EtOAc−hexane, 1:1) to yield 6 (106.4 mg; 78.23%); mp 68−70 °C.
1H NMR (DMSO-d6) δ 2.04 (d, 6H, OAc), 2.13 (s, 3H, OAc), 3.45
(br s, 4H, piperazine CH2), 4.25−4.43 (m, 7H, H-4′, H-5′, piperazine
CH2), 5.63 (t, 1H, H-3′), 6.03 (t, 1H, H-2′), 6.26 (d, J = 5.6 Hz, 1H,
H-1′), 6.13 (d, Jo = 8.8 Hz, 2H, H-2,6 in phenyl), 7.54 (d, Jo = 8.4 Hz,
2H, H-3,5 in phenyl), 8.33 (s, 1H, H-8), 8.46 (s, 1H, H-2). 13C NMR
(DMSO-d6) δ 20.87, 21.03, 21.17 (3 × CH3), 44.50, 47.61 (CH2 in
piperazine), 63.47 (CH2-5′), 70.73 (CH-3′), 72.58 (CH-2′), 80.11
(CH-4′), 86.24 (CH-1′), 115.05, 118.78 (q), 120.23, 124.29 (C in
phenyl), 126.87 (q) (CF3), 139.91 (C-5), 150.93 (C-8), 152.89 (C-6),
153.76 (C-2), 153.81 (C-4), 169.96, 170.14, 170.72 (3 × CO). HRMS
(ESI+) m/z calcd for C27H30F3N6O7 (M + H)+ 607.2128, found
607.2115. Anal. Calcd for C27H29F3N6O7·0.3EtOAc: C, 53.51; H, 5.00;
N, 13.28. Found C, 53.88; H, 5.04; N, 13.03.
1
191 °C. H NMR (DMSO-d6) δ 3.21 (t, 4H, piperazine CH2), 3.56
6-[4-(3,4-Dichlorophenyl)piperazine-1-yl]-9-(2,3,5-tri-O-acetyl-β-
D-ribofuranosyl)-9H-purine (7). The compound was prepared from
and 3.68 (2 × dd, 2H, CH2-5′), 3.97 (q, 1H, H-4′), 4.16 (t, 1H, H-2′),
3063
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Journal of Medicinal Chemistry
Article
4.38 (br s, 4H, piperazine CH2), 4.59 (t, 1H, H-3′), 5.23 (br s, 1H, 3′−
OH), 5.48 (br s, 1H, 5′−OH), 5.94 (d, 1H, 1′-H), 7.01−7.10 (m, 4H,
H-2,3,5,6 in phenyl), 8.28 (s, 1H, H-8), 8.45 (s, 1H, H-2). 13C NMR
(DMSO-d6) δ 45.16, 49.96 (CH2 in piperazine), 62.15 (CH2-5′), 71.14
(CH-3′), 74.22 (CH-2′), 86.42 (CH-4′), 88.45 (CH-1′), 115.91,
116.13, 118.37 (2), 120.33 (C in phenyl), 139.67 (C-5), 148.51 (C-8),
150.99 (C-6), 152.47 (C-2), 153.79 (C-4). HRMS (ESI+) m/z calcd
for C20H24FN6O4 (M + H)+ 431.1843, found 431.1846. Anal. Calcd
for C20H23FN6O4·0.5H2O: C, 54.66; H, 5.50; N, 19.12. Found C,
54.62; H, 5.32; N, 19.25.
acids, and 1% penicillin (Biochrome). It was incubated at 37 °C with
5% CO2. DMSO (Sigma) was used as a solvent for the compounds.
The concentration of DMSO was always less than 1% in the cell
culture medium. The cytotoxic drugs (Camptothecin, 5FU, doxor-
ubicin, and MeSAdo) used as positive controls were from Calbiochem.
Sulforhodamine B (SRB) Assay for Cytotoxicity Screening.
Huh7, HCT116, MCF7, HepG2, Mahlavu, and FOCUS cells were
inoculated (2000−10000 cells/well in 200 μL) in 96-well plates. The
next day, the media were refreshed and the compounds dissolved in
DMSO were applied in concentrations between 1 and 40 μM in
parallel with DMSO-only treated cells as negative controls. At the
72nd hour of treatment with compounds 9−13 and the other drugs,
the cancer cells were fixed with 100 μL of 10% (w/v) trichloroacetic
acid (TCA) and kept at +4 °C in the dark for one hour. TCA fixation
was terminated by washing the wells with ddH2O five times. Air-dried
plates were stained with 0.4% sulphorhodamine B (SRB) dissolved in
1% acetic acid solution for 10 min in the dark and at room
temperature. The protein-bound and dried SRB dye was then
solubilized with 10 mM Tris-Base pH 8. The absorbance values
were obtained at 515 nm in a microplate reader. The data normalized
against DMSO only treated wells, which were used as controls in serial
dilutions. In all experiments, a linear response was observed, with serial
dilutions of the compounds and the drugs; R2 ≥ 0.9.
Real-Time Cell Electronic Sensing (RT-CES) for Cytotoxicity
Profiling. The proliferation of primary liver cancer cell lines was
monitored in real-time cell electronic sensing (RT-CES) (xCELLi-
gence-Roche Applied Science), and the cell index (CI) was measured
every 30 min for 96 h. Huh7, HepG2, Mahlavu, and FOCUS cells were
inoculated (2000 cells/well in 200 μL) in the 96 well E-plate on the
xCELLigence station in 5% CO2 at 37 °C. The CI values were
recorded every 30 min. The next day, 150 μL of medium from each
well was replaced with 100 μL of fresh medium containing compound
11 applied in the indicated concentrations. The CI values were
recorded every 30 min to monitor real-time drug response. DMSO-
only and medium-only wells were also included in the monitoring to
account for their possible solitary effects on cancer cells.
Kinase Assay. Kinase-Glo-Plus luminescence kinase activity assay
(Promega) was performed according to the manufacturer’s protocol.
First, the Kinase-Glo reaction buffer (40 mM Tris-HCl pH7.6, 20 mM
MgCl2, 0.1 mg/mL BSA) was placed in the wells of a 96-well Elisa
plate. Then, Huh7 cells treated with 11, Staurosporin (STS), and
MeSAdo for 72 h. Next, lysates from 80000 cells were placed into the
wells. DMSO used as negative control. The total volume of the lysates
and the kinase reaction buffer was 50 μL. Then, 50 μL of Kinase-Glo
reagent (Kinase-Glo plus substrate + Kinase-Glo-Plus buffer) was
applied. After 10 min of dark incubation at room temperature, the
luminescence was detected with a luminometer. If kinase activity is
diminished, ATP concentration will increase and so will luminescence.
Senescence Associated-β-gal Assay and BrdU Proliferation
Co-staining. Huh7 cells (5000cell) were inoculated in two identical
six-well plates on coverslips. The next day, compound 11 and
doxorubicin at the indicated concentrations, and their corresponding
DMSO controls were applied to the plates. On day three and the day
six, the senescence-associated-β-gal (SAβgal) assay and BrdU
costaining were performed as described previously.23,24
Western Blot Analysis. Proteins from Huh7 cells treated with
compound 11 and doxorubicin for three and six days were separated
on a 10% SDS-PAGE, transferred onto nitrocellulose membranes, and
visualized, as described previously.19 The total homogenates from the
cells were resuspended in 50 mM Tris-HCl pH 7.4, 1 mM EDTA, 1
mM EGTA, 150 mM NaCl, 1% Triton X-100, and 0.1% SDS with a
protease−inhibitor cocktail and phosphatase inhibitors. p15, Rb and
pRb were detected by Western blotting. Actin protein was used for
equal loading control.
6-[4-(4-Trifluoromethylphenyl)piperazine-1-yl]-9-(β-D-ribofura-
nosyl)-9H-purine (11). The compound was prepared from (6) (106.4
mg, 0.17 mmol) at room temperature for 1 h according to the general
procedure and was purified by column chromatography (EtOAc) to
1
yield 11 (15 mg; 17.8%); mp 108−110 °C. H NMR (DMSO-d6) δ
3.45 (t, 4H, piperazine CH2), 3.52−3.60 and 3.64−3.72 (2m, 2H,
CH2-5′), 3.96−4.17 (m, 2H, H-2′, 3′), 4.38 (br s, 4H, piperazine CH2),
4.59 (q, 1H, H-4′), 5.21 (d, 1H, 3′-OH), 5.33 (t, 1H, 5′-OH), 5.48 (d,
1H, 2′-OH), 5.93 (d, 1H, 1′-H), 7.14 (d, Jo = 8.8 Hz, 2H, H-2,6), 7.54
(d, Jo = 8.8 Hz, 2H, H-3,5), 8.29 (s, 1H, H-8), 8.46 (s, 1H, H-2). 13C
NMR (DMSO-d6) δ 44.87, 47.65 (CH2 in piperazine), 62.15 (CH2-
5′), 71.14 (CH-3′), 74.24 (CH-2′), 86.42 (CH-4′), 88.46 (CH-1′),
115.06, 118.79 (q), 120.37, 124.29 (C in phenyl), 126.87 (q) (CF3),
139.73 (C-5), 151.01 (C-8), 152.48 (C-6), 153.79 (C-2), 153.82 (C-
4). HRMS (ESI+) m/z calcd for C21H24F3N6O4 (M + H)+ 481.1811,
found 481.1810. Anal. Calcd for C21H23F3N6O4·0.6H2O: C, 51.34; H,
4.96; N, 17.10. Found C, 51.65; H, 4.74; N, 16.73.
6-[4-(3,4-Dichlorophenyl)piperazine-1-yl]-9-(β-D-ribofuranosyl)-
9H-purine (12). The compound was prepared from (7) (166.2 mg,
0.27 mmol) at room temperature for 12 h according to the general
procedure and was purified by column chromatography (EtOAc−
hexane,4:1 and then EtOAc) to yield 12 (90 mg; 68.7%); mp 211 °C.
1H NMR (DMSO-d6) δ 3.34 (t, 4H, piperazine CH2), 3.51−3.60 and
3.65−3.73 (2m, 2H, CH2-5′), 3.92−4.63 (m, 7H, H-2′,3′,4′, piperazine
CH2), 5.21 (d, 1H, 3′-OH), 5.33 (t, 1H, 5′-OH), 5.47 (d, 1H, 2′-OH),
5.94 (d, J = 5.6 Hz, 1H, 1′-H), 7.00 (d, Jo = 7.2 Hz, 1H, H-6 in
phenyl), 7.21 (s, 1H, H-2 in phenyl), 7.29 (d, J = 9.2 Hz, 1H, H-5 in
phenyl), 8.29 (s, 1H, H-8), 8.45 (s, 1H, H-2). 13C NMR (DMSO-d6) δ
29.69 (CH2 in piperazine), 48.30 (CH2 in piperazine), 62.18 (CH2-5′),
71.16 (CH-3′), 74.25 (CH-2′), 86.44 (CH-4′), 88.47 (CH-1′), 116.26
(CH in phenyl), 117.25 (CH in phenyl), 120.37 (CH in phenyl),
120.60 (C in phenyl), 131.21 (C in phenyl), 132.24 (C in phenyl),
139.76 (C-5), 151.03 (C-8), 151.31 (C-6), 152.50 (C-2), 153.81 (C-
4). HRMS (ESI+) m/z calcd for C20H23Cl2N6O4 (M)+ 481.1158,
found 481.1156. Anal. Calcd for C20H22Cl2N6O4·0.2C6H14: C, 51.07;
H, 5.01; N, 16.86. Found C, 51.46; H, 4.88; N, 16.56.
6-(4-(Diphenylmethyl)piperazine-1-yl)-9-(β-D-ribofuranosyl)-9H-
purine (13). The compound was prepared from (8) (166.8 mg, 0.26
mmol) at room temparature for 8 h according to the general
procedure and was purified by column chromatography (EtOAc and
then EtOAc-MeOH, 6:1)) to yield 13 (60.5 mg; 45.5%): mp 117−119
1
°C. H NMR (DMSO-d6) δ 2.43 (br s, 4H, piperazine CH2), 3.50−
3.58 ve 3.62−3.69 (2m, 2H, CH2-5′), 3.92−4.38 (m, 7H, piperazine
CH2, CH, H-2′, 3′), 4.55 (q, 1H, H-4′), 5.19 (d, J = 4.4 Hz, 1H, 3′−
OH), 5.31 (t, J = 5.2 Hz, 1H, 5′−OH), 5.46 (d, J = 6 Hz, 1H, 2′−OH),
5.90 (d, J = 5.6 Hz, 1H, H-1′), 7.21 (t, Jo = 7.2 Hz, 2H, H-4 in phenyl),
7.32 (t, Jo = 7.2 Hz, 4H, H-3,5 in phenyl), 7.45 (d, Jo = 8 Hz, 4H, H-
2,6 in phenyl), 8.22 (s, 1H, H-8), 8.39 (s, 1H, H-2). 13C NMR
(DMSO-d6) δ 52.18 (CH2 in piperazine), 62.14 (CH), 71.13 (CH2-
5′), 74.20 (CH-3′), 75.48 (CH-2′), 86.39 (CH-4′), 88.42 (CH-1′),
120.27 (CH in phenyl), 127.65 (CH in phenyl), 128.35 (CH in
phenyl), 129.26 (C in phenyl), 139.54 (C-5), 143.12 (C-8), 150.93
(C-6), 152.41 (C-2), 153.82 (C-4). HRMS (ESI+) m/z calcd for
C27H31N6O4 (M + H)+ 503.2407, found 503.2406. Anal. Calcd for
C27H30N6O4·0.2EtOAc·1.0H2O: C, 62.03; H, 6.29; N, 15.61. Found C,
61.72; H, 6.05; N, 15.21.
AUTHOR INFORMATION
■
Cells and Culture. The human primary liver cancer cell lines
(Huh7, HepG2, Mahlavu, and FOCUS) were grown in Dulbecco’s
Modified Eagle’s Medium (DMEM) (Invitrogen GIBCO), with 10%
fetal bovine serum (FBS) (Invitrogen GIBCO), nonessential amino
Corresponding Author
*For M.T.: phone, +90 312 203 3071; fax, +90 312 213 1081;
E-mail, tuncbile@pharmacy.ankara.edu.tr. For R.C.A.: phone,
3064
dx.doi.org/10.1021/jm3001532 | J. Med. Chem. 2012, 55, 3058−3065

Journal of Medicinal Chemistry
Article
T.; Longerich, T.; Kubicka, S.; Zender, L. Pro-senescence therapy for
cancer treatment. Nature 2011, 479, 547−551.
+90 312 290 2503; fax, +90 312 266 5097; E-mail: rengul@
bilkent.edu.tr.
(16) Irmak, M. B.; Ince, G.; Ozturk, M.; Cetin-Atalay, R. Acquired
tolerance of hepatocellular carcinoma cells to selenium deficiency: a
selective survival mechanism? Cancer Res. 2003, 63, 6707−6715.
(17) Finn, R. S. Sorafenib use while waiting for liver transplant: We
still need to wait. J. Hepatol. 2012, 56, 723−725.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
(18) Andrzejewska, M.; Kaminski, J.; Kazimierczuk, Z. Microwave
induced synthesis of ribonucleosides on solid support. Nucleosides,
Nucleotides, Nucleic Acids 2002, 21, 73−78.
This work was supported by the Scientific and Technological
Research Council of Turkey-TUBITAK (TBAG-109T987), the
KANILTEK Project from the State Planning Organization of
Turkey (DPT), and Bilkent University funds. We thank to
Professor Hakan Goker and Dr. Mehmet Alp from Central
Instrumentation Laboratory of Faculty of Pharmacy, Ankara
University for NMR and elemental analyses and to Dr. Murat
K. Sukuroglu from Faculty of Pharmacy, Gazi University for
HRMS. We thank R. Nelson for editing the English of the final
version of our manuscript.
(19) Buontempo, F.; Ersahin, T.; Missiroli, S.; Senturk, S.; Etro, D.;
Ozturk, M.; Capitani, S.; Cetin-Atalay, R.; Neri, M. L. Inhibition of Akt
signaling in hepatoma cells induces apoptotic cell death independent
of Akt activation status. Invest. New Drugs 2011, 29, 1303−1313.
(20) Gobeil, S.; Boucher, C. C.; Nadeau, D.; Poirier, G. G.
Characterization of the necrotic cleavage of poly(ADP-ribose)
polymerase (PARP-1): implication of lysosomal proteases. Cell
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(22) Hanahan, D.; Weinberg, R. A. Hallmarks of cancer: the next
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(23) Dimri, G. P.; Lee, X.; Basile, G.; Acosta, M.; Scott, G.; Roskelley,
C.; Medrano, E. E.; Linskens, M.; Rubelj, I.; Pereira-Smith, O. A
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