Investigation on the ZBG-functionality of phenyl-4-yl-acrylohydroxamic acid derivatives as histone deacetylase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2015.09.006

A series of alternative Zn-binding groups were explored in the design of phenyl-4-yl-acrylohydroxamic acid derivatives as histone deacetylase (HDAC) inhibitors. Most of the synthesized compounds were less effective than the parent hydroxamic acid. However

Full text of DOI:10.1016/j.bmcl.2015.09.006

Bioorganic & Medicinal Chemistry Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Investigation on the ZBG-functionality of phenyl-4-yl-
acrylohydroxamic acid derivatives as histone deacetylase inhibitors
a
a
b
b
c
Loana Musso , Raffaella Cincinelli , Valentina Zuco , Franco Zunino , Alessandra Nurisso ,
c
d
d
d,y
a,
⇑
Muriel Cuendet , Giuseppe Giannini , Loredana Vesci , Claudio Pisano , Sabrina Dallavalle
a
Department of Food, Environmental and Nutritional Sciences, University of Milan, Via Celoria 2, 20133 Milan, Italy
Fondazione IRCCS Istituto Nazionale Tumori, Via Amadeo 42, 20133 Milan, Italy
School of Pharmaceutical Sciences, University of Geneva, University of Lausanne, Quai Ernest-Ansermet 30, 1211 Geneva 11, Switzerland
R&D Sigma-Tau Industrie Farmaceutiche Riunite S.p.A., Via Pontina Km 30,400, I-00040 Pomezia, Roma, Italy
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of alternative Zn-binding groups were explored in the design of phenyl-4-yl-acrylohydroxamic
acid derivatives as histone deacetylase (HDAC) inhibitors. Most of the synthesized compounds were less
effective than the parent hydroxamic acid. However, the profile of activity shown by the analog bearing a
hydroxyurea head group, makes this derivative promising for further investigation.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 9 July 2015
Revised 1 September 2015
Accepted 2 September 2015
Available online xxxx
Keywords:
Histone deacetylase (HDAC) inhibitors
Zn-binding group
Antiproliferative activity
Synthesis
Over the last 10 years the reversible acetylation of histones and
other proteins has emerged as an important regulatory mechanism
implicated in cell proliferation and has been identified as a valu-
able target for anticancer drug design.
modelling studies involving the histone deacetylase-like protein
(HDLP), these molecules appear to bind in a pocket of the HDAC
active site in such a way that the metal binding moiety (a hydrox-
amic acid for SAHA, Belinostat and Panobinostat, a thiol derivative
for Romidepsin and a benzamide for Chidamide), coordinates the
catalytic zinc ion at the end of the pocket. Binding of the Zn ion
has proven to be crucial for the activity.
Since the discovery of SAHA, structural studies to identify novel
HDAC inhibitors have mainly focused on modifications of the lin-
ker or the cap group, while less attention has been paid to the
metal binding group.
Acetylation is executed by histone acetyltransferases and is
reversed by histone deacetylases (HDACs).1 Eighteen mammalian
HDAC enzymes have been identified so far, which can be subdi-
2+
2+
vided into two families according to a cofactor (Zn for Classes I,
+
2
II and IV or NAD for Class III) for activity.
A number of HDAC inhibitors reached clinical trials, and five of
them were approved by authorities like the US FDA: Vorinostat
(
Zolinza; 2006), Romidepsin (Istodax; 2009) both for use in periph-
Hydroxamic acid derivatives represent the most studied and
successful class of HDAC inhibitor drug candidates. However,
although very powerful, they suffer from poor metabolic stability
and a rapid excretion. The different clinical applications of this
class of drugs, such as CNS, inflammatory and HIV diseases, require
a renewed effort in the search for new ZBG.
eral T-cell lymphoma (CTCL) with the latter also for use in periph-
eral T-cell lymphoma (CTCL) (2011). Belinostat (Beleodaq; 2014)
and Chidamide (Epidaza; 2015) both for use in CTCL and, more
recently, Panobinostat (Farydak; 2015) for use in multiple
5
,6
3
myeloma. They all fit in the widely accepted HDAC pharma-
cophore model for HDAC inhibitors characterized by a cap group,
a linker chain and a zinc binding group (ZBG). Based on molecular
Several efforts have been made to replace the hydroxamic acid
with alternative chemical moieties that could also further improve
the isoform selectivity of HDAC inhibitors.
4
7
–11
As part of a previous research program aimed at studying new
HDAC inhibitors, we have developed a series of hydroxamic acid-
based compounds, characterized by a cinnamic spacer capped with
⇑
Corresponding author. Tel.: +39 02 50316818; fax: +39 02 50316801.
E-mail address: sabrina.dallavalle@unimi.it (S. Dallavalle).
Present address: Biogem SCARL, Via Camporeale, I-83031 Ariano Irpino (AV),
1
2
a substituted phenyl group (1, Fig. 1). Extensive research has
y
been conducted on the derivatization of the cap group, to optimize
1
2–14
Italy.
drug–target interactions.
http://dx.doi.org/10.1016/j.bmcl.2015.09.006
0
960-894X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Musso, L.; et al. Bioorg. Med. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.bmcl.2015.09.006

2
L. Musso et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
SAHA.16 To investigate the effect of the hydroxyurea group on
the 4-vinylbiphenyl scaffold, compound 6 was synthesized starting
from acid 2 (weak Zn coordinator), which was tested for its
HDAC inhibitory activity as well. The acid was quantitatively
converted into the corresponding isocyanate 4 by treatment with
CONHOH
2
+
R
1
1a R = H
diphenylphosphorylazide (DPPA) in dry CH
reacted with hydroxylamine in toluene to give the hydroxyurea
2 2
Cl . This latter was
Figure 1. General structure of compounds 1.
6
7
. The same strategy was used to prepare the O-benzyl derivative
(Scheme 1).
Recently, we have initiated the search for moieties that would
Isocyanate 4 was also used as an intermediate to prepare semi-
retain the zinc-binding properties and enzyme inhibitory activity of
the hydroxamic acid group. In this Letter we report the attempts
madeto modifythe ZBG-functionalityofthe 4-vinylbiphenylscaffold.
A series of analogs incorporating non-hydroxamic groups was
designed to probe the potential of various ZBGs. Indeed, very strin-
gent constraints should be satisfied in order to have zinc binding
carbazide 5, based on the consideration that a SAHA semicarbazide
1
6
derivative was found to be active as HDAC inhibitor. As other
derivatives with a hydrazide group showed HDAC inhibitory activ-
1
8
ity, we also replaced the hydroxamic acid moiety of reference
compound 1a with a hydrazide counterpart. Accordingly, com-
pound 3 was prepared by condensation of acid 2 with hydrazine
(
as with the hydroxamic acid group), and also interactions with
1
9
in the presence of HOBt and EDC (Scheme 1).
amino acid residues of the enzymes to have a strong binding affin-
1
5
Other groups were selected based on the concept that it is pos-
sible for them to bind to the zinc ion. Sulfur ligands are well known
to bind tightly to Zn containing enzymes. In particular, HDAC inhi-
ity. The N-hydroxyurea group appears to be an interesting possi-
ble replacement for the hydroxamate group commonly found in
HDAC inhibitors, because both groups display an identical binding
2
0
1
5–17
bitors with
a-thioacetate as well as a-thioacetoxyketone moiety
mode.
SAHA N-hydroxyurea derivative was found to show
in place of the hydroxamic acid showed higher inhibition of the
anti-HDAC activity even though it was much less effective than
2
1,22
enzyme than SAHA.
analogs harboring an
Based on these results, the synthesis of
-thioacetoxyketone as a ZBG on the phe-
a
COOH
CONHNH
2
nyl-4-yl-acryloyl scaffold was performed. Heck condensation
b
between 4-bromobiphenyl 8 and but-3-en-2-one afforded ketone
9
that was brominated with pyrrolidone hydrotribromide to obtain
2
3
5
10. Treatment of this latter with potassium thioacetate gave the
desired thioacetoxyketone 11 in good yield (Scheme 2).
We also speculated that another possible ZBG could be the thio-
cyanate group, which is known to bind the Zn ion through the
nitrogen atom. Thus, compound 14 was prepared from 4-bromo-
biphenyl 8, which was coupled with 3,3-diethoxypropene to obtain
a
NCO
NHCONHNH
2
c
23
4
4
aldehyde 12 in 78% yield. Reduction by NaBH , followed by
d
3
treatment with PBr afforded the bromide 13 in 88% overall yield.
NHCONHOH
Reaction with silica gel-supported potassium thiocyanate24
afforded compound 14 in good yield (Scheme 2).
e
During the past years, Cohen’s group has investigated several
new cyclic ketones and lactones as ZBGs for Zn-dependant
enzymes. Among them, they proposed some 6-membered cyclic
6
H
N
H
N
O
2
5
hydroxamic and thiohydroxamic acids. The purpose to investi-
gate the effect of a five-membered cyclic hydroxamic acid, which
should be the closest structural analog of hydroxamic acid,
prompted us to synthesize compound 17 (Scheme 3). When this
work was already in an advanced stage, the same group was
O
7
Scheme 1. Synthesis of compounds 2–7. Reagents and conditions: (a) DPPA, TEA,
CH Cl , reflux, 7 h, 100%; (b) HOBT, EDC, NH NH O, cyclohexene, acetonitrile, 0–
ꢀH
0 °C, 10 min, 77%; (c) NH NH O, toluene, 80 °C, 1 h, 63%; (d) NH OH, toluene,
2
6
2
2
2
2
2
reported as a novel ZBGs for HDAC inhibitors.
1
2
2
ꢀH
2
2
The synthesized compounds were tested for their inhibitory
activity towards HDAC2 isoform (Table 1), and for cell growth
reflux, 1 h, 53%; (e) O-benzylhydroxylamine, toluene, reflux, 3.5 h, 23%.
O
O
O
Br
SCOCH
3
b
c
9
10
1
1
a
Br
CHO
Br
SCN
d
e,f
g
1
2
13
14
8
Scheme 2. Synthesis of compounds 11 and 14. Reagents and conditions: (a) but-3-en-2-one, palladium acetate, tri(o-tolyl)phosphine, TEA, 100–110 °C, 12 h, 42%; (b)
pyrrolidone hydrotribromide, THF, rt, 24 h, 41%; (c) potassium thioacetate, DMF, rt, 2.5 h, 56%; (d) 3,3-diethoxypropene, Bu NOAc, K CO , KCl, Pd(OAc) , DMF, 100 °C, 5 h,
8%; (e) NaBH , EtOH, rt, 30 min, 98%; (f) PBr , Et O, rt, 1 h, 90%; (g) potassium thiocyanate–SiO , hexane, 50 °C, 3.5 h, 78%.
4
2
3
2
7
4
3
2
2
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L. Musso et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
3
O
O
CHO
OH
N
O
a
b
1
5
16
17
Scheme 3. Synthesis of compound 17. Reagents and conditions: (a) butyrolactone, NaOMe, toluene, ꢁ10 °C for 5 min, then rt for 1.5 h, 89%; (b) NH
.5 h, 20%.
2
OHꢀHCl, EtONa, reflux,
3
inhibition against a panel of tumor cell lines of different tissue ori-
gin: NB4 (human acute promyelocytic leukemia), H460 (human
lung carcinoma), HCT116 (human colon cancer), IGROV-1 and its
subline resistant to cisplatinum IGROV-1/Pt1 (human ovarian
carcinoma) (Table 2).
Table 1
HDAC2 inhibitory activity of synthesized compounds
HDAC2a (IC50
Compd
SAHA
lM)
0.15 ± 0.02
0.82 ± 0.14
>5
>5
1
2
3
5
6
7
1
1
1
a
Most of the synthesized compounds were characterized by a
low activity (compared to SAHA) in terms of both HDAC inhibition
and cell growth inhibition. In spite of a lack of enzymatic activity,
compounds 3, 5, 11, and 14 showed a moderate cell growth inhibi-
tion, most likely related to the inhibition of multiple targets. In
fact, there appears to be a common opinion that inhibitors contain-
ing metal binding groups can be prone to widespread off-target
enzymatic inhibition, due to non-specific metal binding.27
The introduction of an O-benzyl hydroxyurea moiety (com-
pound 7) and of a five-membered cyclic hydroxamic acid (com-
pound 17) led to a loss of activity Only compound 6, bearing a
hydroxyurea as ZBG, exhibited activity comparable to the parent
compound 1a, both in HDAC2 and cell growth inhibitions.
Thus, the profiling of 6 was carried out also against HDAC1 and
HDAC6 isoforms, using SAHA as a reference compound. The
>5
1.05 ± 0.62
>5
>5
>5
>5
1
4
7
a
HDAC activity was determined using HDAC2 from HeLa cells as indicated in SI.
Table 2
Cell growth inhibition (IC50, lM) of synthesized compounds and reference HDAC
inhibitor (SAHA) against a panel of tumor cell lines
Compd
IGROV-1
IGOV/Pt1
HCT116
NB4
H460
IC50a
results (HDAC1: compound 6, IC50 1.31 ± 0.21
.12 ± 0.01 M; HDAC6: compound 6, IC50 1.05 ± 0.09
IC50 0.07 ± 0.01 M) suggested that the compound was a multi-
HDAC inhibitor, its activity against HDAC1 and 6 being comparable
to the activity against HDAC2 (see SI for details).
l
M. SAHA, IC50
(lM)
0
l
lM. SAHA,
SAHA
2.2 ± 0.3
3.5 ± 0.6
—
7.6 ± 0.2
14.5 ± 0.3
3.5 ± 0.3
40 ± 1
26.4 ± 0.8
3.8 ± 0.6
ꢂ60
2.2 ± 0.2
4.0 ± 0.4
—
1.2 ± 0.1
1.6 ± 0.3
>20
11.9 ± 1.2
16.4 ± 3.0
4.0 ± 0.7
40 ± 3
14.0 ± 1.5
12.6 ± 3.8
>20
0.70 ± 0.03
0.9 ± 0.1
>10
7.1 ± 0.7
9.8 ± 0.9
1.96 ± 0.1
>20
>20
>20
>20
3.4 ± 0.8
5.4 ± 0.1
>20
>20
>20
9.5 ± 0.4
>20
>20
9.4 ± 1.6
>20
l
1
2
3
5
6
7
1
1
1
a
25.1 ± 5.9
26.7 ± 0.3
6.7 ± 0.2
ꢂ50
A structural rationalization study was conducted in order to
understand the main interaction features of compounds 1a and 6
with the enzyme. The molecular docking protocol adopted here
has been validated in the past through the re-docking of the co-
1
4
7
18.7 ± 0.6
58.1 ± 2.6
ꢂ60
28
crystallized SAHA within the HDAC2 protein structure. Both the
a
hydroxamate and N-hydroxyurea groups of compound 1a
(ChemPLP score of the best ranked pose: 69.8; 100% convergent
docking solutions) and 6 (ChemPLP score of the best ranked pose:
Cells were exposed for 72 h. Cell growth inhibitory activity was measured by
cell counting and expressed as concentration required for 50% inhibition of cell
growth (IC50). IC50 values were determined by dose–response curves. Results are
the means ± SD of three independent experiments.
Figure 2. HDAC2 in complex with compound 1a (best-ranked pose, purple balls and sticks, A), and compound 6 (best-ranked pose, black balls and sticks, B). The protein is
reported as gray ribbons. Zinc ion is indicated as a gray ball. Key residues for ligands stabilization are labeled.
Please cite this article in press as: Musso, L.; et al. Bioorg. Med. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.bmcl.2015.09.006

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L. Musso et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
8. Ononye, S. N.; VanHeyst, M. D.; Oblak, E. Z.; Zhou, W.; Ammar, M.; Anderson, A.
7
5.4; 100% convergent docking solutions), respectively, were
C.; Wright, D. L. ACS Med. Chem. Lett. 2013, 4, 757.
involved in zinc chelation. Polar interactions of these chelating
groups with neighbor residues (His146, Tyr308) were also
observed. The additional nitrogen atom present in compound 6
made a hydrogen bond with Gly154. The zinc binding groups of
both molecules were connected to two phenyl rings, the first one
stabilized by hydrophobic interactions with Phe105, Phe210, and
Leu276, the second one solvent exposed in both cases (Fig. 2).
In conclusion, we explored alternative ZBGs in the design of
phenyl-4-yl-acrylohydroxamic acid derivatives as HDAC inhibitors.
Most of the synthesized compounds were less effective than the
parent hydroxamic acid 1a. However, the profile of activity shown
by compound 6, bearing a hydroxyurea head group, makes this
derivative a promising candidate for further investigation, also tak-
ing into account the potential improvement in terms of bioavail-
ability/metabolism of hydroxyurea over hydroxamate.2
9
.
Dallavalle, S.; Pisano, C.; Zunino, F. Biochem. Pharmacol. 2012, 84, 756.
10. Madsen, A. S.; Kristensen, H. M. E.; Lanz, G.; Olsen, C. A. ChemMedChem 2014, 9,
614.
1
1
1. Suzuki, T.; Miyata, N. Curr. Med. Chem. 2005, 12, 2867.
2. Dallavalle, S.; Cincinelli, R.; Nannei, R.; Merlini, L.; Morini, G.; Penco, S.; Pisano,
C.; Vesci, L.; Barbarino, M.; Zuco, V.; De Cesare, M.; Zunino, F. Eur. J. Med. Chem.
2009, 44, 1900.
1
1
1
1
1
1
1
3. Cincinelli, R.; Musso, L.; Giannini, G.; Zuco, V.; De Cesare, M.; Zunino, F.;
Dallavalle, S. Eur. J. Med. Chem. 2014, 79, 251.
4. Giannini, G.; Brunetti, T.; Pezzi, R.; Battistuzzi, G.; Di Marzo, M.; Marzi, M.;
Cabri, W.; Vesci, L.; Pisano, C. Bioorg. Med. Chem. Lett. 2009, 19, 2346.
5. Vanommeslaeghe, K.; Loverix, S.; Geerlings, P.; Tourw e´ , D. Bioorg. Med. Chem.
2005, 13, 6070.
6. Suzuki, T.; Nagano, Y.; Kouketsu, A.; Matsuura, A.; Maruyama, S.; Kurotaki, M.;
Nakagawa, H.; Miyata, N. J. Med. Chem. 2005, 48, 1019.
7. Hanessian, S.; Vinci, V.; Auzzas, L.; Marzi, M.; Giannini, G. Bioorg. Med. Chem.
Lett. 2006, 16, 4784.
8. Hou, X.; Du, J.; Liu, R.; Zhou, Y.; Li, M.; Xu, W.; Fang, H. J. Chem. Inf. Model. 2015,
55, 861.
9. Zhang, X.; Breslav, M.; Grimm, J.; Guan, K.; Huang, A.; Liu, F.; Maryanoff, C. A.;
Palmer, D.; Patel, M.; Qian, Y.; Shaw, C.; Sorgi, K.; Stefanick, S.; Xu, D. J. Org.
Chem. 2002, 67, 9471.
9,30
Acknowledgments
2
2
0. Giannini, G.; Vesci, L.; Battistuzzi, G.; Vignola, D.; Milazzo, F. M.; Berardino
Guglielmi, M.; Barbarino, M.; Santaniello, M.; Fanto, N.; Mor, M.; Rivara, S.;
Pala, D.; Taddei, M.; Pisano, C.; Cabri, W. J. Med. Chem. 2014, 57, 8358.
1. Wash, P. L.; Hoffman, T. Z.; Wiley, B. M.; Bonnefous, C.; Smith, N. D.; Sertic, M.
S.; Lawrence, C. M.; Symons, K. T.; Nguyen, P.-M.; Lustig, K. D.; Guo, X.;
Annable, T.; Noble, S. A.; Hager, J. H.; Hassig, C. A.; Malech, J. W. Bioorg. Med.
Chem. Lett. 2008, 18, 6482.
This work was performed within the framework of COST
Actions CM1106 (Chemical Approaches to Targeting Drug Resis-
tance in Cancer Stem Cells). The Authors are grateful to Professor
Nadine Martinet for her contribution to the network coordination
and for creating the compound library.
2
2. Gu, W.; Nusinzon, I.; Smith, R. D., Jr.; Horvath, C. M.; Silverman, R. B. Bioorg.
Med. Chem. 2006, 14, 3320.
Supplementary data
23. Diaz, A.; Massacesi, M.; Ponticelli, G.; Paschina, G. J. Inorg. Nucl. Chem. 1975, 37,
469.
4. Ando, T.; Clark, J. H.; Cork, D. G.; Fujita, M.; Kimura, T. J. Org. Chem. 1987, 52,
81.
25. Jacobsen, J. A.; Fullagar, J. L.; Miller, M. T.; Cohen, S. M. J. Med. Chem. 2011, 54,
91.
2
2
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2015.09.
6
5
0
06.
26. Mutule, I.; Borovika, D.; Rozenberga, E.; Romanchikova, N.; Zalubovskis, R.;
Shestakova, I.; Trapencieris, P. J. Enzyme Inhib. Med. Chem. 2015, 30, 216.
References and notes
27. Day, J. A.; Cohen, S. M. J. Med. Chem. 2013, 56, 7997.
2
2
3
8. Zwick, V.; Chatzivasileiou, A.-O.; Deschamps, N.; Roussaki, M.; Simoes-Pires, C.;
Nurisso, A.; Denis, I.; Blanquart, C.; Martinet, N.; Carrupt, P.-A.; Detsi, A.;
Cuendet, M. Bioorg. Med. Chem. Lett. 2014, 24, 5497.
9. Campestre, C.; Agamennone, M.; Tortorella, P.; Preziuso, S.; Biasone, A.;
Gavuzzo, E.; Pochetti, G.; Mazza, F.; Hiller, O.; Tschesche, H.; Consalvi, V.;
Gallina, C. Bioorg. Med. Chem. Lett. 2006, 16, 20.
0. Stewart, A. O.; Bhatia, P. A.; Martin, J. G.; Summers, J. B.; Rodriques, K. E.;
Martin, M. B.; Holms, J. H.; Moore, J. L.; Craig, R. A.; Kolasa, T.; Ratajczyk, J. D.;
Mazdiyasni, H.; Kerdesky, F. A. J.; DeNinno, S. L.; Maki, R. G.; Bouska, J. B.;
Young, P. R.; Lanni, C.; Bell, R. L.; Carter, G. W.; Brooks, C. D. W. J. Med. Chem.
1
2
3
4
5
.
.
.
.
.
Grunstein, M. Nature 1997, 389, 349.
Mottamal, M.; Zheng, S.; Huang, T. L.; Wang, G. Molecules 2015, 20, 3898.
Falkenberg, K. J.; Johnstone, R. W. Nat. Rev. Drug Disc. 2014, 13, 673.
Paris, M.; Porcelloni, M.; Binaschi, M.; Fattori, D. J. Med. Chem. 2008, 51, 1505.
Summers, J. B.; Gunn, B. P.; Mazdiyasni, H.; Goetze, A. M.; Young, P. R.; Bouska,
J. B.; Dyer, R. D.; Brooks, D. W.; Carter, G. W. J. Med. Chem. 1987, 30, 2121.
Kelly, W. K.; Richon, V. M.; O’Connor, O.; Curley, T.; MacGregor-Curtelli, B.;
Tong, W.; Klang, M.; Schwartz, L.; Richardson, S.; Rosa, E.; Drobnjak, M.;
Cordon-Cordo, C.; Chiao, J. H.; Rifkind, R.; Marks, P. A.; Scher, H. Clin. Cancer Res.
6
.
1997, 40, 1955.
2
003, 9, 3578.
Sodji, Q. H.; Patil, V.; Kornacki, J. R.; Mrksich, M.; Oyelere, A. K. J. Med. Chem.
013, 56, 9969.
7
.
2
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