Synthesis, activity evaluation, and docking analysis of barbituric acid aryl hydrazone derivatives as RSK2 inhibitors

Article abstract of DOI:10.3109/14756366.2012.681651

The 90 kDa ribosomal S6 kinases (RSKs), especially RSK2, have attracted attention for the development of new anticancer agents. Through structural optimization of the hit compound 1 from our previous study, a series of barbituric acid aryl hydrazone analogues were designed and synthesized as potential RSK2 inhibitors. The most potent one, compound 9, showed a higher activity against RSK2 with an IC50 value of 1.95 μM. To analyze and elucidate their structure-activity relationship, the homology model of RSK2 N-terminal kinase domain was built and molecular docking simulations were performed, which provide helpful clues to design new inhibitors with desired activities.

Full text of DOI:10.3109/14756366.2012.681651

Journal of Enzyme Inhibition and Medicinal Chemistry, 2013; 28(4): 747–752
© 2013 Informa UK, Ltd.
ISSN 1475-6366 print/ISSN 1475-6374 online
DOI: 10.3109/14756366.2012.681651
research arTIcLe
Synthesis, activity evaluation, and docking analysis of
barbituric acid aryl hydrazone derivatives as
RSK2 inhibitors
Mengzhu Xue, Minghao Xu, Weiqiang Lu, Jin Huang, Honglin Li, Yufang Xu, Xiaofeng Liu, and
Zhenjiang Zhao
Shanghai Key Laboratory of New Drug Design, Shanghai Key Laboratory of Chemical Biology, State Key Laboratory of
Bioreactor Engineering, School of Pharmacy, East China University of Science and Technology, Shanghai, China
abstract
The 90 kDa ribosomal S6 kinases (RSKs), especially RSK2, have attracted attention for the development of new
anticancer agents. Through structural optimization of the hit compound 1 from our previous study, a series of
barbituric acid aryl hydrazone analogues were designed and synthesized as potential RSK2 inhibitors. The most
potent one, compound 9, showed a higher activity against RSK2 with an IC50 value of 1.95 μM.To analyze and elucidate
their structure-activity relationship, the homology model of RSK2 N-terminal kinase domain was built and molecular
docking simulations were performed, which provide helpful clues to design new inhibitors with desired activities.
Keywords: RSK2 inhibitors, barbituric acid derivatives, SAR, homology modeling, molecular docking
Introduction
a C-terminal kinase domain (CTKD) and an N-terminal
e 90 kDa ribosomal S6 kinases (RSKs) are serine/
threonine kinases and lie downstream of Ras/mitogen-
activated protein kinase (MAPK) pathway, which are
responsible for activating some important cellular com-
kinase domain (NTKD) connected by a linker region.
CTKD is believed to be activated by ERK and serves to
regulate the phosphorylation of the linker region12, and
the activated NTKD is believed to be responsible for
phosphorylation of other substrates13. However, it has
been found that the linker region could be activated in
the absence of CTKD, and CTKD-independent activation
mechanisms of physiological RSK2 remain unknown2.
erefore, present inhibitor studies against RSK2 mainly
focus on NTKD.
ponents including transcription factors and histones1–8
.
is family consists of four human isoforms (RSK1-4)
with each being a product of a separate gene9, among
which RSK2 has attracted more attention, for its over
expression and aberrant activation have been linked to
many human diseases, including breast cancer, prostate
cancer10. ese studies suggest that RSK2 is a potential
therapeutic target in the treatment of metastatic cancers,
which is of both research and clinical interest to develop
potent RSK2 inhibitors11. In particular, RSK2 possesses
To date, a number of potent RSK2 inhibitors have
been reported (Figure 1)14. ere are mainly three RSK
specific inhibitors identified and widely applied in RSK2
mediated biologic process as chemical probes. SL0101
e first three authors contributed equally to this work
Address for Correspondence: Xiaofeng Liu, Shanghai Key Laboratory of New Drug Design, Shanghai Key Laboratory of Chemical Biology,
State Key Laboratory of Bioreactor Engineering, School of Pharmacy, East China University of Science and Technology, Shanghai, China.
Tel/Fax: +86-21-64250213. E-mail: xfliu@ecust.edu.cn; Zhenjiang Zhao, Shanghai Key Laboratory of New Drug Design, Shanghai Key
Laboratory of Chemical Biology, State Key Laboratory of Bioreactor Engineering, School of Pharmacy, East China University of Science and
Technology, Shanghai, China. Tel/Fax: +86-21-64250213. E-mail: zhjzhao@ecust.edu.cn
(Received 31 December 2011; revised 21 March 2012; accepted 27 March 2012)
747

748 M. Xue et al.
Figure 1. Selected structures of reported RSK2 inhibitors.
is the first reported RSK2 specific inhibitor which was
modification of a series of derivatives of compound
1 to improve the inhibitory potency against RSK2, of
which compounds 9 and 14 showed improved inhibi-
tory activities against RSK2 with IC50 values of 1.95 μM
and 2.19 µM, respectively. Finally, molecular docking
simulations were performed using a reliable RSK2
homology model to elucidate the relationship between
SAR and binding mode.
extracted from the tropical plant Forsteronia refracta and
exhibits excellent inhibitory activity (IC50 = 89 nM)15,16
.
BI-D1870 is another specific inhibitor of the RSK family
(RSK1-4) with an in vitro enzyme IC50 of 10–30 nM17.
Unlike SL0101 and BI-D1870, Fmk is an irreversible
inhibitor of RSK2 with an IC50 of 15 nM by binding
to the CTKD18. Moreover, two bisindoylmaleimide
compounds GF109203X and Ro31–8220, which
were both potent but nonspecific RSK2 inhibitors,
were obtained from kinase screening assays of PKC
inhibitors19. Although RSK2 is proved to be a promising
cancer therapeutic target, the crystal structure of
NTKD of human RSK2 has not yet been determined.
erefore, Nguyen et al. identified two novel weakly
binding inhibitors (NSC356821 and NSC51023) of RSK2
through homology modeling and virtual screening20.
ese inhibitors were widely applied for elucidating
the context of the RSK2 signaling pathway as chemical
probes, but none of them are applicable in clinical use.
erefore, there are ever-increasing needs to discover
novel RSK2 inhibitors for further development into
therapeutic candidates for cancer treatment.
Using SHAFTS program, an in-house 3D similar-
ity based computational virtual screening procedure
combined with shape and chemical feature superposi-
tion, several RSK2 inhibitors with novel scaffolds and
moderate inhibitory activities were identified in our
previous study21. One of the hit compounds 1, a barbi-
turic acid aryl hydrazine derivative, proved to have an
inhibitory ratio of 47% at 10 μM against RSK2. In this
study, we further report the synthesis and structural
Methods
Chemistry
Preparation of target compounds was carried out from
the coupling of corresponding aniline diazonium salt and
barbituric acid derivatives (Scheme 1). In detail, aniline
(20.0 mmol) dissolved in 6.7 mL concentrated hydrochlo-
ride was mixed with 21 mL water, stirred and cooled to
0°C. en a solution of sodium nitrite (25 mmol) in 6 mL
water was added carefully to form a diazonium solution.
Separately, a mixture of NaOAc (9.2 g, 0.1 mol) dissolved
in 16 mL water and barbituric acid (20.0 mmol) dissolved
in ethanol (62 mL). e diazonium solution was added
dropwise when the mixture was cooled below 10°C.
After stirring overnight, the resulting precipitation was
filtered and washed by ethanol/water (1/1). e product
was separated and purified by recrystallization or silica
gel chromatography and the structure was identified by
1H-NMR and MS.
5-(2-(4-hydroxyphenyl)hydrazono)pyrimidine-2,4,6
(1H,3H,5H)-trione (9). Brown solid; mp > 300°C; 1H
NMR (400 MHz, DMSO-d6) δ (ppm): 14.34 (s, 1H), 11.39
(s, 1H), 11.17 (s, 1H), 9.79 (d, J = 3.2 Hz, 1H), 7.43 (d, J
Journal of Enzyme Inhibition and Medicinal Chemistry

Barbituric acid derivatives as RSK2 inhibitors 749
⎛
⎞
F
590,cmpd.
Inhibition % = 1−
× 100%
⎜
⎟
⎟
⎜
F
590,control
⎝
⎠
IC50 values were determined from the results of at least
three independent tests and calculated from the inhibition
curves. e accuracy of the in vitro screening assays was
confirmed by the IC50 value of control inhibitor Ro31–8220
at 10 nM, which is in compliance with the reported value19.
Scheme 1. General synthesis route of the barbituric acid aryl
hydrazone derivatives.
=8.8 Hz, 2H), 6.85 (d, J = 8.8 Hz, 2H); HRMS (ESI, m/z):
247.0474 (M-H+).
Molecular modeling
5-(2-(4-methoxyphenyl)hydrazono)pyrimidine-2,4,6
(1H,3H,5H)-trione (12). Yellow solid; mp 291–292°C; H
1
To investigate the binding poses of these compounds and
then elucidate corresponding relationship with SAR, the
homology model of RSK2 NTKD built in our previous study
was used for molecular docking22. Compounds 1, 9 and 13
were picked out for the docking studies and prepared by
the Ligprep module in Maestro (Schrödinger Inc, version
7.5). en Glide program in extra-precision (XP) mode
was used to perform the molecular docking simulations
with default parameter sets. e grid-enclosing box was
centered on the centroid of aligned ligand (Staurosporine)
and defined so as to enclose residues located within 20 Å
around the ATP binding site, and a scaling factor of 1.0 was
set to van der Waals (VDW) radii of those receptor atoms
with the partial atomic charge less than 0.25. e top 10
docked poses of each compound ranked by GlideScore
were remained for further analysis.
NMR (400 MHz, DMSO-d6) δ (ppm): 14.28 (s, 1H), 11.42
(s, 1H), 11.20 (s, 1H), 7.55 (d, J = 9.2 Hz, 2H), 7.04 (d, J = 9.2
Hz, 2H), 3.78 (s, 3H); HRMS (ESI, m/z): 261.0629 (M-H+).
5-(2-(4-morpholinophenyl)hydrazono)pyrimi-
dine-2,4,6 (1H,3H,5H)-trione (13). Red solid; mp 279–
1
280°C; H NMR (400 MHz, DMSO-d6) δ (ppm): 14.39
(s, 1H), 11.38 (s, 1H), 11.17 (s, 1H), 7.48 (d, J =8.8 Hz,
2H), 7.04 (d, J = 8.8 Hz, 2H), 3.74 (t, J =4.8 Hz, 2H), 3.15
(t, J =4.8 Hz, 2H); MS (ESI, m/z): 318 (M-H+).
5-(2-(4-hydroxyphenyl)hydrazono)-dihydro-2-
thioxopyrimidine-4,6 (1H,5H)-dione (14). Brown solid;
1
mp > 300°C; H NMR (400 MHz, DMSO-d6) δ (ppm):
14.49 (s, 1H), 12.50 (s, 1H), 12.33 (s, 1H), 9.88 (s, 1H), 7.48
(d, J =9.2 Hz, 2H), 6.86 (d, J = 9.2 Hz, 2H); HRMS (ESI,
m/z): 263.0242 (M-H+).
5-(2-(4-morpholinophenyl)hydrazono-dihydro-
2-thioxopyrimidine-4,6 (1H,5H)-dione (15). Brown solid;
1
results and discussion
mp 242–243°C; H NMR(400 MHz, DMSO-d6) δ (ppm):
14.55 (s, 1H), 12.49 (s, 1H), 12.32 (s, 1H), 7.53 (d, J = 9.2
Hz, 2H), 7.05 (d, J = 9.2 Hz, 2H), 3.74 (t, J = 4.4 Hz, 2H),
3.18 (t, J = 4.4 Hz, 2H); HRMS (ESI, m/z): 332.0811 (M-H+).
SAR analysis
e structures of synthesized compounds and
corresponding potencies against RSK2 are listed in
Table 1. Compound 1 was obtained through the molecular
3D similarity (shape-densities and chemical feature
overlaps) method SHAFTS in our previous study using
NSC 51023 as the query template19, suggesting NSC 51023
and compound 1 may present similar binding modes in
the RSK2 NTKD ATP site. e docking results revealed that
the amide moieties of barbituric acid ring can form pivotal
contacts with the hinge region of RSK2 NTKD by H-bonds
interaction. Comparing with compound 1 as shown in
Table 1, the enzyme inhibitory activity of di-methylated
derivate (compound 2) decreases significantly with both
1, 3-positions of the barbituric acid ring methylated,
highlighting the pivotal role of the polar hydrogen atoms
on the barbituric acid scaffold in forming putative
H-bonds. erefore, in the subsequent investigations, the
polar hydrogen atoms in 1, 3-positions were retained, and
various substituted groups were introduced to different
positions on the benzene ring to further optimize the
inhibitory activity. In general, the inhibitory potencies
were more sensitive to structural optimizations at the
para-position of the benzene ring. Introduction of halogen
atoms including F, Cl, Br and I on the benzene ring in
any position (compound 3–8) did not contribute to the
Biological assay
e inhibition assays against RSK2 in vitro were deter-
mined by ADP Quest® (DiscoveRx) in 96-well flat-bottom
plates according to the manufacturer’s instructions21. e
kinase (20 ng millipore) in 40 μL assay buffer (15 mM
HEPES, pH 7.4, 20 mM NaCl, 1 mM EGTA, 0.02% Tween
20, 10 mM MgCl2, and 0.1 mg/mL BGG) containing 25 μM
S6 peptide (AKRRRLSSLRA, Anaspec) was incubated for
20 min at room temperature with indicated concentrations
of the compounds to be tested. Reactions were initiated by
adding 10 μL of ATP to a final ATP concentration of 10 μM
and terminated after 60 min at room temperature by adding
20 μL ADP Reagent A and 40 μL ADP Reagent B. Ro31–8220
was used as a positive control. Compounds dilutions were
prepared from stock in DMSO and diluted with assay buf-
fer for inhibition assay. e fluorescence signal detecting
the amount of ADP produced as a result of enzyme activity
was recorded on the Synergy™ 2 Multi-Mode Microplate
Reader (BioTek) at an excitation wavelength of 530 nm
and an emission wavelength of 590 nm at 30 min after the
addition of the ADP Reagent B. e inhibition rate (%) was
calculated using the following equation:
© 2013 Informa UK, Ltd.

750 M. Xue et al.
improvement of inhibitory activities, which was even
deteriorated with the meta-position substitute (compound
6). Meanwhile, introduction of electron-withdrawing
groups such as nitro, cyano, carboxyl and sulfonamide
greatly decreases the inhibitory activities (compounds
16, 19, 22 and 23), in which the para-nitro substitute
led to a complete ablation of the inhibitory activity. Only
electron-donating groups like hydroxyl or morpholinyl at
the para-position such as compounds 9, 13–15, can make a
positive contribution to the inhibitory activity remarkably,
suggesting that electron factor on the conjugation structure
may affect the strength of the hydrogen bonds formed with
the active site. Similarly, electron-donating groups (e.g.
OH, compound 10) are more favored at the meta-position
over electron-withdrawing groups (e.g. Cl, compound 6).
At the ortho-position, moderate electron-withdrawing
groups like CONH2 of compound 21 or Cl of compound 5
were tolerated while more electron-negative groups such
as NO2 and CN or electron-donating groups like OH would
further reduce the activities as in compounds 11, 17 and
18. Furthermore, it also may be that the ortho CONH2
forms a planar conformation through internal H-bonding
to the hydrazone which may form an additional H-bonding
with the hinge, whereas the ortho NO2 or CN substitute
disrupts the planarity and undermines the extra H-bond
with the hinge. Replacement of the carbonyl at 2-position
to a thiocarbonyl was also tolerated, which only led to a
slight reduction in activity by comparisons of 9 with 14 and
13 with 15.
Homology model and binding modes
e RSK2 homology model used in this study proved
reliable for elucidating the binding modes and inhibi-
tory activity relationship for known RSK2 inhibitors in
our previous study. As shown in Figure 2, the domains
of these two kinases are generally similar, since the resi-
due Ser 72 in RSK1 has changed to a larger Phe 79 in the
same position of RSK2, the ATP binding pocket of RSK2
appears more closed at the bottom compared with RSK1.
e potential binding modes of docking result for
compounds 1, 9 and 13 are shown in Figure 3. Similar to
the other well-known kinase inhibitors23, barbituric acid
inhibitors reported here bind to active conformation
of ATP-binding site of RSK2; Like Staurosporine in the
homology model, the barbituric acid core of compound
1 (Figure 3A) occupies the adenine binding pocket of the
enzyme and interacts with the kinase hinge region by two
H-bond interactions including amide moiety of barbituric
acid with the backbone oxygen atom of Asp 148 (2.76 Å)
and the backbone nitrogen atom of Leu 150 (2.61 Å), whilst
the extended hydrazone stretches along the hinge with the
terminal phenyl ring to a solvent exposed region. However,
comparing with compound 1, compounds 9 and 13 show
alternative conformations of the hydrazone (Figure 3B and
3C), which coincide with the report of similar compounds
as GSK3β inhibitors by Arnost et al.24. For compounds 9
and 13 the H-bonds interactions with the hinge region
are conserved but additional H-bond contacts are formed
with Asp154 by the para hydroxyl or morpholinyl, which
presumably, contribute to the improved the inhibitory
potency. As for the ortho- and meta-substituent analogues
such as 10 and 11, the deterioration of activities may also
result from the introduction to inappropriate positions
as well except for the electron effects, which hinders the
hinge H-bonds formation by steric clash and abolishes
the H-bond with Asp154 by an inappropriate distance
and dihedral angle.
Table 1. e structures of babituric acid aryl hydrone derivatives
and their inhibitory activities against RSK2.
Inhibition IC50
Compound
X
O
O
O
O
O
O
O
O
O
O
O
O
O
S
R1
H
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
R
H
(%)a
46.53
6.13
(μM)b
ND
ND
ND
ND
ND
ND
ND
ND
1.95
ND
ND
ND
5.11
2.19
9.23
ND
ND
ND
ND
ND
ND
ND
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
H
4-F
40.85
39.20
40.88
9.97
4-Cl
2-Cl
3-Cl
4-Br
30.01
37.34
87.05
45.14
11.80
42.15
84.35
71.37
51.44
0.00
4-I
4-OH
3-OH
2-OH
4-OCH3
4-morpholinyl
4-OH
S
4-morpholinyl
4-NO2
2-NO2
2-CN
O
O
O
O
O
O
O
1.21
15.49
46.77
21.04
48.88
24.4
4-COOH
4-CO2CH3
2-CONH2
4-piperidinyl
sulfonyl
conclusion
23
O
H
4-N,N-dimethyl
aminosulfonyl
42.02
ND
In summary, through structural optimization of the hit
compound 1 from our previous study, a series of barbi-
turic acid aryl hydrazone analogues were designed and
synthesized as RSK2 inhibitors. e SAR was analyzed to
highlight the pivotal role of the barbituric acid scaffold,
thus the assumed interaction mode was verified. Among
24
Ro31–8220
98.21
0.01
aInhibition rate was determined at the concentration of 10 μM
sample; bIC50 values were made if the inhibition rate at 10 μM was
larger than 50%. IC50 values are determined from the results of
at least three independent tests of RSK2 kinase activity. ND, Not
determined.
Journal of Enzyme Inhibition and Medicinal Chemistry

Barbituric acid derivatives as RSK2 inhibitors 751
Figure 2. Superimpose RSK1 NTD complexed with Staurosporine (blue) onto the modeled RSK2 NTD (golden). e hydrogen bond
interactions are shown with red dashed lines.
Figure 3. Docking results of compounds (A) 1, (B) 9, and (C) 13 against RSK2 NTD. e protein model and Staurosporine are colored in
golden, and the docked inhibitors are colored in green. e hydrogen bonds interactions between RSK2 NTD and inhibitors are shown
with red dashed lines.
them, four compounds exhibited inhibitory rates more
than 50% at 10 μM concentration, and the most potent
of which, compound 9 inhibited RSK2 with an IC50 value
of 1.95. e potential binding modes were explored to
further elucidate the SAR, which was also consistent
with the previously proposed interaction mode by a
ligand-similarity method. To our knowledge, although
hydrazone-containing compounds with different scaf-
folds were reported as kinase inhibitors, especially as
moderate RSK2 inhibitors25, this is the first report that
barbituric acid aryl hydrazone derivatives can act as
RSK2 inhibitors. ese inhibitors might serve as new lead
compounds for further development of therapeutics to
treat cancer, especially to suppress cell migration.
Declaration of interest
is work was supported by the Fundamental Research
Funds for the Central Universities, the National Natural
Science Foundation of China (grants 21173076,
© 2013 Informa UK, Ltd.

752 M. Xue et al.
12. Smith JA, Poteet-Smith CE, Malarkey K, Sturgill TW.
Identification of an extracellular signal-regulated kinase (ERK)
docking site in ribosomal S6 kinase, a sequence critical for
activation by ERK in vivo. J Biol Chem 1999;274:2893–2898.
13. Cho YY, Yao K, Pugliese A, Malakhova ML, Bode AM, Dong
Z. A regulatory mechanism for RSK2 NH(2)-terminal kinase
activity. Cancer Res 2009;69:4398–4406.
14. Nguyen TL. Targeting RSK: an overview of small molecule
inhibitors. Anticancer Agents Med Chem 2008;8:710–716.
15. Smith JA, Maloney DJ, Clark DE, Xu Y, Hecht SM, Lannigan DA.
Influence of rhamnose substituents on the potency of SL0101,
an inhibitor of the Ser/r kinase, RSK. Bioorg Med Chem
2006;14:6034–6042.
16. Smith JA, Poteet-Smith CE, Xu Y, Errington TM, Hecht SM,
Lannigan DA. Identification of the first specific inhibitor of
p90 ribosomal S6 kinase (RSK) reveals an unexpected role
for RSK in cancer cell proliferation. Cancer Res 2005;65:
1027–1034.
17. Sapkota GP, Cummings L, Newell FS, Armstrong C, Bain J,
Frodin M et al. BI-D1870 is a specific inhibitor of the p90 RSK
(ribosomal S6 kinase) isoforms in vitro and in vivo. Biochem J
2007;401:29–38.
81102375 and 81102420), the Special Fund for Major
State Basic Research Project (grant 2009CB918501), the
Shanghai Committee of Science and Technology (grants
11DZ2260600, 09dZ1975700 and 10431902600), the
Innovation Program of Shanghai Municipal Education
Commission (grant 10ZZ41), and the 863 Hi-Tech
Program of China (Grant 2012AA020308). H. L. is also
sponsored by Shanghai Rising-Star Program (grant
10QA1401800) and Program for New Century Excellent
Talents in University (grant NCET-10-0378). X. L. is also
sponsored by Research Fund for the Doctoral Program of
Higher Education (20110074120009).
references
1. Doehn U, Hauge C, Frank SR, Jensen CJ, Duda K, Nielsen JV et al.
RSK is a principal effector of the RAS-ERK pathway for eliciting a
coordinate promotile/invasive gene program and phenotype in
epithelial cells. Mol Cell 2009;35:511–522.
18. Cohen MS, Zhang C, Shokat KM, Taunton J. Structural
bioinformatics-based design of selective, irreversible kinase
inhibitors. Science 2005;308:1318–1321.
2. Anjum R, Blenis J. e RSK family of kinases: emerging roles in
cellular signalling. Nat Rev Mol Cell Biol 2008;9:747–758.
3. Carriere A, Ray H, Blenis J, Roux PP. e RSK factors of activating
the Ras/MAPK signaling cascade. Front Biosci 2008;13:4258–4275.
4. Cho YY, Yao K, Bode AM, Bergen HR 3rd, Madden BJ, Oh
SM et al. RSK2 mediates muscle cell differentiation through
regulation of NFAT3. J Biol Chem 2007;282:8380–8392.
5. Xing J, Ginty DD, Greenberg ME. Coupling of the RAS-MAPK
pathway to gene activation by RSK2, a growth factor-regulated
CREB kinase. Science 1996;273:959–963.
19. Alessi DR. e protein kinase C inhibitors Ro 318220 and GF
109203X are equally potent inhibitors of MAPKAP kinase-1beta
(Rsk-2) and p70 S6 kinase. FEBS Lett 1997;402:121–123.
20. Nguyen TL, Gussio R, Smith JA, Lannigan DA, Hecht SM,
Scudiero DA et al. Homology model of RSK2 N-terminal kinase
domain, structure-based identification of novel RSK2 inhibitors,
and preliminary common pharmacophore. Bioorg Med Chem
2006;14:6097–6105.
6. Yang X, Matsuda K, Bialek P, Jacquot S, Masuoka HC, Schinke
T et al. ATF4 is a substrate of RSK2 and an essential regulator of
osteoblast biology; implication for Coffin-Lowry Syndrome. Cell
2004;117:387–398.
21. Lu W, Liu X, Cao X, Xue M, Liu K, Zhao Z et al. SHAFTS: a hybrid
approach for 3D molecular similarity calculation. 2. Prospective
case study in the discovery of diverse p90 ribosomal S6 protein
kinase 2 inhibitors to suppress cell migration. J Med Chem
2011;54:3564–3574.
22. Li S, Zhou Y, Lu W, Zhong Y, Song W, Liu K et al. Identification
of inhibitors against p90 ribosomal S6 kinase 2 (RSK2) through
structure-based virtual screening with the inhibitor-constrained
refined homology model. J Chem Inf Model 2011;51:2939–2947.
23. Noble ME, Endicott JA, Johnson LN. Protein kinase
inhibitors: insights into drug design from structure. Science
2004;303:1800–1805.
24. Arnost M, Pierce A, ter Haar E, Lauffer D, Madden J, Tanner K
et al. 3-Aryl-4-(arylhydrazono)-1H-pyrazol-5-ones: Highly
ligand efficient and potent inhibitors of GSK3beta. Bioorg Med
Chem Lett 2010;20:1661–1664.
25. Andreani A, Granaiola M, Leoni A, Locatelli A, Morigi R,
Rambaldi M et al. Imidazo[2,1-b]thiazole guanylhydrazones as
RSK2 inhibitors. Eur J Med Chem 2011;46:4311–4323.e first
three authors contributed equally to this work.
7. Peng C, Cho YY, Zhu F, Xu YM, Wen W, Ma WY et al. RSK2
mediates NF-{kappa}B activity through the phosphorylation of
IkappaBalphaintheTNF-R1pathway. FASEBJ2010;24:3490–3499.
8. Cho YY, Yao K, Kim HG, Kang BS, Zheng D, Bode AM et al.
Ribosomal S6 kinase 2 is a key regulator in tumor promoter
induced cell transformation. Cancer Res 2007;67:8104–8112.
9. Jones SW, Erikson E, Blenis J, Maller JL, Erikson RL. A Xenopus
ribosomal protein S6 kinase has two apparent kinase domains
that are each similar to distinct protein kinases. Proc Natl Acad
Sci USA 1988;85:3377–3381.
10. Kang S, Elf S, Lythgoe K, Hitosugi T, Taunton J, Zhou W et al. p90
ribosomal S6 kinase 2 promotes invasion and metastasis of
human head and neck squamous cell carcinoma cells. J Clin
Invest 2010;120:1165–1177.
11. Kang S, Chen J. Targeting RSK2 in human malignancies. Expert
Opin er Targets 2011;15:11–20.
Journal of Enzyme Inhibition and Medicinal Chemistry