Synthesis and structure of the β-carboline derivatives and their binding intensity with cyclin-dependent kinase 2

Article abstract of DOI:10.1248/cpb.60.435

Series of 3-substituted of 6-aminosulfonyl-β-carbolines were designed and synthesized. In addition, the binding mode of these β-carboline derivatives with cyclin-dependent kinase 2 (CDK2) was studied by means of fluorescence measurements and molecular doc

Full text of DOI:10.1248/cpb.60.435

April 2012
Regular Article
Chem. Pharm. Bull. 60(4) 435–441 (2012)
435
Synthesis and Structure of the β-Carboline Derivatives and Their
Binding Intensity with Cyclin-Dependent Kinase 2
Yue Wang,a,c Qiaomei Jin,a Guowu Lin,b Taotao Yang,a Zhanwei Wang,a Yi Lu,a Yimin Tang,a
,a
,a,c
Lifang Liu,* and Tao Lu*
c
a State Key Laboratory of Natural Medicines, China Pharmaceutical University; Laboratory of Molecular
Design and Drug Discovery, College of Basic Science, China Pharmaceutical University; Nanjing 210009, P.R.
b
China: and Department of Structural Biology, University of Pittsburgh School of Medicine; Pittsburgh, PA 15260,
U.S.A. Received August 18, 2011; accepted December 11, 2011; published online January 25, 2012
Series of 3-substituted of 6-aminosulfonyl-β-carbolines were designed and synthesized. In addition, the
binding mode of these β-carboline derivatives with cyclin-dependent kinase 2 (CDK2) was studied by means
of fluorescence measurements and molecular docking calculation. The results showed that replacement of
3-cyclohexylmethoxy group will increase the hydrophobic binding interaction with the deep hydrophobic
pocket of CDK2 correlate to the higher binding intensity.
Key words cyclin-dependent kinase 2; β-carboline; synthesis; molecular docking; fluorescence
It is well known that the phases of the cell cycle are driven
by cyclin-dependent kinases (CDKs). Upon complexation with
Experimental
Synthesis The target compounds were prepared using the
its activating proteins, cyclin E or cyclin A, cyclin-dependent reaction sequence. All the chemical structures of the synthe-
kinase 2 (CDK2) modulates the activity of many cellular sub- sized compounds were confirmed by spectroscopic methods,
strates via phosphorylation on serine (Ser) and/or threonine and exact stereostructure of compound 5 has been determined
(Thr) residues.1–4) Abnormal CDK control of the cell cycle has by X-ray crystal structure analysis.
been strongly linked to the molecular pathology of cancer.5)
Materials Unless otherwise specified, reagents were
The importance of CDK2 for cell cycle progression has led to purchased from commercial suppliers and used without fur-
an active pursuit of small molecule inhibitors of this enzyme ther purification. CDK2/cyclin A was purchased from Carna
as a possible treatment against cancer and other hyper-pro- Biosciencens Inc. and used without purification. All other
liferative disorders.6–9) All CDK inhibitors, identified so far, chemicals were of analytical grade. Doubly distilled water was
function by competing with ATP for binding to the catalytic used throughout.
site. Actually several CDK inhibitors have entered clinical
Apparatus Reaction progress was monitored using ana-
evaluation for the treatment of cancer, including flavopiridol, lytical thin layer chromatography (TLC) on percolated Merck
amino-thiazole compound.10) In our program to develop CDK2 silica gel Kiesegel 60 F254 plates, and the spots were detected
inhibitors, we recently investigate the β-carboline alkaloids under UV light (254nm). Melting points were determined
containing a planar tricyclic system as a large group of natu- with a digital melting point apparatus and are reported un-
rally-occurring and synthetic alkaloids,11–14) which has a broad corrected. 1H-NMR spectra was recorded at 300MHz on a
spectrum of biochemical effects and pharmaceutical proper- Bruker ARX 300 spectrometers. IR spectra were measured
ties. These compounds have been shown to intercalate into on a Jasco FT/IR-430 spectrophotometer. Mass spectra were
DNA, to inhibit CDK, topisomerase and monoamine oxidase, recorded on an a Quattro microMS Micromass UK mass
and to interact with benzodiazepine receptors (BZ), 5-hydroxy spectrometer, and were recorded on an electrospray ionization
serotonin receptors (5-HT), dopamine (DA) and imidazoline mass spectrometer as the value m/z. The X-ray measurements
receptors.15–23)
were made on a Nonius Cad4 diffractometer with a graphite
Inspired by these results, our research group recently fo- monochromatised MoKα radiation (λ=0.71069Å) using ω scan
cused on first of all, using a structure-guided strategy based mode. And all fluorescence spectra were recorded on RF-
on CDK2 as appropriate means to generate potent CDK2 5301PC Spectrofluorimeter (Shimadzu, Japan) equipped with
inhibitors; then to synthesize disubstituted β-carbolines and 0.2cm quartz cells, the widths of both the excitation slit and
to complete their biological evaluation study. As a part of the emission slit were set to 5.0nm.
our systematic work, in this paper, structure modification of
β-carboline based on increasing its hydrophobic nature has
been adopted. Herein we reported the design and synthesis of
series of 3-substituted of 6-aminosulfonyl-β-carbolines. Series
of 3-substituted compounds with different hydrophobic groups
(Chart 1) were synthesized and one of their structures was elu-
cidated with X-ray crystal analysis. The new derivatives were
prepared following the reaction sequences depicted in Charts
2–4. Their binding intensity with CDK2 was measured by
fluorescence spectroscopy.
1-Carbamoyl-6-aminosulfonyl-β-carboline (1) For 1,
Chart 1. Chemical Structures of the Title Compounds
*To whom correspondence should be addressed. e-mail: liulifang69@126.com; lut163@163.com
© 2012 The Pharmaceutical Society of Japan

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Vol. 60, No. 4
Chart 2. Synthesis of 6-Substituted β-Carboline Compounds 1 and 5
Chart 3. Synthesis of 3,6-Substituted β-Carboline Compound 2
tryptamine was chosen as the starting material for the study, cm−1: 3495, 3458, 3381, 3352, 3259, 1674. MS m/z 291 (M+1)+.
which was converted into 1,2,3,4-tetrahydro-β-carboline-1- Anal. Calcd for C12H10N4O3S: C, 49.65; H, 3.47; N, 19.30;
carboxylic acid (1a) according to the Pictet–Spengler reac- Found: C, 49.61; H, 3.53; N, 19.26.
tion in good yield.24,25) And the treatment of 1a with excess
6-Aminosulfonyl-1-carbamoyl-3-(1H-benzo[d]imidazol-
thionyl chloride in anhydrous EtOH at reflux for 6h gave 2-yl)-β-carboline (2) Compound 2 was prepared following
ethyl 1,2,3,4-tetrahydro-β-carboline-1-carboxylate (1b) as the reaction sequences depicted in Chart 3. ( )-Tryptophan
a white solid.26) Then, ethyl β-carboline-1-carboxylate (1c) was chosen as the starting material for the study, which
was carried out by dehydrogenation with KMnO4 in N,N- was converted into 1,2,3,4-tetrahydro-β-carboline-3-carboxylic
dimethylformamide (DMF) at room temperature for 24h.27) acid (2a) according to the Pictet–Spengler reaction in good
1-Amino-β-carboline (1d) was prepared by the aminolysis of yield.24,25) And the treatment of 2a with excess thionyl
(1c).28) The key intermedium (1d) was converted into the tar- chloride in anhydrous EtOH for 6h at reflux gave ethyl
get molecule (1) in 45–80% yield through chlorosulfonation in 1,2,3,4-tetrahydro-β-carboline-3-carboxylate (2b) as a white
dry chlorosulfonic acid and subsequent aminolysis with amine solid in 88% yield.27) Then, ethyl β-carboline-3-carboxylate
solution28) (Chart 2).
(2c) was dehydrogenated with KMnO4 in DMF at room tem-
Yield 48%. mp 217–219 C (MeOH). 1H-NMR (300MHz, perature for 24h.27) The β-carboline-3-carboxylic acid (2d)
DMSO-d6) δ: 12.03 (1H, s, indole), 8.78 (1H, s, ArH), 8.50 was obtained through both the hydrolysis of 2c in a solution
(1H, m, –CONH2), 8.30 (1H, s, ArH), 8.02 (1H, d, J=7.7Hz, of EtOH/NaOH at reflux and followed by acidation with hy-
ArH), 7.91 (1H, d, J=7.7Hz, ArH), 7.80 (1H, s, ArH), 7.27 drochloric acid.29) One of the most important procedures in
(2H, s, –SO2NH2). 13C-NMR (300MHz, DMSO-d6) δ: 167.6 our experiment was to synthesize 3-(1H-benzo[d]imidazol-
(s), 142.8 (s), 137.5 (s), 135.7 (d), 135.2 (s), 133.4 (s), 130.4 (s), 2-yl)-β-carboline (2e), which was achieved by the condensa-
126.1 (d), 120.2 (d), 119.2 (s), 118.3 (d), 113.2 (d). IR (KBr) tion of o-phenylenediamine with β-carboline-3-carboxylic
°

April 2012
437
Chart 4. Synthesis of 3,6-Substituted β-Carboline Compounds 3 and 4
°
acid in the presence of polyphosphoric acid (PPA) in 180 C carboline (4) 3-Amino-β-carboline (3d) was prepared
for 6h in 45% yield.30) Subsequently, we treated 2e with by a Curtius-Rearrangement reaction starting with ethyl
FeSO4 and H2O2 in different reagents (including formamide β-carboline-3-carboxylate (2c). Thus, refluxing of 2c with
°
or N-methylformamide), under mild conditions (10–15 C) via 85% hydrazine hydrate in MeOH gave the hydrazide 3b
Minisci-Reaction, giving the expected products 1-carbamoyl- which, after treatment with sodium nitrite, yielded the azide
3-(1H-benzo[d]imidazol-2-yl)-β-carboline (2f) in 55% yield 3c.27) When 3c was refluxed for several minutes in HCl
(Chart 3).31–36) The title compound 2 was achieved from 2f solution, rearrangement to the amine 3d occurred in good
by the method analogus with 1f prepared from 1d mentioned yield.27) Treatment of 3d with methanol (or cyclohexylmetha-
above.
nol) and H2SO4 in the presence of isoamyl nitrite provided
Yield 60%. mp 321–323 C (MeOH). 1H-NMR (300MHz, 4e.27) Subsequently, we treated 4e with FeSO4 and H2O2 in
DMSO-d6) δ: 13.12 (1H, s, benzimidazole), 12.27 (1H, s, the presence of H2SO4 in formamide under mild conditions
°
°
indole), 9.34 (1H, s, ArH), 9.06 (1H, s, ArH), 8.95 (1H, d, (10–15 C) via Minisci-Reaction, giving the expected products
J=7.7Hz, ArH), 8.07 (1H, d, J=7.7Hz, ArH), 7.59–7.74 (2H, 1-carbamoyl-3-cyclohexylmethoxy-β-carboline (4f).37) The key
m, –CONH2), 7.30 (2H, s, –SO2NH2), 7.29–7.24 (4H, m, ArH). intermediate 4f was converted into the target molecules 4 in
13C-NMR (300MHz, DMSO-d6) δ: 167.2 (s), 151.4 (s), 144.2 45–80% yields through both the chlorosulfonylation in dry
(s), 143.4 (s), 137.1 (s), 136.2 (s), 135.4 (s), 134.6 (s), 132.3 (s), chlorosulfonic acid and subsequent aminolysis with amine
131.7 (d), 126.5 (d), 122.8 (d), 121.7 (d), 120.6 (d), 119.5 (s), solution (Chart 4).
1
119.1 (d), 115.6 (d), 113.5 (d), 111.3 (s). IR (KBr) cm−1: 3415,
Yield 65%. mp 304–306 C (MeOH). H-NMR (300MHz,
°
3280, 3124, 3111, 1725. MS m/z: 408 (M+1)+. Anal. Calcd for DMSO-d6) δ: 11.64 (1H, s, indole), 8.70 (1H, s, ArH), 8.07
C19H14N6O3S: C, 56.15; H, 3.47; N, 20.68; Found: C, 56.18; H, (1H, s, ArH), 7.97 (1H, d, J=7.9Hz, ArH), 7.89 (1H, m, ArH),
3.36; N, 20.70.
7.69–7.81 (2H, m, –CONH2), 7.22 (2H, s, –SO2NH2), 4.23 (2H,
6-Aminosulfonyl-1-carbamoyl-3-methoxy-β-carboline (3) d, J=5.2Hz, –CH2O), 1.0–2.0 (11H, m, cyclohexyl). 13C-NMR
For compound 3, the same method with compound 4 as fol- (300MHz, DMSO-d6) δ: 167.3 (s), 155.6 (s), 144.4 (s), 135.1
low is used, only the cyclohexanol group was introduced from (s), 134.9 (s), 132.1 (d), 128.2 (s), 126.6 (d), 120.6 (d), 118.8 (s),
intermedium 3d.27)
112.8 (d), 104.1 (s), 71.3 (t), 37.2 (d), 29.4 (t), 26.1 (t), 25.3 (t).
Yield 57%. mp 165–167 C (MeOH). 1H-NMR (300MHz, IR (KBr) cm−1: 3465, 3346, 3319, 1672. MS m/z: 402 (M+1)+.
DMSO-d6) δ: 11.68 (1H, s, indole), 8.72 (1H, s, ArH), 8.12 (1H, Anal. Calcd for C19H22N4O4S: C, 56.70; H, 5.51; N, 13.92;
s, ArH), 7.96 (1H, d, J=7.8Hz, ArH), 7.8 (2H, m, –CONH2), Found: C, 56.67; H, 5.55; N, 13.90.
°
7.23 (2H, s, –SO2NH2), 7.1 (1H, d, J=7.8Hz, ArH), 4.02 (3H,
6-Aminosulfonyl-1-N-methylcarbamoyl-β-carboline (5)
s, –OCH3). 13C-NMR (300MHz, DMSO-d6) δ: 167.2 (s), 155.6 Compound 5 was obtained by the similar process as com-
(s), 144.5 (s), 135.2 (s), 134.9 (s), 132.2 (d), 128.1 (s), 126.6 pound 1 mentioned above via 5d and 5e37) (Chart 2).
1
°
(d), 120.7 (d), 118.8 (s), 112.8 (d), 104.1 (s), 53.8 (q). IR (KBr)
Yield 68%. mp 256–257 C (MeOH). H-NMR (300MHz,
cm−1: 3470, 3350, 3304, 1672. MS m/z: 321 (M+1)+. Anal. DMSO-d6) δ: 12.09 (1H, s, indole), 8.90 (1H, d, J=4.8Hz,
Calcd for C13H12N4O4S: C, 48.74; H, 3.78; N, 17.49; Found: C, ArH), 8.78 (1H, s, ArH), 8.50 (1H, m, ArH), 8.00 (1H,
48.70; H, 3.84; N, 17.51.
m, ArH), 7.90 (1H, d, J=8.4Hz, –CONH–), 7.20 (2H, s,
6-Aminosulfonyl-1-carbamoyl-3-cyclohexylmethoxy-β- –SO2NH–), 2.90 (3H, d, J=4.8Hz, –CONCH3). 13C-NMR

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Vol. 60, No. 4
Table 1. Crystal and Experimental Data of Compound 5
(300MHz, DMSO-d6) δ: 167.2 (s), 142.8 (s), 137.5 (d), 135.7
(s), 135.0 (s), 133.4 (s), 130.4 (s), 126.1 (d), 120.2 (d), 119.2
(s), 118.3 (d), 113.2 (d). 30.5 (q). IR (KBr) cm−1: 3495, 3458,
3381, 3352, 3259, 1674. MS m/z: 305 (M+1)+. Anal. Calcd for
C13H12N4O3S: C, 51.31; H, 3.97; N, 18.41; Found: C, 51.36; H,
3.89; N, 18.40.
Empirical formula
Formula weight
Wavelength
Crystal system
Space group
a (Å)
C13H12N4O3S
304.33
0.71073
Monoclinic
P21/C
Fluorescence Measurements β-Carbolines were obtained
by the methods mentioned in the text. Each solution of the
compound was prepared in methanol, respectively. On the
basis of the results of the biological activity by calculation,
in this study, four compounds with the different substituents
at 3-position of β-carbolines (1–4) were investigated. CDK2/
cycA2 solution which shows intrinsic fluorescence emission
with the excitation nearby 280nm was prepared in pH 7.5
Tris–HCl buffer solution (containing 50nM Tris–HCl, 150nM
NaCl, pH=7.5). Titrations were done manually by using
micro-injector. The fluorescence spectra were then measured
(excitation at 282nm and emission at 290–550nm) at room
temperature. All measurements were carried out keeping
the concentration of CDK2 fixed at 1.0×10−7 M and that of
β-carbolines varying from 1.25×10−10 M to 2.0×10−7 M for
all systems. Five minutes later after adding the compounds,
record the fluorescence intensity of the mixture, respectively.
All the measurements were carried out at room temperature
11.455 (2)
12.538 (3)
9.2690 (19)
100.04 (3)
1310.9 (5)
4
b (Å)
c (Å)
°
β ( )
V (Å)
Z
Crystal size (mm)
Shape
0.2×0.2×0.1
Block
Colour
D calc (g/cm−3
Yellow
)
1.542
Reflections collected/unique
Unique reflections
Absorption coefficient (mm−1
F(000)
2494/2372
1815
)
0.264
632
R (F2>2σ (F2))
0.0456
wR (F2)
0.1288
Goodness of fit
0.971
No. of variables
197
Program system
SHELXL 97
Direct method
°
(25 0.5 C).
Structure determination
X-Ray Crystallography A suitable pale brown crystal of
the compound 5 (0.20mm×0.20mm×0.20mm) crystallized
from methanol/DMF was selected and mounted on the top of
a glass fiber. The data was collected by a Nonius cad4 dif-
fractometer equipped with a graphite-monochromatized MoKα
radiation at 293K. The structure was solved and refined with
the SHELXS-97 and SHELXL-97 programs. The structure
was refined on F2 by successive full-matrix least-squares
techniques. All hydrogen atoms were located in a difference
Fourier map and their geometry was idealized, and refined
by a riding model. (CCDC 740985 contains the supplemen-
tary crystallographic data of compound 5. These data can be
obtained free of charge from the Cambridge Crystallographic
Data Centre via http://www.ccdc.cam.ac.uk/data_request/
cif.)38)
°
Table 2. Selected Bond Distances (Å), Angles ( ), and Torsion Angles
°
( ) for 5
Bond lengths
Bond angles
N1–C2 1.325 (3)
N3–C12–C11 129.0
O3–S–O2 118.94 (13)
(3)
O1–C2 1.235 (3)
C9–S 1.772 (3)
O2–S 1.437 (2)
C6–C7–C8 134.2 (2)
C5–C6–C7 135.6 (3)
C1–N1–C2 122.1 (3)
C8–C9–S 119.5 (2)
O3–S–N4 107.55 (14)
C13–N13–C12 109.1
(2)
O3–S 1.426 (2)
N4–S 1.618 (3)
N3–C13 1.369 (3)
N2–C3–C2 118.9 (2)
C3–C2–O1 119.6 (2)
N1–C2–O1 123.7 (3)
O3–S–C9 107.87 (13)
C3–N2–C4 118.9 (2)
N4–S–O2 106.01 (14)
Results and Discussion
Crystal Structure of Compound 5 The crystallographic
data of 5 are summarized in Table 1. The basic scaffold of
title compound 5 is nearly planar, with a largest deviation of
0.0815(3)Å from the carboline plane by N3 atom. The selected
bond lengths, angles and torsion angles are given in Table 2.
The ORTEP drawings of the compound 5 are illustrated in
Fig. 1.
Fluorescence Quenching of CDK2 The binding intensity
of CDK2 and β-carbolines were evaluated by the measurement
of intrinsic fluorescence intensity of CDK2 before and after
addition of compounds. The effect of β-carbolines on CDK2
fluorescence intensity was shown in Fig. 2. When the excita-
tion was set at 282nm, the intrinsic fluorescence of CDK2 at
332nm was observed, whereas β-carbolines had no intrinsic
fluorescence. With the addition of compound 1 into the CDK2
Fig. 1. ORTEP Drawing of the X-Ray Crystal Structure of Compound
5
Displacement ellipsoids were drawn at 50% probability level.
solution, a remarkable decrease of fluorescence intensity of ing tended to equilibrium. And the other three compounds
CDK2 was observed, which indicated that 1 can interact with exhibited the similar results. The maximum wavelength of
CDK2. Furthermore, with the concentration of compound 1 intrinsic fluorescence of four compounds from 1, 2, 3 to 4 was
increasing to 10−7 M in the mixture, the fluorescence quench- 367nm, 424nm, 424nm and 434nm, respectively. As compari-

April 2012
439
Fig. 2. The Effect of 1 on CDK2 Fluorescence Intensity
son, the red shift of maximum wavelength of compounds 2, 3
and 4 mainly owed to their structure with higher conjugacy
degree when inducing ether group in compound 3 and 4, and
the benzimidazoyl group in compound 2 at 3-position.
The Comparison of the Four Compounds 1–4
Fluorescence quenching is an important method to study the Fig. 3. Comparison of Fluorescence Quenching of Compounds 1–4
interaction of small molecule with protein because of its accu-
racy, sensitivity, rapidity, and convenience of usage. It can re-
veal accessibility of quenchers to protein fluorophores, help to
understand the binding mechanisms between small molecules
and protein and provides clues to the nature of the binding
phenomenon.39–41) In our study, the quantitative analysis of
the binding intensity of β-carbolines to CDK2 was carried out
using the fluorescence quenching at 332nm at various concen-
trations of the former (Fig. 3). The fluorescence intensity of
system gradually decreased with the increase of β-carbolines
concentration and with the further addition of β-carbolines,
the fluorescence intensity of system decreased tardily in each
Table 3. Docking Scour for Each Compound in the Test Set Mapped
with Active Site of CDK2
Docking value (kcal/
Compound No.
C log P
mol)
4
1
3
2
−8.507
−8.08
−6.36
−6.327
3.53
0.46
0.88
2.34
titration curve which indicates the beginning of saturation of Score, which was composed by electrostatic, hydrogen bond,
the CDK2 binding site. In Fig. 3, compound 4 show the stron- hydrophobic, and van der Waals etc., was applied as the scor-
gest fluorescence quenching ability.
ing function to prioritize the test set compounds. Then the
The ability of fluorescence quenching is: 4>1>3>2. For the compounds were all flexibly docked into the rigid binding
interaction between small molecule and protein mainly oc- pocket of the target protein.
curs in the small molecule and amino acid residue within the
The docking results suggested that all compounds make the
hydrophobic pocket, herein, the 3-cyclohexylmethoxy group same hydrogen binding interaction with LEU 83, GLU 81 and
in 4 may be extending to such hydrophobic field in the CDK2 ASP 86, provided by the necessary group N–H, 1-carbamoyl
structure. Although the benzimidazoyl group in compound 2 group and 6-aminosulfonyl group, respectively, indicated by
at 3-position is hydrophobic group, 2 exhibited lower ability dashed line in Fig. 4. Moreover, replacement of 3-cyclohexyl-
of fluorescence quenching compared with compound 4. The methoxy group with the highest ClogP value will increase the
existence of benzimidazoyl group decreased the flexibility of hydrophobic binding interaction with the deep hydrophobic
branched chain, which lowered the ability to bind with the pocket correlate to the higher docking scours (Table 3). From
hydrophobic field. Owing to the absence of hydrophobic group the docking scour in Table 3, the intensity of interaction be-
of compound 1 at 3-position, the interaction between 1 and tween four compounds and CDK2 was: 4>1>3>2. The result
CDK2 was weaker than 4. Otherwise, the methoxyl group in was consistent with our research result using fluorescence
compound 3 had no influence in proving the intensity of fluo- spectroscopy which induced the binding mode to be reason-
rescence quenching.
able.
Molecular Docking Studies and Binding Conformation
Molecular docking acted as an additional tool for pharmaco-
phore-based virtual screening, the concurrent use of which
Conclusion
In conclusion, the desired 3-substituted of 6-aminosulfonyl-
is believed to make the discovery of potent CDK2 kinase β-carbolines were prepared and their structures were char-
inhibitors more efficient. All dock runs were conducted using acterized. From the data of binding intensity with CDK2 by
Glide with extra precision. The performance of it on CDK2 means of fluorescence measurements and molecular docking
inhibitors was evaluated by re-docking co-crystallized ligand, studies, compound 4 with 3-cyclohexylmethoxy group showed
showing a good reproducibility. The crystal complex structure the highest binding ability. To assess the potentials of these
of inhibitor CDK2 with a resolution of 1.95 was prepared new compounds as possible CDK2 inhibitors, further antitu-
using Protein Preparation Wizard workflow. The compound mor evaluation studies are needed. The results obtained from
set to be docked was prepared with LigPre module. Glide this study can be used as guidelines for further development.

440
Vol. 60, No. 4
Fig. 4. The Docking Mode of Four Compounds with CDK2
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Acknowledgments We are grateful to the National
Natural Science Foundation of China (Program No.
30973609), the Fundamental Research Funds for the Central
Universities (Program No. 2J10004) and the Natural Science
Foundation of Jiangsu Province (Program No. BK2009303) for
financial support. This study was also supported by the Project
Program of State Key Laboratory of Natural Medicines, China
Pharmaceutical University (Program No. JKGP201105).
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