Substituted aminobenzimidazole pyrimidines as cyclin-dependent kinase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2005.02.076

A series of aminobenzimidazole-substituted pyrimidines were synthesized and evaluated for biochemical activity against CDK1. A high-speed parallel synthesis approach enabled the identification of a potent lead series having improved potency in the CDK1 assay (IC₅₀ < 10 nM). Cell cycle analysis showed that the compounds induced a G2/M block. Docking studies were carried out with a CDK1 homology model, and provide a rationale for the observed activities.

Full text of DOI:10.1016/j.bmcl.2005.02.076

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1973–1977
Substituted aminobenzimidazole pyrimidines as
cyclin-dependent kinase inhibitors
Sharad Verma,a,* Dhanapalan Nagarathnam,a Jianxing Shao,a Lei Zhang,a Jin Zhao,a
Yamin Wang,a Tindy Li,a Eric Mull,a Istvan Enyedy,a Chunguang Wang,a Qingming Zhu,a
Martha Altieri,b Jerold Jordan,b Thu-Thi-Anh Dangb and Sanjeeva Reddyb
aDepartment of Chemistry Research, Bayer Research Center, 400 Morgan Lane, West Haven, CT 06516, USA
bDepartment of Cancer Research, Bayer Research Center, 400 Morgan Lane, West Haven, CT 06516, USA
Received 5 January 2005; revised 18 February 2005; accepted 24 February 2005
Abstract—A series of aminobenzimidazole-substituted pyrimidines were synthesized and evaluated for biochemical activity against
CDK1. A high-speed parallel synthesis approach enabled the identification of a potent lead series having improved potency in the
CDK1 assay (IC50 < 10 nM). Cell cycle analysis showed that the compounds induced a G2/M block. Docking studies were carried
out with a CDK1 homology model, and provide a rationale for the observed activities.
Ó 2005 Elsevier Ltd. All rights reserved.
H
N
The cyclin-dependent kinases (CDKs) represent a family
of serine–threonine protein kinases, which control cell
cycle progression in proliferating eukaryotic cells.1
CDK activity is dependent on the presence of cyclin
partners, whose levels are regulated in a sequential man-
ner to ensure that the phases of the cell cycle proceed in
the correct order.2 Perturbations in CDK function and
consequent loss of cell cycle regulation have been di-
rectly linked to the molecular pathology of cancer.3 As
a result of the strong association between the CDKs
and cancer biology, efforts have been aimed towards
the development of CDK inhibitors for the treatment
of cancer.4
X
HN
H
N
4
R
1
N3
5
6
X
2
N
N
H
1
X = N
X = C
a) R = CF3, IC50 = 10 nM e) R = Br, IC50 > 1,000 nM
b) R = Br, IC50 = 14 nM
c) R = F, IC50 = 430 nM
f) R = CF3, IC50 > 1,000 nM
g) R = F, IC50 > 1,000 nM
d) R = H, IC50 > 1,000 nM h) R = H, IC50 > 1,000 nM
Figure 1. Aminobenzimidazole substituted pyrimidine based CDK1
inhibitors.
The pyrimidine scaffold is found in several kinase inhib-
itor classes,5 and the use of 2,4,5-trisubstituted pyrimi-
dines as inhibitors of CDKs reported.6 In this context,
the design, synthesis, and testing of a number of kinase
biased analogs was executed, and the bis-2,4-amino-
benzimidazole pyrimidines 1a and 1b were identified as
potent inhibitors of CDK1 (Fig. 1). It quickly became
apparent that the presence of an aminobenzimidazole
group at either the C-2 and/or C-4 positions is critical
for CDK1 inhibitory activity. For example, replacement
of the aminobenzimidazoles at C-2 and C-4 with an
aminoindole results in IC50 > 1000 nM. Furthermore,
it was observed that trifluoromethyl or bromo groups
were optimal as C-5 substituents, while replacement
with hydrogen or fluorine resulted in 200- and 10-fold
decreases in potency, respectively.
CDK1 inhibition is further exemplified in a cell func-
tional assay by compound 1a, which displays a G2/M
cell cycle arrest in proliferating cells (Fig. 2). When
MDA-MB-231 cells were treated with 3 lM of com-
pound 1a for 24 h, the cells showed a complete block
in the G2/M phase of the cell cycle.4c
*
Corresponding author. Tel.: +1 203 812 6574; fax: +1 203 812
3655; e-mail: sharad.verma.b@bayer.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.02.076

1974
S. Verma et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1973–1977
A). Likewise, reversal of these steps as shown in route
B furnished analogs having an aminobenzimidazole
group attached at C-2, as in compound 6. It should be
noted that preparation of the lead analogs 1a and 1b
could be accomplished in one step by heating of chloro-
pyrimidine 2 with 2 equiv of 5-aminobenzimidazole in n-
BuOH at 110 °C for 12 h. For the preparation of
analogs containing an aminobenzimidazole group at
the C-2 position wherein X = CF3, route C was utilized.
This method first involved conversion of 5-aminobenz-
imidazole to its corresponding bis–Boc derivative 8,
obtained as a mixture of regioisomers with respect to
N-Boc substitution on the imidazole ring. Subsequent
coupling of compound 8 with dichloropyrimidine 2
(X = 5-CF3) occurs regioselectively at the C-2 position
to afford chloropyrimidine 9.7 Treatment of chloropyr-
imidine 9 with arylamines as above followed by depro-
tection furnished analogs of type 6. Likewise, the
N-linker could be replaced with either O or S by treat-
ment of chloropyrimidine 9 with the appropriate phe-
nols or thiols.
Figure 2. G2/M phase cell cycle arrest of 1a in MDA-MB-231 cells.
By virtue of their observed activity and potential for effi-
cient analoging, compounds 1a and 1b were selected as
leads for further optimization. Preparation of substi-
tuted pyrimidines was generally straightforward, as de-
scribed in Figure 3. Modification of either the C-2 or
C-4 positions of the lead structure core 1 entailed the
use of a two-step synthetic protocol. Treatment of com-
mercially available dichloropyrimidines of type 2 with 5-
aminobenzimidazole and sodium carbonate in ethanol
at room temperature for 16 h affords C-4 substituted
pyrimidines of type 3.7 Subsequent coupling of arylam-
ines at the C-2 position was then accomplished by heat-
First, we replaced the aminobenzimidazole group at the
C-2 position with anilines while retaining the amino-
benzimidazole group at the C-4 position of the pyrimi-
dine ring, and the results are summarized in Table 1.
Analogs containing a substituted aniline at the C-2 posi-
tion of the pyrimidine displayed good activity in the
CDK1 biochemical assay, as IC50 values <50 nM were
routinely observed.8 Overall, substitution at the 3- or
4-position of the aniline ring is preferred, as placement
of a substituent at the 2-position results in a loss in po-
tency (e.g., 15). The greatest potency was exhibited by
analogs bearing anilines containing a sulfonamide group
such as compound 10, which displays a 100-fold
improvement in potency compared to lead compound
1a. This result is consistent with docking studies, as
shown in Figure 4.9 In our binding model, the C-2
NH group comes in contact with the hinge region at
Leu83 through two hydrogen bonds, an interaction that
would be determined to be crucial for activity.10 The en-
hanced activity displayed by compound 10 results from
the interaction of the sulfonamide group with Lys89,
ing chloropyrimidine
3
with the corresponding
arylamine in n-BuOH under acidic conditions at
110 °C for 12 h, to afford compounds of type 4 (route
H
H
N
N
N
HN
Route A
(i)
N
HN
X
N
(ii)
X
X
N
Ar
N
N
R
N
3
Cl
Cl
R = H, CH3
4
Route B
(iii)
Ar
N
Ar
N
Cl
Y
Y
H
N
X
X
(iv)
N
N
Cl
N
N
N
N
H
2
5
6
X = Br, CF3
Y= N, cond. (vii)
Y= O,S, cond (viii)
Cl
N
X
Table 1. Activities for C-2 modified analogs
N
Route C
(vi)
Boc
N
N
N
N
Boc
N
H
HN
N
9
X
N
Z
H
N
Y
N
(v)
Boc
Compound
X
Y
Z
CDK1 IC50 (nM)
Boc
N
N
N
H
H2N
1a
10
11
12
13
14
15
16
17
18
CF3 NH
CF3 NH
CF3 NH
CF3 NH
3,4-NCNH
4-SO2NH2
4-Br
10
0.1
8
7
11
22
Figure 3. Reagents and conditions: (i) 5-aminobenzimidazole, Na2CO3,
EtOH, rt, 16 h, 68%; (ii) Ar–NHR, n-BuOH, cat. HCl, 110 °C, 12 h, 20–
60%; (iii) Ar–NHR, Na2CO3, EtOH, rt, 16 h, 85–99%; (iv) 5-amino-
benzimidazole, n-BuOH, 110 °C, 12 h, 20–60%; (v) (Boc)2O, THF,
Et3N, rt, 16 h, 62%; (vi) n-BuLi, THF, À78 °C to rt, compound 8, 24 h,
55%; (vii) (1) Ar–NH2, n-BuOH, cat. HCl, 110 °C, 12 h, (2) TFA, 45–
55% overall; (viii) (1) K2CO3, CH3CN, Ar–OH or Ar–SH, (2) TFA, 35–
55% overall.
3-Cl
CF3 NCH3 3-Cl
Br
>1000
9
NH
3-Cl
2-Cl
CF3 NH
CF3 NH
CF3 NH
>1000
15
15
3-CH3
4-CF3
Br
NH
4-O-(3-Pyridyl) 248

S. Verma et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1973–1977
Table 2. Activities for C-4 modified analogs
1975
Z
Y
N
X
N
N
N
H
N
H
Compound
X
Y
Z
CDK1 IC50 (nM)
1a
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
CF3 NH
CF3 NH
CF3 NH
CF3 NH
3,4-NCNH
2-Cl
3-Cl
10
0.9
3
4-Cl
4-F
4
Br NH
CF3 NH
4
4-OCH3
3,4-OCH2O
4-O-(3-Pyridyl)
8
Br
Br
NH
NH
9
34
Figure 4. Docking of compound 10 with CDK1.
Br
NHCH2 2,3-Dichloro
126
4
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
O
S
2-CH3
2-CH3
4-F
Asp86, and solvent. Furthermore, the CF3 group occu-
pies a hydrophobic region defined by Phe80, Val64,
Leu135, and Ala145. The benzimidazole fragment forms
a hydrogen bond with Gln132 and with the main-chain
carbonyl of Glu12.
135
32
O
O
O
O
O
S
4-Cl
4-Br
252
562
>1000
160
>1000
4-CF3
3-OCH3
3-OCH3
In order to validate the importance of the C-2 NH
group, compound 13 which has an N-methyl group
substituted at C-2 was synthesized and tested. As antici-
pated, compound 13 was found to be inactive. The
inability of this analog to hydrogen bond in the hinge re-
gion, crucial for this class of inhibitors, is reasoned to
account for this observation (Fig. 5). Likewise, analogs
having a sterically encumbering group substituted to
the aniline were found to be less potent (e.g., 18). These
results would suggest that potency for the C-2 modified
series is limited to those groups (i.e., benzimidazole, ani-
lines), which are capable of adopting an appropriate
hydrogen-bonding pattern for hinge binding.
anilines tolerated substitution at the 2-position of the
ring (e.g., 19). Among the groups at the C-4 position
which were also found to demonstrate potency in the
biochemical assay were anilines bearing groups which
are sterically encumbering (e.g., 24–25). Groups having
an N-methylene linker (e.g., 26) displayed activity in
the CDK1 biochemical assay as well. Compounds hav-
ing an O-linker at C-4 were also tolerated, and observed
to be more potent than those having an S-linker (e.g., 27
vs 28, 33 vs 34). Substitution of the ortho position of the
phenyl ether ring is tolerated, as well as for the meta and
para positions.
We then replaced the aminobenzimidazole group at the
C-4 position while retaining the aminobenzimidazole
group at the C-2 position of the pyrimidine ring, and
the results are summarized in Table 2. The C-4 position
of the molecule tolerated modification to a greater ex-
tent than the C-2 position, as a highly diverse group
of compounds were observed to be potent. The main
reason for this difference as suggested by docking studies
is that the benzimidazole fragment attached at C-2 can
adopt a hydrogen bonding pattern similar to that of
10 (Fig. 4). Unlike the C-2 modified series, C-4 bound
Compound 35 was made with the intention of providing
an analog that would be isosteric with compound 26,
and was observed to be potent in the biochemical assay.
Modeling of compound 35 suggests two hydrogen bonds
between the benzimidazole fragment and Asp86 and
Figure 5. Inability of compound 13 to bind to the hinge region.
Figure 6. Docking of compound 35 with CDK1.

1976
S. Verma et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1973–1977
Table 4. Activities against other kinases
Lys89, while the phenyl from the benzimidazole group is
between Phe82 and Leu135 (Fig. 6). The 5-Br group
binds in the same hydrophobic region as the CF3 group
in compound 10. Likewise, hydrophobic groups at the 4-
position on the pyrimidine ring come in contact with
Val118. Longer side chains such as in compound 35
can also potentially interact with Tyr15.
Compound
IC50 (nM)
CDK1 CK2 ETK PDGFR p38
ERK1
1a
11
19
23
10
11
89
91
400 110
1200 190
>1000
>1000
300
750
0.9
8
510
11
>1000 290
33
>1000 >1000
>1000 >1000
230
However, increasing the size of the five-membered ring
of the aminoindane group of compound 35 to the corre-
sponding tetrahydronaphthalene of compound 36, re-
sulted in a drop in potency (Fig. 7). A similar result
was observed for compounds 29–32, wherein potency
decreases as the size of the para-substituent on the C-4
substituted aniline increases (Table 2). These results
would suggest that although larger groups are tolerated
with respect to C-4 substitution of the molecule, there
are spatial limits. One possible explanation could be that
the increasingly larger groups at C-4 disrupt the stabiliz-
ing hydrophobic interaction between the 5-Br (or CF3)
group and Val118, thereby resulting in reduced potency.
Table 5. Activities against other CDKs
Compound
CDK1
IC50 (nM)
CDK2
CDK4
1a
11
19
23
10
11
19
73
20
31
260
260
29
0.9
8
57
specificity for the CDKs in comparison to the other
kinases, with a preference for CDK1.
Interestingly, replacement of the substituted phenyl
groups with alkyl groups within the O-linked series
was well tolerated in the CDK1 assay (e.g., 37–40, Table
3). Analogs within this series were generally tested below
100 nM, with some less than 5 nM (e.g., 39). Thus, there
is room to further expand this series with respect to po-
tency and structural diversity.
In summary, a modular, high-speed route to aminobenz-
imidazole-substituted pyrimidines and their biological
evaluation in the CDK1 biochemical assay has been de-
scribed. The G2/M cell cycle block displayed by com-
pound 1a exemplifies the anti-proliferative properties
for the compounds reported herein. Docking studies uti-
lizing a CDK1 homology model provides a rationale for
the observed activities. Potency in the C-2 modified ser-
ies is limited to those groups, which are capable of
adopting a critical hydrogen bonding pattern to the
hinge region, for example, aminobenzimidazoles and
anilines. The C-4 position can tolerate a greater number
of groups without loss of activity. The presence of a tri-
fluoromethyl or bromo group at C-5 is preferred, while
replacements with fluorine or hydrogen result in loss of
activity. Further pharmacological evaluation of selected
compounds from these series will be reported in due
course.
A subset of compounds from these studies were tested
against CDK2, CDK4, and other kinases (e.g., CK2,
ETK, PDGFR, p38, ERK1), and the results are summa-
rized in Tables 4 and 5. The compounds tested showed
HN
N
HN
N
H
N
H
N
Br
Br
N
N
N
N
N
H
N
H
Acknowledgements
35
CDK1 IC50 = 34 nM
36
The authors would like to thank Anthony Paiva, Tim
He, Joanne Tarnoski, Peter Demou, Jeff Chin, and
Romulo Romero for their LC/MS, 1H NMR, and chro-
matographic separation support. We are also grateful to
Jacques Dumas and William Scott for helpful
discussions.
CDK1 IC50 = 392 nM
Figure 7. Biochemical potencies of compound 35 and 36.
Table 3. Activities for C-4 O-linked analogs
R
O
References and notes
F3C
N
N
1. (a) Harper, J. W.; Adams, P. D. Chem. Rev. 2001, 101,
2511–2526; (b) Mani, S.; Wang, S.; Wu, K.; Francis, R.;
Pestell, R. Expert Opin. Invest. Drugs 2000, 9(8), 1849–
1870; (c) Webster, K. R.; Kimball, S. D. Emerg. Drugs
2000, 5(1), 45–59; (d) Garrett, M. D.; Fattaey, A. Curr.
Opin. Genet. Dev. 1999, 9, 104–111; (e) Morgan, D. O.
Annu. Rev. Cell Dev. Biol. 1997, 13, 261–291; (f) Sherr, C.
J. Science 1996, 274, 1672–1677.
N
H
N
N
H
Compound
R
CDK1 IC50 (nM)
37
38
39
40
O-Ethyl
77
25
2
O-Isopropyl
O-Isobutyl
O-Cyclohexyl
43
2. Fischer, P. M. Cell Cycle 2004, 3, e9–e13.

S. Verma et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1973–1977
1977
3. Malumbres, M.; Barbacid, M. Natl. Can. Rev. 2001, 1,
222–231.
6. Dahmann, G.; Himmelsbach, F.; Wittneben, H.; Pautsch,
A.; Prokopowicz, A. S.; Krist, B.; Schnapp, G.; Steegma-
ier, M.; Lenter, M.; Schoop, A.; Steurer, S.; Spevak, W.
WO 2003 032997.
4. (a) Misra, R. N.; Xiao, H.-Y.; Kim, K. S.; Lu, S.; Han,
W.-C.; Barbosa, S. A.; Hunt, J. T.; Rawlins, D. B.; Shan,
W.; Ahmed, S. Z.; Qian, L.; Chen, B.-C.; Zhao, R.;
Bednarz, M. S.; Kellar, K. A.; Mulheron, J. G.; Batorsky,
R.; Roongta, U.; Kamath, A.; Marathe, P.; Ranadive, S.
A.; Sack, J. S.; Tokarski, J. S.; Pavletich, N. P.; Lee, F. Y.
F.; Webster, K. R.; Kimball, S. D. J. Med. Chem. 2004,
47, 1719–1728; (b) Marie Knockaert, M.; Greengard, P.;
Laurent, ; Meijer, L. Trends Pharm. Sci. 2002, 23, 417–
425; (c) Yue, E. W.; Higley, C. A.; DiMeo, S. V.; Carini,
D. J.; Nugiel, D. A.; Benware, C.; Benfield, P. A.; Burton,
C. R.; Cox, S.; Grafstrom, R. H.; Sharp, D. M.; Sisk, L.
M.; Boylan, J. F.; Muckelbauer, J. K.; Smallwood, A. M.;
Chen, H.; Chang, C.-H.; Seitz, S. P.; Trainor, G. L. J.
Med. Chem. 2002, 45, 5233–5248; (d) Kim, K. S.; Kimball,
S. D.; Misra, R. N.; Rawlins, D. B.; Hunt, J. T.; Xiao,
H.-Y.; Lu, S.; Qian, L.; Han, W.-C.; Shan, W.; Mitt, T.;
Cai, Z.-W.; Poss, M. A.; Zhu, H.; Sack, J. S.; Tokarski, J.
S.; Chang, C. Y.; Pavletich, N.; Kamath, A.; Humphreys,
W. G.; Marathe, P.; Bursuker, I.; Kellar, K. A.; Roongta,
U.; Batorsky, R.; Mulheron, J. G.; Bol, D.; Fairchild, C.
R.; Lee, F. Y.; Webster, K. R. J. Med. Chem. 2002, 45,
3905–3927; (e) Hardcastle, I. R.; Golding, B. T.; Griffin,
R. J. Annu. Rev. Pharmacol. Toxicol. 2002, 42, 325–348; (f)
Bramson, H. N.; Corona, J.; Davis, S. T.; Dickerson, S.
H.; Edelstein, M.; Frye, S. V.; Gampe, R. T., Jr.; Harris,
P. A.; Hassell, A.; Holmes, W. D.; Hunter, R. N.; Lackey,
K. E.; Lovejoy, B.; Luzzio, M. J.; Montana, V.; Rocque,
W. J.; Rusnak, D.; Shewchuk, L.; Veal, J. M.; Walker, D.
H.; Kuyper, L. F. J. Med. Chem. 2001, 44, 4339–4358; (g)
Kelland, L. R. Expert Opin. Invest. Drugs 2000, 9(12),
2903–2911; (h) Barvian, M.; Boschelli, D. H.; Cossrow, J.;
Dobrusin, E.; Fattaey, A.; Fritsch, A.; Fry, D.; Harvey,
P.; Keller, P.; Garrett, M.; La, F.; Leopold, W.; McNa-
mara, D.; Quin, M.; Trumpp-Kallmeyer, S.; Toogood, P.;
Wu, Z.; Zhang, E. J. Med. Chem. 2000, 43, 4606–4616; (i)
Arris, C. E.; Boyle, F. T.; Calvert, A. H.; Curtin, N. J.;
Endicott, J. A.; Garman, E. F.; Gibson, A. E.; Golding, B.
T.; Grant, S.; Griffin, R. J.; Jewsbury, P.; Johnson, L. N.;
Lawrie, A. M.; Newell, D. R.; Noble, M.; Sausville, E. A.;
Schultz, R.; Yu, W. J. Med. Chem. 2000, 43, 2797–2804; (j)
Sielecki, T. M.; Boylan, J. F.; Benfield, P. A.; Trainor, G.
L. J. Med. Chem. 2000, 43, 1–18; (k) Kath, J. C.; Luzzio,
M. J. WO 2004 056786.
7. Regiochemical assignments were confirmed by 2D NMR
studies. Substitution of dichloropyrimidine
2 occurs
regioselectively at C-4 when X = Br. When X = CF3, a
minor amount of C-2 substituted regioisomer is observed
(3:1, in favor of the C-4 isomer), while couplings with
compound 7 occur specifically at the C-2 position.
8. CDK1 assay was performed according to the manufac-
turerÕs protocol (New England Biolabs) with the minor
modification of incubating the reaction for 2 h at room
temperature. Indurubin 3-monoxime (IC50 = 180 nM) was
used as a reference inhibitor of CDK1 activity.
9. The structure of human CDK1 was built through homo-
11
logy modeling using MODELLER
in INSIGHTII. The
template used for building the model was the structure
of CDK2 in complex with 4-[(6-amino-4-pyrimidi-
nyl)amino]benzene sulfonamide.12 The template has 65%
sequence identity, 78% sequence similarity and 3% gaps
when aligned with the CDK1 sequence. Compounds 10,
13, and 35 were docked into CDK1 using GLIDE.13 The
obtained protein–ligand complex was optimized using
500 ps molecular dynamics simulation using TIP3P water
14
for solvation. The simulation was done using CHARMM
with constant dielectric of 1, temperature of 300 K, and
1 fs time step.
10. Engh, R. A.; Bossemeyer, D. Pharmacol. Ther. 2002, 93,
99–111.
11. (a) Sali, A.; Blundell, T. L. J. Mol. Biol. 1993, 234, 779–
815; (b) Sali, A.; Potterton, L.; Yuan, F.; van Vlijmen, H.;
Karplus, M. Proteins: Structure, Function, Genetics 1995,
23, 318–326.
12. (a) Berman, H. M.; Westbrook, J.; Feng, Z.; Gilliland, G.;
Bhat, T. N.; Weissig, H.; Shindialov, I. N.; Bourne, P. E.
Nucleic Acids Res. 2000, 28, 235–242; (b) Bernstein, F.;
Koetzle, T. F.; Williams, G. J. B.; Meyer, E. F., Jr.; Brice,
M. D.; Rodgers, J. R.; Kennard, O.; Schimanouchi, T.;
Tasumi, M. J. J. Mol. Biol. 1977, 112, 535–542.
13. Friesner, R. A.; Banks, J. L.; Murphy, R. B.; Halgren, T.
A.; Klicic, J. J.; Mainz, D. T.; Repasky, M. P.; Knoll, E.
H.; Shelley, M.; Perry, J. K.; Shaw, D. E.; Francis, P.;
Shenkin, P. S. J. Med. Chem. 2004, 47, 1739–1749.
14. Brooks, B. R.; Bruccoleri, R. E.; Olafson, B. D.; States, D.
J.; Swaminathan, S.; Karplus, M. J. Comput. Chem. 1983,
4, 187–217.
5. Bridges, A. J. Chem. Rev. 2001, 101, 2541–2571.