Fragment-based discovery of 7-azabenzimidazoles as potent, highly selective, and orally active CDK4/6 inhibitors

Article abstract of DOI:10.1021/ml200241a

Herein, we describe the discovery of potent and highly selective inhibitors of both CDK4 and CDK6 via structure-guided optimization of a fragment-based screening hit. CDK6 X-ray crystallography and pharmacokinetic data steered efforts in identifying compound 6, which showed >1000-fold selectivity for CDK4 over CDKs 1 and 2 in an enzymatic assay. Furthermore, 6 demonstrated in vivo inhibition of pRb-phosphorylation and oral efficacy in a Jeko-1 mouse xenograft model.

Full text of DOI:10.1021/ml200241a

Letter
pubs.acs.org/acsmedchemlett
Fragment-Based Discovery of 7-Azabenzimidazoles as Potent, Highly
Selective, and Orally Active CDK4/6 Inhibitors
,
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‡
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†
Young Shin Cho,* Hayley Angove, Christopher Brain, Christine Hiu-Tung Chen, Hong Cheng,
‡
†
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‡
‡
‡
Robert Cheng, Rajiv Chopra, Kristy Chung, Miles Congreve, Claudio Dagostin, Deborah J. Davis,
Ruth Feltell, John Giraldes, Steven D. Hiscock, Sunkyu Kim, Steven Kovats, Bharat Lagu,
Kim Lewry, Alice Loo, Yipin Lu, Michael Luzzio, Wiesia Maniara, Rachel McMenamin,
Paul N. Mortenson, Rajdeep Benning, Marc O'Reilly, David C. Rees, Junqing Shen, Troy Smith,
Yaping Wang, Glyn Williams, Alison J.-A. Woolford, Wojciech Wrona, Mei Xu, Fan Yang,
and Steven Howard*
‡
†
‡
†
†
†
‡
†
†
†
†
‡
‡
‡
‡
‡
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†
,‡
†Novartis Institutes for Biomedical Research, 250 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
‡Astex Pharmaceuticals Inc., 436 Cambridge Science Park, Milton Road, Cambridge, CB4 0QA, United Kingdom
*
S Supporting Information
ABSTRACT: Herein, we describe the discovery of potent and
highly selective inhibitors of both CDK4 and CDK6 via
structure-guided optimization of a fragment-based screening
hit. CDK6 X-ray crystallography and pharmacokinetic data
steered efforts in identifying compound 6, which showed
>
1000-fold selectivity for CDK4 over CDKs 1 and 2 in an
enzymatic assay. Furthermore, 6 demonstrated in vivo
inhibition of pRb-phosphorylation and oral efficacy in a Jeko-1 mouse xenograft model.
KEYWORDS: CDK4/6, pRb phosphorylation, mantle cell lymphoma, fragment-based screening, structure-guided optimization
rogression through the cell cycle is a highly regulated
process. In the absence of appropriate growth signals, a
family of pocket proteins including retinoblastoma protein
Previously, we reported on the identification and structure-
guided optimization of a series of 4-(pyrazol-4-yl) pyrimidines
as selective CDK4/6 inhibitors. Several potency and selectivity
determinants were established based on the X-ray crystallo-
graphic analysis of representative compounds bound to
CDK6. Herein, we describe the structure-guided optimization
of a CDK6 fragment-based screening hit leading to the
discovery of compound 6, a potent, selective, and orally
bioavailable CDK4/6 inhibitor.
P
(
pRb) prevents cells from entering the DNA replication phase
1,2
(
S phase). The replication cycle begins when mitogens
18
trigger signal transduction pathways, leading to increase of
cellular levels of D-cyclins. D-cyclins, in turn, activate cyclin-
dependent kinases 4/6 (CDK4/6), which phosphorylate and
1
,3,4
inactivate pRb.
Uncontrolled cell proliferation is one of the hallmarks of
cancer, and pRb inactivation is the key event that enables tumor
cells to progress through the cell cycle unchecked. While some
tumors delete the pRb gene itself, the majority maintain a
functional pRb and instead activate CDK4/6 kinase activ-
The starting point for this work was provided by the CDK6
fragment-based screening hit A [Scheme 1, CDK6 IC = 7.2
5
0
19
μM, ligand efficiency (LE) = 0.44]. The X-ray crystallo-
graphic structure of CDK6 in complex with fragment A (Figure
1a) was solved using a novel back-soakable form of CDK6. The
crystal structure revealed that the ligand binds in the ATP
pocket and forms hydrogen bonds with the backbone NH and
the backbone carbonyl of Val101. Further inspection of the
crystal structure suggested that the two heterocyclic rings
offered multiple “growth” vectors and therefore the opportunity
to interact with many of the residues within the ATP pocket
5
−11
ity.
Ablation of CDK4 kinase activity led to complete tumor
growth inhibition in CDK4/cyclin D1-dependent tumors such
12
as those found in breast cancer that overexpresses Her2/neu.
Furthermore, normal fibroblast cells were shown to overcome
the absence of CDK4/6 due to compensation by CDK1, whose
13
absence is not tolerated. Taken together, this evidence
suggests that a selective inhibitor of CDK4/6 may have a wider
therapeutic window than pan-CDK inhibitors.
1
8,20−22
known to be important for potency and selectivity.
Some of these key residues are highlighted in Figure 1. In
particular, His100 (CDK6) and Thr107 (CDK6) are replaced
A variety of CDK inhibitors have been evaluated preclinically
1
,14,15
and clinically.
Given the evidence described above, many
research groups have embarked on the discovery of a CDK4/6
selective inhibitor, with the most well-documented being PD-
Received: December 2, 2011
Accepted: April 16, 2012
1
6,17
0332991.
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ml200241a | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
Scheme 1. Identification of Benzimidazole 1, Starting from
a
Fragment A
a
IC50 values are given as the average of two or more determinations
unless otherwise indicated by an asterisk (*) (where n = 1).
with Phe and Lys, respectively, in CDKs 1 and 2. Exploiting
these two differences is important for establishing selectivity
over CDK1/2. Hydrogen bonding between the ligands and the
side chain of Lys43 and the backbone of Asp163 also seems to
21,23
contribute to a potent and selective CDK4/6 profile.
Examination of the structure in Figure 1 suggested that
optimal growth vectors to target Lys43 would be provided by
replacing the pyrrole with a pyridine ring. The pyridine
analogue B (Scheme 1, 57% inhibition at 30 μM, LE = ca. 0.36)
retained acceptable LE and provided a better starting point for
further optimization.
Structure-based design and previous experience with CDK4/
1
8,20−22
6
described above
enabled rapid elaboration of the
scaffold as exemplified by compound 1 (CDK6 IC = 1.1 μM,
5
0
2
4
Figure 1. (a) X-ray crystal structure of benzimidazole fragment A
bound to CDK6, showing hydrogen bonds to the kinase hinge region
(dotted black lines). (b) Model of compound 1 (magenta) bound to
CDK6 (yellow) overlaid with key amino acid differences from CDK2
(1hck; shown in cyan). The pyridin-3-yl nitrogen aims to hydrogen
bond with the side chain of Lys43 (CDK6). The 4-dimethylamino-
piperidine group is designed to make unfavorable electrostatic and
1
8,20−22
steric repulsion with Lys89 (CDK2) but to be tolerated by Thr107
25,26
(CDK6). Docking performed using GOLD.
(
Table 1). Compound 2 also exhibited good potency against
CDK4 (IC50 = 0.006 μM, LE = 0.31) and moderate selectivity
over CDKs 1 and 2.
The next strategy to improve selectivity relied on the CDK6
hinge residue His100, an amino acid that is rarely conserved in
other kinases. Previously reported structural studies with
selective inhibitors bound to CDK6 propose an aromatic sp2
nitrogen in proximity to His100 (His92 in CDK4) as an
Figure 2a. As with fragment A, two hydrogen-bonding
interactions are observed between 3 and the backbone NH/
carbonyl of Val101. The pyridine forms a favorable edge-to-face
aromatic−aromatic contact with the gatekeeper residue Phe98.
The isoquinoline ring nitrogen is positioned toward the side
chain of Lys43, indicating a hydrogen bond.
Interestingly, the isoquinoline ring also shows ca. 30° twist
relative to the plane of the pyridine, which appears to provide a
better orientation for hydrogen bonding and the resulting
increased potency of the isoquinoline analogues 2 and 3
compared to the pyridyl analogue 1. A close examination of the
structure in Figure 2a also revealed an opportunity to form a
hydrogen bond with the backbone of NH of Asp163 (part of
the DFG kinase motif) by placing a nitrile group in the back of
1
6,18,22
important determinant for selectivity over other kinases.
Analysis of the structural information in Figure 1 suggested
that replacing the benzimidazole with 7-azabenzimidazole
would provide an opportunity to test this hypothesis. Indeed,
the 7-azabenzimidazole analogue 3 was the first compound to
exhibit greater than 10-fold selectivity for CDKs 4 and 6 over
CDKs 1 and 2. The structure of 3 bound to CDK6 is shown in
B
dx.doi.org/10.1021/ml200241a | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
a,b
Table 1. IC50 values for compounds in the CDK enzyme assays
IC50 (μM)
X1
X2
CDK4/Cyclin D1
CDK6/Cyclin D3
CDK1/Cyclin B
CDK2/Cyclin A
2
3
4
5
6
N
CH
N
0.006 ± 0.001
0.012 ± 0.002
0.003 ± 0.000
0.25 ± 0.03
0.049 ± 0.006
0.3 ± 0.1
0.026 ± 0.003
NT
1.2 ± 0.2
>15
0.27 ± 0.06
6.2 ± 0.4
2.4 ± 0.2
>15
>15
N
CCN
N
N
5.6 ± 0.4
>15
CH
N
CCN
0.015 ± 0.001
0.12
>15
a
Results are expressed as the mean ± standard deviation of 1−2 IC50 determinations. For each determination, concentration−inhibition curves were
b
obtained in duplicate and then averaged to afford a single IC50 curve with a ≥95% confidence interval. 70% homology shared between CDK4 and -6
contributes to comparable SAR profiles of two enzymes.
benzonitrile motif gave compound 4, which showed increased
potency for CDK4/6 as well as improved selectivity over CDKs
1
and 2.
While demonstrating good potency and modest selectivity,
the isoquinoline analogues typically showed limited oral
exposure. In an attempt to improve the pharmacokinetic
profile, analogues of 2 [plasma clearance (CLp) = 144 mL/
min/kg at 2 mg/kg iv, AUC 250 nM h at 5 mg/kg po,
Sprague−Dawley rats] were prepared, as these provided the
most convenient way to explore the effect of replacing the
isoquinoline ring with alternative heterocycles. This led to the
identification of the trimethylpyrazole 5 (Table 1), which
exhibited moderate potency (CDK4 IC 0.25 μM, LE = 0.26),
50
reduced in vivo clearance (27 mL/min/kg), and improved oral
exposure (AUC 2526 nM h, 5 mg/kg po) in rat.
Experience gained with compounds 2−4 was then used to
further optimize potency and selectivity. Thus, introducing the
azabenzimidazole group and replacing the pyridine motif with
benzonitrile led to the identification of compound 6 (CDK4
IC50 = 0.015 μM, LE = 0.30), which exhibited a ≥1000-fold
selectivity for CDK4 over CDKs 1 and 2. Furthermore, 6
demonstrated an oral bioavailability of 26% in rat (AUC 1147
nM h, 5 mg/kg po) with acceptable plasma clearance (37 mL/
min/kg, 1 mg/kg iv).
Compound 6 was also shown to be selective for CDK4/6
over a panel of 35 serine-threonine and tyrosine kinases
(
Supporting Information). These results indicated a compara-
ble or improved selectivity profile relative to currently
1
6,18
published CDK4/6 selective inhibitors.
A critical aim of this project was to generate compounds with
a high degree of selectivity for CDK4/6 over CDK1/2. During
the optimization process, certain modifications were made to
the molecule, not solely to increase CDK4/6 potency but to
drive the high selectivity over CDK1/2. Examples include the
(
dimethylamino)piperidine group and the azabenzimidazole
modification, which actually reduces CDK4/6 potency
compare 2 and 3). One consequence of this is that compound
has a lower LE (CDK4 LE = 0.31, CDK6 LE = 0.26) than the
Figure 2. (a) X-ray crystal structure of compound 3 bound to CDK6.
b) Model of compound 4 bound to CDK6 created using GOLD-
based software. Projection is rotated (∼90°) and expanded to illustrate
the putative hydrogen bond between the nitrile and the backbone NH
of Asp163. Lys43 has been removed for clarity.
(
(
6
starting fragment A (CDK6 LE = 0.44).
Selective CDK4/6 inhibition prevents phosphorylation of
pRb, consequently blocking the G1 to S phase transition.
Compound 6 demonstrated a normalized cellular IC50 value
of 0.26 μM in a Jeko-1 mantle cell lymphoma assay measuring
18
the pocket. This strategy is illustrated by the docking model in
Figure 2b. Replacing the pyridine ring of compound 3 with a
C
dx.doi.org/10.1021/ml200241a | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
9
,27
inhibition of pRb phosphorylation.
Fluorescence-activated
cell sorting (FACS) demonstrated a dose-dependent increase in
exclusive G1 arrest with increasing concentrations of
compound (Figure 3). Clean G1 block was maintained over a
range of 0.625−2.5 μM. At a higher concentration of 10 μM,
block in non-G1 phases of the cell cycle emerged.
Figure 3. Cell cycle flow cytometry analyses of compound 6.
This favorable balance of PK properties with biochemical and
cellular potency/selectivity profile led us to evaluate 6 in Jeko-1
xenograft tumor-bearing severe combined immunodeficiency
(
SCID) mice. Modulation of pRb phosphorylation was
examined at several time points (0, 1, 4, and 7 h post the last
dose) after orally administering 6 for 2.5 days twice daily at 125
and 250 mg/kg (Figure 4a,b). Dose-dependent increases in
Cmax and inhibition of pRb were observed. A dose level of 250
mg/kg/day with 6 resulted in a maximum 80% inhibition of
phosphorylation, which declined to 60% at 7 h. At the 500 mg/
kg/day dose level, the pharmacodynamic (PD) readout reached
1
00% 1 h postdosing and was sustained above 85% over 7 h.
We then verified the maximum tolerated dose (MTD)
following 7 days of oral treatment of naive SCID mice at
100, 200, and 400 mg/kg/day bid. The MTD was established as
the maximum dose that resulted in less than 15% mean body
weight loss and no treatment-related deaths. On the basis of
these criteria, the MTD of 6 was determined to be between 200
and 400 mg/kg/day. Efficacy was then assessed in a Jeko-1
xenograft model, where tumor-bearing mice were treated with 6
twice daily for 10 days (Figure 4c). Results were reported as T/
C, determined by the ratio of the change in tumor volume of
the treated animals to the change in tumor volume of the
control animals. Following oral dosing at 75, 150, and 250 mg/
kg/day, compound 6 exhibited dose-related antiproliferative
effects with T/C values of 41, 25, and 4%, respectively. These
results indicated that 6 was able to demonstrate dose-
dependent tumor growth inhibition, with the 250 mg/kg/day
dose significantly blocking tumor growth. The 250 mg/kg/day
dose was well tolerated and did not exceed the MTD criteria
described above. Coupled with the PD data obtained, the
CDK4/6 selective inhibitor 6 exhibited sufficient in vivo
exposure to block CDK4/6 specific pRb phosphorylation in the
tumors and a robust antitumor activity in this mouse tumor
model.
Figure 4. (a and b) Inhibition of phosphorylation of pRb in Jeko-1
xenograft tumor bearing SCID mice treated with 6. (c) Efficacy of
compound 6 in Jeko-1 xenograft tumor bearing SCID mice.
apparent in both biochemical assays and cellular functional
readouts. Multiday treatment of Jeko-1 tumors in mice
demonstrated dose-dependent inhibition of pRb phosphor-
ylation as well as tumor growth delay.
ASSOCIATED CONTENT
■
*
S
Supporting Information
Enzyme assay conditions for CDK1, -2, -4, and -6, CDK4
cellular assay conditions, fluorescence-activated cell-sorting
conditions, crystallography conditions, in vivo experiments
method, experimental details for the synthetic procedures and
characterization data of A, B, and 1−6, and further in vitro and
in vivo pharmacokinetic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
In summary, we have identified a potent, highly selective, and
orally bioavailable CDK4/6 inhibitor 6, starting from
benzimidazole fragment A. CDK6 structure-guided design
and pharmacokinetic data were utilized for optimization toward
compound 6. CDK4/6 specific activity of this compound was
D
dx.doi.org/10.1021/ml200241a | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
(
17) Fry, D. W.; Harvey, P. J.; Keller, P. R.; Elliott, W. L.; Meade, M.;
AUTHOR INFORMATION
■
*
Trachet, E.; Albassam, M.; Zheng, X.; Leopold, W. R.; Pryer, N. K.;
Toogood, P. L. Specific inhibition of cyclin-dependent kinase 4/6 by
PD 0332991 and associated antitumor activity in human tumor
xenografts. Mol. Cancer Ther. 2004, 3, 1427−1438.
(18) Cho, Y. S.; Borland, M.; Brain, C.; Chen, C. H.-T.; Cheng, H.;
Chopra, R.; Chung, K.; Groarke, J.; He, G.; Hou, Y.; Kim, S.; Kovats,
Corresponding Author
Tel: 1-617-871-3495. Fax: 1-617-871-4081. E-mail: youngshin.
cho@novartis.com (Y.S.C.). Tel: 44-1223-226220. Fax: 44-
223-226201. E-mail: Steven.Howard@astx.com (S.H.).
1
Notes
̈
S.; Lu, Y.; O'Reilly, M.; Shen, J.; Smith, T.; Trakshel, G.; Vogtle, M.;
Xu, M.; Xu, M.; Sung, M. J. 4-(Pyrazol-4-yl)-pyrimidines as selective
The authors declare no competing financial interest.
inhibitors of cyclin-dependent kinase 4/6. J. Med. Chem. 2010, 53,
7
(
938−7957.
ACKNOWLEDGMENTS
■
19) LE = −ΔG/HAC ≈ −RT ln(IC )/HAC. Hopkins, A. L.;
50
We thank Dr. Timothy Ramsey and Dr. Chris Johnson for
helpful suggestions in the preparation of the manuscript.
Groom, C. R.; Alex, A. Ligand efficiency: A useful metric for lead
selection. Drug Discovery Today 2004, 9, 430−431.
(
20) Day, P. J.; Cleasby, A.; Tickle, I. J.; O'Reilly, M.; Coyle, J. E.;
Holding, F. P.; McMenamin, R. L.; Yon, J.; Chopra, R.; Lengauer, C.;
Jhoti, H. Crystal structure of human CDK4 in complex with a D-type
cyclin. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 4166−4170.
REFERENCES
■
(
1) Malumbres, M.; Barbacid, M. Cell cycle, CDKs and cancer: a
changing paradigm. Nature Rev. Cancer 2009, 9, 153−166.
2) Weinberg, R. A. The retinoblastoma protein and cell-cycle
control. Cell 1995, 81, 323−330.
3) Sherr, C. J.; Roberts, J. M. CDK inhibitors: positive and negative
regulators of G1-phase progression. Genes Dev. 1999, 13, 1501−1512.
4) Lundberg, A. S.; Weinberg, R. A. Functional inactivation of the
retinoblastoma protein requires sequential modification by at least two
distinct cyclin-cdk complexes. Mol. Cell. Biol. 1998, 18, 753−761.
5) Shapiro, G. I. Cyclin-dependent kinase pathways as targets for
cancer treatment. J. Clin. Oncol. 2006, 24, 1770−1783.
6) Ortega, S.; Malumbres, M.; Barbacid, M. Cyclin D-dependent
kinases, INK4 inhibitors and cancer. Biochim. Biophys. Acta, Rev. Cancer
002, 1602, 73−87.
7) Shapiro, G. I.; Park, J. E.; Edwards, C. D.; Mao, L.; Merlo, A.;
Sidransky, D.; Ewen, M. E.; Rollins, B. J. Multiple mechanisms of p16
Ink4A) inactivation in non-small-cell lung-cancer cell-lines. Cancer
Res. 1995, 55, 6200−6209.
8) Kamb, A.; Gruis, N. A.; Weaverfeldhaus, J.; Liu, Q. Y.; Harshman,
(21) Pratt, D. J.; Bentley, J.; Jewsbury, P.; Boyle, F. T.; Endicott, J. A.;
(
Noble, M. E. M. Dissecting the determinants of cyclin-dependent
kinase 2 and cyclin-dependent kinase 4 inhibitor selectivity. J. Med.
Chem. 2006, 49, 5470−5477.
(
(
22) Lu, H.; Schulze-Gahmen, U. Toward understanding the
(
structural basis of cyclin-dependent kinase 6 specific inhibition. J.
Med. Chem. 2006, 49, 3826−3831.
(23) Honma, T.; Yoshizumi, T.; Hashimoto, N.; Hayashi, K.;
(
Kawanishi, N.; Fukasawa, K.; Takaki, T.; Ikeura, C.; Ikuta, M.; Suzuki-
Takahashi, I.; Hayama, T.; Nishimura, S.; Morishima, H. A novel
approach for the development of selective Cdk4 inhibitors: Library
design based on locations of Cdk4 specific amino acid residues. J. Med.
Chem. 2001, 44, 4628−4640.
(
2
(
(
24) See the Supporting Information for a detailed preparation of
compounds A, B, and 1−6.
25) Jones, G.; Willett, P.; Glen, R. C. Molecular recognition of
(
(
receptor sites using a genetic algorithm with a description of
(
desolvation. J. Mol. Biol. 1995, 245, 43−53.
K.; Tavtigian, S. V.; Stockert, E.; Day, R. S.; Johnson, B. E.; Skolnick,
M. H. A cell-cycle regulator potentially involved in genesis of many
tumor types. Science 1994, 264, 436−440.
(26) Jones, G.; Willett, P.; Glen, R. C.; Leach, A. R.; Taylor, R.
Development and validation of a genetic algorithm for flexible docking.
J. Mol. Biol. 1997, 267, 727−748.
(
9) Amin, H. M.; McDonnell, T. J.; Medeiros, L. J.; Rassidakis, G. Z.;
(27) Tchakarska, G.; Lan-Leguen, A.; Roth, G.; Sola, B. The targeting
Leventaki, V.; O’Connor, S. L.; Keating, M. J.; Lai, R. Characterization
of 4 mantle cell lymphoma cell lines - Establishment of an in vitro
study model. Arch. Pathol. Lab. Med. 2003, 127, 424−431.
10) Sutherland, R. L.; Musgrove, E. A. Cyclins and breast cancer. J.
Mammary Gland Biol. Neoplasia 2004, 9, 95−104.
11) Sirvent, N.; Coindre, J. M.; Maire, G.; Hostein, I.; Keslair, F.;
of the sole cyclin D1 is not adequate for mantle cell lymphoma and
myeloma therapies. Haematologica 2009, 94 (12), 1781−1782.
(
(
Guillou, L.; Ranchere-Vince, D.; Terrier, P.; Pedeutour, F. Detection
of MDM2-CDK4 amplification by fluorescence in situ hybridization in
2
00 paraffin-embedded tumor samples: Utility in diagnosing
adipocytic lesions and comparison with immunohistochemistry and
real-time PCR. Am. J. Surg. Pathol. 2007, 31, 1476−1489.
(
12) Yu, Q. Y.; Sicinska, E.; Geng, Y.; Ahnstrom, M.; Zagozdzon, A.;
Kong, Y.; Gardner, H.; Kiyokawa, H.; Harris, L. N.; Stal, Q.; Sicinski,
P. Requirement for CDK4 kinase function in breast cancer. Cancer Cell
2
(
006, 9, 23−32.
13) Santamaria, D.; Barriere, C.; Cerqueira, A.; Hunt, S.; Tardy, C.;
Newton, K.; Caceres, J. F.; Dubus, P.; Malumbres, M.; Barbacid, M.
Cdk1 is sufficient to drive the mammalian cell cycle. Nature 2007, 448,
8
11−818U8.
14) Malumbres, M.; Pevarello, P.; Barbacid, M.; Bischoff, J. R. CDK
inhibitors in cancer therapy: What is next? Trends Pharmacol. Sci.
007, 29, 16−21.
15) Hirai, H.; Kawanishi, N.; Iwasawa, Y. Recent advances in the
(
2
(
development of selective small molecule inhibitors for cyclin-
dependent kinases. Curr. Top. Med. Chem. 2005, 5, 167−179.
(
16) Toogood, P. L.; Harvey, P. J.; Repine, J. T.; Sheehan, D. J.;
VanderWel, S. N.; Zhou, H.; Keller, P. R.; McNamara, D. J.; Sherry,
D.; Zhu, T.; Brodfuehrer, J.; Choi, C.; Barvian, M. R.; Fry, D. W.
Discovery of a potent and selective inhibitor of cyclin-dependent
kinase 4/6. J. Med. Chem. 2005, 48, 2388−2406.
E
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