2252
K. F. Byth et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2249–2252
Table 3. PK summary of 2c and 4c following 2 mg/kg po to micea
References and notes
Compd
AUC 0–6 h, Cmax
,
T
max, h
Approx. T1=2, h
1. Byth, K. F.; Culshaw, J. D.; Green, S.; Oakes, S.; Thomas,
A. P. Bioorg. Med. Chem. Lett. 2004, 14, preceding paper.
2. Sherr, C. J. Science 1996, 274, 1672–1677.
3. Chen, Y.-N. P.; Sharma, S. K.; Ramsey, T. M.; Jiang, L.;
Martin, M. S.; Baker, K.; Adams, P. D.; Bair, K. W.;
Kaelin, W. G., Jr. Proc. Natl. Acad. Sci. U.S.A. 1999, 96,
4325–4329.
4. Thomas, A. P. PCT Int. Application WO 2002-066481.
5. Podergajs, S.; Stanovnik, B.; Tisler, M. Synthesis 1984,
263–265.
6. Abarbri, M.; Thibonnet, J.; Berillon, L.; Dehmel, F.;
Mario Rottlander, M.; Knochel, P. J. Org. Chem. 2000,
65, 4618–4634.
7. Schulze-Gahmen, U.; DeBondt, H. L.; Kim, S.-H. J. Med.
Chem. 1996, 39, 4540–4546.
8. Davies, T. G.; Tunnah, P.; Meijer, L.; Marko, D.;
Eisenbrand, G.; Endicott, J. A.; Noble, M. E. M.
Structure 2001, 9, 389–397.
9. Anderson, M.; Beattie, J. F.; Breault, G. A.; Breed, J.;
Byth, K. F.; Culshaw, J. D.; Ellston, R. P. A.; Green, S.;
Minshull, C. A.; Norman, R. A.; Pauptit, R. A.; Stanway,
J.; Thomas, A. P.; Jewsbury, P. J. Bioorg. Med. Chem.
Lett. 2003, 13, 3021–3026.
lM h
lM
2c
4b
9.4
20
3.2
9.9
0.5
0.5
3.0
3.3
the imidazo[1,2-a]pyridine series1 indicated this H-bond
interaction to be important for CDK2 activity, however
the edge-to-face interaction of the pyridazine ring of 2e
with the phenyl group of Phe80 that is gained must
provide compensatory binding free energy. The change
in binding conformation of the imidazo[1,2-b]pyridazine
2e compared to the imidazo[1,2-a]pyridine 5 can be
understood in terms of the electrostatic repulsion that
would exist between the N4 of the imidazo[1,2-b]pyrid-
azine and the N3 of the pyrimidine ring of 2e if it were to
adopt the binding mode of imidazo[1,2-a]pyridine such
as 5. While the absence of a peri-hydrogen at the 5-
position of the 5,6-ring system in imidazo[1,2-b]pyrid-
azines, such as 2e, also makes the adopted conformation
(Fig. 1) more favourable.
For both the imidazo[1,2-b]pyridazine 2e and imida-
zo[1,2-a]pyridine 5, the sulfonamide group forms
hydrogen bonds with the Asp86 backbone NH and
its carboxylic side chain and also with the side chain
NH of Lys89 (Figs. 1 and 2). Though some subtle dif-
ferences in binding and orientation are apparent, the
cause of the difference in dependence on sulfonamide
group for binding potency shown by the two series is not
clear.
10. For details of methods and crystallographic statistics for
structure of CDK2 complex with compd 5, see Ref. 9 and
references cited therein.
11. Protein and crystals were obtained according to estab-
lished procedures.12;13 Diffraction data were collected on
beamline PX9.6 at SRS, Daresbury, at 100 K. Data
processing, data reduction and structure solution by
molecular replacement were carried out using programs
from the CCP4 suite.14 Compound 2e was modeled into
electron density using QUANTA.15 The protein complex
model was refined using CNX,16 and the final structure17
have been deposited in the Protein Data Bankwith
deposition code 1urw together with structure factors and
detailed experimental conditions.
12. Lawrie, A. M.; Noble, M. E.; Tunnah, P.; Brown, N. R.;
Johnson, L. N.; Endicott, J. A. Nat. Struct. Biol. 1997, 4,
796–801.
13. Legraverend, M.; Tunnah, P.; Noble, M.; Ducrot, P.;
Ludwig, O.; Grierson, D. S.; Leost, M.; Meijer, L.;
Endicott, J. J. Med. Chem. 2000, 43, 1282–1292.
14. CCP4 Acta Crystallogr. 1994, D50, 760–763.
15. Quanta2000, Accelrys.
The properties and activities of compounds 2c and 4b
have been characterised in more detail. Both compounds
show selectivity for CDK2 over CDK1, though this
appears to be greater for 4b (Table 2). Examining
activity against a broader range of kinases, 2c gave an
IC50 of >10 lM against the following kinases: Csk,
EGFR, FAK, FGFR-1, IGF-1R, JAK3, Src, vAbl,
Zap70, p38a, JNK1 and PKA. Significantly, compounds
from the imidazo[1,2-b]pyridazine series show improved
blood levels following oral dosing over those shown by
the imidazo[1,2-a]pyridine series. Both compounds 2c
and 4b show significant blood levels in mice following
oral dosing (Table 3).
16. CNX version 2000.1, Accelrys.
17. Crystallographic statistics for compd 2e are: space group
ꢀ
ꢀ
P212121, unit cell 54.2, 72.9, 73.3 A, resolution 1.6 A,
33,057 reflections from 72,225 observations give 85%
completeness with Rmerge ¼ 5:4% and mean I=rðIÞ of 13.2.
The final model containing 2153 protein, 212 water and 32
inhibitor atoms has an R-factor of 22.0% (Rfree using 5% of
the data 25.5%). Mean temperature factors for the protein
In conclusion, imidazo[1,2-b]pyridazines CDK2 inhibi-
tors have been discovered and optimised. Compounds
2c and 4b have been characterised as potent and selec-
tive inhibitors of CDK enzymes, which show significant
plasma levels following oral dosing. These compounds
provide useful leads for the discovery and development
of orally active CDK inhibitors.
2
ꢀ
and the ligand are 23.7 and 20.4 A , respectively.
18. Esnouf, R. J. Mol. Graphics 1997, 156, 132–134.
19. Merritt, E. A.; Murphy, M. E. P. Acta Crystallogr. 1994,
D50, 869–873.