Imidazo[2,1-b]thiazoles: Multitargeted inhibitors of both the insulin-like growth factor receptor and members of the epidermal growth factor family of receptor tyrosine kinases

Article abstract of DOI:10.1016/j.bmcl.2010.03.015

The design and enzyme activities of a novel class of imidazo[2,1-b]thiazoles is presented.

Full text of DOI:10.1016/j.bmcl.2010.03.015

Bioorganic & Medicinal Chemistry Letters 20 (2010) 2452–2455
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Imidazo[2,1-b]thiazoles: Multitargeted inhibitors of both the insulin-like
growth factor receptor and members of the epidermal growth factor family
of receptor tyrosine kinases
*
Steve D. Fidanze , Scott A. Erickson, Gary T. Wang, Robert Mantei, Richard F. Clark, Bryan K. Sorensen,
Nwe Y. Bamaung, Peter Kovar, Eric F. Johnson, Kerren K. Swinger, Kent D. Stewart, Qian Zhang,
Lora A. Tucker, William N. Pappano, Julie L. Wilsbacher, Jieyi Wang, George S. Sheppard, Randy L. Bell,
Steven K. Davidsen, Robert D. Hubbard
Global Pharmaceutical Research and Development, Abbott Laboratories, Abbott Park, IL 60064, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The design and enzyme activities of a novel class of imidazo[2,1-b]thiazoles is presented.
Received 7 December 2009
Revised 1 March 2010
Accepted 4 March 2010
Available online 7 March 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
IGF-1R
EGFR
ErbB2
Kinase
Inhibitor
Imidazo[2,1-b]thiazoles
Recent literature has suggested that simultaneous inhibition of
the insulin-like growth factor receptor (IGF-IR) and members of the
epidermal growth factor family (EGFR or ErbB2) with combinations
of either small molecules or antibodies affords enhanced inhibition
of cellular proliferation relative to the single agents.1 In particular,
the combination of IGF-IR and EGFR antibodies showed improved
activity versus an A549 mouse tumor xenograft.2 Combinations
of gefitinib and NVP-ADW742 (selective EGFR and IGF-IR inhibi-
tors, respectively) caused synergistic decreases in cellular prolifer-
ation across a diverse set of cancer cell lines.3 The impressive
combination data provides rationale for a small molecule discovery
program targeting simultaneous inhibition of IGF-IR and EGFR/
ErbB2. We have previously published on multitargeted pyrazolo-
pyrimidines as inhibitors of IGF-IR, EGFR and ErbB2.4 Herein, we
disclose a novel scaffold that possesses the desired activities, sim-
ilar to those highlighted above.
appropriately substituted imidazothiazoles inhibited members of
the ErbB-family of receptor tyrosine kinases (RTKs). According to
the known SAR surrounding the ErbB-families of RTKs, the pres-
ence of a large hydrophobic head group is crucial for optimal activ-
ity versus both EGFR and ErbB2 (Fig. 1).7 In contrast, a majority of
R1
O
F
O
N
N
N
N
S
S
NH
N
H
N
N
N
N
N
During a screen of Abbott’s compound collection, we identified
a scaffold, the imidazo[2,1-b]thiazoles, that were prepared as puta-
tive p38 kinase inhibitors.5,6 Cross-screening of this class of com-
pounds revealed that in addition to p38 inhibitory properties,
R2
HN
SB-203580 (1)
3
N
2
* Corresponding author. Tel.: +1 847 937 9547; fax: +1 847 935 5165.
E-mail address: steve.fidanze@abbott.com (S.D. Fidanze).
Figure 1. Structural comparison of SB-203580 (1), a literature IGF-IR inhibitor (2),
and the desired imidazothiazole scaffold 3.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.03.015

S. D. Fidanze et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2452–2455
2453
Table 1
p38 scaffolds disclosed to date, like the prototypical p38 inhibitor,
Kinase inhibitory activities of various hinge-binding motifs
SB-203580 (1) require small substituents (like fluorine) for p38
inhibitory activity.8 A survey of the literature revealed that proper
modulation of the pendant functionality surrounding a reported
p38 scaffold afforded compound 2, which reportedly possessed en-
zyme activity versus IGF-IR.9
R1
N
S
N
R
Pyrazole 2, in combination with our screening data (vide supra),
provided the impetus to execute a lead-optimization program
geared toward targeting IGF-IR and members of the ErbB family
of RTKs using a p38 scaffold as our lead. In doing so, we needed
to develop a novel, flexible synthetic route toward highly substi-
tuted imidazothiazoles that would be required to afford a scaffold
like 3. From preliminary modeling studies, we envisioned that the
R1-substituent, head-piece, on 3 would access the hydrophobic
pocket, and R2-substituent, tail portion, would extend toward the
solvent front of the kinase.
Analog
R=
R1=
Enzyme (IC50, nM)
IGF-IR
EGFRa
ErbB2
Cl
NH
4
>12,500
>12,500
>12,500
O
N
NH
NH2
The initial set of analogs was prepared to probe the effect of
putative hinge-binders on the kinase activity profile. The benz-
imidazole moiety in 4, which has been postulated to serve as a
hinge-binder for a PLK1 scaffold10 was found to be inactive versus
the targets of interest.11 Replacement of the benzimidazole with a
4-pyridinyl group, as shown in 1, provided analog 5. This com-
pound showed improved kinase inhibition versus the ErbB family
of RTKs, however, was still inactive against IGF-IR. The use of
substituted anilinopyrimidines provided improved potency against
EGFR and ErbB2 activity, but still lacked IGF-IR activity (6–7). As
expected from known SAR in the ErbB family, a simple nitro in
the selectivity pocket abolished most ErbB2 activity (8). Finally,
elaboration of the phenyl group by addition of a 4-morpholino-
subsitutent provided 9, the first analog to display enzyme inhibi-
tion versus all three targets of interest.
The initial SAR presented in Table 1 indicated that we needed a
synthesis that was flexible enough to systematically vary both the
amide head-group and the aniline tail-group. The synthesis of the
imidazothiazole analogs proceeded from the condensation of
2-aminothiazole with the commercially available bromoketone
10 to afford 11 (Scheme 1). Freidel–Crafts acylation with acetic
anhydride provided the methyl ketone 12.12 Subsequent treatment
with DMF di-tert-butyl acetal provided enamine 13, which was
subjected to a three-step sequence entailing cyclization with gua-
nidine, diazotization/hydrolysis, and chlorination with POCl3 to
provide chloropyrimidine 14. Displacement of the chloride with
substituted anilines, followed by nitro reduction and acylation
afforded the fully elaborated imidazothiazole analogs (6–9, 16–30).
With the 2-anilinopyrimidine chosen as the preferred hinge
binder, we examined a variety of groups in the amide position of
our lead series (Table 2). Interestingly, 2,6-difluorobenzamide 16
was shown to be highly potent against ErbB2, and particularly
against EGFR. However, benzamides generally inhibited IGF-IR
fairly weakly. Cyclohexylmethyl (17) and cyclopentylmethyl (18)
amides showed considerably weaker activity against EGFR and
ErbB2. Phenyl urea (19) and phenyl carbamate (20) analogs exhib-
ited reasonably balanced, albeit weaker, potency against all three
enzymes. Phenylacetamide 21 demonstrated balanced and potent
activity against all three enzymes, although it is less active against
ErbB family kinases than 16.
Cl
NH
5
6
>12,500
>12,500
240
83
240
35
N
O
O
N
N
N
N
Cl
NH
N
NH
NH
NH
NH
7
8
N
N
N
>12,500
>12,500
55
6.3
O
NH
NO2
230
6000
O
NH
9
1500
7.9
4.8
N
O
a
EGFR kinase construct possessed the L858R activating mutation.
ied between EGFR/ErbB2 and IGF-IR, these side chains are all polar.
Polar ligand–protein interactions in this region might modulate
enzyme selectivity, but accurate prediction would not be possible
because of the difficulty in quantitating polar interactions and
desolvation effects in a solvent exposed environment. Thus IGF-
IR potency could possibly be improved by addition of a polar tail
to the 2,6-difluorobenzamides. Furthermore, the presence of the
crystal structure confirmed that the most potent IGF-IR amide,
the phenylacetamide, was tolerated by the ErbB-family of RTKs,
confirming that this amide was worthy of further SAR exploration
(vide infra).
In order to assist in further SAR development, we obtained an
EGFR X-ray co-crystal structure with analog 21, which possessed
a phenylacetamide head group. As shown in Figure 2, the amide
head group lies deep in the protein in the selectivity/ hydrophobic
pocket. The anilinopyrimidine forms two hydrogen bond interac-
tions with the backbone of Met 769 in the hinge region of the pro-
tein. The morpholine tail of analog 21 extends to a solvent exposed
region of the protein and close to Asp 776 and Glu 780 side chains.
These two residues of EGFR correspond to Asp 786 and Glu 790 of
ErbB2 and Ser 1059 and Ser 1063 of IGF-IR, respectively. While var-
Exploration of a variety of tail groups, as illustrated by com-
pounds 22–25 in Table 3 showed that a compound with a neutral
group (22), exhibits low nM potency for EGFR and ErbB2, but only
micromolar affinity for IGF-IR. Empirically, we found that cationic

2454
S. D. Fidanze et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2452–2455
NO2
NO2
NO2
c
a
b
N
N
N
O
S
S
N
Br
O
10
11
12
NO2
NO2
NO2
N
N
N
d
e
S
S
N
S
N
N
N
N
N
N
O
N
14
15
R1
13
Cl
NH
S
N
N
O
R2
N
f
HN
NH
Figure 2. X-ray crystal structure of 21 in EGFR.13
N
R1
6-9, 16-30
Scheme 1. Reagents and conditions: (a) 2-aminothiazole, NMP, 85 °C, 90%; (b)
H2SO4, Ac2O, 140 °C, 85%; (c) DMF-di-tert-butyl acetal, NMP, 90 °C, 89%; (d) 1-
guanidineÁHCl, K2CO3, NMP, 100 °C, 85%: 2-NaNO2, AcOH, water, 60 °C, 90%; 3-
POCl3, 80 °C, 83%; (e) ArNH2, HCl, 2-propanol, 35–90%; (f) 1-iron powder, NH4Cl,
EtOH, water, 90 °C, 50–88%; 2-RC(O)Cl, NMP, 45–99%
Table 3
Kinase inhibitory activities of 2,6-difluorobenzamides
S
N
N
F
N
Table 2
NH
HN
O
Kinase inhibitory activities of 4-morpholino imidazothiazoles
N
R2
R1
O
F
S
N
Analog
R1=
R2=
Enzyme (IC50, nM)
EGFR
N
N
IGF-IR
3800
ErbB2
86
N
NH
HN
R
N
O
O
N
22
23
H
6.2
Analog
R=
Enzyme (IC50, nM)
H
H
210
0.78
4.3
IGF-IR
1500
EGFR
7.9
ErbB2
4.8
N
9
Ph
N
F
F
24
25
230
100
0.96
2.3
14
N
16
1300
1.9
86
7.2
17
18
1100
580
870
660
1400
1300
HN
19
20
NHPh
OPh
590
450
100
190
170
170
aniline (24) afforded a compound of similar potency to 23. Incorpo-
ration of a fused ring secondary amine (25) further improved the
IGF-IR potency twofold over 23, while maintaining excellent EGFR
and ErbB2 potency. Overall, the presence of an appropriately
substituted aniline such as 25 provided an increase of 13-fold in
potency against IGF-IR, and a 12-fold improvement against ErbB2
as compared to 16, without a loss of enzyme activity versus EGFR.
Unfortunately, this series of analogs did not afford the balanced en-
zyme profile that was desired.
21
130
63
370
tail groups, as shown in compounds 23–25, improved the potency
for all three enzymes by approximately an order of magnitude. In-
deed, addition of an (N,N-dimethyl)ethylamine at the 4-position of
the aniline (23) provided a >6-fold improvement in potency
against IGF-IR compared to 16, and a >15-fold potency improve-
ment against ErbB2. An ethylpiperazine at the 4-position of the
We then returned to analogs possessing the phenylacetamide
head group, which had previously been shown in compound 21
to provide a balanced, albeit weaker enzyme activity profile.

S. D. Fidanze et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2452–2455
2455
Table 4
Table 5
Kinase inhibitory activities of phenylacetamides
Inhibition of cellular phosphorylation
a
Analog
IC50 (nM)
S
pIGF-1R
p-EGFR
p-ErbB2
N
N
23
24
26
330
200
140
220
50
610
27
69
680
O
N
NH
HN
N
28
30
Erlotinib
Lapatinib
OSI-906
41
180
>10,000
>10,000
18
350
1000
34
52
>10,000
310
120
5200
100
>10,000
R2
R1
Analog
R1=
R2=
Enzyme (IC50, nM)
EGFR
a
IGF-IR
280
ErbB2
73
pIGF-1R and pEGFR were performed in A431 epidermoid carcinoma cells.
pErbB2 was performed in N87 gastric cancer cells.
N
O
26
27
H
56
57
pound 21 bound to EGFR, the incorporation of an appropriately
functionalized amine tail provided the desired activity profile. Fur-
ther improvements in potency and selectivity in this and related
series will be reported in due course.
81
52
160
170
N
N
N
28
H
16
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.03.015.
29
30
H
46
27
24
150
55
N
References and notes
6.4
NH
1. (a) Dempke, W. C. M.; Heinemann, V. Eur. J. Cancer 2009, 45, 1117; (b) Quanri, J.
J. E. F. J.; Esteva, F. J. J. Mammary Gland Biology and Neoplasia 2008, 13, 485.
2. Goetsch, L.; Gonzalez, A.; Leger, O.; Beck, A.; Pauwels, P. J.; Haeuw, J. F.; Carvaia,
N. Int. J. Cancer 2005, 113, 316.
3. Wilsbacher, J. L.; Zhang, Q.; Tucker, L. A.; Hubbard, R. D.; Sheppard, G. S.;
Bamaung, N. Y.; Fidanze, S. D.; Wang, G. T.; Hu, X.; Davidsen, S. K.; Bell, R. L.;
Wang, J. J. Biol. Chem. 2008, 283, 23721.
4. Hubbard, R. D.; Bamaung, N. Y.; Fidanze, S. D.; Erickson, S. A.; Palazzo, F.;
Wilsbacher, J. L.; Zhang, Q.; Tucker, L. A.; Hu, X.; Kovar, P.; Osterling, D. J.;
Johnson, E. F.; Bouska, J.; Wang, J.; Davidsen, S. K.; Bell, R. L.; Sheppard, G. S.
Bioorg. Med. Chem. Lett. 2009, 19, 1718.
5. Borhani, D. W.; Calderwood, D. J.; Frank, K. E.; Davis, H. M.; Josephsohn, N. S.;
Skinner, B. S. WO 2008/063287.
Optimization of the tail position, as in the benzamide series (Table
3), provided the analogs in Table 4. A fused ring tertiary amine (27)
provided a small improvement in IGF-IR and ErbB2 potency com-
pared to 21, without loss of EGFR activity. An ethylpiperazine in
the 4-position of the aniline (28) provided a further boost to IGF-
IR and EGFR potency. The secondary amine 30 provided a uniform
increase in potency compared to 21 of at least fivefold, also provid-
ing a compound with balanced activity of <100 nM against all three
enzymes.
Selected compounds were further characterized by their ability
to prevent target phosphorylation in cellular systems.11 As shown
in Table 5, inhibition of intracellular phosphorylation correlated
well with kinase inhibitory activity. Several compounds were ac-
tive in the cellular assay at <400 nM across all three kinases. Sur-
prisingly, although the phenylacetamides (26,14 28, 30) appear to
have a more balanced activity profile in the enzyme assay, the
2,6-difluorobenzamides (23, 24) were as balanced in the cellular
assay. Furthermore, compound 28 approaches the cellular activity
of OSI-906 against IGF-1R, and compounds 23 and 24 are as potent
as lapatinib and erlotinib against EGFR and ErbB2. It should also be
noted that while the EGFR kinase assays were performed using the
L858R mutation, the A431 cells used in the cellular assay possess
wild-type EGFR.
6.
A similar scaffold displaying IGF1R activity has also been reported
independently of this work: Emmitte, K. A.; Wilson, B. J.; Baum, E. W.;
Emerson, H. K.; Kuntz, K. W.; Nailor, K. E.; Salovich, J. M.; Smith, S. C.; Cheung,
M.; Gerding, R. M.; Stevens, K. L.; Uehling, D. E.; Mook, R. A., Jr.; Moorthy, G. S.;
Dickerson, S. H.; Hassell, A. M.; Leesnitzer, M. A.; Shewchuk, L. M.; Groy, A.;
Rowand, J. L.; Anderson, K.; Atkins, C. L.; Yang, J.; Sabbatini, P.; Kumar, R. Bioorg.
Med. Chem. Lett. 2009, 19, 1004.
7. Lackey, K. E. Curr. Top. Med. Chem. 2006, 6, 435.
8. (a) Wang, Z.; Canagarajah, B. J.; Boehm, J. C.; Kassisa, S.; Cobb, M. H.; Young, P.
R.; Abdel-Meguid, S.; Adams, J. L.; Goldsmith, E. J. Structure 1998, 6, 1117; (b)
Boehm, J. C.; Adams, J. L. Expert Opin. Ther. Patents 2000, 10, 25.
9. Garcia-Echeverria, C. WO 2005/068452.
10. Lansing, T. J.; McConnell, R. T.; Duckett, D. R.; Spehar, G. M.; Knick, V. B.;
Hassler, D. F.; Noro, N.; Furuta, M. Mol. Cancer Ther. 2007, 6, 450.
11. The kinase activities of disclosed compounds were assayed by a homogeneous
time-resolved fluorescence assay as previously described. The inhibition of
intracellular phosphorylation by selected compounds was assayed using ELISA
as previously described: Hubbard, R. D.; Bamaung, N. Y.; Palazzo, F.; Zhang, Q.;
Kovar, P.; Osterling, D. J.; Hu, X.; Wilsbacher, J. L.; Johnson, E. F.; Bouska, J.;
Wand, J.; Bell, R. L.; Davidsen, S. K.; Sheppard, G. S. Bioorg. Med. Chem. Lett.
2007, 17, 5406.
In conclusion, we have disclosed that suitably functionalized
imidazothiazole scaffolds provide potent and balanced enzyme
and cellular activity against IGF-IR, EGFR, and ErbB2. The selectiv-
ity profile has been shown to be highly dependent upon the amide
head group. In addition, as suggested by a crystal structure of com-
12. Stevens, K. L.; Jung, D. K.; Alberti, M. J.; Badiang, J. G.; Peckham, G. E.; Veal, J. M.;
Cheung, M.; Harris, P. A.; Chamberlain, S. D.; Peel, M. R. Org. Lett. 2005, 7, 4753.
13. The refined crystallographic coordinates have been deposited in the Protein
Data Bank (www.rcsb.org) with entry code 3LZB.
14. Compound 26 has been tested against a panel of 56 kinases for selectivity. Data
can be found in Supplementary data.