N-substituted 2′-(aminoaryl)benzothiazoles as kinase inhibitors: Hit identification and scaffold hopping

Article abstract of DOI:10.1016/j.bmcl.2009.01.054

Starting with a hit from vHTS attained by a docking procedure of virtual compounds into ATP pockets of different kinases applying the 4SCan technology, variations of the adenine mimic resulted in the identification of promising scaffolds, giving rise to in vitro IC₅₀ values in the nanomolar range on different kinases down to 63 nM.

Full text of DOI:10.1016/j.bmcl.2009.01.054

Bioorganic & Medicinal Chemistry Letters 19 (2009) 1349–1356
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
N-substituted 20-(aminoaryl)benzothiazoles as kinase inhibitors: Hit
identification and scaffold hopping
b
Stefan Tasler a, Oliver Müller a, Tanja Wieber a, Thomas Herz a, Rolf Krauss a, , Frank Totzke ,
*
Michael H. G. Kubbutat b, Christoph Schächtele b
a 4SC AG, Am Klopferspitz 19a, 82152 Planegg-Martinsried, Germany
b ProQinase GmbH, Breisacher Str. 117, 79106 Freiburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Starting with a hit from vHTS attained by a docking procedure of virtual compounds into ATP pockets of
different kinases applying the 4SCanÒ technology, variations of the adenine mimic resulted in the iden-
tification of promising scaffolds, giving rise to in vitro IC50 values in the nanomolar range on different
kinases down to 63 nM.
Received 16 December 2008
Revised 14 January 2009
Accepted 15 January 2009
Available online 22 January 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Aurora
EGFR
Protein kinase
Molecular modeling
vHTS
Protein kinases play a vital role in cell signal transduction and
proliferation of cells. Focusing on cancer, deregulation of activity
or expression levels of certain kinases is implicated and linked to
different stages of tumor genesis, promotion, survival and metasta-
sis (for a selection, see Table 1).1–3 Small molecule kinase inhibitors
are usually addressing the ATP binding site,1,3–7 which is highly
conserved among the 518 kinases being encoded in the human
genome. Thus, selectivity for certain kinases might only be achiev-
able by addressing side pockets of the binding site or inactive state
conformations.1,3,4,7–10 A different approach aims at multi-target
inhibitors, which might even be advantageous for successfully
tackling cancer cells in terms of efficacy and avoiding quick devel-
opment of resistances.1,4,6,11–16 With around 165 kinases being
identified to be of potential relevance in cancer cells,1,17 the ap-
proach described in here aimed at identifying small molecule
inhibitors with an interesting inhibition profile on selected kinases,
which displayed a scattered activity within different tumor stages
as depicted in Table 1. The inhibition of kinases whose functional-
ity is essential for non-tumor cells, like the insulin receptor (InsR)
had to be avoided. In short: selectively unselective kinase inhibi-
tors were envisaged. An initial screen was performed usually on
a set of 16 kinases (Table 1) and InsR.18
as deduced from crystal structures of four different kinases out of
the proliferation, survival and angiogenesis section (Aurora A,
Akt1, IGF1R, VEGFR2) using the 4SCanÒ technology.19 From a data-
base of 3.3 Mio commercially available compounds, around 250
were selected for biological testing according to their docking
scores and simultaneously covering structural diversity. With an
initial hit rate of 16% in an in vitro kinase screen (residual activity
at 10 lM below 50% on at least one of the tested kinases), several
different entities were identified as potential hit structures, includ-
ing acyl-thiourea derivatives. These were next submitted to a vir-
tual 2D similarity search within the same database for a hit
validation, resulting in the identification of compound 1 displaying
several excellent inhibitory activities with IC50 values below 1
even down to 190 nM on the Src kinase (Fig. 1).
lM,
Besides the excellent inhibitory activity at Src and, similarly, at
EGFR and IGF1R, several derivatives of this class of compounds dis-
played inhibitory activities to the InsR in a low micromolar range,
compound 1 even with an IC50 of 900 nM. Furthermore, the scaf-
fold of 1 incorporated a pharmacologically unfavourable thiourea
linker.20 Thus, our approach was a suitable replacement of this
unit, keeping the benzothiazole moiety constant. Certain N-unsub-
stituted (aminophenyl)benzothiazoles like 2 and 3 (Fig. 1) are
known from literature to be able to unfold an antitumor activity,
even though not by inhibition of protein kinases but by preferen-
tially activating metabolic pathways via the aryl hydrocarbon
receptor within sensitive tumor cells.21 For our synthetic purposes,
several differently substituted 20-(3-aminophenyl)- and 20-(4-ami-
Hit finding was accomplished by virtual high throughput
screening (vHTS) based on docking experiments into the active site
* Corresponding author. Tel.: +49 89 700 763 0; fax: +49 89 700 763 29.
E-mail address: rolf.krauss@4sc.com (R. Krauss).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.01.054

1350
S. Tasler et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1349–1356
Table 1
as present in the initial hit molecule. Thus the effect of several such
Selection of kinases for initial screening
groups was evaluated for both benzothiazole scaffolds with the
amino group being located in the 3- or 4-position of the aryl ring.
Synthetic realization of such derivatives is only depicted for the
linear 4-aminoaryl series, though, but was similar for the second,
bent 3-aminorayl series (Schemes 1 and 2). Guided by detailed
scaffold analyses in the literature,8,11,23–26 focus was laid on the
incorporation of pyrimidines, purines, quinolines and quinazolines.
Further variations of these core fragments evolved from a superpo-
sition of structures with the suggested 5-point pharmacophore for
promiscuous kinase inhibitors,9 which allowed for a rough estima-
tion of possible different binding modes within the hinge region
and thus for a generation of further substitution ideas (Fig. 2). In
most cases, attachment of the heterocycles envisaged was
achieved by treating the (aminophenyl)benzothiazoles 2 and 3
with the respective chlorinated heterocycle (Schemes 1 and 2),
which was realized in DMF at 160 °C for 4-chloropyridines 4 and
in ethylene glycol at 110–160 °C for mono-chlorinated pyrimi-
dine-based heterocycles 11, 13, and 4,7-dichloroquinoline (not
shown, yields: 5–45%) with optional acceleration of reaction rates
by addition of HCl in dioxane or NaI. For substrates 2,4-dichloropy-
rimidine (9) or –quinazoline 15, a regioselective twofold amination
was based on different reactivities in the 2- and 4-position of these
substrates27: amination of the more reactive 4-position was
achieved with the corresponding amine in alcoholic solution (eth-
anol or n-butanol) and added base at ambient temperatures (for
pyrimidines) or moderate heating (for quinazolines), the second
amination at the 2-position succeeded under similar conditions
but now without any base present and at elevated temperatures,
either with or without purification of the intermediates formed
Proliferation
Survival
Angiogenesis
Metastasis
Aurora A
Aurora B
CDK2
CDK4
EGFR
ErbB2 (HER2)
PDGFRb
PLK1
Akt1
IGF1R
VEGFR2 (KDR)
VEGFR3 (FLT4)
TIE2
FAK
Src
EphB4
nophenyl)benzothiazoles 2 and 3 were prepared (Fig. 1; R = Me, Cl,
OH and OMe, varying positions) as building blocks and initially
tested in vitro for their inhibitory activity on 16 kinases, also
including DF 203 (3, R = 3-Me) – one of the above mentioned, most
potent compounds with antiproliferative activity on MCF-7 cells21
– only to verify that these compounds were not remarkably potent
on these targets, with IC50 values ranging around 10 lM at best.
The syntheses of 20-(3-aminophenyl)- or 20-(4-aminophenyl)ben-
zothiazole derivatives as well as 20-(4-bromophenyl)benzothiazole
(7, Scheme 1) followed a literature procedure, in which condensa-
tion of 2-aminothiophenols with differently substituted amino- or
bromobenzoic acids, respectively, was achieved in polyphosphoric
acid at 185 °C.22
According to literature data it was assumed that the attachment
of an ‘‘adenine mimic” at the aniline nitrogen of building blocks 2
and 3 should result in improved inhibitory activities, due to its
capability to effectively bind into the hinge region of the protein23
– which is most likely not adequately addressed by a thiourea unit
Figure 1. IC50 values [lM] on selected kinases for the initial hit molecule, and structures of building blocks 2 and 3.
Scheme 1. Reagents and conditions: (i) H2NMeꢀHCl, EDC, DIEA, DMF, r.t., 48 h; (ii) DMF, 160 °C, 8 h; optional for acceleration of otherwise sluggish reactions: HCl in dioxane
(4.0 M, 1.0 equiv); (iii) Pd2dba3 (0.04 equiv), Xantphos (0.12 equiv), K3PO4 (1.4 equiv), dioxane, 100 °C, 20 h; (iv) Pd(OAc)2 (0.1 equiv), JohnPhos (0.2 equiv), K3PO4 (1.4 equiv),
DME, 100 °C, 20 h; (v) Pd2dba3 (0.05 equiv), JohnPhos (0.2 equiv), NaOtPent (1.4 equiv), toluene, 110 °C, 20 h; (vi) 5-methyl-1H-pyrazol-3-amine (1.5 equiv), HCl in dioxane
(4.0 M, 1.0 equiv), ethylene glycol, 160 °C, 24 h; yields are not optimized.

S. Tasler et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1349–1356
1351
Scheme 2. Reagents and conditions: (i) aliphatic amine or 5-methyl-1H-pyrazol-3-
amine, DIEA, EtOH, r.t., 20 h; for 5-methyl-1H-pyrazol-3-amine, additional 4 h at
50 °C; (ii) 20-(4-aminopheny)benzothiazole, ethylene glycol, 160 °C, 3–6 h; (iii)
ethylene glycol, 110 °C, 3 h; optional for acceleration of otherwise sluggish
reactions: HCl in dioxane (4.0 M, 1.0 equiv), additional 3 h at 120 °C; (iv) DIEA
(1.0 equiv), nBuOH, 120 °C, 20 h, then aliphatic amine or 5-methyl-1H-pyrazol-3-
amine (2.0 equiv), 140 °C, 3 h; yields are not optimized.
upon first amination. For a few substrates, palladium-catalyzed
amination reactions had to be conducted: substituted 4-amino-
pyridines 6 were arylated with 20-(4-bromophenyl)benzothiazole
(7) using Xantphos28 or biphenyl-2-yl-di-tert-butyl-phosphine
(JohnPhos)29 as ligand, in poor yields though following either var-
iant (<10%; Scheme 1).30 Interestingly, 2-chloro-4-(40-methyl-pip-
erazin-10-yl)-pyrimidine – obtained from 2,4-dichloropyrimidine
(9) and N-methylpiperazine – could not be converted into the
corresponding diaminated substrates 10 applying a second nucle-
ophilic substitution step as for all other similar substrates, the
second amination had to be replaced by a Pd-catalyzed amination
reaction as well.30
With an attachment of 6-membered heterocycles to the linear or
bent (aminophenyl)benzothiazole fragments 2 and 3, both of the
series examined displayed some inhibitory activities with a focus
on Aurora A (only the linear series is depicted in Table 2, for a bent
derivative, see Table 5). For attaining inhibitory activities with IC50
Figure 2. Reference compounds in superposition with the pharmacophore for
promiscuous kinase inhibitors (green)9,33 and discussion of binding modes for
representatives of the arylbenzothiazole series. Lined green circles = hydrogen bond
donors within the ligand, dotted green circles = hydrogen bond acceptor, full green
circle = aromatic region;9 blue areas = simplified pocket nomenclature according to
Liao.33
mide (entry 2) – a moiety also found in Sorafenib12,31 (Fig. 2). Obvi-
ously, best interaction within the hinge region was attained among
the 6-membered heterocycles by the addition of an aminopyrazole
unit – as present in the Aurora inhibitor VX-68032 (Fig. 2) – which
resulted in best inhibitory activities for both, linear and bent 20-
values around 1 lM, the presence of a hydrogen bond donor for
binding to the hinge region was required within the adenine mimic
but not a sufficient precondition, though (cf. entry 4). The only
example with highlighted inhibitory activities within the EGFR/
ErbB2 group (IC50 = 1100 nM on ErbB2) was the pyridinecarboxa-

1352
S. Tasler et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1349–1356
Table 2
In vitro IC50 values [lM] on selected kinases out of a panel of 16; n.d. = not determined
Entry
R1
R2
H
Proliferation
Aurora B
Angiogenesis
Aurora A
>10
EGFR
>10
ErbB2
>10
VEGFR2
VEGFR3
InsR
>10
1
2
>10
>10
>10
6.000
H
>10
3.600
>10
>10
2.000
>10
1.100
>10
3
4
H
H
8.800
8.900
6.500
6.900
>10
1.800
1.300
>10
>10
>10
>10
9.700
5
6
H
H
>10
>10
>10
>10
>10
n.d.
>10
>10
>10
n.d.
>10
>10
1.500
>10
7
8
H
>10
>10
>10
>10
5.200
>10
0.840
H
7.000
3.100
>10
>10
n.d.
>10
>10
>10
n.d.
>10
>10
>10
0.370
0.320
9
2-OMe
0.5 µM
0.501–2.0
µM
(aminophenyl)benzothiazoles, with IC50s down to 320 nM and a
clear Aurora selectivity for the former (Table 2, entries 8 and 9)
and even 160 nM for the latter (Table 5, entry 31), which proved
to be less selective (also 190 nM at Src). According to literature,26,33
incorporation of such a hinge region binding motif would position
the arylbenzothiazole from the front cleft towards the back cleft,
occupying the intersection of binding pockets I and II (BP-I and
BP-II, respectively; Fig. 2) according to the nomenclature estab-
lished by Liao,33 an area wide enough to host either linear or bent
arylbenzothiazole moieties. Attachment of an otherwise unsubsti-
tuted 4-pyridyl resulted in compounds offering only one possibility
for hydrogen bonding to the hinge region, either by the pyridine it-
self (mode I, X = N; Fig. 2) or by the anilinic NH (mode II, X = N) –
which is apparently not sufficient within this structural class of
compounds (no inhibitory activities detected).
for the methylamino group being positioned between the two
endocyclic nitrogens of a pyrimidinyl (around 6 M, entry 4), but
clearly better (by a factor of 4) for the N-shifted pyrimidinyl of en-
try 5 with an IC50 of 1.5 M on Aurora A. The gain in inhibitory
l
l
activity as compared to simply pyridyl might be explained by bind-
ing mode I (Fig. 2), in which the additional methylamino group
would match a hydrogen bond donor unit of the 5-point pharma-
cophore model. Comparing compounds of entries 3 and 8, both
containing the aminopyrazole unit enabling strong interactions
within the hinge region, a 4-pyridyl unit was again less favourable
by a factor of 4 as compared to the pyrimidinyl: IC50 value of
1300 nM at Aurora A for the former (entry 3) vs. around 350 nM
for the latter derivatives (entries 8, 9). Consequently, a heterocyclic
nitrogen in 60-position seemed to be essential for attaining opti-
mized inhibitory activities (cf. entries 1, 3 and 4 with 5, 8 and 9).
Incorporation of an endocyclic nitrogen next to the anilinic attach-
ment site was identified to be a privileged substructure for kinase
inhibitors, no statement, however, was made on which of the two
For linear (aminophenyl)benzothiazoles, an additional 30-meth-
ylamino group on the pyridyl resulted in some inhibitory activity
on VEGFR3 (entry 1), which was found to be in a similar range

S. Tasler et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1349–1356
1353
possible positions (20 or 60) to be preferred.26 Even though struc-
tures of compounds of entries 8 and 9 possess an array probably
coinciding with all areas of the postulated 5-point pharmacophore
of promiscuous kinase inhibitors9 (cf. superposition with VX-680
in Fig. 2, wherein the benzothiazole replaces the cyclopropylamide
unit, with the nitrogen of the thiazole unit matching the hydrogen
bond acceptor region of the pharmacophore), these derivatives dis-
played high selectivity for Aurora A, which, however, was less
emphasized for the bent variant of entry 31 (Table 5).
When the scaffold offered no hydrogen bond donor properties
within the adenine mimic (entries 6, 7 and also 4-pyridyl), a com-
plete loss of inhibitory activity was found, with 4-piperazinyl-
pyrimidin-2-yl being an exception (IC50 = 840 nM at Aurora A;
entry 7). Incorporation of a piperazine unit was likewise deduced
from VX-680, in our case, however, generating a substrate which
might accomplish hinge region binding only by its anilinic NH. This
fact may be compensated by a decent orientation of the arylbenzo-
thiazole within the adenine pocket towards BP-II in the back cleft
and positioning of the piperazine within the secondary hydropho-
bic patch E0 (mode II, Fig. 2).33
(entry 13) and TIE2 for purine (entry 15). With the pyrrolopyrimi-
dine as core fragment, the methoxy group R2 (entry 11) seemed to
be the decisive selectivity factor, as its unsubstituted (aminophe-
nyl)benzothiazole analog (entry 10) was rather equi-active on
EGFR, IGF1R, VEGFR2, TIE2 and Src. Incorporation of thieno[3,2
-d]pyrimidin-4-yl resulted in rather selective Aurora B inhibitors
depending on an appropriate choice of the substituent within the
arylbenzothiazole unit (entries 16–19): the fluoro-substituted
derivative (entry 19) displayed a good inhibitory activity with an
IC50 value of 290 nM with almost complete selectivity.
Investigating the influence on inhibitory activity for quinoline/
quinazoline derivatives, the starting scaffold was a 70-chloroquino-
line,36 to which were added additional substituents and heteroat-
oms (Table 4). Using the former adenine mimic (entries 20–23),
substituents at the aryl unit of the 20-(4-aminophenyl)benzothia-
zole showed a drastic effect, in that either no inhibitory activity
was detectable (entry 21, R2 = 2-OMe) or a broadly scattered inhi-
bition profile was identified with several sub-micromolar IC50 val-
ues. Generally, these compounds were rather inactive at Aurora A,
a slight preference for Src, VEGFR2 (cf. Ref. 37) and Aurora B was
deducible, with best IC50 values around 500 nM (entries 22 and
23).
Next, bicyclic heteroaromatics were investigated for their abil-
ity to bind within the hinge region: attachment of pyrrolopyrimi-
dines,34 purines35 and thienopyrimidines11 to the (aminoaryl)
benzothiazole fragments (Table 3). With the (all N)-heterocycle
scaffolds, a methoxy substitution of the arylbenzothiazole proved
to be advantageous, resulting in generally better inhibitory activi-
ties (entries 10, 12, 14 vs. 11, 13, 15). Variation of the heterocycle
resulted in different kinases to be the preferred target with IC50
Keeping the chloro-substituent in 70-position but adding a
methoxy group and an additional heterocyclic nitrogen (quinazo-
line framework)38 emphasized the preference for Src (now
300 nM) and resulted in a certain selectivity over other kinases
(entry 24). Replacement of 70-Cl by an additional methoxy yielded
rather selective kinase inhibitors with good IC50 values below
400 nM on different kinases, depending on the substituent at-
values around 1 l
M (R2 = 2-OMe): EGFR for an unsubstituted pyr-
rolopyrimidine (entry 11), VEGFR2 for methoxypyrrolopyrimidine
tached to the arylbenzothiazole (entries 25–28): Aurora
B
Table 3
In vitro IC50 values [lM] on selected kinases out of a panel of 16; n.d. = not determined; for InsR, all IC50 > 10 lM, except for entry 10: IC50 = 6.95 lM
Entry
R1
R2
Proliferation
Survival
IGF1R
Angiogenesis
VEGFR3
Metastasis
Src
Aurora A
Aurora B
EGFR
ErbB2
VEGFR2
TIE2
10
11
H
8.000
>10
>10
>10
3.100
1.600
2.100
>10
1.500
>10
1.300
0.720
1.500
3.600
1.900
8.600
1.200
>10
2-OMe
12
H
>10
>10
7.700
>10
4.400
10.000
>10
8.700
6.400
13
14
2-OMe
H
>10
>10
>10
>10
5.100
7.800
>10
n.d.
2.900
>10
7.700
n.d.
2.500
n.d.
2.100
>10
0.960
>10
15
16
2-OMe
H
5.800
2.800
>10
2.600
>10
>10
>10
>10
>10
9.200
>10
3.400
>10
1.300
>10
>10
>10
0.720
17
18
19
2-OMe
2-CI
3-F
>10
>10
4.000
>10
>10
0.290
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
4.400
4.100
>10
>10
>10
0.5 µM
0.501–2.0 M
µ

1354
S. Tasler et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1349–1356
Table 4
In vitro IC50 values [lM] on selected kinases out of a panel of 16; n.d. = not determined
Entry
R1
R2
Proliferation
Aurora B
Survival
IGF1R
Angiogenesis
TIE2
Metastasis
Src
InsR
Aurora A
EGFR
VEGFR2
EphB4
20
21
22
23
H
5.700
>10
>10
5.300
>10
0.800
>10
7.000
>10
>10
2.700
>10
>10
0.860
>10
0.800
1.100
0.640
>10
0.600
0.800
1.400
>10
0.800
1.000
0.630
>10
0.500
0.700
0.750
>10
>10
2-OMe
2-CI
3-F
>10
>10
0.510
1.000
24
H
2.700
>10
>10
3.300
>10
>10
>10
0.300
1.000
25
26
27
28
H
>10
>10
>10
>10
6.800
3.400
6.900
>10
n.d.
0.840
n.d.
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
0.350
4.400
>10
0.135
3.700
4.200
>10
1.500
0.240
2.200
3-Me
2-OH
2-OMe
>10
n.d.
0.063
29
30
H
H
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
0.630
6.000
>10
3.300
3.000
2.150
1.650
0.5 µM
0.501–2.0 M
µ
(IC50 = 135 nM; entry 25) or EGFR (IC50 = 240 nM and excellent
63 nM; entries 26 and 28). With a 2-methoxy substituent (entry
28), complete selectivity for EGFR was achieved within the panel
of kinases tested. The 40-(arylamino)-60,70-dialkoxyquinoline pat-
tern represents a privileged scaffold for EGFR inhibition8,11,25 with
one of the most prominent representatives being Iressa (Fig. 2),
which is already FDA approved for the treatment of metastatic
non-small cell lung cancer as a selective EGFR/ErbB2 inhibi-
tor.8,11,39 Binding mode of the compounds of Table 4 bearing the
60,70-dimethoxyquinazoline (entries 25–28) should be comparable
to that of Iressa, with an additional stretching into the intersection
area of BP-I and -II (Fig. 2). Based on such a binding mode, knowl-
edge of the 5-point pharmacophore for promiscuous kinase bind-
ing9 would suggest an attachment of an additional hydrogen
bond donor within the 20-position of the quinazoline to probably
allow for enhanced binding within the hinge region (mode III
and IV, Fig. 2). However, such substitutions proved to be detrimen-
tal to inhibitory activities: with a 20-methylamino substitution in
place, only an IC50 of 950 nM was attained for PDGFRb, paralleled
by an IC50 of 630 nM for InsR (entry 29). With aminopyrazole being
attached as a self-sustaining unit for hinge region binding, a medi-
ocre IC50 value of 1650 nM was achieved on IGF1R (entry 30), and
again, inhibition of the InsR was found to be within the same range
(IC50 = 2150 nM). Such a general inhibition level is comparable to
the aminopyrazole-pyridine derivative of entry 3 (Table 2), which
was likewise found to display IC50 values only around 1.5 M (on
Aurora A though), but clearly unfavourable to those from the
l
pyrimidine series (Table 2, entries 8 and 9).
All adenine mimics depicted in Tables 2–4 were likewise at-
tached to the bent 20-(3-aminophenyl)benzothiazole derivatives
2. Utilizing pyridine and pyrimidine moieties, results were rather
comparable to those of the linear series (cf. Table 2), giving rise
to moderate IC50 values around 1 lM especially on Aurora A, but
now sporadically also on Aurora B and VEGFR3, with the most
promising variation implementing an aminopyrazole unit (Table
5, entry 31). Incorporation of the latter unit proved to be manda-
tory for attaining any moderate inhibitory activity at all for com-
pounds based on the different quinazoline frameworks (entry
32), resulting in a broad inhibition panel with several IC50 values
around 1 lM – but clearly being disadvantageous as compared to
the results of the corresponding pyrimidine (IC50 = 160 nM on Aur-
ora A, entry 31). Similarly, bent derivatives of the purine-derived
adenine mimics (cf. Table 3) displayed rather poor inhibitory activ-
ities, only for a pyrrolopyrimidine and a purine variation IC50 val-
ues around 1 lM were attained: on TIE2 in the former (entry 33)
and rather non-selectively on IGF1R, VEGFR2, EphB4 and Src in
the latter case (entry 34). Within the bent series, however, selectiv-
ity over the InsR regularly became an issue, the insulin receptor

S. Tasler et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1349–1356
1355
Table 5
In vitro IC50 values [
lM] on selected kinases out of a panel of 16; n.d. = not determined; IC50 values [
l
M] for InsR: 1.6 (entry 31), 0.94 (entry 32), >10 (entries 33, 34)
Entry
31
R
Proliferation
Aurora B
Survival
IGF1R
Angiogenesis
Metastasis
Src
Aurora A
EGFR
PDGFRb
VEGFR2
7.500
VEGFR3
TIE2
EphB4
>10
2.000
1.900
>10
2.500
1.600
>10
0.160
0.1000
0.190
0.960
32
33
5.500
>10
3.300
1.900
4.400
0.780
0.810
1.100
>10
>10
6.400
>10
>10
>10
n.d.
>10
>10
>10
n.d.
>10
>10
1.200
34
1.700
n.d.
1.200
0.870
0.840
1.100
0.5 µM
0.501–1.5 M
µ
was likewise inhibited with IC50 values around 1–3
cases (selected data shown in Table 5).
lM in several
References and notes
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B), 6.6 (CDK4), 1.6 (EGFR), 7.5 (IGF1R), 7.9 (Src).