2-Arylimidazo[2,1-b]benzothiazoles: A new family of amyloid binding agents with potential for PET and SPECT imaging of Alzheimer's brain

Article abstract of DOI:10.1016/j.bmcl.2011.03.052

We designed and synthesized a small series of 2-aryl-imidazo[2,1-b] benzothiazole, representing a combination of motifs from the two most potent amyloid imaging agents, PIB and IMPY. The binding affinity of the new compounds ranged from 6 to 133 nM. Among the best compounds, 3b (K_i = 6 nM) can be labeled with ¹¹CH₃ for PET imaging whereas 3j (K _i = 10.9 nM) can be labeled with ¹²³I for SPECT imaging.

Full text of DOI:10.1016/j.bmcl.2011.03.052

Bioorganic & Medicinal Chemistry Letters 21 (2011) 2966–2968
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
2-Arylimidazo[2,1-b]benzothiazoles: A new family of amyloid binding
agents with potential for PET and SPECT imaging of Alzheimer’s brain
⇑
David Alagille , Herve DaCosta, Ronald M. Baldwin, Gilles D. Tamagnan
Institute for Neurodegenerative Disorders, 60 Temple St., Suite 8A, New Haven, CT 06510, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We designed and synthesized a small series of 2-aryl-imidazo[2,1-b]benzothiazole, representing a com-
bination of motifs from the two most potent amyloid imaging agents, PIB and IMPY. The binding affinity
of the new compounds ranged from 6 to 133 nM. Among the best compounds, 3b (Ki = 6 nM) can be
labeled with 11CH3 for PET imaging whereas 3j (Ki = 10.9 nM) can be labeled with 123I for SPECT imaging.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 21 January 2011
Revised 11 March 2011
Accepted 15 March 2011
Available online 21 March 2011
Keywords:
2-Aryl-imidazo[2,1-b]benzothiazole
b-Amyloid
Alzheimer’s disease
PET imaging
SPECT imaging
Alzheimer’s disease (AD) is a progressive and irreversible neu-
rodegenerative disorder affecting an estimated 35 million people
worldwide.1 AD is characterized by three structural changes: a dif-
fuse neuron loss, intracellular deposit of neurofibrillary tangles
In an attempt to develop more promising amyloid imaging
agents, we synthesized a new series of 2-aryl-imidazo[2,1-b]ben-
zothiazole derivatives and tested their binding to amyloid Ab1–40
.
This series was chosen since it combines key structural motifs
shared by the most effective current agents, PIB and IMPY, possess-
ing the benzothiazole part of PIB fused with the 2-arylimidazo part
of IMPY (Fig. 1). We report here our initial results in the synthesis
and in vitro evaluation of this new series of potential amyloid
imaging agent.
(NFT) consisting mainly of hyper phosphorylated
s protein, and
extracellular deposit of senile plaque consisting mainly of amyloid
b peptide (Ab). The etiology of the disease is largely unknown, but
robust evidences (mainly genetic observations) implicate amyloid
cascade as a central event in the development of the disease.
Familial mutation within the presenilin 1 and 2 genes, which are
The general synthesis of 2-aryl-imidazo[2,1-b]benzothiazole,
outlined in Scheme 1, followed the classical16,17 condensation of
commercially available 2-aminobenzothiazole (1) with 2-bromo-
acetophenone (2) in the presence of sodium hydrogen carbonate,
to provide, upon cooling and filtration, the expected compound 3
in good to excellent yield. 40-Iodo-2-bromoacetophenone was pre-
pared by simple bromination of 4-iodoacetophenone, while
40-dimethylamino-2-bromoacetophenone was obtained in two
steps following a previously published methodology.18 The amino
derivatives were obtained by reduction of the nitro analogues
using tin chloride in good yield (a relatively large amount of EtOH
is needed since both reagent and product have a relatively low sol-
ubility in EtOH). Selective monomethylation was achieved in one
step and in excellent yield using the methodology developed by
Barluenga et al.19 All compounds were characterized by 1H and
13C NMR, HR-mass spectrometry, elemental analysis, and their
purity was confirmed by HPLC and exceeds 96%.
critical for the catalytic activity of c-secretase, cause an extremely
aggressive and early form of AD.2–4 Duplication of the amyloid pre-
cursor protein (APP) locus in extremely rare family mutation of
chromosome 21 also induces early onset of AD,5,6 The load of amy-
loid deposit in the brain correlates rather poorly with the cognitive
deficit in AD7,8 and a general consensus is emerging that soluble Ab
oligomers rather than plaque deposit9–11 make up the toxic sub-
stance associated with cognitive impairment. Imaging amyloid
deposits in living brain has stimulated much research in the past
20 years, with the goal of providing a reliable diagnostic for AD
during the patient life (previously, only postmortem autopsy was
available). Since this goal has been achieved with the discovery
of [11C]PIB12, [123I]IMPY13, and other amyloid probes14,15 such as
[
18F]florbetaben and [18F]FDDNP (Fig. 1), the new goal in AD imag-
ing is its early diagnosis and better quantification for monitoring
the amyloid burden in the brain.
All compounds were evaluated for their affinity to synthetic
amyloid plaque (Ab1–40) in a competitive binding assay against
[3H]BTA-1 ([3H]PIB) at a concentration of 1 nM; for comparison
⇑
Corresponding author.
E-mail address: dalagille@mnimaging.com (D. Alagille).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.03.052

D. Alagille et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2966–2968
2967
11CH3
NH
N
N
S
N
N
123I
HO
[11C]PIB
[
123I]IMPY
benzothiazol part
2-arylimidazo part
Y
N
X
N
S
2-aryl-imidazo[2,1-b]benzothiazole
O
O
NC
CN
18F
O
18F
N
H
N
[
18F]Florbetaben
[
18F]FDDNP
Figure 1.
O
i
N
S
Y
Br
X
N
NH2
+
N
X
S
Y
3
X = OMe, Me, F, Br
Y = NO2, I, Br, NMe2
1
2
v
X=I
Y = NH2
vi
Y = NHMe
iv
ii
O
O
iii
X=NMe2
Br
Br
Y
N
Scheme 1. Synthesis of 2-aryl-imidazo[2,1-b]benzothiazole. aReagents and conditions: (i) EtOH, reflux 2 h, then NaHCO3, reflux 4 h, 45–69%; (ii) Br2, CH2Cl2, RT, 72%; (iii)
H2SO4, Br2, RT, 16 h, 97%; (iv) THF, PO(OEt)2, Et3N, RT, 6 h, 76%; (v) EtOH, SnCl2ꢀ2H2O, reflux, 4 h, 89–65%; (vi) MeOH, MeONa, (HCHO)n, reflux 2 h, then NaBH4, reflux 2 h, 72–
83%.
purpose PIB and IMPY were screened in the same assay. Inhibition
constants (Ki) were calculated from the Cheng–Prusoff equation
(Ki = IC50/(1+[L]/KD)) using IC50 values obtained from a 12-concen-
the anilino substitution (3j–o 9.4 nM 6 Ki 6 41.9 nM). Of particular
interest, compound 3j (Ki = 10.9 nM) presents the advantage of po-
tential dual labeling with either 11C on the benzothiazole ring or
123I on the phenylimidazo side of the molecule (substituent Y);
however, the higher lipophilicity of 3j, due to the absence of the
aniline moiety, might result in increased non-specific binding
and therefore limits its use as in vivo imaging agent. The most po-
tent compound of this series was compound 3b, exhibiting an
affinity intermediate between PIB (Ki = 4.5 nM)20 and IMPY
(Ki = 15.0 nM)21 and should be easily labeled with 11CH3 at the O-
methoxy or N-methyl position.
In conclusion, we developed a novel series of amyloid binding
agents based on a 2-aryl-imidazo[2,1-b]benzothiazole scaffold,
which can be viewed as a condensed mix between PIB and IMPY.
Most of the compounds showed acceptable binding affinity for
synthetic Ab1–40 fibril, ranging from 133 nM to 6.09 nM, and the
best compound in the series (3b) can potentially be labeled with
11CH3 at either the O or N position for in vivo evaluation. These re-
sults support the hypothesis that mix-condensed analogues of PIB-
tration dose–response curve (3 lM-0.01 nM serial dilution), using
KD = 10 nM for [3H]PIB (obtained by saturation experiments). Re-
sults are expressed as the mean SD of at least three independent
experiments with less than 20% in SD and are summarized in Table
1. As a general trend, the monomethyl aniline analogs had better
affinity than their dimethyl or unsubstituted analogues (3b vs
3a,c; 3e vs 3d, f; 3h vs 3i); however, the substituent on the benzo-
thiazole side of the molecule (substituent X) is clearly important,
since the ranking between NH2, NHMe, and NMe2 was reversed be-
tween X = OMe and F, whereas almost equal binding affinities were
observed with X = Br. Among the benzothiazole substituents,
X = OMe led to the best affinity (3b, j, n Ki = 6.09, 10.9, 9.40 nM,
respectively), whereas fluorine systematically showed much lower
binding independently of phenylimidazo substituent Y (3d–f, k,
X = F, Y = NH2, NHMe, NMe2, I). More surprisingly, good to excel-
lent binding affinity values were observed for compounds lacking

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D. Alagille et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2966–2968
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Amyloid binding of 2-aryl-imidazo[2,1-b]benzothiazole derivatives
Y
X
N
N
S
a
Compound
X
Y
Ki (nM)
c Log P
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
PIB
OMe
OMe
OMe
F
F
F
Br
Br
Br
OMe
F
NH2
29.8 2.1
6.09 0.08
58.6 4.7
133 21
38.1 2.6
42.9 5.7
28.8 1.2
34.5 3.5
43.4 5.7
10.9 0.18
41.9 5.2
21.1 0.9
17.7 1.9
9.40 0.07
26.0 0.9
11.0 0.2
4.5b
3.58
3.88
4.66
3.86
4.16
4.95
4.53
4.83
5.62
5.74
6.02
6.69
6.35
5.21
5.82
3.41
NHMe
NMe2
NH2
NHMe
NMe2
NH2
NHMe
NMe2
I
I
I
I
Br
8. Giannakopoulos, P.; Herrmann, F. R.; Bussiere, T.; Bouras, C.; Kovari, E.; Perl, D.
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Br
Me
OMe
Me
–––
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Br
–––
IMPY
–––
–––
9.0 0.9
4.64
15.0b
Inhibition constants were measured on Ab1–40 against [3H]BTA-1 at 1 nM, using
KD = 10 nM. Results are expressed as mean standard deviation (n P3).
a
c Log P values are calculated using ChemDraw Ultra 10.0.
b
Literature values for the reference compounds PIB20 and IMPY.21
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compounds.
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C.; Montesiparallel, M. P.; Sorbi, S.; Rainero, I.; Pinessi, L.; Neestar, L.;