Design, synthesis and structure-activity relationship of rhenium 2-arylbenzothiazoles as β-amyloid plaque binding agents

Article abstract of DOI:10.1016/j.bmcl.2013.01.068

To continue our efforts toward the development of ^99mTc PiB analogs, we have synthesized 24 neutral and lipophilic Re (as a surrogate of ^99mTc) 2-arylbenzothiazoles, and explored their structure-activity relationship for binding to Aβ_1-40 fibrils. These Re complexes were designed and synthesized via the integrated approach, so their ^99mTc analogs would have a greater chance of crossing the blood-brain barrier. While the lipophilicities (log P_C18 = 1.59-3.53) of these Re 2-arylbenzothiazoles were all within suitable range, their binding affinities (K_i = 30-617 nM) to Aβ_1-40 fibrils varied widely depending on the selection and integration of the tetradentate chelator into the 2-phenylbenzothiazole pharmacophore. For potential clinical applications, further refinement to obtain Re 2-arylbenzothiazoles with better binding affinities (<10 nM) will likely be needed. The integrated approach reported here to generate compact, neutral and lipophilic Re 2-arylbenzothiazoles could be applied to other potent pharmacophores as well to convert other current Aβ PET tracers to their ^99mTc analogs for more widespread application via the use of SPECT scanners.

Full text of DOI:10.1016/j.bmcl.2013.01.068

Bioorganic & Medicinal Chemistry Letters 23 (2013) 1720–1726
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and structure–activity relationship of rhenium
2-arylbenzothiazoles as b-amyloid plaque binding agents
Jinhe Pan a, Neale S. Mason b, Manik L. Debnath c, Chester A. Mathis b, William E. Klunk c, Kuo-Shyan Lin a,
⇑
a Department of Molecular Oncology, BC Cancer Agency, Vancouver, BC, Canada V5Z1L3
b Department of Radiology, University of Pittsburgh, Pittsburgh, PA 15213, USA
c Department of Psychiatry, University of Pittsburgh, Pittsburgh, PA 15213, USA
a r t i c l e i n f o
a b s t r a c t
To continue our efforts toward the development of 99mTc PiB analogs, we have synthesized 24 neutral and
lipophilic Re (as a surrogate of 99mTc) 2-arylbenzothiazoles, and explored their structure–activity rela-
tionship for binding to Ab1–40 fibrils. These Re complexes were designed and synthesized via the inte-
grated approach, so their 99mTc analogs would have a greater chance of crossing the blood–brain
barrier. While the lipophilicities (logPC18 = 1.59–3.53) of these Re 2-arylbenzothiazoles were all within
suitable range, their binding affinities (Ki = 30–617 nM) to Ab1–40 fibrils varied widely depending on
the selection and integration of the tetradentate chelator into the 2-phenylbenzothiazole pharmaco-
phore. For potential clinical applications, further refinement to obtain Re 2-arylbenzothiazoles with bet-
ter binding affinities (<10 nM) will likely be needed. The integrated approach reported here to generate
compact, neutral and lipophilic Re 2-arylbenzothiazoles could be applied to other potent pharmaco-
phores as well to convert other current Ab PET tracers to their 99mTc analogs for more widespread appli-
cation via the use of SPECT scanners.
Article history:
Received 18 November 2012
Revised 6 January 2013
Accepted 16 January 2013
Available online 24 January 2013
Keywords:
Technetium-99m
Rhenium
SPECT
PiB
Flutemetamol
b-Amyloid plaques
Alzheimer’s disease
Ó 2013 Elsevier Ltd. All rights reserved.
Alzheimer’s disease (AD) is a progressive and fatal neurodegen-
erative disorder characterized by irreversible memory impairment,
continuous cognitive decline and behavioral disturbances. AD
causes about two thirds of dementia in the elderly.1 It is estimated
that by the year of 2050, there will be 13.2 million cases of AD in
the US.2 At present, there is no medical treatment that cures or pre-
vents AD. The production and accumulation of b-amyloid peptides
(Ab) is believed to be pivotal to the pathogenesis and progression
of AD,3 and therefore, research on the treatment of AD has focused
on the anti-amyloid therapies.4 It is well documented that the for-
mation of Ab plaques precedes the appearance of clinical symp-
toms.5 In order to achieve the best therapeutic outcome, it may
be necessary to identify potential subjects for therapy before neu-
rons are damaged by Ab aggregates. Therefore, the development of
a noninvasive imaging method capable of quantifying the deposi-
tion of Ab plaques could provide a useful tool for identifying pre-
clinical cases of AD as candidates for early intervention and to
follow the effectiveness of anti-amyloid therapy in individual
patients.6
topic in the past two decades.7,8 PET and SPECT are effective nucle-
ar imaging modalities to detect probes that bind saturable binding
sites because their high sensitivity is suitable for extremely low
tracer concentrations. Both modalities are now commonly coupled
with computed tomography (CT) to generate hybrid images that
provide the benefits of both structural and functional/molecular
information. Currently, there are several 11C- and 18F-labeled
Ab PET tracers that have been successfully applied in clinical
research studies of AD ( Fig. 1). Among these imaging agents,
2-(4-[11C]methylaminophenyl)-6-hydroxybenzothiazole (Pittsburgh
compound B, PiB)9 has a high signal-to-noise ratio and has been
adopted to perform AD-related research studies worldwide. Unfor-
tunately, due to its short half-life (20 min), the 11C-label on PiB
limits its use to major academic PET facilities with on-site cyclo-
trons and sophisticated radiochemistry laboratories. Promising
radiotracers labeled with the longer half-life (110 min) radioiso-
tope 18F have been developed. Among them, florbetapir10 has re-
cently been approved by the US food and drug administration
(FDA) for clinical use for ruling out AD. Manufacturers of other
18F-labeled tracers such as florbetaben,11 flutemetamol12 and
NAV469413 are expected to seek FDA approval in the next few
years. These 18F-labeled PET tracers could increase the availability
of Ab imaging to all PET facilities, but this still represents a minor-
ity of modern hospitals with PET scanners. Many more hospitals
have the capacity to perform SPECT imaging. Ab imaging agents
labeled with SPECT radionuclides, particularly inexpensive and
Toward this end, the development of Ab plaque-targeting radio-
tracers for use with positron emission tomography (PET) and single
photon emission tomography (SPECT) has been an active research
⇑
Corresponding author. Address: 675 West 10th Avenue, Rm 4-123, Vancouver,
BC, Canada V5Z1L3. Tel.: +1 604 675 8208; fax: +1 604 675 8218.
E-mail address: klin@bccrc.ca (K.-S. Lin).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.01.068

J. Pan et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1720–1726
1721
Figure 1. PET Ab imaging agents currently under clinical evaluation.
readily available 99mTc will have more widespread clinical applica-
bility especially in developing countries.
40-position (compounds 4–6) or both the 30- and 40-position (com-
pounds 7–24). To further enhance their binding affinity, we also
synthesized analogs with a fluoro or methoxy substitution at the
6-position when a tetradentate chelator is integrated into the phe-
nyl ring, or at the 40-position when a tetradentate chelator is inte-
grated into the benzothiazole ring. The choice of the small halogen
fluorine and a methoxy group is to limit the overall size increase. In
addition, substitution with an aromatic fluoro or methoxy group
will not significantly change the overall lipophilicity.
The synthetic steps for the preparation of Re 2-ABT 1–24 are de-
picted in Schemes 2–9. The SN3 chelator used for the preparation of
1–6 (Schemes 2 and 3) was modified from the thiol-triamide.21,22
chelator as shown in Scheme 1A. Complexation of the original
thiol-triamide chelator with [Tc(V)O]3+ or [Re(V)O]3+ led to metal
complexes with one negative charge due to the loss of four protons
from three amide N–H groups and one thiol S–H group. It is well
documented that charged Tc complexes do not cross the blood–
brain barrier (BBB). In addition, Tc complexes derived from
tetradentate chelators containing amide groups (such as mono-
amide-monoamine-dithiol, MAMA) showed less brain uptake when
compared to those derived from their corresponding amino chelators
(such as diaminedithiol, DADT).23,24 Therefore, we modified the ori-
ginal thiol-triamide chelator by replacing the three amide groups
with three amino groups. In order to obtain neutral Tc/Re complexes,
we also added one small methyl group to one of the aliphatic amino
groups. After such modification (see Scheme 1A), all three protons
were lost after complexation with [Re(V)O]3+, and the overall charge
of 2-ABT 1–5 was balanced, which is in agreement with our previ-
ously reported results from the synthesis of 6.17
Re 2-ABTs 7–9 (Scheme 4), 10–15 (Schemes 5 and 6), and
16–21 (Schemes 7 and 8) were synthesized using modified
semi-rigid SN2X chelators (Scheme 1B, X = NH, O, and S, respec-
tively) integrated into the 30- and 40-position of phenyl ring. This
design was to further reduce their overall molecular weight to
500 daltons or less as suggested by the rule of five,25 and in hopes
that their corresponding 99mTc analogs would show rapid and
high brain entry. These semi-rigid SN2X chelating systems were
modified from previously reported thiol-diamide-X systems
(X = NX, O and S).26 As shown in Scheme 1B, the complexation
of [Re(V)O]3+ with thiol-diamide-X chelating systems led to Re
complexes with one negative charge. Similar to our modification
to the thiol-triamide chelator shown in Scheme 1A, we also
replaced the two amide groups with two amino groups in order
to produce neutral Re complexes and increase brain uptake of
With the success in the development of the 2-arylbenzothiazole
(2-ABT) based PET radiotracers, PiB and flutemetamol, we were
also interested in the development of 99mTc-labeled 2-ABTs for
more widespread application using SPECT scanners. In contrast to
most attempts by other investigators on the development of
99mTc-labeled 2-ABTs using the pendant approach,14–16 we have
previously demonstrated the feasibility of design and synthesis
of three neutral and lipophilic Re (as a surrogate of 99mTc) 2-ABTs
(compounds# 6, 12 and 21 in Table 1) with moderate Ab binding
affinity (30–87 nM) using the integrated approach to minimize
their overall molecular weight (<550 Da).17 Compound 6 was
prepared using
a thiol-triamine SN3 tetradentate chelator
(Scheme 1A), while semi-rigid thiol-diamine-phenol (SN2O) and
thiol-diamine-thiol (SN2S) chelators (Scheme 1B, X = O and S) were
used for the preparation of 12 and 21, respectively. Besides the SN3,
SN2O and SN2S chelators, the semi-rigid thiol-diamine-thioether
(SN2Sether)18 chelator is also commonly used for the design and
synthesis of neutral and lipophilic Tc/Re complexes. While SN3,
SN2X, SN2Sether chelators all form stable Tc/Re complexes, the
preparation of Tc/Re 2-ABTs by the use of different tetradentate
chelators or the same chelator integrated at different positions of
the pharmacophore might generate Tc/Re 2-ABTs with different
binding affinity, lipophilicity and in vivo pharmacokinetics. The
aim of this present work was to systematically explore the poten-
tial of utilizing SN3, SN2X, and SN2Sether chelators to generate
compact, neutral, and lipophilic Re 2-ABTs, and to investigate the
structure–activity relationship for their lipophilicity and binding
affinity to aggregated Ab.
As shown in Table 1, besides 6, 12 and 21 that were reported
earlier, we synthesized an additional 21 Re 2-ABTs with chelators
integrated at different positions of 2-ABT pharmacophore for com-
parison. Our previous results19,20 indicated that substitution on the
2-ABT pharmacophore with an electron-donating group or a halo-
gen significantly increases the binding affinity. For example, the Ki
for an amino, N-methylamino, N,N-dimethylamino, fluoro, bromo,
or iodo substitution at the 40-position of 2-ABT were 37.0, 11.0,
4.0, 43.8, 8.8, and 2.6 nM, respectively. The definition of substitu-
tion positions is depicted on the structure of PiB shown in Figure
1. The integration of a tetradentate chelator into the benzothiazole
ring provides an electron-donating group at the 6-position (com-
pounds 1–3), whereas the integration of a tetradentate chelator
into the phenyl ring provides an electron-donating group at the

1722
J. Pan et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1720–1726
Table 1
Properties of Re 2-phenylbenzothiazoles including molecular weight (MW), binding affinity (Ki) to Ab1–40, and lipophilicity (logPC18
)
Compd #
Structure
R
MW
Ki (nM)
log pc18
1
2
3
H
F
OMe
586 (498a)
604 (516a)
556
617
85
2.54
2.67
2.37
616 (528a)
4
5
6
H
F
OMe
586 (498a)
604 (516a)
378
118
87
2.65
2.84
2.52
616 (528a)
7
8
9
H
F
OMe
558 (470a)
576 (488a)
90
113
61
2.33
2.50
2.21
588 (500a)
10
11
12
H
F
OMe
545 (457a)
563 (475a)
109
64
30
1.70
1.90
1.65
575 (487a)
13
14
15
H
F
OMe
545 (457a)
563 (475a)
280
226
140
1.68
1.87
1.59
575 (487a)
16
17
18
H
F
OMe
561 (473a)
579 (491a)
264
93
132
2.49
2.65
2.45
591 (503a)
19
20
21
H
F
OMe
561 (473a)
579 (491a)
38
31
43
2.41
2.60
2.30
591 (503a)
22
23
24
H
F
OMe
575 (487a)
593 (505a)
200
148
178
3.35
3.53
3.25
605 (517a)
a
Molecular weight of their corresponding Tc analogs.
Scheme 1. Design and synthesis of neutral Re 2-arylbenzothiazoles by modification of reported (A) SN3 chelating system, and (B) SN2X chelating system.

J. Pan et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1720–1726
1723
Scheme 2. Synthesis of 1–3. Reagents and conditions: (a) 4-substituted benzoyl chloride, DMF, 140 °C, 18 h, 57–90%; (b) SnCl2, EtOH, 3 h, reflux, 73–91%; (c) 2-chloroacetyl
chloride, K2CO3, THF, 18 h, rt, 94–99%; (d) 2-(4-methoxybenzylthio)ethyl bromide, CH2Cl2, 18 h, reflux, 81%; (e) NaOH, H2O, MeOH, 4 h, rt, 92%; (f) KI, K2CO3, CH3CN, 14 h,
reflux, 35–68%; (g) LAH, THF, 18 h, rt, 38–80%; (h) (i) TFA, anisole, rt, 1 h; (ii) Hg(OAc)2 0 °C, 1 h; (iii) H2S, EtOH; (iv) ReO(PPh3)2Cl3, NaOAc, MeOH, 18 h, 75 °C, 16–29%.
Scheme 3. Synthesis of 4–6. Reagents and conditions: (a) 4-nitrobenzoyl chloride, DMF, 140 °C, 18 h, 38–80%; (b) SnCl2, EtOH, 3 h, reflux, 88–100%; (c) 2-chloroacetyl
chloride, K2CO3, THF, 18 h, rt, 92–96%; (d) KI, K2CO3, CH3CN, 18 h, reflux, 63–88%; (e) LAH, THF, 18 h, rt, 42–58%; (f) (i) TFA, anisole, rt, 1 h; (ii) Hg(OAc)2 0 °C, 1 h; (iii) H2S,
EtOH; (iv) ReO(PPh3)2Cl3, NaOAc, MeOH, 18 h, 75 °C, 19–41%.
Scheme 4. Synthesis of 7–9. Reagents and conditions: (a) 4-fluoro-3-nitrobenzoyl chloride, DMF, 140 °C, 18 h, 58–92%; (b) K2CO3, DMF, 100 °C, 18 h, 27–81%; (c) SnCl2, EtOH,
3 h, reflux, 10–81%; (d) (i) TFA, anisole, rt, 1 h; (ii) Hg(OAc)2 0 °C, 1 h; (iii) H2S, EtOH; (iv) ReO(PPh3)2Cl3, NaOAc, MeOH, 18 h, 75 °C, 10–39%.
their 99mTc analogs. As expected, after complexation with [Re(V)O]3+
,
complexation reaction using Re(V)O(PPh3)2Cl3.27 In spite of poten-
tial existence of cis- and anti-isomers, similar to our previous re-
sults for the syntheses of 6, 12 and 21,17 only one single isomer
was isolated for each of these additional 21 Re 2-ABTs reported
here, and their identities were confirmed by NMR spectroscopy.28
only three protons were lost. The aliphatic amino N–H group that
has relatively higher pKa value was not de-protonated after Re
complexation reaction, and therefore, the overall charge was
balanced. These results were consistent with our previously re-
ported results for the synthesis of 12 and 21.17
Re 2-ABT 1–24 are moderately lipophilic with logPC18 (PC18
:
Syntheses of 22–24 are shown in Scheme 9 using the previously
reported semi-rigid SN2Sether chelator.18 The methyl thioether
group was chosen to keep the overall molecular weight at mini-
mum. After complexation with [Re(V)O]3+, as expected, neutral
Re 2-ABT 22–24 were obtained resulting from the loss of three pro-
tons (two aromatic N–H and one thiol S–H groups). The final step
in the preparation of Re 2-ABT 1–24 involved a two-stage reaction,
deprotection of p-methoxybenzyl (PMB)/methoxymethyl (MOM)
protecting groups to restore the chelating core followed by Re
estimation of Poct by a reverse-phase HPLC method19) in the range
of 1.59–3.53 (Table 1). Among them, 10–15 derived from the SN2O
chelator displayed the lowest lipophilicity (logPC18 = 1.59–1.90),
whereas 22–24 derived from the SN2Sether chelator had the high-
est lipophilicity (logPC18 = 3.25–3.53). Replacing the phenol of the
SN2O chelator in 10–15 with a thiol resulted in Re SN2S derivatives
16–21 with an average increase of 0.75 in their logPC18 values
(10–12 vs 19–21; 13–15 vs 16–18). The Re 2-ABTs derived from
the same tetradentate chelator (SN2O or SN2S) but with different

1724
J. Pan et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1720–1726
Scheme 5. Synthesis of 10–12. Reagents and conditions: (a) 3-hydroxy-4-nitro benzaldehyde, DMSO, 125 °C, 18 h, 15–65%; (b) MOMCl, DIPEA, THF, 18 h, reflux, 51–71% (c)
NaBH4, Cu(OAc)2, EtOH, rt, 18 h, 96–100%; (d) 2-chloroacetyl chloride, K2CO3, THF, 18 h, rt, 64–82%; (e) 2-(4-methoxybenzylthio)ethylamine, KI, K2CO3, CH3CN, 18 h, reflux,
80–98%; (f) LAH, THF, 18 h, rt, 43–45%; (g) (i) TFA, anisole, rt, 1 h; (ii) Hg(OAc)2 0 °C, 1 h; (iii) H2S, EtOH; (iv) ReO(PPh3)2Cl3, NaOAc, MeOH, 18 h, 75 °C, 17–49%.
Scheme 6. Synthesis of 13–15. Reagents and conditions: (a) 4-hydroxy-3-nitro benzaldehyde, DMSO, 125 °C, 18 h, 28-35%; (b) MOMCl, DIPEA, THF, reflux, 18 h, 98–99%; (c)
NaBH4, Cu(OAc)2, EtOH, rt, 18 h, 92–97%; (d) 2-chloroacetyl chloride, K2CO3, THF, 18 h, rt, 79–91%; (e) 2-(4-methoxybenzylthio)ethylamine, KI, K2CO3, CH3CN, 18 h, reflux,
40–96%; (f) LAH, THF, 17 h, rt, 43–59%; (g) (i) TFA, anisole, rt, 1 h; (ii) Hg(OAc)2 0 °C, 1 h; (iii) H2S, EtOH; (iv) ReO(PPh3)2Cl3, NaOAc, MeOH, 18 h, 75 °C, 20–41%.
Scheme 7. Synthesis of 16–18. Reagents and conditions: (a) 4-methoxy-a-toluenethiol, K2CO3, DMF, 18 h, 100 °C, 35–77%; (b) SnCl2, THF, EtOH, 3 h, reflux, 57–95%; (c)
2-chloroacetyl chloride, K2CO3, THF, 18 h, rt, 90–93%; (d) 2-(4-methoxybenzylthio)ethylamine, KI, K2CO3, CH3CN, 18 h, reflux, 43–84%; (e) LAH, THF, 18 h, rt, 24–53%; (f) (i)
TFA, anisole, rt, 1 h; (ii) Hg(OAc)2 0 °C, 1 h; (iii) H2S, EtOH; (iv) ReO(PPh3)2Cl3, NaOAc, MeOH, 18 h, 75 °C, 25–36%.
integration patterns into the phenyl ring (the amino group substi-
tuted at the 30- or 40-position) had similar lipophilicity (10–12 vs
13–15; 16–18 vs 19–21). Compared with 1–6 derived from the
SN3 chelating system with only one lateral amino group of the che-
lator integrated into the 2-ABT backbone, 7–9 derived from the
semi-rigid SN3 chelator with two amino groups integrated into
the phenyl ring had lower lipophilicity due to the presence of an
amino N–H proton at the 30-position and one less ethylene moiety
in the overall structure. If comparing the Re 2-ABTs with the same
tetradentate chelator but with different substitution (H, F and
OMe) at the 6- or 40-position, the methoxy-substituted 2-ABTs
had the lowest logPC18 values with an average of 0.10 and 0.25
lower than those of their respective un-substituted and fluoro-
substituted Re 2-ABTs.
Re 2-ABTs 1–24 bind Ab1–40 fibrils with moderate to poor affin-
ity (Ki = 30–617 nM, Table 1) as determined by previously pub-
lished in vitro competition binding assays19,29 using [3H]BTA-1 as
the radioactive control compound. In general, compared to the
un-substituted Re 2-ABTs, substitution with a fluoro or a methoxy
group at the 6- or 40-position enhanced their binding affinity to Ab
fibrils. The integration of an SN3 chelator into the phenyl ring
1–40
(in 4–6) rather than the benzothiazole ring (in 1–3) resulted in Re

J. Pan et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1720–1726
1725
Scheme 8. Synthesis of 19–21. Reagents and conditions: (a) 3-fluoro-4-nitrobenzoyl chloride, DMF, 140 °C, 18 h, 73–99%; (b) 4-methoxy-
a-toluenethiol, K2CO3, DMF, 18 h,
95 °C, 55–96%; (c) SnCl2, THF, EtOH, 3 h, reflux, 67–79%; (d) 2-chloroacetyl chloride, K2CO3, THF, 18 h, rt, 81–98%; (e) 2-(4-methoxybenzylthio)ethylamine, KI, K2CO3, CH3CN,
18 h, reflux, 75–93%; (f) LAH, THF, 18 h, rt, 56–81%; (g) (i) TFA, anisole, rt, 1 h; (ii) Hg(OAc)2 0 °C, 1 h; (iii) H2S, EtOH; (iv) ReO(PPh3)2Cl3, NaOAc, MeOH, 18 h, 75 °C, 12–23%.
Scheme 9. Synthesis of 22–24. Reagents and conditions: (a) 2-(4-methoxybenzylthio)ethylamine, K2CO3, DMF, 100 °C, 18 h, 41–96%; (2) SnCl2, THF, EtOH, 3 h, reflux, 55–
98%; (c) 2-chloroethyl methyl sulfide, KI, K2CO3, CH3CN, 18 h, reflux, 40–64%; (d) (i) TFA, anisole, rt, 1 h; (ii) Hg(OAc)2 0 °C, 1 h; (iii) H2S, EtOH; (iv) ReO(PPh3)2Cl3, NaOAc,
MeOH, 18 h, 75 °C, 25–26%.
2-ABTs with better binding affinities. Replacing the free rotating
Re-SN3 complex in 4–6 with a semi-rigid Re-SN3 complex in 7–9
further enhance the binding affinity. As discussed above, different
integration patterns of the same tetradentate chelator (SN2O or
SN2S) had little effects on the overall lipophilicity of the resulted
Re 2-ABTs (10–12 vs 13–15; 16–18 vs 19–21). However, the
binding affinities of these 2-ABTs were strongly influenced by the
integration patterns of the tetradentate chelator. When comparing
the 2-ABTs 10–15 with the semi-rigid SN2O chelator, 10–12
(Ki = 30–109 nM) with an amino group of the chelator substituted
at the 40-position had 2.6- to 4.7-fold better binding affinity than
their corresponding analogs 13–15 (Ki = 140–280 nM) with the
amino group substituted at the 30-position. Similar results were
obtained when comparing the binding affinity of Re 2-ABTs 16–
21 with the semi-rigid SN2S chelator. The binding affinities of
19–21 (Ki = 31–43 nM) with an amino group of the SN2S chelator
substituted at the 40-position were 3.1- to 6.9-fold better than
those of their corresponding analogs 16–18 (Ki = 93–264 nM)
with the amino group substituted at the 30-position. These results
are in agreement with the fact that most of the promising PET trac-
ers derived from the 2-ABT pharmacophore including [11C]PiB,
lators were integrated into the 2-ABT pharmacophore. Based on
the binding affinity data, we have identified two promising semi-
rigid chelators, SN2O and SN2S. The Re 2-ABTs 12 and 20 derived
from the SN2O and SN2S chelators, respectively, had fairly good
binding affinity (Ki ꢀ 30 nM) to Ab1–40 fibrils. However, before
translation into their 99mTc analogs and for potential clinical appli-
cation, further modification to obtain Re 2-ABTs with even better
binding affinity will likely be needed since most of the clinical Ab
imaging agents have binding affinities less than 10 nM. This might
be achievable by optimizing the substitution at the 6-position of
the 2-ABT pharmacophore with other potent electron-donating
groups, and/or by the 3D-QSAR analysis as recently reported by
Kim et al.30 and Yang et al.31 The integrated approach reported
here to generate compact, neutral and lipophilic Re 2-arylbenzo-
thiazoles could be applied to generate Re complexes of other po-
tent pharmacophores, including stilbene and benzoxazole. Once
potent Re complexes are obtained, their 99mTc analogs will have
great potential to extend current Ab imaging practices from PET
to SPECT.
Acknowledgement
[
11C]AZD2184, and [18F]flutemetamol (see Fig. 1) have an amino
group substituted at the 40-position.
This work was supported by the US National Institutes of Health
(R21EB009497).
In summary, we have synthesized twenty-four neutral and
compact Re 2-ABTs, and measured their lipophilicity and binding
affinity to aggregated Ab. These Re 2-ABTs were designed and pre-
pared via the integrated approach, so their 99mTc analogs would
have a greater chance of crossing the BBB, and bind to Ab plaques
deposited in the brain parenchyma. While the lipophilicities of
these 2-ABTs were within suitable range (logPC18 = 1–4), their
binding affinities (Ki = 30–617 nM) to Ab1–40 fibrils varied widely
depending on the selection of the chelators, and the ways the che-
References and notes
1. Nussbaum, R. L.; Ellis, C. E. N. Engl. J. Med. 2003, 348, 1356.
2. Hebert, L. E.; Scherr, P. A.; Bienias, J. L.; Bennett, D. A.; Evans, D. A. Arch. Neurol.
2003, 60, 1119.
3. Hardy, J.; Selkoe, D. J. Science 2002, 297, 353.
4. Pangalos, M. N.; Jacobsen, S. J.; Reinhart, P. H. Biochem. Soc. Trans. 2005, 33, 553.

1726
J. Pan et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1720–1726
5. Pike, K. E.; Savage, G.; Villemagne, V. L.; Ng, S.; Moss, S. A.; Maruff, P.; Mathis, C.
A.; Klunk, W. E.; Masters, C. L.; Rowe, C. C. Brain 2007, 130, 2837.
6. Mathis, C. A.; Lopresti, B. J.; Klunk, W. E. Nucl. Med. Biol. 2007, 34, 809.
7. Mathis, C. A.; Wang, Y.; Klunk, W. E. Curr. Pharm. Des. 2004, 10, 1469.
8. Mori, T.; Maeda, J.; Shimada, H.; Higuchi, M.; Shinotoh, H.; Ueno, S.; Suhara, T.
Psychogeriatrics 2012, 12, 106.
9. Klunk, W. E.; Engler, H.; Nordberg, A.; Wang, Y. M.; Blomqvist, G.; Holt, D. P.;
Bergstrom, M.; Savitcheva, I.; Huang, G. F.; Estrada, S.; Ausen, B.; Debnath, M.
L.; Barletta, J.; Price, J. C.; Sandell, J.; Lopresti, B. J.; Wall, A.; Koivisto, P.; Antoni,
G.; Mathis, C. A.; Langstrom, B. Ann. Neurol. 2004, 55, 306.
10. Clark, C. M.; Schneider, J. A.; Bedell, B. J.; Beach, T. G.; Bilker, W. B.; Mintun, M.
A.; Pontecorvo, M. J.; Hefti, F.; Carpenter, A. P.; Flitter, M. L.; Krautkramer, M. J.;
Kung, H. F.; Coleman, R. E.; Doraiswamy, P. M.; Fleisher, A. S.; Sabbagh, M. N.;
Sadowsky, C. H.; Reiman, P. E. M.; Zehntner, S. P.; Skovronsky, D. M. JAMA, J. Am.
Med. Assoc. 2011, 305, 275.
11. Rowe, C. C.; Ackerman, U.; Browne, W.; Mulligan, R.; Pike, K. L.; O’Keefe, G.;
Tochon-Danguy, H.; Chan, G.; Berlangieri, S. U.; Jones, G.; Dickinson-Rowe, K.
L.; Kung, H. F.; Zhang, W.; Kung, M. P.; Skovronsky, D.; Dyrks, T.; Hall, G.;
Krause, S.; Friebe, M.; Lehman, L.; Lindemann, S.; Dinkelborg, L. M.; Masters, C.
L.; Villemagne, V. L. Lancet Neurol. 2008, 7, 129.
12. Nelissen, N.; Van Laere, K.; Thurfjell, L.; Owenius, R.; Vandenbulcke, M.; Koole,
M.; Bormans, G.; Brooks, D. J.; Vandenberghe, R. J. Nucl. Med. 2009, 50, 1251.
13. Cselenyi, Z.; Jonhagen, M. E.; Forsberg, A.; Halldin, C.; Julin, P.; Schou, M.;
Johnstrom, P.; Varnas, K.; Svensson, S.; Farde, L. J. Nucl. Med. 2012, 53, 415.
14. Serdons, K.; Vanderghinste, D.; Van Eeckhoudt, M.; Cleynhens, J.; de Groot, T.;
Bormans, G.; Verbruggena, A. J. Labelled Compd. Radiopharm. 2009, 52, 227.
15. Chen, X.; Yu, P.; Zhang, L.; Liu, B. Bioorg. Med. Chem. Lett. 2008, 18, 1442.
16. Serdons, K.; Verduyckt, T.; Cleynhens, J.; Terwinghe, C.; Mortelmans, L.;
Bormansa, G.; Verbruggen, A. Bioorg. Med. Chem. Lett. 2007, 17, 6086.
17. Lin, K. S.; Debnath, M. L.; Mathis, C. A.; Klunk, W. E. Bioorg. Med. Chem. Lett.
2009, 19, 2258.
18. McBride, B. J.; Baldwin, R. M.; Kerr, J. M.; Wu, J. L.; Schultze, L. M.; Salazar, N. E.;
Chinitz, J. M.; Byrne, E. F. J. Med. Chem. 1993, 36, 81.
19. Mathis, C. A.; Wang, Y. M.; Holt, D. P.; Huang, G. F.; Debnath, M. L.; Klunk, W. E.
J. Med. Chem. 2003, 46, 2740.
20. KlunK, W. E.; Mathis, C. A. U.S. Patent 0142269A1, 2009.
21. Eisenhut, M.; Mohammed, A.; Mier, W.; Schoensiegel, F.; Friebe, M.; Mahmood,
A.; Jones, A. G.; Haberkorn, U. J. Med. Chem. 2002, 45, 5802.
22. Hansen, L.; Cini, R.; Taylor, A., Jr.; Marzilli, L. G. Inorg. Chem. 1992, 31, 2801.
23. Zhuang, Z. P.; Kung, M. P.; Hou, C.; Ploessl, K.; Kung, H. F. Nucl. Med. Biol. 2005,
32, 171.
24. Oya, S.; Plossl, K.; Kung, M. P.; Stevenson, D. A.; Kung, H. F. Nucl. Med. Biol. 1998,
25, 135.
25. Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J. Adv. Drug Delivery Rev.
1995, 23, 3.
26. Le Gal, J.; Tisato, F.; Bandoli, G.; Gressier, M.; Jaud, J.; Michaud, S.;
Dartiguenave, M.; Benoist, E. Dalton Trans. 2005, 23, 3800.
27. General procedures for the synthesis of Re complexes 1–24: A solution of the
respective PMB/MOM-protected precursor (0.2 mmol) in trifluoroacetic acid
(5 mL) and anisole (0.2 mL) was stirred at room temperature for 1 h, and then
cooled in ice/water bath. Hg(OAc)2 (0.48 mmol for the preparation of 16–21,
and 0.24 mmol for the preparation of other Re complexes) was added, and the
resulting solution was stirred at 0 °C for another 1 h. After removing volatile
trifluoroacetic acid under reduced pressure, the residue was washed with
diethyl ether (20 mL), and then dissolved in ethanol (15 mL). To this solution
was bubbled in hydrogen sulfide for 10 min, and the resulting black precipitate
was removed by filtration through celite. The filtrate was concentrated under
reduced pressure. ReO(PPh3)2Cl3 (208 mg, 0.25 mmol) and NaOAc methanolic
solution (1.0 M, 20 mL) were added to the residue, and the resulting solution
was heated at 75 °C for 18 h. After cooling to room temperature, the solution
was diluted with water (50 mL), and extracted with chloroform (50 mL). The
organic phase was dried over anhydrous magnesium sulfate and concentrated
under reduced pressure. The crude product was purified by chromatography
on silica gel eluting with 10:90 methanol/ethyl acetate to give the expected
product in 10–49% yields.
28. NMR spectra were recorded using a Bruker Avance 400inv NMR spectrometer.
Chemical shifts (d) are reported in parts per million (ppm). Compound 1
(DMSO-d6): 2.04–2.16 (m, 1H), 2.71–2.79 (m, 1H), 3.19 (s, 3H), 3.21–3.27 (m,
2H), 3.39–3.45 (m, 1H), 3.49–3.57 (m, 1H), 3.61–3.70 (m, 1H), 3.83–3.90 (m,
1H), 3.84 (s, 3H), 3.91–3.40 (m, 2H), 3.66–4.73 (m, 1H), 7.36 (dd, J = 8.8, 2.4 Hz,
1H), 7.52–7.64 (m, 3H), 7.70 (d, J = 2.4 Hz, 1H), 7.81 (d, J = 8.8 Hz, 1H), 8.02 (dd,
J = 7.8, 2.0 Hz, 2H); Compound 2 (DMSO-d6): 2.06–2.16 (m, 1H), 2.71–2.79 (m,
1H), 3.19 (s, 3H), 3.21–3.27 (m, 2H), 3.39–3.46 (m, 1H), 3.49–3.58 (m, 1H),
3.62–3.71 (m, 1H), 3.83–3.90 (m, 1H), 3.91–3.40 (m, 2H), 3.66–4.73 (m, 1H),
7.33–7.42 (m, 3H), 7.73 (d, J = 2.0 Hz, 1H), 7.81 (d, J = 8.8 Hz, 1H), 8.04–8.09 (m,
2H); Compound 3 (DMSO-d6): 2.06–2.16 (m, 1H), 2.71–2.79 (m, 1H), 3.19 (s,
3H), 3.21–3.29 (m, 2H), 3.39–3.46 (m, 1H), 3.50–3.59 (m, 1H), 3.62–3.70 (m,
1H), 3.83–3.90 (m, 1H), 3.84 (s, 3H), 3.91–3.40 (m, 2H), 3.66–4.73 (m, 1H), 7.10
(d, J = 8.8 Hz, 2H), 7.32 (dd, J = 8.8, 2.4 Hz, 1H), 7.67 (d, J = 2.4 Hz, 1H), 7.76 (d,
J = 8.8 Hz, 1H), 7.96 (d, J = 8.8 Hz, 2H); Compound 4 (DMSO-d6): 2.13–2.20 (m,
1H), 2.53–2.58 (m, 1H), 2.75–2.79 (m, 1H), 3.20 (s, 3H), 3.22–3.40 (m, 2H),
3.43–3.50 (m, 2H), 3.64–3.71 (m, 1H), 3.84–3.90 (m, 2H), 3.94–3.99 (m, 1H),
4.59–4.64 (m, 1H), 7.24 (d, J = 8.8 Hz, 2H), 7.36–7.40 (m, 1H), 7.47–7.51 (m,
1H), 7.79 (d, J = 8.8 Hz, 2H), 7.96 (d, J = 8.1 Hz, 1H), 8.70 (d, J = 7.9 Hz, 1H);
Compound 5 (DMSO-d6): 2.13–2.20 (m, 1H), 2.53–2.58 (m, 1H), 2.75–2.79 (m,
1H), 3.20 (s, 3H), 3.22–3.40 (m, 2H), 3.43–3.50 (m, 2H), 3.64–3.70 (m, 1H),
3.80–3.91 (m, 2H), 3.93–3.99 (m, 1H), 4.60–4.64 (m, 1H), 7.24 (d, J = 8.9 Hz,
2H), 7.32–7.38 (m, 1H), 7.87 (d, J = 8.8 Hz, 2H), 7.95–8.01 (m, 2H); Compound 7
(DMSO-d6): 2.17–2.25 (m, 1H), 3.07–3.23 (m, 3H), 3.30 (s, 3H), 3.74–3.79 (m,
1H), 4.06–4.11 (m, 1H), 4.62–4.69 (m, 1H), 6.88 (d, J = 8.1 Hz, 1H), 7.34–7.38
(m, 2H), 7.46–7.50 (m, 1H), 7.58 (d, J = 1.8 Hz, 1H), 7.95 (d, J = 7.9 Hz, 1H), 8.05
(d, J = 7.5 Hz, 1H), 11.3 (s, 1H); Compound 8 (DMSO-d6): 2.16–2.25 (m, 1H),
3.05–3.23 (m, 3H), 3.29 (s, 3H), 3.73–3.80 (m, 1H), 4.04–4.11 (m, 1H), 4.60–
4.69 (m, 1H), 6.87 (d, J = 8.0 Hz, 1H), 7.30–7.36 (m, 2H), 7.54 (d, J = 1.6 Hz),
7.93–7.98 (m, 2H), 11.30 (s, 1H); Compound 9 (DMSO-d6): 2.15–2.25 (m, 1H),
3.05–3.23 (m, 3H), 3.27 (s, 3H), 3.73–3.80 (m, 1H), 3.83 (s, 3H), 4.02–4.10 (m,
1H), 4.60–4.69 (m, 1H), 6.85 (d, J = 8.0 Hz, 1H), 7.06 (dd, J = 8.8, 2.4 Hz, 1H),
7.29 (dd, J = 8.0, 2.0 Hz, 1H), 7.51 (d, J = 1.6 Hz, 1H), 7.62 (d, J = 2.4 Hz, 1H), 7.83
(d, J = 8.8 Hz, 1H), 11.30 (s, 1H); Compound 10 (DMSO-d6): 2.14–2.28 (m, 1H),
2.84–2.95 (m, 1H), 2.98–3.05 (m, 1H), 3.17–3.27 (m, 1H), 3.61–3.71 (m, 1H),
3.86–4.00 (m, 2H), 4.38–4.46 (m, 1H), 7.00 (d, J = 8.0 Hz, 1H), 7.36–7.41 (m,
1H), 7.47–7.53 (m, 2H), 7.64 (d, J = 1.6 Hz, 1H), 7.98 (d, J = 8.0 Hz, 1H), 8.07 (d,
J = 7.2 Hz, 1H), 9.46 (s, 1H); Compound 11 (DMSO-d6): 2.15–2.23 (m, 1H), 2.83–
2.94 (m, 1H), 2.97–3.05 (m, 1H), 3.18–3.28 (m, 1H), 3.60–3.71 (m, 1H), 3.86–
4.01 (m, 2H), 4.36–4.46 (m, 1H), 6.99 (d, J = 8.0 Hz, 1H), 7.35 (td, J = 8.8, 2.4 Hz,
1H), 7.48 (dd, J = 8.0, 2.0 Hz, 1H), 7.57–7.64 (m, 1H), 7.96–8.02 (m, 2H), 9.46 (s,
1H); Compound 13 (DMSO-d6): 2.15–2.27 (m, 1H), 2.85–2.95 (m, 1H), 2.96–
3.04 (m, 1H), 3.18–3.27 (m, 1H), 3.64–3.74 (m, 1H), 3.91–4.00 (m, 2H), 4.47–
4.57 (m, 1H), 7.12 (d, J = 8.0 Hz, 1H), 7.36–7.42 (m, 1H), 7.44–7.52 (m, 2H), 7.55
(d, J = 1.6 Hz, 1H), 7.98 (d, J = 7.6 Hz, 1H), 8.07 (d, J = 7.6 Hz, 1H), 9.47 (s, 1H);
Compound 14 (DMSO-d6): 2.15–2.27 (m, 1H), 2.85–2.95 (m, 1H), 2.96–3.04 (m,
1H), 3.18–3.29 (m, 1H), 3.63–3.74 (m, 1H), 3.91–4.00 (m, 2H), 4.46–4.55 (m,
1H), 7.12 (d, J = 8.4 Hz, 1H), 7.36 (td, J = 8.8, 2.4 Hz, 1H), 7.44 (dd, J = 8.0, 2.0 Hz,
1H), 7.53 (d, J = 2.0 Hz, 1H), 7.96–8.03 (m, 2H), 9.47 (s, 1H); Compound 15
(DMSO-d6): 2.13–2.28 (m, 1H), 2.80–3.02 (m, 2H), 3.18–3.33 (m, 1H), 3.60–
3.73 (m, 1H), 3.83 (s, 3H), 3.87–4.02 (m, 2H), 4.43–5.02 (m, 1H), 7.02–7.11 (m,
2H), 7.38 (dd, J = 8.1, 1.8 Hz, 1H), 7.49 (d, J = 1.8 Hz, 1H), 7.64 (d, J = 2.4 Hz, 1H),
7.85 (d, J = 8.7 Hz, 1H), 9.46 (s, 1H); Compound 16 (DMSO-d6): 2.06–2.20 (m,
1H), 2.90–3.05 (m, 2H), 3.34–3.42 (m, 2H), 3.92–4.02 (m, 1H), 4.31–4.40 (m,
1H), 4.45–4.54 (m, 1H), 7.42–7.48 (m, 1H), 7.51–7.57 (m, 2H), 7.66–7.72 (m,
2H), 8.04 (d, J = 7.6 Hz, 1H), 8.12 (d, J = 7.6 Hz, 1H), 9.67 (s, 1H); Compound 17
(DMSO-d6): 2.06–2.16 (m, 1H), 2.91–3.04 (m, 2H), 3.27–4.43 (m, 2H), 3.92–
4.01 (m, 1H), 4.30–4.39 (m, 1H), 4.45–4.55 (m, 1H), 7.40 (td, J = 8.8, 2.4 Hz, 1H),
7.52 (dd, J = 8.0, 1.6 Hz, 1H), 7.65–7.70 (m, 2H), 8.02–8.08 (m, 2H), 9.69 (s, 1H);
Compound 18 (DMSO-d6): 2.02–2.18 (m, 1H), 2.90–3.05 (m, 2H), 3.85 (s, 3H),
3.88–4.03 (m, 2H), 4.28–4.53 (m, 2H), 7.12 (dd, J = 7.5, 2.1 Hz, 1H), 7.47 (d,
J = 7.8 Hz, 1H), 7.55–7.75 (m, 3H), 7.92 (d, J = 8.7 Hz, 1H), 9.67 (s, 1H);
Compound 19 (DMSO-d6): 2.05–2.17 (m, 1H), 2.85–3.02 (m, 2H), 3.35–3.46 (m,
2H), 3.90–4.01 (m, 1H), 4.30–4.44 (m, 2H), 7.20 (d, J = 8.4, 1H), 7.37–7.53 (m,
2H), 7.79 (dd, J = 8.4, 2.0 Hz, 1H), 7.99 (d, J = 8.0 Hz, 1H), 8.09 (d, J = 7.6 Hz, 1H),
8.20 (d, J = 1.6 Hz, 1H), 9.66 (s, 1H); Compound 20 (DMSO-d6): 2.06–2.19 (m,
1H), 2.86–3.01 (m, 2H), 3.35–3.48 (m, 2H), 3.91–4.00 (m, 1H), 4.31–4.44 (m,
2H), 7.20 (d, J = 8.8 Hz, 1H), 7.37 (td, J = 8.8, 2.4 Hz, 1H), 7.76 (dd, J = 8.4, 2.0 Hz,
1H), 7.97–8.04 (m, 2H), 8.18 (d, J = 1.6 Hz, 1H), 9.67 (s, 1H); Compound 22
(CDCl3): 2.74–2.81 (m, 1H), 2.82 (s, 3H), 3.45–3.52 (m, 1H), 3.59–3.67 (m, 1H),
3.85–3.92 (m, 1H), 4.30–4.40 (m, 2H), 4.42–4.52 (m, 1H), 4.65–4.75 (m, 1H),
6.95 (d, J = 8.4 Hz, 1H), 7.49–7.56 (m, 2H), 7.63 (t, J = 8.0 Hz, 1H), 7.84 (d,
J = 8.0 Hz, 1H), 8.50 (d, J = 8.4 Hz, 1H), 8.55 (s, 1H); Compound 23 (CDCl3):
2.70–2.82 (m, 1H), 2.83 (s, 3H), 3.45–3.54 (m, 1H), 3.60–3.68 (m, 1H), 3.79–
3.92 (m, 1H), 4.30–4.49 (m, 3H), 4.59–4.69 (m, 1H), 6.96 (d, J = 8.4 Hz, 1H), 7.36
(td, J = 8.8, 2.4 Hz, 1H), 7.49 (dd, J = 8.4, 2.4 Hz, 1H), 7.55 (dd, J = 7.6, 2.4 Hz, 1H),
8.36–8.46 (m, 2H); Compound 24 (CDCl3): 2.60–2.72 (m, 1H), 2.79 (s, 3H),
3.40–3.52 (m, 1H), 3.63–3.75 (m, 1H), 3.80–3.88 (m, 1H), 3.91 (s, 3H), 4.23–
4.41 (m, 2H), 4.43–4.61 (m, 2H), 6.97 (d, J = 8.1 Hz, 1H), 7.10 (dd, J = 8.0, 2.1 Hz,
1H), 7.32 (d, J = 2.1 Hz, 1H), 7.50 (d, J = 8.1 Hz, 1H), 7.80 (s, 1H), 8.02 (d,
J = 8.7 Hz, 1H).
29. Klunk, W. E.; Wang, Y. M.; Huang, G. F.; Debnath, M. L.; Holt, D. P.; Mathis, C. A.
Life Sci. 2001, 69, 1471.
30. Kim, M. K.; Choo, I. H.; Lee, H. S.; Woo, J. I.; Chong, Y. Bull. Korean Chem. Soc.
2007, 28, 1231.
31. Yang, Y.; Zhu, L.; Chen, X. J.; Zhang, H. B. J. Mol. Graph. Model. 2010, 29, 538.