18F-Labeled indole-based analogs as highly selective radioligands for imaging sigma-2 receptors in the brain

Article abstract of DOI:10.1016/j.bmc.2017.05.019

We have designed and synthesized a series of indole-based σ₂ receptor ligands containing 5,6-dimethoxyisoindoline or 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline as pharmacophore. In vitro competition binding assays showed that all ten ligands

Full text of DOI:10.1016/j.bmc.2017.05.019

Accepted Manuscript
18F-Labeled indole-based analogs as highly selective radioligands for imaging
sigma-2 receptors in the brain
Liang Wang, Jiajun Ye, Yingfang He, Winnie Deuther-Conrad, Jinming Zhang,
Xiaojun Zhang, Mengchao Cui, Jörg Steinbach, Yiyun Huang, Peter Brust,
Hongmei Jia
PII:
S0968-0896(17)30681-8
DOI:
http://dx.doi.org/10.1016/j.bmc.2017.05.019
BMC 13737
Reference:
Bioorganic & Medicinal Chemistry
To appear in:
Received Date:
Accepted Date:
31 March 2017
8 May 2017
Please cite this article as: Wang, L., Ye, J., He, Y., Deuther-Conrad, W., Zhang, J., Zhang, X., Cui, M., Steinbach,
18
J., Huang, Y., Brust, P., Jia, H., F-Labeled indole-based analogs as highly selective radioligands for imaging
sigma-2 receptors in the brain, Bioorganic & Medicinal Chemistry (2017), doi: http://dx.doi.org/10.1016/j.bmc.
2017.05.019
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1
18F-Labeled indole-based analogs as highly selective
radioligands for imaging sigma-2 receptors in the brain
Liang Wanga, Jiajun Yea, Yingfang Hea, Winnie Deuther-Conradb, Jinming Zhangc, Xiaojun
Zhangc, Mengchao Cuia,*, Jörg Steinbachb, Yiyun Huangd, Peter Brustb, Hongmei Jiaa,*
aKey Laboratory of Radiopharmaceuticals (Beijing Normal University), Ministry of Education,
College of Chemistry, Beijing Normal University, Beijing 100875, China
bHelmholtz-Zentrum Dresden-Rossendorf, Institute of Radiopharmaceutical Cancer
Research/ Department of Neuroradiopharmaceuticals, 04318 Leipzig, Germany
cNuclear Medicine Department, Chinese PLA General Hospital, Beijing 100853, China
dPET Center, Department of Radiology and Biomedical Imaging, Yale University, New Haven,
CT, USA
*Corresponding author. Tel.: 86-10-58808891. Fax: 86-10-58808891. E-mail address:
hmjia@bnu.edu.cn, cmc@bnu.edu.cn
ABSTRACT
We have designed and synthesized a series of indole-based σ2 receptor ligands containing
5,6-dimethoxyisoindoline
or
6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline
as
pharmacophore. In vitro competition binding assays showed that all ten ligands possessed
low nanomolar affinity (Ki = 1.79–5.23 nM) for σ2 receptors and high subtype selectivity (Ki
(σ2)/Ki (σ1) = 56–708). Moreover, they showed high selectivity for σ2 receptor over the
vesicular acetylcholine transporter (>1000-fold). The corresponding radiotracers [18F]16
and [18F]21 were prepared by an efficient one-pot, two-step reaction sequence with a
home-made automated synthesis module, with 10–15% radiochemical yield and
radiochemical purity of >99%. Both radiotracers showed high brain uptake and σ2 receptor
binding specificity in mice.
Keywords: sigma-2 receptors; 18F; indole-based analogs; brain

2
1. INTRODUCTION
The sigma-2 (σ2) receptor (molecular weight ~ 21.5 kDa1) is believed to play important
roles in the regulation of cellular differentiation.2 It has been shown to be upregulated in
various types of tumor cells,3-7 with approximately 10-fold higher expression in proliferating
than quiescent tumors.8-10 Thus the σ2 receptor has been suggested to be a biomarker for
the proliferative status of solid tumors.11-12 Moreover, the σ2 receptor interacts with Ca2+
channels and participates in various central nervous system (CNS) processes,13-15 with
potential impact on learning and memory.16 Hence it is also considered a therapeutic target
for the treatment of brain disorders.17-19
Over the years, substantial efforts have been directed toward the development of in vivo
imaging agents for the investigation of σ2 receptors in human diseases.11-12 However, up to
date,
N-{4-[6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl]butyl}-2-(2-[18F]fluoroethoxy)-5-methylb
enzamide ([18F]ISO-1) is the only radiotracer that has been tested in humans for tumor
imaging.20-22 [18F]ISO-1 showed high uptake and clear visualization of EMT-6 tumors in
female Balb/c mice. However, it is less suitable for imaging σ2 receptors in the brain due to
its low brain uptake (~ 0.76% injected dose (ID)/g at 2 min).20 Our aim was to develop
radioligands with high affinity for σ2 receptors, good selectivity, high uptake and favorable
kinetics for use in brain imaging.
Figure 1. Design concept for the indole-based compounds.
2-[4-(indol-1-yl)butyl]-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline23 (1, Figure 1) was
reported to have nanomolar affinity and acceptable selectivity for σ2 receptors, and hence
chosen as the lead compound to design a series of indole-based derivatives. The design
concept is shown in Figure 1. At first, a fluoroethoxy group was introduced at different
positions of the indole ring to enable the incorporation of 18F for PET imaging. Second, the
length
of
the
carbon
chain
between
the
indole
ring
and
the

3
6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline moiety was modified to explore the linker
effect
on
pharmacokinetic
properties
of
the
compounds.
Finally,
the
6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline moiety was replaced by another σ2-preferring
5,6-dimethoxyisoindoline moiety to lower the lipophilicity. Herein we report the synthesis of
these novel indole-based derivatives, together with the 18F-labeled radiotracers [18F]16 and
[18F]21 and their evaluation for imaging σ2 receptors in the brain.
2. RESULTS
2.1 Chemistry.
The synthetic routes for the indole-based derivatives 13−22 are depicted in Scheme 1.
Intermediates 5−7 were obtained by reaction of the hydroxyindols 2−4 with
1-bromo-2-fluoroethane under basic conditions. Based on a recently reported method,23
N-alkylation of compounds 5−7 with the corresponding bromide provided compounds 8−12.
Subsequent
reaction
with
6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline
or
5,6-dimethoxyisoindoline then afforded compounds 13−22 in yields of 34−94%.
Scheme 1. Reagents and conditions: (a) 1-bromo-2-fluoroethane, CH3CN, K2CO3, 90 °C, 6
h, for 5, 92%, for 6, 69%, and for 7, 70%; (b) DMF, KOH, Br(CH2)mBr, TBAF, r.t, 2 h, 24−82%;

4
(c) CH3CN, 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline/5,6-dimethoxyisoindoline, K2CO3,
90 °C, 6 h, 34−94%.
1
The target compounds were characterized by H NMR, 13C NMR and high-resolution
mass spectrometry (HRMS). Chemical purities of ≥ 95% were identified by high performance
liquid chromatography (HPLC) analysis (see Supporting Information).
Table 1. Binding affinities of indole-based compoundsa
Compd
m
n
Position
Ki(σ1)
Ki(σ2)
Ki(σ1)/
Ki(σ2)
Ki(VAChT)
Ki(VAChT)/
Ki (σ2)
of
(nM)
(nM)
(nM)
OCH2CH2F
-
1b
4
3
3
4
4
5
5
4
4
4
4
1
0
1
0
1
0
1
0
1
0
1
140.7 5.7
270 17
3.66 0.83
4.51 0.33
5.23 0.57
1.79 0.86
2.40 0.58
3.28 0.18
2.82 0.67
3.27 0.19
2.74 0.17
2.63 0.48
3.24 0.21
20.7 0.07
38.4
59.9
-
-
13
14
15
16
17
18
19
20
21
22
4-
4-
4-
4-
4-
4-
5-
5-
6-
6-
-
-
293
6
56.0
-
-
371 105
1698 548
426 54
207.5
707.7
129.8
92.7
34280 8443
19151
3570 88
1488
-
-
262 134
187 1.41
471 107
376 351
263 107
4.95 1.74
-
-
57.6
-
-
172.1
143.2
81.3
36865 205
13454
35355 7729
13443
-
-
-
-
Haloperidol
4.2
aValues are the mean standard deviation (SD) of at least two experiments performed in
triplicate. bFrom reference23.
2.2 In Vitro Radioligand Competition Binding Assays.
The affinities of compounds 13−22 for the σ1 and σ2 receptors and the vesicular
acetylcholine transporter (VAChT) were determined using radioligand competition binding
assays (σ1 receptor: (+)-[3H]-pentazocine; σ2 receptor: [3H]-1,3-di-o-tolylguanidine ([3H]DTG);
VAChT: (-)-[3H]vesamicol) as reported previously.24-25 The results are provided in Table 1. In
general, the introduction of a fluoroethoxy group maintained the nanomolar affinity of the
indole-based analogs for σ2 receptors (Ki(σ2) = 1.79−5.23 nM) and increased the subtype
selectivity (Ki(σ1)/Ki(σ2) = 56−708). The presence of the fluoroethoxy group at 4-position of
the indole group led to higher subtype selectivity compared with the 5- or 6-position (15 vs
19 and 21, 16 vs 20 and 22). The length of the carbon linker between the indole group and

5
the 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (or 5,6-dimethoxyisoindoline) moiety
significantly influenced the subtype selectivity. The presence of four carbon units is favorable
for σ2 receptor affinity and subtype selectivity (15 vs 13 and 17, 16 vs 14 and 18).
Compounds
with
either
the
5,6-dimethoxyisoindoline
or
6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline moiety displayed similar affinity for σ2
receptors (13 vs 14, 15 vs 16, 21 vs 22).
Considering the σ2 receptor affinity, subtype selectivity, lipophilicity and positions of the
fluoroethoxy substitution, compounds 15, 16, 20 and 21 were further tested for their VAChT
affinity. All four compounds proved to be of high selectivity for σ2 receptors over VAChT
(Ki(VAChT)/Ki(σ2) = 1488−19151). The corresponding radioligands [18F]16 (fluoroethoxy
group at the 4-position) and [18F]21 (fluoroethoxy group at the 6-position) were synthesized
to investigate their binding and kinetic properties in vivo.
2.3 Radiochemistry.
The precursors for radiolabelling were synthesized as outlined in Scheme 2. Protection
of compound 4-hydroxyindole (2) or 6-hydroxyindole (4) with 3,4-dihydro-2H-pyran, followed
by reaction with 1,4-dibromobutane provided compound 24 or 28. N-Alkylation of
6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline and 5,6-dimethoxyisoindoline with 24 and 28
gave compounds 25 and 29, respectively. Deprotection afforded compounds 26 and 30.
Scheme 3 depicts the radiosynthesis of [18F]16 and [18F]21, which were prepared by an
efficient one-pot, two-step reaction sequence with a home-made automated synthesis
module.26 In the first step, 2,2’-(ethane-1,2-diyl)bis(4-methylbenzenesulfonate) reacted with
Kryptofix 2.2.2/K+[18F]F- complex to provide 2-[18F]fluoroethyl-1-tosylate. Alkylation of the
precursors 26 and 30 with 2-[18F]fluoroethyl-1-tosylate (32) in the presence of Cs2CO3 gave
radioligands [18F]16 and [18F]21, respectively. After purification via semi-preparative HPLC,
[18F]16 and [18F]21 were obtained in decay-corrected radiochemical yields of 10−15% (n = 4)
and 16−21% (n = 7), respectively, and radiochemical purity (RCP) of > 99%. Specific
activities were 29 (n = 1) and 26−34 GBq/µmol (n = 2), respectively, for [18F]16 and [18F]21.
In order to identify the radiotracer, [18F]16 and its reference compound 16, and [18F]21
and its reference compound 21 were co-injected and their HPLC profiles assessed using
acetonitrile and water containing 0.1% trifluoroacetic acid (TFA) (45:55, v/v) as mobile phase
at a flow rate of 1 mL/min. The respective HPLC chromatograms are presented in Figure 2.
The retention times were observed to be 13.18 and 13.27 min for 16 and [18F]16, and 10.78

6
and 10.86 min for 21 and [18F]21, respectively. The differences in retention times were in
accordance with the time lag due to the volume and flow rate within the distance between
the UV and radioactivity detectors of our HPLC system, thus indicating co-elution of the
radiolabeled species with the unlabeled reference compounds.
Br
NH
c
b
a
NH
N
OH
OTHP
OTHP
24
2
23
OCH3
OCH3
OCH3
OCH3
N
N
N
d
N
OH
b
26
OTHP
25
H
H
HO
Br
N
THPO
a
THPO
N
N
c
4
27
OCH3
28
OCH3
OCH3
OCH3
N
HO
N
N
d
THPO
N
29
30
Scheme 2. Reagents and conditions: (a) DHP, TFA, CH2Cl2, 1 h，for 23, 36%, and for 27,
83%; (b) DMF, Br(CH2)4Br, KOH, TBAF, r.t, 2 h, for 24, 72%, and for 28, 47%; (c) K2CO3,
6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline/5,6-dimethoxyisoindoline, 90°C, 6 h, for 25,
69%, and for 29, 83%; (d) HCl (1 mol/L), NaOH (1 mol/L), r.t., 1 h, for 26, 33%, and for 30,
33%.

7
Scheme 3. Radiosyntheses of [18F]16 and [18F]21. Reagents and conditions: (a) Kryptofix
2.2.2, Cs2CO3, DMSO, 100 °C, 5 min; (b) 32, DMSO, 140 °C, 25 min.

8
Figure 2. HPLC co-elution profiles of 16 and [18F]16 (A), and 21 and [18F]21 (B), with UV
trace (λ = 254 nm) in black and radiotrace in red. Conditions: CH3CN/H2O (containing 0.1%
TFA) = 45/55, v/v, flow rate = 1 mL/min.
2.4 Evaluation of the Radioligands.
2.4.1 Lipophilicity.
A shake-flask method was employed for the determination of the apparent distribution of
the radiotracers as previously reported.26 The log D7.4 values were measured to be 2.35
0.11 (n = 3) and 2.17 0.13 (n = 3), respectively, for [18F]16 and [18F]21.
2.4.2 In vitro stability.
The in vitro stability of [18F]16 and [18F]21 in saline was evaluated after 4 h of incubation
at room temperature by measuring the RCP, which was maintained at > 99% as shown in
Figure 3.

9
Figure 3. In vitro stability analysis: analytical radio-HPLC chromatograms of [18F]16 and
[18F]21 in saline at room temperature at 4 h after synthesis.
2.4.3 Biodistribution studies in male ICR mice.
To evaluate the kinetics of [18F]16 and [18F]21, biodistribution studies were performed in
male ICR mice at 2, 15, 30, 60 and 120 min after radiotracer injection. The results are
summarized in Tables 2 and 3. Both [18F]16 and [18F]21 showed high initial brain uptake with
3.29 0.51% ID/g and 4.55 0.43% ID/g at 2 min, respectively, followed by a relatively fast
clearance from the brain. The brain-to-blood ratios were highest at 15 min after injection,
with 3.02 and 4.53 for [18F]16 and [18F]21, respectively. Relatively low accumulation of the
radiotracers was observed in the bone, with 4.04 0.60% ID/g and 5.46 1.42% ID/g,
respectively, at 120 min, indicating little defluorination of [18F]16 and [18F]21 in vivo.
To verify the binding specificity of [18F]16 and [18F]21 to σ2 receptors in vivo, blocking
studies were carried out by pre-administration of haloperidol (2.7 µmol/kg, 1 mg/kg, 0.1 mL)
or compound 21 (3 µmol/kg, 1.2 mg/kg, 0.1 mL) at 5 min prior to radiotracer injection.
Results are presented in Figure 4. For [18F]16, pretreatment with haloperidol reduced the
brain uptake by 26% (p = 0.001) and the brain-to-blood ratio by 35% (p = 0.001).
Pretreatment with compound 21 reduced the brain uptake by 24% (p = 0.001) and the
brain-to-blood ratio by 65% (p < 0.001). Moreover, pretreatment with compound 21 reduced
the radiotracer uptake in the liver by 62% (p < 0.001) and the liver-to-blood ratio by 83% (p <
0.001). For [18F]21, pretreatment with haloperidol reduced the brain uptake by 32% (p <
0.001) and the brain-to-blood ratio by 38% (p = 0.002). Pretreatment with compound 21
reduced the brain uptake by 21% (p = 0.004) and the brain-to-blood ratio by 68% (p < 0.001).
Radiotracer uptake in the liver and the liver-to-blood ratio were reduced by 54% (p < 0.001)

10
and 81% (p < 0.001), respectively, by pretreatment with compound 21. These data provide
evidence for the binding specificity of [18F]16 and [18F]21 in vivo.
Table 2. Biodistribution of [18F]16 in male ICR micea
Organ
Blood
Brain
2 min
15 min
30 min
60 min
120 min
2.19 0.25 1.00 0.07
3.29 0.51 3.01 0.24
10.02 1.76 3.21 0.29
8.33 0.83 20.44 2.56
4.56 1.04 8.35 1.01
31.57 4.67 9.77 1.93
18.19 2.33 16.77 1.11
7.31 1.13 11.94 0.81
0.94 0.17
2.19 0.18
2.24 0.34
24.26 3.67
7.59 0.57
7.28 0.9
1.04 0.13 1.05 0.07
1.34 0.13 1.01 0.03
Heart
Liver
1.56 0.16
1.50 0.11
23.41 3.09 17.12 1.82
5.37 0.65 3.76 0.32
3.67 0.90 2.87 0.30
6.63 0.62 4.91 0.49
19.43 2.35 20.97 1.38
Spleen
Lung
Kidney
10.49 0.84
14.5 1.94
Stomachb
Small
1.25 0.22 2.92 0.44
1.98 0.21
3.44 1.15 3.83 0.26
intestineb
Muscle
4.08 0.85 2.79 0.33
2.12 0.40 3.77 0.20
1.78 0.38
4.47 0.70
2.37
1.25 0.12 1.05 0.03
4.53 0.57 4.04 0.60
Bone
Brain/blood
1.50
3.02
1.30
0.96
aData are expressed as percentage of injected dose per gram (% ID/g), means SD, n = 5.
bPercentage of injected dose per organ.
Table 3. Biodistribution of [18F]21 in male ICR micea
Organ
Blood
2 min
15 min
30 min
60 min
120 min
2.15 0.23
4.55 0.43
9.01 0.83
8.87 0.91
6.06 1.44
39.54 8.12
0.75 0.04
3.39 0.29
2.60 0.30
0.63 0.09
2.28 0.16
1.39 0.55
0.54 0.06 0.49 0.04
1.14 0.07 0.97 0.17
1.14 0.26 1.13 0.10
Brain
Heart
Liver
23.30 0.98 31.60 3.93 29.66 2.04 26.24 4.00
Spleen
Lung
8.60 0.29
9.25 1.52
8.26 1.63
6.23 0.94
6.83 0.90
5.32 0.44
5.12 1.65
4.11 0.34
3.11 0.64
Kidney
Stomachb
17.77 1.91 11.07 0.86
7.33 0.57
4.26 0.37 3.87 0.25
12.77 0.85 14.83 1.33 16.62 2.00 21.51 2.60

11
Small
intestineb
Muscle
1.42 0.15
2.64 0.43
2.38 0.39
2.44 0.45 2.48 0.78
3.83 0.38
3.46 0.43
2.12
2.23 0.30
4.73 0.43
4.53
1.63 0.16
5.66 1.12
3.67
0.92 0.13 0.84 0.05
3.70 0.74 5.46 1.42
Bone
Brain/blood
2.12
2.02
aData are expressed as percentage of injected dose per gram (% ID/g), means SD, n = 5
bPercentage of injected dose per organ.
Figure 4. Effects of pretreatment with haloperidol (0.1 mL, 1.0 mg/kg) or compound 21 (0.1
mL, 3 µmol/kg) on organ biodistribution of [18F]16 and [18F]21 at 30 min after intravenous
injection. Student’s t test (independent, two-tailed) was performed, and p < 0.05 (except for
haloperidol in the liver).
2.4.4 Effect of P-gp on brain uptake of [18F]16 and [18F]21.
P-glycoprotein (P-gp) is highly expressed at the blood-brain barrier (BBB) and restricts
the brain entry of its substrates.27 To test whether [18F]16 and [18F]21 are substrates for P-gp
in vivo, cyclosporine A (50 mg/kg, 0.1 mL), an inhibitor of P-gp, was intravenously injected
via the tail vein at 1 h prior to radiotracer injection. Radioligand distribution results are
summarized in Figure 5. Cyclosporine A significantly increased the initial brain uptake of
[18F]16 and [18F]21 by 201% and 51%, respectively, at 2 min postinjection. At the same time,
the accumulation in the blood was maintained at the same level for [18F]16 (2.19 0.49%
ID/g vs 2.13 0.46% ID/g) and [18F]21 (2.15 0.23% ID/g vs 1.95 0.38% ID/g). These data
suggest that [18F]16 and [18F]21 may be substrates for P-gp.

12
Figure 5. Effects of P-gp on brain uptake of [18F]16 and [18F]21 in mice. Student’s t test
(independent, two-tailed) was performed, and p < 0.01 for brain.
Figure 6. Analytical radio-HPLC chromatograms of the mouse brain extracts at 30 min after
injection of [18F]21 (370 µCi, 0.1 mL, 7% ethanol in saline).
2.4.5 Analysis of radiometabolite in the brain.
Avery important step in the development of brain imaging agent is to determine whether
any radiometabolites of the radiotracer can enter the brain and confound the measurement
of binding signal. Since the higher brain uptake and brain-to-blood ratio favors [18F]21, its
radioactive species in the brain were analyzed in brain samples obtained from mice at 30
min after radiotracer injection, as previously reported.26 The results are shown in Figure 6.
In the brain, ≥ 95% of the radioactivity signal represented the parent tracer [18F]21, indicating
no entry of any radioactive metabolites into the brain.

13
3. DISCUSSION
The σ2 receptor has been proposed as a potential biomarker for the proliferative status of
solid tumors.8-12 More recently it has also been suggested to play an important role in
various brain disorders.13-15 and thus as a potential target for noninvasive imaging of
neuronal/synaptic loss in neurodegenerative diseases using positron emission tomography
(PET).28 However, there is no suitable PET radiotracer available for imaging σ2 receptors in
the brain. In this study, we designed and synthesized a series of indole-based σ2 receptor
ligands in an effort to develop 18F-labeled radiotracers with high affinity, selectivity and
suitability for imaging the σ2 receptors in brain.
In vitro binding assays showed that these new compounds with fluoroethoxy moiety
(13−22) possessed nanomolar affinity for σ2 receptors and high subtype selectivity. Moreover,
compounds 15, 16, 20, and 21 displayed extremely high selectivity for σ2 receptors over
VAChT (>1000-fold). Among the indole-based analogues, compound 16 with a
6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline moiety and the fluoroethoxy group at
4-position of the indole ring displayed the highest subtype selectivity. Compound 21 with a
5,6-dimethoxyisoindoline moiety and the fluoroethoxy group at 6-position of the indole ring
displayed nanomolar affinity for σ2 receptors, >100-fold subtype selectivity, and >10000-fold
selectivity for σ2 vs. VAChT. The corresponding radiotracers [18F]16 and [18F]21 were thus
prepared for further in vitro and in vivo evaluation.
Radioligand [18F]21 (log D7.4 = 2.17) displayed lower lipophilicity than [18F]16 (log D7.4
=
2.35), consistent with the lower lipophilicity of the 5,6-dimethoxyisoindoline moiety. The log
D7.4 values of both [18F]16 and [18F]21 are within the desirable range (log D = 1−3) for good
BBB permeability required for brain imaging probes.29 Both [18F]16 and [18F]21 also
displayed excellent stability in vitro.
In biodistribution studies in mice, [18F]16 (3.29 % ID/g at 2 min) and [18F]21 (4.55 % ID/g at
2 min) showed higher initial brain uptake levels than the previously reported σ2 radioligand
[18F]ISO-1 (0.76 % ID/g at 2 min).20 The brain-to-blood activity ratios of [18F]16 and [18F]21 at
15 min after injection were also high. In addition, pretreatment with haloperidol and
compound 21 significantly reduced radiotracer accumulation in the brain and liver, providing
evidence for specific binding of [18F]16 and [18F]21 to σ2 receptors in vivo.
Ex vivo experiments were conducted to determine whether [18F]16 and [18F]21 are
potential substrates for P-gp, as P-gp is highly expressed at the BBB and actively transports
different lipophilic drugs out of the brain capillary endothelial cells. Pretreatment with the

14
P-gp inhibitor cyclosporine A at 60 min prior to radiotracer injection increased brain uptake of
[18F]16 and [18F]21 by approximately 2.0-fold and 0.5-fold, respectively, while levels in the
blood were not changed. These data indicate that [18F]16 and [18F]21 may be substrates for
P-gp. The relatively lower initial brain uptakes of [18F]16 and [18F]21 (less than 5 % ID/g)
could be attributed to the observation that both compounds appear to be weak substrates for
P-gp, which might limit their utility in translational imaging studies. However, it is well known
that a substrate of P-gp is species dependent.30-31 Evaluation of [18F]16 and [18F]21 in higher
animal species is warranted to further investigate this point.
Another important aspect for a brain radiotracer is the absence of radiometabolites able to
cross the BBB into brain. [18F]21 displayed higher brain uptake and brain-to-blood ratios
than [18F]16, and its radioactivity profile in the brain of male ICR mice was determined at 30
min after radioligand injection. The parent tracer [18F]21 was found to be the only radioactive
species in the mouse brain, indicating that radiometabolites generated in the periphery did
not enter the brain.
4. Conclusion
We have successfully designed and synthesized a series of indole-based analogs with
nanomolar affinity for σ2 receptors and high subtype selectivity as well as high selectivity for
σ2 over VAChT. Radioligands [18F]16 and [18F]21 displayed suitable lipophilicity and high
stability in vitro. Biodistribution results in ICR mice indicated that these two 18F-labeled
tracers displayed high brain uptake and high brain-to-blood ratios. Blocking studies
confirmed specific binding of [18F]16 and [18F]21 to σ2 receptors in vivo. In vivo metabolic
studies indicated no presence of radioactive metabolites from [18F]21 in the brain. Taken
together, both of these two new radioligands appear to be suitable in vivo imaging agents for
the σ2 receptors and further development is warranted.
5. Experimental section
5.1. General method
All the chemicals or reagents were obtained from commercial suppliers without further
purification. All synthesized compounds were monitored by thin-layer chromatography (TLC)
1
on silica gel. H NMR spectra were recorded on a Bruker Avance III (400 MHz) NMR
spectrometer in CDCl3 solutions at room temperature and with tetramethylsilane (TMS) as
an internal standard. 13C NMR spectra were recorded on a Bruker Avance III (100 MHz)
NMR spectrometer. Chemical shifts (δ) are reported in ppm downfield from TMS and

15
coupling constants (J) in Hertz (Hz). Mass spectra were acquired on a Quattro micro API
ESI/MS instrument (Waters, USA). High-resolution mass spectrometry (HRMS) was
performed on a LCT Premier XE ESI-TOF mass spectrometer (Waters, USA). High
performance liquid chromatography analysis and purification were performed on a Waters
600 system (Waters Corporation, USA) equipped with a Waters 2489 UV-VIS detector and a
Raytest Gabi NaI (Tl) scintillation detector (Raytest, Germany). Samples were separated on
an Agela Venusil MP C18 column (250 × 4.6 mm, 5 µm) using 40% acetonitrile (containing
0.1% trifluoroacetic acid, TFA) and 60% water (containing 0.1% TFA) as mobile phase at a
flow rate of 4 mL/min. Samples were analyzed using 45% acetonitrile (containing 0.1% TFA)
and 55% water (containing 0.1% TFA) as mobile phase at a flow rate of 1 mL/min.
Normal male ICR mice (22–24 g, 4–5 weeks) were purchased from Vital River
Experimental Animal Technical Co., LTD. All procedures of the animal experiments were
carried out according to the relevant laws and institutional guidelines. The protocol was
approved by the Institutional Animal Care and Use Committee of Beijing Normal University.
5.2. Chemistry
5.2.1. 4-(2-fluoroethoxy)-indole (5)
Compound 2 (202.2 mg, 1.52 mmol) and 1-bromo-2-fluoroethane (347.6 mg, 2.74 mmol)
were dissolved in CH3CN (30 mL), followed by addition of K2CO3 (637.1 mg, 4.61 mmol).
The mixture was heated under reflux and stirred at 90 °C for 6 h. After cooling and filtration,
the solvent was removed under reduced pressure. The residue was purified by silica gel
column chromatography (petroleum ether/ethyl acetate/triethyamine = 10:5:1, v/v/v) to afford
5 (249.9 mg, 92%) as a pale yellow solid, MP: 119.0−120.2 °C. 1H NMR (400 MHz, CDCl3): δ
8.16 (s, 1H, indole N-H), 7.13–7.04 (m, 3H, Ar-H), 6.71–6.69 (m, 1H, Ar-H), 6.52 (dd, J = 7.5,
0.6 Hz, 1H, Ar-H), 4.83 (dt, J = 47.4, 4.2 Hz, 2H, F-CH2), 4.38 (dt, J = 27.6, 4.3 Hz, 2H,
O-CH2). ESI-MS, [M+H]+: m/z = 180.2.
5.2.2. 5-(2-fluoroethoxy)-indole (6)
The procedure described for the synthesis of 5 was applied to compound 3 (215.8 mg,
1.62 mmol), 1-bromo-2-fluoroethane (400.2 mg, 3.15 mmol) and K2CO3 (626.6 mg, 4.53
mmol) to afford 6 (201.7 mg, 69%) as a pink oil. 1H NMR (400 MHz, CDCl3): δ 8.15 (s, 1H,
indole N-H), 7.28–7.16 (m, 3H, Ar-H), 6.97 (dd, J = 8.8, 0.2 Hz, 1H, Ar-H), 6.55–6.53 (m, 1H,
Ar-H), 4.80 (dt, J = 47.5, 4.2 Hz, 2H, F-CH2), 4.27 (dt, J = 27.6, 4.2 Hz, 2H, O-CH2). ESI-MS,

16
[M+H]+: m/z = 180.2.
5.2.3. 6-(2-fluoroethoxy)-indole (7)
The procedure described for the synthesis of 5 was applied to compound 4 (500.0 mg,
3.76 mmol), 1-bromo-2-fluoroethane (732.5 mg, 5.77 mmol) and K2CO3 (1643.5 mg, 11.91
mmol) to afford 7 (470.1 mg, 70%) as a white solid. Mp: 92.3−93.9 °C. 1H NMR (400 MHz,
CDCl3): δ 8.05 (s, 1H, indole N-H), 7.53 (d, J = 8.6, 1H, Ar-H), 7.09 (s, 1H, Ar-H), 6.91 (s, 1H,
Ar-H), 6.84 (d, J = 8.6, 1H, Ar-H), 6.50 (s, 1H, Ar-H), 4.78 (dt, J = 47.5, 4.0 Hz, 2H, F-CH2),
4.25 (dt, J = 27.9, 4.1 Hz, 2H, O-CH2). ESI-MS, [M+H]+: m/z = 180.2.
5.2.4. 1-(3-bromopropyl)-4-(2-fluoroethoxy)-indole (8)
Tetrabutylammonium fluoride (85.0 mg, 0.33 mmol) and potassium hydroxide (278.6 mg,
4.97 mmol) were added to a solution of 5 (178.2 mg, 0.99 mmol) in DMF (2 mL). The
reaction mixture was stirred at 0 °C for 1 h. Then 1,3-dibromopropane was added and the
mixture was stirred for 1 h at room temperature. The solvent was removed. The residue was
washed with saturated sodium chloride, extracted with dichloromethane and dried. The
solvent was removed under reduced pressure. The residue was purified by silica gel
chromatography (petroleum ether/ethyl acetate) = 10:1, v/v) to afford 8 (70.6 mg, 24%) as a
yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.14 (t, J = 8.0 Hz, 1H, Ar-H), 7.07 (d, J = 3.2 Hz, 1H,
Ar-H), 7.04 (d, J = 8.3 Hz, 1H, Ar-H), 6.66 (dd, J = 3.1, 0.7 Hz, 1H, Ar-H), 6.53 (d, J = 7.7 Hz,
1H, Ar-H), 4.84 (dt, J = 47.5, 4.2 Hz, 2H, F-CH2), 4.38 (dt, J = 27.7, 4.3 Hz, 2H, O-CH2), 4.32
(t, J = 6.4 Hz, 2H, indole N-CH2), 3.30 (t, J = 6.1 Hz, 2H, Br-CH2), 2.37–2.31 (m, 2H, CH2).
5.2.5. 1-(4-bromobutyl)-4-(2-fluoroethoxy)-indole (9)
The procedure described for the synthesis of 8 was applied to compound 5 (290.1 mg,
1.62 mmol), tetrabutylammonium fluoride (84.6 mg, 0.33 mmol), potassium hydroxide (278.3
mg, 4.97 mmol) and 1,4-dibromobutane (1071.6 mg, 4.96 mmol) to afford 9 (376.6 mg, 74%)
as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.10 (t, J = 8.0 Hz, 1H, Ar-H), 6.98–6.95 (m, 2H,
Ar-H), 6.63 (d, J = 3.1 Hz, 1H, Ar-H), 6.49 (d, J = 7.7 Hz, 1H, Ar-H), 4.80 (dt, J = 47.5, 4.2 Hz,
2H, F-CH2), 4.33 (dt, J = 27.8, 4.2 Hz, 2H, O-CH2), 4.09 (t, J = 6.8 Hz, 2H, indole N-CH2),
3.32 (t, J = 6.5 Hz, 2H, Br-CH2), 2.00–1.92 (m, 2H, CH2), 1.83–1.76 (m, 2H, CH2). ESI-MS,
[M+H]+: m/z = 314.0.

17
5.2.6. 1-(5-bromopentyl)-4-(2-fluoroethoxy)-indole (10)
The procedure described for the synthesis of 8 was applied to compound 5 (193.4 mg,
1.08 mmol), tetrabutylammonium fluoride (83.9 mg, 0.32 mmol), potassium hydroxide (181.7
mg, 3.24 mmol) and 1,5-dibromopentane (744.4 mg, 3.24 mmol) to afford 10 (293.8 mg,
82%) as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.12 (t, J = 8.0 Hz, 1H, Ar-H), 7.01–6.98
(m, 2H, Ar-H), 6.64 (d, J = 3.1 Hz, 1H, Ar-H), 6.51 (d, J = 7.7 Hz, 1H, Ar-H), 4.83 (dt, J = 47.5,
4.0 Hz, 2H, F-CH2), 4.38 (dt, J = 28.0, 4.0 Hz, 2H, O-CH2), 4.12 (t, J = 7.0 Hz, 2H, indole
N-CH2), 3.37 (t, J = 6.7 Hz, 2H, Br-CH2), 1.90–1.82 (m, 4H, CH2), 1.50–1.42 (m, 2H, CH2).
5.2.7. 1-(4-bromobutyl)-5-(2-fluoroethoxy)-indole (11)
The procedure described for the synthesis of 8 was applied to compound 6 (201.0 mg,
1.12 mmol), tetrabutylammonium fluoride (113.7 mg, 0.44 mmol), potassium hydroxide
(248.6 mg, 4.44 mmol) and 1,4-dibromobutane (1001.4 mg, 4.64 mmol) to afford 11 (162.6
mg, 46%) as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.25 (d, J = 8.6 Hz, 1H, Ar-H), 7.13 (s,
1H, Ar-H), 7.08 (s, 1H, Ar-H), 6.94 (d, J = 7.8 Hz, 1H, Ar-H), 6.43 (s, 1H, Ar-H), 4.78 (d, J =
47.5 Hz, 2H, F-CH2), 4.26 (d, J = 28.1 Hz, 2H, O-CH2), 4.13 (t, J = 4.9 Hz, 2H, indole N-CH2),
3.37 (t, J = 4.0 Hz, 2H, Br-CH2), 1.99 (s, 2H, CH2), 1.85 (s, 2H, CH2). ESI-MS, [M+H]+: m/z =
314.0.
5.2.8. 1-(4-bromobutyl)-6-(2-fluoroethoxy)-indole (12)
The procedure described for the synthesis of 8 was applied to compound 7 (389.6 mg,
2.17 mmol), tetrabutylammonium fluoride (85.1 mg, 0.33 mmol), potassium hydroxide (258.6
mg, 4.62 mmol) and 1,4-dibromobutane (1432.1 mg, 6.63 mmol) to afford 12 (309.4 mg,
45%) as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.51 (d, J = 8.5 Hz, 1H, Ar-H), 6.99 (d, J =
2.4 Hz, 1H, Ar-H), 6.86 (s, 1H, Ar-H), 6.81 (d, J = 8.6 Hz, 1H, Ar-H), 6.43 (d, J = 2.4 Hz, 1H,
Ar-H), 4.80 (dt, J = 47.5, 3.8 Hz, 2H, F-CH2), 4.28 (dt, J = 27.8, 3.9 Hz, 2H, O-CH2), 4.09 (t, J
= 6.8 Hz, 2H, indole N-CH2), 3.38 (t, J = 6.5 Hz, 2H, Br-CH2), 2.04–1.97 (m, 2H, CH2),
1.90–1.82 (m, 2H, CH2). ESI-MS, [M+H]+: m/z = 314.1.
5.2.9. 1-[3-(5,6-dimethoxyisoindolin-2-yl)butyl]-4-(2-fluoroethoxy)-indole (13)
Compound 8 (70.6 mg, 0.24 mmol) and 5,6-dimethoxyisoindoline (68.5 mg, 0.38 mmol)
were dissolved in CH3CN (30 mL), followed by addition of K2CO3 (138.8 mg, 1.00 mmol).
The mixture was heated under reflux and stirred at 90 °C for 6 h. After cooling and filtration,

18
the solvent was removed under reduced pressure. The residue was purified by silica gel
chromatography (petroleum ether/ethyl acetate/triethyamine) = 10:10:1, v/v/v) to afford 13
(56.8 mg, 59%) as a pale yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.15–7.06 (m, 3H, Ar-H),
6.77 (s, 2H, Ar-H), 6.66 (dd, J = 3.1, 0.5 Hz, 1H, Ar-H), 6.52 (d, J = 7.2 Hz, 1H, Ar-H), 4.84
(dt, J = 47.5, 4.2 Hz, 2H, F-CH2), 4.37 (dt, J = 27.8, 4.3 Hz, 2H, O-CH2), 4.27 (t, J = 6.7 Hz,
2H, indole N-CH2), 3.87 (s, 10H, O-CH3, Ar-CH2-N), 2.65 (t, J = 6.7 Hz, 2H, N-CH2),
2.08–2.05 (m, 2H, CH2). 13C NMR (100 MHz, CDCl3) δ (ppm): 152.43, 148.63, 137.94,
131.86, 127.08, 122.29, 119.61, 106.14, 103.84, 100.64, 98.63, 82.35 (J = 169.2, F-CH2),
67.49 (J = 20.6, O-CH2), 59.32, 56.34, 52.75, 44.12, 29.56. ESI-TOF MS calcd for
C23H28FN2O3 [M+H]+: 399.2084; found: 399.2076.
5.2.10.
2-{3-[4-(2-fluoroethoy)-indol-1-yl]propyl}-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (14)
The procedure described for the synthesis of 13 was applied to compound 8 (59.7 mg,
0.20 mmol), 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (56.6 mg, 0.29 mmol) and K2CO3
(81.8 mg, 0.59 mmol) to afford 14 (39.4 mg, 48%) as a yellow oil. 1H NMR (400 MHz, CDCl3):
δ 7.13–7.04 (m, 3H, Ar-H), 6.65 (dd, J = 3.1, 0.3 Hz, 1H, Ar-H), 6.62 (s, 1H, Ar-H), 6.53–6.50
(m, 2H, Ar-H), 4.84 (dt, J = 47.5, 4.2 Hz, 2H, F-CH2), 4.37 (dt, J = 27.8, 4.3 Hz, 2H, O-CH2),
4.24 (t, J = 6.7 Hz, 2H, indole N-CH2), 3.86 (s, 3H, O-CH3), 3.85 (s, 3H, O-CH3), 3.52 (s, 2H,
Ar-CH2-N), 2.85 (t, J = 5.8 Hz, 2H, Ar-CH2), 2.67 (t, J = 5.8 Hz, 2H, N-CH2), 2.43 (t, J = 6.8
Hz, 2H, N-CH2), 2.09 (t, J = 6.8 Hz, 2H, CH2). 13C NMR (100 MHz, CDCl3): δ 152.23, 147.57,
147.26, 137.75, 126.83, 126.70, 126.32, 122.04, 119.41, 111.49, 109.58, 103.68, 100.47,
98.44, 82.16 (J = 169.2, F-CH2), 67.31 (J = 20.5, O-CH2), 55.96, 55.93, 55.63, 54.63, 50.99,
44.03, 28.84, 27.63. ESI-TOF MS calcd for C24H30FN2O3 [M+H]+: 413.2240; found:
413.2236.
5.2.11. 1-[4-(5,6-dimethoxyisoindolin-2-yl)butyl]-4-(2-fluoroethoxy)-indole (15)
The procedure described for the synthesis of 13 was applied to compound 9 (82.0 mg,
0.27 mmol), 5,6-dimethoxyisoindoline (73.6 mg, 0.41 mmol) and K2CO3 (206.1 mg, 1.49
mmol) to afford 15 (73.3 mg, 68%) as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.11 (t, J =
8.0 Hz, 1H, Ar-H), 7.02 (dd, J = 10.8, 5.7 Hz, 2H, Ar-H), 6.72 (s, 2H, Ar-H), 6.64 (dd, J = 3.1,
0.7 Hz, 1H, Ar-H), 6.51 (d, J = 7.6 Hz, 1H, Ar-H), 4.83 (dt, J = 47.4, 4.2 Hz, 2H, F-CH2), 4.37
(dt, J = 27.7, 4.2 Hz, 2H, O-CH2), 4.16 (t, J = 6.9 Hz, 2H, indole N-CH2), 3.89 (s, 4H,
Ar-CH2-N), 3.85 (s, 6H, O-CH3), 2.74 (t, J = 7.4 Hz, 2H, N-CH2), 1.97–1.93 (m, 2H, CH2),

19
1.64–1.60 (m, 2H, CH2). 13C NMR (100 MHz, CDCl3): δ 152.44, 148.57, 137.86, 131.86,
126.67, 122.24, 119.63, 106.09, 103.73, 100.64, 98.66, 82.34 (J = 170.2, F-CH2), 67.43 (J =
20.7, O-CH2), 59.37, 56.32, 55.70, 46.65, 28.27, 26.53. ESI-TOF MS calcd for C24H30FN2O3
[M+H]+: 413. 2240; found: 413. 2236.
5.2.12. 2-{4-[4-(2-fluoroethoy)-indol-1-yl]butyl}-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline
(16)
The procedure described for the synthesis of 13 was applied to compound 9 (47.6 mg,
0.15 mmol), 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (51.2 mg, 0.26 mmol) and K2CO3
1
(123.5 mg, 0.89 mmol) to afford 16 (22.5 mg, 34%) as a yellow oil. H NMR (400 MHz,
CDCl3): δ 7.09 (t, J = 8.0 Hz, 1H, Ar-H), 7.01 (dd, J = 7.3, 5.8 Hz, 2H, Ar-H), 6.63 (d, J = 3.0
Hz, 1H, Ar-H), 6.58 (s, 1H, Ar-H), 6.50 (d, J = 8.9 Hz, 2H, Ar-H), 4.83 (dt, J = 47.4, 4.2 Hz, 2H,
F-CH2), 4.37 (dt, J = 27.7, 4.2 Hz, 2H, O-CH2), 4.14 (t, J = 7.0 Hz, 2H, indole N-CH2), 3.83 (d,
J = 3.0 Hz, 6H, O-CH3), 3.48 (s, 2H, Ar-CH2-N), 2.79 (t, J = 5.8 Hz, 2H, Ar-CH2), 2.64 (t, J =
5.9 Hz, 2H, N-CH2), 2.51–2.47 (m, 2H, N-CH2), 1.93–1.88 (m, 2H, CH2), 1.63–1.55 (m, 2H,
CH2). 13C NMR (100 MHz, CDCl3): δ 151.75, 147.03, 146.72, 137.16, 126.13, 126.00,
125.71, 121.55, 118.96, 110.92, 109.04, 103.07, 99.98, 97.95, 81.65 (J = 170.3, F-CH2),
66.81 (J = 21.0, O-CH2), 57.18, 55.44, 55.42, 55.23, 50.55, 45.98, 28.19, 27.67, 24.07.
ESI-TOF MS calcd for C25H32FN2O3 [M+H]+: 427.2397; found: 427.2394.
5.2.13. 1-[5-(5,6-dimethoxyisoindolin-2-yl)pentyl]-4-(2-fluoroethoxy)-indole (17)
The procedure described for the synthesis of 13 was applied to compound 10 (60.0 mg,
0.18 mmol), 5,6-dimethoxyisoindoline (85.5 mg, 0.47 mmol) and K2CO3 (129.6 mg, 0.94
mmol) to afford 17 (37.1 mg, 48%) as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.11 (t, J =
8.0 Hz, 1H, Ar-H), 7.02 (t, J = 5.9 Hz, 2H, Ar-H), 6.74 (s, 2H, Ar-H), 6.64 (d, J = 3.1 Hz, 1H,
Ar-H), 6.51 (d, J = 7.7 Hz, 1H, Ar-H), 4.83 (dt, J = 47.4, 4.2 Hz, 2H, F-CH2), 4.37 (dt, J = 27.7,
4.2 Hz, 2H, O-CH2), 4.10 (t, J = 7.0 Hz, 2H, indole N-CH2), 3.90–3.85 (m, 10H, Ar-CH2N,
O-CH3), 2.68 (t, J = 7.4 Hz, 2H, N-CH2), 1.90–1.87 (m, 2H, CH2), 1.60–1.58 (m, 2H, CH2),
1.41 (t, J = 7.7 Hz, 2H, CH2). 13C NMR (100 MHz, CDCl3): δ 152.43, 148.58, 137.81, 131.94,
126.64, 122.19, 119.62, 106.07, 103.68, 100.66, 98.57, 82.23 (J = 169.2, F-CH2), 67.42 (J =
20.7, O-CH2), 59.40, 56.30, 56.09, 46.69, 30.37, 28.77, 24.91. ESI-TOF MS calcd for
C25H32FN2O3 [M+H]+: 427.2391; found: 427.2390.
5.2.14.

20
2-{5-[4-(2-fluoroethoy)-indol-1-yl]pentyl}-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (18)
The procedure described for the synthesis of 13 was applied to compound 10 (60.0 mg,
0.18 mmol), 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (75.3 mg, 0.39 mmol) and K2CO3
1
(123.8 mg, 0.89 mmol) to afford 18 (38.2 mg, 47%) as a yellow oil. H NMR (400 MHz,
CDCl3): δ 7.11 (t, J = 8.0 Hz, 1H, Ar-H), 7.01 (t, J = 5.9 Hz, 2H, Ar-H), 6.64 (dd, J = 3.1, 0.5
Hz, 1H, Ar-H), 6.59 (s, 1H, Ar-H), 6.51 (d, J = 7.4 Hz, 2H, Ar-H), 4.83 (dt, J = 47.5, 4.2 Hz, 2H,
F-CH2), 4.47 (dt, J = 27.7, 4.2 Hz, 2H, O-CH2), 4.11 (t, J = 7.1 Hz, 2H, indole N-CH2), 3.84 (s,
6H, O-CH3), 3.53 (s, 2H, Ar-CH2-N), 2.81 (t, J = 5.8 Hz, 2H, Ar-CH2), 2.68 (t, J = 5.9 Hz, 2H,
N-CH2), 2.46 (t, J = 7.4 Hz, 2H, N-CH2), 1.90–1.84 (m, 2H, CH2), 1.65–1.58 (m, 2H, CH2),
1.42–1.36 (m, 2H, CH2). 13C NMR (100 MHz, CDCl3): δ 151.28, 146.56, 146.25, 136.66,
125.70, 125.52, 125.27, 121.06, 118.46, 110.48, 108.61, 102.54, 99.49, 97.43, 81.15 (J =
170.3, F-CH2), 66.32 (J = 21.0, O-CH2), 54.96, 54.93, 54.83, 50.08, 45.53, 29.21, 27.68,
25.88, 23.92. ESI-TOF MS calcd for C26H34FN2O3 [M+H]+: 441.2547; found: 441.2545.
5.2.15. 1-[4-(5,6-dimethoxyisoindolin-2-yl)butyl]-5-(2-fluoroethoxy)-indole (19)
The procedure described for the synthesis of 13 was applied to compound 11 (80.0 mg,
0.26 mmol), 5,6-dimethoxyisoindoline (93.6 mg, 0.52 mmol) and K2CO3 (222.0 mg, 1.61
mmol) to afford 19 (101.8 mg, 94%) as a yellow solid. MP: 91.8−92.5 °C. 1H NMR (400 MHz,
CDCl3): δ 7.24 (s, 1H, Ar-H), 7.11–7.10 (m, 2H, Ar-H), 6.92 (d, J = 8.8 Hz, 1H, Ar-H), 6.72 (s,
2H, Ar-H), 6.41 (d, J = 2.7 Hz, 1H, Ar-H), 4.77 (dt, J = 47.4, 4.0 Hz, 2H, F-CH2), 4.26 (dt, J =
28.1, 4.0 Hz, 2H, O-CH2), 4.15 (t, J = 6.9 Hz, 2H, indole N-CH2), 3.89 (s, 4H, Ar-CH2-N), 3.85
(s, 6H, O-CH3), 2.74 (t, J = 7.3 Hz, 2H, N-CH2), 1.97–1.93 (m, 2H, CH2), 1.64–1.60 (m, 2H,
CH2). 13C NMR (100 MHz, CDCl3): δ 152.84, 148.57, 131.91, 131.80, 129.03, 128.65, 112.59,
110.39, 106.07, 104.40, 100.76, 82.42 (J = 170.0, F-CH2), 68.37 (J = 20.5, O-CH2), 59.36,
56.30, 55.69, 46.63, 28.27, 26.56. ESI-TOF MS calcd for C24H30FN2O3 [M+H]+: 413.2240;
found: 413.2238.
5.2.16. 2-{4-[5-(2-fluoroethoy)-indol-1-yl]butyl}-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline
(20)
The procedure described for the synthesis of 13 was applied to compound 11 (80.0 mg,
0.26 mmol), 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (99.6 mg, 0.52 mmol) and K2CO3
1
(146.1 mg, 1.06 mmol) to afford 20 (85.2 mg, 77%) as a yellow oil. H NMR (400 MHz,
CDCl3): δ 7.24 (s, 1H, Ar-H), 7.11–7.08 (m, 2H, Ar-H), 6.90 (d, J = 8.9 Hz, 1H, Ar-H), 6.58 (s,

21
1H, Ar-H), 6.48 (s, 1H, Ar-H), 6.40 (d, J = 2.7 Hz, 1H, Ar-H), 4.77 (dt, J = 47.4, 3.9 Hz, 2H,
F-CH2), 4.25 (dt, J = 28.0, 4.0 Hz, 2H, O-CH2), 4.14 (t, J = 6.9 Hz, 2H, indole N-CH2), 3.84 (s,
3H, O-CH3), 3.83 (s, 3H, O-CH3), 3.53 (s, 2H, Ar-CH2-N), 2.81 (s, 2H, Ar-CH2), 2.70 (s, 2H,
N-CH2), 2.52 (t, J = 7.2 Hz, 2H, N-CH2), 1.94–1.90 (m, 2H, CH2), 1.64–1.61 (m, 2H, CH2).
13C NMR (100 MHz, CDCl3): δ 152.16, 147.04, 146.72, 131.22, 128.36, 127.98, 126.07,
125.69, 111.89, 110.91, 109.72, 109.02, 103.74, 100.05, 81.73 (J = 170.0, F-CH2), 67.70 (J
= 20.4, O-CH2), 57.14, 55.44, 55.41, 55.23, 50.51, 45.95, 28.16, 27.67, 24.10. ESI-TOF MS
calcd for C25H32FN2O3 [M+H]+: 427.2397; found: 427.2394.
5.2.17. 1-[4-(5,6-dimethoxyisoindolin-2-yl)butyl]-5-(2-fluoroethoxy)-indole (21)
The procedure described for the synthesis of 13 was applied to compound 12 (80.0 mg,
0.26 mmol), 5,6-dimethoxyisoindoline (86.2 mg, 0.48 mmol) and K2CO3 (148.6 mg, 1.08
1
mmol) to afford 21 (66.6 mg, 62%) as a yellow solid. MP: 105.7−106.8 °C. H NMR (400
MHz, CDCl3) δ (ppm): 7.51 (d, J = 8.6 Hz, 1H, Ar-H), 7.02 (d, J = 3.0 Hz, 1H, Ar-H), 6.87 (s,
1H, Ar-H), 6.81 (d, J = 8.6 Hz, 1H, Ar-H), 6.73 (s, 2H, Ar-H), 6.43 (d, J = 3.0 Hz, 1H, Ar-H),
4.75 (dt, J = 47.5, 4.0 Hz, 2H, F-CH2), 4.25 (dt, J = 28.0, 4.0 Hz, 2H, O-CH2), 4.12 (t, J = 7.0
Hz, 2H, indole N-CH2), 3.89 (s, 4H, Ar-CH2-N), 3.85 (s, 6H, O-CH3), 2.75 (t, J = 7.2 Hz, 2H,
N-CH2), 1.97–1.92 (m, 2H, CH2), 1.65–1.61 (m, 2H, CH2). 13C NMR (100 MHz, CDCl3): δ
154.31, 147.91, 136.06, 131.14, 126.63, 123.00, 121.04, 108.92, 105.41, 100.42, 94.37,
81.66 (J = 170.3, F-CH2), 67.51 (J = 20.4, O-CH2), 58.69, 55.63, 54.91, 45.71, 27.24, 25.77.
ESI-TOF MS calcd for C24H30FN2O3 [M+H]+: 413.2240; found: 413.2239.
5.2.18. 2-{4-[5-(2-fluoroethoy)-indol-1-yl]butyl}-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline
(22)
The procedure described for the synthesis of 13 was applied to compound 12 (150.0 mg,
0.48 mmol), 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (151.1 mg, 0.78 mmol) and K2CO3
(216.4 mg, 1.57 mmol) to afford 22 (188.9 mg, 92%) as a yellow solid. MP: 85.1−86.4 °C. 1H
NMR (400 MHz, CDCl3): δ 7.51 (d, J = 8.6 Hz, 1H, Ar-H), 7.02 (d, J = 3.0 Hz, 1H, Ar-H), 6.82
(s, 1H, Ar-H), 6.80 (d, J = 8.6 Hz, 1H, Ar-H), 6.58 (s, 1H, Ar-H), 6.49 (s, 1H, Ar-H), 6.42 (d, J
= 2.9 Hz, 1H, Ar-H), 4.75 (dt, J = 47.4, 4.0 Hz, 2H, F-CH2), 4.25 (dt, J = 27.9, 4.0 Hz, 2H,
O-CH2), 4.10 (t, J = 7.0 Hz, 2H, indole N-CH2), 3.84 (d, J = 2.7 Hz, 6H, O-CH3), 3.52 (s, 2H,
Ar-CH2-N), 2.81 (t, J = 5.4 Hz, 2H, Ar-CH2), 2.70 (t, J = 5.1 Hz, 2H, N-CH2), 2.52 (t, J = 7.3
Hz, 2H, N-CH2), 1.94–1.88 (m, 2H, CH2), 1.67–1.60 (m, 2H, CH2). 13C NMR (100 MHz,

22
CDCl3): δ 154.31, 147.04, 146.73, 136.04, 126.65, 126.04, 125.66, 123.02, 121.04, 110.89,
109.00, 108.91, 100.42, 94.41, 81.66 (J = 170.0, FCH2), 67.70 (J = 20.5, OCH2), 57.10,
55.43, 55.41, 55.24, 50.49, 45.76, 28.16, 27.40, 24.06. ESI-TOF MS calcd for C25H32FN2O3
[M+H]+: 427.2397; found: 427.2396.
5.2.19. 4-[(tetrahydro-pyran-2-yl)oxy]-indole (23)
A mixture of 2 (108.2 mg, 0.81 mmol) and 3,4-dihydro-2H-pyran (189.6 mg, 2.25 mmol)
in CH2Cl2 (25 mL) was added to TFA (19.9 mg, 0.14 mmol). The mixture was heated under
reflux and stirred at 40 °C for 1 h. After cooling and filtration, the solvent was removed under
reduced pressure. The residue was purified by silica gel chromatography (petroleum
ether/ethyl acetate) = 20:1, v/v) to afford 23 (63.4 mg, 36%) as a white solid. MP:
1
63.4−64.9 °C. H NMR (400 MHz, CDCl3): δ 8.16 (s, 1H, indole N-H), 7.15–7.00 (m, 3H,
Ar-H), 6.81–6.51 (m, 2H, Ar-H), 5.63 (t, J = 3.1 Hz, 1H, CH), 4.04–3.98 (m, 1H, O-CH2A),
3.65–3.61 (m, 1H, O-CH2B), 1.99–1.92 (m, 2H, CH2), 1.72–1.53 (m, 4H, CH2). ESI-MS,
[M+H]+: m/z = 218.1.
5.2.20. 1-(4-bromobutyl)-4-[(tetrahydro-pyran-2-yl)oxy]-indole (24)
The procedure described for the synthesis of 8 was applied to compound 23 (210.0 mg,
0.97 mmol), tetrabutylammonium fluoride (109.6 mg, 0.42 mmol), potassium hydroxide
(113.2 mg, 2.02 mmol) and 1,4-dibromobutane (645.9 mg, 2.99 mmol) to afford 24 (245.4
mg, 72%) as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.09–6.52 (m, 5H, Ar-H), 5.65 (t, J =
4.1 Hz, 1H, CH), 4.16 (t, J = 6.8 Hz, 2H, indole N-CH2), 4.06–3.99 (m, 1H, O-CH2A),
3.68–3.62 (m, 1H, O-CH2B), 3.39 (t, J = 6.5 Hz, 2H, Br-CH2), 2.01–1.91 (m, 4H, CH2),
1.89–1.63 (m, 6H, CH2). ESI-MS, [M+H]+: m/z = 352.1.
5.2.21.
6,7-dimethoxy-2-(4-{4-[(tetrahydro-pyran-2yl)oxy]-indol-1-yl}butyl)-1,2,3,4-tetrahydroisoquin
oline (25)
The procedure described for the synthesis of 13 was applied to compound 24 (273.1 mg,
0.78 mmol), 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (222.5 mg, 1.15 mmol) and K2CO3
1
(897.9 mg, 6.50 mmol) to afford 25 (250.0 mg, 69%) as a yellow oil. H NMR (400 MHz,
CDCl3): δ 7.13 (t, J = 7.9 Hz, 1H, Ar-H), 7.04 (dd, J = 8.1, 6.0 Hz, 2H, Ar-H), 6.81 (d, J = 7.7
Hz, 1H, Ar-H), 6.64–6.52 (m, 3H, Ar-H), 5.66 (s, 1H, CH), 4.16 (t, J = 6.8 Hz, 2H, indole

23
N-CH2), 4.03–4.01 (m, 1H, O-CH2A), 3.86 (s, 6H, O-CH3), 3.66–3.64 (m, 1H, O-CH2B), 3.51
(s, 2H, Ar-CH2-N), 2.82 (t, J = 5.4 Hz, 2H, Ar-CH2), 2.67 (t, J = 5.7 Hz, 2H, N-CH2), 2.51 (t, J
= 7.3 Hz, 2H, N-CH2), 2.22–1.95 (m, 4H, CH2), 1.93–1.59 (m, 6H, CH2). ESI-MS, [M+H]+:
m/z = 465.3.
5.2.22. 1-{4-[6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl]butyl}-4-hydroxyindole (26)
To the solution of compound 25 (230.3 mg, 0.50 mmol) in THF (10 mL) was added HCl
(1 mol/L, 30 mL). The mixture was stirred at 40 °C for 30 min, followed by addition of NaOH
(1 mol/L, 35 mL). After cooling and filtration, the solvent was removed under reduced
pressure. The residue was purified by silica gel chromatography (petroleum ether/ethyl
acetate/triethyamine) = 5:10:1, v/v/v) to afford 26 (62.1 mg, 33%) as a yellow solid. MP:
166.6−167.8 °C. 1H NMR (400 MHz, CDCl3): δ 9.28 (s, 1H, indole O-H), 7.16 (d, J = 3.1 Hz,
1H, Ar-H), 6.88–6.86 (m, 2H, Ar-H), 6.62 (s, 1H, Ar-H), 6.56 (s, 1H, Ar-H), 6.44 (d, J = 3.1 Hz,
1H, Ar-H), 6.35 (dd, J = 5.2, 3.1 Hz, 1H, Ar-H), 4.11 (t, J = 6.9 Hz, 2H, indole N-CH2), 3.67 (d,
J = 2.1 Hz, 6H, O-CH3), 3.36 (s, 2H, Ar-CH2-N), 2.66 (t, J = 5.5 Hz, 2H, Ar-CH2), 2.53 (t, J =
5.7 Hz, 2H, N-CH2), 2.39 (t, J = 7.1 Hz, 2H, N-CH2), 1.79–1.75 (m, 2H, CH2), 1.47–1.43 (m,
2H, CH2). ESI-TOF MS calcd for C23H29N2O3 [M+H]+: 381.2172; found: 381.2174.
5.2.23. 6-[(tetrahydro-pyran-2-yl)oxy]-indole (27)
The procedure described for the synthesis of 23 was applied to compound 4 (982.2 mg,
7.37 mmol), 3,4-dihydro-2H-pyran (1892.9 mg, 22.50 mmol) and TFA (220.2 mg, 1.53 mmol)
to afford 27 (230.3 mg, 14%) as a white solid. MP: 55.1−56.6 °C. 1H NMR (400 MHz, CDCl3):
δ 8.03 (s, 1H, indole N-H), 7.52 (d, J = 8.0 Hz, 1H, Ar-H), 7.15–7.09 (m, 2H, Ar-H), 6.90 (d, J
= 8.0 Hz, 1H, Ar-H), 6.49–7.48 (m, 1H, Ar-H), 5.43 (t, J = 3.2 Hz, 1H, C-H), 4.03–3.97 (m, 1H,
O-CH2A), 3.64–3.60 (m, 1H, O-CH2B), 2.01–1.88 (m, 2H, CH2), 1.71–1.56 (m, 4H, CH2).
ESI-MS, [M+H]+: m/z = 218.1.
5.2.24. 1-(4-bromobutyl)-6-[(tetrahydro-pyran-2-yl)oxy]-indole (28)
The procedure described for the synthesis of 8 was applied to compound 27 (420.0 mg,
1.93 mmol), tetrabutylammonium fluoride (108.9 mg, 0.42 mmol), potassium hydroxide
(220.2 mg, 3.93 mmol) and 1,4-dibromobutane (1248.4 mg, 5.78 mmol) to afford 28 (322.3
1
mg, 47%) as a yellow oil. H NMR (400 MHz, CDCl3): δ 6.99 (d, J = 3.2 Hz, 1H, Ar-H),
6.58–6.49 (m, 3H, Ar-H), 6.13 (d, J = 3.1 Hz, 1H, Ar-H), 5.35 (t, J = 3.4 Hz, 1H, CH), 4.25 (t,

24
J = 6.7, 2H, indole N-CH2), 4.19–4.16 (m, 1H, O-CH2A), 3.90–3.88 (m, 1H, O-CH2B), 3.69 (t,
J = 11.3 Hz, 2H, Br-CH2), 2.60–2.45 (m, 6H, CH2), 2.37–2.23 (m, 4H, CH2). ESI-MS, [M+H]+:
m/z = 352.1.
5.2.25. 1-[4-(5,6-dimethoxyisoindolin-2-yl)butyl]-6-[(tetrahydro-pyran-2-yl)oxy]-indole (29)
The procedure described for the synthesis of 13 was applied to compound 28 (208.9 mg,
0.59 mmol), 5,6-dimethoxyisoindoline (184.5 mg, 0.95 mmol) and K2CO3 (247.2 mg, 1.79
mmol) to afford 29 (230.3 mg, 86%) as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 7.50 (d, J =
8.6 Hz, 1H, Ar-H), 7.06–7.02 (m, 2H, Ar-H), 6.89 (dd, J = 8.6, 2.1 Hz, 1H, Ar-H), 6.73 (s, 2H,
Ar-H), 6.42 (d, J = 3.1 Hz, 1H, Ar-H), 5.44 (t, J = 3.2 Hz, 1H, C-H), 4.11 (t, J = 7.0 Hz, 2H,
indole N-CH2), 3.99–3.97 (m, 1H, O-CH2A), 3.86 (s, 6H, O-CH3), 3.84 (s, 4H, Ar-CH2-N),
3.63–3.59 (m, 1H, O-CH2B), 2.71 (t, J = 4.0 Hz, 2H, N-CH2), 1.95–1.57 (m, 10H, CH2).
5.2.26. 1-[4-(5,6-dimethoxyisoindolin-2-yl)butyl]-6-hydroxyindole (30)
The procedure described for the synthesis of 26 was applied to compound 29 (220.2 mg,
0.49 mmol), 1 mol/L HCl (30 mL) and 1 mol/L NaOH (35 mL) to afford 30 (60.0 mg, 33%) as
a white solid. MP: 150.5−151.9 °C.1H NMR (400 MHz, CDCl3): δ 7.44 (d, J = 8.4 Hz, 1H,
Ar-H), 6.98 (d, J = 3.2 Hz, 1H, Ar-H), 6.82 (d, J = 1.9 Hz, 1H, Ar-H), 6.72 (s, 2H, Ar-H), 6.65
(dd, J = 8.5, 2.2 Hz, 1H, Ar-H), 6.41 (d, J = 3.1 Hz, 1H, Ar-H), 4.07 (t, J = 6.9 Hz, 2H, indole
N-CH2), 3.92 (s, 4H, Ar-CH2-N), 3.85 (s, 6H, O-CH3), 2.75 (t, J = 8.0 Hz, 2H, N-CH2),
1.97–1.89 (m, 2H, CH2), 1.69–1.62 (m, 2H, CH2). ESI-TOF MS calcd for C22H27N2O3 [M+H]+:
367.2016; found: 367.2017.
5.3. In vitro radioligand competition studies
All the procedures for the radioligand competition studies for σ receptors and VAChT
were previously described.24-25 Detailed procedures are provided in the Supporting
Information.
5.4. Radiochemistry
The syntheses of radiotracers were performed in a home-made synthesis module.26
Detailed procedures are provided in the Supporting Information. Specific activity was
determined with a HPLC-based method. For animal experiments, the radiotracer was
formulated as a saline solution containing no more than 7% ethanol.

25
5.5. Determination of log D value
The log D value of [18F]16 and [18F]21 was determined by measuring the distribution of
the radiotracer between 1-octanol and potassium phosphate buffer (PBS, 0.05 mol/L, pH
7.4).26 Detailed procedures are provided in the Supporting Information.
5.6. In vitro stability studies
The in vitro stability of [18F]16 and [18F]21 were evaluated by monitoring the
radiochemical purity at different time points according to literature procedures.26 Detailed
procedures are provided in the Supporting Information.
5.7. Biodistribution studies
The biodistribution experiments were performed in normal male ICR mice (22–24 g, 4–5
weeks). Detailed procedures are provided in the Supporting Information.
5.8. Effect of P-gp on the Brain Uptake of [18F]16 and [18F]21 in Mice
To investigation of the effect of P-gp on the brain uptake of [18F]16 and [18F]21, mice
were injected via the tail vein with cyclosporine A (50.0 mg/kg, 0.1 mL) 60 min prior to
radiotracer injection. Detailed information is provided in the Supporting Information.
5.9. In vivo metabolic stability of [18F]21
The in vivo metabolic fate of [18F]21 was performed in male ICR mice. Detailed
procedures are provided in the Supporting Information.
Acknowledgements
Partial support of the studies by the German Research Foundation (DFG, Br 1360/13-1)
and the National Natural Science Foundation of China (No. 21471019) is gratefully
acknowledged.
Supplementary data
Supplementary data associated with this article can be found, in the online version.
References
1. Hellewell SB, Bruce A, Feinstein G, Orringer J, Williams W, Bowen WD. Rat liver and

26
kidney contain high densities of σ1 and σ2 receptors: characterization by ligand binding and
photoaffinity labeling. Eur J Pharmacol. 1994; 268: 9-18.
2. Vilner BJ, de Costa BR, Bowen WD. Cytotoxic effects of sigma ligands: sigma
receptor-mediated alterations in cellular morphology and viability. J Neurosci. 1995; 15:
117-134.
3. Bem WT, Thomas GE, Mamone JY, Homan SM, Levy BK, Johnson FE, Coscia CJ.
Overexpression of σ receptors in nonneural human tumors. Cancer Res. 1991; 51:
6558-6562.
4. Vilner BJ, John CS, Bowen WD. Sigma-1 and sigma-2 receptors are expressed in a
wide variety of human and rodent tumor cell lines. Cancer Res. 1995; 55: 408-413.
5. John CS, Vilner BJ, Geyer BC, Moody T, Bowen WD. Targeting sigma receptor-binding
benzamides as in vivo diagnostic and therapeutic agents for human prostate tumors. Cancer
Res. 1999; 59: 4578-4583.
6. van Waarde A, Rybczynska AA, Ramakrishnan NK, Ishiwata K, Elsinga PH, Dierckx
RAJO. Sigma receptors in oncology: therapeutic and diagnostic applications of sigma
ligands. Curr Pharm Des. 2010; 16: 3519-3537.
7. Megalizzi V, Le Mercier M, Decaestecker C. Sigma receptors and their ligands in cancer
biology: overview and new perspectives for cancer therapy. Med Res Rev. 2012; 32:
410-427.
8. Mach RH, Smith CR, Al-Nabulsi I, Whirrett BR, Childers SR, Wheeler KT. σ2 receptors
as potential biomarkers of proliferation in breast cancer. Cancer Res. 1997; 57: 156-161.
9. Al-Nabulsi I, Mach RH, Wang LM, Wallen CA, Keng PC, Sten K, Childers SR, Wheeler
KT. Effect of ploidy, recruitment, environmental factors, and tamoxifen treatment on the
expression of sigma-2 receptors in proliferating and quiescent tumour cells. Br J Cancer
1999; 81: 925-933.
10. Wheeler KT, Wang LM, Wallen CA, Childers SR, Cline JM, Keng PC, Mach RH. Sigma-2
receptors as a biomarker of proliferation in solid tumours. Br J Cancer 2000; 82: 1223-1232.
11. Mach RH, Zeng C, Hawkins WG. The σ2 receptor: a novel protein for the imaging and
treatment of cancer. J Med Chem. 2013; 56: 7137-7160.
12. Sai KKS, Jones LA, Mach RH. Development of 18F-labeled PET probes for imaging cell
proliferation. Curr Top Med Chem. 2013; 13: 892-908.
13. Zhang H, Cuevas J. Sigma receptors inhibit high-voltage–activated calcium channels in
rat sympathetic and parasympathetic neurons. J Neurophysiol 2002; 87: 2867-2879.
14. Vilner BJ, Bowen WD. Modulation of cellular calcium by sigma-2 receptors: release from

27
intracellular stores in human SK-N-SH neuroblastoma cells. J Pharmacol Exp Ther. 2000;
292: 900-911.
15. Cassano G, Gasparre G, Niso M, Contino M, Scalera V, Colabufo NA. F281, synthetic
agonist of the sigma-2 receptor, induces Ca2+ efflux from the endoplasmic reticulum and
mitochondria in SK-N-SH cells. Cell Calcium 2009; 45: 340-345.
16. Bezprozvanny I. Calcium signaling and neurodegerative diseases. Treads Mol Med.
2009; 15: 89-100.
17. Izzo NJ, Xu J, Zeng C, Kirk MJ, Mozzoni K, Silky C, Rehak C, Yurko R, Look G, Rishton
G, Safferstein H, Cruchaga C, Goate A, Cahill MA, Arancio O, Mach RH, Craven R, Head E,
LeVine H, Spires-Jones TL, Catalano SM. Alzheimer's therapeutics targeting amyloid beta
1-42 oligomers II: Sigma-2/PGRMC1 receptors mediate Abeta 42 oligomer binding and
synaptotoxicity. PLoS One 2014; 9: e111899.
18. Sahlholm K, Liao F, Holtzman DM, Xu J, Mach RH. Sigma-2 receptor binding is
decreased in female, but not male, APP/PS1 mice. Biochem Biophys Res Commun. 2015;
460: 439-445.
19. Yi B, Sahn JJ, Ardestani PM, Evans AK, Scott LL, Chan JZ, Iyer S, Crisp A, Zuniga G,
Pierce JT, Martin SF, Shamloo M. Small molecule modulator of sigma 2 receptor is
neuroprotective and reduces cognitive deficits and neuroinflammation in experimental
models of Alzheimer's disease. J Neurochem. 2017;140: 561-575.
20. Tu Z, Xu J, Jones LA, Li S, Dumstorff C, Vangveravong S, Chen DL, Wheeler KT, Welch
MJ, Mach RH. Fluorine-18-labeled benzamide analogues for imaging the σ2 receptor status
of solid tumors with positron emission tomography. J Med Chem. 2007; 50: 3194-3204.
21. Dehdashti F, Laforest R, Gao F, Shoghi KI, Aft RL, Nussenbaum B, Kreisel FH, Bartlett
NL, Cashen A, Wagner-Johnson N, Mach RH. Assessment of cellular proliferation in tumors
by PET using 18F-ISO-1. J Nucl Med. 2013; 54: 350-357.
22. Shoghi KI, Xu J, Su Y, He J, Rowland D, Yan Y, Garbow JR, Tu Z, Jones LA,
Higashikubo R. Quantitative receptor-based imaging of tumor proliferation with the sigma-2
ligand [18F]ISO-1. PloS one 2013; 8: e74188.
23. Mesangeau C, Amata E, Alsharif W, Seminerio MJ, Robson MJ, Matsumoto RR,
Poupaert JH, McCurdy CR. Synthesis and pharmacological evaluation of indole-based
sigma receptor ligands. Eur J Med Chem. 2011; 46: 5154-5161.
24. Fan C, Jia H, Deuther-Conrad W, Brust P, Steinbach J, Liu B. Novel 99mTc labeled σ
receptor ligand as a potential tumor imaging agent. Sci China Ser B 2006; 49: 169-176.
25. Sorger D, Scheunemann M, Großmann U, Fischer S, Vercouille J, Hiller A, Wenzel B,

28
Roghani A, Schliebs R, Brust P, Sabri O, Steinbach J. A new 18F-labeled
fluoroacetylmorpholino derivative of vesamicol for neuroimaging of the vesicular
acetylcholine transporter. Nucl Med Biol. 2008; 35: 185-195.
26. Chen YY, Wang X, Zhang JM, Deuther-Conrad W, Zhang XJ, Huang Y, Li Y, Ye JJ, Cui
MC, Steinbach J, Brust P, Liu BL, Jia HM. Synthesis and evaluation of a 18F-labeled
spirocyclic piperidine derivative as promising σ1 receptor imaging agent. Bioorg Med Chem.
2014; 22: 5270-5278.
27.Kreisl WC, Liow JS, Kimura N, Seneca N, Zoghbi SS, Morse CL, Herscovitch P, Pike VW,
Innis RB. P-glycoprotein function at the blood-brain barrier in humans can be quantified with
the substrate radiotracer 11C-N-desmethyl-loperamide. J Nucl Med. 2010; 51: 559-566.
28. Zeng C, Garg N, Mach RH. The PGRMC1 protein level correlates with the binding
activity of a sigma-2 fluorescent probe (SW120) in rat brain cells. Mol Imaging Biol. 2016; 18:
172-179.
29. Patel S, Gibson R. In vivo site-directed radiotracers: a mini-review. Nucl Med Biol. 2008;
35: 805-815.
30. Yamazaki M, Neway WE, Ohe T, Chen I, Rowe JF, Hochman JH, Chiba M, Lin JH. In
vitro substrate identification studies for P-glycoprotein-mediated transport: species
difference and predictability of in vivo results. J Pharmacol Exp Ther. 2001; 296: 723-735.
31. Syvänen S, Lindhe O, Palner M, Kornum BR, Rahman O, Långström B, Knudsen GM,
Hammarlund-Udenaes M. Species differences in blood-brain barrier transport of three
positron emission tomography radioligands with emphasis on P-glycoprotein transport. Drug
Metab Dispos. 2009; 37: 635-643.

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