A new single-photon emission computed tomography (SPECT) imaging agent for serotonin transporters: [125I]Flip-IDAM, (2-((2-((dimethylamino) methyl)-4-iodophenyl)thio)phenyl)methanol

Article abstract of DOI:10.1016/j.bmcl.2012.11.043

New ligands for in vivo brain imaging of serotonin transporter (SERT) with single photon emission tomography (SPECT) were prepared and evaluated. An efficient synthesis and radiolabeling of a biphenylthiol, FLIP-IDAM, 4, was accomplished. The affinity of FLIP-IDAM was evaluated by an in vitro inhibitory binding assay using [¹²⁵I]-IDAM as radioligand in rat brain tissue homogenates (K_i = 0.03 nM). New [¹²⁵I]Flip-IDAM exhibited excellent binding affinity to SERT binding sites with a high hypothalamus to cerebellum ratio of 4 at 30 min post iv injection. The faster in vivo kinetics for brain uptake and a rapid washout from non-specific regions provide excellent signal to noise ratio. This new agent, when labeled with ¹²³I, may be a useful imaging agent for mapping SERT binding sites in the human brain.

Full text of DOI:10.1016/j.bmcl.2012.11.043

Bioorganic & Medicinal Chemistry Letters 23 (2013) 869–872
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
A new single-photon emission computed tomography (SPECT) imaging agent
for serotonin transporters: [125I]Flip-IDAM, (2-((2-((dimethylamino)methyl)-4-
iodophenyl)thio)phenyl)methanol
Pinguan Zheng a, Brian P. Lieberman a, Karl Ploessl a, Laetitia Lemoine a, Sara Miller a, Hank F. Kung a,b,
⇑
a Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA
b Department of Pharmacology, University of Pennsylvania, Philadelphia, PA 19104, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
New ligands for in vivo brain imaging of serotonin transporter (SERT) with single photon emission
tomography (SPECT) were prepared and evaluated. An efficient synthesis and radiolabeling of a
biphenylthiol, FLIP-IDAM, 4, was accomplished. The affinity of FLIP-IDAM was evaluated by an in vitro
inhibitory binding assay using [125I]-IDAM as radioligand in rat brain tissue homogenates (Ki = 0.03 nM).
New [125I]Flip-IDAM exhibited excellent binding affinity to SERT binding sites with a high hypothalamus
to cerebellum ratio of 4 at 30 min post iv injection. The faster in vivo kinetics for brain uptake and a rapid
washout from non-specific regions provide excellent signal to noise ratio. This new agent, when labeled
with 123I, may be a useful imaging agent for mapping SERT binding sites in the human brain.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 23 August 2012
Revised 7 November 2012
Accepted 12 November 2012
Available online 2 December 2012
Keywords:
Serotonin transporter
Diphenyl sulfide
Single photon emission computed
tomography
SPECT
Serotonin transporter (SERT) is located exclusively on the pre-
synaptic terminals on serotonergic neurons in the brain. It plays
a crucial role in the regulation of the level of serotonin concentra-
tion in the synaptic cleft, as it rapidly transports the neurotrans-
mitter serotonin from the synaptic cleft to the neuron terminal.
SERT binding sites are major drug targets for the treatment of
depression. By blocking the re-uptake serotonin the SSRIs improve
the availability of serotonin in the synaptic cleft. Many selective
serotonin re-uptake inhibitors (SSRIs), approved by the FDA, have
been prescribed to treat millions of patients with depression. SSRIs
also find efficacy in the treatment of anxiety disorders and eating
disorders.1,2 In addition, alterations of SERT could be related to
other neuropsychiatric disorders, such as epilepsy, Alzheimer’s dis-
ease, Parkinson’s disease and dementia with Lewy bodies, as well
as addictive behaviors, such as cocaine abuse and alcoholism.3–7
Therefore, a successful molecular imaging agent for SERT will be
able to measure the drug occupancy of antidepressants and in turn
provide personalized dosage of drugs. A SERT imaging agent will
also help elucidate the function of serotonin transmission and
alterations in the pathophysiology of various neuropsychiatric
conditions.
biological functions in the diseases. Even though rapid growths
of PET in the clinical setting, SPECT is also widely used in clinical
applications. Compared with PET, production of SPECT tracers is
easier and less costly for those long half-life SPECT radionuclides
allowing off-site production of radiotracers and efficient distribu-
tion with a national supply network. Therefore, we are interested
in the development of SERT-selective SPECT imaging agents la-
beled with I-123 (T1/2, 13 h).
Several structural classes of SERT ligands have been studied in
the past two decades, including nitroquipazine,8 tropanes,9–11
isoquinoline,12 as well as diphenyl sulfides.13–19 Nitroquipazine li-
gands generally possess high SERT potency (Ki = 0.05–0.3 nM).
Molecular imaging techniques, such as positron emission
tomography (PET) and single photon emission computed tomogra-
phy (SPECT), provide powerful, noninvasive tools for visualizing
⇑
Corresponding author. Tel.: +1 215 662 3096; fax: +1 215 349 5035.
E-mail address: kunghf@sunmac.spect.upenn.edu (H.F. Kung).
Figure 1. Structure of some SPECT imaging agent for SERT.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.11.043

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P. Zheng et al. / Bioorg. Med. Chem. Lett. 23 (2013) 869–872
Table 1
Comparison of inhibition constants of selected ligands for serotonin transporter
Starting material
Conditions
Product
Yield (%)
1
2
3
4
5
6a
6b
6c
6b
6b
NaOMe, DMA, 100 °C, 72 h
NaOMe, DMA, 110 °C, 68 h
Pd2(dba)3, dppf, TEA, NMP 60 °C, 2.5 h
CuI, glycol, K2CO3, i-PrOH, 80 °C, 68 h
CuI, 1,10-phen, DIPE Tol, 110 °C, 68 h
7a
7b
7c
7b
7b
0
0
0
83
71
However, the modest target to non-target ratios and the slow phar-
macokinetics make this series of tracers unsuitable for PET/SPECT
imaging. Hypothalamus an area in the brain which is rich in sero-
tonin transporters, is commonly used as an indicator for SERT spe-
cific uptake (target area). Cerebellum (another area in the brain) on
the other hand, is devoid of SERT and thus making it a good refer-
ence as a background (non-target area). Tropane derivatives, such
as bFEpZIENT, displayed good SERT binding affinity (Ki = 0.08 nM)
and excellent selectivity over DAT and NET. In vivo distribution
studies of bFEpZIENT revealed a ratio of 2.5 for hypothalamus to
cerebellum.9 Isoquinoline scaffold, such as (+)-McN5652, was also
under intensive studies in 1990s. However, further biological stud-
ies of these ligands were impeded by several limitations, which in-
clude low signal to noise ratio and high non-specific binding.8,12
Diphenyl sulfide ligands, derived from 403U76, emerged as an
interesting SERT ligands.20 Due to its high binding affinity for SERT
and ease of synthesis, a large number of diphenyl sulfide ligands
have been synthesized and tested in biological studies (Fig. 1). In
1999, our group reported [123I]IDAM as a SPECT ligand in this ser-
ies. [123I]IDAM displayed good binding affinity towards SERT
(Ki = 0.097 nM) and excellent selectivity (DAT/SERT >100,000;
NET/SERT >2400).13–15,19 Biodistribution study in rat showed
favorable pharmacokinetics and good ratio of 2.76 at 60 min for
hypothalamus to cerebellum.21 Further structure–activity studies
led to the discovery of [123I]ADAM. [123I]ADAM binds to SERT with
high affinity (Ki = 0.013 nM). A peak signal to noise of 4.78 was ob-
served at 120 min post-injection. Since then, [123I]ADAM has be-
come a leading SPECT imaging agent and it is useful for SPECT
imaging studies in humans.22 With an aim to further improve the
kinetics of [123I]ADAM, we prepared a novel ligand [123I]Flip-
ADAM. A switch of iodo position from ring A to ring B in ADAM
led to comparable initial brain uptake and faster brain clearance,
albeit a lower target-background ratio than ADAM.23 Encouraged
by the improved biological properties demonstrated by [123I]
Flip-ADAM, we herein document a new member of SPECT ligand,
123I]Flip-IDAM. We envision that a similar switching of iodo sub-
stitution position in IDAM would maintain the high brain uptake
while improving the target-background ratio of IDAM.
[
Synthesis of the desired Flip-IDAM (4) started with an investi-
gation of C–S coupling between aryl thiols and aryl halides
(Table 1). Nucleophilic aromatic substitution of aryl halide did
not yield any desired product in the presence of sodium methoxide
(entries 1 and 2). Attracted by elegant work by Migita et al.24 and
Ortar and co-workers25 on the palladium-catalyzed C–S coupling,
we subjected 5 and 6c to the catalytic system comprising Pd2(dba)3
and dppf. However, no desired coupling product was obtained (en-
try 3). A homo-coupling product (aryl disulfide) was generated
exclusively.
Encouraged by elegant organic synthetic works by Buchwald
and co-workers26 and Sawada et al.,27 we tested copper catalytic
system in our synthesis of 7b. To our delight, thiol 5 was success-
fully coupled with aryl iodide 6b to afford 7b in good yield (entries
4 and 5). It is noteworthy that Buchwald’s catalytic system deliv-
ered a higher yield of desired product (entries 4 and 5).
With key intermediate 7b in hand, 7b was further converted to
amide 8 in 76% yield. Borane reduction of ester and amide afforded
final Flip-IDAM 4 in 43% yield (Scheme 1).
Palladium-catalyzed stannylation delivered tin precursor 9 with
75% yield and 98% purity (Scheme 2).
Trace iodide impurity can be removed via HPLC purification.
Brief treatment of tin precursor 9 with an ethanolic solution of
[
125I]NaI, HCl and 3% H2O2, yielded [125I]Flip-IDAM in 83% radio-
chemical yield and >99% radiochemical purity. The radiochemical
identity was verified by a co-injection of cold Flip-IDAM, which
showed an identical retention time using HPLC analysis (Fig. 2).
The in vitro studies were carried out using literature reported
procedures.28 Membrane homogenates containing serotonin
transporter (LLC-SERT) was prepared and used to measure binding
Scheme 1. Synthesis of Flip-IDAM, 4.

P. Zheng et al. / Bioorg. Med. Chem. Lett. 23 (2013) 869–872
871
Scheme 2. Synthesis of labeling precursor and radiolabeling.
Figure 2. HPLC profiles of [125I]Flip-IDAM and Flip-IDAM, 4.
affinity. Inhibition constant of Flip-IDAM was obtained through the
displacement experiment of [125I]IDAM in the competitive binding
of serotonin transporter. As shown in Table 2, Flip-IDAM displayed
an excellent affinity to SERT with a Ki value of 0.03 nM, which is
comparable to ADAM (entries 1 and 4).
subnanomolar SERT affinity in vitro, good brain uptake, selective
regional brain localization, high target to non-target ratio, as well
as favorable kinetics. We are currently pursuing further biological
studies.
Furthermore, Flip-IDAM showed 3-fold and 33-fold higher
affinities than IDAM and Flip-ADAM towards the binding of seroto-
nin transporter (entries 2–4).
In summary, we have developed a novel SPECT ligand, Flip-
IDAM, for mapping serotonin transporter (SERT) in the brain.
Preliminary biological evaluation showed that Flip-IDAM displayed
Acknowledgment
This work was supported by a Grant awarded by the National
Institutes of Health (RO1-MH068782).
Supplementary data
Table 2
Supplementary data (experimental procedures, characteriza-
tion data, as well as biological studies) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.bmcl.2012.11.043.
Comparison of inhibition constants of selected ligands for serotonin transporter
Entry
Compound
Kia (nM)
1
2
3
4
ADAM, 1
Flip-ADAM, 2
IDAM, 3
0.01 0.003b
1.00 0.06c
0.10 0.01b
0.03 0.007c
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