Highly efficient synthesis and imaging studies of an arylpiperazine derivative as a 5-HT1A receptor imaging agent

Article abstract of DOI:10.1246/cl.2006.1304

A novel and straightforward microwave synthesis of a new arylpiperazine derivative 3 which has an N₂S₂ moiety has been developed and radiolabeled with an optimum radionuclide of technetium in the presence of a borohydride exchange re

Full text of DOI:10.1246/cl.2006.1304

1304
Chemistry Letters Vol.35, No.11 (2006)
Highly Efficient Synthesis and Imaging Studies
of an Arylpiperazine Derivative as a 5-HT1A Receptor Imaging Agent
Sang Hyun Park,ꢀ1 Hui Jeong Gwon,1 Seung Ho Jang,1 and Hyosun Lee2
1Radiation Application Research Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea
2Department of Chemistry, Kyungpook National University, Daegu 702-701, Korea
(Received August 18, 2006; CL-060950; E-mail: parksh@kaeri.re.kr)
A novel and straightforward microwave synthesis of a new
The labeling of a derivative of WAY100635 with 99mTc
was, at first, achieved by conjugating the WAY100635 deriva-
tive, 1-(2-methoxyphenyl)-4-(2-(2-pyridylamino)ethyl) pipera-
zine (1) and 8-(3-chloropropyl)-[1,2,5,8]dithiadiazecan-6-one
(2), which has a chloropropyl spacer, through MWI (Scheme
1). Experimentally, a solution of 1 (0.62 g, 0.2 mmol) and 2
(0.52 g, 0.2 mmol) in DMF (10 mL) was added into a 50-mL
round bottom flask containing an excess of K2CO3. The mixture
was heated under reflux for 12 h. The progress of the reaction
was monitored by TLC analyses. Although the compounds 1
and 2 were successfully prepared (yield 75 and 61%, respective-
ly) by a literature method,6 the coupling of 1 and 2 was unsuc-
cessful. However, the conjugation of 1 and 2 was successfully
achieved by the new method using a microwave irradiation with
a yield of 20%. A solution of 1 (0.62 g, 0.2 mmol) and 2 (0.52 g,
0.2 mmol) in DMF (10 mL) was added into a 50-mL glass vessel
containing an excess of K2CO3. The reaction vessel was capped
with a TFM teflon cover and placed in a rotor in a microwave
reactor. The mixture was irradiated for 30 min at 130 ꢂC in a
300 W and cooled to room temperature. The reaction mixture
was evaporated, dissolved in water, and extracted with dichloro-
methane (100 mL ꢃ 2). The organic layer was washed succes-
arylpiperazine derivative 3 which has an N2S2 moiety has been
developed and radiolabeled with an optimum radionuclide of
technetium in the presence of a borohydride exchange resin
(BER) as a reducing agent. According to the present study this
new radiolabeled compound 4 could have the potential for a
diagnostic application as an imaging agent of a serotonin neuro-
receptor.
A neurotransmitter system includes the cholinergic nervous
system which releases acetylcholine and the adrenergic nervous
system which releases noradrenaline. Acetylcholine and nor-
adrenaline are released by a stimulation in the central and
peripheral nervous systems. Additionally, many important
neurotransmitters such as dopamine, serotonin, and inhibitory
GABA (ꢀ-aminobutyric acid) exist in the central nervous
system. Among them, the serotonergic system in the brain,
specifically, various receptor subtypes of it, is an important
neurotransmitter that controls emotion-related actions including
worry, anxiety, and physiological functions. Thus, serotonergic
nervous system is closely related to mental illnesses such as
anxiety, manic-depression, and melanocholia.1 Serotonin recep-
tors activated by 5-HT,2 which is one of the serotonin receptor
subtypes, have been divided into at least seven classes (5-
HT1ꢁ7), and each class has been further subdivided into different
subtypes (A, B, . . .).
OCH3
N
O
Cl
H
N
N
NHN
N
+
S
S
So far, from extensive research studies on agonists and
antagonists for 5-HT1A receptors, WAY100635 (Figure 1),
which is an arylpiperazine compound, is known as a typical an-
tagonist. It was identified that WAY100635 is a crucial ligand
for imaging 5-HT1A receptor.3
1
2
DMF, K2CO3,
Reflux, 12 h
DMF, K2CO3, MWI,
30 min, 300 W
X
When WAY100635 derivatives were synthesized by a con-
ventional heat treatment, a large quantity of solvent is required,
the reaction time is fairly long and the yield is low.4 Overcoming
these inefficencies, the radioligand of WAY100635 derivatives
were synthesized by a microwave irradiation (MWI) that induces
a direct coupling between the reaction molecules and shows a
thermal conductivity leading to a rapid rise in the reaction tem-
perature. MWI has outstanding advantages of a shorter reaction
time, higher yield, easier work-up and cleaner reaction due to
less side reactions over a conventional heating in organic synthe-
sis.5–8
O
N
N
OCH3
H
N
N
S
S
N
N
3
Na99mTcO4, SnCl2 ·2H2O
Na99mTcO
DMSO, 30 min, RT
4, BER
X
DMSO, 30 min, 70°C
O
N
N
O
OCH3
OCH3
99mTc
N
S
N
N
N
H
S
N
N
N
N
4
O
Scheme 1. Preparation of the 99mTc-labeled arylpiperazine
derivative.
Figure 1. WAY100635 as an antagonist for 5-HT1A receptors.
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.11 (2006)
1305
sively with water and brine, and dried over an anhydrous sodium
sulfate. Evaporation of the solvent gave the crude product, which
was purified by column chromatography, yielding 0.07 g (20%)
of the desired product as an oily form. TLC (ethylacetate:n-
hexane:MeOH = 1:1:2, v/v): 1, Rf ¼ 0:561, 2, Rf ¼ 0:929, 3,
Rf ¼ 0:789 (Scheme 1). The complexation of 99mTc with the
prepared chelating ligand 3 was followed by using MWI as
shown in Scheme 1.
Interestingly, depending on the reducing agent, the labeling
yield was clearly different. In the case of using a conventional
reducing agent such as tin(II) chloride, we observed no techne-
tium-labeled complex. Experimentally, an aqueous solution of
sodium pertechnetate, Na99mTcO4 (185 MBq) was injected into
a vial containing a solution of 3 (0.07 g, 0.13 mmol) in DMSO
(2 mL) and tin(II) chloride dihydrate (0.05 mg) in 5 mmol HCl
(0.1 mL). After a stirring for 30 min, no technetium-labeled
complex was obtained. Further the mixture was then heated in
boiling water for 30 min and cooled to room temperature, no
technetium-labeled complex was obtained. Alternatively, by
using a tetrahydroborate exchange resin (BER), which was
newly introduced by us, was very successful in the labeling.9–11
To a vial containing 5 mg of BER, 0.1 mL of Na99mTcO4
(185 MBq), and a solution of 3 (0.07 g, 0.13 mmol) in DMSO
(2 mL) were added at a time. After stirring the mixture at room
temperature for 30 min under N2, it was filtrated by a membrane
filter (0.22 mm) before the instrumented analyses. It should
be noted that the reduction of pertechnetate as well as the disul-
fide bond S–S in ligand 3 was occurred during the labeling of
99mTc. To determine the labeling yield of the 99mTc-complex 4,
ITLC-SG (silica gel) was performed by using MEK and saline
as a development solvent. The ITLC-SG of 4 usinꢁg MEK gave
no peak at the solvent front where 99mTcO4 would be
expected. Some reduced 99mTcO2 fragments were observed at
the origin after an elution of 4 with saline. These results indicate
Figure 2. Image scan of a rabbit after i.v. injection of the
99mTc-complex 4.
Diacam gamma camera (Simens, Germany) with a low-energy
collimetor was positioned. Energy gate and window width were
set to 140 keV and 10%, respectively. Rabbits were injected with
3.7 MBq of 4 per head through the left ear vein. The static image
of 4 in the male New Zealand White rabbit at 5 and 180 min post
injection is shown in Figure 2. At 5 min post injection
image, high activity was found in the liver and a trace amount
of 4 seemed to have remained in the brain (Figure 2).
In summary, 99mTc-8-[3-({2-[4-(2-methoxyphenyl)pipera-
zine-1-yl]ethyl}pyridin-2-yl-amino)propyl]-[1,2,5,8]dithiadi-
azecan-6-one (4) was prepared by the reaction of radioactive
99mTc with 3 that is a WAY100635 derivative with a labeling
yield of 99%. In addition, the facile synthesis of the arylpipera-
zine derived ligand 3 containing the N2S2 binding site for the
metals was achieved by a new MWI method with a high yield.
The chelating ligand 3 has a high affinity for 5-HT1A receptor an-
tagonist thus it is suitable for being developed as a radiopharma-
ceutical agent especially as an imaging agent for a neutrotrans-
mitter receptor.
This work was supported by the mid- and long-term research
development project from Korea Ministry of Science and
Technology.
that
99mTc-8-[3-({2-[4-(2-methoxyphenyl)piperazine-1-yl]-
ethyl}pyridine-2-yl-amino)propyl]-[1,2,5,8]dithiadiazecan-6-one
(4) with a 99% labeling efficiency was formed with the use of
BER.
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Radiochemical purity of 4 was determined by HPLC
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3
4
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Additionally, a filtrated preparation was applied to a paper
(20 ꢃ 200 mm) for electrophoresis that was eluted for 45 min
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and xylazine. Rabbit was placed in a posterior posture. The
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Published on the web (Advance View) October 21, 2006; doi:10.1246/cl.2006.1304