[18F]FMDAA1106 and [18F]FEDAA1106: two positron-emitter labeled ligands for peripheral benzodiazepine receptor (PBR).

Article abstract of DOI:10.1016/S0960-894X(02)00886-7

We synthesized and evaluated N-(5-fluoro-2-phenoxyphenyl)-N-(2-[(18)F]fluoromethyl-5-methoxybenzyl)acetamide ([(18)F]-FMDAA1106) and N-(5-fluoro-2-phenoxyphenyl)-N-(2-[(18)F]fluoroethyl-5-methoxybenzyl)acetamide ([(18)F]FEDAA1106) as two potent radioligan

Full text of DOI:10.1016/S0960-894X(02)00886-7

Bioorganic & Medicinal Chemistry Letters 13 (2003) 201–204
[18
18
Labeled Ligands for Peripheral Benzodiazepine Receptor (PBR)
F]FMDAA1106 and [ F]FEDAA1106: Two Positron-Emitter
Ming-Rong Zhang, * Jun Maeda, Kenji Furutsuka,a,b Yuichiro Yoshida,a,b
a,b,
a,b
Masanao Ogawa, Tetsuya Suharaa,c and Kazutoshi Suzukia
a,b
aDepartment of Medical Imaging, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
bSHI Accelerator Service Co. Ltd., 5-9-11 Kitashinagawa, Shinagawa-ku, Tokyo 141-8686, Japan
cCREST, Japan Sciences and Technology Corporation, 4-1-8 Honmachi, Kawaguchi 332-0012, Japan
Received 2 September 2002; accepted 12 October 2002
1
8
18
Abstract—We synthesized and evaluated N-(5-fluoro-2-phenoxyphenyl)-N-(2-[ F]fluoromethyl-5-methoxybenzyl)acetamide ([ F]-
1
8
18
FMDAA1106) and N-(5-fluoro-2-phenoxyphenyl)-N-(2-[ F]fluoroethyl-5-methoxybenzyl)acetamide ([ F]FEDAA1106) as two
1
8
18
potent radioligands for peripheral benzodiazepine receptors (PBR). [ F]FMDAA1106 and [ F]FEDAA1106 were respectively
1
8
18
2 2 2
synthesized by fluoroalkylation of the desmethyl precursor DAA1123 with [ F]FCH I and [ F]FCH CH Br. Ex vivo auto-
1
8
18
radiograms of [ F]FMDAA1106 and [ F]FEDAA1106 binding sites in the rat brains revealed that a high radioactivity was pre-
sent in the olfactory bulb, the highest PBR density region in the brain.
#
2002 Elsevier Science Ltd. All rights reserved.
Benzodiazepine receptors are divided into two types:
central and peripheral benzodiazepine receptors.
Although peripheral benzodiazepine receptor (PBR)
was initially identified in the peripheral system, it
became clear that its density in the brain regions can
equal or exceed the density of central benzodiazepine
been great interest in developing radioligands that could
be used to visualize the distribution of PBR in a living
human brain using positron emission tomography
6
,7
(PET).
Recently, N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phen-
oxyphenyl)acetamide (DAA1106) (Scheme 1) has been
1À3
receptor (CBR) in the corresponding regions.
Recent
8
,9
studies have shown that PBR might mediate physio-
logical responses in the central nervous system and may
be involved in certain pathophysiological events, such as
anxiety, by stimulating the production of neuroactive
reported as a potent and selective ligand for PBR.
DAA1106 displayed a high affinity for PBR in mito-
chondrial fractions of rat (K =0.043 nM) and monkey
i
8
(K =0.188 nM) brains. Moreover, DAA1106 showed
i
4
,5
steroids in glial cells in the brain. Therefore, the PBR
ligands may become effective anxiolytics without the side
weak affinities (IC =10,000 nM) for melanin, Kappa
1
5
0
and GABAA receptors, and negligible affinities (IC50
>10,000 nM) for 54 others including receptors, ion
3
,5
effects sometimes seen with classical benzodiazepines.
However, in contrast to the clarification of CBR at the
molecular level, the pharmacological characterization of
9
channels, uptake/transporter and second messenger.
To develop a PET tracer that would provide selective
imaging of PBR in vivo, and to elucidate the pharma-
cological role of PBR in the brain, we have labeled
DAA1106 by reacting the corresponding desmethyl
precursor N-(5-fluoro-2-phenoxyphenyl)-N-(2-hydroxy-5-
3
,4
PBR in primate brain has not been fully elucidated.
Moreover, the precise physiological significance of
events mediated through PBR in the brain and the
therapeutic potential of PBR antagonists in the pathol-
ogy and/or etiology of central nervous system disorders
11
10
methoxybenzyl)acetamide (DAA1123) with [ C]CH I in
3
3À5
are still subjects of controversy.
As a result there has
an excellent radiochemical yield as shown in Scheme 1.
Moreover, we have determined a high specific binding
1
1
of [ C]DAA1106 to PBR in the mouse brain. Now,
1
1
[ C]DAA1106 is being used for investigating PBR in
the human brain.
*
3
Corresponding author. Tel.: +81-43-206-4041; fax: +81-43-206-
261; e-mail: zhang@nirs.go.jp
0
960-894X/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00886-7

202
M.-R. Zhang et al. / Bioorg. Med. Chem. Lett. 13 (2003) 201–204
ꢀ
ꢀ
Scheme 1. Structures, chemical synthesis and radiosynthesis: (a) NaH, DMF, 30 C, 5 min, 81%; (b) 60 C, 80–100 mm Hg, 10 h, 15%; (c) NaH,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
DMF, 0 C, 1 h, 78%; (d) 100 C, 10 min, 8%; (e) NaH, DMF, 15 C, 10 min, 86%; (f) pyridine, 25 C, 4 h, 48%; (g) NaH, DMF, 25 C, 8 h, 37%;
(h) o-dichlorobenzene, 130 C, 5 min, 57%; (i) NaH, DMF, 130 C, 10 min, 29%.
ꢀ
ꢀ
This success prompted us to design two 18F-labeled ana-
DAA1123 with FCH I in the presence of NaH at 0 C
2
14
ꢀ
was accomplished rapidly to give FMDAA1106 in a
1
1
logues of [ C]DAA1106: N-(5-fluoro-2-phenoxyphenyl)-
18
18
N-(2-[ F]fluoromethyl-5-methoxybenzyl)acetamide ([ F]-
FMDAA1106) and N-(5-fluoro-2-phenoxyphenyl)-N-
78% yield. Reaction of DAA1123 with NaH and
1-fluoro-2-tosyloxyethane (FCH CH OTs), which was
prepared from 2-fluoroethanol and p-toluenesulfonyl
2
2
1
8
18
(
2-[ F]fluoroethyl-5-methoxybenzyl)acetamide ([ F]-
FEDAA1106) as putative PET ligands for PBR
Scheme 1). First, because of the molecular similarity
and bioisoteric property of O–CH F and O–CH CH F
1
4
chloride, gave FEDAA1106 in a 75% yield.
(
1
8
18
[ F]FMDAA1106 and [ F]FEDAA1106 were synthe-
sized as shown in Scheme 1. The labeling intermediate
2
2
2
with O–CH group, the two compounds may display
3
1
8
18
18
similar affinities for PBR with DAA1106. Second, com-
pared with DAA1106, FMDAA1106 and FEDAA1106
were more lipophilic and may readily pass through the
blood-brain barrier, which is necessary for a suitable
[ F]fluoromethyl iodide ([ F]FCH I) or 2-[ F]fluoro-
2
1
8
ethyl bromide ([ F]FCH CH Br) for the radiosynthesis
2
was prepared by the reaction of [ F]F with CH I or
2
2
1
8
À
2
2-bromoethyl triflate (BrCH CH OTf) using a newly
2
2
15 18
1
8
18
PET tracer over the brain. Third, the [ F]fluoroalkyl
substitution may lead to significant improvement in
tracer behavior including pharmacokinetics, as can be
developed automated system. [ F]FCH I or [ F]-
2
FCH CH Br was purified by distillation and trapped in
2
2
a solution of DMF containing DAA1123 (1 mg) and
ꢀ
1
1,12
seen in some examples of PET tracers.
Finally, since
F has advantage over C, with a longer half-life
110min vs 20min) and a lower positron energy (650KeV
NaH (6–8 mL, 0.5 g/20 mL DMF) at À15 C. After the
1
8
11
radioactive reagent trapping ended, the fluoromethyl-
ation finished perfectly, whereas the fluoroethylation
(
1
8
ꢀ
was purified by HPLC using a reversed phase semi-pre-
vs 960 KeV), F is convenient for long-time storage and
long-distance transportation and could give a higher
quality of images with higher spatial resolution.
required a further 10 min at 130 C. The desired product
parative YMC J’sphere ODS-H80 column (10 mm ID Â
2
50 mm) and a mixture of CH CN/H O (5.5/4.5 for
3 2
1
8
18
In this paper, we report: (1) synthesis, binding affinities
of FMDAA1106 and FEDAA1106 to PBR and CBR;
[ F]FMDAA1106 and 6/4 for [ F]FEDAA1106).
Using CAPCELL PAK C18 analytic column (4.6 mm ID
 250 mm) and a mixture of CH CN/H O (7/3), the
1
8
18
(
2) radiosynthesis of [ F]FMDAA1106 and [ F]
3
18
2
1
8
FEDAA1106 and ex vivo autoradiograms of their
binding sites in the rat brain.
identity of [ F]FMDAA1106 or [ F]FEDAA1106 was
confirmed by co-injecting with the corresponding non-
radioactive sample. The retention time was 6.1 min for
1
8
18
The non-radioactive FMDAA1106 and FEDAA1106
were prepared according to Scheme 1. The fluoro-
methylating agent fluoromethyl iodide (FCH I) was
[ F]FMDAA1106 and 6.3 min for [ F]FEDAA1106 at
a flow rate of 2 mL/min, respectively. In the final pro-
duct solution, no significant DAA1123 peak was deter-
mined by HPLC. The radiochemical purity of
2
prepared by reacting diiodomethane (CH I ) with HgF
2
2
2
1
3
18
18
under reduced pressure in a 15% yield. Reaction of
[ F]FMDAA1106 or [ F]FEDAA1106 was higher

M.-R. Zhang et al. / Bioorg. Med. Chem. Lett. 13 (2003) 201–204
203
Figure 1. Ex vivo autoradiographic localizations or [18F]FMDAA1106 and [18F]FEDAA1106 in the rat brains at 30 min postinjection (20 MBq,
2
0.1 nmol). Quantified values (fmol/mm ) of the ex vivo bindings in the olfactory bulb, cerebellum and frontal cortex (n=8) were calculated by the
method.
16,17
than 98% and the specific activity was >120 GBq/mmol
as determined from the mass measured from the HPLC
UV analysis. The radiochemical purities of two radio-
ligands remained >95% after maintenance of the pre-
parations at 25 C for 4 h, and they were stable for
performing evaluation.
FEDAA1106 and FMDAA1106 for CBR in the rat brain
4
were >10 mM, 10 -fold lower than for PBR. These findings
revealed that FEDAA1106 and FMDAA1106 exhibited
high potency for PBR and negligible affinities for CBR.
18
ꢀ
The distribution coefficients (log Ps) of [ F]FMDAA1106
and [ F]FEDAA1106 were determined in the water/octa-
nol system at pH 7.4 using the shaking method according
18
18
The in vitro binding (IC ) studies of FMDAA1106 and
FEDAA1106 to PBR were performed using
to the previous procedure (Table 1).
50
11
16,17
[
C]DAA1106 according to the previous method.
As
Figures 1 and 2 show ex vivo autoradiographic locali-
1
8
18
shown in Table 1, FEDAA1106 displayed 2-fold higher
affinity for PBR than DAA1106, whereas FMDAA1106
displayed similar affinity with DAA1106. This result
showed that substituting O–CH group with O–CH CH F
zations of [ F]FMDAA1106 and [ F]FEDAA1106
binding sites in the rat brains at 30 min postinjection.
As can be seen in comparison with the sagittal sections
of the control rat brain (Figs. 1 and 2a and b), both of
these radioligands showed a considerably high brain
uptake, indicating that they can pass through the blood-
brain barrier, which is a prerequisite for a promising
PET tracer. This finding was compatible with their
3
2
2
group augmented the binding affinity, whereas substituting
with O–CH F group did not obviously affect the affinity
2
for PBR. FEDAA1106 was 10-fold more potent than
PK11195, the most commonly used ligand for PBR.
Competition binding site analyses were also used to eval-
uate the comparative affinities of FMDAA1106 and
1
1
FEDAA1106 for CBR labeled by [ C]flumazenil (a
selective CBR ligand) (Table 1). The IC50 values of
Table 1. In vitro binding affinity (IC50) for PBR and CBR, and
octanol/water distribution coefficient (log P)
IC50 (nM)a
Ligand
[11C]DAA1106 [11C]Flumazenil
displacement (PBR) displacement (CBR)
Log P
FMDAA1106
FEDAA1106
DAA1106
1.71
0.77
1.62
8.26
>10,000
>10,000
>10,000
>10,000
3.70
3.81
3.65
No test
Figure 2. Ex vivo autoradiogram of [18F]FMDAA1106 and
18
PK11195
[ F]FEDAA1106 in the sagittal sections of rat brains at 30 min post-
injection (20 MBq, 0.1 nmol):
18
(a)
[
F]FMDAA1106;
(b)
aIC50 was obtained from 9 concentrations of compound using at least
slices of rat brains (n=3).
[
[
18
F]FEDAA1106; (c) [ F]FMDAA1106+DAA1106 (1 mg/kg), (d)
F]FEDAA1106+DAA1106 (1 mg/kg).
18
18
8

204
M.-R. Zhang et al. / Bioorg. Med. Chem. Lett. 13 (2003) 201–204
lipophilicities as shown in Table 1. A significantly-high
radioactivity was observed in the olfactory bulb, the
References and Notes
8
,19
high PBR density area in the rat brain.
Followed by
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chem. 1992, 58, 1589.
the olfactory bulb, a moderate radioactivity level was
observed in the cerebellum, whereas a low uptake was seen
in the other brain regions such as frontal cortex. The
uptake pattern of radioactivity was not only consistent
3
305.
. Zisterer, D. M.; Williams, D. C. Gen. Pharmac. 1997, 29,
3
3
with [ H]DAA1106 and [ H]PK11195 binding sites in the
8,19
4. Papadopoulos, V.; Amri, H.; Li, H.; Yao, Z.; Brown, R. C.;
Vidic, B.; Culty, M. J. Pharmacol. Exp. Ther. 2001, 299, 793.
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pharmacology 1995, 118, 65.
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10. Suzuki, K.; Inoue, O.; Hashimoto, K.; Yamasaki, T.;
Kuchiki, M.; Tamate, K. Appl. Radiat. Isot. 1985, 36, 971.
11. Chi, D.-Y.; Kilbourn, M. R.; Katzenellenbogen, J. A.;
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2. Mach, R. H.; Ehrenkaufer, R. L. E.; Greenberg, J. H.;
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3. Zheng, L.; Berridge, M. S. Appl. Radiat. Isot. 2000, 52,
5.
4. FMDAA1106: white powder; mp: 71–72 C; IR (Nujol):
rat brain,
distribution of PBR in the brain.
activity in the olfactory bulb to that in the frontal cortex
was 3.5 for [ F]FEDAA1106 and 1.6 for [ F]-
FMDAA1106. The difference between the radioactivity
distributions of the two tracers in the olfactory bulb and
frontal cortex may be due to their stabilities in the rat
brain. Metabolite analysis for the brain homogenate dis-
but was also in accordance with the regional
1À3
The ratio of radio-
5
18
18
6
18
played that [ F]FEDAA1106 was not metabolized,
18
whereas [ F]FEDAA1106 was decomposed in the brain at
0 min postinjection. Co-injection with non-radioactive
DAA1106 (1 mg/kg) exhibited a significant reduction of
F]FMDAA1106 or [ F]FEDAA1106 concentration in
the brain regions when compared with the control groups
3
9
18
18
[
18
(
Figs. 1 and 2c and d). As for [ F]FMDAA1106, the
radioactivity levels were reduced to 30–50% of control in
the brain regions including the olfactory bulb. As for
18
[
F]FEDAA1106, the most significantly reduced uptake
less than 20% of control) was found in the olfactory bulb,
whereas a modest decrease (40–60%) was observed in the
1
1
(
1
8
other regions. These findings revealed that [ F]
1
8
FMDAA1106 and [ F]FEDAA1106 displayed high spe-
cific bindings in the rat brain, especially in the olfactory
bulb. Since FMDAA1106 and FEDAA1106 had potent
affinities for PBR, these radioligands may have high
binding sites to PBR in the rat brain.
1
5
1
ꢀ
À1
1
1680 cm ; H NMR (200 MHz, CDCl ) d: 7.18–7.40 (2H, m),
3
6.71–7.13 (8H, m), 6.28–6.44 (1H, m), 5.30 (1H, d, J=49 Hz),
4
.71 (2H, dd, J=7, 46 Hz), 3.71 (3H, s), 2.12 (3H, s); FABMS
+
1
8
18
23 21 2 4
(m/z): 414.2 (M +1). Anal. (C H F NO ) C, H, N.
In conclusion, [ F]FMDAA1106 and [ F]FEDA-
A1106 were designed, synthesized and evaluated as two
potent radioligands for PBR. They showed high specific
bindings to PBR in the rat brain and may become pro-
mising PET tracers for PBR. Further investigation into
ꢀ
FEDAA1106: white powder; mp: 54–56 C; IR (Nujol):
À1
1
1
6
685 cm ; H NMR (200 MHz, CDCl ) d: 7.26–7.59 (2H, m),
3
.22–7.19 (9H, m), 4.88 (2H, dt, J=4, 41 Hz), 4.65 (2H, dd,
J=7, 46 Hz), 4.12 (2H, dt, J=4, 27 Hz), 3.80 (3H, s), 2.15 (3H,
+
23 2 4
s); FABMS (m/z): 427.2 (M +1). Anal. (C24H F NO ) C,
1
8
18
the binding of [ F]FMDAA1106 and [ F]FEDAA1106
to PBR in the primate brains is currently underway.
H, N.
1
5. Zhang, M.-R.; Tsuchiyama, A.; Haradahira, T.; Yoshida,
Y.; Furutsuka, K.; Suzuki, K. Appl. Radiat. Isot. 2002, 9, 335.
6. Sihver, S.; Sihver, W.; Bergstrom, M.; Hoglund, U.; Sjo-
1
Acknowledgements
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18. Zhang, M.-R.; Tsuchiyama, A.; Haradahira, T.; Fur-
utsuka, K.; Yoshida, Y.; Kida, T.; Noguchi, J.; Irie, T.;
The authors are grateful to Dr. A. Nakazoto (Taisho
Pharmaceutical Co., Ltd.) for giving us the samples
(
DAA1123 and DAA1106) and helpful suggestions. We
also thank the crew of the Cyclotron Operation Section
and Radiopharmaceutical Chemistry Section of
National Institute of Radiological Sciences (NIRS) for
their support in the operation of the cyclotron and pro-
duction of radioisotopes.
Suzuki, K. Nucl. Med. Biol. 2002, 29, 463.
9. Funakoshi, T.; Chaki, S.; Okuyama, S.; Okubo, T.;
1
Nakazato, A.; Nagamine, M.; Tomisawa, K. Res. Commun.
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