Exploration of the structure-activity relationship of a novel tetracyclic class of TSPO ligands - Potential novel positron emitting tomography imaging agents

Article abstract of DOI:10.1016/j.bmcl.2013.02.057

A series of novel TSPO ligands based on the tetracyclic class of translocator protein (TSPO) ligands first described by Okubo et al. was synthesised and evaluated as potential positron emitting tomography (PET) ligands for imaging TPSO in vivo. Fluorine-18 labelling of the molecules was achieved using direct radiolabelling or synthon based labelling approaches. Several of the ligands prepared have promising profiles as potential TSPO PET imaging ligands.

Full text of DOI:10.1016/j.bmcl.2013.02.057

Bioorganic & Medicinal Chemistry Letters 23 (2013) 2368–2372
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Exploration of the structure–activity relationship of a novel
tetracyclic class of TSPO ligands—Potential novel positron emitting
tomography imaging agents
Dennis O’Shea a, Rabia Ahmad a, Erik Årstad a, Michelle Avory a, Wai-Fung Chau a, Clare Durrant a,
Ella Hirani a, Paul A. Jones a, Imtiaz Khan a, Sajinder K. Luthra a, Dimitrios Mantzilas b,
Véronique Morisson-Iveson a, Joanna Passmore a, Edward G. Robins a, Bo Shan a, Harry Wadsworth a,
Sarah Walton a, Yongjun Zhao a, William Trigg a,
⇑
a GE Healthcare, The Grove Centre, White Lion Road, Amersham, Bucks HP7 9LL, UK
b GE Healthcare AS, Nycoveien 1-2, Postboks 4220, Nydalen, Oslo 0401, Norway
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel TSPO ligands based on the tetracyclic class of translocator protein (TSPO) ligands first
described by Okubo et al. was synthesised and evaluated as potential positron emitting tomography
(PET) ligands for imaging TPSO in vivo. Fluorine-18 labelling of the molecules was achieved using direct
radiolabelling or synthon based labelling approaches. Several of the ligands prepared have promising pro-
files as potential TSPO PET imaging ligands.
Received 9 January 2013
Revised 4 February 2013
Accepted 11 February 2013
Available online 24 February 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
TSPO
PBR
PET
Neuroinflammation
Synthesis
O
O
O
O
O
N
N
Translocator protein (TSPO) formerly known as the peripheral
benzodiazepine receptor (PBR) is an established target for the imag-
ing of inflammation using PET and SPECT. [11C]PK11195 has been
used clinically for more than 20 years.1,2 Whilst [11C]PK11195 has
provided valuable insight into the presence of inflammation in a
wide variety of neurological and peripheral diseases, it is not an
ideal PET tracer due to high non-specific binding and the carbon-
11 radiolabel that limits its use to PET centres with an on-site
cyclotron. The need to discover improved PET tracers for TSPO
imaging has led to the development of many new PET tracer candi-
dates including the carbon-11 labelled DAA1106,3 DPA7134 and
PBR285 as well as the fluorine-18 labelled FE-DAA1106,6 DPA7147
and PBR06 (Fig. 1).8
N
O
F
N
O
N
O
N
N
O
O
N
N
2. DAA1106
3. DPA-713
4. PBR28
F
Cl
O
F
O
O
O
N
N
N
O
N
O
F
1. PK11195
N
O
O
O
F
O
N
5. FE-DAA1106
6. DPA-714
7. PBR06
Figure 1. Example PET ligands for TSPO.
For our own discovery programme we aimed to find a high
affinity pharmacophore which was suitable for radiolabelling with
fluorine-18 using a direct labelling strategy, which would be both
high yielding and compatible with an automated radiosynthesis
platform such as FASTlab™.
R
R
N
O
X1
Y
C
8
1
Y = S, SO2 or CH2
X1/X2 = H, F, Cl, Me
R = alkyl or cycloalkyl
In 2004, Okubo et al.9 described a tetracyclic indole based phar-
macophore (Fig. 2) with a robust synthetic route and high affinity
B
2
7
X2
A
N
H
3
6
5
4
8
Figure 2. Phamacophore from Okubo et al. 20049 with ring notation.
⇑
Corresponding author.
E-mail address: william.trigg@ge.com (W. Trigg).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.02.057

D. O’Shea et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2368–2372
2369
COOH
S
COCl
S
F
i
ii
SH
F
O
O
O
N
N
+
O
O
N
O
O
R
S
O
S
R
R
S
N
(
N
H
N
)n
iii
10
Not isolated
F
9a n=1
9b n=2
9c n=3
CONR'
S
CONR'
S
CONR'
S
R''
R''
v
iv
O
N
X
N
R'''
X
N
H
O
N
S
R
O
R
R
X = C or N
S
N
O
Scheme 1. Reagents and conditions: (i) (a) NEt3, toluene, 50–70 °C; (b) AlCl3, 10 °C,
8–94% yield; (ii) oxalyl chloride, DCM (quantitative); (iii) amines, DCM, 46–91%
yield; (iv) phenylhydrazines, H2SO4, EtOH, refluxed, 12–80%; (v) NaH or LiHMDS,
DMF then alkylating agent 18–80% yield.
N
F
F
N
N
N
12
11
Figure 3. Initial compounds prepared to evaluate fluorination.
Table 2
Structures of A ring modified compounds and measured Ki data for TSPO in rat heart
and estimated logP13
Table 1
Structures of compounds prepared to evaluate radiolabelling approaches and
measured Ki data for TSPO in rat heart and clogP10
N
O
Compound
Ki (nM)
clogP
S
R1
N
X
N
Y
O
S
Compound
R1
X
Y
Ki (nM)
elogP
0.68
4.93
13
9a
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
H
H
CH
CH
CH
CH
CH
CH
CH
N
H
0.76
0.68
1.09
0.37
0.40
0.93
0.23
3.92
8.44
19.00
59.20
0.47
0.49
0.34
0.26
0.19
0.52
4.33a
3.76
4.92a
3.77
3.82
3.85
4.17
3.12
3.87
3.19
3.45a
4.23
3.85
3.96
4.03
3.60
3.88
N
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
H
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
1-OMe
2-OMe
3-OMe
4-OMe
2-Cl
F
9a
N
O
S
H
0.36
0.52
5.00
5.58
3-OMe
2-OH
2-NHAc
2-Br
N
N
CH
CH
CH
CH
CH
CH
CH
CH
F
9b
2-F
N
O
2-Me
2-OCF3
2-CN
2-OEt
S
N
a
Calculated logP rather than elogP.
F
9c
10
F
N
which we first synthesised to evaluate the introduction of fluorine
in a position which was amenable to a radiolabelling procedure.
Table 1 shows the TSPO affinity and lipohilicity of the com-
pounds shown in Figure 3.
Introduction of the fluorine via an alkyl group on the indole
nitrogen (9a–c) proved straightforward and had no significant ef-
fect on the affinity for TSPO but increasing the chain length did in-
crease the lipohilicity as expected. An attempt to introduce fluorine
in the amide side chain did not prove trivial with concurrent oxida-
tion of the ring system occurring leading to the isolation of com-
O
S
297
5.48
4.92
N
N
O
S
0.35
O
N
F
11
N
pound 10 which had very poor affinity. Introduction via
a
O
S
fluoralkoxy was also successfully achieved with no significant im-
pact on affinity (compound 11) but this approach was not highly
favoured due to the multiple variations possible and the relatively
lower expected stability of the final product to metabolism. An at-
tempt to introduce fluorine using a click type approach (compound
12) also did not give a high affinity compound. We have previously
shown11 that the fluoroethyl derivative provides robust radio-
chemistry and also enabled a flexible approach to be used in com-
pound screening and so this was selected as our preferred labeling
method.
63.6
4.36
N
F
N
N
N
12
for TSPO. This pharmacophore has been used as the starting point
in our synthetic chemistry programmes.
The initial efforts in our tracer discovery programme were tar-
geted at determining which was the most favourable position to
introduce the fluorine-18 label. Figure 3 shows the compounds
Scheme 1 highlights the general synthetic route employed to
synthesise the unlabelled compounds for affinity testing.

2370
D. O’Shea et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2368–2372
O
Table 3
N
O
Structures of B ring modified compound and measured Ki data for TSPO in rat heart
and e-logP
S
N
S
N
i
N
H
N
O
OTBDMS
S
R2
N
ii
O
O
F
N
N
S
S
Compound
R2
Ki (nM)
elogP
iii
N
29
30
31
32
33
34
35
36
37
38
39
5-OMe
6-OMe
7-OMe
8-OMe
6-F
7-F
8-F
6-OH
7-OH
1.07
0.30
0.30
0.33
0.39
0.32
0.31
0.33
0.55
0.39
1.35
3.67
3.65
3.91
3.67
3.84
3.87
3.79
2.70
4.67a
3.15
3.13
N
OMs
OH
O
N
S
iv
O
N
H
N
S
7-NHAc
7-N(CHCH2F)Ac
N
O
a
N
Calculated logP rather than elogP.
S
18F
v
N
Table 4
OMs
Structures of C ring modified compound and measured Ki data for TSPO in rat heart
and estimated logP
Scheme 2. Reagents and conditions: (i) (2-bromoethoxy)(tert-butyl)dimethylsi-
lane, NaH, DMF, 77% yield; (ii) TBAF, 71% yield; (iii) mesyl chloride, pyridine, 71%
yield; (iv)
K2CO3.
[ [
18F] 2-fluoroethyl 4-methylbenzenesulfonate, NaH; (v) 18F], K222,
N
N
O
O
Y
X
OMe
N
N
F
Z
oxidised to the corresponding sulfones (compounds 41 and 42).
Introduction of a methyl was not well tolerated and combination
of the sulfone with the methyl in place had very low affinity.
One of the two five-membered C-rings showed good affinity but
the other had low affinity.
45
Compound
X
Y
Z
Ki (nM)
elogP
40
41
42
43
44
45
S
CH3
H
CH3
H
–CH2CH2F
–CH2CH2F
–CH2CH2F
–CH2CH2F
H
13.10
8.79
410
1.13
0.80
154
4.00
3.00
3.10
3.69
3.74a
4.60a
SO2
SO2
CH2
—
Overall our exploration of the SAR of this series has shown that
many modifications can be made to the core pharmacophore with-
out impacting the affinity. In general however the modifications
allowable were relatively subtle and evaluation of affinity and lip-
ohilicity alone were not able to differentiate the compounds.
Radiolabelling was carried out using either a direct or indirect
labeling process (Scheme 2).16,17 Utilisation of the indirect process
enabled a more rapid evaluation of the compounds but the direct
labeling approach affords a more robust and reliable radiosynthe-
sis and compounds which progressed through the screening cas-
cade were generally radiolabelling using the direct labeling
approach. Direct labeling precursors were synthesised using anal-
ogous chemistry (Scheme 2) to that used to prepare the unlabeled
molecules for affinity assessment and the free indole precursors
were intermediates in the synthesis described in Scheme 1.
Non-decay corrected end of synthesis yields for both radiolabel-
ling strategies were typically 5–35%.17 The radiolabelled molecules
were purified by HPLC prior to the biological evaluations.
In the uninjured human CNS the expression of TSPO is very
low18 and the same is true for the rat with the exception of the
olfactory bulbs which have a high expression of TSPO compared
to other brain areas.19 As previously described,11 we used this dif-
ference in tracer retention between the olfactory bulbs (OB) and
the low TSPO expressing striata (Str) as the basis for screening
and prioritisation of our novel compounds.
H
As above
a
Calculated logP rather than elogP.
Starting from a range of commercially available thiols and
phenylhydrazines it was possible to rapidly evaluate a series of
compounds where the substitution on the A and B rings are
introduced.
Table 2 details the compounds we prepared using the strategy
outlined in Scheme 1 in which the A ring was modified.12
From these data it is clear that small relatively hydrophobic
substituents are well tolerated with no significant loss of affinity
for a wide range of substituents. Substitution at the 2-position
was most rigorously examined as only 1 regioisomer was formed
and no significant difference in affinity was noted in the methoxy
series. Introduction of an amide or phenol group was not well tol-
erated in the examples prepared (compounds 21 and 22). The pyr-
idyl examples (compounds 19 and 20) showed modest affinity loss.
Table 3 details the compounds we prepared using the strategy
outlined in Scheme 1 the B-ring was modified.14
From these data it is clear that substitution of the B-ring is well
tolerated with essentially no loss of activity regardless of the
substituent.
The biodistribution of the 18F-labelled compounds was evalu-
ated in naïve Wistar rats. The key brain data for the compounds
in our screening programme (those which relate to ability to image
neuroinflammation in the brain) are detailed in Table 5. The
corresponding data for [11C]PK11195 is included for comparison.
Table 4 details the compounds we prepared using the strategy
outlined in Scheme 1 the C-ring was modified.15
Changes to the C-ring are less well tolerated with a moderate
loss of activity when the sulfur is replaced by carbon (compound
43) but a more significant loss of affinity when the sulfur is

D. O’Shea et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2368–2372
2371
Table 5
Table 6
Regional brain data from the biodistribution data of 18F-compounds naïve Wistar rats
Brain data from in vivo metabolism study of 18F-compounds naïve Wistar rats (n = 1)
(n = 3)
Compound
Brain 10 min
pi (% parent)
Brain 30 min
pi (% parent)
Brain 60 min
pi (% parent)
Compound
Whole brain
2 min pi (% id/g
(SD))
OB 30 min pi OB:Str
Str
2:30 min
pi ratio
(% id/g (SD))
30 min pi
9a
15
99
98
95
97
Not tested
96
ratio
9a
14
15
16
17
18
20
21
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
40
41
44
—
0.17
0.29
0.31
0.28
0.10
0.22
0.19
0.03
0.23
0.29
0.26
0.35
0.25
0.24
0.16
0.34
0.42
0.23
0.26
0.52
0.38
0.13
0.05
0.14
0.17
0.05
0.21 (0.02)
1.42
2.42
2.07
3.50
2.00
1.57
2.11
1.00
1.35
1.16
1.73
1.46
1.93
2.67
1.78
1.37
1.40
1.53
1.37
2.26
2.92
1.44
1.25
0.88
2.13
1.00
2.10
2.16
2.00
1.73
4.75
5.00
1.29
4.67
1.33
1.53
1.60
2.00
1.29
2.23
2.89
3.00
1.42
1.17
1.07
2.11
1.65
2.54
1.11
0.75
2.75
1.75
5.80
3.20
16
99
100
98
100
98
100
0.28
0.32
0.42
0.31
0.29
0.44
0.05
0.29
0.45
0.35
0.34
0.34
0.30
0.30
0.35
0.42
0.21
0.44
0.46
0.39
0.13
0.04
0.45
0.16
0.30
[3H]PK11195
Table 7
Affinity data for enantiomers of 9a, 15 and 40
Compound
More active enantiomer
(nM)
Less active enantiomer
(nM)
Ratio
9a
15
40
0.22
0.29
16.8
5.24
1.17
30.2
23.8
4.0
1.8
BioScan
PBR
Retention Time
Area Percent
14
14
12
10
8
12
10
8
11C]PK11195 0.28–0.43
6
6
[
4
4
2
2
0.6
0.5
0.4
0.3
0.2
0.1
0.0
[18F]15
0
0
Striatum
Cerebellum
Thalamus
OB
0
2
4
6
8
10
Minutes
12
14
16
18
20
Figure 5. Chiral analysis of the active enantiomer of compound 9a.
BioScan
PBR
Retention Time
Area Percent
0.15
0.10
0.05
0.00
0.15
0.10
0.05
0.00
-0.05
-0.10
0
10
20
30
40
50
60
Time post-injection (min)
[18F]16
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Striatum
Cerebellum
Thalamus
OB
-0.05
-0.10
0
2
4
6
8
10
Minutes
12
14
16
18
20
Figure 6. Chiral analysis of the active enantiomer of compound 9a following
plasma incubation at 37 °C for 15 min.
0
10
20
30
40
50
60
Time post-injection (min)
Compounds 15 and 16 showed the best overall biodistribution
profiles (Fig. 4).
Figure 4. Regional brain biodistribution for compounds 15 and 16.
In vivo metabolic stability was also assessed in brain and plas-
ma using HPLC. For an imaging agent targeted at the CNS it is
important that the signal in the brain is attributable to the parent
molecule and not a radiolabelled metabolite. In our programme we
were targeting >90% parent at 60 min post injection (pi). Table 6
shows the data from the parent compound and the two com-
pounds which showed most promise in the naïve rat biodistribu-
tion and [11C]PK11195.
From these data it is clear that relatively modest changes in
structure can lead to significant changes in in vivo performance.
More polar compounds such as the phenols 21, 36 and 37 and
the sulfones 41 and 42 have decreased overall brain uptake. Alkoxy
substituents appear to be most favoured on the A-ring with F-sub-
stitution favoured on the B-ring.

2372
D. O’Shea et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2368–2372
Arlicot, N.; Vercouillie, J.; Ribeiro, M. J.; Tauber, C.; Venel, Y.; Baulieu, J. L.; Maia,
All initial evaluations in this series were carried out on the race-
S.; Corcia, P.; Stabin, M. G.; Reynolds, A.; Kassiou, M.; Guilloteau, D. Nucl. Med.
Biol. 2012, 39, 570.
mic mixtures but subsequently compounds 9a, 15 and 40 were re-
solved using chiral supercritical fluid chromatography (SFC).20
Table 7 shows the affinity data for the enantiomers. For com-
pounds 9a and 15 whilst there is a significant difference in affinity
the inactive enantiomer retains high affinity. For compound 40,
both enantiomers exhibit relatively low affinity.
The proton at the chiral centre in this class is relatively acidic
and has the potential for epimerisation when treated to the highly
basic conditions employed during labelling. The mesylate precur-
sor for compound 9a was resolved using and initial labelling exper-
iments did show that epimerisation occurred but it was found that
by the reducing reaction temperature to 90 °C and changing from
Kryptofix™ to 18-crown-6 the chirality could be retained. Figure
5 shows the chiral analysis of a radiolabeled 9a with 96% retention
of stereochemistry.
The possibility for in vivo epimerisation was preliminary evalu-
ated by looking at the chiral stability of 9a in plasma. Figure 6
shows the chiral analysis of 9a after 15 min of incubation in plasma
which shows only 83% active enantiomer which suggest that there
is a relatively rapid epimerisation in plasma. This suggest that
whilst it is technically feasible to deliver the tracer as a single
enantiomer that this would not be necessary as the compound
readily epimerises. Compound 40, where the epimerisation would
not be possible, unfortunately does not have a suitable affinity for a
PET tracer.
Overall our evaluation of the tetracyclic class of TSPO ligands as
potential PET ligands shows that a very subtle SAR exists with min-
or modifications in structure having relatively dramatic impact on
the in vivo performance of the compounds. Compounds 15 and
especially compound 16 have good profiles for an in vivo imaging
agent of TSPO and are suitable for further evaluation.
8. Imaizumi, M.; Briard, E.; Zoghbi, S. S.; Gourley, J. P.; Hong, J.; Musachio, J. L.;
Gladding, R.; Pike, V. W.; Innis, R. B.; Fujita, M. Synapse 2007, 61, 595.
9. Okubo, T.; Yoshikawa, R.; Chaki, S.; Okuyama, S.; Nakazato, A. Bioorg. Med.
Chem. 2004, 12, 3569.
10. Assay carried out using homogenated rat heart tissue with [3H]PK11195 as the
radioligand. Ki measurements are averaged from an 2 experiments across 6
concentrations (see Supplementary data for details).
11. Wadsworth, H.; Jones, P. A.; Chau, W.; Durrant, C.; Fouladi, N.; Passmore, J.;
O’Shea, D.; Wynn, D.; Morisson-Iveson, V.; Ewan, A.; Thaning, M.; Mantzilas,
D.; Gausemel, I.; Khan, I.; Black, A.; Avory, M.; Trigg, W. Bioorg. Med. Chem. Lett.
2012, 22, 1308.
12. Example analytical data (compound 9a): 1H NMR (300 MHz, CDCl3) dH 1.12 (3H,
t, J = 7 Hz, N(CH2CH3)a), 1.37 (3H, t, J = 7 Hz, N(CH2CH3)b), 3.28–3.68 (4H, m,
N(CH2CH3)2), 4.50–4.79 (2H, m, NCH2CH2F), 4.92 (2H, dt, J = 47 and 6 Hz,
NCH2CH2F), 5.13 (1H, s, CHCONEt2), 7.12–7.31 (4H, m, ArH), 7.37–7.50 (3H, m,
ArH), 7.63 (1H, d, J = 8 Hz, ArH); 13C NMR (75 MHz, CDCl3) dC 12.9, 14.9, 37.0,
41.1, 42.5, 45.5 (d, JCF = 23 Hz), 82.2 (d, JCF = 172 Hz), 110.2, 110.5, 118.1, 120.7,
123.0, 124.3, 124.9, 126.5, 127.2, 127.6, 128.8, 132.2, 136.4, 139.0, 168.0.
13. Donovan, S. F.; Pescatore, M. C. J. Chromatogr. A 2002, 952, 47.
14. Example analytical data (compound 29): 1H NMR (300 MHz, DMSO3) dH 1.00
(3H, t, J = 6 Hz, N(CH2CH3)a), 1.23 (3H, t, J = 6 Hz, N(CH2CH3)b), 3.18–3.34 (2H,
m, N(CH2CH3)a), 3.42–3.59 (2H, m, N(CH2CH3)b), 3.89 (3H, s, OCH3), 4.44–4.98
(4H, m, NCH2CH2F), 5.35 (1H, s, CHCONEt2), 7.01–7.24 (5H, m, ArH), 7.45 (1H, d,
J = 9 Hz, ArH), 7.61 (1H, d, J = 9 Hz, ArH).
15. Example analytical data (compound 40): 1H NMR (300 MHz, CDCl3) dH 0.74 (3H,
t, J = 7 Hz, N(CH2CH3)a), 1.23 (3H, t, J = 7 Hz, N(CH2CH3)b), 1.82 (3H, s, CH3),
3.07–3.25 (1H, m, N(CHaHdCH3)a), 3.35–3.55 (2H, m, N(CH2CH3)b), 3.91–4.07
(1H, m, N(CHcHdCH3)a), 4.50–5.00 (4H, m, NCH2CH2F), 7.05–7.12 (1H, m, ArH),
7.16–7.29 (3H, m, ArH), 7.36–7.46 (3H, m, ArH), 7.56 (1H, d, J = 8 Hz, ArH).
16. Example analytical data (9a mesylate precursor): 1H NMR (300 MHz, CDCl3) 1.02
(3H, t, J = 7 Hz, N(CH2CH3)a), 1.39 (3H, t, J = 7 Hz, N(CH2CH3)b), 2.65 (3H, s,
SO2CH3), 3.17–3.72 (4H, m, N(CH2CH3)2), 4.50–4.85 (4H, m, NCH2CH2O), 5.50
(1H, s, CHCONEt2), 6.53 (1H, d, J = 8 Hz, 8-CH), 7.01 (1H, d, J = 8 Hz, 10-CH),
7.11–7.20 (2H, m, 9-CH and ArH), 7.27 (1H, td, J = 8 and 2 Hz, ArH), 7.42 (1H,
dd, J = 8 and 1 Hz, ArH), 7.64 (1H, dd, J = 8 and 1 Hz, ArH).
17. Direct labelling: (radiosynthesis of compound 15): KrytofixÒ2.2.2 (4 mg,
10.6
l
mol), potassium bicarbonate (0.1 mol dmÀ3, 100
l
L, 10 mg, 10
l
mol)
and acetonitrile (0.5 mL) was added to [18F]fluoride/H2O (ca. 100–200
l
L) and
heated at 100 °C under a stream of nitrogen for 20–25 min. The labelling
mixture containing the corresponding mesylate precursor for compound 15
(2–3 mg, 4–6 lmol) in acetonitrile (1 mL) was added and heated at 100 °C/
Supplementary data
10 min. After 10 min, the reaction was cooled and then purified by semi-
preparative HPLC then formulated in ethanol/PBS for biological studies (tR ca.
11.2 min). Radiochemical yield of [18F] compound 15, was 18 4% (n = 11),
non-decay corrected yield. The total process time was ca. 90 min. Indirect
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2013.02.
057.
labelling: (radiosynthesis of compound 15): KrytofixÒ2.2.2 (4 mg, 10.6
l
mol),
mol) and
acetonitrile (0.5 mL) was added to [18F]fluoride/H2O (ca. 100–200
L) and
heated at 100 °C under a stream of nitrogen for 20–25 min. Ethylene glycol-
potassium bicarbonate (0.1 mol dmÀ3
,
100 lL, 10 mg, 10 l
l
References and notes
ditosylate (3–5 mg, 8–13.5 lmol) in acetonitrile (1 mL) was added and heated
at 100 °C/10 min. The crude intermediate, [18F](CH2)2OTs, was purified by
preparative HPLC. The purified [18F](CH2)2OTs intermediate was trapped onto a
conditioned light t-C18 sep pak and dried on a high flow N2 line for 15–20 min.
1. Chauveau, F.; Boutin, H.; Van Camp, N.; Dollé, F.; Tavitian, B. Eur. J. Nucl. Med.
Mol. Imaging 2008, 35, 2304.
2. Katsifis, A.; Mattner, F.; Zhang, Z.; Dikic, B.; Papazian, V. J. Labelled Compd.
Radiopharm. 2000, 43, 385.
3. Zhang, M.-R.; Kida, T.; Noguchi, J.; Furutsuka, K.; Maeda, J.; Suhara, T.; Suzuki,
K. Nucl. Med. Biol. 2003, 30, 513.
4. James, M. L.; Fulton, R. R.; Henderson, D. J.; Eberl, S.; Meikle, S. R.; Thomson, S.;
Allan, R. D.; Dollé, F.; Fulham, M. J.; Kassiou, M. Bioorg. Med. Chem. 2005, 13,
6188.
5. Imaizumi, M.; Kim, H.-J.; Zoghbi, S. S.; Briard, E.; Hong, J.; Musachio, J. L.;
Ruetzler, C.; Chuang, D.-M.; Pike, V. W.; Innis, R. B.; Fujita, M. Neurosci. Lett.
2007, 411, 200; Brown, A. K.; Fujita, M.; Fujimura, Y.; Liow, J. S.; Stabin, M.; Ryu,
Y. H.; Imaizumi, M.; Hong, J.; Pike, V. W.; Innis, R. B. J. Nucl. Med. 2007, 48, 2072.
6. Zhang, M.-R.; Maeda, J.; Furutsuka, K.; Yoshida, Y.; Ogawa, M.; Suhara, T.;
Suzuki, K. Bioorg. Med. Chem. Lett. 2003, 13, 201; Takano, A.; Gulyás, B.; Varrone,
A.; Karlsson, P.; Sjoholm, N.; Larsson, S.; Jonsson, C.; Odh, R.; Sparks, R.; Al
Tawil, N.; Hoffmann, A.; Zimmermann, T.; Thiele, A.; Halldin, C. Eur. J. Nucl. Med.
Mol. Imaging 2011, 38, 2058.
The corresponding indole precursor for compound 15 (10 mg, 27
(250 L) was added to a second reaction vessel and purged with N2 for 5 min.
After 5 min, with continued N2 flow, NaH (1 mg, 42 mol) in anhydrous DMF
(2 Â 250 L) was added. The reaction vessel was placed in a heater and left at
45 °C/0.5–1 h. The purified
18F](CH2)2OTs intermediate was eluted with
anhydrous DMF (500 L) into the second reaction vessel. The RV was sealed
lmol) in DMF
l
l
l
[
l
and heated at 100 °C/10 min. After 10 mins, the reaction was cooled and
removed, the RV was rinsed with water (1 mL). The solution was filtered thru a
5 mL syringe barrel containing a cotton wool plug. The reaction was purified by
semi-preparative HPLC then formulated in ethanol/PBS for biological studies
(tR ca. 11.2 min). The non-decay corrected radiochemical yield of [18F] 15 was
4% (n = 1). The total process time was ca. 120–150 min.
18. Scarf, A. M.; Kassiou, M. J. Nucl. Med. 2011, 52, 677.
19. Casellas, P.; Galiegue, S.; Basile, A. S. Neurochem. Int. 2002, 40, 475.
20. Taylor, L. T. J. Supercrit. Fluids 2009, 47, 566.
7. James, M. L.; Fulton, R. R.; Vercoullie, J.; Henderson, D. J.; Garreau, L.; Chalon, S.;
Dollé, F.; Selleri, S.; Guilloteau, D.; Kassiou, M. J. Nucl. Med. 2008, 49, 814;