New iodinated quinoline-2-carboxamides for SPECT imaging of the translocator protein

Article abstract of DOI:10.1016/j.bmcl.2009.12.061

With the aim of developing new SPECT imaging agents for the translocator protein (TSPO), a small library of iodinated quinoline-2-carboxamides have been prepared and tested for binding affinity with TSPO. N,N-Diethyl-3-iodomethyl-4-phenylquinoline-2-carboxamide was found to have excellent affinity (K_i 12.0 nM), comparable to that of the widely used TSPO imaging agent PK11195.

Full text of DOI:10.1016/j.bmcl.2009.12.061

Bioorganic & Medicinal Chemistry Letters 20 (2010) 954–957
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
New iodinated quinoline-2-carboxamides for SPECT imaging
of the translocator protein
Louise Stevenson a, Adriana A. S. Tavares b, Aurélie Brunet a, Fiona I. McGonagle a,
Deborah Dewar b, Sally L. Pimlott c, Andrew Sutherland a,
*
a WestCHEM, Department of Chemistry, The Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, UK
b Division of Clinical Neuroscience, Faculty of Medicine, University of Glasgow, Glasgow G12 8QQ, UK
c West of Scotland Radionuclide Dispensary, University of Glasgow and North Glasgow University Hospital NHS Trust, Glasgow G11 6NT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
With the aim of developing new SPECT imaging agents for the translocator protein (TSPO), a small library
of iodinated quinoline-2-carboxamides have been prepared and tested for binding affinity with TSPO.
N,N-Diethyl-3-iodomethyl-4-phenylquinoline-2-carboxamide was found to have excellent affinity (Ki
12.0 nM), comparable to that of the widely used TSPO imaging agent PK11195.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 18 November 2009
Revised 14 December 2009
Accepted 14 December 2009
Available online 22 December 2009
Keywords:
SPECT
Imaging
Translocator protein
Quinoline-2-carboxamides
Heterocyclic synthesis
The translocator protein 18 kDa (TSPO, formerly known as the
peripheral benzodiazepine receptor) was initially characterised as a
highaffinitybindingsitefordiazepamandhasbeenshowntobefunc-
tionally and structurally different from the central benzodiazepine
receptor (CBR).1 TSPO is found in lung, heart, spleen, liver, kidney,
brain, adrenals, macrophages and mast cells and its function while
tissue specific, revolves primarily around steroid synthesis.2 Within
the brain, TSPO is expressed by reactive glial cells and is implicated
in a number of nervous system disorders such as cerebral ischaemia,
epilepsy, nerve injury, and neurodegenerative diseases as well as im-
mune system diseases such as cancer.3 Expression of brain TSPO is
markedly increased during chronic neurodegeneration and following
acute brain injury4 and thus, is an attractive target for molecular
imaging of neuroinflammationinhumanneurodegenerativediseases
such as Alzheimer’s disease5 and in stroke-induced brain injury.6
A number of positron emission tomography (PET) and single
positron emission computed tomography (SPECT) ligands for the
molecular imaging of TSPO have been developed.7 The most widely
used ligand is the isoquinoline carboxamide PK11195 1.7a How-
ever, there are a number of limitations in using radiolabelled ana-
logues of PK11195 1 such as high nonspecific binding, high plasma
protein binding and relatively low brain uptake.8 In the search for
new radioligands for the molecular imaging of TSPO, the Cappelli
group identified a family of quinoline-2-carboxamides (e.g., 2)
which have exceptionally high affinity for TSPO and showed prom-
ise in biodistribution experiments.9 This work led to the develop-
ment of various [11C]-quinoline-2-carboxamides for PET imaging
of TSPO.9b,e As yet, there are no reported examples of radioiodin-
ated quinoline-2-carboxamides of this type developed for SPECT
imaging of TSPO. In this Letter, we report the preparation of a small
library of iodinated quinoline-2-carboxamides (e.g., 3 and 4) and
the screening of these compounds for affinity with TSPO.
O
O
N
N
N
N
Cl
F
1
2
O
O
N
N
R2
N
N
I
R1
I
* Corresponding author. Tel.: +44 0141 330 5936; fax: +44 0141 330 4888.
E-mail address: andrews@chem.gla.ac.uk (A. Sutherland).
3
4
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.12.061

L. Stevenson et al. / Bioorg. Med. Chem. Lett. 20 (2010) 954–957
NH2
955
N
Our first aim was the synthesis of 3-iodomethylquinoline-2-
carboxamide 11, an analogue bearing the same side-chain as
PK11195 1 (Scheme 1). The quinoline carboxamide core was pre-
pared in two steps by condensation of aniline 5 with diethyl oxal-
propionate 6 followed by acid-mediated ring closure of the
resulting imine which gave the 4-hydroxyquinoline 7 in good yield
over the two steps.10 The phenyl group was then introduced by
bromination of 7 with phosphorus oxybromide followed by a Suzu-
ki reaction with phenylboronic acid which gave 8 in excellent
yields.11 The PK11195 side-chain was then introduced using a
standard series of transformations involving hydrolysis of the es-
ter, formation of the acid chloride, reaction with sec-butyl amine
and finally methylation which gave quinoline-2-carboxamide 10.
The final stage involved bromination of 10 with NBS in the pres-
ence of dibenzoyl peroxide followed by nucleophilic substitution
with sodium iodide to give 3-iodomethylquinoline-2-carboxamide
11 in good overall yield.9e
Cl
O
Cl
a
O
+
CO2Et
CO2Et
12
13
14
b
N
N
OH
OH
O
O
c
16
15
d
While this initial approach allowed the synthesis of 3-iodom-
ethylquinoline-2-carboxamide 11, problems were encountered
during the NBS bromination stage of quinoline-2-carboxamides
with more bulky amide side-chains resulting in either low yields
or no reaction. Thus, for the general synthesis of 3-iodomethyl-
quinoline-2-carboxamides of this type a second approach was
developed as shown in Scheme 2. A chlorotrimethylsilane-medi-
ated Friedländer condensation was used for the efficient synthesis
of quinoline 14.12 Lactonisation was then followed by reduction
with lithium aluminium hydride which gave diol 16.13 Regioselec-
tive oxidation with manganese dioxide gave the key lactone inter-
mediate 17 in 80% yield. Lactone 17 was then reacted with a series
of secondary amines in the presence of trimethylaluminium which
gave the corresponding 3-hydroxymethylquinoline-2-carboxam-
ides in good yields. Chlorination of the hydroxymethyl group with
thionyl chloride followed by reaction with sodium iodide gave 3-
iodomethylquinoline-2-carboxamides 18, 19 and 20.
O
O
N
N
NR1R2
O
e, f
I
18, R1= R2= CH2CH3
19, R1= CH3, R2= CH2Ph
20, R1= CH3, R2= CH2CH2Ph
17
Scheme 2. Reagents and conditions: (a) TMSCl, DMF, 100 °C (sealed tube), 85%; (b)
6 M HCl, EtOH, , 91%; (c) (i) LiAlH4, THF; (ii) 10% Pd/C, MeOH, 67%; (d) MnO2,
CHCl3, 80%; (e) diethylamine, N-methylbenzylamine or N-methylphenethylamine,
Me3Al, CH2Cl2, , (65%, 59%; 53%, respectively); (f) (i) SOCl2, CH2Cl2; (ii) NaI, MeCN,
, (18, 56%; 19, 39%; 20, 53%).
D
D
D
Having prepared a series of 3-iodomethylquinoline-2-carbox-
amides with various amide side-chains, we wanted to explore
the TSPO binding affinity of 3-methylquinoline-2-carboxamides
with iodine incorporated in the amide side-chain. Accordingly,
two analogues containing a 3- and 4-iodobenzyl amide side-chain
were prepared (Scheme 3). The previously prepared quinoline car-
boxylate 9 was reacted with oxalyl chloride to give the acid chlo-
ride and this was then reacted with either 3- or 4-iodobenzyl
amine. Methylation using iodomethane and sodium hydride gave
targets 23 and 24.
O
CO2Et
N
CO2Et
NH2
a
+
EtO2C
OH
5
6
7
b, c
N
CO2H
N
CO2Et
Finally, a series of desmethyl quinoline-2-carboxamides were
prepared to investigate the affinity of less rigid compounds
(Scheme 4). 4-Hydroxyquinoline 26 was prepared in two steps
by condensation of aniline 5 with diethyl oxalacetate 25. Bromin-
ation followed by Suzuki reaction with phenylboronic acid gave
27 in excellent yield. Formation of the amide side-chain was then
carried out incorporating either 3- or 4-iodobenzyl groups as
described above to give targets 35 and 36 in good overall yields.
d
8
9
e, f
N
O
O
O
N
N
CO2H
N
R1
N
N
N
g, h
a, b
I
11
10
23, R1= 3-I-PhCH2
24, R1= 4-I-PhCH2
9
Scheme 1. Reagents and conditions: (a) (i) p-TsOH, cyclohexane,
55%; (b) POBr3, K2CO3, MeCN, , 98%; (c) phenylboronic acid, Pd(PPh3)4, K3PO4,
DMF, , 99%; (d) NaOH, EtOH/H2O (1:1), 80 °C, 93%; (e) (i) (COCl)2, DMF, CH2Cl2,
50 °C; (ii) sec-butyl amine, CH2Cl2, 50 °C, 70%; (f) MeI, NaH, THF, , 82%; (g) NBS,
dibenzoyl peroxide, CCl4, , 45%; (h) NaI, 18-crown-6, MeCN, 76%.
D; (ii) PPA, 120 °C,
D
D
Scheme 3. Reagents and conditions: (a) (i) (COCl)2, DMF, CH2Cl2, 50 °C; (ii) 3-
iodobenzylamine or 4-iodobenzylamine, CH2Cl2, 50 °C, (21, R1 = 3-I–PhCH2, 80%; 22,
D
D
R D, (23, 76%; 24, 90%).
1 = 4-I–PhCH2, 53%); (b) MeI, NaH, THF,

956
L. Stevenson et al. / Bioorg. Med. Chem. Lett. 20 (2010) 954–957
Table 1
O
CO2Et
N
CO2Et
NH2
a
Binding affinity of quinoline-2-carboxamide analogues 11, 18, 19, 20, 23, 24 and 35–
38 with the translocator protein
+
EtO2C
O
O
O
OH
N
N
N
5
25
NR1R2
NR1R2
NR1R2
26
b, c
N
CO2H
N
CO2Et
I
d
X
23, 24
11, 18, 19, 20
35, 36, 37, 38
log P
Ki (nM)a
Compound
R1
R2
—
X
X
X
PK11195 1
—
Me
Et
—
—
—
—
—
—
—
H
H
F
3.85
5.17
4.76
5.39
5.41
5.03
5.12
5.12
5.11
5.16
5.21
9.8 1.6
173 35
12.0 1.3
26.1 4.7
411 62
455 51
>1000
>1000
>1000
>1000
>1000
29, X= H
30, X= F
27, X= H
28, X= F
11
18
19
20
23
24
35
36
37
38
sec-Butyl
Et
PhCH2
PhCH2CH2
3-I–PhCH2
4-I–PhCH2
3-I–PhCH2
4-I–PhCH2
3-I–PhCH2
4-I–PhCH2
Me
Me
Me
Me
Me
Me
Me
Me
e
O
O
N
R1
N
R1
N
H
N
f
F
a
Calculated from three independent experiments.
X
X
In summary, a small library of quinoline-2-carboxamides have
31, X= H, R1= 3-I-PhCH2
32, X= H, R1= 4-I-PhCH2
33, X= F, R 1= 3-I-PhCH2
34, X= F, R 1= 4-I-PhCH2
35, X= H, R1= 3-I-PhCH2
36, X= H, R1= 4-I-PhCH2
37, X= F, R 1= 3-I-PhCH2
38, X= F, R 1= 4-I-PhCH2
been synthesised and their affinity for TSPO determined. N,N-
Diethyl-3-iodomethyl-4-phenylquinoline-2-carboxamide 18 was
found to have excellent affinity for TSPO with a Ki value compara-
ble to that of PK11195 1. It should be noted that diethyl analogue
18 contains an iodomethyl moiety, a well-known alkylating group.
However, due to bulky substitution at both positions ortho to the
iodomethyl group, diethyl analogue 18 is relatively stable under
physiological conditions.18 In general, the binding affinity results
and the log P values determined for this library of compounds
show that for the successful development of a SPECT imaging agent
for TSPO, quinoline-2-carboxamides with small amide groups are
required. Further pharmacological analysis of 18 to determine its
potential as a SPECT imaging agent for TSPO is currently underway.
Scheme 4. Reagents and conditions: (a) (i) p-TsOH, cyclohexane,
48%; (b) POBr3, K2CO3, MeCN, , 99%; (c) phenylboronic acid or 4-fluorophenylbo-
ronic acid, Pd(PPh3)4, K3PO4, DMF, , (27, 92%; 28, 69%); (d) NaOH, EtOH/H2O (1:1),
80 °C, (29, 90%; 30, 96%); (e) (i) (COCl)2, DMF, CH2Cl2, 50 °C; (ii) 3-iodobenzylamine
or 4-iodobenzylamine, CH2Cl2, 50 °C, (31, 94%; 32, 73%; 33, 84%; 34, 79%); (f) MeI,
D; (ii) PPA, 120 °C,
D
D
NaH, THF,
D, (35, 74%; 36, 56%; 37, 66%; 38, 64%).
A further series of compounds were prepared incorporating fluo-
rine at the para-position of the phenyl ring. These analogues have
the potential to be used for either SPECT (123I) or PET (18F) imaging.
Compounds 37 and 38 were prepared as described for 35 and 36
except using 4-fluorophenylboronic acid during the Suzuki
reaction.
Acknowledgments
Financial support from BBSRC (DTA award to L.S.) and SINAPSE
(Studentship to A.A.S.T) is gratefully acknowledged.
Thebindingaffinity of thelibraryof compoundsat TSPO waseval-
uated in vitro using homogenates of rat brain (Table 1).14 The bind-
ing affinity of PK11195 1 was also determined so that a direct
comparison could be made with the new compounds. Two com-
pounds from the 3-iodomethylquinoline-2-carboxamide series
were found to have excellent affinity for TSPO. Benzyl analogue 19
was found to have a Ki value of 26.1 nM,15 while diethyl analogue
18 has binding affinity (Ki 12.0 nM) comparable to that of PK11195
1. As shown by the results for compounds 20, 23, 24 and 35–38, sub-
stantially increasing the size of the side-chain of the amide prevents
these compounds from effectively binding to TSPO resulting in poor
affinity. The higher affinity of 3-methyl analogue 23 (Ki 455 nM)
compared to the desmethyl analogue 35 (Ki >1000 nM) confirms
the observations of others, that semi-rigid compounds (restricted
rotation of the amide carbonyl) bind with higher affinity to TSPO.9a
The log P value for these compounds was also determined using an
HPLC method involving C18 chromatography.16 The analogues bear-
ing a benzyl group in the amide side-chain all have log P values too
high to be considered as suitable imaging agents. While diethyl ana-
logue 18 also has a relatively high log P value (4.76), this value is
comparable to other SPECT imaging agents known to cross the blood
brain barrier.17
Supplementary data
Supplementary data (experimental procedures and spectro-
scopic data for all compounds synthesised as well as details for
competition binding assays) associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.2009.12.061.
References and notes
1. (a) Braestrup, C.; Squires, R. Proc. Natl. Acad. Sci. U.S.A. 1977, 74, 3805; (b) Gavish,
M.; Bachman, I.; Shoukrun, R.; Katz, Y.; Veenman, L.; Weisinger, G.; Weizman, A.
Pharmacol. Rev. 1999, 51, 629; (c) Papadopoulos, V.; Baraldi, M.; Guilarte, T. R.;
Knudsen, T. B.; Lacapère, J. J.; Lindemann, P.; Norenberg, M. D.; Nutt, D.;
Weizman, A.; Zhang, M. R.; Gavish, M. Trends Pharmacol. Sci. 2006, 27, 402.
2. Scarf, A. M.; Ittner, L. M.; Kassiou, M. J. Med. Chem. 2009, 52, 581. and references
therein.
3. Wang, M.; Gao, M.; Hutchins, G. D.; Zheng, Q.-H. Eur. J. Med. Chem. 2009, 44,
2748.
4. (a) Benavides, J.; Fage, D.; Carter, C.; Scatton, B. Brain Res. 1987, 421, 167; (b)
Kuhlmann, A. C.; Guilarte, T. R. J. Neurochem. 2000, 74, 1694; (c) Chen, M. K.;
Baidoo, K.; Verina, T.; Guilarte, T. R. Brain 2004, 127, 1379.
5. (a) Maeda, J.; Ji, B.; Irie, T.; Tomiyama, T.; Maruyama, M.; Okauchi, T.;
Staufenbiel, M.; Iwata, N.; Ono, M.; Saido, T. C.; Suzuki, K.; Mori, H.; Higuchi,

L. Stevenson et al. / Bioorg. Med. Chem. Lett. 20 (2010) 954–957
957
M.; Suhara, T. J. Neurosci. 2007, 27, 10957; (b) Venneti, S.; Wang, G.; Nguyen, J.;
Wiley, C. A. J. Neuropathol. Exp. Neurol. 2008, 67, 1001; (c) Yasuno, F.; Ota, M.;
Kosaka, J.; Ito, H.; Higuchi, M.; Doronbekov, T. K.; Nozaki, S.; Fujimura, Y.;
Koeda, M.; Asada, T.; Suhara, T. Biol. Psychiatry 2008, 64, 835.
Menziani, M. C.; Langer, T.; Hagen, B.; Manzoni, C.; Bourguignon, J.-J. J. Med.
Chem. 2001, 44, 1134; (d) Cappelli, A.; Mohr, G. P.; Galleli, A.; Giuliani, G.;
Anzini, M.; Vomero, S.; Fresta, M.; Porcu, P.; Maciocco, E.; Concas, A.; Biggio, G.;
Donati, A. J. Med. Chem. 2003, 46, 3568; (e) Cappelli, A.; Matarrese, M.; Moresco,
R. M.; Valenti, S.; Anzini, M.; Vomero, S.; Turolla, E. A.; Belloli, S.; Simonelli, P.;
Filannino, M. A.; Lecchi, M.; Fazio, F. Bioorg. Med. Chem. 2006, 14, 4055.
10. Abbott, P. A.; Bonnert, R. V.; Caffrey, M. V.; Cage, P. A.; Cooke, A. J.; Donald, D.
K.; Furber, M.; Hill, S.; Withnall, J. Tetrahedron 2002, 58, 3185.
11. Stevenson, L.; Pimlott, S. L.; Sutherland, A. Tetrahedron Lett. 2007, 48, 7137.
12. Ryabukhin, S. V.; Volochnyuk, D. M.; Plaskon, A. S.; Naumchik, V. S.; Tolmachev,
A. A. Synthesis 2007, 1214.
13. Anzini, M.; Cappelli, A.; Vomero, S. Heterocycles 1994, 38, 103.
14. A [3H]-PK11195 displacement assay was used for TSPO. See Supplementary
data for full experimental details.
6. (a) 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; (b) Rojas, S.; Martín, A.; Arranz, M. J.; Pareto, D.; Purroy, J.;
Verdaguer, E.; Llop, J.; Gómez, V.; Gispert, J. D.; Millán, O.; Chamorro, A.; Planas,
A. M. J. Cereb. Blood Flow Metab. 2007, 27, 1975.
7. (a) Chauveau, F.; Boutin, H.; Van Camp, N.; Dollé, F.; Tavitian, B. Eur. J. Nucl.
Med. Mol. Imaging 2008, 35, 2304 and references therein; (b) Yu, W.; Wang, E.;
Voll, R. J.; Miller, A. H.; Goodman, M. M. Bioorg. Med. Chem. 2008, 16, 6145; (c)
Wang, H.; Pullambhatla, M.; Guilarte, T. R.; Mease, R. C.; Pomper, M. G. Biochem.
Biophys. Res. Commun. 2009, 389, 80; (d) Yanamoto, K.; Yamasaki, T.; Kumata,
K.; Yui, J.; Odawara, C.; Kawamura, K.; Hatori, A.; Inoue, O.; Yamaguchi, M.;
Suzuki, K.; Zhang, M.-R. Synapse 2009, 63, 961.
15. Compound 19 has been prepared previously and shown to have an IC50 value of
0.4 nM with TSPO. See Ref. 9e for details.
8. (a) Petit-Taboué, M.-C.; Baron, J.-C.; Barré, L.; Travére, J.-M.; Speckel, D.;
Camsonne, R.; MacKenzie, E. T. Eur. J. Pharmacol. 1991, 200, 347; (b) Shah, F.;
Hume, S. P.; Pike, V. W.; Ashworth, S.; McDermott, J. Nucl. Med. Biol. 1994, 21,
573; (c) Lockhart, A.; Davis, B.; Matthews, J. C.; Rahmoune, H.; Hong, G.; Gee,
A.; Earnshaw, D.; Brown, J. Nucl. Med. Biol. 2003, 30, 199; (d) Maeda, J.; Suhara,
T.; Zhang, M.-R.; Okauchi, T.; Yasuno, F.; Ikoma, Y.; Inaji, M.; Nagai, Y.; Takano,
A.; Obayashi, S.; Suzuki, K. Synapse 2004, 52, 283.
9. (a) Cappelli, A.; Anzini, M.; Vomero, S.; De Benedetti, P. G.; Menziani, M. C.;
Giorgi, G.; Manzoni, C. J. Med. Chem. 1997, 40, 2910; (b) Matarrese, M.; Moresco,
R. M.; Cappelli, A.; Anzini, M.; Vomero, S.; Simonello, P.; Verza, E.; Magni, F.;
Sudati, F.; Soloviev, D.; Todde, S.; Carpinelli, A.; Kienle, M. G.; Fazio, F. J. Med.
Chem. 2001, 44, 579; (c) Anzini, M.; Cappelli, A.; Vomero, S.; Seeber, M.;
16. (a) Valkó, K.; Bevan, C.; Reynolds, D. Anal. Chem. 1997, 69, 2022; (b) Valkó, K. J.
Chromatogr., A 2004, 1037, 299; (c) Valkó, K. LCGC North America 2007, 25, 284.
17. The log P value for CNS 1261, a known SPECT imaging agent for the NMDA
receptor (Owens, J.; Tebbutt, A. A.; McGregor, A. L.; Kodama, K.; Magar, S. S.;
Perlman, M. E.; Robins, D. J.; Durant, G. J.; McCulloch, J. Nucl. Med. Biol. 2000, 27,
557) was determined using our HPLC method and was found to be 4.94.
18. Compound 18 was incubated in phosphate buffered saline at 37 °C. The
decomposition of 18 was then monitored using HPLC. Decomposition was only
observed at around 24 h. See Supplementary data for full experimental details.
A patient, on average is normally scanned 2–4 h after injection of a SPECT
imaging agent.