Development of [18F]-PSS223 as a PET tracer for imaging of metabotropic glutamate receptor subtype 5 (mGluR5)

Article abstract of DOI:10.2533/chimia.2012.201

Involvement of metabotropic glutamate receptor subtype 5 (mGluR5) in physiological and pathophysiological processes in the brain has been demonstrated, and hence mGluR5 has emerged as an important drug target. [ ¹¹C]-ABP688 is clinically the mo

Full text of DOI:10.2533/chimia.2012.201

Laureates: awards and Honors, sCs FaLL Meeting 2011
CHIMIA 2012, 66, No. 4 201
doi:10.2533/chimia.2012.201
Chimia 66 (2012) 201–204 © Schweizerische Chemische Gesellschaft
Development of [18F]-PSS223 as a PET
Tracer for Imaging of Metabotropic
Glutamate Receptor Subtype 5 (mGluR5)
Selena Milicevic Sephton§, Patrick Dennler, Dominique S. Leutwiler, Linjing Mu, Roger Schibli,
Stefanie D. Krämer, and Simon M. Ametamey*
§SCS-Metrohm Foundation Award for best oral presentation
Abstract: Involvement of metabotropic glutamate receptor subtype 5 (mGluR5) in physiological and
pathophysiological processes in the brain has been demonstrated, and hence mGluR5 has emerged as an
important drug target. [11C]-ABP688 is clinically the most successful mGluR5 positron emission tomography (PET)
tracer to date and it allows visualization and quantification of mGluR5. Due to the short half-life of carbon-11,
clinical use of [11C]-ABP688 is limited to facilities with an on-site cyclotron and a fluorine-18 (half-life 110 min)
analogue would be more practical. Based on the [11C]-ABP688 structural motif, a novel derivative [18F]-PSS223
was prepared and evaluated as a PET tracer for imaging of mGluR5 in vitro and in vivo. Our results show
favourable in vitro binding properties; however rapid defluorination of [18F]-PSS223 does not allow visualization
of mGluR5 in the rat brain.
Keywords: [11C]-ABP688 · [18F]-FDEGPECO · [18F]-PSS223 · mGluR5 · PET imaging
a seven transmembrane domain G-protein is limited to facilities with an on-site cy-
coupled postsynaptically situated recep- clotron due to the extremely short half-
1. Introduction
tor. Studies have implicated mGluR5 with life of carbon-11 (20 min). In order to
The function of glutamate, a major
processes of learning and memory, but also overcome this limitation scientific efforts
involvement of mGluR5 in several disor- were made towards the development of an
ders of the central nervous system (CNS) analogous fluorine-18 radiotracer which
such as Alzheimer’s and Parkinson’s dis- would have a physical half-life of 110 min.
eases, schizophrenia, depression and anxi- [18F]-SP203[15–17] and [18F]-FPEB[18,19] are
ety.[9–11] Although the precise mechanisms two radiotracers for imaging of mGluR5
governing the involvement of mGluR5 in developed by the Pike and Hamill groups,
pathophysiological processes in the brain respectively; however they both have
are not fully understood, mGluR5 is con- shortcomings, first due to defluorination
sidered an important drug target and imag- in human subjects albeit in modest amount
ing of mGluR5 in vivo arose as a challenge and the latter due to tedious radiosynthesis
neurotransmitter in the mammalian brain,
is mediated through two types of recep-
tors: ionotropic (e.g. NMDA, AMPA or
kainate) and metabotropic (e.g. mGluR
receptor family). Metabotropic glutamate
receptors (mGluR) were identified in
1991 and they are divided in three groups
based on sequence homology, receptor
pharmacology and signal transduction:
group I (mGluR1 and mGluR5), group
II (mGluR2 and mGluR3), and group III
(mGluR4 and mGluR6-8).[1–8] While iono-
tropic glutamate receptors are mainly in-
volved with fast excitatory neurotransmis-
sion, mGluRs are responsible for subtle
changes in neurotransmission. mGluR5 is
to PET community.
(Fig. 1). Efforts from the Ametamey group
Positron emission tomography (PET)
were aimed at developing a fluorine-18
is a powerful non-invasive imaging tech- analogue of [11C]-ABP688 by the least
nique which is employed in the quantifica- number of structural changes which led
tion of biochemical and pharmacodynamic to (E)-3-(pyridin-2-ylethynyl)cyclohex-
processes in healthy and diseased states 2-enone O-(2-(2-18F-fluoroethoxy)ethyl)
and is particularly important for the drug oxime ([18F]-FDEGPECO, Fig. 1).[20,21]
development as it facilitates deeper under- The main advantages of [18F]-FDEGPECO
standing of drug-target interactions in vivo in comparison to other fluorine-18 radio-
and monitoring of effects of drug candi- tracers are based on the ease with which
dates on the progression of a disease.[12] [18F]-FDEGPECO is produced in a single
For these reasons, development of a PET
radiotracer for mGluR5 is advantageous stability in vivo (i.e. no defluorination was
and several mGluR5 PET tracers have observed in the dynamic PET scan); how-
high-yielding radiochemical step and good
been developed to date. Clinically the ever although quantification of mGluR5
most successful radiotracer for imaging of was possible, quality of the images was
mGluR5 is (E)-3-((6-methylpyridin-2-yl) reduced by significant background.
*Correspondence: Prof. Dr. S. M. Ametamey
Center for Radiopharmaceutical Sciences of ETH, PSI
and USZ
Department of Chemistry and Applied Biosciences
ETH-Hönggerberg
Wolfgang-Pauli Strasse 10
CH-8093 Zürich
Tel.: +41 1 633 7463
Fax: +41 1 633 1367
ethynyl)cyclohex-2-enone-O-11C-methyl
One of the important criteria in design-
oxime ([11C]-ABP688),[13,14] developed ing new brain tracers is their lipophilic-
by the Ametamey group and exhibited ity, which among other parameters deter-
excellent in vitro and in vivo properties; mines the successful passage across the
however the application of [11C]-ABP688 blood brain barrier (BBB).[22] For mGluR5
E-mail: simon.ametamey@pharma.ethz.ch

202 CHIMIA 2012, 66, No. 4
Laureates: awards and Honors, sCs FaLL Meeting 2011
Fig. 1. Selected
philic substitutions of common precursor
1 with the corresponding nucleophilic
8
1 F
carbon-11 and
fluorine-18 PET trac-
ers for imaging of
mGluR5.
S
fluoride sources (Scheme 1). The suitably
functionalized intermediate 1 would be ac-
cessed via a coupling of oxime 3 and bro-
N
N
N
N
moether 2.
8
1 F
To form bromoether 2 the commercial-
n
ly available 1,3-propanediol (4) was mono-
8
N
O
N
C
F N
C
1 F
N
O
protected to yieldTBS silyl ether 5. Several
attempts to obtain the coupling partner 7
directly using dibromoethane failed and
a two-step approach via bromoester 6 af-
O
11CH3
11
18
18
18
[ C]-ABP688 [ F]-SP203 [ F]-FPEB
[ F]-FDEGPECO
18 (n =1)
forded bromoether 7 after reduction of
[ F]-PSS223
(n =2)
the ester functionality with triethylsilane/
indium(iii)-bromide[23] (Scheme 2) in 7%
yield over three steps.[24] Interestingly,
ethyl) oxime ([18F]-PSS223, Fig. 1) could
in turn lead to a better signal-to-noise ratio
our group established an optimal range
of logD values (1.7–2.5)[20] and [18F]-
FDEGPECO exhibited a logD slightly
lower than desired. We hypothesized that
by extending the lipophilic side chain in
[18F]-FDEGPECO by one methylene unit,
the increased lipophilicity of this novel de-
rivative (E)-3-(pyridin-2-ylethynyl)cyclo-
hex-2-enone O-(2-(3-18F-fluoropropoxy)
7 was isolated as a mixture of TBS- and
TES-silylethers 7a and 7b, respectively,
in the PET image.
presumably from the exchange of silyl-
ether groups. Although it was possible to
separate 7a and 7b via column chroma-
tography, for the next step the mixture of
silylethers was successfully employed.
2. Synthesis of PSS223
The syntheses of both PSS223 and
Oxime 3 was synthesized from com-
[18F]-PSS223 were envisioned via nucleo-
mercially available 3-ethoxy-cyclohex-
2-enone (8) which was converted to acet-
ylene 9 in 93% yield and further reacted
Scheme 1.
with hydroxylamine to give a 3:1 ratio of
Retrosynthetic analy-
N
N
N
sis for the synthesis
of PSS223; PG – pro-
tecting group; LG –
leaving group.
E- and Z-oximes (E)-10 and (Z)-10, respec-
tively.[20,25] The mixture was successfully
OP
G
+
separated by column chromatography and
the major isomer (E) was coupled to bro-
mopyridine under Sonogashira reaction
conditions (Scheme 3).
The mixture of bromoethers 7a and 7b
wasthenreactedwithoxime3andthecrude
mixture of silylethers was immediately de-
F
OLG
O
O
Br
N
O
O
N
O
N
OH
2
PSS223
1
3
protected using a TBAF·THF complex to
Scheme 2. Synthesis
of bromoether 7.
furnish alcohol 11 with an overall yield of
28% (five steps). With alcohol 11 in hand,
we next sought an optimal leaving group.
Initially, 11 was converted to tosylate 12 in
65% yield, however attempts to substitute
the tosyl group in 12 with fluoride resulted
only in decomposition of 12 (Scheme 4).
Alcohol 11 was then converted to mesyl-
ate 13 with a higher yield (85%), reduced
reaction time and successfully substituted
to form PSS223 with potassium fluoride in
70% yield (Scheme 4).
O
Br
BrC(O)C 2
H Br,EtN
3
O
CH2Cl2,DMA
P
OTBS
0°Ctort, 16 h, 36%
6
OH
OH
TBSCl, DIEA
InBr ,EtSiH
3
3
D
MF,rt, 6h
C
HCl3,60°C, 5h,42%
OTBS
OH
49%
Br
4
5
Br
O
1or2eq.
Br
RO
NaH or iPr EtN
CH2Cl2 or D2 MF
7a R=TB
7b R=TE
S
S
3. Radiolabelling of 10
Scheme 3. Synthesis
O
Et
of alcohol 11.
C
C
MgBr,THF
NH2OH·H O
Mesylate 13 was also used to ob-
2
+
tain [18F]-PSS223 in one step using an
OH
O
O
N
(E)-10
N
(Z)-10
rt, 19 h, 93%
Pyr,rt, 16.5 h
aqueous solution of 18F– complexed with
O
H
86%, E:Z 3:1
8
9
Kryptofix-222® and K2CO as a nucleo-
philic source of fluoride at390 °C over 10
2-Bromopyridine
min (Scheme 5).[24] [18F]-PSS223 was pu-
Pd(PPh ) ,EtN
3
CuI, D 3 4
MF,rt, 23 h
rified via semi-preparative HPLC and the
collected product was then trapped on a
N
73%
C18 cartridge to remove acetonitrile used
7a and 7b
NaH, DMF,rt, 1h,then
N
for HPLC purification. The labelled prod-
N
OH
N
O
OH
uct was eluted with ethanol and the etha-
nolic solution of [18F]-PSS223 was diluted
with PEG:H2O 1:1 to give 5–6 GBq (20%
O
TBAF·THF,rt, 1h,48%
11
3

Laureates: awards and Honors, sCs FaLL Meeting 2011
CHIMIA 2012, 66, No. 4 203
Scheme 4. Synthesis
of cold PSS223.
cay corrected yield. In vitro evaluation of
[18F]-PSS223 revealed excellent binding
potential of our novel tracer; however in-
cubation of [18F]-PSS223 with rat micro-
somal enzymes suggested fast metabolism
and the in vivo analysis confirmed rapid
defluorination as reflected by accumula-
tion of radioactivity in the skull and jaws.
Further efforts are ongoing to discover an
18F-labelled alternative to [11C]-ABP688
with favourable in vitro and improved in
vivo profile suitable for clinical use.
N
N
TsCl, DMA
P
MsCl, Et N
3
Et N, CH2Cl2
THF
3
11
OTs
O
OMs
0°C, 18 h
65%
0°C, 22 min
85%
N
O
N
O
O
K 2,KF
K 2,KF
2
2
22
12
13
P S223
S
MeCN, 80 °C
90 min, 0%
MeCN, 80 °C
40 min, 70%
5. in vivo Evaluation of [18F]-PSS223
Acknowledgements
S. M. S. would like to acknowledge the
Swiss Chemical Society and Metrohm AG for
K 2,K2CO3
N
N
2
2
8 –
1 F ,DMF
We next explored [18F]-PSS223 in an
8
O
Ms
1 F
in vivo dynamic PET
scan. Time–activity
the award. The authors would like to acknowl-
edge Mrs. Claudia Keller and Mrs. Petra Wirth
for technical assistance in the PET
90 °C,10min
curves obtained from the in vivo imaging
are depicted in Fig. 3. Significant accumu-
lation of radioactivity was observed in the
jaws over the duration of the scan (90 min)
suggesting rapid defluorination of [18F]-
PSS223. Likely mechanisms for the rapid
metabolism of [18F]-PSS223 include oxi-
dation by cytochrome P450.[24]
20% RCY
lab. Prof.
N
O
O
N
O
O
P. A. Schubiger, Dr. Cindy Wanger-Baumann,
Miss Cindy Fischer and Dr. Thomas Betzel are
acknowledged for support and many fruitful
discussions.
1
13
[ 8F]-P
SS223
Scheme 5. Radiolabelling of mesylate 13 to
yield [18F]-PSS223.
Received: January 9, 2012
decay corrected radiochemical yield) of
>97% pure formulated product with spe-
cific activity ranging between 100–320
GBq/µmol.
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6. Conclusion
P. Rao, S. D. Hess, C. Liaw, A. Urrutia, C.
In conclusion, we have successfully
synthesized PSS223, a novel 18F-fluorine
containing derivative based on theABP688
structural motif which showed excellent
binding affinity for mGluR5. Employing
the same key intermediate 13 as the pre-
cursor, [18F]-PSS223 was prepared in a
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PSS223
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single radiochemical step with 20% de-
involved determining the binding affinity
and in vitro autoradiography with rat brain
slices. For CNS tracers the ideal binding
affinity is below 2 nM with a tolerance of 5
nM, to comply with the criteria of Bmax/Kd
Fig. 2. In vitro au-
toradiography of
[18F]-PSS223 on
horizontal rat brain
slices: a) incubation
of slices with 1 nM
[18F]-PSS223; b) incu-
bation of slices with
1 nM [18F]-PSS223
and 100 nM ABP688.
>>1.[22] The Kd for [18F]-PSS223 was es-
timated from the saturation binding assay
as 3.34±2.05 nM,[24] suggesting excellent
binding of [18F]-PSS223 to mGluR5.
Invitroautoradiographywasperformed
on horizontal rat brain slices with a 1 nM
solution of [18F]-PSS223 and resulted in
heterogeneous distribution of radioactivity
with highest signals in the regions where
mGluR5 is highly expressed (i.e. striatum,
hippocampus and cortex). MGluR5 shows
low expression in the cerebellum[1,3,5] and
the radioactivity in this region was at the
Fig. 3. Time-activity
curves for [18F]-
PSS223 in different
rat brain regions and
the jaws.
background level (Fig. 2a). To establish
specificity of [18F]-PSS223 for mGluR5,
incubation of brain slices was conducted
with a solution containing 1 nM of radio-
tracer and 100 nM of cold ABP688, which
specifically binds to mGluR5. A complete
blocking of the radioactivity signals in
mGluR5 brain regions was observed (Fig.
2b). Stability of [18F]-PSS223 towards rat
liver microsomal enzymes was investigat-
ed and it was found that 90% of the parent
radiotracer was metabolized over one hour
suggesting rapid metabolism in vivo.

204 CHIMIA 2012, 66, No. 4
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