Efficient microwave-assisted direct radiosynthesis of [18F]PR04.MZ and [18F]LBT999: Selective dopamine transporter ligands for quantitative molecular imaging by means of PET

Article abstract of DOI:10.1016/j.bmc.2009.09.054

PR04.MZ 8-(4-fluoro-but-2-ynyl)-3-p-tolyl-8-aza-bicyclo[3.2.1]octane-2-carboxylic acid methyl ester (1) and LBT999 8-((E)-4-fluoro-but-2-enyl)-3b-p-tolyl-8-aza-bicyclo[3.2.1]octane-2β-carboxylic acid methyl ester (2) are selective dopamine reuptake inhibi

Full text of DOI:10.1016/j.bmc.2009.09.054

Bioorganic & Medicinal Chemistry 17 (2009) 7630–7634
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Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Efficient microwave-assisted direct radiosynthesis of [18F]PR04.MZ
and [18F]LBT999: Selective dopamine transporter ligands for
quantitative molecular imaging by means of PET
*
Patrick J. Riss , Frank Roesch
Institute of Nuclear Chemistry, Johannes Gutenberg-University, Fritz-Straßmann-Weg 2, D-55128 Mainz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2009
Revised 24 September 2009
Accepted 30 September 2009
Available online 4 October 2009
PR04.MZ 8-(4-fluoro-but-2-ynyl)-3-p-tolyl-8-aza-bicyclo[3.2.1]octane-2-carboxylic acid methyl ester (1)
and LBT999 8-((E)-4-fluoro-but-2-enyl)-3b-p-tolyl-8-aza-bicyclo[3.2.1]octane-2b-carboxylic acid methyl
ester (2) are selective dopamine reuptake inhibitors, derived from cocaine. Compounds 1 and 2 were
labelled with fluorine-18 at their terminally fluorinated N-substituents employing microwave enhanced
direct nucleophilic fluorination. K[18F]FÀ KryptofixÒ222 cryptate, tetrabutyl ammonium [18F]fluoride and
caesium [18F]fluoride were compared as fluoride sources under conventional and microwave enhanced
conditions. Fluorination yields were remarkably increased under microwave irradiation for all three fluo-
Keywords:
Direct aliphatic radiofluorination
Microwave irradiation
Dopamine transporter
PET
ride salts. Radiochemically pure (>98%) [18F]PR04.MZ (0.95–1.09 GBq, 42–135 GBq/
lmol) was obtained
within 34–40 min starting from 3.0 GBq [18F]fluoride ion in 32–36% non-decay-corrected overall yield
using K[18F]FÀKryptofixÒ222 cryptate in MeCN.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
potency tropane-analogue DAT-inhibitor LBT999 and the recently
described PR04.MZ (Fig. 1).6
Dopaminergic pathologies are related to several neurodegener-
ative and psychiatric disorders.1 As a consequence, molecular
imaging of the dopaminergic signalling has attracted considerable
attention.2 In particular, the availability of the presynaptic dopa-
mine transporter (DAT), a transmembrane protein belonging to
the family of neurotransmitter–sodium symporters, can be used
as an indicator on the integrity of the dopaminergic system.2h
Cocaine-analogue radioligands, labelled with short-lived posi-
tron emitters have shown to be the most promising lead for the
development of selective probes for the DAT.3 Although state-of-
the-art in fluorine-18 labelling of tropane-analogue DAT-ligands is
N-alkylation with an appropriate secondary labelling synthon, for
example, 2-[18F]fluoroethyl tosylate,4a,b,5b recent studies elucidated
direct nucleophilic aliphatic fluorination routes with analogue com-
pounds.5a,c Nevertheless, radio-labelling of cocaine-analogue radio-
ligands with fluorine-18 was often complicated by (a) low N-
alkylation yields, (b) poor accessibility of sp2-carbon-bound fluo-
Initial direct aliphatic nucleophilic radiofluorination of PR04.MZ
was performed under conventional conditions. Unfortunately, only
low radiochemical yields of 19 3% corrected for decay were
obtained after 60–70 min radiosynthesis.6c Similar yields have also
been reported in
a simple one-step fluorine-18-labelling of
LBT999,basedon achlorine-for-fluorinenucleophilicsubstitution.4b
Reaction of potassium K222Òcryptate [18F]FÀ complex ([KꢀK222]+
[
18F]FÀ) with the chlorinated precursor at 165 °C in DMSO for
10 min followed by C-18 solid phase extraction and final semipre-
parative HPLC separation afforded [18F]2 in 10–16% non-decay-cor-
rected radiochemical yield after ꢁ65 min.
Inordertoincreasethe radiochemicalyield, weconsideredthe use
of microwave enhanced conditions for nucleophilic aliphatic radio-
fluorination.7 Most striking with regard to radiosynthesis employing
rine, resulting in laborious reaction pathways, and (c) a/b-epimeri-
sation at the C-2 position in the tropane skeleton,5 due to the basic
reaction conditions required for nucleophilic [18F]F-labelling.
The present report is concerned with a reliable, microwave
enhanced high yield direct fluorination method for the moderate
* Corresponding author. Tel.: +4961313925302.
E-mail address: pr340@wbic.cam.ac.uk (P.J. Riss).
Figure 1. PR04.MZ (1) and LBT999 (2).
0968-0896/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2009.09.054

P. J. Riss, F. Roesch / Bioorg. Med. Chem. 17 (2009) 7630–7634
7631
Scheme 1. Synthesis of precursors 5 and 7 for LBT999 and PR04.MZ, respectively. Reagents and conditions: (a) DiPEA, 4-hydroxybut-2-ene-1-yl bromide, MeCN; (b) Ms2O,
Yb(OTf)3, CH2Cl2, 0 °C, 1 d; (c) DiPEA, MeCN, 4-hydroxybut-2-yne-1-yl chloride; (d) MsCl, NEt3, CH2Cl2, 0 °C, 25 min.
short-lived isotopes, is the high and direct transfer of activation en-
ergy to the reactants, which mostly results in remarkably shortened
reactions times.7 Microwave-assisted synthesis has thus evolved as
a powerful tool in the development of fast, chemo-selective synthetic
methods.7 Compared to conventional heating, energy transfer does
not proceed gradually from the outside of the vessel to the inside.
Instead, the microwave directly interacts with dipoles or charged
species inside the reaction mixture. Thus, the direct energy transfer
toa reaction center, a reactant or an intermediate is possible. Further-
more, the amount of activation energy, necessary for the conversion
of a given molar amount of reactants can be provided within a shorter
time, due to the high performance of focussed microwaves. The high
energy influx facilitates different reaction pathways. In opposite to
conventional conditions, thermodynamic reaction routes are
preferred and kinetic side reactions are disfavoured.7
2-carboxylic acid methyl ester (7), was synthesised according to
Scheme 1. Compound 3 was reacted with 4-chlorobut-2-ynyl alco-
hol to afford 6 in 95% yield. Subsequent mesylation in dichlorometh-
ane using methanesulfonyl chloride and triethylamine afforded 7 in
94% yield.
2.2. Radiochemistry
[
18F]PR04.MZ ([18F]1) was obtained by reacting
7 with
[KꢀK222]+[18F]FÀ, using a CEM DiscoverÒ focussed microwave
reactor. The maximum pressure during the labelling reaction was
20 bar, the maximum temperature at the end of microwave irradi-
ation was 175 °C (Scheme 2).
The optimal microwave performance was found to be 255 W
(Fig. 2). Labelling precursor 7 was exposed to cyclotron-produced
In general, commercially available microwave-reactors provide
[
[
18F]fluoride ion as n.c.a. [K+ꢀK222][18F]FÀ, tetrabutyl ammonium
monomodal electromagnetic radiation (k = 12.2 cm,
m
= 2450
18F]fluoride complex ([18F]TBAF) or caesium [18F]fluoride salt
MHz), representing the energy spectrum of molecular rotation.
Therefore, it is unlikely that microwave irradiation results in cleav-
age of covalent bonds.7
([18F]CsF), cf. Figure 4. Labelling was performed under constant
microwave irradiation at 255 W for up to 45 s in a pressure-tight
microwave reaction vial. The maximum pressure during the label-
ling reaction was 1.8 Â 106 Pa. After heating for various time-peri-
ods (5 s, 10 s, 20 s, 45 s), an aliquot of the solution was analysed by
TLC. A mixture of diethyl ether (Et2O) and hexanes containing 10%
of triethylamine (NEt3) was used as mobile phase. The relative
reaction yield was determined on Silica Gel 60 coated aluminium
plates. The conversion to the desired fluorine-18 labelled com-
pound was expressed as percentage of total radioactivity in the
TLC lane. Volatile side products were neglected, because the head
space of the vial did not contain any radioactivity. The radiochem-
ical yield (RCY) after 45 s of heating was 60–80%. The reaction mix-
2. Results and discussion
2.1. Chemistry
Reference compounds 1, 2 and nortropane 3 were synthesised
from natural cocaine as published somewhere else.8 The LBT999
labelling precursor 8-(4-methanesulfonyloxy-but-2-enyl)-3-exo-p-
tolyl-8-aza-bicyclo[3.2.1] octane-2-exo-carboxylic acid methyl
ester (6) was obtained from nortropane 4 via alkylation with 4-
bromo-but-2-ene-1-ol in 74% yield, followed by Lewis-acid cata-
lysed esterification with methanesulfonyl anhydride in 67% yield
(Scheme 1). The labelling precursor for PR04.MZ, namely 8-(4-meth-
anesulfonyloxy-but-2-ynyl)-3-p-tolyl-8-aza-icyclo[3.2.1]octane-
ture was then taken into a automated syringe containing 300 ll of
sterile water and directly transferred into the sample loop of a
semipreparative HPLC system. Three to ten percent of the starting
activity routinely remained in the microwave reaction vessel.
18F
MsO
18F
O
O
N
O
N
N
b
O
O
O
9 - 17 %
60 - 80 %
18F]1
[
18F]1
7
[
Scheme 2. [18F]Fluorination of PR04.MZ precursor 7; conventional conditions. Reagents and conditions: (a) 120 °C, MeCN, K2CO3, [K+ꢀK222][18F]FÀ, 3 min; (b) 175 °C, 18 bar,
MeCN, K2CO3, [K+ꢀK222][18F]FÀ, 0.75 min.

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P. J. Riss, F. Roesch / Bioorg. Med. Chem. 17 (2009) 7630–7634
tonic PBS was used for elution of the product. A sterile filter was
150 W
200 W
serially connected to the cartridge and the [18F]PR04.MZ contain-
ing PBS was collected in a multi injection vial. Absorption of radio-
activity on the filter pad was negligible. At this point an aliquot of
the sterile filtered solution was guided into a screw-cap vial, con-
taining 1 ml of 30% ammonium formate buffer in MeCN. This sam-
ple was used for quality control. Less than 5% of the trapped
radioactivity remained on the SCX-cartridge. The radiotracer for-
mulation was a colourless, clear solution with a pH-value of 7.3.
Quality control by analytical HPLC proved, that [18F]1 was >98%
radiochemically pure (1, tR: 10 min). Chemical impurities were be-
CsF 250 W
255 W
TBAF 250 W
80
60
40
20
0
low 2.4
tected in the tracer formulation. Specific radioactivity was
89 45 GBq
molÀ1 at end of synthesis (EOS). The radiotracer re-
mained stable in this solution for more than 240 min.
18F]LBT999 ([18F]2) was obtained under the same conditions in
lg/ml. The non-radioactive labelling precursor was not de-
l
[
27 2% non-decay-corrected yield. The retention time for prepara-
tive purification was 8 min, and the radiochemical purity was com-
parable to the radiochemical purity of [18F]1. Interestingly, the
total radioactivity incorporation was remarkably lower in this case.
2.3. Comparison of fluoride sources
0
10
20
30
40
50
Time / s
Initially, the microwave enhanced method was optimised using
the well established potassium KryptofixÒcryptate [18F]FÀ complex
([K+ꢀK222]18FÀ). The reaction outcome, as percentage of total 18F
radioactivity that was converted to the desired product, was deter-
mined as a function of reaction time, precursor amount, microwave
energy and temperature. During these investigations it was found,
that the reaction between the [18F]fluoride ion and precursors 5
and 7 proceeded very fast. Almost quantitative incorporation of
the employed fluoride occurred within the first 45 s. Upon pro-
longed heating, the product degraded to a polar side product. The
latter has also been found with thermal heating. Dollé et al. de-
scribe a similar effect within the radiosynthesis of LBT999 with
prolonged heating leading to product degradation. 5a In the present
case, a maximum radioactivity incorporation of 80% was observed
with [K+ꢀK222][18F]FÀ.
Figure 2. Labelling kinetics of [18F]PR04.MZ in a CEM DiscoverÒ focussed micro-
wave reactor (errors are 1 S.D.: n = 6–9). Settings: Nmax = 175 °C, pressure reaction,
pmax = 1.8 Â 106 Pa, moderate stirring, no cooling.
Later on it was found, that the bulk of n.c.a. fluoride had been
incorporated into the molecule, at this point. Prolonged heating re-
sulted in thermal degradation to form a polar side product, cf.
Figure 3. The latter eluted shortly after the HPLC inject-signal
and displayed up to 20% of starting radioactivity. [18F]1 eluted after
a reasonable purification time of tR: 14.0 0.5 min. The product
fraction did not contain chemical impurities in concentrations
above 2.4 lg/ml. In some runs, a non-significant by-product was
formed which was assigned to the saponified acid of [18F]PR04.MZ.
For concentration and isolation of the radiotracer, the product
fraction was diluted with water and passed through a proton-
loaded Merck-Lichrolut SCX strong cation exchanger. About 95%
of the total radioactivity were retained on the SCX resin. Subse-
quently, the cartridge was washed with 3 ml of water. Sterile, iso-
The reactions with [18F]CsF and [18F]TBAF were performed
analogously to the conditions described above. For comparison,
labelling was performed under thermal conditions with all three
fluoride sources. Representative results are summarised in
Figure 4.
Virtually, the use of [18F]CsF gave the highest radiochemical
yields. Unfortunately, [18F]CsF proved to be less stable with regard
to the reaction outcome than [18F]TBAF and K[18F]FÀ Krypto-
70
60
50
40
30
20
100
microwave
conventional
80
60
40
20
0
0
1
2
3
4
5
6
7
Time / min
CsF
TBAF
K222
Figure 3. [18F]1 product degradation under prolonged reaction time (errors are
S.D.: n = 3).
1
Figure 4. Comparison of maximum yields ( 1 S.D.) of [18F]1 after of microwave
irradiation (1 min, n = 6–9) and conventional heating (20 min, n = 4) in MeCN.

P. J. Riss, F. Roesch / Bioorg. Med. Chem. 17 (2009) 7630–7634
7633
fixÒ222 cryptate. Thus, although the highest yields were ascribed
to CsF, this salt also claimed the lowest reliability and low
reproducibility.
3.03–2.92 (m, 2H), 2.90–2.77 (m, 2H), 2.89 (s, 3H, SO2CH3), 2.56
(dt, J = 12.5 Hz, J = 2.9 Hz, 1H), 2.28 (s, 3H), 2.10–1.90 (m, 2H),
1.78–1.55 (m, 3H). 13C NMR: (100 MHz, CDCl3) d (in ppm): 172.1,
140.0, 135.2, 134.5, 128.6, 127.2, 126.2, 125.2, 65.8, 62.3, 61.3,
55.0, 52.7, 50.9, 38.1, 34.1, 33.8, 26.2, 26.0, 21.0. Anal. Calcd C,
61.89; H, 7.17; N, 3.44. Found: C, 61.86; H, 6.89; N, 3.40. MS (FD)
407.2 (100); C21H29NO5S requires 407.1766.
[
18F]TBAF proved a better reliability, in addition the reaction
worked well in low boiling THF and almost equal results were
found in MeCN. However, the total reaction yields were substan-
tially lower than with the standard [18F]fluoride source K[18F]FÀ
KryptofixÒ222 cryptate. For these reasons, we conclude, that
K[18F]FÀ KryptofixÒ222 cryptate is the most efficient source in
microwave-assisted radiofluorinations of tropane-analogue DAT
ligands.
4.3. 2-exo-Carboxymethyl-3-(4-methylphenyl)-8–(4’-hydroxy-
but-2-yne-1-yl)-8-azabicyclo [3.2.1]octane, 6
Nortropane 3 (260 mg, 1 mmol) was added to 1.05 equiv of Di-
PEA, dissolved in acetonitrile (5 ml). 4-Hydroxybut-2-yne-1-yl
chloride was added subsequently and the reaction mixture was
heated to 75 °C for 13 h. After completion of the reaction the reac-
3. Conclusions
[
18F]PR04.MZ and [18F]LBT999 have been prepared using a no-
vel, highly efficient and time effective microwave-assisted label-
tion mixture was concentrated to approximately 500 ll, which
ling method. Radiosynthesis of
[
18F]PR04.MZ was performed
were directly transferred to a silical gel column (20 g) and eluted
with MeOH/CHCl3 1:6, 89% yield. 1H NMR: (300 MHz, CDCl3) d
(in ppm): 7.16 (d, J = 8 Hz, 2H, ArH), 7.08 (d, J = 8 Hz, 2H, ArH),
4.26 (t, J = 2 Hz, 2H), 3.92 (br s, 1H), 3.53 (s, 3H, OCH3), 3.51 (br
s, 1H), 3.21 (dt, J = 2 Hz, J = 16 Hz, 1H), 3.09 (dt, J = 2 Hz, J = 16 Hz,
1H), 3.02 (dt, J = 12.7 Hz, J = 5 Hz, 1H), 2.94 (‘t’, J = 4 Hz, 1H), 2.62
(td, J = 2.9 Hz, J = 12.5 Hz, 1H), 2.30 (s, 3H, CH3), 2.20–2.09 (m,
1H), 2.07–1.96 (m, 1H), 1.83–1.60 (m, 3H). 13C NMR: (100 MHz,
CDCl3) d (in ppm): 172.1, 139.7, 135.3, 129.0, 128.6, 127.2, 82.8,
81.7, 62.6, 60.9, 52.8, 51.2, 50.9, 42.9, 34.1, 33.7, 25.9, 25.8, 21.0.
Anal. Calcd C, 73.37; H, 7.70; N, 4.28. Found: C, 73.71; H, 7.78; N,
4.5. MS (FD) 327.2 (100); C20H25NO3 requires 327.1834.
under pressure in a CEM discoverÒ focussed microwave reactor.
The use of direct molecular heating significantly increased the
radiochemical yield and remarkably reduced the reaction time.
[
18F]1 was thus rapidly formed within the first 45 s of microwave
irradiation. Interestingly, when a high microwave-intensity was
maintained the product degraded. The formulated tracer was ob-
tained in a non-decay-corrected yield of 34 2% in a radiochemical
purity >98% and a specific activity of 89 45 GBq l .
molÀ1
Similar conditions have been applied to synthesise [18F]LBT999
[
18F]2. The yields of 27 2% obtained are significantly less com-
pared to [18F]1, but much higher compared to the data for [18F]2
described earlier.4b,5a
4.4. 2-exo-Carboxymethyl-3-(4-methylphenyl)-8–(4’-methane-
sulfonyloxybut-2-yne-1-yl)-8-azabicyclo[3,2,1]octane, 7
4. Experimental
Alcohol 6 was added to 1.05 equiv of triethylamine dissolved in
dry dichloromethane (1 ml/mmol) and cooled to 0 °C. After stirring
at 0 °C for 30 min, neat methanesulfonyl chloride 1 equiv was added
drop by drop without interruption. After completion of the addition
the reaction mixture was stirred for 5 additional minutes when all
alcohol had been consumed (TLC monitoring). The reaction was
quenched by the addition of cold water (1 ml/mmol) with vigorous
stirring. Subsequently, thereactionmixturewas dilutedwithdichlo-
romethane (1 ml/mmol). After separation of the aqueous phase, the
reaction mixture was washed with 5% sodium carbonate solution,
dried over potassium carbonate and concentrated in vacuo to leave
a residue that was quickly chromatographed on silica gel to yield
precursor 8 in 90% yield. 1H NMR: (300 MHz, CDCl3) d (in ppm):
7.15 (d, J = 8 Hz, 2H, ArH), 7.08 (d, J = 8 Hz, 2H, ArH), 4.23 (t,
J = 2 Hz, 2H), 3.91 (br s, 1H), 3.52 (s, 3H, OCH3), 3.50 (br s, 1H), 3.22
(dt, J = 2 Hz, J = 16 Hz, 1H), 3.10 (dt, J = 2 Hz, J = 16 Hz, 1H), 3.02 (dt,
J = 12.5 Hz, J = 5 Hz, 1H), 2.93 (‘t’, J = 4 Hz, 1H), 2.89 (s, 3H, SO2CH3),
2.61 (td, J = 2.9 Hz, J = 12.5 Hz, 1H), 2.29 (s, 3H, CH3), 2.21–2.10 (m,
1H), 2.07–1.95 (m, 1H), 1.83–1.59 (m, 3H).13C NMR: (100 MHz,
CDCl3) d (in ppm): 172.1, 139.7, 135.3, 129.0, 128.6, 127.2, 82.7,
81.7, 62.5, 60.9, 52.8, 51.2, 50.9, 42.9, 37.6, 34.1, 33.7, 25.85, 25.8,
21.0. Anal. Calcd C, 62.20; H, 6.71; N, 3.45; O, 19.73; S, 7.91. Found:
C, 61.9; H, 6.91; N, 3.24. MS (FD) 405.2 (100); C21H27NO5S requires
405.1610.
4.1. Methyl 8-((E)-4-hydroxybut-2-enyl)-3-p-tolyl-8-aza-
bicyclo[3.2.1]octane-2-carboxylate, 5
DiPEA (129.1 mg, 1 mmol) was added to nortropane 3 (260 mg,
1 mmol) and 4-bromobut-2-enol dissolved in acetonitrile (5 ml).
The reaction mixture was heated to 45 °C for 3 h. After completion
of the reaction, the reaction mixture was concentrated to approxi-
mately 500 ll, which were directlytransferred to a silical gel column
(20 g) and eluted with MeOH/CHCl3 1:6, 79% yield. 1H NMR:
(300 MHz, CDCl3) d (in ppm): 7.13 (d, J = 8 Hz, 2H, ArH), 7.05 (d,
J = 8 Hz, 2H, ArH), 5.78–5.57 (m, 2H), 4.1 (t, J = 5,5 Hz, 2H), 3.65 (br
s, 1H), 3.47 (s, 1H, OCH3), 3.40 (br s, 1H), 3.03–2.92 (m, 2H), 2.90–
2.77 (m, 2H), 2.57 (dt, J = 12.5 Hz, J = 2.9 Hz, 1H), 2.27 (s, 3H), 2.10–
1.91 (m, 2H), 1.77–1.55 (m, 3H). 13C NMR: (100 MHz, CDCl3) d (in
ppm): 172.0, 139.9, 135.2, 134.4, 128.6, 127.2, 126.3, 126.0, 65.8,
62.3, 61.3, 54.9, 52.7, 50.9, 34.1, 33.8, 26.1, 25.9, 21.0. Anal. Calcd
C, 72.92; H, 8.26; N, 4.25; O, 9.71. Found: C, 72.79; H, 8.03; N, 4.53.
MS (FD) 329.2 C20H27NO3 requires 329.1991.
4.2. (E)-4-(2-(Methoxycarbonyl)-3-p-tolyl-8-aza-bicyclo[3.2.1]-
octan-8-yl)but-2-enyl methane sulfonate, 5
Alcohol 4 was dissolved in dichloromethane and methanesulfo-
nyl anhydride was added. The reaction was initiated by the addition
of 1 mol % of ytterbiumtriflate. Afterstirringthereaction mixturefor
5 h, the reaction was terminated and the reaction mixture was
concentrated in vacuo. The residue was re-dissolved in a small
amount of dichloromethane and purified by flash chromatography
on a silica gel column (hexanes/diethylether, 8:2, 10% triethyl-
amine). Compound 5 (63%) was obtained as colourless crystals.
1H NMR: (300 MHz, CDCl3) d (in ppm): 7.15 (d, J = 8 Hz, 2H,
ArH), 7.08 (d, J = 8 Hz, 2H, ArH), 5.80–5.61 (m, 2H), 4.08 (t,
J = 6 Hz, 2H), 3.66 (br s, 1H), 3.49 (s, 1H, OCH3), 3.40 (br s, 1H),
4.5. Representative protocol for microwave-assisted
fluorination of tropanes
[
18O]H2O containing [18F]fluoride was passed through a waters
accel plus light QMA strong anion exchange cartridge, precondi-
tioned with 1 M potassium carbonate solution (10 ml) followed
by sterile water (20 ml). TBAHCO3 and CsCO3 were used to obtain
TBA[18F]F and Cs[18F]F. The trapped fluoride was eluted in an ace-
tonitrile solution containing cryptand KryptofixÒ222 (15 mg,

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P. J. Riss, F. Roesch / Bioorg. Med. Chem. 17 (2009) 7630–7634
40
lmol) and 1 M potassium carbonate solution (15
ll, 15
lmol),
ethylamine. An aliquot of 20 ll was withdrawn from the sterile fil-
directly into a glass micro reaction vessel (SupelcoÒ reactivial,
5 ml). A reduced pressure of 100 mbar was applied to the tightly
closed vial in an oil bath at 90 °C while a stream of nitrogen
(300 ml/min) was applied. After the complete evaporation addi-
tional MeCN (1 ml) was added (two times). After evaporation of
the third amount of MeCN, the nitrogen inlet was closed and full
vacuum was applied to the reaction vial for 3 min. Afterwards,
the dried solution was re-dissolved in MeCN (1 ml) and stirred at
room temperature for 5 min.
tered formulated tracer solution and diluted with eluent.
Approximately 50 kBq of this solution were injected into the HPLC.
The product which was >98% radiochemically pure eluted after
10 min. The clear, colourless tracer formulation did not contain
chemical impurities above 2.4 lg/ml.
Acknowledgements
This work was financially supported by DFG-grant RO985/21.
The authors are grateful to the ‘Fonds der Chemischen Industrie’
for the donation of various chemicals and solvents.
This stock solution was transferred into a CEM pressure-reac-
tion vial (PyrexÒ) containing PR04.MZ labelling precursor
(4.5 mg, 11 lmol) and an 8 mm stirring magnet. The vial was
placed inside the cavity of a CEM DiscoverÒ laboratory microwave.
After an irradiation with microwaves at 250 W for approximately
45 s the preset maximum pressure and temperature were reached.
After cooling down of the vial for 90 s in a stream of air, the reac-
tion mixture was taken up into a syringe containing cold water
References and notes
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Curr. Pharm. Des. 2000, 6, 1611; (d) Laakso, A.; Bergman, J.; Haaparanta, M.;
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(300 ll) and directly injected into a Dionex p680 HPLC-system
equipped with a Dionex UVD 170U UV-detector (k = 254 nm) and
a Raytest Gabi StarÒ radioactivity detector. Dionex Chromeleon
software was used for UV-data analysis and Raytest Gina-star soft-
ware was used for radioactivity detection. A semipreparative Phe-
nomenex Luna RP 18 10
l HPLC column was used as stationary
phase (250 Â 10 mm). Ammonium acetate buffer (0.1 M) (pH
4.7)/acetonitrile (4:6) at a flow rate of 4.7 ml/min was used as mo-
bile phase. The product fraction was collected after a total reten-
tion time of 14 min, taken up into a syringe containing water
(10 ml) and passed through a Merck Lichrolute SCX strong cation
exchange cartridge (200 mg), preconditioned with 1 M hydrochlo-
ric acid (5 ml) and neutralised with water (20 ml). Subsequently,
the cartridge was washed with water (3 ml). At this point usually
>95% of radioactivity were trapped on the cartridge.
4.6. Formulation of [18F]PR04.MZ
The radiotracer was eluted with sterile phosphate buffered sal-
ine (5 ml). A MillexÒ-GS 0.22
lm sterile filter (‘blue’) was serially
5. (a) Dollé, F.; Hinnen, F.; Emond, P.; Mavel, S.; Mincheva, Z.; Saba, W.; Schöllhorn-
Peyronneau, M.-A.; Valette, H.; Garreau, L.; Chalon, S.; Halldin, C.; Helfenbein, J.;
Legaillard, J.; Madelmont, J.-C.; Deloye, J.-B.; Bottlaender, M.; Guilloteau, D. J.
Labelled Compd. Radiopharm. 2006, 49, 687; (b) Klok, R. P.; Klein, P. J.; Herscheid,
J. D. M.; Windhorst, A. D. J. Labelled Compd. Radiopharm. 2006, 49, 77; (c) Wuest,
F.; Berndt, M.; Strobel, K.; van den Hoff, J.; Peng, X.; Neumeyer, J. L.; Bergmann,
R. Bioorg. Med. Chem. 2007, 15, 4511.
connected to the cartridge, to obtain the injectable radiotracer for-
mulation containing [18F]PR04.MZ in a non-decay-corrected yield
of 32–36% calculated from [18F]fluoride ion starting activity. The
specific activity of product [18F]1 at end of synthesis was 42–
135 GBq/l
mol, depending on the status of [18F]fluoride ion.
6. (a) Chalon, S.; Hall, H.; Saba, W.; Garreau, L.; Dollé, F.; Halldin, C.; Emond, P.;
Bottlaender, M.; Deloye, J.-B.; Helfenbein, J.; Madelmont, J.-C.; Bodard, S.;
Mincheva, Z.; Besnard, J.-C.; Guilloteau, D. J. Pharmacol. Exp. Ther. 2006, 317, 147;
(b) Riss, P. J.; Hummerich, R.; Schloss, P. Org. Biomol. Chem. 2009, 7, 2688; (c)
Riss, P. J.; Debus, F.; Hummerich, R.; Schmidt, U.; Lueddens, H.; Schloss, P.;
Roesch, F. ChemMedChem 2009, 4, 1480; (d) Riss, P. J.; Hooker, M. J.; Alexoff, D.;
Kim, S.-W.; Fowler, J. S.; Rösch, F. Bioorg. Med. Chem. Lett. 2009, 19, 4343.
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239; (b) Dollé, F. Curr. Pharm. Des. 2005, 11, 3221; (c) Hayes, B. Microwave
Synthesis; CEM Publishing: Matthews, 2002; (d) Lidström, P.; Tierney, J.;
Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225.
4.7. Quality control of [18F]PR04.MZ
The radiochemical purity and the specific radioactivity were
determined using a Sykam S1100 HPLC-pump, a Berthold Flow Star
LB 513 radioactivity detector and a Knauer K 2501 UV-detector
(254 nm). A Phenomenex Luna RP 18 5
l
(250 Â 4.6 mm) analytical
HPLC column was used as stationary phase at a flow rate of 1 ml/
min. The mobile phase consisted of 30% 0.05 M ammonium acetate
buffer at pH 4.6 in acetonitrile. The results were validated using
7 mM ammonia/acetonitrile 1:4 at 1 ml/min on the same column
and by silica gel TLC: hexanes/diethylether, 4:1 containing 10% tri-
8. (a) Meltzer, P. C.; Liang, A. Y.; Brownell, A. L.; Elmaleh, D. R.; Madras, B. K. J.
Med. Chem. 1993, 36, 855; (b) Xu, L.; Trudell, M. L. J. Heterocycl. Chem. 1996,
33, 2037.