18F-labelling of a potent nonpeptide CCR1 antagonist: Synthesis of 1-(5-chloro-2-{2-[(2r)-4-(4-[18F]fluorobenzyl)-2-methylpiperazin- 1-yl]-2-oxoethoxy}phenyl)urea in an automated module

Article abstract of DOI:10.1002/jlcr.1023

The synthesis of 1-(5-chloro-2-{2-[(2R)-4-(4-[¹⁸F]fluorobenzyl)- 2-methylpiperazin-1-yl]-2-oxoethoxy}phenyl)urea ([¹⁸F]4), a potent nonpeptide CCR1 antagonist, is described as a module-assisted two-step one-pot procedure. The final p

Full text of DOI:10.1002/jlcr.1023

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2006; 49: 253–262.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.1023
Research Article
18F-labelling of a potent nonpeptide CCR1 antagonist:
synthesis of 1-(5-chloro-2-{2-[(2R)-4-(4-
[18F]fluorobenzyl)-2-methylpiperazin-1-yl]-2-
oxoethoxy}phenyl)urea in an automated module
P. Mading1,*, F. Fuchtner1, B. Johannsen1, J. Steinbach2, C. S. Hilger3,
¨
¨
M. Friebe3, M. Halks-Miller4, R. Horuk4 and R. Mohan5
1 Institut fur Bioanorganische und Radiopharmazeutische Chemie, Forschungszentrum
¨
Rossendorf, Postfach 510119, 01314 Dresden, Germany
2 Institut fur Interdisziplinare Isotopenforschung, Permoserstrasse 15, 04318 Leipzig,
¨
¨
Germany
3 Schering AG, Mullerstr. 170-178, 13342 Berlin, Germany
¨
4 Berlex Biosciences, Departments of Chemistry, Pharmacology and Immunology,
15049 San Pablo Avenue, Richmond, CA 94806, USA
5 X-Ceptor Therapeutics, Incorporated, 4757 Nexus Centre Drive, Suite 200, San Diego,
CA 92121, USA
Summary
The synthesis of 1-(5-chloro-2-{2-[(2R)-4-(4-[18F]fluorobenzyl)-2-methylpiperazin-1-
yl]-2-oxoethoxy}phenyl)urea ([18F]4), a potent nonpeptide CCR1 antagonist, is
described as a module-assisted two-step one-pot procedure. The final product was
obtained utilizing the reductive amination of the formed 4-[18F]fluorobenzaldehyde (2)
with a piperazine derivative 3 and sodium cyanoborohydride. After HPLC
purification of the final product [18F]4, its solid phase extraction, formulation and
sterile filtration, the isolated (not decay-corrected) radiochemical yields of [18F]4 were
between 7 and 13% ðn ¼ 28Þ. The time of the entire manufacturing process did not
exceed 95 min. The radiochemical purity of [18F]4 was higher than 95%, the chemical
purity 560% and the enantiomeric purity >99.5%. The specific radioactivity was in
the range of 59–226 GBq/mmol at starting radioactivities of 23.6–65.0 GBq
[18F]fluoride. Copyright # 2006 John Wiley & Sons, Ltd.
Key Words: positron emission tomography; Alzheimer’s disease; 4-[18F]fluoro-
benzaldehyde; reductive amination; CCR1 antagonist; automated module synthesis
*Correspondence to: P. Mading, Forschungszentrum Rossendorf, Abt. FWBP, Postfach 510119, 01314
¨
Dresden, Germany. E-mail: p.maeding@fz-rossendorf.de
Copyright # 2006 John Wiley & Sons, Ltd.
Received 15 March 2005
Revised 5 June 2005
Accepted 15 October 2005

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P. MADING ET AL.
254
Introduction
The hydrochloric acid salt of 1-(5-chloro-2-{2-[(2R)-4-(4-fluorobenzyl)-2-
methyl-piperazin-1-yl]-2-oxoethoxy}phenyl)urea with the abbreviation BX
471 was described as a potent, selective and orally active nonpeptide
antagonist of the CC chemokine receptor-1 (CCR1).1 The CCR1 is a prime
therapeutic target for treating autoimmune diseases, e.g. chronic inflammatory
diseases. It was further discovered that CCR1 is upregulated in dystrophic
neurites found in and around the amyloid plaques of Alzheimer’s disease.2
These CCR1-positive plaques were seen in very early stages of dementia and
were also found to increase as the degree of dementia increased.2 In healthy
brain, expression of CCR1 was negligible.2
These data have encouraged us to develop a radiolabelled derivative of the
CCR1 antagonist BX 471, mentioned above as a possible imaging agent for
the reliable and early diagnosis of Alzheimer’s disease by positron emission
tomography (PET).3 Accordingly, we have searched for a method for labelling
BX 471 with the positron-emitting radionuclide 18F ðt1=2 ¼ 109:8 minÞ, which
can be carried out as one-pot reaction in an automated module. The 18F-
labelled compound 1-(5-chloro-2-{2-[(2R)-4-(4-[18F]fluorobenzyl)-2-methylpi-
perazin-1-yl]-2-oxoethoxy}phenyl)urea ([18F]4) has been evaluated in a human
study for its potential to image Alzheimer’s disease by PET.
Results and discussion
Starting from a suitable precursor, the 1-(5-chloro-2-{2-[(2R)-2-methylpiper-
azin-1-yl]-2-oxoethoxy}phenyl)urea (3)3, [18F]4 was synthesized using a
published method of reductive amination of 4-[18F]fluorobenzaldehyde (2)4,5
with the piperazine derivative 3 and sodium cyanoborohydride according to
Scheme 1.
The first step of this labelling way is the synthesis of the important 18F-
labelling moiety 2 by aromatic nucleophilic substitution of [18F]fluoride on
CH2
H
C
NH
N
18F
O
O
N
O
O
N
O
18F
2
H
N
H
N
NH2
NaBH3CN; AcOH
NH2
O
O
Cl
Cl
3
[
18F]4
Scheme 1. Synthesis of [18F]4
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18F-LABELLING OF A POTENT NONPEPTIDE CCR1 ANTAGONIST
255
H
C
H
C
18F-/ K222/K2CO3
+
18F
O
NMe3
O
TfO-
2
1
Scheme 2. Synthesis of 4-[18F]fluorobenzaldehyde (2)
O
O
Cl
Cl
i-Pr2NEt
CH2Cl2
HN
N
O
HN
Cl
(BOC)2O
+
i-Pr2NEt, CH2Cl2
NBOC
NBOC
NH
OH
O
NH
NH2
Cl
O
O
N
N
O
HCl
NBOC
NH
NH
NH
NH
Cl
EtOAc Cl
K2CO3, DMSO
· HCl
O
NH2
O
O
O
N
NH3/H2O
EtOAc
NH
NH
NH2
Cl
O
3
OH
OH
NH
H2O
TMSNCO
+
THF
NH2
Cl
Cl
O
NH2
Scheme 3. Synthesis of the piperazine derivative 3
4-trimethylammonium-benzaldehyde triflate (1) according to Scheme 2 using
the phase transfer catalyst Kryptofix 222 and potassium carbonate.4–7
The starting material 1 was prepared by conversion of 4-dimethylamino-
benzaldehyde with methyl triflate as given by Wilson et al.,4 with slight
modifications.
The synthesis of 2 and its reductive amination with a secondary amine can
be carried out as a one-pot process. Such conversions are described for 18F-
labelling of fluorodexetimides4 and fluorotropaprides5. We adapted this
method for our purposes.
The second precursor, the piperazine derivative 3, was prepared by a multi-
step synthesis starting from (R)-(ꢀ)-2-methylpiperazine (Scheme 3)3 via the
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P. MADING ET AL.
256
hydrochloric acid salt of 3, followed by liberation of the free base 3 by
treatment with ammonium hydroxide solution.
Manual labelling procedures
Similar to the conditions of the one-pot experiments given by Wilson et al.4
(reactions of 7–8.6 mmol 1 with dried [18F]fluoride/K2CO3/K222 mixtures) we
have tested the reductive amination of the formed 2 with the piperazine
precursor 3 using sodium cyanoborohydride and acetic acid in DMSO (1208C,
10 min reaction time). Unfortunately, the reaction did not yield [18F]4 and 2
remained unchanged.
DMF was the solvent of choice for the two-step one-pot procedure, i.e. the
optimization of the reaction conditions was carried out in DMF. DMF was
also successfully used by Hamacher et al.8 for the synthesis of 2 using
electrochemically separated [18F]fluoride, Kryptofix 222/K2CO3 and 4-
nitrobenzaldehyde as precursor.
To reach satisfactory yields of 2, the nucleophilic substitution had to be
carried out using a relatively large amount of 1 (27–34 mmol) in 1 ml of DMF
and heating at 1208C for 10 min. In this way 76–78% 2 were obtained. The
crude 2 thus produced was subsequently reductively aminated with the
piperazine precursor 3, using sodium cyanoborohydride and acetic acid, in the
same vessel at 1208C for 10 min. Starting from 76–78% 2, the radiochemical
yield of [18F]4 in the reaction mixture was up to 52–53%. An equimolar ratio
between precursor 1 and 3 proved to be favourable for higher yields. Probably
the aldehyde group of the unconverted 1 is also reductively aminated with the
piperazine 3 as competitive reaction. Henceforth this procedure was adapted
to an automated module.
Labelling procedures in the module
A scheme of the automated module used for the synthesis of [18F]4 is
illustrated in Figure 1. The capability of this module includes the radio-
synthesis of 2 and its reductive amination with the piperazine derivative 3 to
[18F]4, the HPLC purification of the final product [18F]4, its solid phase
extraction and formulation.
In order to improve HPLC purification, the amounts of the precursors 1 and
3 were reduced to 22–23 mmol compared with the manual synthesis. The
amounts of the solvents for precursor 3 (HOAc) and NaBH3CN (DMF) were
increased to reduce the losses of the transfer to the reactor (see Experimental).
Best purification of crude [18F]4 was obtained with a semipreparative C18
Kromasil column (Knauer) using a mixture of MeCN : EtOH : phosphate
buffer (pH7)=45 : 5 : 50 as eluent. The separated HPLC fraction of [18F]4
was diluted with water and collected by solid phase extraction9 on an
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18F-LABELLING OF A POTENT NONPEPTIDE CCR1 ANTAGONIST
257
Figure 1. Schematic diagram of the module for synthesizing [18F]4
RP-18-cartridge. After washing the cartridge with water, [18F]4 was eluted with
ethanol. Then the ethanolic solution of [18F]4 and a saline solution were
transferred successively through a sterile filter to obtain the formulated final
solution of [18F]4. The colourless, sterile and pyrogene-free solution was ready
for injection.
The isolated (not decay-corrected) radiochemical yield of [18F]4 ranged from
7 to 13% ðn ¼ 28Þ in its final formulation. The synthesis time of the entire
manufacturing process did not exceed 95 min. The radiochemical purity of
[18F]4 was higher than 95%. The chemical purity ranged from 60 to 90% based
on UV absorption at 240 nm. Nontoxicity of [18F]4 solutions with such a
composition was confirmed by a toxicity study. The specific radioactivity was
in the range of 59–226 GBq/mmol at starting radioactivities of 23.6–65.0 GBq
[18F]fluoride.
1-(5-chloro-2-{2-[(2R)-2-methylpiperazin-1-yl]-2-oxoethoxy}phenyl)urea (3)
was used as starting material to yield the R-enantiomerically pure [18F]4.
Although racemization was not expected during the reductive amination of 2
with the piperazine derivative 3, we checked this issue by HPLC with a special
chiral column. Therefore, the S-enantiomer of 4 (1-(5-chloro-2-{2-[(2S)-4-(4-
fluorobenzyl)-2-methylpiperazin-1-yl]-2-oxoethoxy}-phenyl)urea) was synthe-
sized and used as reference substance in addition to the desired R-enantiomer
of 4 (1-(5-chloro-2-{2-[(2R)-4-(4-fluorobenzyl)-2-methylpiperazin-1-yl]-2-oxo-
ethoxy}phenyl)urea). Briefly, both 4 (R and S) were synthesized by 4-
fluorobenzylation of the piperazine precursor 3 in its R-(ꢀ) or S-(+) form
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P. MADING ET AL.
258
starting form (R)-(ꢀ)-2- or (S)-(+)-2-methylpiperazine which were commer-
cially available.3,10
No racemization was observed after synthesis of [18F]4. The enantiomeric
purity of the final [18F]4 was >99.5%.
Experimental
1H-NMR spectra were recorded on a Bruker AC 300 system. IR-data were
collected on a Nicolet NIC 710 IR spectrometer with KBr compound pellets.
MS spectra were measured on a VG Micromass Autospec system.
To determine the extent of the reaction conversion, the radiochemical and
chemical purity of the reaction products and the specific radioactivity of [18F]4,
an HPLC system (JASCO) was used, including a pump, a Rheodyne injector
with a 20 ml loop, a LiChrospher WP300 RP-18 column (5 mm, 250 mm ꢁ 3 mm
+ precolumn 4 mm ꢁ 4 mm, Merck) and a UV detector (240 nm) coupled in
series with a radioactivity detector FLO-ONE\Beta 150TR (Canberra
Packard). The HPLC analyses were carried out at a flow rate of 0.5 ml/min
with the following eluent: 0–20 min: 0.2% TFA in MeCN:water=23:77, 20–
30 min: 0.2% TFA in MeCN. The specific radioactivity of [18F]4 was
calculated by means of a calibration curve using different concentrations of
the nonradioactive R-enantiomer of 4 in relation to their UV absorbance
response and the final radioactivity of [18F]4.
Enantiomeric purity of [18F]4 was determined using a CHIRALCEL OJ-R
column (5 mm, 150 mm ꢁ 4.6 mm + precolumn 10 mm ꢁ 4 mm, Daicel) eluted
isocratically with 0.01 M HCOONH4 in MeCN:water=35:65 at a flow rate of
0.5 ml/min.
A module for nucleophilic fluorination from GE Medical Systems (former
Nuclear Interface, Munster, Germany) was modified in terms of program and
¨
hardware and used for the study. This module, outlined in Figure 1, is an
automatically operating, remote-controlled, closed system. It has been
installed in a hot cell to process highest radioactivity levels of [18F]fluoride.
Chemistry
1-(5-chloro-2-{2-[(2R)-2-methylpiperazin-1-yl]-2-oxoethoxy}phenyl)urea
(3). The hydrochloric acid salt of 3 was synthesized according to the
literature procedure,3 starting from enantiomerically pure (R)-(ꢀ)-2-methyl-
piperazine. The free base 3 was generated as follows.
1-(5-chloro-2-(2-[(2R)-2-methylpiperazin-1-yl]-2-oxoethoxy)phenyl)urea hy-
drochloride (200 mg, 0.554 mmol) was suspended in 150 ml of water.
Ammonium hydroxide solution (20 ml, 22%) was added and the aqueous
phase was extracted three times with 50 ml of ethyl acetate. The combined
organic extracts were dried over sodium sulphate, filtered and the solvent was
evaporated. The dry precipitate was re-dissolved in acetonitrile and
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18F-LABELLING OF A POTENT NONPEPTIDE CCR1 ANTAGONIST
259
recrystallized. Crystallization yielded 171 mg, 0.526 mmol (95%) of 3 as white
1
precipitate. Melting point: 139.0–141.58C; MS (CI): 327 (100%) M+H, H-
NMR (CDCl3, d, ppm): 1.2 (d, 3H, -CH-CH3), 2.5–2.9 (m, 2H + 2H, N-CH2-
CH2NH-), 3.0 (t, 1H, N-CH2-CH2NH-, ax), 3.75 (d, 1H, N-CH2-CH2NH-,
eq), 4.2 (q, 1H, N-CH-CH3), 4.8 (s, 2H, -O-CH2-CO), 5.9 (s, 2H, -NH2), 6.82
(dd, 1H, aryl), 6.88 (d, 1H, aryl), 7.95 (s,1H, -NH), 8.12 (s, 1H, -NH), 8.13 (s,
1H, aryl). IR (KBr, cmꢀ1): 3300–3500 (-NH2 and –NH), 1670 (carbonyl
(urea)), 1655 (carbonyl (amide)).
1-(5-chloro-2-{2-[(2R or 2S)-4-(4-fluorobenzyl)-2-methylpiperazin-1-yl]-2-ox-
oethoxy}-phenyl)urea (4 (R) and 4 (S) form). Compound 4 (R and S) was
synthesized according to the literature procedures described in,3,10 starting
from the enantiomerically pure (R)-(ꢀ)-2- or (S)-(+)-2-methylpiperazine via
precursor 3 (R or S form). The conversion of 3 (R or S form) with 4-
fluorobenzyl chloride yielded 4 (R and S):
To a solution of 1-(5-chloro-2-{2-[(2R or 2S)-2-methylpiperazin-1-yl]-2-
oxoethoxy}phenyl)urea (3 (R) or 3 (S)) (150 mg, 0.46 mmol) in 20 ml CH2Cl2
was added diisopropylethylamine (119 mg, 0.92 mmol), sodium iodide (68 mg,
0.46 mmol) and 4-fluorobenzyl chloride (67 mg, 0.46 mmol). The resulting
mixture was stirred at ambient temperature for 15 h. After the reaction was
completed, the reaction mixture was washed with water (3 ꢁ 20 ml) and brine,
then dried over Na2SO4, filtered and concentrated in vacuum to afford the raw
product as a white solid. The material was taken up in methylene chloride/
methanol and purified by column chromatography on silica gel (Mesh 60) and
methylene chloride /methanol (v/v 95:5) as the mobile phase. Purification
yielded a white solid (105 mg, 52% 4 as R-enantiomer); 101 mg, 51% 4 as S-
enantiomer)).
1
4, R-enantiomer: MS (EI): 434 (100%) M+, H-NMR (DMSO-D6, d,
ppm): 1.23 (d, 3H, -CH-CH3), 2.0 (t,d, 1H, N-CH2-CH2NH-, ax), 2.12 (d,d,
1H, N-CH2-CH2NH-, ax), 2.6 (d, 1H, N-CH2-CH2NH-, eq), 2.78 (d, 1H, N-
CH2-CH2NH-, eq), 3.12 (t, 1H, N-CH2-CH2NH-, ax), 3.41 (d, 1H, N-CH2-
C(aryl)), 3.5 (d, 1H, N-CH2-C(aryl)), 3.87 (d, 1H, N-CH2-CH2NH-, eq), 4.3
(q, 1H, N-CH-CH3), 4.82 (s, 2H, -O-CH2-CO), 6.0 (s, 2H, -NH2), 6.8 (dd, 1H,
aryl), 6.88 (d, 1H, aryl), 7.1 (t, 1H + 1H, aryl), 7.33 (d,d, 1H + 1H, aryl) 8.0
(s,1H, -NH), 8.12 (s, 1H, -NH), 8.13 (s, 1H, aryl), 8.15 (d, 1H, aryl). IR (KBr,
cmꢀ1): 3300–3500 (-NH2 and –NH), 1690 (carbonyl (urea)), 1655 (carbonyl
(amide)).
1
4, S-enantiomer: MS (EI): 434 (100%) M+, H-NMR (CDCl3, d, ppm):
1.2–1.4 (m, 3H, -CH-CH3), 2.0–2.2 (m, 1H + 1H, N-CH2-CH2NH-, ax + ax),
2.65 (d, 1H, N-CH2-CH2NH-, eq), 2.83 (br, 1H, N-CH2-CH2NH-, eq), 3.0–3.8
(m, 2H, N-CH2-C(aryl)), (m, 1H, N-CH2-CH2NH-), 4.3–4.8 (m, 2H, -O-CH2-
CO), (m, 1H, N-CH-CH3), 5.0 (s, 2H, -NH2), 6.8 (d, 1H, aryl), 6.85 (d,d, 1H,
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260
aryl), 7.0 (t, 1H + 1H, aryl), 7.3 (d,d, 1H + 1H, aryl), 8.27 (d, 1H, aryl), 9.0
(s, 1H, -NH). IR (KBr, cmꢀ1): 3300–3500 (-NH2 and –NH) 1690 (carbonyl
(urea)), 1655 (carbonyl (amide)).
Radiochemistry
Production of 18F. [18F]Fluoride was produced utilizing the PET cyclotron
‘Cyclone 18/9’ (IBA, Belgium). [18O]H2O was irradiated with protons
exploiting the 18O(p,n)18F nuclear reaction in a 1.5 ml target. The incident
energy of the protons was 18 MeV, the beam current was about 30 mA. After
irradiation, the [18O]H2O containing [18F]Fꢀ was transferred in a pneumatic
transport system from the cyclotron to the radiopharmaceutical laboratory.
The [18F]Fꢀ was separated from the [18O]H2O using an anion exchange
cartridge (Sep-Pak1 Light Waters AccellTM Plus QMA cartridge) in the
HCOꢀ3 form.
Manual synthesis of ½18Fꢂ4. The [18F]fluoride was eluted from the anion
exchange cartridge into a 5 ml reaction vessel using a solution of Kryptofix 222
(15 mg; 40 mmol) and potassium carbonate (2.77 mg; 20 mmol) in aqueous
acetonitrile (1.5 ml; 66% acetonitrile). The radioactive eluate was dried by
means of a vacuum and a nitrogen stream at 958C. The reaction mixture was
carefully dried by repeated addition and evaporation of anhydrous acetonitrile
(3 ꢁ 1 ml). After addition of a solution of 1 (8.5–10.8 mg; 27.1–34.5 mmol) in
1 ml anhydrous DMF, the well-sealed vessel was heated at 1208C for 10 min to
yield 2. Subsequently, the piperazine precursor 3 (9.2–12.4 mg; 28.1–38.0 mmol)
in 200 ml of acetic acid and the reducing agent NaBH3CN (4.0 mg; 64 mmol) in
100 ml of DMF were added to the reaction vial. The reductive amination was
performed by heating the reaction mixture at 1208C for 10 min.
Module-assisted one-pot synthesis of drug substance ½18Fꢂ4. Before starting the
one-pot synthesis, a cleaning program for the module was carried out. Then
the storage vessels 1–9 and the mixing vessel were supplied with the needed
starting materials. The starting activity of the [18F]fluoride deposited on the
anion exchange cartridge was measured. The synthesis program was started
with the elution of the [18F]fluoride from the anion exchange cartridge into the
reaction vessel using a solution of Kryptofix 222 (15 mg; 40 mmol) and
potassium carbonate (2.77 mg; 20 mmol) in aqueous acetonitrile (1.5 ml; 66%
acetonitrile). The radioactive eluate was dried by means of a vacuum and a
nitrogen stream at 958C. The reaction mixture was carefully dried by addition
and evaporation of 3 ml anhydrous acetonitrile. After addition of a solution of
1 (7.0 mg; 22.3 mmol) in 1 ml anhydrous DMF, the mixture was heated at
1208C for 10 min. Then the solution of the piperazine precursor 3 (ZK258394;
7.5 mg; 23 mmol) in 500 ml acetic acid and a solution of NaBH3CN (4.0 mg;
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18F-LABELLING OF A POTENT NONPEPTIDE CCR1 ANTAGONIST
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64 mmol) in 500 ml DMF were added in succession. After heating at 1208C for
10 min the reaction mixture was neutralized with 6 ml 1 M sodium hydroxide
and transferred to the HPLC injection loop. The purification was carried out
by means of a semipreparative HPLC system (JASCO), including a pump, a
Rheodyne injector with a 10 ml loop, a Knauer C18 column (Kromasil-100,
7 mm, Vertex, 300 mm ꢁ 8 mm with precolumn 30 mm ꢁ 8 mm) with the eluent
MeCN : EtOH : phosphate buffer (pH7)=45 : 5 : 50 (c[NaH2PO4]=2.6 mM;
c[Na2HPO4]=5.1 mM) at a flow rate of 2.5 ml/min and based on a UV
(240 nm) and a radioactivity detector.
The separated fraction of [18F]4 (retention time: about 36 min) from the
HPLC purification was transferred into the mixing vessel containing 20 ml
water. This water-diluted solution was passed through the reversed phase C18
cartridge (Chromafix C18 ec; Macherey-Nagel) for solid phase extraction of
[18F]4. After washing the cartridge with 15 ml water, [18F]4 was eluted with
1 ml ethanol into the elution vessel.
Formulation of the radiopharmaceutical under aseptic conditions. The ethanolic
solution of [18F]4 (1 ml) and a solution of 19 ml sterile, isotonic NaCl were
passed successively through a sterile filter ‘Millex-GP’ (Millipore) yielding the
final solution of [18F]4 (product vessel) as a clear, colourless, sterile, pyrogen-
free, isotonic NaCl solution with a total volume of 20 ml containing 5%
ethanol.
Acknowledgements
The authors would like to thank Mrs A. Hentges and Mr T. Krauꢀ for
technical assistance. Thanks are also due to the crew of the PET cyclotron,
Mr S. Preusche and Mr H. Roꢀ, for radionuclide production.
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