Automated synthesis of18F-labelled analogs of metomidate, vorozole and harmine using commercial platform

Article abstract of DOI:10.1002/jlcr.1742

Three ¹⁸F-labelled PET tracers, 2-[¹⁸F]fluoroethyl 1-[(1R)-1-phenylethyl]-1H-imidazole-5-carboxylate ([¹⁸F]FETO), 6-[(S)-(4-chlorophenyl)-( 1H)-1,2,4-triazol-1-yl)methyl]-1-(2-[ ¹⁸F]fluoroethyl)-1H-benzotriazole

Full text of DOI:10.1002/jlcr.1742

Research Article
Received 16 September 2009,
Revised 11 December 2009,
Accepted 14 December 2009
Published online 19 January 2010 in Wiley Interscience
(www.interscience.wiley.com) DOI: 10.1002/jlcr.1742
18
Automated synthesis of F-labelled analogs
of metomidate, vorozole and harmine using
commercial platform
Ã
Obaidur Rahman,a Maria Erlandsson,b Elisabeth Blom,b and
b
˚
Bengt Langstrom
¨
Three 18F-labelled PET tracers, 2-[18F]fluoroethyl 1-[(1R)-1-phenylethyl]-1H-imidazole-5-carboxylate ([18F]FETO), 6-[(S)-(4-chloro-
phenyl)-(1H)-1,2,4-triazol-1-yl)methyl]-1-(2-[18F]fluoroethyl)-1H-benzotriazole ([18F]FVOZ) and 7-[2-(2-[18F]fluoroethoxy)ethoxy]-
1-9H-b-carboline ([18F]FHAR) were synthesized by a one-step nucleophilic fluorination using the automated commercial
platform TRACERLab FXFN. The labelled products were obtained with 16–20% isolated decay corrected radiochemical yields
after 70–75 min synthesis time. The radiochemical and chemical purities were more than 98% in all cases. The synthesis using
commercial platform may make these tracers more accessible for clinical research.
Keywords: PET; 18F-labelling; [18F]FETO; [18F]FVOZ; [18F]FHAR; automated synthesis
platform TRACERLab FXFN (Figure 2). The radiochemical and
chemical purities were over 98% in all cases. Nucleophilic
Introduction
fluorination of the corresponding tosylate (for [18F]FETO,
Ethyl 1-[(1R)-1-phenylethyl]-1H-imidazole-5-carboxylate (etomidate),
6-[(S)-(4-chlorophenyl)-(1H)-1,2,4-triazol-1-yl)methyl]-1-methyl-1H-
[18F]FVOZ) or bromide (for [18F]FHAR) was used to synthesize
the compounds. The reaction was performed in an aprotic polar
benzotriazole (vorozole) and 7-methoxy-1-methyl-9H-b-carbo-
line (harmine) are inhibitors of three different enzymes
11b-hydroxylase,1 aromatase2 and monoamine oxidase-A,3
respectively. 11C-Labelled metomidate and etomidate have
shown to be valuable for PET imaging of the adrenal cortex
and its tumors4 and especially [11C]metomidate has been
successfully used to visualize adrenal cortical tumors.5,6,7,
11C-Labelled vorozole was used to study the in vivo bindings
of this compound in whole body8 and brain9 of rhesus monkey.
11C-Labelled analog of harmine was used to detect the high
expression of MAO-A in neuroendocrine gastroenteropancreatic
tumor.10 18F-Labelled analogs of these tracers, metomidate,11,12
vorozole13 and harmine14 have recently been developed and
evaluated as PET tracers. The previous papers described the
syntheses on the semi-automated platform Synthia15 developed
at Uppsala University PET center. Automated synthesis is
important for making PET tracer clinically useful. This paper
describes the fully automated synthesis of these three PET
tracers by one-step nucleophilic 18F-fluorination using the
18F
N
18F
N
O
O
N
N
N
N
N
N
Cl
[
18F]FVOZ
N
[
18F]FETO
18F
O
O
N
H
[
18F]FHAR
commercially available platform TRACERLab FXFN
.
Figure 1. Structures of [18F]FETO, [18F]FVOZ and [18F]HAR.
Results and discussion
aUppsala Applied Science Laboratory, GE Healthcare, SE-752 28 Uppsala,
Sweden
Three potential PET tracers 2-[18F]fluoroethyl 1-[(1R)-1-phenyl-
ethyl]-1H-imidazole-5-carboxylate ([18F]FETO), 6-[(S)-(4-chloro-
phenyl)-(1H)-1,2,4-triazol-1-yl)methyl]-1-(2-[18F]fluoroethyl)
-1H-benzotriazole ([18F]FVOZ) and 7-[2-(2-[18F]fluoroethoxy)-
ethoxy]-1-9H-b-carboline ([18F]FHAR) (Figure 1) were prepared
by one-step nucleophilic 18F-fluorination with radiochemical
yields of 16–20% (Table 1) using an automated commercial
bDepartment of Biochemistry and Organic Chemistry, Uppsala University,
Box 576, Husargatan 3, BMC, S-751 23 Uppsala, Sweden
*Correspondence to: Obaidur Rahman, Uppsala Applied Science Laboratory,
GE Healthcare, SE-752 28 Uppsala, Sweden.
E-mail: obaidur.rahman@ge.com
J. Label Compd. Radiopharm 2010, 53 169–171
Copyright r 2010 John Wiley & Sons, Ltd.

O. Rahman et al.
Table 1. Reaction conditions and radiochemical yields (RCY) of FETO, FVOZ and FHAR
Ã
Target compounds
Reaction conditions
Total synthesis time (min)
RCY (%), (n)
[18F]FETO
[18F]FVOZ
[18F]FHAR
1101C for 10 min
1401C for 15 min
1601C for 15 min
70
70
75
2073 (7)
1773 (2)
1671 (5)
Ã
Decay corrected isolated radiochemical yield calculated from the starting radioactivity of 18FÀ; n, number of runs.
Reservoirs 7 -9
Reservoir for
for reagents
crude product
Reservoirs 1 -6
for reagents
Reservoir for
final product
Reactor
Reservoir for HPLC
fraction
Figure 2. Graphical display of the monitor of the TRACERLab FXFN.
solvent DMF in the presence of potassium carbonate and transfer of the radioactivity was traced and recorded with inbuilt
kryptofix K2.2.2. Biological samples were prepared by dissolving radioactivity detector. After completion of the reaction the crude
the purified product in sterile phosphate buffer with 10% product was diluted with a 1:1 mixture of water and acetonitrile
absolute ethanol. The final product was filtered through a and the mixture was transferred into the intermediate reservoir for
DynaGard syringe filter of 0.2 mm pore size. The presence of crude product from where it was loaded into the preparative HPLC
residual solvents such as DMF and acetonitrile was investigated for purification. The HPLC solvent from the collected fraction was
by GC and no trace of DMF or acetonitrile was found, i.e., the removed using SPE technique. The final product was formulated
concentrations of acetonitrile and DMF in the final product were into the reservoir of the platform (Figure 2) and finally sterile
lower than 10 and 30 ppm, respectively, which were the lowest filtered and transferred into a sterile vial.
concentrations that could be detected by the instrument. The
The synthesis was developed previously on different platform
presence of kryptofix 2.2.2 was checked by LC-MS and no trace and some optimization was thus needed to obtain the best
of this reagent was found, i.e., the concentration of kryptofix results using the platform. Nucleophilic fluorination using 18FÀ
residue was lower than 2 mg/mL that was the lowest concentra- may be tricky and the radiochemical yield varies while running
tion that could be detected by the instrument. The syntheses of the synthesis at different times. It is thus important to clean and
precursors were published previously.11,13,14
dry the platform just before starting the synthesis and using the
A simple time sequence program was applied to perform the same procedure every time. A time sequence program was
synthesis. All valves of the TRACERLab FXFN were controlled developed to clean and dry the platform that gave reproducible
according to the preprogrammed time intervals to transfer the results. Moreover, the preparation of reactive 18FÀ is also very
reagents from one part to another of the instruments. Helium important to obtain high radiochemical yield. The effect of
pressure and vacuum pump were used to transfer the reagents. temperatures and reaction times were investigated. The best
The reagents were loaded into the reservoirs 1–6 and the result was obtained when 18FÀ was dried in two steps, first at
formulation liquids were loaded into the reservoirs 7–9 of the 601C for 7 min and then at 1201C for 5 min. Moreover a gentle
platform according to the graphical display showed in Figure 2. flow of helium gas was flushing through the reactor at the same
The heating and cooling of the reagents, switch on/off of helium time while it was being heated at 601C. Different reaction
gas and pump were also performed according to the program. The temperatures and times for final reaction of dried 18FÀ with
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J. Label Compd. Radiopharm 2010, 53 169–171

O. Rahman et al.
substrates were investigated and the best-obtained results are (5.0 mg in 500 mL anhydrous DMF) were loaded into reservoirs 1,
presented in Table 1.
2 and 3, respectively. Reservoirs 4, 7 and 8 were filled with a 1:1
mixture of water and acetonitrile (5 mL), water (10 mL) and
absolute ethanol (1 mL), respectively. The target water contain-
ing 18FÀ was passed through a preconditioned QMA cartridge
where the 18F- was trapped. The 18FÀ was released from QMA
cartridge by passing K2CO3 solutions from reservoir 1 through
the cartridge and allowed to enter into the reactor. Kyrptofix
solution from reservoir 2 was added into the reactor and the
whole mixture was dried first at 601C for 7 min and then at
1201C for 5 min. The precursor solution from reservoir 3 was
added with the dried 18FÀ ion complexed with potassium and
kryptofix. The reaction mixture was heated at 1101C for 10 min
([18F]FETO). In case of [18F]FVOZ and [18F]FHAR, the reaction was
performed at 1401C for 15 min and 1601C for 15 min,
respectively. The mixture was cooled to 501C and diluted with
the solvent mixture from reservoir 4 and loaded into the in-built
preparative HPLC system of the platform. The appropriate
fraction was collected, diluted with water (25 mL) and passed
through a C18 SPE cartridge. The purified product was trapped
on C18 SPE cartridge, washed with water from reservoir 7 and
eluted with absolute ethanol (1 mL) from reservoir 8 and
collected into the reservoir for final product containing
injectionable phosphate buffer solution. Finally the product
was transferred from that reservoir into a vial through the
syringe filter DynaGard and the radioactivity of the final product
was measured.
Conclusion
The implementation of the synthesis of three potential PET
tracers [18F]FETO, [18F]FVOZ and [18F]FHAR into a commercial
platform TRACERLab FXFN is described. This implementation will
make them more accessible for production in clinical research.
Experimental
General
All chemicals and reagents were purchased from Aldrich or
Lancaster. The 18F-trapping cartridge (QMA cartridge) and C18
SPE cartridge were purchased from Waters. Syringe filter
DynaGard was obtained from Microgon, Inc. Laguna Hills,
California. The QMA cartridge was conditioned by passing
10 mL of 0.5 M aqueous K2CO3 solution and 10 mL of water,
respectively, through it. The noncarrier added 18F was produced
as 18F– ion by the PETtrace cyclotron from GE Medical System
using the nuclear reaction 18O(p,n)18F. The isotopically enriched
[18O]H2O was used in the target and irradiated with a 16 MeV
proton beam. The synthesis of precursors and authentic
references for FETO,11 FVOZ13 and FHAR14 were described
previously. The identifications of the 18F-labeled products were
performed by analytical HPLC using authentic nonradioactive
compounds as the references. The analysis was performed with
a Shimadzu analytical HPLC system with variable-wavelength UV
detector in series with a Bioscan b1-flow detector. The
following mobile phases were used: 25 mM aqueous sodium
dihydrogen phosphate (A), acetonitrile (B1) and methanol (B2).
For analytical LC, an ACE reverse phase C18, 5 mm, 4.6 Â 250 mm
column was used with a flow of 1.0 mL/min. For preparative LC,
TRACERLab’s built-in HPLC system with Beckman’s Ultrasphere
reverse phase C18, 4 mm, 250 Â 10 mm (i.d.) (for [18F]FETO) or
MACHEREY NAGEL’s VrioPrep reverse phase C18, 250 Â 21 mm
(i.d.) (for [18F]FVOZ and [18F]FHAR) was used with a flow of 6 mL/
min. An isocratic HPLC method with 50:50 mixture of mobile
phases A and B1 was used for purification. For the analysis of
[18F]FETO and [18F]FVOZ an isocratic HPLC method with 30:70
mixture of mobile phases A and B2 were used. For the analysis
of [18F]FHAR an isocratic HPLC method with 65:35 mixture of
mobile phases A and B1 were used. Radioactivity of products
was measured in an ion chamber, Veenstra Instrumenten bv,
VDC-202. For coarse estimations of radioactivity during the
production, a portable dose-rate meter from Alnor OY Finland
was used. Nonradioactive compounds were characterized by 1H
and 13C NMR spectroscopy and GC-MS. NMR spectra were
recorded on a Varian Unity-400 NMR spectrometer. Chloroform-
d was used as the internal standard. GC-MS was performed
using a Finnigan GCQ mass spectrometer coupled to a Finnigan
Q-GC. The presence of solvent residue and Kryptofix residue in
the final product was checked by Varian 3380 GC and Finnigan
AQA LC-MS, respectively.
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