WO 2012/089594 PCTYEP2011/073670
ELUENT SOLUTION
Technical Field of the Invention
The present invention relates to the field o radiopharmaceuticals, and in particular to
the preparation of compounds suitable for use in positron emission tomography (PET).
A method useful in the synthesis o compounds labelled with 1 F s provided. Also
provided by the present invention is a radiof!uorination reaction which comprises the
method f the invention and as a cassette for conveniently carrying out the method and
the radiofluorination reaction of the invention.
Description of Related Art
Nucleophilic substitution with [ F]fluoride ( F ) is currently the most important route
in obtaining [ F]-labelled tracers for PET imaging (Schubiger et al, Eds "PET
Chemistry: The Driving Force of Molecular Imaging" (In: Ernst Schering Res Found
Workshop; 2007: 62); 2007 Springer GmbH).
1 F is normally produced as an aqueous solution from the nuclear reaction 180(p,n) 8F
by proton irradiation of [I 0 ]water (Ruth and Wolf, Radiochim. Acta 979; 26: 2 1) . It
s well-known that F in aqueous form is not very reactive and a number of
manipulations are necessary in order to provide a reactive nucleophilic reagent. One
important step is the addition of a cationic counterion (e.g. the cationic complex of
Kryptofix and potassium or TBA+). Typically, the aqueous solution of F is first
adsorbed onto a anion exchange resin (Schlyer et al, Appl Rad Isotop 990; 41: 531),
followed by elution with an aqueous acetonitrile solution containing a carbonate salt
such as K2CO3, or KHCO3 accompanied by a cryptand such as Kryptofix™ (K222) or
tetrabutyl ammonium (Hamacher et al, J Nucl Med 986; 27: 235; Brodack et al App
Rad Isotop 1988; 39: 699). Alternatively, the F can be eluted fro the anion
exchange column with the carbonate salt and addition of this to a solution of cryptand in
acetonitrile as described by McConathy et al (Appl Rad Isotop 2003; 58: 657-666).
Acetonitrile is the solvent of choice for the eluent solution primarily because of the
excellent solubility of K[ ' F]/Kryptofix or tetrab utylammonium F therein. Also,
- ] o given that the next step in making F reactive generally involves use of acetonitrile to

during storage at 5 °C, 25 °C and 40 °C (n 2-3).
Figure 3 shows the RCY of [I F]FACBC after eluent stored at 30°C (·), 40°C (¨) and
RCY of [ F]FDG after eluent stored at eluent at 25°C (■), 40°C (A).
Figure 4 illustrates the RCY of [1 F]FACBC after eluent with methanol (MeOH) stored
at 30°C (A), 50°C (·) and RCY of [ F]FDG after eluent with acetonitrile (MeCN)
solution from the nuclear reaction 0(p,n) F is passed through. Preferably, said ionexchange
cartridge is an anion exchange cartridge, most preferably a quaternary
methylammonium (QMA) cartridge.
The term " F~ eluent" refers to the solution comprising F and the eluent solution
obtained when the eluent solution is passed through the ion exchange column.
Said "eluent solution" is free of acetonitrile, and preferably consists of said cationic
counterion in said suitable solvent.
A "cationic counterion" in the context of the present invention is a positively-charged
counterion that acts to improve the reactivity of F~ when combined therewith.
Examples of suitable cationic counterions for use i the method of the present invention
include large but soft metal ions such as rubidium, caesium, potassium complexed with
a cryptand, or tetraalkylammonium salts. A preferred cationic counterion is a metal
complex of a cryptand, most preferably wherein said metal is potassium and wherein
said cryptand is Kryptofix 222.
The "suitable solvent" for the eluent solution docs not comprise any acetonitrile.
Preferably, said suitable solvent i an alkanol, and is preferably ethanol o methanol,
most preferably methanol. Said suitable solvent is either 100% alkanol, or is
alternatively an "aqueous solution of an alkanol". For example said suitable solvent
may comprise a ratio of alkanokwater in the range 60:40 to 100:0, preferably i the
range 80:20 to 100:0 and most preferably 90: 0 to 100:0. A certain amount of water
can help with consistent elution of F but it is preferable to have as little water as
possible as the percentage of water is directly proportional to subsequent drying time.
The method of the invention is most advantageous where the eluent solution is for
convenience prepared as a bulk solution and/or in prefilled vials for storage. As noted
in the description of the prior art, use of prefi lled vials permits more well defined,
reliable and reproducible synthesis processes (Hjelstuen et al, Eur J Pharm Biopharm
201 , 78: 307), and prefilled vials can be made with a low bioburden and a documented
shelf life, which serves as a better starting point for good manufacturing practice (GMP)
quality manufacture compared to manually mixed solutions.
The method of the invention may optionally comprise the additional step:
(in) drying said F~ eluted from said column in step (ii).
The term "drying" refers to the evaporation of the suitable solvent (as described above)

subject preferentially binds to a particular target within said subject in order that the
target may be imaged by detecting emissions from F external to said subject using
PET imaging. The term "PET imaging" refers to the nuclear medicine imaging
technique that produces a three-dimensional image or picture of functional processes i
the body. The technique detects pairs of gamma rays emitted indirectly by a positronemitting
radionuclide such as fluorine-18, which is introduced into the body as part of a
PET tracer. Three-dimensional images of tracer concentration within the body are then
constructed by computer analysis.
A "precursor compound" comprises a non-radioactive derivative of an F-Iabelled PET
tracer designed so that chemical reaction with F occurs site-specifically, can be
conducted in the minimum number of steps (ideally a single step) and without the need
for significant purification (ideally no further purification), to give the F-Iabelled PET
tracer. Such precursor compounds are synthetic and can conveniently be obtained in
good chemical purity.
Suitable "protecting groups" are well-known in the art and are discussed in more detail
by Theodora W. Greene and Peter G. M. Wuts i "Protective Groups i Organic
Synthesis" (Fourth Edition, John Wiley & Sons, 2007).
It will be appreciated by the skilled person that the inventive methods described herein
can be applied for the preparation of any 1 F-labelled PET tracer that can be prepared
using nucleophilic radio fluorination with F Non-limiting examples of such 1 Flabelled
PET tracers includes those set out in Table 1below:

Reactor during labelling 1000 3495 1795
End-product 26000 0.2-0.5 nm
[ F]FDG synthesis
Eluent vial 825 8700 3100
Reactor before drying 377 6265 2120
Reactor during labelling 1600 844 597
End-product 15000 0.2-0.4 nm
As compared with [ F]FDG, in the synthesis of [ F]FACBC more eluent ( 105m1vs.
825m1) and hence more acetate is introduced to the reaction vessel. The difference is
enhanced during labelling because the volume used for labelling for [ FJFACBC is
smaller (1.0 ml vs. 1.6 ml). These coincidental factors like smaller volume of eluent
and larger volume of labelling solvent made the synthesis o [ F]FDG as described
herein more resistant to eluent storage compared to the [ F]FACBC reaction. t may
well be that [ F]FDG synthesis setups elsewhere could be more prone to eluent storage.
This could be equally true in the case of other 8F-labelled PET tracers such as those
listed above, and the present invention is thereby a solution that is easy to implement
and is not detrimental o the quality of the eventual product.
It is most preferred that the radiofluorination reaction of the invention is automated,
most preferably on an automated radiosynthesis apparatus as suitable and preferably
described above.
n yet another aspect, the present invention provides a cassette for carrying out the
radiofluorination reaction on an automated synthesis apparatus wherein said cassette
comprises:
(i) an anion exchange column suitable for trapping an aqueous solution of F ,
wherein said anion exchange column i as defined herein;
(ii) a first vessel containing an eluent solution as defined herein;
(iii) a second vessel containing a precursor compound which upon reaction with
1 F results in an ! F-labelled PET tracer as defined herein, wherein said
F is obtained by the method as defined herein.
The suitable and preferred embodiments of any features of the method of the invention
and/or the radiofluorination reaction of the invention that are common to the cassette of
the invention also apply to the cassette of the invention .
Brief Description of the Examples
Example 1 describes an analysis of prior art eluent solutions that were stored.
Example 2 describes the synthesis of [ F]FACBC and [, F]FDG with stored vs.
freshly-prepared prior art eluent.
Example 3 describes the synthesis of [, F]FACBC with stored vs. freshly-prepared
eluent of the present invention.
List of Abbreviations used in the Examples
ATR attenuated total reflectance
DTGS deuterated triglycine sulphate
[1 F]FACBC 1-amino-3-[ '8F]fluorocyclobutane- 1-carboxylic acid
[1 F]FDG 2-deoxy-2-[ l F]fluoro-D-glucose
FT-IR Fourier transform infrared
K222 Kryptofix 222
MeCN acetonitrile
MeOH methanol
QMA quaternary methyl ammonium
RCY radiochemical yield
SPE solid-phase extraction
synthesis
The eluent solutions were as follows:
The vials were stored i darkness in an up-right position using storage temperatures of
5, 25, 30, 40 and 50°C. Both eluents were stored over a nine-month period, during
which time levels of acetamide and acetate were measured. Acetamide was quanti fied
by infrared spectroscopy using a Perkin Elmer Spectrum 2000 Explorer FT-IR
spectrometer with a DTGS detector and a single reflection diamond ATR
(DuraSamplIR I from ScnsIR Technologies). Acetate was quantified by liquid
chromatography with UV detection (Agilent 1100 series).
Notable levels (mg/ml) of acetamide and acetate were generated during a nine-month
period of storage as seen in Figure 1 (acetamide generated in FACBC and FDG eluent
vials during storage at 5°C, 25°C and 40°C; n = 2-3) and Figure 2 (acetate generated m
FACBC and FDG eluent vials during storage at 5°C, 25°C and 40°C. n = 2-3).
Example 2: Synthesis of I F FACBC and [1 FIFDG with Stored vs. Freshlyprepared
Prior Art Eluent
The synthesis of [ F]FACBC and [ F]FDG was tested with both freshly prepared and
stored eluents to investigate the impact of generated levels of acetamide and ammonium
acetate on the RCY.
No-carrier-added [, F]fluoride was produced via the 0(p,n) 8F nuclear reaction o a
GE PETtrace 6 cyclotron (Norwegian Cyclotron Centre, Oslo). Irradiations were
performed using a dual-beam, 30mA current on two equal Ag targets with HAVAR foils
using 16.5 MeV protons. Each target contained 1.6 m of > 96% [ O]water (Marshall
Isotopes). Subsequent to irradiation and delivery to a hotcell, each target was washed
with .6 ml of [ 0]water (Merck, water for GR analysis), giving approximately 2-5
Gbq in 3.2 ml of [, 0]water.
All radiochemistry was performed on a commercially available GE FASTlab with
single-use cassettes. Each cassette is built around a one-piece-moulded manifold with
25 three-way stopcocks, all made of polypropylene. Briefly, the cassette includes a 5 ml
reactor (cyclic olefin copolymer), one 1m syringe and two 5 m syringes, spikes for
connection with five prefilled vials, one water bag (100 ml) as well as various SPE
cartridges and filters. Fluid paths are controlled with nitrogen purging, vacuum and the
three syringes. The fully automated system is designed for single-step fluoridations
with cyclotron-produced [ F]fluoride. The FASTlab was programmed by the software
package i a step-by-step time-dependent sequence of events such as moving the
syringes, nitrogen purging, vacuum, and temperature regulation. Synthesis of [ FjFDG
and [ F]FACBC were customized on separate cassettes, but both synthesis followed the
three general steps: (a) [1 F]fluorination, (b) hydrolysis o f protection groups and (c) SPE
purification.
Prior Art Synthesis of f FlFDG
Vial A contained K222 (43.7 mg, 17 mh o ), K2C0 3 (7.8 mg, 56.7 Lmo ) in 79.5%
(v/v) MeCN(aq) (825 m ) . Vial B contained the precursor (39 mg, 8 .2 tmo ) in 2.0 ml of
MeCN with 1700 ppm water. Vial C contained of MeCN (4.1 ml).Vial D contained 2 M
NaOH (4.1 ml). Vial E contained 2.3 M phosphoric acid (4. 1 ml). Aqueous
[ F]fluoride ( 1 ml, 100-200 Mbq) was passed through the QMA and into the 1 0-H 20
recovery vial. The trapped [ F]fluoride was eluted into the reactor using eluent from
vial A (450 m ) and then concentrated to dryness by azeotropic distillation with
acetonitrile (80 mΐ , vial C). Approximately 1.6 ml of precursor solution (corresponds to
3 1.2 mg; 65 mh o precursor) from vial B was added to the reactor and heated at 25 °C
for 2 min. The reaction mixture was diluted with water and sent through the tC18
cartridge. Reactor was washed with water and sent through the tC18 cartridge. The
labelled intermediate, fixed on the tC18 cartridge was first washed with water, then
incubated with 2 NaOH (2.0 ml) for 2 min. The crude mixture was mixed with water
(1.5 ml) and 2.3 M phosphoric acid (1.5 ml) and passed through the HLB and Alumina
cartridges into the product vial made of glass (30 ml). Water (9 ml) was then sent
through the HLB and Alumina cartridges and into the product vial. The purified
formulation of [' F]FDG contained a final volume of 15 ml. Radiochemical purity was
tested by radio-TLC using a mixture of MeCN:H20 (95:5) as the mobile phase. The
radiochemical yield (RCY) was expressed as the amount of radioactivity in the
[ FjFDG fraction divided by the total used [ F]fluoride activity (decay corrected).
Total synthesis time was 22 min.

after eluent stored at 30°C (·), 40°C (¨) and RCY of [, F]FDG after eluent stored at
eluent at 25°C (■ ), 40°C (A). The RCY of [i F]FACBC dropped from 62.5% to 44.7%
when the FACBC eluent was stored at 30 °C for 12 months and from 62.5% to 33.6%
when stored at 40 °C for 6 months. It was therefore observed a negative correlation
between degradation of acetonitrile and reduction in RCY of [1 F]FACBC. The RCY
for [1 F]FDG was observed to fall from 86.8% to 66.7% for [ F]FDG when the eluent
solution was stored at 50 °C for 3 months (n=3).
Example 3: Synthesis of f l F]FACBC with Stored vs. Freshly-preparedEluent of the
Present Invention
FACBC eluent vials in which acetonitrile was replaced by methanol was stored for
predetermined time points and tested in the synthesis of [ F]FACBC. Figure 4
illustrates the RCY of [ F]FACBC after eluent with MeOH stored at 30°C (A ), 50°C
(·) and RCY of [18F]FDG after eluent with MeCN stored at eluent at 30°C (¨), 40°C
(■ ) . While the acetonitrile based eluent resulted a gradual decrease in RCY with
increasing storage time, the RCY remained unchanged with the methanol-based eluent
even when stored at 50°C for 6 months.
Amethod for preparation of F for use in a radio fluonnation reaction wherein said
method comprises:
(i) trapping an aqueous solution of F onto an ion exchange column; and,
(ii) passing an eluent solution through said ion exchange column on which said
F is adsorbed to obtain an F~ eluent, wherein said eluent solution
comprises a cationic counterion i a suitable solvent with the proviso that
said eluent solution does not comprise acetonitrile.
The method as defined in Claim wherein said anion exchange column is an anion
exchange column.
The method as defined i Claim 2 wherein said anion exchange column is a
quaternary methylammonium (QMA) column.
The method as defined in any one of Claims 1-3 wherein said cationic counterion is
a metal complex of a cryptand.
The method as defined in Claim 4 wherein said metal of said metal complex of a
cryptand s potassium.
The method as defined in either Claim 4 or Claim 5 wherein said cryptand of said
metal complex of a cryptand is Kryptofix 222 (K222).
The method as defined in any one of Claims 1-6 wherein said suitable solvent
comprises an alkanol.
The method as defined in Claim 7 wherein said suitable solvent is an aqueous
solution of an alkanol.
The method as defined in either Claim 7 or Claim 8 wherein said alkanol is
methanol.
(10) The method as defined in any one of Claims 1-9 which comprises a further step:
(m) drying said 8F eluted from said column in step (ii).
( 11) The method as defined in any one of Claims 1-10 which s automated.
(12) The method as defined in Claim which is carried out o a cassette suitable for
use with a automated radiosynthesis apparatus.
(13) A radiofluorination reaction to obtain an F-labelled positron-emission tomography
(PET) tracer wherein said radiofluorination reaction comprises reaction of a
precursor compound with 1 F . wherein said precursor compound may comprise
one or more protecting groups, and wherein said ! F~is obtained by the method as
defined in any one of Claims 1-1 1.
(14) The radiofluorination reaction as defined i Claim 1 wherein said F-labelled
PET tracer is 2-deoxy-2-[ 1 F]fluoro-D-glucose, [, F]fluorothymidine,
[i F]fluoronitroimidazole, 6-[ , F]fluoroDOPA, [ F]setoperone, [1 F]altanserin,
[' F]N-methylspiperone, 6-[1 F]fluorodopamine, (-)6-[ , F]fluoro-norcpinephrine,
16a-[ , F]fluoroestradiol, [ F]fleroxacin or [ F]fluconazole.
(15) The radiofluorination reaction as defined in Claim 14 wherein said F-labelled
PET tracer is 2-deoxy-2-[ F] fluoro-D-glucose, [ F]fluorothymidine or
[ FJfluoronitroimidazole.
(16) The radiofluorination reaction as defined in any one of Claims 13-15 which is
automated.
(17) The radiofluorination reaction as defined i Claim 6 which is carried out on a
cassette suitable for use with an automated radiosynthesis apparatus.
(18) A cassette for carrying out the radiofluorination reaction as defined in Claim 17
comprising:
(i) an anion exchange column suitable for trapping an aqueous solution of F~
wherein said anion exchange column s as defined in any one ofClaims 1-3;