OF 18F- LABELLED COMPOUNDS COMPRISING HYDROLYTIC DEPROTECTION
STEP AND SOLID PHASE EXTRACTION
Technical Field of the Invention
The present invention relates to a method for the synthesis of l F-labelled compounds
and in particular l F-labelled compounds that are useful as positron emission
tomography (PET) tracers.
Description of Related Art
The radioisotopes suitable for detection in positron emission tomography (PET) have
notably short half-lives. Carbon-1 1 ( C) has a half-life of about 20 minutes, nitrogen-
13 ( l N) has a half-life of about 10 minutes, oxygen-15 ( 0 ) has a half-life of about 2
minutes and fluorine-1 8 (1 F) has a half-life of about 110 minutes. Synthetic methods
for the production of compounds labelled with these radionuclides need to be as quick
and as high yielding as possible. This is particularly important in the case of
compounds destined to be used for in vivo imaging, commonly known as PET tracers.
Furthermore, the step of adding the radioisotope to the compound should be as late as
possible in the synthesis, and any steps taken following the addition of radioisotope for
the work up and purification of the radioisotope-labelled compounds should be
completed with as little time and effort as possible.
PET tracers, and [ F]-radiotracers in particular, are now often conveniently prepared by
means of an automated radiosynthesis apparatus, e.g. Tracerlab™ and Fastlab™ from
GE Healthcare Ltd. A disposable cassette in which the radiochemistry is performed is
fitted to the apparatus. The cassette normally includes fluid pathways, a reaction vessel,
and ports for receiving reagent vials as well as any solid phase extraction (SPE)
cartridges used in post-radiosynthetic clean up steps. A well-developed automatic
synthesis method provides advantages of speed, convenience and a generally reliable
routine supply of the PET tracer. Furthermore and importantly, radiation burden to the
operator is reduced to a minimum.
The synthesis of a number of F-labelled PET tracers comprises l F labelling of a
protected precursor compound, with subsequent removal of the protecting groups by
acidic or alkaline hydrolysis. Examples of such 1 F-labelled PET tracers include 1 Ffluorodeoxyglucose
( 1 F-FDG), 6-[ 1 F]-L-fluorodopa ( 1 F-FDOPA), F-fluorothymidine
( 1 F-FLT), l-H-l-(3-[ F]fluoro-2-hydroxypropyl)-2-nitroimidazole ( 1 F-FMISO), 1 Fl-(
5-fluoro-5-deoxy-a-arabinofuanosyl)-2-mitroimidazole ( i F-FAZA), 16-a-[ 1 F]-
fluoroestradiol ( 1 F-FES), and 6-[ 1 F]-fluorometarminol ( 1 F-FMR) (see for example
chapters 6 and 9 of "Handbook of Radiopharmaceuticals" 2003; Wiley: by Welch and
Redvanly, and chapter 8 o f "Basics of PET Imaging, 2nd Edition" 2010; Springer: by
Saha).
Taking [ 1 F]FMISO as an example, Oh et al (2005 Nuc Med Biol; 32: 899-905)
describes an automated method for its synthesis. On a TracerLab Mx [ 1 F]FDG
synthesis module (GE Healthcare) and using modified disposable [ 1 F]FDG cassettes, a
solution of the precursor compound l-(2'-nitro -r-imidazolyl)-2-O-tetrahydrofuranyl-3-
O-toluenesulfonylpropanediol in acetonitrile (MeCN) was reacted with [1 F]fluoride
( , F ) at 95-120°C for 300-600 seconds and at 75°C for 280 seconds, then hydrolysed at
105°C for 300 seconds with IN HC1 following solvent removal, and neutralised using
NaOH. The neutralised [ F]FMISO crude solution was purified using highperformance
liquid chromatography (HPLC) to result in [ 1 F]FMISO having decaycorrected
end of synthesis (EOS) radiochemical yields of 58.5 ± 3.5%. The reported
synthesis time was 60.0 ± 5.2 minutes.
Frank et al (2009 Appl Radiat Isotop; 67(6): 1068-1070) report the synthesis of
[ FjFMISO using an automated synthesiser. The precursor compound l-(2'-nitro-
1'imidazolyl)-2 -O-tetrahydropyranyl-3-0-toluenesulfonylpropanediol (NITTP) was
labelled with 1 F~ in acetonitrile at 120°C for 10 minutes, deprotected with IN HC1 at
105°C for 5 minutes and neutralised with IN NaOH. The neutralised crude product
reaction mixture was purified using HPLC. The decay-corrected yields were reported to
be 20-30%.
The above-described automated methods for the production of [ 1 F]FMISO both use
purification by HPLC. It is preferred that a purification method taking up less time and
space is used, such as solid-phase extraction (SPE). Chang et al (2007 App Rad Isotop;
65: 682-686) describe an automated method for the synthesis of [ 1 F]FMISO using a
Scanditronix Anatech RB III robotic system. The precursor compound (2'-nitro-l'-
imidazolyl)-2-0-acetyl-3-0-tosylpropanol in acetonitrile was labelled with 1 F at 95°C
for 10 minutes, hydrolysed using IN HC1 at 90°C for 10 minutes following solvent
removal and neutralised with a solution of NaOH. The neutralised crude reaction
product was purified by first passing through a CI8 Sep-Pak cartridge and then a neutral
alumina Sep-Pak cartridge. The uncorrected EOS radiochemical yields reported were
30 ± 5%, and the synthesis time was 65 minutes. Radiochemical yield was reduced and
no apparent advantage in synthesis time was provided by this method as compared with
the earlier method including HPLC purification disclosed by Oh et al (referenced
above).
There is therefore scope for the provision of an automated method for the production of
[I F]FMISO, and other 1 F-labelled compounds wherein production comprises a
hydrolytic deprotection step, that improves upon the methods known in the art.
Brief Description of the Drawings
Figure 1 is a schematic diagram of a cassette according to the present invention.
Figure 2 is a schematic illustration of one way of carrying out the diluting and trapping
steps comprised in the method of the present invention, as described in more detail in
Example 1.
Figure 3 is a workflow diagram of showing how the method of the present invention
may be carried out and is described in more detail in Example 1.
Summary of the Invention
The present invention provides an improved method to prepare an l F-labelled
compound where the synthesis comprises a hydrolytic deprotection step. Specifically,
the method of the invention permits neutralisation of an acidic or basic crude product
without using any neutralising chemicals. Instead, the product is trapped on an SPE
column and then thoroughly rinsed with water. As a consequence of this process
simplification, the method of the invention can more readily be carried out on an
automated synthesiser. In addition to the radiofluorination method of the invention, the
present invention provides a cassette designed to carry out the method on an automated
synthesiser.
Detailed Description of Preferred Embodiments
The present invention therefore provides in one aspect a method comprising:
(i) labelling a protected precursor compound with F;
(ii) deprotecting the 1 F-labelled compound obtained in step (i) by hydrolysis;
(iii) diluting the deprotected 1 F-labelled compound obtained in step (ii) with
water;
(iv) trapping the deprotected F-labelled compound on a solid-phase extraction
(SPE) column by passing the diluted solution obtained in step (iii) through
said column;
(v) eluting the deprotected 1 F-labelled compound obtained in step (iv) from
the SPE column;
with the proviso that no neutralising step is carried out following the deprotection step.
An " 1 F-labelled compound" in the context of the present invention is a chemical
compound comprising at least one l F atom. Preferably, an 1 F-labelled compound of
the present invention comprises only one 1 F atom.
The term "labelling" in the context of the present invention refers to the radiochemical
steps involved to add 1 F to a compound. The precursor compound is reacted with a
suitable source of 1 F to result in the 1 F-labelled compound. A "suitable source of 1 F" is
typically either F-fluoride or an F-labelled synthon. F-fluoride is normally obtained as
an aqueous solution from the nuclear reaction 1 0(p,n) 1 F. In order to increase its reactivity
and to avoid hydroxylated by-products resulting from the presence of water, water is
typically removed from 1 F-fluoride prior to the reaction, and fluorination reactions are
carried out using anhydrous reaction solvents (Aigbirhio et al 1995 J Fluor Chem; 70: 279-
87). The removal of water from F-fluoride is referred to as making "naked" F-fluoride.
A further step that is used to improve the reactivity of F-fluoride for radiofluorination
reactions is to add a cationic counterion prior to the removal of water. Suitably, the
counterion should possess sufficient solubility within the anhydrous reaction solvent to
maintain the solubility of the 1 F-fluoride. Therefore, counterions that are typically used
include large but soft metal ions such as rubidium or caesium, potassium complexed with a
cryptand such as Kryptofix™, or tetraalkylammonium salts, wherein potassium complexed
with a cryptand such as Kryptofix™, or tetraalkylammonium salts are preferred.
The term "precursor" refers to a compound that when reacted with a suitable source of
F results in the desired F-labelled compound. The term "protected " refers to the
presence of one or more protecting groups on the precursor whose presence is required
for site-directed incorporation of 1 F. The terms "protecting group" and "deprotectinfi"
are well-known in the art. The use of protecting groups is described in 'Protective
Groups in Organic Synthesis', by Greene and Wuts (Fourth Edition, John Wiley &
Sons, 2007). The step of deprotecting is typically carried out by hydrolysis, either using
an acid or a base. The deprotecting step of the present invention is preferably carried
out by acid hydrolysis.
The term "diluting" is well-known in the art and refers to the process of reducing the
concentration of a solute in solution by mixing with more solvent. In the context of the
present invention the solvent used in the diluting step is water. The purpose of the
diluting step is to increase the polarity of the reaction mixture in order to permit high
and reproducible trapping of the product on an apolar (also commonly termed "reversephase")
SPE column.
The term "trapping" in the present invention refers to the retention of the deprotected
F-labelled compound on the SPE column by interactions between the deprotected 1 Flabelled
compound and the sorbent of the SPE column. These interactions are solventdependent.
The term "solid-phase extraction" (SPE) refers to the chemical separation technique that
uses the affinity of solutes dissolved or suspended in a liquid (known as the mobile phase)
for a solid through which the sample is passed (known as the stationary phase or sorbent) to
separate a mixture into desired and undesired components. The result is that either the
desired analytes of interest or undesired impurities in the sample are retained on the
sorbent, i.e. the trapping step as defined above. The portion that passes through the sorbent
is collected or discarded, depending on whether it contains the desired analytes or
undesired impurities. If the portion retained on the sorbent includes the desired analytes,
they can then be removed from the sorbent for collection in an additional step, in which the
sorbent is rinsed with an appropriate eluent. The sorbent is typically packed between two
porous media layers within an elongate cartridge body to form the "solid-phase extraction
(SPE) column ". High-performance liquid chromatography (HPLC) is specifically excluded
from the definition of SPE in the context of the present invention.
The term "neutralising " as used herein refers to the process of adjusting the pH of a
solution to bring it back to pH 7, or as close as possible to pH 7. Therefore, an acidic
solution can be neutralised by adding a suitable amount of an alkali such as NaOH, and
an alkaline solution can be neutralised by adding a suitable amount of an acid such as
HC1.
The term "eluting " refers to the process of removing the desired compound from the
SPE column by passing a suitable solvent through the column. The suitable solvent for
eluting is one in which the interactions between the sorbent of the SPE column and the
desired compound are broken thereby allowing the compound to pass through the
column and be collected.
In the method of the present invention, a distinct neutralisation step is not carried out.
Rather, the step of diluting serves both to bring the pH to neutrality and to prepare the
reaction mixture for SPE purification. As compared to the prior art methods, the
method of the present invention is therefore simplified by removal of the neutralisation
step, which makes the method more straightforward to carry out and to automate.
The method of the invention may be applied to the synthesis of any 1 F-labelled PET
tracer that comprises 1 F labelling of a precursor compound that comprises protecting
groups and subsequent removal of the protecting groups by acid or alkaline hydrolysis.
Non-limiting examples of such l F-labelled PET tracer include 1 F-fluorodeoxyglucose
(1 F-FDG), 6-[ 1 F]-L-fluorodopa ( 1 F-FDOPA), 1 F-fluorothymidine ( F-FLT), 1-H-l-
(3-[ F]fluoro-2-hydroxypropyl)-2-nitroimidazole ( 1 F-FMISO), 1 F-l-(5-fluoro-5-
deoxy-a-arabinofuanosyl)-2-mitroimidazole ( 1 F-FAZA), 6-a-[ 1 F]-fluoroestradiol
( F-FES), and 6-[ , F]-fluorometarminol (1 F-FMR). Said F-labelled compound is
preferably 1 F-fluorodeoxyglucose (1 F-FDG), 6-[ 1 F]-L-fluorodopa ( 1 F-FDOPA), Ffluorothymidine
( F-FLT), or F-fluoromisonidazole ( F-FMISO), and most
preferably 1 F-fluorothymidine ( 1 F-FLT) or 1 F-fluoromisonidazole (1 F-FMISO). The
known synthesis of each of these PET tracers includes a deprotection step and a
neutralisation step (see for example chapters 6 and 9 of "Handbook of
Radiopharmaceuticals" 2003; Wiley: by Welch and Redvanly, and chapter 8 of "Basics
of PET Imaging, 2nd Edition" 2010; Springer: by Saha). The method of the invention is
carried out to obtain any of these PET tracers in purified form in a straightforward
manner by omitting the neutralisation step and carrying out the diluting, trapping and
eluting steps as defined herein.
Examples of PET tracers which may be synthesised by the method of this aspect of the
present invention include [1 F]-fluorodeoxyglucose ([1 F]-FDG), [1 F]-
fluorodihydroxyphenylalanine ([18F]-F-DOPA), [1 F]-fluorouracil, [1 F]-l-amino-3-
fluorocyclobutane-l-carboxylic acid ([1 F]-FACBC), [, F]-altanserine, [1 F]-
fluorodopamine, 3'-deoxy-3'- F-fluorothymidine [ F-FLT] and [ F]-
fluorobenzothiazoles.
The structures of various 1 F-labelled protected precursor compounds obtained in step
(i) of the method of the present invention are as follows (wherein P1 to P4 are each
independently hydrogen or a protecting group):
[, F]-FDG
[ F]-F-DOPA
[, F]-altanserine
[ F]-fluorodopamine
[ F]-fluorobenzothiazole*
[ F]-FLT
* R1 is selected from hydrogen, C . alkyl, C1-6 hydroxyalkyl, and C|_ haloalkyl; R2 to R9 are
independently selected from hydrogen, halo, Ci. alkyl, Ci_ haloalkyl, Ci. hydroxyalkyl, C, alkoxy, C
haloalkoxy, hydroxy, cyano, and nitro.
In one embodiment, the method of the invention is used for the synthesis of F-FMISO:
When 1 F-FMISO is the 1 F-labelled compound obtained by the method of the present
invention, a preferred protected precursor compound is a compound of Formula I :
wherein:
R is a protecting group for the hydroxyl function; and,
R2 is a leaving group.
Rl of Formula I is preferably selected from acetyl, benzoyl, dimethoxytrityl (DMT), b-
methoxyethoxymethyl ether (MEM), methoxymethyl ether (MOM), and
tetrahydropyranyl (THP), and is most preferably THP.
R of Formula I is a leaving group, wherein the term "leaving group" refers to a moiety
suitable for nucleophilic substitution and is a molecular fragment that departs with a
pair of electrons in heterolytic bond cleavage. R is preferably selected from CI, Br, I,
tosylate (OTs), mesylate (OMs) and inflate (OTf), most preferably selected from OTs,
OMs and OTf, and is most especially preferably OTs.
A most preferred precursor compound for the synthesis of 1 F-FMISO is l-(2'-nitro-l'-
imidazolyl)-2-0-tetrahydropyranyl-3-0-tosyl-propanediol, i.e. a compound of Formula I
wherein R1 is tetrahydropyranyl and R2 is OTs.
In a preferred embodiment of the invention, the diluting step comprises:
(a) adding a first volume of water to said deprotected 1 F-labelled compound to
obtain a first diluted solution, and,
(b) adding subsequent volumes of water to aliquots of said first diluted solution
to obtain subsequent diluted solutions.
It is intended that the diluting step will result in a reaction mixture having a polarity
suitable to permit high and reproducible trapping on an apolar SPE column. Ideally, the
diluted reaction mixture should not have more than around 10-15% organic solvent in
water in order to achieve this aim. Aliquots of the diluted solution are passed through
the SPE column so as to trap the deprotected 1 F-labelled compound onto the column.
Optionally, once all the diluted solutions has been passed through the SPE column, an
additional step of washing the column with water may be carried out prior to the eluting
step.
Preferably, the eluting step is carried out using a solution of aqueous ethanol. In the
case of l F-FMISO, it is preferred that the eluting step is carried out with an aqueous
ethanol solution comprising 2-20% ethanol, most preferably 5-10% ethanol.
The sorbent of the SPE column for the present invention can be any silica- or
polymeric-based apolar sorbent. Non-limiting examples of suitable apolar SPE
columns include polymer-based Oasis HLB or Strata X SPE columns, or silica-based
C2, C4, C8, CI 8, tC18 or C30 SPE columns. The SPE column of the invention is
preferably selected from Oasis HLB, tCl 8, and Strata X.
l F-labelled PET tracers are now often conveniently prepared on an automated
radiosynthesis apparatus. Therefore, in a preferred embodiment, the method of the
present invention is an automated synthesis. The term "automated synthesis" refers to a
chemical synthesis that is performed without human intervention. In other words, it
refers to a process that is driven and controlled by at least one machine and that is
completed without the need of manual interference.
There are several commercially-available examples of such apparatus, including
Tracerlab™ and Fastlab™ (GE Healthcare Ltd). Such apparatus commonly comprises a
"cassette", often disposable, in which the radiochemistry is performed, which is fitted to the
apparatus in order to perform a radiosynthesis. The cassette normally includes fluid
pathways, a reaction vessel, and ports for receiving reagent vials as well as any solid-phase
extraction cartridges used in post-radiosynthetic clean up steps. The automation of
synthesis of PET tracers performed on a synthesiser platform is limited by the number of
available reagent slots. The method of the present invention permits a reduction in the
number of chemicals required by removing the neutralising agent.
In another aspect, the present invention provides a cassette for carrying out the method of
the invention, said cassette comprising:
(i) a vessel containing said protected precursor compound as defined herein;
(ii) means for eluting the vessel containing said protected precursor compound
with a suitable source of F as defined herein;
(iii) means for deprotecting the F-labelled compound obtained following
elution of the vessel containing said protected precursor compound with a
suitable source of 1 F; and,
(iv) an SPE column as defined herein suitable for trapping the deprotected I Flabelled
compound;
with the proviso that a vessel containing a neutralisation agent suitable for neutralising the
pH of said deprotected 1 F-labelled compound is neither comprised in or in fluid
connection with said cassette.
In the context of the cassette of the invention, a "neutralising agent" is an acidic or an
alkaline solution designed to neutralise the pH of, respectively an alkaline or an acidic
solution comprising deprotected labelled 1 F-labelled compound.
All the suitable, preferred, most preferred, especially preferred and most especially
preferred embodiments of the precursor compound of Formula la, 1 F-fluoride and the SPE
cartridges that are presented herein in respect of the method of the invention also apply to
the cassette of the invention.
The cassette of the invention may furthermore comprise:
(iv) an ion-exchange cartridge for removal of excess [ F]-fluoride.
Brief Description of the Examples
Example 1 describes how 1 F-FMISO was obtained according to the method of the
invention.
List of Abbreviations used in the Examples
EtOH ethanol
1 F fluoride
F-FMISO l-H-l-(3-[ 1 F]fluoro-2-hydroxypropyl)-2-nitroimidazole
ID internal diameter
NITTP 1-(2'-Nitro-l '-imidazolyl)-2-0-tetrahydropyranyl-3-0-toluenesulfonylpropanediol
MeCN acetonitrile
QMA quaternarymethylammonium
THP tetrahydropyranyl
Examples
Example 1: Synthesis of' sF-FMISO
A cassette as illustrated in Figure 1 was fitted to a FASTlab synthesiser (GE
Healthcare).
[ F]Fluoride was supplied from GE Healthcare on a GE PETrace cylcotron. The initial
activity was transferred via the activity inlet of the FASTlab cassette using vacuum.
The activity was transferred from the activity inlet to the (pre-treated) QMA cartridge
where the [1 F] was trapped and the water passed through to the 1 0 water recovery vial,
using a combination of N2 to push and vacuum to pull.
After the transfer of the eluent containing the F- activity into the reaction vessel, the
solvents were evaporated until dryness. During the drying process, a small amount of
acetonitrile (80 mΐ) was added to the reaction vessel. The evaporation was carried out
with heating under nitrogen flow and under vacuum.
The 1-(2'-Nitro- 1'-imidazolyl)-2-0-tetrahydropyranyl-3-0-toluenesulfonyl-propanediol
precursor (also called NITTP) was added to the dry residue. Nucleophilic substitution
at 110°C was carried out in the closed reaction vessel, in which the tosylate group of the
precursor was replaced by the 1 F- ions. After labelling, the solution is cooled down to
60°C.
The tetrahydropyranylated (THP) compound was converted into F-FMISO by
removing the THP protecting group. This deprotection was carried out in the reaction
vessel at 90°C by means of 1ml of 0.6M H3PO4 for about 5 min. This acid
concentration was obtained by dilution of ~ 360 mΐ 2.29M H3P0 4 with ~ 840m1water.
The resulting 18F-FMISO was obtained in an organic/water mixture. The organic
solvent (MeCN) was removed by flushing nitrogen through right hand side connector
combined with vacuum (-10 kPa (-100 mBar)) during 8 minutes at 90°C.
The crude FMISO was mixed in a syringe with 3.5 ml of water, and sent back to the
reaction vessel. This solution (B) was then diluted with water in 3 portions. 1.5 ml of
this solution (B) was diluted with 5.0 ml of water (solution C) and then passed through
the reverse phase cartridge (Oasis® HLB). This operation was done 3 times with the
remaining solution in the reaction vessel. The FMISO was trapped onto the cartridge.
Solvents, unreacted 1 F ions and impurities were washed off into the external waste
bottle with 7 ml of water. Figure 2 is a schematic diagram of this dilution and trapping
process.
The trapped FMISO was rinsed prior the elution with a full syringe of water (~ 7 ml).
The elution of the FMISO was performed by dilution of absolute ethanol with water to a
ratio of 5 to 6% of EtOH. This dilution was performed in the middle syringe by
withdrawing ~ 350 mΐ of EtOH first then about 6.5 ml of water and repeated 3 times.
The FMISO was eluted from the Oasis® HLB cartridge trough an acidic alumina light
cartridge to the product collection vial.
At the end of the elution, 2 full syringes of nitrogen were flushed trough the transfer
tube followed by 30 sec of direct nitrogen flush (HF; 100 kPa (1000 mbar)) in order to
allow a transfer trough a 15 m long tubing (min ID 1 mm).
Non polar by-products were retained on the Oasis HLB cartridge and the polar, such as
last traces of unreacted 18F , on the Alumi*na. The solution was finally passed through a
vented 0.22 mhi filter.
The final volume of F-FMISO was 20mL ± 0.5mL.
A schematic of the entire process is set out in Figure 3. The process took less than 57
minutes in total and resulted in uncorrected yields of around 35%.
A method comprising:
(i) labelling a protected precursor compound with 1 F;
(ii) deprotecting the 1 F-labelled compound obtained in step (i) by hydrolysis;
(iii) diluting the deprotected F-labelled compound obtained in step (ii) with
water;
(iv) trapping the deprotected 1 F-labelled compound on a solid-phase extraction
(SPE) column by passing the diluted solution obtained in step (iii) through
said column;
(v) eluting the deprotected 1 F-labelled compound from the SPE column;
the proviso that no neutralising step is carried out following the deprotection step.
The method as defined in Claim 1wherein said deprotecting step (ii) is carried out
by acid hydrolysis.
The method as defined in Claim 1 wherein said 1 F-labelled compound is 1 Ffluorodeoxyglucose
(1 F-FDG), 6-[ 1 F]-L -fluorodopa ( F-FDOPA), 1 Ffiuorothymidine
( F-FLT), 1 F-fluoromisonidazole ( F-FMISO), , F-l-(5-fluoro-
5-deoxy-a-arabinofuanosyl)-2-mitroimidazole (1 F-FAZA), 16- -[1 F]-
fluoroestradiol (1 F-FES), or 6-[ 1 F]-fluorometarminol ( 1 F-FMR).
The method as defined in Claim 1 wherein said 1 F-labelled compound is 1 Ffluorodeoxyglucose
(1 F-FDG), 6-[ 1 F]-L -fluorodopa (1 F-FDOPA), 1 Ffluorothymidine
( 1 F-FLT), or 1 F-fIuoromisonidazole (1 F-FMISO).
The method as defined in Claim 1 wherein said 1 F-labelled compound is 1 Ffluorothymidine
( 1 F-FLT) or 1 F-fluoromisonidazole ( 1 F-FMISO).
The method as defined in Claim 1wherein said 1 F-labelled compound is l-H-l-(3-
[1 F]fluoro-2-hydroxypropyl)-2-nitroimidazole ( 1 F-FMISO):
The method as defined in Claim 6 wherein said protected precursor compound is a
compound of Formula I :
wherein:
R1 is a protecting group for the hydroxyl function; and,
R2 is a leaving group.
The method as defined in any one of Claims 1-7 wherein said diluting step
comprises:
(a) adding a first volume of water to said deprotected 1 F-labelled compound to
obtain a first diluted solution, and,
(b) adding subsequent volumes of water to aliquots of said first diluted solution
to obtain subsequent diluted solutions.
(9) The method as defined in any one of Claims 1-8 wherein said SPE cartridge is
selected from Oasis HLB, tC18, and Strata X.
(10) The method as defined in any one of Claims 1-9 which is automated.
A cassette for carrying out the method as defined in Claim 10, said cassette
comprising:
(i) a vessel containing said protected precursor compound as defined in Claims
1 and 7;
(ii) means for eluting the vessel containing said protected precursor compound
with a suitable source of F;
(iii) means for deprotecting the F-labelled compound obtained following
elution of the vessel containing said protected precursor compound with a
suitable source of 1 F; and,
(iv) an SPE column as defined in Claims 1 and 9 suitable for trapping the
deprotected F-labelled compound;
with the proviso that a vessel containing a neutralisation agent suitable for
neutralising the pH of said deprotected l F-labelled compound is neither comprised
in or in fluid connection with said cassette.