Radiosynthesis of 3-[18F]fluoropropyl and 4-[ 18F]fluorobenzyl triarylphosphonium ions

Article abstract of DOI:10.1002/jlcr.835

3-[¹⁸F]Fluoropropyl-, 4-[¹⁸F]fluorobenzyl- triphenylphosphonium and 4-[¹⁸F]fluorobenzyltris-4- dimethylaminophenylphosphonium cations were synthesized in multistep reactions from no carrier added (nca) [¹⁸F]fluo

Full text of DOI:10.1002/jlcr.835

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2004; 47: 469–476.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.835
Research Article
Radiosynthesis of 3-[18F]fluoropropyl and
4-[18F]fluorobenzyl triarylphosphonium ions
Hayden T. Ravert*, Igal Madar and Robert F. Dannals
Division of Nuclear Medicine, Department of Radiology, The Johns Hopkins Medical
Institutions, 600 North Wolfe Street, Baltimore, Maryland 21287-0750, USA
Summary
3-[18F]Fluoropropyl-, 4-[18F]fluorobenzyl-triphenylphosphonium and 4-[18F]fluoro-
benzyltris-4-dimethylaminophenylphosphonium cations were synthesized in multi-
step reactions from no carrier added (nca) [18F]fluoride. The time for synthesis,
purification, and formulation was 56, 82, and 79 min with an average radiochemical
yield of 12, 6 and 15%, respectively (not corrected for decay). The average specific
radioactivity for the three radiolabeled compounds was 14.9 GB q/mmole (403 mCi/
mmole) at end of synthesis (EOS). Copyright # 2004 John Wiley & Sons, Ltd.
Key Words: phosphonium; fluorine-18; positron emission tomography
Introduction
Recent studies using lipophilic cations including phosphonium cations indicate
these cations accumulate in some cancer cells to a higher degree than in normal
cells.1,2 This phenomenon is attributed to the higher than normal electro-
chemical membrane potential of the mitochondria in cancer cells.3,4
Furthermore, loss of mitochondrial membrane potential is an early event in
cell death caused by pro-apoptotic agents.5 Cumulative evidence suggests that
mitochondria-controlled apoptosis underlies cell loss in heart failure.6 Tumor
and cardiac mitochondria membrane potential has been evaluated with tritium
labeled phosphonium cations and fluorescent dyes, such as, the [3H]tetra-
phenylphosphonium1 and [3H]methyltriphenylphosphonium7–9 cations, and
rhodamine 123.10
Positron emission tomographic (PET) animal brain tumor studies indicated
the lipophilic cation radiotracer, [11C]methyltriphenylphosphonium cation,
had a tumor to normal brain uptake ratio of nearly 48:1 and a prolonged
*Correspondence to: H. T. Ravert, Division of Nuclear Medicine, Department of Radiology, The Johns
Hopkins Medical Institutions, 600 North Wolfe Street, Baltimore, Maryland 21287-0750, USA. E-mail:
htr@jhu.edu
Copyright # 2004 John Wiley & Sons, Ltd.
Received 10 January 2004
Revised 13 April 2004
Accepted 13 April 2004

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retention time.11 Similarly, animal cardiac PET studies with the same
radiotracer show heart to lung and heart to blood ratios of 14:1 and greater
than 46:1, respectively, with a non-infarcted to infarcted myocardium ratio of
12:1 and prolonged retention.12
To provide a tracer similar to the [11C]methyltriphenylphosphonium cation
but with a longer half-life, the syntheses of 18F labeled phosphonium cations
were investigated. This paper describes the synthesis, purification, formula-
tion, and characterization of 3-[18F]fluoropropyltriphenylphosphonium,
4-[18F]fluorobenzyltriphenylphosphonium and 4-[18F]fluorobenzyltris-4-di-
methylaminophenylphosphonium cations from [18F]fluoride. Reported pre-
liminary animal data on the 4-[18F]fluorobenzyltriphenylphosphonium cation
indicates a rapid blood clearance and myocardial accumulation with a heart to
lung ratio of 15:1, prolonged retention and low non-specific binding in
canines.13,14 In a canine pacing model for heart failure, myocardium showed a
global decrease of 4-[18F]fluorobenzyltriphenylphosphonium cation uptake of
16% and regional apoptic site decreases of greater than 40%.15 Also, mice
bearing human breast carcinoma tumors showed a 45% reduced uptake of the
4-[18F]fluorobenzyltriphenylphosphonium cation 48 h after administration of
taxotere with no affect on heart, lung or liver uptake; and, low uptake in sites
of inflammation.16,17
Results and discussion
The 3-[18F]fluoropropyltriphenylphosphonium cation radiosynthesis (Figure 1)
was performed in a single vessel. After drying the nca [18F]fluoride in the
presence of Kryptofix and potassium carbonate (K2CO3), propylene glycol
ditosylate in acetonitrile was added and the solution heated for 4 min.
Triphenylphosphine in toluene was subsequently added. After the solution was
heated to boiling on a heat gun for 5 min, the toluene was evaporated, and
high pressure liquid chromatography (HPLC) solvent added while cooling the
mixture to room temperature. The mixture was filtered to remove the
Figure 1. 3-[18F]fluoropropyltriphenylphosphonium cation radiosynthesis
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RADIOSYNTHESIS OF 3-
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unreacted triphenylphosphine precipitate prior to injection on to the semi-
preparative HPLC column. The radioproduct was collected, the solvent was
evaporated, and residue was dissolved in normal saline to yield a radio-
chemically pure solution (>99%), as determined by analytical HPLC, in 12%
yield (EOS) from starting [18F]fluoride. The synthesis, purification and
formulation were completed in 56 min (n ¼ 10) at an average specific
radioactivity of 15.5 GBq/mmole (420 mCi/mmole EOS). Authentic 3-fluor-
opropyltriphenylphosphonium bromide was synthesized and used to deter-
mine the identity, purity and specific radioactivity of the final product.
The radiosynthesis of the 4-[18F]fluorobenzyltriphenylphosphonium cation
involved the synthetic steps shown in Figure 2 and follows a reaction scheme
previously investigated.18,19 After drying the [18F]fluoride in the presence of
Kryptofix and K2CO3, the 4-trimethylammoniumbenzaldehyde trifluoro-
methanesulfonate in dimethylsulfoxide (DMSO) was added. The solution
was heated in a sealed vial for 10 min. The cooled reaction mixture was diluted
with water and passed through a reverse phase solid phase extraction (SPE)
cartridge. After blowing with argon gas for 3–4 min, the SPE cartridge was
eluted with ether through a column containing sodium borohydride and
K2CO3 to yield the 4-[18F]fluorobenzyl alcohol. The eluant was immediately
added to a vial containing triphenylphosphine dibromide in methylene
chloride (CH2Cl2). After 5 min the mixture containing 4-[18F]fluorobenzyl
Figure 2. 4-[18F]fluorobenzyltriphenylphosphonium cation radiosynthesis
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H. T. RAVERT ET AL.
bromide was eluted through a silica SPE cartridge into a vial containing
triphenylphosphine in toluene. The solution was evaporated to a low volume,
tightly capped and heated to boiling for 5 min. The workup of the toluene
solution was the same as described in the 3-[18F]fluoropropyltriphenylphos-
phonium synthesis above. A radiochemically pure product (>99%) was
obtained in 6% yield (EOS) with an average synthesis time of 82 min (n ¼ 20)
and an average specific radioactivity of 16.7 GBq/mmole (451 mCi/mmole
EOS). Authentic 4-[18F]fluorobenzyltriphenylphosphonium bromide was
synthesized and used to determine the identity, purity and specific radio-
activity of the final product.
The radiosynthesis, purification and formulation of the 4-[18F]fluorobenzyl-
tris-4-dimethylaminophenylphosphonium cation was performed in the same
manner as the 4-[18F]fluorobenzyltriphenylphosphonium cation described
above except tris-dimethylaminophosphine was substituted for triphenylphos-
phine in the last synthetic step (Figure 2). The radiochemically pure
radioproduct (>99%) was obtained in 15% yield (EOS) with an average
synthesis time of 80 min (n ¼ 6) and an average specific radioactivity of
12.6 GBq/mmole (340 mCi/mmole EOS). Authentic p-fluorobenzyltris-4-di-
methylaminotriphenylphosphonium bromide was synthesized and used to
determine the identity, purity and specific radioactivity of the final product.
Experimental
[18F]Fluoride was produced by 18 MeV proton bombardment of a high
pressure 18O-water target using a GE PETtrace biomedical cyclotron. All
reagents were ACS or HPLC purity. The 4-trimethylammoniumbenzaldehyde
trifluoromethanesulfonate was synthesized as previously described.20 DMSO
was distilled under vacuum from barium oxide prior to use. MALDI-Mass
Spectra were performed by Anaspec, Inc. (San Jose, CA). Melting points were
determined on a Mel-Temp (Thermolyne) melting point apparatus and are
uncorrected. Elemental analyses were obtained from Atlantic Microlab
(Norcross, GA). NMR spectra were performed on a Varian 400 MHz
instrument. Reverse phase HPLC analysis and purification were performed
with two Waters 610 HPLC pumps, a Waters 441 fixed wavelength (254 nm)
UV detector, and a Bioscan Flow Count PIN diode radioactivity detector.
All HPLC chromatograms were recorded with a Rainin Dynamax dual
channel control/interface module connected to a Macintosh computer
with appropriate program software (Dynamax, version 1.4). Reverse phase
HPLC semipreparative purification were performed on an Waters Novapak
6 mm C-18 column (7.8 ꢀ 300 mm) using the mobile phase described in the
syntheses below at a flow rate of 7 ml/min. Chemical and radiochemical
purity were determined using a Waters Novapak 4 mm C-18 HPLC column
(3.9 ꢀ 150 mm) using the mobile phase described in the syntheses below at a
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RADIOSYNTHESIS OF 3-
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flow rate of 3 ml/min. A dose calibrator (Capintec 15R) was used for all
radioactivity measurements.
Synthesis of 3-fluoropropyltriphenylphosphonium bromide
To 0.93 g (3.5 mmole) of triphenylphosphine (Aldrich) dissolved in 15 ml
acetonitrile (CH3CN) was added dropwise 0.32 ml (0.5 g, 3.5 mmole) 1-bromo-
3-fluoropropane (Aldrich) in 15 ml of CH3CN. The solution was refluxed 16 h.
After rotary vacuum evaporation of the solvent, the white solid was
recrystallized from methylene chloride (CH2Cl2). The solid was further
purified on a short silica (EM Sciences, silica gel 60) column using 5%
methanol in CH2Cl2 (1.21 g, 85%). mp: 232–2358C. 1H NMR (CDCl3):
d 1.81–2.17 (m, 2 H), 4.01–4.11 (m, 2 H), 4.72–4.75 (m, 1 H), 4.87–4.90 (m, 1 H),
7.69–7.88 (m, 15 H). MS: m=z 323.3 (C+). Elemental analysis calculated for
C21H21BrFP: C, 62.55; H, 5.25; F, 4.71. Found: C, 62.26; H, 5.24; F, 4.47.
Synthesis of 4-fluorobenzyltriphenylphosphonium bromide
To 1.0 g (4 mmole) of triphenylphosphine in 15 ml CH3CN was added
dropwise 0.5 ml (0.75 g, 4 mmole) 4-fluorobenzyl bromide (Aldrich) in 10 ml
CH3CN. The solution was stirred 2 days at room temperature. The white
precipitate was collected and recrystallized from CH3CN (1.44 g, 80%). mp:
1
313–3158C [lit: 280–2828C21]. H NMR (D6-DMSO): d 5.17–5.21 (m, 2 H),
6.99–7.13 (m, 4 H), 7.67–7.94 (m, 15 H). MS: m=z 371.2 (C+). Elemental
analysis calculated for C25H21BrFP: C, 66.53; H, 4.69; F, 4.21. Found: C,
66.55; H, 4.65; F, 4.15.
Synthesis of 4-fluorobenzyltris-4-dimethylaminophenylphosphonium bromide
To 1.6 g (4 mmole) tris(4-dimethylamino)phosphine (Organometallics, Inc.)
dissolved in 160 ml of CH3CN and 170 ml dimethylformamide (DMF) was
added 0.5 ml (0.75 g, 4 mmole) 4-fluorobenzyl bromide in 10 ml DMF over
25 min. The solution was stirred 36 h. The solution was evaporated to a purple
solid and recrystallized from CH3CN to yield a white solid (1.88 g, 81%). mp:
302–3048C. 1H NMR (CD3OD): d 3.08 (s, 18 H), 4.36, 4.40 (d, 2 H), 6.85–7.26
(m, 16 H). MS: m=z 500.3 (C+). Elemental analysis calculated for
C31H36BrFN3P: C, 64.14; H, 6.25; F, 3.27. Found: C, 64.18; H, 6.35; F, 3.11.
Radiosynthesis and purification of 3-[18F]fluoropropyltriphenylphosphonium
cation
Aqueous [18F]fluoride (approx. 1 ml) was passed through a resin column [Trap
and Release column (DW-TRC), D and W, Inc., Oakdale, TN, USA)]. The
trapped [18F]fluoride was eluted from the resin column with 2.3 mg
(0.02 mmole) of K2CO3 (Aldrich, 99%) in 0.3 ml of water (Fluka) into a
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5 ml crimp top v-vial containing Kryptofix (Aldrich, 4,7,13,16,21,24-hexaoxa-
1,10-diazabicyclo [8,8,8]-hexacosane) (13 mg, 0.03 mmole). The vented vial was
heated to 1208C under argon flow and CH3CN (1 ml) was added in aliquots to
azeotropically remove water. After evaporation of water, the vial was heated
under argon flow for 3 min. The flow was stopped and the vent was removed.
Propylene glycol ditosylate (Aldrich) (15 mg, 0.036 mmole) in 0.2 ml CH3CN
was added to the vial and the solution heated at 808C for 4 min.
Triphenylphosphine (21 mg, 0.08 mmole) in 0.5 ml of toluene was added to
the vial and the solution heated to boiling on a heat gun for 5 min. The vial
was vented to remove the toluene and then cooled to room temperature.
HPLC buffer [0.5 ml of 35:65 CH3CN:H2O (0.1 M ammonium formate (AF))]
was added, the mixture filtered through a 0.45 mm teflon filter (Alltech, 13 mm)
and injected onto the preparative HPLC column. After collection and
evaporation to dryness, the product (TR=5.3 min, k0 ¼ 3:1) was dissolved in
3 ml of sterile normal saline and filtered through a sterile 0.2 mm filter (Gelman
Acrodisc, 25 mm) into a sterile evacuated vial (Mallinckrodt Medical, 20 ml).
An aliquot was removed aseptically to determine chemical and radiochemical
purity of the final solution (TR=1.7 min, k0 ¼ 1:8) by analytical HPLC [40:60
CH3CN:H2O (0.1 M AF)].
Radiosynthesis and purification of 4-[18F]fluorobenzyltriphenylphosphonium
cation
To the dried mixture of [18F]fluoride ion, K2CO3 and Kryptofix prepared as
described in the synthesis above was added 4-trimethylammoniumbenzalde-
hyde trifluoromethanesulfonate (7 mg, 0.02 mmole) in 0.2 ml of DMSO. The
solution was heated at 1208C for 10 min. After cooling, 5 ml of HPLC water
was added and the solution was passed through an activated C-18 solid phase
extraction cartridge (SPE) (Waters Sep-Pak Plus). The C-18 SPE was washed
with 10 ml of HPLC water and blown with argon flow for 3–4 min. Ether
(2 ml) was passed through the C-18 SPE and a connected column containing
300 mg of sodium borohydride on alumina (Aldrich) above 300 mg K2CO3
into an open 5 ml v-vial containing triphenylphosphine dibromide
(100 mg, 0.2 mmole) in 1 ml CH2Cl2. The vial was allowed to sit at room
temperature for 5 min. The mixture was then passed through a silica SPE
(Waters Sep-Pak Classic) followed by 1 ml CH2Cl2 into a vial containing
triphenylphosphine (21 mg, 0.08 mmole) in 0.4 ml toluene. The volume was
reduced to approximately 0.5 ml with argon and the vial was capped tightly.
The solution was heated to boiling on a heat gun for 5 min. The vial was
vented to remove the toluene and cooled to room temperature. HPLC buffer
[0.5 ml of 50:50 CH3CN:H2O (0.1 M AF)] was added, the mixture filtered
through a 0.45 mm teflon filter and injected onto the preparative HPLC
column. After collection (TR=3.9 min, k0 ¼ 3:2) and evaporation to dryness,
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RADIOSYNTHESIS OF 3-
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the product was dissolved in 3 ml of sterile normal saline and filtered through a
sterile 0.2 m filter into a sterile evacuated vial. The chemical and radiochemical
purity (TR=3.9 min, k0 ¼ 5:5) of the final solution were determined as
described previously by analytical HPLC [40:60 CH3CN:H2O (0.1 M AF)].
Radiosynthesis and purification of 4-[18F]fluorobenzyltris-4-dimethylamino-
phenyltriphenylphosphonium cation
The synthesis was performed following the procedure above for the 4-
[18F]fluorobenzyltriphenylphosphonium cation up to and including the
bromination and silica SPE step. The eluant from the silica SPE was added
to tris(4-dimethylaminophenyl)phosphine (21 mg, 0.05 mmole) in 0.4 ml to-
luene. After reducing volume and capping tightly, the solution was heated to
boiling for 5 min on a heat gun. The mixture was vented, cooled, diluted and
filtered as described above. After injection onto the preparative HPLC [70:30
CH3CN:H2O (0.1 M AF)] and collection, the product (TR=4.3 min, k0 ¼ 2:3)
was dissolved in 3 ml of sterile normal saline and filtered through a sterile 0.2 m
filter into a sterile evacuated vial. The chemical and radiochemical purity
(TR=4.7 min, k0 ¼ 6:8) of the final solution were determined as described
previously by analytical HPLC [60:40 CH3CN:H2O (0.1 M AF)].
Conclusion
The radiosyntheses of the no carrier added 3-[18F]fluoropropyl-, 4-[18F]fluoro-
benzyltriphenylphosphonium and 4-[18F]fluorobenzyltris-4-dimethylamino-
phenylphosphonium cations were achieved in modest radiochemical yields
and specific radioactivities. Although the procedure has not been optimized
and is being modified to a more remote and automated process for handling
large amounts of radioactivity, the yields and specific radioactivities are
sufficient for in vivo animal PET studies.
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