[11C]Fluoroform, a Breakthrough for Versatile Labeling of PET Radiotracer Trifluoromethyl Groups in High Molar Activity

Article abstract of DOI:10.1002/chem.201701701

Positron-emission tomography (PET) is an immensely important imaging modality in biomedical research and drug development but must use selective radiotracers to achieve biochemical specificity. Such radiotracers are usually labeled with carbon-11 (t_1/2=20 min) or fluorine-18 (t_1/2=110 min), but these are only available from cyclotrons in a few simple chemical forms. [¹⁸F]Fluoroform has emerged for labeling tracers in trifluoromethyl groups but is severely limited in utility by low radioactivity per mass (low molar activity). Here, the synthesis of [¹¹C]fluoroform is described, based on CoF₃-mediated fluorination of cyclotron-produced [¹¹C]methane. This process is efficient and repetitively reliable. [¹¹C]Fluoroform shows versatility for labeling small molecules in very high molar activity (>200 GBq μmol^?1), far exceeding that possible by using [¹⁸F]fluoroform. Therefore, [¹¹C]fluoroform represents a major breakthrough for labeling prospective PET tracers in trifluoromethyl groups at high molar activity.

Full text of DOI:10.1002/chem.201701701

DOI: 10.1002/chem.201701701
Communication
&
Radiochemistry
[11C]Fluoroform, a Breakthrough for Versatile Labeling of PET
Radiotracer Trifluoromethyl Groups in High Molar Activity
Mohammad B. Haskali and Victor W. Pike*[a]
may result in far too high occupancy of the target by carrier
Abstract: Positron-emission tomography (PET) is an im-
with consequent violation of the tracer principle, or even an
mensely important imaging modality in biomedical re-
obliteration of any target-specific signal.[1] Minimal occupancy
search and drug development but must use selective radi-
of the target by carrier may also be needed to avoid unwanted
otracers to achieve biochemical specificity. Such radiotrac-
pharmacological effects.[2]
ers are usually labeled with carbon-11 (t1/2 =20 min) or flu-
The most popular methods for labeling PET radiotracers at
orine-18 (t1/2 =110 min), but these are only available from
high molar activity use [11C]methyl iodide or [18F]fluoride ion as
cyclotrons in a few simple chemical forms. [18F]Fluoroform
labeling agents.[1b] [11C]Methyl iodide is produced from cyclo-
has emerged for labeling tracers in trifluoromethyl groups
tron-produced [11C]methane or [11C]carbon dioxide, whereas
but is severely limited in utility by low radioactivity per
[18F]fluoride ion is produced directly from a cyclotron.[1b] How-
mass (low molar activity). Here, the synthesis of
ever, the use of these labeling agents or of others restricts the
[11C]fluoroform is described, based on CoF3-mediated fluo-
kind of groups that might be labeled in radiotracers, for exam-
rination of cyclotron-produced [11C]methane. This process
ple to methyl (Me) groups for [11C]methyl iodide[1b,3] and to
is efficient and repetitively reliable. [11C]Fluoroform shows
monofluoro (CF) groups for [18F]fluoride ion.[1b,4]
versatility for labeling small molecules in very high molar
Many drugs and potential PET radiotracers contain trifluoro-
activity (>200 GBqmmolÀ1), far exceeding that possible by
methyl (CF3) groups. A methyl, chloro, or other substituent can
using [18F]fluoroform. Therefore, [11C]fluoroform represents
often be replaced with a CF3 group with good retention of
a major breakthrough for labeling prospective PET tracers
physicochemical and pharmacological properties.[5] Also, the
in trifluoromethyl groups at high molar activity.
CF3 group is generally considered to be metabolically stable.
Consequently, the pharmaceutical industry develops many
drugs with CF3 groups. In parallel, academic groups are devel-
oping methods for labeling such groups with fluorine-18,[6]
Positron-emission tomography (PET) is immensely important
for biomedical research and for drug discovery and develop-
ment. The value of PET for imaging molecular targets in
a living human or animal subject depends on the availability of
biochemically specific radiotracers, in which the radiolabel is
usually one of the short-lived cyclotron-produced positron-
with the most recent methods based on conversion of
[18F]fluoride ion into [18F]fluoroform,[7] and then in situ genera-
tion of the reactive derivative [18F]CuCF3 (Figure 1).[8] These
[18F]fluoroform production methods at best deliver only mod-
erate molar activity (<32 GBqmmolÀ1) owing to [18F]fluoride
ion dilution with carrier fluoride ion in the solution reaction
systems. Generally, the radiotracer molar activities that are
needed for PET imaging of low-density protein targets are sev-
eral-fold higher.[1b,9] The range of useful PET radiotracers that
may be produced from [18F]fluoroform or [18F]CuCF3 that is
produced by the best-performing methods is therefore limited
to the low proportion not requiring such high molar activities.
We noted that 11C-labeling of the trifluoromethyl group has
never been achieved at any molar activity. [11C]Methane is pro-
duced in high activity in numerous PET research facilities,
either directly by the 14N(p,a)11C nuclear reaction on nitrogen-
10% hydrogen or by reduction of cyclotron-produced
[11C]carbon dioxide. By either route, [11C]methane typically has
very high molar activity that well exceeds that of cyclotron
sources of fluorine-18.[10,11] We reasoned that if [11C]fluoroform
could be produced from readily accessible [11C]methane, very
high molar activity might be retained and well exceed that cur-
rently achievable for [18F]fluoroform or [18F]CuCF3. Moreover,
[11C]fluoroform would be expected to participate in labeling re-
actions without any further dilution with carrier, and therefore
emitters, carbon-11 (t1/2 =20.4 min) or fluorine-18 (t1/2
=
109.8 min). The label position is often critical for avoiding trou-
blesome radiometabolites that may confound attempts to
quantify radiotracer interaction with the imaging target.[1]
Therefore, in many cases it is preferable to label in one part of
the structure rather than in another. A further very important
consideration is the molar activity of the radiotracer, namely
the radioactivity [Bq] per total mass of tracer [mol], in which
the latter is predominantly the accompanying non-radioactive
tracer known as carrier. For low-density imaging targets, such
as enzymes, transporters, receptors, and plaques, the radiotrac-
er molar activity needs to be very high. A low molar activity
[a] Dr. M. B. Haskali, Dr. V. W. Pike
Molecular Imaging Branch, National Institute of Mental Health,
National Institutes of Health, Building 10, Rm B3 C346A
10 Center Drive, Bethesda, MD 20892-1003 (USA)
E-mail: pikev@mail.nih.gov
Supporting Information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under https://doi.org/10.1002/
chem.201701701.
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We noted that heated cobalt(III) fluoride (CoF3) has been
used to fluorinate hydrocarbons to yield mixtures of fluorinat-
ed products on a macro-scale.[12] An unappealing feature of
a typical fluorination process was that the CoF3 needed to be
generated in situ by passing fluorine gas over heated CoF2.
Nonetheless, we noted that CoF3 is now commercially available
and might be used to avoid any need for noxious and highly
reactive fluorine. Passage of [11C]methane over heated CoF3
therefore seemed an attractive possibility for producing
[11C]fluoroform, provided that neither over-fluorination of the
sub-micromole amount of carrier methane nor the formation
of other hydrocarbon byproducts would be a major issue.
Therefore, we set out to test the feasibility of using commer-
cially available CoF3 for producing [11C]fluoroform in useful
yield.
Herein, we report that heated CoF3 successfully mediates ef-
ficient conversion of [11C]methane into [11C]fluoroform. We fur-
ther report a remotely controllable and reliable apparatus for
[11C]fluoroform production and illustrate the utility of the
[11C]fluoroform for labeling model compounds and examples
of drug-like compounds by various chemical routes, some
known and some new. These labeled compounds show very
high molar activities that match those to be expected[6] from
cyclotron-produced [11C]methane in the absence of further di-
lution with carrier and far exceed those previously attained for
[18F]fluoroform or [18F]CuCF3. In particular, these molar activities
rival those for PET radiotracers commonly used for imaging
low-density protein targets, such as neurotransmitter recep-
tors, transporters, enzymes, and plaques.
Figure 1. Methods for the synthesis of [18F]fluoroform or the derivative
[18F]CuCF3 for conversion into [18F]trifluoromethylarenes.
Our initial experiments showed that [11C]methane readily
passed through a heated column of commercial CoF3 without
any hold-up of radioactivity or any restriction of flow and with
some production of [11C]fluoroform. Figure 2 illustrates the ap-
paratus that we finally developed for routinely preparing
[11C]fluoroform after further experimentation.
the labeling of PET radiotracers in trifluoromethyl groups at
high molar activity would then become possible. Therefore, we
aimed to produce [11C]fluoroform form [11C]methane.
To meet our objective, a rapid and efficient fluorination
method was required. To maintain high molar activity in the
[11C]fluoroform, the method would need to be applicable to
[11C]methane in the presence of only a low amount of carrier
methane (typically !1 mmol) and without introducing an ap-
preciable amount of extra carrier. Moreover, the method would
need to be amenable to easy remote-control within a lead-
shielded “hot-cell” for radiation safety to personnel.
The temperature dependence of the conversion of
[11C]methane into [11C]fluoroform was investigated in the appa-
ratus with carrier He flow set at 20 mLminÀ1. Radioactivity
trapped in cold (ꢀÀ948C) ethanol was used to calculate the
yield (radioactivity breakthrough was relatively low). High
yields of [11C]fluoroform were obtained between 260 and
Figure 2. Apparatus for producing [11C]fluoroform. Cyclotron-produced 11CH4 in N2-10% H2 is passed first through a cold (À1868C) stainless-steel U-tube and
then another cold U-tube (À1868C) containing Porapak Q. Waste gas goes to a collection bag. The U-tube is then raised and allowed to warm to RT over
4 min while the tube is also flushed with He gas (20 mLminÀ1) to transfer the 11CH4 over Sicapent and into a heated (2708C) stainless-steel tube containing
CoF3 (17 g). The effluent passes through MeCN–dry ice (ꢀÀ418C) to trap any trace HF (b.p. 19.58C) and then into a trap containing EtOH cooled with
hexane–liquid N2 (ꢀÀ948C) or DMF cooled with MeCN–dry ice (ꢀÀ418C) to trap the generated [11C]CHF3. See the Supporting Information for complete ap-
paratus construction and operation details.
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2908C (Figure 3). The average yield of [11C]fluoroform in this
range was 53Æ4%, based on 11 experiments conducted with
two different CoF3 columns. The overall process for producing
[11C]fluoroform from [11C]methane required only 7 min from
the end of a cyclotron irradiation and was thus much less than
one half-life of carbon-11.
fluorine to afford [11C]fluoromethane, with this replacement
process repeated until [11C]fluoroform is formed.
The reactivity of [11C]fluoroform was tested on model sub-
strates (Figure 5). When [11C]fluoroform in DMF was added to
a solution of benzophenone (1) and tBuOK at RT, [11C]a-(tri-
Figure 3. Dependence of the yield of [11C]fluoroform from [11C]methane on
the temperature. Values are means Æ standard deviation (n>3).
Although we observed that heating a CoF3 column to 3508C
many times resulted in severely impaired performance, initial
conditioning of a newly installed CoF3 column by heating it
once to 3508C while sealed under He subsequently resulted in
optimal yields of [11C]fluoroform at lower temperatures. More-
over, such a heat-conditioned CoF3 column showed remarkably
consistent performance over a very large number (>80) of
[11C]fluoroform productions (Figure 4). Therefore, once set up
Figure 5. Preparation and use of [11C]fluoroform for labeling model organic
compounds with trifluoromethyl groups.
fluoromethyl)-benzhydrol ([11C]2) was obtained quantitatively
in just 5 min. Treatment of 4-nitrophenylboronic acid (3) with
[11C]fluoroform under copper(I)-mediated conditions[7b,8c] for
1 min at RT afforded [11C]1-nitro-4-(trifluoromethyl)benzene
([11C]4) in 99% yield with a molar activity of 551 GBqmmolÀ1.
This molar activity is over 20-fold higher than that reported for
[18F]4 prepared from [18F]fluoroform. Similarly, copper(I)-mediat-
ed treatment of 3-fluorophenylboronic acid (5) with
[11C]fluoroform yielded [11C]1-fluoro-3-(trifluoromethyl)benzene
Figure 4. Dependence of [11C]fluoroform yield on the number of CoF3
column uses. Data are for the column operating at 2708C with He flow set
at 20 mLminÀ1
.
and conditioned, the apparatus required no significant mainte-
nance other than to be kept filled with He. Additionally, the
apparatus was easily adapted for automation and remote-con-
trol to ensure radiation protection to personnel (see the Sup-
porting Information).
([11C]6) in 98% yield and with
a
molar activity of
242 GBqmmolÀ1. Together, these results confirmed that
[11C]fluoroform was produced from [11C]methane without ap-
preciable dilution of molar activity.
The robustly repeatable performance of a conditioned CoF3
column suggests that the fluorination process does not
depend on prior decomposition of CoF3 to CoF2 and fluorine.
More likely is that the [11C]methane interacts directly and stoi-
chiometrically with CoF3 to replace one hydrogen atom with
We also investigated the reactivity of [11C]fluoroform towards
other substrates. Thus, under copper(I)-mediated conditions,[5b]
ethyl 4-iodobenzoate (7) and 4-iodobenzonitrile (9) were con-
verted rapidly into ethyl [11C]4-(trifluoromethyl)benzoate
([11C]8) and [11C]4-(trifluoromethyl)benzonitrile ([11C]10) in 88
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and 90% yield, respectively. These high yields attest to the ab-
sence of troublesome impurities in the collected
[11C]fluoroform; only low levels of minor unreactive fluorinated
11C species were ever observed as contaminants.
The easy access to [11C]fluoroform enabled us to explore the
development of new labeling chemistries. Thus, we showed
that treatment of a commercially available “wet” diazonium
salt 11 with [11C]CuCF3 afforded [11C]4 in high yield approach-
ing that from the boronic acid precursor 3. Moreover, we
showed the versatility of [11C]fluoroform for labeling diverse
functional groups through its reaction with the diaryldisulfane
12, which afforded the labeled trifluoromethylsulfur arene
[11C]13 rapidly (<10 min) in 29% non-optimized yield.
Finally, we demonstrated the utility of [11C]fluoroform for la-
beling trifluoromethyl groups with carbon-11 in drug-like or
PET radiotracer-type molecules. For example, we used
[11C]fluoroform to label three known drugs, namely the antian-
drogen flutamide (Eulexin) (14), the antirheumatic drug leflu-
nomide (Arava) (15), and the antidepressant fluoxetine (Prozac)
(16) (Figure 6). [11C]14 was obtained in 76% yield from the cor-
Experimental Section
Production of [11C]fluoroform: The apparatus (Figure 2) was con-
structed, set-up, and operated as detailed in the Supporting Infor-
mation. The collected [11C]fluoroform was analyzed with HPLC (see
the Supporting Information).
Conversion of [11C]fluoroform into [11C]CuCF3: In a glovebox,
tBuOK in DMF (0.3m, 50 mL, 15 mmol) was added to CuBr (0.7 mg,
5 mmol) in a 1 mL vial. The vial was septum-sealed and removed
from the glovebox. [11C]Fluoroform (185–555 MBq) in DMF (50–
300 mL) was added to the vial, mixed, and left at RT for 1 min. A so-
lution of Et3N·3HF in DMF (1.64% v/v; 5 mL) was then added. The
mixture was mixed thoroughly and allowed to stay at RT for anoth-
er minute.
Labeling reactions: For details of labeling reactions and product
analyses see the Supporting Information.
Acknowledgements
This work was supported by the Intramural Research Program
of the National Institutes of Health (NIMH; ZIA MH002793). We
are grateful to Mr. George Dold and colleagues (NIMH) for ap-
paratus construction and the NIH Clinical Center (Chief: Dr. P.
Herscovitch) for cyclotron production of carbon-11.
Conflict of interest
This work is the subject of a patent application.
Keywords: [11C]fluoroform
tomography · radiochemistry · trifluoromethyl
· carbon-11 · positron-emission
Figure 6. 11C-Labeled drugs prepared from no-carrier-added [11C]fluoroform.
[1] a) V. W. Pike, Trends Pharmacol. Sci. 2009, 30, 431; b) V. W. Pike, Curr.
Med. Chem. 2016, 23, 1818.
[2] a) S. E. Lapi, M. J. Welch, Nucl. Med. Biol. 2012, 39, 601; b) S. P. Hume,
R. N. Gunn, T. Jones, Eur. J. Nucl. Med. 1998, 25, 173; c) E. M. Jagoda, J. J.
Vaquero, J. Seidel, M. V. Green, W. C. Eckelman, Nucl. Med. Biol. 2004, 31,
771.
[3] F. Wuest, M. Berndt, T. Kniess in PET chemistry, The Driving Force in Mo-
lecular Imaging (Eds.: P. A. Schubiger, L. Lehmann, M. Friebe), Springer-
Verlag, Heidelberg, 2007, pp. 183–213.
[4] L. Cai, S. Lu, V. W. Pike, Eur. J. Org. Chem. 2008, 2853.
[5] a) W. Zhu, J. Wang, S. Wang, Z. Gu, J. L. AceÇa, K. Izawa, H. Liu, V. A. So-
loshonok, J. Fluorine Chem. 2014, 167, 37; b) M. Bassetto, S. Ferla, F. Per-
tusati, Future Med. Chem. 2015, 7, 527.
[6] V. T. Lien, P. J. Riss, BioMed. Res. Int. 2014, 2014, 380124.
[7] a) D. van der Born, J. D. M. Herscheid, R. V. A. Orru, D. J. Vugts, Chem.
Commun. 2013, 49, 4018; b) D. van der Born, C. Sewing, J. D. M. Hersc-
heid, A. D. Windhorst, R. V. A. Orru, D. J. Vugts, Angew. Chem. Int. Ed.
2014, 53, 11046; Angew. Chem. 2014, 126, 11226.
[8] a) M. Huiban, M. Tredwell, S. Mizuta, Z. Wan, X. Zhang, T. L. Collier, V.
Gouverneur, J. Passchier, Nat. Chem. 2013, 5, 941; b) T. Rꢁhl, W. Rafique,
V. T. Lien, P. Riss, Chem. Commun. 2014, 50, 6056; c) P. Ivashkin, G. Lem-
onnier, J. Cousin, V. Grꢂgoire, D. Labar, P. Jubault, X. Pannecoucke,
Chem. Eur. J. 2014, 20, 9514.
[9] K. Madsen, L. Marner, M. Haahr, N. Gillings, G. M. Knudsen, Nucl. Med.
Biol. 2011, 38, 1085.
[10] D. R. Christman, R. D. Finn, K. I. Karlstrom, A. P. Wolf, Int. J. Appl. Radiat.
Isot. 1975, 26, 435.
responding iodo-precursor (17) and [11C]fluoroform under cop-
per(I)-mediated conditions. This yield compares very favorably
with that of [18F]14 prepared with the much lower molar activi-
ty method of Huiban et al.[8a] (55%). [11C]15 was labeled in
93% yield and with a high molar activity of 400 GBqmmolÀ1
(corrected to end of radionuclide production) from the cop-
per(I)-mediated reaction on the corresponding boronic acid
precursor (19). Finally, [11C]16 was obtained in 45% yield from
a copper(I)-mediated reaction on a Boc-protected iodo precur-
sor (20) followed by deprotection.
In conclusion, [11C]fluoroform is readily and simply produced
in usefully high yield and in high molar activity from cyclotron-
produced [11C]methane by passage over heated CoF3. The pro-
cess is rapid, repetitively robust, and low-maintenance.
[11C]Fluoroform represents a breakthrough as an agent for la-
beling PET radiotracers at trifluoromethyl groups in high molar
activity sufficient for imaging low-density targets in human
subjects. We now envisage access to an enhanced range of ex-
citing radiotracers for PET applications based on adapting the
known richly diverse chemistry of fluoroform[13] to
[11C]fluoroform for unprecedented 11C-labeling at trifluorometh-
yl groups.
[11] a) K. R. Buckley, J. M. Huser, S. Jivan, K. S. Chun, T. J. Ruth, Radiochim.
Acta 2000, 88, 201; b) J. Andersson, P. Truong, C. Halldin, Appl. Radiat.
Isot. 2009, 67, 106; c) H. Bjçrk, K. Dahlstrçm, J.-O. Bergstrçm, P. Truong,
C. Halldin in 10th Workshop on Targetry and Target Chemistry, Madison,
&
&
Chem. Eur. J. 2017, 23, 1 – 6
www.chemeurj.org
4
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Communication
WI, 2004, Abstract A02, p. 12; d) V. Gꢃmez-Vallejo, V. Gaja, J. Koziorow-
ski, J. Llop in Positron Emission Tomography—Current Clinical and Re-
search Aspects (Ed.: C.-H. Hsieh), InTech, Rijeka, 2012, pp. 183–210.
[12] a) V. S. Asovich, V. V. Kornilov, R. Kostyaev, V. N. Maksimov, Russ. J. Appl.
Chem. 1994, 67, 94; b) V. V. Kornilov, R. A. Kostyaev, B. N. Maksimov, B. A.
Mel’nichenko, T. Fedorova, Russ. J. Appl. Chem. 1995, 68, 1227.
[13] a) A. Lishchynskyi, B. Guillaume, V. V. Grushin, Chem. Commun. 2014, 50,
10237; b) J. Russell, N. Roques, Tetrahedron 1998, 54, 13771; c) B. Fol-
lꢂas, I. Marek, J.-F. Normant, L. Saint-Jalmes, Tetrahedron 2000, 56, 275;
d) G. K. S. Prakash, P. V. Jog, P. T. D. Batamack, G. A. Olah, Science 2012,
338, 1324; e) H. Kawai, Z. Yuan, E. Tokunaga, N. Shibata, Org. Biomol.
Chem. 2013, 11, 1446.
Manuscript received: April 17, 2017
Accepted Article published: May 20, 2017
Final Article published: && &&, 0000
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COMMUNICATION
&
Radiochemistry
M. B. Haskali, V. W. Pike*
&& – &&
[11C]Fluoroform, a Breakthrough for
Versatile Labeling of PET Radiotracer
Trifluoromethyl Groups in High Molar
Activity
Putting a label on it: Cobalt(III) fluoride
mediates efficient transformation of cy-
clotron-produced [11C]methane into
[11C]fluoroform with high molar activity.
[11C]Fluoroform shows broad utility as
an efficient labeling synthon for pro-
spective positron emission tomography
(PET) radiotracers.
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