Direct [11C]Methylation of Amines from [11C]CO2 for the Synthesis of PET Radiotracers

Article abstract of DOI:10.1002/ejoc.201500924

[¹¹C]Methylation (t_1/2 = 20.4 min) is a main labeling strategy for the development of PET (positron emission tomography) radiotracers. A straight radiomethylation of amines with cyclotron-produced ¹¹CO₂ has been developed to obtain various radiolabeled compounds with good radiochemical yields. This strategy is a new approach to simplifying radiolabeling processes and transpositions onto automatic synthesizers and has been applied to the synthesis of radiolabeled drugs and a PET radiotracer used in the study of Alzheimer's disease. From producer to consumer, no intermediate: Direct [¹¹C]methylation of amines with cyclotron-produced ¹¹CO₂, and without intermediate transformations, provides a simpler entry to various PET radiotracers, such as β-amyloid radioligand [¹¹C]PIB.

Full text of DOI:10.1002/ejoc.201500924

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500924
Direct [11C]Methylation of Amines from [11C]CO2 for the Synthesis of PET
Radiotracers
François Liger,[a] Ture Eijsbouts,[a,b] Florence Cadarossanesaib,[a] Christian Tourvieille,[a]
Didier Le Bars,[a,b] and Thierry Billard*[a,b]
Keywords: Radiochemistry / Isotopic labeling / Radiotracers / Alkylation / Carbon-11 / Positron emission tomography
[
11C]Methylation (t1/2 = 20.4 min) is a main labeling strategy
yields. This strategy is a new approach to simplifying radio-
labeling processes and transpositions onto automatic synthe-
sizers and has been applied to the synthesis of radiolabeled
drugs and a PET radiotracer used in the study of Alzheimer’s
disease.
for the development of PET (positron emission tomography)
radiotracers. A straight radiomethylation of amines with
cyclotron-produced 11CO2 has been developed to obtain
various radiolabeled compounds with good radiochemical
by reaction with Grignard and organolithium reagents[3a,6]
or, more recently, by cross-coupling reactions with boronic
acids.[7] The synthesis of 11C-labeled ureas and carbamates
have also been described.[8]
Introduction
PET (positron emission tomography) imaging is a nu-
clear medicine technology increasingly used not only in rou-
tine clinical diagnosis[1] but also in biomedical research and
drug development.[2] These growing applications of PET
imaging require the development of more and more radio-
tracers. However, the short-lived positron emitters impose
specific synthetic methods, easily transferable onto auto-
matic synthesizers.[1c,3]
Methylation reactions of heteroatoms, in particular
nitrogen, is one of the more classic approaches used for the
11C-labeling of molecules,[3a,6] as illustrated by the radio-
synthesis of the β-amyloid imaging radioligand [11C]PIB.[9]
These radiomethylations require preliminary transforma-
tion of 11CO2 into very reactive species, 11CH3I or
11CH3OTf.[3a,6,10] These reagents are often used under basic
conditions in a strictly anhydrous environment. Such condi-
tions can require additional protection and deprotection
steps of incompatible functional groups that substantially
prolong and complicate the radiolabeling process. Further-
more, from a technical point of view, the simplest radio-
synthesis, with a minimum of steps, will lead to an easier
automation.[10,11] Consequently, the direct radiomethylation
of amines using 11CO2, generated in a cyclotron, appears
very attractive. Some previous work in this area has been
described,[12] however, these strategies require a multistep
procedure: 11CO2 is first fixed with silylated amines (pre-
viously prepared) to give O-silyl carbamates, which are sub-
sequently reduced in situ by LiAlH4 (a harsh reducer).
Therefore, a more direct and milder procedure could be
beneficial.
In general, 18F-labeled molecules are preferred for pro-
duction and use in routine clinical imaging due to the
longer half-life of fluorine-18, as illustrated by the recent
development of various methods for radiofluorination.[4]
Nevertheless, despite the short half-life of carbon-11 (t1/2
= 20.4 min), 11C-labeled molecules still represent valuable
targets for research studies. Indeed, easy-to-introduce
carbon substituents are present in a large majority of drugs
and bioactive compounds. Consequently, a direct radio-
labeled equivalent of a molecule is easier to envisage.
The radioisotope carbon-11 is usually produced in a
cyclotron by the 14N(p,α)11C nuclear reaction in the form
of either of two precursors, 11CH4 or 11CO2. The direct use
of these two compounds is quite limited and they are usu-
ally converted into more reactive species. Some direct syn-
thetic applications of 11CO2 have previously been de-
scribed[5] for the production of 11C-labeled carboxylic acids
Results and Discussion
[a] CERMEP - in vivo imaging, Groupement Hospitalier Est,
59 Bd Pinel, 69677 Lyon, France
www.cermep.fr
Recently, some groups described the application of CO2
as a C1 building block for the catalytic methylation of
amines.[13] Such elegant work appeared in perfect adequacy
with our effort to develop a simpler and more direct ap-
proach to the [11C]methylation of amines. This approach
would eliminate the preliminary time-consuming steps,
[b] Institute of Chemistry and Biochemistry (ICBMS - UMR
CNRS 5246), Université de Lyon, Université Lyon 1, CNRS,
43 Bd du 11 novembre 1918, 69622 Lyon, France
E-mail: Thierry.billard@univ-lyon1.fr
www.FMI-Lyon.fr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500924.
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Eur. J. Org. Chem. 2015, 6434–6438

[11C]Methylation of Amines
which are also sometimes the source of failure during the tion time of 10 min, the radiochemical yield was halved (en-
automatic radiolabeling, required for the preparation of re- try 4), even with twice the amount of catalyst (entry 5). The
active methylating reagents. We focused our attention on presence of both ZnCl2 and IPr proved to be essential (en-
the strategy developed by Cantat and co-workers who em- tries 6–8). However, preliminary complex formation
ployed a standard and simple catalytic system {ZnCl2, IPr [ZnCl2·IPr][14] appears to be deleterious for radiolabeling,
[IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imid- in contrast with the work of Cantat and co-workers (en-
azol-2-ylidene]} without the use of additional hydrogen try 9).[13b] To increase 11CO2 concentration in solution to
gas.[13b] As in their article, N-methylaniline (1a) was selected potentially accelerate the reaction, TMEDA (N,N,NЈ,NЈ-
as a model substrate (Table 1).
tetramethylethylenediamine) as CO2 trap was added, as de-
scribed by Pike and co-workers.[7] However, no reaction was
then observed: TMEDA certainly competes with 1a to
catch CO2 (entry 10). Finally, microwave irradiation also
appears to be detrimental for the reaction; the increased
molecular agitation contributes to breaking the amine–CO2
interaction, which seems to be essential to initiate the meth-
ylation (entry 11). Furthermore, Dyson and co-workers de-
scribed a similar methylation under metal-free conditions
in the presence of another carbene, namely NHC [1,3-
bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-yl-
idene].[13e] Unfortunately, under our conditions, the ex-
pected radiolabeling was not observed (entry 12).
Moreover, the trapping efficiency of cyclotron-produced
11CO2 was measured under the optimal conditions (Table 1,
entry 3) at a trapping flow rate of 10 mL/min. Untrapped
11CO2 was confined within an Ascarite® cartridge attached
to the reactor outlet. Up to 80% of the delivered 11CO2
remained inside the reactor at the end of entrapment (trap-
ping time: 60 s). More precisely, the trapping efficiency de-
pends on the basicity of the amine: 80% with 1d, 70% with
1a, and 65% with 1e.
Table 1. Direct [11C]methylation of 1a with 11CO2.[a]
Entry [1a] [m]
Solvent
T [°C]
t [min]
RCY[b] [%]
1
2
3
4
0.231
0.231
0.023
0.023
0.023
0.023
0.023
0.023
0.023
0.023
0.023
0.023
THF
80
20
20
20
10
10
20
20
20
20
20
5
3
40
43
23
27
0
0
3
6
0
diglyme
diglyme
diglyme
diglyme
diglyme
diglyme
diglyme
diglyme
diglyme
diglyme
diglyme
150
150
150
150
150
150
150
150
150
150
150
5[c]
6[d]
7[e]
8[f]
9[g]
10[h]
11[i]
12[j]
4
0
20
With the optimal reaction conditions in hand (Table 1,
entry 3), the scope of this new radiolabeling strategy was
investigated using various amines (Scheme 1).
[a] Reagents and conditions: Solvent (400 μL), ZnCl2 (1.3 mg), IPr
(3.5 mg), PhSiH3 (23 μL). [b] Radiochemical yields (RCY) were
estimated from trapped 11CO2 within the reactor and are decay-
corrected from the end of 11CO2 trapping inside the reactor. [c]
ZnCl2 (3.5 mg), IPr (7 mg). [d] Without ZnCl2 and IPr. [e] Without
IPr. [f] Without ZnCl2. [g] With pre-formed complex [IPr·ZnCl2]
(6 mg). [h] With addition of TMEDA (70 μL). [i] Under microwave
irradiation (140 W). [j] Without ZnCl2, with NHC (2 mg) instead
of IPr, and Ph2SiH2 (23 μL) instead of PhSiH3.
These [11C]methylation reactions generally gave satisfac-
tory radiochemical yields with aromatic or aliphatic amines.
Primary amines were selectively monomethylated and, gen-
erally, only a small amount of dimethylation was observed.
With anilines, the presence of electron-withdrawing or
-donating substituents seemed to have no relevant influence
With a similar catalytic system in the same solvent and (3b–3h). In accordance with the supposed mechanism
at the same temperature as used in the optimal described (Scheme 2), electron-withdrawing groups should favor the
conditions, only 3% of the expected methylation was ob- first step of CO2 trapping by the amine, whereas electron-
served in 20 min (entry 1). Such a disappointing result can donating substituents should more favor the second and
be rationalized by the short reaction time, imposed by the third reduction steps. The reaction is also compatible with
11C half-life, compared with the 20 hours required in the the ester function (3f). In the case of the cyano group, a
original publication. To increase the reaction kinetics, a sol- more moderate yield was observed, maybe due to the par-
vent with similar properties and polarity but with a higher tial chelation of ZnCl2 by the nitrogen atom partially
boiling point was selected to increase the reaction tempera- quenching the reaction. Starting from p-bromoaniline (1h),
ture. Thus, in diglyme at 150 °C, a satisfactory radiochemi- a satisfactory yield of the expected product was obtained,
cal yield of 40% was obtained (entry 2). Because of the despite the formation of 3b, which arises from the partial
small molar amount of 11CO2 generated by the cyclotron reduction of the C–Br bond. This radiolabeling also ap-
(e.g., 1 Ci, i.e., 37 GBq, corresponds to 108 pmol), the pre- pears to be chemoselective because in the case of amino
cursor (1a) quantity was decreased to match the standard alcohol 1k, only N-methylation was observed (see 3k).
amount (ca. 1–2 mg) used in classic radiolabeling without However, to achieve similar radiochemical yields compared
any significant change in yield (entry 3). It should be noted with other amines, an excess of the amino alcohol (vs.
that, for practical reasons (weighing), the same amount of ZnCl2) was required. This could be explained by chelate
catalyst was preserved, bringing the catalyst amount to a formation between the amino alcohol and zinc, which could
stoichiometric level (relative to 1a). With a shortened reac- deactivate the nitrogen atom and thus would disfavor the
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T. Billard et al.
SHORT COMMUNICATION
Scheme 1. Direct [11C]methylation of amines with 11CO2. RCY values are mean (n = 3). They were estimated on the basis of trapped
11CO2 within the reactor and are decay-corrected from the end of 11CO2 trapping inside the reactor. [a] Ratio 3h/3b = 60:40. [b] With
2 equiv. of 1k vs. ZnCl2.
trapping of CO2. Finally, this methodology can be applied ported.[15] Under our conditions, the CO2–amine interac-
to the synthesis of radiolabeled drugs, such as ephedrine tion appeared to be determining. Indeed, an increase of mo-
(sympathomimetic amine, 3k), imipramine (tricyclic antide- lecular agitation by microwave irradiation[16] disfavored the
pressant, 3j), and the β-amyloid radiotracer [11C]PIB (Alzh- reaction by breaking this interaction. In addition, starting
eimer disease diagnostic, 3l).
from imipramine hydrochloride 3j, no labeling was ob-
served due to the impossibility of the CO2–amine interac-
tion. During the radiomethylation of 1a, the corresponding
labeled formamide could be identified by HPLC. These ob-
servations led us to propose the pathway shown in
Scheme 2: formation of the CO2–amine complex, then first
reduction to formamide, and thereafter second reduction to
the expected methylamine.
This direct 11C-labeling strategy was applied to the effec-
tive production of the β-amyloid radiotracer [11C]PIB[17]
(3l) with HPLC purification and a final formulation step
(Scheme 3). [11C]PIB was produced in sufficient radioactive
quantity (57 mCi, RCY = 38%) with a high radiochemical
and chemical purity. Such a result is very encouraging and
demonstrates the possibility of extrapolating this new strat-
egy to a “productive scale” of radiotracers. Specific radioac-
Scheme 2. Hypothesized mechanism for the [11C]methylation with
11CO2.
In terms of mechanism, Cantat and co-workers proposed tivity, however, remained weak (SA = 15 GBq/μmol), cer-
the over-reduction of an intermediate formamide tainly due to excess of environmental CO2 which dilutes
(Scheme 2, steps 2 and 3).[13b] Furthermore, radiomethyl- 11CO2. Nevertheless, in a classical production of [11C]PIB
ation by reduction of a formamide has been previously re- (the [11C]methyl triflate method) with the same synthesizer
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[11C]Methylation of Amines
Scheme 3. Production of β-amyloid radiotracer [11C]PIB. The RCY value was estimated on the basis of trapped 11CO2 within the reactor
and is decay-corrected from the end of 11CO2 trapping inside reactor.
and a similar production of 11CO2 by the same cyclotron,
PET Imaging: Physics, Chemistry, and Regulations, Springer,
New York, 2010.
[11C]PIB was obtained with a similar radiochemical yield of
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Experimental Section
Synthesis of [11C]PIB (3l): Cyclotron-produced 11CO2, trapped
(953 mCi) on a column of molecular sieves (4 Å), was released by
purging the heated column (350 °C) with He gas and bubbled,
through Teflon® lines, into a reactor containing ZnCl2 (1.3 mg),
IPr (3.5 mg), and 1l (1.7 mg) in diglyme (400 μL) and PhSiH3
(23 μL) cooled to 0 °C (11CO2 traps: 711 mCi). The Teflon® lines
were removed when the radioactivity content reached a maximum
and the reacting mixture was heated at 150 °C for 20 min. After
cooling, the HPLC mobile phase (2.2 mL) was added to the mix-
ture and the resulting solution was purified by HPLC on a Waters
Symmetry-Prep C18 column (7 μm, 7.8ϫ300 mm) at a flow rate of
4 mL/min (H2O/MeCN, 60:40, v/v). The [11C]PIB fraction (tR
=
11 min) was collected, diluted in water (40 mL), and formulated by
solid-phase extraction (SPE). After rinsing with water (10 mL), the
purified product was released from the Sep-Pak (Waters Plus tC18)
with ethanol (1 mL) and water (2 mL) in a sterile vial (57 mCi).
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Acknowledgments
This work was supported by the European Union (EU) through
the project Radiochemistry for Molecular Imaging (project number
EU FP7-PEOPLE-2012-ITN-RADIOMI).
Dr.
Anis
Tlili
[ICBMS – UMR, Centre National de la Recherche Scientifique
(CNRS) 5246] is thanked for fruitful discussions. Frédéric Bonnefoi
and Thibaut Iecker (CERMEP - in vivo imaging) are acknowl-
edged for their technical assistance. The authors wish to thank the
Centre National de la Recherche Scientifique (CNRS) for financial
support.
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Published Online: September 3, 2015
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