[18F] Fluorination of o-carborane via nucleophilic substitution: Towards a versatile platform for the preparation of 18F-labelled BNCT drug candidates

Article abstract of DOI:10.1039/c3cc46695g

The mono-[¹⁸F]fluorination of o-carborane via nucleophilic substitution is reported. The new radiochemical transformation uses cyclotron produced [¹⁸F]F^- and a carboranyl iodonium salt. Further derivatization of the ¹⁸F-labelled carborane is achieved by formation of the C_c-lithio salt and reaction with an aldehyde.

Full text of DOI:10.1039/c3cc46695g

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[18F]Fluorination of o-carborane via nucleophilic
substitution: towards a versatile platform for the
preparation of F-labelled BNCT drug candidates†
Cite this: Chem. Commun., 2013,
49, 11491
18
Received 2nd September 2013,
Accepted 11th October 2013
a
a
b
a
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Kiran B. Gona, Vanessa Gomez-Vallejo, Daniel Padro and Jordi Llop*
DOI: 10.1039/c3cc46695g
www.rsc.org/chemcomm
The mono-[18F]fluorination of o-carborane via nucleophilic substitution
is reported. The new radiochemical transformation uses cyclotron
produced [18F]FÀ and a carboranyl iodonium salt. Further derivatization
of the 18F-labelled carborane is achieved by formation of the Cc-lithio
salt and reaction with an aldehyde.
Fig. 1 From left to right, chemical structure of o-, m- and p-carborane. J Stands
for B–H; ꢀ stands for C–H.
Boron Neutron Capture Therapy (BNCT) is a binary approach to
cancer therapy based on the ability of the stable isotope 10B to noninvasive way, the boron concentration in a specific region of
capture thermal neutrons via the nuclear reaction 10B(n, a, g)7Li. the body.
4
This reaction produces 478 keV gamma-rays, He particles and
Positron Emission Tomography (PET) and Single Photon
7Li recoil ions, the latter two having high linear energy transfer Emission Computerized Tomography (SPECT) are noninvasive,
(LET) properties.1 When 10B is selectively delivered to tumour in vivo molecular imaging techniques able to determine the
tissue, cellular damage and death can be triggered by external temporal distribution of a radiotracer after administration into
neutron irradiation.
a living organism. The suitability of these techniques for the
A great deal of effort in developing BNCT agents has focused determination of pharmacokinetic properties of drugs or new
on compounds containing multiple boron atoms.2 Within this chemical entities has been extensively reported in the literature.5
context, dicarba-closo-dodecaboranes, colloquially known as However, their application requires previous incorporation of a
carboranes and which exist as ortho (o-), meta (m-) and para radionuclide (i.e. a positron or a gamma emitter) into the
( p-) isomers (Fig. 1), are ideally suited candidates, because they chemical structure of the molecule under investigation.
have 10 boron atoms in their structure and present unique
To date, radiolabelling of carborane clusters with the radio-
structural features, high stability and rich chemistry, the latter nuclide covalently attached to the carborane cage has been
facilitating incorporation of targeting moieties covalently achieved using different iodine isotopes, i.e. Iodine-1256 and
attached to the carborane cage.
Iodine-131.7 Iodine-125 has a long half life (59.41 days) and
Although BNCT was first described in 1936 by Locher,3 up decays by electron capture,8 with subsequent emission of low
to date only two compounds (i.e. sodium borocaptate and energy gamma rays (35.5 keV). These physical characteristics
boronophenylalanine) have been clinically approved;4 the failure make the isotope attractive for in vitro studies, while in vivo
of BNCT as a routine treatment in clinical practice might be investigation is constrained due to effective attenuation in the
attributed to two main factors: (i) the complexity associated tissues. On the other hand, Iodine-131 emits higher energy
with the development of agents able to deliver a sufficient boron gamma rays (364 keV), which are more convenient for in vivo
load in the tumour without peripheral toxicity and (ii) the lack of imaging. However, bÀ particles are simultaneously emitted and
a technique able to determine, in vivo, in real time and in a a greater radiation dose is released to tissue.
Alternatively, attachment of the positron emitter Fluorine-18
(18F) to one of the boron atoms of the carborane cage can be
a Radiochemistry Department, Molecular Imaging Unit, CIC biomaGUNE, Paseo
Miramon 182, Parque Tecnologico de San Sebastian, 20009, San Sebastian, Spain. envisaged. 18F is an attractive radionuclide because it can be
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E-mail: jllop@cicbiomagune.es; Fax: +34 943 00 53 00; Tel: +34 943 00 53 33
produced in large quantities in biomedical cyclotrons, it has a
reasonably long half life (109.7 min) and a small positron
range. Consequently, good resolution and quantifiable PET
images can be obtained in vivo with unparalleled sensitivity;
for this reason, the incorporation of an 18F atom into one of the
b Nuclear Magnetic Resonance Platform, CIC biomaGUNE, San Sebastian, Spain
† Electronic supplementary information (ESI) available: Experimental prepara-
tion of 2, 3, [18F]4, 5 and [18F]5. 1H-, 1H{11B}-, 11B-, 11B{1H}-, {11C}- and 19F-NMR
and 1H,13C-HSQC NMR spectra of 5. UPLC/ESI-MS chromatogram and the MS
spectrum of 5. Elemental analysis of 5. See DOI: 10.1039/c3cc46695g
c
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Table 1 Fluorination of 3 to yield [18F]4
Entry
CP (mg mLÀ1
a
)
RT (1C)
Solvent
RCCb (%)
1
2
3
4
5
6
7
8
10
20
10
20
10
20
10
20
80
80
100
100
80
80
100
100
MeCN
MeCN
MeCN
MeCN
2M2B–MeCN (9/1)
2M2B–MeCN (9/1)
2M2B–MeCN (9/1)
2M2B–MeCN (9/1)
12 Æ 3
18 Æ 7
22 Æ 6
27 Æ 5
41 Æ 9
44 Æ 9
65 Æ 3
75 Æ 8
a
Concentration of the precursor 3. In all reactions, 1 mL of solution
b
was used. Radiochemical conversion, as determined by radio-HPLC;
Scheme 1 Synthesis of 9-[18F]fluoro-o-carborane ([18F]4). (i) ICl–AlCl3 in CH2Cl2,
reflux, 4 h; (ii) NaBO3–Ac2O, RT, 90 min; then toluene–H2SO4 (c), RT, 16 h;
(iii) kriptofix222–K2CO3, solvent.
mean Æ standard deviation, n = 3. 2M2B: 2-methyl-2-butanol.
[18F]FÀ and minute amounts of p-[18F]fluorotoluene (o5%). A
purification step based on solid phase extraction (SPE) was imple-
mented before approaching Cc-functionalization. The reaction mix-
ture was diluted with water (20 mL) and flushed through a C-18
cartridge. After rinsing with water, [18F]4 could be eluted with 1 mL
CH2Cl2 with overall decay-corrected radiochemical yield of 55%.
Albeit radiochemical purity as determined by radio-HPLC was
>98%, the precursor could not be removed quantitatively. Thus, a
semi-preparative radio-HPLC method was implemented and [18F]4
was prepared in 44% decay corrected radiochemical yield in an
overall synthesis time of 60 minutes. This radiochemical yield
significantly exceeds the one previously reported using electrophilic
substitution.9
boron atoms of the carborane cage would be a convenient
platform for the synthesis and in vivo evaluation of 18F-labelled
BNCT drug candidates.
The incorporation of an 18F atom into one of the boron atoms of
the carborane cage was reported as an abstract.9 The methodology
was based on direct reaction of cyclotron produced [18F]F2 with
o-carborane to yield 9-fluoro-o-carborane in 21% radiochemical
yield. Nevertheless, despite recent advances,10 the production of
[18F]F2 is still challenging and thus the [18F]-fluorination of carboranes
using the labelling agent [18F]FÀ, which can be produced at a multi
gigabecquerel level, is anticipated to be more convenient.
Further Cc derivatization of the 18F-labelled carborane was
investigated first in cold (non-radioactive) mode using 9-fluoro-
o-carborane as starting material, following the synthetic route
shown in Scheme 2. Despite the well known difficulties asso-
ciated with the monosubstitution of o-carborane,14 the reaction
of the latter with aldehydes via formation of the lithium salt
under controlled conditions is known to yield mainly mono-
substituted derivatives.15 In our case, 9-fluoro-o-carborane (4)
was treated with n-BuLi (THF solution, 1 : 3 molar ratio) at low
temperature and the resulting salt was reacted in situ with
4-methoxybenzaldehyde (MBA). The reaction of Cc-lithio
(unsubstituted) o-carborane with MBA has been reported pre-
viously.16 In this previous work, the formation of two enantio-
mers was described. In our case, the formation of four different
compounds could be anticipated due to the presence of the
In the current communication we report the unprecedented
[18F]-monofluorination of 1,2-dicarba-closo-dodecaborane (o-carborane)
using [18F]FÀ as a labelling agent. The attachment of the radio-
nuclide to one of the boron atoms of the carborane cage (B9 or
B12), which is based on the previously reported nucleophilic
substitution using phenyl(B-carboranyl)iodonium salts,11 leaves
the cluster carbon atoms (Cc) available for further derivatization
and incorporation of a targeting moiety. Generation of the
Cc-lithio derivative and subsequent reaction with an aldehyde
is also reported as a proof of concept of the potential preparation
of [18F]-fluorinated Cc-substituted carboranes.
The synthesis of 9-[18F]fluoro-o-carborane ([18F]4) was envisioned
through a three step sequence (see Scheme 1). 9-Iodo-o-carborane
(2) was synthesized by reaction of o-carborane with iodine mono-
chloride in the presence of AlCl3, following a previously reported
method (yield = 74%).12 The reaction of 2 with toluene in the
presence of NaBO3 and under strong acidic conditions yielded
p-tolyl-(9-o-carboranyl) iodonium bromide (3) in 30% yield.13 Finally,
[18F]4 was prepared from [18F]FÀ (aqueous solution) generated by
proton irradiation of [18O]H2O; [18F]FÀ was first trapped in an anion
exchange resin and subsequently eluted with K2CO3–H2O and
Kryptofix2.2.2–MeCN. After azeotropic evaporation of the solvent,
the dried potassium [18F]fluoride-Kryptofix2.2.2 complex was dis-
solved in dry MeCN and the resulting solution was reacted with 3.
Different experimental conditions (reaction temperature, reaction
time and solvent) were investigated for the 18F-fluorination step
(Table 1) and radiochemical conversion (RCC) was calculated from
chromatographic profiles. Higher RCC values were obtained when
a mixture of 2-methyl-2-butanol–MeCN (9/1) was used as a solvent.
At 100 1C, RCCs of 75 Æ 8% were obtained when 20 mg of 3 were
Scheme 2 Syntheses of 19F- and 18F-labelled Cc substituted o-carborane by
reaction of 9-fluoro- and 9-[18F]fluoro-o-carborane with MBA (4-methoxybenz-
used. Undesired radiolabeled by-products were identified as free aldehyde). (i) n-BuLi, THF, D; then MBA in THF. J Stands for B–H; ꢀ stands for C/C–H.
c
11492 Chem. Commun., 2013, 49, 11491--11493
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one of the boron atoms of the carborane cluster (B9), further
functionalization of the Cc atoms can be achieved. The for-
mation of the Cc-lithio derivatives and further reaction with
4-methoxybenzaldehyde to yield the corresponding alcohol as a
mixture of positional isomers and enantiomers is only a proof
of concept that the methodology reported here can be applied
to the preparation of 18F-labelled Cc-substituted carboranes.
This methodology might be extended to the reaction of
18F-labelled carboranes with other aldehydes, e.g. 1,2 : 3,4-di-
O-isopropylidene-a-D-galacto-hexadialdo-1,5-pyranose, which may
act as targeting moieties towards tumour tissue. Thus, the use
of Positron Emission Tomography for the evaluation of newly
developed BNCT drug candidates incorporating one or more
carborane cages is foreseen. This work is currently being con-
ducted in our laboratory.
We gratefully acknowledge Dr. Javier Calvo for assistance in
Fig. 2 (a) 1H{11B}-NMR spectrum of 5; (b and c) HPLC profiles of 5 (UV detector),
(b) and [18F]5 (radiometric detector), (c) obtained using a chiral column.
´
MS data interpretation and the Ministerio de Ciencia e Innovacion
(Grant number CTQ2009-08810) for financial support.
Notes and references
fluorine atom: two positional isomers each one corresponding to
the mixture of two enantiomers (5a–5d, Scheme 2). Compounds
5a–d eluted as a single fraction under preparative chromatographic
conditions (30% yield). Analysis of the purified fraction by 1H{11B}-
NMR (Fig. 2a) and by 19F-NMR (see ESI†) suggested the presence of
two positional isomers in a 1 : 1.5 molar ratio (Fig. 2a). Analytical
HPLC using a chiral column confirmed that compound 5 was
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1H-, 1H{11B}-, 11B-, 11B{1H}-, {13C}-NMR and 1H,13C-HSQC analyses
corroborated the presence of two positional isomers.
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trivial. [18F]FÀ is produced in cyclotrons with high specific
activity (in our hands, B200 GBq mmolÀ1 at the end of the
irradiation). Consequently, only nanomolar amounts of [18F]4
were obtained at the end of the synthetic procedure shown in
Scheme 1. Under these conditions, precise control of the
desired molar ratio between compound [18F]4 and n-BuLi was
anticipated to be unachievable; to overcome this problem,
[18F]4 was diluted with 4 before approaching the synthesis of
[18F]5. Practically, the solution of [18F]4 resulting from elution
of the C-18 cartridge with CH2Cl2 was first evaporated to
dryness for complete removal of residual water. To the dry
residue, compound 4 (10 mg, 62 mmol) was added and the solid
was dried again under nitrogen flow (50 1C, 5 min). Treatment
with n-BuLi (2.6 M solution in THF, 71 mL, 186 mmol) and
further reaction with MBA at 50 1C for 10 minutes yielded a
major radioactive species (B80% RCC), which could be purified
using radio-HPLC. Analysis of the collected fraction by chiral
radio-HPLC confirmed the presence of 4 radioactive species,
which co-eluted with non-radioactive 5a–5d (Fig. 2c).
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 11491--11493 11493