Stable aluminium fluoride chelates with triazacyclononane derivatives proved by X-ray crystallography and 18F-labeling study

Article abstract of DOI:10.1039/c1cc13151f

A single crystal structure of an aluminium-fluoride complex of a model compound (NODA-benzyl) was studied to understand the co-ordination chemistry. Series of ligands with an extra carboxylic acid linker for biomolecule conjugation were studied for improv

Full text of DOI:10.1039/c1cc13151f

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COMMUNICATION
Stable aluminium fluoride chelates with triazacyclononane derivatives
18
proved by X-ray crystallography and F-labeling studyw
Dinesh Shetty,ab Soo Young Choi, Jae Min Jeong,* Ji Youn Lee,ab Lathika Hoigebazar,ab
c
ab
ab
a
ab
a
Yun-Sang Lee, Dong Soo Lee, June-Key Chung, Myung Chul Lee and
c
Young Keun Chung
Received 28th May 2011, Accepted 11th July 2011
DOI: 10.1039/c1cc13151f
1
2–14
18
It was an important progress in F-labeling
A single crystal structure of an aluminium-fluoride complex of a
model compound (NODA-benzyl) was studied to understand
the co-ordination chemistry. Series of ligands with an extra
carboxylic acid linker for biomolecule conjugation were studied
labeling.
chemistry, but structural studies on complexes of aluminium
fluoride and NOTA derivatives were not conducted to determine
how the chelating agent structures are related to labeling
efficiency.
1
8
for improved F-labeling applications.
3
+
ꢀ
It has been reported that Al binds with F more strongly
1
than sixty other metal ions tested, and forms a much
5
Positron emission tomography (PET) is an important imaging
modality for assessing neurologic, oncologic and cardiologic
ꢀ
16
stronger complex with F than with any other halide.
Generally, most halides form more stable complexes with
1
–3 18
abnormalities.
because of excellent imaging properties, and thus, the development
F is the most widely used PET radioisotope
3
+
3+
than with Al , but F binds to Al
ꢀ
3+
Fe
strongly than to Fe
10 times more
1
of F-labeled bioactive molecules has become an important
8
3+ 17
.
4
–7
area.
Most established F-labeling procedures start with the
In the present study, we determined the chemical structure
19
of an Al– F-1,4,7-triazacyclononane-1,4-diiacetic acid
1
8
1
8
trapping of the [ F]fluoride ion on an anion-exchange resin
and subsequent elution with an organic/aqueous solution
8
containing base that can act as a phase-transfer catalyst. The
(NODA)-benzyl complex (6) using X-ray crystallography,
and synthesized various NODA derivatives containing different
linkers suitable for subsequent conjugation to biomolecules
(Fig. 1, Scheme S1, ESIw).z In vitro and in vivo stabilities and
biodistribution studies were performed on the labeled complexes
using mice.
1
8
18
eluate is then dried to activate the [ F]fluoride. F-labeling is
performed using a nucleophilic substitution reaction in organic
solvent with heating in the presence of a base catalyst. Finally,
the organic solvent is removed for clinical application.
However, this general procedure has inherent problems in
the labeling of biomolecules, because radiolabeling in organic
solvent at high temperature detrimentally affects biomolecules
such as peptides, proteins, and nucleic acids. Furthermore, the
drying steps required are time consuming and make automation
of the procedure difficult. Thus, the developments of new
The common building block compound 1 was synthesized
from 1,4,7-triazacyclononane by reacting with tert-butyl-
bromoacetate. Purification of reaction products by column
chromatography gave a mixture of mono-, di-, and tri-
substituted products. This mixture was purified using a pH
controlled workup method. NODA was obtained by acid
9
–11
methods without drying steps are actively being pursued.
18
A novel method for the one step F-labeling of peptides using
1
Al– F) complex formation with 1,4,7-triazacyclononane-
,4,7-triacetic acid (NOTA) derivatives has been reported,
which did not need a drying step neither before nor after the
8
+2
(
1
a
Department of Nuclear Medicine, Department of Radiation Applied
Life Science and Institute of Radiation Medicine,
Seoul National University College of Medicine, Seoul, Korea.
E-mail: jmjng@snu.ac.kr
Clinical Research Institute and Cancer Research Institute,
Seoul National University Hospital, Seoul, Korea
b
c
Intelligent Textile System Research Center and Department of
Chemistry, Seoul National University, Seoul, Korea
w Electronic supplementary information (ESI) available: Experimental
methods, spectroscopic data and detailed crystallographic data.
CCDC 813724. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc13151f
Fig. 1 Chemical structures of NOTA, NODA and derivatives.
This journal is c The Royal Society of Chemistry 2011
9
732 Chem. Commun., 2011, 47, 9732–9734

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hydrolysis of 1. Nucleophilic substitution of 1 with benzyl
bromide and succinic anhydride followed by acid hydrolysis
gave 3 and 4 as hydrochloride salts, respectively. Furthermore,
a series of derivatives containing various acid linkers (5a–c),
which could be used for conjugation to biomolecules via active
ester formation, were synthesized to determine relations with
labeling yields. Compound 5a was prepared by reacting 1 with
N(2)–Al(1)–F(1) and N(3)–Al(1)–O(1) were 165.61(8)1, 174.28(7)1
and 165.71(7)1, respectively. The average Al–N bond
˚
length was 2.083 A, which is slightly longer than that in the
1
8
˚
Al–NOTA complex (average Al–N = 2.06 A), possibly due
to bonding between the aluminium and the fluorine atom. The
˚
average Al–O bond length was 1.86 A, which is comparable to
˚
that of Al–NOTA complex (average Al–O distance = 1.84 A).
˚
The Al(1)–F(1) bond length was 1.709(14) A, which is similar
to the calculated bond length for the ground state of diatomic
3
-bromopropionic acid and subsequent acid hydrolysis,
whereas 5b and 5c were synthesized by reacting 1 with
protected 4-bromobutyric acid or 5-bromovaleric acid and
subsequent hydrolysis using lithium hydroxide, respectively.
We examined the solid state structure of complex 6 by single
crystal X-ray analysis. The model complex 6 was obtained by
reacting 3 with AlCl3 in sodium acetate buffer (pH 3.5),
followed by addition of NaF under heating (Fig. S1, ESIw).
Complex formation was confirmed by a molecular ion peak by
1
9
Al–F. This value indicates the strong coordination bond
formation between the aluminium and fluoride. Three intra-
and inter-molecular hydrogen bonds were observed (Fig. S3,
˚
ESIw) between O(1) and O(11) (2.8718(31) A), O(2) and
˚
O(12) (2.8653(38) A), and O(11) and neighboring O(12_1)
˚
(2.8114(38) A).
1
To study F labeling, we first compared NOTA and NODA
8
+
18
ꢀ
mass/ion spray positive ionization (MS/ESI ) analysis and the
at different concentrations and pH.
F
was prepared as
was prepared by
in 1.0 mL of sodium
1
previously described, and ( F–Al)
0
18
2+
complex was purified by reverse phase high performance liquid
chromatography (RP-HPLC). Single crystals were obtained
by evaporating a solution of pure 6 from a water:EtOH
mixture (1 : 9, v/v) at 4 1C. X-Ray crystallography data are
summarized in Table S1 (ESIw).
3+
mixing Al (11 to 180 nmol) with
18
ꢀ
F
acetate buffer (0.1 M, pH 4) at room temperature for 10 min.
2+
1
8
Each ligand (12.5 to 200 nmol) was mixed with the ( F–Al)
solution and reactions were monitored at room temperature
and 110 1C after 10 min. Labeling efficiencies were determined
using Instant Thin Layer Chromatography-Silica Gel (ITLC-SG)
and labeled products were purified using an Alumina-N
cartridge (Fig. 3a). Purities were also checked by autoradio-
graphy (Fig. S2, ESIw). The labeling efficiencies of NOTA and
NODA at room temperature were only 0 and 1.28%, respectively,
2
+
The (Al–F)
ion fits well inside the nitrogen/oxygen
coordination cavity of 3. The results obtained confirmed that
five 5-membered rings are composed around the coordinated
central aluminium atom on which a fluorine atom is bonded
covalently (Fig. 2). The complex forms a discrete monomer
and was found to have a distorted geometry due to the
restricted fixation of the chelating ligands. The nitrogen atom
attached to the benzyl group was also found to participate in
the chelation. The coordination includes three nitrogen atoms
occupying one face and two oxygen atoms of carboxylate
occupying the opposite face. Distortion of the coordination
from the regular octahedral shape was evidenced by the
compression of N–Al(1)–N angles (83.301) and the expansions
of O(1)–Al(1)–O(3) (96.94(7)1), F(1)–Al(1)–O(3) (98.56(7)1)
and F(1)–Al(1)–O(1) (93.69(7)1) angles from the expected
9
01. As a result, the trans angles of N(1)–Al(1)–O(3),
˚
Fig. 2 Crystal structure of complex 6. Selected bond lengths [A] and
angles [1]: Al(1)–N(1) 2.0497(18), Al(1)–N(2) 2.0863(19), Al(1)–N(3)
2
1
.1125(18), Al(1)–O(1) 1.8856(16), Al(1)–O(3) 1.8446(16), Al(1)–F(1)
.7090(14), N(1)–Al(1)–N(2) 82.89(7), N(1)–Al(1)–N(3) 83.66(7),
Fig. 3 Comparative labeling efficiencies under different reaction con-
ditions. (a) At different pH values and reaction temperatures. (b) At
different chelator concentrations.
N(2)–Al(1)–N(3) 83.34(7), O(3)–Al(1)–O(1) 96.94(7), F(1)–Al(1)–
O(1) 93.69(7), F(1)–Al(1)–O(3) 98.56(7).
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9732–9734 9733

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1
8
a
18
18
Table 1 Al– F labeling of various ligands
for F–Al–NODA, 0.06 ꢁ 0.01% for F–Al–3, and 0.79 ꢁ
18
.12% for F–Al–5b). This result suggests the possibility of
0
using these compounds for in vivo studies.
b
Labeling efficiency (%)
Labeled compound
R group
–H
18
18
NODA
89.3 ꢁ 2.3
83.0 ꢁ 7.7
32.9 ꢁ 6.1
23.6 ꢁ 7.7
86.1 ꢁ 2.1
77.8 ꢁ 3.4
10.9 ꢁ 0.4
Biodistribution studies on F–Al–3 and F–Al–5b in balb/c
mice showed low bone uptakes, which confirmed stabilities in vivo
3
4
–CH
2 2 2
–CO(CH ) CO H
2
Ph
18
18
(
Fig. S5 and S6, ESIw). F–Al–3 and F–Al–5b were rapidly
5a
5b
5c
–(CH
–(CH
–(CH
2
)
2
)
2
)
2
3
4
CO
CO
CO
2
H
2
H
2
H
cleared from blood (B9% at 60 min) and showed low residual
activities in tissues. The high uptake and low retention in kidneys
18
indicated excretion via the renal route. However, F–Al–3
NOTA
–CH
2
2
CO H
a
18 2+
NODA derivatives were labeled with (Al– F) at a concentration
showed high uptakes in the liver and kidneys, indicating excretion
via both the renal and hepatobiliary routes. It is clear that the
of 50 nM in sodium acetate buffer (pH 4) at 110 1C for 10 min.
Determined by ITLC: expressed as radioactivity percentage of the
product areas versus total areas.
b
18
negative charge of F–Al–5b and the lipophilic benzyl group of
18
F–Al–3 make these differences in biodistribution.
In summary, this study shows how fluoride is bound to
aluminium in F–Al–NODA using X-ray crystallography, and
but these increased up to 16% and 89% at 110 1C, respectively.
The optimal pH range for labeling was between 4.1 and 4.4
18
describes the results of F-labeling studies using newly synthe-
sized ligands. It demonstrates that the existence of a competing
intra-molecular ligand which can form a 5- or 6-membered ring is
an important factor for binding fluoride to aluminium.
We acknowledge support from Converging Research Center
Program (2009-0082087) and NRL grant (R0A-2008-000-
20116-0) from MEST.
(
Fig. 3a). Ligand concentration also was an important factor
for labeling efficiency (Fig. 3b). High concentrations of 2 and
NOTA (165 mM) gave maximum labeling yields (95% and
0%, respectively), and these decreased to 52% and 3%,
1
respectively, when their concentrations were reduced to 11 mM.
NODA consistently showed higher labeling efficiency than
NOTA demonstrating that the presence of the third carboxylic
group in NOTA compared to NODA interferes the binding of
fluoride to aluminium.
Notes and references
z Crystal data for C17
H
23Al
1
F
1
N
3
4
O ꢂ2H
2
O (293 K): M = 415.40,
˚
/a, a = 13.9007(6) A, b = 7.1927(5) A,
Five more NODA derivatives were synthesized to identify
ligands that improve labeling yield (Scheme S1, ESIw and
Table 1). Using the procedure described above, these ligands
˚
monoclinic, space group P2
1
3
˚
˚
c = 19.9931(12) A, b = 106.473(3)1, V = 1916.93(19) A , Z = 4,
ꢀ3
ꢀ1
r
calc. = 1.439 g cm , absorption coefficient = 0.156 mm , total
reflections collected 7515, unique 4371 (Rint = 0.0471), GOF = 1.008,
= 0.0499, R = 0.1051 (I > 2s(I)).
1
8
(
50 nM) were labeled with F at pH 4. Compound 5a, which
R
1
w
has an ethyl spacer between the backbone ring and the
carboxylic acid group showed lower labeling efficiency
1
2
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23.6%) than 5b having a propyl (86.1%) or 5c having a butyl
77.8%) spacer. The low yield of 5a is probably due to
(
interference of the propionic carboxylic acid during complex
3
3
+
¨
formation. Thus, Al
hinders the coordination bonding between
can form stable complex 5a, which
ꢀ
1
8
F and chelated
6
Al. A similar phenomenon is observed for NOTA. This
observation suggests that butyric or valeric acid substitutes
would give high labeling efficiencies.
7
8
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the 21 amine of NODA can form a 5- or 6-membered ring with
3
+
3+
Al , then fluoride binding yield to Al decreased. However,
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supported by the fact that the labeling efficiencies of 2 (no
substituent) and 3 (benzyl substituent) are high because their
substituents cannot form rings with aluminium (Table 1).
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despite the fact that it cannot form a 6-membered ring. We
believe that this is due to the presence of a carbonyl group
adjacent to a substituted nitrogen atom, which could give a
negative effect for the formation of a stable complex due to the
electron-withdrawing effect.
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1
1
1
8
18
Both F–Al–NODA and F–Al–3 were found to be stable
in human serum at 37 1C and in sodium acetate buffer (pH 4)
at room temperature for at least 2 h (Fig. S4, ESIw). Protein
binding studies were performed after incubating the labeled
compounds with human serum at 37 1C. All the compounds
studied showed very low protein binding at 60 min (0.23 ꢁ 0.06%
1
1
629–630.
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9
734 Chem. Commun., 2011, 47, 9732–9734
This journal is c The Royal Society of Chemistry 2011