Rhodium-mediated [11C]Carbonylation: A library of N-phenyl-N′-{4-(4-quinolyloxy)-phenyl}-[11C]-urea derivatives as potential PET angiogenic probes

Article abstract of DOI:10.1002/jlcr.1582

As part of our ongoing investigation into the imaging of angiogenic processes, a small library of eight vascular endothelial growth factor receptor-2 (VEGFR-2)/platelet-derived growth factor receptor β dual inhibitors based on the N-phenyl-N′-4-(4-quinolyloxy)-phenyl-urea was labelled with ¹¹C (β⁺, t_1/2 = 20.4 min) in the urea carbonyl position via rhodium-mediated carbonylative cross-coupling of an aryl azide and different anilines. The decay-corrected radiochemical yields of the isolated products were in the range of 38-81% calculated from [ ¹¹C]carbon monoxide. Starting with 10.7±0.5 GBq of [ ¹¹C]carbon monoxide, 1-[4-(6,7-dimethoxy-quinolin-4-yloxy)-3-fluoro- phenyl]-3-(4-fluoro-phenyl)-[¹¹C]-urea (2.1 GBq) was isolated after total reaction time of 45 min with a specific activity of 92±4 GBq μmol^-1. Copyright

Full text of DOI:10.1002/jlcr.1582

Research Article
Received 5 August 2008,
Revised 18 December 2008,
Accepted 19 December 2008
Published online 26 January 2009 in Wiley Interscience
(www.interscience.wiley.com) DOI: 10.1002/jlcr.1582
Rhodium-mediated [11C]Carbonylation:
a library of N-phenyl-N0-{4-(4-quinolyloxy)-
phenyl}-[11C]-urea derivatives as potential
PET angiogenic probes
a
b
b,c
Ã
Ohad Ilovich, Ola Aberg, Bengt Langstrom, and Eyal Mishania
˚
˚
¨
As part of our ongoing investigation into the imaging of angiogenic processes, a small library of eight vascular endothelial
growth factor receptor-2 (VEGFR-2)/platelet-derived growth factor receptor b dual inhibitors based on the N-phenyl-N0-4-(4-
quinolyloxy)-phenyl-urea was labelled with 11C (b1, t1/2 = 20.4 min) in the urea carbonyl position via rhodium-mediated
carbonylative cross-coupling of an aryl azide and different anilines. The decay-corrected radiochemical yields of the
isolated products were in the range of 38–81% calculated from [11C]carbon monoxide. Starting with 10.770.5 GBq of
[
11C]carbon monoxide, 1-[4-(6,7-dimethoxy-quinolin-4-yloxy)-3-fluoro-phenyl]-3-(4-fluoro-phenyl)-[11C]-urea (2.1 GBq) was
isolated after total reaction time of 45 min with a specific activity of 9274 GBq mmolꢀ1
.
Keywords: carbonylation; carbon-11; angiogenesis; VEGFR-2; PET
should be taken into account regarding the labelling position
within the chemical structure of the suggested tracer and the
selection of the radioisotope.10–13 Recently, the design and
synthesis of a series of N-phenyl-N0-4-(4-quinolyloxy)-phenyl-
urea derivatives as VEGFR-2/PDGFR-b selective and potent
inhibitors was reported.14 These derivatives offer different
labelling approaches at various positions and with different
positron emitters such as 18F (b1, t1/2 = 109.8 min) or
11C (b1, t1/2 = 20.4 min) (Figure 1). General 18F-labelling
procedure of diaryl-urea containing molecules using either
4-nitrophenyl chloroformate15 or triphosgene16 was developed.
In the latter article, complete chemical characterization, evalua-
tion of receptor phosphorylation inhibitory potency and
selectivity toward other RTKs were described. One of the
advantages of labelling with 18F is that it offers a longer in vivo
surveillance time period than with 11C; on the other hand,
however, the shorter half-life of 11C offers unique opportunities
for multiple in vivo examinations in the same subject over a
shorter period of time. Moreover, since 11C, as opposed to 18F,
can be incorporated into various positions of the chemical
Introduction
Angiogenesis, the process by which new capillaries are created
from pre-existing blood vessels, is essential for the growth of
solid tumors.1,2 The vascular endothelial growth factor receptor-
2 (VEGFR-2/KDR/flk-1) plays a key role in angiogenic processes
and overexpression of both the VEGFR-2 protein and mRNA was
found in tumor-associated endothelial cells, yet not in the
vasculature surrounding normal tissues.3–5 Inhibition of the
VEGFR-2 has been shown both to induce tumor regression and
reduce metastatic potential in preclinical models.6 The platelet-
derived growth factor receptor b (PDGFR-b) has also been
associated with angiogenic processes. The pericytes that coat
the angiogenic blood vessels confer stability to the capillary
walls and participate in assembling the basal lamina. These
pericytes require PDGFR-b signalling, which, when inactivated,
reduces pericyte coverage of tumor blood vessels, render
endothelial cells more sensitive to the damaging effect of
cytotoxic drugs and can inhibit tumor growth.7 In addition,
therapeutic regimes involving both receptor tyrosine kinase
(RTKs) inhibitors functioning in synergism were found to be
more efficacious than selective inhibition of either receptor
alone.8 Since angiogenesis plays a central role in the growth of
most solid tumors, both VEGFR-2 and PDGFR-b appear to be
promising imaging targets for positron emission tomography
(PET).
aCyclotron/Radiochemistry Unit, Nuclear Medicine Department, Hadassah/
Hebrew University Hospital, Jerusalem 91120, Israel
bDepartment of Biochemistry and Organic Chemistry, BMC, Box 576,
Husargatan 3, S-751 23 Uppsala, Sweden
cUppsala Applied Science Lab, GE Healthcare, Box 967, S-751 09 Uppsala,
Sweden
To perform in vivo quantitative molecular imaging using PET,
a ‘high-resolution’ PET biomarker is required. The prerequisites
for adequate in vivo imaging of biological/physiological
*Correspondence to: Eyal Mishani, Cyclotron/Radiochemistry Unit, Nuclear
Medicine Department, Hadassah/Hebrew University Hospital, Jerusalem 91120,
Israel.
E-mail: eyalmi@ekmd.huji.ac.il
`
processes vis-a-vis the labelled biomarker and its respective
target have been widely discussed in the literature.9 Some of
these prerequisites deal with the different considerations that
J. Label Compd. Radiopharm 2009, 52 151–157
Copyright r 2009 John Wiley & Sons, Ltd.

O. Ilovich et al.
[
11C]O
H
H
[
18F-]
F
N
N
[
18F-]
O
O
R
O
O
[
11C]H3I
a: R - 4-F
b: R - 2-F
N
c: R - 2,4-di-F
1a-c
Figure 1. VEGFR-2/PDGFR-b inhibitors and potential positron emitter labelling positions via available labelled precursors.
NH2
N3
1) 2M HCl, NaNO2
2) NaN3
F
F
3
F
NH2
F
N3
4-fluoroaniline
O
O
1) 2M HCl, NaNO2
2) NaN3
O
O
O
O
N
N
2
4
Scheme 1. Synthesis of the azide precursors 3 and 4.
structure of a certain class of compounds, it may yield a superior nitrogen from the azide.18,24 The [11C]-urea bond formation may
PET agent.
then proceed via the reaction between [11C]carbon monoxide
[11C]Carbon monoxide has proven to be a valuable and and the coordinated nitrene. In laboratory scale, in the absence
versatile precursor in labelling chemistry and its chemistry has of a nucleophile, the isocyanate is the major product, which
been extensively explored.17 Rhodium-catalyzed carbonylation upon treatment with an amine forms the corresponding urea.
of phenyl azides and amines to form the corresponding ureas Obviously, a one-step reaction is desirable in 11C-labelling
has been used in laboratory-scale synthesis.18–21 Recently, a chemistry due to the short half-life of the radionuclide. When
method using a rhodium-mediated reaction for labelling N,N0- the starting solution contains an amine, the amine can react
diphenyl-[11C]-urea in the carbonyl position was reported.22 either directly with the metal–carbonyl complex or with the
Herein, we describe the importance of choosing an appropriate in situ produced isocyanate to form the urea.
radiosynthetic pathway for the urea-labelling reaction and the
The presence of two nitrogen atoms in the urea moiety, one
11C-labelling at the urea position of eight different N-phenyl-N0- originating from the amine nucleophile and the other from the
4-(4-quinolyloxy)-phenyl-urea derivatives.
azide, opens up two possible routes for the synthesis of
unsymmetrical 11C-labelled ureas. Azides
and were
3
4
synthesized from 4-fluoroaniline and 2, respectively, using
sodium nitrite and sodium azide under conventional conditions
(Scheme 1).
Results and discussion
Labelling chemistry
In the first route, we used the less sterically hindered 4-fluoro-
Reaction of carbon monoxide with an organic azide and an phenyl moiety 3 as the azide and the more sterically hindered
amine nucleophile to form a urea moiety has been shown to 4-(6,7-dimethoxy-quinolin-4-yloxy)-3-fluoro-phenylamine as
2
occur both at the mass and tracer levels under high the amine nucleophile (Scheme 2). Within the ranges tested,
temperatures and pressures even without the presence of any the concentrations of azide, catalyst and ligand did not have any
catalyst.23 However, mild conditions are desirable in order to significant impact on the radiochemical yield of 5 and low yields
avoid unwanted side reactions with sensitive functional groups were obtained (Table 1, entries 1–4). Unfortunately, it was not
present in a complex drug/tracer-like chemical skeleton. With possible to increase the concentration of the nucleophile 2 to
the use of rhodium-mediated carbonylative cross-coupling, the more than approximately 50 mM due to its low solubility in THF,
temperature could be significantly decreased while simulta- thus an alternative nucleophilic derivative of 2 was explored.
neously increasing the radiochemical yield.22 While the mechan- The use of an organic lithium base such as n-BuLi to form
ism of this reaction is not yet fully understood, it may involve the lithium salt of an amine has previously been a successful
the formation of a nitrene–metal complex via loss of molecular approach when using weak nucleophiles in transition
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Copyright r 2009 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2009, 52 151–157

O. Ilovich et al.
F
NH
2
H
H
N
F
N
*
O
O
O
O
O
F
N
N
O
O
3
2
[
11C]O, Rh-complex,
solvent, heat
* = 11
C
F
N
5
3
Scheme 2. Rhodium-mediated carbonylative cross-coupling between 4-fluoro-phenylazide and an amine 2 using [11C]carbon monoxide.
Table 1. [11C]-urea labelling of a VEGFR-2/PDGFR-b inhibitor using 2 as nucleophile
Azide 3
(mM)
Aniline 2
(mM)
[Rh(cod)Cl]2
(mM)
Ligand
(mM)
Conversion of
[11C]O (%)a
Isolated
rcy (%)bc
Entry
T (1C)
1
2
3
4
5d
6e
36
146
146
48
48
48
48
48
48
37
37
37
0.39
0.39
3.04
0.39
0.39
0.39
dppe, 0.78
dppe, 0.78
dppe, 6.27
PPh3, 0.78
dppe, 0.78
dppe, 0.78
110
110
110
110
110
110
46
16
42
37
86
33
6
5
3
7
0
0
Standard conditions were 200 mL solution of [Rh(cod)Cl]2, ligand, 4-fluoro-phenylazide 3 and aniline 2 in THF.
aDecay-corrected conversion yield of [11C]carbon monoxide to non-volatile products remaining in the reaction mixture after
removal of solvent.
bDecay-corrected radiochemical yield based on the initial amount of radioactivity at the start of the synthesis and the
radioactivity of the isolated product. When the reaction mixture was transferred from the micro-autoclave to an evacuated vial,
the radioactivity in the vial was measured. The radioactive residues left in the micro-autoclave were estimated to be less than 1%.
Hence, the amount of initial radioactivity of [11C]carbon monoxide could be determined.
cRadiochemical purity was 497% in all experiments.
dn-BuLi 1.1 eq to the aniline added, solvent mixture of DMF (50 mL) and THF (150 mL).
eSolvent mixture of DMF (50 mL) and THF (150 mL).
H
H
N
F
N3
F
N
*
NH2
O
O
O
F
O
O
O
O
F
[
11C]O, Rh-complex,
solvent, heat
* = 11
C
N
N
5
4
Scheme 3. Rhodium-mediated carbonylative cross-coupling between an azide 4 and 4-fluoroaniline using [11C]carbon monoxide forming an urea.
metal-mediated carbonylation with [11C]carbon monoxide.25,26 desired product (Table 2, entry 7). In addition, the higher
However, the lithium salt of 2 had an even lower solubility in solubility of 4-fluoroaniline in THF compared with 2 allowed us
THF than that of 2 itself. Although addition of DMF as a co- to increase the concentration of nucleophile, resulting in even
solvent effectively dissolved the salt, DMF was found to be higher yields (Table 2, entries 3–6). Lowering the reaction
incompatible with the reaction (Table 1, entries 5 and 6). The temperature from 140 to 801C increased the conversion of
conversion of [11C]carbon monoxide to non-volatile products [11C]carbon monoxide from 84 to 95% and improved the
was significantly higher in the lithium amide reaction, but the radiochemical yield (Table 2, entries 2, 4 and 9). With regard to
radioactivity was mainly composed of highly polar products and the choice of the Rh ligand, triphenylphosphane (PPh3) was
the desired urea product 5 could not be obtained.
favored over [1,2-bis(diphenylphosphino)ethane] at all tempera-
In the second route, the less sterically hindered, more soluble tures tested (Table 2, entries 1,3 and 8 versus 2, 4 and 9,
4-fluoroaniline acts as a nucleophile and the more sterically respectively). When keeping the amine concentration constant
hindered 4-(4-azido-2-fluoro-phenoxy)-6,7-dimethoxy-quinoline at 371 mM and gradually decreasing the rhodium complex and
4 as an azide (Scheme 3). This approach modestly improved the azide concentrations down to 0.1 and 9.4 mM, respectively,
[11C]carbon monoxide conversion and decreased formation of the conversion of [11C]carbon monoxide and radiochemical yield
by-products resulting in a higher radiochemical yield of the were preserved (Table 2, entries 9 and 10 versus 12 and 13). The
J. Label Compd. Radiopharm 2009, 52 151–157
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www.jlcr.org

O. Ilovich et al.
Table 2. 11C-labelling of a VEGFR-2/PDGFR-b inhibitor using 4-fluoroaniline as nucleophile
Conversion
of [11C]O
(%)a,g
Azide 4
(mM)
Aniline
(mM)
[Rh(cod)Cl]2
(mM)
Ligand
(mM)
Analytical rcy
(%)f,g
Isolated rcy
(%)b,c,g
Entry
T (1C)
1
2
3
4
5
6
7
8
37
37
37
37
19
37
48
37
37
19
19
9.4
4.7
9.4
371
371
371
371
186
74
0.39
0.39
0.39
0.39
0.19
0.39
0.39
0.39
0.39
0.19
0.19
0.10
0.05
0.19
dppe, 0.39
PPh3, 0.78
dppe, 0.78
PPh3, 0.78
dppe, 0.38
PPh3, 0.78
PPh3, 0.78
dppe, 0.78
PPh3, 0.78
PPh3, 0.39
PPh3, 0.39
PPh3, 0.20
PPh3, 0,10
PPh3, 0.39
140
140
110
110
110
110
110
80
80
80
80
80
71
70
45
62710 (2)
49
50
50
3577 (2)
27
44
81
7871 (2)
72
7370 (2)
51
8370 (2)
79
84
8173 (2)
67
71
81
69
7475 (2)
4675 (2)
84
9571 (2)
95 4 (2)
92
9071 (2)
77710 (2)
96
6178 (2)
4179 (2)
77
9171 (2)
9076 (2)
87
8673 (2)
71710 (2)
32
37
371
371
371
186
371
371
74
9
10
11
12
13
14h
80
80
Standard conditions were 200 mL solution of [Rh(cod)Cl]2, ligand, azide 4 and 4-fluoroaniline in THF. For explanations see Table 1.
fDecay-corrected analytical radiochemical yield was determined from the conversion yield multiplied by the decay-corrected
radiochemical purity assessed with analytical HPLC. gThe numbers in brackets indicate the number of experiments. hn-BuLi 0.9 eq
to aniline.
lithium salt of 4-fluoroaniline was highly soluble in THF and gave
good conversion of [11C]carbon monoxide, however, to a
complex mixture of labelled compounds. The desired product
was co-eluted with several labelled by-products and could not
be isolated from the crude mixture (Table 2, entry 14). The most
optimal conditions for obtaining maximal radiochemical yield
combined with minimal concentration of starting materials are
indicated in Table 2, entry 10.
In traditional radiochemistry, the amount of transition
metal complex exceeds both the amount of [11C]carbon
monoxide and the obtained labelled product by a factor of
approximately 50.27 The amount of the rhodium complex in
entry 12 is approximately half of the amount of the product
formed suggesting that the reaction is catalytic in nature.
The use of [11C]carbon monoxide enables the production of a
broad spectrum of substituted 11C-labelled ureas in a fast and
systematic manner. Using the most optimal reaction conditions
(Table 2, entry 10), a small library of eight urea derivatives was
synthesized with decay-corrected radiochemical yields (dc rcy) in
the range of 38–78% (Table 3). Tracer libraries may accelerate
the research of new radiopharmaceutical structure–activity
relationships by enabling the synthesis of a multitude of tracers
using the same chemical system with only minor modifications.
Specific activity
Carbon monoxide is present at low levels in the atmosphere
(0.5–5 ppm), while carbon dioxide is present at much higher
concentrations (380 ppm). Thus, with the use of [11C]carbon
monoxide, the isotopic dilution can be kept at a minimal level
resulting in higher specific activity.17 Further, throughout this
investigation, the amount of product formed was determined
after each synthesis and ranged from 15 to 175 nmol with a
median of approximately 40 nmol and was unrelated to the
amount of radioactivity produced in the cyclotron target.
The most important factor for isotopic dilution was found to
be the conditioning of the zinc-oven used for reduction of
[11C]O2 to [11C]O. After a service where the zinc-oven was loaded
with fresh zinc granules, the oven needed to be flushed for 3 h
with a flow of helium (20 mL minꢀ1) at 4001C prior to synthesis
for low isotopic dilution. When higher quantities of activity are
produced, higher specific activity may be obtained. A 10 mA h
bombardment gave 10.770.5 GBq of [11C]carbon monoxide
and, after 45 min, 2371 nmol of product 5 was isolated as
determined by HPLC-UV using a calibration curve. This 23 nmol
of product corresponds to 2.1 GBq giving a specific radioactivity
of 9274 GBq mmolꢀ1 at 45 min from EOB.
Characterization
Experimental
The radiolabelled products (Table 3, entries 1–3) were character-
ized using HPLC by co-eluting with an authentic sample16 and
also by analyzing them with ESI1MS/MS. References as well as
labelled compounds all showed similar fragmentation patterns.
The molecular ion [M1H] and two primary fragmentation ions
that arose and differed by m/z = 26 were observed. This type of
fragmentation is characteristic of 1,3-diphenyl-urea derivatives
where the corresponding aniline (m/z = 315) and isocyanate
(m/z = 341) are formed.28–30 The other compounds (Table 3,
entries 4–8) were characterized by only using ESI1MS/MS and all
gave the expected molecular weight and fragmentations.
General
11C was prepared by the 14N(p,a)11C nuclear reaction using 17 MeV
proton beam produced by a Scanditronix MC-17 Cyclotron at
Uppsala Imanet, GE Healthcare, and obtained as [11C]carbon
dioxide. The target gas used was nitrogen (AGA Nitrogen 6.0)
containing 0.05% oxygen (AGA Oxygen 4.8). The carbonylation
reactions were carried out in a 200 mL Teflon-coated micro-
autoclave according to a previously described method.27,31
THF was freshly distilled over sodium and benzophenone
under a nitrogen atmosphere. All chemicals were purchased
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O. Ilovich et al.
Table 3. 11C-labelling of a small library of VEGFR-2/PDGFR-b inhibitors using substituted anilines, azide 4, Rh–PPh3 complex, in
THF, 801C, 5 min
Compound
Aniline substituent(s)
Conversion of [11C]O [%]a,g
Analytical rcy (%)f,g
Isolated rcy (%)b,c,g
5
6
4-F
2-F
95 4 (2)
6577 (2)
9076 (2)
5373 (2)
7871 (2)
31710 (2)
7
8
9
10
11
12
2,4-diF
4-Cl
2-OMe
4-OMe
2-Me
4-Me
79
86
92
63
90
93
59
86
91
58
90
89
48
70
77
50
74
76
For explanations see Tables 1 and 2.
from Aldrich/Fluka and used without further purification. The 135.9 (d, J = 2.9 Hz), 120.4 (d, J = 8.3 Hz), 116.7 (d, J = 24 Hz) ppm.
identities of the 11C-labelled compounds (Table 3, entries 1–3) IR l: 2100 (azide), 1496, 1301, 1223, 825 cmꢀ1. GC-MS for
were determined by analytical HPLC using authentic samples as C6H4FN3 Rt = 4.8 min, m/z 137 (90), 123 (31), 109 (100).
references. Analytical HPLC was performed on a Beckman
system, equipped with a Beckman 126 pump, a Beckman 166 4-(4-Azido-2-fluoro-phenoxy)-6,7-dimethoxy-quinoline (4)
UV detector in series with a Bioscan b1-flow count detector and
4-(6,7-Dimethoxy-quinolin-4-yloxy)-3-fluoro-phenylamine (2)
a Beckman Ultrasphere ODS dp 5 mm column (250 ꢁ 4.6 mm).
(0.500 g, 1.59 mmol) was dissolved in 2 M HCl (40 mL) under gentle
Mobile phase: (A) 50 mM ammonium formate pH 3.5, (B)
warming. The solution was cooled to 01C and a solution of NaNO2
(0.346 g, 5.30 mmol) in water (15 mL) was added while keeping the
temperature under 11C. The reaction mixture was stirred at 01C for
30 min, and a saturated solution of NaN3 (5 mL) was added slowly
while keeping the temperature under 11C with stirring continued
for another 30 min during which a white precipitate was formed.
acetonitrile; 50% B for 10 min then 95% B for 15 min. A Gilson
231 XL autoinjector was used. Further characterization of the
purified labelled products was made using a Waters Quattro
Premier triple quadrupole mass spectrometer with electrospray
ionization operated in positive mode. Purification with semi-
preparative HPLC was performed on a similar Beckman system
equipped with a Genesis C18 120 4 mm (250 ꢁ 10 mm). Mobile
The mixture was diluted with water and extracted with dichlor-
omethane. The combined organic extracts were dried over MgSO4
phase: (A) 50 mM ammonium formate pH 3.5, (B) acetonitrile.
and evaporated to give an orange solid. The solid was purified
1H- and 13C-NMR spectra were recorded on a Varian 400 or
using flash chromatography with EtOAc as eluent to give the
product of 0.37 g (68%) as an orange oil, which crystallized upon
using CHCl3 as the internal standard. GC-MS analyses were
500 MHz spectrometer and chemical shifts are given in ppm (d)
standing cold overnight. This azide proved to be stable when kept
performed on a Finnigan MAT Thermoquest GCQ mass spectro-
in the dark at room temperature under normal atmosphere for
several months. Melting point 131–1331C. 1H-NMR (400 MHz, CDCl3,
meter operated in EI1mode, equipped with a non-polar column
SE-54, using a temperature gradient of 70–2501C over 10min. IR
spectra were recorded on a Perkin Elmer Spectrum 100 FT/IR
251C): 8.51 (s, 1H), 7.57 (s, 1H), 7.47 (s, 1H), 7.28 (m, 1H), 6.93 (m, 2H),
6.40 (m, 1H), 4.07 (s, 3H), 4.06 (s, 3H) ppm. 13C-NMR (100 MHz, CDCl3,
spectrometer as neat compounds. Melting point was recorded on a
251C): 160.0 (d, J= 1.2 Hz), 156.5, 154.0, 153.3, 150.0, 148.9, 147.3,
139.0 (d, J= 8.4 Hz), 138.6 (d, J= 12.5 Hz), 124.7 (d, J= 1.9 Hz), 115.6
Bibby Stuart melting point apparatus and is uncorrected. Elemental
analysis was preformed by Mikro kemi AB (Uppsala, Sweden).
(d, J= 3.7 Hz), 108.8 (d, J= 22.1 Hz), 108.3, 102.5, 99.6, 56.3,
56.2 ppm. IR l: 2984, 2120 (azide), 1737, 1372, 1231, 1043 cmꢀ1
.
¼
19F-NMR (376 MHz, CDCl3, 251C): 125.9 (unresolved dd, JF;H
Synthesis of precursors
9:0; 10:0 Hz) ppm. Direct inlet EI1 for C17H13FN4O3 m/z: 340 (83),
312 (100), 277 (75). Elementary analysis for C17H13FN4O3, expected C
60.0%, H 3.85%, N 16.46%, found C 59.8%, H 4.0%, N 16.2%.
4-Fluoro-phenylazide (3)32
4-Fluoroaniline (1.00 g, 9.00 mmol) was dissolved in 2 M HCl
(10 mL) and cooled in an ice bath. To the cold solution, a
solution of sodium nitrite (2.29 g, 33.3 mmol) in water (10 mL)
was added dropwise while keeping the temperature below 31C.
The mixture was stirred for another 15 min and a saturated
solution of sodium azide (7.60 g, 117 mmol) in water (18 mL) was
added dropwise while keeping the solution temperature
Labelling chemistry
1-[4-(6,7-Dimethoxy-quinolin-4-yloxy)-3-fluoro-phenyl]-3-(4-fluoro-
phenyl)-[11C]-urea (5)
below 31C. The solution was stirred for an additional 15 min. In a typical experiment, chloro(1,5-cyclooctadiene)rhodium(I)
After neutralization with saturated NaHCO3, the reaction mixture dimer ([Rh(cod)Cl]2) (0.038 mg, 0.076 mmol) and triphenylphos-
was extracted three times with diethyl ether and the combined phane (0.041 mg, 0.16 mmol) was added as a solution in
organic extracts were washed with brine and dried over MgSO4 100 mL THF to a capped 1-mL vial under argon. To this solution,
and purified on SiO2 using flash chromatography eluting with 4-(4-azido-2-fluoro-phenoxy)-6,7-dimethoxy-quinoline (2.55 mg,
pentane to give the product as a pale yellow oil (0.8 g, 66%). The 7.50 mmol) dissolved in 100 mL THF and 4-fluoro-phenylaniline
product was kept in darkness and under nitrogen at 41C. (16.5 mg, 148 mmol) dissolved in 200 mL THF were added. The
1H-NMR (500 MHz, CDCl3, 251C): d = 7.04 (m, 2H), 6.98 (m, resulting 400 mL solution was degassed with argon and 200 mL of
2H) ppm. 13C-NMR (125 MHz, CDCl3, 251C): 160.1 (d, J = 248 Hz), the solution was loaded onto an injection loop. THF was
J. Label Compd. Radiopharm 2009, 52 151–157
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O. Ilovich et al.
pumped through the loop and the reagents were transferred 1-[4-(6,7-Dimethoxy-quinolin-4-yloxy)-3-fluoro-phenyl]-3-p-tolyl-
[11C]-urea (12)
into a 200 mL Teflon-coated stainless steel micro-autoclave
containing [11C]carbon monoxide and helium immersed in an
Similar procedure as for 5. p-Toluidine (15.9 mg, 148 mmol). Dc
oil bath. The reaction mixture was kept at 801C for 5 min and
rcy 76%. Analytical Rt = 7.8 min. ESI1MS/MS C25H22FN3O4 m/z
then transferred into an evacuated capped 3-mL vial and the
[M1H]: 448, 341, 315.
radioactivity was measured (2.4 GBq at 20.0 min from EOB). The
vial was purged with a stream of nitrogen for 1 min at 801C and
the radioactivity was measured (1.9 GBq, 25 min and 30 s from
EOB) giving the conversion yield of [11C]carbon monoxide 92%.
The crude product was transferred to a 0.8 mL vial and the 3 mL
Determination of specific activity
The synthesis of 1-[4-(6,7-dimethoxy-quinolin-4-yloxy)-3-fluoro-
phenyl]-3-(4-fluoro-phenyl)-[11C]-urea (5) was performed as
vial was rinsed with acetonitrile, 300 mL, and then water, 200 mL,
and in this manner 97% of the radioactivity was transferred.
A small sample of the solution was withdrawn and analyzed by
reversed-phase HPLC for determination of the radiochemical
purity of the crude product. The crude product was purified on a
reversed-phase semi-preparative HPLC, eluent 60% B, flow:
described above. A cyclotron bombardment of 10 mA h gave
10.770.5 GBq of [11C]carbon monoxide at 18–20 min after EOB.
After 45 min, 2.1 GBq of the product was isolated. The product
fraction had a volume of 6.2 mL as measured with a syringe.
A calibration curve was made from a series of three calibration
standards, three injections each (1, 5 and 15 mM), which were
analyzed using analytical reversed-phase HPLC coupled to a UV
detector (l = 254 nm), inj. volume was 50 mL. A sample from the
isolated product was analyzed in a similar manner. The
concentration of the analyte was 3.67 mM corresponding to an
amount of 23 nmol of product, giving a specific activity of
9274 GBq mmolꢀ1 at 45 min after EOB.
4 mL minꢀ1
, Rt = 7.6 min. The radioactivity of the purified
product was measured (0.79 GBq at 45 min and 30 s from EOB)
to determine the dc rcy (78%) calculated from [11C]carbon
monoxide. Analytical HPLC was used to assess the identity of the
labelled compound as compared with the reference compound
(Rt = 6.6 min). In addition, ESI1MS/MS was used for identification
of the purified product, cone voltage 45 V, source temperature
1201C, desolvation temperature 3501C showing peaks for
C24H19F2N3O4 at m/z [M1H]: 452 and fragments at 341 and 315.
Conclusions
11C-labelling of a series of asymmetric phenyl-ureas has been
performed via rhodium-mediated carbonylation reaction with
an azide 4, [11C]carbon monoxide and various anilines.
1-[4-(6,7-Dimethoxy-quinolin-4-yloxy)-3-fluoro-phenyl]-3-(2-fluoro-
phenyl)-[11C]-urea (6)
The reaction conditions were explored by altering reagent
concentrations, temperature and Rh ligands. High concentra-
tions of the amine nucleophile were found to be the key factor
Similar procedure as for 5. 2-Fluoroaniline (16.5 mg, 148 mmol).
Dc rcy 38%. Analytical Rt = 8.3 min. ESI1MS/MS C24H19F2N3O4
m/z: 452, 341, 315.
for obtaining high radiochemical yields. After
bombardment, 2.08 GBq of the product 5 was isolated at
a 10 mA h
1-(2,4-Difluoro-phenyl)-3-[4-(6,7-dimethoxy-quinolin-4-yloxy)-3-
45 min from EOB with a specific activity of 9274 GBq mmolꢀ1
.
fluoro-phenyl]-[11C]-urea (7)
The 11C-labelled VEGFR-2/PDGFR-b dual inhibitors are now
being further explored as potential PET angiogenic probes.
Similar procedure as for 5. 2,4-Difluoroaniline (19.1 mg,
148 mmol). Dc rcy 48%. Analytical Rt = 8.3 min. ESI1MS/MS
C24H18F3N3O4 m/z: 470, 341, 315.
Acknowledgement
1-(4-Chloro-phenyl)-3-[4-(6,7-dimethoxy-quinolin-4-yloxy)-3-fluoro-
phenyl]-[11C]-urea (8)
Uppsala Applied Science Lab, GE Healthcare and the Israel
Science Foundation (Grant ]445/07 to E. M.) are acknowledged
for financial support of this work.
Similar procedure as for 5. 4-Chloroaniline (18.9 mg, 148 mmol).
Dc rcy 70%. Analytical Rt = 9.4 min. ESI1MS/MS C24H19ClFN3O4
m/z: 468, 341, 315.
1-[4-(6,7-Dimethoxy-quinolin-4-yloxy)-3-fluoro-phenyl]-3-(2-methoxy-
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