A tin precursor for the synthesis of no-carrier-added [*I]MIBG and [211At]MABG

Article abstract of DOI:10.1002/jlcr.1243

Radioiodinated MIBG has shown considerable promise as an imaging agent for cardiac and oncologic applications, and also as a targeted radio therapeutic for treating patients with neuroendocrine tumors. This radiolabeled agent, synthesized at a no-carrier-

Full text of DOI:10.1002/jlcr.1243

Journal of Labelled Compounds and Radiopharmaceuticals
J Label Compd Radiopharm 2007; 50: 177–182.
Published online in Wiley InterScience
JLCR
(www.interscience.wiley.com). DOI: 10.1002/jlcr.1243
Research Article
A tin precursor for the synthesis of no-carrier-added ½*IꢀMIBG
and ½211AtꢀMABG
GANESAN VAIDYANATHAN*, DONNA J. AFFLECK, KEVIN L. ALSTON and MICHAEL R. ZALUTSKY
Department of Radiology, Duke University Medical Center, Durham, NC 27710, USA
Received 17 November 2006; Revised 21 December 2006; Accepted 22 December 2006
Abstract: Radioiodinated MIBG has shown considerable promise as an imaging agent for cardiac and oncologic
applications, and also as a targeted radiotherapeutic for treating patients with neuroendocrine tumors. This
radiolabeled agent, synthesized at a no-carrier-added level, has demonstrated advantages over the carrier-added
preparation in preliminary clinical studies. Earlier we developed a silicon precursor from which both radioiodinated
MIBG and the a-particle-emitting 211At analog [211At]MABG could be synthesized at a no-carrier-added level. In
order to increase the practicality of this approach, we have developed a synthesis of a tin precursor in two steps from
a readily available starting material. This tin precursor, N, N0-bis(tert-butyloxycarbonyl)-3-(trimethylstannyl)ben-
zylguanidine (Bis-Boc MTMSBG) was evaluated for the synthesis of n.c.a. [*I]MIBG and [211At]MABG via
halodestannylation. The radiochemical yields were 83 ꢁ 9% ðn ¼ 7Þ, 30 ꢁ 21% ðn ¼ 2Þ, 77 ꢁ 2% ðn ¼ 2Þ, and 66 ꢁ
7% ðn ¼ 4Þ for labeling with 131I, 124I, 125I, and 211At, respectively. Copyright # 2007 John Wiley & Sons, Ltd.
Keywords: MIBG; tin precursor; radioiodine; Astatine-211
Introduction
responsible for MIBG transport is saturable8,9 and
second, the high amounts of unlabeled MIBG present
in therapy formulations can cause pharmacological
effects. To avert these potential problems, we have
developed a method to synthesize radioiodinated MIBG
at a no-carrier-added (n.c.a.) level from a silicon
precursor.10 It was possible to synthesize meta-
Radioiodinated meta-iodobenzylguanidines (MIBG) are
one of the most prominent types of radiopharmaceu-
ticals with more than 2400 research articles involving
MIBG having been published since their discovery in
1980. MIBG labeled with 123I and 131I has been
routinely used for diagnostic imaging of the pathophy-
siology of heart and neuroendocrine tumors such as
neuroblastoma.1,2 The synthesis of [124I]MIBG, useful
for positron emission tomography (PET), has also been
[
211At]astatobenzylguanidine ([211At]MABG), a MIBG
analog tagged with the a-particle-emitting heavy halo-
gen 211At, using the same precursor.11 Other methods
to synthesize n.c.a. [*I]MIBG have been described
subsequently.12–15
reported.3 In addition,
[
131I]MIBG has been used
extensively in the treatment of neuroendocrine tu-
mors,4 and the feasibility of using 125I-labeled MIBG
for therapy has been explored.5
Our original method involves the radiohalogenation
of a silicon precursor, which was synthesized in multi-
ple steps.10 Although better radiochemical yields can
be obtained from a tin precursor,16 at that time, our
attempts to synthesize a tin precursor of MIBG were not
successful. However, within the last 10–15 years,
methods for the synthesis of protected guanidine
derivatives have been developed17 and these methods
have made the synthesis of guanidine-containing
compounds very facile. Unlike the free guanidines,
protected derivatives are nonpolar, and therefore, are
amenable to normal-phase chromatographic purifica-
tions. For example, we have reported the synthesis of N,
N0-bis(tert-butyloxycarbonyl)-3-iodobenzylguanidine
Radiolabeled MIBG preparations currently used in
the clinic are synthesized by the isotopic exchange
method.6,7 This preparation method has several in-
trinsic disadvantages. First, the uptake-1 mechanism
*Correspondence to: Ganesan Vaidyanathan, Department of Radiology,
Duke University Medical Center, Durham, NC 27710, USA. E-mail:
Ganesan.v@duke.edu
Contract/grant sponsor: National Institutes of Health; contract/grant
numbers: CA78417, CA93371
Contract/grant sponsor: Pediatric Brain Foundation
Copyright # 2007 John Wiley & Sons, Ltd.

178 G. VAIDYANATHAN ET AL.
Figure 1 Scheme for the synthesis of N, N0-bis(tert-butyloxycarbonyl)-3-(trimethylstannyl)benzylguanidine.
(Bis-Boc MIBG; 1; Figure 1), which is a protected
derivative of MIBG.18 The current paper extends this
approach to the synthesis of the corresponding
tin precursor, N, N0-bis(tert-butyloxycarbonyl)-3-(tri-
methylstannyl)benzylguanidine (Bis-Boc MTMSBG; 2;
Figure 1). We describe its conversion to n.c.a. MIBG
labeled with 131I, 125I, and 124I and to [211At]MABG.
might be easier using the protected MIBG described in
this study. It should be pointed out that Samnick
et al.13 have reported the synthesis of n.c.a. [123I]MIBG
from a tin precursor; however, they refer to another
publication for the synthesis of the tin precursor. This
publication is a meeting abstract and actually de-
scribes the synthesis of some fatty acid tin deriva-
tives.24
Results and discussion
There are a number of ways by which the tin
precursor 2 can be synthesized. The synthesis of both
3-tributylstannylbenzyl alcohol25 and 3-tributylstan-
nylbenzylamine10 has been reported. The tin precursor
Several clinical trials have been undertaken to evaluate
n.c.a. [*I]MIBG.13,19–22 A recent report lamented the
limited availability of n.c.a. [*I]MIBG as a possible
cause for the lack of interest in exploiting the potential
advantages of this tracer.23 Molecular Insight Pharma-
ceuticals, Inc. (MIP; Boston, MA) in collaboration with
MDS Nordion (Vancouver, Canada) is optimizing the
synthesis of n.c.a. [*I]MIBG on a large scale (personal
communications), and MIP has just initiated a phase I
clinical study to determine its dosimetry, metabolism
and pharmacokinetics. Nonetheless, n.c.a. [*I]MIBG is
currently not available to most research groups
wishing to utilize this tracer for studies in cell culture,
animal models and clinical studies.
2
could have been prepared from these by the
guanidinylation methods reported.26,27 However, we
opted to convert 1 to 2 by the palladium-catalyzed
stannylation (Figure 1). A yield of 57% was obtained for
this conversion and the NMR and mass spectral data
were consistent with the structure of 2. Lack of stability
does not seem to be an issue because a batch that was
synthesized more than 2 years ago and stored at 48C is
still intact.
Radiochemical yields for the radioiododestannylation
of 2 (Figure 2) were dependent on the radionuclide
(Table 1) presumably due to differences in factors
including source, specific activity, oxidation state,
impurities and age. With 131I, the yields were very
good, the average yield being 83 ꢁ 9% ðn ¼ 7Þ, compar-
able to yields reported for the synthesis of n.c.a.
In the work reported herein, we have presented an
alternative strategy for the synthesis of n.c.a. [*I]MIBG
and [211At]MABG. In addition to its ease of radio-
halogenation compared with labeling via a silicon
precursor, there was another motivation for exploring
the synthesis of MIBG from a tin precursor. This tin
precursor can be made from commercially available
materials in two steps compared to the four steps used
for the synthesis of silicon precursor we reported
earlier.10 The method used by MIP to synthesize n.c.a.
[
131I]MIBG from the silicon precursor.10 However, the
reaction time is longer for the current method. We used
a reaction time of 10 min in this study and have not
explored the possibility of using shorter time periods.
Although Boc groups are expected to be removed under
the initial reaction conditions namely at 70oC in acetic
acid, to ensure their complete removal the reaction
mixture was additionally treated with TFA necessitat-
ing the use of an extra 10 min. Only two reactions each
were conducted for 125I and 124I. The radiochemical
yield for [125I]MIBG synthesis (77 ꢁ 2%; n ¼ 2) was
slightly lower than that obtained for [131I]MIBG; how-
ever, the HPLC of its reaction mixture showed, like in
the case of 131I, mainly one radioactive peak corre-
sponding to MIBG (Figure 3). Radiochemical yields for
[
131I]MIBG is that of Hunter and Zhu12 using the resin-
bound tin precursor. While labeled MIBG can be
prepared with ease using this precursor, the precursor
is not commercially available and its synthesis may be
too difficult for many laboratories. On the other hand,
the tin precursor described herein can be made easily.
In addition, we speculate that the synthesis of a solid
support-immobilized tin precursor such as those
described by Hunter and Zhu12 and Valliant et al.15
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 177–182
DOI: 10.1002.jlcr

MIBG FROM TIN PRECURSOR 179
Figure 2 Radiohalogenation of the tin precursor to [*I]MIBG and [211At]MABG.
reactions 14–17 is 66 ꢁ 7%. This is lower than that
reported for the synthesis of [211At]MABG from the
silicon precursor.11 This result, and to a certain degree
that obtained with 131I, is counterintuitive but the
nature of the solvent might have a role in the observed
difference. The solvent used for radiohalodesilylation
was trifluoroacetic acid and we have observed higher
yields for radioiododesilylation with this solvent com-
pared to acetic acid.10 However, the lability of carbon–
tin bonds to trifluoroacetic acid precludes the use of
this solvent in halodestannylation reactions. Radio-
chemical purity of all labeled MIBG derivatives was
more than 95% as demonstrated by HPLC (Figure 4)
and radio-TLC (data not shown). No significant peaks
were seen in the UV profiles of these HPLC runs.
Table 1 Radiochemical yields for the synthesis of [*]MIBG
and [211At]MABG from tin precursor
Serial
number
Isotope
Starting
radioactivity
(mCi)
%Radiochemical
yielda
1
2
3
4
5
6
7
8
Iodine-131
}
880
763
845
1059
792
125
1331
310
279
2800
1110
1824
435
3204
6486
3087
3634
86
65
91
80
91
79
86
75
78
44
15
35
45
66
70
56
70
}
}
}
}
}
Iodine-125
}
9
10
11
12
13
14
15
16
17
Iodine-124
}
Astatine-211
}
}
}
}
}
Experimental
General
aNot corrected for decay.
All chemicals were purchased from Sigma-Aldrich
unless otherwise noted. Sodium [125I]iodide and so-
dium [131I]iodide with specific activities of 2200 and
1200 Ci/mmol, respectively, were obtained from Perkin
Elmer Life and Analytical Sciences (Boston, MA).
124I were 44 and 15% for the two reactions. Contrary to
125I runs, the HPLC of the crude reaction mixture was
rather messy (Figure 3). Lower radiochemical yields
(20–40%) for 124I-labeling, compared to 125I (60–70%),
have been reported for the synthesis of N-hydroxysuc-
cinimidyl 4-[*I]iodobenzoate.28 Addition of carrier so-
dium iodide to improve the radiochemical synthesis of
124I-labeled Bolton–Hunter reagent has been recom-
mended.29 More reactions need to be done in order to
optimize the synthesis of n.c.a. [124I]MIBG for routine
use in PET.
Sodium
[
124I]iodide with
a
specific activity of
ꢂ2800 Ci/mmol was obtained from IBA Molecular
(Sterling, VA). N, N0-bis(tert-butyloxycarbonyl)-3-iodo-
benzylguanidine (bis-Boc-MIBG; 1) was synthesized as
reported before.18
Aluminum-backed sheets (Silica gel 60 F254) were
used for analytical TLC, and normal-phase column
chromatography was performed using silica gel 60,
both obtained from EM Science (Gibbstown, NJ).
Column chromatographic fractions were collected
The radiochemical yields for labeling 2 with 211At
were 57 ꢁ 15% (n ¼ 7; not decay corrected). However, it
should be noted that in reaction 12 (Table 1), there was
a radioactive peak with a retention time corresponding
using
a Gilson model 203 microfraction collector
(Middleton, WI) or an ISCO Foxy 200 fraction collector
(Lincoln, NE), and the products were identified by TLC.
In some cases, an ISCO UA-6 UV-visible detector was
placed between the column outlet and the fraction
collector to identify products. Preparative thick layer
chromatography was performed using 20 ꢃ 20 cm,
to
1 and 50% of the injected radioactivity was
associated with this. Also for reaction 13, the HPLC of
crude reaction mixture showed one major radioactive
peak corresponding to MABG. Average yields for
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 177–182
DOI: 10.1002.jlcr

180 G. VAIDYANATHAN ET AL.
Figure 3 Typical HPLC radiochromatograms of reaction mixtures obtained from the radiohalodestannylation of 2.
N, N0-bis(tert-butyloxycarbonyl)-3-(trimethylstannyl)-
benzylguanidine (bis-Boc-TMSBG; 2)
1000 mm plates (Whatman, Clifton, NJ). Before apply-
ing the sample, the plates were run in ethyl acetate to
remove any adsorbed impurities. Radio-TLC was in-
itially analyzed using a System 200 Imaging Scanner
(BioScan, Washington, DC); sheets were then cut into
strips and counted using an automated gamma
counter (LKB 1282, Wallac, Finland). High-pressure
liquid chromatography was performed using a Beck-
man System Gold HPLC equipped with a Model 126
programmable solvent module, a Model 168 diode
array detector, a Model 170 radioisotope detector, and
a Model 406 analog interface module. Reversed-phase
chromatography was performed using a Waters XTerra
C18 column (4:6 ꢃ 250 mm, 5m). Proton NMR spectrum
A mixture of 1 (101 mg; 0.21 mmol), hexamethylditin
(550 mg; 1.7 mmol), and (Ph3P)2PdCl2 (17 mg;
0.024 mmol) in 10 ml of dry dioxane was heated at
1008C for 1–2 h. The precipitated palladium was
removed by filtration through a bed of Celite1, and
the Celite1 bed was washed with ethyl acetate.
Solvents were removed from the combined filtrate by
rotary evaporation. The residual orange oil was loaded
onto a bed of silica gel, and the nonpolar byproducts
were eluted with hexanes. The silica bed was further
flushed with ethyl acetate to elute the product and
other more polar byproducts. Ethyl acetate fractions
were concentrated and the resultant crude mixture was
subjected to preparative thin layer chromatography
using 10% (v/v) ethyl acetate/hexanes as the eluent to
yield 61 mg (57%) of 2 as a colorless oil: 1H NMR
(CDCl3): 0.27 (s, 9H [119Sn-H d]), 1.35 (s, 9H), 1.49 (s,
9H), 5.18 (s, 2H), 7.18–7.30 (m, 2H), 7.38 (dd, 1H), 7.42
(d, 1H), 9.44 (br s, 2H). MS (LCMS) m/z: cluster peaks
was obtained on
a Varian Mercury 300 spectro-
meter. Chemical shifts are reported in d units; solvent
peaks are referenced appropriately. Mass spectral
data were obtained using an Agilent LCMS system
(Agilent 1100 LC/MSD Trap SL; Agilent Technologies,
Palo Alto, CA). High-resolution mass spectral data were
obtained using JEOL SX-102 high-resolution mass
spectrometer.
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 177–182
DOI: 10.1002.jlcr

MIBG FROM TIN PRECURSOR 181
Figure 4 Radiochromatograms of purified [*I]MIBG and [211At]MABG. No detectable peaks were seen in the UV profile.
at 514.1 (MH+), 458.1 (MH–C4H8)+; 402.0 (MH–
2C4H8)+. HRMS (FAB+) calcd. for C21H36N3O1418Sn
(MH+): 512.1716. Found: 512:1718 ꢁ 0:0005 ðn ¼ 4Þ.
concentrated by solid-phase extraction as described
before.30 125I]MIBG and [124I]MIBG were synthesized
[
similarly. Iodine-124 came as a more dilute solution,
and therefore, it was necessary to evaporate the radio-
activity solution (25–35 ml) with an argon stream to a
volume less than 2 ml before subjecting it to the reaction
conditions.
*
No-carrier-added ½ IꢀMIBG
N-chlorosuccinimide (10 ml of 0.3 M in acetic acid) and 2
(50 mg in 10 ml acetic acid) were added to a 2-dram vial
containing ꢂ1 mCi of 131I in 1–2 ml of 0.1N NaOH. The
vial was capped and after thoroughly mixing the
contents by vortexing, it was heated at 708C in an oil
bath for 10 min. Trifluoroacetic acid (100 ml) was added
to the reaction mixture, which was further heated at
708C for 10 min. The solvents were evaporated with a
gentle stream of argon. Further co-evaporation with
3 ꢃ 25 ml MeOH was done to ensure complete removal of
residual acetic acid. The radioactivity was taken in m
of MeOH and injected onto a reversed-phase HPLC
column that was eluted isocratically with 0.1% TFA in
80:20 water:acetonitrile at a flow rate of 1 ml/min. The
fractions containing the radioactivity peak correspond-
ing to MIBG ðtR ¼ 16:8 minÞ was collected, and
½
211AtꢀMABG
The 211At activity was produced on the Duke University
Medical Center CS-30 cyclotron via the 209Bi(a,
2n)211At reaction by bombarding natural bismuth
metal targets with 28 MeV a-particles using an MIT-1
internal target system (Cyclotron Inc., Napa, CA).31 The
radioactivity was condensed into
a
Teflon tube32
immersed in a dry ice/ethanol bath and subsequently
extracted from the tube into methanol. Because 211At is
carrier free (no stable astatine isotopes), its specific
activity is assumed to be the theoretical value (ꢂ435 Ci
per mmol).
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 177–182
DOI: 10.1002.jlcr

182 G. VAIDYANATHAN ET AL.
N-chlorosuccinimide (3 mmol in 10 ml of acetic acid)
and the tin precursor (0.2 mg in 10 ml of acetic acid) were
added to a solution of 211At in methanol (3–7mCi in
11. Vaidyanathan G, Zalutsky MR. Bioconjug Chem
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1
about 50 ml) in a -dram vial. The vial was vortexed and
2
heated in an oil bath at 708C for 10 min. The methanol
was evaporated with a gentle stream of argon and TFA
(100ml) was added. The vial was further heated at 708C
for another 10 min and TFA was evaporated from the
reaction mixture with an argon stream. The radioactive
residue was reconstituted in 25 ml of methanol and
injected onto HPLC. The remainder of the procedure
was the same as described above for [*I]MIBG.
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We have described a synthesis for a tin precursor that
was prepared in two steps from commercially available
materials. This could be converted to radioiodinated
MIBG and [211At]MABG in reasonable yields and excel-
lent radiochemical purity. This novel tin precursor
provides an additional tool for the synthesis of radio-
halogenated MIBG derivatives at a no-carrier-added level.
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This work was supported by Grants CA78417 and
CA93371 from the National Institutes of Health and a
Grant from the Pediatric Brain Foundation.
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