A new strategy for preparing molecular imaging and therapy agents using fluorine-rich (fluorous) soluble supports

Article abstract of DOI:10.1021/ja0600375

A convenient new strategy for producing radiolabeled compounds in high effective specific activity was developed using soluble fluorous supports. The reported methodology involves a fluorous linker group that is released from the substrate of interest upon reaction with radioiodine. The desired product can then be selectively separated from unreacted starting material and reaction byproducts using a simple fluorous solid-phase extraction procedure. The utility of this approach was demonstrated by labeling a series of benzoic acid derivatives which are commonly used to prepare molecular imaging agents. All compounds were produced in high radiochemical yields, purities, and effective specific activities. The strategy was further elaborated in that it was used to prepare a small collection of radiolabeled benzamides as a way of demonstrating the potential utility of this method for creating libraries of molecular imaging agents. Copyright

Full text of DOI:10.1021/ja0600375

Published on Web 02/25/2006
A New Strategy for Preparing Molecular Imaging and Therapy Agents Using
Fluorine-Rich (Fluorous) Soluble Supports
†
†
†
†
‡
,†
Amanda Donovan, Jane Forbes, Peter Dorff, Paul Schaffer, John Babich, and John F. Valliant*
Departments of Chemistry and Medical Physics & Applied Radiation Sciences, McMaster UniVersity,
Hamilton, ON, Canada L8S 4M1, and Molecular Insight Pharmaceuticals Inc., Cambridge, Massachusetts 02142
Received January 16, 2006; E-mail: valliant@mcmaster.ca
The increasing use of molecular radioimaging for diagnostic
medicine, drug development, and biological research1 has created
a need for new labeling methods that afford radiotracers in high
effective specific activity without having to employ time-consuming
purification protocols. In the case of tracers which utilize targeting
vectors for delivering the radionuclides to cell surface receptors,
the key impurity that must be removed is the excess substrate
present during labeling. Unlabeled starting material can compete
with the small amount of tracer for the target which typically leads
to reduced image quality. It is particularly imperative that residual
starting material be removed when tracers are being employed in
small animal imaging studies, so that sufficient levels of activity
can be administered in order to be able to acquire realistic imaging
data without violating the tracer principle or causing unwanted
biological effects.2
Scheme 1
,2
Saponification of the esters using NaOH led to the desired products
in excellent yields. Compounds 3a and 3b were fully characterized
and their purities (>98% in both cases) determined by HPLC. The
fact that precursors can be characterized by traditional methods and
analyzed (or purified if necessary) by HPLC is a major advantage
of the fluorous labeling method over an analogous solid-phase
technique.
To verify that the tin-aryl bond was still reactive toward iodo-
demetalation and that the fluorine-rich starting material and reaction
byproducts could be effectively separated from the desired products,
To achieve “precursor-free” formulations without having to resort
to HPLC, which is not an attractive option particularly for
radionuclides with short half-lives, a number of groups have
developed solid-phase labeling strategies.3 In solid-phase labeling,
a substrate is bound to an insoluble support in such a manner that
it will only be released into solution if it reacts with a radionuclide.
Unreacted material remains bound to the support and is removed
by simple filtration. One of the disadvantages common to all
reactions that are performed on insoluble supports is that preparation
of ultrahigh purity resin-bound substrates can be problematic. If
reactions used to prepare precursors are not quantitative upon
labeling, impurities will be formed, which must then be removed
using traditional purification methods.
-8
2
compounds 3a and 3b were iodinated using 0.5 equiv of “cold” I .
Purification of the products was accomplished by passing the
reaction mixtures down fluorous solid-phase extraction (FSPE)
cartridges following dilution of the reaction mixtures with water.
FSPE cartridges contain modified reversed-phase silica that selec-
tively retain fluorine-rich compounds, which in the case presented
here includes unreacted 3a/3b and the tin support cleaved during
labeling. The cartridges were washed with water to remove residual
An alternative approach is to use soluble supports in place of
cross-linked resins. This would allow for purification and charac-
terization of precursors using standard solution-phase techniques,
while retaining the ability to separate labeled compounds from the
corresponding starting materials. The approach described here
2
salts followed by 80% MeOH-H O, where the collected fractions
showed only one major peak according to HPLC, which matched
retention times of authentic standards. ESMS, which is very
sensitive to the presence of the fluorous tin compounds, showed
that the fractions containing the desired products (4a/4b) were not
contaminated with any residual fluorous tin derivatives.
employed fluorous supports, which have been used successfully
as soluble synthetic platforms.9
-14
The first target was a system
In light of the success of the cold experiments, compounds 3a
based on an iodo-demetalation reaction using a fluorine-rich tin
support. This is directly analogous to the solid-phase labeling system
developed by Hunter and co-workers.4
125
and 3b were combined with [ I]NaI in the presence of iodogen.
After 3 min, the reactions were diluted with water and the mixtures
,6
17
passed down FSPE cartridges. Washing with water eluted residual
To investigate the feasibility of the fluorous labeling strategy,
meta and para benzoic derivatives were attached to a “heavy”
fluorous support through a tin linker (Scheme 1). In an attempt to
2
salts and iodide, while washing with 80% MeOH-H O selectively
eluted the desired products, which were both obtained in 85%
radiochemical yield and 98% radiochemical purity. Compounds 3a
and 3b could be recovered by elution with THF. FSPE in
conjunction with fluorous scavengers has likewise been employed
1
5
prepare the target compounds, a fluorous tin bromide reported
by Curran and co-workers was reacted with the dilithio salt of
p-bromobenzoic acid. Using this approach, 3b could only be isolated
in 40% yield. A superior approach involved reacting the fluorous
tin bromide with organozinc derivatives of benzoic acid ethyl ester,
which are commercially available.16 Compounds 2a and 2b were
isolated by simple extraction into perfluorinated hexanes (FC-72).
35
18
to purify S-radioligands.
Benzoic acid derivatives were selected as the model substrates
because they are useful starting materials for the preparation of a
range of different types of labeled compounds, including benza-
mides, which are currently being investigated as agents for imaging
melanomas.1
9-22
To demonstrate their potential utility, compounds
†
McMaster University.
Molecular Insight Pharmaceuticals.
‡
3a and 3b were converted to propyl benzamides 6a and 6b (R′ )
3536
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J. AM. CHEM. SOC. 2006, 128, 3536-3537
10.1021/ja0600375 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
labeled compounds, as mentioned previously, FLS also enables
precursors to be purified and characterized by techniques used for
small molecule pharmaceuticals. This point is further illustrated
by the fact that we were able to obtain an X-ray structure of 3a
(Figure 1). For compounds which are destined for clinical use, the
ability to unequivocally establish the purity and structure of
precursors is a key aspect of getting regulatory approval.
In conclusion, a convenient new strategy for radiolabeling and
purifying radiopharmaceuticals was developed. We are currently
expanding the reported technique to include developing methods
for labeling libraries of fluorous-tagged substrates with a range of
nuclides beyond solely isotopes of iodine. We are also developing
an automated labeling and purification system based on the FLS
that is suited to routine production using high levels of activity.
Figure 1. ORTEP drawing of 3a (30% thermal probability ellipsoids).
Scheme 2. Synthesis and Labeling of Fluorous-Tagged
Benzamides
Acknowledgment. We would like to acknowledge NSERC of
Canada, CFI, OIT, and ORDCF for funding, and The McMaster
Nuclear Reactor for the generous donations of 1 I[NaI].
25
Supporting Information Available: X-ray data along with syn-
thetic procedures and characterization data for 2a, 2b, 3a, 3b, 4a, 4b,
5
a, 5b, 6a-j, and 7a-j. This material is available free of charge via
the Internet at http://pubs.acs.org.
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(
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