A simplified route to the synthesis of new 99mTc-specific tetradentate ligands

Article abstract of DOI:10.1016/S0040-4039(02)02385-7

The synthesis of two new N₂S₂ or N₂SO tetradentate ligands and the preparation of their technetium-99m complexes are reported. Each ligand leads to a unique ^99mTc-complex with an excellent radiochemical yield. T

Full text of DOI:10.1016/S0040-4039(02)02385-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9295–9297
A simplified route to the synthesis of new 99mTc-specific
tetradentate ligands
Julien Le Gal,a Eric Benoist,a,* Marie Gressier,a Yvon Coulaisb and Miche`le Dartiguenavea
aLaboratoire de Chimie Inorganique, Universite´ Paul Sabatier, Bat. IIR1, 118, route de Narbonne, 31062 Toulouse, France
bService de Me´decine Nucle´aire, Hoˆpital Purpan, Place du docteur Baylac, 31059 Toulouse, France
Received 24 September 2002; accepted 23 October 2002
Abstract—The synthesis of two new N2S2 or N2SO tetradentate ligands and the preparation of their technetium-99m complexes
are reported. Each ligand leads to a unique 99mTc-complex with an excellent radiochemical yield. The in vivo stability of these
complexes was determined by serum stability and cysteine challenge studies. © 2002 Elsevier Science Ltd. All rights reserved.
The appropriate physical properties (T1/2=6 h, 140
KeV gamma emitter) and widespread availability of
99mTc continue to make it the most attractive candidate
for use in formulating diagnostic radiopharmaceuticals
for SPECT imaging studies in patients.1 Efforts to
design new chelate systems for this nuclide and for
rhenium for subsequent uses, respectively, in diagnostic
and therapy have led us2 and others3 to the develop-
ment of new N2S2 tetradentate ligands. These ligand
systems are one of the most commonly used ligands
which form stable technetium(V) or rhenium(V)
complexes.
The ligands L1 and L2 were synthesised with a benzoyl
or a triphenylmethyl (trityl) protected mercapto group,
in order to inhibit the oxidation of the thiol functional-
ity upon a long storage. They can be easily removed
during the 99mTc radiolabelling reaction. Ligands were
prepared by a multi-step synthesis, starting from o-sub-
stituted anilines as outlined in Schemes 1 and 2. Ligand
L1 was synthesised according to a previously reported
procedure3 but with major modifications. The first step
involves a conventional carbodiimide amide coupling of
N-carbobenzyloxyglycine and 2-aminophenol followed
by the removal of the carbobenzyloxy protecting group
by catalytic hydrogenation using cyclohexene as a
source of hydrogen and palladium on charcoal (80% for
the two steps).6 Acylation with the appropriate synthe-
sised activated ester 5a7 or 5b8 afforded the desired
ligands L1a and L1b, respectively, in 40 and 55% yield.
One of the most important criteria in radiopharmaceu-
tical design is the in vivo stability of radiopharmaceuti-
cals. In order to limit metal dissociation, we initially
reported the synthesis of a new class of tetradentate
ligands incorporating an aromatic ring to induce a
rigidity to the cavity and facilitate complexation nota-
bly by reducing the reaction entropy.4 The rigidity of
the ligands was introduced to improve the stability of
the corresponding complexes.5 But, our first stability
investigations showed us a fairly stability in human
serum after 2 h but a significative dissociated 99mTc
ratio after 6 h (:50%). The objective of the present
study was to develop and to test two new aromatic
bridged diamide–dithiol (DADT) or diamide–thiol–
alcohol (DATA) chelators which, while still maintain-
ing a simplicity of synthesis, will be able to increase the
in vivo stability of 99mTc-complexes.
Heterogeneous catalysts like palladium on charcoal are
usually poisoned by small amounts of sulfur such as
thiols or sulfides. The obtention of the sulfur analog of
compound 3 is impossible using a catalytic hydrogena-
tion step. To circumvent this problem, we adopted a
different synthetic way to obtain L2 (Scheme 2). This
route needs to use a base-stable sulfur protecting group.
Thus, we firstly synthesised the succinimidyl-S-
triphenylmethylthioglycolate 5b.7 Reaction of ethyl gly-
cinate with 5b in THF yielded to the compound 6
(85%). Hydrolysis of the ester function of 6 in basic
media (NaOH in methanol) afforded the corresponding
acid 7 in excellent yield (92%). Then, L2 was obtained
by a classical peptidic coupling of the acid 7 with the
2-(S-trityl)aniline 89 in the presence of dicyclohexylcar-
bodiimide. The same reaction attempted with 2-
* Corresponding author. Fax: 33 561 55 61 18; e-mail: benoist@
chimie.ups-tlse.fr
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02385-7

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J. Le Gal et al. / Tetrahedron Letters 43 (2002) 9295–9297
Scheme 1. Synthetic pathway for L1.
Scheme 2. L2 synthesis.
aminothiophenol instead of 2-(S-trityl)aniline failed,
giving a mixture of unseparable products.
ical yield of 90%. Purification was accomplished by
C-18 reverse phase HPLC and resulted in only a com-
ponent for each ligand. The large difference obtained in
the retention time of the two complexes could result in
the formation of a L2M species for 99mTc-L2.11 Rhe-
nium chemistry is under investigation to confirm this
hypothesis.
The new ligands L1a, L1b and L2, obtained with a good
overall yield, have been satisfactorily characterised by
usual spectroscopic methods.10 Thiol deprotection and
99mTc-labelling of the ligands were performed in situ in
a mixture methanol/buffer solution pH 8.6 (1/4) by
direct reduction of sodium pertechnetate in the presence
of tin chloride at 80°C during 30 min with a radiochem-
The radiolabelled chelators were subject to metathesis
in the presence of an excess of cysteine as an assay of

J. Le Gal et al. / Tetrahedron Letters 43 (2002) 9295–9297
9297
label stability.12 After 12 h (two 99mTc periods), about
45% of 99mTc dissociated were observed for 99mTc-L2
while 99mTc-L1 exhibited a great stability (less than 3%
of 99mTc dissociated). The excellent behaviour for the
latter compound was confirmed by a serum stability
study. After 12 h of incubation in fresh human serum,
no significative technetium dissociation of the complex
was observed.
Hoyos, O. L.; Watkinson, M. New J. Chem. 2000, 24,
235–241.
7. Schneider, R. F.; Subramanian, G.; Feld, T. A.; McAfee,
J. G.; Zapf-longo, C.; Palladino, E.; Thomas, F. D. J.
Nucl. Med. 1984, 25, 223–229.
8. Bell, R. A.; Hughes, D. W.; Lock, C. J. L.; Valliant, J. F.
Can. J. Chem. 1996, 74, 1503–1511.
9. Noveron, J. C.; Olmstead, M. M.; Mascharak, P. K. J.
Am. Chem. Soc. 2001, 123, 3242–3259.
10. Only selected data are cited below for L1a and L2 charac-
terisation: L1a: 13C{1H} NMR (50.32 MHz, CDCl3
JMOD) lC (ppm): 37.7 (SCH2), 48.6 (NCH2), 120.7
(CHAr), 124.3 (CHBz), 127.1, 129.9 (2CHAr), 131.2 (CAr),
132.3, 134.5 (4CHBz), 139.5 (CHAr), 141.3 (CBz), 152.9
(CAr), 172.8, 172.9 (2CO), 195.6 (COS). IR (KBr):
wNHCꢀO=1674 cm−1. m/z (DCI NH3): 345 (M+H+); L2:
13C{1H} NMR (50.32 MHz, CDCl3 JMOD) lC (ppm):
35.5 (SCH2), 43.7 (NCH2), 60.5, 67.9 (2C-S), 119.4
(CHAr), 120.4 (CAr), 123.6 (CHAr), 127.1, 127.2, 127.9,
128.3, 129.5, 129.6 (30CHTr), 131.2, 137.4 (2CHAr), 141.5
(CAr), 143.6, 143.9 (6CTr), 165.5, 168.4 (2CO). IR (KBr):
wNHCꢀO=1677 cm−1. m/z (DCI NH3): 741 (M+H+).
11. HPLC conditions: Satisfaction RP18AB column, eluent:
MeOH/H2O/TFA: 45/55/0.1; u=270 nm; Tr (99mTc-L1)=
9.40 min; Tr (99mTc-L2)=17.00 min. Electrophoresis of
each purified complex showed that both are negatively
charged complexes.
In conclusion, a simplified route has been developed for
the synthesis of two new tetradentate ligands. Their
corresponding 99mTc-complexes were achieved with a
good radiochemical yield. Serum stability and cysteine
challenge assay showed a great stability for one of the
two 99mTc-complexes, which makes it a promising chela-
tor of technetium. Biodistribution studies are currently
under progress.
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12. Cysteine challenge assay: In each case, an aliquot of a
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