Rhenium(I) and technetium(I) fac-M(NSO)(CO)3 (M = Re, 99mTc) tricarbonyl complexes, with a tridentate NSO bifunctional agent: Synthesis, structural characterization, and radiochemistry

Article abstract of DOI:10.1016/j.poly.2009.10.009

The synthesis and structural characterization of the neutral rhenium complex fac-[Re(NSO)(CO)₃], Re-1, where (NSO) is a tridentate bifunctional chelating agent, 3-(carboxymethylthio)-3-(1H-imidazol-4-yl)propanoic acid (1), is presented. The complex crystallized from methanol-water and its structure was assigned by IR and ¹H, ¹³C NMR spectroscopies and X-ray crystallography. Furthermore, the analogous technetium complex fac-[^99mTc(NSO)(CO)₃], ^99mTc-1, was synthesized in high yield by reacting ligand 1 with the fac-[^99mTc(OH₂)₃(CO)₃]⁺ precursor for 30 min at 85 °C. The tracer complex was found to be more than 95% stable in the L-histidine challenge experiment. Our data indicate that the bifunctional NSO chelating agent 1 can be successfully applied for the development of potential ^99mTc-radiopharmaceuticals.

Full text of DOI:10.1016/j.poly.2009.10.009

Polyhedron 29 (2010) 876–880
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Rhenium(I) and technetium(I) fac-M(NSO)(CO)3 (M = Re, 99mTc) tricarbonyl
complexes, with a tridentate NSO bifunctional agent: Synthesis, structural
characterization, and radiochemistry
a
b
c
Dionysia Papagiannopoulou a, , George Makris , Charalambos Tsoukalas , Catherine P. Raptopoulou ,
*
Aris Terzis c, Maria Pelecanou d, Ioannis Pirmettis b, Minas S. Papadopoulos b
a Department of Medicinal Chemistry, School of Pharmacy, Aristotle University of Thessaloniki, 54 124 Thessaloniki, Greece
b Institute of Radioisotopes–Radiodiagnostic Products, National Centre for Scientific Research ‘‘Demokritos”, 15310 Athens, Greece
c Institute of Materials Science, National Centre for Scientific Research ‘‘Demokritos”, 15310 Athens, Greece
d Institute of Biology, National Centre for Scientific Research ‘‘Demokritos”, 15310 Athens, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and structural characterization of the neutral rhenium complex fac-[Re(NSO)(CO)3], Re-1,
where (NSO) is a tridentate bifunctional chelating agent, 3-(carboxymethylthio)-3-(1H-imidazol-4-
yl)propanoic acid (1), is presented. The complex crystallized from methanol–water and its structure
was assigned by IR and 1H, 13C NMR spectroscopies and X-ray crystallography. Furthermore, the analo-
gous technetium complex fac-[99mTc(NSO)(CO)3], 99mTc-1, was synthesized in high yield by reacting
ligand 1 with the fac-[99mTc(OH2)3(CO)3]+ precursor for 30 min at 85 °C. The tracer complex was found
to be more than 95% stable in the L-histidine challenge experiment. Our data indicate that the bifunc-
tional NSO chelating agent 1 can be successfully applied for the development of potential 99mTc-
radiopharmaceuticals.
Received 21 July 2009
Accepted 8 October 2009
Available online 13 October 2009
Keywords:
Rhenium
Technetium
Tricarbonyl complexes
Bifunctional
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
capable of highly specific in vivo localization in target tissues
[7,8]. This strategy involves the development of a suitable bifunc-
Technetium (99mTc) is the most significant radiometal in Nucle-
ar Medicine and 99mTc-radiopharmaceuticals, routinely employed
in Single Photon Emission Computerized Tomography (SPECT)
diagnostic imaging [1–4]. Hexacoordinated 99mTc–tricarbonyl
complexes of the fac-[99mTcL(CO)3] type are an attractive approach
to the design of novel 99mTc-radiopharmaceuticals, because of their
high stability and their convenient, high yield and high specific
activity preparation in aqueous media [5]. Furthermore, the use
of the beta-emitter 188Re in therapeutic radiopharmaceuticals has
been gaining considerable ground and the development of analo-
gous to 99mTc complexes of the type fac-[188ReL(CO)3] could be em-
ployed for targeted radiotherapy. The ligand L should be preferably
tridentate because it results in complexes with more favourable
pharmacokinetics – compared to bidentate ones – and it may have
a variety of donor atoms, like N, S, O [6]. The bifunctional strategy
for the development of technetium and rhenium radiopharmaceu-
ticals has become the most widely used method for producing
well-defined technetium and rhenium labeled receptor ligands
tional chelating agent (BFCA) which is used for the chelation of
the radionuclide (99mTc or 188Re) and the conjugation of the tar-
get-specific moiety. An ideal BFCA is that which is able to form sta-
ble and inert 99mTc or 188Re complexes in high yield, at low
concentration.
We have focused on the study of 3-(S-carboxymethylthio)-3-
(1H-imidazol-4-yl)propanoic acid (1, Fig. 1), which is a natural
metabolite of L-histidine [9], as a potential bifunctional chelating
agent. This compound contains the N(p)-imidazolyl nitrogen, a
thioether S, and a carboxylate O for charge-neutralizing purposes
and can therefore act as a tridentate NSO ligand to generate neu-
tral [M(NSO)(CO)3] complexes with the fac-[M(CO)3]+ core
(M = Re, 99mTc). Aromatic nitrogen atoms like the N(
p)-pyridinyl
or N(
p)-imidazolyl are potent donors and a series of chelating
agents used for the stabilization of the fac-[M(CO)3]+ core contain
either of those [6,10–19]. Moreover, in addition to the NSO donor
atom system, ligand 1 contains a second carboxylic group that
can function as an anchor to conjugate amine containing
biomolecules.
In the present work, we describe the synthesis and character-
ization of the products (Fig. 1) of the reaction of the [NEt4]2[Re-
(CO)3Br3] and [99mTc(CO)3(H2O)3]+ precursors with ligand 1.
* Corresponding author.
E-mail address: papagd@pharm.auth.gr (D. Papagiannopoulou).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.10.009

D. Papagiannopoulou et al. / Polyhedron 29 (2010) 876–880
877
8
COOH
7
COOH
S
2
3
4
5
S
HN
(NEt4)2[ReBr3(CO)3]
COOH
N
HSCH2COOH
NaOH
O
6
COOH
1
HN
O
M
N
HN
fac-[99mTc(OH2)3(CO)3]+
OC
N
CO
OC
1
M=Re, Re-1
M=99mTc, 99mTc-1
Fig. 1. Synthetic scheme of ligand 1 and Re-1, 99mTc-1 complexes.
was dissolved in 40 mL methanol containing 2 mL NaOH 2 M. To
this solution, thioglycolic acid (12 mmoL, 800 L) was added and
2. Experimental
l
the mixture was refluxed overnight. The next day, the mixture
was cooled and the pH was adjusted to 4 by addition of HCl 2 M.
The mixture was subsequently condensed to afford a white solid
which was recrystallized from a pH 4 aqueous solution. Yield:
460 mg (50%). 1H NMR, d (300 MHz, d6-DMSO): 2.76–2.96 (m,
2H), 3.07–3.24 (m, 2H), 4.36 (tr, 1H), 6.98 (s, 1H), 7.65 (s, 1H).
13C NMR, d (300 MHz, d6-DMSO): 32.71, 37.81, 39.15, 115.71,
135.2, 137.35, 171.44, 171.8. Anal. Calc. for C8H10N2O4S: C, 41.73;
H, 4.38; N, 12.17. Found: C, 42.05; H, 4.68; N, 12.35%.
2.1. Materials and methods
All chemicals were reagent grade and were used as such unless
otherwise noted. Rhenium was purchased from Aldrich as
Re2(CO)10 and was converted to (NEt4)2[ReBr3(CO)3] as previously
reported [20].
For the 99mTc labeling a kit containing 5.5 mg NaBH4, 4 mg
Na2CO3 and 10 mg Na–K tartarate, was purged with CO gas prior
to addition of Na99mTcO4, as described elsewhere [5].
Elemental analyses were performed on a Perkin–Elmer 2400
automated analyzer. IR spectra were recorded as KBr pellets on a
Perkin–Elmer 1600 FT-IR spectrophotometer in the region 4000–
2.3. Synthesis of fac-[Re(NSO)(CO)3], Re-1
500 cmÀ1 1H NMR spectra were recorded on a Bruker 500 MHz
.
0.1 mmol (77 mg) of (NEt4)2[ReBr3(CO)3] was reacted with
0.1 mmol (29 mg) 3-(carboxymethylthio)-3-(1H-imidazol-4-yl)
propanoic acid in refluxing water, pH 6 for 3 h. The complex pre-
cipitated from water and was collected as a whitish solid. Crystals
suitable for X-ray crystallography were obtained by slow evapora-
tion from a methanol–water solution. Yield: 31 mg (62%), tR (min)
15.60, IR (cmÀ1, KBr): 2033, 1895, 1725, 1607, NMR data in Table 1,
Anal. Calc. for C11H9N2O7ReS: C, 26.45; H, 1.82; N, 5.61. Found: C,
26.81; H, 2.05; N, 5.82%.
Avance DRX spectrometer and are reported in Table 1. HPLC anal-
ysis was performed on a Waters 600 chromatography system cou-
pled to both a Waters 2487 Dual k Absorbance detector and a Gabi
gamma detector from Raytest. Separations were achieved on a
Nucleosil C18 (10
lm, 250 mm  4 mm) column eluted with a bin-
ary gradient system at a 1 mL minÀ1 flow rate. Mobile phase A was
methanol containing 0.1% trifluoroacetic acid and mobile phase B
was water containing 0.1% trifluoroacetic acid. The elution profile
was 0–1 min 100% B, followed by a linear gradient to 30% B in
10 min; this composition was held for 10 min more. After a column
wash with 95% A for 5 min, the column was re-equilibrated by
applying the initial conditions (100% B) for 15 min prior to the next
injection.
2.4. X-ray crystal structure determination of Re-1
Crystals of Re-1 suitable for X-ray analysis were mounted in
air on a Crystal Logic Dual Goniometer diffractometer using
2.2. Synthesis of 3-(S-carboxymethylthio)-3-(1H-imidazol-4-
yl)propanoic acid (1)
graphite monochromated Mo Ka radiation. Unit cell dimensions
were determined by using the angular settings of 25 automati-
cally centered reflections in the range 11 < 2h < 23° and they
appear in Table 2. Intensity data were recorded using a h–2h
scan. Three standard reflections monitored every 97 reflections
showed less than 3% variation and no decay. Lorentz, polariza-
tion and psi-scan absorption corrections were applied using
CRYSTAL LOGIC software. The structures were solved by direct meth-
ods using SHELXS-97 [23] and refined by full-matrix least squares
techniques on F2 using SHELXL-97 [24]. Further crystallographic
details of Re-1: 2hmax = 50°, scan speed 2.5°/min, scan range
1.6 + a1a2 separation, reflections collected/unique/used 2535/
Ligand 1 was synthesized by reaction of thioglycolic acid with
trans-urocanic acid according to a literature procedure [9,21,22]
with small modifications. Trans-urocanic acid (560 mg, 4 mmol)
Table 1
1H and 13C NMR chemical shifts (ppm) for complex Re-1 in DMSO-d6 at 25 °C. The
numbering of the atoms is shown in Fig. 1.
H-1
H-2
H-4
H-5
H-7
NH
8.36
7.28
4.73
3.62, 3.42
2.82, 2.73
13.17
C-1
C-2
C-3
C-4
C-5
C-6
C-7
C-8
C„O
139.36
115.02
141.75
46.13
34.62
177.80
40.31
170.50
2388 [Rint = 0.0365]/2388, 227 parameters refined,
[
D
q
]
/
max
[D
q
]
min = 1.801/À1.131 e/Å3,
[D/r]max = 0.002, R1/wR2 (for all
data) = 0.0353/0.0837. Hydrogen atoms were located by differ-
ence maps and were refined isotropically, except that of the
imidazolic group which was introduced at calculated position
as riding on bonded atom. All non-H atoms were refined
anisotropically.
196.00, 195.49, 193.75

878
D. Papagiannopoulou et al. / Polyhedron 29 (2010) 876–880
Table 2
Summary of crystal, intensity collection and refinement data.
Re-1
Empirical formula
Formula weight
Temperature
Wavelength
Space group
a (Å)
C11H9N2O7ReS
499.47
298
Mo Ka 0.710730
P21/c
9.518(4)
10.390(4)
14.981(6)
108.01(2)
1408.9(10)
4
b (Å)
c (Å)
b (°)
V (Å3)
Z
Dcalcd (Mg mÀ3
)
2.355
8.810
Absorption coefficient
l
(mmÀ1
)
F(0 0 0)
944
1.077
Goodness-of-fit (GOF) on F2
R indices
R1 = 0.0308a
wR2 = 0.0807a
a
For 2152 reflections with I > 2
r(I).
Fig. 2. 1H–13C long-range correlation spectrum (HMBC, range dY 4.9–2.5, range dC
185.9–29.1) of complex Re-1 in DMSO-d6 at 25 °C. The numbering of protons is
shown in Fig. 1.
2.5. Synthesis of fac-[99mTc(NSO)(CO)3], 99mTc-1
1 mL of an aqueous mixture of [99mTc(OH2)3(CO)3]+ (ꢀ0.5 GBq/
protons on C-5 and C-7 are diastereotopic, due to the asymmetry of
the complex, and appear at different chemical shifts.
mL) and 100 lg of 1 at pH 6.5 reacted at 85 °C for 30 min. The reac-
tion mixture was analyzed by RP HPLC to estimate the yield and
the complex was purified by isolating the radioactive peak from
the column. tR (min): 16.2. Yield > 90%.
3.2. Description of the structure
Histidine challenge of 99mTc-1: 0.2 mL of the reaction mixture of
complex 99mTc-1 was mixed with 0.8 mL solution of 1 mM histi-
dine in 0.1 M potassium phosphate buffer solution, pH 7.4 and
the mixture was incubated at 37 °C for 4 h. The mixture was ana-
lyzed by HPLC.
An ORTEP diagram of Re-1 is given in Fig. 3a and selected bond
distances and angles are listed in Table 3. The distorted octahedral
environment of the Re atom in the structure of Re-1 is defined by
the three facially bound CO groups, and the NSO donor atom set of
the tridentate ligand. The Re–carbonyl bond distances, 1.884(7)–
1.927(8) Å, are consistent which those found in other Re–tricar-
bonyl complexes [25,26]. The Re–S and Re–Ocarb bond distances
(2.479(2) and 2.162(4) Å, respectively) are consistent with those
found in the analogous complex [Re(CO)3L] (L = the monoanion of
3. Results and discussion
3.1. Synthesis and characterization of complex Re-1
S-(carboxymethyl)-L-cysteine; Re–S = 2.469, 2.459 Å and Re–
Ocarb = 2.148, 2.158 Å for the two crystallographically independent
molecules) [26]. The Re–Nimid bond distance (2.163(6) Å) in Re-1 is
significantly shorter than the Re–Namine bond length in [Re(CO)3L]
(2.244, 2.265 Å) [26] as expected for the sp2 vs. sp3 hybridization of
the nitrogen atoms, respectively. There are two five-membered
rings in the coordination sphere which are almost perpendicular
to each other forming a dihedral angle of 83.3°. One of the five-
membered rings is planar and is defined by the atoms Re, S, C6,
C5, O5 (largest deviation ꢀ0.08 Å for C6) and the second one, de-
fined by the atoms Re, N1, C1, S, adopts the envelope configuration
with C7 being 0.55 Å out of the best mean plane of the remaining
four atoms. The protonated carboxylate entity of the tridentate li-
gand is hydrogen bonded to the uncoordinated oxygen atom of the
3-(Carboxymethylthio)-3-(1H-imidazol-4-yl)propanoic acid (1)
provides two possible modes of complexation; the linear mode
where the N(p)-imidazolyl nitrogen, the thioether S, and the car-
boxylate O of the thioglycolate moiety form the tridentate NSO do-
nor atom system, and the tripodal mode, where the NSO system
involves the coordination of the propionate carboxylate group in-
stead (Fig. 1). However, HPLC analysis of the reaction mixture of li-
gand
1 with fac-(NEt4)2[ReBr3(CO)3] precursor showed the
formation of a single product, under the conditions employed.
The product was isolated as white crystalline solid and character-
ized by elemental analysis and spectroscopic methods. The NMR
data confirmed the existence of a single product which was shown
by X-ray crystallography to be the one with the linearly coordi-
nated ligand (Re-1 in Fig. 1). Re-1 is soluble in DMSO, slightly sol-
uble in methanol and ethanol and insoluble in water, and proved
stable in the solid state and in solution for a period of months.
Infrared spectroscopy of the complex Re-1 revealed the charac-
second
carboxylato
moiety
[O2ÁÁÁO4
(1 + x,
0.5 À y,
0.5 + z) = 2.592 Å, HO2ÁÁÁO4 = 1.438 Å, O2–HO2ÁÁÁO4 = 163.42°],
and the imidazolic group is hydrogen bonded to the coordinated
carboxylate
oxygen
atom
O5
[N2ÁÁÁO5
(Àx,
0.5 + y,
0.5 À z) = 2.769 Å, H2 NÁÁÁO5 = 2.005 Å, N2–H2 NÁÁÁO5 = 147.3°],
teristic stretching bands of facially coordinated CO with m(CO) at
2033 and 1895 cmÀ1. Furthermore, the two bands at 1725 and
1607 cmÀ1 indicate the presence of one free carboxylate group
and one coordinated, respectively.
thus forming a 2D network (Fig. 3b).
3.3. Radiochemistry
1H and 13C chemical shift assignments for complex Re-1 were
based on 1H–1H and 1H–13C correlation spectra (Fig. 2) and are re-
ported in Table 1. Upon coordination, downfield shifts (2–6 ppm)
are noted for all carbons and protons (0.2–0.7 ppm) of Re-1 com-
pared to 1, with the exception of the atoms of the free carboxyl
chain that are affected to a lesser extend, as expected. The geminal
Complex fac-[99mTc(NSO)(CO)3], 99mTc-1 was synthesized in
high yield (>90%) by the reaction of the fac-[99mTc(H2O)3(CO)3]+
precursor with ligand 1 at 85 °C for 30 min at pH 5–6. HPLC anal-
ysis of the reaction mixture showed the formation of a single prod-
uct. The reaction yield was almost quantitative. The structure of

D. Papagiannopoulou et al. / Polyhedron 29 (2010) 876–880
879
Fig. 3. (a) Labeled plot of the molecular structure of Re-1. (b) A small fragment of the 2D network in the structure of Re-1 due to intermolecular hydrogen bonding
interactions (dashed lines). Color code: Re, black; S, dark grey; O, medium grey; N, large white; C, small white; H, small black.
the tracer complex 99mTc-1 was assigned by HPLC comparison of
its retention time to that of the authentic well-characterized Re-
Table 3
Selected bond distances (Å) and angles (°) for Re-1.
1, by applying parallel radiometric and photometric detection.
The retention times of the tracer complex 99mTc-1 and the Re-1
were found to be t R = 16.2 and 15.6 min, respectively (Fig. 4), pro-
viding a strong indication of their structural analogy. The tracer
complex 99mTc-1 was about 100% stable for at least 4 h in the
presence of 1 mM histidine in a 0.1 M potassium phosphate buffer
at physiological pH 7.4 and 37 °C. This result indicates that the
tracer complex will be stable against the endogenous histidine
concentrations.
Distances
Re(1)–C(11)
Re(1)–C(12)
Re(1)–C(13)
1.886(7)
1.911(8)
1.928(8)
Re(1)–O(5)
Re(1)–N(1)
Re(1)–S
2.162(4)
2.163(6)
2.479(2)
Angles
C(11)–Re(1)–C(12)
C(11)–Re(1)–C(13)
C(12)–Re(1)–C(13)
C(11)–Re(1)–O(5)
C(12)–Re(1)–O(5)
C(13)–Re(1)–O(5)
C(11)–Re(1)–N(1)
C(12)–Re(1)–N(1)
87.7(3)
86.1(3)
90.9(4)
175.2(2)
97.0(3)
94.4(3)
94.8(2)
172.2(3)
C(13)–Re(1)–N(1)
O(5)–Re(1)–N(1)
C(11)–Re(1)–S
C(12)–Re(1)–S
C(13)–Re(1)–S
O(5)–Re(1)–S
96.6(3)
80.4(2)
99.4(2)
92.9(3)
173.4(2)
79.8(1)
79.4(1)
N(1)–Re(1)–S
5. Conclusions
Ligand 3-(carboxymethylthio)-3-(1H-imidazol-4-yl)propanoic
acid (1), coordinates in tridentate fashion with the rhenium(I)–tri-
carbonyl metal fragment with formation of a neutral [Re(NSO)
(CO)3] complex in high yield, leaving the propionate carboxylate
group free. The formation of only one complex demonstrates that li-
gand 1, coordinates preferably by the linear mode and formation of
two five-membered chelate rings (Fig. 1). Apparently this type of
coordination renders higher stability to the complex compared to
formation of three rings (two five-membered and one seven-mem-
bered) expected from the non-linear mode, as also reported in the
literature [26]. The analogous tracer complex fac-[99mTc(NSO)(CO)3]
is also formed in high yield and is stable in high histidine concentra-
tions. The presence of a free carboxylate group in these metal che-
lates can be used to further anchor a bioactive moiety, e.g., via the
formation of an amide bond, targeting thus specific biological sub-
strates. Therefore, this bifunctional chelating agent can be applied
for the development of target-specific 99mTc and 186/188Re radiophar-
maceuticals for imaging and therapeutic applications, respectively.
99mTc-1
Re-1
0
10
20
30
Supplementary data
minutes
CCDC 738387 contains the supplementary crystallographic data
for Re-1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Fig. 4. Comparative high performance liquid chromatography (HPLC) chromato-
grams for complexes Re-1 (carrier: photometric detection) and 99mTc-1 (tracer:
radiometric detection).

880
D. Papagiannopoulou et al. / Polyhedron 29 (2010) 876–880
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