2
20
K. Yu et al.
2
1
00
50
Acute Toxicity
One of these dimeric Gd(iii) complexes, Gd2·4a, which has the
highest relaxivity in the DTPA series, was chosen for acute tox-
icity testing. It was conducted by single intravenous (trail vein)
injection of a 0.2 mol L solution at a rate of 2 mL min in
mice (20 ± 2 g, male and female). The animals were regularly
observed for 7–14 days after the injection.[ The acute toxicity
test using Gd2·4a showed no obvious toxicity.
−
1
−1
100
50
0
12]
M
4a 4b 4c 4d 4e 4f 4g 4h
Complex
Magnetic Resonance Imaging
The comparison of MR images using Gd2·4a and Gd-DTPA
was carried out by performing imaging experiments on a 4.7T
spectrometer (Bruker Biospec 47/30). The Wistar rats (weigh-
ing 170 ± 20 g, three rats each group) were anaesthetized with a
Fig. 1. Relative enhancement of complexes (measured at 200 MHz)
Gd2·4a–Gd2·4h and in comparison to Magnivest (M).
−1
1
g kg urethane i.p.Axial images of 2 mm slice thickness were
acquired by spin-echo imaging pulse sequence with the follow-
ing parameters: time of echoTE 13 ms, time of repeatTR 500 ms,
number of averages NA 4. After the rat received an injection of
the ligand occupies eight binding sites at the metal centre,
and the ninth coordination site is occupied by a solvent water
molecule. In EDTA-lysine(OR)-EDTA, each EDTA unit pro-
vides six monodentate donors. The ligand occupies six binding
sites at the metal centre and three coordination sites are occu-
pied by three solvent water molecules. Increasing the number
of water molecules located in the inner sphere of the com-
plex almost invariably increases the water relaxivity. Therefore,
higher relaxivities of Gd-EDTA-lysine(OR)-EDTA-Gd com-
plexes compared with those of Gd-DTPA-lysine(OR)-DTPA-Gd
complexes are a natural consequence of the increased molecule
sizes of the novel contrast agents.
−
1
Gd2·4a or Gd-DTPA (dose 0.1 mmol kg ) intravenously, the
images of the same slice were acquired every 5 min over 1 h.
Results and Discussion
l-Lysine esters were chosen in order to obtain molecules con-
taining ester and amide function groups that contribute to the
complexing ability of the synthesized structures. The ligands
contain long-chain alkyl groups that cause a size increase of
molecules so that the molecules do not readily pass through
the pores of blood vessels. Because the anhydride groups in
DTPA monoanhydride and EDTA monoanhydride are easily
hydrolyzed, we used their solutions directly without separation.
Amidation reaction of anhydride group with amine group was
performed at high temperature because of steric hindrance due to
the existence of large groups such as long-chain alkyl in l-lysine
esters.
In T1-weighted magnetic resonance imaging experiments
on live animals, the water proton signal intensities of the
liver (parenchyma) were found to be increased after the intra-
venous injection of Gd2·4a and Gd-DTPA, respectively (dose
−
1
0.1 mmol kg ). Representative images acquired before and
after injection of Gd2·4a are shown in Fig. 2. The averaged time
courses of signal intensity changes (three rats) are plotted in
Fig. 3. The first data points in each series (0 min, Fig. 1a) were
measured before injection of the contrast agents, and the values
were used as the reference (100%) to scale the signals acquired
at later time points.
The effects of paramagnetic ions on the T1 relaxation nuclear
[
13–15]
spins were first formulated by Bloembergen and Solomon
and subsequently extended by several authors.[
3,4]
On the basis
of this theory, the longitudinal relaxation timeT1 in the presence
of Gd complex is given by[
3,4]
As shown in these figures, the amplitudes of signal enhance-
ment increased gradually from 0 to 35 min after the injection
of Gd2·4a (Fig. 2b,c). The largest enhancements were found
between 20 and 50 min (>20%) after injection, correspond-
ing to an average enhancement of 125%. The highest signal
enhancement was found at ∼35 min after injection. The average
signal enhancement decreased to 117% at 60 min after injection
(Fig. 2d). Between 10 and 60 min after injection, the average
signal enhancement was 123%.
1/T1,obsd = 1/T1,d + R1[M]
whereT1,d is theT1 relaxation time in the absence of gadolinium,
M] denotes the concentration of gadolinium ion in the measured
[
solution, and R1 is a constant defining the relaxivity of the con-
trast agent, which is the most important parameter in evaluating
a contrast agent.
The relaxivities of all the new contrast agents we synthesized
were calculated by measuring the longitudinal relaxation times
In the case of Gd-DTPA, the signal intensity of the liver
increased in the first 25 min after injection, reached a plateau
at 20–40 min (>18%) after injection, corresponding to an aver-
age enhancement of 119%. The largest enhancement was found
at ∼25 min after injection. The enhancement decreased to 111%
at 60 min after injection. Between 10 and 60 min after injec-
tion, the average signal enhancement was 115%. These results
show that, compared with Gd-DTPA, Gd2·4a provides stronger
relaxation enhancement in the liver and the enhancement lasts
longer.
To conclude, eight novel dimeric Gd(iii) complexes were
synthesized by treating GdCl3·6H2O with diamino-ester lig-
ands which contain long-chain alkyl esters of l-lysine and two
DTPA or EDTA chelate units. The longitudinal relaxivity (R1)
of all these dimeric Gd(iii) complexes are invariably higher than
(T1) of the water protons. It is found that the relaxivities of all
the gadolinium binuclear complexes produced in our present
work are higher than that of Gd-DTPA (see Fig. 1). Relaxivi-
ties of Gd-EDTA-lysine(OR)-EDTA-Gd complexes, where R is
an alkyl group, are 45–65% higher than that of Gd-DTPA (per
Gd(iii) atom). Relaxivities of Gd-DTPA-lysine(OR)-DTPA-Gd
complexes are 14–30% higher than Gd-DTPA (per Gd(iii) atom).
Comparing the relaxivities of the binuclear complexes with
eachother, itisfoundthat, asanticipated, increasingthemolecule
size by introducing a side alkyl group R causes an increase in the
residence and correlation times,[3,4] enhancing the relaxivity. In
DTPA-lysine(OR)-DTPA, each DTPA unit provides eight mono-
dentatedonorsandGd(iii)hasninecoordinationsites.Therefore,