Synthesis and in vitro studies of Gd-DTPA derivatives as new potential MRI contrast agents

Article abstract of DOI:10.1016/j.tetlet.2010.02.138

A new type of dendritic molecules, Gd-DTPA derivatives, which work as a functionalized ligand coordinating gadolinium(III) ion at the center of their frameworks with different terminal moieties on the molecular surfaces, was readily synthesized with high

Full text of DOI:10.1016/j.tetlet.2010.02.138

Tetrahedron Letters 51 (2010) 2431–2433
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Tetrahedron Letters
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Synthesis and in vitro studies of Gd–DTPA derivatives as new potential
MRI contrast agents
Arigala Uma Ravi Sankar a,b, Mitsuji Yamasitha a, , Kambam Srinivasulu a,b, Nobuhisa Ozaki a, Takashi Aoki a,
*
Michio Fujie a,c, Keisuke Ogawa a, Shingo Okada a, Manubu Yamada a, Yasutaka Tanaka d, Motohiko Kimura d,
Mitsuo Toda d
a Graduate School of Science and Technology, Shizuoka University, Naka-Ku, Hamamatsu 432-8561, Japan
b Japan Association for the Advancement of Medical Equipment (JAAME) Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
c Faculty of Medicine, Hamamatsu University School of Medicine, Kita-Ku, Hamamatsu 431-3192, Japan
d Department of Materials Science and Chemical Engineering, Faculty of Engineering, Shizuoka University, Hamamatsu 432-8561, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new type of dendritic molecules, Gd–DTPA derivatives, which work as a functionalized ligand coordi-
nating gadolinium(III) ion at the center of their frameworks with different terminal moieties on the
molecular surfaces, was readily synthesized with high yield. The structures were established by 1H, 13C
NMR, and mass spectral studies. In vitro studies showed them to have enhanced r1 value in albumin med-
ium and good potentiality as MRI contrast agent.
Received 23 December 2009
Revised 20 February 2010
Accepted 23 February 2010
Available online 26 February 2010
Ó 2010 Published by Elsevier Ltd.
Keywords:
Dendritic ligands
Gd-complexes
MRI contrast agents
T1 and r1 values
Over the past two decades Magnetic Resonance Image (MRI) has
becomea verypowerfultoolof diagnosticmedicine.1,2 Paramagnetic
materialshave beeninvestigated as MRI contrastagents (CAs). These
materials enhance the contrast of the image indirectly by remark-
ably shortening the magnetic relaxation time of water protons coor-
dinated by comparison with protons of the surrounding tissues.3 The
most frequently used CAs are stable gadolinium(III) complexes with
hydrophilic poly(aminocarboxylate) ligands resulting in rapid
extracellular distribution and renal elimination. Gd(III) is preferred
because of its favorable magnetic properties. Gd-complexes with
amphiphilic properties have previously been prepared and evalu-
ated as blood-pool and liver-imaging agents. Long chain amides
and esters of Gd–DTPA are the most common.4
The condensation of terminal 2a–e and DTPA dianhydride 36 re-
sults in the polyamine ligand 47 which on further reaction with
GdCl3ꢀ6H2O forms compound (1a–e)8 (Fig. 1 and Scheme 1).
COO-
-OOC
H
COO-
H
N
N
N
R
N
C
O
C
O
N
R
Gd+3
O
H
H
1
In continuation of our work on MRI contrast agents5 we designed
a more water-soluble molecules 1a–e (Fig. 1). The terminal groups of
diethylenetriaminepentaacetic acid (DTPA) have a specific target
and combine only with asialoglycoprotein receptor (ASGPR) on the
surface of hepatocyte, and also improve the water solubility of con-
trast agent. So DTPA was used as a core, and free amine was used as a
biofunctional group to prepare a series of dendritic Gd-complexes
for use as novel MRI contrast agents that improve blood-pool prop-
erty and enhance T1 and r1 values in albumin medium.
NH2
1d :
R =
COOH
1a :
R =
1e : R =
COOH
COOH
1b : R =
COOH
HO
1c :
R =
* Corresponding author. Tel./fax: +81 53 478 1144.
E-mail address: tcmyama@ipc.shizuoka.ac.jp (M. Yamasitha).
Figure 1. The structure of Gd–DTPA-complexes 1a–e.
0040-4039/$ - see front matter Ó 2010 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2010.02.138

2432
A. U. R. Sankar et al. / Tetrahedron Letters 51 (2010) 2431–2433
O
COH
O
HOC
O
COH
HOOC
N
b
a
N
N
1
O
N
O
N
H2N
R +
C
O
N
NH
R
HN
C
O
R
O
O
O
O
4
2
3
NH2
4d : R =
COOH
4a : R =
4e : R =
COOH
COOH
R =
4b :
COOH
HO
4c : R =
Scheme 1. Reagents and conditions: (a) DMF, 60 °C, 24 h; (b) GdCl3ꢀ6H2O, pyridine, H2O, 60 °C, 24 h.
Table 1
on their metal complexes as new potential candidates for MRI con-
trast media are now in progress.14
Comparison of r1 relaxivity of Gd–DTPA and Gd-complexes 1a–e
Gd-complexes
r1 (sꢂ1 mMꢂ1
)
In vitro evaluation: Relaxivity r1 that divided relaxation time T1
by gadolinium concentration is used as a guide to contrast intensi-
fication of MRI contrast agent because the relaxation time depends
on the gadolinium concentration of MRI contrast agent. Because
gadolinium ion that did not form complexes has influence on mea-
surements of relaxation time, free gadolinium ion were removed
by adjusting pH 7.0 in water, adding ChelexÒ100 Resin, and stirring
at room temperature for six hours. The removal of free gadolinium
ion was confirmed with color test by xylenol orange. Gadolinium
concentration was measured with ICP-AES because relaxation time
depended on gadolinium concentration of contrast agents. T1 was
measured by TD-NMR of 0.47 T at 37 °C. T1 was measured not only
in water but also in serum albumin, the most existing protein in
blood. The r1 values were calculated by the following expression,
the values are shown in Table 1.
In H2O
In albumin
Gd–DTPA
3.5
8.0
5.8
4.8
5.5
5.4
3.5
10.25
10.00
6.2
6.4
9.5
1a
1b
1c
1d
1e
These ligands are composed of DTPA6 and terminal units 2a–e,
which may immobilize gadolinium ion at the focal points by eight
coordination sites, allowing one water molecule to chelate and
encapsulates the metal ions inside the glycoside clusters. Like the
previous reports on Gd(III)–DTPA complexes9,10 our new Gd-com-
plex has showed good solubility in aqueous solutions, although the
acetylated free amines might reinforce their own hydrophobic fea-
tures. The Gd–DTPA-complexes 1a–e with different terminal func-
tionalities were designed to have a rigid frame work in the
molecule and to facilitate the protein recognition with the amines
as well. Hence Gd–DTPA-derived complexes 1a–e are expected to
possess better r1 values in water and enhanced r1 values in
albumin.
The r1 values obtained for 1a–e in water and albumin are shown
in Table 1. As expected, the r1 value in albumin medium was en-
hanced moderately but not remarkably. Along with the ‘glycoside
cluster effect’11 the carbohydrate aggregation may offer a potential
advantage for site-specific delivery of the contrast agents at a
molecular level since carbohydrates play significant roles in recog-
nition processes on cell surface.11–13 Therefore, the combined con-
tribution of amines and carbohydrates to the enhancement of r1
value of Gd-complexes by hybridization of the terminal of free
amine will be preferable.14
We developed the synthesis of new dendritic molecules having
functionalized ligands useful for the preparation of Gd-complexes.
This synthetic methodology can be scaled up for multigram for
practical use as radiopharmaceuticals. The chelates with higher-
molecular weight groups are indispensable for the prevention of
their diffusion from the intravascular space during MRI examina-
tions.15 Accordingly, these gadolinium(III) chelates after creation
and structural modifications may fulfill many criteria for superior
contrast agents. Following the intensive investigations on a wide
variety of carbohydrate-modified ligands, the feasibility studies
2O
ꢁ 1000 ꢂ rH1
1
T1
r1
¼
½Gd3þꢃ
2O
where rH1 is the relaxivity (sꢂ1 mMꢂ1); T1 is the relaxation time
(ms); r1 is the water of relaxivity (sꢂ1 mMꢂ1); and [Gd3+] is the gad-
olinium concentration (mM).
We have succeeded in the synthesis of a new gadolinium com-
plex, Gd–DTPA derivatives as an MRI CAs. Their higher accumula-
tion in the blood vessel and higher tumor-selectivity indicate
that they have potential as MRI angiography agents and for early
diagnosing medical treatments of tumors.
Acknowledgments
We would like to thank the Ministry of Health, Labor, and Wel-
fare, Japan, and JAMME, Japan Association for the Advancement of
Medical Equipment Japan; the Headquarters of Japan Science and
Technology Agency (JST); Shizuoka Prefecture Industry Creating
Agency; and Grants-in-Aid for Scientific Research by Japan Society
for the promotion of Science (JSPS) for providing financial support.
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