Synthesis and magnetic relaxation properties of new Gd(III) complexes derived from DTPA-bis(amide) conjugates of arylpiperazinyl amines

Article abstract of DOI:10.1016/j.inoche.2015.03.042

Two new Gd(III) complexes 1 and 2 of the type [Gd(L)H₂O]·nH₂O were synthesized from DTPA-bis(amide) conjugates of arylpiperazinyl amines. The relaxivity (R₁) of these complexes was measured in deionized water, which reveal

Full text of DOI:10.1016/j.inoche.2015.03.042

Inorganic Chemistry Communications 56 (2015) 5–7
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and magnetic relaxation properties of new Gd(III) complexes
derived from DTPA-bis(amide) conjugates of arylpiperazinyl amines
c
Abdullah O. Ba-Salem a, M. Nasiruzzaman Shaikh b, Nisar Ullah a, , Mohamed Faiz
⁎
a
Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
b
Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
c
Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new Gd(III) complexes 1 and 2 of the type [Gd(L)H2O]·nH2O were synthesized from DTPA-bis(amide)
conjugates of arylpiperazinyl amines. The relaxivity (R1) of these complexes was measured in deionized water,
which revealed that complex 2 had a higher relaxivity than 1 and Omniscan®, a commercially available MRI con-
trast agent. The cytotoxicity studies of 1 and 2 indicated that they are non-toxic which warrant their physiological
suitability as potential contrast agents for MRI. All the synthesized ligands and complexes were characterized
with the aid of analytical and spectroscopic methods including elemental analysis, 1H NMR, FT-IR, XPS and fast
atom bombardment (FAB) mass spectrometry.
Received 11 February 2015
Received in revised form 14 March 2015
Accepted 22 March 2015
Available online 24 March 2015
Keywords:
DTPA-bis(amide)
Arylpiperizine
Gd(III)-complexes
Relaxivity
© 2015 Elsevier B.V. All rights reserved.
MRI contrast agents
Magnetic Resonance Imaging (MRI) is at present one of the safest
and efficient non-invasive imaging modality in clinical diagnosis due
to no use of harmful high-energy radiation compared to competing
radio-diagnostic methods [1–3]. The diagnostic potential of MRI is enor-
mous since, in addition to the assessment of anatomical changes, MRI
can also be used for monitoring of organ functions for instance following
the functions of the human brain on a real time-scale by functional-MRI
(fMRI). In addition, MRI often represents the only reliable diagnostic
method for, e.g. cranial abnormalities or multiple sclerosis [4,5]. With
the advent of clinical MRI scanners, MRI contrast agents are used to
study tissue function for instance to track vascular flow for large arteries
and veins, vascular perfusion and permeability in capillary networks [6].
MRI contrast agents (CAs) are diagnostic magneto-pharmaceuticals
which usually constitute low molecular weight Gd(III) chelates with
an acyclic or macrocyclic ligand [7]. The efficacy of the CAs, known
as relaxivity, is measured by its ability to transmit the paramagnetic
properties into the bulk water proton and thereby shortening the T1
relaxation times of water proton in the body that in turn provide im-
pressive anatomical information [2]. The prominent position of Gd(III)
in most MRI CAs relies on its favorable combination of a large magnetic
moment (spin-only μeff = 1/4 7.94 BM, from seven half-filled f orbitals)
and long electron spin relaxation time (10−8 to 10−9 s, from symmetric
S electronic state) [8].
Anionic gadolinium complexes, for example Gd(DTPA)2−, are gen-
erally associated with high osmolality under physiological conditions
giving rise to some adverse effects. In addition, they suffer from limita-
tions including limited utility in focal lesion detection. In order to allevi-
ate such liabilities and improve the tissue and/or organ-specificity,
preparation of neutral complexes based on the Gd macrocycle, aspiring
for highly efficient (“optimized”) CAs has been the subject of intensive
research in the last 20 years. [9–12]. Based on earlier reports, incorpora-
tion of alkyl and aromatic groups in the side arm of the diethylene
triamine pentaacetic acid (DTPA) ligand rendered excellent relaxivity
and water solubility [13–15]. In light of the above, we have designed
and synthesized two novel ligands 13 and 14 from the reaction of
diethylene triamine pentaacetic acid dianhydride (DTPAA) with suit-
ably modified arylpiperazines. These ligands were then transformed to
their corresponding Gd(III) complexes 1 and 2. Herein, we wish to dis-
close the synthesis, characterization, relaxivity (R1) and cytotoxic stud-
ies of these complexes.
Ligands 13 and 14 were synthesized as outlined Scheme 1. Alkylation
of commercially available fluoronitrophenol 3 produced 4 and 5, which
were condensed with piperazine 6 to furnish 7 and 8. The basic hydrolysis
of latter under heating generated 9 and 10, respectively. Initially, we
treated 4 and 5 with 1-boc-piperazine to obtain the corresponding con-
densed products but the acid induced removal of boc group led us to
the deprotection of methoxymethyl group. Michael addition of amine 9
with acrylonitrile produced intermediate 11 (for details, see Supplemen-
tary information). The reduction of nitro group of 11 by hydrogenation
using Pd–C or Ra–Ni as catalysts was problematic, leading to complex
⁎
Corresponding author.
E-mail address: nullah@kfupm.edu.sa (N. Ullah).
http://dx.doi.org/10.1016/j.inoche.2015.03.042
1387-7003/© 2015 Elsevier B.V. All rights reserved.

6
A.O. Ba-Salem et al. / Inorganic Chemistry Communications 56 (2015) 5–7
NO2
HN
N Bz
NO2
OR
NO2
OR
OH
RX/K2CO3, DMF
40 oC, overnight
3M KOH
6
K2CO3, DMF, 60 oC, overnight
60 oC, 8 hrs
N
F
F
3
N
4
(R = CH3, 98%)
5 (R = CH3OCH2, 85%)
Bz
7
(R = CH3, 99%)
8 (R = CH3OCH2, 97%)
NO2
NO2
OR
NH2
OR
OR
DTPAA, DMF, 80 o
16 hrs
C
CN, Et3N, EtOH
55 oC, 3 hrs
Ammonium formate
Pd-C, MeOH, 75 oC, 3 hrs
N
N
N
N
N
N
H
9
(R = CH3, 75%)
10 (R = CH3OCH2, 80%)
CN
(R = CH3, 71%)
12 (R = CH3OCH2, 59%)
CN
(R = CH3, 97%)
14 (R = CH3OCH2, 99%)
11
13
O
HO
O
O
OH
N
HO
N
O
O
N
N
N
O
N
Gd
HN
NH
NH
NH
O
O
O
O
H
RO
OR
O
OR
OR
Gd2O3, H2O
95 oC, 8 hrs
O
O
O
H
N
N
N
N
N
N
N
N
CN
CN
CN
CN
1
(R = CH3, 89%)
2 (R = CH3OCH2, 83%)
15
(R = CH3, 51%)
16 (R = CH3OCH2, 63%)
Scheme 1. Synthesis of gadolinium complexes (1) and (2).
mixture of products, which were difficult to resolve. Therefore, the reduc-
tion of nitro group was conceived via transfer hydrogenation in methanol
with ammonium formate and Pd–C as catalyst to produce aminonitrile 13
in high yield (for details, see Supplementary information). DTPAA [16]
was condensed with 13 to construct intermediate 15, which in turn was
heated with Gadolinium(III) oxide to produce complex 1 (Scheme 1).
Likewise, complex 2 was synthesized by similar synthetic strategy as
employed for 1 (Scheme 1).
Synthesis of 1: To a solution of compound 15 (0.100 g, 0.114 mmol)
in deionized water (10 mL) was added Gd2O3 (0.021 g, 0.057 mmol)
and the reaction was stirred at 90 °C for 8 h. The mixture was filtered
through a pad of celite while hot and the filtrate was concentrated
Fig. 1. Survey XPS spectrum of (1) and (2), showing all elements except hydrogen.

A.O. Ba-Salem et al. / Inorganic Chemistry Communications 56 (2015) 5–7
7
Table 1
Relaxivity and cytotoxicity data of complexes 1 and 2 compared with Omniscana.
Acknowledgment
The authors would like to acknowledge the support provided by King
Abdulaziz City for Science and Technology (KACST) through the Science &
Technology Unit at King Fahd University of Petroleum & Minerals
(KFUPM) for funding this work through project No. 11-BIO2138-04 as a
part of the National Science, Technology and Innovation Plan.
CA
T1
(ms)
R1
In vitro cytotoxicity
(IC50 μg/mL)
(mM−1 s−1
)
1
2
293.51 1.21
235.67 1.01
3.40
4.24
3.2
1
N50
2
N50
Omniscan®b
Cyclohexamide
0.13 0.02
a
Data measured under same condition.
Data obtained from reference [20].
b
Appendix A. Supplementary data
under reduced pressure. The residue was then added drop wise to cold
acetone (25 ml) and the precipitated product was filtered, dried under
reduced pressure to afford compound 1 as a light blue solid. Yield:
89%. Anal. Calcd for C42H56GdN11O10·4H2O: C, 45.68; H, 5.84; and N,
13.95. Found: C, 45.62; H, 5.90; and N, 13.89. FAB-MS (m/z): calcd for
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2015.03.042.
References
C
42H56GdN11O10, 1032.35 ([MH]+). Found: 1032.35 ([MH]+). Melting
[1] P.C. Lauterbur, Image formation by induced local interactions: examples employing
nuclear magnetic resonance, Nature 242 (1973) 190–191.
[2] P. Caravan, J.J. Ellison, T.J. McMurry, R.B. Lauffer, Gadolinium (III) chelates as MRI
contrast agents: structure, dynamics, and applications, Chem. Rev. 99 (1999)
2293–2352.
[3] K. Aneta, Z. Thomas, Bioorganic synthesis of some recent tactical application of
bioisosteres in drug design, Med. Chem. 54 (8) (2011) 2529–2591.
[4] S. Ogawa, T.-M. Lee, A.S. Nayak, P. Glynn, Oxygenation-sensitive contrasts in
magnetic resonance image of rodent brain at high magnetic fields, Magn. Reson.
Med. 14 (1990) 68–78.
[5] J. Belliveau, B. Rosen, H. Kantor, R. Rzedzian, D. Kennedy, R. McKinstry, J. Vevea, M.
Cohen, I. Pykett, T. Brady, Functional cerebral imaging by susceptibility-contrast
NMR, Magn. Reson. Med. 14 (1990) 538–546.
point: N300 °C. IR (KBr): 3444, 3030, 2927, 2250, 1611, 1456, 1399,
1323, 1250, 1204, 1135, 1093 cm−1
.
Synthesis of 2: Following the same procedure adopted for the synthe-
sis of 1, condensation of ligand 16 with Gd2O3 afforded compound 2 as
light purple solid. Yield: 83%. Anal. Calcd for C44H60GdN11O12·4H2O: C,
45.39; H, 5.89; and N, 13.23. Found: C, 45.32; H, 5.95; and N, 13.26. FAB-
MS (m/z): calcd for C44H60GdN11O12, 1092.37 ([MH]+). Found: 1092.37
([MH]+). Melting point: N300 °C. IR (KBr): 3486, 3012, 2845, 2253,
1616, 1403, 1322, 1246, 1123, 1090, 990 cm−1
.
The structures of all the intermediates and final complexes 1 and 2
were established by spectroscopic techniques such as elemental analysis,
1H and 13C NMR, FAB mass spectrometry. Complexes 1 and 2 were highly
hygroscopic and isolated as a hydrated solid. The appearance of the ν
(OH) band from the water of crystallization at 3444 and 3486 cm−1 sup-
ported this observation [13]. Likewise, the disappearance of carbonyl
stretching bands of free ligands 15 and 16 at 1706 and 1729 cm−1 in
both 1 and 2 complexes indicated the participation of the carbonyl groups
in coordination [17].
The metal contents and chemical composition of 1 and 2 were deter-
mined by X-ray photoelectron spectroscopy (XPS) (Figs. 1, 2 in Supple-
mentary information). In compound 1, C 1 s region fitted with three
peaks (eV) at 284.8 (C–C), 286.0 (C–N, C–O) and 288.2 (C_C, C–O)
[18]. Whereas Gd 4d region was fitted with four peaks, showing multi-
plet structure, N 1 s region was fitted with one peak at 399.7 eV. Like-
wise, O 1 s region was fitted with peaks (eV) at 531.4 (C–O) and 532.8
(C_O) (Fig. 2) [18]. The XPS measurement of compound 2 showed
similar peaks pattern like in the case of 1 (Fig. 3, Supplementary
information).
Relaxivities (R1) were calculated as an inverse of relaxation times
per mM based on the earlier report [19]. The R1 data (Table 1) revealed
that 2 had a higher relaxivity than 1 and Omniscan®, a commercially
available MRI contrast agent. This higher relaxivity of 2 could be attrib-
uted to its relatively higher molecular weight along with the dangling
alkylnitrile moiety, attached to the piperazine functionality, which
may render stability through hydrogen bonding to the water molecule
that is coordinated to the Gd(III).
[6] D.V. Hingorania, A.S. Bernsteinb, M.D. Pagela, A review of responsive MRI contrast
agents: 2005–2014, Contrast Media Mol. Imaging (2014)http://dx.doi.org/10.
1002/cmmi.1629.
[7] K.W.-Y. Chan, W.-T. Wong, Small molecular gadolinium(III) complexes as MRI
contrast agents for diagnostic imaging, Coord. Chem. Rev. 251 (2007) 2428–2451.
[8] M.T. Vlaardingerbroek, J.A. den Boer, Magnetic Resonance Imaging. Theory and
practice, Springer Verlag, Germany, 1996.
[9] H. Schmitt-Willich, M. Brehm, C.L.J. Evers, G. Michl, A. Muller-Fahrnow, O. Petrov, J.
Platzek, B. Raduchel, D. Sulzle, Synthesis and physicochemical characterization of a
new gadolinium chelate: the liver-specific magnetic resonance imaging contrast
agent Gd-EOB-DTPA, Inorg. Chem. 38 (1999) 1134–1144.
[10] H.J. Weinmann, R.C. Brash, W.R. Press, Characteristics of gadolinium-DTPA complex:
a potential NMR contrast agent, AJR Am. J. Roentgenol. 142 (1984) 619–622.
[11] C.A. Chang, Magnetic resonance imaging contrast agents. Design and physicochem-
ical properties of gadodiamide, Investig. Radiol. 28 (Suppl. 1) (1993) S21–S27.
[12] S.K. Mark, C.D. William, B.L. David, N.R. Kenneth, C.Q. Steven, M.R. Scott, Gadolinium
complexation by a new DPTA-amide ligand. Amide oxygen coordination, J. Inorg.
Chem. 29 (1990) 1488–1491.
[13] S. Dutta, S.-K. Kim, E.J. Lee, T.-J. Kim, D.-S. Kang, Y. Chang, S.O. Kang, W.-S. Han, Syn-
thesis and magnetic relaxation properties of paramagnetic Gd-complexes of new
DTPA-bis-amides. The X-ray crystal structure of [Gd(L)(H2O)] · 3H2O (L = DTPA-
bis(4-carboxylicphenyl)amide), Bull. Korean Chem. Soc. 27 (2006) 1038–1042.
[14] Y.-M. Wang, T.-H. Cheng, G.-C. Liu, R.-S. Sheu, Synthesis of some N,N″-bis(amide)
derivatives of diethylenetriaminepentaacetic acid and the stabilities of their com-
plexes with Gd3+, Ca2+, Cu2+ and Zn2+, J. Chem. Soc. Dalton Trans. 5 (1997)
833–837.
[15] S. Dutta, J.-A. Park, J.-C. Jung, Y. Chang, T.-J. Kim, Gd-complexes of DTPA-bis(amide)
conjugates of and its with high relaxivity and stability for magnetic resonance
imaging, Dalton Trans. (2008) 2199–2206.
[16] D.J. Hnatowich, W.W. Layne, R.L. Childs, The preparation and labeling of DTPA-
coupled albumin, Appl. Radiat. Isot. 33 (1982) 327–332.
[17] D. Zhang, X. Zhang, Z. Linb, H. Zhang, Y. Chen, S. He, Synthesis and relaxation properties
of two non-ion complexes of gadolinium(III) and manganese(II) with derivatives from
diethylene triamine pentaacetic acid and isoniazid, Inorg. Chem. Commun. 40 (2014)
66–68.
Toxicity of compounds 1 and 2 was analyzed on adherent 3T3 mouse
embryonic fibroblastic cells by MTT assay following earlier procedures
with some modification [21]. The cytotoxicity assay indicated that com-
pounds 1 and 2 were non-toxic, which warrant their physiological suit-
ability as potential contrast agents for MRI.
In conclusion, two new non-ionic complexes of type [Gd(L)H2O]·
nH2O have been synthesized and evaluated them for relaxivity and cy-
totoxicity. The relaxivity of these complexes turned out to be higher or
comparable to Omniscan®. The cytotoxicity assay revealed that these
complexes were non-toxic and hence make them prospective candidate
as potential contrast agents for MRI.
[18] D. Briggs, M.P. Seah, Practical surface analysis, second ed.vol. 1 Wiley, New York,
1990.
[19] K.-H. Jung, H.-K. Kim, G.H. Lee, D.-S. Kang, J.-A. Park, K.M. Kim, Y. Chang, T.-J. Kim, Gd
Complexes of macrocyclic diethylenetriaminepentaacetic acid (DTPA) biphenyl-
2,20-bisamides as strong blood-pool magnetic resonance imaging contrast agents,
J. Med. Chem. 54 (2011) 5385–5394.
[20] M. Rohrer, H. Bauer, J. Mintorovitch, M. Requardt, H.-J. Weinmann, Comparison of
magnetic properties of MRI contrast media solutions at different magnetic field
strengths, Investig. Radiol. 40 (2005) 715–724.
[21] M.A. Mesaik, Z. Haq, S. Murad, Z. Ismail, N.R. Abdullah, H.K. Gill, A. Rahman, M.
Yousaf, R.A. Siddiqui, A. Ahmad, M.I. Choudhary, Biological and molecular docking
studies on coagulin-H: human IL-2 novel natural inhibitor, Mol. Immunol. 43
(2006) 1855–1863.