EuII-containing cryptates as contrast agents for ultra-high field strength magnetic resonance imaging

Article abstract of DOI:10.1039/c1cc15219j

The relaxivity (contrast-enhancing ability) of Eu^II-containing cryptates was found to be better than a clinically approved Gd ^III-based agent at 7 T. These cryptates are among a few examples of paramagnetic substances that show an in

Full text of DOI:10.1039/c1cc15219j

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COMMUNICATION
EuII-containing cryptates as contrast agents for ultra-high field strength
magnetic resonance imagingw
Joel Garcia,a Jaladhar Neelavalli,b E. Mark Haackeb and Matthew J. Allen*a
Received 22nd August 2011, Accepted 20th October 2011
DOI: 10.1039/c1cc15219j
The relaxivity (contrast-enhancing ability) of EuII-containing
cryptates was found to be better than a clinically approved GdIII
We hypothesized that EuII-containing [2.2.2]cryptates in
aqueous solution could be used as the basis for efficient positive
contrast agents for ultra-high field strength MRI because EuII
and GdIII are isoelectronic but differ in other molecular para-
meters that influence relaxivity. This hypothesis is supported
by studies of EuII-containing complexes: for example, EuII-
containing [2.2.2]cryptate, 1, has been reported to have a fast
-
based agent at 7 T. These cryptates are among a few examples
of paramagnetic substances that show an increase in longitudinal
relaxivity, r1, at ultra-high field strength relative to lower field
strengths.
water-exchange rate.8 The water-exchange rate of 1 is 3 ꢀ 108 sꢁ1
,
Contrast-agent-enhanced magnetic resonance imaging (MRI)
is a widely used, noninvasive imaging technique for clinical
and preclinical research as well as diagnostic medicine. Most
clinical imaging is performed at high field strengths (1.5 or 3 T);
however, a shift to ultra-high field strength MRI ( Z 7 T) is
desirable because of the increased spatial resolution and shorter
acquisition times enabled at these fields, which offer the potential
to lead to early diagnoses of diseases and more accurate imaging
in vivo.1–4 Consequently, preclinical research relies heavily on
ultra-high field strengths. A major limitation is that common
T1-reducing (positive) contrast agents at lower field strengths,
such as GdIII-containing complexes, become inefficient at
ultra-high field strengths.2,5 Multiple parameters—including
the water-exchange rate, the number of inner-sphere water
molecules, the rotational correlation time, and the relaxation
times of the electron spins of GdIII—are in a complex interplay
that is crucial to increasing relaxivity, r1, which is a measure of
contrast-enhancing ability of a contrast agent.2–4 An alterna-
tive to GdIII is EuII, which is isoelectronic with GdIII but has
a faster water-exchange rate due to its lower charge density.
However, until recently, research on EuII chemistry in aqueous
solution was limited because of its tendency to oxidize to
which is faster than most GdIII-based complexes and is very
close to the optimal value (B108 sꢁ1 at 7 T) for developing
efficient MRI contrast agents at ultra-high fields.2,8 Attempts
to increase the water-exchange rate of GdIII-containing com-
plexes have been made by altering the coordination number of
ligands for GdIII or by increasing steric bulk at the site where
inner-sphere water binds.10 However, because of the compli-
cated interplay between water-exchange rate and other para-
meters that affect relaxivity, there is a need to simultaneously
optimize all factors to increase the efficacy of contrast agents.
In addition to its optimal water-exchange rate, 1 has two inner-
sphere water molecules in a ten-coordinate complex, which
should enable higher relaxivity than an equivalent complex with
only one inner-sphere water molecule according to Solomon–
Bloembergen–Morgan (SBM) theory. These favorable molecular
properties prompted us to hypothesize that EuII-containing
cryptates would be candidates for contrast agents at ultra-high
field MRI.
To test our hypothesis, we investigated the imaging (longi-
tudinal relaxivity, r1, at 3 and 7 T) and physical (water-exchange
rate, electron-spin relaxation rate, and number of inner-sphere
water molecules) properties of a series of EuII-containing
[2.2.2]cryptates (Fig. 1). Relaxivity was determined using
multiple flip angles in gradient echo imaging experiments perfor-
med between 19 and 20 1C at 3 and 7 T (Table 1) and inversion
recovery experiments at either 20 or 37 1C at 1.4 and 11.7 T.11
At all of these field strengths, EuII-containing cryptates 2 and 3
EuIII 6–8
We have used modified [2.2.2]cryptands to increase
.
the oxidative stability of EuII,6 and here, we report the mole-
cular properties and imaging profiles of a series of EuII-containing
[2.2.2]cryptates (1–3 in Fig. 1) that are, to the best of our
knowledge, among a few reported contrast agents, and the first
lanthanide-based contrast agents, that are more effective at
ultra-high field strengths than at lower fields.9
a Department of Chemistry, Wayne State University,
5101 Cass Avenue, Detroit, MI 48202, USA.
E-mail: mallen@chem.wayne.edu; Fax: 1 313 577 8822;
Tel: 1 313 577 2070
b Department of Radiology, Wayne State University, Detroit,
MI 48201, USA
w Electronic supplementary information (ESI) available: Experimental
procedures and NMR spectra. See DOI: 10.1039/c1cc15219j
Fig. 1 Structures of GdDOTA (DOTA = 1,4,7,10-tetraazacyclo-
dodecane-N,N0,N0 0,N0 0 0-tetraacetate) and EuII-containing cryptates
1–3. Coordinated water molecules have been omitted for clarity.
c
12858 Chem. Commun., 2011, 47, 12858–12860
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Table 1 r1 values of GdDOTA and EuII-containing cryptates
r1 at 1.4 T
(mMꢁ1 sꢁ1
T = 37 1C
r1 at 11.7 T
(mMꢁ1 sꢁ1
T = 37 1C
r1 at 3 T
(mMꢁ1 sꢁ1
T = 19.8 1C
r1 at 7 T
(mMꢁ1 sꢁ1
T = 19 1C
r1 at 11.7 T
(mMꢁ1 sꢁ1
)
T = 20 1C
)
)
)
)
GdDOTA
3.00 ꢂ 0.06
2.09 ꢂ 0.02
3.67 ꢂ 0.09
4.39 ꢂ 0.10
2.12 ꢂ 0.02
2.65 ꢂ 0.04
3.34 ꢂ 0.02
4.80 ꢂ 0.06
3.69 ꢂ 0.06
3.94 ꢂ 0.12
4.84 ꢂ 0.08
6.31 ꢂ 0.07
3.73 ꢂ 0.01
5.01 ꢂ 0.02
6.47 ꢂ 0.14
7.17 ꢂ 0.03
2.89 ꢂ 0.004
3.99 ꢂ 0.14
4.81 ꢂ 0.06
6.35 ꢂ 0.01
1
2
3
All complexes were in phosphate buffered saline (PBS, pH = 7.4). Results are reported as mean ꢂ standard error.
displayed higher r1 than clinically approved GdDOTA (up to 46,
71, 92, and 120% higher at 1.4, 3, 7, and 11.7 T, respectively).
Furthermore, cryptate 1 displayed a higher relaxivity than
GdDOTA at 7 and 11.7 T. We found that the r1 values of
GdDOTA were not different at 3 and 7 T (Student t test) but
decreased by 29% from 1.4 to 11.7 T. This result was expected
based on what is known about fast rotating GdIII-containing
complexes.3 However, EuII-containing complexes 1–3 showed an
increase in r1 values at 7 relative to 3 T.12 Also, cryptates 1 and 3
displayed an increase in r1 values at 11.7 relative to 1.4 T. The
r1 value of cryptate 2 was slightly lower at 11.7 T compared to
1.4 T. Additionally, the r1 values of cryptates 1–3 increased as a
function of molecular weight at all field strengths.
associated with the best correlation coefficient are reported in
Table 2. The electron-spin relaxation rates of the EuII ion in
cryptates 1–3 were in the range of 106–107 sꢁ1 and were not
limiting as evidenced by the change in relaxivity with molecular
weight. Another key parameter in increasing relaxivity at ultra-
high fields is the number of inner-sphere water molecules. EuII
cryptates have two inner-sphere water molecules, which corres-
ponds to higher efficiency of a contrast agent relative to com-
plexes with fewer inner-sphere water molecules. In general, the
superiority of the EuII cryptates over GdDOTA in enhancing
relaxivity at ultra-high field strengths is likely due to a combi-
298
nation of factors. We have determined kex298, q, and T1e for
cryptates 1–3 in an attempt to explain the molecular basis for our
observations; however, our results cannot be satisfactorily inter-
preted using SBM theory.
To understand our observations of r1 values, we used variable
temperature 17O NMR spectroscopy to determine the mole-
cular parameters expected to influence relaxivity (Table 2): the
residence lifetime of bound water molecules, tm298, where water-
exchange rate, kex298, is equal to 1/tm298; the longitudinal electronic
relaxation time, T1e298; and the number of inner-sphere water
molecules, q. To attain the highest relaxivity at ultra-high field
strengths, water-exchange rate must be B108 sꢁ1.2 The water-
exchange rates of the EuII complexes are 6–94 times faster than
GdDOTA and, consequently, are closer to being optimal at ultra-
high field strengths as described by SBM theory. However, our
observed water-exchange rate does not limit the relaxation
enhancement for fast rotating complexes even at ultra-high
fields. As seen from the data in Tables 1 and 2, r1 values were
larger for cryptates 2 and 3 relative to 1 despite 2 and 3 having
slower water-exchange rates than 1. The relaxivity differences
among cyptates 1–3 are likely due to differences in molecular
weight because increases in molecular weight correspond to
decreases in rotational correlation rates for structurally similar
monomeric complexes.13
To demonstrate the consequence of having higher relaxivity,
we obtained phantom images (images of solutions) of GdDOTA
and EuII-containing complexes 1–3 using T1-weighted imaging at
7 T (Fig. 2). The MR images of EuII cryptates 1–3 showed higher
signal intensity relative to PBS. This observation is indicative that
EuII cryptates are effective at influencing the relaxation of water
protons in solution. The signal intensities of the EuII complexes
are different from each other (Student t test) with 3 giving rise to
the highest signal intensity. The 1/T1 values were between 29 and
89% higher for cryptates 1–3 relative to GdDOTA. An increase
SBM theory also describes the dependence of relaxivity on
the electron spin relaxation, which we investigated with 17O
NMR studies. Data acquired from variable temperature 17O
NMR experiments were refitted using 63 unique values of
T1e298 in the range of 10ꢁ8–10ꢁ3 s, and the T1e298 values that were
Table 2 Relaxivity parameters (based on 17O NMR data) and
molecular weights of GdDOTA and cryptates 1–3
GdDOTA
1
2
3
298
tm (ns)
Fig. 2 (a) T1-weighted MR images of solutions of GdDOTA (1.0 mM
in PBS) and 1–3 (1.0 mM in PBS) at 7 T and 19 1C. The diameter of the
tubes that were used for imaging was 6 mm. Imaging parameters were
TR = 21 ms; TE = 3.26 ms; and resolution = 0.27 ꢀ 0.27 ꢀ 2 mm3.
(b) Comparison of 1/T1 values of the samples from (a) at 7 T and 19 1C.
Error bars represent standard error of the mean.
287 ꢂ 10
0.035
1
3.0 ꢂ 0.4
3.3
2
120
11.7 ꢂ 0.3
48 ꢂ 4
0.21
2
1100
49 ꢂ 5
731
kex (108 sꢁ1
)
0.85
2
298
q
T1e (ns)
298
29
70 ꢂ 2
41
34 ꢂ 1
DH (kJ molꢁ1
MW (Da)
)
60 ꢂ 8
591
564
657
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12858–12860 12859

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in 1/T1 corresponds to higher signal intensity and, therefore, is
advantageous for imaging. These imaging experiments demon-
strate that cryptates 1–3 are effective contrast agents at ultra-high
field strengths.
L. V. Elst, H. A. Mayer, N. K. Logothetis and G. Angelovski, Eur. J.
Inorg. Chem., 2009, 3298.
6 N.-D. H. Gamage, Y. Mei, J. Garcia and M. J. Allen, Angew.
Chem., Int. Ed., 2010, 49, 8923.
´
7 (a) E. To
´
216–217, 363; (b) P. Caravan, E. Toth, A. Rockenbauer and
´
th, L. Burai and A. E. Merbach, Coord. Chem. Rev., 2001,
´
We found that EuII-based complexes 1–3 are effective con-
trast agents at ultra-high field strengths likely because of the
interplay of water-exchange rate, rotational correlation rate,
and the presence of two inner-sphere water molecules. To the
best of our knowledge, these complexes are among a few
reported examples of contrast agents that display increased
r1 values at ultra-high field strengths relative to lower fields.
Further studies on the physical properties of EuII-containing
cryptates are being performed in our laboratory to better
understand these results. We expect that our findings will be
a step toward addressing the lack of positive contrast agents
for ultra-high field strength MRI, and we are currently study-
ing EuII cryptates in more detail to obtain a more complete
understanding of the structure–function relationships as well as
the thermodynamic and kinetic stabilities of these complexes.
This research was supported by startup funds from Wayne
State University (WSU) and a Pathway to Independence
Career Transition Award (R00EB007129) from the National
Institute of Biomedical Imaging and Bioengineering of the
National Institutes of Health. J. G. was supported by a
Thomas C. Rumble Graduate Fellowship from WSU, and
M. J. A. gratefully acknowledges a Schaap Faculty Scholar
Award. We thank Prabani Dissanayake for acquiring induc-
tively coupled plasma mass spectrometry data, Jeremiah
Moore for helpful discussions, and Latif Zahid and Yimin
Shen for performing imaging experiments.
A. E. Merbach, J. Am. Chem. Soc., 1999, 121, 10403; (c) P. Caravan
and A. E. Merbach, Chem. Commun., 1997, 2147.
´
´
8 L. Burai, R. Scopelliti and E. Toth, Chem. Commun., 2002, 2366.
9 (a) J. Kowalewski, A. Egorov, A. Laaksonen, S. Nikkhou Aski,
G. Parigi and P.-O. Westlund, J. Magn. Reson., 2008, 195, 103;
(b) D. Kruk and J. Kowalewski, J. Chem. Phys., 2002, 116, 4079;
(c) T. Nilsson, G. Parigi and J. Kowalewski, J. Phys. Chem. A,
2002, 18, 4476; (d) J. Svoboda, T. Nilsson, J. Kowalewski, P.-O.
Westlund and P. T. Larsson, J. Magn. Reson., Ser. A, 1996,
121, 108.
10 (a) E. J. Werner, S. Avedano, M. Botta, B. P. Hay, E. G. Moore,
S. Aime and K. N. Raymond, J. Am. Chem. Soc., 2007, 129, 1870;
(b) S. Torres, J. A. Martins, J. P. Andre
´
E. Brucher, L. Helm, E. To
´
, G. A. Pereira, R. Kiraly,
´
th and C. F. G. C. Geraldes, Eur. J.
¨
Inorg. Chem., 2007, 5489; (c) V. C. Pierre, M. Botta and K. N.
Raymond, J. Am. Chem. Soc., 2005, 127, 504; (d) S. Laus,
´
´
R. Ruloff, E. Toth and A. E. Merbach, Chem.–Eur. J., 2003,
9, 3555; (e) D. M. J. Doble, M. Botta, J. Wang, S. Aime, A. Barge
and K. N. Raymond, J. Am. Chem. Soc., 2001, 123, 10758;
(f) S. M. Cohen, J. Xu, E. Radkov and K. N. Raymond, Inorg.
Chem., 2000, 39, 5747; (g) S. Aime, A. Barge, A. Borel, M. Botta,
S. Chemerisov, A. E. Merbach, U. Muller and D. Pubanz, Inorg.
Chem., 1997, 36, 5104.
¨
11 E. M. Haacke, R. W. Brown, M. R. Thompson and R. Venkatesan,
Magnetic Resonance Imaging Physical Principles and Sequence Design,
John Wiley & Sons, Inc., New York, 1999, p. 654.
12 (a) The temperatures at 3 and 7 T differed by 0.8 1C. To examine
the influence of this temperature variation on relaxivity, we
measured cryptate 3 at 11.7 T at temperatures of 19 and 19.8 1C.
The relaxivity values were 6.82 ꢂ 0.01 and 6.72 ꢂ 0.01, respec-
tively; (b) D. J. Averill, J. Garcia, B. N. Siriwardena-Mahanama,
S. M. Vithanarachchi and M. J. Allen, J. Vis. Exp., 2011,
53, e2844.
13 (a) W. C. Floyd III, P. J. Klemm, D. E. Smiles, A. C. Kohlgruber,
V. C. Pierre, J. L. Mynar, J. M. J. Frechet and K. N. Raymond,
´
Notes and references
1 (a) E. Moser, World J. Radiol., 2010, 2, 37; (b) D. Pitt, A. Boster,
W. Pei, E. Wohleb, A. Jasne, C. R. Zachariah, K. Rammohan,
M. V. Knopp and P. Schmalbrock, Arch. Neurol., 2010, 67, 812;
(c) N. Blow, Nature, 2009, 458, 925.
2 L. Helm, Future Med. Chem., 2010, 2, 385.
3 P. Caravan, Chem. Soc. Rev., 2006, 35, 512.
4 J. Costa, R. Ruloff, L. Burai, L. Helm and A. E. Merbach, J. Am.
Chem. Soc., 2005, 127, 5147.
5 (a) J. B. Livramento, C. Weidensteiner, M. I. M. Prata, P. R.
Allegrini, C. F. G. C. Geraldes, L. Helm, R. Kneuer, A. E. Merbach,
J. Am. Chem. Soc., 2011, 133, 2390; (b) A. Kundu, H. Peterlik,
M. Krssak, A. K. Bytzek, I. Pashkunova-Martic, V. B. Arion,
T. H. Helbich and B. K. Keppler, J. Inorg. Biochem., 2011,
105, 250; (c) S. Dumas, V. Jacques, W.-S. Sun, J. S. Troughton,
J. T. Welch, J. M. Chasse, H. Schmitt-Willich and P. Caravan,
Invest. Radiol., 2010, 45, 600; (d) V. C. Pierre, M. Botta, S. Aime
and K. N. Raymond, J. Am. Chem. Soc., 2006, 128, 9272;
(e) K. W.-Y. Chan and W.-T. Wong, Coord. Chem. Rev., 2007,
251, 2428; (f) P. Caravan, N. J. Cloutier, M. T. Greenfield, S. A.
McDermid, S. U. Dunham, J. W. M. Bulte, J. C. Amedio Jr.,
R. J. Looby, R. M. Supkowski, W. D. Horrocks Jr., T. J.
McMurry and R. B. Lauffer, J. Am. Chem. Soc., 2002,
124, 3152; (g) D. E. Reichert, R. D. Hancock and M. J. Welch,
Inorg. Chem., 1996, 35, 7013; (h) F. E. Armitage, D. E. Richardson
and K. C. P. Li, Bioconjugate Chem., 1990, 1, 365.
´
A. C. Santos, P. Schmidt and E. To
2006, 1, 30; (b) J. B. Livramento, L. Helm, A. Sour, C. O’Neil,
´
th, Contrast Media Mol. Imaging,
´
A. E. Merbach and E. To
C. Cannizzo, C. Prestinari, F. Berriere and L. Helm, Inorg. Chem.,
2008, 47, 8357; (d) I. Mamedov, P. Taborsky, P. Lubal, S. Laurent,
´
th, Dalton Trans., 2008, 1195; (c) L. Moriggi,
´
´
c
12860 Chem. Commun., 2011, 47, 12858–12860
This journal is The Royal Society of Chemistry 2011