Tuning the coordination number of hydroxypyridonate-based gadolinium complexes: Implications for MRI contrast agents

Article abstract of DOI:10.1021/ja057805x

Eight-coordinate hydroxypyridinone/terephthalamide Gd^III complexes display high relaxivities due to their two inner sphere water molecules. This relaxivity can be further increased by functionalizing the terephthalamide moiety with an amine. A

Full text of DOI:10.1021/ja057805x

Published on Web 04/04/2006
Tuning the Coordination Number of Hydroxypyridonate-Based Gadolinium
Complexes: Implications for MRI Contrast Agents1
Vale´rie C. Pierre,† Mauro Botta,‡ Silvio Aime,§ and Kenneth N. Raymond*,†
Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460, Dipartimento di Scienze
dell’Ambiente e della Vita, UniVersita` del Piemonte Orientale “A. AVogadro”, Via Bellini 25/G, I-15100
Alessandria, Italy, and Dipartimento di Chimica I.F.M., UniVersita` di Torino, Via P. Giuria 7, I-10125 Torino, Italy
Received November 16, 2005; E-mail: raymond@socrates.berkeley.edu
With seven unpaired electrons and a long electronic relaxation
time, GdIII is ideal for use as a relaxation agent in Magnetic
Resonance Imaging (MRI). Its high toxicity, however, requires that
it be complexed by a strong chelator for in vivo applications.
Current commercial poly(amino carboxylate)-based chelates have
only one water molecule coordinated, which exchanges too slowly
with the bulk solvent such that it limits the image enhancing
capability (relaxivity, r1p) of macromolecular derivatives.2 The
Figure 1. Gd-TREN-bisHOPO-TAM-Me (Gd-Me) and the amine deriva-
tives Gd-N1, Gd-N2, and Gd-N3.
development of second generation agents, for example, site-specific,
requires much higher relaxivity. As predicted by the Solomon-
Bloembergen-Morgan theory, this can be achieved by increasing
the number of water molecules coordinated to the GdIII without
destabilizing the complex.2
We have previously reported hydroxypyridonate (HOPO)-based
GdIII chelates which display high relaxivity while maintaining high
stability.3 In these complexes, the GdIII centers are eight-coordinate
with a bicapped trigonal prism geometry.4 Since the ligands are
hexadentate, water molecules occupy two of the coordination sites
of GdIII, resulting in a relaxivity double that of commercial agents.
These complexes exchange water rapidly (kex ∼ 108 s-1) through
an associative interchange mechanism, suggesting that the eight
and nine coordination states are close in energy.5 This is supported
by the structure of the LaIII analogue of TREN-1-Me-3,2-HOPO
which crystallized as a dimer, with one La being eight-coordinate
and the other nine.6 The crystal structure of Gd-TREN-1-Me-3,2-
HOPO indicates that filling the open coordination site of GdIII with
a third water molecule would result in minimal distortion of the
complex.4 This indicates that it would be possible to stabilize the
nine coordination state to achieve q ) 3 complexes which would
still maintain high stability. One possibility to achieve this is to
graft a hydrogen bond acceptor, such as an amine, on the
terephthalamide moiety. A significant hydrogen bonding interaction
between the amine and another water molecule close to the GdIII
could facilitate its coordination.
Three derivatives of Gd-TREN-bisHOPO-TAM-Me (Gd-Me)
bearing either one, two, or three pendant amines were synthesized
(Figure 1). Their 1/T1 NMRD profiles (Figure 2) indicate that the
relaxivity of the smaller Gd-N1 is noticeably higher than that of
the intermediate Gd-N2, which is, in turn, higher than that of the
larger Gd-N3. The refinement parameters (Table 1) indicate that
all three complexes have similar electronic relaxation times (∆2
and τv), and that their rotational correlation times, τR, are
proportional to their molecular weight. The large difference in
relaxivity observed, which is inversely proportional to the size of
the complexes, is therefore not due to a change in τR, but rather to
an increase in the number of inner sphere water molecules, q, for
Figure 2. 1/T1 NMRD profile at 298 K and pH 7.4 of Gd-N1 (9), Gd-N2
(4), Gd-N3 (b), and Gd-Me (O).
Table 1. Stability and Refinement Parameters of the 1/T1 NMRD
Profiles of Gd-N1, Gd-N2 and Gd-N3
Gd-Me6
Gd-N1
Gd-N2
Gd-N3
pM
20.1
8.7
19.5
11.1
20.0
9.7
18.8
9.0
298
r
1p
(mM-1 s-1
)
a
q
1.94 ( 0.09 2.82 ( 0.08 2.39 ( 0.07 1.98 ( 0.05
τ
R (ps)
∆2 (1020 s-2
τv (ps)
m (ns)
110 ( 2
110 ( 5
114 ( 3
1.4 ( 0.1
19.2 ( 0.4
3c
127 ( 3
)
1.2 ( 0.2
17.1 ( 0.3
8.0 ( 0.9b
1.3 ( 0.2
18.7 ( 0.9
2.6 ( 0.9b
1.3 ( 0.1
20.0 ( 0.9
2.1 ( 0.8b
τ
a At 20 MHz. b Determined from the temperature dependence of the
paramagnetic contribution to the water 17O NMR r2p. c Fixed.
the smaller Gd-N1.2 Refinements of the NMRD profiles indicate
that Gd-N3, like the parent methyl derivative, Gd-Me, bears two
inner sphere water molecules, whereas Gd-N1 coordinates three.
The intermediate Gd-N2 displays q ) 2.4((0.1), indicating that at
room temperature the eight and nine coordination states are present
in almost equal proportions.7
Further evidence of the 8.5 coordination number of Gd-N2 (at
298 K) was obtained by temperature dependence of the paramag-
netic contribution to the water 17O NMR transverse relaxation rate
(Supporting Information). Unlike Gd-N3, Gd-N2 does not display
the exponential decrease of r2p with temperature, typical of
† University of California, Berkeley.
‡ Universita` del Piemonte Orientale.
§ Universita` di Torino.
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J. AM. CHEM. SOC. 2006, 128, 5344-5345
10.1021/ja057805x CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 5. Proposed model for the pH dependence of the hydrogen bonding
network of Gd-N2.
Figure 3. Proposed hydrogen bonding network of Gd-N1 and Gd-N3.
ammonium can no longer hydrogen bond to, and stabilize, the
coordination of a third water molecule. The resulting complex is
only eight-coordinate, with lower relaxivity.
The stabilities of the three GdIII complexes were measured by
spectrophotometric titration against DTPA. All complexes are highly
stable, with pM values11 comparable to those of the commercial
agents Gd-DTPA (19.2) and Gd-DOTA (19.4).2 In particular, the
stability of Gd-N1 (pM ) 19.5) is similar to that of the parent
Gd-Me (20.1).6 This indicates that coordination of a third water
molecule does not destabilizes the GdIII complex.
In summary, this report describes the increase of the hydration
number of HOPO-based GdIII complexes by substituting the TAM
podand with a moiety capable of hydrogen bonding a third water
molecule close to the metal center. The resulting nine-coordinate
GdIII complex is the first displaying three inner sphere water
molecules and high stability. The ensuing high relaxivity of this
small complex renders it a promising candidate for the development
of second generation contrast agents.
Figure 4. The pH dependence of the longitudinal relaxivity (r1p) of Gd-
N1 (black squares), Gd-N2 (open circles), and Gd-N3 (gray triangles).
complexes with fast water exchange.2 The “S” shaped profile
observed, characteristic of two overlapping decays, indicates a
change in coordination number with temperature. The nine-
coordinate complex is predominant at high temperature, and the
eight-coordinate one at low temperature.
Since no change in electronic parameters is observed, we do not
believe that the difference in q is due to a change in the ligands
coordinating the GdIII. The increase in coordination number from
Gd-N3 to Gd-N1 may, however, be rationalized by the hydrogen
bonding network created by the terminal moiety (Figure 3). The
ethylene bridge to the terminal primary amine of Gd-N1 positions
the nitrogen such that it may intramolecularly hydrogen bond to
the proton of the amide, which is also bonded to the catechol
oxygen. In our proposed model, the terminal amine hydrogen bonds
a solvent water molecule, thereby bringing it close to the open
coordination site of the GdIII and facilitating its coordination. In
Gd-N3, however, the tertiary nitrogen is probably intramolecularly
hydrogen bonded to the two primary amines. This is favored over
an intermolecular bond to a water molecule, such that the complex
remains eight-coordinate. Such hydrogen bonding networks have
already been observed in several crystal structures of lanthanide
poly(amino carboxylates) and HOPO-based complexes.4,8-10
The hydrogen bonding network and, hence, the relaxivity of the
complexes are pH dependent (Figure 4) and in agreement with
previous solution thermodynamic studies.11 Between pH 13 and 4,
Acknowledgment. This research was supported (UCB) by NIH
Grant HL69832 and NATO Travel Grant PST.CLG.980380. An
unrestricted gift from Schering AG is acknowledged, and we thank
Eric Werner for his assistance.
Supporting Information Available: Detailed experimental pro-
cedures and characterization data for the synthesis of Gd-N1, Gd-N2,
and Gd-N3; spectrophotometric titration data, pH dependence of the
hydrogen bonding network of Gd-N3, temperature dependence of the
paramagnetic contribution to the water 17O NMR transverse relaxation
rate (r2p) for Gd-N2, Gd-N1, and Gd-N3. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) Paper no. 16 in the series “High Relaxivity MRI Agents”; for the previous
paper in the series, see: Thompson, M. K.; Misselwitz, B.; Tso, L.; Doble,
D. M. D. J.; Schmitt-Willich, H.; Raymond, K. R. J. Med. Chem. 2005,
48, 3874-3877.
(2) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B. Chem. ReV.
1999, 99, 2293-2352.
(3) Raymond, K. N.; Pierre, V. C. Bioconjugate Chem. 2005, 16, 3-8.
(4) Xu, J.; Franklin, S. J.; Whisenhunt, D. W.; Raymond, K. N. J. Am. Chem.
Soc. 1995, 117, 7245-7246.
r1p of Gd-N2 and Gd-N3 remains constant and corresponds to q )
2 complexes. At pH 3, the protonated TAM podand dissociates
the GdIII and is replaced by two water molecules, resulting in a
2-fold increase in r1p. The relaxivity then decreases toward pH 1
as the complex completely dissociates. The relaxivity of Gd-N1
displays different pH dependence (Figure 5). It remains constant
between pH 13 and 7 corresponding to the nine-coordinate, q ) 3,
complex. It decreases by about a third, to presumably a q ) 2
complex at pH 5, corresponding to the pKa of the terminal amine,
and increases again at pH 3 to the partially dissociated q ) 4 form.
As previously observed with the tertiary nitrogen of the TREN
backbone, favorable hydrogen bonding stabilizes the deprotonated
form of the pendant amine and consequently significantly decreases
its pKa to 5.8,11 Our interpretation is that, at this pH, the terminal
(5) Thompson, M. K.; Botta, M.; Nicolle, G.; Helm, L.; Aime, S.; Merbach,
A. E.; Raymond, K. N. J. Am. Chem. Soc. 2003, 125, 14274-14275.
(6) Cohen, S. M.; Xu, J.; Radkov, E.; Raymond, K. N.; Botta, M.; Barge, A.;
Aime, S. Inorg. Chem. 2000, 39, 5747-5756.
(7) The fractional q number allows for a good fit of the data with physically
reasonable values of the parameters τR, τV, and ∆2. If q is fixed to either
2 or 3, unrealistic values of these parameters are obtained.
(8) Cohen, S. M.; O’Sullivan, B.; Raymond, K. N. Inorg. Chem. 2000, 39,
4339-4346.
(9) Xu, J.; Churchill, D. G.; Botta, M.; Raymond, K. N. Inorg. Chem. 2004,
43, 5492-5494.
(10) Dickins, R. S.; Aime, S.; Batsanov, A. S.; Beeby, A.; Botta, M.; Bruce,
J.; Howard, J. A. K.; Love, C. S.; Parker, D.; Peacock, R. D.; Puschmann,
H. J. Am. Chem. Soc. 2002, 124, 12697-12705.
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Pierre, V. C.; Raymond, K. N. Inorg. Chem. 2003, 42, 4930-4937.
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