Efficient relaxivity enhancement in dendritic gadolinium complexes: Effective motional coupling in medium molecular weight conjugates

Article abstract of DOI:10.1039/b413536a

Enhancement of the relaxivities of Gd-based MRI contrast agents is achieved by placing the metal ion at the barycentre of the molecular complex in order to improve motional coupling.

Full text of DOI:10.1039/b413536a

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Efficient relaxivity enhancement in dendritic gadolinium complexes:
effective motional coupling in medium molecular weight conjugates{
David A. Fulton,a Mark O’Halloran,a David Parker,*a Kanthi Senanayake,a Mauro Bottab and Silvio Aimec
Received (in Cambridge, UK) 3rd September 2004, Accepted 8th October 2004
First published as an Advance Article on the web 17th December 2004
DOI: 10.1039/b413536a
spherical complex possessing an overall density maximum at the
centre, is provided by a hydrophilic dendritic structure,7 packed
with second sphere water molecules, with the Gd ion at the focal
point. Globular dendrimers based on polyethers,8 PAMAMS9 and
hyper-branched polymeric analogues10 conform to this model and
possess a maximal intrinsic viscosity at intermediate MW, i.e. in
the range 3 to 6 kDa.
The RRRR (or SSSS) [GdgDOTA(H2O)]52 complex,
[Gd?1(OH2)]52 possesses a fast water exchange rate (15 6
106 s21, 298 K) and therefore serves as a useful core for dendrimer
synthesis by functionalising the C4-related peripheral carboxylate
groups.11 By analogy with Frechet’s development of hydrophilic
PEO dendrimer analogues,12 we set out to link [Gd?1(OH2)]52 to
the new amino-substituted dendrons 2–5, thereby generating a set
of four gadolinium complexes of intermediate molecular weight i.e.
1804, 3100, 2028 and 3548. Complexes formed by amide bond
formation of 5 with [Gd?1(OH2)]52, possess 32 more methylene
groups than the analogue derived from 4, and the difference
between tetraamide complexes derived from 2 vs. 3 is 16 methylene
groups.
Enhancement of the relaxivities of Gd-based MRI contrast
agents is achieved by placing the metal ion at the barycentre of
the molecular complex in order to improve motional coupling.
A key objective in MRI contrast agent research is to devise
covalently-linked or non-covalently bound conjugates that effi-
ciently catalyse the rate of relaxation of the bulk water proton
signal.1 It is generally appreciated that in order to devise such a
high relaxivity MRI contrast agent based on gadolinium, it is
necessary to couple effectively the local motion of the Gd–OH2
vector with the rotational motion of the whole complex, whilst
maintaining fast water exchange on and off the Gd ion.2
Theoretical predictions suggest that maximal relaxivity enhance-
ments will occur when the water exchange rate is in the range 10 to
50 6 106 s21 (310 K) and when the rotational correlation time, tr,
defining the motion of the Gd–OH2 vector, is of the order of a few
nanoseconds.3
Previous attempts to devise such systems have met with limited
success. The most useful relaxivity enhancements have been
reported in non-covalently bound systems,4 such as those based on
binding of the Gd complex to serum albumin (MW y 67 kDa
tr y 10 ns). Under physiological conditions, the protein-bound
complex of Epix’s MS-325 system possesses a limiting relaxivity of
the order of 25 mM21 s21 (65 MHz, 310 K, 0.1 mM complex) at
the current clinically used imaging field.5 Every attempt to devise a
high relaxivity covalently-linked high MW conjugate has failed
either because of inefficient motional coupling or a slow water
exchange rate that quenches the theoretical relaxivity gain. For
example, addition of linear polymeric chains (e.g. poly(ethylene
oxide) (PEO)) to Gd complexes gives virtually no increase in
relaxivity as the rapid segmental motion of a given chain precludes
any concerted macromolecular tumbling.6 Similarly, various Gd
complexes have been covalently attached to putative biocompa-
tible polymers (e.g. polylysine, polyethyleneimine) or around the
periphery of a spherical object such as a higher order dendrimer,
but in each case relaxivity gains were either limited by a slow water
exchange rate or by independent motion of the Gd complex
moiety, with respect to the overall tumbling motion of the
macromolecule.1,2
The amines 2–5 were prepared from the reported12 ketal or
O-benzyl protected alcohols either via a Mitsunobu reaction
followed by ketal (H2O/dioxan/0.1 M HCl) or O-benzyl (Pd(OH)2/
C : H2O–dioxan) deprotection, or via the derived azide followed by
hydrogenation. The azides were prepared via standard mesylation
(MsCl/Et3N/CH2Cl2) and azide formation steps (DMF/NaN3).{
The formation of the tetraamide complexes was undertaken using
standard coupling methods, e.g. EDC/HOBt/H2O–THF for
linking 3 and 5 and HBTU/DMF for linking 2 and 4.
Complexes were purified using gel permeation chromatography
Our approach was based upon the premise that when a Gd ion
lies at the barycentre of a macromolecular structure, it will lie upon
the axis of any re-orientational motion. A simple example of a
{ Electronic supplementary information (ESI) available: representative
syntheses are given in the ESI, which also details complex characterisation,
selected VT 17O NMR analyses and certain NMRD profiles. See http://
www.rsc.org/suppdata/cc/b4/b413536a/
*david.parker@durham.ac.uk
474 | Chem. Commun., 2005, 474–476
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The rate of water exchange at the Gd centre was measured by a
VT 17O NMR study, measuring the rate of transverse relaxation of
each complex at 2.1 or 11.7 T. Complex proton relaxivities
(NMRD profiles) were determined at 288, 298 and 310 K over the
field range 0.001 to 20 MHz, with additional measurements at 60,
65 and 200 MHz. Data were treated using standard fitting
procedures and ‘best-fit’ analyses are reported in Table 1, with
results compared to the methylamide parent 6a, MW 912, for
which tm 5 42 ns (298 K), and r1P 5 5.8 mM21 s21 (298 K,
20 MHz). For those cases where the inner sphere water exchange
rate remains fast, i.e. 6a–6d, the inner sphere contribution scales
with the increase in molecular volume (Fig. 1: upper) The less
hydrophilic complex 6e with the largest number of methyl and
methylene groups, gave a relaxivity that was limited by a slow
water exchange rate, tm 5 0.57 ms (298 K). Indeed the observed
relaxivity was dominated by a large second sphere contribution
that continues to scale with molecular volume (Fig. 1: lower). The
linearity of relaxivity vs. MW (molecular volume) for those
complexes maintaining a fast inner sphere water exchange rate,
coupled with the bonus of a rising second sphere contribution that
may be independent of the inner sphere exchange dynamics,
suggests that such hydrophilic dendritic conjugates of intermediate
MW may afford an efficient and pragmatic means to devise
relatively high relaxivity contrast agents in the field range 1.5 to
3 T, that is used in 21st Century MRI scanners.
Table 1 Relaxivities (298 K, 20 MHz) and relaxation times derived
by analysis of VT 17O NMR analyses and by the fitting of NMRD
profiles at 288, 298 and 310 K
[Gd?1(H2O)] 6a
6b
6c
6d
6e
FW
860
7.1
10.0
94
3.1
68
1
3.00
2
912
5.8
1804 2028 3100 3548
13.9 15.0 19.6 19.7
r1P/mM21 21a
tv/psa
tr/ps
s
10.0 14.9 14.3 15.8 14.9
100
5.6
42
1
220
4.3
45
1
240
4.2
88
1
330
4.4
85
1
357
4.3
570
1
D2/1019 s22a
tm/nsa
q
r/A
˚
3.00 3.00 3.00 3.00 3.00
—
—
q9b
4
4
8
8
3.80 3.80 4.00 4.00
c
˚
r9/A
3.80
a
r1P is the paramagnetic relaxivity, tm the water exchange lifetime,
tv is the correlation time of the modulation of the transient zerofield
b
splitting, expressed by the square of its trace value (D2). Number of
second sphere water molecules. Mean Gd–H(water) distance of the
second sphere waters.
c
and characterised by negative ion ESMS (ESI{) or MALDI-
TOFMS (matrix: trihydroxyacetophenone/ammonium citrate).
The percentage Gd in sample solutions was verified using atomic
emission spectroscopy (ICP-OES), using analytically pure
H3O+[Gd?1(OH2)]2?2H2O as the standard.
We thank EPSRC (DF, KS), the EPSRC-National MS Service
for assistance and ESF-COST-Action-D-18 for support.
David A. Fulton,a Mark O’Halloran,a David Parker,*a
Kanthi Senanayake,a Mauro Bottab and Silvio Aimec
aDepartment of Chemistry, University of Durham, South Road, Durham,
UK DH1 3LE. E-mail: david.parker@durham.ac.uk
bDipartimento di Scienze dell’ Ambiente e delle Vita, Universita` del
Piemonte Orientale ‘Amedeo Avogadro’, Spalto Morengo 33, I-15100,
Alessandria, Italy
cDipartimento di Chimica IFM, Universita degli Studi di Torino, 7 via P.
Giuria, 10125, Torino, Italy
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Fig. 1 Variation with molecular weight of the inner sphere (upper) and
second sphere (lower) contributions to the overall relaxivity of gadolinium
complexes (298 K, 20 MHz).
MRI contrast agents based on
a stereoisomeric mixture of
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Chem. Commun., 2005, 474–476 | 475

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476 | Chem. Commun., 2005, 474–476
This journal is ß The Royal Society of Chemistry 2005