Accelerating water exchange for GdIII chelates by steric compression
around the water binding site†
Robert Ruloff,a Éva Tóth,a Rosario Scopelliti,a Raphaël Tripier,b Henri Handelb and André E.
Merbach*a
a Ecole Polytechnique Fédérale de Lausanne, ICMB, BCH, Lausanne, Switzerland.
E-mail: andre.merbach@epfl.ch; Fax: 41 21 693 9875; Tel: 41 21 693 9871
b Université de Bretagne Occidentale, 6 av. Le Gorgeu, BP 809, 29285, Brest, France.
E-mail: Henri.Handel@univ-brest.fr; Fax: 33 298 01 65 94; Tel: 33 298 01 79 27
Received (in Cambridge, UK) 7th August 2002, Accepted 4th October 2002
First published as an Advance Article on the web 23rd October 2002
The water exchange process was accelerated for nine-
coordinate, monohydrated macrocyclic GdIII complexes by
inducing steric compression around the water binding site;
the increased steric crowding was achieved by replacing an
ethylene bridge of DOTA42 by a propylene bridge;‡ in
addition to the optimal water exchange rate, the stability of
[Gd(TRITA)(H2O)]2 is sufficiently high to ensure safe
medical use which makes it a potential synthon for the
development of high relaxivity, macromolecular MRI con-
trast agents.
Among macrocyclic poly(amino carboxylates), the 12-mem-
bered DOTA42 is known to form a monohydrated GdIII chelate
which has the highest thermodynamic and kinetic stability of all
MRI contrast agents.§ The 14-membered TETA42 forms a less
stable GdIII complex which, due to steric constraints induced by
the larger cycle, has no inner sphere water molecule. Conse-
quently, the intermediate 13-membered macrocycle, TRITA42
,
is likely to form a monohydrated GdIII complex with suffi-
ciently high steric crowding to have fast water exchange.
Importantly, the thermodynamic stability of Gd(TRITA)2 is
high enough for medical use (log b = 19.2).2 It is more
apprioriate to compare pGd values: the slightly lower pGd for
Gd(TRITA)2 shows that the stability is not much decreased
Some currently emerging applications in magnetic resonance
imaging require contrast agents of particularly high efficiency.
For instance, in molecular imaging the amount of contrast agent
delivered to a target site can be very much limited by biological
constraints (low receptor concentration in receptor targeting,
etc.), therefore only high relaxivity agents can allow visual-
izeation the specific site.§ The Solomon–Bloembergen–Mor-
gan theory, which relates the observed paramagnetic relaxation
rate enhancement to microscopic properties, predicts maximum
proton relaxivities for GdIII complexes ( > 100 cf. 4–5 mM21
s21 of commercial agents) when the three most important
influencing factors, rotation, electron paramagnetic relaxation
and water exchange are simultaneously optimised.1 Namely, the
rotation has to be slow enough, which can be achieved by using
macromolecules. The electron spin relaxation, despite the
recent theoretical developments, is difficult to modify on a
rational basis. The tuning of the water exchange rate to the
compared to the commercial agents: pGd
=
14.6
(Gd(TRITA)2); 15.8 (Gd(DTPA-BMA); 19.1 (Gd(DTPA)22
)
or 19.2 (Gd(DOTA)2) at pH 7.4; cGd = 1 mM; clig = 10 mM.
The kinetic inertness, as important for non-toxicity as thermo-
dynamic stability, is also expected to be high for this
macrocyclic chelate, though it is likely lower than for
[Gd(DOTA)(H2O)]2.
We synthesized TRITA via the bis-aminal methodology,3
and TRITA-bz-NO2 by modifying the synthesis described by
Maecke and coworkers.4 The bifunctional derivative is intended
to be covalently linked to macromolecules to optimise rotation
(Scheme 1).
Longitudinal and transverse 17O relaxation rates and chem-
ical shifts were measured as a function of the temperature on
aqueous solutions of [Gd(TRITA)(H2O)]2 and [Gd(TRITA-bz-
NO2)(H2O)]2¶ and on a diamagnetic reference solution
(HClO4, pH 4) at B = 9.4 T. The experimental data were
analysed with the Swift–Connick equations to yield parameters
describing water exchange and rotation.1 In the whole tem-
perature range, the transverse relaxation rates, 1/T2r, decrease
with increasing temperature, indicating that the system is in the
fast exchange regime. Under such conditions, the transverse
relaxation rate is determined by both the water exchange rate
and the longitudinal electron spin relaxation. Hence, informa-
tion on electronic relaxation is indispensable to calculate the
water exchange rate. We have performed variable temperature,
multiple field (0.34, 2.7, 5.4 and 8.1 T) EPR measurements on
[Gd(TRITA)(H2O)]2. The EPR linewidths at all magnetic
fields are very similar to those for [Gd(DOTA)(H2O)]2. For this
latter, a complete analysis of multiple field EPR data has already
been performed with the recently developed Rast–Borel
theory.5 Therefore as a quick but reliable estimation of 1/T1e for
[Gd(TRITA)(H2O)]2 and [Gd(TRITA-bz-NO2)(H2O)]2, we
used the value calculated for [Gd(DOTA)(H2O)]2 at the NMR
field B = 9.4 T (1/T1 = 8 3 107 s21). (A rigorous analysis of
the EPR spectra by using the recently developed theory has been
undertaken and will be reported in due course). The parameters
obtained from 17O NMR are given in Table 1 and the fitted data
shown in the ESI†).
optimal value of around kex
=
108 s21 has also been
problematic. While it is relatively easy to slow down the water
exchange process as compared to the currently used GdIII-based
contrast agents (kex ≈ 106 s21), it is much more difficult to
accelerate, especially if high thermodynamic and kinetic
stability has to be retained to ensure non-toxicity. Here we
report a GdIII chelate which is the first to be selected on a
rational basis to present optimal water exchange rate. In
addition, this complex is sufficiently stable to be applied as a
diagnostic agent.
Nine-coordinate GdIII poly(amino carboxylates), including
all commercial GdIII-based MRI contrast agents, undergo a
dissociative, D, or dissociative interchange, Id, water exchange,
in contrast to the mechanism, A, on [Gd(H2O)8]3+.1 The rate of
such dissociative exchange processes is primarily determined
by the overall charge of the chelate (more negative charge leads
to faster exchange) and by the steric crowding around the bound
water site. An increased steric compression around the inner
sphere water molecule will facilitate its leaving which, in a
dissociative process, constitutes the rate determining step.
Therefore, our objective was to induce steric crowding around
the water binding site in GdIII poly(amino carboxylates) in order
to increase the exchange rate without loosing complex stability.
The value of the scalar coupling constant, A/H, is typical of
GdIII chelates and proves unambigously the presence of one
inner sphere water molecule. The water exchange rate, kex298, is
† Electronic supplementary information (ESI) available: synthesis of
TRITA and TRITA-bz-NO2-17O relaxation rates and chemical shifts for
GdIII chelates. See http://www.rsc.org/suppdata/cc/b2/b207713b/
2630
CHEM. COMMUN., 2002, 2630–2631
This journal is © The Royal Society of Chemistry 2002