A Class of FeIII Macrocyclic Complexes with Alcohol Donor Groups as Effective T1 MRI Contrast Agents

Article abstract of DOI:10.1002/anie.201912273

Early studies suggested that Fe^III complexes cannot compete with Gd^III complexes as T₁ MRI contrast agents. Now it is shown that one member of a class of high-spin macrocyclic Fe^III complexes produces more intense contrast in mice kidneys and liver at 30 minutes post-injection than does a commercially used Gd^III agent and also produces similar T₁ relaxivity in serum phantoms at 4.7 T and 37 °C. Comparison of four different Fe^III macrocyclic complexes elucidates the factors that contribute to relaxivity in vivo including solution speciation. Variable-temperature ¹⁷O NMR studies suggest that none of the complexes has a single, integral inner-sphere water that exchanges rapidly on the NMR timescale. MRI studies in mice show large in vivo differences of three of the Fe^III complexes that correspond, in part, to their r₁ relaxivity in phantoms. Changes in overall charge of the complex modulate contrast enhancement, especially of the kidneys.

Full text of DOI:10.1002/anie.201912273

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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International Edition
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Accepted Article
Title: A new class of Fe(III) macrocyclic complexes with alcohol donor
groups as effective T1 MRI contrast agents
Authors: Eric M Snyder, Didar Asik, Samira M Abozeid, Ariel Burgio,
Gage Bateman, Steven G Turowski, Joseph Spernyak, and
Janet Ruth Morrow
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912273
Angew. Chem. 10.1002/ange.201912273
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10.1002/anie.201912273
Angewandte Chemie International Edition
RESEARCH ARTICLE
A new class of Fe(III) macrocyclic complexes with alcohol donor
groups as effective T1 MRI contrast agents.
Eric M Snyder,[a] Didar Asik,[a] Samira M Abozeid,[a] Ariel Burgio,[a] Gage Bateman,[a] Steven G.
Turowski,[b] Joseph A. Spernyak,[b] Janet R Morrow*[a]
Abstract: Early studies suggested that Fe(III) complexes cannot
compete with Gd(III) complexes as T1 MRI contrast agents. Here we
show that one member of a new class of high spin macrocyclic Fe(III)
complexes produces more intense contrast in mice kidneys and liver
at 30 minutes post injection than does a commercially used Gd(III)
agent and also produces similar T1 relaxivity in serum phantoms at
4.7 T and 37 oC. Comparison of four different Fe(III) macrocyclic
complexes elucidates the factors that contribute to relaxivity in vivo
including solution speciation. Variable temperature 17O NMR studies
suggest that none of the complexes has a single, integral inner-sphere
water that exchanges rapidly on the NMR time scale. MRI studies in
mice show large in vivo differences of three of the Fe(III) complexes
that correspond, in part, to their r1 relaxivity in phantoms. Changes in
overall charge of the complex modulate contrast enhancement,
especially of the kidneys.
closely match the properties of clinically used Gd(III) agents in
terms of biodistribution and pharmacokinetic clearance.
One of the barriers to the development of Fe(III)
coordination complexes as contrast agents is their complicated
aqueous solution chemistry. The complexes reported to date
feature mostly hexadentate chelates with diamino-backbones and
carboxylate and/or phenolate donors to give seven-coordinate
complexes.[7] There are also tris-complexes of bidentate
catechols that lack an innersphere water ligand as well as Fe(III)
complexes with an azamacrocycle and bridging oxo groups to
form a cluster.[8] Most of these reported Fe(III) complexes have
ill-defined solution chemistry and unknown redox potentials. More
recently, two studies of Fe(III) complexes of EDTA, DTPA or a
related hexadentate polyaminocarboxylate ligand have been
used for MRI of tumors in mice, albeit at three-fold higher
concentrations than used for Gd(DTPA).[9] With the exception of
these recent reports, the solution speciation and water ligand
exchange rate constants of Fe(III) contrast agents have not been
reported. Another property that has been overlooked involves the
need to tune the redox potential of the Fe(III)/Fe(II) center so that
Fe(III) is stabilized in the reducing biological environment.[10]
The small size of the Fe(III) ion and its correspondingly high
Lewis acidity make it difficult to develop complexes with rapidly
exchanging inner-sphere water ligands. However, these
disadvantages may be offset by the small size of the Fe(III) that
imparts strong second-sphere water interactions. The relaxation
of the protons of water by a paramagnetic complex is described
by the relaxivity, r1, which is obtained from a plot of the 1/T1 (rate
constant) versus concentration of the contrast agent. Importantly,
the r1 term has contributions from directly bound water (inner-
sphere), from water bound to ligands in the coordination sphere
(second-sphere) and from water diffusing in close proximity to the
paramagnetic center (outer-sphere). The outer-sphere
contribution in contrast agents can be quite substantial, although
the focus has been on inner-sphere contributions.[11]
Introduction
Early studies of T1 MRI contrast agents focused on
complexes of several metal ions, most prominently Gd(III), Mn(II),
and to a lesser extent, high spin Mn(III) and Fe(III).[1] Since these
early days, Gd(III) MRI contrast agents have seen remarkable
success with approximately 40% of all MRI scans incorporating
Gd(III) contrast agents.[2] However, in response to concerns
about patients who cannot tolerate Gd(III) contrast agents[3] as
well as more recent reports of Gd(III) retention in the body,[4] the
development of Mn(II) contrast agents has experienced a
renaissance aimed at producing alternatives to Gd(III).[5] Fe(III)
coordination complexes have been much less explored, despite
the important status of iron as a metal ion that the human body
stores and recycles. To address the need for alternatives, iron-
based nanoparticles such as superparamagnetic iron oxides
(SPIONS) are under continued investigation as T2 and T1
agents.[6] However, SPIONS demonstrate bio-distribution profiles
that are characteristic of nanoparticles, and are cleared slowly
through the liver. Fe(III) coordination complexes may more
푟 = 푟ꢁꢂ + 푟ꢂꢂ + 푟ꢃꢂ
Eq. 1
ꢀ
ꢀ
ꢀ
ꢀ
Other properties to consider in the design of Fe(III) T1 agents
include the relaxation time of bound water (T1M) as described in
Eq. 2.[12] T1m depends on the Bohr magneton (µ0), the
gyromagnetic ratio of proton (γH), the electron g factor (ge), the
spin quantum number (S), metal proton distance (rMH), the proton
Lamor frequency (ωH) and the correlation time (τc), as the time
constant for the fluctuating magnetic dipole that mediates proton
relaxation. The spin quantum number of S = 5/2 for high spin
Fe(III) is the same as that in high spin Mn(II) complexes, but lower
than that of Gd(III) at S = 7/2. However, the much shorter M-H
bond radius in Fe(III) complexes may partially compensate for
these other factors. The correlation time will be dominated by the
[a]
[b]
E. M. Snyder, D. Asik, S. M. Abozeid, A. Burgio, G. Bateman,
Prof. Dr. J. R. Morrow
Department of Chemistry, University at Buffalo, State University of
New York,
Amherst New York 14260, United States.
*E-mail: jmorrow@buffalo.edu
S. G. Turowski, Dr. J. A. Spernyak
Department of Cell Stress Biology, Roswell Park Comprehensive
Cancer Center,
Buffalo New York 14263, United States.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201912273
Angewandte Chemie International Edition
RESEARCH ARTICLE
fastest process (Eq. 3), whether it is electron spin relaxation (T1e),
the lifetime of bound water (τm), or the rotational motion of the
complex (τR). In this regard, the lifetime of a bound inner-sphere
water is generally longer than the rotational correlation time for a
low molecular weight complex, so that rotation is the limiting
factor.[2a] However, second-sphere water lifetimes can be very
short and thus limiting, making the creation of strong second-
sphere interactions an important goal. Electron spin relaxation of
Fe(III) is more rapid than Gd(III) or Mn(II) at low fields, but is less
of a limiting factor at high field strengths (>3T).[13]
ꢐ
ꢏ
ꢐ
ꢐ
(
)
ꢀ
ꢇ
ꢑ
µ
ꢕ
ꢂ ꢂꢔꢀ
ꢙꢚ
ꢛ
ꢐ
ꢉµꢊ ꢍ ꢎ
ꢘ
ꢀꢔꢜ ꢚ
ꢛ
ꢏ
ꢐꢝ
Eq. 2
ꢒ
ꢓ
=
ꢗ
ꢖꢏ
ꢄ
ꢀꢈ ꢋꢌ
ꢅꢆ
ꢀ
ꢀ
ꢀ
ꢀ
=
+
+
ꢚ
ꢆ
Eq. 3
ꢚ
ꢚ
ꢄ
ꢅꢒ
ꢛ
ꢞ
Here we present a new class of Fe(III) macrocyclic
complexes as T1 MRI contrast agents which are studied in
phantoms and in mice. The complexes have remarkable solution
chemistry, a stabilized Fe(III) oxidation state and inertness to
dissociation in acid or with anions present in the blood including
carbonate and phosphate. The T1 relaxivity at 4.7 T in serum
phantoms of one of the Fe(III) complexes is similar to that of
Gd(DTPA), (Magnevist®) a clinically used agent. This Fe(III)
agent shows kidney-specific enhancement in mice MRI studies,
producing an increase in T1 relaxation rates which is higher than
that of Gd(DTPA) at thirty minutes post injection. Slight changes
in functional groups to modulate lipophilicity modulate the
biodistribution and clearance of the agent as studied by dynamic
MRI. These studies constitute some of the first structure activity
studies on Fe(III) macrocyclic complexes towards their further
development as T1 MRI contrast agents.
Scheme 1. Four complexes studied here shown with predominant
protonation state at pH 7.4. Alternatively, Fe(L1) or Fe(L3) may have
deprotonated water instead of deprotonated alcohol pendents. Note that
Fe(L) designates all iron species in solution unless otherwise specified.
The Fe(III) complexes were best prepared by stirring solutions of
the Fe(II) bromide or chloride salts in ethanol with ligand in air.
Solids were characterized for iron content by ICP-MS and
analyzed as [Fe(L)X]X where L is the singly deprotonated ligand
and X is Cl- or Br-. Solution studies confirmed high spin Fe(III) as
shown by Evan’s method with ueff values ranging from 5.6-5.9
(TableS1).
The electronic spectra of the four Fe(III) complexes showed
an absorbance peak at 330 nm with extinction coefficients ranging
from 3100 M-1cm-1 to 4800 M-1cm-1 that was useful for monitoring
the dissociation of the complexes. Remarkably, in 100 mM HCl
the complexes remained largely intact over 4 hours, but had
dissociated by 20% to 40% over 72 hours (Table S2). In solutions
with 25 mM carbonate and 0.40 mM phosphate, none of the
complexes tested, Fe(L1), Fe(L2), Fe(L3), or Fe(L4) dissociated
to any measurable degree over a period of 72 hours at 37 °C (Fig.
S21-S28).
The pH-potentiometric titrations of the intact complexes
showed one or two ionizations over the pH range of 3 to 11 (Fig.
S30-S33). For Fe(L1), either an alcohol pendent and or a water
ligand may ionize at neutral pH. Analogously, Fe(L3) has a nearly
identical coordination sphere and we assign the ionization at near
neutral pH as either an alcohol pendent or water ligand, and the
ionization at acidic pH as the carboxylic acid. Fe(L2) appears to
Results and Discussion
The triazacyclononane (TACN) macrocycle was used as the
preferred ligand framework for binding the small Fe(III) ion.[14]
Pendent alcohol groups were chosen as potentially anionic
oxygen donors with chiral hydroxyl-propyl arms to rigidify the
complex. Two macrocyclic ligands with a non-coordinating
pendent (L1 and L3) were prepared to produce Fe(III) complexes
with a coordination site for an inner-sphere water ligand. The
triazole group was added as a versatile pendent to add
functionality to the complex (Scheme 1). Triazole pendents have
been shown to coordinate to trivalent metal ion centers,[15] and
were chosen here to produce a coordinatively saturated complex.
Ligands were synthesized in several steps from TACN by
using the orthoamide protecting group and monoalkylation with a
benzyl pendent. Benzyl TACN was directly alkylated with (S)-(-)-
propylene oxide to give L1. Alternatively, the benzyl group was
removed by using catalytic hydrogenation, and the triazole or
benzoate groups were added (Schemes S2, S3). This route
proved superior to adding the ancillary pendent directly to
protected TACN. The triazole pendents were clicked prior to
attachment to TACN to avoid complexation of Cu(II) (Scheme S1).
Table 1. Protonation constants from potentiometric titrations at 37°C, in
0.10 M NaCl (Eqs. S5, S6) and Log P octanol/water partition coefficients
calculated from Eq. S4
Compound
Fe(L1)
Log K1
Log K2
Log P
-0.13
-1.32
-1.53
-1.87
7.05 ± 0.06
6.75 ± 0.08
6.91 ± 0.05
7.44 ± 0.09
-
Fe(L2)
-
Fe(L3)
3.31 ± 0.01
4.01 ± 0.13
Fe(L4)
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10.1002/anie.201912273
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RESEARCH ARTICLE
Figure 2. A perspective ORTEP view of the molecular structure (50%
ellipsoid probability level) of [Fe(L2-H+)Cl]+, where hydrogen atoms are
omitted for clarity. The benzyl group shows rotational disorder.
Figure 1. Cyclic voltammogram of Fe(L2) at acidic and neutral pH at a
scan rate of 100 mV/s. Solutions contained 1.0 mM Fe(L2) and 100 mM
KCl as the supporting electrolyte.
coordinated to the Fe(III) and there is a coordinated chloride
completing the coordination sphere. The trigonal plane of the
three nitrogens and the trigonal plane formed by the two alcohols
and chloride produce a twist angle of 47°, consistent with a
distorted octahedral geometry. Previously reported Fe(III)
complexes of TACN with pendent hydroxyethyl alcohol groups
also feature six-coordinate Fe(III) with all alcohols bound and
partial deprotonation of the alcohol groups.[18] 17O NMR
spectroscopy and T1 relaxivity measurements were studied to
understand solution speciation and water interactions of the
complexes.
lack a water molecule at neutral pH as shown by relaxivity studies
below, which leads us to assign the ionization with a pKa of 6.8 to
an alcohol pendent. Fe(L4) shows values at 7.4 and at 4.0,
consistent with deprotonation of a pendent alcohol group at near-
neutral pH and the carboxylic acid at acidic pH. Based on this
data, the predominate species at pH 7.4 are shown in Scheme 1
and speciation diagrams are shown in Figs S30-33. Additional
experiments with Fe(L1) and Fe(L2) using UV-vis spectroscopy
showed evidence of the loss of halide from the inner-coordination
sphere upon addition of water (Fig. S29). Assignments of solution
species and ionizing groups will be discussed further below in
context with the 17O NMR and relaxivity experiments. The 1-
octanol/water partition coefficients (Log P) are shown in Table 1
for comparison of the lipophilicity of the complexes.
The pH dependent change in speciation was mirrored in
cyclic voltammetry experiments. At pH 3, all complexes exhibited
a reversible wave at approximately 300 mV versus NHE. At
neutral pH, a new wave at more negative potentials was produced.
For example, Fe(L2) produces a reversible wave at pH 7 at -274
mV versus NHE which is shifted by nearly 600 mV from acidic pH
(Fig. 1). The other three complexes, Fe(L1), Fe(L3), and Fe(L4)
Variable temperature 17O NMR spectroscopy experiments
were recorded on the Fe(III) complexes. Swift-Connick plots for
Fe(L1) Fe(L2), and Fe(L3) show a curved plot as typically
observed for a rapidly exchanging inner-sphere water whereas
Fe(L4) gives a linear plot (Fig. S36). However, a plot of the 17O
NMR peak width as a function of inverse temperature (Fig. 3A) in
comparison to Fe(CDTA), a well-studied complex with a q = 1,
suggests otherwise.[19] These data show that the Fe(III)
macrocyclic complexes produce much less broadening of the 17
O
resonance compared to that of Fe(CDTA). The peak width of the
17O resonance for Fe(L1) and Fe(L2) at pH 3 reaches about 21%
and 14%, respectively of the maximum peak width of Fe(CDTA).
At pH 6.5, the peak width of Fe(L1) is about 15% of Fe(CDTA)
whereas Fe(L2), Fe(L3), Fe(L4) show virtually no broadening of
the 17O resonance at this pH (Fig. S37). This data suggests that
the macrocyclic complexes do not have an inner-sphere water (q
= 1) that exchanges sufficiently rapidly on the NMR time scale to
substantially broaden and shift the 17O water resonance.[20] To
further study the q number, the shift of the 17O NMR resonance
as a function of Fe(III) complex concentration was plotted (Fig.
3B). In this study, Fe(CDTA) and Fe(DTPA) are compared to
Fe(L1) and Fe(L2). Fe(L1) and Fe(L2) have slopes much closer
to that of Fe(DTPA) which has no bound water than to Fe(CDTA).
The small shift and broadening of the 17O NMR resonance by the
macrocyclic Fe(III) complexes is attributed to second sphere
interactions, or alternatively, to a small percentage of a species
with a bound water.
showed
a similar shift in peak potential but had waves
characteristic of quasi-reversible processes (Fig. S34). Notably,
there was broadening of the oxidation peak in the voltammograms
of both complexes that contain a carboxylate group, Fe(L3) and
Fe(L4) at neutral pH, perhaps signifying the presence of an
additional species in solution. The shift towards more negative
potential reflects the stabilization of Fe(III) by the deprotonation of
the macrocyclic ligand, most likely an alcohol pendent. In blood,
the redox potential of the Fe(III)/Fe(II) center should be less than
-200 mV to avoid reduction to Fe(II) and the reactions that may
be promoted by Fe(II).[16] For example, when the complexes
reported here were incubated with ascorbate as an important
biological reductant and peroxide to assay for Fenton
chemistry,[17] there was no detectable hydroxylation of aromatics
in comparison to Fe(EDTA), which has a redox potential of 300
mV versus NHE (Fig. S35).
The r1 and r2 relaxivity of the four complexes in buffer, HSA
and serum on a 4.7 Tesla MRI scanner are given in Table 2. In
buffer, there is a five-fold difference in r1 between the best
(Fe(L1)) and worst complex (Fe(L4)). The r2 values vary by a
factor of nearly four. The complexes with triazole pendent
produce the lowest relaxivity. Both triazole complexes have r1
The crystal structure of [Fe(L3)(Cl)]Cl) showed a six-
coordinate Fe(III) center with one alcohol pendent deprotonated
and one protonated as shown by the difference in Fe-O bond
lengths of 0.25 Å. In this structure, the pendent triazole was not
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10.1002/anie.201912273
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RESEARCH ARTICLE
hydrogen-bonding network between alcohol pendents and bound
water that produces effective proton exchange.[11b] Clearly, this
network is missing for
Fe(L2) or Fe(L4).
It is curious that the relaxivity of the carboxylate containing
complexes, Fe(L3) and Fe(L4) are lowered compared to their
benzyl analogs. One explanation is that the carboxylate groups
modulate the electronics at the Fe(III) center, which seems
reasonable for Fe(L1) versus Fe(L3), but not for Fe(L2) versus
Fe(L4) which contains a more remote aryl group. In the presence
of human serum albumin (HSA), the r1 relaxivity of Fe(L1) and
Fe(L3) increase by about 10-30%. The triazole complexes Fe(L2)
and Fe(L4) increase r1 relaxivity by 40% and 100%, respectively,
in the presence of HSA. This data suggests that the iron
complexes bind to HSA, and that complexes containing a
carboxylate bind more strongly. Notably, in blood serum, Fe(L1)
has r1 values that are similar to that of Gd(DTPA).
Mice were injected via tail vein with the Fe(III) complexes at
the relatively low dose of 0.05 mmol/kg. Fe(L1), Fe(L3) and
Fe(L4) were tolerated well by the mice at this dose. However,
mice who received Fe(L2) at doses as low as 0.025 mmol/kg
showed signs of distress and this complex was not studied further.
Interestingly, the addition of the carboxylate group to Fe(L2) to
give Fe(L4) ameliorated adverse effects in mice, suggesting that
the charge of the complex has a role in toxicity.
Figure 3. Comparison of Fe(L1), Fe(L2), Fe(DTPA) and Fe(CDTA);
(A) 17O NMR resonance broadening at pH 6.5 as a function of temperature.
(B) 17O NMR resonance shift as a function of Fe(III) complex concentration.
The three Fe(III) complexes were studied in mice on a small
animal 4.7 T MRI scanner. Dynamic MRI was used to examine
biodistribution and clearance of the complexes in mice. Fig. 4
shows T1 weighted images of kidney and liver of mice at 45
minutes and 4 h post-administration of Fe(L1). Exceptional
enhancement of kidneys was observed at 30-45 minutes. At four
hours, enhancement of the gall bladder indicates a portion of the
agent is cleared through the hepatobiliary system.
The three complexes showed distinctly different dynamic
MRI profiles in mice. Consistent with its lower relaxivity in
phantoms, Fe(L4) produced barely detectable changes in T1
weighted contrast in blood, liver or kidneys, but was visualized in
the bladder after 7 minutes (Fig. S39). Fe(L3) showed slightly less
contrast enhancement in the vena cava blood vessel than Fe(L1),
and produced less kidney and liver contrast enhancement (Fig.
S38). This difference may be linked to the higher lipophilicity of
Fe(L1) compared to Fe(L3) that may favor better retention in
relaxivities that are similar to Fe(III) complexes that have no
bound water including Fe(DTPA) at 0.57 mM-1s-1 at 4.7 T.[21] It is
interesting that Fe(L1) and Fe(L3) have r1 relaxivities that are
several times higher than those of Fe(L2) or Fe(L4). In fact,
Fe(L1) and Fe(L3) both have higher relaxivity than Fe(III)
complexes with a single innersphere water molecule, such as
Fe(EDTA) (1.34 mM-1s-1 at 4.7 T).[21] As discussed above, Fe(L1)
and Fe(L3) have one deprotonated alcohol ligand or hydroxide at
neutral pH. Given the higher r1 proton relaxivity and apparent lack
of exchangeable inner-sphere water, a hydroxide ligand may be
responsible for strong second sphere water interactions, similar
to a reported Fe(III) porphyrin with fluoride and no bound water
ligands.[22] Alternatively, the coordination sphere may have a
Table 2. Relaxivity values for Fe(L1), Fe(L2), Fe(L3), and Fe(L4) measured at 4.7T, pH 7.2 and 37°C compared to Gd(DTPA).
Complex
r1 (mM-1sec-1)
r2 (mM-1sec-1)
r1 (mM-1sec-1) in
HSA
r2 (mM-1sec-1) in
HSA
r1 (mM-1sec-1) in
serum
r2 (mM-1sec-1) in
serum
Fe(L1)
Fe(L2)
2.2 ± 0.3
0.81 ± 0.07
1.7 ± 0.1
4.5 ± 1.1
1.5 ± 0.1
5.0 ± 0.5
1.5 ± 0.05
3.9 ± 0.39
2.5 ± 0.1
1.1 ± 0.02
2.2 ± 0.01
0.96 ± 0.02
3.2 ± 0.32
4.2 ± 0.7
1.6 ± 0.04
3.6 ± 0.2
2.0 ± 0.2
4.0 ± 0.40
3.8 ± 0.38
6.2 ± 0.62
-
-
Fe(L3)
1.8 ± 0.18
-
3.0 ± 0.30
-
Fe(L4)
0.42 ± 0.2
3.1 ± 0.31
Gd(DTPA)
4.1 ± 0.41
5.0 ± 0.50
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10.1002/anie.201912273
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RESEARCH ARTICLE
Figure 5. Change in ∆R1 for Fe(L1) at 0.05 mmol/kg in kidneys, liver and
blood of Balb/C mice (n =3) over time.
TACN complexes of Fe(III) are low spin.[25] The highly polarizing
Fe(III) ion does not produce rapidly exchanging water ligands in
the six-coordinate complexes presented here. Instead, strong
second-sphere interactions are likely, especially through groups
that have hydrogen-bonding capabilities such as alcohols,
alkoxides or hydroxide ligands. These trivalent Fe(III) macrocyclic
complexes have the advantage of being more kinetically inert in
comparison to divalent metal ions.[26]
The two Fe(III) complexes Fe(L1) and Fe(L3) that lack a
rapidly exchanging inner-sphere water still have r1 relaxivity
values that are higher than complexes that do have an
exchangeable water, such as Fe(EDTA)(OH2) under similar
conditions.[21] We propose that strong second-sphere water
interactions are formed that may be mediated by alcohol or
hydroxide ligands or by a network involving both. Studies are
underway to better understand structure activity relationships for
modulation of biodistribution and clearance and to utilize the
specific kidney contrast enhancement that is a remarkable
property of Fe(L1).
Figure 4. T1-weighted MRI of a healthy Balb/C mouse at 4.7 T at a dose
of 0.05 mmol/kg Fe(L1). Top row: before and 45 min post-injection images
(a
& b) showing strong enhancement of kidneys (arrow). Kidney
enhancement was observed at 4 h post-injection (c). Bottom: liver and gall
bladder images before (d) and 45 min after injection (e). Enhancement of
liver (L) was observed at early timepoints and substantial enhancement of
gall bladder (arrow head) was observed at 4 h post-injection (f).
blood.[23] The log P value for Fe(L1) is close to zero (-0.13),
signifying a degree of lipophilicity not observed for the other
complexes. The log P octanol constants for Fe(L3) or Fe(L4) are
very negative (Table 1), as is Gd(DTPA) (-3.16).[2a]
Notably, the change in the 1/T1 (∆R1) versus control for
Fe(L1) was 6-fold higher in the kidney than for Gd(DTPA) at 30
minutes (Fig. S40). The basis for the large T1 contrast
enhancement in kidneys is not clear. Specific kidney contrast
enhancement by Fe(L1) may be linked to organic cation
transporters which have been implicated in kidney secretion of
lipophilic cationic drugs.[24] This is supported by the fact that the
enhanced kidney contrast for the neutral Fe(L3) complex is not as
pronounced as that of Fe(L1). The large difference in the
lipophilicity of the two complexes may also have a role in the
different pharmacokinetic profiles of the two complexes. As
shown, Fe(L1) was cleared from the kidneys and liver at 24 hours
(Fig. 5).
Conflict of interest
JRM is a co-founder of Ferric Contrast, Inc.
Acknowledgements
JRM acknowledges the NSF (CHE-1710224) and JAS and JRM
acknowledge the NIH (1R03EB02536901) for support of this work.
The authors would like to thank the Chemistry Instrument Center
(CIC), University at Buffalo. This work utilized ICP-MS and FTMS
that was purchased with funding from a NSF Major Research
Instrumentation Program (NSF CHE-0959565) and National
Institutes of Health (S10 RR029517).
Conclusion
The development of Fe(III) complexes as T1 MRI contrast
agents presents unique opportunities and challenges. For
example, it is challenging to produce an iron coordination complex
with a Fe(III)/Fe(II) redox potential of less than -200 mV that also
has a water ligand. Spin state is a consideration as well, as some
Keywords: imaging agent • iron • macrocyclic ligand • NMR
spectroscopy
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10.1002/anie.201912273
Angewandte Chemie International Edition
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10.1002/anie.201912273
Angewandte Chemie International Edition
RESEARCH ARTICLE
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High spin iron(III)
complexes with an open
coordination site for solvent
interactions are effective T1
MRI contrast agents in
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phantoms and in live mice. A
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coordinated water ligand with
the deprotonation of an
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alcohol pendent stabilize the
trivalent iron center that
produces strong second-
sphere water interactions.
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