Fe-HBED Analogs: A Promising Class of Iron-Chelate Contrast Agents for Magnetic Resonance Imaging

Rigid Fe(III) Chelate with Phosphonate Pendants: A Stable and Effective Extracellular MRI Contrast Agent

A novel Fe(III) complex, Fe-tBPCDTA, was synthesized and explored as a potential contrast agent for MRI. Compared to established agents like Fe-EDTA and Fe-tCDTA, Fe-tBPCDTA exhibited moderate relaxivity (r1 = 1.17 s-1·mmol-1) due to its enhanced second-sphere mechanism. It also displayed improved kinetic inertness, lower cytotoxicity, and enhanced redox stability. In vivo studies demonstrated its function as an extracellular fluid agent, providing tumor contrast comparable to that of Gd-DTPA at a higher dosage. Complete renal clearance occurred within 24 h. These findings suggest Fe-tBPCDTA as a promising candidate for further development as a safe and effective extracellular MRI contrast agent.

Naturally biocompatible melanin based iron-complex nanoparticles for pH-responsive magnetic resonance imaging

The sensitivity and diagnostic accuracy of magnetic resonance imaging mainly depend on the relaxation capacity of contrast agents (CAs) and their accumulated amount at the pathological region. Due to the better biocompatibility and high-spin capacity, Fe-complexes have been studied widely as an alternative to replace popular Gd-based CAs associated with potential biotoxicity. Compared with a variety of Fe complex-based CAs, such as small molecular, macrocyclic, multinuclear complexes, the form of nanoparticle exhibits outstanding longitudinal relaxation, but the clinical transformation was still limited by the inconspicuous difference of contrast between tumor and normal tissue. The enhanced effect of contrast is a positive relation as relaxation of CAs and their concentration in desired region. To specifically improve the amount of CAs accumulated in the tumor, pH-responsive polymer poly(2-ethyl-2-oxazoline) (PEOz) was modified on melanin, a ubiquitous natural pigment providing much active sites for chelating with Fe(III). The Fe(III)-Mel-PEOz we prepared could raise the tumor cell endocytosis efficiency via switching surface charge from anion to cation with the stimuli of the decreasing pH of tumor microenvironment. The change of pH has negligible effect on ther1of Fe(III)-Mel-PEOz, which is always maintained at around 1.0 mM-1s-1at 0.5 T. Moreover, Fe(III)-Mel-PEOz exhibited low cytotoxicity, and satisfactory enhancement of positive contrast effectin vivo. The excellent biocompatibility and stable relaxation demonstrate the high potential of Fe(III)-Mel-PEOz in the diagnosis of tumor.

Novel Biodegradable Chelating Agents for Micronutrient Fertilization

Serious concerns about the negative impact of ethylenediaminetetraacetic acid (EDTA) on the environment resulted in severe restrictions imposed on this compound in many countries. One of the main concerns is related to the use of EDTA in agriculture as a chelator in microelement fertilizers: being introduced directly into the sawing fields, it penetrates into groundwater, with no chance to be captured/recycled. Respectively, there is an active search for environmentally friendly, biodegradable alternatives for this chelator. In this study, we proposed a biodegradable chelating agent, 2-((1,2-dicarboxyethyl)amino)pentanedioic acid (IGSA). It was synthesized in accordance with the principles of "green chemistry" from readily available nonhazardous precursors using water as a solvent; in addition, the method yields literally no waste. The synthesized chelator in the form of the crude reaction mixture was further used for preparing a multicomponent micronutrient fertilizer (B, Zn, Fe, Cu, Mn, and Mo). The fertilizer was shown to be highly biodegradable (72% in 28 days), while the EDTA-based product degraded only by 13%. The plant growing efficiency was tested on lettuce in the greenhouse experiments. The results were compared against the known commercial fertilizers based on EDTA and iminodisuccinic acid (IDS). The newly developed IGSA-based fertilizer significantly outperformed the EDTA-based fertilizer in lettuce biomass (1.4 and 1.6 times for root and foliar application, respectively). The total mineral uptake was almost two times higher (1.9 and 1.8 times for root and foliar treatments, respectively) compared to the EDTA-based complex and even slightly higher (1.2 and 1.1 times, respectively) compared to the IDS-based complex. Our work opens the doors for the industrial scale production and application of this fully "green", inexpensive microelement fertilizer that has the potential to replace the EDTA-based products.

Fe(III) T1 MRI Probes Containing Phenolate or Hydroxypyridine-Appended Triamine Chelates and a Coordination Site for Bound Water

Fe(III) complexes containing a triamine framework and phenolate or hydroxypyridine donors are characterized and studied as T1 MRI probes. In contrast to most Fe(III) MRI probes of linear chelates reported to date, the ligands reported here are pentadentate to give six-coordinate complexes with a coordination site for inner-sphere water. The crystal structure of the complex containing unsubstituted phenolate donors, Fe(L1)Cl, shows a six-coordinate iron center and contains a chloride ligand that is displaced in water. Two additional derivatives are sufficiently water-soluble for study as MRI probes, including a complex with a hydroxypyridine group, Fe(L2), and a hydroxybenzoic acid group, Fe(L3). The pH potentiometric titrations give protonation constants of 7.2 and 7.5 for Fe(L2) and Fe(L3), respectively, which are assigned to deprotonation of the bound water. Changes in the electronic absorbance spectra of the complexes as a function of pH are consistent with the deprotonation of phenol pendants at acidic pH values. However, the inner-sphere water ligand of Fe(L2) and Fe(L3) does not exchange rapidly on the NMR timescale at pH 6.0 or 7.4, as shown by variable-temperature 17O NMR spectroscopy. The pH-dependent proton relaxivity profiles show a maximum in relaxivity at a near-neutral pH, suggesting that exchange of the protons of the bound water is an important contribution. Competitive binding studies with ethylenediaminetetraacetic acid (EDTA) show effective stability constants for Fe(L2) and Fe(L3) at pH 7.4 with log K values of 21.1 and 20.5, respectively. These two complexes are kinetically inert in carbonate phosphate buffer at 37 °C for several hours but transfer iron to transferrin. Fe(L2) and Fe(L3) show enhanced contrast in T1-weighted imaging analyses in BALB/c mice. These studies show that Fe(L2) clears through mixed renal and hepatobiliary routes, while Fe(L3) has a similar pharmacokinetic clearance profile to a macrocyclic Gd(III) contrast agent.

Characterization of the Fe(III)-Tiron System in Solution through an Integrated Approach Combining NMR Relaxometric, Thermodynamic, Kinetic, and Computational Data

The Fe(III)-Tiron system (Tiron = 4,5-dihydroxy-1,3-benzenedisulfonate) was investigated using a combination of ¹H and ¹⁷O NMR relaxometric studies at variable field and temperature and theoretical calculations at the DFT and NEVPT2 levels. These studies require a detailed knowledge of the speciation in aqueous solution at different pH values. This was achieved using potentiometric and spectrophotometric titrations, which afforded the thermodynamic equilibrium constants characterizing the Fe(III)-Tiron system. A careful control of the pH of the solution and the metal-to-ligand stoichiometric ratio allowed the relaxometric characterization of [Fe(Tiron)₃]^9-, [Fe(Tiron)₂(H₂O)₂]^5-, and [Fe(Tiron)(H₂O)₄]^- complexes. The ¹H nuclear magnetic relaxation dispersion (NMRD) profiles of [Fe(Tiron)₃]^9- and [Fe(Tiron)₂(H₂O)₂]^5- complexes evidence a significant second-sphere contribution to relaxivity. A complementary ¹⁷O NMR study provided access to the exchange rates of the coordinated water molecules in [Fe(Tiron)₂(H₂O)₂]^5- and [Fe(Tiron)(H₂O)₄]^- complexes. Analyses of the NMRD profiles and NEVPT2 calculations indicate that electronic relaxation is significantly affected by the geometry of the Fe³⁺ coordination environment. Dissociation kinetic studies indicated that the [Fe(Tiron)₃]^9- complex is relatively inert due to the slow release of one of the Tiron ligands, while the [Fe(Tiron)₂(H₂O)₂]^5- complex is considerably more labile.

Low-Molecular-Weight Fe(III) Complexes for MRI Contrast Agents

Fe(III) complexes have again attracted much attention for application as MRI contrast agents in recent years due to their high thermodynamic stability, low long-term toxicity, and large relaxivity at a higher magnetic field. This mini-review covers the recent progress on low-molecular-weight Fe(III) complexes, which have been considered as one of the promising alternatives to clinically used Gd(III)-based contrast agents. Two kinds of complexes including mononuclear Fe(III) complexes and multinuclear Fe(III) complexes are summarized in sequence, with a specific highlight of the structural relationships between the complexes and their relaxivity and thermodynamic stability. In additional, the future perspectives for the design of low-molecular-weight Fe(III) complexes for MRI contrast agents are suggested.

Distinct Coordination Chemistry of Fe(III)-Based MRI Probes

ConspectusContrast agents are used in approximately 40% of all magnetic resonance imaging (MRI) procedures to improve the quality of the images based on the distribution and dynamic clearance of the agent. To date, all clinically approved contrast agents are Gd(III) coordination complexes that serve to shorten the longitudinal (T₁) and transverse (T₂) proton relaxation times of water. Recent interest in replacing Gd with biologically relevant metal ions such as Mn or Fe has led to increased interest in the aqueous coordination chemistry of their complexes. In this Account, we focus on high-spin Fe(III) complexes that have been recently reported as MRI contrast agents or probes in our laboratory.The highly Lewis acidic Fe(III) center has distinct coordination chemistry in aqueous solutions, facilitating alternative strategies in the design of MRI probes. To illustrate this, we describe different classes of Fe(III) MRI probes with a focus on macrocyclic complexes and multinuclear complexes such as self-assembled metal organic polyhedra (MOP). Our initial efforts focused on macrocyclic complexes of Fe(III) in order to tune spin and oxidation states with the goal of stabilizing high-spin Fe(III) in reducing biological environments. Our probes feature six-coordinate Fe(III) complexes of 1,4,7-triazacyclononane with hydroxypropyl, phosphonate, or carboxylate pendant groups to produce Fe(III) complexes that shorten proton T₁ times predominantly from second-sphere or outer-sphere interactions at neutral pH. Analogues with pentadentate macrocyclic ligands have an inner-sphere water that does not exchange rapidly on the NMR time scale, yet these complexes are effective relaxation agents. Fe(III) macrocyclic complexes in this class can be modified to modulate their biodistribution and pharmacokinetic clearance in mice. The goal of these studies is for the Fe(III) agents to clear as extracellular fluid agents and produce profiles similar to those of Gd agents. Finally, studies of multimeric Fe(III) complexes are of interest to produce probes that give large proton relaxivity. In this approach the two Fe(III) centers are connected through aryl linkers as demonstrated for several macrocyclic complexes. Even more tightly connected Fe(III) centers are produced in a Fe(III) self-assembled cage with relaxivity of 21 mM^-1 s^-1 at 4.7 T, 37 °C in the presence of serum albumin to which it is tightly bound. This cage enhances contrast of the vasculature as a blood pool agent and accumulates in tumors. Finally, we present our perspectives on the further development of Fe(III) complexes for various applications in MRI.

Metal-Organic Polyhedron with Four Fe(III) Centers Producing Enhanced T1 Magnetic Resonance Imaging Contrast in Tumors

A metal-organic polyhedron (MOP) with four paramagnetic Fe(III) centers was studied as a magnetic resonance imaging (MRI) probe. The MOP was characterized in solution by using electron paramagnetic resonance (EPR), UV-visible (UV-vis) spectroscopies, Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry, and in the solid state with single-crystal X-ray diffraction. Water proton T1 relaxation properties were examined in solution and showed significant enhancement in the presence of human serum albumin (HSA). The r1 relaxivities in the absence and presence of HSA were 8.7 mM-1 s-1 and 21 mM-1 s-1, respectively, per molecule (2.2 mM-1 s-1 and 5.3 mM-1 s-1 per Fe) at 4.7 T, 37 °C. In vivo studies of the iron MOP show strong contrast enhancement of the blood pool even at a low dose of 0.025 mmol/kg with prolonged residence in vasculature and clearance through the intestinal tract of mice. The MOP binds strongly to serum albumin and shows comparable accumulation in a murine tumor model as compared to a covalently linked Gd-HSA contrast agent.

Enzyme Control Over Ferric Iron Magnetostructural Properties

Fe³⁺ complexes in aqueous solution can exist as discrete mononuclear species or multinuclear magnetically coupled species. Stimuli-driven change to Fe³⁺ speciation represents a powerful mechanistic basis for magnetic resonance sensor technology, but ligand design strategies to exert precision control of aqueous Fe³⁺ magnetostructural properties are entirely underexplored. In pursuit of this objective, we rationally designed a ligand to strongly favor a dinuclear μ-oxo-bridged and antiferromagnetically coupled complex, but which undergoes carboxylesterase mediated transformation to a mononuclear high-spin Fe³⁺ chelate resulting in substantial T₁ -relaxivity increase. The data communicated demonstrate proof of concept for a novel and effective strategy to exert biochemical control over aqueous Fe³⁺ magnetic, structural, and relaxometric properties.

Bioimaging agents based on redox-active transition metal complexes

Detecting the fluctuation and distribution of various bioactive species in biological systems is of great importance in determining diseases at their early stages. Metal complex-based probes have attracted considerable attention in bioimaging applications owing to their unique advantages, such as high luminescence, good photostability, large Stokes shifts, low toxicity, and good biocompatibility. In this review, we summarized the development of redox-active transition metal complex-based probes in recent five years with the metal ions of iron, manganese, and copper, which play essential roles in life and can avoid the introduction of exogenous metals into biological systems. The designing principles that afford these complexes with optical or magnetic resonance (MR) imaging properties are elucidated. The applications of the complexes for bioimaging applications of different bioactive species are demonstrated. The current challenges and potential future directions of these probes for applications in biological systems are also discussed.

This review summarizes transition metal complexes as bioimaging agents in optical and magnetic resonance imaging.

Liposomal Fe(III) Macrocyclic Complexes with Hydroxypropyl Pendants as MRI Probes

Paramagnetic liposomes containing Fe(III) complexes were prepared by incorporation of mononuclear (Fe(L1) or Fe(L3)) or dinuclear (Fe₂(L2)) coordination complexes of 1,4,7-triazacyclononane macrocycles containing 2-hydroxypropyl pendant groups. Two different types of paramagnetic liposomes were prepared. The first type, LipoA, has the mononuclear Fe(L1) complex loaded into the internal aqueous core. The second type, LipoB, has the amphiphilic Fe(L3) complex inserted into the liposomal bilayer and the internal aqueous core loaded with either Fe(L1) (LipoB1) or Fe₂(L2) (LipoB2). LipoA enhances both T₁ and T₂ water proton relaxation rates. Treatment of LipoA with osmotic gradients to produce a nonspherical liposome produces a liposome with a chemical exchange saturation transfer effect as shown by an asymmetry analysis but only at high osmolarity. LipoB1, which contains an amphiphilic complex in the liposomal bilayer, produced a broadened Z-spectrum upon treatment of the liposome with osmotic gradients. The r₁ relaxivity of LipoB1 and LipoB2 were higher than the r₁ relaxivity of LipoA on a per Fe basis, suggesting an important contribution from the amphiphilic Fe(III) center. The r₁ relaxivities of paramagnetic liposomes are relatively constant over a range of magnetic field strengths (1.4-9.4 T), with the ratio of r₂/r₁ substantially increasing at high field strengths. MRI studies of LipoB1 in mice showed prolonged contrast enhancement in blood compared to the clinically employed Gd(DOTA), which was injected at a 2-fold higher dose per metal than the Fe(III)-loaded liposomes.

Visualizing stem cells in vivo using magnetic resonance imaging

Stem cell (SC) therapies displayed encouraging efficacy and clinical outcome in various disorders. Despite this huge hype, clinical translation of SC therapy has been disheartening due to contradictory results from clinical trials. The ability to monitor migration and engraftment of cells in vivo represents an ideal strategy in cell therapy. Therefore, suitable imaging approach to track MSCs would allow understanding of migratory and homing efficiency, optimal route of delivery and engraftment of cells at targeted location. Hence, longitudinal tracking of SCs is crucial for the optimization of treatment parameters, leading to improved clinical outcome and translation. Magnetic resonance imaging (MRI) represents a suitable imaging modality to observe cells non-invasively and repeatedly. Tracking is achieved when cells are incubated prior to implantation with appropriate contrast agents (CA) or tracers which can then be detected in an MRI scan. This review explores and emphasizes the importance of monitoring the distribution and fate of SCs post-implantation using current contrast agents, such as positive CAs including paramagnetic metals (gadolinium), negative contrast agents such as superparamagnetic iron oxides and ¹⁹ F containing tracers, specifically for the in vivo tracking of MSCs using MRI. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Fe(deferasirox)2: An Iron(III)-Based Magnetic Resonance Imaging T1 Contrast Agent Endowed with Remarkable Molecular and Functional Characteristics

The search for alternatives to Gd-containing magnetic resonance imaging (MRI) contrast agents addresses the field of Fe(III)-bearing species with the expectation that the use of an essential metal ion may avoid the issues raised by the exogenous Gd. Attention is currently devoted to highly stable Fe(III) complexes with hexacoordinating ligands, although they may lack any coordinated water molecule. We found that the hexacoordinated Fe(III) complex with two units of deferasirox, a largely used iron sequestering agent, owns properties that can make it a viable alternative to Gd-based agents. Fe(deferasirox)2 displays an outstanding thermodynamic stability, a high binding affinity to human serum albumin (three molecules of complex are simultaneously bound to the protein), and a good relaxivity that increases in the range 20-80 MHz. The relaxation enhancement is due to second sphere water molecules likely forming H-bonds with the coordinating phenoxide oxygens. A further enhancement was observed upon the formation of the supramolecular adduct with albumin. The binding sites of Fe(deferasirox)2 on albumin were characterized by relaxometric competitive assays. Preliminary in vivo imaging studies on a tumor-bearing mouse model indicate that, on a 3 T MRI scanner, the contrast ability of Fe(deferasirox)2 is comparable to the one shown by the commercial Gd(DTPA) agent. ICP-MS analyses on blood samples withdrawn from healthy mice administered with a dose of 0.1 mmol/kg of Fe(deferasirox)2 showed that the complex is completely removed in 24 h.

Dinuclear Fe(III) Hydroxypropyl-Appended Macrocyclic Complexes as MRI Probes

Four high-spin Fe(III) macrocyclic complexes, including three dinuclear and one mononuclear complex, were prepared toward the development of more effective iron-based magnetic resonance imaging (MRI) contrast agents. All four complexes contain a 1,4,7-triazacyclononane macrocyclic backbone with two hydroxypropyl pendant groups, an ancillary aryl or biphenyl group, and a coordination site for a water ligand. The pH potentiometric titrations support one or two deprotonations of the complexes, most likely deprotonation of hydroxypropyl groups at near-neutral pH. Variable-temperature ¹⁷O NMR studies suggest that the inner-sphere water ligand is slow to exchange with bulk water on the NMR time scale. Water proton T₁ relaxation times measured for solutions of the Fe(III) complexes at pH 7.2 showed that the dinuclear complexes have a 2- to 3-fold increase in r₁ relaxivity in comparison to the mononuclear complex per molecule at field strengths ranging from 1.4 T to 9.4 T. The most effective agent, a dinuclear complex with macrocycles linked through para-substitution of an aryl group (Fe₂(PARA)), has an r₁ of 6.7 mM^-1 s^-1 at 37 °C and 4.7 T or 3.3 mM^-1 s^-1 per iron center in the presence of serum albumin and shows enhanced blood pool and kidney contrast in mice MRI studies.

Iron (III)-Quercetin Complex: Synthesis, Physicochemical Characterization, and MRI Cell Tracking toward Potential Applications in Regenerative Medicine

In cell therapy, contrast agents T1 and T2 are both needed for the labeling and tracking of transplanted stem cells over extended periods of time through magnetic resonance imaging (MRI). Importantly, the metal-quercetin complex via coordination chemistry has been studied extensively for biomedical applications, such as anticancer therapies and imaging probes. Herein, we report on the synthesis, characterization, and labeling of the iron (III)-quercetin complex, "IronQ," in circulating proangiogenic cells (CACs) and also explore tracking via the use of a clinical 1.5 Tesla (T) MRI scanner. Moreover, IronQ had a paramagnetic T1 positive contrast agent property with a saturation magnetization of 0.155 emu/g at 1.0 T and longitudinal relaxivity (r1) values of 2.29 and 3.70 mM-1s-1 at 1.5 T for water and human plasma, respectively. Surprisingly, IronQ was able to promote CAC growth in conventional cell culture systems without the addition of specific growth factors. Increasing dosages of IronQ from 0 to 200 μg/mL led to higher CAC uptake, and maximum labeling time was achieved in 10 days. The accumulated IronQ in CACs was measured by two methodologies, an inductively coupled plasma optical emission spectrometry (ICP-EOS) and T1-weighted MRI. In our research, we confirmed that IronQ has excellent dual functions with the use of an imaging probe for MRI. IronQ can also act as a stimulating agent by favoring circulating proangiogenic cell differentiation. Optimistically, IronQ is considered beneficial for alternative labeling and in the tracking of circulation proangiogenic cells and/or other stem cells in applications of cell therapy through noninvasive magnetic resonance imaging in both preclinical and clinical settings.

Rational Ligand Design Enables pH Control over Aqueous Iron Magnetostructural Dynamics and Relaxometric Properties

Complexes of Fe³⁺ engage in rich aqueous solution speciation chemistry in which discrete molecules can react with solvent water to form multinuclear μ-oxo and μ-hydroxide bridged species. Here we demonstrate how pH- and concentration-dependent equilibration between monomeric and μ-oxo-bridged dimeric Fe³⁺ complexes can be controlled through judicious ligand design. We purposed this chemistry to develop a first-in-class Fe³⁺-based MR imaging probe, Fe-PyCy2AI, that undergoes relaxivity change via pH-mediated control of monomer vs dimer speciation. The monomeric complex exists in a S = 5/2 configuration capable of inducing efficient T₁-relaxation, whereas the antiferromagnetically coupled dimeric complex is a much weaker relaxation agent. The mechanisms underpinning the pH dependence on relaxivity were interrogated by using a combination of pH potentiometry, ¹H and ¹⁷O relaxometry, electronic absorption spectroscopy, bulk magnetic susceptibility, electron paramagnetic resonance spectroscopy, and X-ray crystallography measurements. Taken together, the data demonstrate that PyCy2AI forms a ternary complex with high-spin Fe³⁺ and a rapidly exchanging water coligand, [Fe(PyCy2AI)(H₂O)]⁺ (ML), which can deprotonate to form the high-spin complex [Fe(PyCy2AI)(OH)] (ML(OH)). Under titration conditions of 7 mM Fe complex, water coligand deprotonation occurs with an apparent pK_a 6.46. Complex ML(OH) dimerizes to form the antiferromagnetically coupled dimeric complex [(Fe(PyCy2AI))₂O] ((ML)₂O) with an association constant (K_a) of 5.3 ± 2.2 mM^-1. The relaxivity of the monomeric complexes are between 7- and 18-fold greater than the antiferromagnetically coupled dimer at applied field strengths ranging between 1.4 and 11.7 T. ML(OH) and (ML)₂O interconvert rapidly within the pH 6.0-7.4 range that is relevant to human pathophysiology, resulting in substantial observed relaxivity change. Controlling Fe³⁺ μ-oxo bridging interactions through rational ligand design and in response to local chemical environment offers a robust mechanism for biochemically responsive MR signal modulation.

Modulating the Properties of Fe(III) Macrocyclic MRI Contrast Agents by Appending Sulfonate or Hydroxyl Groups

Complexes of Fe(III) that contain a triazacyclononane (TACN) macrocycle, two pendant hydroxyl groups, and a third ancillary pendant show promise as MRI contrast agents. The ancillary group plays an important role in tuning the solution relaxivity of the Fe(III) complex and leads to large changes in MRI contrast enhancement in mice. Two new Fe(III) complexes, one with a third coordinating hydroxypropyl pendant, Fe(L2), and one with an anionic non-coordinating sulfonate group, Fe(L1)(OH₂), are compared. Both complexes have a deprotonated hydroxyl group at neutral pH and electrode potentials representative of a stabilized trivalent iron center. The r₁ relaxivity of the Fe(L1)(OH₂) complex is double that of the saturated complex, Fe(L2), at 4.7 T, 37 °C in buffered solutions. However, variable-temperature ¹⁷O-NMR experiments show that the inner-sphere water of Fe(L1)(OH₂) does not exchange rapidly with bulk water under these conditions. The pendant sulfonate group in Fe(L1)(OH₂) confers high solubility to the complex in comparison to Fe(L2) or previously studied analogues with benzyl groups. Dynamic MRI studies of the two complexes showed major differences in their pharmacokinetics clearance rates compared to an analogue containing a benzyl ancillary group. Rapid blood clearance and poor binding to serum albumin identify Fe(L1)(OH₂) for development as an extracellular fluid contrast agent.