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

Macrocyclic complexes of Fe(III) with mixed hydroxypropyl and phenolate or amide pendants as T1 MRI probes

High-spin Fe(III) complexes of 1,4,7-triazacyclononane (TACN) with mixed oxygen donor pendants including hydroxypropyl, phenolate or amide groups are prepared for study as T1 MRI probes. Complexes with two hydroxypropyl pendants and either amide (Fe(TOAB)) or phenolate (Fe(PTOB)) groups are compared to an analog with three hydroxypropyl groups (Fe(NOHP)), in order to study the effect of the third pendant on the coordination sphere as probed by solution chemistry, relaxivity and structural studies. Solution studies show that Fe(PTOB) has two ionizations with the phenol pendant deprotonating with a pKa of 1.7 and a hydroxypropyl pendent with pKa of 6.3. The X-ray crystal structure of [Fe(PTOB)]Br2 features a six-coordinate complex with two bound hydroxypropyl groups, and a phenolate in a distorted octahedral geometry. The Fe(TOAB) complex has a single deprotonation, assigned to a hydroxypropyl group with a pKa value of 7.0. Both complexes are stabilized as high-spin Fe(III) in solution as shown by their effective magnetic moments and Fe(III)/Fe(II) redox potentials of -390 mV and -780 mV versus NHE at pH 7 and 25 °C for Fe(TOAB) and Fe(PTOB) respectively. Both Fe(PTOB) and Fe(TOAB) are kinetically inert to dissociation under a variety of challenges including phosphate/carbonate buffer, one equivalent of ZnCl2, two equivalents of transferrin or 100 mM HCl, or at basic pH values over 24 h at 37 °C. The r1 relaxivity of Fe(TOAB) at 1.4 T, pH 7.4 and 33 °C is relatively low at 0.6 mM-1 s-1 whereas the r1 relaxivity of Fe(PTOB) is more substantial and shows an increase of 2.5 fold to 2.5 mM-1 s-1 at acidic pH. The increase in relaxivity at acidic pH is attributed to protonation of the phenolate group to provide an additional pathway for proton relaxation.

Endeavor toward Redox-Responsive Transition Metal Contrast Agents Based on the Cross-Bridge Cyclam Platform

We present the synthesis and characterization of a series of Mn(III), Co(III), and Ni(II) complexes with cross-bridge cyclam derivatives (CB-cyclam = 1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) containing acetamide or acetic acid pendant arms. The X-ray structures of [Ni(CB-TE2AM)]Cl2·2H2O and [Mn(CB-TE1AM)(OH)](PF6)2 evidence the octahedral coordination of the ligands around the Ni(II) and Mn(III) metal ions, with a terminal hydroxide ligand being coordinated to Mn(III). Cyclic voltammetry studies on solutions of the [Mn(CB-TE1AM)(OH)]2+ and [Mn(CB-TE1A)(OH)]+ complexes (0.15 M NaCl) show an intricate redox behavior with waves due to the MnIII/MnIV and MnII/MnIII pairs. The Co(III) and Ni(II) complexes with CB-TE2A and CB-TE2AM show quasi-reversible features due to the CoIII/CoII or NiII/NiIII pairs. The [Co(CB-TE2AM)]3+ complex is readily reduced by dithionite in aqueous solution, as evidenced by 1H NMR studies, but does not react with ascorbate. The [Mn(CB-TE1A)(OH)]+ complex is however reduced very quickly by ascorbate following a simple kinetic scheme (k0 = k1[AH-], where [AH-] is the ascorbate concentration and k1 = 628 ± 7 M-1 s-1). The reduction of the Mn(III) complex to Mn(II) by ascorbate provokes complex dissociation, as demonstrated by 1H nuclear magnetic relaxation dispersion studies. The [Ni(CB-TE2AM)]2+ complex shows significant chemical exchange saturation transfer effects upon saturation of the amide proton signals at 71 and 3 ppm with respect to the bulk water signal.

Design and synthesis of chiral DOTA-based MRI contrast agents with remarkable relaxivities

Due to the adverse effects of de-metallation in past concerning FDA-approved gadolinium-based contrast agents (GBCAs), researchers have been focusing on developing safer and more efficient alternatives that could avoid toxicity caused by free gadolinium ions. Herein, two chiral GBCAs, Gd-LS with sulfonate groups and Gd-T with hydroxyl groups, are reported as potential candidates for magnetic reasonance imaging (MRI). The r1 relaxivities of TSAP, SAP isomers of Gd-LS and SAP isomer of Gd-T at 1.4 T, 37 °C in water are 7.4 mM-1s-1, 14.5 mM-1s-1 and 5.2 mM-1s-1, respectively. Results show that the hydrophilic functional groups introduced to the chiral macrocyclic scaffold of Gd-T and Gd-LS both give constructive influences on the second-sphere relaxivity and enhance the overall r1 value. Both cases indicate that the design of GBCAs should also focus on the optimal window in Solomon-Bloembergen-Morgan (SBM) theory and the effects caused by the second-sphere and outer-sphere relaxivity.

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.

Liposomal MRI probes containing encapsulated or amphiphilic Fe(III) coordination complexes

Liposomes containing high-spin Fe(III) coordination complexes were prepared towards the production of T1 MRI probes with improved relaxivity. The amphiphilic Fe(III) complexes were anchored into the liposome with two alkyl chains to give a coordination sphere containing mixed amide and hydroxypropyl pendant groups. The encapsulated complex contains a macrocyclic ligand with three phosphonate pendants, [Fe(NOTP)]3-, which was chosen for its good aqueous solubility. Four types of MRI probes were prepared including those with intraliposomal Fe(III) complex (LipoA) alone, amphiphilic Fe(III) complex (LipoB), both intraliposomal and amphiphilic complex (LipoC) or micelles formed with amphiphilic complex. Water proton relaxivities r1 and r2 were measured and compared to a small molecule macrocyclic Fe(III) complex containing similar donor groups. Micelles of the amphiphilic Fe(III) complex had proton relaxivity values (r1 = 2.6 mM-1 s-1) that were four times higher than the small hydrophilic analog. Liposomes with amphiphilic Fe(III) complex (LipoB) have a per iron relaxivity of 2.6 mM-1 s-1 at pH 7.2, 34 °C at 1.4 T whereas liposomes containing both amphiphilic and intraliposomal Fe(III) complexes (lipoC) have r1 of 0.58 mM-1 s-1 on a per iron basis consistent with quenching of the interior Fe(III) complex relaxivity. Liposomes containing only encapsulated [Fe(NOTP)]3- have a lowered r1 of 0.65 mM-1 s-1 per iron complex. Studies show that the biodistribution and clearance of the different types liposomal nanoparticles differ greatly. LipoB is a blood pool agent with a long circulation time whereas lipoC is cleared more rapidly through both renal and hepatobiliary pathways. These clearance differences are consistent with lower stability of LipoC compared to LipoB.

An Fe complex for 19F magnetic resonance-based reversible redox sensing and multicolor imaging

We report a first-in-class responsive, pentafluorosulfanyl (-SF₅)-tagged ¹⁹F MRI agent capable of reversibly detecting reducing environments via an Fe^II/III redox couple. In the Fe^III form, the agent displays no ¹⁹F MR signal due to paramagnetic relaxation enhancement-induced signal broadening; however, upon rapid reduction to Fe^II with one equivalent of cysteine, the agent displays a robust ¹⁹F signal. Successive oxidation and reduction studies validate the reversibility of the agent. The -SF₅ tag in this agent enables 'multicolor imaging' in conjunction with sensors containing alternative fluorinated tags and this was demonstrated via simultaneous monitoring of the ¹⁹F MR signal of this -SF₅ agent and a hypoxia-responsive agent containing a -CF₃ group.

Metal-Assembled Collagen Peptide Microflorettes as Magnetic Resonance Imaging Agents

Magnetic resonance imaging (MRI) is a medical imaging technique that provides detailed information on tissues and organs. However, the low sensitivity of the technique requires the use of contrast agents, usually ones that are based on the chelates of gadolinium ions. In an effort to improve MRI signal intensity, we developed two strategies whereby the ligand DOTA and Gd(III) ions are contained within Zn(II)-promoted collagen peptide (NCoH) supramolecular assemblies. The DOTA moiety was included in the assembly either via a collagen peptide sidechain (NHdota) or through metal–ligand interactions with a His-tagged DOTA conjugate (DOTA-His6). SEM verified that the morphology of the NCoH assembly was maintained in the presence of the DOTA-containing peptides (microflorettes), and EDX and ICP-MS confirmed that Gd(III) ions were incorporated within the microflorettes. The Gd(III)-loaded DOTA florettes demonstrated higher intensities for the T1-weighted MRI signal and higher longitudinal relaxivity (r1) values, as compared to the clinically used contrast agent Magnevist. Additionally, no appreciable cellular toxicity was observed with the collagen microflorettes loaded with Gd(III). Overall, two peptide-based materials were generated that have potential as MRI contrast agents.

Redox-Responsive MRI Probes Based on First-Row Transition-Metal Complexes

The presence of multiple oxidation and spin states of first-row transition-metal complexes facilitates the development of switchable MRI probes. Redox-responsive probes capitalize on a change in the magnetic properties of the different oxidation states of the paramagnetic metal ion center upon exposure to biological oxidants and reductants. Transition-metal complexes that are useful for MRI can be categorized according to whether they accelerate water proton relaxation (T₁ or T₂ agents), induce paramagnetic shifts of ¹H or ¹⁹F resonances (paraSHIFT agents), or are chemical exchange saturation transfer (CEST) agents. The various oxidation state couples and their properties as MRI probes are summarized with a focus on Co(II)/Co(III) or Fe(II)/Fe(III) complexes as small molecules or as liposomal agents. Solution studies of these MRI probes are reviewed with an emphasis on redox changes upon treatment with oxidants or with enzymes that are physiologically important in inflammation and disease. Finally, we outline the challenges of developing these probes further for in vivo MRI applications.

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.

Comparison of phosphonate, hydroxypropyl and carboxylate pendants in Fe(III) macrocyclic complexes as MRI contrast agents

Fe(III) macrocyclic complexes containing a macrocycle and three pendant groups including phosphonate (NOTP =1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid), carboxylate (NOTA = 1,4,7 - triazacyclononane - N,N',N″ - triacetate) or hydroxypropyl (NOHP =(2S,2'S,2"S)-1,1',1″-(1,4,7-triazonane-1,4,7-triyl)tris(propan-2-ol)) were studied in order to compare the effect of these donor groups on solution chemistry and water proton relaxivity. All three complexes, Fe(NOTP), Fe(NOHP) and Fe(NOTA), display a large degree of kinetic inertness to dissociation in the presence of phosphate and carbonate, under acidic conditions of 100 mM HCl or 1 M HCl or to trans-metalation with Zn(II). The r1 proton relaxivity of the complexes at 1.4 T, 33 °C is compared over the pH range of 1 to 10. At pH 7.4, 33 °C, 1.4 T, Fe(NOHP) has the largest relaxivity (1.5 mM-1 s-1), Fe(NOTP) is second at 1.0 mM-1 s-1, whereas Fe(NOTA) is the lowest at 0.61 mM-1 s-1. Fe(NOTP), Fe(NOHP) and Fe(NOTA) all show an increase in relaxivity at very acidic pH values (< 3) that is consistent with an acid-catalyzed process. Variable temperature 17O NMR studies at near neutral pH are consistent with the absence of an inner-sphere water molecule for Fe(NOTP) and Fe(NOHP), supporting second-sphere or outer-sphere water contributions to proton relaxation. Fe(NOTP) shows contrast enhancement in T1 weighted MRI studies in mice and clears through a renal pathway.

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.

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.

Defining the conditions for the development of the emerging class of FeIII-based MRI contrast agents

Fe(iii) complexes are attracting growing interest in chemists developing diagnostic probes for Magnetic Resonance Imaging because they leverage on an endogenous metal and show superior stability. However, in this case a detailed understanding of the relationship between the chemical structure of the complexes, their magnetic, thermodynamic, kinetic and redox properties and the molecular parameters governing the efficacy (relaxivity) is still far from being available. We have carried out an integrated ¹H and ¹⁷O NMR relaxometric study as a function of temperature and magnetic field, on the aqua ion and three complexes chosen as reference models, together with theoretical calculations, to obtain accurate values of the parameters that control their relaxivity. Moreover, thermodynamic stability and dissociation kinetics of the Fe(iii) chelates, measured in association with the ascorbate reduction behaviour, highlight their role and mutual influence in achieving the stability required for use in vivo.

An integrated ¹H and ¹⁷O NMR relaxometric study on model systems allowed to highlight that the Fe(III) complexes might represent the best alternative to Gd-based MRI contrast agents at the magnetic fields of current and future clinical scanners.

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)-tCDTA derivatives as MRI contrast agents: Increased T1 relaxivities at higher magnetic field strength and pH sensing

PURPOSE

Low molecular weight iron(III) complex-based contrast agents (IBCA) including iron(III) trans-cyclohexane diamine tetraacetic acid [Fe(tCDTA)]^- could serve as alternatives to gadolinium-based contrast agents in MRI. In search for IBCA with enhanced properties, we synthesized derivatives of [Fe(tCDTA)]^- and compared their contrast effects.

METHODS

Trans-cyclohexane diamine tetraacetic acid (tCDTA) was chemically modified in 2 steps: first the monoanhydride of Trans-cyclohexane diamine tetraacetic acid was generated, and then it was coupled to amines in the second step. After purification, the chelators were analyzed by high-performance liquid chromatography, mass spectrometry, and NMR spectrometry. The chelators were complexed with iron(III), and the relaxivities of the complexes were measured at 0.94, 1.5, 3, and 7 Tesla. Kinetic stabilities of the complexes were analyzed spectrophotometrically and the redox properties by cyclic voltammetry.

RESULTS

Using ethylenediamine (en) and trans-1,4-diaminocyclohexane, we generated monomers and dimers of tCDTA: en-tCDTA, en-tCDTA-dimer, trans-1,4-diaminocyclohexane-tCDTA, and trans-1,4-diaminocyclohexane-tCDTA-dimer. The iron(III) complexes of these derivatives had similarly high stabilities as [Fe(tCDTA)]^- . The iron(III) complexes of the trans-1,4-diaminocyclohexane derivatives had higher T₁ relaxivities than [Fe(tCDTA)]^- that increased with increasing magnetic field strengths and were highest at 6.8 L·mmol^-1 ·s^-1 per molecule for the dimer. Remarkably, the relaxivity of [Fe(en-tCDTA)]⁺ had a threefold increase from neutral pH toward pH6.

CONCLUSION

Four iron(III) complexes with similar stability in comparison to [Fe(tCDTA)]^- were synthesized. The relaxivities of trans-1,4-diaminocyclohexane-tCDTA and trans-1,4-diaminocyclohexane-tCDTA-dimer complexes were in the same range as gadolinium-based contrast agents at 3 Tesla. The [Fe(en-tCDTA)]⁺ complex is a pH sensor at weakly acidic pH levels, which are typical for various cancer types.

Alternatives to Gadolinium-Based Contrast Agents

Gadolinium-based contrast agents have been used in hundreds of millions of patients in the past 30 years, with an exemplary safety record. However, assumptions made at their inception have been recently challenged, rekindling innovation efforts. This critical review outlines the motivations, technical obstacles, problems, and the most recent published progress toward the creation of alternatives to the existing gadolinium-based contrast agent.

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.

Co(II) Macrocyclic Complexes Appended with Fluorophores as paraCEST and cellCEST Agents

Four high-spin macrocyclic Co(II) complexes with hydroxypropyl or amide pendants and appended coumarin or carbostyril fluorophores were prepared as CEST (chemical exchange saturation transfer) MRI probes. The complexes were studied in solution as paramagnetic CEST (paraCEST) agents and after loading into Saccharomyces cerevisiae yeast cells as cell-based CEST (cellCEST) agents. The fluorophores attached to the complexes through an amide linkage imparted an unusual pH dependence to the paraCEST properties of all four complexes through of ionization of a group that was attributed to the amide NH linker. The furthest shifted CEST peak for the hydroxypropyl-based complexes changed by ∼90 ppm upon increasing the pH from 5 to 7.5. At acidic pH, the Co(II) complexes exhibited three to four CEST peaks with the most highly shifted CEST peak at 200 ppm. The complexes demonstrated substantial paramagnetic water proton shifts which is a requirement for the development of cellCEST agents. The large shift in the proton resonance was attributed to an inner-sphere water at neutral pH, as shown by variable temperature ¹⁷O NMR spectroscopy studies. Labeling of yeast with one of these paraCEST agents was optimized with fluorescence microscopy and validated by using ICP mass spectrometry quantitation of cobalt. A weak asymmetry in the Z-spectra was observed in the yeast labeled with a Co(II) complex, toward a cellCEST effect, although the Co(II) complexes were toxic to the cells at the concentrations necessary for observation of cellCEST.

Saccharomyces cerevisiae and Candida albicans Yeast Cells Labeled with Fe(III) Complexes as MRI Probes

The development of MRI probes is of interest for labeling antibiotic-resistant fungal infections based on yeast. Our work showed that yeast cells can be labeled with high-spin Fe(III) complexes to produce enhanced T2 water proton relaxation. These Fe(III)-based macrocyclic complexes contained a 1,4,7-triazacyclononane framework, two pendant alcohol groups, and either a non-coordinating ancillary group and a bound water molecule or a third coordinating pendant. The Fe(III) complexes that had an open coordination site associated strongly with Saccharomyces cerevisiae upon incubation, as shown by screening using Z-spectra analysis. The incubation of one Fe(III) complex with either Saccharomyces cerevisiae or Candida albicans yeast led to an interaction with the β-glucan-based cell wall, as shown by the ready retrieval of the complex by the bidentate chelator called maltol. Other conditions, such as a heat shock treatment of the complexes, produced Fe(III) complex uptake that could not be reversed by the addition of maltol. Appending a fluorescence dye to Fe(TOB) led to uptake through secretory pathways, as shown by confocal fluorescence microscopy and by the incomplete retrieval of the Fe(III) complex by the maltol treatment. Yeast cells that were labeled with these Fe(III) complexes displayed enhanced water proton T2 relaxation, both for S. cerevisiae and for yeast and hyphal forms of C. albicans.