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

Comprehensive Investigation of [Fe(EDTA)]--Functionalized Derivatives and their Supramolecular Adducts with Human Serum Albumin

In recent years, the coordination chemistry of high-spin Fe(III) complexes has increasingly attracted interest due to their potential as effective alternatives to Gd(III)-based MRI contrast agents. This paper discusses the results from our study on Fe(III) complexes with two EDTA derivatives, each modified with either one (EDTA-BOM) or two (EDTA-BOM2) benzyloxymethylene (BOM) groups on the acetic arm(s). These pendant hydrophobic groups enable the complexes to form noncovalent adducts with human serum albumin (HSA), leading to an observed increase in relaxivity due to the reduction in molecular tumbling. Our research involved detailed relaxometric measurements and analyses of both 1H and 17O NMR data at varying temperatures and magnetic field strengths, which is conducted with and without the presence of a protein. A significant finding of this study is the effect of electronic relaxation time on the effectiveness of [Fe(EDTA-BOM)(H2O)]- and [Fe(EDTA-BOM2)(H2O)]- as diagnostic MRI probes. By integrating these relaxometric results with comprehensive thermodynamic, kinetic, and electrochemical data, we have thoroughly characterized how structural modifications to the EDTA base ligand influence the properties of the complexes.

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.

Co(II) Macrocyclic Complexes with Amide-Glycinate Pendants as ParaCEST and Liposomal CEST Agents

Macrocyclic Co(II) complexes with appended amide-glycinate groups were prepared to develop paramagnetic Co(II) chemical exchange saturation transfer (CEST) agents of reduced overall charge. Complexes with reduced charge and lowered osmolarity are important for their loading into liposomes and to provide complexes that are highly water soluble and well tolerated in animals. Co(L1) has two non-coordinating benzyl groups and two amide-glycinate pendants, whereas Co(L2) has two unsubstituted amide pendants and two amide-glycinate pendants on cyclam (1,4,8,11-tetraazacyclododecane). The 1H NMR spectrum of Co(L1) is consistent with a single cis-pendant isomer with both amide protons in the trans-configuration, as supported by an X-ray crystal structure. Co(L2) has a mixture of different isomers in solution, including the trans-1,4 and 1,8 pendant isomers. The Z-spectrum of Co(L1) shows one highly-shifted CEST peak, whereas Co(L2) exhibits six CEST peaks. Encapsulation of 40 mM Co(L1) in a liposome with osmotically-induced shrinking at 300 mOsm/L produces a liposomal CEST agent with saturation frequency offset of 3 ppm. Addition of the amphiphilic 1,4,7-triazacyclononane-based complex Co(L5) to the liposomal bilayer at 18 mM with Co(L1) encapsulated in the liposome at 50 mM changes the sign and increases the magnitude of the saturation frequency offset to -7.5 ppm at 300 mOsm/L.

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.

Kilogram-Scale Synthesis of Extremely Small Gadolinium Oxide Nanoparticles as a T1-Weighted Contrast Agent for Magnetic Resonance Imaging

Magnetic resonance imaging contrast agents are frequently used in clinics to enhance the contrast between diseased and normal tissues. The previously reported poly(acrylic acid) stabilized exceedingly small gadolinium oxide nanoparticles (ES-GdON-PAA) overcame the problems of commercial Gd chelates, but limitations still exist, i.e., high r2/r1 ratio, long blood circulation half-life, and no data for large scale synthesis and formulation optimization. In this study, polymaleic acid (PMA) is found to be an ideal stabilizer to synthesize ES-GdONs. Compared with ES-GdON-PAA, the PMA-stabilized ES-GdON (ES-GdON-PMA) has a lower r2/r1 ratio (2.05, 7.0 T) and a lower blood circulation half-life (37.51 min). The optimized ES-GdON-PMA-9 has an exceedingly small particle size (2.1 nm), excellent water dispersibility, and stability. A facile, efficient, and environmental friendly synthetic method is developed for large-scale synthesis of the ES-GdONs-PMA. The weight of the optimized freeze-dried ES-GdON-PMA-26 synthesized in a 20 L of reactor reaches the kilogram level. The formulation optimization is also finished, and the concentrated ES-GdON-PMA-26 formulation (CGd = 100 mm) after high-pressure steam sterilization possesses eligible physicochemical properties (i.e., pH value, osmolality, viscosity, and density) for investigational new drug application.

Nitric oxide nano-reactor DNMF/PLGA enables tumor vascular microenvironment and chemo-hyperthermia synergetic therapy

BACKGROUND

Breast cancer ranks first among malignant tumors, of which triple-negative breast cancer (TNBC) is characterized by its highly invasive behavior and the worst prognosis. Timely diagnosis and precise treatment of TNBC are substantially challenging. Abnormal tumor vessels play a crucial role in TNBC progression and treatment. Nitric oxide (NO) regulates angiogenesis and maintains vascular homeostasis, while effective NO delivery can normalize the tumor vasculature. Accordingly, we have proposed here a tumor vascular microenvironment remodeling strategy based on NO-induced vessel normalization and extracellular matrix collagen degradation with multimodality imaging-guided nanoparticles against TNBC called DNMF/PLGA.

RESULTS

Nanoparticles were synthesized using a chemotherapeutic agent doxorubicin (DOX), a NO donor L-arginine (L-Arg), ultrasmall spinel ferrites (MnFe2O4), and a poly (lactic-co-glycolic acid) (PLGA) shell. Nanoparticle distribution in the tumor was accurately monitored in real-time through highly enhanced magnetic resonance imaging and photoacoustic imaging. Near-infrared irradiation of tumor cells revealed that MnFe2O4 catalyzes the production of a large amount of reactive oxygen species (ROS) from H2O2, resulting in a cascade catalysis of L-Arg to trigger NO production in the presence of ROS. In addition, DOX activates niacinamide adenine dinucleotide phosphate oxidase to generate and supply H2O2. The generated NO improves the vascular endothelial cell integrity and pericellular contractility to promote vessel normalization and induces the activation of endogenous matrix metalloproteinases (mainly MMP-1 and MMP-2) so as to promote extravascular collagen degradation, thereby providing an auxiliary mechanism for efficient nanoparticle delivery and DOX penetration. Moreover, the chemotherapeutic effect of DOX and the photothermal effect of MnFe2O4 served as a chemo-hyperthermia synergistic therapy against TNBC.

CONCLUSION

The two therapeutic mechanisms, along with an auxiliary mechanism, were perfectly combined to enhance the therapeutic effects. Briefly, multimodality image-guided nanoparticles provide a reliable strategy for the potential application in the fight against TNBC.

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.

Multifunctional Fe-based coordination polymer nano-bomb modified with β-lapachone and CaO2 for targeted tumor dual chemodynamic therapy with enhanced ferroptosis and H2O2 self-supply

Chemodynamic therapy (CDT) is seriously limited by the inadequacy of exogenous catalytic ions and endogenous H2O2 in tumors. Herein, a multifunction nano-bomb integrated with calcium peroxide (CaO2) and β-lapachone as donors of H2O2 and GSH-sensitive Fe-based coordination polymer as provider of catalytic ions was constructed for dual cascade-amplified tumor CDT. This hyaluronic acid (HA)-modified nano-bomb could be specially endocytosed by breast cancer cells through a targeting pathway, degraded and released cargoes in response to the GSH-rich cytoplasm. Furthermore, the released CaO2 and β-lapachone could significantly self-generated sufficient H2O2, which could dual-cascade amplify CDT and induce severe oxidative to tumors via cooperating with the delivered iron ions from nano-bombs. Moreover, the unloaded iron and calcium ions could further accelerate tumor damage by overloading Ca2+ and ferroptosis, as accompanied by good magnetic resonance imaging (MRI). In vitro and in vivo studies collectively reveal that this nano-bomb not only self-initiates double cascade-amplified CDT via self-generation of H2O2, but also efficiently activates ferroptosis and initiates Ca2+ overloading, consequently significantly tumor growth suppression. This study offers a novel tumor-initiated nano-bomb for dual cascade-amplified CDT and bioimaging with activated ferroptosis and self-supplying H2O2.

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.

Gelothermal Synthesis of Monodisperse MIL-88A Nanoparticles with Tunable Sizes and Metal Centers for Potential Bioapplications

Developing novel synthetic strategies to downsize metal-organic frameworks (MOFs) from polydisperse crystals to monodisperse nanoparticles is of great importance for their potential bioapplications. In this work, a novel synthetic strategy termed gelothermal synthesis is proposed, in which coordination polymer gel is first prepared and followed by a thermal reaction to give the monodisperse MOF nanoparticles. This novel synthetic strategy successfully leads to the isolation of Materials of Institute Lavoisier (MIL-88), Cu(II)-fumarate MOFs (CufumDMF), and Zeolitic Imidazolate Frameworks (ZIF-8) nanoparticles. Focused on MIL-88A, the studies reveal that the size can be well-tuned from nanoscale to microscale without significant changes in polydispersity index (PDI) even in the case of in situ metal substitution. A possible mechanism is consequently proposed based on extensive studies on the gelothermal condition including sol-gel chemistry, thermal condition, kinds of solvents, and so on. The unique advantages of monodisperse MIL-88A nanoparticles over polydisperse ones are further demonstrated in terms of in vitro magnetic resonance imaging (MRI), cellular uptake, and drug-carrying properties.

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.

Magnetic Resonance Diagnosis of Early Triple-Negative Breast Cancer Based on the Ionic Covalent Organic Framework with High Relaxivity and Long Retention Time

Patients with triple-negative breast cancer (TNBC) have dismal prognoses due to the lack of therapeutic targets and susceptibility to lymph node (LN) metastasis. Therefore, it is essential to develop more effective approaches to identify early TNBC tissues and LNs. In this work, a magnetic resonance imaging (MRI) contrast agent (Mn-iCOF) was constructed based on the Mn(II)-chelated ionic covalent organic framework (iCOF). Because of the porous structure and hydrophilicity, the Mn-iCOF has a high longitudinal relaxivity (r₁) of 8.02 mM^-1 s^-1 at 3.0 T. For the tumor-bearing mice, a lower dose (0.02 mmol [Mn]/kg) of Mn-iCOF demonstrated a higher signal-to-noise ratio (SNR) value (1.8) and longer retention time (2 h) compared to a 10-fold dose of commercial Gd-DOTA (0.2 mmol [Gd]/kg). Moreover, the Mn-iCOF can provide continuous and significant MR contrast for the popliteal LNs within 24 h, allowing for accurate evaluation and dissection of LNs. These excellent MRI properties of the Mn-iCOF may open new avenues for designing more biocompatible MRI contrast agents with higher resolutions, particularly in the diagnosis of TNBC.

Alkyl Chain Appended Fe(III) Catecholate Complex as a Dual-Modal T1 MRI-NIR Fluorescence Imaging Agent via Second Sphere Water Interactions

The C₁₂-alkyl chain-conjugated Fe(III) catecholate complex [Fe(C₁₂CAT)₃]^3-, Fe(C₁₂CAT)₃ [C₁₂CAT = N-(3,4-dihydroxyphenethyl)dodecanamide], was synthesized and characterized, reported as a dual-modal T₁-MRI and an optical imaging probe. The DFT-optimized structure of Fe(C₁₂CAT)₃ reveals a distorted octahedral coordination geometry around the high spin Fe(III) center. The formation constant (-log K) of Fe(C₁₂CAT)₃ was calculated as 45.4. The complex exhibited r₁-relaxivity values of 2.31 ± 0.12 and 1.52 ± 0.06 mM^-1 s^-1 at 25 and 37 °C, respectively, on 1.41 T at pH 7.3 via second-sphere water interactions. The interaction of Fe(C₁₂CAT)₃ with human serum albumin showed concomitant enhancement of r₁-relaxivity to 6.44 ± 0.15 mM^-1 s^-1. The MR phantom images are significantly brighter and directly correlate to the concentration of Fe(C₁₂CAT)₃. Adding an external fluorescent marker IR780 dye to Fe(C₁₂CAT)₃ leads to the formation of self-assembly by C₁₂-alkyl chains. It resulted in the fluorescence quenching of the dye, and its critical aggregation concentration was calculated as 70 μM. The aggregated matrix of Fe(C₁₂CAT)₃ and IR780 dye is spherical, with an average hydrodynamic diameter of 189.5 nm. This self-assembled supramolecular system is found to be non-fluorescent and was "turn-on" under acidic pH via dissociation of aggregates. The r₁-relaxivity is found to be unchanged during the matrix aggregation and disaggregation. The probe showed MRI ON and fluorescent OFF under physiological conditions and MRI ON and fluorescent ON under acidic pH. The cell viability experiments showed that the cells are 80% viable at 1 mM probe concentration. Fluorescence experiments and MR phantom images showed that Fe(C₁₂CAT)₃ is a potential dual model imaging probe to visualize the acidic pH environment of the cells.

Tumor microenvironment responsive T1-T2 dual-mode contrast agent Fe3O4@ZIF-8-Zn-Mn NPs for in vivo magnetic resonance imaging

Activated T₁-T₂ contrast agents can effectively improve the sensitivity and diagnosis accuracy of magnetic resonance imaging (MRI), but the construction of such contrast agents still remains a great challenge. In this work, a pH- and glutathione (GSH)-responsive T₁-T₂ dual-mode contrast agent, Fe₃O₄@ZIF-8-Zn-Mn nanoparticles (NPs), with simple components was constructed via simply assembly of paramagnetic Mn²⁺ ions (as T₁ contrast agent) and Fe₃O₄ NPs (as T₂ contrast agent) into a pH- and GSH-sensitive Zn-zeolitic imidazole framework (ZIF-8) matrix. Under neutral conditions, Fe₃O₄@ZIF-8-Zn-Mn NPs show good stability and weak T₁-T₂ dual-mode MRI contrast effect (r₁ = 0.82 mM^-1 s^-1, r₂ = 21.28 mM^-1 s^-1) due to the magnetic interference between Fe₃O₄ NPs and paramagnetic Mn²⁺ ions. In contrast, under acidic environment (pH = 6.5-5.5) and in the present GSH (0-4 mM), Fe₃O₄@ZIF-8-Zn-Mn NPs can be disassembled and release Fe₃O₄ NPs and paramagnetic Mn²⁺ ions, which causes simultaneous recovery of T₁ and T₂ imaging performances with enhanced r₁ and r₂ relaxation values up to 6.9 and 9.9 times, respectively. Moreover, in vivo MRI experiments showed that after the intravenous injection of Fe₃O₄@ZIF-8-Zn-Mn NPs for about one hour, the T₁-weighted imaging of the tumor site becomes brighter with T₁ signal enhanced by about 31%, while the T₂-weighted imaging of the tumor site becomes darker with T₂ signal enhanced by nearly 30%, suggesting the great potential of Fe₃O₄@ZIF-8-Zn-Mn NPs to be used as a tumor microenvironment-responsive T₁-T₂ dual-mode contrast agent for sensitive tumor imaging.

A tumor-targeting and ROS-responsive iron-based T1 magnetic resonance imaging contrast agent for highly specific tumor imaging

T₁ contrast agents (CAs) exhibit outstanding capacity in enhancing the magnetic resonance imaging (MRI) contrast between tumor tissues and normal tissues for generating bright images. However, the clinical application of representative gadolinium(III) chelate-based T₁ CAs is limited due to their potential toxicity and low specificity for pathological tissues. To obtain MRI CAs with a combination of low toxicity and high tumor specificity, herein, we report a reactive oxygen species (ROS)-responsive T₁ CA (GA-Fe(II)-PEG-FA), which was constructed by chelating Fe(II) with gallic acid (GA), and modified with tumor-targeted folic acid (FA). The resultant CA could accumulate in tumor tissues via the affinity between FA and their receptors on the tumor cell membrane. It realized the switch from Fe(II) to Fe(III), and further enhancing the longitudinal relaxation rate (r₁) under the stimuli of ROS in the tumor microenvironment. The r₁ of GA-Fe(II)-PEG-FA on a 0.5 T nuclear magnetic resonance analyzer increased to 2.20 mM^-1 s^-1 under ROS stimuli and was 5 times greater than the r₁ (0.42 mM^-1 s^-1) before oxidation. The cell and in vivo experiments demonstrated that GA-Fe(II)-PEG-FA exhibited good biocompatibility and significant targeting specificity to tumor cells and tumor tissues. Furthermore, in vivo MRI studies demonstrated that the enhanced T₁ contrast effect against tumors could be achieved after injecting the CA for 3 h, indicating that GA-Fe(II)-PEG-FA has the potential as an ideal tumor MRI CA to increase the contrast and improve the diagnostic precision.

Copper(II) complexes of cyclams with N-(2,2,2-trifluoroethyl)-aminoalkyl pendant arms as potential probes for 19F magnetic resonance imaging

A series of Cu(II) complexes with cyclam-based ligands containing two N-(2,2,2-trifluoroethyl)-aminoalkyl pendant arms in 1,8-positions (L1: 1,2-ethylene spacer, L2: 1,3-propylene spacer; L3: 1,4-butylene spacer) was studied in respect to potential use as contrast agents for ¹⁹F magnetic resonance imaging (MRI). A number of structures of the complexes as well as of several organic precursors were determined by single-crystal X-ray diffraction analysis. Geometric parameters (especially distances between fluorine atoms and the central metal ion) were determined for each complex and the identity of isomeric complex species present in solution was established. The NMR longitudinal relaxation times (T₁) of ¹⁹F nuclei in the ligands at clinically relevant fields and temperatures (1-2 s) were significantly shortened upon Cu(II) binding to 7-10 ms for [Cu(L1)]²⁺, 20-30 ms for [Cu(L2)]²⁺ and 20-50 ms for [Cu(L3)]²⁺. The trend of the relaxation time shortening is in accordance with the distance and number of chemical bonds between fluorine atoms and the Cu(II) ion. The signals show promising T₂*/T₁ ratios in the range 0.25-0.55, assuring their good applicability to ¹⁹F NMR/MRI. The results show that even the Cu(II) ion, with a small magnetic moment, causes significant relaxation enhancement with a long-range effect and can be considered as a highly suitable metal ion for efficient ¹⁹F MRI contrast agents.

MRI Contrast Agents in Glycobiology

Molecular recognition involving glycoprotein-mediated interactions is ubiquitous in both normal and pathological natural processes. Therefore, visualization of these interactions and the extent of expression of the sugars is a challenge in medical diagnosis, monitoring of therapy, and drug design. Here, we review the literature on the development and validation of probes for magnetic resonance imaging using carbohydrates either as targeting vectors or as a target. Lectins are important targeting vectors for carbohydrate end groups, whereas selectins, the asialoglycoprotein receptor, sialic acid end groups, hyaluronic acid, and glycated serum and hemoglobin are interesting carbohydrate targets.

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.