A water-soluble and water-coordinated Mn(II) complex: synthesis, characterization and phantom MRI image study

Folate Receptor Targeting Mn(II) Complex Encapsulated Porous Silica Nanoparticle as an MRI Contrast Agent for Early-State Detection of Cancer

Cancer is recognized as one of the major causes of mortality, however, early-stage detection can increase the survival chance greatly. It is recognized that folate receptors are gradually overexpressed in the cellular membrane with the progress of cancer from stage 1 to stage 4. Utilizing the fact, herein, developed a porous silica nanoparticle system C1@SiO2-FA-NP; A) impregnated with thermodynamically stable Mn(II) complex (1) molecules within the core of the nanoparticle, and B) surface functionalized with folate units. It exhibited a high longitudinal relaxivity value r1 = 21.45 mM-1s-1 that substantially increased to r1 = 40.97 mM-1s-1 in the presence of 0.67 mM concentration of BSA under the physiological condition. The in vitro fluorescent images after surface conjugation of C1@SiO2-FA-NP with FITC (fluorescein isothiocyanate) buttressed the inclusion of the nanoparticle exclusively within the cancerous HeLa cells than that of healthy HEK293 cells. The importance of the surface-bound folate unit in the nanoparticle is further established by comparing the fluorescent images of HeLa cells in the absence of the group. Finally, the applicability of C1@SiO2-FA-NP as the T1-weighted MRI contrast agent for early-stage cancer diagnosis is established within C57BL/6 mice after infecting the mice with HeLa cells.

A Bis(Aquated) Mn(II)-Based MRI Contrast Agent with a Rigid Hydroquinazoline Unit: Synthesis, Characterization, and in Vivo MR Imaging Study

Since the finding of nephrogenic systemic fibrosis (NFS) in patients with renal impairment and the long-term accumulation of Gd(III) ions in the central nervous system, the search for nongadolinium ion-based MRI contrast agents made of nutrient metal ions has drawn paramount attention. In this context, the development of Mn(II)-based MRI contrast agents has been a subject of interest for the last few decades. Herein, we report a pentadentate ligand (Li2[BenzPic2]) composed of two picolinate moieties and a rigid 1,2,3,4-tetrahydroquinazoline unit and the corresponding bis(aquated) Mn(II) complex (Complex 1). The complex exhibited high thermodynamic stability (log Kcond = 11.62) and kinetic inertness similar to that of the clinically approved Gd(III)-based contrast agent Magnevist. Complex 1 exerted longitudinal relaxivity (r1) of 5.32 mM-1 s-1 at 1.41 T, 37 °C, pH 7.4, and it increased by 3.6-fold in the presence of serum albumin protein, confirming a substantial rigidifying interaction (albumin association constant KA = 1.66 × 103 M-1) between the protein and the amphiphilic (log P = -0.45) contrast agent. An intravenous dose of 0.08 mmol/kg in a healthy mouse, excellent MRI signal intensity enhancement in the vasculature of the mouse liver, and brightened images of the gallbladder, kidney, and liver were realized.

Thermodynamic Stability of Mn(II) Complexes with Aminocarboxylate Ligands Analyzed Using Structural Descriptors

We present a quantitative analysis of the thermodynamic stabilities of Mn(II) complexes, defined by the equilibrium constants (log K_MnL values) and the values of pMn obtained as −log[Mn]_free for total metal and ligand concentrations of 1 and 10 μM, respectively. We used structural descriptors to analyze the contributions to complex stability of different structural motifs in a quantitative way. The experimental log K_MnL and pMn values can be predicted to a good accuracy by adding the contributions of the different motifs present in the ligand structure. This allowed for the identification of features that provide larger contributions to complex stability, which will be very helpful for the design of efficient chelators for Mn(II) complexation. This issue is particularly important to develop Mn(II) complexes for medical applications, for instance, as magnetic resonance imaging (MRI) contrast agents. The analysis performed here also indicates that coordination number eight is more common for Mn(II) than is generally assumed, with the highest log K_MnL values generally observed for hepta- and octadentate ligands. The X-ray crystal structure of [Mn₂(DOTA)(H₂O)₂], in which eight-coordinate [Mn(DOTA)]^2– units are bridged by six-coordinate exocyclic Mn(II) ions, is also reported.

We present empirical relationships that allow estimating the log K and pMn values of Mn(II) complexes relevant as contrast agents for magnetic resonance imaging (MRI). The prediction of complex stability with these expressions relies on structural descriptors, providing a very powerful tool to aid with ligand design.

Porous Silica Nanospheres with a Confined Mono(aquated) Mn(II)-Complex: A Potential T1-T2 Dual Contrast Agent for Magnetic Resonance Imaging

Magnetic resonance imaging has emerged as an indispensable imaging modality for the early-stage diagnosis of many diseases. The imaging in the presence of a contrast agent is always advantageous, as it mitigates the low-sensitivity issue of the measurements and provides excellent contrast in the acquired images even in a short acquisition time. However, the stability and high relaxivity of the contrast agents remained a challenge. Here, molecules of a mononuclear, mono(aquated), thermodynamically stable [log K_MnL = 14.80(7) and pMn = 8.97] Mn(II)-complex (1), based on a hexadentate pyridine-picolinate unit-containing ligand (H₂PyDPA), were confined within a porous silica nanosphere in a noncovalent fashion to render a stable nanosystem, complex 1@SiO₂NP. The entrapped complex 1 (complex 1@SiO₂) exhibited r₁ = 8.46 mM^-1 s^-1 and r₂ = 33.15 mM^-1 s^-1 at pH = 7.4, 25 °C, and 1.41 T in N-(2-hydroxyethyl)piperazine-N'-ethanesulfonic acid buffer. The values were about 2.9 times higher compared to the free (unentrapped)-complex 1 molecules. The synthesized complex 1@SiO₂NP interacted significantly with albumin protein and consequently boosted both the relaxivity values to r₁ = 24.76 mM^-1 s^-1 and r₂ = 63.96 mM^-1 s^-1 at pH = 7.4, 37 °C, and 1.41 T. The kinetic inertness of the entrapped molecules was established by recognizing no appreciable change in the r₁ value upon challenging complex 1@SiO₂NP with 30 and 40 times excess of Zn(II) ions at pH 6 and 25 °C. The water molecule coordinated to the Mn(II) ion in complex 1@SiO₂ was also impervious to the physiologically relevant anions (bicarbonate, biphosphate, and citrate) and pH of the medium. Thus, it ensured the availability of the inner-coordination site of complex 1 for the coordination of water molecules in the biological media. The concentration-dependent changes in image intensities in T₁- and T₂-weighted phantom images and uptake of the nanoparticles by the HeLa cell put forward the biocompatible complex 1@SiO₂NP as a potential dual-mode MRI contrast agent, an alternative to Gd(III)-containing contrast agents.

Understanding the Effect of the Electron Spin Relaxation on the Relaxivities of Mn(II) Complexes with Triazacyclononane Derivatives

Investigating the relaxation of water ¹H nuclei induced by paramagnetic Mn(II) complexes is important to understand the mechanisms that control the efficiency of contrast agents used in diagnostic magnetic resonance imaging (MRI). Herein, a series of potentially hexadentate triazacyclononane (TACN) derivatives containing different pendant arms were designed to explore the relaxation of the electron spin in the corresponding Mn(II) complexes by using a combination of ¹H NMR relaxometry and theoretical calculations. These ligands include 1,4,7-triazacyclononane-1,4,7-triacetic acid (H₃NOTA) and three derivatives in which an acetate group is replaced by sulfonamide (H₃NO2ASAm), amide (H₂NO2AM), or pyridyl (H₂NO2APy) pendants. The analogue of H₃NOTA containing three propionate pendant arms (H₃NOTPrA) was also investigated. The X-ray structure of the derivative containing two acetate groups and a sulfonamide pendant arm [Mn(NO2ASAm)]^- evidenced six-coordination of the ligand to the metal ion, with the coordination polyhedron being close to a trigonal prism. The relaxivities of all complexes at 20 MHz and 25 °C (1.1-1.3 mM^-1 s^-1) are typical of systems that lack water molecules coordinated to the metal ion. The nuclear magnetic relaxation profiles evidence significant differences in the relaxivities of the complexes at low fields (<1 MHz), which are associated with different spin relaxation rates. The zero field splitting (ZFS) parameters calculated by using DFT and CASSCF methods show that electronic relaxation is relatively insensitive to the nature of the donor atoms. However, the twist angle of the two tripodal faces that delineate the coordination polyhedron, defined by the N atoms of the TACN unit (lower face) and the donor atoms of the pendant arms (upper face), has an important effect in the ZFS parameters. A twist angle close to the ideal value for an octahedral coordination (60°), such as that in [Mn(NOTPrA)]^-, leads to a small ZFS energy, whereas this value increases as the coordination polyhedron approaches to a trigonal prism.

Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference

We report two macrocyclic ligands based on a 1,7-diaza-12-crown-4 platform functionalized with acetate (tO2DO2A²⁻) or piperidineacetamide (tO2DO2AM^Pip) pendant arms and a detailed characterization of the corresponding Mn(II) complexes. The X−ray structure of [Mn(tO2DO2A)(H₂O)]·2H₂O shows that the metal ion is coordinated by six donor atoms of the macrocyclic ligand and one water molecule, to result in seven-coordination. The Cu(II) analogue presents a distorted octahedral coordination environment. The protonation constants of the ligands and the stability constants of the complexes formed with Mn(II) and other biologically relevant metal ions (Mg(II), Ca(II), Cu(II) and Zn(II)) were determined using potentiometric titrations (I = 0.15 M NaCl, T = 25 °C). The conditional stabilities of Mn(II) complexes at pH 7.4 are comparable to those reported for the cyclen-based tDO2A²⁻ ligand. The dissociation of the Mn(II) chelates were investigated by evaluating the rate constants of metal exchange reactions with Cu(II) under acidic conditions (I = 0.15 M NaCl, T = 25 °C). Dissociation of the [Mn(tO2DO2A)(H₂O)] complex occurs through both proton− and metal−assisted pathways, while the [Mn(tO2DO2AM^Pip)(H₂O)] analogue dissociates through spontaneous and proton-assisted mechanisms. The Mn(II) complex of tO2DO2A²⁻ is remarkably inert with respect to its dissociation, while the amide analogue is significantly more labile. The presence of a water molecule coordinated to Mn(II) imparts relatively high relaxivities to the complexes. The parameters determining this key property were investigated using ¹⁷O NMR (Nuclear Magnetic Resonance) transverse relaxation rates and ¹H nuclear magnetic relaxation dispersion (NMRD) profiles.

Ultrasmall Ternary FePtMn Nanocrystals with Acidity-Triggered Dual-Ions Release and Hypoxia Relief for Multimodal Synergistic Chemodynamic/Photodynamic/Photothermal Cancer Therapy

Multimodal imaging-guided synergistic anticancer strategies have attracted increasing attention for efficient diagnosis and therapy of cancer. Herein, a multifunctional nanotheranostic agent FePtMn-Ce6/FA (FPMCF NPs) is constructed by covalently anchoring photosensitizer chlorin e6 (Ce6) and targeting molecule folic acid (FA) on ultrasmall homogeneous ternary FePtMn nanocrystals. Response to tumor microenvironment (TME), FPMCF NPs can release Fe²⁺ to catalyze H₂ O₂ into •OH by Fenton reaction and simultaneously catalyze hydrogen peroxide (H₂ O₂ ) into O₂ to overcome the tumor hypoxia barrier. Released O₂ is further catalyzed into ¹ O₂ under 660 nm laser irradiation with Ce6. Thus, the FPMCF NPs exhibit superior dual-ROS oxidization capability including ferroptosis chemodynamic oxidization and ¹ O₂ -based photodynamic oxidization. Interestingly, FPMCF NPs reveal strong photothermal conversion efficiency exposed to an 808 nm laser, which can assist dual-ROS oxidization to suppress solid tumor remarkably. Additionally, Mn²⁺ can be released from FPMCF NPs to enhance longitudinal relaxivity (T₁ -weighted magnetic resonance (MR) imaging) and Fe-synergistic transverse relaxivity (T₂ -weighted MR imaging), which is convenient for diagnosis of solid tumors. Meanwhile, the fluorescent/photothermal (FL/PT) imaging function of FPMCF NPs can also accurately monitor tumor location. Therefore, FPMCF NPs with multimodal MR/FL/PT imaging-guided synergistic chemodynamic/photodynamic/photothermal cancer therapy capability have potential bioapplication in bionanomedicine field.

FePt@MnO-Based Nanotheranostic Platform with Acidity-Triggered Dual-Ions Release for Enhanced MR Imaging-Guided Ferroptosis Chemodynamic Therapy

Reactive oxygen species (ROS)-based anticancer therapy methods were heavily dependent on specific tumor microenvironments such as acidity and excess hydrogen peroxide (H₂O₂). In this work, an acidity-sensitive nanotheranostic agent (FePt@MnO)@DSPE-PEG5000-FA (FMDF NPs)  was successfully constructed for MR imaging guided ferroptosis chemodynamic therapy (FCDT) of cancer. The FMDF NPs could specifically target folic acid (FA) receptor-positive tumor cells (HeLa etc.) and induce ferroptosis efficiently by rapidly releasing active Fe²⁺ to catalyze intracellular H₂O₂ into ROS based on Fenton reaction. On the other hand, the Mn²⁺ could also be released due to acidity  and further coordinate with GSH to enhance the longitudinal-transverse relaxivity (T₁/T₂-weighted MR imaging), which could obviously strengthen the contrast distinction between solid tumors and the surrounding tissue to accurately real-time monitor the tumor location. Furthermore, the in vivo anticancer study revealed that the growth of solid tumor models could be suppressed remarkably after treating with FMDF NPs and no obvious damage to other major organs. Therefore, the FMDF NPs were competent simultaneously as an enhanced imaging diagnosis contrast agent and efficient therapy agent for promoting more precise and effective treatment in the bionanomedicine field.

Controlling water exchange rates in potential Mn2+-based MRI agents derived from NO2A2

We report a series of pentadentate ligands based on a 1,4,7-triazacyclononane-1,4-diacetic acid (H2NO2A) containing different substituents attached to the third nitrogen atom of the macrocyclic unit. Detailed 1H Nuclear Magnetic Relaxation Dispersion (NMRD) characterisation of the corresponding Mn2+ complexes suggests the formation of six-coordinate species in solution containing an inner-sphere water molecule. This was confirmed by recording the transverse 17O relaxation time and chemical shift measurements. The water exchange rate of the coordinated water molecule was found to be strongly influenced by the nature of the substituent R at position 7 of the triazacyclononane unit (R = Me, k298ex = 62.6 × 107 s-1; R = Bz, k298ex = 4.4 × 107 s-1; R = 1-phenylethyl, k298ex = 2.6 × 107 s-1). The decreasing exchange rates are explained by the increasing bulkiness of the substituent, which hinders the approach of the entering water molecule in an associatively activated water exchange mechanism. This is supported by DFT calculations (M062X/TZVP), which confirm the associative nature of the water exchange reaction. A potentially decadentate ligand containing two NO2A units linked by a xylenyl spacer in the meta position was also synthesised. The corresponding binuclear Mn2+ complex contains two metal ions with different hydration numbers, as evidenced by 1H NMRD and 17O NMR measurements. DFT calculations show that this is related to the presence of a bridging bidentate μ-η1-carboxylate group connecting the two metal centers. The results reported in this work provide a straightforward strategy to control the exchange rate of the coordinated water molecule in this family of MRI contrast agent candidates.

Janus nanoparticles for contrast enhancement of T1–T2 dual mode magnetic resonance imaging

MnFe₂O₄@MnO Janus nanoparticles efficiently enhances contrast of both T₁ and T₂ based MR Images.

Synthesis and Evaluation of Manganese(II)-Based Ethylenediaminetetraacetic Acid-Ethoxybenzyl Conjugate as a Highly Stable Hepatobiliary Magnetic Resonance Imaging Contrast Agent

In this study, we designed and synthesized a highly stable manganese (Mn²⁺)-based hepatobiliary complex by tethering an ethoxybenzyl (EOB) moiety with an ethylenediaminetetraacetic acid (EDTA) coordination cage as an alternative to the well-established hepatobiliary gadolinium (Gd³⁺) chelates and evaluated its usage as a T₁ hepatobiliary magnetic resonance imaging (MRI) contrast agent (CA). This new complex exhibits higher r₁ relaxivity (2.3 mM^-1 s^-1) than clinically approved Mn²⁺-based hepatobiliary complex Mn-DPDP (1.6 mM^-1 s^-1) at 1.5 T. Mn-EDTA-EOB shows much higher kinetic inertness than that of clinically approved Gd³⁺-based hepatobiliary MRI CAs, such as Gd-DTPA-EOB and Gd-BOPTA. In addition, in vivo biodistribution and MRI enhancement patterns of this new Mn²⁺ chelate are comparable to those of Gd³⁺-based hepatobiliary MRI CAs. The diagnostic efficacy of the new complex was demonstrated by its enhanced tumor detection sensitivity in a liver cancer model using in vivo MRI.

Magnetic resonance imaging contrast enhancement in vitro and in vivo by octanuclear iron-oxo cluster-based agents

A water-soluble octanuclear cluster, [Fe₈], was studied with regard to its properties as a potential contrast enhancing agent in magnetic resonance imaging (MRI) in magnetic fields of 1.3, 7.2 and 11.9 T and was shown to have transverse relaxivities r₂ = 4.01, 10.09 and 15.83 mM s^-1, respectively. A related hydrophobic [Fe₈] cluster conjugated with 5 kDa hyaluronic acid (HA) was characterized by ⁵⁷Fe-Mössbauer and MALDI-TOF mass spectroscopy, and was evaluated in aqueous solutions in vitro with regard to its contrast enhancing properties [r₂ = 3.65 mM s^-1 (1.3 T), 26.20 mM s^-1 (7.2 T) and 52.18 mM s^-1 (11.9 T)], its in vitro cellular cytotoxicity towards A-549 cells and COS-7 cells and its in vivo enhancement of T₂-weighted images (4.7 T) of a human breast cancer xenografted on a nude mouse. The physiologically compatible [Fe₈]-HA conjugate was i.v. injected to the tumor-bearing mouse, resulting in observable, heterogeneous signal change within the tumor, evident 15 min after injection and persisting for approximately 30 min. Both molecular [Fe₈] and its HA-conjugate show a strong magnetic field dependence on r₂, rendering them promising platforms for the further development of T₂ MRI contrast agents in high and ultrahigh magnetic fields.

Effect of the Nature of Donor Atoms on the Thermodynamic, Kinetic and Relaxation Properties of Mn(II) Complexes Formed With Some Trisubstituted 12-Membered Macrocyclic Ligands

During the past few years increasing attention has been devoted to Mn(II) complexes as possible substitutes for Gd(III) complexes as contrast agents in MRI. Equilibrium (log KMnL or pMn value), kinetic parameters (rates and half-lives of dissociation) and relaxivity of the Mn(II) complexes formed with 12-membered macrocyclic ligands were studied. The ligands were selected in a way to gain information on how the ligand rigidity, the nature of the donor atoms in the macrocycle (pyridine N, amine N, and etheric O atom), the nature of the pendant arms (carboxylates, phosphonates, primary, secondary and tertiary amides) affect the physicochemical parameters of the Mn(II) complexes. As expected, decreasing the denticity of DOTA (to afford DO3A) resulted in a drop in the stability and inertness of [Mn(DO3A)]- compared to [Mn(DOTA)]2-. This decrease can be compensated partially by incorporating the fourth nitrogen atom into a pyridine ring (e.g., PCTA) or by replacement with an etheric oxygen atom (ODO3A). Moreover, the substitution of primary amides for acetates resulted in a noticeable drop in the stability constant (PC3AMH), but it increased as the primary amides (PC3AMH) were replaced by secondary (PC3AMGly) or tertiary amide (PC3AMPip) pendants. The inertness of the Mn(II) complexes behaved alike as the rates of acid catalyzed dissociation increased going from DOTA (k1 = 0.040 M-1s-1) to DO3A (k1 = 0.45 M-1s-1). However, the rates of acid catalyzed dissociation decreased from 0.112 M-1s-1 observed for the anionic Mn(II) complex of PCTA to 0.0107 M-1s-1 and 0.00458 M-1s-1 for the cationic Mn(II) complexes of PC3AMH and PC3AMPip ligands, respectively. In spite of its lower denticity (as compared to DOTA) the sterically more hindered amide complex ([Mn(PC3AMPip)]2+) displays surprisingly high conditional stability (pMn = 8.86 vs. pMn = 9.74 for [Mn(PCTA)]-) and excellent kinetic inertness. The substitution of phosphonates for the acetate pendant arms (DOTP and DO3P), however, resulted in a noticeable drop in the conditional stability as well as dissociation kinetic parameters of the corresponding Mn(II) complexes ([Mn(DOTP)]6- and [Mn(DO3P)]4-) underlining that the phosphonate pedant should not be considered as a suitable building block for further ligand design while the tertiary amide moiety will likely have some implications in this respect in the future.

A highly stable l-alanine-based mono(aquated) Mn(ii) complex as a T1-weighted MRI contrast agent

The synthesized lithium (S)-6,6'-(1-carboxyethylazanediyl)bis(methylene)dipicolinate (Li₃cbda) is a new chiral, alanine-based ligand bearing two picolinate functionalities. The trianionic form of the ligand [(cbda)^3-] constitutes a seven-coordinate, water-soluble, pentagonal bipyramidal Mn(ii) complex (1). The structural analysis reveals the presence of a water coordinating site in the complex. The complex is thermodynamically very stable, and the stability is not affected by the presence of physiological anions (HCO₃^-, PO₄^3-, and F^-). The pH of the medium exerts a small effect on the stability of the complex. The r₁ relaxivity of 3.02 mM^-1 s^-1 is exhibited by the complex at 1.41 T, pH ∼7.4, and 25 °C. Phantom images obtained via a clinical MRI BRIVO MR355 system established concentration-dependent signal enhancement by the complex. The cytotoxicity test confirmed complex 1 as a biocompatible potential T₁-weighted MRI contrast agent.

Developing the family of picolinate ligands for Mn2+ complexation

We have reported here a series of ligands containing pentadentate 6,6'-(azanediylbis(methylene))dipicolinic acid units that differ in the substituent present at the amine nitrogen atom (acetate: H₃DPAAA; phenyl: H₂DPAPhA; dodecyl: H₂DPAC12A; 4-hexylphenyl: H₂DPAC6PhA). The protonation constants of the hexadentate DPAAA^3- and pentadentate DPAPhA^2- ligands and the stability constants of their Mn²⁺ complexes were determined using pH-potentiometry (25 °C, 0.15 M NaCl). The mono-hydrated [Mn(DPAAA)]^- complex (log K_MnL = 13.19(5)) was found to be considerably more stable than the bis-hydrated [Mn(DPAPhA)] analogue (log K_MnL = 9.55(1)). A detailed ¹H and ¹⁷O NMR relaxometric study was carried out to determine the parameters that govern the proton relaxivities of these complexes. The [Mn(DPAC12A)] complex, which contains a dodecyl lipophilic chain, forms micelles in solution characterized by a critical micellar concentration (cmc) of 96(9) μM. The lipophilic [Mn(DPAC6PhA)] and [Mn(DPAC12A)] derivatives form rather strong adducts with Human Serum Albumin (HSA) with association constants of 7.1 ± 0.1 × 10³ and 1.3 ± 0.4 × 10⁵ M^-1, respectively. The X-ray structure of the complex {K(H₂O)₄}{[Mn(DPAAA)(H₂O)]}₂ shows that the Mn²⁺ ion in [Mn(DPAAA)]^- is coordinated to the six donor atoms of the ligand, a coordinated water molecule completing the pentagonal bipyramidal coordination environment.

A New Bis(aquated) High Relaxivity Mn(II) Complex as an Alternative to Gd(III)-Based MRI Contrast Agent

Disclosed here are a piperazine, a pyridine, and two carboxylate groups containing pentadentate ligand H₂pmpa and its corresponding water-soluble Mn(II) complex (1). DFT-based structural optimization implied that the complex had pentagonal bipyramidal geometry where the axial positions were occupied by two water molecules, and the equatorial plane was constituted by the ligand ON₃O donor set. Thus, a bis(aquated) disc-like Mn(II) complex has been synthesized. The complex showed higher stability compared with Mn(II)-EDTA complex [log K_MnL = 14.29(3)] and showed a very high r₁ relaxivity value of 5.88 mM^-1 s^-1 at 1.41 T, 25 °C, and pH = 7.4. The relaxivity value remained almost unaffected by the pH of the medium in the range of 6-10. Although the presence of 200 equiv of fluoride and bicarbonate anions did not affect the relaxivity value appreciably, an increase in the value was noticed in the presence of phosphate anion due to slow tumbling of the complex. Cell viability measurements, as well as phantom MR images using clinical MRI imager, consolidated the possible candidature of complex 1 as a positive contrast agent.

Physico-chemical properties of MnII complexes formed with cis- and trans-DO2A: thermodynamic, electrochemical and kinetic studies

SYNOPSIS

Mn^II complexes formed with cis- and trans-DO2A (DO2A=1,4,7,10-tetraazacyclododecane-1,4 (or 1,7) -diacetic acid) chelators were investigated by pH-potentiometry, ¹H relaxometry, UV-vis spectrophotometry and cyclic voltammetry. The physico-chemical characteristics of Mn^II complexes of these structure isomers do not differ dramatically, however the cis-DO2A platform has better potential for further development. Manganese (Mn^II) is a promising alternative to gadolinium (Gd^III) as a magnetic resonance imaging (MRI) agent. Unlike gadolinium, this biogenic metal might be better tolerated by the body, reducing the risk of toxicity associated with dissociation of the complex. Herein we report detailed equilibrium and kinetic studies performed with Mn^II complexes of 1,4,7,10-tetraazacyclododecane-1,4-diacetic acid (1,4-DO2A or cis-DO2A) and 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (1,7-DO2A or trans-DO2A). The protonation constants of the ligands as well as stability constants of their Mn^II complexes have been determined by pH-potentiometry. The stability constants of [Mn(cis-DO2A)] are slightly higher than those of [Mn(trans-DO2A)] (log K_MnL=15.68 and 15.22, respectively). Cyclic voltammetric (CV) experiments performed on [Mn(cis-DO2A)] and [Mn(trans-DO2A)] revealed quasireversible systems with a half-wave potential of +636 and +705mV versus Ag/AgCl, respectively. These values indicate that the Mn^II ion in these complexes is more stabilized against the oxidation than in [Mn(EDTA)]^2-. The kinetic inertness of the complexes has been studied in transmetallation reactions with Cu^II or Zn^II ions. Kinetic measurements indicate that both Mn^II complexes primarily undergo acid catalyzed dissociation and positions of the acetate pendant arms do not influence kinetic inertness. The inertness of these complexes is comparable to that of [Mn(NOTA)]^- (NOTA=1,4,7-triazacyclononane-1,4,7-triacetic acid) and about twenty times lower than that of [Mn(DOTA)]^2- (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). In conclusion, [Mn(cis-DO2A)] displays some very interesting features (thermodynamic and redox stability as well as kinetic inertness) which makes this complex a promising platform for the development of more efficient Mn^II complexes as alternatives to Gd-based MRI agents.