Mn-Based MRI Contrast Agents: An Overview

MRI contrast agents are required in the clinic to detect some pathologies, such as cancers. Nevertheless, at the moment, only small extracellular and non-specific gadolinium complexes are available for clinicians. Moreover, safety issues have recently emerged concerning the use of gadolinium complexes; hence, alternatives are urgently needed. Manganese-based MRI contrast agents could be one of these alternatives and increasing numbers of studies are available in the literature. This review aims at synthesizing all the research, from small Mn complexes to nanoparticular agents, including theranostic agents, to highlight all the efforts already made by the scientific community to obtain highly efficient agents but also evidence of the weaknesses of the developed systems.

Synthesis of novel DOTA-/AAZTA-based bifunctional chelators: Solution thermodynamics, peptidomimetic conjugation, and radiopharmaceutical evaluation

Bifunctional chelators (BFCs), which link metallic radionuclide and a targeting vector, are some of the most crucial components of metallic radionuclide-based radiopharmaceuticals for positron-emission computed tomography (PET) imaging. In this study, we designed and synthesized two versatile BFCs, p-NCS-Ph-DE4TA and p-NCS-Ph-AAZ4TA, and we conjugated them with a prostate-specific membrane antigen (PSMA) inhibitor. These two chelators showed high affinity for Ga (III) according to a study of the thermodynamics and kinetics and DFT calculations. The labeled PSMA targeted probes, [68Ga]Ga-p-NCS-Ph-DE4TA-PSMA and [68Ga]Ga-p-NCS-Ph-AAZ4TA-PSMA, maintained excellent stability in vitro, and they exhibited high specific activity when binding to PSMA. A PET/CT imaging study in mice bearing SMMC-7721 hepatocellular carcinoma xenografts demonstrated clear visualization of tumors with a high tumor uptake and low background level, indicating the excellent performance in vivo and specific activity when targeting hepatocellular carcinomas. In summary, p-NCS-Ph-DE4TA and p-NCS-Ph-AAZ4TA are leading developmental candidates for PET imaging for tumor diagnosis.

Current advancement in the development of manganese complexes as magnetic resonance imaging probes

Emerging non-invasive molecular imaging modalities can detect a pathophysiological state at the molecular level before any anatomic changes are observed. Magnetic resonance imaging (MRI) is preferred over other nuclear imaging techniques owing to its radiation-free approach. Conventionally, most MRI contrast agents employed predominantly involve lanthanide metal: Gadolinium (Gd) until the discovery of associated severe nephrogenic toxicity issues. This limitation led a way to the development of manganese-based contrast agents which offer similar positive contrast enhancement capability. A vast quantity of experimental data has been accumulated over the last decade to define the physicochemical characteristics of manganese chelates with various ligand scaffolds. One can now observe how the ligand configurations, rigidity, and donor-acceptor characteristics impact the stability of the complex. This review covers the current trends in the development of manganese-based MRI contrast agents, the mechanisms they are based on and design considerations for newer manganese-based contrast agents with higher diagnostic strength along with better safety profiles.

Molecular Parameters Promoting High Relaxivity in Cluster-Nanocarrier Magnetic Resonance Imaging Contrast Agents

We have investigated the mechanism of relaxivity for two magnetic resonance imaging contrast agents that both employ a cluster-nanocarrier design. The first system termed Mn8Fe4-coPS comprises the cluster Mn8Fe4O12(L)16(H2O)4 or Mn8Fe4 (1) (L = carboxylate) co-polymerized with polystyrene to form ∼75 nm nanobeads. The second system termed Mn3Bpy-PAm used the cluster Mn3(O2CCH3)6(Bpy)2 or Mn3Bpy (2) where Bpy = 2,2'-bipyridine, entrapped in ∼180 nm polyacrylamide nanobeads. Here, we investigate the rate of water exchange of the two clusters, and corresponding cluster-nanocarriers, in order to elucidate the mechanism of relaxivity in the cluster-nanocarrier. Swift-Connick analysis of O-17 NMR was used to determine the water exchange rates of the clusters and cluster-nanocarriers. We found distinct differences in the water exchange rate between Mn8Fe4 and Mn8Fe4-coPS, and we utilized these differences to elucidate the nanobead structure. Using the transverse relaxivity from O-17 NMR line widths, we were able to determine the hydration state of the Mn3Bpy (2) cluster as well as Mn3Bpy-PAm. Using these hydration states in the Swift-Connick analysis of O-17 NMR, we found the water exchange rate to be extremely close in value for the cluster Mn3Bpy and cluster-nanocarrier Mn3Bpy-PAm.

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.

AAZTA-Derived Chelators for the Design of Innovative Radiopharmaceuticals with Theranostic Applications

With the development of 68Ga and 177Lu radiochemistry, theranostic approaches in modern nuclear medicine enabling patient-centered personalized medicine applications have been growing in the last decade. In conjunction with the search for new relevant molecular targets, the design of innovative chelating agents to easily form stable complexes with various radiometals for theranostic applications has gained evident momentum. Initially conceived for magnetic resonance imaging applications, the chelating agent AAZTA features a mesocyclic seven-membered diazepane ring, conferring some of the properties of both acyclic and macrocyclic chelating agents. Described in the early 2000s, AAZTA and its derivatives exhibited interesting properties once complexed with metals and radiometals, combining a fast kinetic of formation with a slow kinetic of dissociation. Importantly, the extremely short coordination reaction times allowed by AAZTA derivatives were particularly suitable for short half-life radioelements (i.e., 68Ga). In view of these particular characteristics, the scope of this review is to provide a survey on the design, synthesis, and applications in the nuclear medicine/radiopharmacy field of AAZTA-derived chelators.

Rigid versions of PDTA4- incorporating a 1,3-diaminocyclobutyl spacer for Mn2+ complexation: stability, water exchange dynamics and relaxivity

Rigid derivatives of the acyclic ligand PDTA4- (H₄PDTA = propylenediamine-N,N,N',N'-tetraacetic acid) were prepared by functionalization of a 1,3-diaminocyclobutyl spacer. The new ligands contain either four acetate groups attached to the central scaffold (H4L1) or incorporate pyridyl (H2L2) or propylamide (H2L3) units replacing two of the carboxylate groups. The ligand protonation constants and the stability constants of their Mn²⁺ complexes were determined using potentiometric and spectrophotometric titrations. The stability of the [Mn(L1)]^2- complex was found to be significantly higher than that of the flexible [Mn(PDTA)]^2- derivative (log K_MnL = 10.78 and 10.01, respectively). A detailed study of the ¹H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles and ¹⁷O NMR measurements evidence that the [Mn(L1)]^2- and [Mn(L2)] complexes display a hydration equilibrium in solution involving a seven-coordinate species with an inner-sphere water molecule and a six-coordinate species that lacks a coordinated water molecule. As a result the ¹H relaxivities of these complexes are somewhat lower than that of [Mn(EDTA)]^2- and related systems. The introduction of propylamide groups in [Mn(L3)] shifts the hydration equilibrium to the seven-coordinate species, which results in a ¹H relaxivity (r_1p = 3.7 mM^-1 s^-1 at 22 MHz and 25 °C) exceeding that of [Mn(EDTA)]^2- (r_1p = 3.3 mM^-1 s^-1 at 22 MHz and 25 °C). The parameters that control the relaxivities in this family of complexes were determined by simultaneous fitting of the experimental ¹H NMRD and ¹⁷O NMR data (transverse relaxation rates and chemical shifts), with the aid of computational studies performed at the DFT and CASSCF/NEVPT2 levels. These studies provide detailed insight of the parameters that control the efficiency of these relaxation agents at the molecular level.

Selective functionalization of 6-amino-6-methyl-1,4-perhydrodiazepine for the synthesis of a library of polydentate chelators

Polydentate chelators are an important part of an imaging probe, which consists of an agent that usually produces signals for imaging purposes connected to a targeting moiety. The goal of this study was to set up a generic protocol to prepare a library of polydentate ligands having a 6-amino-6-methyl-1,4-perhydrodiazepine (AMPED) core and able to chelate metal ions of interest for various diagnostic imaging techniques, including Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET) and Single-Photon Emission Computed Tomography (SPECT). These ions, among which we can include Mn(ii), Cu(ii), Al(iii) or Ga(iii), require penta- or hexa-dentate chelators for this purpose, and the AMPED scaffold has considerable potential to support various pendant arms for coordination of such ions. AMPED already has three amino nitrogen donors; thus, only two or three additional arms should be introduced to obtain penta- or hexa-dentate systems. This condition implies that symmetrical or asymmetrical structures have to be developed, depending on the functionalization of cyclic and exocyclic amines. Starting from easily available materials, we have designed a convenient protocol for the preparation of multiple AMPED-based ligands endowed with different characteristics, several of which were synthesized as examples.

Mild and efficient 64Cu labeling of perhydro-1, 4-diazepine derivatives for potential use with large peptides, proteins and antibodies

DATA (6-Amino-1,4-diazapine-triacetate) and AAZTA (6-Amino-1,4-diazapine-tetracetate) chelators represent a novel approach representing hybrid-chelates: possessing significant cyclic and acyclic character. It is believed that flexibility of the acyclic part facilitates rapid complexation, whilst the preorganized cyclic part minimizes the energy barrier to complexation and inhibits decomplexation processes. So far, these chelators have been used exclusively with 44Sc and 68Ga only. Recent results with natCu predict high stabilities for Cu-AAZTA, yet no radioactive labeling of AAZTA or DATA with 64Cu or any additional radioactive isotope has been reported. We present the one pot synthesis of the bifunctional derivatives AAZTA5OMe and DATA5mOMe and their labeling with 64Cu. In addition, in vitro stability of the respective complexes are presented.

Mn(II) compounds as an alternative to Gd-based MRI probes

Mn(II) has several favorable physicochemical characteristics and a good toxicity profile, which makes it a viable alternative to the Gd(III)-based MRI contrast agents currently used in clinics. Although many studies have been undertaken in the last 10 years, this is a field of investigation still in rapid and continuous development. This review aims to critically discuss the chemical and magnetic properties of Mn(II) compounds relevant as MRI probes, both small complexes and nanosystems containing a large number of metal centers, the possible approaches for optimizing their efficiency by understanding the role of various molecular parameters that control the relaxation processes, and the most important issues related to stability and kinetic inertness.

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.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

A pentadentate member of the picolinate family for Mn(ii) complexation and an amphiphilic derivative

We report a pentadentate ligand containing a 2,2'-azanediyldiacetic acid moiety functionalized with a picolinate group at the nitrogen atom (H3paada), as well as a lipophylic derivative functionalized with a dodecyloxy group at position 4 of the pyridyl ring (H3C12Opaada). The protonation constants of the paada3- ligand and the stability constant of the Mn(ii) complex were determined using a combination of potentiometric and spectrophotometric titrations (25 °C, 0.15 M NaCl). A detailed relaxometric characterisation was accomplished by recording 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles and 17O chemical shifts and relaxation rates. These studies provide detailed information on the microscopic parameters that control their efficiency as relaxation agents in vitro. For the sake of completeness and to facilitate comparison, we also characterised the related [Mn(nta)]- complex (nta = nitrilotriacetate). Both the [Mn(paada)]- and [Mn(nta)]- complexes turned out to contain two inner-sphere water molecules in aqueous solution. The exchange rate of these coordinated water molecules was slower in [Mn(paada)]- (k298ex = 90 × 107 s-1) than in [Mn(nta)]- (k298ex = 280 × 107 s-1). The complexes were also characterised using both DFT (TPSSh/def2-TZVP) and ab initio CAS(5,5) calculations. The lipophylic [Mn(C12Opaada)]- complex forms micelles in solution characterised by a critical micellar concentration (cmc) of 0.31 ± 0.01 mM. This complex also forms a rather strong adduct with Bovine Serum Albumin (BSA) with an association constant of 5.5 × 104 M-1 at 25 °C. The enthalpy and entropy changes obtained for the formation of the adduct indicate that the binding event is driven by hydrophobic interactions.

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.

Optimising the relaxivities of Mn2+ complexes by targeting human serum albumin (HSA)

We report two novel macrocyclic ligands based on the 1,4-DO2AM platform (1,4-DO2AM = 2,2'-(1,4,7,10-tetraazacyclododecane-1,4-diyl)diacetamide) and containing two benzyl groups attached either to the nitrogen atoms of the macrocyclic unit (1,4-BzDO2AM) or to the amide pendant arms (1,4-DO2AMBz). The protonation constants of the ligands and the stability constants of their Mn²⁺ complexes were determined using pH potentiometry. The introduction of benzyl groups results in a slight decrease of the stability constants of the Mn²⁺ complexes and a slight increase of their acid-catalysed dissociation reactions. A detailed relaxometric characterisation of the complexes using nuclear magnetic dispersion relaxation (NMRD) and ¹⁷O NMR studies indicated that the increase in molecular weight associated with the presence of benzyl groups results in a remarkable increase of proton relaxivities r_1p, which take values of 3.8, 3.5 and 2.5 mM^-1 s^-1 for [Mn(1,4-BzDO2AM)]²⁺, [Mn(1,4-DO2AMBz)]²⁺ and [Mn(1,4-DO2AM)]²⁺ (at 25 °C and 20 MHz). The [Mn(1,4-BzDO2AM)]²⁺ and [Mn(1,4-DO2AMBz)]²⁺ complexes form relatively strong adducts with Human Serum Albumin (HSA) with association constants of (3.9 ± 0.6) × 10³ and (2.0 ± 0.3) × 10³ M^-1, respectively. The interaction with the protein slows down the rotational tumbling of the complex in solution, which results in adducts endowed with remarkably high proton relaxivities (r_1p^b = 18.5 ± 0.7 and 27.4 ± 1.4 mM^-1 s^-1 for [Mn(1,4-BzDO2AM)]²⁺ and [Mn(1,4-DO2AMBz)]²⁺, respectively).

Chemistry and engineering of cyclodextrins for molecular imaging

Cyclodextrins (CDs) are naturally occurring cyclic oligosaccharides bearing a basket-shaped topology with an "inner-outer" amphiphilic character. The abundance of hydroxyl groups enables CDs to be functionalized with multiple targeting ligands and imaging elements. The imaging time, and the payload of different imaging elements, can be tuned by taking advantage of the commercial availability of CDs with different sizes of the cavity. This review aims to offer an outlook of the chemistry and engineering of CDs for the development of molecular probes. Complexation thermodynamics of CDs, and the corresponding implications for probe design, are also presented with examples demonstrating the structural and physiochemical roles played by CDs in the full ambit of molecular imaging. We hope that this review not only offers a synopsis of the current development of CD-based molecular probes, but can also facilitate translation of the incremental advancements from the laboratory to real biomedical applications by illuminating opportunities and challenges for future research.

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.

Current advances in ligand design for inorganic positron emission tomography tracers 68Ga, 64Cu, 89Zr and 44Sc

A key part of the development of metal based Positron Emission Tomography probes is the chelation of the radiometal. In this review the recent developments in the chelation of four positron emitting radiometals, ⁶⁸Ga, ⁶⁴Cu, ⁸⁹Zr and ⁴⁴Sc, are explored. The factors that effect the chelation of each radio metal and the ideal ligand system will be discussed with regards to high in vivo stability, complexation conditions, conjugation to targeting motifs and complexation kinetics. A series of cyclic, cross-bridged and acyclic ligands will be discussed, such as CP256 which forms stable complexes with ⁶⁸Ga under mild conditions and PCB-TE2A which has been shown to form a highly stable complex with ⁶⁴Cu. ⁸⁹Zr and ⁴⁴Sc have seen significant development in recent years with a number of chelates being applied to each metal - eight coordinate di-macrocyclic terephthalamide ligands were found to rapidly produce more stable complexes with ⁸⁹Zr than the widely used DFO.

Supramolecular assemblies based on amphiphilic Mn2+-complexes as high relaxivity MRI probes

The Mn²⁺ complexes of amphiphilic derivatives of EDTA and 1,4-DO2A ligands show a strong increase in relaxivity upon micellar aggregation and human serum albumin binding.

Equilibrium, Kinetic and Structural Properties of Gallium(III) and Some Divalent Metal Complexes Formed with the New DATAm and DATA5m Ligands

The development of ⁶⁸ Ge/⁶⁸ Ga generators has made the positron-emitting ⁶⁸ Ga isotope widely accessible and raised interest in new chelate complexes of Ga³⁺ . The hexadentate 1,4-di(acetate)-6-methyl[amino(methyl)acetate]perhydro-1,4-diazepane (DATA^m ) ligand and its bifunctional analogue, 1,4-di(acetate)-6-pentanoic acid[amino(methyl)acetate]perhydro-1,4-diazepane (DATA^5m ), rapidly form complexes with ⁶⁸ Ga in high radiochemical yield. The stability constants of DATA^m and DATA^5m complexes formed with Ga³⁺ , Zn²⁺ , Cu²⁺ , Mn²⁺ and Ca²⁺ have been determined by using pH potentiometry, spectrophotometry (Cu²⁺ ) and ¹ H and ⁷¹ Ga NMR spectroscopy (Ga³⁺ ). The stability constants of Ga(DATA^m ) and Ga(DATA^5m ) complexes are slightly higher than those of Ga(AAZTA). The species distribution calculations indicated the predominance of Ga(L)OH mixed-hydroxo complexes at physiological pH. The ¹ H and ⁷¹ Ga NMR spectroscopy studies provided information about the coordinated functional groups of ligands and on the kinetics of exchange between the Ga(L) and Ga(L)OH complexes. The transmetalation reactions between the Ga(L) complexes and Cu²⁺ citrate (6<pH<8.5) occur through both spontaneous and OH^- -assisted dissociation of the Ga(L)OH species. At pH 7.4 and 25 °C, the half-lives of the dissociation of Ga(DATA^m ), Ga(DATA^5m ) and Ga(AAZTA) were 11, 44 and 24 h, respectively. Similar half-lives have been obtained for the ligand-exchange reactions between the Ga(L)OH complexes and transferrin. The equilibrium and kinetic data indicate that the Ga(DATA^5m ) complex is a good ⁶⁸ Ga-based radiodiagnostic candidate.

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.

Mono-, bi-, and trinuclear bis-hydrated Mn(2+) complexes as potential MRI contrast agents

We report a series of ligands containing pentadentate 6,6′-((methylazanediyl)bis(methylene))dipicolinic acid binding units that form mono- (H2dpama), di- (mX(H2dpama)2), and trinuclear (mX(H2dpama)3) complexes with Mn2+ containing two coordinated water molecules per metal ion, which results in pentagonal bipyramidal coordination around the metal ions. In contrast, the hexadentate ligand 6,6′-((ethane-1,2-diylbis(azanediyl))bis(methylene))dipicolinic acid (H2bcpe) forms a complex with distorted octahedral coordination around Mn2+ that lacks coordinated water molecules. The protonation constants of the ligands and the stability constants of the Mn2+, Cu2+, and Zn2+ complexes were determined using potentiometric and spectrophotometric titrations in 0.15 M NaCl. The pentadentate dpama2– ligand and the di- and trinucleating mX(dpama)24– and mX(dpama)36– ligands provide metal complexes with stabilities that are very similar to that of the complex with the hexadentate ligand bcpe2–, with log β101 values in the range 10.1–11.6. Cyclic voltammetry experiments on aqueous solutions of the [Mn(bcpe)] complex reveal a quasireversible system with a half-wave potential of +595 mV versus Ag/AgCl. However, [Mn(dpama)] did not suffer oxidation in the range 0.0–1.0 V, revealing a higher resistance toward oxidation. A detailed 1H NMRD and 17O NMR study provided insight into the parameters that govern the relaxivity for these systems. The exchange rate of the coordinated water molecules in [Mn(dpama)] is relatively fast, kex298 = (3.06 ± 0.16) × 108 s–1. The trinuclear [mX(Mn(dpama)(H2O)2)3] complex was found to bind human serum albumin with an association constant of 1286 ± 55 M–1 and a relaxivity of the adduct of 45.2 ± 0.6 mM–1 s–1 at 310 K and 20 MHz.

Structure-redox-relaxivity relationships for redox responsive manganese-based magnetic resonance imaging probes

A library of 10 Mn-containing complexes capable of switching reversibly between the Mn(II) and Mn(III) oxidation states was prepared and evaluated for potential usage as MRI reporters of tissue redox activity. We synthesized N-(2-hydroxybenzyl)-N,N',N'-ethylenediaminetriacetic acid (HBET) and N-(2-hydroxybenzyl-N,N',N'-trans-1,2-cyclohexylenediaminetriacetic acid (CyHBET) ligands functionalized (-H, -OMe, -NO2) at the 5-position of the aromatic ring. The Mn(II) complexes of all ligands and the Mn(III) complexes of the 5-H and 5-NO2 functionalized ligands were synthesized and isolated, but the Mn(III) complexes with the 5-OMe functionalized ligands were unstable. (1)H relaxivity of the 10 isolable complexes was measured at pH 7.4 and 37 °C, 1.4 T. Thermodynamic stability, pH-dependent complex speciation, hydration state, water exchange kinetics of the Mn(II) complexes, and pseudo-first order reduction kinetics of the Mn(III) complexes were studied using a combination of pH-potentiometry, UV-vis spectroscopy, and (1)H and (17)O NMR measurements. The effects of ligand structural and electronic modifications on the Mn(II/III) redox couple were studied by cyclic voltammetry. The Mn(II) complexes are potent relaxation agents as compared to the corresponding Mn(III) species with [Mn(II)(CyHBET)(H2O)](2-) exhibiting a 7.5-fold higher relaxivity (3.3 mM(-1) s(-1)) than the oxidized form (0.4 mM(-1) s(-1)). At pH 7.4, Mn(II) exists as a mixture of fully deprotonated (ML) and monoprotonated (HML) complexes and Mn(II) complex stability decreases as the ligands become more electron-releasing (pMn for 10 μM [Mn(II)(CyHBET-R')(H2O)](2-) decreases from 7.6 to 6.2 as R' goes from -NO2 to -OMe, respectively). HML speciation increases as the electron-releasing nature of the phenolato-O donor increases. The presence of a water coligand is maintained upon conversion from HML to ML, but the water exchange rate of ML is faster by up to 2 orders of magnitude (k(ex)(310) for H[Mn(II)(CyHBET)(H2O)](-) and [Mn(II)(CyHBET)(H2O)](2-) are 1.2 × 10(8) and 1.0 × 10(10) s(-1), respectively). The Mn(II/III) redox potential can be tuned over a range of 0.30 V (E(1/2) = 0.27-0.57 V) through electronic modifications to the 5-substituent of the aromatic ligand component. However, care must be taken in tuning the ligand electronics to avoid Mn(III)-ligand autoredox. Taken together, these results serve to establish criteria for optimizing Mn(III) versus Mn(II) relaxivity differentials, complex stability, and Mn(II/III) redox potential.

Picolinate-containing macrocyclic Mn2+ complexes as potential MRI contrast agents

We report the synthesis of the ligand Hnompa (6-((1,4,7-triazacyclononan-1-yl)methyl)picolinic acid) and a detailed characterization of the Mn(2+) complexes formed by this ligand and the related ligands Hdompa (6-((1,4,7,10-tetraazacyclododecan-1-yl)methyl)picolinic acid) and Htempa (6-((1,4,8,11-tetraazacyclotetradecan-1-yl)methyl)picolinic acid). These ligands form thermodynamically stable complexes in aqueous solution with stability constants of logKMnL = 10.28(1) (nompa), 14.48(1) (dompa), and 12.53(1) (tempa). A detailed study of the dissociation kinetics of these Mn(2+) complexes indicates that the decomplexation reaction at about neutral pH occurs mainly following a spontaneous dissociation mechanism. The X-ray structure of [Mn2(nompa)2(H2O)2](ClO4)2 shows that the Mn(2+) ion is seven-coordinate in the solid state, being directly bound to five donor atoms of the ligand, the oxygen atom of a coordinated water molecule and an oxygen atom of a neighboring nompa(-) ligand acting as a bridging bidentate carboxylate group (μ-η(1)-carboxylate). Nuclear magnetic relaxation dispersion ((1)H NMRD) profiles and (17)O NMR chemical shifts and transverse relaxation rates of aqueous solutions of [Mn(nompa)](+) indicate that the Mn(2+) ion is six-coordinate in solution by the pentadentate ligand and one inner-sphere water molecule. The analysis of the (1)H NMRD and (17)O NMR data provides a very high water exchange rate of the inner-sphere water molecule (kex(298) = 2.8 × 10(9) s(-1)) and an unusually high value of the (17)O hyperfine coupling constant of the coordinated water molecule (AO/ℏ = 73.3 ± 0.6 rad s(-1)). DFT calculations performed on the [Mn(nompa)(H2O)](+)·2H2O system (TPSSh model) provide a AO/ℏ value in excellent agreement with the one obtained experimentally.

Direct measurement of the Mn(II) hydration state in metal complexes and metalloproteins through 17O NMR line widths

Here we describe a simple method to estimate the inner-sphere hydration state of the Mn(II) ion in coordination complexes and metalloproteins. The line width of bulk H2(17)O is measured in the presence and absence of Mn(II) as a function of temperature, and transverse (17)O relaxivities are calculated. It is demonstrated that the maximum (17)O relaxivity is directly proportional to the number of inner-sphere water ligands (q). Using a combination of literature data and experimental data for 12 Mn(II) complexes, we show that this method provides accurate estimates of q with an uncertainty of ±0.2 water molecules. The method can be implemented on commercial NMR spectrometers working at fields of 7 T and higher. The hydration number can be obtained for micromolar Mn(II) concentrations. We show that the technique can be extended to metalloproteins or complex:protein interactions. For example, Mn(II) binds to the multimetal binding site A on human serum albumin with two inner-sphere water ligands that undergo rapid exchange (1.06 × 10(8) s(-1) at 37 °C). The possibility of extending this technique to other metal ions such as Gd(III) is discussed.

Hyperfine coupling constants on inner-sphere water molecules of a triazacyclononane-based Mn(II) complex and related systems relevant as MRI contrast agents

We report the synthesis of the ligand H2MeNO2A (1,4-bis(carboxymethyl)-7-methyl-1,4,7-triazacyclononane) and a detailed experimental and computational study of the hyperfine coupling constants (HFCCs) on the inner-sphere water molecules of [Mn(MeNO2A)] and related Mn(2+) complexes relevant as potential contrast agents in magnetic resonance imaging (MRI). Nuclear magnetic relaxation dispersion (NMRD) profiles, (17)O NMR chemical shifts, and transverse relaxation rates of aqueous solutions of [Mn(MeNO2A)] were recorded to determine the parameters governing the relaxivity in this complex and the (17)O and (1)H HFCCs. DFT calculations (TPSSh model) performed in aqueous solution (PCM model) on the [Mn(MeNO2A)(H2O)]·xH2O and [Mn(EDTA)(H2O)](2-)·xH2O (x = 0-4) systems were used to determine theoretically the (17)O and (1)H HFCCs responsible for the (17)O NMR chemical shifts and the scalar contributions to (17)O and (1)H NMR relaxation rates. The use of a mixed cluster/continuum approach with the explicit inclusion of a few second-sphere water molecules is critical for an accurate calculation of HFCCs of coordinated water molecules. The impact of complex dynamics on the calculated HFCCs was evaluated with the use of molecular dynamics simulations within the atom-centered density matrix propagation (ADMP) approach. The (17)O and (1)H HFCCs calculated for these complexes and related systems show an excellent agreement with the experimental data. Both the (1)H and (17)O HFCCs (A(iso) values) are dominated by the spin delocalization mechanism. The A(iso) values are significantly affected by the distance between the oxygen atom of the coordinated water molecule and the Mn(2+) ion, as well as by the orientation of the water molecule plane with respect to the Mn-O vector.

1H and 17O NMR relaxometric and computational study on macrocyclic Mn(II) complexes

Herein we report a detailed 1H and 17O relaxometric investigation of Mn(II) complexes with cyclen-based ligands such as 2-(1,4,7,10-tetraazacyclododecan-1-yl)acetic acid (DO1A), 2,2'-(1,4,7,10-tetraazacyclododecane-1,4-diyl)diacetic acid (1,4-DO2A), 2,2'-(1,4,7,10-tetraazacyclododecane-1,7-diyl)diacetic acid (1,7-DO2A), and 2,2',2"-(1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (DO3A). The Mn(II) complex with the heptadentate ligand DO3A does not have inner sphere water molecules (q = 0), and therefore, the metal ion is most likely seven-coordinate. The hexadentate DO2A ligand has two isomeric forms: 1,7-DO2A and 1,4-DO2A. The Mn(II) complex with 1,7-DO2A is predominantly six-coordinate (q = 0). In aqueous solutions of [Mn(1,4-DO2A)], a species with one coordinated water molecule (q = 1) prevails largely, whereas a q = 0 form represents only about 10% of the overall population. The Mn(II) complex of the pentadentate ligand DO1A also contains a coordinated water molecule. DFT calculations (B3LYP model) are used to obtain information about the structure of this family of closely related complexes in solution, as well as to determine theoretically the 17O and 1H hyperfine coupling constants responsible for the scalar contribution to 17O and 1H NMR relaxation rates and 17O NMR chemical shifts. These calculations provide 17O A/ħ values of ca. 40 × 10(6) rad s(-1), in good agreement with experimental data. The [Mn(1,4-DO2A)(H2O)] complex is endowed with a relatively fast water exchange rate (k(ex)298 = 11.3 × 10(8) s(-1)) in comparison to the [Mn(EDTA)(H2O)]2- analogue (k(ex)298 = 4.7 × 10(8) s(-1)), but about 5 times lower than that of the [Mn(DO1A)(H2O)]+ complex (k(ex)298 = 60 × 10(8) s(-1)). The water exchange rate measured for the latter complex represents the highest water exchange rate ever measured for a Mn(II) complex.

Mn2+ complexes of 1-oxa-4,7-diazacyclononane based ligands with acetic, phosphonic and phosphinic acid pendant arms: stability and relaxation studies

A new class of macrocyclic ligands based on 1-oxa-4,7-diazacyclononane was synthesized and their Mn(2+) complexes were investigated with respect to stability and relaxation properties. Each ligand has two pendant arms involving carboxylic (H(2)L(1)--1-oxa-4,7-diazacyclononane-4,7-diacetic acid), phosphonic (H(4)L(2)--1-oxa-4,7-diazacyclononane-4,7-bis(methylenephosphonic acid)), phosphinic (H(2)L(3)--1-oxa-4,7-diazacyclononane-4,7-bis(methylenephosphinic acid)) or phenylphosphinic (H(2)L(4)--1-oxa-4,7-diazacyclononane-4,7-bis[methylene(phenyl)phosphinic acid]) acid moieties. H(2)L(3) and H(2)L(4) were synthesized for the first time. The crystal structure of the Mn(2+) complex with H(2)L(4) confirmed a coordination number of 6 for Mn(2+). The protonation constants of all ligands and the stability constants of their complexes with Mn(2+) and some biologically or biomedically relevant metal ions were determined by potentiometry. The protonation sequence of H(2)L(3) was followed by (1)H and (31)P NMR titration and the second protonation step was attributed to the second macrocyclic nitrogen atom. The potentiometric data revealed a relatively low thermodynamic stability of the Mn(2+) complexes with all ligands investigated. For H(2)L(3) and H(2)L(4), full Mn(2+) complexation cannot be achieved even with 100% ligand excess. The transmetallation of MnL(1) and MnL(2) with Zn(2+) was too fast to be followed at pH 6. Variable temperature (1)H NMRD and (17)O NMR measurements have been performed on MnL(1) and MnL(2) to provide information on water exchange and rotational dynamics. The (17)O chemical shifts indicate hydration equilibrium between mono- and bishydrated species for MnL(1), while MnL(2) is monohydrated. The water exchange is considerably faster on MnL(1) (k(ex)(298) = 1.2 × 10(9) s(-1)) than on MnL(2) (k(ex)(298) = 1.2 × 10(7) s(-1)). Small endogenous anions (phosphate, carbonate, citrate) do not replace the coordinated water in either of the complexes, but they induce their slow decomposition. All Mn(2+) complexes are stable toward air-oxidation.