H4octapa: highly stable complexation of lanthanide(III) ions and copper(II)

Synthesis of new acyclic chelators H4aPyta and H6aPyha and their complexes with Cu2+, Ga3+, Y3+, and Bi3

In this article, we present the synthesis and characterization of new acyclic pyridine-containing polyaminocarboxylate ligands H4aPyta and H6aPyha, which differ in structural rigidity and the number of chelating groups. Their abilities to form complexes with Cu2+, Ga3+, Y3+, and Bi3+ cations, as well as the stability of the complexes, were evaluated by potentiometric titration method, radiolabeling with the corresponding radionuclides, in vitro studies, mass spectrometry, and HPLC. The structures of the resulting complexes were determined using NMR spectroscopy and DFT calculations. The results obtained made it possible to evaluate the influence of the structural features of the complexes on their stability. The developed chelators H4aPyta and H6aPyha were proved to be promising for further research in the field of radiopharmaceuticals.

Bipyridil-based chelators for Gd(III) complexation: kinetic, structural and relaxation properties

In the last 20 years, research in the field of MRI (magnetic resonance imaging) contrast agents (CAs) has been intensified due to the emergence of a disease called nephrogenic systemic fibrosis (NSF). NSF has been linked to the in vivo dissociation of certain Gd(III)-based compounds applied in MRI as CAs. To prevent the dechelation of the probes after intravenous injection, the improvement of their in vivo stability is highly desired. The inertness of the Gd(III) chelates can be increased through the rigidification of the ligand structure. One of the potential ligands is (2,2',2'',2'''-(([2,2'-bipyridine]-6,6'-diylbis(methylene))bis(azanetriyl))tetraacetic acid) (H4DIPTA), which has been successfully used as a fluorescent probe for lanthanides; however, it has never been considered as a potential chelator for Gd(III) ions. In this paper, we report the thermodynamic, kinetic and structural features of the complex formed between Gd(III) and DIPTA. Since the solubility of the [Gd(DIPTA)]- chelate is very low under acidic conditions, hampering its thermodynamic characterization, we can only assume that its stability is close to that determined for the structural analogue [Gd(FENTA)]- (H4FENTA: (1,10-phenanthroline-2,9-diyl)bis(methyliminodiacetic acid)), which is similar to that determined for the agent [Gd(DTPA)]2- routinely used in clinical practice. Unfortunately, the inertness of [Gd(DIPTA)]- is significantly lower (t1/2 = 1.34 h) than that observed for [Gd(EGTA)]- and [Gd(DTPA)]2- as a result of its spontaneous dissociation pathway during dechelation. The relaxivity values of [Gd(DIPTA)]- are comparable with those of [Gd(FENTA)]- and somewhat higher than the values characterizing [Gd(DTPA)]2-. Luminescence lifetime measurements indicate the presence of one water molecule (q = 1) in the inner sphere of the complex with a relatively high water exchange rate (k298ex = 43(5) × 106 s-1). DFT calculations suggest a rigid distorted tricapped trigonal prismatic polyhedron for the Gd(III) complex. On the basis of these results, we can conclude that the bipyridine backbone is not favourable with respect to the inertness of the chelate.

Tuning aminopolycarboxylate chelators for efficient complexation of trivalent actinides

The complexation of trivalent lanthanides and minor actinides (Am3+, Cm3+, and Cf3+) by the acyclic aminopolycarboxylate chelators 6,6'-((ethane-1,2-diylbis-((carboxymethyl)azanediyl))bis-(methylene))dipicolinic acid (H4octapa) and 6,6'-((((4-(1-(2-(2-(2-hydroxyethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)pyridine-2,6-diyl)bis-(methylene))bis-((carboxymethyl)azanediyl))bis-(methylene)) dipicolinic acid (H4pypa-peg) were studied using potentiometry, spectroscopy, competitive complexation liquid-liquid extraction, and ab initio molecular dynamics simulations. Two studied reagents are strong multidentate chelators, well-suited for applications seeking radiometal coordination for in-vivo delivery and f-element isolation. The previously reported H4octapa forms a compact coordination packet, while H4pypa-peg is less sterically constrained due to the presence of central pyridine ring. The solubility of H4octapa is limited in a non-complexing high ionic strength perchlorate media. However, the introduction of a polyethylene glycol group in H4pypa-peg increased the solubility without influencing its ability to complex the lanthanides and minor actinides in solution.

Rigid H4OCTAPA derivatives as model chelators for the development of Bi(III)-based radiopharmaceuticals

Octadentate ligands containing ethyl (H₄OCTAPA), cyclohexyl (H₄CHXOCTAPA) or cyclopentyl (H₄CpOCTAPA) spacers were assessed as chelators for Bi(III)-based radiopharmaceuticals. The H₄CHXOCTAPA chelator displays excellent properties, including ^205/206Bi-nuclide radiolabelling under mild conditions, excellent stability in serum and in the presence of competing cations or H₅DTPA. The poor performance of H₄CpOCTAPA appears to be related to the stereochemical activity of the Bi(III) lone pair.

Physico-Chemical Characterization of a Highly Rigid Gd(III) Complex Formed with a Phenanthroline Derivative Ligand

The discovery of the nephrogenic systemic fibrosis (NSF) and its link with the in vivo dissociation of certain Gd(III)-based contrast agents (CAs) applied in the magnetic resonance imaging (MRI) induced a still growing research to replace the compromised agents with safer alternatives. In recent years, several ligands were designed to exploit the luminescence properties of the lanthanides, containing structurally constrained aromatic moieties, which may form rigid Gd(III) complexes. One of these ligands is (1,10-phenanthroline-2,9-diyl)bis(methyliminodiacetic acid) (H₄FENTA) designed and synthesized to sensitize Eu(III) and Tb(III) luminescence. Our results show that the conditional stability of the [Gd(FENTA)]^- chelate calculated for physiological pH (pGd = 19.7) is similar to those determined for [Gd(DTPA)]^2- (pGd = 19.4) and [Gd(DOTA)]^- (pGd = 20.1), routinely used in the clinical practice. The [Gd(FENTA)]^- complex is remarkably inert with respect to its dissociation (t_1/2 = 872 days at pH = 7 and 25 °C); furthermore, its relaxivity values determined at different field strengths and temperatures (e.g., r_1p = 4.3 mM^-1s^-1at 60 MHz and 37 °C) are ca. one unit higher than those of [Gd(DTPA)]^2- (r_1p = 3.4 mM^-1 s^-1) and [Gd(DOTA)]^- (r_1p = 3.1 mM^-1 s^-1) under the same conditions. Moreover, significant improvement on the relaxivity was observed in the presence of serum proteins (r_1p = 6.9 mM^-1 s^-1 at 60 MHz and 37 °C). The luminescence lifetimes recorded in H₂O and D₂O solutions indicate the presence of a water molecule (q = 1) in the inner sphere of the complex directly coordinated to the metal ion, possessing a relatively high water exchange rate (k_ex²⁹⁸ = 29(2) × 10⁶ s^-1). The acceleration of the water exchange can be explained by the steric compression around the water binding site due to the rigid structure of the complex, which was supported by DFT calculations. On the basis of these results, ligands containing a phenanthroline platform have great potential in the design of safer Gd(III) agents for MRI.

H2ampa─Versatile Chelator for [203Pb]Pb2+, [213Bi]Bi3+, and [225Ac]Ac3

A new decadentate chelator, H₂ampa, was designed to be a potential radiopharmaceutical chelator component. The chelator involves both amide and picolinate functional groups on a large non-macrocyclic, ether-bridged backbone. With its large scaffold, H₂ampa was paired with [^nat/203Pb]Pb²⁺, [^nat/213Bi]Bi³⁺, and ^natLa³⁺/[²²⁵Ac]Ac³⁺ ions. Nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry were used to study the non-radioactive metal complexes. A single crystal of [Bi(ampa)](NO₃) was obtained; its asymmetric, 10-coordinate complex structure was revealed by X-ray diffraction. Optimal conformations of the metal complexes were assessed by density functional theory studies to provide further structural information. Solution studies providing thermodynamic insights into metal complex formation revealed H₂ampa coordinated Bi³⁺, Pb²⁺, and La³⁺ ions to obtain pM values of 26, 14.8, and 15.1, respectively. Preliminary concentration-dependent radiolabeling experiments were carried out between H₂ampa and three different radiometals to evaluate their compatibility for radiopharmaceutical applications. The chelator radiolabeled [²⁰³Pb]Pb²⁺, [²¹³Bi]Bi³⁺, and [²²⁵Ac]Ac³⁺ in short reaction times (7-30 min), at dilute concentrations, and under mild conditions. Thus, H₂ampa was proven to be a versatile chelator able to well coordinate a small range of radiometals frequently considered to be alpha therapeutic candidates.

Rigidified Derivative of the Non-macrocyclic Ligand H4OCTAPA for Stable Lanthanide(III) Complexation

The stability constants of lanthanide complexes with the potentially octadentate ligand CHXOCTAPA^4–, which contains a rigid 1,2-diaminocyclohexane scaffold functionalized with two acetate and two picolinate pendant arms, reveal the formation of stable complexes [log K_LaL = 17.82(1) and log K_YbL = 19.65(1)]. Luminescence studies on the Eu³⁺ and Tb³⁺ analogues evidenced rather high emission quantum yields of 3.4 and 11%, respectively. The emission lifetimes recorded in H₂O and D₂O solutions indicate the presence of a water molecule coordinated to the metal ion. ¹H nuclear magnetic relaxation dispersion profiles and ¹⁷O NMR chemical shift and relaxation measurements point to a rather low water exchange rate of the coordinated water molecule (k_ex²⁹⁸ = 1.58 × 10⁶ s^–1) and relatively high relaxivities of 5.6 and 4.5 mM^–1 s^–1 at 20 MHz and 25 and 37 °C, respectively. Density functional theory calculations and analysis of the paramagnetic shifts induced by Yb³⁺ indicate that the complexes adopt an unprecedented cis geometry with the two picolinate groups situated on the same side of the coordination sphere. Dissociation kinetics experiments were conducted by investigating the exchange reactions of LuL occurring with Cu²⁺. The results confirmed the beneficial effect of the rigid cyclohexyl group on the inertness of the Lu³⁺ complex. Complex dissociation occurs following proton- and metal-assisted pathways. The latter is relatively efficient at neutral pH, thanks to the formation of a heterodinuclear hydroxo complex.

A non-macrocyclic ligand containing a rigid cyclohexyl spacer forms thermodynamically stable complexes with the lanthanide(III) ions in aqueous solution. The complexes also show remarkable kinetic inertness, though a structural change facilitates dissociation through the metal-assisted mechanism for the small lanthanides. The Gd(III) complex displays a relatively high relaxivity due to the presence of a water molecule coordinated to the metal ion, while the Eu(III) and Tb(III) analogues display strong metal-centered luminescence.

Phosphonate Chelators for Medicinal Metal Ions

A family of phosphonate-bearing chelators was synthesized to study their potential in metal-based (radio)pharmaceuticals. Three ligands (H₆phospa, H₆dipedpa, H₆eppy; structures illustrated in manuscript) were fully characterized, including X-ray crystallographic structures of H₆phospa and H₆dipedpa. NMR spectroscopy techniques were used to confirm the complexation of each ligand with selected trivalent metal ions. These methods were particularly useful in discerning structural information for Sc³⁺ and La³⁺ complexes. Solution studies were conducted to evaluate the complex stability of 15 metal complexes. As a general trend, H₆phospa was noted to form the most stable complexes, and H₆eppy associated with the least stable complexes. Moreover, In³⁺ complexes were determined to be the most stable, and complexes with La³⁺ were the least stable, across all metals. Density functional theory (DFT) was employed to calculate structures of H₆phospa and H₆dipedpa complexes with La³⁺ and Sc³⁺. A comparison of experimental ¹H NMR spectra with calculated ¹H NMR spectra using DFT-optimized structures was used as a method of structure validation. It was noted that theoretical NMR spectra were very sensitive to a number of variables, such as ligand configuration, protonation state, and the number/orientation of explicit water molecules. In general, the inclusion of an explicit second shell of water molecules qualitatively improved the agreement between theoretical and experimental NMR spectra versus a polarizable continuum solvent model alone. Formation constants were also calculated from DFT results using potential-energy optimized structures. Strong dependence of molecular free energies on explicit water molecule number, water molecule configuration, and protonation state was observed, highlighting the need for dynamic data in accurate first-principles calculations of metal-ligand stability constants.

Pyclen-Based Ligands Bearing Pendant Picolinate Arms for Gadolinium Complexation

We report the synthesis of two pyclen-based regioisomer ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene) functionalized with picolinic acid pendant arms either at positions 3,9-pc2pa (L5) or 3,6-pc2pa (L6) of the macrocyclic fragment. The ligands were prepared by the regiospecific protection of one of the amine nitrogen atoms of the macrocycle using Boc and Alloc protecting groups, respectively. The X-ray structure of the Gd(III) complex of L5 contains trinuclear [(GdL5)₃(H₂O)₃]³⁺ entities in which the monomeric units are joined by μ₂-η¹:η¹-carboxylate groups. However, the ¹H and ⁸⁹Y NMR spectra of its Y(III) analogue support the formation of monomeric complexes in solution. The Tb(III) complexes are highly luminescent, with emission quantum yields of up to 28% for [TbL5]⁺. The luminescence lifetimes recorded in H₂O and D₂O solutions indicate the presence of a water molecule coordinated to the metal ion, as also evidenced by the ¹H relaxivities measured for the Gd(III) analogues. The Gd(III) complexes present very different exchange rates of the coordinated water molecule (k_ex²⁹⁸ = 87.1 × 10⁶ and 1.06 × 10⁶ s^-1 for [GdL5]⁺ and [GdL6]⁺, respectively). The very high water exchange rate of [GdL5]⁺ is associated with the steric hindrance originating from the coordination of the ligand around the water binding site, which favors a dissociatively activated water exchange process. The Gd(III) complexes present rather high thermodynamic stabilities (log K_GdL = 20.47 and 19.77 for [GdL5]⁺ and [GdL6]⁺, respectively). Furthermore, these complexes are remarkably inert with respect to their acid-assisted dissociation, in particular the complex of L5.

pH-Dependent Hydration Change in a Gd-Based MRI Contrast Agent with a Phosphonated Ligand

The heptadentate ligand L was shown to form an extremely stable Gd complex at neutral pH with a pGd value of 18.4 at pH 7.4. The X-ray crystal structures of the complexes formed with Gd and Tb displayed two very different coordination behaviors being, respectively, octa- and nonacoordinated. The relaxometric properties of the Gd complex were studied by field-dependent relaxivity measurements at various temperatures and by ¹⁷ O NMR spectroscopy. The pH-dependence of the longitudinal relaxivity profile indicated large changes around neutral pH leading to a very large value of 10.1 mm^-1 ⋅s^-1 (60 MHz, 298 K) at pH 4.7. The changes were attributed to an increase of the hydration number from one water molecule in basic conditions to two at acidic pH. A similar trend was observed for the luminescence of the Eu complex, confirming the change in hydration state. DOSY experiments were performed on the Lu analogue, pointing to the absence of dimers in solution in the considered pH range. A breathing mode of the complex was postulated, which was further supported by ¹ H and ³¹ P NMR spectroscopy of the Yb complex at varying pH and was finally modeled by DFT calculations.

Hydrogen Peroxide Disproportionation with Manganese Macrocyclic Complexes of Cyclen and Pyclen

The catalase family of enzymes, which include a variety with a binuclear manganese active site, mitigate the risk from reactive oxygen species by facilitating the disproportionation of hydrogen peroxide into molecular oxygen and water. In this work, hydrogen peroxide disproportionation using complexes formed between manganese and cyclen or pyclen were investigated due to the spectroscopic similarities with the native MnCAT enzyme. Potentiometric titrations were used to construct speciation diagrams that identify the manganese complex compositions at different pH values. Each complex behaves as a functional mimic of catalase enzymes. UV-visible spectroscopic investigations of the H₂O₂ decomposition reaction yielded information about the structure of the initial catalyst and intermediates that include monomeric and dimeric species. The results indicate that rigidity imparted by the pyridine ring of pyclen is a key factor in increased TON and TOF values measured compared to cyclen.

Oxyaapa: A Picolinate-Based Ligand with Five Oxygen Donors that Strongly Chelates Lanthanides

Coordination compounds of the lanthanide ions (Ln³⁺) have important applications in medicine due to their photophysical, magnetic, and nuclear properties. To effectively use the Ln³⁺ ions for these applications, chelators that stably bind them in vivo are required to prevent toxic side effects that arise from localization of these ions in off-target tissue. In this study, two new picolinate-containing chelators, a heptadentate ligand OxyMepa and a nonadentate ligand Oxyaapa, were prepared, and their coordination chemistries with Ln³⁺ ions were thoroughly investigated to evaluate their suitability for use in medicine. Protonation constants of these chelators and stability constants for their Ln³⁺ complexes were evaluated. Both ligands exhibit a thermodynamic preference for small Ln³⁺ ions. The log K_LuL = 12.21 and 21.49 for OxyMepa and Oxyaapa, respectively, indicating that the nonadentate Oxyaapa forms complexes of significantly higher stability than the heptadentate OxyMepa. X-ray crystal structures of the Lu³⁺ complexes were obtained, revealing that Oxyaapa saturates the coordination sphere of Lu³⁺, whereas OxyMepa leaves an additional open coordination site for a bound water ligand. Solution structural studies carried out with NMR spectroscopy revealed the presence of two possible conformations for these ligands upon Ln³⁺ binding. Density functional theory (DFT) calculations were applied to probe the geometries and energies of these conformations. Energy differences obtained by DFT are small but consistent with experimental data. The photophysical properties of the Eu³⁺ and Tb³⁺ complexes were characterized, revealing modest photoluminescent quantum yields of <2%. Luminescence lifetime measurements were carried out in H₂O and D₂O, showing that the Eu³⁺ and Tb³⁺ complexes of OxyMepa have two inner-sphere water ligands, whereas the Eu³⁺ and Tb³⁺ complexes of Oxyaapa have zero. Lastly, variable-temperature ¹⁷O NMR spectroscopy was performed for the Gd-OxyMepa complex to determine its water exchange rate constant of k_ex²⁹⁸ = (2.8 ± 0.1) × 10⁶ s^-1. Collectively, this comprehensive characterization of these Ln³⁺ chelators provides valuable insight for their potential use in medicine and garners additional understanding of ligand design strategies.

A simple time-resolved fluorescence assay for quantitative determination of DOTA chelator

DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) is one of the preeminent metal chelator applied for diagnostic and therapeutic purposes, however to date there is no versatile and reliable nonradioisotopic method for its precise determination. In this technical note, we present a novel and sensitive fluorimetric assay for quantitative determination of DOTA based on the luminescence quenching of the highly luminescent europium ions complex with trioctyl phosphine oxide and naphthoyl trifluoroacetone sensitizing activators. The assay is carried out in two simple steps and enables the determination of DOTA in the nanomolar range providing a superior tool compared to commonly applied spectrophotometric assay with Arsenazo-III reagent.

Gadolinium Complexes of Highly Rigid, Open-Chain Ligands Containing a Cyclobutane Ring in the Backbone: Decreasing Ligand Denticity Might Enhance Kinetic Inertness

In an effort to explore novel ligand scaffolds for stable and inert lanthanide complexation in magnetic resonance imaging contrast agent research, three chiral ligands containing a highly rigid (1S,2S)-1,2-cyclobutanediamine spacer and different number of acetate and picolinate groups were efficiently synthesized. Potentiometric studies show comparable thermodynamic stability for the Gd³⁺ complexes formed with either the octadentate (L3)^4- bearing two acetate or two picolinate groups or the heptadentate (L2)^4- analogue bearing one picolinate and three acetate groups (log K_GdL = 17.41 and 18.00 for [Gd(L2)]^- and [Gd(L3)]^-, respectively). In contrast, their dissociation kinetics is revealed to be very different: the monohydrated [Gd(L3)]^- is considerably more labile, as a result of the significant kinetic activity of the protonated picolinate function, as compared to the bishydrated [Gd(L2)]^-. This constitutes an uncommon example in which lowering ligand denticity results in a remarkable increase in kinetic inertness. Another interesting observation is that the rigid ligand backbone induces an unusually strong contribution of the spontaneous dissociation to the overall decomplexation process. Thanks to the presence of two inner-sphere water molecules, [Gd(L2)]^- is endowed with high relaxivity (r₁ = 7.9 mM^-1 s^-1 at 20 MHz, 25 °C), which is retained in the presence of large excess of endogenous anions, excluding ternary complex formation. The water exchange rate is similar for [Gd(L3)]^- and [Gd(L2)]^-, while it is 1 order of magnitude higher for the trishydrated tetraacetate analogue [Gd(L1)]^- (k_ex²⁹⁸ = 8.1, 10, and 127 × 10⁶ s^-1, respectively). A structural analysis via density functional theory calculations suggests that the large bite angle imposed by the rigid (1S,2S)-1,2-cyclobutanediamine spacer could allow the design of ligands based on this scaffold with suitable properties for the coordination of larger metal ions with biomedical applications.

Cyclobutane-Containing Scaffolds as Useful Intermediates in the Stereoselective Synthesis of Suitable Candidates for Biomedical Purposes: Surfactants, Gelators and Metal Cation Ligands

Efficient and versatile synthetic methodologies are reported for the preparation of products that are suitable candidates to be used as surfactants, gelators for hydroxylic solvents or metal cation ligands, with potential use in several fields including biomedical applications. The common structural feature of all the synthesized products is the presence of a cis or trans-1,2- or cis-1,3-difunctionalized cyclobutane ring. In the two first cases, the key intermediates including enantiomerically pure 1,3-diamines and 1,3-amino alcohols have been prepared from β-amino acid derivatives obtained, in turn, from a chiral half-ester. This compound is also precursor of γ-amino esters. Furthermore, two kind of polydentate ligands have also been synthesized from a symmetric 1,5-diamine obtained from norpinic acid, which was easily prepared from commercial verbenone.

Ligand design strategies to increase stability of gadolinium-based magnetic resonance imaging contrast agents

Gadolinium(III) complexes have been widely utilised as magnetic resonance imaging (MRI) contrast agents for decades. In recent years however, concerns have developed about their toxicity, believed to derive from demetallation of the complexes in vivo, and the relatively large quantities of compound required for a successful scan. Recent efforts have sought to enhance the relaxivity of trivalent gadolinium complexes without sacrificing their stability. This review aims to examine the strategic design of ligands synthesised for this purpose, provide an overview of recent successes in gadolinium-based contrast agent development and assess the requirements for clinical translation.

Influence of a Pre-organized N-Donor Group on the Coordination of Trivalent Actinides and Lanthanides by an Aminopolycarboxylate Complexant

The thermodynamic influence of a pre-organized N-donor group on the coordination of trivalent actinides and lanthanides by an aqueous aminopolycarboxylate complexant has been investigated. The synthesized reagent, N-2-methylpicolinate-ethylenediamine-N,N',N'-triacetic acid (EDTA-Mpic), resembles ethylenediamine-N,N,N',N'-tetraacetic acid (EDTA) with a single acetate pendant arm replaced by a 6-carboxypyridin-2-ylmethyl group. The rigid N-donor picolinate functionality has a profound impact on ligand protonation and trivalent f element complexation equilibria, as demonstrated by potentiometric, spectroscopic, and liquid/liquid metal-partitioning studies as well as by molecular dynamics calculations. Relative to diethylenetriamine-N,N,N',N'',N''-pentaacetic acid (DTPA), the ability to preferentially bind trivalent actinides over trivalent lanthanides was moderately lowered due to the presence of the N-(6-carboxypyridin-2-ylmethyl) substituent. The structural modification substantially amplifies the total ligand acidity of EDTA-Mpic. As a result the complexant sustains the metal complexation and efficient An³⁺ /Ln³⁺ differentiation in aqueous mixtures of unprecedented acidity for this class of reagents.

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.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

Composed in the f-block: solution structure and function of kinetically inert lanthanide(iii) complexes

An induction to the wonders of lanthanides, and a call for standardised methods for characterisation of lanthanide complexes in solution.

p-NO2-Bn-H4neunpa and H4neunpa-Trastuzumab: Bifunctional Chelator for Radiometalpharmaceuticals and 111In Immuno-Single Photon Emission Computed Tomography Imaging

Potentially nonadentate (N₅O₄) bifunctional chelator p-SCN-Bn-H₄neunpa and its immunoconjugate H₄neunpa-trastuzumab for ¹¹¹In radiolabeling are synthesized. The ability of p-SCN-Bn-H₄neunpa and H₄neunpa-trastuzumab to quantitatively radiolabel ¹¹¹InCl₃ at an ambient temperature within 15 or 30 min, respectively, is presented. Thermodynamic stability determination with In³⁺, Bi³⁺, and La³⁺ resulted in high conditional stability constant (pM) values. In vitro human serum stability assays have demonstrated both ¹¹¹In complexes to have high stability over 5 days. Mouse biodistribution of [¹¹¹In][In(p-NO₂-Bn-neunpa)]^-, compared to that of [¹¹¹In][In(p-NH₂-Bn-CHX-A″-diethylenetriamine pentaacetic acid (DTPA))]^2-, at 1, 4, and 24 h shows fast clearance of both complexes from the mice within 24 h. In a second mouse biodistribution study, the immunoconjugates ¹¹¹In-neunpa-trastuzumab and ¹¹¹In-CHX-A″-DTPA-trastuzumab demonstrate a similar distribution profile but with slightly lower tumor uptake of ¹¹¹In-neunpa-trastuzumab compared to that of ¹¹¹In-CHX-A″-DTPA-trastuzumab. These results were also confirmed by immuno-single photon emission computed tomography (immuno-SPECT) imaging in vivo. These initial investigations reveal the acyclic bifunctional chelator p-SCN-Bn-H₄neunpa to be a promising chelator for ¹¹¹In (and other radiometals) with high in vitro stability and also show H₄neunpa-trastuzumab to be an excellent ¹¹¹In chelator with promising biodistribution in mice.

Strongly Circularly Polarized Emission from Water-Soluble Eu(III)- and Tb(III)-Based Complexes: A Structural and Spectroscopic Study

Water-soluble Eu(III) and Tb(III) complexes with N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N'-diacetic acid (H₂bpcd) have been synthesized and characterized in their racemic and enantiopure forms. The ligand has been designed to bind Ln(III) ions, providing a dissymmetric environment able to solicit strong chiroptical features while at the same time leaving a few coordination sites available for engaging further ancillary ligands. Potentiometric studies show that Ln(III) complexes have a relatively good stability and that at pH 7 the [Ln(bpcd)]⁺ species is largely dominant. DFT calculations carried out on the (S,S)-[Y(bpcd)(H₂O)₅]⁺ complexes (the closed-shell equivalents of [Eu(bpcd)(H₂O)₅]⁺ and [Tb(bpcd)(H₂O)₅]⁺) indicate that the two trans-O,O and trans-N_py,N_py configurations are equally stable in solution and present two coordinated water molecules. This is in agreement with the hydration number ∼2.6 determined by luminescence lifetime measurements on Tb(III) and Eu(III) complexes. A detailed optical and chiroptical spectroscopic characterization has been carried out and reveals that the complexes display an efficient luminescence in the visible spectral range accompanied by a strong CPL activity. A value for g_lum (around 0.1 on the top of the 546 nm band) for the Tb-based complex has been found. This is one of the highest g_lum values measured up to now for chiral Tb complexes. These results suggest that in principle Tb(bpcd)Cl is suitable to be employed as a CPL bioprobe for relevant analytes in aqueous media.

H4octapa: synthesis, solution equilibria and complexes with useful radiopharmaceutical metal ions

H₄octapa is synthesized and complexed to nine metals of medicinal interest. Crystal structures of the ligand and its La complex were obtained. Solution equilibria for the ligand and several lanthanide complexes were investigated.

Optical and relaxometric properties of monometallic (EuIII, TbIII, GdIII) and heterobimetallic (ReI/GdIII) systems based on a functionalized bipyridine-containing acyclic ligand

The photophysical and relaxometric properties, in aqueous solution, of Ln-BPMNTA complexes and a derived Re^I/Gd^III dinuclear complex are reported in this paper.