TRASUTA: The Effect of the Structural Rigidity of a Mesocyclic AAZTA-like Chelating Agent on the Thermodynamic, Kinetic, and Structural Properties of Some Divalent Metal and Ga3+ Complexes

Mesocyclic chelating agents such as AAZTA and its derivatives have been recently reported to overcome the relatively low thermodynamic stability of metal complexes of acyclic chelating agents and the slow complexation kinetics of macrocyclic chelating agents. This work reports the preparation of a spirobicyclic hexadentate AAZTA-like chelating agent (TRASUTA) and the investigation of the thermodynamic, kinetic, and structural properties of the corresponding chelates with the PET-relevant Ga3+ and selected metal ions. A combination of analytical techniques allowed identification of a coordination isomerization process, involving the coordinating side arms and the inversion of a nitrogen atom and leading to lower thermodynamic and kinetic inertness with respect to mononuclear mesocyclic analogues. The bicyclic system of TRASUTA retains significant dynamics despite the conformational constraint imposed by the spiro-fusion, resulting in a lower stability of the corresponding metal chelates.

The effects of novel macrocyclic chelates on the targeting properties of the 68Ga-labeled Gastrin releasing peptide receptor antagonist RM2

BACKGROUND

The gastrin-releasing peptide receptor (GRPr) is a molecular target for the visualization of prostate cancer. Bombesin (BN) analogs are short peptides with a high affinity for GRPr. RM2 is a bombesin-based antagonist. It has been demonstrated that RM2 have superior in vivo biodistribution and targeting properties than high-affinity receptor agonists. This study developed new RM2-like antagonists by introducing the novel bifunctional chelators AAZTA⁵ and DATA^5m to RM2.

RESULTS

The effects of different macrocyclic chelating groups on drug targeting properties and the possibility of preparing ⁶⁸Ga-radiopharmaceuticals in a kit-based protocol were investigated using ⁶⁸Ga-labeled entities. Both new RM2 variants were labelled with ⁶⁸Ga³⁺ resulting in high yields, stability, and low molarity of the ligand. DATA^5m-RM2 and AAZTA⁵-RM2 incorporated ⁶⁸Ga³⁺ nearly quantitatively at room temperature within 3-5 min, and the labelling yield for ⁶⁸Ga-DOTA-RM2 was approximately 10% under the same conditions. ⁶⁸Ga-AAZTA⁵-RM2 showed stronger hydrophilicity according to partition coefficient. Although the maximal cellular uptake values of the three compounds were similar, ⁶⁸Ga-AAZTA⁵-RM2 and ⁶⁸Ga-DATA^5m-RM2 peaked more rapidly. Biodistribution studies showed high and specific tumor uptake, with a maximum of 9.12 ± 0.81 percentage injected activity per gram of tissue (%ID/g) for ⁶⁸Ga-DATA^5m-RM2 and 7.82 ± 0.61%ID/g for ⁶⁸Ga-AAZTA⁵-RM2 at 30 min after injection.

CONCLUSIONS

The conditions for complexation of DATA^5m-RM2 and AAZTA⁵-RM2 with gallium-68 are milder, faster and require less amount of precursors than DOTA-RM2. Chelators had an evident influence on the pharmacokinetics and targeting properties of ⁶⁸Ga-X-RM2 derivatives. Positively charged ⁶⁸Ga-DATA^5m-RM2 provided a high tumor uptake, high image contrast and good capability of targeting GRPr.

Modern Developments in Bifunctional Chelator Design for Gallium Radiopharmaceuticals

The positron-emitting radionuclide gallium-68 has become increasingly utilised in both preclinical and clinical settings with positron emission tomography (PET). The synthesis of radiochemically pure gallium-68 radiopharmaceuticals relies on careful consideration of the coordination chemistry. The short half-life of 68 min necessitates rapid quantitative radiolabelling (≤10 min). Desirable radiolabelling conditions include near-neutral pH, ambient temperatures, and low chelator concentrations to achieve the desired apparent molar activity. This review presents a broad overview of the requirements of an efficient bifunctional chelator in relation to the aqueous coordination chemistry of gallium. Developments in bifunctional chelator design and application are then presented and grouped according to eight categories of bifunctional chelator: the macrocyclic chelators DOTA and TACN; the acyclic HBED, pyridinecarboxylates, siderophores, tris(hydroxypyridinones), and DTPA; and the mesocyclic diazepines.

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.

Development and in vitro evaluation of new bifunctional 89Zr-chelators based on the 6-amino-1,4-diazepane scaffold for immuno-PET applications

INTRODUCTION

Combination of hydroxamate bearing side chains with the 6-amino-1,4-diazepane scaffold provides a promising strategy for fast and stable 89Zr-labeling of antibodies. Following this approach, we hereby present the development, labeling kinetics and in vitro complex stability of three resulting bifunctional chelator derivatives both stand-alone and coupled to a model protein in comparison to different linear deferoxamine (DFO) derivatives.

METHODS

The novel 89Zr-chelator Hy3ADA5 was prepared via amide-coupling of separately synthesized 6-amino-1,4-diazepane-6-pentanoic acid and hydroxamate-containing side chains. Two further bifunctional derivatives were synthesized by extending the resulting system with either a squaramide- or p-isothiocyanatophenyl moiety for simplified binding to proteins. After coupling to a model antibody and purification, the resulting immunoconjugates as well as the unbound chelator derivatives were 89Zr-labeled at room temperature (RT) and neutral pH. For comparison, different DFO derivatives were analogously coupled, purified and radiolabeled. In vitro complex stability of the resulting radioconjugates was investigated in phosphate buffered saline (PBS) and human serum at 37 °C over a period of 7 days.

RESULTS

89Zr-labeling of the novel unbound Hy3ADA5 derivatives indicated rapid complexation kinetics resulting in high radiochemical conversions (RCC) of 84-94% after 90 min. Similar or even faster radiolabeling with slightly increased maximum yields was obtained using the DFO-analogues. Initially, [89Zr]Zr-DFO*-p-Ph-NCS showed a delayed formation, nevertheless reaching almost quantitative complexation. Radiolabeling of the corresponding immunoconjugates Hy3ADA5-SA-mAb and Hy3ADA5-p-Ph-NCS-mAb resulted in 82.0 ± 1.1 and 89.2 ± 0.7% RCC, respectively after 90 min representing high but slightly lower labeling efficiency compared to the DFO- and DFO*-functionalized analogues. All examined radioimmunoconjugates showed very high in vitro complex stability both in human serum and PBS, providing no significant release of the radiometal. In the case of unbound chelators, however, the p-Ph-NCS-functionalized derivatives indicated considerable instability in human serum already after 1 h.

CONCLUSION

The novel chelator derivatives based on hydroxamate-functionalized 6-amino-1,4-diazepane revealed fast and high yielding 89Zr-labeling kinetics as well as high in vitro complex stability both stand-alone and coupled to an antibody. Therefore, Hy3ADA5 represents a promising tool for radiolabeling of biomolecules such as antibodies at mild conditions for immuno-PET applications.

68Ga, 44Sc and 177Lu-labeled AAZTA5-PSMA-617: synthesis, radiolabeling, stability and cell binding compared to DOTA-PSMA-617 analogues

BACKGROUND

The AAZTA chelator and in particular its bifunctional derivative AAZTA⁵ was recently investigated to demonstrate unique capabilities to complex diagnostic and therapeutic trivalent radiometals under mild conditions. This study presents a comparison of ⁶⁸Ga, ⁴⁴Sc and ¹⁷⁷Lu-labeled AAZTA⁵-PSMA-617 with DOTA-PSMA-617 analogues. We evaluated the radiolabeling characteristics, in vitro stability of the radiolabeled compounds and evaluated their binding affinity and internalization behavior on LNCaP tumor cells in direct comparison to the radiolabeled DOTA-conjugated PSMA-617 analogs.

RESULTS

AAZTA⁵ was synthesized in a five-step synthesis and coupled to the PSMA-617 backbone on solid phase. Radiochemical evaluation of AAZTA⁵-PSMA-617 with ⁶⁸Ga, ⁴⁴Sc and ¹⁷⁷Lu achieved quantitative radiolabeling of > 99% after less than 5 min at room temperature. Stabilities against human serum, PBS buffer and EDTA and DTPA solutions were analyzed. While there was a small degradation of the ⁶⁸Ga complex over 2 h in human serum, PBS and EDTA/DTPA, the ⁴⁴Sc and ¹⁷⁷Lu complexes were stable at 2 h and remained stable over 8 h and 1 day. For all three compounds, i.e. [^natGa]Ga-AAZTA⁵-PSMA-617, [^natSc]Sc-AAZTA⁵-PSMA-617 and [^natLu]Lu-AAZTA⁵-PSMA-617, in vitro studies on PSMA-positive LNCaP cells were performed in direct comparison to radiolabeled DOTA-PSMA-617 yielding the corresponding inhibition constants (K_i). K_i values were in the range of 8-31 nM values which correspond with those of [^natGa]Ga-DOTA-PSMA-617, [^natSc]Sc-DOTA-PSMA-617 and [^natLu]Lu-DOTA-PSMA-617, i.e. 5-7 nM, respectively. Internalization studies demonstrated cellular membrane to internalization ratios for the radiolabeled ⁶⁸Ga, ⁴⁴Sc and ¹⁷⁷Lu-AAZTA5-PSMA-617 tracers (13-20%IA/10⁶ cells) in the same range as the ones of the three radiolabeled DOTA-PSMA-617 tracers (17-20%IA/10⁶ cells) in the same assay.

CONCLUSIONS

The AAZTA⁵-PSMA-617 structure proved fast and quantitative radiolabeling with all three radiometal complexes at room temperature, excellent stability with ⁴⁴Sc, very high stability with ¹⁷⁷Lu and medium stability with ⁶⁸Ga in human serum, PBS and EDTA/DTPA solutions. All three AAZTA⁵-PSMA-617 tracers showed binding affinities and internalization ratios in LNCaP cells comparable with that of radiolabeled DOTA-PSMA-617 analogues. Therefore, the exchange of the chelator DOTA with AAZTA⁵ within the PSMA-617 binding motif has no negative influence on in vitro LNCaP cell binding characteristics. In combination with the faster and milder radiolabeling features, AAZTA⁵-PSMA-617 thus demonstrates promising potential for in vivo application for theranostics of prostate cancer.

PIDAZTA: Structurally Constrained Chelators for the Efficient Formation of Stable Gallium-68 Complexes at Physiological pH

Two structurally constrained chelators based on a fused bicyclic scaffold, 4-amino-4-methylperhydro-pyrido[1,2-a][1,4]diazepin-N,N',N'-triacetic acids [(4R*,10aS*)-PIDAZTA (L1) and (4R*,10aR*)-PIDAZTA (L2)], were designed for the preparation of Ga^III -based radiopharmaceuticals. The stereochemistry of the ligand scaffold has a deep impact on the properties of the complexes, with unexpected [Ga(L2)OH] species being superior in terms of both thermodynamic stability and inertness. This peculiar behavior was rationalized on the basis of molecular modeling and appears to be related to a better fit in size of Ga^III into the cavity of L2. Fast and efficient formation of the Ga^III chelates at room temperature was observed at pH values between 7 and 8, which enables ⁶⁸ Ga radiolabeling under truly physiological conditions (pH 7.4).

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.

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.

Prospective of 68Ga Radionuclide Contribution to the Development of Imaging Agents for Infection and Inflammation

During the last decade, the utilization of ⁶⁸Ga for the development of imaging agents has increased considerably with the leading position in the oncology. The imaging of infection and inflammation is lagging despite strong unmet medical needs. This review presents the potential routes for the development of ⁶⁸Ga-based agents for the imaging and quantification of infection and inflammation in various diseases and connection of the diagnosis to the treatment for the individualized patient management.

Comparison of macrocyclic and acyclic chelators for gallium-68 radiolabelling

Gallium-68 (68Ga) is a positron-emitting isotope used for clinical PET imaging of peptide receptor expression. 68Ga radiopharmaceuticals used in molecular PET imaging consist of disease-targeting biomolecules tethered to chelators that complex 68Ga3+. Ideally, the chelator will rapidly, quantitatively and stably coordinate 68Ga3+ at room temperature, near neutral pH and low chelator concentration, allowing for simple routine radiopharmaceutical formulation. Identification of chelators that fulfil these requirements will facilitate development of kit-based 68Ga radiopharmaceuticals. Herein the reaction of a range of widely used macrocyclic and acyclic chelators with 68Ga3+ is reported. Radiochemical yields have been measured under conditions of varying chelator concentrations, pH (3.5 and 6.5) and temperature (25 and 90 °C). These chelators are: 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), 1,4,7-triazacyclononane macrocycles substituted with phosphonic (NOTP) and phosphinic (TRAP) groups at the amine, bis(2-hydroxybenzyl)ethylenediaminediacetic acid (HBED), a tris(hydroxypyridinone) containing three 1,6-dimethyl-3-hydroxypyridin-4-one groups (THP) and the hexadentate tris(hydroxamate) siderophore desferrioxamine-B (DFO). Competition studies have also been undertaken to assess relative complexation efficiencies of each chelator for 68Ga3+ under different pH and temperature conditions. Performing radiolabelling reactions at pH 6.5, 25 °C and 5-50 μM chelator concentration resulted in near quantitative radiochemical yields for all chelators, except DOTA. Radiochemical yields either decreased or were not substantially improved when the reactions were undertaken at lower pH or at higher temperature, except in the case of DOTA. THP and DFO were the most effective 68Ga3+ chelators at near-neutral pH and 25 °C, rapidly providing near-quantitative radiochemical yields at very low chelator concentrations. NOTP and HBED were only slightly less effective under these conditions. In competition studies with all other chelators, THP demonstrated highest reactivity for 68Ga3+ complexation under all conditions. These data point to THP possessing ideal properties for rapid, one-step kit-based syntheses of 68Ga-biomolecules for molecular PET imaging. LC-MS and 1H, 13C{1H} and 71Ga NMR studies of HBED complexes of Ga3+ showed that under the analytical conditions employed in this study, multiple HBED-bound Ga complexes exist. X-ray diffraction data indicated that crystals isolated from these solutions contained octahedral [Ga(HBED)(H2O)], with HBED coordinated in a pentadentate N2O3 mode, with only one phenolic group coordinated to Ga3+, and the remaining coordination site occupied by a water molecule.

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.

DATATOC: a novel conjugate for kit-type 68Ga labelling of TOC at ambient temperature

Background

The widespread acceptance and application of ⁶⁸Ga-PET depends on our ability to develop radiopharmaceuticals that can be prepared in a convenient and suitable manner. A kit-type labelling protocol provides such characteristics and requires chelators that can be radiolabelled under exceptionally mild conditions. Recently the DATA chelators have been introduced that fulfil these requirements. In continuing their development, the synthesis and radiolabelling of the first DATA bifunctional chelator (BFC) and peptide conjugate are described.

Results

A BFC derived from the DATA ligand (2,2'-(6-((carboxymethyl)amino)-1,4-diazepane-1,4-diyl)diacetic acid) has been synthesised in five steps from simple building blocks, with an overall yield of 8 %. DATA^M5-3^tBu (5-[1,4-Bis-tert-butoxycarbonylmethyl-6-(tert-butoxycarbonylmethyl-methyl-amino)-[1, 4]diazepan-6-yl]-pentanoic acid) has been coupled to [DPhe¹][Tyr³]-octreotide (TOC) and the resulting peptide conjugate (DATATOC) radiolabelled with purified ⁶⁸Ga derived via four different ⁶⁸Ge/⁶⁸Ga generator post-processing (PP) methods. The stability and lipophilicity of the radiotracer have been assessed and a kit-type formulation for radiolabelling evaluated. ⁶⁸Ga-DATATOC has been prepared with a > 95 % radiochemical yield (RCY) within 1 (fractionated and acetone-PP) and 10 min (ethanol- and NaCl-PP) at 23 °C (pH 4.2-4.9, 13 nmol). The radiolabelled peptide is stable in the presence of human serum. Lipophilicity of ⁶⁸Ga-DATATOC was calculated as logP = -3.2 ± 0.3, with a HPLC retention time (t_R = 10.4 min) similar to ⁶⁸Ga-DOTATOC (logP = -2.9 ± 0.4, t_R = 10.3 min). Kit-type labelling from a lyophilised solid using acetone-PP based labelling achieves > 95 % RCY in 10 min at 23 °C.

Conclusions

The favourable labelling properties of the DATA chelators have been retained for DATATOC. High radiochemical purity can be achieved at 23 °C in less than 1 min and from a kit formulation. The speed, reliability, ease, flexibility and simplicity with which ⁶⁸Ga-DATATOC can be prepared makes it a very attractive alternative to current standards.

Approaching 'Kit-Type' Labelling with (68)Ga: The DATA Chelators

The DATA chelators are a novel class of tri-anionic ligands based on 6-amino-1,4-diazepine-triacetic acid, which have been introduced recently for the chelation of (68)Ga. Compared with macrocyclic chelators based on the cyclen scaffold (i.e., DOTA, DO3A, and DO2A derivatives), DATA chelators undergo quantitative radiolabelling more rapidly and under milder conditions. In this study, a systematic evaluation of the labelling of four DATA chelators--DATA(M), DATA(P), DATA(Ph), and DATA(PPh)--with (68)Ga is presented. The results highlight the extraordinary potential of this new class of chelators for application in molecular imaging using (68)Ga positron emission tomography (PET).

Cation exchange-based post-processing of (68)Ga-eluate: a comparison of three solvent systems for labelling of DOTATOC, NO2AP(BP) and DATA(m.)

Interest in (68)Ga has led to a number of innovations for its provision suitable for clinical application. Several post-processing methods are available to reduce eluate volume and remove metal trace impurities. In this work three cation exchange resin based post-processing methods (acetone, ethanol and NaCl) have been compared, using three model precursors (DOTATOC, NO2AP(BP) and DATA(m)), in terms of labelling yield and reproducibility. The acetone and ethanol based methods provided greater reproducibility and yields that makes subsequent purification unnecessary.

AMPED: a new platform for picolinate based luminescent lanthanide chelates

Anchoring three picolinate units on a AMPED scaffold affords a strongly pre-organized ligand for the formation of luminescent lanthanide complexes.

Chemical, radiochemical and biological studies of new gallium(iii) complexes with hexadentate chelators

New Ga³⁺ complexes with hexadentate chelators were prepared using natural gallium and radiogallium, and showed some favorable features for radiopharmaceutical research.

Matching chelators to radiometals for radiopharmaceuticals

Radiometals comprise many useful radioactive isotopes of various metallic elements. When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, e.g.(67)Ga, (99m)Tc, (111)In, (177)Lu) and positron emission tomography (PET, e.g.(68)Ga, (64)Cu, (44)Sc, (86)Y, (89)Zr), as well as therapeutic applications (e.g.(47)Sc, (114m)In, (177)Lu, (90)Y, (212/213)Bi, (212)Pb, (225)Ac, (186/188)Re). A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems. The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging. A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed. The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals. Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents.

Formation and base hydrolysis of oxidimethaneamine bridges in CoIII-amine complexes

cis-[CoL2](3+) (1a(3+)), trans-[CoL2](3+) (2a(3+)), cis-[Co(MeL)2](3+) (1b(3+)), and trans-[Co(MeL)2](3+) (2b(3+)), L = 1,4-diazepan-6-amine (daza) and MeL = 6-methyl-1,4-diazepan-6-amine (Medaza), were allowed to react as templates in acetonitrile with paraformaldehyde and triethylamine. Several Co(III) complexes, where two adjacent amino groups of two ligand moieties are interlinked by an oxidimethaneamine bridge, were obtained. Connection of a primary with a secondary amino group (prim-sec bridging) was found to be predominant. The singly and doubly bridged daza- and Medaza-derivatives 7a(3+), 9a(3+) and 7b(3+), 9b(3+) were characterized by crystal-structure analysis. The bridging process resulted in a slight lengthening of the mean Co-N distance, a red shift of the A1g-T1g transition, and an increase of the Co(III)/Co(II) reduction potential. Several minor components, which could be only partially separated by chromatographic methods, were also formed. The daza-derivatives 6a(3+) (prim-prim bridged) and 10a(3+) (bidentate coordination of one daza frame) formed in small quantities. The Medaza derivatives 3b(3+) and 4b(3+) (trans configuration of the Medaza frames, with additional pending carbinolamino groups), and 8b(3+) (with a methylideneimino group) represent intermediates of the condensation process. Their structure was again corroborated by X-ray diffraction. All bridged species (6a(3+), 7a(3+), 7b(3+), 8b(3+), 9a(3+), 9b(3+), and 10a(3+)) exhibited exclusively a cis orientation of the two diazepane frames, even if the trans configured 2a(3+) or 2b(3+) were used as starting materials. Molecular mechanics calculations indicate that in the bridged species with a trans configuration steric strain is substantially more pronounced. In alkaline aqueous media, 9a(3+) and 9b(3+) revealed a complete degradation of the bridges whereby the original 1a(3+) and 1b(3+) reformed. The pseudo-first-order rate constant k(obs) of the degradation reaction was found to depend linearly on OH(-) concentration. The degradation of the first bridge is about 100 times faster than the degradation of the second. The mechanism of formation and degradation of such oxidimethaneamine bridges is discussed.