Transition metal complexes of the (2,2,2-trifluoroethyl)phosphinate NOTA analogue as potential contrast agents for 19F magnetic resonance imaging

A new hexadentate 1,4,7-triazacyclononane-based ligand bearing three coordinating methylene-(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and trivalent (Cr3+, Fe3+, Co3+) transition metal ions was studied. The ligand forms stable complexes with late divalent transition metal ions (from Co2+ to Zn2+) and the complexes of these metal ions are formed above pH ∼3. A number of complexes with divalent metal ions were structurally characterized by means of single-crystal X-ray diffraction. The complex of the larger Mn2+ ion adopts a twisted trigonally antiprismatic geometry with a larger coordination cavity and smaller torsion of the pendant arms, whereas the smaller ions Ni2+, Cu2+ and Zn2+ form octahedral species with a smaller cavity and larger pendant arm torsion. In the case of the Co2+ complexes, both coordination arrangements were observed. The complexes with paramagnetic metal ions were studied from the point of view of potential utilization in 19F magnetic resonance imaging. A significant shortening of the 19F NMR longitudinal relaxation times was observed: a sub-millisecond range for complexes of Cr3+, Mn2+ and Fe3+ with symmetric electronic states (t2g3 and HS-d5), the millisecond range for the Ni2+ and Cu2+ complexes and tens of milliseconds for the Co2+ complex. Such short relaxation times are consistent with a short distance between the paramagnetic metal ion and the fluorine atoms (∼5.5-6.5 Å). Among the redox-active complexes (Mn3+/Mn2+, Fe3+/Fe2+, Co3+/Co2+, Cu2+/Cu+), the cobalt complexes show sufficient stability and a paramagnetic-diamagnetic changeover with the redox potential lying in a physiologically relevant range. Thus, the Co3+/Co2+ complex pair can be potentially used as a smart redox-responsive contrast agent for 19F MRI.

Inorganic Chemistry of the Tripodal Picolinate Ligand with Gallium(III) and Radiolabeling with Gallium-68

We report here the improved synthesis of the tripodal picolinate chelator Tpaa, with an overall yield of 41% over five steps, in comparison to the previously reported 6% yield. Tpaa was investigated for its coordination chemistry with Ga(III) and radiolabeling properties with gallium-68 (68Ga). The obtained crystal structure for [Ga(Tpaa)] shows that the three picolinate arms coordinate to the Ga(III) ion, fully occupying the octahedral coordination geometry. This is supported by 1H NMR which shows that the three arms are symmetrical when coordinated to Ga(III). Assessment of the thermodynamic stability through potentiometry gives log KGa-Tpaa = 21.32, with a single species being produced across the range of pH 3.5-7.5. Tpaa achieved >99% radiochemical conversion with 68Ga under mild conditions ([Tpaa] = 6.6 μM, pH 7.4, 37 °C) with a molar activity of 3.1 GBq μmol-1. The resulting complex, [68Ga][Ga(Tpaa)], showed improved stability over the previously reported [68Ga][Ga(Dpaa)(H2O)] in a serum challenge, with 32% of [68Ga][Ga(Tpaa)] remaining intact after 30 min of incubation with fetal bovine serum.

Transition metal complexes of cyclam with two 2,2,2-trifluoroethylphosphinate pendant arms as probes for 19F magnetic resonance imaging

A new cyclam-based ligand bearing two methylene(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent transition metal ions [Co(II), Ni(II), Cu(II), Zn(II)] was studied. The ligand was found to be very selective for the Cu(II) ion according to the common Williams-Irving trend. Complexes with all the studied metal ions were structurally characterized. The Cu(II) ion forms two isomeric complexes; the pentacoordinated isomer pc-[Cu(L)] is the kinetic product and the octahedral trans-O,O'-[Cu(L)] isomer is the final (thermodynamic) product of the complexation reaction. Other studied metal ions form octahedral cis-O,O'-[M(L)] complexes. The complexes with paramagnetic metal ions showed a significant shortening of 19F NMR longitudinal relaxation times (T1) to the millisecond range [Ni(II) and Cu(II) complexes] or tens of milliseconds [Co(II) complex] at the temperature and magnetic field relevant for 19F magnetic resonance imaging (MRI). Such a short T1 results from a short distance between the paramagnetic metal ion and the fluorine atoms (∼6.1-6.4 Å). The complexes show high kinetic inertness towards acid-assisted dissociation; in particular, the trans-O,O'-[Cu(L)] complex was found to be extremely inert with a dissociation half-time of 2.8 h in 1 M HCl at 90 °C. Together with the short relaxation time, it potentially enables in vitro/in vivo utilization of the complexes as efficient contrast agents for 19F MRI.

Inorg Chem 2022;61:17059-17067. [PMID: 36251390 DOI: 10.1021/acs.inorgchem.2c01992] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The chelator Bn₂DT3A was used to produce a novel ⁶⁸Ga complex for positron emission tomography (PET). Unusually, this system is stabilized by a coordinated hydroxide in aqueous solutions above pH 5, which confers sufficient stability for it to be used for PET. Bn₂DT3A complexes Ga³⁺ in a hexadentate manner, forming a mer-mer complex with log K([Ga(Bn₂DT3A)]) = 18.25. Above pH 5, the hydroxide ion coordinates the Ga³⁺ ion following dissociation of a coordinated amine. Bn₂DT3A radiolabeling displayed a pH-dependent speciation, with [⁶⁸Ga][Ga(Bn₂DT3A)(OH)]^- being formed above pH 5 and efficiently radiolabeled at pH 7.4. Surprisingly, [⁶⁸Ga][Ga(Bn₂DT3A)(OH)]^- was found to show an increased stability in vitro (for over 2 h in fetal bovine serum) compared to [⁶⁸Ga][Ga(Bn₂DT3A)]. The biodistribution of [⁶⁸Ga][Ga(Bn₂DT3A)(OH)]^- in healthy rats showed rapid clearance and excretion via the kidneys, with no uptake seen in the lungs or bones.

Cyclam with a phosphinate-bis(phosphonate) pendant arm is a bone-targeting carrier of copper radionuclides

Ligands combining a bis(phosphonate) group with a macrocycle function as metal isotope carriers for radionuclide-based imaging and for treating bone metastases associated with several cancers. However, bis(phosphonate) pendant arms often slow down complex formation and decrease radiochemical yields. Nevertheless, their negative effect on complexation rates may be mitigated by using a suitable spacer between bis(phosphonate) and the macrocycle. To demonstrate the potential of bis(phosphonate) bearing macrocyclic ligands as a copper radioisotope carrier, we report the synthesis of a new cyclam derivative bearing a phosphinate-bis(phosphonate) pendant (H₅te1P^BP). The ligand showed a high selectivity to Cu^II over Zn^II and Ni^II ions, and the bis(phosphonate) group was not coordinated in the Cu^II complex, strongly interacting with other metal ions in solution. The Cu^II complex formed quickly, in 1 s, at pH 5 and at a millimolar scale. The complexation rates significantly differed under a ligand or metal ion excess due to the formation of reaction intermediates differing in their metal-to-ligand ratio and protonation state, respectively. The Cu^II-te1P^BP complex also showed a high resistance to acid-assisted hydrolysis (t_1/2 2.7 h; 1 M HClO₄, 25 °C) and was effectively adsorbed on the hydroxyapatite surface. H₅te1P^BP radiolabeling with [⁶⁴Cu]CuCl₂ was fast and efficient, with specific activities of approximately 30 GBq ⁶⁴Cu per 1 μmol of ligand (pH 5.5, room temperature, 30 min). In a pilot experiment, we further demonstrated the excellent suitability of [⁶⁴Cu]Cu^II-te1P^BP for imaging active bone compartments by dedicated small animal PET/CT in healthy mice and subsequently in a rat femoral defect model, in direct comparison with [¹⁸F]fluoride. Moreover, [⁶⁴Cu]Cu^II-te1P^BP showed a higher uptake in critical bone defect regions. Therefore, our study highlights the potential of [⁶⁴Cu]Cu^II-te1P^BP as a PET radiotracer for evaluating bone healing in preclinical and clinical settings with a diagnostic value similar to that of [¹⁸F]fluoride, albeit with a longer half-life (12.7 h) than ¹⁸F (1.8 h), thereby enabling extended observation times.

Cross-Bridged Cyclam with Phosphonate and Phosphinate Pendant Arms: Chelators for Copper Radioisotopes with Fast Complexation

Cross-bridged cyclam derivatives bearing two phosphonate (H₄L¹), bis(phosphinate) (H₄L²), or phosphinate (H₂L³) pendant arms were synthesized and studied with respect to their application as copper radioisotope carriers in nuclear medicine. The ligands show high macrocycle basicity (pK₁ > 14) and high Cu(II) complex stability (log K = 20-24). The complexation and dissociation kinetics of the Cu(II) complexes were studied by ultraviolet-visible spectroscopy. Phosphonate Cu(II)-H₄L¹ and bis(phosphinate) Cu(II)-H₄L² complexes form very quickly, reaching quantitative formation within 1 s at pH ∼6 and millimolar concentrations. Conversely, the formation of the phosphinate complex Cu(II)-H₂L³ is much slower (9 min at pH ∼6) due to the low stability of the out-of-cage reaction intermediate. All studied complexes are highly kinetically inert, showing half-lives of 120, 11, and 111 h for Cu(II)-H₄L¹, Cu(II)-H₄L², and Cu(II)-H₂L³ complexes, respectively, in 1 M HClO₄ at 90 °C. The high thermodynamic stability, fast formation, and extreme kinetic inertness of Cu(II) complexes indicate that phosphonate and bis(phosphinate) derivatives are promising ligands for nuclear medicine.

Al(iii)-NTA-fluoride: a simple model system for Al–F binding with interesting thermodynamics

Unsaturated Al^III complex shows a fast exchange of water molecules, hydroxide and fluoride anions in the coordination sphere, highly pH-dependent fluoride binding and release of fluorides at high pH or at high phosphate anion concentrations.

Improved Conjugation, 64-Cu Radiolabeling, in Vivo Stability, and Imaging Using Nonprotected Bifunctional Macrocyclic Ligands: Bis(Phosphinate) Cyclam (BPC) Chelators

Bifunctional derivatives of bis(phosphinate)-bearing cyclam (BPC) chelators bearing a carboxylate, amine, isothiocyanate, azide, or cyclooctyne in the BP side chain were synthesized. Conjugations required no protection of phosphinate or ring secondary amine groups. The ring amines were not reactive (proton protected) at pH < ∼8. For isothiocyanate coupling, oligopeptide N-terminal α-amines were more suitable than alkyl amines, e.g., Lys ω-amine (p K_a ∼7.5-8.5 and ∼10-11, respectively) due to lower basicity. The Cu-64 labeling was efficient at room temperature (specific activity ∼100 GBq/μmol; 25 °C, pH 6.2, ∼100 ligand equiv, 10 min). A representative Cu-64-BPC was tested in vivo showing fast clearance and no nonspecific radioactivity deposition. The monoclonal anti-PSCA antibody 7F5 conjugates with thiocyanate BPC derivative or NODAGA were radiolabeled and studied in PC3-PSCA tumor bearing mice by PET. The radiolabeled BPC conjugate was accumulated in the prostate tumor with a low off-target uptake, unlike Cu-64-labeled NODAGA-antibody conjugate. The BPC chelators have a great potential for theranostic applications of the Cu-64/Cu-67 matched pair.

NOTA Complexes with Copper(II) and Divalent Metal Ions: Kinetic and Thermodynamic Studies

H₃nota derivatives are among the most studied macrocyclic ligands and are widely used for metal ion binding in biology and medicine. Despite more than 40 years of chemical research on H₃nota, the comprehensive study of its solution chemistry has been overlooked. Thus, the coordination behavior of H₃nota with several divalent metal ions was studied in detail with respect to its application as a chelator for copper radioisotopes in medical imaging and therapy. In the solid-state structure of the free ligand in zwitterionic form, one proton is bound in the macrocyclic cavity through a strong intramolecular hydrogen-bond system supporting the high basicity of the ring amine groups (log K_a = 13.17). The high stability of the [Cu(nota)]^- complex (log K_ML = 23.33) results in quantitative complex formation, even at pH <1.5. The ligand is moderately selective for Cu(II) over other metal ions (e.g., log K_ML(Zn) = 22.32 and log K_ML(Ni) = 19.24). This ligand forms a more stable complex with Mg(II) than with Ca(II) and forms surprisingly stable complexes with alkali-metal ions (stability order Li(I) > Na(I) > K(I)). Thus, H₃nota shows high selectivity for small metal ions. The [Cu(nota)]^- complex is hexacoordinated at neutral pH, and the equatorial N₂O₂ interaction is strengthened by complex protonation. Detailed kinetic studies showed that the Cu(II) complex is formed quickly (millisecond time scale at c_Cu ≈ 0.1 mM) through an out-of-cage intermediate. Conversely, conductivity measurements revealed that the Zn(II) complex is formed much more slowly than the Cu(II) complex. The Cu(II) complex has medium kinetic inertness (τ_1/2 46 s; pH 0, 25 °C) and is less resistant to acid-assisted decomplexation than Cu(II) complexes with H₄dota and H₄teta. Surprisingly, [Cu(nota)]^- decomplexation is decelerated in the presence of Zn(II) ions due to the formation of a stable dinuclear complex. In conclusion, H₃nota is a good carrier of copper radionuclides because the [Cu(nota)]^- complex is predominantly formed over complexes with common impurities in radiochemical formulations, Zn(II) and Ni(II), for thermodynamic and, primarily, for kinetic reasons. Furthermore, the in vivo stability of the [Cu(nota)]^- complex may be increased due to the formation of dinuclear complexes when it interacts with biometals.

Lanthanide(iii) complexes of monophosphinate/monophosphonate DOTA-analogues: effects of the substituents on the formation rate and radiolabelling yield

Highly acidic additional coordinating groups in the pendant arms increase the radiolabelling yield of DOTA-like complexes.

Complexes of phosphonate and phosphinate derivatives of dipicolylamine

Phospho(i)nate derivatives of dipicolylamine show excellent selectivity for Zn(ii) over alkaline earth metal ions.

DOTA analogues with a phosphinate-iminodiacetate pendant arm: modification of the complex formation rate with a strongly chelating pendant

The formation of highly stable out-of-cage complexes slows down the transfer of the metal ion into the cavity of the macrocyclic ligand.

Amino acid based gallium-68 chelators capable of radiolabeling at neutral pH

Herein we show a flexible synthesis for bifunctional chelators based on amino acids that rapidly complex ⁶⁸Ga under physiological conditions.

Interaction of the Zn(ii)–cyclen complex with aminomethylphosphonic acid: original simultaneous potentiometric and 31P NMR data treatment

The interaction of aminomethylphosphonic acid with the Zn(ii)–cyclen complex was evaluated by an original combination of potentiometry and multinuclear NMR.

A DOTA based bisphosphonate with an albumin binding moiety for delayed body clearance for bone targeting

Radiolabeled bisphosphonates are commonly used in the diagnosis and therapy of bone metastases. Blood clearance of bisphosphonates is usually fast and only 30%-50% of the injected activity is retained in the skeleton, while most of the activity is excreted by the urinary tract. A longer blood circulation may enhance accumulation of bisphosphonate compounds in bone metastases. Therefore, a chemically modified macrocyclic bisphosphonate derivative with an additional human albumin binding entity was synthesized and pharmacokinetics of its complex was evaluated. The DOTA-bisphosphonate conjugate BPAMD was compared against the novel DOTAGA-derived albumin-binding bisphosphonate DOTAGA(428-d-Lys)M^BP (L1). The ligands were labeled with ⁶⁸Ga(III) and were evaluated in in vitro binding studies to hydroxyapatite (HA) as well as to human serum albumin. The compounds were finally compared in in vivo PET and ex vivo organ distribution studies in small animals over 6h. Binding studies revealed a consistent affinity of both bisphosphonate tracers to HA. Small animal PET and ex vivo organ distribution studies showed longer blood retention of [⁶⁸Ga]L1. [⁶⁸Ga]BPAMD is initially more efficiently bound to the bone but skeletal accumulation of the modified compound and [⁶⁸Ga]BPAMD equalized at 6h p.i. Ratios of femur epiphyseal plate to ordinary bone showed to be more favorable for [⁶⁸Ga]L1 than for [⁶⁸Ga]BPAMD due to the longer circulation time of the new tracer. Thus, the chemical modification of BPAMD toward an albumin-binding bisphosphonate, L1, resulted in a novel PET tracer which conserves advantages of both functional groups within one and the same molecule. The properties of this new diagnostic tracer are expected to be preserved in ¹⁷⁷Lu therapeutic agent with the same ligand (a theranostic pair).

(177)Lu-labelled macrocyclic bisphosphonates for targeting bone metastasis in cancer treatment

Background

Metastatic bone lesion is a common syndrome of many cancer diseases in an advanced state. The major symptom is severe pain, spinal cord compression, and pathological fracture, associated with an obvious morbidity. Common treatments including systemic application of bisphosphonate drugs aim on pain reduction and on improving the quality of life of the patient. Particularly, patients with multiple metastatic lesions benefit from bone-targeting therapeutic radiopharmaceuticals. Agents utilizing beta-emitting radionuclides in routine clinical praxis are, for example, [⁸⁹Sr]SrCl₂ and [¹⁵³Sm]Sm-EDTMP. No-carrier-added (n.c.a.) ¹⁷⁷Lu is remarkably suitable for an application in this scope.

Methods

Five 1,4,7,10-tetraazacyclododecane N,N′,N′′,N′′-tetra-acetic acid (DOTA)- and DO2A-based bisphosphonates, including monomeric and dimeric structures and one 1,4,7-triazacyclononane-1,4-diacetic acid (NO2A) derivative, were synthesized and labelled with n.c.a. ¹⁷⁷Lu. Radio-TLC and high-performance liquid chromatography (HPLC) methods were successfully established for determining radiochemical yields and for quality control. Their binding to hydroxyapatite was measured in vitro. Ex vivo biodistribution experiments and dynamic in vivo single photon computed tomography (SPECT)/CT measurements were performed in healthy rats for 5 min and 1 h periods. Data on %ID/g or standard uptake value (SUV) for femur, blood, and soft-tissue organs were analyzed and compared with [¹⁷⁷Lu]citrate.

Results

Radiolabelling yields for [¹⁷⁷Lu]Lu-DOTA and [¹⁷⁷Lu]Lu-NO2A monomeric bisphosphonate complexes were >98 % within 15 min. The dimeric macrocyclic bisphosphonates showed a decelerated labelling kinetics, reaching a plateau after 30 min of 60 to 90 % radiolabelling yields. All ¹⁷⁷Lu-bisphosphonate complexes showed exclusive accumulation in the skeleton. Blood clearance and renal elimination were fast. SUV data (all for 1 h p.i.) in the femur ranged from 3.34 to 5.67. The bone/blood ratios were between 3.6 and 135.6, correspondingly. ¹⁷⁷Lu-bisphosphonate dimers showed a slightly higher bone accumulation (SUV_femur = 4.48 ± 0.38 for [¹⁷⁷Lu]Lu-DO2A(P^BP)₂; SUV_femur = 5.41 ± 0.46 for [¹⁷⁷Lu]Lu-DOTA(M^BP)₂) but a slower blood clearance (SUV_blood = 1.25 ± 0.09 for [¹⁷⁷Lu]Lu-DO2A(P^BP)₂; SUV_blood = 1.43 ± 0.32 for [¹⁷⁷Lu]Lu-DOTA(M^BP)₂).

Conclusions

Lu-complexes of macrocyclic bisphosphonates might become options for the therapy of skeletal metastases in the near future, since they show high uptake in bone together with a very low soft-tissue accumulation.

Scandium(iii) complexes of monophosphorus acid DOTA analogues: a thermodynamic and radiolabelling study with 44Sc from cyclotron and from a 44Ti/44Sc generator

The influence of the phosphonic/phosphinic acid pendant arm in DOTA derivatives on properties of their Sc³⁺ complexes and efficiency of their ⁴⁴Sc labelling were investigated.

PiB-Conjugated, Metal-Based Imaging Probes: Multimodal Approaches for the Visualization of β-Amyloid Plaques

In an effort toward the visualization of β-amyloid plaques by in vivo imaging techniques, we have conjugated an optimized derivative of the Pittsburgh compound B (PiB), a well-established marker of Aβ plaques, to DO3A-monoamide that is capable of forming stable, noncharged complexes with different trivalent metal ions including Gd(3+) for MRI and (111)In(3+) for SPECT applications. Proton relaxivity measurements evidenced binding of Gd(DO3A-PiB) to the amyloid peptide Aβ1-40 and to human serum albumin, resulting in a two- and four-fold relaxivity increase, respectively. Ex vivo immunohistochemical studies showed that the DO3A-PiB complexes selectively target Aβ plaques on Alzheimer's disease human brain tissue. Ex vivo biodistribution data obtained for the (111)In-analogue pointed to a moderate blood-brain barrier (BBB) penetration in adult male Swiss mice (without amyloid deposits) with 0.36% ID/g in the cortex at 2 min postinjection.

Towards MRI contrast agents responsive to Ca(II) and Mg(II) ions: metal-induced oligomerization of dota-bisphosphonate conjugates

In magnetic resonance imaging (MRI), paramagnetic complexes are utilized as contrast agents. Much attention has been paid to the development of new contrast agents responsive to pH, temperature or concentration of various components of body liquids. We report a new type of MRI probe sensing the concentrations of calcium and magnesium in biological media. The ligand do3ap(BP) combines a dota-like chelator with a bisphosphonate group. In the complex, the Gd(III) ion is entrapped in the macrocyclic cavity whereas the bisphosphonate group is not coordinated and therefore is available for coordination with endogenous metal ions. In the presence of metal ions, Gd-do3ap(BP) appears to show formation of coordination oligomers leading to an unprecedented increase in r(1) up to 200-500%. The extremely high relaxivity response makes this type of compound interesting for further studies as MRI ion-responsive probes for biomedical research.