Bispidine Chelators for Radiopharmaceutical Applications with Lanthanide, Actinide, and Main Group Metal Ions

Targeted bismuth-based materials for cancer

The use of bismuth and its compounds in biomedicine has developed rapidly in recent years. Due to their unique properties, there are great opportunities for the development of new non-invasive strategies for the early diagnosis and effective treatment of cancers. This perspective highlights key fabrication methods to generate well-defined and clinically relevant bismuth materials of varying characteristics. On the one hand, this opens up a wide range of possibilities for unimodal and multimodal imaging. On the other hand, effective treatment strategies, which are increasingly based on combinatorial therapies, are given a great deal of attention. One of the biggest challenges remains the selective tumour targeting, whether active or passive. Here we present an overview on new developments of bismuth based materials moving forward from a simple enrichment at the tumour site via uptake by the mononuclear phagocytic system (MPS) to a more active tumour specific targeting via covalent modification with tumour-seeking molecules based on either small or antibody-derived molecules.

Bispidine coordination chemistry

Bispidines are extremely rigid ligands, easy to prepare in a large variety, with denticities of four to ten, various donor sets and charges, for mono- and oligonuclear transition metal, main group and rare earth complexes. In the last approx. 20 years significantly more than 50 new bispidine based ligands were prepared and their coordination chemistry studied. Biological probes and medicinal applications is one main area in bispidine coordination chemistry, where fast complex formation, high stability, metal ion selectivity and inertness are of utmost importance. Oxygen activation and oxidation catalysis is another main focus in bispidine coordination chemistry, with catalyst efficiency and stability as well as product selectivity as important requirements. Particularly successful applications in these areas are presented and discussed in detail, in addition to fundamental principles that show the importance of ligand rigidity, cavity size and shape as overarching fundamental properties.

Metal Complexes of Bispidine Derivatives: Achievements and Prospects for the Future

Multidentate bispidine ligands, including tetra-, penta-, hexa-, hepta-, and octadentate variants, exhibit strong coordination tendencies due to their intrinsic rigidity, significant reorganization potential, and ability to efficiently encapsulate metal ions. These structural attributes profoundly influence the thermodynamic stability, metal ion selectivity, redox behavior, and spin-state configuration of the resulting complexes. Metal ions, in turn, serve as highly suitable candidates for coordination due to their remarkable kinetic inertness, rapid complex formation kinetics, and low redox potential. This review focuses on ligands incorporating the bispidine core (3,7-diazabicyclo[3.3.1]nonane) and provides an overview of advancements in the synthesis of metal complexes involving p-, d-, and f-block elements. Furthermore, the rationale behind the growing interest in bispidine-based complexes for applications in radiopharmaceuticals, medicinal chemistry, and organic synthesis is explored, particularly in the context of their potential for diagnostic and catalytic drug development.

Octadentate Bispidine Chelators for Tb(III) Complexation: Pyridine Carboxylate versus Pyridine Phosphonate Donors

With their rigid and preorganized skeleton, bispidine (3,7-diazabicyclo[3.3.1]nonane) chelators are very appealing for the preparation of metal complexes with high kinetic inertness. With the aim to develop new Tb(III)-based medical imaging probes, this study describes the synthesis and physicochemical properties of two novel terbium(III) complexes with octadentate bispidine-based ligands substituted with either pyridine-phosphonate (H6L1) or picolinate (H4L2) subunits. Thermodynamic stability constants of the corresponding Tb(III) complexes have been determined by potentiometric, UV-visible absorption spectrophotometric and spectrofluorimetric methods. Despite their apparent similarity, these two octadentate ligands differ in their most stable conformation: chair-chair conformation for H4L2 and boat-chair conformation for H6L1, as confirmed by 1H NMR studies and suggested by physicochemical investigations. This conformational change induces different protonation schemes for the two ligands. The kinetic inertness of the Tb complexes has been studied in various media and assessed by transmetalation and transchelation experiments. In particular, Tb(L2) displayed a remarkable kinetic inertness with no measurable dissociation over two months in mouse serum at 10-5 M concentration. The complex was also very inert in the presence of a 50-fold excess of Zn(II) in H2O at pH = 7.4 (7% of dissociation over two months). The complexes with ligand L1 are significantly less inert, emphasizing the influence of the ligand conformation on the kinetic inertness of the Ln(III) complexes. Finally, the luminescence properties of the isolated complexes have also been investigated. A bright green luminescence was observed, especially for Tb(L2), which displays a high quantum yield value of 50% in H2O (60% in D2O; λexc = 263 nm). In addition, luminescence lifetimes of 1.9(2) and 1.7(2) ms have been measured for Tb(L1) and Tb(L2), respectively, hence confirming the formation of nona-coordinated complexes with one inner-sphere water molecule. These data on a bispidine scaffold pave the way for developing bright, inert luminescent probes for bioimaging and for radiolabeling applications with Tb(III) radioisotopes.

Chelation of [111In]In3+ with the dual-size-selective macrocycles py-macrodipa and py2-macrodipa

Indium-111 (111In) is a diagnostic radiometal that is important in nuclear medicine for single-photon emission computed tomography (SPECT). In order to apply this radiometal, it needs to be stably chelated and conjugated to a targeting vector that delivers it to diseased tissue. Identifying effective chelators that are capable of binding and retaining [111In]In3+in vivo is an important research area. In this study, two 18-membered macrocyclic chelators, py-macrodipa and py2-macrodipa, were investigated for their ability to form stable coordination complexes with In3+ and to be effectively radiolabeled with [111In]In3+. The In3+ complexes of these two chelators were characterized by NMR spectroscopy, X-ray crystallography, and density functional theory calculations. These studies show that both py-macrodipa and py2-macrodipa form 8-coordinate In3+ complexes and attain an asymmetric conformation, consistent with prior studies on this ligand class with small rare earth metal ions. Spectrophotometric titrations were carried out to determine the thermodynamic stability constants (log KML) of [In(py-macrodipa)]+ and [In(py2-macrodipa)]+, which were found to be 18.96(6) and 19.53(5), respectively, where the values in parentheses are the errors of the last significant figures obtained from the standard deviation from three independent replicates. Radiolabeling studies showed that py-macrodipa and py2-macrodipa can quantitatively be radiolabeled with [111In]In3+ at 25 °C within 5 min, even at ligand concentrations as low as 1 μM. The in vitro stability of the radiolabeled complexes was investigated in human serum at 37 °C, revealing that ∼90% of [111In][In(py-macrodipa)]+ and [111In][In(py2-macrodipa)]+ remained intact after 7 days. The biodistribution of these radiolabeled complexes in mice was investigated, showing lower uptake in the kidneys, liver, and blood at the 24 h mark compared to [111In]InCl3. These results demonstrate the potential of py-macrodipa and py2-macrodipa as chelators for [111In]In3+, suggesting their value for SPECT radiopharmaceuticals.

Scalability study on [133La]LaCl3 production with a focus on potential clinical applications

BACKGROUND

In recent years, targeted alpha therapy has gained importance in the clinics, and in particular, the alpha-emitter 225Ac plays a fundamental role in this clinical development. Nevertheless, depending on the chelating system no real diagnostic alternative has been established which shares similar chemical properties with this alpha-emitting radionuclide. In fact, the race to launch a diagnostic radionuclide to form a matched pair with 225Ac is still open, and 133La features attractive radiation properties to claim this place. However, in order to enable its translation into clinical use, upscaling of the production of this PET radionuclide is needed.

RESULTS

A study on optimal irradiation parameters, separation conditions and an exhaustive product characterization was carried out. In this framework, a proton irradiation of 2 h, 60 µA and 18.7 MeV produced 133La activities of up to 10.7 GBq at end of bombardment. In addition, the performance of four different chromatographic resins were tested and two optimized purification methods presented, taking approximately 20 min with a 133La recovery efficiencies of over 98%, decay corrected. High radionuclide purity and apparent molar activity was proved, of over 99.5% and 120 GBq/µmol, respectively, at end of purification. Furthermore, quantitative complexation of PSMA-617 and mcp-M-PSMA were obtained with molar activities up to 80 GBq/µmol. In addition, both 133La-radioconjugates offered high stability in serum, of over (98.5 ± 0.3)% and (99.20 ± 0.08)%, respectively, for up to 24 h. A first dosimetry estimation was also performed and it was calculated that an 133La application for imaging with between 350 and 750 MBq would only have an effective dose of 2.1-4.4 mSv, which is comparable to that of 18F and 68Ga based radiopharmaceuticals.

CONCLUSIONS

In this article we present an overarching study on 133La production, from the radiation parameters optimization to a clinical dose estimation. Lanthanum-133 activities in the GBq range could be produced, formulated as [133La]LaCl3 with high quality regarding radiolabeling and radionuclide purity. We believe that increasing the 133La availability will further promote the development of radiopharmaceuticals based on macropa or other chelators suitable for 225Ac.

Extension of the D3 and D4 London dispersion corrections to the full actinides series

Efficient dispersion corrections are an indispensable component of modern density functional theory, semi-empirical quantum mechanical, and even force field methods. In this work, we extend the well established D3 and D4 London dispersion corrections to the full actinides series, francium, and radium. To keep consistency with the existing versions, the original parameterization strategy of the D4 model was only slightly modified. This includes improved reference Hirshfeld atomic partial charges at the ωB97M-V/ma-def-TZVP level to fit the required electronegativity equilibration charge (EEQ) model. In this context, we developed a new actinide data set called AcQM, which covers the most common molecular actinide compound space. Furthermore, the efficient calculation of dynamic polarizabilities that are needed to construct CAB6 dispersion coefficients was implemented into the ORCA program package. The extended models are assessed for the computation of dissociation curves of actinide atoms and ions, geometry optimizations of crystal structure cutouts, gas-phase structures of small uranium compounds, and an example extracted from a small actinide complex protein assembly. We found that the novel parameterizations perform on par with the computationally more demanding density-dependent VV10 dispersion correction. With the presented extension, the excellent cost-accuracy ratio of the D3 and D4 models can now be utilized in various fields of computational actinide chemistry and, e.g., in efficient composite DFT methods such as r2SCAN-3c. They are implemented in our freely available standalone codes (dftd4, s-dftd3) and the D4 version will be also available in the upcoming ORCA 6.0 program package.

PYTA: a universal chelator for advancing the theranostic palette of nuclear medicine

To clinically advance the growing arsenal of radiometals available to image and treat cancer, chelators with versatile binding properties are needed. Herein, we evaluated the ability of the py2[18]dieneN6 macrocycle PYTA to interchangeably bind and stabilize 225Ac3+, [177Lu]Lu3+, [111In]In3+ and [44Sc]Sc3+, a chemically diverse set of radionuclides that can be used complementarily for targeted alpha therapy, beta therapy, single-photon emission computed tomography (SPECT) imaging, and positron emission tomography (PET) imaging, respectively. Through NMR spectroscopy and X-ray diffraction, we show that PYTA possesses an unusual degree of flexibility for a macrocyclic chelator, undergoing dramatic conformational changes that enable it to optimally satisfy the disparate coordination properties of each metal ion. Subsequent radiolabeling studies revealed that PYTA quantitatively binds all 4 radiometals at room temperature in just minutes at pH 6. Furthermore, these complexes were found to be stable in human serum over 2 half-lives. These results surpass those obtained for 2 state-of-the-art chelators for nuclear medicine, DOTA and macropa. The stability of 225Ac-PYTA and [44Sc]Sc-PYTA, the complexes having the most disparity with respect to metal-ion size, was further probed in mice. The resulting PET images (44Sc) and ex vivo biodistribution profiles (44Sc and 225Ac) of the PYTA complexes differed dramatically from those of unchelated [44Sc]Sc3+ and 225Ac3+. These differences provide evidence that PYTA retains this size-divergent pair of radionuclides in vivo. Collectively, these studies establish PYTA as a new workhorse chelator for nuclear medicine and warrant its further investigation in targeted constructs.

Experimental and Theoretical Insight into the Ionic Liquid-Mediated Complexation of Trivalent Lanthanides with β-Diketone and Its Fluorinated Analogue

A multitechnique approach with theoretical insights has been employed to understand the complexation of trivalent lanthanides with two β-diketones, viz. 1-phenyl-1,3-butanedione (L1) and 4,4,4-trifluoro-1-phenyl-1,3-butanedione (L2), in an ionic liquid (C6mim·NTf2). UV-vis spectral analysis of complexation using Nd3+ revealed the predominance of ML2+ and ML4- species. The stability constants for the PB complexes were higher (β2 ∼ 10.45 ± 0.05, β4 ∼ 15.51 ± 0.05) than those for the TPB (β2 ∼ 7.56 ± 0.05, β4 ∼ 13.19 ± 0.06). The photoluminescence titration using Eu3+ corroborated the same observations with slightly higher stability constants, probably due to the higher ionic potential of Eu3+. The more asymmetric (AL2ML4 ∼ 5.2) Eu-L2 complex was found to contain one water molecule in the primary coordination sphere of Eu3+ with more covalency of the Eu3+-O bond (Ω2L1 = 8.5 × 10-20, Ω4L1 = 1.3 × 10-20) compared to the less asymmetric Eu-L1 complex (AL1ML4 ∼ 3.5) with two water molecules having less Eu-O covalency (Judd-Offelt parameters: Ω2L1 = 7.3 × 10-20, Ω4L1 = 1.0 × 10-20). Liquid-liquid extraction studies involving Nd3+ and Eu3+ revealed the formation of the ML4- complex following an 'anion exchange' mechanism. The shift of the enol peak from 1176 to 1138 cm-1 on the complexation of the β-diketones with Eu3+ was confirmed from the FTIR spectra. 1H NMR titration of the β-diketones with La(NTf2)3 evidenced the participation of α-H of the β-diketones and protons at C2, C4, and C5 positions of the methylimidazolium ring. For the ML2 complex, 4 donor O atoms are suggested to coordinate to the trivalent lanthanides with bond distances of 2.3297-2.411 Å for La-O, 2.206-2.236 Å for Eu-O, and 2.217-2.268 Å for Nd-O, respectively, while for the ML4 complex, 8 donor O atoms were coordinated with bond lengths of 2.506-2.559 Å for La-O, 2.367-2.447 Å for Eu-O, and 2.408-2.476 Å for Nd-O. The Nd3+ ion was higher by 9.7 kcal·mol-1 than that of the La3+ ion for the 1:4 complex. The complexation energy with L1 was quite higher than that with L2 for both 1:2 and 1:4 complexes. Using cyclic voltammetry, the redox behavior of trivalent lanthanides Eu and Gd with β-diketonate in ionic liquid medium was probed and their redox energetic and kinetic parameters were determined.