Rearmed Bifunctional Chelating Ligand for 225Ac/155Tb Precision-Guided Theranostic Radiopharmaceuticals─H4noneunpaX.

Superior bifunctional chelating ligands, which can sequester both α-emitting radionuclides (225Ac, 213Bi) and their diagnostic companions (155Tb, 111In), remain a formidable challenge to translating targeted alpha therapy, with complementary diagnostic imaging, to the clinic. H4noneupaX, a chelating ligand with an unusual diametrically opposed arrangement of pendant donor groups, has been developed to this end. H4noneunpaX preferentially complexes Ln3+ and An3+ ions, forming thermodynamically stable (pLa = 17.8, pLu = 21.3) and kinetically inert complexes─single isomeric species by nuclear magnetic resonance and density functional theory. Metal binding versatility demonstrated in radiolabeling [111In]In3+, [155Tb]Tb3+, [177Lu]Lu3+, and [225Ac]Ac3+ achieved high molar activities under mild conditions. Efficient, scalable synthesis enabled in vivo evaluation of bifunctional H4noneunpaX conjugated to two octreotate peptides targeting neuroendocrine tumors. Single photon emission computed tomography/CT and biodistribution studies of 155Tb-radiotracers in AR42J tumor-bearing mice showed excellent image contrast, good tumor uptake, and high in vivo stability. H4noneunpaX shows significant potential for theranostic applications involving 225Ac/155Tb or 177Lu/155Tb.

Production and radiochemistry of antimony-120m: Efforts toward Auger electron therapy with 119Sb.

Targeted Meitner-Auger Therapy (TMAT) has potential for personalized treatment thanks to its subcellular dosimetric selectivity, which is distinct from the dosimetry of ß- and α particle emission based Targeted Radionuclide Therapy (TRT). To date, most clinical and preclinical TMAT studies have used commercially available radionuclides. These studies showed promising results despite using radionuclides with theoretically suboptimal photon to electron ratios, decay kinetics, and electron emission spectra. Studies using radionuclides whose decay characteristics are considered more optimal are therefore important for evaluation of the full potential of Meitner-Auger therapy; 119Sb is among the best such candidates. In the present work, we develop radiochemical purification of 120Sb from irradiated natural tin targets for TMAT studies with 119Sb.

Toward 68Ga and 64Cu Positron Emission Tomography Probes: Is H2dedpa-N,N'-pram the Missing Link for dedpa Conjugation?

H2dedpa-N,N'-pram (H2L1), a new chelator derived from the hexadentate ligand 1,2-bis[[(6-carboxypyridin-2-yl)methyl]amino]ethane (H2dedpa), which incorporates 3-propylamine chains anchored to the secondary amines of the ethylenediamine core of the latter, has emerged as a very promising scaffold for preparing 68Ga- and 64Cu-based positron emission tomography probes. This new platform is cost-effective and easy to prepare, and the two pendant primary amines make it versatile for the preparation of bifunctional chelators by conjugation and/or click chemistry. Reported herein, we have also included the related H2dedpa-N,N'-prpta (H2L2) platform as a simple structural model for its conjugated systems. X-ray crystallography confirmed that the N4O2 coordination sphere provided by the dedpa2- core is maintained at both Ga(III) and Cu(II). The complex formation equilibria were deeply investigated by a thorough multitechnique approach with potentiometric, NMR spectrometric, and UV-vis spectrophotometric titrations, revealing effective chelation. The thermodynamic stability of the Ga(III) complexes at physiological relevant conditions is slightly higher than that of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), the common and clinically approved chelator used in the clinic [pGa = 19.5 (dedpa-N,N'-pram) and 20.8 (dedpa-N,N'-prpta) versus 18.5 (DOTA) at identical conditions], and significantly higher for the Cu(II) complexes [pCu = 21.96 (dedpa-N,N'-pram) and 22.8 (dedpa-N,N'-prpta) versus 16.2 (DOTA)], which are even more stable than that of the parent ligand dedpa2- (pCu = 18.5) and that of 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) (pCu = 18.5). This high stability found for Cu(II) complexes is related to the conversion of the secondary amines of the ethylenediamine core of dedpa2- into tertiary amines, whereby the architecture of the new H2L1 chelator is doubly optimal in the case of this metal ion: high accessibility of the primary amine groups and their incorporation via the secondary amines, which contributes to a significant increase in the stability of the metal complex. Quantitative labeling of both chelators with both radionuclides ([68Ga]Ga3+ and [64Cu]Cu2+) was observed within 15 min at room temperature with concentrations as low as 10-5 M. Furthermore, serum stability studies confirmed a high radiochemical in vitro stability of all systems and therefore confirmed H2L1 as a promising and versatile chelator for further radiopharmaceutical in vivo studies.

H3TPAN-Triazole-Bn-NH2: Tripicolinate Clicked-Bifunctional Chelate for [225Ac]/[111In] Theranostics.

A new, high-denticity, bifunctional ligand─H3TPAN-triazole-Bn-NH2─has been synthesized and studied in complexation with [225Ac]Ac3+ and [111In]In3+ for radiopharmaceutical applications. The bifunctional chelator is readily synthesized, using a high-yielding four-step prep, which is highly adaptable and allows for straightforward incorporation of different covalent linkers using CuI-catalyzed alkyne-azide cycloaddition (click) chemistry. Nuclear magnetic resonance (NMR) studies of H3TPAN-triazole-Bn-NH2 with La3+ and In3+ metal ions show the formation of a single, asymmetric complex with each ion in solution, corroborated by density functional theory (DFT) calculations. Radiolabeling studies with [225Ac]Ac3+ and [111In]In3+ showed highly effective complexation, achieving quantitative radiochemical conversions at low ligand concentrations (<10-6 M) under mild conditions (RT, 10 min), which is further accompanied by high stability in human serum. The bioconjugate─H3TPAN-triazole-Bn-Aoc-Pip-Nle-CycMSHhex─was prepared for targeting of MC1R-positive tumors, and the corresponding 111In-radiolabeled tracer was studied in vivo. SPECT/CT and biodistribution studies in C57BL/6J mice bearing B16-F10 tumors were performed, with the radiotracer showing good in vivo stability; tumor uptake was achieved. This work highlights a new promising and versatile bifunctional chelator, easily prepared and encouraging for 225Ac/111In theranostics.

H4picoopa─Robust Chelate for 225Ac/111In Theranostics.

The nuclear decay characteristics of 225Ac (Eα = 5-8 MeV, linear energy transfer (LET) = ∼100 keV/µm, t1/2 = 9.92 days) are well recognized as advantageous for the treatment of primary and metastatic tumors; however, suitable chelation systems are required, which can accommodate this radiometal. Since 225Ac does not possess any suitable low-energy, high abundance γ-ray emissions for nuclear imaging, there is a clear need for the development of other companion radionuclides with similar coordination characteristics and comparable half-lives, which can be applied in diagnostics. H4picoopa was designed and executed as a high-denticity ligand for chelation of [225Ac]Ac3+, and the complexation characteristics have been explored through nuclear magnetic resonance (NMR) spectroscopy, solution thermodynamic stability studies, and radiolabeling. The ligand shows highly favorable complexation with La3+ (pM = 17.6), Lu3+ (pM = 21.3), and In3+ (pM = 31.2) and demonstrates effective radiolabeling of both [225Ac]Ac3+ and [111In]In3+ ions achieving quantitative radiochemical conversions (RCCs) under mild conditions (RT, 10 min), accompanied by high serum stability (>97% radiochemical purity (RCP) over 6 days). A bifunctional analogue of H4picoopa was synthesized and conjugated to the Pip-Nle-CycMSHhex peptide for targeting of MC1R positive melanoma tumors. In vivo single-photon emission computed tomography (SPECT) and biodistribution studies of the 111In-radiolabeled bioconjugate in mice bearing B16-F10 tumors showed good radiotracer stability, although improved tumor targeting could not be achieved for imaging purposes. This work highlights H4picoopa as a very promising platform for application of [225Ac]Ac3+ and [111In]In3+ as a theranostic pair and allows great versatility for the incorporation of other directing vectors. The logical synthetic approach reported here for bifunctional H4picoopa, involving an azide-functionalized covalent linker and CuI-catalyzed alkyne-azide cycloaddition, allows for ease of optimization of bioconjugate pharmacokinetics and will be valuable for further radiopharmaceutical applications moving forward.

Toward Bifunctional Chelators for Thallium-201 for Use in Nuclear Medicine.

Auger electron therapy exploits the cytotoxicity of low-energy electrons emitted during radioactive decay that travel very short distances (typically <1 µm). 201Tl, with a half-life of 73 h, emits ∼37 Auger and other secondary electrons per decay and can be tracked in vivo as its gamma emissions enable SPECT imaging. Despite the useful nuclear properties of 201Tl, satisfactory bifunctional chelators to incorporate it into bioconjugates for molecular targeting have not been developed. H4pypa, H5decapa, H4neunpa-NH2, and H4noneunpa are multidentate N- and O-donor chelators that have previously been shown to have high affinity for 111In, 177Lu, and 89Zr. Herein, we report the synthesis and serum stability of [nat/201Tl]Tl3+ complexes with H4pypa, H5decapa, H4neunpa-NH2, and H4noneunpa. All ligands quickly and efficiently formed complexes with [201Tl]Tl3+ that gave simple single-peak radiochromatograms and showed greatly improved serum stability compared to DOTA and DTPA. [natTl]Tl-pypa was further characterized using nuclear magnetic resonance spectroscopy (NMR), mass spectroscopy (MS), and X-ray crystallography, showing evidence of the proton-dependent presence of a nine-coordinate complex and an eight-coordinate complex with a pendant carboxylic acid group. A prostate-specific membrane antigen (PSMA)-targeting bioconjugate of H4pypa was synthesized and radiolabeled. The uptake of [201Tl]Tl-pypa-PSMA in DU145 PSMA-positive and PSMA-negative prostate cancer cells was evaluated in vitro and showed evidence of bioreductive release of 201Tl and cellular uptake characteristic of unchelated [201Tl]TlCl. SPECT/CT imaging was used to probe the in vivo biodistribution and stability of [201Tl]Tl-pypa-PSMA. In healthy animals, [201Tl]Tl-pypa-PSMA did not show the myocardial uptake that is characteristic of unchelated 201Tl. In mice bearing DU145 PSMA-positive and PSMA-negative prostate cancer xenografts, the uptake of [201Tl]Tl-pypa-PSMA in DU145 PSMA-positive tumors was higher than that in DU145 PSMA-negative tumors but insufficient for useful tumor targeting. We conclude that H4pypa and related ligands represent an advance compared to conventional radiometal chelators such as DOTA and DTPA for Tl3+ chelation but do not resist dissociation for long periods in the biological environment due to vulnerability to reduction of Tl3+ and subsequent release of Tl+. However, this is the first report describing the incorporation of [201Tl]Tl3+ into a chelator-peptide bioconjugate and represents a significant advance in the field of 201Tl-based radiopharmaceuticals. The design of the next generation of chelators must include features to mitigate this susceptibility to bioreduction, which does not arise for other trivalent heavy radiometals.

Trastuzumab-conjugated oxine-based ligand for [89Zr]Zr4+ immunoPET.

A new, bifunctional chelating ligand for immuno-Positron Emission Tomography (PET) was designed, synthesized, and conjugated to Trastuzumab for a proof-of-concept study with 89Zr. H4neunox was synthesized from the tris(2-aminoethyl)amine backbone, decorated with 8-hydroxyquinoline moieties, and utilizes a primary amine for functionalization. A maleimide moiety extends the chelator to create H4neunox-mal for antibody conjugation via maleimide-thiol click chemistry. Preliminary 89Zr radiolabeling of H4neunox indicated quantitative radiolabeling at 1 × 10-5 M, but improved inertness towards human serum (96% intact at 7 d) and Fe3+ (92% intact at 24 h) compared to the previously synthesized H5decaox. The chelator was successfully conjugated to the monoclonal antibody, Trastuzumab, and used in preliminary radiolabeling reactions (37 °C, 2 h) with 89Zr. Radiochemical assessments of the new H4neunox-Trastuzumab conjugate include 89Zr radiolabeling, spin filter purification, cell-binding immunoreactivity, and in vivo PET imaging and biodistribution in SKOV-3 tumour bearing nude mice, performed in comparison with the desferrioxamine B analog, DFO-Trastuzumab. The [89Zr]Zr(neunox-Trastuzumab) showed lowered inertness towards serum (76% intact at 24 h) as well as demetallation in vivo through bone uptake (21% ID/g) in PET imaging and biodistribution studies when compared to [89Zr]Zr(DFO-Trastuzumab). Although the combination of the chelator and antibody had detrimental effects on their intended purposes, nonetheless, the primary amine platform of H4neunox developed here provides an oxine-based bifunctional ligand for further derivatizations with other targeting vectors.

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.

Bis(amido)bis(oxinate)diamine Ligands for theranostic radiometals.

With the interest in radiometal-containing diagnostic and therapeutic pharmaceuticals increasing rapidly, appropriate ligands to coordinate completely and stably said radiometals is essential. Reported here are two novel, bis(amido)bis(oxinate)diamine ligands, H2amidohox (2,2'-(ethane-1,2-diylbis(((8-hydroxyquinolin-2-yl)methyl)azanediyl))diacetamide) and H2amidoC3hox (2,2'-(propane-1,3-diylbis(((8-hydroxyquinolin-2-yl)methyl)azanediyl))diacetamide), that combine two 8-hydroxyquinoline and amide donor groups and differ by one carbon in their 1,2-ethylenediamine vs. 1,3-diaminopropane backbones, respectively. Both ligands have been thoroughly studied via metal complexation, solution thermodynamics and radiolabeling with three radiometal ions: [nat/64Cu]Cu2+, [nat/111In]In3+, and [nat/203Pb]Pb2+. X-ray crystallography determined the structures of the hexacoordinated Cu2+-ligand complexes, indicating a better fit of Cu2+ to the H2amidohox binding pocket. Concentration dependent radiolabeling with [64Cu]Cu2+ was successfully quantitative as low as 1 µM with H2amidohox and 10 µM with H2amidoC3hox within 5 min at room temperature. However, [64Cu][Cu(amidohox)] maintained higher kinetic inertness against a superoxide dismutase enzyme-challenge assay and ligand challenges compared to the [64Cu][Cu(amidoC3hox)] counterpart. Similarly, H2amidohox had significantly higher radiochemical conversion with both [111In]In3+ (97% at 1 µM) and [203Pb]Pb2+ (97% at 100 µM) under mild conditions compared to H2amidoC3hox (76% with [111In]In3+ at 1 µM and 0% with [203Pb]Pb2+). By studying non-radioactive and radioactive complexation with both ligands, a comprehensive understanding of the coordination differences between two- and three­carbon diamine backbones is discussed. Overall, the ethylenediamine backbone of H2amidohox proves to be superior in rapid, mild radiolabeling and kinetic inertness towards competing ligands and proteins.

[213Bi]Bi3+/[111In]In3+-neunpa-cycMSH: Theranostic Radiopharmaceutical Targeting Melanoma─Structural, Radiochemical, and Biological Evaluation.

With the emergence of [225Ac]Ac3+ as a therapeutic radionuclide for targeted α therapy (TAT), access to clinical quantities of the potent, short-lived α-emitter [213Bi]Bi3+ (t1/2 = 45.6 min) will increase over the next decade. With this in mind, the nonadentate chelator, H4neunpa-NH2, has been investigated as a ligand for chelation of [213Bi]Bi3+ in combination with [111In]In3+ as a suitable radionuclidic pair for TAT and single photon emission computed tomography (SPECT) diagnostics. Nuclear magnetic resonance (NMR) spectroscopy was utilized to assess the coordination characteristics of H4neunpa-NH2 on complexation of [natBi]Bi3+, while the solid-state structure of [natBi][Bi(neunpa-NH3)] was characterized via X-ray diffraction (XRD) studies, and density functional theory (DFT) calculations were performed to elucidate the conformational geometries of the metal complex in solution. H4neunpa-NH2 exhibited fast complexation kinetics with [213Bi]Bi3+ at RT achieving quantitative radiolabeling within 5 min at 10-8 M ligand concentration, which was accompanied by the formation of a kinetically inert complex. Two bioconjugates incorporating the melanocortin 1 receptor (MC1R) targeting peptide Nle-CycMSHhex were synthesized featuring two different covalent linkers for in vivo evaluation with [213Bi]Bi3+ and [111In]In3+. High molar activities of 7.47 and 21.0 GBq/µmol were achieved for each of the bioconjugates with [213Bi]Bi3+. SPECT/CT scans of the [111In]In3+-labeled tracer showed accumulation in the tumor over time, which was accompanied by high liver uptake and clearance via the hepatic pathway due to the high lipophilicity of the covalent linker. In vivo biodistribution studies in C57Bl/6J mice bearing B16-F10 tumor xenografts showed good tumor uptake (5.91% ID/g) at 1 h post-administration with [213Bi][Bi(neunpa-Ph-Pip-Nle-CycMSHhex)]. This study demonstrates H4neunpa-NH2 to be an effective chelating ligand for [213Bi]Bi3+ and [111In]In3+, with promising characteristics for further development toward theranostic applications.

Harnessing α-Emitting Radionuclides for Therapy: Radiolabeling Method Review.

Targeted α-therapy (TAT) is an emerging powerful tool treating late-stage cancers for which therapeutic options are limited. At the core of TAT are targeted radiopharmaceuticals, where isotopes are paired with targeting vectors to enable tissue- or cell-specific delivery of α-emitters. DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and DTPA (diethylenetriamine pentaacetic acid) are commonly used to chelate metallic radionuclides but have limitations. Significant efforts are underway to develop effective stable chelators for α-emitters and are at various stages of development and community adoption. Isotopes such as 149Tb, 212/213Bi, 212Pb (for 212Bi), 225Ac, and 226/227Th have found suitable chelators, although further studies, especially in vivo studies, are required. For others, including 223Ra, 230U, and, arguably 211At, the ideal chemistry remains elusive. This review summarizes the methods reported to date for the incorporation of 149Tb, 211At, 212/213Bi, 212Pb (for 212Bi), 223Ra, 225Ac, 226/227Th, and 230U into radiopharmaceuticals, with a focus on new discoveries and remaining challenges.

Glypican-3-Targeted 227Th α-Therapy Reduces Tumor Burden in an Orthotopic Xenograft Murine Model of Hepatocellular Carcinoma.

Hepatocellular carcinoma (HCC) is a significant cause of morbidity and mortality worldwide, with limited therapeutic options for advanced disease. Targeted α-therapy is an emerging class of targeted cancer therapy in which α-particle-emitting radionuclides, such as 227Th, are delivered specifically to cancer tissue. Glypican-3 (GPC3) is a cell surface glycoprotein highly expressed on HCC. In this study, we describe the development and in vivo efficacy of a 227Th-labeled GPC3-targeting antibody conjugate (227Th-octapa-αGPC3) for treatment of HCC in an orthotopic murine model. Methods: The chelator p-SCN-Bn-H4octapa-NCS (octapa) was conjugated to a GPC3-targeting antibody (αGPC3) for subsequent 227Th radiolabeling (octapa-αGPC3). Conditions were varied to optimize radiolabeling of 227Th. In vitro stability was evaluated by measuring the percentage of protein-bound 227Th by γ-ray spectroscopy. An orthotopic athymic Nu/J murine model using HepG2-Red-FLuc cells was developed. Biodistribution and blood clearance of 227Th-octapa-αGPC3 were evaluated in tumor-bearing mice. The efficacy of 227Th-octapa-αGPC3 was assessed in tumor-bearing animals with serial measurement of serum α-fetoprotein at 23 d after injection. Results: Octapa-conjugated αGPC3 provided up to 70% 227Th labeling yield in 2 h at room temperature. In the presence of ascorbate, at least 97.8% of 227Th was bound to αGPC3-octapa after 14 d in phosphate-buffered saline. In HepG2-Red-FLuc tumor-bearing mice, highly specific GPC3 targeting was observed, with significant 227Th-octapa-αGPC3 accumulation in the tumor over time and minimal accumulation in normal tissue. Twenty-three days after treatment, a significant reduction in tumor burden was observed in mice receiving a 500 kBq/kg dose of 227Th-octapa-αGPC3 by tail-vein injection. No acute off-target toxicity was observed, and no animals died before termination of the study. Conclusion:227Th-octapa-αGPC3 was observed to be stable in vitro; maintain high specificity for GPC3, with favorable biodistribution in vivo; and result in significant antitumor activity without significant acute off-target toxicity in an orthotopic murine model of HCC.

[nat/89Zr][Zr(pypa)]: Thermodynamically Stable and Kinetically Inert Binary Nonadentate Complex for Radiopharmaceutical Applications.

H4pypa is a nonadentate nonmacrocyclic chelator, which previously demonstrated high affinity for scandium-44, lutetium-177, and indium-111. Herein, we report the highly stable binary [Zr(pypa)] complex; the nonradioactive complex was synthesized and characterized in detail using high-resolution electrospray-ionization mass spectroscopy (HR-ESI-MS) and various nuclear magnetic resonance spectroscopies (NMR), which revealed C2v symmetry of the complex. The geometry of [Zr(pypa)] was further detailed via X-ray crystallography and compared with the structure of [Fe(Hpypa)]. Despite a slow complexation rate with an association half-life of 31.4 h at pH 2 and room temperature, the [Zr(pypa)] complex is thermodynamically stable (log KML = 38.92, pZr = 39.4). Radiochemical studies demonstrated quantitative radiolabeling achieved at 10 µM chelator concentration within 2 h at 40 °C and pH = 7, antibody-compatible conditions. Of the utmost importance, [89Zr][Zr(pypa)] is highly kinetically inert upon challenge with excess EDTA and DFO ligands, superior to [89Zr][Zr(DFO)]+, and maintains inertness toward human serum.

Inorganic radiopharmaceutical chemistry of oxine.

8-Hydroxyquinoline (8-HQ, oxine) is a small, monoprotic, bicyclic aromatic compound and its relative donor group orientation imparts impressive bidentate metal chelating abilities that have been exploited in a vast array of applications over decades. 8-HQ and its derivatives have been explored in medicinal applications including anti-neurodegeneration, anticancer properties, and antimicrobial activities. One long established use of 8-HQ in medicinal inorganic chemistry is the coordination of radioactive isotopes of metal ions in nuclear medicine. The metal-oxine complex with the single photon emission computed tomography (SPECT) imaging isotope [111In]In3+ was developed in the 1970s and 1980s to radiolabel leukocytes for inflammation and infection imaging. The [111In][In(oxine)3] complex functions as an ionophore: a moderately stable lipophilic complex to enter cells; however, inside the cell environment [111In]In3+ undergoes exchange and remains localized. As new developments have progressed towards radiopharmaceuticals capable of both imaging and therapy (theranostics), 8-HQ has been re-explored in recent years to investigate its potential to chelate larger radiometal ions with longer half-lives and different indications. Further, metal-oxine complexes have been used to study liposomes and other nanomaterials by tracking these nanomedicines in vivo. Expanding 8-HQ to multidentate ligands for highly thermodynamically stable and kinetically inert complexes has increased the possibilities of this small molecule in nuclear medicine. This article outlines the historic use of metal-oxine complexes in inorganic radiopharmaceutical chemistry, with a focus on recent advances highlighting the possibilities of developing higher denticity, targeted bifunctional chelators with 8-HQ.

Getting a lead on Pb2+-amide chelators for 203/212Pb radiopharmaceuticals.

Amide-based chelators DTPAm, EGTAm and ampam were synthesized to investigate which chelator most ideally coordinates [nat/203Pb]Pb2+ ions for potential radiopharmaceutical applications. 1H NMR spectroscopy was used to study each metal-ligand complex in the solution state. The 1H NMR spectrum of [Pb(DTPAm)]2+ revealed minimal isomerization and fluxional behaviour compared to [Pb(EGTAm)]2+ and [Pb(ampam)]2+, both of which showed fewer spectral changes indicative of less static behaviour. The solid-state coordination properties of each complex were also examined from single crystal structures that were studied by X-ray diffraction (XRD). In the solid-state, octadentate DTPAm coordinated Pb2+ to form an eight-coordinate hemidirected complex; octadentate EGTAm coordinated Pb2+ forming a ten-coordinate holodirected complex with a bidentate NO3- ion also coordinated to the metal centre; decadentate ampam completely encapsulated the Pb2+ ion to form a ten-coordinate holodirected complex with a C2 axis of symmetry. Potentiometric titrations were carried out to assess the thermodynamic stability of each metal-ligand complex. The pM values obtained for [Pb(DTPAm)]2+, [Pb(EGTAm)]2+ and [Pb(ampam)]2+ were 9.7, 7.2 and 10.2, respectively. The affinity of each chelator for Pb2+ ions was tested by [203Pb]Pb2+ radiolabeling studies to evaluate their prospects as chelators for [203/212Pb]Pb2+-based radiopharmaceuticals. DTPAm radiolabeled [203Pb]Pb2+ ions achieving molar activities as high as 3.5 MBq µmol-1 within 15 minutes, at 25 °C, whereas EGTAm and ampam produced lower molar activities of 0.25 MBq µmol-1 within 30 minutes, at 37 °C. EGTAm and ampam were therefore deemed unsuitable for [203/212Pb]Pb2+-based radiopharmaceutical applications, while DTPAm warrants further studies.

H4HBEDpa: Octadentate Chelate after A. E. Martell.

H4HBEDpa, a new octadentate chelator inspired by the 1960s ligand HBED of Arthur E. Martell, has been investigated for a selection of trivalent metal ions useful in diagnostic and therapeutic applications (Sc3+, Fe3+, Ga3+, In3+, and Lu3+). Complex formation equilibria were thoroughly investigated using combined potentiometric and UV-vis spectrophotometric titrations which revealed effective chelation and high metal-sequestering capacity, in particular for Fe3+, log KFeL = 36.62, [Fe(HBEDpa)]-. X-ray diffraction study of single crystals revealed that the ligand is preorganized and forms hexa-coordinated complexes with Fe3+ and Ga3+ at acidic pH. Density functional theory (DFT) calculations were applied to probe the geometries and energies of all the possible conformers of [M(HBEDpa)]- (M = Sc3+, Fe3+, Ga3+, In3+, and Lu3+). DFT calculations confirmed the experimental findings, indicating that [Fe(HBEDpa)]- is bound tightly in an asymmetric pattern as compared to the symmetrically bound and more open [Ga(HBEDpa)]-, prone to hydrolysis at higher pH. DFT calculations also showed that a large metal ion such as Lu3+ fully coordinates with HBEDpa4-, forming a binary octadentate complex in its lowest-energy form. Smaller metal ions form six or seven coordinate complexes with HBEDpa4-.

H2pyhox - Octadentate Bis(pyridyloxine).

A new versatile chelating ligand for intermediate size and softness radiometals [64Cu]Cu2+ and [111In]In3+, H2pyhox, was synthesized by introducing pyridine as a new donor moiety to complement 8-hydroxyquinoline on an ethylenediamine backbone. The combination of pyridine and oxine as donor sets was explored through structural analysis, and crystals of the three metal complexes with Cu2+, La3+, and In3+ demonstrate how the ligand adapts to accommodate metal ions of different sizes and charge. Exhaustive in-batch UV solution studies characterized the protonation constants of the free ligand as well as the formation constants of the metal complexes with Cu2+, In3+, and La3+. Preliminary concentration-dependent radiolabeling studies with [111In]In3+ and [64Cu]Cu2+ show the robustness of H2pyhox to successfully coordinate both radiometals under mild conditions (<15 min, room temperature, pH 6). H2pyhox is the first oxinate ligand to successfully radiolabel [225Ac]Ac3+, albeit only at high concentrations (0.1-1 mM) with gentle heating to 37 °C. Whole serum, protein, and ligand challenge assays further demonstrate the kinetic inertness of the [111In]In3+ and [64Cu]Cu2+ radiometal-ligand complexes, confirming H2pyhox to be a promising versatile radiopharmaceutical chelator.

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.

Evaluation of La(XT), a novel lanthanide compound, in an OVX rat model of osteoporosis.

PURPOSE: The purpose of this study was to evaluate the efficacy and toxicity of a novel lanthanum compound, La(XT), in an ovariectomized (OVX) rat model of osteoporosis. METHODS: Twenty-four ovariectomized female Sprague Dawley rats were divided into 3 groups receiving a research diet with/without treatment compounds (alendronate: 3 mg/kg; La(XT) 100 mg/kg) for three months. At the time of sacrifice, the kidney, liver, brain, lung and spleen were collected for histological examination. The trabecular bone structure of the tibiae was evaluated using micro-CT and a three-point metaphyseal mechanical test was used to evaluate bone failure load and stiffness. RESULTS: No significant differences were noted in plasma levels of calcium, phosphorus, creatinine, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) between the La(XT) treatment compared to the non-treated OVX group. Alendronate-treated animals (positive control) showed higher BV/TV, Tb.N and lower Tb.Th and Tb.Sp when compared to the non-treated OVX group. Mechanical analysis indicated that stiffness was higher in the alendronate (32.88%, p = 0.04) when compared to the non-treated OVX group. Failure load did not differ among the groups. CONCLUSIONS: No kidney or liver toxicities of La(XT) treatments were found during the three-month study. The absence of liver and kidney toxicity with drug treatment for 3 months, as well as the increased trabecular bone stiffness are encouraging for the pursuit of further studies with La(XT) for a longer duration of time.

Chemical Promiscuity of Non-Macrocyclic Multidentate Chelating Ligands for Radiometal Ions: H4neunpa-NH2 vs H4noneunpa.

A comparative investigation of two structurally related potentially nonadentate chelating ligands, H4neunpa-NH2 and H4noneunpa, has been undertaken to examine the influence of bifunctionalization on their coordination chemistry and metal ion selectivity. Significantly improved synthetic routes for each compound have been developed, employing straightforward high-yielding strategies. Radiolabeling studies with [44Sc]Sc3+, [111In]In3+, [177Lu]Lu3+, and [225Ac]Ac3+ revealed a sharp contrast between the affinity of each chelator for large radiometal ions. H4noneunpa demonstrated highly effective coordination of [177Lu]Lu3+ and [225Ac]Ac3+ achieving quantitative radiochemical yields (>98%) at ligand concentrations of 10-6 M (room temperature (RT), 10 min), with excellent stability when challenged in human serum, while H4neunpa-NH2 was unable to complex either metal ion effectively. Nuclear magnetic resonance (NMR) spectroscopy was employed to explore the coordination chemistry of each chelating ligand with nonradioactive metal ions, spanning a range of ionic radii and coordination numbers. A comprehensive conformational analysis of each metal complex was undertaken using density functional theory (DFT) calculations to explore the coordination geometries and explain the discrepancy in binding characteristics. Theoretical simulations revealed notable differences in the coordination geometry and apparent denticity of each ligand, which together account for the observed selectivity in metal binding and have important implications for the future design of complexes based upon this framework to target large radiometal ion coordination.