Two diphosphonates, etidronic acid (HEDP) and zoledronic acid (ZOL), were radiolabelled with ¹⁶¹Tb and evaluated as potential bone-targeting radiopharmaceuticals. Radiolabeling was performed at pH 7, achieving high radiolabeling yields (greater than 98%) and demonstrating excellent in vitro stability in saline and human serum. Both radiolabeled complexes exhibited hydrophilic behavior, a strong binding affinity to hydroxyapatite, and moderate to high plasma protein binding. Biodistribution studies in healthy Wistar rats demonstrated that ¹⁶¹Tb-HEDP and ¹⁶¹Tb-ZOL achieve high and stable skeletal uptake with rapid blood clearance and minimal soft tissue accumulation. ¹⁶¹Tb-HEDP favored higher initial bone localization, while ¹⁶¹Tb-ZOL showed lower renal and hepatic accumulation, indicating higher safety and selectivity. Compared to unchelated ¹⁶¹TbCl₃, both diphosphonate complexes exhibited significantly higher bone-to-kidney and bone-to-liver ratios, resulting in superior targeting. Complementary experiments with non-radioactive terbium were performed to investigate the redox behavior and confirm complex formation, providing valuable insight into the stability and binding modes of the ligands. Both terbium and the ligands displayed well-defined redox behavior within the potential range of −1 to 1.7 V, with complex formation evidenced by shifts in the oxidation peaks. Density functional theory (DFT) calculations further supported these findings, showing that both phosphonate groups of a ligand coordinate to Tb³⁺, while the hydroxyl groups in HEDP enable intermolecular hydrogen bonding, contributing to additional structural stabilization. Results encourage further investigations of ¹⁶¹Tb-labeled diphosphonates as promising candidates for radionuclide therapy of bone metastases and other skeletal diseases.

Background: Bone-seeking radiopharmaceuticals based on bisphosphonates enable targeted therapy of skeletal metastases. They are ideal carriers for therapeutic radionuclides such as Terbium-161 (¹⁶¹Tb), a β-emitter that additionally releases short-range conversion and Auger electrons, which may enhance radiation dose delivery to small lesions. This study aimed to investigate the potential of the specific DOTA conjugated bisphosphonate - BPAMD (4-{[(bis(phosphonomethyl))carbamoyl]methyl}-7,10-bis(carboxymethyl)-1,4,7,10 tetraazacyclododec-1-yl)acetic acid) radiolabeled with ¹⁶¹Tb as a next-generation of bone-targeted radiopharmaceutical. Methods: BPAMDwas radiolabeled with ¹⁶¹Tb and ¹⁷⁷Lu under mild conditions (pH 4.5, 95°C, 30min); subsequently the radiochemical purity was assessed by radio-TLC and radioelectrophoresis. Physicochemical properties (charge, lipophilicity, protein binding), in vitro stability (saline and human serum, 48h), and hydroxyapatite binding were evaluated for ¹⁶¹Tb-BPAMD. Biodistribution was studied in healthy Wistar rats (n=3 per time point) at 2h, 24h, and 7 days post-injection. Complementary DFT calculations explored the coordination chemistry of Tb³⁺ and Lu³⁺ with BPAMD. Results: Both complexes achieved >98% radiochemical yield. ¹⁶¹Tb-BPAMD exhibited negative charge, high hydrophilicity (logP = –3.92±0.13), low protein binding (19.07±1.01%), excellent radiochemical stability under simulated physiological conditions (>97% at 48h), and strong hydroxyapatite affinity (>98% with ≥10 mg HAP). Biodistribution showed high, stable bone uptake (8.06% ID/g at 2h; 6.70% ID/g at 24h; 5.31% ID/g at 7d) with rapid blood clearance (<0.001% ID/g at 24h), and low non-target retention. To contextualize its performance, ¹⁶¹Tb-BPAMD was compared with ¹⁷⁷Lu-BPAMD, which demonstrated similarly strong skeletal retention (8.74% ID/g at 2h; 8.08% ID/g at 24h; 5.25% ID/g at 7d) but comparatively higher non-target organ uptake. Complementary DFT calculations indicate that both Tb³⁺ and Lu³⁺ favor octa-coordinated BPAMD complexes. Conclusions: ¹⁶¹Tb-BPAMD exhibits excellent radiochemical and pharmacokinetic properties, with enhanced biodistribution selectivity over ¹⁷⁷Lu-BPAMD. Combined with the radiobiological advantages of ¹⁶¹Tb, it represents a promising theranostic candidate for targeted therapy of bone metastases.

Background: Bone-seeking radiopharmaceuticals based on bisphosphonates enable targeted therapy of skeletal metastases. They are suitable carriers for therapeutic radionuclides such as terbium-161 (¹⁶¹Tb), a β⁻ emitter that additionally releases short-range conversion and Auger electrons, which may enhance radiation dose delivery to small lesions. This study explored the potential of the well-established DOTA conjugated bisphosphonate BPAMD (4-{[(bis(phosphonomethyl))carbamoyl]methyl}-7,10-bis(carboxymethyl)-1,4,7,10 tetraazacyclododec-1-yl)acetic acid) radiolabeled with ¹⁶¹Tb as a bone-targeted radiopharmaceutical, focusing on the theranostic and radiophysical advantages conferred by the radionuclide. Methods: BPAMD was radiolabeled with ¹⁶¹Tb and ¹⁷⁷Lu under mild conditions (pH 4.5, 95 °C, 30 min); subsequently, the radiochemical purity was assessed by radio-TLC. Physicochemical properties (charge, lipophilicity, protein binding), in vitro stability (saline and human serum, 48 h), and hydroxyapatite (HAP) binding were evaluated for [¹⁶¹Tb]Tb-BPAMD. Biodistribution was investigated in healthy Wistar rats (n = 3 per time point) at 2 h, 24 h, and 7 days post-injection. Computational density functional theory (DFT) analyses were performed to explore the coordination chemistry of Tb³⁺ and Lu³⁺ with BPAMD. Results: Both complexes achieved a radiochemical yield of greater than 98%. [¹⁶¹Tb]Tb-BPAMD exhibited negative charge, high hydrophilicity (logP = −3.92 ± 0.13), low protein binding (19.07 ± 1.01%), excellent radiochemical stability under simulated physiological conditions (>97% at 48 h), and strong hydroxyapatite affinity (>98% with ≥10 mg HAP). Biodistribution showed high, stable bone uptake (8.06% ID/g at 2 h; 6.70% ID/g at 24 h; 5.31% ID/g at 7 d) with rapid blood clearance (<0.001% ID/g at 24 h) and low non-target retention. To contextualize its performance, [¹⁶¹Tb]Tb-BPAMD was compared with [¹⁷⁷Lu]Lu-BPAMD, which demonstrated similarly strong skeletal retention (8.74% ID/g at 2 h; 8.08% ID/g at 24 h; 5.25% ID/g at 7 d) but comparatively higher non-target organ uptake. DFT calculations indicate that both Tb³⁺ and Lu³⁺ favor octa-coordinated BPAMD complexes. Conclusions: [¹⁶¹Tb]Tb-BPAMD exhibits excellent radiochemical and pharmacokinetic properties, with enhanced biodistribution selectivity over [¹⁷⁷Lu]Lu-BPAMD. Combined with the radiobiological advantages of ¹⁶¹Tb, it represents a promising theranostic candidate for targeted therapy of bone metastases.

Background/Objectives: Effective intratumoral radionuclide therapy requires nanocarriers that enable stable radionuclide coordination and prolonged tumor retention. Integrating these properties with efficient magnetic heating within a single nanoplatform remains challenging because optimization of one property may compromise another. This study investigated whether manganese substitution could improve the magnetic heating performance of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP)-functionalized maghemite nanoparticles while preserving their suitability for radiopharmaceutical applications. Methods: Three Mn_xFe_2−xO₃ compositions (x = 0, 0.221, and 0.480) were evaluated in terms of their structural and physicochemical properties, colloidal stability, and magnetic heating performance. The most promising Mn-substituted formulation and the unsubstituted reference were evaluated by ¹⁷⁷Lu radiolabeling, in vitro radiochemical stability testing, whole-body imaging, ex vivo biodistribution analysis, and therapeutic efficacy assessment in 4T1 tumor-bearing mice. Results: Moderate Mn substitution (x = 0.221) provided the most favorable combination of magnetic heating performance (SAR = 360 W g⁻¹; ILP = 5.65 nH m² kg⁻¹) and colloidal stability. HEDP functionalization enabled direct radiolabeling with ¹⁷⁷Lu, with labeling yields of 86–92% and radiochemical stability exceeding 94% after 96 h in saline and human serum. Following intratumoral administration, qualitative whole-body imaging showed persistent tumor-associated radioactivity for both nanoparticle formulations on days 5 and 14. On day 14, tumor-associated activity was 91.68 ± 0.73%ID and 84.53 ± 2.26%ID for [¹⁷⁷Lu]Lu-NP1 and [¹⁷⁷Lu]Lu-NP2, respectively, compared with 17.49 ± 8.06%ID for free [¹⁷⁷Lu]LuCl₃. Both nanoparticle formulations inhibited tumor growth compared with saline, with a more pronounced response and partial tumor regression in animals bearing smaller tumors at treatment initiation. Conclusions: Moderate Mn substitution improved magnetic heating performance and colloidal stability while preserving the radiochemical stability, intratumoral retention, and therapeutic efficacy of the ¹⁷⁷Lu-labeled HEDP-functionalized nanocarriers. This nanoplatform warrants further investigation for intratumoral ¹⁷⁷Lu therapy and its potential integration with magnetic hyperthermia.