[¹⁶¹Tb]Tb-BPAMD as a High-Affinity Agent for Skeletal Targeting: Radiochemical and Biodistribution Insights

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.