[ 213 Bi]Bi 3+ /[ 111 In]In 3+ -neunpa-cycMSH: Theranostic Radiopharmaceutical Targeting Melanoma─Structural, Radiochemical, and Biological Evaluation.

With the emergence of [ 225 Ac]Ac 3+ as a therapeutic radionuclide for targeted α therapy (TAT), access to clinical quantities of the potent, short-lived α-emitter [ 213 Bi]Bi 3+ ( t 1/2 = 45.6 min) will increase over the next decade. With this in mind, the nonadentate chelator, H 4 neunpa-NH 2 , has been investigated as a ligand for chelation of [ 213 Bi]Bi 3+ in combination with [ 111 In]In 3+ 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 H 4 neunpa-NH 2 on complexation of [ nat Bi]Bi 3+ , while the solid-state structure of [ nat Bi][Bi(neunpa-NH 3 )] 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. H 4 neunpa-NH 2 exhibited fast complexation kinetics with [ 213 Bi]Bi 3+ 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-CycMSH hex were synthesized featuring two different covalent linkers for in vivo evaluation with [ 213 Bi]Bi 3+ and [ 111 In]In 3+ . High molar activities of 7.47 and 21.0 GBq/μmol were achieved for each of the bioconjugates with [ 213 Bi]Bi 3+ . SPECT/CT scans of the [ 111 In]In 3+ -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 [ 213 Bi][Bi(neunpa-Ph-Pip-Nle-CycMSH hex )]. This study demonstrates H 4 neunpa-NH 2 to be an effective chelating ligand for [ 213 Bi]Bi 3+ and [ 111 In]In 3+ , with promising characteristics for further development toward theranostic applications.