An Evaluation of Cage Amine Sarcophagine Ligands for the Synthesis of Cobalt Radiopharmaceuticals

Positron-emitting cobalt-55 (t_1/2 = 17.53 h, β⁺ = 77%, Eγ = 931.1 keV, Iγ = 75%) has the potential to be used for PET imaging and Auger electron-emitting cobalt-58m (t_1/2 = 9.10 h, IC= 100%) has the potential to be used in targeted radionuclide therapy. Sarcophagines are a family of macrobicyclic cage amine ligands that form stable complexes with cobalt(III). This work investigates the potential of a derivative of sarcophagine with a pendent carboxylic acid functional group, 5-(8-methyl-3,6,10,13,16,19-hexaaza-bicyclo[6.6.6]icosan-1-ylamino)-5-oxopentanoic acid (known as MeCOSar), to serve as a ligand for cobalt-55. The cobalt(III) complex, [Co^III(MeCOSar)]³⁺, was synthesized and characterized as well as a complex where the MeCOSar ligand is conjugated to tumor-targeting peptide Tyr³-ocreotate, [Co^III(SarTATE)]³⁺. The MeCOSar and SarTATE ligands were radiolabeled with cobalt-55 to give [⁵⁵Co][Co^III(MeCOSar)]ⁿ⁺ and [⁵⁵Co][Co^III(SarTATE)] respectively and their biodistribution was evaluated in non tumor-bearing mice. The tumor uptake of [⁵⁵Co][Co^III(SarTATE)] was then further evaluated in AR42J tumor-bearing mice and compared to the copper-64 complex, [⁶⁴Cu][Cu^II(SarTATE)]. Radiolabeling SarTATE with cobalt-55 requires higher temperatures when compared to copper-64, and [⁵⁵Co][Co^III(SarTATE)] has lower tumor uptake and higher liver uptake in tumor-bearing mice when compared to [⁶⁴Cu][Cu^II(SarTATE)].