Stability Estimation of Gallium Complexes of DOTA Derivatives for Radiotheranostic Applications

[Image: see text] 1,4,7,10-Tetraazacyclododecane-1,4,7-triacetic acid (DO3A) has been used to prepare (68)Ga-labeled probes for the diagnostic counterpart of radiotheranostic applications. While DO3A provides stable complexes with therapeutic radionuclides such as (90)Y, (177)Lu, and (225)Ac, further improvement of the in vivo stability of the Ga-DO3A complex is required. Considering the high stability of an intact Ga-DOTA complex, the stability of Ga complexes of DOTA and DO3A derivatives, including benzyl-DOTA (Bn-DOTA), was evaluated to gain fundamental knowledge for developing the next-generation radiotheranostic probes using (68)Ga as a diagnostic counterpart. Following the complexation reaction to prepare (67)Ga-labeled DOTA and DO3A derivatives, the stability of the resulting (67)Ga-labeled compounds was evaluated in murine plasma and apo-transferrin challenge. [(67)Ga]Ga-Bn-DOTA produced two isomers, and one of the isomers exhibited the highest stability among the tested complexes. The X-ray crystallography showed that the less stable isomer of Ga-Bn-DOTA suggested an N(3)O(3) coordination geometry, while Ga-DOTA and Ga-Bn-DO3A show N(4)O(2) coordination. To further evaluate the stability, a synthetic somatostatin analogue, [Tyr(3)]octreotide (TOC), was used as a model peptide, and p-COOH-Bn-DOTA and DO3A were conjugated with TOC to prepare DOTA-Bn-TOC and DOTATOC. [(67)Ga]Ga-DOTA-Bn-TOC also yielded two isomers with varying stability, and one isomer exhibited significantly higher stability than [(67)Ga]Ga-DOTATOC both in vitro and in vivo. These findings indicate that para-substituted Bn-DOTA would constitute a suitable chelating agent for developing next-generation radiotheranostic probes, although high-performance liquid chromatography purification is needed. Thus, further chemical modification on the Bn-DOTA molecule is also needed to avoid the formation of a Ga complex with the N(3)O(3) configuration.