Fundamental Study of Radiogallium-labeled Aspartic Acid Peptides Introducing Octreotate Derivatives

Objective: Somatostatin receptors are highly expressed in neuroendocrine tumors, and many radiolabeled somatostatin analogs for diagnosis and treatment have been developed. To simultaneously detect not only primary cancer but also bone metastases, this study aimed to develop a positron emission tomography probe using generator-produced nuclide Gallium-68 (T = 68 min), in which a carrier for primary cancer, a carrier for bone metastases lesions, and a stable gallium complex are introduced into the one molecule. Based on this strategy, the somatostatin receptor-targeted peptide, [Tyr]-octreotate (TATE), aspartic acid peptide (D) with high binding affinity for hydroxyapatite, and Ga-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) as a stable gallium complex were selected. The novel complexes, Ga-DOTA-D-TATE (n = 0, 2, 5, 8, or 11), were designed, synthesized, and evaluated. The radiogallium complexes were prepared using the easy-to-handle radioisotope Ga due to relatively long half-life.

Methods: The radiogallium complex precursor DOTA-D-TATE was synthesized by the Fmoc-based solid-phase method and by the air oxidation method to form the disulfide bond. [Ga]Ga-DOTA-D-TATE was synthesized by reacting DOTA-D-TATE and Ga. Hydroxyapatite binding assays, in vitro cellular uptake experiments in AR42J tumor cells, in biodistribution experiments in AR42J tumor-bearing mice, were performed using [Ga]Ga-DOTA-D-TATE.

Results: The radiochemical purities of [Ga]Ga-DOTA-D-TATE were > 96.0%. In in vitro and in vivo experiments, [Ga]Ga-DOTA-D-TATE had a high affinity for hydroxyapatite and highly accumulated in bone. However, the uptake of [Ga]Ga-DOTA-D-TATE into somatostatin receptor-positive AR42J cells was lower than that of [Ga]Ga-DOTA-TATE, and the accumulation of [Ga]Ga-DOTA-D-TATE in tumor was significantly low.

Conclusion: Ga-DOTA-D-TATE may not be recognized by somatostatin receptor by the introduction of D, and the charge adjustment may be important for somatostatin receptor-positive cell uptake.