Synthesis of novel ⁶⁸Ga-labeled amino acid derivatives for positron emission tomography of cancer cells

Objectives: We developed amino acid derivatives of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (DO2A) and 1,4,7,10-tetraazacyclododecane-1,4,7,-triacetic acid (DO3A) that can be labeled with ⁶⁸Ga, and we investigated their basic biological properties. Materials and methods: Alanine derivatives of DO2A and DO3A were synthesized by regiospecific nucleophilic attack of DO2tBu and DO3tBu on the β-position of Boc-l-serine-β-lactone, followed by acid hydrolysis. Also, homoalanine derivatives were synthesized by reacting with the protected bromo derivative of homoalanine, which was synthesized from N-Cbz-l-homoserine lactone. Further catalytic reduction and acid cleavage of protected groups resulted in the required products. All derivatives were labeled with ⁶⁸Ga. Cell uptake assays were carried out in Hep3B (human hepatoma) and U87MG (human glioma) cell lines at 37°C. Positron emission tomography (PET) imaging studies were performed using balb/c mice xenografted with CT-26 (mouse colon cancer). Results: All compounds were labeled with >97% efficiency. According to in vitro studies, the labeled amino acid derivatives showed significantly greater uptakes than the control (⁶⁸Ga 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) in cancer cells. Small animal PET images for labeled compounds showed high tumor uptake, as well as kidney and bladder uptakes, at 30 min postinjection. ⁶⁸Ga-DO3A-homoalanine showed the highest standardized uptake value ratio (3.9±0.3), followed by ⁶⁸Ga-DO2A-alanine (3.1±0.2), ⁶⁸Ga-DO3A-alanine (2.8±0.2) and ⁶⁸Ga-DO2A-homoalanine (2.3±0.2). Conclusion: These derivatives were found to have high labeling efficiencies, high stabilities, high tumor cell uptakes, high tumor/nontumor xenograft uptakes and low nonspecific uptake in normal organs, except for the kidneys. However, the uptake mechanism of these derivatives remains unclear, and uptake via specific amino acid transporters needs to be demonstrated.