Freeze-Dried Radiolabeled Cyclic Peptide Kits

A freeze-dried kit formulation was developed for both cyclic peptide conjugates to provide ready to use kits for the labelling with ¹⁷⁷Lu. The kit formulation was based on an ascorbic acid buffer (Sigma Aldrich, St. Louis, MO, catalogue no. 95212,) for pH adjustment [25 (link)]. Gentisic acid (Sigma Aldrich, catalogue no. 85707) and L-Met (Sigma Aldrich, catalogue no. M8439) were added to the kit composition to increase the RCP and decrease Met-oxidation [26 (link)]. The lyophilized kits were composed of 50 µg DOTA-cyclo-MG1 or DOTA-cyclo-MG2, 25 mg ascorbic acid, 5.2 mg L-Met and 0.53 mg Gentisic acid. Radiolabelling was performed by addition of ~2 GBq ¹⁷⁷LuCl₃ in 1 mL of diluted HCl and heating at 80°C for 15-25 min. RCP was monitored by HPLC. The stability of the kits during storage for up to 6 months, as well as the stability of the radiolabelled peptides for up to 48 h after preparation, was evaluated at different storage conditions. For isocratic HPLC analysis a Phenomenex Kinetex 5 µm C18 100 Å column (4.6 × 150 mm) with mobile phase of 28% acetonitrile/0.1% trifluoroacetic acid/water, flow rate of 1 mL/min, UV-detection at 215 nm and radiodetection was used.