From octreotide to shorter analogues: synthesis, radiolabeling, stability.

This study aims at analyzing complexation properties of two new short somatostatin analogues, their synthesis, radiolabeling with Sc-44, Bi-207, and Eu-152 and stability in vitro. Short tetrapeptide Phe-d-Trp-Lys-Thr and pentapeptide Thz-Phe-d-Trp-Lys-Thr were first conjugated with the DOTA macrocyclic chelator. These conjugates were radiolabeled with Sc-44, Bi-207, and Eu-152 and characterized by thin-layer chromatography (TLC) and HPLC. The radiochemical purity was measured using digital autoradiography and gamma spectrometry. Optimum conditions of DOTA-conjugate labeling were found: 0.1mM, pH 8.0 to 8.4 at 90 degrees C for DOTA-tetrapeptide complexes with Bi-207 and Eu-152; 0.05mM, pH 4.0 to 5.0 at 90 degrees C for Sc-44-DOTA-tetrapeptide; 0.2mM, pH 4.0 to 5.0 at 90 degrees C for Sc-44-DOTA-pentapeptide. Complexes of DOTA-pentapeptide with Bi-207 and Eu-152 of radiochemical purity more than 95% were probably unstable at temperature higher than 37 degrees C and were obtained at 37 degrees C, pH 8.0 to 8.4 within 4 days. Mass spectra of the Eu-DOTA-pentapeptide revealed the presence of small fragments of the pentapeptide conjugate in the complex solution. in vitro stability studies were performed in saline in the presence of serum proteins and biologically relevant metal cations. All complexes demonstrated no cation release in vitro within 1 to 4 hours.