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 ⁴⁴Sc, ²⁰⁷Bi, and ¹⁵²Eu 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 ⁴⁴Sc, ²⁰⁷Bi, and ¹⁵²Eu 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°C for DOTA‐tetrapeptide complexes with ²⁰⁷Bi and ¹⁵²Eu; 0.05mM, pH 4.0 to 5.0 at 90°C for ⁴⁴Sc‐DOTA‐tetrapeptide; 0.2mM, pH 4.0 to 5.0 at 90°C for ⁴⁴Sc‐DOTA‐pentapeptide. Complexes of DOTA‐pentapeptide with ²⁰⁷Bi and ¹⁵²Eu of radiochemical purity more than 95% were probably unstable at temperature higher than 37°C and were obtained at 37°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.