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.

Somatostatin analogues play an important role in the therapy of neuroendocrine tumors by binding to somatostatin receptors on the surface of cancer cells. In this work, we analyze the receptor-binding affinity and in vitro stability of a novel ultra-short somatostatin analogue Thz-Phe-D-Trp-Lys-Thr-DOTA (DOTA-P4). This conjugate is successfully radiolabeled with 44 Sc, 90 Y, 152 Eu, and 207 Bi, characterized and validated by thin layer and high-performance liquid chromatography. The optimum conditions for M-DOTA-P4 labeling are found. In vitro stability studies are performed in saline, in the presence of serum proteins, and with biologically relevant metal cations. All complexes demonstrate no cation release in vitro within 4-24 h. The conformations of DOTA-conjugates are studied by circular dichroism spectroscopy. The circular dichroism spectra of DOTA-P4 conjugates show a negative peak at 225 nm, which may correspond to the required β-sheet conformation. The binding to somatostatin receptors of types 2 and 5 is performed with the IMR-32 cells at 4 C, with non-specific binding representing 26% of the total binding. A two-line approximation of the Scatchard plot results in the apparent dissociation constants of 0.10 and 2.25 nM. It is shown that the chelator position with respect to the amino acid sequence significantly affects the labeling conditions with cations of different ionic radii. For the first time, the binding of a linear type ultra-short peptide conjugate with DOTA to somatostatin receptors is demonstrated. The obtained results are promising for experiments with DOTA-P4