Labeling and receptor affinity of an ultra-short somatostatin analogue Thz-Phe-D-Trp-Lys-Thr-DOTA.

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 Sc, Y, Eu, and 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 in vivo in mice with inoculated tumors.