From octreotide to shorter analogues: Synthesis, radiolabeling, stability

Abstract

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.

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

“…DISCUSSIONDue to the promising therapeutic efficiency of Thz-containing peptides, we earlier evaluated their potential to be used as a part of a radiopharmaceutical. However, we have shown that the conditions for the effective labeling of DOTA-P1 and DOTA-P2 with 44 Sc, 90 Y, 152 Eu, and 207 Bi cations significantly differ and are not mainly defined by the hydrolytic behavior of cations 14,32. Bi 3+ is the mostF I G U R E 3Time dependence of the labeling yield of DOTA-P4 with different cations: (A) 44 Sc 3+ , (B) 90 Y 3+ , (C) 152 Eu 3+ , and (D) 207 Bi 3+ at 90 C hydrolysable among the cations studied; however, the maximum labeling yield for DOTA-P1 and DOTA-P2 was observed at pH 8.…”

“…30 For 90 Y 3+ , the optimal labeling conditions (pH 4, 0.1mM of DOTA-P4) were the same as for 90 Y-DOTA-P2. 14 Due to the high similarity of the chemical structures of DOTA-P2…”

“…12,13 We have demonstrated earlier that DOTA-Phe-D-Trp-Lys-Thr (DOTA-P1) and DOTA-Thz-Phe-D-Trp-Lys-Thr (DOTA-P2) conjugates form in vitro stable complexes with 44 Sc 3+ , 152 Eu 3+ and 207 Bi 3+ cations but optimized labeling conditions of DOTA-P2 with "large" 152 Eu 3+ and 207 Bi 3+ were inappropriate for the synthesis of radiopharmaceuticals. 14 At the same time, complexes with DOTA-P2 have shown improved stability in vitro in comparison with DOTA-P1. Thus, it is reasonable to keep five amino acid peptide lengths but to alter the position of the DOTA chelator due to the possible spatial blocking of DOTA near the rigid -Thz residue (Figure 1).…”

The Crystal Structure Elucidation of a Tetrapeptide Analog of Somatostatin DOTA-Phe-D-Trp-Lys-Thr-OMe

“…Previously, we have synthesized chimeric molecules containing in their structure the pharmacophore sequence -Phe-D-Trp-Lys-Thr-OMe, which selectively acts on human lung adenocarcinoma (A549) (IC50(Human embryonic fibroblasts (HEF))/IC50(A-549) > 9) [7]. In addition, we studied the labeling of the conjugate described in this work with Sc 3+ , Bi 3+ and Eu 3+ cations and showed the stability of these complexes in solutions with biologically relevant ions and in bovine serum at 37 • C [8]. In this work, we attempted to study the conformation of the -DOTA-Phe-D-Trp-Lys-Thr-OMe conjugate to understand the structureactivity relationship using X-ray structural analysis (XRD) data and quantum chemistry methods.…”