Direct labeling of a somatostatin receptor antagonist via peptide cyclization with Re, Tc and Re metal centers: Radiochemistry and in vitro evaluation.

INTRODUCTION

With the long-term goal of developing a diagnostic (Tc) and therapeutic (Re) agent pair for targeting somatostatin receptor (SSTR)-positive neuroendocrine tumors (NETs), we developed novel metal-cyclized peptides through direct labeling of the potent SSTR2 antagonist Ac-4-NO-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH (1) with Re (in Re-1), Tc (in [Tc]Tc-1) and Re (in [Re]Re-1).

METHODS

Re-1 was characterized by LC-ESI-MS and HR-ESI-MS and was tested for receptor affinity in SSTR-expressing cells (AR42J). Radiolabeling of the peptide was achieved via ligand exchange from Tc-labeled glucoheptonate or [Re]ReOCl(PPh), yielding [Tc]Tc-1 or [Re]Re-1, respectively. In vitro stability of [Tc]Tc-1/[Re]Re-1 in PBS (10 mM) at pH 7.4 and 37 °C was determined by HPLC analysis. Moreover, [Tc]Tc-1 stability was tested in cysteine (1 mM) and rat serum under the same conditions.

RESULTS

Re-1 consisted of two isomers, confirmed by LC-ESI-MS, with good SSTR2 affinity (IC = 43 ± 6 nM). Optimization of the Tc labeling through varying reaction parameters such as pH, reaction time, and Sn and ligand concentrations resulted in high radiochemical yield (RCY ≥92%). Similarly, [Re]Re-1 was prepared in reasonable RCY (≥50%). Both Tc/Re-tracers consisted of two product isomers as identified by HPLC co-injection with Re-1. While [Tc]Tc-1 was sufficiently stable in vitro (≥71% intact through 4 h in PBS, cysteine and rat serum), [Re]Re-1 exhibited more moderate in vitro stability (58% intact after 1 h in PBS).

CONCLUSIONS

Novel Tc/Re-cyclized SSTR2 antagonist peptides were synthesized and characterized using the Re-cyclized analogue as a reference. Due to the nanomolar SSTR2 affinity of Re-1 and good in vitro stability of [Tc]Tc-1, the latter shows early promise for development as a radiodiagnostic agent for SSTR-expressing NETs.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The Tc-cyclized complex showed promising in vitro properties, and future in vivo studies will determine the potential for translating such a design into the human clinic.