Synthesis, characterization and in vitro evaluation of new oxorhenium- and oxotechnetium-CCK4 derivatives as molecular imaging agents for CCK2-receptor targeting

The goal of this study is to design new 99mTc-radiolabelled shortened CCK derivatives that might be suitable for the molecular imaging of cholecystokinin-2 receptors (CCK2-R), these receptors being over-expressed in a number of neuroendocrine tumors such as medullary thyroid cancer and small-cell lung cancer. For this purpose, we designed several modified CCK4 analogs bearing an ON2S tetradentate chelating agent at the N-terminus, the CCK4 sequence representing the minimal peptide sequence that presents nanomolar affinity and activity towards the CCK2-R. Four peptide conjugates of general formula (Trt)SN2OPh–(X)n–CCK4 (X = β-alanine or 6-aminohexanoic acid spacers; n = 0, 2, 4) and their oxorhenium peptide conjugates have been synthesized and characterized. In vitro evaluation of these compounds showed a close relationship between the nature and the length of the spacer and the corresponding binding affinity values. The most promising oxorhenium complex 5-Re exhibited potent CCK2-receptor agonist properties in promoting the production of inositol phosphate in COS-7 cells (EC50 = 5.17 nM). Preliminary 99mTc-radiolabelling studies with peptide conjugates 3 or 5 led exclusively to the corresponding 99mTcO-complexes 3-Tc and 5-Tc, which exhibited high resistance towards an excess of cysteine and satisfactory stabilities in human serum. To conclude, the promising in vitro characteristics of compounds 5-Re, 5-Tc illustrate the feasibility to develop stable radiolabelled shortened CCK4 derivatives with a nanomolar CCK2-R affinity.

In the present study, we demonstrated for the first time the feasibility to develop a CCK4 peptide-based oxorhenium complex which exhibited a nanomolar affinity for CCK2-receptor.Figure optionsDownload as PowerPoint slide