Synthesis and preliminary evaluation of Tc-99m-labeled somatostatin analog (RC-160) using "3+1" mixed ligand approach.

The success of (111)In-pentetreotide as a cancer-imaging agent has given impetus to the search for other peptide-based radiopharmaceuticals. The labeling with Tc-99m has become even more attractive because of the ready availability and near ideal physical properties. Additionally, the kinetics of the peptide-receptor interactions favors the radiolabeling with technetium-99m. A somatostatin analog RC-160 has been labeled with Tc-99m using the "3+1" mixed ligand approach utilizing the NNS/S coordination sites. The ternary complex was formed in greater than 95% within 30 min by simultaneous reduction and complexation of technetium-99m pertechnetate. The Tc-99m and the surrogate rhenium complexes showed similar chromatographic behavior. The complex was evaluated by in vitro receptor binding studies carried out on HTB-121 breast cancer cell line and biodistribution studies performed in normal mice. Our findings suggest that RC-160 can be labeled by the mixed ligand approach with the complex retaining its biological activity and warrants further studies.