Evaluation of single amino acid chelate (SAAC) derivatives for the regioselective radiolabeling of peptides with technetium-99m

Objectives The aim of this work was to investigate the relative radiolabeling kinetics and affinity of a series of SAAC-type ligands for the [^99mTc(CO)₃]⁺ core. This information could then be used to select a suitable SAAC unit for radiolabeling peptide-based targeting vectors in high yield under mild conditions.

Methods A series of protected lysine derivatives were synthesized via reductive amination reactions and complexes of [Re(CO)₃]⁺ were prepared as characterization standards. The SAAC ligands were labeled with ^99mTc under a variety of conditions; competitive labeling experiments were also conducted in the presence of histidine and Gly-His-Gly.

Results Three SAAC derivatives were capable of forming complexes with ^99mTc at ambient temperature. Residual ligand was removed in minutes using a resin-capture approach. Competition experiments indicated that the bis-thiazolyl and bis-pyridyl derivatives label preferentially over internal histidine residues, while only the bis-thiazolyl compound labeled significantly in the presence of free histidine. Incorporation of the 2-pyridyl-SAAC enabled the rapid, high yield labeling of a uPAR-targeting cyclic peptide without interfering with the essential disulfide bridge.

Conclusions The lead agent is the bis-thiazole SAAC derivative which demonstrated superior affinity for [^99mTc(CO)₃]⁺, though the bis-(2-pyridine) analogue displayed comparable efficiency in several instances and is generated from more readily available starting materials. A further benefit of this work was that the resin capture purification method has potential for the rapid generation of ^99mTc-labeled peptides in high effective specific activity.