[Development of new peptide-based chelating agents for site-specific radiolabeling with 64Cu]

Radiolabeled monoclonal antibodies (mAbs) are very useful molecular probes for nuclear imaging technique due to their high-target specificity and high-stability in the bloodstream. MAbs are generally labeled by indirect method using the combination of relatively long-lived metallic radionuclides (including (64)Cu, (89)Zr, and (111)In) and bifunctional chelating agents which are capable of binding both antibodies and radio metals. The indirect radiolabeling method has some advantages such as high labeling efficiency and long-term retention ability within their target cells. However, this conventional labeling method can potentially lead to low-target affinity of the mAb probes, because of the non-site-specific introduction of the bifunctional chelators into the active site of the mAbs. To overcome the shortcoming, we proposed a new direct labeling method utilizing fusion proteins comprising mAbs linked to metal binding peptides at the N- or C-terminus. In this study, we synthesized new peptide derivatives possessing an N-terminal tripeptide sequence (Xaa-Yaa-His) called the amino terminal Cu(2+)- and Ni(2+)-binding (ATCUN) motif as (64)Cu binding peptides for the proposed labeling method. Moreover, we studied the stability constants of Cu(2+)-ATCUN peptide complexes by pH titration. From these studies, we found that a low basicity of the N-terminal amine in the peptide resulted in a high stability constant of the complex. This finding may provide valuable guidelines in designing the ATCUN peptide with high-binding affinity toward (64)Cu.