Enhanced stability of Cu²⁺-ATCUN complexes under physiologically relevant conditions by insertion of structurally bulky and hydrophobic amino acid residues into the ATCUN motif

Copper complexes formed by an amino terminal Cu²⁺- and Ni²⁺-binding (ATCUN) motif have attracted attention as metallodrug candidates that cleave DNA or RNA and inactivate enzymes. Although the stability of the Cu²⁺-ATCUN complex under physiologically relevant conditions is a key factor for medical applications, it has remained unclear. Here we prepared a series of ATCUN peptides by inserting various amino acid residues into positions 1 and 2, and investigated the stability of the Cu²⁺-ATCUN complexes in aqueous solution, blood plasma, and living animals. Systematic pH titration showed that the low basicity of the N-terminal amine of the peptide stabilized the Cu²⁺-ATCUN complex in aqueous solution. Interestingly, the stability of ⁶⁴Cu-labeled ATCUN complexes in blood plasma was significantly enhanced by the structural bulkiness and hydrophobicity of the amino acid residues at positions 1 and 2. To validate the in vivo stability, six ATCUN motifs (YYH, VVH, NNH, TTH, GGH, and DDH) were conjugated to a tumor-targeting peptide, octreotide (Oct). The stability of the ⁶⁴Cu-ATCUN-Oct complexes in blood plasma showed a similar trend to that of the ⁶⁴Cu-ATCUN complexes. The ⁶⁴Cu-YYH-Oct complex exhibited the highest stability in blood plasma. According to the positron emission tomography and competitive blocking studies of a tumor-bearing mouse model, ⁶⁴Cu-YYH-Oct specifically accumulated in tumors, suggesting that the complex was sufficiently stable to reach its target in vivo. The results show that the structural bulkiness and hydrophobicity of the residues at positions 1 and 2 are key parameters for designing metallodrugs on the basis of the Cu²⁺-ATCUN complex.