Influence of Peptide‐Based Chelator Sequences on HER2‐Targeting Radiopeptide

In our pursuit of developing HER2‐targeting radiolabeled peptides for detection and therapy of HER2‐positive breast cancer, the previously reported rL‐A9 peptide had shown promising potential. In the present work, a different variant of rL‐A9 was designed by incorporation of peptide‐based chelators for 99m Tc‐labeling. Cysteine‐based amino acids (CGGG, CSSG, CEEG) were conjugated at the N‐terminus of the peptide and were radiolabeled with [ 99m Tc]NaTcO 4 . Of the three radiopeptides, [ 99m Tc]Tc‐CGGG‐rL‐A9 exhibited high stability, whereas [ 99m Tc]Tc‐CSSG‐rL‐A9 and [ 99m Tc]Tc‐CEEG‐rL‐A9 decomposed rapidly. Hence, only CGGG‐incorporated radiopeptide was further investigated in vitro in human breast carcinoma SKBR3 cells and in vivo in tumor xenografted mice. Cell‐binding studies demonstrated high binding affinity [K d : 5.82 ± 0.92 nM] and specificity (63% reduction in blocking study) of [ 99m Tc]Tc‐CGGG‐rL‐A9 toward HER2‐expressing cells. In vivo studies showed moderate tumor uptake (3.56 ± 0.81% ID/g, 1 h) with 82.8% retention at 3 h (3.16 ± 0.22% ID/g). Moderately high tumor‐to‐background ratios, along with washout from the other non‐target organs, were observed. Inhibition studies revealed 52.6% reduction in the tumor activity, signifying high HER2‐receptor specificity of the presently studied radiopeptide, [ 99m Tc]Tc‐CGGG‐rL‐A9.