Synthesis and ¹⁷⁷Lu Labeling of the First Retro Analog of the HER2-Targeting A9 Peptide: A Superior Variant

The retro analog of the HER2-targeting A9 peptide was synthesized by coupling amino acids in a reverse fashion and switching the N-terminal in the original sequence of the L-A9 peptide (QDVNTAVAW) to the C-terminal in rL-A9 (WAVATNVDQ). Modification in the backbone resulted in higher conformational stability of the retro peptide as evident from CD spectra. Molecular docking analysis revealed a higher HER2 binding affinity of [¹⁷⁷Lu]Lu-DOTA-rL-A9 than the original radiopeptide [¹⁷⁷Lu]Lu-DOTA-L-A9. Enormously enhanced metabolic stability of the retro analog led to significant elevation in tumor uptake and retention. SPECT imaging studies corroborated biodistribution results demonstrating a remarkably higher tumor signal for [¹⁷⁷Lu]Lu-DOTA-rL-A9. The presently studied retro probe has promising efficiency for clinical screening.