Synthesis and chemoselective intramolecular crosslinking of a HER2-binding affibody

The human epidermal growth factor receptor HER2 has emerged as an important target for molecular imaging of breast cancer. This article presents the design and synthesis of a HER2-targeting affibody molecule with improved stability and tumor targeting capacity, and with potential use as an imaging agent. The 58 aa three-helix bundle protein was assembled using solid-phase peptide synthesis, and a chemoselective ligation strategy was used to establish an intramolecular thioether bond between the side chain thiol group of a cysteine residue, positioned in the loop between helices I and II, and a chloroacetyl group on the side chain amino group of the C-terminal lysine residue. The tethered protein offered an increased thermal stability, with a melting temperature of 64 degrees C, compared to 54 degrees C for the linear control. The ligation did not have a major influence on the HER2 binding affinity, which was 320 and 380 pM for the crosslinked and linear molecules, respectively. Biodistribution studies were performed both in normal and tumor-bearing mice to evaluate the impact of the crosslinking on the in vivo behavior and on the tumor targeting performance. The distribution pattern was characterized by a low uptake in all organs except kidney, and rapid clearance from blood and normal tissue. Crosslinking of the protein resulted in a significantly increased tumor accumulation, rendering the tethered HER2-binding affibody molecule a valuable lead in the development of superior HER2 imaging agents.