Design, synthesis and biological evaluation of a multifunctional HER2-specific Affibody molecule for molecular imaging

Purpose The purpose of this study was to design and evaluate a novel platform for labelling of Affibody molecules, enabling both recombinant and synthetic production and site-specific labelling with 99m Tc or trivalent radiometals. Methods The HER2-specific Affibody molecule PEP05352 was made by peptide synthesis. The chelator sequence SECG (serine-glutamic acid-cysteine-glycine) was anchored on the C-terminal to allow 99m Tc labelling. The cysteine can alternatively serve as a conjugation site of the chelator DOTA for indium labelling. The resulting 99m Tc- and 111 In-labelled Affibody molecules were evaluated both in vitro and in vivo. Results Both conjugates retained their capacity to bind to HER2 receptors in vitro and in vivo. The tumour to blood ratio in LS174T xenografts was 30 at 4 h post-injection for both conjugates. Biodistribution data showed that the 99m Tc-labelled Affibody molecule had a fourfold lower kidney accumulation compared with the 111 In-labelled Affibody molecule while the accumulation in other organs was similar. Gamma camera imaging of the conjugates could clearly visualise the tumours 4 h after injection. Conclusion Incorporation of the C-terminal SECG sequence in Affibody molecules provides a general multifunctional platform for site-specific labelling with different nuclides (technetium, indium, gallium, cobalt or yttrium) and for a flexible production (chemical synthesis or recombinant).