Purpose: Affibody molecules are low molecular weight proteins (7 kDa), which can be selected to bind to tumour-associated target proteins with subnanomolar affinity. Because of rapid tumour localisation and clearance from nonspecific compartments, Affibody molecules are promising tracers for molecular imaging. Earlier, (99m)Tc-labelled Affibody molecules demonstrated specific targeting of tumour xenografts. However, the biodistribution was suboptimal either because of hepatobiliary excretion or high renal uptake of the radioactivity. The goal of this study was to optimise the biodistribution of Affibody molecules by chelator engineering.
Materials and methods: Anti-HER2 Z(HER2:342) Affibody molecules, carrying the mercaptoacetyl-glutamyl-seryl-glutamyl (maESE), mercaptoacetyl-glutamyl-glutamyl-seryl (maEES) and mercaptoacetyl-seryl-glutamyl-glutamyl (maSEE) chelators, were prepared by peptide synthesis and labelled with (99m)Tc. The tumour-targeting capacity of these conjugates was compared with each other and with the best previously available conjugate, (99m)Tc-maEEE-Z(HER2:342,) in nude mice bearing SKOV-3 xenografts. The tumour-targeting capacity of the most promising conjugate, (99m)Tc-maESE-Z(HER2:342,) was compared with radioiodinated Z(HER2:342).
Results: All novel conjugates demonstrated successful tumour targeting and a low degree of hepatobiliary excretion. The renal uptakes of serine-containing conjugates, 33 +/- 5, 68 +/- 21 and 71 +/- 10%IA/g, for(99m)Tc-maESE-Z(HER2:342), (99m)Tc-maEES-Z(HER2:342) and (99m)Tc-maSEE-Z(HER2:342), respectively, were significantly reduced in comparison with (99m)Tc-maEEE-Z(HER2:342) (102 +/- 13%IA/g). For (99m)Tc-maESE-Z(HER2:342), a tumour uptake of 9.6 +/- 1.8%IA/g and a tumour-to-blood ratio of 58 +/- 6 were reached at 4 h p.i.
Conclusions: A combination of serine and glutamic acid residues in the chelator sequence confers increased renal excretion and relatively low renal uptake of (99m)Tc-labelled Affibody molecules. In combination with preserved targeting capacity, this improved imaging of targets in abdominal area.