Purpose: Affibody molecules are a novel class of tumour-targeting proteins, which combine small size (7 kDa) and picomolar affinities. The Affibody molecule Z(HER2:342) has been suggested for imaging of HER2 expression in order to select patients for trastuzumab therapy. When optimizing chelators for (99m)Tc-labelling, we have found that synthetic Z(HER2:342) conjugated with mercaptoacetyl-glycyl-glycyl-glycyl (maGGG) and mercaptoacetyl-glycyl-seryl-glycyl (maGSG) chelators provides relatively low renal uptake of radioactivity and could be suitable for therapy.
Methods: maGGG-Z(HER2:342) and maGSG-Z(HER2:342) were labelled with (186)Re and their biodistribution was studied in normal mice. Dosimetric evaluation and tumour targeting to HER2-overexpressed xenografts (SKOV-3) by (186)Re-maGSG-Z(HER2:342) were studied.
Results: Gluconate-mediated labelling of maGGG-Z(HER2:342) and maGSG-Z(HER2:342) with (186)Re provided a yield of more than 95% within 60 min. The conjugates were stable and demonstrated specific binding to HER2-expressing SKOV-3 cells. Biodistribution in normal mice demonstrated rapid blood clearance, low accumulation of radioactivity in the kidney and other organs, accumulating free perrhenate. Both (186)Re-maGGG-Z(HER2:342) and (186)Re-maGSG-Z(HER2:342) demonstrated lower renal uptake than their (99m)Tc-labelled counterparts. (186)Re-maGSG-Z(HER2:342) provided the lowest uptake in healthy tissues. Biodistribution of (186)Re-maGSG-Z(HER2:342) in nude mice bearing SKOV-3 xenografts showed specific targeting of tumours. Tumour uptake 24 h after injection (5.84+/-0.54%ID/g) exceeded the concentration in blood by more than 500-fold, and uptake in kidneys by about 8-fold. Preliminary dosimetric evaluation showed that dose-to-tumour should exceed dose-to-kidney by approximately 5-fold.
Conclusion: Optimization of chelators improves biodistribution properties of rhenium-labelled small scaffold proteins and enables selection of promising radiotherapeutic agents based on the Affibody molecule.