¹⁸⁶Re-maSGS-Z_HER2:342, a potential Affibody conjugate for systemic therapy of HER2-expressing tumours

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 ^99mTc-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 ¹⁸⁶Re and their biodistribution was studied in normal mice. Dosimetric evaluation and tumour targeting to HER2-overexpressed xenografts (SKOV-3) by ¹⁸⁶Re-maGSG-Z_HER2:342 were studied. Results: Gluconate-mediated labelling of maGGG-Z_HER2:342 and maGSG-Z _HER2:342 with ¹⁸⁶Re 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 ¹⁸⁶Re-maGGG-Z _HER2:342 and ¹⁸⁶Re-maGSG-Z_HER2:342 demonstrated lower renal uptake than their ^99mTc-labelled counterparts. ¹⁸⁶Re-maGSG-Z_HER2:342 provided the lowest uptake in healthy tissues. Biodistribution of ¹⁸⁶Re-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.