Anti-MUC1 aptamers: radiolabelling with 99mTc and biodistribution in MCF-7 tumour-bearing mice

Introduction: Aptamers previously selected against the protein core (AptA) or the tumour glycosylated (AptB) MUC1 glycoprotein have been conjugated to MAG2 and labelled with 99mTc, for the potential use as radiopharmaceuticals for diagnostic imaging of breast cancer. Methods: The conjugation was achieved in high yield using standard peptide coupling reactions between an amino modification on the aptamer and the activated carboxylic group on the ligands. The retention of the affinity of the MAG2 modified AptA for the MUC1 protein core was confirmed using a fluorescent intercalator displacement binding assay. The labelled aptamers were separated from free 99mTc using ultrafiltration and monitored by high-performance liquid chromatography at all stages, to ensure that only radiolabelled aptamers were produced. The biodistribution properties of the two aptamer-radionuclide conjugates were analysed in MCF-7 tumour bearing mice and compared. Results: Efficient and convenient labelling of the two aptamers with 99mTc was achieved as the last step of the synthesis (post-conjugation labelling). Both the aptamer-chelator conjugates had strong 99mTc binding properties and the resulting complexes were stable in vivo, both in terms of nuclease degradation and leaking of the metal. The radiolabelled aptamers showed a high renal clearance and a high uptake in the intestine. Conclusions: AptA and AptB have been successfully conjugated in high yield to the ligand MAG2 and labelled with 99mTc. The radiolabelled aptamers showed different tumour uptake and clearance, but will require further development prior to diagnostic use.