Figure 2.
Schematic representation of the pretargeting agents. The trivalent bispecific antibody construct, TF2, binds divalently to CEACAM5, the tumour-associated antigen that is overexpressed on the cell surface of colorectal tumour cells. After the bsMAb has localised the tumour and cleared from the blood, a radiolabelled divalent peptide is given, substituted with the hapten, histamine-succinyl-glycine (HSG). This is rapidly targeted to the tumours and bound by high affinity to the anti-HSG Fab fragment of the bsMAb. Due to its bivalency, it has the ability to crosslink the bsMAbs at the tumour surface, forming a stable complex. The peptide is conjugated with the chelator, DOTA that can be labelled with a variety of radionuclides.