A modified procedure for rapid labelling of low concentrations of bioactive proteins with indium-111

We describe the conjugation of DTPA to 100-500 micrograms of protein in concentrations of 0.6-1.0 mg/mL utilizing the mixed anhydride method. Free DTPA is removed by minicolumn gel filtration and centrifugation with minimal protein dilution. Radiolabelling at any selected pH can be achieved easily by diluting the protein in the desired buffer. The radiolabelling process can be monitored by instant thin layer chromatography. Any radiochemical impurity detected can be eliminated either by additional minicolumn filtration or further chelation with more conjugated protein. In citrate buffer at pH 6 with minicolumn gel chromatography we prepared 111In-DTPA-D3 (3.0 microCi/micrograms) monoclonal antibody and used it to image hepatocarcinoma in guinea pigs.