Electrophoretic method for the quantitative determination of a benzyl-DTPA ligand in DTPA monoclonal antibody conjugates.

A simple electrophoretic IEF procedure was developed for the quantitation of bifunctional DTPA ligand molecules in DTPA-protein conjugates. From a calibration plot of pI versus substitution ratios of reference conjugates, the concentrations of DTPA conjugated to protein were determined. Molar ratios of DTPA to protein agreed satisfactorily with the ratios obtained by a spectrophotometric technique using a colored yttrium(III) complex of arsenazo III. The IEF method was successfully applied on preparations of benzyl-DTPA to mAbs MOPC-21, SC-20 (aCEA), and human serum albumin.