Labeling of monoclonal antibody conjugates with 90Y

An anti-carcinoembryonic antigen (CEA) antibody, NP-4, was labeled with 90Y using p-isothiocyanatobenzyl DTPA (SCN-Bz-DTPA) and its derivatives 1-(p-isothiocyanatobenzyl)-3-methyl-DTPA (1B3M), 2-(p-isothiocyanatobenzyl)-4-methyl-DTPA (1M3B), 1-(2)-methyl-4- isothiocyanatobenzyl-DTPA (MX-DTPA) as the chelating agents. The 90Y conjugates were purified from unbound 90Y by two different methods, HPLC or acrylamide size exclusion gel chromatography, in order to evaluate the best purification method. Labeling efficiency, reaction kinetics and immunoreactivity were compared to the same antibodies labeled with [111In]citrate. Labeling efficiency, as determined by either HPLC or ITLC (instant thin layer chromatography), was consistently higher by ITLC than HPLC for 90Y-labeled MAb, but equal for 111In-labeled MAbs. Discrepancies between the 2 methods were linked to impurities in the 90Y that remained at the origin of ITLC plates. After purification by acrylamide gel filtration, recovery was 50-60% of loaded 90Y activity, but was more than 87% for the 111In compounds. Using HPLC, the recovery measured 85% for 90Y-labeled MAb and more than 93% for 111In-labeled conjugates. Immunoreactivity of the [90Y]MAb was comparable to the 111In-labeled conjugates. These studies indicate that HPLC purification of the [90Y]MAbs improves recovery of activity, and suggests that impurities found in the 90Y and metal-binding properties of acrylamide may have contributed to the poor recoveries from acrylamide gels.