Radiolabeled
antibodies have produced encouraging remissions in patients with
chemotherapy-resistant
hematological malignancies; however, the selection of therapeutic
radionuclides for clinical trials remains controversial. In this study, we compared the internalization, lysosomal targeting, metabolism, and cellular retention of radiolabeled murine and
humanized monoclonal antibodies targeting the
CD33 antigen (
monoclonal antibodies mP67 and hP67, respectively) on
myeloid leukemia cell lines (HEL and HL-60) and of anti-
carcinoma antibodies (monoclonal antibodies hCTM01 and hA33) targeting
breast cancer and
colorectal carcinoma cell lines (MCF7 and Colo 205, respectively). Each antibody was labeled with 125I (by the
IodoGen method) and with 111In and 90Y using macrocyclic chelation technology. Targeted
tumor cells were analyzed for retention and metabolism of
radioimmunoconjugates using cellular-radioimmunoassays,
Percoll gradient fractionation of cell organelles, SDS-PAGE, and TLC of cell lysates and culture supernatants. Our results suggest that
antibodies are routed to lysosomes after endocytosis, where they are proteolytically degraded. [125I]
monoiodotyrosine is rapidly excreted from cells after lysosomal catabolism of
antibodies radioiodinated by conventional methods, whereas small molecular weight 111In and 90Y catabolites remain trapped in lysosomes. As a consequence of the differential disposition of small molecular weight catabolites, 111In and 90Y conjugates displayed superior retention of radioactivity compared with 125I conjugates when
tumor cells were targeted using rapidly internalizing antibody-
antigen systems (e.g., hP67 with HEL cells and hCTM01 with MCF7 cells). When
tumor cells were targeted using antibody-
antigen systems exhibiting slow rates of endocytosis (e.g., hP67 on HL-60 cells and hA33 on Colo 205 cells), little differences in cellular retention of radioactivity was observed, regardless of whether 125I, 111In, or 90Y was used.