Comparative metabolism and retention of iodine-125, yttrium-90, and indium-111 radioimmunoconjugates by cancer cells.

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.

Degradation of ricin A chain by endosomal and lysosomal enzymes--the protective role of ricin B chain.

We investigated the role of intracellular processing of ricin A chain (RTA) by proteolytic enzymes on the expression of its ribosome inhibitory activity. Endosomal and lysosomal proteases extracted from Jurkat cells and purified cathepsins B, D and G were incubated with RTA, resulting in generation of a 28-kDa fragment by proteolytic cleavage. This process was reminiscent of the nicking of Pseudomonas exotoxin and Diphtheria toxin by intracellular proteases to produce functionally active toxin fragments. However, the ribosome inhibitory activity of the purified 28-kDa fragment of RTA was 11,000-fold less than that of native RTA, suggesting that such cleavage is not an essential step in the cytotoxic activity of the toxin. Addition of ricin B chain (RTB) in degradation assays resulted in the protection of RTA from proteolytic activities of lysosomes and cathepsins. However, RTB did not protect another RNA acting protein, RNAase; nor did excess amounts of unlabeled RTA or IgG protect labelled RTA from degradation, suggesting that the protective effect of RTB was specific to its interaction with RTA. Such a protective role for RTB may partially account for the higher toxicity of immunotoxins (ITs) containing whole ricin compared to ITs containing only RTA.

Retention of B-cell-specific monoclonal antibodies by human lymphoma cells.

The rates of endocytosis, intracellular degradation, and cell-surface shedding of 125I-labeled monoclonal antibodies (MoAbs) HD-37 (anti-CD19), B1 (anti-CD20), MB-1 (anti-CD37), BC8 (anti-CD45), and DA4-4 (anti-mu) by B-lymphoma cells were compared by cellular radioimmunoassay, ultrastructural autoradiography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and thin layer chromatography using biopsy specimens from 12 patients with non-Hodgkin's lymphomas. 125I-BC8 was stably retained on the surface of lymphoma cells without appreciable internalization or shedding, whereas 125I-DA4-4 underwent rapid endocytosis and degradation. 125I-B1 was not internalized or degraded by tumor cells, but rapidly dissociated from the cell surface in intact form. Moderate rates of endocytosis, intracellular metabolism, and cell-surface shedding were shown by 125I-HD37 and 125I-MB-1. The 3 patients with diffuse, small cleaved-cell lymphomas internalized and degraded antibodies more slowly than did patients with other histologic subtypes. These kinetic differences may be important in the selection of MoAbs for immunotoxin and radioimmunoconjugate therapy of B-cell malignancies.