Comparison of the radiotoxicity of two alpha-particle-emitting immunoconjugates, terbium-149 and bismuth-213, directed against a tumor-specific, exon 9 deleted (d9) E-cadherin adhesion protein
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We investigated the effects of the a-particle emitters Tb-149 and Bi-213 coupled to a tumor-specific antibody targeting the mutated delta 9 E-cadherin (d9 E-Cad) on single cells and cell pellets. The d9 mutation of the adhesion molecule E-cadherin is found in 10% of diffuse-type gastric cancers and is not expressed in normal tissue. Human breast cancer cells (MDA-MB-435S) transfected with d9 E-Cad or the wild-type E-cadherin gene were used to study the effects of anti-d9 E-Cad MAb coupled to Tb-149 and Bi-213 (Tb-149-d9 MAb and Bi-213-d9 MAb). The density of binding sites determined on transfected MDA tumor cells by Scatchard analysis and flow cytometry varied from 4 x 10(4) to 6 x 10(4) antigens per cell. Internalization of radioimmunoconjugates by cells expressing d9 E-Cad was less than 10% of bound antibody within 240 min. The effect of the radioimmunoconjugates on cell suspensions and cell pellets was quantified by [H-3]thymidine incorporation, and the dose to the cell nuclei was d...etermined using microdosimetric calculations. Tb-149 and Bi-213 immunoconjugates affected cells in suspension similarly. Significant differences in the proliferation capacity of d9 E-cadherin- and wildtype E-cadherin-expressing cells were observed at activity concentrations around 185 kBq/ml, corresponding to antibody concentrations between 200 ng/ml and 1000 ng/ml. Proliferation after incubation with Bi-213-d9 MAb was 50% greater in pelleted wild-type E-Cad-expressing cells compared to wildtype E-Cad cells in suspension. In contrast, the proliferation of pelleted d9 E-Cad cells was similar to that of d9 E-Cad cells in suspension. For Tb-149-d9 MAb, no significant difference was found between pelleted cells and cells in suspension for low activity concentrations. However, at high activity concentrations, Tb-149-d9 MAb had only a small effect on pelleted cells. These in vitro studies demonstrate different effects of Tb-149 and Bi-213 conjugated to a tumor-specific antibody toward single cells and tumor cell pellets. Microdosimetric simulation of single cell survival after a-particle irradiation modeled the experimental results with reasonable accuracy. (C) 2003 by Radiation Research Society.