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

We investigated the effects of the α-particle emitters ¹⁴⁹Tb and ²¹³Bi 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 ¹⁴⁹Tb and ²¹³Bi (¹⁴⁹Tb-d9 MAb and ²¹³Bi-d9 MAb). The density of binding sites determined on transfected MDA tumor cells by Scatchard analysis and flow cytometry varied from 4 × 10⁴ to 6 × 10⁴ 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 [³H]thymidine incorporation, and the dose to the cell nuclei was determined using microdosimetric calculations. ¹⁴⁹Tb and ²¹³Bi immunoconjugates affected cells in suspension similarly. Significant differences in the proliferation capacity of d9 E-cadherin- and wild-type 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 ²¹³Bi-d9 MAb was 50% greater in pelleted wild-type E-Cad-expressing cells compared to wild-type 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 ¹⁴⁹Tb-d9 MAb, no significant difference was found between pelleted cells and cells in suspension for low activity concentrations. However, at high activity concentrations, ¹⁴⁹Tb-d9 MAb had only a small effect on pelleted cells. These in vitro studies demonstrate different effects of ¹⁴⁹Tb and ²¹³Bi conjugated to a tumor-specific antibody toward single cells and tumor cell pellets. Microdosimetric simulation of single cell survival after α-particle irradiation modeled the experimental results with reasonable accuracy.