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

Abstract

We investigated the effects of the alpha-particle emitters (149)Tb and (213)Bi 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 (149)Tb and (213)Bi ((149)Tb-d9 MAb and (213)Bi-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 [(3)H]thymidine incorporation, and the dose to the cell nuclei was determined using microdosimetric calculations. (149)Tb and (213)Bi 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 (213)Bi-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 (149)Tb-d9 MAb, no significant difference was found between pelleted cells and cells in suspension for low activity concentrations. However, at high activity concentrations, (149)Tb-d9 MAb had only a small effect on pelleted cells. These in vitro studies demonstrate different effects of (149)Tb and (213)Bi conjugated to a tumor-specific antibody toward single cells and tumor cell pellets. Microdosimetric simulation of single cell survival after alpha-particle irradiation modeled the experimental results with reasonable accuracy.

Targeted therapy of osteosarcoma with radiolabeled monoclonal antibody to an insulin-like growth factor-2 receptor (IGF2R)

“…Since efficacy of RIT is directly proportional to the mAb’s binding constant, high RIT efficacy was predicted. It should be noted that in RIT studies where similar antigen levels on the surface of the targeted cells were observed in vitro [22,23], the RIT was nevertheless successful in killing the targeted cells. In the future, use of αemitters should also be explored for IGF2R-targeted RIT of osteosarcoma to counteract the relatively low number of binding sites.…”

Fractionated Locoregional Low-Dose Radioimmunotherapy Improves Survival in a Mouse Model of Diffuse-Type Gastric Cancer Using a 213Bi-Conjugated Monoclonal Antibody

“…disseminated gastric cancer (3). The specific binding of the labeled carrier to single tumor cells or tumor cell clusters and the short path length of a-particles emitted by 213 Bi of only several cell diameters result in a high cytotoxic potential (4,5). 213 Bi (t 1/2 = 46 minutes) is eluted from a 225 Ac/ 213 Bi generator and rapidly coupled to mAbs by the chelator CHX-A…”

Targeted use of Alpha Particles: Current Status in Cancer Therapeutics

“…It has been conjugated to antibodies and used in mice with colorectal cancer, melanoma as well as other malignancies [17,18]. When compared to 213 Bi in preclinical studies the results were indecisive [19]. Polonium (Po), emits x-rays and can be used in external imaging for bio-distribution studies [20].…”

“…213 Bi has a half-life of 45.6 minutes and emits an alpha particle of 8 MeV. Additionally, a 440 keV photon emission accompanies 26.5% of 213 Bi decays, allowing detailed biodistribution and dosimetry studies to be performed [19,24,26].…”