Radiotoxicity of Bismuth-213 Bound to Membranes of Monolayer and Spheroid Cultures of Tumor Cells

Monoclonal antibody 13A to murine CD44 was used to bind the α-particle emitter ^{213}{\rm Bi}$ to cell surfaces of cultured EMT-6 or Line 1 tumor cells. Data on kinetics and saturation of binding, cell shape and nuclear size were used to calculate the absorbed dose to the nuclei. Treatment of monolayer cells with $[{}^{213}{\rm Bi}]{\rm MAb}$ 13A produced a classical exponential survival curve with no apparent shoulder. Microdosimetry analyses indicated that 1.4-1.7 Gy produced a 37% surviving fraction (D0). Multicellular spheroids were shown to bind $[{}^{213}{\rm Bi}]{\rm MAb}$ 13A mainly on the outer cell layer. Relatively small amounts of activity added to the spheroids resulted in relatively large absorbed doses. The result was that 3-6-fold less added radioisotope was necessary to kill similar fractions of cells in spheroids than in monolayer cells. These data are consistent with the interpretation that the α particles from a single ^{213}{\rm Bi}$ atom bound to one cell can penetrate and kill adjacent cells. Flow cytometry was used to sort cells originating from the periphery or from the interior of spheroids. Cells from the outside of the $[{}^{213}{\rm Bi}]{\rm MAb}$ 13A exposed spheroids had a lower surviving fraction per administered activity than cells from the interior. Cells were killed efficiently in spheroids up to 20-30 cells in diameter. The data support the hypothesis that α-particle emitters should be very efficient at killing cells in micrometastases of solid tumors.