Abstract P169: Dynamic cell-level modeling of antibody binding and internalization for radiosensitivity assessements in alpha-emitter radiopharmaceutical therapy
Authors: Bastiaannet Remco , Liatsou Ioanna , Hobbs Robert , Sgouros George 
Published: December 2021

Abstract
The radiobiological response to alpha-emitter radiopharmaceutical therapy (αRPT) is often studied in cell monolayers. The geometric models which are currently used to estimate the relationship between isotope activity concentration and cell survival probability typically idealize cells as spheres without considering cellular biological processes. This results in inaccurate and non-generalizable results, which could hamper the rigorous study of the underlying radiobiology. The purpose of this study was to create accurate absorbed dose models by combining Monte Carlo simulations with 3D measurements of cell clustering and geometries, as well as dynamic carrier molecule binding and trafficking in individual cells of cell monolayers. This allows for a more accurate way to model cell survival in these αRPT experiments. Experimental conditions of previous cell survival experiments with 212Pb on NT2.5 HER2+ breast cancer cells were replicated. Live cells were imaged on a confocal microscope. Nuclei were stained with Hoechst and the media was stained with labelled dextran, creating a negative template of the cells. A relevant antibody (Ab) was tagged with AF488. 3D time lapses of membrane binding kinetics and internalization were recorded. Photobleaching was modelled and corrected for. All cells were segmented into nuclei, membrane and cytosol compartments using a purpose-build algorithm. The temporal antibody signals were used to fit pharmacokinetic models, which enabled interpolation and validation with experimental binding assays. The segmentations were used in a Monte Carlo code. S-values for every compartment and time frame were calculated using the Ab distribution directly, capturing the effect of Ab trafficking. Absorbed doses were calculated for each cell and were used to model previously obtained cell survival curves. Statistics were calculated for &gt;100 cells. We observed a large range in absorbed doses (coefficient of variation 0.74). Absorbed doses to the nucleus per unit decay on the membrane, which are mainly determined by cellular geometries, agreed with the geometric model (error &lt;6%). S-values for intercellular decays increased &gt;50% over time, which corresponds to perinuclear trafficking of Abs. The dose contribution of neighboring cells was high (46% of total dose; 6x geometric model), highlighting the importance of cell clustering. Applying this to previous cell survival data yielded an estimated radiosensitivity kappa of 7.1 (geometric model: 2.8). Cell clustering has a larger, and cell geometry has a smaller impact than is assumed in current models. Perinuclear trafficking of internalized Ab positively impacts cell nucleus absorbed dose, which is typically ignored. Dose variability should be included in radiosensitivity modeling. We intend to use such rigorous and highly detailed, cell-level analyses to arrive at simplifications that are generalizable and whose accuracy is better understood. For example, based on our findings a better accounting of cell clustering would substantially improve geometric model calculations.
Citation Format: Remco Bastiaannet, Ioanna Liatsou, Robert Hobbs, George Sgouros. Dynamic cell-level modeling of antibody binding and internalization for radiosensitivity assessements in alpha-emitter radiopharmaceutical therapy [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P169.