@article {Winter1294, author = {Gordon Winter and Anja Vogt and Nusrat Begum and Christoph Solbach and Ambros Beer and Gerhard Glatting and Peter Kletting}, title = {Determination of the internalization rate of radiolabeled ligands by mathematical modeling}, volume = {59}, number = {supplement 1}, pages = {1294--1294}, year = {2018}, publisher = {Society of Nuclear Medicine}, abstract = {1294Objectives: bjectives Targeting the individual tumor characteristics of each patient, an individualized molecular radiotherapy is particularly suitable for an optimal therapeutic treatment. In comparison to simpler models, treatment planning based on complex physiologically-based pharmacokinetic (PBPK) models and pretherapeutic measurements of biokinetics results in better prediction accuracy for absorbed doses. These models require accurately determined physiological parameters, e.g. binding kinetics with substance and tumor cell-specific association rate kon and dissociation rate koff. Also, the internalization rate parameter is of essential relevance for the model. These values need to be determined by in vitro methods to reduce the number of unknown parameters in a complex PBPK model. Here we present a method how to determine objectively these parameters for PSMA-specific peptides. Material and methods The kinetic parameters for association (kon) and for dissociation (koff) rates were obtained by surface plasmon resonance (SPR) spectroscopy. The prostate cancer cell lines LNCaP C4-2 and PC-3 were grown in 12-well plates for internalization experiments. Regarding specificity to PSMA the inhibitor 2-PMPA was added in excess. Peptide concentrations of 0.09 nM, 0.9 nM, 8.8 nM and 77 nM at 37{\textdegree}C were used for the following time points: 10 min, 60 min, 120 min and 180 min. The experimental protocol included two washing steps. Afterwards the surface-bound and internalized fraction were collected separately, and the activity measured using a gamma counter (COBRA II, Perkin Elmer, USA). A specific compartmental model was developed for the cell-based measurement and implemented in SAAM II software (The Epsilon Group, USA, ver. 2.2). The internalization rate was fitted to the experimental data for a fixed protein recycling rate. Results An internalization rate of ((0.025{\textpm}0.0016) min-1) was computed by the implementation of the determined parameters for PSMA-11 with kon = (2.8{\textpm}2.0 x 106) (Ms)-1 and koff = (17{\textpm}9.8 x10-5) s-1 and for PSMA-617 with kon = (3.9{\textpm}2.7 x 106) (Ms)-1 and koff = (5.4{\textpm}4.7 x10-5) s-1. Conclusions: The developed method can be used to accurately and objectively determine important peptide-specific kinetic parameters in vitro. The observed internalization rate appeared to be independent of the applied concentration. Integration of the obtained parameters into patient-specific PBPK models will reduce the number of unknown parameters, enhance the model accuracy and accordingly improve the individual treatment planning for therapy.}, issn = {0161-5505}, URL = {https://jnm.snmjournals.org/content/59/supplement_1/1294}, eprint = {https://jnm.snmjournals.org/content}, journal = {Journal of Nuclear Medicine} }