In our study, the absorbed dose of 188Re was sufficient for a significant selective killing effect of 78.7% using 188Re in an in vitro clonogenic assay, while U87-0 cells showed a non-selective killing effect of approximately 31.3%. In this context, it is also important to mention that the in vitro monolayer system is an artificial system and does not allow the full assessment of the therapeutic efficacy of a radionuclide due to the lack of a three-dimensional structure, which requires TCS 46b further exploration in in vivo xenotransplant models. Dinglia et al. suggested that therapy depends on adequate retention of the isotope in the tumor. In the absence of iodide organification isotope trapping is a dynamic process either due to slow efflux or re-uptake of the isotope by cells expressing NIS. With sufficient NIS expression, iodide efflux is a zero-order process and iodide organification is insignificant. In our in vivo imaging study, 188Re remained in the U87-hNIS tumor even 48 hours after administration. In the following therapy study, there was a significant difference in tumor size between U87-hNIS mice and U87-0 mice treated with 55.5 MBq 188Re for 4 weeks. Higher dose of 188Re did not increase therapeutic effect. Unlike thyroid cells, U87-hNIS cells are not polarized and therefore should express hNIS over all regions of their plasma membrane. In vivo, U87-hNIS tumors have a three-dimensional S 25585 structure that places tumor cells in close proximity to each other. This geometry may allow rapid re-uptake of any isotope that leaks from one cell by surrounding cells and serve as a mechanism for isotope trapping by the tumor, which is in part responsible for the observed therapeutic effect of hNIS and 188Re in xenograft models. Therefore, cell arrangement can influence cytotoxicity. Studies with hNIS cDNA transfected human glioma cells also showed increased cytotoxicity of 131I if cells were grouped in a three-dimensional spheroid culture compared to a monolayer culture. This was believed to be due to bystander toxicity, which is maximized in a three-dimensional model. As a corollary, to maximize the therapeutic effect of hNIS, high level transduction and expression are required. Thus, aiming for high level expression of hNIS makes sense not only for maximal radioisotope uptake but also to ensure adequate retention if the isotope is to have its desired effect. In our study, little 188Re is retained in the thyroid gland, as 188Re cannot be organified by this organ.