Introduction: Cancer is a disease or a set of diseases characterized by uncontrolled growth and cell division. Prostate cancer is the most common cancer in men. Treatment can be surgical, irradiation, hormonal, combined or delayed. The most common is irradiation. The effectiveness of irradiation can be improved by the use of drugs called radiosensitizers. Purpose: The purpose of the diploma work was to determine whether GnRHRGemicitabin (Gemcitabine bound to a peptide binding to a gonadolinberin receptor (GnRHR)) is a targeted drug that can treat cancerous prostate cells and if it is possible to radiosensibilize only cancer cells and not healthy cells. Methods: We performed a clonogenic assay to determine the number of cells that maintained reproductive capacity. We used two cell lines of human prostate cancer PC-3 and DU145, and one murine cell line of colon cancer MC-38. We first determined the expression of GnRH receptor in all three cell lines with quantitative real time PCR. Then we either irradiated the cells lines or irradiate them in combination with Gemcitabine or GnRHR-Gemcitabine. After 14 days, the cells were fixed and stained and visible colonies counted. Results: We showed that all three cell lines were sensitive to irradiation, and their radiosensitivity increased in combination with Gemcitabine. The use of GnRHR-Gemcitabine increased radiosensitivity only in the PC-3 and Du145 cell lines, while it remained the same as for the irradiated cells in the MC-38 cell line. In the murine cell line MC-38 we showed that the GnRH receptor on which GnRHR-Gemcitabine is bound is not expressed, while in the cell lines PC-3 and Du145 it is expressed, which explains the differences in the effect of GnRHR-Gemcitabine in combination with irradiation. Discussion and conclusion: The addition of Gemcitabine increased the radiosensibility of all three cell lines, whereas the addition of GnRHR-Gemcitabine increased only the radiosensibility of PC-3 and Du145 cell lines, but not the MC-38 cell line. Thus, confirming that GnRHR-Gemcitabin is a targeted drug and a radiosenzitizor.