The rational search and verification of the action of DNA damage sensitizers – modified nucleosides (MNs) (for radiotherapy and photodynamic therapy of cancers) was the main goal of this research project. A number of new potential sensitizers were designed using a quantum-chemical model (QM). These compounds were further chemically synthesized, and a wide range of physicochemical methods were used to study the dissociative electron attachment (DEA) process both in solution (stationary radiolysis coupled with high performance liquid chromatography HPLC and mass spectrometry LC/MS, as well as pulse radiolysis) and gas phase (DEA induced in crossed electron-molecular beams experiment). The sensitizing properties of MNs were also tested in the target environment of the sensitizer, i.e. in the DNA molecule, where a series of labeled oligonucleotide sequences were obtained using the on-site developed enzymatic-chemical DNA labeling method. Finally, the activity of the tested derivatives was verified in healthy and cancer cells, determining the cytotoxicity of MN and their radiosensitizing properties in the clonogenic assay.