Heptamethine dyes from the group of cyanine dyes have fluorescent properties and work in the near-infrared region. Due to their low phototoxicity, they have a potential application for diagnostic purposes. Thanks to their photosensitivity, they can also be used as part of photodynamic therapy in the treatment of certain types of tumors. During the preparation of this thesis, the biological properties of three Cy7 dyes were investigated: SB-35, SB-53 and SB-55, newly synthesized phenanthridine derivatives with amino acid substituents (glycine, lysine) and a guanidinocarbonylpyrrole unit. The effect of Cy7 dyes on cell metabolism, as a measure of viability, their accumulation in the cell and phototoxicity was tested using cervical cancer cells (HeLa), alveolar adenocarcinoma (A549), melanoma (RPMI-7951) and normal fibroblasts. The possible selectivity of the compounds, considering cell drug resistance, was tested on four additional cell lines: high-grade serous ovarian cancer cells (parental MES-OV and carboplatin-resistant MES-OV CBP) and ovarian adenocarcinoma cells (parental SK-OV-3 and carboplatin resistant line SK-OV-3 CBP). Using the 3-(4,5-methylthiazol-2-yl)-2,5-diphenyltetrazolium bromide test, it was determined that the weak cytotoxicity of Cy7 dyes, but also the selectivity considering the resistance of cells to drugs and their origin, indicate their possible use in diagnostics, while the simultaneous increase in cytotoxicity by photoactivation indicates their possible use in therapy. Differences in the accumulation of Cy7 dyes considering the type of cells are most likely caused by different expressions of the OATP transporter which was analyzed by the Western blot method. Preliminary results, obtained using the sulforadamine S assay, indicate a possible effect of Cy7 dyes on protein synthesis.