Objectives: To examine the effects of thienopyridine compound on two breast carcinoma cell lines. Materials and methods: (3-amino-N-(3-chloro-2-methylphenyl)-5-oxo-5,6,7,8-tetrahydrothieno- -[2,3-b]quinoline-2-carboxamide (Compound 1) was used to treat two breast carcinoma cell lines; MDA-MB-231 and MCF-7. To assess the viability of the cells we used MTT test. By identifying Annexin-V+PI- subpopulation and Annexin-V+PI+ subpopulation we assessed the rate of early, late and total apoptosis. Using flow cytometry we examined the expression of glycosphingolipids, which were determined with the fluorescent antibodies. Results: Treatment with Compound 1 resulted in a statistically significant reduction in cancer stem cells in the MDA-MB-231 line from 89.9% (control) to 55.5% (Compound 1). The number of cancer stem cells was also significantly reduced in the MCF-7 line from 48.3% (control) to 19.4% (Compound 1). The inhibitory concentration for the MDA-MB-231 cell line after 48h of treatment was 2.082 μM, while for the MCF-7 cell line it was 8.086 μM. In the MCF-7 cell line, the percentage of GalNAc-GM1b-positive cells increased significantly, while the percentage of Gg3Cer-positive cells decreased in both populations, cancer stem cells and non-cancer stem cells. Conclusions: Compound 1 reduced viability, induced apoptosis, decreased the number of cancer stem cells, and altered glycosphingolipid expression in both cell lines, with more pronounced differences in the MDA-MB-231 cell line. Compound 1 exhibited cytotoxic effects, especially on triple-negative breast cancer cells, and warrants further investigation using in vitro and in vivo models.