Objectives: The aim of this study was to demonstrate the effect of a synthetically derived inhibitor from the thieno[2,3-b]pyridine group on cell apoptosis of MCF7 breast cancer cell line and MDA-MB-231 triple-negative breast cancer cell line. It is hypothesized that after the treatment of tumor cell lines with an inhibitor, the percentage of cells in early, late, and early and late apoptosis will increase. Materials and methods: Using MTT, we obtained required concentration of inhibitor with whom we treated breast cancer cells of the MCF7 and MDA-MB-231 cell lines for 48 hours. After the addition of Annexin V and propidium iodide, the cells were analyzed by flow cytometer to obtain the percentage of cells in early and late apoptosis in treated and untreated cancer cells. Results: They are presented as the percentage of early and late apoptosis and surviving cells. The effect of inhibitor on MCF7 cell line showed a decrease in the number of cells in early apoptosis compared to the control, which is compensated by the statistically significant increase in percentage of the cells in late and total early and late apoptosis compared to untreated cells. In this study, the inhibitor showed a mild, but statistically significant increase in the percentage of MDA-MB-231 cells, a triple-negative breast cancer cell line, in early apoptosis. The inhibitor also showed a significant increase in cells in late and total early and late apoptosis in observed cell lines. Conclusions: Newly synthesized thieno[2,3-b]pyridine led to a significant increase in the percentage of breast cancer cells which enter into apoptosis. The action shown by this inhibitor could be significant in the treatment of triple-negative breast cancer.