Background <p>Triple-negative breast cancer (TNBC) is characterized by high invasiveness, high metastatic potential, and poor prognosis. TNBC is not sensitive to endocrine therapy or HER2-targeted treatment, highlighting the need for the development of standardized TNBC treatment regimens. Thus, the development of new TNBC treatment strategies has become an urgent need. MIM1 – BH3 mimetic, which inhibits Mcl-1 antiapoptotic protein, may be an efficacious molecule able to induce apoptosis. Previously, we found that moxifloxacin (MOXI) can modulate the Mcl-1 protein expression. Therefore, in the current study, we assessed the impact of MIM1 and MOXI/MIM1 mixtures on the viability and apoptosis in MDA-MB-231 breast cancer cells.</p> Methods <p>The viability of cells was assessed by the WST-1 assay. The proapoptotic activity of the tested compounds was determined by cytometric technique.</p> Results <p>The results showed that MIM1 exerted high cytotoxic and proapoptotic potential. In the case of two-component models, we have demonstrated that the use of MOXI and MIM1 mixtures resulted in a significant intensification of both cytotoxic and proapoptotic activity – shown as a modulatory effect on mitochondrial membrane potential, cell cycle distribution, or DNA fragmentation, toward MDA-MB-231 cells when compared with MIM1 or MOXI alone.</p> Conclusions <p>We have reported, for the first time, the high proapoptotic activity of MIM1 toward MDA-MB-231 cells pointing to the Mcl-1 protein as an important molecular target. Moreover, the observed potential synergistic mode of action (expressed as a significant enhancement of the induction of apoptosis process) detected for MOXI and MIM1 in the two-component model may provide a new direction for further in vitro and in vivo experiments concerning the role of Mcl-1 protein in the treatment of TNBC.</p> Clinical trial number <p>Not applicable.</p>