The development of new drugs or drug candidates based on the phenothiazine system (10H-dibenzo-[b,e]-1,4-thiazine) is a promising approach in view of the diverse biological activity of this tricyclic system, which is present in traditional drugs (chlorpromazine. thioridazine, trifluoperazine, trifluopromazine) with antipsychotropic, antihistamine and antimuscarinic activities. In practical medicine, these drugs are used as antagonists of dopamine and other neurotransmitter receptors for the treatment of schizophrenia and bipolar disorders. Ongoing studies on the synthesis and biological screening of various phenothiazine derivatives in recent years have revealed other important biological effects of these compounds, among which their antitumor effects are of great interest. This work reports the synthesis of a novel N-substituted phenothiazine analog bearing a mitochondria-directed cationic group (E)-4-(1H-indol-3-ylvinyl)-pyridinium (F16) linked to the nitrogen atom of the phenothiazine core by a butane bridge. The lipophilic cationic F16 fragment was used as a means to enhance transmembrane transport and selective delivery of the hybrid molecule into the mitochondria of cancer cells. In tests on the BT474 breast cancer cell line, the phenothiazine-F16 hybrid demonstrated significant cytotoxic activity. The cytotoxic effect of the compound was noticeable at a concentration of 5 μМ and further increased dose-dependently, leading to complete tumor cell death at a concentration of 50 μМ (IC₅₀ 3.3 μM). The F16-derivative of phenotzine showed marked mitochondrial targeting. In experiments on isolated rat liver mitochondria, the tested agent already at a concentration of 5 μМ significantly decreased the membrane potential of succinate-energized organelles. Increasing the concentration of phenothiazine hybrid to 20 μМ resulted in complete dissipation of the potential. The obtained result of antitumor activity against BT-474 cell culture and significant effect on the reduction of mitochondrial membrane potential allows us to consider phenothiazine-F16 hybrid as a new promising antitumor drug.