Background <p>Triple-negative breast cancer (TNBC) lacks targeted therapies and is commonly treated with doxorubicin, whose efficacy is limited by toxicity and chemoresistance. Bioactive peptides have emerged as potential adjuvants due to their membrane activity and antitumor properties. To address the potential of the combined treatment, we explored the cellular effects and underlying mechanisms of Ctn-2 peptide and doxorubicin in TNBC cells.</p> Methods and Results <p>MDA-MB-231 cells were treated with Ctn-2 (15 µM), doxorubicin (1.25–5 µM), and their combination. Membrane interactions were evaluated using FTIR spectroscopy in model lipid systems. Intracellular reactive oxygen species (ROS) were measured by DCFDA fluorescence, and cell cycle distribution was analyzed by DNA content. FTIR analysis suggested peptide–membrane interactions in tumor-like lipid systems. Ctn-2 alone did not significantly affect ROS production or cell cycle distribution. Doxorubicin induced a dose-dependent increase in ROS and cell death. The combined treatment enhanced cell death, as evidenced by an increased sub-G1 population, without further increasing ROS levels or inducing phase-specific cell cycle arrest.</p> Conclusions <p>The effect of the combined treatment appears to be associated with increased cell death rather than ROS accumulation or cell cycle arrest. Membrane perturbation induced by Ctn-2 may contribute to this effect by facilitating doxorubicin activity. Although preliminary, these findings suggest the potential of peptide-based combination strategies in TNBC and warrant further mechanistic investigation.</p>