Ketogenic Diet and Black Seed Oil (Nigella sativa) Combination Overcomes Doxorubicin Resistance in Breast Cancer: An In Vitro and In Vivo Study

Background: This study evaluated the effects of black seed oil (BSO), a simulated ketogenic metabolic condition consisting of a low-glucose medium supplemented with 5 mM β-hydroxybutyrate (BHB), and their combination in EMT6/P and EMT6/DOX cells. The in vivo effects of a ketogenic diet, BSO, and their combination were also investigated in tumor-bearing mice. Methods: Cytotoxicity of black seed oil (BSO), a simulated ketogenic metabolic condition consisting of low-glucose medium supplemented with 5 mM β-hydroxybutyrate (BHB), their combination, and doxorubicin (DOX) was evaluated in EMT6/P and EMT6/DOX cells using the MTT assay. Combination index (CI) analysis was used to determine the interaction between BSO and the simulated ketogenic metabolic condition. Caspase-3 activity, vascular endothelial growth factor VEGF expression, and P-glycoprotein (P-gp) expression were evaluated in EMT6/DOX cells. In vivo, mice bearing EMT6/P and EMT6/DOX tumors received a ketogenic diet, BSO, their combination, DOX, and control treatment, and tumor growth, tumor weight, tumor-free rate, and serum biochemical parameters were assessed. Results: BSO half-maximal inhibitory concentration IC₅₀ was 14.5-fold higher in EMT6/DOX than in EMT6/P, and DOX IC50 was 39.5-fold higher. The simulated ketogenic metabolic condition consisting of a low-glucose medium supplemented with 5 mM BHB + BSO combination gave a CI of 0.1244 in EMT6/P and 0.0239 in EMT6/DOX, indicating synergism in both lines. The combination produced the highest caspase-3 activity (1.54-fold vs. control), the lowest VEGF level (268.34 pg/mL vs. 953.34 pg/mL in control), and the lowest P-gp level (1.6 ng/mL vs. 3.7 ng/mL in control) among the treatments tested. In vivo, the combination reduced tumor volume by 87.15% in EMT6/P and 58.12% in EMT6/DOX, and gave a tumor-free rate of 66.6% in EMT6/P and 50% in EMT6/DOX. No consistent treatment-related change was observed in liver enzymes, renal markers, or lipid profile compared with healthy controls. Conclusions: The combination of a simulated ketogenic metabolic condition and BSO showed synergistic cytotoxic activity in vitro, while the ketogenic diet and BSO combination produced enhanced antitumor activity in vivo. These effects were associated with increased caspase-3 activity and reduced VEGF and P-gp expression in EMT6/DOX cells, without evidence of hepatic, renal, or metabolic toxicity. These findings support further investigation of this combination in doxorubicin-resistant breast cancer.