In Vitro Anticancer Efficacy of Metformin Combination Therapy in Breast Cancer: A Systematic Review

Metformin, a widely used antidiabetic drug, has attracted increasing interest as a potential anticancer agent owing to its ability to modulate cellular metabolism and oncogenic signaling pathways. This systematic review evaluated the in vitro anticancer efficacy of metformin-based combination therapies in breast cancer, focusing on synergistic activity, molecular mechanisms, and subtype-specific responses. A systematic search of the Scopus database was conducted in accordance with PRISMA 2020 guidelines, resulting in the inclusion of 23 eligible studies published between 2016 and May 2026. The studies investigated combinations of metformin with chemotherapeutic agents, targeted therapies, repurposed drugs, and natural compounds across estrogen receptor-positive (ER+), HER2-positive, and triple-negative breast cancer (TNBC) cell lines. Most combinations demonstrated synergistic or potentiating effects, characterized by enhanced apoptosis, cell-cycle arrest, metabolic disruption, and inhibition of tumor-promoting signaling pathways. Mechanistically, recurrent modulation of the AMPK/mTOR, PI3K/Akt, MAPK/ERK, and mitochondrial bioenergetic networks was observed across multiple therapeutic combinations. Several studies also reported reversal of drug resistance, suppression of cancer stemness, and inhibition of epithelial–mesenchymal transition-associated phenotypes. However, the interpretation and translational relevance of these findings are limited by the widespread use of supra-physiological metformin concentrations and considerable methodological heterogeneity among studies. Overall, the available evidence supports metformin as a metabolic sensitiser capable of enhancing the efficacy of diverse anticancer agents in breast cancer models. Future research should prioritize clinically relevant exposure conditions, advanced in vitro models, and predictive biomarker development to facilitate translational application.