Metformin and oxyphotodynamic therapy as a novel treatment approach for triple-negative breast cancer

BACKGROUND: Treatment for triple-negative breast cancer (TNBC) remains a significant challenge due to a lack of targeted therapies. While photodynamic therapy (PDT) has been utilized as a treatment approach for several types of cancer, oxyphotodynamic therapy (OPDT) is a novel method that improves treatment efficacy by increasing local oxygen concentration. Metformin (MET) has been demonstrated utility as an anti-tumor agent by acting through the adenosine monophosphate-activated protein kinase (AMPK) pathway. We hypothesized that MET in combination with heme, a byproduct of 5-aminolevulinic acid (ALA), may increase cytotoxicity for cancer treatment. This study aimed to investigate the synergistic effect of MET and ALA with PDT or OPDT on TNBC tumorigenic cells. METHODS: The treatment efficacy and phototoxicity of PDT or OPDT were determined using a cell viability assay. PDT/OPDT experiments were carried out in nine groups based on different combinations and concentrations of ALA and/or MET. To calculate the synergistic effect by compuSyn soft for different groups, cells were incubated with ALA and/or MET at the following concentrations (0, 0.25, 0.5,1, 2, 4, 8, 16, 24, and 32 mM). The fluorescence of ALA-induced protoporphyrin IX (PpIX) and MitoTracker Green were observed under a confocal microscope. RESULTS: The optimized therapeutic concentration ratio of ALA and MET was determined to be 1:1. The inhibition of cancer growth (IC(50)) for each group was 14.03, 10.62, 7.71, 18.27, 22.09, 23.96, 4.57, 10.20, and 8.18 mM, respectively. The combination index (CI) values (fa =0.5) of the last three combination groups (groups 7, 8, and 9) were 0.44, 1.70, and 1.47, respectively. PpIX fluorescence intensity of group 9 (ALA-MET-OPDT group) remained the highest among all groups, indicating an enhanced therapeutic effect. CONCLUSIONS: This study introduces OPDT as a novel anti-tumor therapy for TNBC. Furthermore, the combined use of ALA and MET had a synergistic anti-tumor effect in TNBC cells when combined with OPDT.