The aim of our study was to assess the effect of the combined treatment of Metformin (Metf) and 5, 10, 15, 20-tetra-sulfophenyl-porphyrin (TSPP)–mediated photodynamic therapy (PDT) on an in vivo tumour model. Wistar male rats were divided in 6 groups: group 1, treated with TSPP; groups 2 and 4 treated with TSPP and Metf, respectively, and irradiated 24 h thereafter; group 3 was treated with Metf and the last two groups received the combined treatment, Metf administered prior (group 5) or after (group 6) irradiation. 72 h from the start of the treatment, tumour tissue was sampled for the investigation of oxidative and nitrosative stress. The apoptotic rate, inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX)-2 expressions and matrix metalloproteinases activities were also quantified. Malondialdehyde and glutathione levels were significantly elevated in the groups treated with combined therapy (p < 0.05). Metf associated with TSPP–PDT reduced iNOS and COX-2 expressions and enhanced nitrotyrosine levels in both therapeutic regimens. Peroxynitrate formation and its cytotoxic effect on tumour cells were related to an elevated index of apoptosis and necrosis. Moreover, MMP-2 activity reached a minimum in the groups which received combined therapy. Our results confirmed that the association of Metf with PDT might prove a new and promising oncological approach.
