Hypoxia negatively affcts the effctiveness of all types of anticancer therapy, in particular photodynamic therapy (PDT). In this regard, various approaches to overcome the limitations associated with hypoxia are widely discussed in the literature, one of them is the use of photosensitizers (PS) operating through the fist mechanism of the photodynamic reaction, such as methylene blue (MB). Previously, we have demonstrated that MB can have a positive effect on tumor oxygenation. In this work, we investigated the photodynamic activity of MB and a combination of MB with chlorin e6 on a tumor in vivo using a model of Ehrlich carcinoma. PDT was studied with the joint and separate administration of chlorin e6 and MB. The accumulation and localization of MB and its combination with chlorin e6 in vivo was assessed using video ˛uorescence and spectroscopic methods, and the effect of laser exposure on accumulation was analyzed. After the PDT with chlorin e6, MB and a combination of MB with chlorin e6, a good therapeutic effect and a decrease in the tumor growth rate were observed compared to the control, especially in groups with PDT with MB and with the simultaneous administration of chlorin e6 and MB. The level of tumor oxygenation on days 3 and 5 after PDT was higher for groups with irradiation, the highest oxygenation on the 5th day after PDT was observed in the group with PDT only with MB. Phasor diagrams of tumors after PDT show a deviation from the metabolic trajectory and a shift towards a longer lifetimes compared to the control tumor, which indicates the presence of lipid peroxidation products. Thus, tumor regression after PDT is associated with the direct destruction of tumor cells under the in˛uence of reactive oxygen species formed during PDT. Thus, the effectiveness of PDT with the combined use of MB and chlorin e6 has been demonstrated, and the main mechanisms of the antitumor effect of the combination of these PS have been studied.
