Cancer is one of the major problems of modern medicine. The solution to this problem requires the development of new approaches to diagnosis and treatment, among which photodynamic therapy (PDT) play an important role [1]. The properties of the photosensitizer are one of the most important factors determining the antitumor efficacy of PDT. PS should fulfill have a high quantum yield of ROS. It should provide effective selective destruction of all tumor compartments, have no dark toxicity and rapid clearance from the skin and other organs. The photoactive substance should be pharmaceutically pure, can be prepared from available standardized inexpensive raw materials via a short synthetic route involving simple technique) with high yield, stable and readily soluble in injectable solvents [2].

One of the reasons for the insufficiently high efficiency of therapeutic approaches and methods of treating oncological diseases is cancer stem cells (CSC). CSC induce invasive growth, metastasis and cancer recurrence [3]. CSC are resistant to chemotherapy and radiation therapy [4]. Approaches and methods of cancer treatment aimed at the destruction of CSCs and their niches, are under research [5].

It has been shown that CSCs are sensitive to photodynamic effect [6,7]. The results of in vitro studies demonstrating the possibility of CSC inhibition by photodynamic action on cells of lung cancer, cervical cancer, and breast cancer using sodium talaporfin, aluminum sulfophthalocyanine, and other photosensitizers (PS) are described in [8], [9], [10].

It is important to note that effective CSC inhibition is achieved only at sufficiently high levels of photodynamic exposure [8,10], and to take into account that the lungs are full-blooded organs with high light absorption by non-sensitized tissue in the red region of the spectrum. Since the area of aggressive growth and invasion of the tumor into the adjacent normal tissue is located mainly in the depth of the tumor, light passes from the irradiated surface of the lung to this zone through the entire thickness of the tumor. Therefore, to ensure high levels of photodynamic exposure in this zone, which are necessary for the destruction of CSCs, it is important to use photosensitizers with high extinction in the near infrared range (700–800 nm), where the intrinsic absorption of the tumor tissue is minimal (the so-called “spectral window of biological tissue transparency”) [2]. Promising photosensitizers for this purpose are polycationic derivatives of synthetic bacteriochlorin, which do not aggregate in a wide range of concentrations and demonstrated in vitro high efficiency against lung cancer cells, including CSC [11], and low cytotoxicity in the "dark" mode. The phototoxicity of the octacationic derivative and its efficacy against CSCs are higher than those of the tetracationic derivative, and also significantly higher compared to the anionic photosensitizers described in [8,10].

The aim of this work was to study in vivo the antitumor efficacy of photosensitizers based on polycationic derivatives of synthetic bacteriochlorin in a mouse model of lung cancer, and to analyze the mechanisms of tumor destruction induced by PDT with such photosensitizers, including the photodynamic effect on CSCs and tumor neovascularization.