Light intensity effect on the mechanisms of tumor damage photosensitized by a substituted Zn(II)-naphthalocyanine

Cytotoxicity induced by photodynamic therapy (PDT) is connected with the phenomena of photo-oxidation. Generation of singlet oxygen and free radicals (superoxide or hydroxide) is accepted as a mechanism for the photo-oxidation action of PDT. Very little is however known about the validity of metabolitic and biochemical events observed in cell culture systems to in vivo tumor shrinkage following PDT. In the present work using the well-studied tetrabenzamido-substituted zinc (II)-naphthalocyanine (ZnNc) including towards pigmented melanoma, we accessed its efficacy for apoptotic processes during PDT of Lewis lung carcinoma (LLC) in mice in dependence on light intensity. Early photodynamic therapy responses were examined at 1, 3, 6, 10 and 24 h after coherent 774 nm illumination of the tumors applied 24 h after intraperitoneal (i.p.) injection of dipalmitoylphosphatidylcholine (DPPC)-liposome-incorporated 0.5 mg kg^-1 b.w. ZnNc. Fluence rates of 260, 380 and 500 mW cm^-2 at a fluence of 360 J cm^-2 were used. Macroscopic observations showed that tumor reduction (and its eventual elimination) depends on optimal conditions for the occurring of photochemical reaction during PDT. At the same time, electron microscopy (EM) assays demonstrated strongly expressed dependence of apoptotic processes on the applied light intensities. Features of apoptotic processes were most clearly expressed at the highest used fluence rate.