Review. Photooxygenation, photodegradation and antioxidative activity of platonin, a cyanine photosensitizing dye.

Platonin (4,4'-dimethyl-3,3'-di-n-heptyl-8-[2-(4methyl-3-n-heptylthiazole)] -2,2'-dicarbocyanine diiodine) is one of the photosensitive trithiazolepentamethine cyanine dyes. Visible light (VL)-promoted photodegradation products of platonin in an aqueous environment were identified as 3-heptyl4-methylthiazoline-2 -carbaldehyde (1), tetradecane-7-thiol (2), 1-nonene (3), heptylamine (4), 3-heptyl-4-methyl-2-thiazolone (5), 3-heptyl-4-methyl-2-thiothiazolone (6), 5-[2-(3-heptyl-4-methylthiazolidene)]-2-penten-l-aldehyde (7), gamma-(3-heptyl-4-methyl-2-thiazolidene)crotonic acid (8) and 3,5-di (4-methyl-2-thiazolyl)-2,4-pentadienic acid (9). The quantum yield of singlet oxygen (1O2 (1deltag)) derived from VL-promoted platonin formed heptyl and heptyl cation radicals together with the photodegradation products described above. In isolated rat hepatocytes, platonin was cytotoxic under VL irradiation, whereas non-irradiated platonin was less cytotoxic and improved cell viability. The effects of oxygen uptake and cell viability of photolysis photoproducts of platonin, 3-heptyl-2,4-dimethylthiazolium iodide (HDT) and 3-heptyl-4-methylthiazolium iodide (HMT) were compared with those of platonin. These compounds, particularly the former, showed greater cytotoxicity and brought about less oxygen uptake than the latter. Radical-scavenging activities of platonin using an induction period method demonstrated that fully oxidized platonin had a stoichiometric factor (n) of 4. Platonin was a potent peroxy-radical scavenger. The dual modulation activity of platonin as a prooxidant and an antioxidant under VL irradiation was revealed by monitoring the oxygen uptake in isolated rat hepatocytes. This antioxidant/prooxidant activity of platonin may induce diverse effective pharmacological activities in biological systems. In the light of recent developments in studies of platonin and related compounds, the VL-promoted photooxygenation, photodegradation, antioxidant activity and biological activity of platonin are discussed.