Identification and characterization of phyllochlorin sodium as a novel chlorin-derived photosensitiser

We identified Phyllochlorin sodium as a novel chlorin-derived photosensitising agent, formed via the decarboxylation of Chlorin e4 in solution. Purified Phyllochlorin sodium had similar spectral properties to other chlorin-based photosensitising agents Chlorin e4 disodium (Ce4) and Talaporfin sodium (Talaporfin) and with Soret band at 406 nm and a single fluorescence emission peak at 668 nm. In vitro Phyllochlorin sodium exhibited almost exclusively singlet oxygen production when activated, with superior rate and yield of reactive oxygen species (ROS) generation compared to Talaporfin and Ce4. Against a broad array of cancer cells Phyllochlorin sodium had phototoxic IC90 in the low nM range, and was well tolerated by cells (up to µM concentrations) in the absence of light-induced activation. Illumination at 660 nm induced rapid cell death in a dose-dependent manner, with caspase-3/7 activation preceding phosphatidylserine externalisation at higher doses suggesting the involvement of multiple cell death processes, including apoptotic and non-apoptotic mechanisms under intense oxidative stress. Intracellular localisation was observed primarily to the ER indicating this as the primary site of oxidative stress induced following ROS production. Our data suggest Phyllochlorin sodium as a promising candidate for next-generation PDT.