Photosensitized Oxidative Damage from a New Perspective: The Influence of Before-Light and After-Light Reaction Conditions.

Photooxidative damage is heavily influenced by the presence of bioactive agents. Conversely, bioactive agents influence the local environment, which in turn is perturbed by photooxidative damage. These sorts of processes give rise to a version of the "chicken-and-egg" quandary. In this Perspective, we probe this issue by referring to photooxidative damage in one direction as the light-dark (L-D) sequence and in a second direction as the dark-light (D-L) sequence with a reversed cause and effect. The L-D sequence can lead to the downstream production of reactive molecular species (RMS) in the dark, whereas the D-L sequence can be a pre-irradiation period, such as an additive to limit cellular iron levels to enhance biosynthesized amounts of a protoporphyrin sensitizer. A third direction comes from L-D or D-L sequences, or both simultaneously, which can also be useful for optimizing photodynamics. Photodynamic optimization will benefit from understanding and quantitating unidirectional L-D and D-L pathways, and bidirectional L-D/D-L pathways, for improved control over photooxidative damage. Photooxidative damage, which occurs during anticancer photodynamic therapy (PDT), will be shown to involve RMS. Such RMS include persulfoxides (RSOO), NO, peroxynitrate (ONOO), OOSCN, SO, selenocyanogen [(SeCN)], the triselenocyanate anion [(SeCN)], I, I, I, and HOOI, as well as additives to destabilize membranes (e.g., caspofungin and saponin A16), inhibit DNA synthesis (5-fluorouracil), or sequester iron (desferrioxamine). In view of the success that additive natural products and repurposed drugs have had in PDT, a Perspective of additive types is expected to reveal mechanistic details for enhanced photooxidation reactions in general. Indeed, strategies for how to potentiate photooxidations with additives remain highly underexplored.