(Photo-)chemical roadmap to strategic antimicrobial photodynamic and photothermal therapies

Antibiotics have become increasingly less effective due to development of microbial resistances. About a century ago, photodynamic therapy started out as anti-tumour therapy then later, it became clinically used to treat a wide range of medical conditions, such as non-malignant diseases, e.g., dermatological, ophthalmic and cardiovascular diseases, malignant diseases, e.g., various cancers, and infectious diseases caused by microbial pathogens, such as bacteria and viruses. The predominantly accepted mechanism of action to kill microbial pathogens is the generation of reactive oxygen species after irradiation of suitable photosensitizers (photo-dynamic therapy), but conversion of light to heat (photo-thermal therapy) is considered increasingly important. In both cases, photosensitizers will probably evoke less microbial resistance than conventional antibiotics, since they are only effective during irradiation. Many classes of pigments, either as rather simple, low-molecular free compounds or as parts of high-molecular photosensitizer-containing complexes, are promising for application as photosensitizer in both antimicrobial photodynamic and photothermal therapy (aPDT and aPTT, respectively). Here, we present a strategic roadmap for the introduction of anti-microbial light-dependent therapies. This also includes the possible emergence of future aPDT- and aPTT-resistances, and strategic blueprints regarding applications, chemical technology and future clinical use.