As defined by Ben L. Feringa et al., the principle of photopharmacology consists in introduction of a reversibly photoisomerizable unit into the molecular structure of bioactive compounds. Photoisomerizable drugs thus obtained can be used to treat localized diseases by light-activation of drugs precisely where and when they are required, leaving the rest of the treated organism unaffected, as schematically illustrated in the figure. Localized activation might reduce the toxicity burden imposed by the drugs on the patients. Moreover, the drugs can be “switched off” by light after the treatment to further minimize the side-toxicity and reduce the probability of the resistance to develop during the therapy.

The current treatment modality where the photoisomerizable drugs might find immediate use is called photodynamic therapy. Current photodynamic treatment strategies are based on the idea born more than a century ago: in situ light-activation of molecular oxygen, mediated by photosensitizers, leads to generation of reactive oxygen species – the major cause of cell death. It is therefore applicable where the cytotoxic action is the main treatment paradigm: antimicrobial and anti-cancer therapy.

Photoisomerizable drugs provide for oxygen-independent photodynamic therapy, where the photosensitive molecules themselves are the actual treatment agents. Their therapeutic potential can be based not only on killing the malignant or microbial cells – there is an intriguing possibility to activate/deactivate by light wide variety of molecular targets involved in pathological processes to cure localized diseases.