Photopharmacology is an emerging multidisciplinary field that combines photochemistry and pharmacology. Built upon the ability of light to change the pharmacokinetics and pharmacodynamics of bioactive molecules, it aims at regulating the activity of drugs in vivo by using light. The light-based modulation is achieved by incorporating molecular photoswitches such as azobenzene and diarylethenes or photocages such as o-nitrobenzyl, coumarin, and BODIPY compounds into the pharmacophore. This selective activation of the biomolecules helps prevent or minimize off-target activity and systemic side effects. Moreover, light being the regulatory element offers additional advantages such as the ability to be delivered with high spatiotemporal precision, low to negligible toxicity, and the ability to be controlled both qualitatively and quantitatively by tuning its wavelength and intensity. 
Though photopharmacology is a relatively new field, the concept of using light in therapeutic applications came into practice a few decades ago. Photodynamic therapy (PTD) is a well-established clinically practiced protocol in which photosensitizers are used to produce singlet oxygen for destroying diseased or damaged cells or tissues. Optogenetics is another method that relies on light for dynamically controlling biological functions especially brain and neural. Though this approach has proven useful as a research tool, its clinical implementation is limited by the requirement for genetic manipulation. Mainly, these two techniques laid the foundation for photopharmacology. Today, it is a rapidly evolving field with diverse applications in both basic research and clinical medicine which has the potential to overcome some of the challenges limiting the range of applications of the other light-guided therapies. 
Figure 1. Schematic representation of the mechanism of (a) photopharmacology (b) photodynamic therapy, and (c) optogenetics 
The discovery of natural photoreceptors such as rhodopsins in the eye inspired the biomedical and pharmacology research community to engineer light-sensitive proteins for therapeutic applications. The development of synthetic photoswitchable molecules is the most significant milestone in the history of light-delivery systems. Scientists are continuing with their efforts to explore new photoswitches and delivery strategies with enhanced performance to target different biological molecules such as ion channels, nucleic acid, and enzyme receptors. Photopharmacology research progressed from in vitro to in vivo studies in a significantly short period of time yielding promising results in both forms. Clinical trials are underway to assess the safety and efficacy of these photopharmacological therapies further and validate their potential as an innovative drug delivery approach.