Photosensitive compounds for use in a method of treating a disease or condition are described. The photosensitive compounds have the formula R—Y, wherein R is a ruthenium complex and Y is at least one sulphur-containing photoreleasable group, and the compounds comprise at least one ruthenium-sulphur bond; or a pharmaceutically acceptable salt, solvate, ester or amide, such that upon influence of visible or near infra-red light (400-1400 nm) in vivo, said at least one ruthenium-sulphur bond is broken, thereby generating a pharmacologically active compound.