Photodynamic therapy (PTD) is a widely used treatment for cancer. The approach conventionally uses a light source of a specific wavelength from outside the body to activate a photosensitiser drug inside a tumour, so that the photosensitiser gains enough energy to react with molecular oxygen already present in tissue. This leads to the production of reactive oxygen species (ROS), which induce the destruction of tumour cells. This technique works well, is minimally invasive and has very few side effects, thus having the potential to supersede more conventional therapies. However, since visible and ultraviolet light can only penetrate flesh to a depth of around one centimetre, the treatment is only suitable for small, localised tumours that are on or just under the skin, or on the lining of internal organs that can be reached by probe or endoscope. Alternatively, the present technology uses the light-emitting mechanisms of deep ocean creatures that can help to overcome the current limitations of photodynamic cancer therapy. Namely, novel photosensitizer drugs were developed, which are self-activated intracellularly and tumour-selectively without the need for external light sources. This achieved by basing the new photosensitizers on the chemiluminescent chemical reaction of Coelenterazine, a common substrate present on marine species. By eliminating the need for light-induced activation, the new potential drugs allow photodynamic therapy to be used on tumours irrespective of their size and localization in the body.