# Advances in Purpurin 18 Research: On Cancer Therapy
###### Abstract  
How to make cancer treatment more efficient and enhance the patient's outcome? By multimodal therapy, theranostics, or personalized medicine? These are questions asked by scientists and doctors worldwide. However, finding new unique approaches and options for cancer treatment as well as new selective therapeutics is very challenging. More frequently, researchers \"go back in time\" and use already known and well-described compounds/drugs, the structure of which further derivatize to \"improve\" their properties, extend the use of existing drugs to new indications, or even to obtain a completely novel drug. Natural substances, especially marine products, are a great inspiration in the discovery and development of novel anticancer drugs. These can be used in many modern approaches, either as photo- and sonosensitizers in photodynamic and sonodynamic cancer therapy, respectively, or in tumor imaging and diagnosis. This review is focused on a very potent natural product, the chlorophyll metabolite purpurin 18, and its derivatives, which is well suitable for all the mentioned applications. Purpurin 18 can be easily isolated from green plants of all kinds ranging from seaweed to spinach leaves and, thus, it presents an economically feasible source for a very promising anticancer drug.  
cancer therapy; gold nanoparticles; marine products; natural products; photosensitizer; purpurin 18; reactive oxygen species; seaweed +
Footnote †: journal: Medical  
waves do not serve as drug activators but as a measurement output. In the case of PAI, the drug is activated by a pulsed laser, and based on the energy conversion, ultrasonic waves are subsequently generated, which are monitored to form an image. Owing to the high resolution of PAI and the possibility of deep penetration into the tissues, this method is a promising technique, especially in terms of tumor diagnosis. However, for all these methods, very specific compounds, e.g., of natural origin, must be sought.  
Natural compounds for cancer therapies have often a plant origin, but many of them come also from marine organisms and seaweed [3]. Many substances isolated from seaweed have been shown to have significant health benefits and are thus often used in the cosmetics or food and pharmaceutical industries. Seaweed can be classified as an important source of polyunsaturated fatty acids, vitamins, enzymes but also natural pigments such as chlorophyls, carotenoids, and quinones [4; 5; 6; 7; 8]. Evidence suggests that the most important are green pigments--chlorophyls. These occur in brown, red, and green seaweed (_Phaeophyceae_, _Rhodophyceae_, and _Chlorophyceae_) [9]. Due to the high content of chlorophyll (Chl; 565 to 2000 mg\(\cdot\)kg\({}^{-1}\) of dried biomass) in seaweed [8] and undemanding cultivation methods on a large scale, seaweed is a favorable source for Chl production. Dried seaweed biomass can be used to extract Chl utilizing organic solvents or supercritical extraction [10]. The Chl thus obtained can then be chemically modified to give its derivatives and metabolites, one of which is purpurin 18 (pu18), which is well known for its unique properties and great potential in anticancer therapy.  
This review aims to detail the most recent advances in pu18 anticancer research, with emphasis especially on PDT, PTT, and PAI. Moreover, biosynthesis of pu18 and novel anticancer derivatives of pu18 are also discussed.