**4. Hypericin – high potential for PD and PDT**

Hypericin, a Naphtodianthron is a naturally occurring compound of the plant *Hypericum perforatum*, better known as St. John's wort. St. John's wort is a plant that has been used since the Middle Ages to treat wounds and depression. It grows bushy and has its peak season between May and August, see figure 2. St. John's wort contains the following active ingredients: essential oils, flavonoids (Biapigenin, hyperoside, Isoquercitin, rutin), tannins, glycosides, resins, Naphtodianthrone (hypericin, Pseudohyericin) and phloroglucinol (hyperforin). In 1942 by Pace a pronounced sensitization of the skin by light for grazing animals taken St. John's wort containing feed was described. The changes in the skin were reversible after the animals were protected from sun exposure. The phenomenon was described as hypericism.(Pace 1942) Hypericin is a lipophilic molecule that is incorporated into the phospholipid bilayer of cell membranes and has already in the dark versatile pharmacological activities. These include antiviral, anticancer and antiangiogenic properties. Takahashi et al. could show an inhibitory effect on proteinkinase C, which is involved in cell proliferation.(Takahashi et al., 1989) Malignant gliomas have, compared to glial cells, a high proteinkinase C activity.(Couldwell et al. 1991) Hypericin has excellent properties as a PS. It has a high triplet quantum yield and a high efficiency in the formation of ROS.(Diwu & Lown 1993;Ehrenberg, Anderson, & Foote 1998;Hadjur et al. 1996)

The excessive production of the so called ROS leads to oxidative stress to many biomolecules, e.g. proteins, causing cell death by induction of apoptosis, necrosis or autophagy associated cell death.(Buytaert, Dewaele, & Agostinis 2007)

Fig. 2. *St. John's wort, also known as Hypericum perforatum*; [www.awl.ch/heilpflanzen/hypericum-perforatum/index.htm].
