Photosensitizers

Substances that increase sensitivity to light. They are widespread in nature and can be found in tars, tar derivatives (e.g. rivanol), plants (e.g. furanocoumarins, which can be found as fragrances in cosmetics) and medicines.

Photosensitizers can trigger diseases (e.g. light dermatoses), whereby almost all triggers of photoallergic reactions also have phototoxic potential.

Basically, a distinction can be made between two types of photosensitivity:

• Photoallergy (photoallergic reaction)
• Phototoxicity (phototoxic reaction)

Photoallergy is an allergic reaction of the skin that usually occurs a few days after exposure to the sun.

Phototoxicity is a non-immunological reaction that occurs at the first contact with the photosensitizer and in every person if the dose of the substance is sufficient and is followed by appropriate UV irradiation. It can occur within a few hours of exposure to the sun.

Triggers can be systemic or topically applied medications. But also herbal remedies (see below furocoumarins; see belowphototoxic dermatitis). The most important representative of the angular 7,8-furanocoumarins is pimpinellin. (see Pimpinellae Radix below) and angelicin, which can be found in Apiaceae, Rutaceae and Fabaceae. The highly toxic and carcinogenic mycotoxins found in certain molds (especially Aspergillus flavus), the aflatoxins, also have furanocoumarin structures.

Photosensitization starts with the absorption of a photon by a molecule of the respective drug. The molecule then enters a short-lived, high-energy singlet state, which can then dissociate into free radicals. The photosensitizer then changes to a lower energy state, which can change to the ground state or a longer-lasting triplet state through heat release or energy transfer to other molecules. In this state, reactions then occur with biological systems such as cell membranes, lysosomes, lipids, proteins and DNA.

1. Epstein JH (1999) Phototoxicity and photoallergy. Seminar Cutan Med Surg 18: 274-284
2. Nigen S et al (2003) Drug eruptions: approaching the diagnosis of drug-induced skin diseases. J Drugs Dermatol 2: 278-299