Photosensitizers

Substances that increase sensitivity to light. They are widespread in nature and are found, among other places, in tars, tar derivatives (e.g., Rivanol), plants (e.g., furocoumarins, which may be used as fragrances in cosmetics), and in numerousmedications.

Photosensitizers can trigger or exacerbate certain conditions (e.g., light-induced skin diseases). Nearly all triggers of photoallergic reactions may also have phototoxic potential.

Generally speaking, there are two types of photosensitivity:

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

Photoallergy is an allergic reaction of the skin that usually occurs several days after sun exposure. It requires prior sensitization to a specific substance. 

Phototoxicity is a non-immunological reaction that occurs upon the first contact with the photosensitizer and affects everyone if the dose of the substance is sufficient and is followed by exposure to UV radiation. It can occur within a few hours after sun exposure.

Triggers can include both systemically and topically administered medications, as well as herbal remedies (see below: furocoumarins; see below:phototoxic dermatitis). The most important representatives of the angular 7,8-furanocoumarins are pimpinellin (see below:Pimpinellae Radix) and angelicin. These are found in species of the Apiaceae, Rutaceae, and Fabaceae families. The highly toxic and carcinogenic mycotoxins—aflatoxins—found in certain molds (especially Aspergillus flavus) 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