Drug Induced Photosensitivity

Abstract

Photoreactions induced by drugs can be phototoxic, clinically characterized by erythema and whealing or an exaggerated sunburn reaction, or photoallergic showing eczematous dermatitis in a photodistribution. The term systemic chemical photosensitivity has been used for reactions which mechanisms are uncertain.

In phototoxicity cell alterations at the molecular level are induced by photoac-tivation of the sensitizer. Damage of DNA, cell membranes and lysosomes may occur. If oxygen is required, the reaction is designated photodynamic. In this process singlet oxygen, superoxide anion and hydroxy radical are generated as activated species. Benoxaprofen has been shown to induce such radicals. The photoaddition of a photosensitizer, e. g., psoralen, to DNA is non-photodynamic. Photo-products of a photosensitizer, which can disrupt cell membranes, may also be generated in absence of oxygen. This may in part be the mechanism of tetracycline phototoxicity. In some instances, more than one mechanism may be involved, as has been shown for chlorpromazine.

Photoallergy requires reaction of a chemical with radiation (UVA) inducing an immune mechanism by which finally a type-IV-reaction is elicited. The method of choice to identify photoallergens is the photopatch test or, if this fails, provocative phototesting. An effort has been made to standardize substances and photo-test procedures in multicenter studies in Scandinavia as well as in Germany, Switzerland and Austria. Most frequently identified photoallergic drugs are pheno-thiazines, nonsteroidal antiinflammatory drugs, sulfonamides and hydrochlorothiazide.

Distinction between phototoxic and photoallergic test reactions and many seemingly non-relevant reactions in the photopatch test represent as yet unsolved problems.