Mutagenicity and carcinogenicity of methoxsalen plus UV-A

The combination of methoxsalen and long-wave UV radiation (UV-A) is being used on a large scale in clinical practice to treat a variety of dermatoses, particularly psoriasis. Theoretically, this combination may be mutagenic and carcinogenic, since methoxsalen binds covalently with DNA under the influence of UV-A. Mutagenicity has been studied in microorganisms and cell cultures. Arguments for possible carcinogenicity come from animal experiments and also from long-term follow-up studies of patients treated with psoralens and UV-A (PUVA). Transplantation studies in animals, allergic and irritative skin reaction experiments, and clinical observations indicate that PUVA also has an immunosuppressive effect. Short-term risk of skin cancer from PUVA therapy seems to be rather low if careful patient selection is made. However, for all patients, long-term maintenance treatment should be avoided as much as possible.