Two cases of mequitazine-induced photosensitivity reactions

Mequitazine is a phenothiazine derivative antihistamine. We experienced 2 cases of mequitazine-induced photosensitivity reaction in patients who took mequitazine for their dermatologic problems. Case no. 1 showed features of photoallergy such as 1-month incubation period, decreased MED to UVA, strong positive photopatch test result, cross reaction to chlorpromazine, and angry back phenomenon found in allergic contact dermatitis. In addition, an immediate erythematous macule was observed on the photopatch test site of mequitazine directly after UV exposure which was similar to the immediate erythema noted in chlorpromazine photoallergy. During the follow-up period of two months, there was transiently decreased MED to UVB, which became normalized after 1 month treatment with cyclophosphamide. In case no. 2, the pathogenic mechanism seemed to be persistent light reaction preceded by systemic photoallergy, as he had taken mequitazine for 6 months, and there were strong positive photopatch test results with immediate erythema reaction, cross-reaction to promethazine, decreased MED to both UVA and UVB, and persistence of the photosensitivity over a 3-year follow-up period after discontinuation of the mequitazine. Photopatch tests to 1% mequitazine with 5 J/cm2 of UVA in 30 normal subjects were all negative, which means that 1% concentration may be appropriate for the photopatch test. Mequitazine seemed to play a part similar to chlorpromazine, and absence of mequitazine-induced photosensitivity may be due to a relatively low dosage of the drug. However, dermatologists should be cautious about a photosensitivity reaction induced by mequitazine or other phenothiazine-derivative drugs.