Phototoxic and photoprotective effects of topical isothipendyl.

Isothipendyl, an H1-receptor antagonist, is an antihistamine used as an antiallergic drug. Its molecular structure is close to the phenothiazines, but it has different photobiological properties. Like phenothiazines, isothipendyl is phototoxic with ultraviolet A (UVA) but it reduces the erythema response to UVB radiation. Isothipenyl seems to have the same protective properties as a sunscreen because its UVB protective properties are effective only if the substance is applied prior to exposure.