Psoralen phototoxicity: correlation with serum and epidermal 8-methoxypsoralen and 5-methoxypsoralen in the guinea pig.

Serum and epidermal concentrations of 8-methoxypsoralen and 5-methoxypsoralen 2 hours after oral administration to guinea pigs were determined by high-performance liquid chromatography. A linear relation was found between the serum and epidermal concentrations of 8-methoxypsoralen. In addition, a relation was found between serum concentrations of 5-methoxypsoralen and 8-methoxypsoralen and the appearance of phototoxicity. The lower phototoxicity of orally administered 5-methoxypsoralen as compared to 8-methoxypsoralen in the guinea pig appears to be due to its reduced concentrations in the epidermis, the primary site of the phototoxic events.