Difference in topical and systemic reactivity of psoralens: determinations of epidermal and serum levels

Topical application of 8- and 5-methoxypsoralen (8-MOP, 5-MOP), psoralen, and 4,5',8-trimethylpsoralen (TMP) to the epilated backs of albino guinea pigs followed by UV irradiation at 320-400 nm resulted in elicitation of similar phototoxicity. However, the systemic phototoxicity determined after oral administration of these psoralen derivatives differed significantly. Serum and epidermal levels of 8-MOP, 5-MOP, and TMP were determined 2 hours post administration. We found a linear relationship between serum and epidermal levels of 8-MOP, but the levels of the other psoralen derivatives were significantly lower. We believe that the reduced oral phototoxicity of 5-MOP and TMP relative to 8-MOP in the guinea pig is due to its reduced levels in the epidermis.