Serum concentration and phototoxic effect of methoxsalen in patients with psoriasis

The correlation between serum concentration of methoxsalen (8-methoxypsoralen; 8-MOP) and phototoxic effect using long-wave ultraviolet light has been studied in 5 psoriatic patients. The maximum serum concentration occurred between 0.5 and 2 hr after oral administration of 0.6 mg/kg of methoxsalen. The lowest value for the minimum phototoxic dose (MPD), i.e., highest photosensitivity, was obtained between 1 and 2 hr. There was a significant negative correlation between the logarithm of the serum concentration and minimum phototoxic dose (MPD) (r = 0.780). Hence, the degree of photosensitivity appears to be related to the serum level of methoxsalen.