Plasma and skin concentration of 5-methoxypsoralen in psoriatic patients after oral administration

The aim of this work was to investigate the distribution of 5-methoxypsoral en in the skin after oral administration of the drug and to examine the cor relation between skin and plasma concentrations. 5-Methoxypsoralen skin con centration was measured in both healthy and psoriatic sites of 10 psoriatic patients after single and multiple oral doses. The results obtained show t hat 5-methoxypsoralen accumulates at higher levels in the more external lay ers of the skin after oral administration. The high affinity of drug for th e stratum corneum was confirmed by in vitro skin affinity measurements. The concentration of 5-methoxypsoralen in the skin was similar in both psoriat ic and healthy sites, indicating that the pathology does not influence drug distribution in the skin. After single dose administration, a linear corre lation was found between skin and plasma drug concentration. After multiple dose administration, drug concentration in the skin was fairly constant de spite the variable plasma concentrations in different subjects.