Concentration of trimethylpsoralen in blood and skin after oral administration.

Trimethylpsoralen (TMP) concentrations were determined in blood and skin of 21 patients given oral TMP therapy and were compared to data obtained from 5 patients treated with TMP baths. The quantitative determination was performed by gas chromatography with selected ion monitoring. Concentrations of up to 5.6 ng/ml were detected in whole blood from orally TMP treated patients, with about the same concentrations in patients given TMP baths. Whole skin biopsies from patients ingesting TMP showed concentrations in 14 patients that ranged from 30 to 1250 ng/g skin, median value 85 ng/g. Large interindividual variations were observed. Stripped skin from 5 patients after TMP baths showed a somewhat higher median value of 160 ng/g and their entire skin had fairly high concentrations, with a median value of 390 ng/g, probably mostly bound to stratum corneum. Different concentrations in different parts of the skin may explain the difference in phototoxic capacity when the drug is given locally.