Intraindividual variations of 8-methoxypsoralen plasma levels.

Intraindividual variations of 8-methoxypsoralen (8-MOP) plasma levels were investigated. On at least 3 consecutive days of treatment 8-MOP levels were determined 2 h following the administration of 0.56-0.82 mg/kg b.w. On day 1 a complete plasma level-time profile was obtained. Interindividually, 8-MOP plasma levels varied from 0-654 ng/ml. Intraindividual variation was considerably less being 1.2-5.3-fold. In one patient who showed a lower absorption rate of 8-MOP plasma levels varied from 0-126 ng/ml. There was a rather close correlation (r = 0.81) of the variation of 8-MOP plasma levels (determined as the ratio of highest to lowest plasma level) and mean plasma level, intraindividual variation decreasing exponentially with increasing 8-MOP plasma levels. Parameters determining variation of 8-MOP plasma levels are the intrinsic hepatic clearance, liver blood flow, and possibly the absorption of the drug from gastrointestinal tract. To keep intraindividual variation as low as possible ingestion of the drug together with a standardized meal is suggested. Drugs interfering with hepatic metabolism or liver blood flow should be avoided.