8-MOP plasma levels in PUVA problem cases with psoriasis.

8-MOP plasma levels of psoriatic patients poorly responsive to PUVA treatment (PUVA problem cases) (N = 14) and of psoriatic patients with adequate response to photochemotherapy (N = 7) were measured for 8 h after oral ingestion of 0.6-0.8 mg/kg body weight, using a gas chromatographic method. We investigated whether there are any differences in the course of the plasma kinetics between the two groups. Problem patients showed significantly lower 8-MOP plasma levels than the control group. Furthermore, the 8-MOP plasma levels increased more rapidly in the control group than in these problem patients. Deviations in time of the maximum 8-MOP plasma levels from the expected 2 h peak could be observed in 50% of the problem cases compared to only 14% of the control patients. There is no correlation between the dose and the 8-MOP plasma level achieved in the two groups, i.e. higher doses do not result in high levels. In individual cases there is not always a correlation between the plasma maximum of 8-MOP at the time of UV-A irradiation and the response to treatment. Adjustment of the UV-A irradiation to coincide with the maximum plasma levels led to an improvement in therapeutic results for three problem patients.