8-Methoxypsoralen serum levels in poor responders to photochemotherapy. Importance of drug formulation and individual factors.

8-Methoxypsoralen (8-MOP) serum levels of psoriatic patients poorly responsive to photochemotherapy (PUVA) treatment (problem cases) were determined by the HPLC method with 11 single blood probes over 8 hours. Abnormally low or deviated serum levels were found in 7 of 11 PUVA problem patients. There was a great interindividually different first-pass effect for 8-MOP in dependence on the galenic formulation of the 8-MOP brand; therefore, the change to another 8-MOP brand with a modern galenic formulation led only to a slight increase of serum levels, and consequently an increase in the 8-MOP dosage was necessary. It is important to be cautious at this point because patients may show an unproportional increase of 8-MOP serum levels due to the individually limited capacity of biotransformation. The studies of the authors reaffirm the necessity of the determination of 8-MOP serum levels in problem cases of PUVA therapy.