Monitoring of 8-methoxypsoralen during extracorporeal photochemotherapy: evidence for a "first-dose" effect.

Therapeutic monitoring of 8-methoxypsoralen (8-MOP) was studied in 12 patients (age range 43-85 years, weight range 48-76 kg) treated for Sezary's syndrome by extracorporeal photochemotherapy (ECP) for 14-41 months. Before the beginning of each ECP cycle (2 sessions on consecutive days at about 4-week intervals), a blood sample was drawn to determine the 8-MOP plasma concentration 2 h after drug ingestion. Plasma 8-MOP levels were measured using a high-performance liquid chromatography method with spectrophotometric detection. Monitoring parameters (dose, 2-h plasma 8-MOP concentration) showed important interindividual and intraindividual variations. The 8-MOP dose ranged from 0.57 to 1.04 mg/kg. Intraindividual variations of 2-h 8-MOP levels ranged from 21% to 75%. Of the 652 measurements, 13% were < 100 ng/ml, the therapeutic threshold for effective ECP; in three of the seven patients, increasing the dose obtained levels exceeding the therapeutic threshold, i.e, the absorption was not saturable. Orthogonal regression analysis between plasma 8-MOP concentrations measured in two consecutive ECP sessions showed a first-dose effect: the 2-h plasma 8-MOP concentration was significantly lower after the first administration than after the second (approximately 1.26-fold). Because 8-MOP has been proven to be a potent suicide inhibitor of drug metabolism in rats and humans, it is possible that 8-MOP had an inhibitory effect on its own metabolism within the therapeutic dose range for ECP, which would explain in part the inter- and intraindividual variability in 8-MOP kinetics and first-dose effect.