Inhibition of caffeine metabolism by 5‐methoxypsoralen in patients with psoriasis

Abstract
Eight patients with psoriasis were given 200 mg caffeine orally with or without 1.2 mg kg−1 of 5-methoxypsoralen. Blood and urine samples were collected over a 2-day period. During 5-methoxypsoralen coadministration, the apparent volume of distribution of caffeine remained unchanged, but oral clearance (CLp.o.) decreased from 9.5±3.8 (mean±s.d.) to 3.2±0.5 l h−1 (P−1 h (Pp.o. with increased AUC was consistent with a CYP1A2-dependent inhibition of caffeine N-demethylation which was further supported by significant decreases in the (AFMU+1U+1X)/17U and (AFMU+1U+1X)/17X urinary metabolic ratios.

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