METABOLIC AND ANALYTICAL INTERACTIONS OF GRAPEFRUIT JUICE AND 1,5-BENZOPYRONE (COUMARIN) IN MAN

Objective: Grapefruit juice is known to inhibit mammalian cytochrome P 450 isozymes such as CYP3A4. The aim of this study was to investigate the influence of the juice on the fate of coumarin (1,2-benzopyrone) m etabolized by CYP2A6 in man. Its potentially inhibitory effect was exa mined when low and high amounts of grapefruit juice were taken. Method s: In crossover studies, doses of 10 mg coumarin (Venalot) were given orally to a healthy male volunteer. The drug was taken either with wat er or with grapefruit juice, at different volumes (300 mi or 4 x 250 m i at intervals of 30 min). Urine samples were collected up to 24 h aft er dosing. After in vitro hydrolysis they were analysed fluorimetrical ly for umbelliferone, the metabolite of coumarin, and cumulative excre tion curves were established. HPLC and TLC served to identify fluoresc ent metabolites from the juice. Results: If coumarin is given in water its excretion is complete after 6 h and 70% of the dose is recovered. Grapefruit juice (300 mi) given simultaneously slightly retards the a ppearance of the fluorescent metabolite in the urine within the first few hours. The recovery of coumarin remains unaffected. One litre of j uice enhances the delay and increases the recovery of coumarin to near ly 100%. Respective controls with grapefruit juice alone lead to remar kable excretions of a fluorescent material identified as conjugated sc opoletin, which strongly interferes with the analysis of the coumarin experiment. The precursor of scopoletin is widely present at different concentrations in commercially available grapefruit juices. However, the auto-inhibition of the juice is correlated neither to the concentr ation of naringin nor to that of scopoletin. Conclusion: Only grapefru it juice given at high doses (1 L) retards the appearance of the main metabolite of coumarin administered orally but increases its recovery. Due to scopoletin formed from the grapefruit juice, experiments espec ially with coumarin are strongly affected.