Lack of correlation between in vitro and in vivo studies on the effects of tangeretin and tangerine juice on midazolam hydroxylation

Background: Tangeretin is a flavonoid that stimulates the catalytic activity of cytochrome P450 3A4 (CYP3A4) and is found in high levels in tangerine juice. Methods: The effect of tangeretin on hydroxylation of midazolam, a CYP3A4 probe, was examined in vitro with human Liver microsomes and recombinant CYP3A4, In addition, the effect of tangerine juice on the pharmacokinetics and pharmacodynamics of orally administered midazolam (15 mg) and its active 1'-hydroxymetabolite was studied in a randomized crossover study in eight healthy volunteers. Results: In microsomes from three human livers, tangeretin (1 to 100 mu mol/L) increased 1'-hydrorymidazo-lam formation (12.5 mu mol/L midazolam) by up to 212%. In complementary deoxyribonucleic acid-expressed CYP3A4, a 52% stimulation of midazolam 1'-hydroxylation was reached at 50 mu mol/L tangeretin with no effect on midazolam 4-hydroxylation. In the pharmacokinetic-pharmacodynamic study 200 mL tangerine juice reduced the area under the concentration versus time curve to 1.5 hours [AUC(0-1.5h)] of midazo- lam and 1'-hydroxymidazolam by 39% and 46%, respectively, and prolonged the time to reach peak concentration (P < .05) without affecting the total AUC values, elimination half-life values, or AUC ratios ( 1'-hydroxymidazolam/midazolam). These findings are consistent with a small delay in the absorption of midazolam and lack of effect on midazolam 1'-hydroxylation. Accordingly, tangerine juice slightly postponed the maximum pharmacodynamic effects of midazolam (P < .05), Conclusion: Tangeretin is a potent regioselective stimulator of midazolam 1'-hydroxylation by human Liver microsomes and complementary deoxyribonucleic acid-expressed CYP3A4. However, tangerine juice is unlikely to have any appreciable effect on CYP3A4 in humans. Further studies are required to assess whether in vitro stimulators of CYP3A4 can influence drug metabolism in vivo.