St John's Wort: effect on CYP3A4 activity

St John's Wort is a widely used herbal product. Information regarding its potential for drug interactions is required for responsible treatment of patients using St John's Wort. CYP3A4 is a metabolic enzyme implicated in most clinically significant drug-drug interactions.

To determine the in vivo effect of reagent-grade St John's Wort extract on CYP3A4 activity through evaluation of urinary 6-beta-hydroxycortisol/cortisol ratios.

Thirteen subjects ranging in age from 18 to 25 years participated in this unblinded, multiple-dose, single-treatment before-after trial conducted in a university-based pharmacokinetics and biopharmaceutics laboratory. Each subject ingested a 300-mg tablet of reagent-grade St John's Wort extract standardized to 0.3% hypericin three times a day for 14 days. Baseline and posttreatment CYP3A4 activity was assessed with the urinary 6-beta-hydroxycortisol/cortisol ratio after a 24-hour urine collection.

The mean +/- SD urinary 6-beta-hydroxycortisol/cortisol ratio significantly increased (P = .003) from a baseline value of 7.1 +/- 4.5 to 13 +/- 4.9. The mean +/- SD percentage increase was 114% +/- 95%, with a range from -25% to 259%. All but one subject had an increase in the ratio.

Treatment with St John's Wort for 14 days resulted in significant increases in the urinary 6-beta-hydroxycortisol/cortisol ratio. This finding suggests that St John's Wort is an inducer of CYP3A4.