Research: GURLEY and others,

Listed in Issue 129

Abstract

GURLEY and others, Department of Pharmaceutical Sciences, University of Arkansas for Medical Sciences, Little Rock, AR 72205, USA, have investigated the interactions of milk thistle and black cohosh with the heart medicine digoxin.

Background

The modulation by herbal remedies of drug transporters in the blood may underlie many herb-drug interactions. The aim of this study was to determine whether supplementation with milk thistle or black cohosh would modify the activity of P-glycoprotein, the main transporter in human blood.

Methodology

16 healthy volunteers were randomized to standardized milk thistle (900 mg daily) or black cohosh (40 mg daily) supplement for 14 days, followed by a 30-day washout period. Subjects were also randomized to receive rifampin (600 mg daily, 7 days) and clarithromycin (1000 mg daily, 7 days) as positive controls for P-glycoprotein induction and inhibition, respectively. Digoxin (0.4 mg) was administered orally before and at the end of each supplementation and control period. Serial digoxin serum concentrations were obtained over 24 hours and analyzed by chemiluminescent immunoassay.

Results

No statistically significant effects on digoxin pharmacokinetics were observed following supplementation with either milk thistle or black cohosh, although digoxin AUC(0-3) and AUC(0-24) approached significance (p = 0.06) following milk thistle administration. When compared with rifampin and clarithromycin, supplementation with these specific formulations of milk thistle or black cohosh did not appear to affect digoxin pharmacokinetics.

Conclusion

These results suggest that milk thistle and black cohosh are not potent modulators of P-glycoprotein in vivo.

References

Gurley BJ, Barone GW, Williams DK, Carrier J, Breen P, Yates CR, Song PF, Hubbard MA, Tong Y, Cheboyina S. Effect of milk thistle (Silybum marianum) and black cohosh (Cimicifuga racemosa) supplementation on digoxin pharmacokinetics in humans. Drug Metabolism & Disposition 34 (1): 69-74, Jan 2006.

Comment

The importance of collecting and publishing these data cannot be overstated – they provide the information on the safety of herbal medicines that both consumers and health professionals need very badly.

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