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ISSUE 2 APRIL
Letters to the Editor

Warfarin, St John’s wort and INR

  • Angela Cattanach, Siboniso Sibindi, Gregory W Roberts

The Editorial Executive Committee welcomes letters, which should be less than 250 words. Before a decision to publish is made, letters which refer to a published article may be sent to the author for a response. Any letter may be sent to an expert for comment. When letters are published, they are usually accompanied in the same issue by their responses or comments. The Committee screens out discourteous, inaccurate or libellous statements. The letters are sub-edited before publication. Authors are required to declare any conflicts of interest. The Committee's decision on publication is final.

 

Letter to the Editor

In regards to the article on the management of warfarin therapy,1 the statement on page 46 ‘drugs that may increase INR – macrolide antibiotics, imidazole antifungals, sulfamethoxazole/trimethoprim, amiodarone, statins, some non-steroidal anti-inflammatory drugs and some complementary medicines such as St John’s wort’ may not be correct.

In the literature, St John’s wort decreases the INR through induction of cytochrome P450 (CYP)-mediated metabolism of warfarin and increases warfarin clearance.2-9

Angela Cattanach
Siboniso Sibindi
Clinical pharmacists
Lismore Base Hospital
Lismore, NSW

 

Author's comments

Gregory Roberts, one of the authors of the article, comments:

Thank you for pointing out the error in the article.1 St John’s wort induces CYP enzymes with a resultant increase in warfarin clearance and decrease in INR, not a possible increase in INR as described in the article. Decreases of 20% in AUC (area under the curve) have been noted in single warfarin dose studies,2 so while prudent INR monitoring should be undertaken, the interaction is likely to be of clinically minor importance.

 

References

  1. Tideman PA, Tirimacco R, St John A, Roberts GW. How to manage warfarin therapy. Aust Prescr 2015;38:44-8.
  2. Jiang X, Williams KM, Liauw WS, Ammit AJ, Roufogalis BD, Duke CC. Effect of St John\u2019s wort and ginseng on the pharmacokinetics and pharmacodynamics of warfarin in healthy subjects. Br J Clin Pharmacol 2004;57:592-9.
  3. Jiang X, Blair EY, McLachlan AJ. Investigation of the effects of herbal medicines on warfarin response in healthy subjects: a population pharmacokinetic-pharmacodynamic modeling approach. J Clin Pharmacol 2006;46:1370-8.
  4. Yue QY, Bergquist C, Gerd\u00e9n B. Safety of St John\u2019s wort (Hypericum perforatum). Lancet 2000;355:576-7.
  5. Zhou S, Chan E, Pan SQ, Huang M, Lee EJ. Pharmacokinetic interactions of drugs with St John\u2019s wort. J Psychopharmacol 2004;18:262-76.
  6. Anticoagulants + Herbal medicines; St John\u2019s wort ( Hypericum perforatum ). In: Baxter K, editor. Stockley\u2019s Drug Interactions. London: RPS Publishing; 2015.
  7. Kaminsky LS, Zhang ZY. Human P450 metabolism of warfarin. Pharmacol Ther 1997;73:67-74.
  8. Moore LB, Goodwin B, Jones SA, Wisely GB, Serabjit-Singh CJ, Willson TM. St. John\u2019s wort induces hepatic drug metabolism through activation of the pregnane X receptor. Proc Natl Acad Sci USA 2000;97:7500-2.
  9. Nebel A, Schneider BJ, Baker RK, Kroll DJ. Potential metabolic interaction between St. John\u2019s wort and theophylline. Ann Pharmacother 1999;33:502.
RELATED ARTICLES
Correction
How to manage warfarin therapy Aust Prescr 2016;39:66
RELATED ARTICLES
Correction
How to manage warfarin therapy Aust Prescr 2016;39:66

Angela Cattanach

Clinical pharmacist, Lismore Base Hospital, Lismore, NSW

Siboniso Sibindi

Clinical pharmacist, Lismore Base Hospital, Lismore, NSW

Gregory W Roberts