According to results recently presented at the annual meeting of the American Association for Cancer Research, the herbal medicine St. Johns wort may reduce the efficacy of the chemotherapy agent Camptosar® when taken together.

Camptosar® (irinotecan, CPT-11) is a commonly used chemotherapy agent for the treatment of colorectal cancer and is being evaluated in clinical trials for many different types of cancer. St. Johns wort is an herbal medicine that is used extensively to treat depression, particularly in Europe. Because St. Johns wort is labeled as an herb, it is not regulated by the Food and Drug Administration in the United States and physicians practicing western medicine are often not thoroughly versed in alternative medicines such as herbs. Furthermore, patients often do not inform their physicians of any herbs or supplements they may be taking. Studies have revealed that many patients often combine western and alternative medicines; although not a lot is known of the interactions between particular agents in these two forms of medicine.

Since herbal remedies are becoming more commonly used among patients, researchers have been trying to identify interactions between herbs and commonly used medications. Recently, researchers from the Netherlands evaluated the interactions between St. John’s wort (SJW) and Camptosar®. Three patients were given one course of Camptosar® alone followed a second course consisting of SJW plus Camptosar® or vice versa. Blood samples were taken through both courses of therapy and tested for the active form of Camptosar®, SN-38. SN-38 levels decreased by greater than 50% when SJW was given with Camptosar®. Inactive byproducts formed from the breakdown of Camptosar® increased with the addition of SJW.

These results indicate that SJW may interfere with the metabolism of Camptosar®, rendering the drug less effective. Although this study only involved three patients, the researchers conducting this study concluded that patients receiving Camptosar® for the treatment of cancer may wish to refrain from simultaneously taking St. John’s wort. All patients should talk to their physician about any herbs or supplements they are taking. (Mathimssen R, Verweij J, Bruijn P, et al. Modulation of irinotecan (CPT-11) metabolism by St. John’s wort in cancer patients. Proceedings from the annual meeting of the American Association for Cancer Research 2002; Abstract 2443.)

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