Drug Interactions

In two of the five spontaneous bleeding episodes described in Heading 4, medications that can affect platelet function or prothrombin time (PT) (i.e., aspirin and warfarin) were involved. Because GB is known to be an inhibitor of PAF (41), in theory GB could interact with antiplatelet drugs (e.g., aspirin, nonsteroidal anti-inflammatory drugs, clopidogrel, ticlopidine, dipyridamole) or anticoagulants (e.g., warfarin, heparin). EGb 761 was shown to potentiate the antiplatelet effect of ticlopidine in rats (42). However, in two studies in humans, the coadministration of GB with warfarin had no effect on either international normalized ratio or warfarin metabolism (43,44).

With respect to the effect of GB on cytochrome P450 drug metabolizing enzymes, in vitro studies have demonstrated that GBE and components have minimal effect on CYP2C9, CYP3A4, CYP1A2, and CYP2D6 (45-47). This lack of effect on these particular cytochrome P450 enzymes has been confirmed by in vivo studies using probe drug substrates (48). However, coadministration of GB with omeprazole (a CYP2C19 substrate) demonstrated significant induction of omeprazole metabolism, resulting in reduced AUC (49). Finally, GB coadministration did not have an effect on donepezil pharmacokinetics (50). A study in which 400 mg of EGb was administered to 24

healthy volunteers for 13 days demonstrated that GB is not an inducer of other hepatic microsomal enzymes (51).

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