Drug interactions with phytotherapeutics in oncology

Introduction: Because 30 to 70% of tumour patients use complementary and alternative medicines; herb-drug combinations are particularly frequent in this population. Some of these combinations can critically alter exposure of anti-neoplastic and palliative treatment. Areas covered: This review summarises pharmacokinetic drug interactions caused by the herbal products most frequently used by tumour patients (garlic, ginkgo, ginseng, echinacea and St John's wort [SJW]). Expert opinion: Herb-drug interactions, in general, and some interactions in particular (e.g., transporters, Phase II metabolism enzymes) are still poorly investigated and are difficult to evaluate because mixtures are administered with variable and often unspecified amounts of ingredients. Current evidence suggests that garlic and ginkgo can be safely co-administered, whereas CYP2C9 substrates (e.g., warfarin) should be monitored closely when ginseng therapy is started. Echinacea can induce drug metabolism mediated by CYP3A, but most likely relevant when administered with substances with a narrow therapeutic index or low oral bioavailability. The most relevant herbal perpetrator drug is SJW, which has substantial impact on CYP3A4- and CYP2C9-mediated metabolism and P-glycoprotein-mediated transport. This may lower exposure of co-administered drugs by up to 70%. Such an interaction is expected to occur with most of the tyrosine kinase inhibitors, but current evidence is limited.