Background: Many cancer patients use additional herbs or supplements in combination with their anti-cancer therapy. Green tea—active ingredient epigallocatechin-3-gallate (EGCG)—is one of the most commonly used dietary supplements among breast cancer patients. EGCG may alter the metabolism of tamoxifen. Therefore, the aim of this study was to investigate the influence of green tea supplements on the pharmacokinetics of endoxifen; the most relevant active metabolite of tamoxifen. Methods: In this single-center, randomized cross-over trial, effects of green tea capsules on endoxifen levels were evaluated. Patients treated with tamoxifen for at least 3 months were eligible for this study. After inclusion, patients were consecutively treated with tamoxifen monotherapy for 28 days and in combination with green tea supplements (1 g twice daily; containing 300 mg EGCG) for 14 days (or vice versa). Blood samples were collected on the last day of monotherapy or combination therapy. Area under the curve (AUC0–24h), maximum concentration (Cmax) and minimum concentration (Ctrough) were obtained from individual plasma concentration–time curves. Results: No difference was found in geometric mean endoxifen AUC0–24h in the period with green tea versus tamoxifen monotherapy (− 0.4%; 95% CI − 8.6 to 8.5%; p = 0.92). Furthermore, no differences in Cmax (− 2.8%; − 10.6 to 5.6%; p = 0.47) nor Ctrough (1.2%; − 7.3 to 10.5%; p = 0.77) were found. Moreover, no severe toxicity was reported during the whole study period. Conclusions: This study demonstrated the absence of a pharmacokinetic interaction between green tea supplements and tamoxifen. Therefore, the use of green tea by patients with tamoxifen does not have to be discouraged.

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Keywords Breast cancer, Epigallocatechin-3-gallate (EGCG), Green tea extract, Herb–drug interaction, Pharmacokinetics, Tamoxifen, Toxicities
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Journal Breast Cancer Research and Treatment
Braal, C.L. (C. Louwrens), Hussaarts, K.G.A.M, Seuren, L. (Lieke), Oomen - de Hoop, E, de Bruijn, P.J, Buck, S.A.J. (Stefan A. J.), … Mathijssen, A.H.J. (2020). Influence of green tea consumption on endoxifen steady-state concentration in breast cancer patients treated with tamoxifen. Breast Cancer Research and Treatment. doi:10.1007/s10549-020-05829-6