- Methanolic and aqueous extracts prepared from representative green teas were screened for inhibitory effects on intestinal UGT activity (4-methylumbelliferone glucuronidation by human intestinal microsomes). All extracts showed clear concentration-dependent inhibition. Five abundant green tea catechins [(+)-catechin, (-)-epicatechin, and (-)-epigallocatechin, (-)-epicatechin gallate, and (-)-epigallocatechin gallate] were subsequently tested as inhibitors of intestinal UGT activity using raloxifene as a clinically relevant in vitro object drug.
- (-)-Epicatechin gallate and (-)-epigallocatechin gallate competitively inhibited raloxifene-4’-glucuronide formation with Ki values (mean ± S.D.) of 0.8 ± 0.2 and 2.0 ± 0.3 µM, respectively. (-)-Epicatechin gallate and (-)-epigallocatechin gallate competitively inhibited raloxifene-6-glucuronide formation with Ki values (mean ± S.D.) of 1.0 ± 0.1 and 2.0 ± 0.3 µM, respectively. (+)-Catechin, (-)-epicatechin, and (-)-epigallocatechin demonstrated relatively weak inhibition (20-50%) of raloxifene glucuronidation when tested at 100 μM.
- Cytochrome P450 induction potential of a methanolic extract of the selected green tea product was evaluated in cryopreserved sandwich-cultured human hepatocytes. No significant (≥ 20% of adjusted positive control response) induction of CYP1A2, CYP2B6, and CYP3A4 mRNA content or activity were observed.
- Transporter induction by a methanolic extract of the selected green tea extract was evaluated in cryopreserved sandwich-cultured human hepatocytes. No significant (≥ 20% of adjusted positive control response) induction of SLCO1B3 (OATP1B3) and ABCB1 (Pgp, MDR1) mRNA content were observed.
In vitro to in vivo extrapolation (IVIVE)
- A mechanistic model was applied to predict raloxifene area under the plasma vs. concentration time curve (AUC) change in the presence of (-)-epicatechin gallate or (-)-epigallocatechin 3-gallate. Intestinal lumen concentrations of ECG and EGCG were predicted to be 4.4 and 15.2 μM, respectively, after administration a bag of the selected green tea product. Using Ki values of 1 and 2 µM for (-)-epicatechin gallate or (-)-epigallocatechin 3-gallate, respectively, the AUCi/AUC ratio was 4.4 and 6.1, respectively. The average maximum enterocyte concentrations of ECG and EGCG were estimated to be 0.18 and 0.54 µM, respectively, after administration a bag of the selected green tea product, producing an AUCi/AUC ratio of 1.2 and 1.3, respectively.
Healthy volunteers (n=58) aged 18-65 years were pre-screened to assess eligibility and gauge interest in clinical study participation. Forty-three individuals were eligible based upon pre-screening and twenty eligible individuals were consented, screened, and met enrollment criteria. Four subjects were withdrawn during the study, resulting in sixteen individuals that completed all study phases. A pharmacokinetic interaction was observed between a well-characterized green tea product and the intestinal UGT substrate raloxifene, as reflected by the geometric mean ratio raloxifene AUC0-96h and Cmax decreasing to below the pre-defined no effect range (0.75-1.33). The unaltered raloxifene and glucuronide terminal half-lives, combined with unaltered ratios of glucuronide-to-raloxifene AUC0-96h, in the presence of green tea, suggested inhibition of intestinal UGT activity was not responsible for the observed interaction, different from the IVIVE prediction. The greater decrease in raloxifene geometric mean Cmax relative to AUC0-96h further suggested that green tea alters primarily processes in the intestine, which could include permeability, transport and/or physicochemical processes involved in raloxifene absorption.