Intestinal efflux transport kinetics of green tea catechins in Caco-2 monolayer model

The bioavailability of green tea catechins (GTCs), including epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG) and epicatechin (EC) is low in both animals and humans. The contribution of intestinal efflux to this low bioavailability has been suggested by previous studies. The objective of the present study was to investigate the kinetics of efflux transport of the four major GTCs in Caco-2 cell lines, to provide comparison on the efflux transport between each GTC. The basal-to-apical transport of each GTC at concentrations ranging from 15 to 265 microM was examined using the Caco-2 cell monolayer model. Transported amount of GTC was measured by high-performance liquid chromatography with electrochemical detection. Kinetic parameters, V(max), K(m) and V(max)/K(m) were determined and compared among the four studied GTCs. The extent of basal-to-apical transport was, in descending order, EC > EGC > ECG approximately EGCG. Kinetic studies indicated that active and saturable efflux transport of EC took place in Caco-2 cells, with a K(m) of 131 microM, a V(max) of 0.0249 nmol min cm(-2) and an intrinsic clearance (V(max)/K(m)) of 0.19 microL min cm(-2). No saturation could be observed for the efflux transport of EGC, ECG and EGCG even at concentrations up to about 200 microM, which may be due to their low affinity towards the transporters at the concentration range studied. In conclusion, the extent of efflux transport of GTCs in Caco-2 cells was, in descending order, EC > EGC > ECG approximately EGCG, which may reflect the order of elimination occurring in the intestine. The kinetic studies showed the importance of efflux transporters in basal-to-apical transport of EC and suggests their role in the limited oral bioavailability of EC.