Phyllanthin and hypophyllanthin inhibit function of P-gp but not MRP2 in Caco-2 cells

OBJECTIVES: The purposes of this study were to investigate the inhibitory effects of two lignans, phyllanthin and hypophyllanthin, on the function of P-glycoprotein (P-gp) and multidrug resistance protein 2 (MRP2), using the in-vitro model of Caco-2 cells. In addition, the effect of prolonged exposure to these two compounds on the expression of active P-gp was also determined. METHODS: The activity of P-gp and MRP2 was determined in the uptake assays by monitoring the intracellular accumulation of their specific substrates (calcein acetoxymethyl ester and 5(6)-carboxy-2',7'-dichlorofluorescein diacetate, respectively) with fluorescence spectroscopy. KEY FINDINGS: Hypophyllanthin and phyllanthin inhibited P-gp function with comparable potencies, but neither compound affected MRP2 activity. When the lignans were washed out before addition of substrate, the inhibitory action of both compounds against P-gp function was lost. These results suggested the reversibility of the inhibition. Moreover, prolonged exposure of the Caco-2 cells to both lignans (up to 7 days) had no effect on P-gp function. CONCLUSIONS: Phyllanthin and hypophyllanthin directly inhibited P-gp activity and did not interfere with MRP2 activity. It was likely that both phyllanthin and hypophyllanthin could reversibly inhibit P-gp function.