Estradiol 17-(β-D-glucuronide) (E217G) is an estrogen metabolite formed in the liver and subsequently excreted in bile.1It acts as a substrate of the multidrug resistance protein 2 (MRP2; Km= 75 μM), and through MRP2-mediated transport, functions as a cholestatic agent, decreasing bile flow.1,2In addition to binding to the MRP2 transport site, E217G has been shown to bind to an allosteric site that through positive cooperativity activates its own transportviaMRP2 and the transport of other MRP2 substrates, including the non-cholestatic estrogen metabolite, estradiol 3-(β-D-glucuronide) .2,3E217G has also been reported to be transported by MDR1, MRP1, MRP3, MRP4, MRP7, ABCG2 (a breast cancer resistance protein transporter), and the rat organic anion-transporting polypeptides 1-4.2 1.Loe, D.W., Almquist, K.C., Cole, S.P., et al.ATP-dependent 17β-estradiol 17-(β-D-glucuronide) transport by multidrug resistance protein (MRP). Inhibition by cholestatic steroidsThe Journal of Biological Chemisty271(16)9683-9689(1996) 2.Gerk, P.M., Li, W., and Vore, M.Estradiol 3-glucuronide is transported by the multidrug resistance-associated protein 2 but does not activate the allosteric site bound by estradiol 17-glucuronideDrug Metabolism and Disposition32(10)1139-1145(2004) 3.Gerk, P.M., Li, W., Megaraj, W., et al.Human multidrug resistance protein 2 transports the therapeutic bile salt tauroursodeoxycholateJournal of Pharmacology and Experimental Therapeutics320(2)893-899(2007)