β-estradiol 3-(β-d-glucuronide) (E23G) is a noncholestatic regioisomer of the estrogen metabolite, E217G (β-Estradiol 17-(β-d-glucuronide)) [1]. E23G functions as a substrate for multidrug resistance associated protein 2 (MRP2) [2].

MRP2 is a member of the MRP subfamily, belongs to the superfamily of ATP-binding cassette (ABC) transporters. MRP2 has been expressed in the canalicular (apical) part of the hepatocyte involved in multi-drug resistance, and functions in biliary transport [3].

In vitro: In Sf9 cells infected with the recombinant baculovirus of which baculovirus genome containing rat Mrp2, E23G (0.4-400 μM) completely and competitively inhibited E217G transport with an IC50 value of 14.2 μM. Doses of E217G at 0.01-250 μM inhibited only 53% of E23G transport with an IC50 of 33.4 μM [1]. It has been reported E23G inhibited E217G transport through rat organic anion-transporting polypeptide 1 with a Ki value of 9.7 μM. E23G was a low-affinity inhibitor of both MRP4 and MRP7 with IC50s of ~ 100 μM. The noncholestatic E23G behaved as an Mrp2 substrate which could compete with E217G for transport, but did not activate the allosteric site. In Sf9 cell membranes expressing multidrug resistance protein 2 (MRP2), E23G functioned as a substrate for MRP2 with Km of 122 μM, competing with E217G for MRP2-mediated transport [3].

References:
[1] Gerk P M, Li W, 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-glucuronide[J]. Drug metabolism and disposition, 2004, 32(10): 1139-1145.
[2] Gerk P M, Li W, Megaraj V, et al.? Human multidrug resistance protein 2 transports the therapeutic bile salt tauroursodeoxycholate[J]. Journal of Pharmacology and Experimental Therapeutics, 2007, 320(2): 893-899.
[3] Borst P, Evers R, Kool M, et al.? The multidrug resistance protein family[J]. Biochimica et Biophysica Acta (BBA)-Biomembranes, 1999, 1461(2): 347-357.