Ursodiol (UDCA) is useful for treating several cholestatic hepatic maladies, including intrahepatic cholestasis of pregnancy. Its taurine amidate (TUDC), which accumulates in the bile salt pool, could interact with ABCG2 (ATP-binding cassette transporter isoform G2), which is expressed in various tissues including the canalicular membrane of the hepatocyte and in the apical membrane of the placental syncytiotrophoblast. The purpose of this study was to determine the interaction between TUDC and ABCG2. ABCG2 was expressed in Sf9 cells, and ABCG2-mediated ATP-dependent transport was determined in sucrose-fractionated Sf9 membrane vesicles. The transport of estrone 3-sulfate (E1S) into ABCG2-expressing membranes was ATP-dependent and was much greater in membrane vesicles expressing ABCG2 versus the negative control (empty virus lacking the ABCG2 coding region). To determine whether TUDC affects ABCG2-mediated ATP-dependent transport of E1S, transport activity in the presence of TUDC (20-500 microM) was measured. No significant changes were observed in the ABCG2-mediated ATP-dependent E1S transport. Furthermore, ABCG2-mediated TUDC transport was undetectable. Thus, TUDC does not affect ABCG2-mediated E1S transport and is not an ABCG2 substrate.