Gadoxetate, a magnetic resonance imaging contrast agent, is eliminated into bile. Gadoxetate geometrical isomers are chromatographically classified into two groups by differences between their ionic states (GIs-I and GIs-II; 65:35 w/w); however, the elimination mechanism of each isomer in vivo remains controversial. Thus, the contribution of carrier-mediated transport systems on the biliary elimination of gadoxetatewas examined. Gadoxetate was injected intravenously into rats, and the time courses of the plasma concentrations and biliary elimination of GIs-I and GIs-II were examined by high-performance liquid chromatography techniques. The results showed that 34.7% of GIs-I (GIs-I(s); 22.6% of gadoxetate) was quickly eliminated into bile within 30 min after injection. The contents of the residual GIs-I (GIs-I(r)) and GIs-II in plasma similarly decreased according to a first-order elimination process (t1/2 = 23-27 min), and 64.0% of GIs-I(r) and GIs-II (49.6% of gadoxetate) was eliminated into the bile within 2 h after injection. There was no significant difference between the elimination half-lives of GIs-I(r) and GIs-II in rats. In conclusion, the geometrical isomer with specific conformation corresponding to 22.6% of gadoxetate was eliminated into bile in rats via a carrier-mediated transport system no later than 30 min after intravenous injection.
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