Biliary excretion and pharmacokinetics of a gadolinium chelate used as a liver-specific contrast agent for magnetic resonance imaging in the rat

The introduction of a lipophilic moiety into the gadolinium chelate Gd-DTPA (dimeglumine gadopentetate, Magnevist) yielded Gd-EOB-DTPA (short form), which has potential as a magnetic resonance contrast agent for liver mass screening. The pharmacokinetics of Gd-EOB-DTPA in rats is nonlinear because after correction for the 10-fold difference in dose, the area under the curve of plasma concentration versus time from time zero to infinity after single intravenous application of two different doses were not superimposable, and the amounts excreted renally and extrarenally differed significantly. However, for both dose groups tested, the values of renal clearance (9.96 and 11.1 mL/min.kg, respectively) were close to the value of glomerular filtration in the rat. Michaelis-Menten kinetics in the extrarenal elimination was therefore considered as the rate-limiting process of Gd-EOB-DTPA, the binding to plasma protein of which is small (10.3 +/- 1.4%). Thus, biliary elimination was significantly inhibited by the intravenous coadministration of sulfobromophthalein (a decrease from 39.5 +/- 3.17 to 30.7 +/- 5.30% of the dose was observed from 0 to 90 min postinoculation under coadministration of the inhibitor), whereas tauroglycocholate revealed no effect, indicating the involvement of the so-called organic anion plasma membrane transport system for the hepatic uptake. The transport of Gd-EOB-DTPA from the cytoplasm to the bile is mainly determined by the capacity of the transport protein glutathione-S-transferase as demonstrated by in vitro binding studies. A hepatobiliary transport maximum of 9.2 mumol/min.kg was evaluated by infusion studies. No metabolites were detected either in the bile or in the urine, and enterohepatic circulation can be excluded.