Direct evidence of the temperature dependence of Gd-BOPTA transport in the intact rat liver

Abstract

The aim was to study the influence of temperature on the transport of the hepatobiliary contrast agent Gadobenate dimeglumine (Gd-BOPTA). Rat livers were isolated and perfused with Gd-BOPTA at 12, 25, 30, 36 and 38 °C. After the perfusion period, biopsies were collected and the MR signal intensity was measured. Uptake and biliary excretion were quantified with radiolabeled Gd-BOPTA. MR signal intensity decreased with temperature of perfusion. This phenomenon was appropriately quantified with ¹⁵³Gd and ¹⁵³Sm labeling, in contrast to ⁶⁷Ga.

Introduction

Gadobenate dimeglumine (Gd-BOPTA) is a new hepatobiliary contrast agent for magnetic resonance imaging (MRI). Its structure differs from the long used extracellular contrast agent gadopentetate dimeglumine (Gd-DTPA) by an additional lipophilic moiety (Fig. 1). With this lipophilic substituent, Gd-BOPTA is taken up specifically by hepatocytes and is excreted into the bile. Consequently, Gd-BOPTA has a dual imaging capability. On the one hand, it is used as an extracellular contrast agent in the immediate postinjection phase of contrast enhancement to reveal hypervascular regions, and on the other hand, it enters into hepatocytes and facilitates MRI detection and characterization of hepatic diseases in a later delayed phase (Hamm et al., 1999; Kuwatsuru et al., 2001; Manfredi et al., 1998; Schneider et al., 2003).

After diffusion into the extracellular space, part of Gd-BOPTA is taken up into hepatocytes and, after trafficking across the cell, is excreted into the bile without biotransformation (Lorusso et al., 1999; Schuhmann-Giampieri et al., 1993). In rats, 50% of the dose injected is excreted into the bile, the remaining being excreted into urine (De Haën et al., 1996; Lorusso et al., 1999). In contrast, only 2–7% of the dose is excreted into the bile in humans (Lorusso et al., 1999; Spinazzi et al., 1998). The transport of Gd-BOPTA into hepatocytes is not fully understood. Evidence exists that Gd-BOPTA enters rat hepatocytes through a transporter belonging to the organic anion transporting polypeptide family (Oatps) localized on the sinusoidal membrane of hepatocytes (Clément et al., 1998; Hahn and Saini, 1998; Pastor et al., 2003). After its intracellular transport, Gd-BOPTA is eliminated into the bile through the ATP-dependent multidrug resistance-associated protein 2 (Mrp2) at the canalicular membrane of the hepatocyte (De Haën et al., 1996; Pascolo et al., 2001).

In the isolated perfused rat liver, it was shown that the increase in MR signal intensity during Gd-BOPTA perfusion can be recorded over time with a direct visualization of the absence of Gd-BOPTA entry in hepatocytes when livers were co-perfused with Gd-BOPTA and bromosulfophthalein (pharmacological inhibition) (Pastor et al., 2003). In a hollow-fiber bioreactor containing freshly isolated rat hepatocytes, we showed that the transport of Gd-BOPTA into hepatocytes can be successfully described by compartmental analysis of the MR signal intensity recorded over time and supports the hypothesis of a transporter-mediated uptake (Planchamp et al., 2004a, Planchamp et al., 2004b).

However, information is lacking on the long- and short-term regulation of Gd-BOPTA transport in hepatocytes. Although a temperature dependence of transport through Oatps and Mrps has been previously reported (Payen et al., 2000; Ruiz-Garcia et al., 2002; Zamek-Gliszczynski et al., 2003), its influence on the MR signal is unknown. Consequently, the aim of our study was to determine, in the isolated perfused rat liver model, the influence of temperature on Gd-BOPTA transport and its consequences on the MR signal. Because the MR signal does not allow exact quantification of Gd-BOPTA concentration in tissues, the transport of radiolabeled Gd-BOPTA was also measured.