Stephanie van Hoppe

154 Chapter 6 to be secreted back into the blood, and taken up into a downstream hepatocyte, where it has another chance of being excreted into the bile, and so on, and so forth. This added flexibility in the hepatobiliary excretion process of bilirubin glucuronide might reduce the risk of toxic damage due to glucuronide accumulation in upstream hepatocytes (Iusuf et al., 2012b, c; van de Steeg et al., 2012; van de Steeg et al., 2010). 3 . 2 . Hepa t o c y t e hopp i ng o f s o r a f en i b g l u c u r on i de It has been theorized that this same hepatocyte hopping process might apply to many drugs that are extensively glucuronidated in the liver as well, as the transporters involved (ABCC2, ABCC3, OATP1B1 and OATP1B3 in human, Abcc2, Abcc3 and Oatp1a/1b proteins in mouse) are all known to have very broad substrate specificities, that include the glucuronide conjugates of many compounds (Iusuf et al., 2012c; van de Steeg et al., 2010). Especially as the glucuronide conjugates of several drugs are thought to be important in yielding reactive intermediates that could cause direct or indirect liver damage and toxicity (Zhou et al., 2005), it would be very important to have a mechanism that can relieve the intracellular exposure level of the hepatocyte to these glucuronides also when canalicular excretion is compromised. The findings with sorafenib glucuronide described above suggested that sorafenib might present an example of a drug (conjugate) that is strongly affected by hepatocyte hopping, analogous to bilirubin. In order to test this hypothesis, the transport and pharmacokinetics of sorafenib and its conjugate were studied in vitro and in a number of single and combination knockout mouse strains, in a collaboration between the groups of S. Baker and A.H. Schinkel (Vasilyeva et al., 2015). In vitro transport assays indicated that sorafenib glucuronide is efficiently transported by mouse, rat, and human Abcc2/ ABCC2, but also by humanABCC3 andABCC4.Together with the demonstrated transport by Oatp1a/1b and OATP1B1 and OATP1B3, this means that all transport elements minimally necessary for the hepatocyte hopping process are available for sorafenib glucuronide. In vivo , after oral administration of sorafenib to Abcc2 knockout mice, the plasma AUC of parental sorafenib was hardly altered, but the plasma AUC of sorafenib glucuronide surged from practically undetectable in wild-type mice to more than 200- fold higher levels in the Abcc2 knockout. The sorafenib glucuronide AUC was also at least 5-fold higher than that of parental sorafenib. As expected, there was a dramatic decrease in biliary excretion of sorafenib glucuronide, from 26% of the administered sorafenib dose over a limited time span in wild-type mice to less than 2.5% in the Abcc2 knockout mice. Sorafenib glucuronide is thus normally extensively excreted into bile, primarily by Abcc2. Accordingly, liver levels of sorafenib glucuronide were 3- to 8-fold increased in the Abcc2 knockout mice (Vasilyeva et al., 2015). To assess whether Abcc3 and/or Abcc4 were involved in secretion of sorafenib