Stephanie van Hoppe

155 The impact of OATPs on disposition and toxicity of antitumor drugs; insights from KO and humanized mice glucuronide from the hepatocyte back into blood, sorafenib and sorafenib glucuronide pharmacokinetics were studied in various single and combination Abcc3, Abcc4, Oatp1a/1b, and Abcc2 knockout strains, after oral sorafenib administration. Single Abcc3, Abcc4, or Abcc3/4 combination knockout strains displayed no significant effect on plasma AUCs of sorafenib or sorafenib glucuronide, but both the Abcc3-deficient strains had an about 2-fold higher liver-to-plasma ratio of sorafenib glucuronide than wild-type and Abcc4 knockout mice, suggesting a role of Abcc3 in limiting hepatic accumulation of sorafenib glucuronide. Indeed, when the Abcc3 deficiency was combined with either the Oatp1a/1b deficiency, or a combined Oatp1a/1b;Abcc2 deficiency, this resulted in, respectively, a 1.9-fold reductionor a 29%reduction inplasma AUCs of sorafenib glucuronide. This clearly demonstrated that Abcc3 is substantially involved in the transport of sorafenib glucuronide from the hepatocyte back into blood. Liver-to-plasma ratios of sorafenib glucuronide in these combination knockout strains further supported this interpretation. Collectively, these findings provide strong support for the idea that also sorafenib glucuronide is subject to the process of hepatocyte hopping. The fact that sorafenib glucuronidation capacity in the mouse intestine is very small compared to the hepatic glucuronidation capacity (Zimmerman et al., 2013) further strenghens the interpretation that primarily sorafenib glucuronide formed in hepatocytes is subject to the hepatocyte hopping process. Figure 4 shows a schematic diagram of the hepatocyte hopping process for sorafenib glucuronide, with the legend providing more detail on the separate steps undergone by sorafenib and sorafenib glucuronide. It should be noted that, in addition to Abcc3, there must be at least one or more other sinusoidal export processes for sorafenib glucuronide, as the reversal of plasma levels of sorafenib glucuronide was far from complete in the various Abcc3 combination knockout strains. Based on the results in the Abcc4-deficient strains it seems unlikely that Abcc4 makes a significant contribution, so the nature of the alternative sinusoidal sorafenib glucuronide exporter(s) still remains to be elucidated. It could well be that the relative contribution of these alternative export processes increases at the elevated hepatocyte concentrations of sorafenib glucuronide that occur in the Abcc2- and/or Abcc3-deficient mice, for instance due to a high(er) Km for transport. Since the hepatocyte hopping proces has now been documented for at least two quite divergent compounds (bilirubin glucuronide and sorafenib glucuronide), it stands to reason that it will also apply to many other compounds that are conjugated in the liver, and that are substrates of ABCC2, ABCC3, and the sinusoidal OATP1 proteins. Given the very broad and often overlapping substrate specificity of all of these transporters, encompassing glucuronide-, glutathione-, and sulfoconjugates of many endobiotic and xenobiotic compounds, including many drugs, it seems likely that a large number