Stephanie van Hoppe

135 The impact of OATPs on disposition and toxicity of antitumor drugs; insights from KO and humanized mice level of the Oatp1a/1b proteins. A later, independent study by Nieuweboer et al. (2014) found quantitatively similar effects of a single Oatp1b2 knockout on the plasma AUC and liver accumulation of i.v. paclitaxel dosed at 5 mg/kg in mice. This could indicate that most of the effects on paclitaxel observed in the Oatp1a/1b(-/-) mice were primarily mediated by Oatp1b2 deficiency, although differences in genetic background strain (DBA/1lacJ vs. FVB), exact paclitaxel formulation used, laboratory and even period of experimentation, may all affect the detailed outcome of such studies (e.g. Nieuweboer et al., 2014; Sparreboom and Mathijssen, 2014). In a follow-up study, the pharmacokinetics of paclitaxel was analyzed in“humanized” Oatp1a/1b(-/-) mice with transgenic expression of human OATP1B1, OATP1B3, or OATP1A2 in the liver parenchyme cells (van de Steeg et al., 2013). The transgenic promoter/enhancer used was chosen to obtain preferential expression of the human proteins in liver parenchyme cells, which was successful, although minor OATP1B1 and OATP1B3 transgene expression was also found in kidney, but not small intestine. Based on protein immunoblot analysis and protein mass spectrometry, the hepatic level of transgenic OATP1B1 was in the same order as that observed in human liver samples, and that of transgenic OATP1B3 somewhat higher, with estimated humanized/human ratios of 0.5- to 1-fold and ~3-fold for OATP1B1 and OATP1B3, respectively (Higgins et al., 2014; Salphati et al., 2014). Transgenic OATP1A2 expression was much higher than in human liver, but this relates mostly to the fact that OATP1A2 in human liver is only expressed in cholangiocytes, whereas in the transgenic mice it is expressed in the far more abundant liver parenchyme cells. This also means that the OATP1A2 humanized mouse strain does not represent a physiologically correct model of the normal OATP1A2 function in human liver. However, it does allow an assessment of the in vivo functioning of OATP1A2 in uptake of drugs and other compounds from plasma. This may, inter alia, be relevant as OATP1A2 is substantially expressed in the human blood-brain barrier, in apical membranes of kidney tubules, and in a variety of human tumors (van de Steeg et al., 2013 and references therein). Immunohistochemically, transgenic OATP1B1 was found in the sinusoidal membrane of hepatocytes and expressed throughout the liver lobule, albeit with stronger staining around the portal vein in human liver, whereas this transporter is variously reported to be expressed throughout the liver lobule, or primarily in centrolobular hepatocytes (van de Steeg et al., 2009; van de Steeg et al., 2013). Transgenic OATP1B3 was likewise found in the hepatocyte sinusoidal membrane, showing somewhat dispersed distribution throughout the liver lobules (van de Steeg et al., 2013). In human liver OATP1B3 is preferentially found in the sinusoidal membranes of centrolobular hepatocytes. Although the transgenic OATP1B1 and OATP1B3 therefore do not exactly reflect the lobular subdistribution of OATPs in human liver, subsequent studies revealed good functionality of these proteins (as well as transgenic OATP1A2) in