Stephanie van Hoppe

131 The impact of OATPs on disposition and toxicity of antitumor drugs; insights from KO and humanized mice Although this has not been systematically analyzed so far in the various Oatp knockout strains, we have observed modest physiological changes in Oatp1a/1b(-/-) mice in mixed Ola/129/FVB background, that disappeared upon further backcrossing into an FVB background (unpublished data). Also the environmental conditions (e.g., diet, bedding, intestinal microflora) may at times be important factors in the penetrance of certain phenotypes. For instance, we have observed that FVB background Oatp1a/1b(-/-) mice analyzed in our facility (The Netherlands Cancer Institute, NKI) had a marked induction of carboxylesterase genes relative to wild-type FVB mice (Iusuf et al., 2014). However, the very same strain of mice obtained by other groups through a commercial supplier (Taconic) did not show a difference in carboxylesterase expression with wild-type mice (Salphati et al., 2014). Further analysis suggested that this was because the wild-type FVB mice obtained from Taconic already had a much higher endogenous hepatic carboxylesterase expression compared to FVBmice kept at the NKI, most likely because of environmental differences. The Oatp1a/1b knockout from “Taconic” circumstances did not further increase carboxylesterase expression. Interestingly, humanizing the Oatp1a/1b knockout mice with the OATP1B1 transgene did reverse the high carboxylesterase expression in both NKI and Taconic mice (Iusuf et al., 2014; Salphati et al., 2014), indicating that this could apparently overrule any environmental factors causing the different carboxylesterase expression in FVB wild-type mice. On the other hand, transgenic OATP1B3 could fully reverse the carboxylesterase overexpression in the NKI facility, but not elsewhere. This further supports that, whatever the factor(s) pushing up carboxylesterase expression in the Oatp1a/1b(-/-) (and FVB) mice, they were more prominent in the other facilities than in the NKI facility. It is important to keep such complications in mind when comparing the results in geneticallymodifiedmouse strains obtainedby different groups. Moreover, as environmental conditions in mouse facilities cannot always be kept under complete control, it is also possible that shifts in phenotypes occur over time even within one facility. Thirdly, also gender may substantially affect results obtained with Oatp1a/1b genetically modified mice. For instance, Oatp1a1, Oatp1a4, and Oatp1b2 are all substantially expressed in the basolateral (sinusoidal) membrane of the mouse liver. Judged by RNA level, Oatp1a1 is more abundant in male than in female liver (about 2-fold), and Oatp1a4 in female liver (also about 2-fold), whereas Oatp1b2 is similarly expressed in both genders (Cheng et al., 2005). This can directly affect whether certain pharmacokinetic effects can be detected in knockout strains. For instance, Gong et al. (2011) found that Oatp1a1 deficiency substantially reduced the clearance of the diagnostic dye dibromosulfophthalein in male mice, but not in female mice. In some cases, authors have asserted that the knockout of mouse Oatp1b2, as the