Stephanie van Hoppe

145 The impact of OATPs on disposition and toxicity of antitumor drugs; insights from KO and humanized mice whether Oatp1a/1b proteins play any significant role in the intestinal uptake of orally administered substrate drugs. However, there can be many causes of negative results in this respect, including the possibility of extensive redundancy with one or more other intestinal uptake transport systems. Further experimentation will be needed to resolve the intriguing question what transport systems are primarily involved in the intestinal uptake of a wide variety of relatively polar or charged drugs. The Oatp1a/1b(-/-) mice can also be used to assess the efficacy and specificity of OATP-inhibiting drugs, for instance as a cause of drug-drug interactions. For this purpose rifampicin, a known OATP inhibitor, was administered i.v. 3 min before i.v. methotrexate to wild-type and Oatp1a/1b(-/-) mice, and methotrexate plasma and liver concentrations were determined 15 min later (van de Steeg et al., 2010). The methotrexate plasma concentration was increased threefold by rifampicin treatment in wild-type mice, to the same levels as seen in Oatp1a/1b(-/-) mice. In the knockout mice, rifampicin treatment had no effect on the plasma levels of methotrexate. The liver concentration of methotrexate in wild-type mice was 4-fold decreased by rifampicin, to about 9% of the dose. The liver concentration in Oatp1a/1b knockout mice was still lower (~1% of the dose), and not affected by rifampicin treatment. These results indicate that rifampicin could largely, but not completely, inhibit Oatp1a/1b-mediated methotrexate uptake into the liver, and thus its associated plasma clearance. The lack of effect of rifampicin on methotrexate in the Oatp1a/1b(-/-) mice indicates that it did not significantly affect other methotrexate clearance mechanisms, testifying to its specificity under these conditions. 2 . 3 . 2 . Me t ho t re xa te pha rma co k i n e t i c s i n OAT P- human i ze d mi ce In a follow-up study, methotrexate pharmacokinetics was analyzed in OATP1B1-, OATP1B3- and OATP1A2-humanized Oatp1a/1b(-/-) mice (van de Steeg et al., 2013). As explained elsewhere in this review (section 2.1.1), these three transgenes are primarily expressed in liver parenchyme cells of these mice, and the proteins are situated in the basolateral (sinusoidal) membrane. For OATP1B1 and OATP1B3 this is the physiologically relevant localization, but for OATP1A2 it is not, as in human liver it is found primarily in cholangiocytes, the epithelial cells lining the bile ducts. Still, inclusion of the latter strain allows analysis of the in vivo functioning of OATP1A2 in drug uptake, which can be relevant for assessment of any drug uptake that it may mediate in other tissues and in tumor cells. Methotrexate was administered i.v. at two different dosages (10 and 2 mg/kg) to wild-type, Oatp1a/1b(-/-), and the three humanized mouse strains, and plasma, liver, and intestine (tissue plus contents) levels of methotrexate were measured 15 minutes after administration (van de Steeg et al., 2013). As observed previously, at 10 mg/kg