Stephanie van Hoppe

144 Chapter 6 Hodgkin’s lymphoma). It is also used, albeit at usually lower dosage, and mostly orally, to treat other diseases such as rheumatoid arthritis and psoriasis (van Outryve et al., 2002; Wessels et al., 2008). Methotrexate is a bicarboxylic organic anion, that is known to be transported in vitro by a number of OATP1A/1B proteins, including human OATP1B1, OATP1B3, and OATP1A2 (Abe et al., 2001; Badagnani et al., 2006; Sasaki et al., 2004). In addition, a genome-wide association study showed that SNPs in SLCO1B1 associated with reduced transport activity were linked to decreased plasma clearance and decreased gastrointestinal toxicity in children with acute lymphoblastic leukemia (ALL) treated with high-dose i.v. methotrexate infusions (Trevino et al., 2009). 2 . 3 . 1 . Me t ho t re xa te pha rma co k i n e t i c s i n Oa t p1a / 1b k no c kou t mi ce Initial characterization of i.v. methotrexate pharmacokinetics in Oatp1a/1b(-/-) mice revealed dramatic effects on plasma clearance and liver uptake of the drug (van de Steeg et al., 2010). The plasma AUC was increased 5-fold in Oatp1a/1b(-/-) mice, and liver concentrations were about 20-fold reduced. Hepatic methotrexate uptake was very rapid, with >50% of the dose accumulating in the wild-type liver within 3.5 min after i.v. dosing, whereas only ~2% accumulated in the Oatp1a/1b(-/-) liver. As could be expected from the dramatically reduced liver uptake of methotrexate, the total intestinal content of i.v. methotrexate, which mostly derives from biliary excretion, was about 17-fold reduced in the Oatp1a/1b(-/-) mice. Accordingly, the amount of unchanged methotrexate excreted in the feces was reduced from ~20% to ~2% of the dose. In contrast, the urinary excretion rose from ~40% of the dose in wild-type mice to ~100% in Oatp1a/1b(-/-) mice, indicating a substantial rerouting from hepatobiliary to renal excretion. As some Oatp1a/1b proteins are also expressed in the mouse small intestine (Slco1a4, and to a lower extent Slco1a5 and Slco1a6 RNA was detected (Cheng et al., 2005)), an oral methotrexate study was also performed to assess possible changes in oral availability. However, whereas the systemic plasma and liver concentrations of methotrexate followed similar patterns as seen after i.v. administration, short-term hepatic portal vein sampling didn’t provide any indication for a reduced rate of intestinal uptake of methotrexate in the Oatp1a/1b(-/-) mice. Apparently other intestinal uptake systems are primarily involved in the uptake of this charged compound. In fact, in broader terms, despite extensive efforts, to our knowledge no one has so far been able to directly demonstrate for any OATP substrate drug that its intestinal uptake is detectably dependent on Oatp1a/1b proteins in available mouse models. Directly tested drugs include, in addition to methotrexate, fexofenadine, pravastatin, and rosuvastatin (Iusuf et al., 2012a; Iusuf et al., 2013; van de Steeg et al., 2010), as well as a number of other, as yet unpublished, drugs. This raises the more general question