Stephanie van Hoppe

148 Chapter 6 anthracycline antibiotic, commonly used in the treatment of several cancers, including Hodgkin’s and non-Hodgkin’s lymphoma, multiple myeloma, leukemia, sarcoma and breast, ovarian, thyroid, gastric and lung cancers (Cortes-Funes and Coronado, 2007; Gewirtz, 1999). Various mechanisms of action have been described for doxorubicin, of which two major mechanisms are intercalation of DNA that leads to inhibition of the topoisomerase II enzyme, required for smooth DNA transcription and replication, and, perhaps more controversially, generation of free radicals that leads to DNA and cell membrane damage (Gewirtz, 1999). Cardiotoxicity is the major and dose-limiting side effect of doxorubicin both in adult and pediatric cancer patients (Grenier and Lipshultz, 1998). Thus, recent efforts to improve doxorubicin treatment focus on improved anti- tumor efficacy and decreased cardiotoxicity by developing tumor-targeting strategies such as liposomal formulations of doxorubicin or developing new doxorubicin analogs (Cortes-Funes and Coronado, 2007; Minotti et al., 2004). Another strategy for lowering the toxicity and increasing the efficacy of doxorubicin is to understand the factors affecting its tissue disposition, such as its interactions with drug transporters, and utilize these insights to modify the pharmacokinetics, possibly with transporter inhibitors. Although doxorubicin has been used in the clinic for a very long time, information on its interaction with drug uptake transporters has been very limited. Okabe et al. (2005) for the first time suggested that doxorubicinmight enter cells via organic cation transporter (OCT6)-mediated active transport. Very recently, it was shown that both mouse and human OATP1A/1B family members can mediate the cellular uptake of doxorubicin (Durmus et al., 2014). This was rather surprising, as its structure, with properties of a weak base, was not considered a typical OATP substrate. In human OATP1A2-overexpressing HEK293 cells, the uptake of doxorubicin was ~2-fold increased compared to control cells, but there was no noticeable transport by humanOATP1B1 or OATP1B3 in vitro . Mouse Oatp1a/1b proteins transported doxorubicin in vivo , as evidenced by up to ~2-fold increased plasma concentrations and up to 4-fold decreased liver-to-plasma ratios in Oatp1a/1b(-/-) mice after i.v. doxorubicin administration ( Figure 3E and F) (Durmus et al., 2014). Moreover, the doxorubicin levels in the small intestinal content were also reduced in the knockout mice, suggesting a decreased biliary output, probably due to the lower liver levels. Interestingly, addition of one of the human OATP1A or 1B proteins in transgenic mice (on a mouse Oatp1a/1b knockout background) rescued the altered levels seen in the knockout mice to various extents ( Figure 3E and F ). Transgenic liver-specific expression of human OATP1A2 could completely revert the hepatic uptake and intestinal excretion of doxorubicin back to wild-type levels, whereas that of human OATP1B1 or OATP1B3 resulted in partial, but substantial rescue of these phenotypes (Durmus et al., 2014). Only the OATP1B1 or OATP1B3 proteins alone were present in these transgenic