Stephanie van Hoppe

114 Chapter 5 D I S C U S S I ON We found that ponatinib is transported by hABCB1 and mAbcg2 in vitro , and that this transport can be inhibited with specific inhibitors. In vivo , we did not observe a clear limiting effect of mAbcb1a/1b or Abcg2 on the oral availability of ponatinib in mice. However, the brain accumulation of ponatinib was modestly increased in both single transporter knockout strains, and highly increased in the Abcb1a/1b;Abcg2 -/- combination knockout strain. No marked differences were observed for ponatinib pharmacokinetics in the liver of the transporter knockout strains. Cyp3a deficiency resulted in a modest (1.4-fold) increase in plasma levels of ponatinib, suggesting increased oral availability of ponatinib. However, no changes were observed in the plasma levels of DMP, one of the active ponatinib metabolites formed by CYP3A in humans. Our data further indicate that DMP is substantially transported in vivo by mAbcg2 andmAbcb1a/1b, resulting inmarkedly (3- to 4-fold) increased plasma levels in both Abcg2-deficient strains, and strongly increased brain accumulation in combination (but not single) transporter knockout mice. Our in vitro transport results are in line with the ATPase experiments of Sen et al ., which showed the ability of ponatinib to stimulate hABCG2 and hABCB1 ATPase activity, and indicate that this reflected their ability to transport this drug [23]. Combining all available in vitro and in vivo data, including from the literature, it seems very likely that mAbcb1a/1b, mAbcg2, hABCB1, and probably also hABCG2, can transport ponatinib. However, it is possible that substantial expressionof hABCG2 is needed inorder to readily detect ponatinib transport. Expression of ABCG2 and ABCB1 has been associated with resistance to chemotherapy for a range of different drugs in several cancers, including leukemia [37]. Our data suggest that (over)expression of these transporters in cancer cells may also confer ponatinib resistance. This would suggest the possible usefulness of inhibiting these transporters when treating patients with ponatinib in order to reverse such tumor resistance. Moreover, possible drug-drug interactions affecting systemic effects of ABC transporters could also alter treatment efficacy. Our results for the brain accumulation studies (Figures 3 and 4) clearly show that both mAbcg2 and mAbcb1a/1b can restrict brain accumulation of ponatinib in mice. Whereas single deficiency of Abcg2 or Abcb1a/1b results in only small increases in brain accumulation, the combineddeficiency of both transporters causes a disproportionately large rise in brain accumulation. At the same time, ponatinib concentration and accumulation in for instance liver were not markedly altered between the strains (Figures 3 and 4), illustrating the unique behavior of brain in this respect. It appears that the liver primarily reflected the plasma concentration(s) of ponatinib, which suggests relatively easy translocation of this drug across the basolateral (sinusoidal)