Stephanie van Hoppe

101 Brain accumulation of ponatinib and its active metabolite is limited by ABCB1 and ABCG2 I N T R ODU C T I ON Ponatinib (Iclusig®, previously AP24524) is a third-generation oral drug for treatment of chronic myeloid leukemia (CML) and Philadelphia chromosome positive (Ph+) acute lymphoblastic leukemia (ALL) [1, 2]. It was initially FDA approved in December 2012 and after a temporary suspension due to life-threatening risks, it was re-approved in December 2013 [3, 4]. Ponatinib is a tyrosine kinase inhibitor (TKI) designed to bind and inhibit BCR-ABL1 and especially its most prominent mutant form, T315I, which confers resistance to other tyrosine kinase inhibitors such as dasatinib, imatinib, nilotinib and bosutinib [5]. Ponatinib also inhibits some other kinases important for the pathogenesis of other malignancies such as vascular endothelial growth factor receptors (VEGFR 1 and 2), platelet-derived growth factor receptor (PDGFR), fibroblast growth factor receptors 1-4 (FGFR1-4), RET, c-KIT, MEKK2, FLT3, RIPK1 and RIPK3 [5-11]. This makes ponatinib a promising and interesting therapeutic drug. The plasma protein binding of ponatinib is high (99.9%) and its half-life (t 1/2 ) is about 24 h in humans [1]. The drug is mainly excreted via feces after phase I and II biotransformation. In humans CYP3A4 is the predominant P450 isoenzyme involved in themicrosomal biotransformationofTKIs such as imatinib, nilotinib, bosutinib, dasatinib and also ponatinib [1, 12-15]. The main ponatinib metabolite, however, is the inactive carboxylic acid AP24600, formed by esterase- and/or amidase-mediated hydrolysis [1, 16]. The pharmacodynamically active N -desmethyl (DMP, AP24567) and (minor) N-oxide (AP24734) metabolites are additionally formed by Cytochrome P450 (mainly CYP3A4)- mediated biotransformation, as schematically shown in Supplementary Figure 1 [1, 17, , 18]. DMP has a 4-fold reduced potency compared to ponatinib [2], and its abundance in patients appears to be modest, with 8.3% of the ponatinib dose retrieved as DMP from feces [18]. Multidrug efflux transporters of the ATP-binding cassette (ABC) protein family affect the disposition of a wide variety of endogenous and exogenous compounds, including numerous anticancer drugs. ABCB1 (P-glycoprotein) and ABCG2 (BCRP) are expressed in the apical membrane of epithelia in a number of organs which are essential for absorption and elimination of drugs like liver, small intestine and kidney. They are also found in luminal membranes of barriers protecting sanctuary tissues like the blood- placenta, blood-testis and blood–brain barriers (BBB). At these barriers ABCB1 and ABCG2 substrates are immediately pumped out of the epithelial or endothelial cells back into the blood. As a consequence, only small amounts of drug can accumulate in, for instance, the brain. This can compromise treatment of (micro-)metastases that are present behind a functionally intact BBB [19-21]. Many anticancer drugs including TKIs are transported substrates of ABCG2 or ABCB1 or both. As a result, these transporters