Stephanie van Hoppe

46 Chapter 3 in the apical membrane of epithelia in organs that are central to the absorption and elimination of drugs like kidney, liver, and small intestine. They are also found in blood- facing luminal membranes of barrier tissues protecting pharmacological sanctuary compartments like the blood-placenta, blood-testis, and blood–brain barriers (BBB). At these barriers ABCB1 and ABCG2 pump their substrates immediately out of the epithelial or endothelial cells back into the blood. Consequently, only limited amounts of drug can accumulate in, for instance, the brain to treat (micro) metastases that are located behind a functional BBB (23-25). Many anticancer drugs including TKIs are transported by ABCB1, ABCG2, or both. These transporters can therefore significantly modulate the pharmacokinetics of these drugs, and hence their therapeutic efficacy and toxicity profile (26). Several studies have shown that the oral availability of TKIs and their tissue (especially brain) penetration can be restricted due to interaction with ABCB1 and ABCG2 transporters (27-31). Moreover, pharmacological inhibition of these ABC transporters can markedly enhance the brain accumulation of these drugs (e.g., (27-31). Some studies as well as the FDA documentation indicate that osimertinib can inhibit ABCB1 and ABCG2, and may possibly be transported by them (32-34). If these transporters can also efficiently transport osimertinib in vivo , this might lead to decreased accumulation of osimertinib in transporter-expressing cancer cells, and thus tumor pharmacokinetic resistance. A recent study using an ABCB1-overexpressing multidrug-resistant KBv200 cell xenograft model in nude mice suggested that osimertinib-mediated inhibition of ABCB1 could enhance the tumor response against other ABCB1-transported drugs (34). Moreover, NSCLC can metastasize to other parts of the body, including the brain. Upon initial diagnosis of NSCLC, brain metastases are observed in 20% of patients, with numbers increasing to 40–50% in those with stage III lung adenocarcinoma (4, 6). The brain is also a common site for disease relapse in patients previously treated with TKIs in about 30–60% of EGFR-mutated NSCLCs (5). Osimertinib could potentially be a more successful candidate drug for these patients, as it is better targeted against thesemutations. However, given the high ABCB1 and ABCG2 expression in the BBB, these transporters could potentially limit brain accumulation of osimertinib, which might reduce therapeutic efficiency against NSCLC CNS metastases. In this study we therefore investigated whether osimertinib and AZ5104 are transported by ABCB1 and ABCG2 in vitro or in mouse models, and how this might affect their oral plasma pharmacokinetics and brain penetration. Furthermore, since osimertinib appears to be predominantly metabolized by human CYP3A4 [35, 36], we also studied the influence of CYP3A on the oral systemic availability and tissue exposure of osimertinib.