Stephanie van Hoppe

45 Brain accumulation of osimertinib and its active metabolite is restricted by ABCB1 & ABCG2 I N T R ODU C T I ON Non-small cell lung cancer (NSCLC) is one of the leading causes of cancer death in the world, and it accounts for approximately 85% of all lung cancer diagnoses [1]. Advanced stage (IV) NSCLC is known to metastasize to a number of organs including the brain [2-6]. The identification of epidermal growth factor receptor (EGFR) mutations as one of the driving factors in NSCLC allowed the development of targeted therapy for NSCLC patients. EGFRtargeting tyrosine kinase inhibitors (TKIs) like gefitinib, erlotinib and several others display promising clinical activity in advanced NSCLC patients whose tumors harbor recurrent somatic activating mutations in EGFR (EGFRm + ) [7-12]. Unfortunately, although most of the EGFRm + NSCLC patients initially respond to these TKIs, there also is a high frequency of acquired resistance. The mechanism of acquired resistance for more than 50% of the patients is the acquisition of an additional EGFR mutation, EGFR-T790M [13-15]. The search for novel therapeutic strategies targeted against this mutation has yielded a potent TKI, osimertinib (AZD9291, Tagrisso). Osimertinib covalently and irreversibly binds to cysteine 797 in the ATP binding site of EGFR, exhibiting 200 times greater potency toward both EGFRm + and T790M variants compared to the wild-type EGFR (16). Osimertinib has been approved by the US FDA in April 2018 for the first- line treatment of metastatic NSCLC patients with epidermal growth factor (EGFR) exon 19 deletions or exon 21 L858R mutations in their tumors (17). In clinical trials (phase II AURA), osimertinib has demonstrated an overall objective response rate (ORR) of 62% and median progression-free survival (PFS) greater than 12 months with manageable toxicity. In patients with central nervous system (CNS) metastases the ORR for osimertinib was still 64%, but a shorter median PFS of 7 months was observed compared to patients without CNS metastases [18]. The absolute oral bioavailability of osimertinib is 69.8%, suggesting that it is well absorbed [19]. Additionally, previous reports have shown that osimertinib undergoes minimal first-pass metabolism with low clearance and is highly distributed to organs [19-21]. However, osimertinib is metabolized, mainly by cytochrome-P450 (CYP) 3A, into the active metabolites AZ5104 and AZ7550 (Supplemental Figure 1), each amounting to ~10% of the overall systemic exposure. Interestingly, whereas AZ7550 showed a similar potency to osimertinib, AZ5104 showed greater potency than osimertinib against exon 19 deletion and T790M mutants (~8-fold) and wild-type (~15-fold) EGFR [20, 22]. Multidrug efflux transporters of the ATP-binding cassette (ABC) protein family can influence the disposition of a wide variety of endogenous and exogenous compounds, including many anti-cancer drugs. ABCB1 (P-glycoprotein) and ABCG2 (BCRP) occur