Stephanie van Hoppe

85 ABCB1 restricts brain penetration of the BTK inhibitor ibrutinib while CYP3A limits its oral bioavailability affect the therapeutic efficacy of this drug against brain malignancies, either primary lesions or (micro-)metastases, that are positioned in whole or in part behind a functional blood-brain barrier. However, quantitative proteomic research has demonstrated that humans possess an ~4-fold more abundant BBB expression of ABCG2 relative to ABCB1 compared to mice [35]. It can therefore not be excluded that in humans there may still be a role of ABCG2 in limiting ibrutinib brain distribution, even though we did not find a clear impact of Abcg2 in the mouse BBB. Conversely, as the human BBB ABCB1 expression is 2.3-fold lower than that of the mouse BBB Abcb1a, the impact of ABCB1 in human brain penetration of ibrutinib might be somewhat smaller than in mice. It is worth noting that the intrinsic brain distribution of ibrutinib is not very low, with brain-to-plasma ratios of 8% in WT mice, increasing to 37-48% in the absence of Abcb1 and Abcg2 (Table 1). Even though most of the primary malignancies (lymphomas) for which ibrutinib is currently prescribed do not often occur in, or metastasize to, the brain this does happen now and then. For instance, in Bing Neel syndrome, malignant lymphoplasmacytic cells from Waldenström’s macroglobulinemia infiltrate the CNS, mantle cell lymphoma can disseminate to the CNS, and primary CNS lymphoma also occurs [30-34]. Interestingly, ibrutinib is already considered a potential treatment option for all these disorders, as its intrinsic brain penetration is thought to be fair based on CSF drug levels [33]. Moreover, brain metastases are common for a number of malignancies, including non-small cell lung cancer, for which ibrutinib is currently in clinical trials. Considering the broad interest in applying ibrutinib to a range of other cancers as well, this will further increase the likelihood of BBB function interfering with optimal therapeutic efficacy of this drug. Moreover, the observation that hABCB1 can actively extrude ibrutinib opens up the possibility that, when substantially expressed in lymphoma or tumor cells themselves (as is often the case [36, 37]), it can significantly contribute directly to resistance against this drug, which acts against an intracellular target. Given these considerations, it might be worthwhile to consider administering ibrutinib together with a strong ABCB1 inhibitor under some circumstances in order to increase efficacy against brain lesions and malignancies resistant due to ABCB1 overexpression. From our data ibrutinib-DiOH also appears to be a transported substrate of Abcb1 in vivo , which is in line with EMA documentation mentioning in vitro transport by hABCB1 [29]. While its brain distribution is strongly restricted by mAbcb1 (Supplemental Figure 5 and Supplemental Table 1), the absolute brain distribution of ibrutinib-DiOH relative to ibrutinib is still very low, even in Abcb1-deficient mice (brain-to-plasma ratio of ~7%). Since ibrutinib-DiOH is also much less (10/15-fold) pharmacodynamically active than ibrutinib, it seems very unlikely that it would substantially contribute to the therapeutic efficacy against CNS lesions.