Impact of apical ABC transporters on pharmacokinetics of targeted anticancer drugs

Abstract

Tyrosine kinase inhibitors (TKIs) are small molecule inhibitors that selectively interfere with the intrinsic tyrosine kinase activity of proteins in cellular signal transduction pathways and thereby block receptor autophosphorylation and activation of downstream signal transducers. TKIs are increasingly used in anticancer pharmacotherapy, often resulting in substantial improvements in overall survival and quality of life of patients. However, their usually limited accumulation in brain tissue may limit their efficacy towards malignancies in the brain. An important factor for this limited brain accumulation could be due to the presence of active drug efflux transporters such as P-glycoprotein (P-gp; ABCB1) and breast cancer resistance protein (BCRP; ABCG2) at the blood-brain barrier (BBB). By utilizing mouse models deficient of ABCB1 and/or ABCG2, we aimed to increase the brain accumulation of TKIs in mice using several approaches, including coadministration of elacridar, a dual inhibitor of Abcb1 and Abcg2, or saturation of ABCB1 and ABCG2 with high initial plasma concentrations of drug.

In this thesis, we have demonstrated the usefulness of single and combination transporter knockout mouse models to study the impact of ABCB1 and ABCG2 on oral availability and brain accumulation of drugs, including sunitinib, crizotinib and everolimus in vivo. We have also shown that concomitant use of elacridar can effectively increase brain penetration of sunitinib, N-desethyl sunitinib and crizotinib in wild-type mice, albeit slightly less effectively for N-desethyl sunitinib. In addition, we also demonstrated that a high initial plasma concentration of sunitinib can lead to partial and complete saturation of Abcb1 and Abcg2 transport activities, respectively, in the BBB of wild-type mice. Consequently, this approach resulted in highly increased brain concentrations of sunitinib in wild-type mice.

By using mice heterozygous for Abcb1 and Abcg2 gene disruptions, we found that halving the amount of Abcb1 and Abcg2 at the BBB only resulted in very small (<2-fold) or no significant increases in brain accumulation of dasatinib, sorafenib and sunitinib, whereas homozygous knockout mice displayed highly increased brain accumulation of these TKIs. These results have provided additional experimental support for the theoretical pharmacokinetic model of Kodaira et al. (J Pharmacol Exp Ther. 2010, 333:788-96), which describes that the seemingly disproportionate effect of simultaneous removal of both Abcb1 and Abcg2 on the accumulation of their shared substrates in the brain can be explained by their separate contributions to the net efflux at the BBB, without postulating any direct or indirect interaction between Abcb1 and Abcg2.

It is important to note that upregulation of carboxylesterase (CES) enzymes in some of the knockout strains that we have developed may cause complications with a drug that can bind to, or be hydrolyzed by, these enzymes. Therefore, great caution is needed when interpreting results obtained for such drugs in these mouse strains. In addition, we have identified everolimus as an inhibitor of human CES1 and CES2, albeit slightly less potent for CES1. This would mean that inhibition of hepatic CES1 and especially intestinal CES2 by everolimus might play a role in drug-drug interactions with coadministered drugs.