The pharmacokinetics of targeted anticancer drugs and food toxins: Roles of ABC efflux and OATP uptake transporters

Abstract

Membrane transporters can play important roles in pharmacokinetic pathways (drug absorption, distribution, metabolism, and excretion). In this thesis we investigated the pharmacological functions of ABC efflux transporters in vitro, by using transwell transport assays and in vivo, by using the single and combination knockout mouse models of ABCB1 and ABCG2. We further used CYP3A knockout and humanized transgenic mouse models to assess the in vivo impact of this multispecific drug-metabolizing complex on the study drugs. We also studied OATP uptake transporters in vivo using several knockout and humanized transgenic mouse strains with liver-specific expression of human OATP1B1 or OATP1B3.

Firstly, 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 several TKIs in vivo. We have shown that dual deficiency of ABCB1 and ABCG2 most effectively increases the brain penetration of the studied tyrosine kinase inhibitors (TKIs). The findings in this thesis suggest that tumors expressing ABCB1 and/or ABCG2 may demonstrate resistance to tivozanib-, quizartinib- and EAI045-based cancer therapy. Inhibiting both these ABC transporters during therapy might therefore be beneficial for the response of these tumors. However, for the BRAFV600E inhibitor encorafenib, it is suggested that it may not be the optimal choice for the treatment of melanoma brain metastases when the blood-brain barrier (BBB) is intact. The oral availability of encorafenib, tivozanib, quizartinib and EAI045, however, does not seem to be noticeably affected by the activity of ABCB1 or ABCG2, at least in mice. On the other hand, the high intrinsic oral availability of encorafenib, if replicated in humans, would present a relatively favorable situation for the clinical application of this anticancer drug.

Secondly, characterization of pharmacological functions of human OATPs by using mouse models suggest that mouse Oatp1a/1b has at best a very modest uptake function for tivozanib and EAI045 into liver, without affecting overall plasma kinetics.

Lastly, we use different mouse models to study the possible effects on oral availability and tissue distribution of Ochratoxin A (OTA), a dietary mycotoxin that can cause nephrotoxicity, hepatotoxicity, and neurotoxicity and may be carcinogenic. We found that in mice OTA is transported by the drug transporters mouse (m)Abcb1 and/or mAbcg2, mOatp1a/1b, and human (h)OATP1B3. The complete deletion of mAbcb1 and mAbcg2 resulted in ~2-fold higher OTA liver and kidney accumulation upon intravenous injection. Upon oral administration, absence of mOatp1a/1b led to a substantial (>3-fold) decrease in hepatic and small intestinal exposure of OTA. Furthermore, in humanized mouse strains, hepatic expression of transgenic hOATP1B3, but not hOATP1B1, partly reversed the reduced liver concentration of OTA in Oatp1a/1b knockout mice. These data indicate that transgenic hOATP1B3 can significantly transport OTA into the liver, and can at least partly compensate for the loss of the mOatp1a/1b transporters. This study shows that some ABC and OATP transporters can substantially affect the pharmacokinetics of OTA, which might have implications for its toxicity behavior.