Abstract
Introduction: The PI3K-Akt-mTOR signaling pathway is activated in several major tumor types and, therefore, many inhibitors of this pathway are in (pre-)clinical development. High-grade glioma (glioblastoma; GBM) patients have a very poor prognosis and better therapies are urgently needed. Because the PI3K-mTor pathway is also commonly activated in GBM, inhibitors that are being developed -primarily for other cancer types- are also considered for testing in GBM patients. However, the delivery of active drug levels to these tumors may be more problematic due to the presence of the blood-brain barrier (BBB). In particular glioma cells that are left behind following gross surgical resection reside in areas of the brain where the BBB is still intact. Consequently, candidate drugs for treatment of glioma should be screened for their ability to penetrate the BBB.
Goal: To establish the impact of the ABC transporters Abcb1 (P-glycoprotein) and Abcg2 (Bcrp1) in the BBB on a series of PI3K and/or mTor inhibitors, namely NVP-BEZ235 (NVP), ZSTK-474, (ZSTK), PI-103 (PI), GDC-0941 (GDC) and rapamycin (RAPA).
Methods: Abcb1 and Abcg2 transport was first assessed by in vitro transwell assays using polarized epithelial cell monolayers. Abcb1 was assessed in LLC-PK1 vs. LLC-Mdr1a and Abcg2 in MDCK-parent vs. MDCK-Bcrp1 cells. Subsequently, we performed in vivo pharmacokinetic studies in WT vs. single and compound Abcb1/Abcg2 knockout (KO) mice. Drug levels were measured by HPLC.
Results: In vitro assays showed that RAPA is a good Abcb1 substrate, whereas ZSTK, NVP and PI were not or only very weak Abcb1 substrates. PI and NVP were transported by Abcg2, whereas ZSTK and RAPA are not substrates of Abcg2. Strikingly, GDC was extremely well transported with almost complete basal-to apical translocation in both Abcb1 and Abcg2 transwells. Moreover, this transport was only partially blocked using 5 uM of potent inhibitors such as zosuquidar and elacridar, whereas these concentrations were sufficient to block ABC-transporter mediatd translocation of the other agents.
Pharmacokinetic studies in WT and KO mice confirmed that Abcb1 and Abcg2 have a very minor effect on the brain penetration of ZSTK. The BBB penetration of RAPA was 3-fold higher in Abcb1 KO mice and was not further increased in compound Abcb1/Abcg2 KO mice. Experiments with NVP and GDC are ongoing.
Conclusion: The affinities of the various PI3K-mTor inhibitors for the ABC transporters vary markedly and this affects their brain penetration. These data should be considered when taking any of these candidates to clinical trial in glioma patients.
Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-301.
