No abstract

Purpose: Little is known about the optimal clinical use of for treatment of glioblastoma. ABT-888 is a PARP inhibitor undergoing extensive clinical evaluation in glioblastoma, because it may synergize with the standard-of-care temozolomide (TMZ). We have elucidated important factors controlling ABT-888 efficacy in glioblastoma.Experimental Design: We used genetically engineered spontaneous glioblastoma mouse models and allograft models that were orthotopically transplanted into wild-type (WT) and Abcb1/Abcg2-deficient (KO) recipients.Results: ABT-888/TMZ is not efficacious against p53;p16;LucR allografts in wild-type recipients, indicating inherent resistance. Abcb1/Abcg2 mediated efflux of ABT-888 at the blood-brain barrier (BBB) causes a 5-fold reduction of ABT-888 brain penetration (P < 0.0001) that was fully reversible by elacridar. Efficacy studies in WT and KO recipients and/or concomitant elacridar demonstrate that Abcb1/ Abcg2 at the BBB and in tumor cells impair TMZ/ABT-888 combination treatment efficacy. Elacridar also markedly improved TMZ/ABT-888 combination treatment in the spontaneous p53;p16 Ink4a /p19 Arf ;K-Ras v12 ; LucR glioblastoma model. Importantly, ABT-888 does enhance TMZ efficacy in Pten deficient glioblastoma allografts and spontaneous tumors, even in Abcb1/Abcg2 proficient wild-type mice. Loss of PTEN occurs frequently in glioblastoma (36%) and in silico analysis on patient with glioblastoma samples revealed that it is associated with a worse overall survival (310 days vs. 620 days, n ¼ 117).Conclusions: The potential of ABT-888 in glioblastoma can best be demonstrated in patients with PTEN null tumors. Therefore, clinical trials with ABT-888 should evaluate these patients as a separate group. Importantly, inhibition of ABCB1 and ABCG2 (by elacridar) may improve the efficacy of TMZ/ABT-888 therapy in all glioblastoma patients. Clin Cancer Res; 20(10); 2703-13. Ó2014 AACR.

No abstract

Diffuse and uncontrollable brain invasion is a hallmark of glioblastoma (GBM), but its mechanism is understood poorly. We developed a 3D ex vivo organotypic model to study GBM invasion. We demonstrate that invading GBM cells upregulate a network of extracellular matrix (ECM) components, including multiple collagens, whose expression correlates strongly with grade and clinical outcome. We identify interferon regulatory factor 3 (IRF3) as a transcriptional repressor of ECM factors and show that IRF3 acts as a suppressor of GBM invasion. Therapeutic activation of IRF3 by inhibiting casein kinase 2 (CK2)-a negative regulator of IRF3-downregulated the expression of ECM factors and suppressed GBM invasion in ex vivo and in vivo models across a panel of patient-derived GBM cell lines representative of the main molecular GBM subtypes. Our data provide mechanistic insight into the invasive capacity of GBM tumors and identify a potential therapy to inhibit GBM invasion.

Cancer cells release extracellular vesicles (EVs) that contain functional biomolecules such as RNA and proteins. EVs are transferred to recipient cancer cells and can promote tumour progression and therapy resistance. Through RNAi screening, we identified a novel EV uptake mechanism involving a triple interaction between the chemokine receptor CCR8 on the cells, glycans exposed on EVs and the soluble ligand CCL18. This ligand acts as bridging molecule, connecting EVs to cancer cells. We show that glioblastoma EVs promote cell proliferation and resistance to the alkylating agent temozolomide (TMZ). Using in vitro and in vivo stem-like glioblastoma models, we demonstrate that EV-induced phenotypes are neutralised by a small molecule CCR8 inhibitor, R243. Interference with chemokine receptors may offer therapeutic opportunities against EV-mediated cross-talk in glioblastoma.

Thus, the brain penetration of palbociclib is restricted by P-gp and BCRP, which may restrict the efficacy against GBM and DIPG. Moreover, preclinical studies with this agent should be conducted at a more clinically relevant dose level.

The anticancer drug temozolomide is the only drug with proven activity against high-grade gliomas and has therefore become a part of the standard treatment of these tumors. P-glycoprotein (P-gp; ABCB1) and breast cancer resistance protein (BCRP; ABCG2) are transport proteins, which are present at the blood-brain barrier and limit the brain uptake of substrate drugs. We have studied the effect of P-gp and BCRP on the pharmacokinetics and pharmacodynamics of temozolomide, making use of a comprehensive set of in vitro transport experiments and in vivo pharmacokinetic and antitumor efficacy experiments using wild-type, Abcg2−/−, Abcb1a/b−/−, and Abcb1a/b;Abcg2−/− mice. We here show that the combined deletion of Abcb1a/b and Abcg2 increases the brain penetration of temozolomide by 1.5-fold compared to wild-type controls (P < .001) without changing the systemic drug exposure. Moreover, the same increase was achieved when temozolomide was given to wild-type mice in combination with the dual P-gp/BCRP inhibitor elacridar (GF120918). The antitumor efficacy of temozolomide against three different intracranial tumor models was significantly enhanced when Abcb1a/b and Abcg2 were genetically deficient or pharmacologically inhibited in recipient mice. These findings call for further clinical testing of temozolomide in combination with elacridar for the treatment of gliomas, as this offers the perspective of further improving the antitumor efficacy of this already active agent.

Enhancer of Zeste Homolog 2 (EZH2) has emerged as a promising therapeutic target for treatment of a broad spectrum of tumors including gliomas. We explored the interactions of five novel, structurally similar EZH2 inhibitors (EPZ005687, EPZ-6438, UNC1999, GSK343 and GSK126) with P-glycoprotein (P-gp/ABCB1) and breast cancer resistance protein (BCRP/ABCG2). The compounds were screened by in vitro transwell assays and EPZ005687, EPZ-6438 and GSK126 were further tested in vivo using wild-type (WT), Abcb1 and/or Abcg2 knockout mice. All EZH2 inhibitors are transported by P-gp and BCRP, although in vitro the transporter affinity of GSK126 was obscured by very low membrane permeability. Both P-gp and Bcrp1 restrict the brain penetration of EPZ005687 and GSK126, whereas the brain accumulation of EPZ-6438 is limited by P-gp only and efflux of EPZ-6438 was completely abrogated by elacridar. Intriguingly, an unknown factor present in all knockout mouse strains causes EPZ005687 and EPZ-6438 retention in plasma relative to WT mice, a phenomenon not seen with GSK126. In WT mice, the GSK126 tissue-to-plasma ratio for all tissues is lower than for EPZ005687 or EPZ-6438. Moreover, the oral bioavailability of GSK126 is only 0.2% in WT mice, which increases to 14.4% in Abcb1;Abcg2 knockout mice. These results are likely due to poor membrane permeability and question the clinical usefulness of GSK126. Although all tested EZH2 inhibitors are substrates of P-gp and BCRP, restricting the brain penetration and potential utility for treatment of glioma, EPZ-6438 would be the most suitable candidate of this series.