Identification of molecular characteristics correlated with glioblastoma sensitivity to EGFR kinase inhibition through use of an intracranial xenograft test panel

PTEN loss does not predict for response to RAD001 (Everolimus) in a glioblastoma orthotopic xenograft test panel

PURPOSE

Hyperactivation of the phosphatidylinositol 3-kinase/Akt signaling through disruption of PTEN function is common in glioblastoma multiforme, and these genetic changes are predicted to enhance sensitivity to mammalian target of rapamycin (mTOR) inhibitors such as RAD001 (everolimus).

EXPERIMENTAL DESIGN

To test whether PTEN loss could be used as a predictive marker for mTOR inhibitor sensitivity, the response of 17 serially transplantable glioblastoma multiforme xenografts was evaluated in an orthotopic therapy evaluation model. Of these 17 xenograft lines, 7 have either genomic deletion or mutation of PTEN.

RESULTS

Consistent with activation of Akt signaling, there was a good correlation between loss of PTEN function and elevated levels of Akt phosphorylation. However, of the 7 lines with disrupted PTEN function, only 1 tumor line (GBM10) was significantly sensitive to RAD001 therapy (25% prolongation in median survival), whereas 1 of 10 xenograft lines with wild-type PTEN was significantly sensitive to RAD001 (GS22; 34% prolongation in survival). Relative to placebo, 5 days of RAD001 treatment was associated with a marked 66% reduction in the MIB1 proliferation index in the sensitive GBM10 line (deleted PTEN) compared with a 25% and 7% reduction in MIB1 labeling index in the insensitive GBM14 (mutant PTEN) and GBM15 (wild-type PTEN) lines, respectively. Consistent with a cytostatic antitumor effect, bioluminescent imaging of luciferase-transduced intracranial GBM10 xenografts showed slowed tumor growth without significant tumor regression during RAD001 therapy.

CONCLUSION

These data suggest that loss of PTEN function is insufficient to adequately predict responsiveness to mTOR inhibitors in glioblastoma multiforme.

miR-124 and miR-137 inhibit proliferation of glioblastoma multiforme cells and induce differentiation of brain tumor stem cells

BACKGROUND

Glioblastoma multiforme (GBM) is an invariably fatal central nervous system tumor despite treatment with surgery, radiation, and chemotherapy. Further insights into the molecular and cellular mechanisms that drive GBM formation are required to improve patient outcome. MicroRNAs are emerging as important regulators of cellular differentiation and proliferation, and have been implicated in the etiology of a variety of cancers, yet the role of microRNAs in GBM remains poorly understood. In this study, we investigated the role of microRNAs in regulating the differentiation and proliferation of neural stem cells and glioblastoma-multiforme tumor cells.

METHODS

We used quantitative RT-PCR to assess microRNA expression in high-grade astrocytomas and adult mouse neural stem cells. To assess the function of candidate microRNAs in high-grade astrocytomas, we transfected miR mimics to cultured-mouse neural stem cells, -mouse oligodendroglioma-derived stem cells, -human glioblastoma multiforme-derived stem cells and -glioblastoma multiforme cell lines. Cellular differentiation was assessed by immunostaining, and cellular proliferation was determined using fluorescence-activated cell sorting.

RESULTS

Our studies revealed that expression levels of microRNA-124 and microRNA-137 were significantly decreased in anaplastic astrocytomas (World Health Organization grade III) and glioblastoma multiforme (World Health Organization grade IV) relative to non-neoplastic brain tissue (P < 0.01), and were increased 8- to 20-fold during differentiation of cultured mouse neural stem cells following growth factor withdrawal. Expression of microRNA-137 was increased 3- to 12-fold in glioblastoma multiforme cell lines U87 and U251 following inhibition of DNA methylation with 5-aza-2'-deoxycytidine (5-aza-dC). Transfection of microRNA-124 or microRNA-137 induced morphological changes and marker expressions consistent with neuronal differentiation in mouse neural stem cells, mouse oligodendroglioma-derived stem cells derived from S100 beta-v-erbB tumors and cluster of differentiation 133+ human glioblastoma multiforme-derived stem cells (SF6969). Transfection of microRNA-124 or microRNA-137 also induced G1 cell cycle arrest in U251 and SF6969 glioblastoma multiforme cells, which was associated with decreased expression of cyclin-dependent kinase 6 and phosphorylated retinoblastoma (pSer 807/811) proteins.

CONCLUSION

microRNA-124 and microRNA-137 induce differentiation of adult mouse neural stem cells, mouse oligodendroglioma-derived stem cells and human glioblastoma multiforme-derived stem cells and induce glioblastoma multiforme cell cycle arrest. These results suggest that targeted delivery of microRNA-124 and/or microRNA-137 to glioblastoma multiforme tumor cells may be therapeutically efficacious for the treatment of this disease.

Preclinical pharmacokinetic/pharmacodynamic models of gefitinib and the design of equivalent dosing regimens in EGFR wild-type and mutant tumor models

Epidermal growth factor receptor (EGFR) inhibitors, such as gefitinib, are examples of targeted anticancer drugs whose drug sensitivity is related to gene mutations that adds a pharmacogenetic (PG) dimension to any pharmacokinetic (PK) and pharmacodynamic (PD) analysis. The goal of this investigation was to cast the combined PG/PK/PD variables into models that could be used to design equivalent PK/PD dosing regimens for gefitinib in genetically distinct tumor models. To this end, groups of mice bearing either s.c. LN229-wild-type EGFR or LN229-EGFRvIII mutant tumors, an EGFR inhibitor-sensitizing mutation, were given gefitinib at doses of 10 mg/kg i.v., 50 mg/kg intraarterially, and 150 mg/kg p.o. In each group, gefitinib plasma and tumor concentrations were quantitated, as were tumoral amounts of phosphorylated extracellular signal-regulated kinase (ERK) 1/2 (pERK), a PD end point that was shown to respond in a dose-dependent manner in each tumor type. Hybrid physiologically based PK/PD models were developed for each tumor type, which consisted of a forcing function describing the plasma drug concentration profile, a tumor compartment depicting drug disposition in tumor, and a mechanistic target-response PD model characterizing pERK in the tumor. Gefitinib showed analogous PK properties in each tumor type yet different PD characteristics consistent with the EGFR status of the tumors. Using the PK/PD model for each tumor type, simulations were done to define multiple-dose regimens for gefitinib that yielded equivalent PD profiles of pERK in each tumor type. The novel concept of PK/PD equivalent dosing regimens could be applied in drug development and to delineate PG differences in drug activity.

Development of a real-time RT-PCR assay for detecting EGFRvIII in glioblastoma samples

PURPOSE

Epidermal growth factor receptor variant III (EGFRvIII) is an oncogenic, constitutively active mutant form of the EGFR that is commonly expressed in glioblastoma and is also detected in a number of epithelial cancers. EGFRvIII presents a unique antigenic target for anti-EGFRvIII vaccines and it has been shown to modulate response to EGFR kinase inhibitor therapy. Thus, detection in clinical samples may be warranted. Existing patents preclude the use of anti-EGFRvIII antibodies for clinical detection. Further, frozen tissue is not routinely available, particularly for patients treated in the community. Thus, detection of EGFRvIII in formalin-fixed paraffin-embedded (FFPE) clinical samples is a major challenge.

EXPERIMENTAL DESIGN

We developed a real-time reverse transcription-PCR (RT-PCR) assay for detecting EGFRvIII in FFPE samples and analyzed 59 FFPE glioblastoma clinical samples with paired frozen tissue from the same surgical resection. We assessed EGFRvIII protein expression by immunohistochemistry using two distinct specific anti-EGFRvIII antibodies and examined EGFR gene amplification by fluorescence in situ hybridization.

RESULTS

The FFPE RT-PCR assay detected EGFRvIII in 16 of 59 (27%) samples, exclusively in cases with EGFR amplification, consistent with the expected frequency of this alteration. The FFPE RT-PCR assay was more sensitive and specific for detecting EGFRvIII than either of the two antibodies alone, or in combination, with a sensitivity of 93% (95% confidence interval, 0.78-1.00) and a specificity of 98% (95% confidence interval, 0.93-1.00).

CONCLUSION

This assay will facilitate accurate assessment of EGFRvIII in clinical samples and may aid in the development of strategies for stratifying patients for EGFRvIII-directed therapies.

Targeting the epidermal growth factor receptor in high-grade astrocytomas

OPINION STATEMENT

High-grade astrocytomas, including glioblastoma multiforme (GBM) and anaplastic astrocytoma (AA), are the most common and aggressive primary malignant brain tumors in adults. Despite improvements in survival with the addition of temozolomide to radiation in the adjuvant setting, the prognosis of patients affected by these tumors remains relatively poor. One approach to improve outcomes in these patients is to target the epidermal growth factor receptor (EGFR). EGFR-targeted therapy is a rational approach since EGFR overexpression and mutant EGFRvIII expression occur in approximately 50% of patients with GBM. Unfortunately, monotherapy with anti-EGFR agents in malignant gliomas has not provided the dramatic results sometimes seen with other targeted therapies, such as imatinib in chronic myelogenous leukemia. Anti-EGFR agents currently being studied in malignant gliomas include the tyrosine kinase inhibitors (TKI), monoclonal antibodies (MAb), and anti-EGFR vaccines. Of all these agents, the tyrosine kinase inhibitors-which include erlotinib and gefitinib-have been the most extensively tested in clinical trials. Retrospective analyses have highlighted co-expression of EGFRvIII and wild-type PTEN (phosphatase and tensin homologue deleted in chromosome 10) as a significant predictor of EGFR TKI response in patients with GBM. As the EGFR signaling pathway is exceptionally complex, newer approaches targeting multiple points in the pathway are being developed to improve treatment efficacy.

Incorporating molecular tools into early-stage clinical trials

The author discusses the implications of a new phase I trial investigating the role of rapamycin in patients with glioblastoma.