The Role of Kinase Signaling in Resistance to Bevacizumab Therapy for Glioblastoma Multiforme

Glioblastoma multiforme (GBM) is the most malignant primary brain tumor and is characterized by vascular hyperplasia, necrosis, and high cell proliferation. Despite current standard therapies, including surgical resection and chemoradiotherapy, GBM patients survive for only about 15 months after diagnosis. Recently, the U.S. Food and Drug Administration (FDA) has approved an antiangiogenesis medication for recurrent GBM-bevacizumab-which has improved progression-free survival in GBM patients. Although bevacizumab has resulted in significant early clinical benefit, it inescapably predisposes tumor to relapse that can be represented as an infiltrative phenotype. Fundamentally, bevacizumab antagonizes the vascular endothelial growth factor A (VEGF_A), which is consistently released on both endothelial cells (ECs) and GBM cells. Actually, VEGF_A inhibition on the ECs leads to the suppression of vascular progression, permeability, and the vasogenic edema. However, the consequence of the VEGF_A pathway blockage on the GBM cells remains controversial. Nevertheless, a piece of evidence supports the relationship between bevacizumab application and compensatory activation of kinase signaling within GBM cells, leading to a tumor cell invasion known as the main mechanism of bevacizumab-induced tumor resistance. A complete understanding of kinase responses associated with tumor invasion in bevacizumab-resistant GBMs offers new therapeutic opportunities. Thus, this study aimed at presenting a brief overview of preclinical and clinical data of the tumor invasion and resistance induced by bevacizumab administration in GBMs, with a focus on the kinase responses during treatment. The novel therapeutic strategies to overcome this resistance by targeting protein kinases have also been summarized.

Experimental approaches for the treatment of malignant gliomas

Malignant gliomas, which include glioblastomas and anaplastic astrocytomas, are the most common primary tumors of the brain. Over the past 30 years, the standard treatment for these tumors has evolved to include maximal safe surgical resection, radiation therapy and temozolomide chemotherapy. While the median survival of patients with glioblastomas has improved from 6 months to 14.6 months, these tumors continue to be lethal for the vast majority of patients. There has, however, been recent substantial progress in our mechanistic understanding of tumor development and growth. The translation of these genetic, epigenetic and biochemical findings into therapies that have been tested in clinical trials is the subject of this review.

Passive antibody-mediated immunotherapy for the treatment of malignant gliomas

Despite advances in understanding the molecular mechanisms of brain cancer, the outcome of patients with malignant gliomas treated according to the current standard of care remains poor. Novel therapies are needed, and immunotherapy has emerged with great promise. The diffuse infiltration of malignant gliomas is a major challenge to effective treatment; immunotherapy has the advantage of accessing the entire brain with specificity for tumor cells. Therapeutic immune approaches include cytokine therapy, passive immunotherapy, and active immunotherapy. Cytokine therapy involves the administration of immunomodulatory cytokines to activate the immune system. Active immunotherapy is the generation or augmentation of an immune response, typically by vaccination against tumor antigens. Passive immunotherapy connotes either adoptive therapy, in which tumor-specific immune cells are expanded ex vivo and reintroduced into the patient, or passive antibody-mediated therapy. In this article, the authors discuss the preclinical and clinical studies that have used passive antibody-mediated immunotherapy, otherwise known as serotherapy, for the treatment of malignant gliomas.

Antiangiogenic therapies for high-grade glioma

High-grade gliomas (HGGs) are vascular tumors that represent attractive targets for antiangiogenic therapies. In this Review, we present the rationale and clinical trial evidence for targeting angiogenesis in HGGs, focusing predominantly on agents that target vascular endothelial growth factor (VEGF) and its receptors. Bevacizumab, a humanized monoclonal antibody against VEGF, was recently approved by the FDA for treatment of recurrent glioblastoma. Bevacizumab prolongs progression-free survival and controls peritumoral edema, but its effects on overall survival remain to be determined. Other inhibitors of VEGF, VEGF receptors and other proangiogenic signaling pathways are being evaluated. Antiangiogenic therapies are well tolerated, although potentially serious adverse events can occasionally occur, and resistance to antiangiogenic therapy inevitably develops. Mechanisms of resistance include upregulation of alternative proangiogenic pathways, and increased perivascular tumor growth. Tumor progression on antiangiogenic agents is a challenging problem for which no effective salvage therapy has been identified. Combining these agents with radiation therapy, cytotoxic chemotherapy, other targeted molecular agents, or anti-invasion therapies could be helpful. The international Response Assessment in Neuro-Oncology Working Group has developed consensus treatment response criteria for HGG that account for the complex effects of antiangiogenic drugs.

Antiangiogenic strategies for treatment of malignant gliomas

Numerous antiangiogenic agents with diverse mechanisms of action are currently under investigation for the treatment of patients with glioblastoma (GBM), a diagnosis that continues to carry a poor prognosis despite maximal conventional therapy. Early clinical trials suggest that antiangiogenic drugs, which target the blood vessels of these highly angiogenic tumors, may have clinical benefit in GBM patients. Antiangiogenic agents have potent antiedema and steroid-sparing effects in patients, and emerging data suggest that these drugs may modestly improve progression-free survival. Although these early results are encouraging, several issues arise regarding the use and efficacy of these agents. Interpretation of the radiographic changes that occur after treatment with antiangiogenic agents presents a major challenge. Still lacking are reliable radiographic and biologic markers that can predict which patients will benefit from treatment and that accurately indicate response and progression during therapy. In addition, most patients treated with antiangiogenic drugs eventually progress, and the mechanisms by which tumors escape from therapy are only beginning to be understood. Larger prospective trials that incorporate correlative biomarker studies will be required to address these challenges. Here, we summarize the clinical experience with antiangiogenic therapy in patients with malignant gliomas (MG), review the major issues concerning the use and development of these agents, and discuss strategies that may build upon the initial gains observed with antiangiogenic agents.

Angiogenesis as a therapeutic target in malignant gliomas

Currently, adult glioblastoma (GBM) patients have poor outcomes with conventional cytotoxic treatments. Because GBMs are highly angiogenic tumors, inhibitors that target tumor vasculature are considered promising therapeutic agents in these patients. Encouraging efficacy and tolerability in preliminary clinical trials suggest that targeting angiogenesis may be an effective therapeutic strategy in GBM patients. However, the survival benefits observed to date in uncontrolled trials of antiangiogenic agents have been modest, and several obstacles have limited their effectiveness. This article reviews the rationale for antiangiogenic agents in GBM, their potential mechanisms of action, and their clinical development in GBM patients. Although challenges remain with this approach, ongoing studies may improve upon the promising initial benefits already observed in GBM patients.

Antiangiogenic therapy in malignant gliomas

PURPOSE OF REVIEW

Antiangiogenic drugs are increasingly used in malignant glioma therapy. This article reviews the rationale for targeting angiogenesis in malignant gliomas, summarizes relevant clinical trial results, and discusses promising avenues of investigation in antiangiogenic therapy.

RECENT FINDINGS

Combination therapy with bevacizumab, the humanized monoclonal antibody against vascular endothelial growth factor, and irinotecan has emerged as the treatment of choice for recurrent malignant gliomas, prolonging progression-free survival markedly in comparison with historical controls. Several small molecule tyrosine kinase inhibitors of the vascular endothelial growth factor receptor are under investigation and show promise as well.

SUMMARY

Antiangiogenic treatments are effective and well tolerated options for recurrent malignant glioma. Future studies will determine whether these drugs have a role in first line therapy. Studies are in progress to elucidate mechanisms of resistance and suggest approaches to further improve survival in patients with these challenging tumors.

Emerging antiangiogenic treatments for gliomas - efficacy and safety issues

PURPOSE OF REVIEW

To review the rationale and recent experience of angiogenesis inhibitors in malignant gliomas and to highlight both the promise and potential complications of these agents.

RECENT FINDINGS

Several new agents targeting angiogenesis in malignant gliomas have become available and have been increasingly used to complement conventional chemotherapy. Specifically, bevacizumab, often in combination with irinotecan, has demonstrated favorable results in achieving significant radiographic responses and in prolonging progression-free survival in patients with recurrent malignant glioma.

SUMMARY

Antiangiogenic drugs have been shown to have promising activity in recurrent malignant gliomas. Investigation of novel antiangiogenic compounds and future clinical trials will determine whether these drugs have a role in first-line therapy. This article reviews the rationale for targeting angiogenesis in malignant brain tumors and summarizes the results of recent clinical trials. In addition, this review will outline potential toxicities associated with angiogenesis inhibition in an attempt to provide practical guidance to physicians treating patients with malignant gliomas.

Editorial: on the road to multi-modal and pluri-disciplinary treatment of glioblastomas

Despite major advances in the management of malignant gliomas of which glioblastomas represent the ultimate grade of malignancy, they remain incurable. Indeed, glioblastoma patients have a median survival expectancy of only 14 months on the current standard treatment of surgical resection to the extent which is feasible, followed by adjuvant radiotherapy plus temozolomide given concomitantly with and after radiotherapy (Lefranc et al., J Clin Oncol 23:2411-2422, 2005; Expert Rev Anticancer Ther 6:719-732, 2006; Stummer et al., Neurosurgery 62:564-576, 2008). Accordingly, the present editorial discusses (1) the high cell motility and resistance to apoptosis which characterise glioblastoma growth and malignancy with respect to the failure of conventional therapy, (2) ways to overcome apoptosis resistance and the real hope offered by temozolomide, (3) targeted chemotherapeutic approaches and the disappointing results obtained in monotherapy but their potential in combination therapy, (4) anti-migratory strategies that could supplement conventional therapy notably by inhibiting a new target; the alpha1 subunit of the sodium pump, (5) dendritic cell therapy, (6) cancer stem cell targeting and finally (7) topical therapies and new surgical approaches for more radical resection which could be used to complement multi-modal treatments within a multi-disciplinary approach.

Localized BCNU chemotherapy and the multimodal management of malignant glioma

BACKGROUND

Gliomas account for 42% of all primary CNS neoplasms and 77% of all malignant primary CNS neoplasms. Unfortunately the high-grade variant of gliomas, glioblastoma multiforme (GBM), is difficult to treat and generally considered incurable. Survival rates are generally poor, and neurological morbidity in the setting of disease progression is high. Fortunately, significant progress has been achieved in the past decade in our understanding of the molecular biology of this aggressive tumour histology and, as a consequence, there is renewed clinical trial activity in this area focused on improving quality of life, treatment-related morbidity and outcomes.

METHODS

A review of literature from June 2005 to June 2008 was conducted on multimodal treatment of malignant glioma (MG) patients, using specific search criteria in Medline, EMBASE, and BIOSIS. Abstracts from relevant US and European medical (cancer) meetings were also evaluated.

RESULTS

The established therapies for MG include surgery, radiotherapy (RT), and local or systemic chemotherapy. However, over the last 10 years only two chemotherapeutic agents have received regulatory approval for treatment of MG: polifeprosan 20 with carmustine (BCNU implant) and temozolomide (TMZ), an imidazotetrazine derivative of dacarbazine. More recent advances in the treatment of brain tumours have been in the development of multimodal approaches. Specific interest in the combination of BCNU implant and TMZ has arisen due to the demonstrable depletion by TMZ of the DNA repair enzyme responsible for resistance to a nitrosourea such as BCNU. Further interest in this combination stems from the observation that there is a difference in the time to peak effect for each agent. Additional emerging data suggest that multimodal therapy with maximal resection and BCNU implants, followed by adjuvant therapy with radiation and TMZ, is effective and well-tolerated in patients with initial high-grade, resectable MG.

CONCLUSIONS

The increasing body of efficacy data suggests that this combination of BCNU implants and TMZ within a multimodal treatment strategy including surgery and RT may provide an enhanced benefit compared with the use of either of these agents alone in select patients with high-grade glioma.

Present and potential future adjuvant issues in high-grade astrocytic glioma treatment

Despite major advances in the management of malignant gliomas of which glioblastomas represent the ultimate grade of malignancy, they remain characterized by dismal prognoses. Glioblastoma patients have a median survival expectancy of only 14 months on the current standard treatment of surgical resection to the extent feasible, followed by adjuvant radiotherapy plus temozolomide, given concomitantly with and after radiotherapy. Malignant gliomas are associated with such dismal prognoses because glioma cells can actively migrate through the narrow extra-cellular spaces in the brain, often travelling relatively long distances, making them elusive targets for effective surgical management. Clinical and experimental data have demonstrated that invasive malignant glioma cells show a decrease in their proliferation rates and a relative resistance to apoptosis (type I programmed cell death) as compared to the highly cellular centre of the tumor, and this may contribute to their resistance to conventional pro-apoptotic chemotherapy and radiotherapy. Resistance to apoptosis results from changes at the genomic, transcriptional and post-transcriptional level of proteins, protein kinases and their transcriptional factor effectors. The PTEN/ PI3K/Akt/mTOR/NF-kappaB and the Ras/Raf/MEK/ERK signaling cascades play critical roles in the regulation of gene expression and prevention of apoptosis. Components of these pathways are mutated or aberrantly expressed in human cancer, notably glioblastomas. Monoclonal antibodies and low molecular-weight kinase inhibitors of these pathways are the most common classes of agents in targeted cancer treatment. However, most clinical trials of these agents as monotherapies have failed to demonstrate survival benefit. Despite resistance to apoptosis being closely linked to tumorigenesis, tumor cells can still be induced to die by non-apoptotic mechanisms such as necrosis, senescence, autophagy (type II programmed cell death) and mitotic catastrophe. Temozolomide brings significant therapeutic benefits in glioblastoma treatment. Part of temozolomide cytotoxic activity is exerted through pro-autophagic processes and also through the induction of late apoptosis. Autophagy, type II programmed cell death, represents an alternative mechanism to overcome, at least partly, the dramatic resistance of many cancers to pro-apoptotic-related therapies. Another way to potentially overcome apoptosis resistance is to decrease the migration of malignant glioma cells in the brain, which then should restore a level of sensitivity to pro-apoptotic drugs. Recent series of studies have supported the concept that malignant gliomas might be seen as an orchestration of cross-talks between cancer cells, microenvironment, vasculature and cancer stem cells. The present chapter focuses on (i) the major signaling pathways making glioblastomas resistant to apoptosis, (ii) the signaling pathways distinctly activated by pro-autophagic drugs as compared to pro-apoptotic ones, (iii) autophagic cell death as an alternative to combat malignant gliomas, (iv) the major scientific data already obtained by researchers to prove that temozolomide is actually a pro-autophagic and pro-apoptotic drug, (v) the molecular and cellular therapies and local drug delivery which could be used to complement conventional treatments, and a review of some of the currently ongoing clinical trials, (vi) the fact that reducing the levels of malignant glioma cell motility can restore pro-apoptotic drug sensitivity, (vii) the observation that inhibiting the sodium pump activity reduces both glioma cell proliferation and migration, (viii) the brain tumor stem cells as a target to complement conventional treatment.