Glioblastoma multiforme: Pathogenesis and treatment

Each year, about 5-6 cases out of 100,000 people are diagnosed with primary malignant brain tumors, of which about 80% are malignant gliomas (MGs). Glioblastoma multiforme (GBM) accounts for more than half of MG cases. They are associated with high morbidity and mortality. Despite current multimodality treatment efforts including maximal surgical resection if feasible, followed by a combination of radiotherapy and/or chemotherapy, the median survival is short: only about 15months. A deeper understanding of the pathogenesis of these tumors has presented opportunities for newer therapies to evolve and an expectation of better control of this disease. Lately, efforts have been made to investigate tumor resistance, which results from complex alternate signaling pathways, the existence of glioma stem-cells, the influence of the blood-brain barrier as well as the expression of 0(6)-methylguanine-DNA methyltransferase. In this paper, we review up-to-date information on MGs treatment including current approaches, novel drug-delivering strategies, molecular targeted agents and immunomodulative treatments, and discuss future treatment perspectives.

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.

Molecularly targeted therapies for malignant gliomas

This review critically evaluates current knowledge of molecularly targeted therapies of malignant gliomas.Various molecularly targeted single-agent therapies, including targeted therapies of growth and survival, have been evaluated in clinical trials but have failed to demonstrate a significant survival benefit compared with standard treatment regimens. The efficacy of multitargeted kinase inhibitors or combinations of single-targeted kinase inhibitors is a promising strategy, but requires additional clinical evaluation before definitive conclusions can be made. Important areas for further research include the assessment of serum or tissue biomarkers, the elucidation of prognostic molecular markers, and the determination of whether the mechanism of action of a drug is appropriate to the genetic alterations observed within individual tumors.

Designer therapies for glioblastoma multiforme

Primary brain tumors account for less than 2% of all cancers in adults; however, they are often associated with neurologic morbidity and high mortality. Glioblastoma multiforme (GBM) has been a focus of new therapy development in neurooncology because it is the most common primary brain tumor in adults. Standard-of-care therapy for newly diagnosed GBM includes surgical resection, radiotherapy, and temozolomide, administered both during and after radiotherapy. However, most patients develop tumor recurrence or progression after this multimodality treatment. Repeat resection and stereotactic radiosurgery upon recurrence may improve outcome only in selected patients. Most salvage chemotherapies offer only palliation. Recent advances in our understanding of the molecular abnormalities of GBM have generated new therapeutic venues of molecularly targeted agents (designer drugs) against key components of cellular pathways critical for cancer initiation and maintenance. Such drugs may offer the potential advantage to increase therapeutic efficacy and decrease systemic toxicity compared with traditional cytotoxic agents. Nonetheless, first-generation targeted agents have failed to demonstrate survival benefits in unselected GBM patient populations. Several mechanisms of treatment failure of the first-generation designer drugs have been proposed, whereas new strategies have been developed to increase effectiveness of these agents. Here we will discuss the recent development and the strategies to optimize the effectiveness of designer therapy for GBM.

Diagnosis and treatment of high-grade astrocytoma

High-grade astrocytomas include the most common adult central nervous system (CNS) tumor, glioblastoma multiforme, and anaplastic astrocytoma--a highly aggressive cancer with short median survival despite maximal multimodality therapy. Diagnosis is by clinical and radiographic findings confirmed by histopathology. Standard-of-care therapy includes surgical resection, radiotherapy, and temozolomide. Nearly all patients who have high-grade astrocytomas develop tumor recurrence or progression after this multimodality treatment. Two treatment challenges are molecular/genetic heterogeneity of tumors and limited CNS tumor delivery. It is probable that targeted therapies will be most effective in combination with one another or with cytotoxic therapies. This article discusses diagnosis and current treatment of high-grade astrocytomas.

New developments in the treatment of malignant gliomas

Malignant gliomas represent an heterogeneous group of brain tumors both in terms of natural history and response to treatment. The standard therapeutic approach for treating glioblastomas is a combination of radiotherapy and concomitant/adjuvant temozolomide, and methylguanine-DNA methyltransferase promoter methylation is now recognized as an important factor for predicting both prognosis and response to alkylating agents. In the future, the discovery of targeted therapies will increasingly allow personalized medical treatments. Anaplastic oligodendroglial tumors display a better prognosis and are more chemosensitive than glioblastomas; the discovery of molecular factors of prognostic significance, such as 1p/19q codeletion, will lead to different treatment strategies for different subgroups of patients. Gliomatosis cerebri is a rare diffuse glioma, and upfront chemotherapy is increasingly being employed instead of whole-brain radiotherapy to avoid/delay cognitive defects in long surviving patients, despite the lack of data to support this.

Molecularly targeted therapy for malignant glioma

Malignant gliomas are relatively uncommon but lethal cancers. Despite recent research efforts in cancer therapy, the prognosis of patients with malignant gliomas has remained dismal. Understanding the molecular pathogenesis of glioma may lead to a rational development of new therapies. Despite the genetic heterogeneity of malignant gliomas, common aberrations in the signaling elements of the growth and survival pathways are found. New treatments have emerged to target molecules in these signaling pathways with the goal to increase specific efficacy and minimize toxicity. Monoclonal antibodies and low molecular-weight kinase inhibitors are the most common classes of agents in targeted cancer treatment. Most clinical trials of these agents as monotherapies have failed to demonstrate survival benefit in unselected malignant glioma patient populations. Several mechanisms of treatment failure have been demonstrated. In response, multitargeted kinase inhibitors and combinations of single-targeted kinase inhibitors have been developed to overcome therapeutic resistance. In addition, multimodality combinations of targeted agents with radiation, chemotherapy, or immunotherapy/vaccines may enhance treatment efficacy. Future development of these agents will require advances in discovery and validation of new molecular targets, improvement of therapeutic delivery, and identification of correlative biomarkers. Novel clinical trial designs and endpoints may increase the efficiency of new drug evaluation. In this review, the authors discussed the current understanding of molecular pathogenesis and the development of molecularly targeted therapies in malignant glioma.

Targeting malignant glioma survival signalling to improve clinical outcomes

Malignant gliomas are common and aggressive brain tumours in adults. Current treatments for glioblastoma multiforme result in a poor median survival of less than 12 months. The blood-brain barrier restricts the delivery of many chemotherapies to the central nervous system, contributing to the failure of treatment. PI3K/Akt and Ras/MAPK pathways have been identified as important oncogenic pathways in these tumours. The PI3K/Akt pathway mediates cell survival and growth, whereas the Ras/MAPK pathway signals cell differentiation, proliferation and anti-apoptosis. Modern targeted therapies include antibodies to circulating growth factors and cell surface receptors, as well as inhibitors of receptor tyrosine kinases and specific intracellular signalling proteins. Monotherapy with most targeted therapies produces only modest efficacy. Better results are achieved in combination with cytotoxic chemotherapies. Future therapeutics should focus on combination therapy with small lipophilic molecules.