Emerging Therapies for Glioblastoma

Glioblastoma is most commonly a primary brain tumor and the utmost malignant one, with a survival rate of approximately 12-18 months. Glioblastoma is highly heterogeneous, demonstrating that different types of cells from the same tumor can manifest distinct gene expression patterns and biological behaviors. Conventional therapies such as temozolomide, radiation, and surgery have limitations. As of now, there is no cure for glioblastoma. Alternative treatment methods to eradicate glioblastoma are discussed in this review, including targeted therapies to PI3K, NFKβ, JAK-STAT, CK2, WNT, NOTCH, Hedgehog, and TGFβ pathways. The highly novel application of oncolytic viruses and nanomaterials in combating glioblastoma are also discussed. Despite scores of clinical trials for glioblastoma, the prognosis remains poor. Progress in breaching the blood-brain barrier with nanomaterials and novel avenues for targeted and combination treatments hold promise for the future development of efficacious glioblastoma therapies.

Glioblastoma Therapy: Past, Present and Future

Glioblastoma (GB) stands out as the most prevalent and lethal form of brain cancer. Although great efforts have been made by clinicians and researchers, no significant improvement in survival has been achieved since the Stupp protocol became the standard of care (SOC) in 2005. Despite multimodality treatments, recurrence is almost universal with survival rates under 2 years after diagnosis. Here, we discuss the recent progress in our understanding of GB pathophysiology, in particular, the importance of glioma stem cells (GSCs), the tumor microenvironment conditions, and epigenetic mechanisms involved in GB growth, aggressiveness and recurrence. The discussion on therapeutic strategies first covers the SOC treatment and targeted therapies that have been shown to interfere with different signaling pathways (pRB/CDK4/RB1/P16ink4, TP53/MDM2/P14arf, PI3k/Akt-PTEN, RAS/RAF/MEK, PARP) involved in GB tumorigenesis, pathophysiology, and treatment resistance acquisition. Below, we analyze several immunotherapeutic approaches (i.e., checkpoint inhibitors, vaccines, CAR-modified NK or T cells, oncolytic virotherapy) that have been used in an attempt to enhance the immune response against GB, and thereby avoid recidivism or increase survival of GB patients. Finally, we present treatment attempts made using nanotherapies (nanometric structures having active anti-GB agents such as antibodies, chemotherapeutic/anti-angiogenic drugs or sensitizers, radionuclides, and molecules that target GB cellular receptors or open the blood-brain barrier) and non-ionizing energies (laser interstitial thermal therapy, high/low intensity focused ultrasounds, photodynamic/sonodynamic therapies and electroporation). The aim of this review is to discuss the advances and limitations of the current therapies and to present novel approaches that are under development or following clinical trials.

Cell Signaling Pathways in Brain Tumors

Primary brain tumors, particularly glioblastoma, are associated with significant morbidity and are often recalcitrant to standard therapies. In recent years, brain tumors have been the focus of large-scale genomic sequencing efforts, providing unprecedented insight into the genomic aberrations and cellular signaling mechanisms that drive these cancers. Discoveries from these efforts have translated into novel diagnostic algorithms, biomarkers, and therapeutic strategies in Neuro-Oncology. However, the cellular mechanisms that drive brain tumors are heterogeneous and complex: applying this new knowledge to improve patient outcomes remains a challenge. Efforts to characterize and target these molecular vulnerabilities are evolving.

Inhibition of the PI3K/AKT/mTOR Pathway in Solid Tumors

The phosphoinositide 3-kinase (PI3K) pathway plays an integral role in many cellular processes and is frequently altered in cancer, contributing to tumor growth and survival. Small molecule inhibitors have been developed that target the three major nodes of this pathway: PI3K, AKT, and mammalian target of rapamycin. However, because oncogenic PI3K pathway activation is achieved in diverse, potentially redundant ways, the clinical efficacy of these inhibitors as monotherapies has, so far, been limited, despite demonstrating promising preclinical activity. Moreover, pathway activation is associated with resistance to other therapies; thus, in combination, PI3K pathway inhibitors could restore therapeutic sensitivity to these agents. To maximize therapeutic benefit, drug combinations and schedules must be explored to identify those with the highest efficacy and lowest toxicity overlap. In addition, defining appropriate patient subpopulations, for both monotherapy and drug combinations, will be important. However, identifying predictive biomarkers remains a challenge.

Immunotherapy for neuro-oncology: the critical rationale for combinatorial therapy

A successful therapeutic paradigm established historically in oncology involves combining agents with potentially complementary mechanisms of antitumor activity into rationally designed regimens. For example, cocktails of cytotoxic agents, which were carefully designed based on mechanisms of action, dose, and scheduling considerations, have led to dramatic improvements in survival including cures for childhood leukemia, Hodgkin's lymphoma, and several other complex cancers. Outcome for glioblastoma, the most common primary malignant CNS cancer, has been more modest, but nonetheless our current standard of care derives from confirmation that combination therapy surpasses single modality therapy. Immunotherapy has recently come of age for medical oncology with exciting therapeutic benefits achieved by several types of agents including vaccines, adoptive T cells, and immune checkpoint inhibitors against several types of cancers. Nonetheless, most benefits are relatively short, while others are durable but are limited to a minority of treated patients. Critical factors limiting efficacy of immunotherapeutics include insufficient immunogenicity and/or inadequate ability to overcome immunosuppressive factors exploited by tumors. The paradigm of rationally designed combinatorial regimens, originally established by cytotoxic therapy for oncology, may also prove relevant for immunotherapy. Realization of the true therapeutic potential of immunotherapy for medical oncology and neuro-oncology patients may require development of combinatorial regimens that optimize immunogenicity and target tumor adaptive immunosuppressive factors.

Tumorsphere as an effective in vitro platform for screening anti-cancer stem cell drugs

Cancer stem cells (CSCs) are a sub-population of cells within cancer tissues with tumor initiation, drug resistance and metastasis properties. CSCs also have been considered as the main cause of cancer recurrence. Targeting CSCs have been suggested as the key for successful treatment against cancer. Tumorsphere cultivation is based on culturing cancer cells onto ultralow attachment surface in serum-free media under the supplementation with growth factors such as epidermal growth factor and basic fibroblast growth factor. Tumorsphere cultivation is widely used to analyze the self-renewal capability of CSCs and to enrich these cells from bulk cancer cells. This method also provides a reliable platform for screening potential anti-CSC agents. The in vitro anti-proliferation activity of potential agents selected from tumorsphere assay is more translatable into in vivo anti-tumorigenic activity compared with general monolayer culture. Tumorsphere assay can also measure the outcome of clinical trials for potential anti-cancer agents. In addition, tumorsphere assay may be a promising strategy in the innovation of future cancer therapeutica and may help in the screening of anti-cancer small-molecule chemicals.

PI3K pathway inhibitors for the treatment of brain metastases with a focus on HER2+ breast cancer

The incidence of breast cancer brain metastases has increased in recent years, largely due to improved control of systemic disease with human epidermal growth factor receptor 2 (HER2)-targeted agents and the inability of most of these agents to efficiently cross the blood-blood barrier (BBB) and control central nervous system disease. There is, therefore, an urgent unmet need for treatments to prevent and treat HER2+ breast cancer brain metastases (BCBMs). Aberrant activation of the phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathway is frequently observed in many cancers, including primary breast tumors and BCBMs. Agents targeting key components of this pathway have demonstrated antitumor activity in diverse cancers, and may represent a new treatment strategy for BCBMs. In preclinical studies, several inhibitors of PI3K and mTOR have demonstrated an ability to penetrate the BBB and down-regulate PI3K signaling, indicating that these agents may be potential therapies for brain metastatic disease. The PI3K inhibitor buparlisib (BKM120) and the mTOR inhibitor everolimus (RAD001) are currently under evaluation in combination with trastuzumab in patients with HER2+ BCBMs.