**Key Principles in Glioblastoma Therapy**

Bartek Jiri Jr.1, Kimberly Ng2, Bartek Jiri Sr.3, Santosh Kesari4, Bob Carter5 and Clark C. Chen1,6 *1Department of Neurosurgery, Karolinska University Hospital, Stockholm 2Department of Radiation Oncology, Dana-Farber Cancer Institute, Boston, MA 3Institute of Cancer Biology and Centre for Genotoxic Stress Research Danish Cancer Society, Copenhagen 4Department of Neurology, Moores Cancer Center, UCSD, San Diego, CA 5Center for Theoretical and Applied Neurosurgery, UCSD, San Diego, CA 6Division of Neurosurgery, Beth Israel Deaconess Medical Center, Boston, MA 1Sweden 2,4,5,6USA* 

#### *3Denmark*

#### **1. Introduction**

52 Advances in the Biology, Imaging and Therapies for Glioblastoma

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Glioblastoma is the most common form of primary brain tumor. The incidence of this tumor is fairly low, with 2-3 cases per 100,000 people in Europe and North America 1. It is one of the most aggressive forms of cancer 2. Without treatment, the median survival is approximately 3 months 3. The current standard of treatment involves maximal surgical resection followed by concurrent radiation therapy and chemotherapy with the DNA alkylating agent, temozolomide 4. With this regimen, the median survival is approximately 14 months. For nearly all affected, the treatments available remain palliative.

The best available evidence suggests that glioblastomas originate from cells that give rise to glial cells5, 6. These glial derived tumors are graded by the World Health Organization (WHO) into 4 categories, termed WHO grade 1 to grade 4. The higher grade denotes histologic features of increased malignancy. WHO 4 glioma is essentially synonymous with glioblastoma7.

Studies carried out over the past three decades suggest that glioblastomas, like other cancers, arise secondary to the accumulation of genetic alterations. These alterations can take the form of epigenetic modifications, point mutations, translocations, amplifications or deletions and modify gene function in ways that deregulate cellular signaling pathways leading to the cancer phenotype 8. The exact number and nature of genetic alterations and deregulated signaling pathways required for tumorigenesis remains an issue of debate9, although it is now clear that CNS carcinogenesis requires multiple disruptions to the normal cellular circuitry. The genetic alteration results in either activation or inactivation of specific gene functions that contribute to the process of carcinogenesis 9. Genes, that when activated, contribute to the carcinogenesis are generally termed proto-oncogenes. The mutated forms of these genes are referred to as oncogenes. Genes, that when inactivated, contribute to the carcinogenesis are generally termed tumor suppressor genes.

Key Principles in Glioblastoma Therapy 55

The term "oncogene addiction" was initially coined by Dr. Bernard Weinstein to describe the phenomenon that some tumors exhibit exquisite dependence on a single oncogenic protein (or pathway) for sustaining growth and proliferation 14. Such dependence has been convincingly demonstrated in both tissue culture and transgenic mice systems for oncogenic versions of MYC 15-17 and RAS 18. Application of this concept to the clinical setting has achieved variable success in various cancer types, including chronic myelogeneous leukemia (CML) harboring the BCR-ABL translocation, Erb2 over-expression breast cancer, and Non-Small Cell Lung Cancer harboring selected EGFR mutations 19, 20. A simplistic application of this concept in glioblastoma would involve identification of the critical "addicted" oncogene followed by the inhibition of such oncogene(s). Unfortunately, the

To understanding this complexity, a careful analysis of the fundamental notion of oncogenic addiction is needed. In some ways, the observation that tumors exhibit dependence on a particular oncogenic pathway at some point in its history is not surprising. However, taken in the context of the plethora of dynamic genetic changes that accumulated during cancer progression 21, it is somewhat anti-intuitive to suspect that any particular pathway would play a prominent role in maintaining cell viability. Moreover, inactivation of the normal counterpart of the addicted oncogenic protein is often tolerated in normal tissue. These observations suggest that the genetic circuitry of the cancer cell have been extensively re-

The molecular nature of this re-programming remains poorly understood. Several hypotheses have been put forward. One hypothesis involves the notion of "genetic streamlining", where genetic instability in cancer cells is thought to mutationally or epigenetically inactivate certain signaling pathways that are operational in a normal cell but not required for growth in the cancer cell. In this "streamlined" state, the tumor cell becomes hyper-dependent on the oncogene driven processes 22. A more generalized form of this explanation involved the notion of synthetic lethality. Two genes are considered synthetically lethal if cells remain viable with inactivation of either gene. Simultaneous inactivation of both genes, on the other hand, results in cell death 23. It is thought that the cancer cells have accumulated mutations that are synthetically lethal with the absence of critical oncogenes. The main difference between this hypothesis and the "streamline" hypothesis is that the mutation in the former can result in a gain or loss of function, whereas the later specifically proposes a loss of function. A third hypothesis suggests that oncogenes reprogrammed the tumor cell by both pro-survival and pro-apoptotic signaling 22. With acute inactivation, the pro-survival signaling decayed faster than the pro-apoptotic

The main reason for revisiting the framework of oncogene addiction is that mechanism by which the cells can evolve to avoid such addiction. For instance, in the context of synthetic lethality, EGFR inhibition may be cytotoxic to glioblastoma cells only in the appropriate genetic context. Indeed, therapeutic effects of EGFR inhibition were observed only in patients with tumors harboring an oncogenic form of EGFR and an intact PTEN tumor suppressor gene 24. To complicate the matter, recent studies demonstrate that glioblastomas harbor activation of multiple oncogenic Receptor Tyrosine Kinases (RTKs), such that inactivation of any single oncogene merely diverts signaling through other active oncogenes 25. In these contexts, it is evident that meaningful therapy will require simultaneous

inhibition of multiple oncogenes or identification of the fitting genetic context.

**3. Concept 2: Oncogene addiction** 

actual biology of glioblastoma is far more complex.

programmed to result in this "addicted" state 14.

signaling, resulting in tumor death.

Despite some progress in the clinical management of glioblastoma, prognosis of patients suffering from this deadly tumor remains dismal, and design of new and more effective therapies for glioblastoma is highly desirable. Arguably the most promising route to discoveries of innovative treatment strategies is to obtain better mechanistic insights into glioblastoma pathogenesis and biology. Indeed, recent research in this area of experimental and clinical oncology has identified the key signaling pathways, critical regulatory nodes, genes and their protein products, as well as their mutual cross-talks, thereby providing a solid molecular basis for selection of candidate therapeutic targets and drug discovery programs. These lines of investigation complement the recent efforts to sequence entire genomes of a growing number of human tumors including glioblastoma, formulation of new concepts and principles in tumor cell biology, and potential exploitation of these major advances for personalized disease management in oncology. Collectively, such efforts have begun to provide exciting leads to conceptual framework that afford innovative therapeutic strategies. This review will aim to review these critical concepts and their relevance for glioblastoma therapeutic development.
