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Gliomas are the most common and aggressive brain tumors and despite advances in their therapeutic management they remain fatal. Radically, different therapeutic approaches are needed to treat this aggressive disease. Oncolytic viruses that intrinsically kill cancer cells or that have been genetically engineered to specifically destroy this population are becoming a viable therapeutic options for gliomas. A number of preclinical studies have demonstrated the efficacy of oncolytic viruses as a therapeutic strategy for gliomas. Those studies have led to a number of clinical trials using oncolytic virus for glioma treatment. In this chapter, we review the different viruses (adenovirus, herpes virus, parvovirus, poliovirus, reovirus, newcastle disease virus, retrovirus, and measles viruses) that have been evaluated preclinically and clinically for the treatment of glioma.

Oncolytic viruses are emerging as a promising alternative for cancer therapy. Among these biologic agents, adenoviruses have been extensively tested as vectors to transfer exogenous genes to cancer cells. Sophisticated modifications of the adenovirus genome to improve infectivity and selectivity have made possible the production of replicating adenoviruses intended to directly destroy tumors by cell lysis. In this chapter, we discuss the origins of virotherapy and early trials with wild-type adenoviruses and other viruses, and the concept underlying the development of replication-deficient adenoviral vectors, with special emphasis on Ad-p53. We then review clinical findings for dl1520 and H101 oncolytic adenoviruses and, finally, the concepts of Delta-24, Delta-24-RGD, and ICOVIR, emphasizing clinical applications. A dramatic increment in clinical trials over the next 5 years will lead to the approval of oncolytic viruses for conventional cancer treatment within the next decade.

Systematic analyses of the expression of angiogenic regulators in cancer models should yield useful information for the development of novel therapies for malignant gliomas. In this study, we analyzed tumor growth, vascularization, and angiopoietin-2 (Ang2) expression during the development of U-87 MG xenografts. We found that tumoral angiogenesis in this model follows a multistage process characterized by avascular, prolific peripheral angiogenesis, and late vascular phases. On day 4, we observed an area of central necrosis, a peripheral ring of Ang2-positive glioma cells, and reactive Ang2-positive vascular structures in the tumor/brain interface. When the tumor had developed a vascular network, Ang2 was expressed only in peripheral vascular structures. Because Ang2 expression was downmodulated in the late stages of development, probably to maintain a stable tumoral vasculature, we next studied whether sustained Ang2 expression might impair vascular development and, ultimately, tumor growth. Ang2 prevented the formation of capillary-like structures and impaired angiogenesis in a chorioallantoic membrane chicken model. Finally, we tested the effect of sustained Ang2 expression on U-87 MG xenograff development. Ang2 significantly prolonged the survival of intracranial U-87 MG tumor-bearing animals. Examination of Ang2treated xenograffs revealed areas of tumor necrosis and vascular damage. We therefore conclude that deregulated Ang2 expression during gliomagenesis hindered successful angiogenesis and that therapies that sustain Ang2 expression might be effective against malignant gliomas.

Replication-competent oncolytic adenoviruses hold considerable promise for treating malignant gliomas. The toxicity of the clinically tested Ell B-55 kDa mutant virus is negligible; however, its full clinical potential is still being evaluated. The purpose of the present study is to compare the antiglioma activity in vitro and in vivo between Delta-24, an Ei A mutant adenovirus, and RA55, an Ell B-55 kDa mutant adenovirus. We selected human glioma cell lines that were tumorigenic in nude mice and express wild-type p53 (U-87 MG, D54 MG) or mutant p53 (U-251 MG, U-373 MG) protein. Our studies demonstrated that Delta-24 induced a more potent antiglioma effect in vitro than RA55. Moreover, Delta-24 replicated markedly more efficiently than RA55 in both wild-type and mutant-p53 scenarios. Importantly, direct intratumoral injection of Delta-24, but not RA55, significantly suppresses tumor growth in intracranial (U-87 MG, U-251 MG) or subcutaneous (D54 MG) animal models. Staining for hexon protein detected replicating adenoviruses in xenografts infected with Delta-24, but not with RA55. Collectively, these data indicate that E1A mutant adenoviruses targeting the Rb pathway are more powerful putative agents for antiglioma therapy than E113 mutant adenoviruses, and suggest that E1A mutant adenoviruses should be tested in the clinical setting for patients with malignant gliomas.

Lessons from the history of cancer therapy inform that a multimodal approach offers the best potential to eradicate the tumor mass and prevent the recurrence after therapy. One of the most promising experimental therapies for cancer is the use of oncolytic adenoviruses as therapeutic agents. These biologic agents exert their function by directly infecting and killing tumor cells. The new progeny generated after the first infection round will spread, generating a therapeutic wave that will optimally eliminate every cancer cell. Recently, modifications in the virus genome have allowed for the designing of viruses that infect with more potency than wild-type adenoviruses, replicate exclusively in tumor cells, selectively target cellular receptors or molecular defects in the cancer cells, and deliver prodrug genes. In addition, these last-generation viruses can be combined with chemotherapy and other forms of cancer therapies to enhance the tumor killing capability. With the steady and fast pace of the progress in our knowledge of the genetics of tumors, we may soon fulfill our hope of reaching the final objective of an adenovirus-based personalized medicine for cancer.

During the last 10 years, gene therapy for brain tumors has known peaks and valleys. The first attempts to induce therapeutic effect, using retrovirus to transduce the HSV-TK gene and adenovirus to transfer wild-type p53 cDNA, failed significantly to improve the survival of the patients. In both cases, the failure was attributed to vector deficiencies, also termed the ‘vector gap’. To address the problem of delivery, investigators have moved from replication-deficient vectors to replication-competent, tumor-selective viruses. These viruses are currently being tested in the clinical setting. In this review, we discuss the progress made with herpes simplex viruses (G207), reoviruses that naturally target Ras pathways, adenoviruses targeted to the p53 pathway (ONYX-015), and third-generation adenovirus targeted to the Rb pathway (Delta-24-RGD). Because cancer frequently requires multiple therapies for curative effect, we also comment on the combination of oncolytic viruses with conventional therapies. Specifically, we mention the combined used of oncolytic viruses and chemotherapy as well as the potential manipulation of the DNA repair machinery to enhance virus replication, virus potency, and antiglioma effect. In the future, oncolytic viruses will be administered to patients in combination with other biological agents such as antibodies and cancer vaccines.

Glioma cells are more likely to respond to therapy through autophagy than through apoptosis. The most efficacious cytotoxic drugs employed in glioma therapy, such as temozolomide and rapamycin, induce autophagy. Oncolytic adenoviruses, which will soon be tested in patients with gliomas at the University of Texas M. D. Anderson Cancer Center, also induce autophagy. Autophagy in gliomas thus represents a promising mechanism that may lead to new glioma therapies. In this chapter, we present the methods for studying autophagy in glioma cells, including assessment of in vitro cellular markers acidic vesicle organelles, and green fluorescent protein (GFP)-LC3 punctation; biochemical markers LC3-I/II conversion, p62 degradation, Atg12–Atg5 accumulation, and p70S6K dephosphorylation; and ultrastucture of the autophagosomes. In addition, we will address how LC3B and Atg5 up-regulation during autophagy can be examined through immunostaining in treated tumors and the potential of these proteins for use as surrogate markers to monitor therapeutic effects in clinical trials. Finally, we will discuss the challenges of studying autophagy in gliomas and the future directions of such use.