Clinical trials of adenoviruses in brain tumors: a review of Ad-p53 and oncolytic adenoviruses

Surgical Adjuncts for Glioblastoma

Although surgical resection of the solid tumor component of glioblastoma has been shown to provide a survival advantage, it will never be a curative procedure. Yet, systemically applied adjuvants (radiation therapy and chemotherapy) also are not curative and their options are limited by the inability of most agents to cross the blood-brain barrier. Direct delivery of adjuvant therapies during a surgical procedure potentially provides an approach to bypass the blood-brain barrier and effectively treat residual tumor cells. This article summarizes the approaches and therapeutics that have been evaluated to date, and challenges that remain to be overcome.

Targeted Brain Tumor Therapy by Inhibiting the MDM2 Oncogene: In Vitro and In Vivo Antitumor Activity and Mechanism of Action

There is a desperate need for novel and efficacious chemotherapeutic strategies for human brain cancers. There are abundant molecular alterations along the p53 and MDM2 pathways in human glioma, which play critical roles in drug resistance. The present study was designed to evaluate the in vitro and in vivo antitumor activity of a novel brain-penetrating small molecule MDM2 degrader, termed SP-141. In a panel of nine human glioblastoma and medulloblastoma cell lines, SP-141, as a single agent, potently killed the brain tumor-derived cell lines with IC₅₀ values ranging from 35.8 to 688.8 nM. Treatment with SP-141 resulted in diminished MDM2 and increased p53 and p21^cip1 levels, G2/M cell cycle arrest, and marked apoptosis. In intracranial xenograft models of U87MG glioblastoma (wt p53) and DAOY medulloblastoma (mutant p53) expressing luciferase, treatment with SP-141 caused a significant 4- to 9-fold decrease in tumor growth in the absence of discernible toxicity. Further, combination treatment with a low dose of SP-141 (IC₂₀) and temozolomide, a standard anti-glioma drug, led to synergistic cell killing (1.3- to 31-fold) in glioma cell lines, suggesting a novel means for overcoming temozolomide resistance. Considering that SP-141 can be taken up by the brain without the need for any special delivery, our results suggest that SP-141 should be further explored for the treatment of tumors of the central nervous system, regardless of the p53 status of the tumor.

Virtual Screening Based on Ensemble Docking Targeting Wild-Type p53 for Anticancer Drug Discovery

The tumor-suppressor function of p53 makes it an attractive drug target. Efforts were mostly put on stabilization of the functional p53 or reactivation of mutated p53. Previous studies have shown that small molecules targeting Loop1/Sheet3 (L1/S3) can reactivate the R175H-p53 and stabilize p53 in vitro. Since the L1/S3 pocket is shared by the mutate and the wild type (WT) p53, virtual screening is introduced to identify natural products targeting the L1/S3 of WT p53. Considering the high flexibility of Loop1, ensemble docking method is utilized for different clusters of the L1/S3. Seven conformations were chosen for docking. As one of the 181 selected candidates, torilin not only improved p53 activity, but also increased p21 protein expression level, which lies downstream of p53, therefore suppressing HCT116 cancer cell growth. Torilin may covalently bind to Cys124 of p53 by 2-methyl-2-butenal (2M2B) group, as torilin derivatives, which do not contain the 2M2B group, were not able to increase the p53 transcription activity. In conclusion, this study demonstrated that L1/S3 of WT-p53 is a druggable pocket, and torilin has a potential cytotoxicity through activating the p53 pathway.

Applications of nanotechnology in drug delivery to the central nervous system

In recent years, the researchers and drug designers have given growing attention to new nanotechnology strategies to improve drug delivery to the central nervous system (CNS). Nanotechnology has a great potential to affect the treatment of neurological disorders, mainly Alzheimer's disease, Parkinson's disease, brain tumors, and stroke. With regard to neurodegeneration, several studies showed that nanomaterials have been successfully used for the treatments of CNS disorders. In this regard, nanocarriers have facilitated the targeted delivery of chemotherapeutics resulting in the efficient inhibition of disease progression in malignant brain tumors. Therefore, the most efficacious application of nanomaterials is the use of these substances in the treatment of CNS disease that enhances the overall effect of drug and highlights the importance of nano-therapeutics. This study was conducted to review the evidence on the applications of nanotechnology in designing drug delivery systems with the ability to cross through the blood-brain barrier (BBB) in order to transfer the therapeutic agents to the CNS.

Biodegradable brain-penetrating DNA nanocomplexes and their use to treat malignant brain tumors

The discovery of powerful genetic targets has spurred clinical development of gene therapy approaches to treat patients with malignant brain tumors. However, lack of success in the clinic has been attributed to the inability of conventional gene vectors to achieve gene transfer throughout highly disseminated primary brain tumors. Here, we demonstrate ex vivo that small nanocomplexes composed of DNA condensed by a blend of biodegradable polymer, poly(β-amino ester) (PBAE), with PBAE conjugated with 5kDa polyethylene glycol (PEG) molecules (PBAE-PEG) rapidly penetrate healthy brain parenchyma and orthotopic brain tumor tissues in rats. Rapid diffusion of these DNA-loaded nanocomplexes observed in fresh tissues ex vivo demonstrated that they avoided adhesive trapping in the brain owing to their dense PEG coating, which was critical to achieving widespread transgene expression throughout orthotopic rat brain tumors in vivo following administration by convection enhanced delivery. Transgene expression with the PBAE/PBAE-PEG blended nanocomplexes (DNA-loaded brain-penetrating nanocomplexes, or DNA-BPN) was uniform throughout the tumor core compared to nanocomplexes composed of DNA with PBAE only (DNA-loaded conventional nanocomplexes, or DNA-CN), and transgene expression reached beyond the tumor edge, where infiltrative cancer cells are found, only for the DNA-BPN formulation. Finally, DNA-BPN loaded with anti-cancer plasmid DNA provided significantly enhanced survival compared to the same plasmid DNA loaded in DNA-CN in two aggressive orthotopic brain tumor models in rats. These findings underscore the importance of achieving widespread delivery of therapeutic nucleic acids within brain tumors and provide a promising new delivery platform for localized gene therapy in the brain.

Adenoviral Vectors Armed with Cell Fusion-Inducing Proteins as Anti-Cancer Agents

Cancer is a devastating disease that affects millions of patients every year, and causes an enormous economic burden on the health care system and emotional burden on affected families. The first line of defense against solid tumors is usually extraction of the tumor, when possible, by surgical methods. In cases where solid tumors can not be safely removed, chemotherapy is often the first line of treatment. As metastatic cancers often become vigorously resistant to treatments, the development of novel, more potent and selective anti-cancer strategies is of great importance. Adenovirus (Ad) is the most commonly used virus in cancer clinical trials, however, regardless of the nature of the Ad-based therapeutic, complete responses to treatment remain rare. A number of pre-clinical studies have shown that, for all vector systems, viral spread throughout the tumor mass can be a major limiting factor for complete tumor elimination. By expressing exogenous cell-fusion proteins, many groups have shown improved spread of Ad-based vectors. This review summarizes the research done to examine the potency of Ad vectors expressing fusogenic proteins as anti-cancer therapeutics.

Evolving lessons on nanomaterial-coated viral vectors for local and systemic gene therapy

Viral vectors are promising gene carriers for cancer therapy. However, virus-mediated gene therapies have demonstrated insufficient therapeutic efficacy in clinical trials due to rapid dissemination to nontarget tissues and to the immunogenicity of viral vectors, resulting in poor retention at the disease locus and induction of adverse inflammatory responses in patients. Further, the limited tropism of viral vectors prevents efficient gene delivery to target tissues. In this regard, modification of the viral surface with nanomaterials is a promising strategy to augment vector accumulation at the target tissue, circumvent the host immune response, and avoid nonspecific interactions with the reticuloendothelial system or serum complement. In the present review, we discuss various chemical modification strategies to enhance the therapeutic efficacy of viral vectors delivered either locally or systemically. We conclude by highlighting the salient features of various nanomaterial-coated viral vectors and their prospects and directions for future research.

A Lentiviral Vector Expressing Desired Gene Only in Transduced Cells: An Approach for Suicide Gene Therapy

Suicide gene therapy is a therapeutic strategy, in which cell suicide inducing transgenes are introduced into target cells. Inserting a toxin-encoding gene into a lentiviral vector leads to decreased efficiency of virus production due to lethal effect of toxin on packaging cells. In this study, we designed and constructed a transfer vector to express the toxin in transduced cells but not in packaging cells. Plasmid pLenti-F/GFP was constructed by cutting out R 5'LTR-R 3'LTR fragment with the AflII restriction endonuclease from a plasmid pLenti4-GW/H1/TO-laminshRNA, followed by ligating R 5'LTR-R 3'LTR fragment, constructed by three PCR stages. The promoter and GFP CDS were inserted in opposite strand. For lentiviral production, the HEK293T cell line was co-transfected with the PMD2G, psPAX2, and pLenti-F/GFP plasmids (envelope, packaging, and transfer plasmids).Viral vector titers were assayed. The HEK293T cell line was transduced with this virus. PCR was performed to confirm the presence of the promoter fragment between the R and U5 in 3'LTR. The lentivirus titers were approximately 2 × 10(5). The GFP expression was seen in 51 % of the HEK293T cells transduced with lentivirus. The PCR product size was 1440 bp confirming the promoter fragment position between the R and U5 in 3'LTR. The strategy enables us to use a broad spectrum of toxin genes in gene therapy and helps avoid the death of the packaging cells with lentiviral vectors carrying a toxin-encoding gene, thereby increasing the efficiency of viral production in packaging cells.

Novel delivery methods bypassing the blood-brain and blood-tumor barriers

Glioblastoma (GBM) is the most common primary brain tumor and carries a grave prognosis. Despite years of research investigating potentially new therapies for GBM, the median survival rate of individuals with this disease has remained fairly stagnant. Delivery of drugs to the tumor site is hampered by various barriers posed by the GBM pathological process and by the complex physiology of the blood-brain and blood-cerebrospinal fluid barriers. These anatomical and physiological barriers serve as a natural protection for the brain and preserve brain homeostasis, but they also have significantly limited the reach of intraparenchymal treatments in patients with GBM. In this article, the authors review the functional capabilities of the physical and physiological barriers that impede chemotherapy for GBM, with a specific focus on the pathological alterations of the blood-brain barrier (BBB) in this disease. They also provide an overview of current and future methods for circumventing these barriers in therapeutic interventions. Although ongoing research has yielded some potential options for future GBM therapies, delivery of chemotherapy medications across the BBB remains elusive and has limited the efficacy of these medications.

Gene therapy for malignant glioma

Glioblastoma multiforme (GBM) is the most frequent and devastating primary brain tumor in adults. Despite current treatment modalities, such as surgical resection followed by chemotherapy and radiotherapy, only modest improvements in median survival have been achieved. Frequent recurrence and invasiveness of GBM are likely due to the resistance of glioma stem cells to conventional treatments; therefore, novel alternative treatment strategies are desperately needed. Recent advancements in molecular biology and gene technology have provided attractive novel treatment possibilities for patients with GBM. Gene therapy is defined as a technology that aims to modify the genetic complement of cells to obtain therapeutic benefit. To date, gene therapy for the treatment of GBM has demonstrated anti-tumor efficacy in pre-clinical studies and promising safety profiles in clinical studies. However, while this approach is obviously promising, concerns still exist regarding issues associated with transduction efficiency, viral delivery, the pathologic response of the brain, and treatment efficacy. Tumor development and progression involve alterations in a wide spectrum of genes, therefore a variety of gene therapy approaches for GBM have been proposed. Improved viral vectors are being evaluated, and the potential use of gene therapy alone or in synergy with other treatments against GBM are being studied. In this review, we will discuss the most commonly studied gene therapy approaches for the treatment of GBM in preclinical and clinical studies including: prodrug/suicide gene therapy; oncolytic gene therapy; cytokine mediated gene therapy; and tumor suppressor gene therapy. In addition, we review the principles and mechanisms of current gene therapy strategies as well as advantages and disadvantages of each.

Molecular neuro-oncology and the development of targeted therapeutic strategies for brain tumors Part 4: p53 signaling pathway

Brain tumors are a diverse group of malignancies that remain refractory to conventional treatment approaches. Molecular neuro-oncology has now begun to clarify the transformed phenotype of brain tumors and identify oncogenic pathways that might be amenable to targeted therapy. Loss of the tumor suppressor gene p53 and its encoded protein are the most common genetic events in human cancer and are a frequent occurrence in brain tumors. p53 functions as a transcription factor and is responsible for the transactivation and repression of key genes involved in cell growth, apoptosis and the cell cycle. Mutation of the p53 gene or dysfunction of its signaling pathway are early events in the transformation process of astrocytic gliomas. The majority of mutations are missense and occur in the conserved regions of the gene, within exons 5 through 8. Molecular therapeutic strategies to normalize p53 signaling in cells with mutant p53 include pharmacologic rescue of mutant protein, gene therapy approaches, small-molecule agonists of downstream inhibitory genes, antisense approaches and oncolytic viruses. Other strategies include activation of normal p53 activity, inhibition of mdm2-mediated degradation of p53 and blockade of p53 nuclear export. Further development of targeted therapies designed to restore or enhance p53 function, and evaluation of these new agents in clinical trials, will be needed to improve survival and quality of life for patients with brain tumors.

[Modern molecular approaches to diagnosis and treatment of high-grade brain gliomas]

The review analyzes the current state of the problem of diagnosis and therapy of high-grade gliomas on the basis of the most promising present-day approaches. The diagnostic and treatment perspectives of the molecular genetic analysis of glioblastoma markers located on the tumor cell surface are considered. Gene therapy and the use of dendritic cells and oncolytic viruses are considered as the most interesting approaches to therapy of high-grade gliomas.

Therapeutic nanomedicine for brain cancer

Malignant brain cancer treatment is limited by a number of barriers, including the blood-brain barrier, transport within the brain interstitium, difficulties in delivering therapeutics specifically to tumor cells, the highly invasive quality of gliomas and drug resistance. As a result, the prognosis for patients with high-grade gliomas is poor and has improved little in recent years. Nanomedicine approaches have been developed in the laboratory, with some technologies being translated to the clinic, in order to address these needs. This review discusses the obstacles to effective treatment that are currently faced in the field, as well as various nanomedicine techniques that have been used or are being explored to overcome them, with a focus on liposomal and polymeric nanoparticles.

Novel delivery strategies for glioblastoma

Brain tumors--particularly glioblastoma multiforme--pose an important public health problem in the United States. Despite surgical and medical advances, the prognosis for patients with malignant gliomas remains grim: current therapy is insufficient with nearly universal recurrence. A major reason for this failure is the difficulty of delivering therapeutic agents to the brain: better delivery approaches are needed to improve treatment. In this article, we summarize recent progress in drug delivery to the brain, with an emphasis on convection-enhanced delivery of nanocarriers. We examine the potential of new delivery methods to permit novel drug- and gene-based therapies that target brain cancer stem cells and discuss the use of nanomaterials for imaging of tumors and drug delivery.

Negative regulation-resistant p53 variant enhances oncolytic adenoviral gene therapy

Intact p53 function is essential for responsiveness to cancer therapy. However, p53 activity is attenuated by the proto-oncoprotein Mdm2, the adenovirus protein E1B 55kD, and the p53 C-terminal domain. To confer resistance to Mdm2, E1B 55kD, and C-terminal negative regulation, we generated a p53 variant (p53VPΔ30) by deleting the N-terminal and C-terminal regions of wild-type p53 and inserting the transcriptional activation domain of herpes simplex virus VP16 protein. The oncolytic adenovirus vector Ad-mΔ19 expressing p53VPΔ30 (Ad-mΔ19/p53VPΔ30) showed greater cytotoxicity than Ad-mΔ19 expressing wild-type p53 or other p53 variants in human cancer cell lines. We found that Ad-mΔ19/p53VPΔ30 induced apoptosis through accumulation of p53VPΔ30, regardless of endogenous p53 and Mdm2 status. Moreover, Ad-mΔ19/p53VPΔ30 showed a greater antitumor effect and increased survival rates of mice with U343 brain cancer xenografts that expressed wild-type p53 and high Mdm2 levels. To our knowledge, this is the first study reporting a p53 variant modified at the N terminus and C terminus that shows resistance to degradation by Mdm2 and E1B 55kD, as well as negative regulation by the p53 C terminus, without decreased trans-activation activity. Taken together, these results indicate that Ad-mΔ19/p53VPΔ30 shows potential for improving p53-mediated cancer gene therapy.

Therapeutic strategies for head and neck cancer based on p53 status

Squamous cell carcinomas of the head and neck (HNSCC) are one of the most common types of cancers worldwide, and despite advances in treatment, they still represent a clinical challenge. Inactivation of one or more components in the p53 signaling pathway is an extremely common event in human neoplasia, including HNSCC. The loss of p53 function is responsible for increased aggressiveness in cancers, while tumor chemoresistance and radioresistance can depend on deleted p53 expression, or on the expression of mutated-p53 proteins. Thus, consideration and manipulation of the p53 status during HNSCC cancer therapy should be considered. This review discusses the p53 signaling pathways activated by various cellular stresses, including exposure to cancer therapies. The recognition of the p53 status in cancer cells is a significant factor and could provide valuable assistance during the selection of an effective therapeutic approach.

Gene therapy and targeted toxins for glioma

The most common primary brain tumor in adults is glioblastoma. These tumors are highly invasive and aggressive with a mean survival time of 15-18 months from diagnosis to death. Current treatment modalities are unable to significantly prolong survival in patients diagnosed with glioblastoma. As such, glioma is an attractive target for developing novel therapeutic approaches utilizing gene therapy. This review will examine the available preclinical models for glioma including xenographs, syngeneic and genetic models. Several promising therapeutic targets are currently being pursued in pre-clinical investigations. These targets will be reviewed by mechanism of action, i.e., conditional cytotoxic, targeted toxins, oncolytic viruses, tumor suppressors/oncogenes, and immune stimulatory approaches. Preclinical gene therapy paradigms aim to determine which strategies will provide rapid tumor regression and long-term protection from recurrence. While a wide range of potential targets are being investigated preclinically, only the most efficacious are further transitioned into clinical trial paradigms. Clinical trials reported to date are summarized including results from conditionally cytotoxic, targeted toxins, oncolytic viruses and oncogene targeting approaches. Clinical trial results have not been as robust as preclinical models predicted; this could be due to the limitations of the GBM models employed. Once this is addressed, and we develop effective gene therapies in models that better replicate the clinical scenario, gene therapy will provide a powerful approach to treat and manage brain tumors.

PED/PEA-15 modulates coxsackievirus-adenovirus receptor expression and adenoviral infectivity via ERK-mediated signals in glioma cells

Glioblastoma multiforme (GBM) is the most aggressive human brain tumor, and is highly resistant to chemo- and radiotherapy. Selectively replicating oncolytic viruses represent a novel approach for the treatment of neoplastic diseases. Coxsackievirus-adenovirus receptor (CAR) is the primary receptor for adenoviruses, and loss or reduction of CAR greatly decreases adenoviral entry. Understanding the mechanisms regulating CAR expression and localization will contribute to increase the efficacy of oncolytic adenoviruses. Two glioma cell lines (U343MG and U373MG) were infected with the oncolytic adenovirus dl922-947. U373MG cells were more susceptible to cell death after viral infection, compared with U343MG cells. The enhanced sensitivity was paralleled by increased adenoviral entry and CAR mRNA and protein levels in U373MG cells. In addition, U373MG cells displayed a decreased ERK1/2 (extracellular signal-regulated kinase-1/2) nuclear-to-cytosolic ratio, compared with U343MG cells. Intracellular content of PED/PEA-15, an ERK1/2-interacting protein, was also augmented in these cells. Both ERK2 overexpression and genetic silencing of PED/PEA-15 by antisense oligonucleotides increased ERK nuclear accumulation and reduced CAR expression and adenoviral entry. Our data indicate that dl922-947 could represent an useful tool for the treatment of GBM and that PED/PEA-15 modulates CAR expression and adenoviral entry, by sequestering ERK1/2.

Oncolytic-adenovirus-expressed RNA interference for cancer therapy

IMPORTANCE OF THE FIELD

RNA interference (RNAi) has generated considerable excitement for its potential cancer therapeutic applications. Because of the difficulties in delivering a large amount of siRNA to cancer cells and the short half-life of siRNA, it is important to choose an efficient delivery system for transduction of siRNA into target cells. Oncolytic adenovirus offers a better platform by virtue of its high transfection efficiency and selective replication in cancer cells.

AREAS COVERED IN THIS REVIEW

This review focuses on the synergism between oncolytic adenovirus and siRNA antitumor responses, and discusses recent progresses in oncolytic-adenovirus-expressed siRNA.

WHAT THE READER WILL GAIN

siRNA-expressing oncolytic adenovirus can generate a significantly enhanced antitumor effect through gene knockdown and viral oncolysis.

TAKE HOME MESSAGE

Due to its potency and target specificity, using siRNA delivery by oncolytic adenovirus has generated much excitement and will open new avenues for treatment of human cancer.

HtrA proteins as targets in therapy of cancer and other diseases

IMPORTANCE OF THE FIELD

The HtrA family proteins are serine proteases that are involved in important physiological processes, including maintenance of mitochondrial homeostasis, apoptosis and cell signaling. They are involved in the development and progression of several pathological processes such as cancer, neurodegenerative disorders and arthritic diseases.

AREAS COVERED IN THIS REVIEW

We present characteristics of the human HtrA1, HtrA2 and HtrA3 proteins, with the stress on their function in apoptosis and in the diseases. We describe regulation of the HtrAs' proteolytic activity, focusing on allosteric interactions of ligands/substrates with the PDZ domains, and make suggestions on how the HtrA proteolytic activity could be modified. Literature cited covers years 1996 - 2010.

WHAT THE READER WILL GAIN

An overview of the HtrAs' function/regulation and involvement in diseases (cancer, neurodegenerative disorders, arthritis), and ideas how modulation of their proteolytic activity could be used in therapies.

TAKE HOME MESSAGE

HtrA2 is the best target for cancer drug development. An increase in the HtrAs' proteolytic activity could be beneficial in cancer treatment, by stimulation of apoptosis, anoikis or necrosis of cancer cells, or by modulation of the TGF-beta signaling cascade; modulation of HtrA activity could be helpful in therapy of neurodegenerative diseases and arthritis.

An ex vivo loop system models the toxicity and efficacy of PEGylated and unmodified adenovirus serotype 5 in whole human blood

Polyethylene glycol coating (PEGylation) of adenovirus serotype 5 (Ad5) has been shown to effectively reduce immunogenicity and increase circulation time of intravenously administered virus in mouse models. Herein, we monitored clot formation, complement activation, cytokine release and blood cell association upon addition of uncoated or PEGylated Ad5 to human whole blood. We used a novel blood loop model where human blood from healthy donors was mixed with virus and incubated in heparin-coated PVC tubing while rotating at 37 degrees C for up to 8 h. Production of the complement components C3a and C5a and the cytokines IL-8, RANTES and MCP-1 was significantly lower with 20K-PEGylated Ad5 than with uncoated Ad5. PEGylation prevented clotting and reduced Ad5 binding to blood cells in blood with low ability to neutralize Ad5. The effect was particularly pronounced in monocytes, granulocytes, B-cells and T-cells, but could also be observed in erythrocytes and platelets. In conclusion, PEGylation of Ad5 can reduce the immune response mounted in human blood, although the protective effects are rather modest in contrast to published mouse data. Our findings underline the importance of developing reliable models and we propose the use of human whole blood models in pre-clinical screening of gene therapy vectors.

Chloroquine activates the p53 pathway and induces apoptosis in human glioma cells

Glioblastoma is the most common malignant brain tumor in adults. The currently available treatments offer only a palliative survival advantage and the need for effective treatments remains an urgent priority. Activation of the p53 growth suppression/apoptotic pathway is one of the promising strategies in targeting glioma cells. We show that the quinoline derivative chloroquine activates the p53 pathway and suppresses growth of glioma cells in vitro and in vivo in an orthotopic (U87MG) human glioblastoma mouse model. Induction of apoptosis is one of the mechanisms underlying the effects of chloroquine on suppressing glioma cell growth and viability. siRNA-mediated downregulation of p53 in wild-type but not mutant p53 glioblastoma cells substantially impaired chloroquine-induced apoptosis. In addition to its p53-activating effects, chloroquine may also inhibit glioma cell growth via p53-independent mechanisms. Our results clarify the mechanistic basis underlying the antineoplastic effect of chloroquine and reveal its therapeutic potential as an adjunct to glioma chemotherapy.

Stem cells as delivery vehicles for oncolytic adenoviral virotherapy

Glioblastoma multiforme is the most common primary intracranial tumor in humans. Despite continued advances in cancer therapy, the outcome for patients diagnosed with this disease remains bleak. Novel treatments involving the use of conditionally replicating adenoviruses (CRAds) to target malignant brain tumors have undergone extensive research and proven to be a promising mode of glioblastoma therapy. CRAds are genetically manipulated to replicate within tumor cells, exhibiting a high degree of infectivity, cytotoxicity, and transgene expression. While the use of various CRAds has been deemed safe for intracranial injection in preclinical trials, a significant therapeutic effect has yet to be seen in patients. This shortcoming stems from the distribution limitations involved with local delivery of virolytic agents. To enhance this modality of treatment, stem cells have been explored as cellular vehicles in virotherapy applications, given that they possess an intrinsic tropism for malignant brain tumors. Stem cell loaded CRAd delivery offers a more specific and effective method of targeting disseminated tumor cells and forms the basis for this review.

Virally mediated immunotherapy for brain tumors

Brain tumors are a leading cause of mortality and morbidity in the United States. Malignant brain tumors occur in approximately 80,000 adults. Furthermore, the average 5-year survival rate for malignant brain tumors across all ages and races is approximately 30% and has remained relatively static over the past few decades, showing the need for continued research and progress in brain tumor therapy. Improved techniques in molecular biology have expanded understanding of tumor genetics and permitted viral engineering and the anticancer therapeutic use of viruses as directly cytotoxic agents and as gene vectors. Preclinical models have shown promising antitumor effects, and generation of clinical grade vectors is feasible. In parallel to these developments, better understanding of antitumor immunity has been accompanied by progress in cancer immunotherapy, the goal of which is to stimulate host rejection of a growing tumor. This article reviews the intersection between the use of viral therapy and immunotherapy in the treatment of malignant gliomas. Each approach shows great promise on its own and in combined or integrated forms.

Adenoviral virotherapy for malignant brain tumors

Glioblastoma multiforme is the most common form of primary brain cancer. In the past decade, virotherapy of tumors has gained credence, particularly in glioma management, as these tumors are not completely resectable and tend to micro-metastasize. Adenoviral vectors have an advantage over other viral vectors in that they are relatively non-toxic and do not integrate in the genome. However, the lack of coxsackie and adenovirus receptors on surface of gliomas provides for inefficient transduction of wild-type adenoviral vectors in these tumors. By targeting receptors that are overexpressed in gliomas, modified adenoviral constructs have been shown to efficiently infect glioma cells. In addition, by taking advantage of tumor-specific promoter elements, oncolytic adenoviral vectors offer the promise of selective tumor-specific replication. This dual targeting strategy has enabled specificity in both laboratory and pre-clinical settings. This review examines current trends in adenoviral virotherapy of gliomas, with an emphasis on targeting modalities and future clinical applications.

Gene- and viral-based therapies for brain tumors

Advances in understanding and controlling genes and their expression have set the stage to alter genetic material to fight or prevent disease with brain tumors being among one of the first human malignancies to be targeted by gene therapy. All proteins are coded for by DNA and most neoplastic diseases ultimately result from the expression or lack thereof with one or more proteins (e.g., coded by oncogenes or tumor suppressor genes, respectively). In theory, therefore, diseases could be treated by expression of the appropriate protein in the affected cells. Gene therapy is an experimental treatment that involves introducing genetic material (DNA or RNA) into cells, and it has made important advances in the past decade. Within this short time span, it has moved from the conceptual laboratory research stage to clinical translational trials for brain tumors. The most efficient approaches for gene delivery are based on viral vectors, which have been proven relatively safe in the CNS, despite occasional cases of morbidity and death in non-neurosurgical trials. However, the human response to various viral vectors can not be predicted in a reliable manner from animal experimentation, nor can size, consistency, and extent of experimental brain tumors in mouse models reflect the large, necrotic, infiltrative nature of malignant gliomas. Furthermore, the problem of delivering genetic vectors into solid brain tumors and the efficiency in situ gene transfer remains one of the most significant hurdles in gene therapy.

RNA interference-mediated knockdown of p21(WAF1) enhances anti-tumor cell activity of oncolytic adenoviruses

The ability of oncolytic adenoviruses to replicate in and lyse cancer cells offers a potential therapeutic approach. However, selectivity and efficacy of adenovirus replication need to be improved. In this study, we present that loss of p21^WAF1 promotes adenovirus replication and more effective cell killing. To test our hypothesis, we took HCT116 colon cancer cell lines carrying deletions of either p21^WAF1 or p53, and infected these cell lines with wild-type adenovirus (WtD) or the oncolytic adenoviruses, ONYX-015 and Delta-24. We found that WtD, ONYX-015 and Delta-24 induced stronger cytopathic effects in HCT116 p21−/− cells compared with HCT116-WT cells. This was accompanied by increased virus production. siRNA-mediated knockdown of p21^WAF1, and similarly of p27^KIP1, in HCT116-WT cells also enhanced replication of and cell killing by these viruses. Furthermore, we found that TE7, an esophageal carcinoma cell line, also showed a strong cell-killing effect and virus production when p21^WAF1 expression was suppressed by RNA interference before adenoviruses infection. Also, H1299 and DU-145 cells transfected with p21^WAF1 siRNA showed higher virus production after ONYX-015 and Delta-24 infections. These observations suggest that p21^WAF1 plays a role in mediating replication of oncolytic viruses with potential implications for adenoviral therapy of cancer.

Treatment of diffuse intrinsic brainstem gliomas: failed approaches and future strategies

Diffuse intrinsic pontine gliomas constitute ~ 60-75% of tumors found within the pediatric brainstem. These malignant lesions present with rapidly progressive symptoms such as cranial nerve, long tract, or cerebellar dysfunctions. Magnetic resonance imaging is usually sufficient to establish the diagnosis and obviates the need for surgical biopsy in most cases. The prognosis of the disease is dismal, and the median survival is < 12 months. Resection is not a viable option. Standard therapy involves radiotherapy, which produces transient neurological improvement with a progression-free survival benefit, but provides no improvement in overall survival. Clinical trials have been conducted to assess the efficacy of chemotherapeutic and biological agents in the treatment of diffuse pontine gliomas. In this review, the authors discuss recent studies in which systemic therapy was administered prior to, concomitantly with, or after radiotherapy. For future perspective, the discussion includes a rationale for stereotactic biopsies as well as possible therapeutic options of local chemotherapy in these lesions.

Ligand-directed cancer gene therapy to angiogenic vasculature

Gene therapy strategies in cancer have remained an active area of preclinical and clinical research. One of the current limitations to successful trials is the relative transduction efficiency to produce a therapeutic effect. While intratumoral injections are the mainstay of many treatment regimens to date, this approach is hindered by hydrostatic pressures within the tumor and is not always applicable to all tumor subtypes. Vascular-targeting strategies introduce an alternative method to deliver vectors with higher local concentrations and minimization of systemic toxicity. Moreover, therapeutic targeting of angiogenic vasculature often leads to enhanced bystander effects, improving efficacy. While identification of functional and systemically accessible molecular targets is challenging, approaches, such as in vivo phage display and phage-based viral delivery vectors, provide a platform upon which vascular targeting of vectors may become a viable and translational approach.

Gene therapy for high-grade glioma: current approaches and future directions

The treatment of high-grade gliomas remains difficult despite recent advances in surgery, radiotherapy and chemotherapy. True advances may emerge from the increasing understanding in molecular biology and discovery of novel mechanisms for the delivery of tumoricidal agents. In an attempt to overcome this formidable neoplasm, molecular approaches using gene therapy have been investigated clinically since 1992. The clinical trials have mainly been classified into three approaches: suicide gene therapy, immune gene therapy and oncolytic viral therapy. In this article, we review these approaches, which have been studied in previous and ongoing clinical trials.

Syncytia formation affects the yield and cytotoxicity of an adenovirus expressing a fusogenic glycoprotein at a late stage of replication

Fusogenic membrane glycoproteins (FMGs) may enhance the cytotoxicity of conditionally replicative adenoviruses. However, expression at early stages of infection impairs virus replication. We have inserted the hyperfusogenic form of the gibbon ape leukemia virus (GALV) envelope glycoprotein as a new splice unit of the major late promoter (MLP) to generate a replication-competent adenovirus expressing this protein. At high multiplicity of infection (MOI), this virus replicated efficiently forming clumps of fused cells and showing a faster release. In contrast, at low MOI, infected cells formed syncytia where only one nucleus contained virus DNA, decreasing total virus production but increasing cytotoxicity.

Molecular strategies for the treatment of malignant glioma--genes, viruses, and vaccines

The standard treatment paradigm of surgery, radiation, and chemotherapy for malignant gliomas has only a modest effect on survival. It is well emphasized in the literature that despite aggressive multimodal therapy, most patients survive approximately 1 year after diagnosis, and less than 10% survive beyond 2 years. This dismal prognosis provides the impetus for ongoing investigations in search of improved therapeutics. Standard multimodal therapy has largely reached a plateau in terms of effectiveness, and there is now a growing body of literature on novel molecular approaches for the treatment of malignant gliomas. Gene therapy, oncolytic virotherapy, and immunotherapy are the major investigational approaches that have demonstrated promise in preclinical and early clinical studies. These new molecular technologies each have distinct advantages and limitations, and none has yet demonstrated a significant survival benefit in a phase II or III clinical trial. Molecular approaches may not lead to the discovery of a "magic bullet" for these aggressive tumors, but they may ultimately prove synergistic with more conventional approaches and lead to a broadening of the multimodal approach that is the current standard of care. This review will discuss the scientific background, therapeutic potential, and clinical limitations of these novel strategies with a focus on those that have made it to clinical trials.

Non-viral gene transfection technologies for genetic engineering of stem cells

The recent rapid progress of molecular biology together with the steady progress of genome projects has given us some essential and revolutionary information about DNA and RNA to elucidate various biological phenomena at a genetic level. Under these circumstances, the technology and methodology of gene transfection have become more and more important to enhance the efficacy of gene therapy for several diseases. In addition, gene transfection is a fundamental technology indispensable to the further research development of basic biology and medicine regarding stem cells. Stem cells genetically manipulated will enhance the therapeutic efficacy of cell transplantation. In this paper, the carrier and technology of gene delivery are briefly overviewed while the applications to the basic researches of biology and medicine as well as regenerative medical therapy are introduced. A new non-viral carrier and the cell culture system are described to efficiently manipulate stem cells.

GCV modulates the antitumoural efficacy of a replicative adenovirus expressing the Tat8-TK as a late gene in a pancreatic tumour model

Replication-competent adenoviruses carrying the herpes simplex thymidine kinase (TK) gene have shown contradictory evidence with regard to their antitumoural efficacy in combination with ganciclovir (GCV) treatment. We generated a replication-competent adenovirus carrying Tat8-TK, a modified form of the TK gene, under the control of the adenoviral major late promoter (AdRGDTat8-TK-L). Pancreatic cancer cell lines with different sensitivity to the TK/GCV system were infected with AdRGDTat8-TK-L, both in the presence and absence of GCV, and tested for treatment efficacy. We observed that, although the presence of GCV reduced viral replication in all infected cell lines, in three out of four GCV significantly enhanced the efficacy of the virotherapy. Interestingly, the cytotoxicity of the AdRGD-Tat8-TK-L/GCV was found more potent than that of a first generation AdTK/GCV system. In tumour xenografts from BxPC-3 and NP-18 pancreatic cells, both AdRGDTat8-TK-L and AdRGDTat8-TK-L/GCV treatment showed antitumoural activity. In BxPC-3 tumours scheduling of virus and prodrug was a key factor to determine the outcome of the therapy. Importantly, the addition of GCV enhanced the antitumoural effect of AdRGDTat8-TK-L only when applied in two rounds of virus+GCV. Interestingly, in spite of interfering with viral replication in vitro, GCV treatment of NP-18 tumours did not compromise the antitumoural efficacy of the AdRGDTat8-TK-L adenovirus. Thus, our results show that the combination therapy of a replicative adenovirus and the Tat8-TK/GCV suicide system can prove beneficial, when the appropriate regimen of virus and GCV is applied.

The status of gene therapy for brain tumors

The advent of gene therapy in the early 1990's raised expectations for brain tumor therapies; however, whereas clinical trials in patients with malignant gliomas provided evidence of safety, therapeutic benefit was not convincing. These early forays resembled the historical introductions of other therapies that seemed promising, only to fail in human trials. Nevertheless, re-study in the laboratory and retesting in iterative laboratory-clinic processes enabled therapies with strong biological rationales to ultimately show evidence of success in humans and become accepted. Examples, such as organ transplantation, monoclonal antibody therapy and antiangiogenic therapy, provide solace that a strategy's initial lack of success in humans provides an opportunity for its further refinement in the laboratory and development of solutions that will translate into patient success stories. The authors herein summarize results from clinical trials of gene therapy for malignant gliomas, and discuss the influence of these results on present thought in preclinical research.

Restoration of wild-type p53 function in human cancer: relevance for tumor therapy

BACKGROUND

In the majority of human cancers, the tumor suppressor activity of p53 is impaired because of mutational events or interactions with other proteins (ie, MDM2). The loss of p53 function is responsible for increased aggressiveness of cancers, while tumor chemoresistance and radioresistance are dependent upon the expression of mutant p53 proteins.

METHODS

Review of the literature indicates that p53 acts primarily as a transcription factor whose function is subject to a complex and diverse array of covalent post-translational modifications that markedly influence the expression of p53 target genes responsible for cellular responses such as growth arrest, senescence, or apoptosis. The ability of p53 to induce apoptosis in cancer cells is believed essential for cancer therapy.

RESULTS

Numerous data indicate that p53 dependent apoptosis is a relevant factor in determining the efficacy of anticancer treatments. Thus, the development of new strategies for restoration of p53 function in human tumors is considered an important issue. Two main approaches for restoration of p53 function have been pursued that impact anticancer treatments: (a) de novo expression of wild-type p53 (wt-p53) through gene therapy and (b) identification of small molecules reactivating wt-p53 function.

CONCLUSIONS

The extensive body of knowledge acquired has identified manipulations of p53 signaling as a relevant issue for successful therapies. In this context, the recognition of p53 status in cancer cells is significant and would help considerably in the selection of an appropriate therapeutic approach. p53 manipulations for cancer therapy have revealed the need for specificity of p53 activation and ability to spare body tissues. Furthermore, the promising results obtained by using molecules competent to reactivate wt-p53 functions in cancer cells provide the basis for the design of new molecules with lower side effects and higher anti-tumor efficiency. The reexpression and reactivation of p53 protein in human cancer cells would increase tumor susceptibility to radiation or chemotherapy enhancing the efficacy of standard therapeutic protocols.

Oncolytic viruses in cancer therapy

Oncolytic virotherapy is a promising form of gene therapy for cancer, employing nature’s own agents to find and destroy malignant cells. The purpose of this review is to provide an introduction to this very topical field of research and to point out some of the current observations, insights and ideas circulating in the literature. We have strived to acknowledge as many different oncolytic viruses as possible to give a broader picture of targeting cancer using viruses. Some of the newest additions to the panel of oncolytic viruses include the avian adenovirus, foamy virus, myxoma virus, yaba-like disease virus, echovirus type 1, bovine herpesvirus 4, Saimiri virus, feline panleukopenia virus, Sendai virus and the non-human coronaviruses. Although promising, virotherapy still faces many obstacles that need to be addressed, including the emergence of virus-resistant tumor cells.

Importance of p53 for cancer onset and therapy

Cancer predisposition, onset and therapeutic response can be critically determined by the integrity of the tumor suppressor p53. The majority of human cancers appear to exhibit either abnormal p53 or disrupted p53 activation pathways. Intervention to restore wild-type p53 activities is an attractive approach for cancer therapy. The manipulation of p53 and its targets is a challenging field that is still in its infancy, but witnessing some notable developments in the areas of p53 gene therapy, mutant reactivation and suppression of the negative p53 regulator Mdm2 using small molecules. In addition, wild-type p53 manipulation in healthy tissues of cancer patients in the context of chemotherapy and radiation therapies is offering the potential of enhanced patient recovery.

Suicide cancer gene therapy using pore-forming toxin, streptolysin O

We cloned the streptolysin O gene from the Streptococcus pyogenes genome and tested the possibility of using it as an anticancer reagent. Transient transfection of the streptolysin O gene efficiently killed 293T cells after 12 hours of transfection as determined by lactate dehydrogenase release and propidium iodide uptake. No caspase activity was observed and necrosis was prominent during streptolysin O-induced cell death. Biochemical analysis of streptolysin O protein revealed that the deletion of only 5 amino acids from the COOH-terminal region of streptolysin O, which is essential for cholesterol binding activity, abolished its cell-killing activity, whereas the NH2-terminal region was more resilient, i.e., up to 115 amino acids could be deleted without changing its cell-killing activity. We generated a streptolysin O-expressing adenovirus and injected it into human cervical cancer cell-derived tumors grown in a nude mouse model. Twenty-one days postinjection, the average size of tumors in the streptolysin O adenovirus-injected group was 29.3% of that of the control PBS-treated group. Our results show that the genes of pore-forming toxins, like streptolysin O protein, have the potential to establish a novel class of suicide gene therapeutic reagents.

Silencing of stathmin induces tumor-suppressor function in breast cancer cell lines harboring mutant p53

Cancers harboring dominant-negative p53 mutations are often aggressive and difficult to treat. Direct attempts to restore wild-type p53 function have produced little clinical benefit. We investigated whether targeting a p53-target gene could induce certain tumor-suppressor characteristics. We found that inhibition of stathmin, a microtubule regulator that can be transcriptionally repressed by wild-type p53, restored certain wild-type functions to cancer cells with mutant p53. Silencing of stathmin by small interfering RNA (siRNA) in mutant p53 cell lines lowered expression to that observed following activation of wild-type p53 by DNA damage in wild-type p53 cell lines. siRNA-induced repression of stathmin decreased cell proliferation, viability and clonogenicity in mutant p53 cell lines. Furthermore, knockdown of stathmin partially restored cell-cycle regulation and activation of apoptosis. Therefore, targeting stathmin, a gene product that is overexpressed in the presence of mutant p53, may represent a novel approach to treating cancers with aberrant p53 function.

Gene therapy and targeted toxins for glioma

The most common primary brain tumor in adults is glioblastoma. These tumors are highly invasive and aggressive with a mean survival time of nine to twelve months from diagnosis to death. Current treatment modalities are unable to significantly prolong survival in patients diagnosed with glioblastoma. As such, glioma is an attractive target for developing novel therapeutic approaches utilizing gene therapy. This review will examine the available preclinical models for glioma including xenographs, syngeneic and genetic models. Several promising therapeutic targets are currently being pursued in pre-clinical investigations. These targets will be reviewed by mechanism of action, i.e., conditional cytotoxic, targeted toxins, oncolytic viruses, tumor suppressors/oncogenes, and immune stimulatory approaches. Preclinical gene therapy paradigms aim to determine which strategies will provide rapid tumor regression and long-term protection from recurrence. While a wide range of potential targets are being investigated preclinically, only the most efficacious are further transitioned into clinical trial paradigms. Clinical trials reported to date are summarized including results from conditionally cytotoxic, targeted toxins, oncolytic viruses and oncogene targeting approaches. Clinical trial results have not been as robust as preclinical models predicted, this could be due to the limitations of the GBM models employed. Once this is addressed, and we develop effective gene therapies in models that better replicate the clinical scenario, gene therapy will provide a powerful approach to treat and manage brain tumors.