Gene therapy for malignant gliomas

Cell therapies for glioblastoma

Malignant gliomas, including the most devastating type, glioblastoma multiforme (GBM), are characterised by their local growth and aggressive infiltration of the normal brain. GBMs result in a profound disability, leading to death in almost all cases. There has been little improvement in outcome despite intensive clinical and laboratory research during recent decades. Interestingly, many researchers have been successful in treating GBM models in animals, but the success has been limited when new treatment principles have been translated into the clinic. One reason for this failure is the lack of appropriate animal models that reflect the behaviour of human GBMs. Therapeutic progress has also been hindered by the limited delivery of effective therapeutic compounds to an extremely heterogenic tumour cell population. This article discusses the present use and limitations of preclinical animal models to study glioma growth and progression. In addition, it focuses on the potential use of cell-based therapies for the treatment of GBMs. This includes aspects of gene therapy, stem cell therapy and immunotherapy. Several of these treatment modalities use the principle of transplanting cells or compounds that either directly or indirectly show therapeutic efficacy. Many of these principles depend on an increased biological knowledge of gliomas. The development of new therapeutic principles based on such knowledge may finally provide glioma patients with an improved survival.

Gene Therapy for Malignant Pleural Mesothelioma

Gene therapy for mesothelioma is currently in its adolescence. The expansion of knowledge regarding molecular aspects of mesothelioma carcinogenesis has facilitated the development of promising gene therapy modalities that target specific oncoproteins and mutant tumor suppressor genes. Although implementation of any of these gene therapy approaches as part of standard medical care for patients who have mesothelioma remains years in the future, the field is finally progressing toward more definitive phase II/III efficacy studies. Unfortunately, the marginal benefits garnered from standard anticancer treatments in mesothelioma argue strongly for continued participation in clinical studies of various experimental approaches, particularly gene therapy. These trials serve multiple purposes: to establish safety, determine proper dosing, evaluate for efficacy, and, in an iterative fashion, guide future avenues of laboratory investigation.

Cancer gene therapy using a novel secretable trimeric TRAIL

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), a member of the TNF family, is a type II transmembrane cytokine molecule. Soluble TRAIL has been shown to induce apoptosis in a wide variety of cancer cells in vitro and to suppress tumor growth specifically without damaging normal cells and tissues in vivo. In our previous report, we have demonstrated that an artificial gene encoding the polypeptide composed of the three functional elements (a secretion signal, a trimerization domain and an apoptosis-inducing moiety of TRAIL gene sequence) expresses and secretes highly apoptotic trimeric TRAIL into the culture supernatant. Here, as an approach to TRAIL-based cancer gene therapy, we developed an adenoviral vector delivering the gene that encodes our secretable trimeric TRAIL (stTRAIL). This adenovirus (Ad-stTRAIL) potently induced apoptosis in vitro in cancer cell lines such as HeLa, MDA-MB-231, A549, HCT116 and U-87MG. In an animal xenograft tumor model bearing a human glioma cell line U-87MG, intratumoral delivery of Ad-stTRAIL dramatically suppressed tumor growth without showing detectable adverse side effects. Histological analysis revealed that Ad-stTRAIL suppresses tumor growth by inducing apoptotic cell death. Contrary to the known rapid clearance of systemically delivered TRAIL protein from the blood circulation, stTRAIL expressed by Ad-stTRAIL in tumor tissues persisted for more than 4 days. In a comparison of tumor suppressor activity between Ad-stTRAIL and Ad-flTRAIL (delivering the full-length TRAIL gene) after mixing infected cells with uninfected cells and implanting these mixed cells in nude mice, Ad-stTRAIL showed higher tumor suppressor activity than that of Ad-flTRAIL. Our data reveal that a gene therapy using Ad-stTRAIL has a promising potential to treat human cancers including gliomas.

Oncolytic viral therapy and immunotherapy of malignant brain tumors: two potential new approaches of translational research

Brain tumors arise at a rate of nearly 5/100,000 in the general population, with over 17,000 U.S. residents being diagnosed each year. Approximately 60% of all brain tumors are gliomas, which are derived from interstitial tissue of the brain, such as astrocytic or ependymal tissue, or oligodendrocytes. The traditional protocols for treatment of malignant gliomas include diagnostic surgery, followed by regimens of radio- and chemotherapies. In the case of chemotherapy, the treatment protocols have remained nearly unchanged for over 30 years despite high mortality rates, and with little to no improvement in outcome. New advances in the fields of molecular biology and immunology have resulted in new possibilities for treating malignant gliomas by targeting cellular and molecular mechanisms of tumor cells, and stand in contrast to traditional forms of treatment. In the field of gene therapy, the possibility of using oncolytic viruses, such as HSV-1, for glioma therapy--specifically, of high grade astrocytomas--is being explored, and trials have begun using a replication-selective mutant strain known as G207. An increased understanding of the role of the cytokine TGF-beta2 has led to developments of anti-sense immunotherapy targeting this factor. The two examples mentioned here are discussed in this review and cited as possible improvements in the treatment of high grade astrocytomas.

Locoregional radioimmunotherapy in selected patients with malignant glioma: experiences, side effects and survival times

Prognosis of malignant glioma is very unfavourable mainly due to minimal tumour remnants in the peritumoural tissue. Intralesionally applied radioimmunotherapy is a possible therapeutical option with the potential to improve survival of patients with malignant glioma. We investigated side effects and survival after surgery, conventional radiotherapy and additional radioimmunotherapy with labelled tenascin-antibodies in patients with malignant glioma.

METHODS

Since 1995, 37 patients were treated with radioimmunotherapy after resection and radiotherapy of a malignant glioma. Patients received antibodies labelled with yttrium-90 and iodine-131 in different doses into the tumour cavity via a previously implanted ommaya-reservoir. Treatment was applied in up to 8 cycles (mean 2.96 cycles) in time intervals of 6-8 weeks. Mean age was 46 years, histology was anaplastic astrocytoma in 13 patients and glioblastoma in 24 patients.

RESULTS

For the whole group median survival time has not yet been reached. For glioblastoma the median survival time is 17 months, 5-year survival probability for anaplastic astrocytoma is 85% approximately. Quality of life was acceptable. Acute side effects following treatment were headache, seizures and worsening of pre-existing neurological symptoms. Late side effects were skin necrosis and, in 1 case, a delayed aphasia probably due to a vascular lesion.

CONCLUSION

Radioimmunotherapy prolonged survival time in a selected group of patients with malignant gliomas as compared to a historical control group. Patients with anaplastic astrocytomas seem to have more benefit from this therapy than patients with glioblastomas.

Melanoma differentiation associated gene-7, mda-7/IL-24, selectively induces growth suppression, apoptosis and radiosensitization in malignant gliomas in a p53-independent manner

Malignant gliomas are extremely aggressive cancers currently lacking effective treatment modalities. Gene therapy represents a promising approach for this disease. A requisite component for improving gene-based therapies of brain cancer includes tumor suppressor genes that exhibit cancer constrained inhibitory activity. Subtraction hybridization identified melanoma differentiation associated gene-7 (mda-7) as a gene associated with melanoma cell growth, differentiation and progression. Ectopic expression of mda-7 by means of a replication-incompetent adenovirus (Ad), Ad.mda-7, induces growth suppression and apoptosis selectively in diverse human cancers, without producing any apparent harmful effect in normal cells. We presently demonstrate that Ad.mda-7 induces growth inhibition and apoptosis in malignant human gliomas expressing both mutant and wild-type p53, and these effects correlate with an elevation in expression of members of the growth arrest and DNA damage (GADD) gene family. In contrast, infection with a recombinant Ad expressing wild-type p53, Ad.wtp53, specifically affects mutant p53 expressing gliomas. When tested in early passage normal and immortal human fetal astrocytes, growth inhibition resulting from infection with Ad.mda-7 or Ad.wtp53 is significantly less than in malignant gliomas and no toxicity is evident in these normal cells. Moreover, infection of gliomas with Ad.mda-7 or treatment with purified GST-MDA-7 protein sensitizes both wild-type and mutant p53 expressing tumor cells to the growth inhibitory and antisurvival effects of ionizing radiation, and this response correlates with increased expression of specific members of the GADD gene family. Since heterogeneity in p53 expression is common in evolving gliomas, the present findings suggest that Ad.mda-7 may, in many instances, prove more beneficial for the gene-based therapy of malignant gliomas than administration of wild-type p53.

Viruses in the treatment of brain tumors

The grave outlook for malignant glioma patients in spite of improvements to current modalities has ushered in new approaches to therapy. Viruses have emerged on the scene and gained attention for their ability to play essentially two roles: first, as vectors for therapeutic gene delivery and second, as engineered infectious agents capable of selectively lysing tumor cells. To date, clinical brain tumor trials using viruses for gene delivery have employed retroviral or adenoviral vectors to introduce ganciclovir susceptibility to tumors in the form of the HSV1-TK gene. Clinical oncolytic studies, on the other hand, have evaluated a conditionally replicating HSV as an antineoplastic agent. Despite some promise afforded by these trials, further studies are warranted; the investigation of additional viruses to play these roles is inevitable and is now precedented.

Gene therapy for brain tumors

"Gene therapy" can be defined as the transfer of genetic material into a patient's cells for therapeutic purposes. To date, a diverse and creative assortment of treatment strategies utilizing gene therapy have been devised, including gene transfer for modulating the immune system, enzyme prodrug ("suicide gene") therapy, oncolytic therapy, replacement/therapeutic gene transfer, and antisense therapy. For malignant glioma, gene-directed prodrug therapy using the herpes simplex virus thymidine kinase gene was the first gene therapy attempted clinically. A variety of different strategies have now been pursued experimentally and in clinical trials. Although, to date, gene therapy for brain tumors has been found to be reasonably safe, concerns still exist regarding issues related to viral delivery, transduction efficiency, potential pathologic response of the brain, and treatment efficacy. Improved viral vectors are being sought, and potential use of gene therapy in combination with other treatments is being investigated.

Gene therapy for brain tumors: the fundamentals

BACKGROUND

Over the past two decades, significant advances have been made in the fields of virology and molecular biology, and in understanding the genetic alterations present in brain tumors. The knowledge gained has been exploited for use in gene therapy.

OBJECTIVE

The purpose of this article is to present an introduction to the field of brain tumor gene therapy for the practicing clinician.

RESULTS

A variety of gene therapy strategies have now been used in the laboratory and in clinical trials for brain tumors. They can be divided into five categories: 1) gene-directed enzyme prodrug ("suicide gene") therapy (GDEPT); 2) gene therapy designed to boost the activity of the immune system against cancer cells; 3) oncolytic virus therapy; 4) transfer of potentially therapeutic genes--such as tumor suppressor genes--into cancer cells; and 5) antisense therapy. GDEPT is the strategy that has been most extensively studied.

CONCLUSIONS

To date, gene therapy has been found to be reasonably safe and concerns related to adverse events such as insertional mutagenesis have not been realized. Although patients have not been cured, the development of this therapy could still be considered to be at an early stage. Current research is addressing factors that could be limiting the successful clinical application of gene therapy, which remains an intriguing experimental option for patients with malignant brain tumors.

Exploitation of immune mechanisms in the treatment of central nervous system cancer

Malignant gliomas are among the most common intrinsic brain tumors of both children and adults, and, because of unique aspects of their biology and anatomic site, they are the most refractory to conventional therapeutic strategies involving surgery, radiotherapy, or chemotherapy. Given the failure of standard therapies to improve the outlook of affected patients, significant attention has been focused on development of alternative treatments, particularly immunotherapy. Attempts have been made to treat gliomas using a variety of immunologically based strategies, including passive immunization, adoptive cellular immunotherapy, local and systemic delivery of biological response modifiers, and vaccination with tumor cells. Although preclinical modeling of these therapies provided an impetus for translation of their results into clinical protocols, these therapies have failed to yield consistently promising results in initial trials. However, significant insights into the immunobiology of the central nervous system (CNS) and gliomas have been gained from these studies, and have established that a number of immunobiological features of the brain and of gliomas themselves may be critical determinants in regulating efficacious treatment of these tumors. These include the following: (1) the presence of a blood-brain barrier that, although partially disrupted by the tumor, functions to exclude elements of the immune system from the tumor or brain parenchyma; (2) a lack of organized secondary lymphatic tissues supporting efficient immune responses locally in the CNS; (3) low levels of expression of major histocompatibility complex proteins in the CNS; (4) an apparent paucity of the most efficient antigen-presenting cells; and (5) glioma-derived immunosuppressive factors, such as transforming growth factor-beta, that interfere with the induction of local as well as systemic immune responses to the tumor. Recognition of these factors, and an appreciation of the underlying need for and validity of developing immunologically based therapies for gliomas, supports continued development of novel immunotherapeutic approaches, particularly those attempting to enhance the immunogenicity of glioma cells. This review addresses the current state of knowledge regarding the immunobiology of gliomas, recent developments in immunotherapy of gliomas, and promising future directions for development and implementation of cellular immunotherapy of gliomas.

Gene transfer technologies for malignant gliomas

Although early clinical gene therapy trials for recurrent central nervous system neoplasms showed the proof-of-principle, they did not fulfill the high expectations suggested by the preclinical experimental data. Insufficient distribution of vectors in human brain tumors and very low transduction efficiency require that we reevaluate gene transfer concepts for brain tumor treatment. Major steps to improve gene transfer into the central nervous system and the efficacy of gene therapy for malignant brain tumors include: 1) the design of more effective vector systems; 2) the development of new or improved prodrug/suicide systems, gene replacement approaches, or strategies targeting the immune response or tumor angiogenesis; 3) the study of new techniques to enhance delivery of genetic vectors into brain tumors and for monitoring gene delivery into tumors; and 4) assessment of the role of gene therapy as part of a combined treatment approach.