Gene therapy for brain tumors: Basic developments and clinical implementation

Glioblastoma multiforme (GBM) is the most common and deadliest of adult primary brain tumors. Due to its invasive nature and sensitive location, complete resection remains virtually impossible. The resistance of GBM against chemotherapy and radiotherapy necessitate the development of novel therapies. Gene therapy is proposed for the treatment of brain tumors and has demonstrated pre-clinical efficacy in animal models. Here we review the various experimental therapies that have been developed for GBM including both cytotoxic and immune stimulatory approaches. We also review the combined conditional cytotoxic immune stimulatory therapy that our lab has developed which is dependent on the adenovirus mediated expression of the conditional cytotoxic gene, Herpes Simplex Type 1 Thymidine Kinase (TK) and the powerful DC growth factor Fms-like tyrosine kinase 3 ligand (Flt3L). Combined delivery of these vectors elicits tumor cell death and an anti-tumor adaptive immune response that requires TLR2 activation. The implications of our studies indicate that the combined cytotoxic and immunotherapeutic strategies are effective strategies to combat deadly brain tumors and warrant their implementation in human Phase I clinical trials for GBM.     

Potential of gene therapy for the treatment of pituitary tumors

Pituitary adenomas constitute the most frequent neuroendocrine pathology, comprising up to 15% of primary intracranial tumors. Current therapies for pituitary tumors include surgery and radiotherapy, as well as pharmacological approaches for some types. Although all of these approaches have shown a significant degree of success, they are not devoid of unwanted side effects, and in most cases do not offer a permanent cure. Gene therapy-the transfer of genetic material for therapeutic purposes-has undergone an explosive development in the last few years. Within this context, the development of gene therapy approaches for the treatment of pituitary tumors emerges as a promising area of research. We begin by presenting a brief account of the genesis of prolactinomas, with particular emphasis on how estradiol induces prolactinomas in animals. In so doing, we discuss the role of each of the recently discovered growth inhibitory and growth stimulatory substances and their interactions in estrogen action. We also evaluate the cell-cell communication that may govern these growth factor interactions and subsequently promote the growth and survival of prolactinomas. Current research efforts to implement gene therapy in pituitary tumors include the treatment of experimental prolactinomas or somatomammotropic tumors with adenoviral vector-mediated transfer of the suicide gene for the herpes simplex type 1 (HSV1) thymidine kinase, which converts the prodrug ganciclovir into a toxic metabolite. In some cases, the suicide transgene has been placed under the control of pituitary cell-type specific promoters, like the human prolactin or human growth hormone promoters. Also, regulatable adenoviral vector systems are being assessed in gene therapy approaches for experimental pituitary tumors. In a different type of approach, an adenoviral vector, encoding the human retinoblastoma suppressor oncogene, has been successfully used to rescue the phenotype of spontaneous pituitary tumors of the pars intermedia in mice. We close the article by discussing the future of molecular therapies. We point out that although, gene therapy represents a key step in the development of molecular medicine, it has inherent limitations. As a consequence, it is our view that at some point, genetic therapies will have to move from exogenous gene transfer (i.e. gene therapy) to endogenous gene repair. This approach will call for radically new technologies, such as nanotechnology, whose present state of development is outlined.     

Pituitary tumorigenesis and hPit-1 cells.

Despite decades of clinical data verifying the success of therapeutic approaches to human pituitary tumors, a significant number of tumors progress and can be life-threatening. The development of better therapeutic strategies for pituitary tumors is complicated by the relative scarcity of human pituitary material for basic experimentation. Human pituitary tissue was used to derive cell cultures, and a cell line, hPIT-1. Molecular and functional analyses were used to further characterize the cells as human pituitary explants in vitro. Functional analyses of the cell cultures indicated that the cells were tumorigenic and of human folliculostellate origin. hPit-1 cells revealed numerous abnormalities of ploidy. Molecular analyses indicated the absence of expression of the following pituitary hormones or hormone subunits by this culture: growth hormone, prolactin, ACTH, FSHbeta, LHbeta, THbeta, and p-glycoprotein. By contrast, the cells expressed uniformly high levels of human follistatin mRNA. Finally, the cells are moderately tumorigenic in immune-deficient mice. Although the precise molecular genetic mechanisms for tumorigenesis in the established cell culture are unknown, the cells serve as a future resource in the study of pituitary tumor initiation, progression, and response to therapy.     

Gene Therapy for Rheumatoid Arthritis

Rheumatoid arthritis (RA) was one of the earliest targets for gene therapy. Since the first clinical trial involving gene therapy in RA was initiated in 1996, eight clinical trials have been conducted assessing gene therapy in RA. Gene therapy has benefited from advances in biologics in terms of the increasing choice of novel, efficient targets to treat RA and also from the optimization of the delivery systems. Several strategies are possible; one of particular interest is local gene therapy directed to rheumatic joints, which avoids systemic vector diffusion. In this review, we discuss (i) gene therapeutic approaches that have been attempted for patients with RA, and (ii) novel strategies that are in development for delivery into patients. We analyze the advantages and disadvantages of the various approaches and how best to optimize them with regard to choosing the most promising vectors and strategies to allow for efficient, long-term, safe delivery of gene therapy in RA.     

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.     

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.     

Targeted gene therapy for ovarian cancer

Despite advances in therapy, advanced ovarian cancer maintains a dismal overall survival of 15-30%. Thus, the need for novel therapeutic modalities exists. Gene therapy represents one such approach and the purpose of this review is to present a logical rationale for the investigation of gene therapy for the treatment of ovarian cancer. The different strategies of gene therapy (molecular chemotherapy (prodrugs), mutation compensation, immunotherapy approaches, altered drug sensitivity, and virotherapy) for cancer treatment are discussed separately with attention to investigations with clinical applicability. Furthermore, the different viral vectors utilized for improvements in targeted therapy are presented. The advancements, discovery, and shortcomings are reviewed which lend itself to future directions. These future directions involve coxsackie-adenovirus receptor (CAR) independent pathways to improve infectivity and specificity to ovarian tumor cells, the potential of utilizing gene therapy as an imaging modality in detecting cancer, and incorporating the recently described technique of RNA interference. Due to the advancements in detection and targeting of ovarian cancer, coupled with the containment to the intraperitoneal cavity, gene therapy remains a promising treatment modality for ovarian cancer.     

Gene Therapy for Brain Tumors

Gene therapy has opened new doors for treatment of neoplastic diseases. This new approach seems very attractive, especially for glioblastomas, since treatment of these brain tumors has failed using conventional therapy regimens. Many different modes of gene therapy for brain tumors have been tested in culture and in vivo. Many of these approaches are based on previously established anti-neoplastic principles, like prodrug activating enzymes, inhibition of tumor neovascularization, and enhancement of the normally weak anti-tumor immune response. Delivery of genes to tumor cells has been mediated by a number of viral and synthetic vectors. The most widely used paradigm is based on the activation of ganciclovir to a cytotoxic compound by a viral enzyme, thymidine kinase, which is expressed by tumor cells, after the gene has been introduced by a retroviral vector. This paradigm has proven to be a potent therapy with minimal side effects in several rodent brain tumor models, and has proceeded to phase 1 clinical trials. In this review, current gene therapy strategies and vector systems for treatment of brain tumors will be described and discussed in light of further developments needed to make this new treatment modality clinically efficacious.     

Adenovirus-based cancer gene therapy

Over the past decade, adenovirus (Ad)-based vectors have been used extensively in the context of cancer gene therapy. Two basic strategies have been pursued for the use of Ad vectors in cancer gene therapy: 1) approaches aimed at direct tumor cell killing through delivery of replicating oncolytic viruses or non-replicating vectors encoding tumor suppressor genes, suicide genes or anti-angiogenic genes, and 2) immunotherapeutic approaches aimed at inducing host anti-tumor immune responses that can destroy tumor cells at both primary and metastatic locations. Both strategies offer the potential of selective tumor cell destruction without damage to normal tissues. Extensive pre-clinical and clinical studies have been conducted based on these strategies. Encouraging results have been obtained but robust clinical efficacy remains elusive. Several obstacles limiting the therapeutic activity of Ad vectors have been encountered, including efficiency of tumor cell transduction and inhibition of efficacy by anti-Ad host immune responses. However, expanding knowledge in the areas of Ad biology and tumor biology continues to lead to increasingly sophisticated approaches to address these issues. A review of various Ad-based cancer gene therapy approaches and recent progress in the area are presented herein.     

Effects of Medical Therapy on Pituitary Tumors

Previously surgery and irradiation were the only available procedures to treat patients with pituitary tumors. During the last few decades, novel drugs such as dopamine agonists and long-acting somatostatin analogs were developed and, an alternative medical therapy emerged. This paper summarizes the effect of medical therapy on the morphologic features of pituitary tumors and illustrates the ultrastructural alterations on electron micrographs. Currently drugs can be used in the management of pituitary tumors secreting GH, PRL, and/or TSH in excess. No medical therapy is available so far for ACTH-, FSH-, LH-, or alpha-subunit-secreting tumors as well as non-hormone-secreting pituitary tumors. Dopamine agonists are effective in the management of PRL-secreting tumors; they cause marked reversible tumor shrinkage in the substantial majority of patients. Long-acting somatostatin analogs are useful in the management of GH- and TSH-secreting pituitary tumors; they lead to mild to moderate tumor shrinkage in approximately 50% of cases. In patients treated with these drugs reduction of elevated blood hormone levels and amelioration of clinical symptoms ensue. It should be emphasized that no permanent cure is obtained. Blood hormone levels increase and the clinical symptoms reappear after discontinuation of treatment. Recently GH receptor blockers (pegvisomant) were introduced in the treatment of GH-producing pituitary adenomas. To the authors' knowledge the effect of these durgs on the morphology of pituitary tumors has not been revealed so far.     

Effects of medical therapy on pituitary tumors

Previously surgery and irradiation were the only available procedures to treat patients with pituitary tumors. During the last few decades, novel drugs such as dopamine agonists and long-acting somatostatin analogs were developed and, an alternative medical therapy emerged. This paper summarizes the effect of medical therapy on the morphologic features of pituitary tumors and illustrates the ultrastructural alterations on electron micrographs. Currently drugs can be used in the management of pituitary tumors secreting GH, PRL, and/or TSH in excess. No medical therapy is available so far for ACTH-, FSH-, LH-, or alpha-subunit-secreting tumors as well as non-hormone-secreting pituitary tumors. Dopamine agonists are effective in the management of PRL-secreting tumors; they cause marked reversible tumor shrinkage in the substantial majority of patients. Long-acting somatostatin analogs are useful in the management of GH- and TSH-secreting pituitary tumors; they lead to mild to moderate tumor shrinkage in approximately 50% of cases. In patients treated with these drugs reduction of elevated blood hormone levels and amelioration of clinical symptoms ensue. It should be emphasized that no permanent cure is obtained. Blood hormone levels increase and the clinical symptoms reappear after discontinuation of treatment. Recently GH receptor blockers (pegvisomant) were introduced in the treatment of GH-producing pituitary adenomas. To the authors' knowledge the effect of these drugs on the morphology of pituitary tumors has not been revealed so far.     

Gene therapy strategies in prostate cancer

Androgen ablation is the choice of treatment for patients with advanced prostate cancer. Although untreated tumors are mostly androgen-dependent, hormone withdrawal is only palliative. The major problem in prostate cancer treatment represents the progression to androgen-independent growth during therapy, rendering current strategies inefficient. Thus, there is an urgent need to develop novel treatments to combat therapy-resistant prostate cancer. Intensive research strongly improved the knowledge about the molecular changes, which are believed to occur during prostate carcinogenesis and progression to androgen-independence. This in turn led to the identification of several interesting genes, which may be useful as targets for prostate cancer gene therapy. In fact, there is a broad range of different gene therapy approaches in the field of prostate cancer, some of which have already progressed to clinical evaluation in patients. Promising data and best benefit for patients currently provide studies where gene therapy strategies are combined with conventional treatments like chemotherapy or radiation. In this review we will give an overview of several interesting gene therapy concepts and delivery systems in prostate cancer and discuss their usefulness in the clinic.     

Effects of Medical Therapy on Pituitary Tumors

Previously surgery and irradiation were the only available procedures to treat patients with pituitary tumors. During the last few decades, novel drugs such as dopamine agonists and long-acting somatostatin analogs were developed and, an alternative medical therapy emerged. This paper summarizes the effect of medical therapy on the morphologic features of pituitary tumors and illustrates the ultrastructural alterations on electron micrographs. Currently drugs can be used in the management of pituitary tumors secreting GH, PRL, and/or TSH in excess. No medical therapy is available so far for ACTH-, FSH-, LH-, or alpha-subunit-secreting tumors as well as non-hormone-secreting pituitary tumors. Dopamine agonists are effective in the management of PRL-secreting tumors; they cause marked reversible tumor shrinkage in the substantial majority of patients. Long-acting somatostatin analogs are useful in the management of GH- and TSH-secreting pituitary tumors; they lead to mild to moderate tumor shrinkage in approximately 50% of cases. In patients treated with these drugs reduction of elevated blood hormone levels and amelioration of clinical symptoms ensue. It should be emphasized that no permanent cure is obtained. Blood hormone levels increase and the clinical symptoms reappear after discontinuation of treatment. Recently GH receptor blockers (pegvisomant) were introduced in the treatment of GH-producing pituitary adenomas. To the authors' knowledge the effect of these durgs on the morphology of pituitary tumors has not been revealed so far.     

Current and Future Gene Therapy for Malignant Gliomas

Malignant gliomas are the most common neoplasm in the centralnervous system. When treated with conventional treatments including surgery, irradiation, and chemotherapy, the average life expectancy of the most malignant type, glioblastoma multiforme is usually less than 1 year. Therefore, gene therapy is expected to be an effective and possibly curative treatment.Many gene therapeutic approaches have demonstrated efficacy in experimental animal models. However, the current clinical trials are disappointing. This review focuses on current therapeutic genes/vectors/delivery systems/targeting strategies in order to introduce updated trends and hopefully indicate prospective gene therapy for malignant gliomas.     

New prospectives of prostate cancer gene therapy: molecular targets and animal models.

Prostate cancer is the most common cancer and the second leading cause of cancer-related death among North American men. The low cure rate for prostate cancer is associated with the fact that many patients have metastatic disease at the time of disease presentation. Currently available therapeutic modalities for prostate cancer, such as surgery, radiation, hormone therapy, and chemotherapy, have failed to cure patients because these therapies are not sufficiently tumor-specific, resulting in dose-limiting toxicity. Therefore, gene therapy may offer great promise in this regard. In this article, we summarize current advances in gene therapy technologies for the treatment of cancer in general, and future prospects for treatment of human prostate cancer metastasis. We specifically emphasize current studies for improvement, both in the efficiency and the specificity of viral and nonviral vectors, and restricted transgene expression in tumors, to achieve selective targeting with minimized host organ toxicity, based on the molecular understanding of potential regulatory differences between normal and tumor cells. Cell and animal models used to study prostate cancer growth, invasion, and metastasis, and their usefulness in preclinical evaluation of therapeutic vectors in the treatment of prostate cancer skeletal metastasis are also discussed.     

Potential of gene therapy for brain tumors

Brain tumors comprise a broad spectrum of biological and clinical entities making it unlikely for any single therapeutic approach to be universally applicable. In particular, malignant glioblastoma multiforme have defied all current therapeutic modalities. Gene therapy offers the potential to augment current neurosurgical, radiation and drug treatments with little increase in morbidity. Many therapeutic transgenes have shown efficacy in experimental models, including generation of toxic compounds, enzymatic activation of pro-drugs, expression of tumor suppressor or apoptotic proteins, inhibition of angiogenesis and enhancement of immune responses to tumor antigens. Vectors have been used as gene delivery vehicles and as cytotoxic agents in their own right by selective replication and lysis of tumor cells, thereby also generating vectors on-site. Brain tumors appear to offer some "Achilles' heels" in that they are usually contained within the brain and represent a unique dividing cell population there. However, the heterogeneous and invasive characteristics of these tumor cells, as well as sequestration of tumor antigens within a relatively immune privileged location present serious problems for effective therapy. This review will focus on current transgene/vector strategies, including novel therapeutic genes, combinational therapies and new delivery modalities, the latter of which appears to be the rate limiting factor for gene therapy of brain tumors in humans.     

Advances in gene therapy for bladder cancer

The efficacy of various currcntly available therapeutic strategies for bladder cancer is not always suffcient, especially for the advanced disease, recurrent superficial cancer, and treatment-resistant carcinoma in situ. Advances in genetic and molecular biology have led to novel approaches for cancer treatment. Gene therapy is currently one of the most promising strategies against various malignancies, and several clinical trials have been approved worldwide. Various strategies for modulating the genetic state have been applied in bladder cancer treatment, and encouraging results have been demonstrated both in vitro and in vivo. Although the therapeutic genes work dramatically when the transgenes are effectively expressed in the targeted cells, however, a sufficient rate of transduction cannot always be achieved. The most significant obstacle for clinical application of cancer gene therapy might be the method for sufficient delivery and expression of the therapeutic genes. Bladder is an easily accessible organ because of its anatomy; however, a glycosaminoglycan (GAG) layer on the bladder mucosa may protect integration of exo-delivered genetic vectors. Various strategies are applied for improving the transduction efficacy of the therapeutic genes into the bladder cancer cells. These strategies include the modification of adenoviral fibers, cotransduction of the materials for enhancing the viral infectivity, and disruption of the GAG layer. Recent advances in the field of gene therapy for bladder cancer are briefly summarized in this review.