Cancer gene therapy

Developments in molecular genetics, immunology, molecular and cellular biology, and tumor biology have given rise to the field of cancer gene therapy. Several gene delivery vehicles have been developed and are being examined in clinical trials. Most cancer gene therapy strategies involve introduction of genes to augment existing therapies. An overview is provided on gene delivery vehicles, gene therapy strategies, and cancer gene therapy clinical trials.     

Noninfectious gene transfer and expression systems for cancer gene therapy

Gene therapy provides a significant opportunity to devise novel strategies for the control or cure of cancer. Success of this modality will ultimately depend on the ability to express a therapeutic gene of interest at high levels, and specific gene delivery to targeted tumor cells will minimize toxicities. Although current gene therapy trials typically use viral-based, infectious vectors to express suitable target genes in human cancer cells, these vectors have significant limitations in their expression characteristics, lack of specificity in targeting tumor cells for gene transfer, and safety concerns regarding induction of secondary malignancies and recombination to form replication-competent virus. These limitations have refocused efforts to develop noninfectious gene transfer technologies for in vivo gene delivery of plasmid-based expression vectors. This article reviews recent developments in non-infectious gene transfer techniques, including liposome and receptor-mediated methods, which can efficiently deliver plasmid vectors into tumor cells in vivo. Additionally, strategies are reviewed for efficiently expressing target genes in tumor cells, including use of tissue-specific promoters, inducible promoters, and replication-control sequences to regulate extrachromosomal amplification of vector DNA in human tumor cells. Optimal coupling of these noninfectious gene transfer and expression technologies have the potential to yield safe and effective gene therapies for patients with cancer.     

Adjuvant medical therapy for colorectal cancer

In the mid-1980s, trials of adjuvant therapy for colon cancer in the United States had a "no treatment" arm, which reflected the belief that effective adjuvant chemotherapy did not exist for patients with surgically resected disease at high risk for recurrence. However, with the observation in the early 1990s that postsurgical adjuvant 5-FU plus levamisole reduced tumor recurrence and ultimately increased overall survival in stage III colon cancer, the potential of effective adjuvant chemotherapy was realized. Questions about the duration of adjuvant chemotherapy, the specifics of chemotherapy schedule/drug selection, and its use in stage II colon cancer are beginning to be clarified in large, randomized adjuvant therapy trials. In rectal carcinomas, combined modality postoperative pelvic irradiation plus chemotherapy for stage II and III disease has been shown to reduce both local and systemic recurrences and to prolong survival compared with that in patients treated with local surgery and radiation. Again, large randomized trials are attempting to clarify both the optimal chemotherapeutic agents and schedules to be used and also whether preoperative combined modality therapy can improve the resectability rate, rate of sphincter preservation, and survival. Future trials will examine new agents shown to be effective in advanced disease as well as monoclonal antibodies, such as MoAb 17-1A, that may have selective activity in minimal disease. Improvement in overall survival remains the ultimate endpoint of future adjuvant therapy trials; however, trials will also critically examine toxicity, quality of life, pharmacoeconomics, and genetic and biologic correlates that may help select more appropriate candidates for adjuvant therapies.     

Targeting gene therapy to cancer: a review

In recent years the idea of using gene therapy as a modality in the treatment of diseases other than genetically inherited, monogenic disorders has taken root. This is particularly obvious in the field of oncology where currently more than 100 clinical trials have been approved worldwide. This report will summarize some of the exciting progress that has recently been made with respect to both targeting the delivery of potentially therapeutic genes to tumor sites and regulating their expression within the tumor microenvironment. In order to specifically target malignant cells while at the same time sparing normal tissue, cancer gene therapy will need to combine highly selective gene delivery with highly specific gene expression, specific gene product activity, and, possibly, specific drug activation. Although the efficient delivery of DNA to tumor sites remains a formidable task, progress has been made in recent years using both viral (retrovirus, adenovirus, adeno-associated virus) and nonviral (liposomes, gene gun, injection) methods. In this report emphasis will be placed on targeted rather than high-efficiency delivery, although those would need to be combined in the future for effective therapy. To date delivery has been targeted to tumor-specific and tissue-specific antigens, such as epithelial growth factor receptor, c-kit receptor, and folate receptor, and these will be described in some detail. To increase specificity and safety of gene therapy further, the expression of the therapeutic gene needs to be tightly controlled within the target tissue. Targeted gene expression has been analyzed using tissue-specific promoters (breast-, prostate-, and melanoma-specific promoters) and disease-specific promoters (carcinoembryonic antigen, HER-2/neu, Myc-Max response elements, DF3/MUC). Alternatively, expression could be regulated externally with the use of radiation-induced promoters or tetracycline-responsive elements. Another novel possibility that will be discussed is the regulation of therapeutic gene products by tumor-specific gene splicing. Gene expression could also be targeted at conditions specific to the tumor microenvironment, such as glucose deprivation and hypoxia. We have concentrated on hypoxia-targeted gene expression and this report will discuss our progress in detail. Chronic hypoxia occurs in tissue that is more than 100-200 microns away from a functional blood supply. In solid tumors hypoxia is widespread both because cancer cells are more prolific than the invading endothelial cells that make up the blood vessels and because the newly formed blood supply is disorganized. Measurements of oxygen partial pressure in patients' tumors showed a high percentage of severe hypoxia readings (less than 2.5 mmHg), readings not seen in normal tissue. This is a major problem in the treatment of cancer, because hypoxic cells are resistant to radiotherapy and often to chemotherapy. However, severe hypoxia is also a physiological condition specific to tumors, which makes it a potentially exploitable target. We have utilized hypoxia response elements (HRE) derived from the oxygen-regulated phosphoglycerate kinase gene to control gene expression in human tumor cells in vitro and in experimental tumors. The list of genes that have been considered for use in the treatment of cancer is extensive. It includes cytokines and costimulatory cell surface molecules intended to induce an effective systemic immune response against tumor antigens that would not otherwise develop. Other inventive strategies include the use of internally expressed antibodies to target oncogenic proteins (intrabodies) and the use of antisense technology (antisense oligonucleotides, antigenes, and ribozymes). This report will concentrate more on novel genes encoding prodrug activating enzymes, so-called suicide genes (Herpes simplex virus thymidine kinase, Escherichia coli nitroreductase, E. (ABSTRACT TRUNCATED)     

Cancer gene therapy: connecting basic research with clinical inquiry

Molecular genetics has spawned an impressive outpouring of insights into the biology of neoplastic transformation and the host-tumor relationship. This deeper understanding of cancer pathogenesis presents a rich opportunity to develop novel therapeutic agents with improved selectivity for cancer cells. One promising approach involves gene therapy, which is the introduction of genetic material into a patient's tissues with the intent to achieve therapeutic benefit. A number of gene transfer systems have been designed that enable the genetic modification of relevant target cells, albeit with varying strengths and limitations. Several strategies to exploit gene transfer as a tool to target specific molecular defects intrinsic to cancer cells, enhance tumor chemosensitivity, and augment tumor immunogenicity are under intensive investigation. A number of these approaches have entered initial clinical testing and already provide intriguing new information about the biology of cancer in patients. In this review, I will highlight the critical issues and controversies that underscore preclinical experiments in cancer gene therapy, discuss some of the preliminary findings from the first wave of clinical trials, and speculate about the prospects that cancer gene therapy will change the way that cancer medicine is practiced.     

Clinical research designs for emerging treatments for Alzheimer disease: moving beyond placebo-controlled trials

The design of clinical trials must evolve as new therapies become available. The demonstrated efficacy and clinical use of donepezil and vitamin E for Alzheimer disease (AD) has shifted the options for AD research design. There is now a compelling case for alternatives to trials that include a treatment arm with no active therapy (ie, a placebo control). With an existing therapy, such as donepezil or vitamin E, new drugs that are clearly superior to those drugs should be sought. Combination therapy is a likely strategy for the future, implying that clinical trials, if possible, should replicate actual practice. The long duration of future AD trials also will make placebo-controlled trials more difficult to justify and more difficult to recruit for. Add-on or active-control designs represent the alternative approaches. We believe that definitive clinical trials of new AD drugs that use one or the other of these designs would be more likely to bring about therapeutic advances than would comparisons with inactive treatments. Our argument is not a general rejection of placebo-control designs. Our recommendations apply only to the circumstances in which the field of AD drug therapy now finds itself.     

From genes to gene medicines: recent advances in nonviral gene delivery

Over the last decade, research in somatic gene therapy has focused on selected approaches to deliver therapeutic genes to cells both ex vivo and in vivo. While most current gene therapy clinical trials are based on cell- and viral-mediated approaches, nonviral gene medicines are emerging as potentially safe and effective in the treatment of a wide variety of genetic and acquired diseases. Nonviral technologies consist of plasmid-based expression systems containing a gene encoding a therapeutic protein and synthetic gene delivery systems. In addition to the therapeutic gene, plasmid-based expression systems contain other genetic sequences to control the in vivo production and secretion of a protein. They may include elements that prolong extrachromosomal gene expression, cell-specific promoters and, optionally, gene switches for enabling drug-regulated gene therapy. Unique gene delivery systems will be required depending upon the biology and (patho)physiology of the target tissue. This review provides a critical view of gene therapy with a major focus on advanced nonviral technologies to control the in vivo location and function of administered genes.     

Identification of proteins that interact with a protein of interest: applications of the yeast two-hybrid system

Colorectal cancer remains a major health problem. Few therapies are effective apart from surgery, and survival has increased little in recent years. This is despite the fact that screening by colonoscopy can potentially remove nearly all colorectal tumours before they become malignant. Molecular genetics has identified some inherited mutations (such as at APC and the mismatch repair loci) that predispose to colon cancer and some somatic mutations (such as at APC and p53) that cause sporadic colon tumours. We review the likely role of these and other genes in colorectal tumorigenesis. We also highlight areas of relative ignorance in colon cancer and emphasise that many important genes, especially those that cause invasion and metastasis, remain to be identified. Colorectal cancer is, however, a well characterised tumour, as regards both its natural history and its histopathology; there are consequently good prospects for advances in colon cancer genetics, with probable benefits for its treatment. We anticipate: (a) that new genes predisposing to colon tumours, including those conferring relatively minor risks, will be characterised; (b) genes and proteins important in invasion and metastasis will be identified; (c) the network of protein interactions in which molecules such as APC are involved will be elucidated; (d) large-scale studies of somatic mutations in tumours will provide accurate predictions of prognosis and suggest optimal therapeutic regimens; and (e) new potential targets for therapy will be identified. Whilst molecular genetics is by no means sufficient for progress in preventing and treating colon cancer, it is a necessary and central part of such advances.     

Peripheral nerve revascularization: a current literature review

Gene therapy of human cancer is likely to be most effective when it is directed at targets that are expressed in cancer cells but are lacking from other cells. Human papillomaviruses can provide such targets, since these viruses are present in many cervical and oral cancers, and are likely to be etiological agents of the tumor. Continued expression of human papillomavirus genes is probably necessary for the growth of these cancers, and effective gene therapy could consist of antisense or ribozyme molecules directed against these genes. Some human papillomavirus gene products are antigenic, and immunotherapy based upon these antigens might prove clinically beneficial. Human papillomaviruses have specific promoters, are linked to toxin genes, the toxin may be selectively expressed by tumor cells where the virus genes are active. Thus, there are several approaches for the development of specific gene therapy for human cancers that contain human papillomaviruses.     

Anaerobic bacteria as a gene delivery system that is controlled by the tumor microenvironment

A fundamental obstacle in gene therapy for cancer treatment is the specific delivery of an anticancer gene product to a solid tumor. Although several strategies exist to control gene expression once a vector is directly introduced into a tumor, as yet no systemic delivery system exists that specifically targets solid tumors. Nonpathogenic, obligate anaerobic bacteria of the genus Clostridium have been used experimentally as anticancer agents because of their selective growth in the hypoxic regions of solid tumors after systemic application. In this report we further describe a novel approach to cancer gene therapy in which genetically engineered clostridia are used as tumor-specific vectors for the delivery of antitumor genes. We have introduced into a strain of C. beijerinckii the gene for an E. coli nitroreductase known to activate the nontoxic prodrug CB 1954 to a toxic anticancer drug. Nitroreductase produced by these clostridia enhanced the killing of tumor cells in vitro by CB 1954, by a factor of 22. To demonstrate the specificity of this approach for tumor targeting, we intravenously injected the inactive spore form of C. beijerinckii, which upon transition to a reproductive state will express the E. coli nitroreductase gene. Nitroreductase activity was detectable in 10 of 10 tumors during the first 5 days after intravenous injection of inactive clostridial spores, indicating a rapid transition from spore to reproductive state. Tumors harboring clostridial spores which did not possess the E. coli nitroreductase gene were devoid of nitroreductase activity. Most importantly, E. coli nitroreductase protein was not found in a large survey of normal mouse tissues following intravenous injection of nitroreductase containing clostridia, strongly suggesting that obligate anaerobic bacteria such as clostridia can be utilized as highly specific gene delivery vectors for cancer therapy.     

Vitamin therapy in cystic fibrosis--a review and rationale

An emphasis on careful surgical staging of adenocarcinoma of the colon has improved the predictive value of tumor staging systems. As a result of improved staging and carefully conducted randomized clinical trials, adjuvant therapy of locally advanced colon cancer, based on 5-fluorouracil chemotherapy, has been proven to substantially reduce recurrence rates and significantly increase overall survival for selected patients. Improved treatments and schedules are currently being studied in randomized trials and may increase the efficacy of this adjuvant therapy. Radiation therapy has not as yet been integrated into the adjuvant treatment of colon carcinoma. The application of a combined approach of surgery and chemotherapy in selected patients with liver metastases may also improve cure rates and long-term survival. The developing understanding of molecular determinants for the biological behavior of these cancers will increase the opportunities to identify, on the one hand, those patients who will benefit from specific therapies, and, on the other hand, new therapeutic strategies and treatments.     

Gene therapy: ovarian carcinoma as the paradigm

The delineation of the molecular basis of cancer in general, and of ovarian carcinoma in particular, allows for the possibility of specific intervention at the molecular level for therapeutic purposes. To this end, three main approaches have been developed: mutation compensation, molecular chemotherapy, and genetic immunopotentiation. For each of these conceptual approaches, human clinical protocols, including those specific for ovarian carcinoma, have entered phase I clinical trials to assess dose escalation, safety, and toxicity issues. However, major problems remain to be solved before these approaches can become effective and commonplace strategies for the treatment of cancer. In this regard, an examination of the applications of gene therapy for ovarian carcinoma can exemplify the rationality and the problems observed in the development of gene therapy, and may illustrate prospects for their solution that are being refined, including current efforts in our laboratory. An overriding obstacle is the basic ability to deliver therapeutic genes quantitatively, and specifically, into tumor cells. As vector technology fulfills these requirements, it is anticipated that promising results already observed in preclinical studies will translate quickly into the clinical setting for amelioration of this life-threatening disease in women.     

Gene therapy for lung cancer

Gene therapy has received considerable attention and some speculation as to its value. Although few patients have been treated, the preliminary results of the phase I lung cancer gene therapy clinical trials are very promising. Clinically relevant basic research in the molecular pathogenesis and immunology of lung cancer is progressing. As improved vector technologies are developed, new opportunities will be available to initiate lung cancer gene therapy trials that are based on a more detailed understanding of lung cancer biology. In conclusion, although important biologic and technical questions remain unanswered, recent research suggests that gene therapy will have a profound impact on lung cancer treatment.     

Lung cancer: therapeutic modalities and cytoprotection

Lung cancer is the leading cancer killer in the United States today. The current methods of treatment, radiation and various chemotherapies, have been used with some success; however, early detection remains the key to successful therapy. Current clinical trials indicate that an improvement of available therapies is needed. Consequently, the development of new approaches to treatment is foremost in the minds of researchers. Advances in molecular medicine have produced new drugs that can protect normal cells from chemotherapy-induced toxicities, resulting in enhanced drug delivery with few dose reductions. This review will discuss some of the advances that have been made in understanding the molecular biology of lung cancer as well as the current and most promising methods of treatment used for various forms of lung cancer. The potential contribution of cytoprotectors to enhance the safety and effectiveness of these therapies will be examined.     

Cancer vaccines. Part 2

Cancer vaccines are being widely studied for the purpose of immune modulation and subsequent antitumor effects. This article cites only a few examples of the many studies underway. Many vaccines have shown efficacy in eliciting systemic responses with minimal toxicities. The use of vaccines as a modality of cancer therapy in combination with chemotherapy, surgery, and radiation therapy is also being investigated. Although the routine use of approved vaccines is still a goal for the future, instituting this fourth modality of cancer therapy is not too distant. Phase III trials with both melanoma and colon cancer vaccines have been completed. Synthetic carbohydrate antigen vaccines have shown efficacy in several tumor types during Phase II trials and are also generating enthusiasm. The potential impact of vaccine therapy on the profession of pharmacy may involve patient counseling regarding management of side effects and possibly also dispensing of vaccine therapy products to patients directly for home administration. As ambulatory sites open in conjunction with pharmacy services and pharmacists obtain prescribing authority, pharmacists' active involvement in vaccine counseling and administration seems likely.     

Association analysis in an evolutionary context: cladistic analysis of the DRD2 locus to test for association with alcoholism

Recent clinical trials of adjuvant therapy for early stage breast cancer support two general observations. First, overall survival is not impacted by the extent of surgery. Low rates of axillary relapse in patients treated with total mastectomy alone combined with the availability of systemic therapy as a substitute for surgical control of the axilla mean that patients can often be spared the morbidity of axillary node dissection. In problematic cases, newer diagnostic approaches, such as sentinel node biopsy, can help in making appropriate treatment decisions. Second, systemic therapy can reduce the clinical manifestations of disease. The incorporation of more sophisticated approaches to predicting outcomes, to varying timing and dose of treatment, and to developing new modalities of treatment, including immunotherapy, will contribute to a general strategy aimed at reducing the tumor to a harmless parasite. These observations support a paradigm shift in our definition of 'adjuvant'. Rather than referring to the use of systemic therapy after the patient's known disease has been surgically removed, adjuvant therapy would be re-defined to refer to local therapy used to eradicate any residual tumor remaining after systemic therapy has been completed.     

Colon carcinogenesis models for chemoprevention studies

Every animal model has its strengths and weaknesses; however, some of these models have proven useful in evaluating hereditary factors, whereas other models were found to be useful in understanding the relationship between nutritional factors and colon cancer. We believe that the results obtained by these models, both genetic based and chemically induced, will contribute to the understanding of genetic and nutritional factors as they relate to colon carcinogenesis.     

Optimal techniques for arterial gene transfer

Cardiovascular gene therapy is becoming a clinical reality due to improved vectors, delivery systems and careful experimental validation studies. Nearly all cardiovascular diseases are amenable to gene therapy, but the optimal combination of vector, delivery system and therapeutic gene is likely to be unique to each application. Currently, the most efficient vectors available are replication-defective adenoviral vectors, but transgene expression is limited in time due to a strong immune response. Conversely, non-viral vectors or plasmid DNA may be used safely but have very limited efficiency. Percutaneous, catheter-based delivery is feasible for most applications. The ultimate issues that will decide of the future of gene therapy are safety of the transfer and delivery techniques as well as cost/effectiveness comparisons with alternative therapies, including local delivery of drugs, proteins and/or mechanical devices.     

Gene therapy: a primer for radiologists

Gene therapy is one of the most rapidly evolving areas in medicine. Radiologists should have an understanding of basic techniques used to identify and clone a gene and insert it into a vector capable of directing expression in mammalian tissues. DNA delivery systems include retroviral vectors (RNA viruses), adenoviral vectors (DNA viruses), and cationic liposomes, along with strategies that involve ultrasound-directed gene transfer, computed tomography-guided gene transfer, and transcatheter gene delivery, in particular via the hepatic artery. Genes being evaluated in preclinical and clinical trials include oncogenes, antioncogenes (tumor suppressor genes), suicide genes, conventional antimetabolites, antiangiogenesis factors, secreted immunostimulatory cytokines such as interleukins and interferons, and immunomodulatory cell surface proteins, including foreign HLA proteins and costimulatory molecules. A foundation in molecular biology is needed for the practicing radiologist interested in but unfamiliar with current gene therapy terminology and experimental strategies. Such a foundation will encourage the dissemination of basic biologic, diagnostic imaging, and interventional oncoradiologic developments and should facilitate integration of the radiologist into the gene therapy team.