Retrovirus-mediated gene transfer in cancer therapy

Colonisation of Clostridium in the body is restricted to hypoxic and necrotic areas of tumours

The use of gene therapy is one of the most recent molecular strategies for the treatment of cancer. It is essential, however, to have an efficient transfer system by which the desired gene can be delivered to the correct environment. The experiments described in this report investigate apathogenic Clostridium as a possible vector to transfer a specific gene product into the extracellular microenvironment of the tumour which is hypoxic/necrotic in parts, using WAG/Rij rats with transplantable rhabdomyosarcomas as a model. Our data show that Clostridium, after systemic administration of at least 10(7) spores, specifically colonises the hypoxic/necrotic areas of our tumour model, the most efficient species being C. acetobutylicum (NI-4082) and C. oncolyticum. Although spores were also detected in normal tissues for up to 4 weeks, they did not germinate in these tissues. We conclude that it seems likely that these bacteria can be used as a selective transfer system into the extracellular environment of tumours which have hypoxic regions. This strategy would be more tumour-specific than various other strategies that are currently being investigated in anti-cancer gene therapy.

Synergistic effect of bromocriptine and tumor necrosis factor-α on reversing hepatocellular carcinoma multidrug resistance in nude mouse MDR1 model of liver neoplasm

AIM: To investigate the effect of bromocriptine (BCT) and tumor necrosis factor-α (TNF-α ) on hepatocellular carcinoma (HCC) multidrug resistance (MDR) in nude mouse MDR model of liver neoplasm.

METHODS: Human hepatocarcinoma cell line HepG₂, drug resistant hepatocarcinoma cell line HepG₂/adriamycin (ADM) and hepatocarcinoma cell line transfected with TNF-α gene HepG₂/ADM/TNF were injected into the liver of nude mice via orthotopic implantation and MDR model of liver neoplasm in vivo was established (HepG₂, ADM, TNF, BCT groups). Among these groups, BCT group and TNF group were treated with BCT through gastric canal. Each group was divided into control group and chemotherapy group. Size and weight of the tumor were measured. Furthermore, tumor histological character and growth of the nude mice were observed and their chemosensitivity was tested. MDR-associated genes and proteins (MRP, LRP) of implanted tumors were detected by immunohistochemistry, reverse transcriptase polymerase chain reaction, and apoptosis rate of hepatocarcinoma cells was detected by TUNEL assay.

RESULTS: The nude mouse model of each cell line was inoculated successfully. The tumor growth rate and weight were significantly different among groups. After chemotherapy, abdominal cavity tumor growth inhibition rate was higher in BCT group (67%) compared to ADM and TNF groups, and similar to HepG₂ group (54%). MDR1 and LRPmRNA could be detected in all groups, but TNF-αwas detected only in TNF and BCT groups. Furthermore, MDR1 and LRP protein expression of tumors in TNF and BCT groups was low similar to HepG2 group. The apoptosis rate of hepatocarcinoma cells was much higher in BCT group than in other groups with TUNEL assay.

CONCLUSION: BCT and TNF-α can reverse HCC MDR in nude mouse MDR1 model of liver neoplasm.

Clinical trials with retrovirus mediated gene therapy--what have we learned?

Retrovirus (RV) has been one of the earliest recombinant vectors to be investigated in the context of cancer gene therapy. Experiments in cell culture and in animal brain tumor models have demonstrated the feasibility of RV mediated gene transduction and killing of glioma cells by toxicity generating transgenes. Phase I and II clinical studies in patients with recurrent malignant glioma have shown a favorable safety profile and some efficacy of RV mediated gene therapy. On the other hand, a prospective randomized phase III clinical study of RV gene therapy in primary malignant glioma failed to demonstrate significant extension of the progression-free or overall survival times in RV treated patients. The failure of this RV gene therapy study may be due to the low tumor cell transduction rate observed in vivo. The biological effects of the treatment may also heavily depend on the choice of transgene/prodrug system and on the vector delivery methods. Retrovirus clinical trials in malignant glioma have nevertheless produced a substantial amount of data and have contributed toward the identification of serious shortcomings of the non-replicating virus vector gene therapy strategy. Novel types of therapeutic virus vector systems are currently being designed and new clinical protocols are being created based on the lessons learned from the RV gene therapy trials in patients with malignant brain tumors.

Specific immunotherapy of cancer in elderly patients

The concept of immunotherapy of cancer is more than a century old, but only recently have molecularly defined therapeutic approaches been developed. In this review, we focus on the most promising approach, active therapeutic vaccination. The identification of tumour antigens can now be accelerated by methods allowing the amplification of gene products selectively or preferentially transcribed in the tumour. However, determining the potential immunogenicity of such gene products remains a demanding task, since major histocompatibility complex (MHC) restriction of T cells implies that for any newly defined antigen, immunogenicity will have to be defined for any individual MHC haplotype. Tumour-derived peptides eluted from MHC molecules of tumour tissue are also a promising source of antigen. Tumour antigens are mostly of weak immunogenicity, because the vast majority are tumour-associated differentiation antigens already 'seen' by the patient's immune system. Effective therapeutic vaccination will thus require adjuvant support, possibly by new approaches to immunomodulation such as bispecific antibodies or antibody-cytokine fusion proteins. Tumour-specific antigens, which could be a more potent target for immunotherapy, mostly arise by point mutations and have the disadvantage of being not only tumour-specific, but also individual-specific. Therapeutic vaccination will probably focus on defined antigens offered as protein, peptide or nucleic acid. Irrespective of the form in which the antigen is applied, emphasis will be given to the activation of dendritic cells as professional antigen presenters. Dendritic cells may be loaded in vitro with antigen, or, alternatively, initiation of an immune response may be approached in vivo by vaccination with RNA or DNA, given as such or packed into attenuated bacteria. The importance of activation of T helper cells has only recently been taken into account in cancer vaccination. Activation of cytotoxic T cells is facilitated by the provision of T helper cell-derived cytokines. T helper cell-dependent recruitment of elements of non-adaptive defence, such as leucocytes, natural killer cells and monocytes, is of particular importance when the tumour has lost MHC class I expression. Barriers to successful therapeutic vaccination include: (i) the escape mechanisms developed by tumour cells in response to immune attack; (ii) tolerance or anergy of the evoked immune response; (iii) the theoretical possibility of provoking an autoimmune reaction by vaccination against tumour-associated antigens; and (iv) the advanced age of many patients, implying reduced responsiveness of the senescent immune system.

Development of herpes simplex virus-1 amplicon-based immunotherapy for chronic lymphocytic leukemia

Herpes simplex virus (HSV)-based vectors have favorable biologic features for gene therapy of leukemia and lymphoma. These include high transduction efficiency, ability to infect postmitotic cells, and large packaging capacity. The usefulness of HSV amplicon vectors for the transduction of primary human B-cell chronic lymphocytic leukemia (CLL) was explored. Vectors were constructed encoding beta-galactosidase (LacZ), CD80 (B7.1), or CD154 (CD40L) and were packaged using either a standard helper virus (HSVlac, HSVB7.1, and HSVCD40L) or a helper virus-free method (hf-HSVlac, hf-HSVB7.1, and hf-HSVCD40L). Both helper-containing and helper-free vector stocks were studied for their ability to transduce CLL cells, up-regulate costimulatory molecules, stimulate allogeneic T-cell proliferation in a mixed lymphocyte tumor reaction, and generate autologous cytotoxic T lymphocytes (CTLs). Although helper-containing and helper-free amplicon stocks were equivalent in their ability to transduce CLL cells, a vigorous T-cell proliferative response was obtained using cells transduced with hf-HSVB7.1 but not with HSVB7.1. CLL cells transduced with either HSVCD40L or hf-HSVCD40L were compared for their ability to up-regulate resident B7.1 and to function as T-cell stimulators. Significantly enhanced B7.1 expression in response to CD40L was observed using hf-HSVCD40L but not with HSVCD40L. CLL cells transduced with hf-HSVCD40L were also more effective at stimulating T-cell proliferation than those transduced with HSVCD40L stocks and were successful in stimulating autologous CTL activity. It is concluded that HSV amplicons are efficient vectors for gene therapy of hematologic malignancies and that helper virus-free HSV amplicon preparations are better suited for immunotherapy.

Generation of high-titer retroviral vector-producing macrophages as vehicles for in vivo gene transfer

The goal of this project was to develop a novel gene transfer system based on macrophages (Mphi) as shuttles of recombinant retroviral vectors carrying therapeutic or marker genes. The murine Mphi cell line WGL5 was used as a source of Mphi for this study. We generated retrovirus-producing Mphi by transducing the WGL5 cells with a replication-defective retroviral vector carrying the enhanced green fluorescent protein (EGFP) reporter gene and the Moloney murine leukemia virus (MoMLV) as helper virus. We demonstrated stable integration of the recombinant retrovirus in the Mphi genome, efficient recombinant retrovirus production, and EGFP gene delivery to different cell lines in vitro. To evaluate Mphi-mediated EGFP gene transfer in vivo, allogeneic mice were injected s.c. with the retrovirus-producing WGL5 Mphi, that gave rise to solid tumor masses at the injection site, highly infiltrated with host leukocytes. We observed EGFP fluorescence in tumor-infiltrating CD4(+) and CD8(+) host T lymphocytes, providing direct evidence of the ability of engineered Mphi to mediate EGFP gene delivery to host cells in vivo. Moreover, we showed that retrovirus-producing Mphi could home to different organs in vivo following i.v. injection into mice. These data demonstrate that Mphi can be engineered as cellular vehicles for recombinant retroviruses carrying heterologous genes and suggest potential applications of this novel vector system for gene therapy.

Purine nucleoside phosphorylases: properties, functions, and clinical aspects

The ubiquitous purine nucleoside phosphorylases (PNPs) play a key role in the purine salvage pathway, and PNP deficiency in humans leads to an impairment of T-cell function, usually with no apparent effects on B-cell function. This review updates the properties of the enzymes from eukaryotes and a wide range of prokaryotes, including a tentative classification of the enzymes from various sources, based on three-dimensional structures in the solid state, subunit composition, amino acid sequences, and substrate specificities. Attention is drawn to the compelling need of quantitative experimental data on subunit composition in solution, binding constants, and stoichiometry of binding; order of ligand binding and release; and its possible relevance to the complex kinetics exhibited with some substrates. Mutations responsible for PNP deficiency are described, as well as clinical methods, including gene therapy, for corrections of this usually fatal disease. Substrate discrimination between enzymes from different sources is also being profited from for development of tumour-directed gene therapy. Detailed accounts are presented of design of potent inhibitors, largely nucleosides and acyclonucleosides, their phosphates and phosphonates, particularly of the human erythrocyte enzyme, some with Ki values in nanomolar and picomolar range, intended for induction of the immunodeficient state for clinical applications, such as prevention of host-versus-graft response in organ transplantations. Methods of assay of PNP activity are reviewed. Also described are applications of PNP from various sources as tools for the enzymatic synthesis of otherwise inaccessible therapeutic nucleoside analogues, as coupling enzymes for assays of orthophosphate in biological systems in the micromolar and submicromolar ranges, and for coupled assays of other enzyme systems.

Gene therapy of brain tumors: problems presented by physiological barriers

The explosion of molecular techniques for gene discovery and their application to a variety of diseases has uncovered numerous gene abnormalities that can result in disease. These discoveries have provided the needed understanding and genetic materials to apply gene therapy approaches in the treatment of several diseases, including those of the central nervous system. A variety of different anticancer complementary DNAs (cDNA) have been shown to possess biological efficacy when used in the appropriate experimental setting. However, efficient and effective delivery of these cDNAs remains a major obstacle for future clinical applications. The focus of this review will be to describe the obstacles that impede the process of gene therapy and oncolytic viral therapy of brain tumors and to describe how important new discoveries derived from other disciplines are being used to address problems encountered in the gene/ viral therapy of this disease.

Therapeutic genes for cancer gene therapy

Cancer still represents a disease of high incidence and is therefore one major target for gene therapy approaches. Gene therapy for cancer implies that ideally selective tumor cell killing or inhibition of tumor cell growth can be achieved using nucleic acids (DNA and RNA) as the therapeutic agent. Therefore, the majority of cancer gene therapy strategies introduce foreign genes into tumor cells which aim at the immunological recognition and destruction, the direct killing of the target cells or the interference with tumor growth. To achieve this goal for gene therapy of cancer, a broad variety of therapeutic genes are currently under investigation in preclinical and in clinical studies. These genes are of very different origin and of different mechanisms of action, such as human cytokine genes, genes coding for immunostimulatory molecules/antigens, genes encoding bacterial or viral prodrug-activating enzymes (suicide genes), tumor suppressor genes, or multidrug resistance genes.

Cancer therapy: new concepts on active immunization

There is increasing evidence that tumors express putative target molecules for a therapeutic immune reaction. Yet, tumor cells lack the prerequisites for appropriate antigen presentation and--hence--the immune system does not respond. This difficulty can probably be circumvented when tumor antigens are processed by conventional antigen presenting cells. Thus, the identification of immunogenic tumor-associated antigens may allow new modes of vaccination with the hope of adding a fourth and hopefully powerful weapon to surgery, radiation and chemotherapy in the fight against cancer.

Protection of hematopoietic stem cells from chemotherapy-induced toxicity by multidrug-resistance 1 gene transfer

An increased chemotherapeutic dose intensity is believed to translate into higher survival rates among cancer patients. Pancytopenia is the dose-limiting toxic result of most anticancer agents. Overexpression of the human multidrug resistance 1 (MDR1) gene in transgenic animals resulted in complete myeloprotection against high doses of cytostatic drugs. Stem cell research, vector development, and experimental pharmacology are uniting their efforts in an attempt to achieve a similar effect in human hematopoietic stem cells. This article gives an overview of the crucial steps involved, from retroviral vector design and optimization of viral titers to vector uptake, gene integration, and expression. The authors' own results are presented with special regard in vitro and in vivo assays for the detection of hematopoietic stem cell transduction.

Psi- vectors: murine leukemia virus-based self-inactivating and self-activating retroviral vectors

We have developed murine leukemia virus (MLV)-based self-inactivating and self-activating vectors to show that the previously demonstrated high-frequency direct repeat deletions are not unique to spleen necrosis virus (SNV) or the neomycin drug resistance gene. Retroviral vectors pKD-HTTK and pKD-HTpTK containing direct repeats composed of segments of the herpes simplex virus type 1 thymidine kinase (HTK) gene were constructed; in pKD-HTpTK, the direct repeat flanked the MLV packaging signal. The generation of hypoxanthine-aminopterin-thymidine-resistant colonies after one cycle of retroviral replication demonstrated functional reconstitution of the HTK gene. Quantitative Southern analysis indicated that direct repeat deletions occurred in 57 and 91% of the KD-HTTK and KD-HTpTK proviruses, respectively. These results demonstrate that (i) deletion of direct repeats occurs at similar high frequencies in SNV and MLV vectors, (ii) MLV psi can be efficiently deleted by using direct repeats, (iii) suicide genes can be functionally reconstituted during reverse transcription, and (iv) the psi region may be a hot spot for reverse transcriptase template switching events.

Gene therapy for neurologic disease: benchtop discoveries to bedside applications. 1. The bench

The overall goal of this review is to provide the pediatric neurologist with a theoretical foundation in gene therapy. Gene therapy became feasible in the early 1970s and the first transfer of a foreign gene into humans was approved by the NIH in 1989. Adenovirus, adeno-associated virus, herpes-simplex virus, retroviruses, and other vectors have been used to efficiently transduce genes into cells in vitro and in vivo. We discuss laboratory experiments that have provided a strong scientific rationale for implementing human clinical trials of gene therapy for neurologic malignancy. The development of viral and nonviral vectors that mediate efficient gene insertion into human cells has created the prospect of using gene therapy for cancer or brain disease. The NIH has approved more than 100 gene therapy protocols since 1989. However, the field will require more research on gene delivery systems before gene therapy becomes an established therapeutic strategy for an array of central nervous system diseases.

Cytokine gene-modified vaccines in the therapy of cancer

Therapeutic strategies based on the insertion of cytokine genes into the genome of tumour cells, followed by vaccination with the resulting genetically modified, cytokine-producing cells, represent a new potential prospect for treatment of cancer patients. In this review, the concept of cytokine gene-modified cancer vaccines is discussed; the discussion is focused on the rationale, characterization, progress in the development, preclinical testing, and first clinical trials. An effort is made to analyse and integrate the results obtained in different experimental model systems in order to determine the needed approaches and directions for further research.

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.